Adsorption of organic molecules on silica surface

Advances in Colloid and Interface Science 121 (2006) 77 – 110 www.elsevier.com/locate/cis

Adsorption of organic molecules on silica surface☆ Sudam K. Parida a , Sukalyan Dash b , Sabita Patel a , B.K. Mishra a,⁎ a

Centre of Studies in Surface Science and Technology, Department of Chemistry, Jyoti Vihar, 768 019, Orissa, India b Department of Chemistry, University College of Engineering, Burla, 768 018, Orissa, India Available online 31 July 2006

Abstract The adsorption behaviour of various organic adsorbates on silica surface is reviewed. Most of the structural information on silica is obtained from IR spectral data and from the characteristics of water present at the silica surface. Silica surface is generally embedded with hydroxy groups and ethereal linkages, and hence considered to have a negative charged surface prone to adsorption of electron deficient species. Adsorption isotherms of the adsorbates delineate the nature of binding of the adsorbate with silica. Aromatic compounds are found to involve the pi-cloud in hydrogen bonding with silanol OH group during adsorption. Cationic and nonionic surfactants adsorb on silica surface involving hydrogen bonding. Sometimes, a polar part of the surfactants also contributes to the adsorption process. Styryl pyridinium dyes are found to anchor on silica surface in flat-on position. On modification of the silica by treating with alkali, the adsorption behaviour of cationic surfactant or polyethylene glycol changes due to change in the characteristics of silica or modified silica surface. In case of PEG-modified silica, adsolubilization of the adsorbate is observed. By using a modified adsorption equation, hemimicellization is proposed for these dyes. Adsorptions of some natural macromolecules like proteins and nucleic acids are investigated to study the hydrophobic and hydrophilic binding sites of silica. Artificial macromolecules like synthetic polymers are found to be adsorbed on silica surface due to the interaction of the multifunctional groups of the polymers with silanols. Preferential adsorption of polar adsorbates is observed in case of adsorbate mixtures. When surfactant mixtures are considered to study competitive adsorption on silica surface, critical micelle concentration of individual surfactant also contributes to the adsorption isotherm. The structural study of adsorbed surface and the thermodynamics of adsorption are given some importance in this review. © 2006 Elsevier B.V. All rights reserved. Keywords: Adsorption; Silica; Surfactants; Polymers; Dyes; Hemimicelle; Biomolecules

Abbreviations: Ac, Area of coverage; AFM, Atomic force microscopy; BDDAB, Benzenedimethyldodecylammonium bromide; BSA, Bovine serum albumin; BTAB, Benzyltrimethylammonium bromide; BTMAB, Benzyltrimethylammonium bromide; C(12)E(9), Polydisperse nonyl ethylene glycol n-dodecyl ether; C(12)G (2), Dodecyl maltoside; C(14)E(6), Hexakis(ethylene glycol) mono-n-tetradecyl ether; CMC, Critical micellar concentration; CPAM, Cationic polyacrylamide; CS, Pyrocarbon silica; CSAC, Critical surface aggregation concentration; CTAB, Cetyltrimethylammonium bromide; CTAC, Cetyltrimethylammonium chloride; DDAO, Dodecyl dimethylamine oxide; DEFUMAC, Diethanol heptadecafluoro-2-undecanol methyl ammonium chloride; DHP, Dihexadecyl phosphate; DLVO, Derjaguin– Landen–Verwey–Overbeek; DME, 1,2-dimethoxy ethane; DMPI, dibutylaminostyryl-1-methylpyridinium iodide; DNA, Deoxyribonucleic acid; DODAB, Dioctadecyldimethylammonium bromide; DP, dibutylaminostyryl-1-(3-sulfopropyl) pyridinium; DPB, Dodecylpyridinium bromide; DPPC, Dipalmitoyl phosphatidyl choline; DSC, Differential scanning calorimetry; DTAB, Dodecyltrimethylammonium bromide; DTAC, Dodecyltrimethylammonium chloride; EFL, Evanescent-field layer; EHEC, Ethyl (hydroxy ethyl) cellulose; EO, Ethylene oxide; EVA, Ethylene vinyl acetate; 19F NMR, Fluorine-19 Nuclear magnetic resonance; FC, Fluorocarbon; FeA, Ferichrome-A; FTICR, Fourier transform ion cyclotron resonance; FTIR, Fourier transform infra red; FT-NMR, Fourier transform nuclear magnetic resonance; 1H NMR, Proton Nuclear magnetic resonance; HC, Hydrocarbon; HDPYC, Hexadecylpyridinium chloride; HDW, High density water; HEC, Hydroxyethylcellulose; HMC, Hemimicellization concentration; ICAFM, Intermittent contact atomic force microscopy; IgG, Immunoglobulin; IR, Infra red; LDW, Low density water; NaPA, Sodium polyacrylate; NaPSS, Sodium polystyrene sulfonate; NMR, Nuclear magnetic resonance; NOESY, Nuclear overhauser effect spectroscopy; NP, Nonyl phenol; PAA, Poly acrylic acid; PC, Phosphatidyl choline; PCS, Photon correlation spectroscopy; PEG, Polyethylene glycol; PEO, Polyethylene oxide; pI, Isoelectric point; PMMA, Polymethyl methacrylate; PO, Propylene oxide; Ppy, Polypyrrole; PS, Polystyrene; PVA, Poly vinyl alcohol; PVAC, Poly vinyl acetate; PVI, Poly vinyl imidazole; PVP, Poly vinyl pyrrolidone; PVPP, Poly 4-vinyl N-propyl pyridinium bromide; RF, Riboflavin; Rnase, Ribonuclease A; RPE, R-Phycoerythrin; SDBS, Sodium dodecylbenzene sulphonate; SDS, Sodium dodecyl sulphate; SFG, Sum frequency generation; TAAB, Tatraalkylammonium bromide; TIRF, Total internal reflection fluorescence; 2,4,6-TMP, 2,4,6-trimethyl pyridine; TSDC, Thermally stimulated depolarization current; TTAB, Tetradecyltrimethylammonium bromide; TX100, Triton-X-100; TX305, Triton-X-305; UV–Vis–NIR, Ultraviolet–Visible–Near infra red; XRD, X-Ray diffraction. ☆ Dedicated to Prof. G.B. Behera on his 66th birthday. ⁎ Coresponding author. E-mail address: [email protected] (B.K. Mishra). 0001-8686/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2006.05.028

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Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Interface characteristics of silica. . . . . . . . . . . . . . . . . 2.1. IR Spectral characteristics of silica surface. . . . . . . . 2.2. Characteristics of water on silica surface. . . . . . . . . 3. Adsorption isotherm . . . . . . . . . . . . . . . . . . . . . . . 3.1. Adsorption of small organic molecules on silica surface . 3.2. Adsorption of dyes on silica surface . . . . . . . . . . . 3.3. Adsorption of surfactants on silica surface . . . . . . . . 3.4. Adsorption of polymers on silica surface . . . . . . . . 3.5. Adsorption of biomolecules on silica surface . . . . . . 3.6. Competitive adsorptions . . . . . . . . . . . . . . . . . 3.6.1. Competition among polymers . . . . . . . . . . 3.6.2. Competition between polymer and surfactant . . 3.6.3. Competition among surfactants . . . . . . . . . 4. Thermodynamics of adsorption . . . . . . . . . . . . . . . . . 5. Adsorbate–adsorbent interactions . . . . . . . . . . . . . . . . 6. Future prospective . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The change in the concentration of a component in the surface layer of the adsorbent in comparison with the bulk phase with respect to unit surface area is termed as adsorption. It involves selective uptake of the adsorbate from the bulk phase onto a phase provided by the adsorbent. In case of adsorption on solid from solution, solute molecules are partitioned from the solution to the adsorbent leading to the change in the concentration of the solution as well as at the site of adsorption. The adsorption phenomenon depends on the interaction between the surface of the adsorbent and the adsorbed species. The interaction may be due to: (1) (2) (3) (4)

chemical bonding, hydrogen bonding, hydrophobic bonding, van der Waals force.

The most important features involved during the investigation of adsorption phenomenon are (i) interface characteristics, (ii) adsorption isotherm, (iii) thermodynamics of adsorption and (iv) the adsorbate–adsorbent interactions. These points are discussed item-wise in the review. Till date a large number of papers have already been published in the area of adsorption of various materials on liquid and solid surfaces. In the present review emphasis is given to the adsorption of a wide class of adsorbates from their solution onto silica surface. The adsorbates mostly are capable of forming hydrogen bonds as donor or acceptor. These are presented according to the complexity of the molecules. 2. Interface characteristics of silica The interface involving silica surface plays an important role in the adsorption process. The surface characteristics of

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78 78 79 79 81 82 84 88 91 93 98 98 99 100 101 103 106 106 106

the adsorbate determine the nature of bonding between adsorbate and adsorbent. Different techniques are in use to characterize the surface of the adsorbents. To investigate the behaviour of porous silica coated with siloxane polymers, Gilpin et al. [1] used nitrogen adsorption technique for surface area and porosity of silica and polymer coated silica; and high-resolution thermo gravimetric technique for thermal behaviour of silica. They found a uniform layer of siloxane polymer on silica surface. On thermal treatment the hydrophilic surface gradually changes to hydrophobic surface by irreversible elimination of a pair of adjacent hydroxyls [2]. The transition temperature depends on the origin and morphology of the sample. In crystalline materials the hydrophobic patches are more stable than in amorphous. Quartz remains as hydrophilic after degassing at 1073 K whereas, pyrogenic silica (Aerosil) can be converted to hydrophobic upon degassing at T b 673 K. The tendency of a given solid to be covered at the surface by a hydrated layer depends on the nature of the solid. Highly hydrated ionic solids easily dissociate water at room temperature restoring completely the hydroxylated surface layer. In contrast, the covalent solids like graphitized carbon interact weakly with water molecules. Silicon dioxide behaves in between. The siloxan bridges produced on thermal condensation owing to the reaction, 2Si–OH→Si–O–Si þ H2 O have poor affinity for water. These species are usually unreactive at room temperature due to stability of silicon– oxygen bond. When calcium chloride, a hygroscopic salt promoter, is impregnated on microporous silica gel taken as a host matrix and the concentration of the aqueous solution of calcium was varied from 0% up to the limit of solubility of CaCl2 in the water, at high humidity, the composite adsorbents obtained with the solutions at concentration greater than or equal to 40% yield

S.K. Parida et al. / Advances in Colloid and Interface Science 121 (2006) 77–110

Fig. 1. Type of silanols on silica surface.

maximum capacity of adsorption approximately doubling their own weight and quadrupling that of the pure silica gel [3]. Likewise, these are found to be desorbing faster compared to the substrate silica gel and thus find application in air-conditioning system. 2.1. IR Spectral characteristics of silica surface IR spectra have been used to characterize divided silica in the region of 3200–3800 cm− 1 corresponding to νOH stretching mode of silanols [4–14]. A broad band appears at around 3530 cm− 1 due to hydrogen-bonded silanol. The appearance of another band at around 3715 cm− 1 suggests the presence of terminal (isolated) silanols (Fig. 1), which behave as proton acceptors and have their own protons free for additional hydrogen bond [9]. However, it is difficult to differentiate between single (isolated) and geminal silanols because their properties are almost similar [15]. Exchange study with heavy water shows that some silanols, called internal silanol, are not accessible to water molecule. Those groups absorb in the range of 3650–3670 cm− 1. Rochester and Trebilco [16] studied the IR spectral characteristics of silica surface by using a suitably designed optical compartment. To avoid the interaction of water with the silanol groups, the spectra were run in presence of nonpolar solvents just after heat treatment (423–1023 K) of silica. The 3750 cm− 1 band due to OH stretching vibration was found to shift to 3705 cm− 1 when preheated (873 K) silica was immersed in heptane or 2,2,4trimethyl pentane. The magnitude of shifting, however, was found to vary for different liquids and thus can be a measure of the strength of the interaction between isolated surface OH groups and organic molecules present at the surface. When silica was heated at 423 K and cooled to room temperature, it exhibited a broad spectrum with absorption maximum at 3535 cm− 1. The band was attributed to lateral hydrogen bonding interactions of adjacent silanols. When silica was immersed in 2,2,4-trimethyl pentane, the absorption band shifted to 3490 cm− 1 indicating the sustaining of lateral hydrogen bonding. Recently Jal et al. [17] synthesised nanosilica and compared the IR spectral characteristics of the silica with the earlier reported data (Table 1). They observed spectral characteristics of molecular water adsorbed on the silica surface. FTIR spectroscopy technique was used to estimate the surface area of porous and non-porous silica powders by comparing the integrated area of the band due to isolated silanol groups on different silica by McCool [39]. Using fumed silica as a calibrant, an accuracy of about 7% in the surface area of several silica materials was obtained when compared to the surface area computed by BET nitrogen adsorption techniques. The principal advantage of this method is that it enables surface area measurements of silica films on

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porous supports. By considering structural parameters, charge distribution, force field and other characteristic features the Si–OH and Si–OD stretching and bending vibrations were determined theoretically by Carteret [40]. The values were reported to be in the range of 790–1030 cm−1 for Si–OH and 790–1010 for Si–OD. 2.2. Characteristics of water on silica surface Due to the presence of silanol group at the surface, silica is mostly embedded with water in multilayer. The first layer of water has different characteristics than the subsequent layers, which is evident from thermogravimetric analysis. IR spectral data also reveal the differential status of water on silica surface. Feng et al. [41] studied the rate of adsorption and desorption of water on silica surface by gravimetric, FTIR and X-ray photoelectron spectroscopy. They heated silica powder in vacuo from 200–1000 °C and analyzed the IR spectra of silica. The silica surface becomes hydrophobic on heating in vacuo but it becomes hydrophilic immediately on contact with water at low temperature. Near-IR spectra of water adsorbed on silica are also used for the study of the surface behaviour of silica [42–46]. Silanols are proposed to be the water adsorption sites [7,41,43,47]. This model can also explain the hydrophobic character of silica with low silanol densities. From intensity measurement of near-IR diffusereflectance spectra, Klier et al. [42] claimed the existence of SiOH–OH2 complexes with about one half of the silanols as “BET monolayer” on a Hisil 233 silica. Additional water adsorption takes place around adsorbed water molecules rather than on free silanols, because such clusters would be energetically more favorable. Yamauchi and Hondo [46] obtained more accurate IR spectra of the silanols and the adsorbed water on a silica gel. They suggested that water would settle only on part of the silanols at first as SiOH–OH2 complexes, where the water hydroxyl would absorb at 5270 cm− 1. A second water molecule absorbing at 5150 cm− 1 would than settle on the previous silanol water (Scheme 1). The surface of divided silica depends on the manufacturing process. Most of the silanols of the fumed silica are randomly distributed on outer surface. Non-nearest neighbor isolated

Table 1 IR spectral characteristics of silica Frequency (cm−1)

Position assignment

Values reported earlier

References

462 800 970–980

Si–O bond rocking OH bending (silanol) Si–OH bond stretching

465–475 800–870 935–980

1102

Asymmetric Si–O–Si stretching in SiO4 tetrahedron O–H bending (molecular water) O–H stretching and adsorbed water OH stretching

1050–1150

[18] [19–25] [18–22, 26–32] [33–37]

1625 3000–3800

[38] [38]

3740–3750

[19,20]

1630 3000–4000 3755

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Scheme 1.

silanols have little affinity for water. Inner surface pore are much more expanded for silica prepared from ethyl silicate. From the study of the surface of fumed silica and silica prepared from aqueous solution by using grafted n-hexadecyl chains as probes, the dependence of the nature of unreacted silanols on the manufacturing processes of initial silica was investigated by Burneau et al. [48]. The grafting yield was determined by the structure of the n-alkyl chains lying as multilayer on the surface. Silanol groups can be distinguished from one another depending on their perturbation and the affinity or accessibility towards water [49]. Free silanols can be divided into three groups: (i) OHiso, which does not have much affinity towards water, absorbs at νOH = 3740–3750 cm− 1, (ii) OHII, which absorbs water according to ….H–O–H or ……OH2 structures and situated on outer surface, absorbs at νOH = 3730–3740 cm− 1 and (iii) OHI, which absorbs water as …..HOH.. having ν OH = 3690– 3730 cm− 1, terminal vicinal silanols absorb at 3715 cm− 1; bound vicinal silanols absorb at 3460–3530 cm− 1; and internal silanols absorb at 3650–3670 cm− 1. From the water adsorption study of the surface of fumed silica and precipitated silica [50] two structures of physisorbed water were proposed for precipitated silica before the formation of cluster. According to this proposal the most stable adsorption of water takes place on inner surface and water absorbing at 5315 cm− 1 is due to less stable adsorption on outer surface. Two types of commercial pyrogenic silica (Degussa and Cabot) were studied for water adsorption on the basis of the chemical shift, homonuclear coupling (T2), and spin-lattice relaxation behavior (T1) using a 1MAS NMR study [51]. Three different types of silanol groups were identified: (i) weakly coupled (long T2), water inaccessible, isolated “internal” silanols at 1.8 ppm; (ii) weakly coupled, external “free” silanols revealed upon dehydration at 2.5 ppm; and (iii) strongly coupled external hydrogen bound silanols with an unresolved broad resonance between 3 and 7 ppm. The resonance of water, whose position between 2.6 and 4.6 ppm depends on water

Scheme 2.

content, corresponded to two unresolved species of slightly different T1. By equating this resonance to the weighted average of two distinct populations of water, the first layer of strongly hydrogen bound water at 2.7 ppm was distinguished from liquid-like water at 5 ppm. The adsorption of water on silica surface may be compared with that of other molecules of similar size. The adsorbate particle may be large enough to occupy more than one adsorbing site [52,53]. The occupancy of same sites by more than one adsorbate particle was studied by Garrone and Ugliergo [54]. Evidences revealed that the main interactions of SiOH to the O end of CO [12,55], to the O end of N2O [56], to the O atom in H2O [42,57], and to the N atom in NH3 [58] are due to the H-bonding (Scheme 2). The silanol may act as a twofold site of adsorption with respect to simple molecules such as water or nitrous oxide [59]. A study of IR spectra of dehydroxylation of fumed and precipitated silica by thermal treatment (973 K) proposed that the first step is always the departure of the more strongly bonded vicinal hydroxyl group [60] (Scheme 3). Yoon and Vivak [61] attempted to identify the forces between fully hydroxylated silica surfaces using atomic force microscopy (AFM). The measurement was conducted using nanopure water in solution containing various organic substances like methanol, ethanol, trifluoroethanol and pyridine. In nanopure water a strong short-range repulsive force at a distance below 15 nm was observed. This non-DLVO (Derjaguin– Landen–Verwey–Overbeek) force was fitted into double experimental force law with its longer decay length of 2.4 nm. The force curve obtained in 15% methanol was fitted into the DLVO theory perfectly, which represents the sum of a repulsive electrostatic interaction and an attractive van der Waals force and indicates no hydration force. This reveals that the hydration forces, originated from water structure in the vicinity of silica, are disrupted in the presence of methanol. Methanol adsorbs on silica by the displacement of water molecules. The study with 10–20% ethanol suggested that ethanol also adsorbs on silica to a lesser extent than methanol. In tetrafluoroethylene and pyridine, the solutes could not displace water molecules from the silica surface. The properties of water near solid surface are known to differ from those of the bulk. Drost-Hansen [62–66] investigated the properties of vicinal water and found that the structure of vicinal water is independent of the specific physico-chemical nature of the surface. The independence of the properties of vicinal water on the specific nature of the surface was termed as the paradoxical effect. Bogdan and Kulmala [67] could show the phase transition of pure water and binary H2O/HNO3 film adsorbed on surface of pyrogenic silica using NMR

Scheme 3.

S.K. Parida et al. / Advances in Colloid and Interface Science 121 (2006) 77–110

spectroscopy. Braun and Drost-Hansen [68] measured the heat capacity of water on silica and found it to be 25–30% greater than that at bulk. An investigation was carried out by Etzier et al. [69] to study the heat capacity of water confined to silica pores from 1.2 to 12.1 nm radii. The results showed that the heat capacity becomes maximum for pores near 7 nm when varied with temperature. From terahertz time domain spectroscopic study of adsorption of water on hydrophilic silica (Aero gel) it was found that the adsorbed water remains in submonolayer form and shows refractive index similar to those of bulk water but has different adsorption properties [70]. The temperature programmed desorption up to 850 K of fumed and precipitated silica was investigated by Dinh et al. [71]. In both the silica, the physisorbed water molecule had activation energy in the range 38–61 kJ/mol. The activation energy of desorption for chemisorbed water varied from ~80 to N 247 kJ/mol for fumed silica and ~96 to 155 kJ/mol for precipitated silica. It was found that while physisorbed water can be taken out from the silica within 1 h at 320 K, chemisorbed water can not be removed even after 100 years. Chemisorbed water with activation energy b 126 kJ/mol can be removed at 500 K and in a high vacuum condition. The adsorbed water and the Si–OH bond on the silica surfaces are the major contributors to the water degassing from this type of silica filled polymers. The intrusion of water into a pore space of silica hydrophobized with alkyl silane was studied by using water porosimetry technique [72]. The advancing and receding angles of water on chemically modified silica surface were used to correlate the structure of a bonded alkyl silane monolayer and its hydrophobicity. Intrusion of water is found to be controlled by bonded alkyl chain length, bonding density, structure of alkyl silane and average pore diameter of silica. The behaviour of water at mosaic hydrophilic/hydrophobic surfaces of different silica was studied in different dispersion media over wide temperature range using 1H NMR spectroscopy with layer-by-layer freezing-out of bulk water (close to 273 K) and interfacial water (180 b Tb 273 K), thermally stimulated depolarization current (TSDC) (90 b Tb 270 K), infrared (IR) spectroscopy, and quantum chemical methods [73]. Bulk water and water bound to hydrophilic/hydrophobic interfaces can be assigned to different structural types. There are (i) weakly associated interfacial water (1H NMR chemical shift δH = 1.1–1.7 ppm) that can be assigned to high-density water (HDW) with collapsed structure, representing individual molecules in hydrophobic pockets, small clusters and interstitial water with strongly distorted hydrogen bonds or without them, and (ii) strongly associated interfacial water (δH = 4–5 ppm) with larger clusters, nano- and microdomains, and continuous interfacial layer with both HDW and low-density water (LDW). The molecular mobility of weakly associated bound water is higher due to weak and distorted hydrogen bonds and their number is smaller than that for strongly associated water whereas that of strongly associated bound water is lower due to strong hydrogen bonds. The difference in the temperature dependences of the 1H NMR spectra at T b 273 K was proposed to be due to difference in the type of interaction existing between water and silica surface.

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The structure and bonding of water molecules around silica surface were simulated by using a modified Monte Carlo technique in a realistic Vycor-like silica mesoporous system at various temperatures. Water–water and water–Vycor pair correlation functions show the existence of strong distortion compared to bulk water due to the influence of the silica surface. From the analysis of the distribution in energy of the attractive sites of the system Puibasset and Pellenq [74,75] proposed that the simulated monolayer of water adsorbed on Vycor is less hydrophilic than the bulk water liquid/gas interface due to the strong distortion in the structure of the adsorbed monolayer. Similarly by using a combination of classical molecular dynamics (MD) and ab initio Car–Parrinello molecular dynamics (CPMD) Mischler et al. [76] have investigated the reaction of water with a free amorphous silica surface and reported that the reaction of a water molecule with a twomembered ring, leading to the formation of two silanol groups on the SiO2 surface, is an exothermic reaction with an activation energy around 0.9 eV. 3. Adsorption isotherm The adsorption phenomenon at the solid/liquid interface involves the change in concentration of the solution. Adsorption isotherm is constructed by measuring the concentration of the adsorbate in the medium before and after adsorption, at a fixed temperature. This is generally used to study the interaction between the adsorbate and the surface of the adsorbent and to know about the structure of the adsorbed layer. The adsorption isotherm of the solution bearing more than one component is termed as composite isotherm and for a twocomponent system, it is described by n-Dx=m ¼ n1 ð1−xÞ−n2 x

ð1Þ

where Δx = decrease in mole fraction x of component 1, when n° mol of original solution are brought into contact with m grams of the adsorbent and n1, n2 are the respective numbers of mole of component 1 and 2 adsorbed per gram of solid. As the above equation contains two unknowns, the individual isotherms for two-component system can be obtained by using supplementary information. For a very dilute solution and for which limiting solubility of the solute occurs at a low value of the mole fraction, even if n2 is large, the product n2x is small compared to n°Δx / m and n1(1 − x) is approximately equal to n1. For this situation, n-Dx=m ¼ n1

ð2Þ

Here the composite isotherm gives an individual isotherm for the solute. van der Waals force of attraction results in weak solute–surface interaction whereas chemisorption (chemical adsorption) results in relatively strong interaction. The shape of isotherm provides qualitative information on the nature of solute–surface interaction (Fig. 2) [77,78]. Physical adsorption is very common and the specific nature of the interaction is usually implied from the chemical nature of

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Fig. 2. Classification of isotherm shapes: S-type with an initial convexing to the concentration axis, Langmuir (L)-type with an initial concavity to the concentration axis, H-type with an intercept on the ordinate and C-type with an initial linear portion. (Ref. [77]).

the materials involved. Energy of adsorption provides important supplementary information. The state of thermodynamic equilibrium in adsorption process on plane solid/liquid interface is characterized by the interfacial mass-transfer, which involves adsorbed and solution species. The adsorption capacity of a sorbent for a solute increases with increase in fluid phase concentration. Similarly, the desorption is promoted by dilution of the solution. The macromolecule adsorption is a complex phenomenon [79]. It is a slow process and the aforementioned type of effect is not generally observed [80]. The interfacial stability is attributed to the thermodynamic and kinetic properties of the adsorbed layer. The existence of multiple contact point between the macromolecule and adsorbent [80,81] leads to a very high adsorption enthalpy. The macromolecule entanglements in the diffused zone constitute a kinetic limit to desorption [79,82]. Different structural characteristics are expected if the macromolecule adsorbs through hydroxyl site binding [83–87].

The adsorption of butane and butene on the silica at different temperatures was used to describe the chemical and physical changes in the system [88]. The adsorption properties of the initial and heat-treated silica samples were correlated with butane and butene adsorption energy distribution. From the above analyses it was suggested that the complex nature of the silica surface is due to its microporosity and the presence of surface hydroxyl group. The adsorption behaviour of butene and butyne on highly dehydrated silica was recently investigated by using IR spectroscopy [89]. The adsorption phenomenon was characterized in terms of involved surface site by in situ FTIR measurement. Using a first principle pseudopotential technique, the adsorption of C2H2 on the Si(001) surface was investigated and it was found that the di–σ–bond configuration is the most stable structure at low temperature from the energetic point of view [90]. C2H2 was found to adsorb preferentially on the alternate dimer sites corresponding to a coverage of 0.5 monolayer. The di–σ adsorbed system was characterized by symmetric C–C bond with length close to the double carbon bond length of ethylene molecule. Energy calculation indicated the possibility of tetra–σ model, which was also supported by high-resolution electronenergy-loss spectroscopy experimental data. The interactions of phenyl acetylene and phenyl acetylene-αd1 with Si(100) − 2 × 1 were studied and a model system to understand the mechanism of adsorption of conjugated π–electron aromatic substitution on silica was proposed [91]. Vibrational signatures and chemical shift of C 1 score level indicate the formation of covalent bonding through [2 + 2] cycloaddition of external C`C and SifSi dimer forming styrene-like conjugation. The experimental results were consistent with density functional theory calculations. Such conjugation can be employed as an intermediate for further organic syntheses and fabrication of molecular architecture for modification and functionalization of Si surface as a monomer for polymerization of Si surface. The adsorption of molecules containing different functional groups was studied using flow microcalorimetry and infrared spectroscopy by McMohan and Rhodes [92]. This technique was used to study the surface structure and adsorptive properties of a series of calcined and uncalcined porous silica. The adsorbates, dl-menthol (1), (r)-(+)-limonene (2), (±)-citronellal (3) and carvone (4) were selected for their functionality that included carbonyl, vinylic and hydroxyl groups.

3.1. Adsorption of small organic molecules on silica surface Adsorption of various small organic molecular species on silica surface was extensively studied to understand the driving force of adsorption. A number of modern techniques were employed to investigate the adsorption mechanism and to enumerate the forces of attraction acting between the adsorbate and adsorbent.

The amounts of probe retained by the silica together with the energy exchange involved in the adsorption/desorption process were determined by flow microcalorimetry. The functional

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Fig. 3. A plausible structure of an ethanol cluster on silica surface based on dichroic analysis of ATR-FTIR spectra. (Ref. [94]).

groups involved in these interactions were studied by means of IR spectroscopy. The strongest interaction with the silica surface was proposed to be through hydrogen bonding onto the surface silanol. The most retentive probes were found to be those with a carbonyl group in their structure. Adsorption onto calcined silica was found to be less energetic than onto the equivalent uncalcined samples. The adsorption densities were compared with theoretical predictions based on molecular models. In all cases, apart from citronellal, monolayer coverages were not observed due to steric effects. 1- and 2-propanol were adsorbed on silica surfaces from their mixture with cyclohexane and study was carried out using a combination of colloidal probe, AFM, adsorption excess isotherm and FTIR spectroscopy in the ATR mode [93]. Adsorption isotherm indicated adsorption of similar amount of each alcohol, and FTIR spectra revealed existence of hydrogen bond between the surface silanol groups and the –OH group of 1-propanol, which is known as surface molecular macrocluster. The contact of adsorption layers of 1-propanol brought about a longer-range attraction than that of 2-propanol due to cyclic aggregation of the later. Similar surface molecular macrocluster was also proposed by the same workers for adsorption of ethanol [94]. They proposed a plausible orientation of ethanol on silica surface as given in Fig. 3. Vibrational sum frequency generation (SFG) spectroscopy was used to study the adsorption of two surface acidity probe molecules, 4-picoline (5) (pKa = 5.94) and piperidine (6) (pKa = 11.24), onto the amorphous SiO2 surface [95].

The adsorption of 4-picoline onto the silica surface occurs by forming weak hydrogen bonds between the nitrogen atoms of 4-picoline molecules and the hydrogen atoms of surface silanol OH groups. Piperidine molecules are strongly chemisorbed onto the SiO2 surface through the protonation of piperidine molecules by surface silanol OH groups. The SFG results indicated that the

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surface acidity constant of silanol OH groups (pKa–(HOSi triple bond)) is in the range of 5.94–11.24 at the air/solid interface. Although this range of surface acidity constants is quite wide, it is possible to narrow it by choosing probe molecules with a smaller pKa range. Together with theoretical prediction methods, adsorption studies using vibrational SFG spectroscopy are found to be capable of quantifying the surface acidity of mineral oxides by carefully choosing the acidity probe molecules. A new force field [96] for the simulation of the adsorption of cycloalkanes in nanoporous silica afforded a significant improvement over any previously employed force field. The simulated isotherms reproduced the most salient features in the experimental isotherms extremely well. The study of cyclopentane,-hexane, and-heptane adsorption in MFI-type silica indicated an inflection for cyclopentane but not for cyclohexane at intermediate pressure. At low pressures, mixture isotherms of cyclohexane and n-hexane showed a temperature dependence on the selectivity in accordance with the results by Fox and Bates [97]. This was attributed to the difference in temperature dependence of the Henry coefficient for both molecules. Furthermore, the influence of the flexibility of the zeolite framework on the adsorption of cyclohexane was found to be as small as with n-alkanes. Aromatic nuclei contain [4n + 2] delocalized π-system and may be associated with varieties of side chains containing O, H, N, S, X etc. The adsorptions of many such compounds on silica surface were studied which could provide an insight into the factors responsible for the cause of adsorption such as hydrogen bonding, attractive force of silica surface and aromatic pisystem, etc. Zhao et al. [98] have constructed the adsorption isotherms for the adsorption of benzyl alcohol (7), benzaldehyde (8), benzoic acid (9), anisole (10) and toluene (11) from cyclohexane on silica. The first four are found to be Langmuir type and the fifth, a linear one. The thickness of the adsorbed layer of the molecules can be

approximately gauged by the horizontal thickness of benzene ring, i.e. 0.37 nm [99]. The adsorption isotherm analysis indicates that hydrogen bond is responsible for the adsorption which may be due to (i) the π-electron of a benzene ring and the hydrogen atom of the hydroxyl group of the silica gel or, (ii) the oxygen atoms of aromatics and the hydrogen atoms of the silanol groups or, (iii) the hydrogen atom attached to the oxygen atom of aromatics and the oxygen atoms of the silanol groups. Since the benzene ring can form a hydrogen bond with the silanol group, the adsorbed aromatics may lie flat on the interface. The adsorption isotherms of cyclohexane and benzoic acid from cyclohexane onto the pretreated silica gel were found to be Langmuir type [100]. Given the similar molecular structure for

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cyclohexane and tetrahydrofuran where the nonpolar segment has good affinity to the nonpolar solvent, it can be supposed that the cyclohexanone is adsorbed vertically as tetrahydrofuran. The thickness of adsorbed layer was reported to be 0.6 nm. The adsorption of vapour of four acetophenones (H:12, 4-Me:13, 4-OMe:14 and 4-NO2:15) were analyzed by the IR technique [101].

Hydrogen bonding between silanol group and carbonyl group is responsible for adsorption. The 4-OMe compound generated SiOH…OMe hydrogen bond via a bridging structure involving two silanol groups perturbed simultaneously by each adsorbed molecule. The adsorption of substituted benzene depends on the electronic effect of the substituent. However, during the adsorption of dioxane on non-porous, mesoporous and microporous silica, it was observed that the adsorption on microporous silica is governed by pore volume and dimension. Bonding interaction between surface silanols and dioxane molecule is due to hydrogen bonding [102]. 1D and 2D NMR spectroscopic investigations on adsorption mechanism of 2,4,6-trimethyl pyridine (2,4,6-TMP:16) were carried out on silica in suspension in water/substituted pyridine mixture by Andrieux et al. [103] and the existence of adsorbed species specific signal was demonstrated.

N-alkyl styryl pyridinium dyes (17–32) were used extensively as probes

17: R = Butyl, X = p-OMe; 18: R = Butyl, X = p-NMe2; 19: R = Amyl, X = p-OMe; 20: R = Amyl, X = p-NMe 2 ; 21: R = Cetyl, X = p-NMe2.

22: n = 3, X = p-OMe; 23: n = 3, X = p-NMe2; 24: n = 3, X = p-Cl; 25: n = 3, X = p-H; 26: n = 4, X = p-OMe; 27: n = 4, X = p-NMe2; 28: n = 4, X = p-Cl; 29: n = 4, X = p-H; 30: n = 6, X = p-OMe; 31: n = 6, X = p-NMe2; 32: n = 6, X = p-Cl. to study their interaction with micellar surface [104–107] as well as silica surface [108–111]. Micelles behave as soft spheres with polar surface whereon, dyes can be oriented according to the alkyl chain length attached to quaternary nitrogen. However, silica has a hard surface embedded with polar hydroxyl groups, which are completely hydrated by forming hydrogen bonds with water. When silica gel was stirred in dye solution the dyes once adsorbed, were not desorbed by treatment with acid nor with 2 alkali [108]. When the silica gel with surface area 137 m was c used, the area of coverage per molecule (A ) on silica surface was 2 reported to be in the range of 1000–3000 A° (Table 2). The styryl pyridinium moiety has a flat structure with a delocalized positive charge and it was proposed that the dye anchors parallel to the silica surface (Fig. 4). Table 2 Area of coverage of N-alkyl styryl pyridinium dyes (17–32) on silica surface Compounds

From the signal under various pH conditions, adsorption via interaction of surface silanol groups and the lone pair of electron of the nitrogen atom was ruled out. The interaction between siloxan oxygen and aromatic pi system was proposed to be responsible for the adsorption. 3.2. Adsorption of dyes on silica surface Styryl pyridinium dyes are found to be active non-linear optical materials. When the monolayer films of these dyes on glass/ silica are tilted against laser beam the activity increases due to a specific orientation of the chromophore. These dyes undergo different types of aggregation or self-association in or on water, which may be of H-or J-type consisting of parallel dye molecules stacked in plane-to-plane or end-to-end manner leading to hypsochromism or bathochromism respectively.

17 18 19 20 22 23 24 25 26 27 28 29 30 31 32 17 ⁎ 18⁎ 21⁎

Area of coverage in A°2 18 °C

28 °C

2775.0 2638.5 2911.5 2802.5 1546.7 1705.9 – – 1751.4 1728.7 – – 1774.2 1774.2 – – – –

2274.6 2388.3 1637.7 1610.6 1059.9 1273.7 1160.0 1410.2 1160.0 1455.7 1319.2 1501.2 1319.2 1592.2 1501.2 227.4 263.8 341.2

⁎ Adsorption from chloroform medium.

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Table 3 Hemimicellization number and log K for Styrylpyridinium dyes on silica gel and alkali-treated silica gel Compound

17 18 19 20 22 23 24 25 26 27

On silica surface

On alkali treated silica surface

n

Log K

n

Log K

3.34 2.08 2.89 1.72 1.10 1.02 1.27 1.28 1.37 1.37

16.17 10.65 13.91 9.01 6.67 6.00 6.97 6.61 7.37 7.74

3.14 5.64 2.70 2.84 3.44 3.40 2.16 2.67 2.51 1.84

15.26 26.48 12.42 13.17 19.83 19.25 12.19 14.48 14.41 10.48

Fig. 4. Orientation of amphiphilic dyes on silica surface. (Ref. [108]).

In nonaqueous medium like chloroform, the dye molecules were found to have greater affinity for the silica surface rather than bulk solvent. In this medium silanol forms hydrogen bond with chlorine of chloroform. The adsorbates had to replace all the chloroform molecules from the silica surface to cover the hydroxyl groups. In water medium the area of coverage (Ac) of the dyes were found to be much more (around ten times) than that in nonaqueous medium. The Ac value for bischromophoric group was reported to be less than that of monochromophoric group. This observation was attributed to the electrical charge on the dye, which contributes to the affinity of the dye towards the silica surface. The increase in Ac values with increasing chain length of the flexible methylene bridge revealed that the two chromophoric groups anchor flat on the silica surface with methylenebridge. The Ac values were found to correlate well with the number of methylene group. Gu and Zhu [112] while investigating adsorption behaviour of some surfactants on silica surface proposed an equation corresponding to S-type adsorption isotherm (Eq. (3)) for determining the hemimicelle aggregation number (n), equilibrium constant (K) and standard free energy of hemimicellization (ΔGhm). Adsorbent þ n Adsorbate K½Adsorbent−ðAdsorbateÞn  log ðs=sl− sÞ ¼ logK þ nlogC

ð3Þ

This good correlation may suggest a higher dependence of hemimicellization on the silica-dye binding than the hydrophobic interaction. The standard free energy of hemimicellization (ΔhmG°) for 1 mol of dye was calculated using Eq. (5). Dhm G- ¼ −ð1=nÞRT lnK

ð5Þ

The values were found to be in the range 27.7–34.0 kJ/mol, which was attributed to a strong interaction of the dyes with the silica surface. Parida and Mishra [109] studied the adsorption behaviour of dyes (17–32) on modified silica surface where silanol groups generate negative charges after treatment with excess alkali. Two clear plateaus were observed in the adsorption isotherms for all bis-chromophoric dyes and the isotherms were of S-type (Fig. 5). The Ac values of the dyes on alkali treated silica (alkali– silica) were found to decrease significantly than that on normal silica. The decrease was more remarkable in case of monochromophoric dyes than bischromophoric dyes. With increasing the number of methylene group in the spacer of bischromophoric dyes, the Ac values were found to increase. In chloroform medium, however, the Ac values were higher than that in aqueous medium. The analysis of hemimicellization number, n (Table 3) suggests that the negatively charged silica surface assists the

This equation is obtained by combining the Langmuir and Freundlich equation where, τ is the amount of adsorbate adsorbed at concentration ‘C’ and τ∞ is the amount adsorbed in the limiting adsorption at high concentration. By using Eq. (3), Parida and Mishra [108] determined the hemimicelle aggregation number (n) values for bischromophoric dyes to be around one. However, with increasing the spacer group between two chromophoric units, the n values increased. In case of monochromophoric dyes the n values varied from 2 to 3 (Table 3). The aggregation number was found to correlate well with log K (R2 = 0.994). Log K ¼ 1:3621 ð4:4385F10:1116Þ n

ð4Þ

Fig. 5. S-type isotherms for bis-chromophoric dyes adsorbed on silica. (Ref. [109]).

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formation of hemimicelle in all the dyes. The n values were found to be more than that of the corresponding dye on normal silica. The log K values were found to be more than the corresponding values obtained for normal silica. In case of bischromophoric dyes the increase in log K did not vary remarkably from threemethylene unit to four-methylene unit, but decreased substantially when the length was changed from four methylene units to six methylene units. This observation led to the suggestion that the electrostatic interaction dominates over the hydrophobic interaction up to four methylene units and beyond this, the reverse is the case. In monochromophoric dyes, the N,N-dimethylamino substituted dyes were found to have higher log K values than the corresponding methoxy substituted dyes whereas reverse was the case for bischromophoric dyes. The area covered by each adsorbate was found to be in the range of 91–489 A°2 at 28 °C. In a subsequent work, the silica surface was modified by treatment with CTAB solution [110]. The surface was assumed to have a coat of cationic surfactant where the cationic head group might be anchoring at the silica surface masking the silanol group. In this condition the hydrophobic groups protrude away from the silica particle. The adsorption of cationic dyes on CTAB-silica surface clearly envisages that at least a part of the adsorbent surface has a monolayer of CTAB where the cationic dyes bind through hydrophobic interaction (Fig. 6). As compared to alkali–silica and normal silica, the ease of desorption in CTAB-silica suggests that the dyes do not bind directly to the silica surface. Within the experimental concentration of the dyes, the adsorption isotherms for all the dyes except methoxy substituted monochromophoric dyes were found to have single plateaus. In case of methoxy substituted monochromophoric dyes, the adsorption isotherms had two plateaus. To explain the adsorption isotherm, both the Langmuir equation (Eq. (6)) and the modified Freundlich equation (Eq. (3)) were treated with the experimental data. Ce =Ae ¼ ð1=Am bÞ þ ðCe =Am Þ

ð6Þ

where Ce is the concentration of the dye at the adsorption equilibrium, Am and b are the Langmuir constants related to the capacity and energy of adsorption respectively. The modified Freundlich equation was found to explain the adsorption behaviour of all the dyes at a high confidence level. The ‘n’ values (number of adsorbate per hemimicelle) were found to be less than 1 which suggests that the formation of hemimicelle in water medium is due to hydrophobic interaction. In nonpolar environment hydrophobic interaction decreases resulting in the inhibition of hemimicellization. From the ease of desorption of the adsorbed dyes it was proposed that the adsorption of dye on CTAB-silica is due to the interaction of hydrophobic group of the dye with the exposed hydrophobic chains of CTAB on the CTABsilica. The CTAB-silica provides a nonpolar environment resulting in inhibition of hemimicellization of the dye and thus the average number of hemimicellization decreases. In case of monochromophoric dyes, ‘n’ value of butyl derivatives were found to be more than the corresponding amyl derivatives.

Fig. 6. Orientation of dyes on CTAB-Silica. (Ref. [110]).

Similarly in case of bischromophoric dyes, the increase in spacer length was found to increase the hydrophobic characteristic, which led to decrease in hemimicellization and consequent decrease in ‘n’ value. The log K values were found to be less [∼1 / 10th] compared to the values obtained during the adsorption of dyes on normal or alkali treated silica. This low value was claimed to be the obvious result of adsorption due to polar bonding. It was further observed that the increase in hydrophobicity decreased the binding constant for each series of dyes with same functional groups. The log K was found to correlate well with `n' (R2 = 0.93). logK ¼ 4:6759F0:465n−0:046

ð7Þ

The Ac values were found to be in the range of 1014.7– 4303.6 A°2 per molecule. These values are very high when compared to the values obtained from the adsorption on normal or alkali treated silica. When CTAB is adsorbed, the silica surface gets a semifluidlike structure due to the exposed flexible cetyl chain of CTAB. Hence the adsorption of the dyes on this semifluid surface can not be treated as a rigid system as in case of normal or alkali treated silica. Further, the adsorption of the dye on CTAB-silica is due to the van der Waal interaction of hydrophobic part of the dye with the hydrophobic tail of CTAB. The orientation of the dyes on the CTAB-silica surface may be assumed to be reversed as in case of normal or alkali treated silica. Thus, the dyes are extended to a great length from the silica surface. When a low coverage is assumed, the extended part have a free motion in the space which would result in a large area of coverage. The proposed orientation of the dyes on CTAB-silica is presented in Fig. 6. Parida and Mishra also investigated the adsorption behaviour of the dyes (17–32) on silica surface in presence of polyethylene glycol (PEG) [111]. PEG binds with the silanol groups through hydrogen bonding, where the ethereal oxygen acts as the hydrogen bond accepter. It is assumed that a part of the PEG

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molecule remains adsorbed on silica surface and the rest part remains protruded into the bulk water. It makes the silica surface hydrophilic. Assuming 100% coverage of PEG on silica surface all the silanol groups are masked by the PEG. Hence the PEGsilica surface has the capability to bind substrates through hydrogen bonds where it behaves as hydrogen acceptor. The adsorption of polar substrates may also occur due to the entrapment of the substrate by the protruded oxyethylene groups exposed to bulk water (Fig. 7). From the analysis of adsorption isotherm it was proposed that the bischromophoric dyes adsorbed more strongly compared to the monochromophoric dyes. Comparison of the regression coefficient obtained from the Langmuir and modified Freundlich equations proposed all the isotherms to be of Langmuir type. There was no significant difference in the Ac values of bischromophoric and monochromophoric dyes. This observation was attributed to the binding of the extended polyethylene units on the silica surface with the dyes. The Ac values were found to be within the range of 1843.2–1873.1 A°2, which are too high for monolayer adsorption on an anchoring site. This high value of Ac on silica surface indicates a low coverage of the dye on PEG-silica surface. In case of normal silica and alkali treated silica, hydrophilic chromophoric groups anchor at the surface in the flat-on position whereas on CTAB-silica the orientation of the dye is completely opposite where the dye binds to the adsorbent surface through hydrophobic interaction. But in case of PEGsilica the adsorbates are entrapped by PEG units and behave as being adsolubilised by PEG-silica particles (Fig. 7). Adsorption of anionic dye such as coccine acid red # 18 (33) on sludge particles was extensively studied by Wang et al. [113].

They found that adsorption of dye is a fast process and attains equilibrium within 30 min. The adsorption of the dye increased with decrease in pH and increase in ionic strength. The adsorption of dye was described using multilayer adsorption model. The area of sludge particle by determining the monolayer adsorption density, the equilibrium constant for the first layer and subsequent layers. By applying evanescent wave cavity ring-down absorption spectroscopy the thermodynamic properties of the surface adsorption for neutral trans-4-[4-(dibutylamino) styryl]-1-(3sulfopropyl) pyridinium (DP:34) and charged trans-4-[4-

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Fig. 7. Entrapment of dyes by oxy-ethylene groups of PEG. (Ref. [111]).

(dibutylamino)styryl]-1-methylpyridinium iodide (DMPI:35) at the silica/CH3CN interface were investigated by Fan et al. [114].

They observed that when the bulk concentration of DP increases the absorbance increases rapidly in the early stage and then reaches a plateau at high concentration (Fig. 8). The interfacial interaction between DP and substrate surface is claimed to be strong, forming a monolayer around the plateau. DMPI also shows a smilar behaviour. The monolayer adsorption behavior was characterized using the Langmuir isotherm model, giving rise to the saturated surface density and the related thermodynamic properties. The DP, adsorbed by forming the hydrogen bonds, shows a smaller equilibrium constant and free energy of adsorption than the DPMI bound by the electrostatic attraction. Surface forces are also probed by addition of triethylamine (TEA), which is competitive with DP for the silanol sites. When the TEA concentration was increased, the DP surface density was found to decrease, whereas the DMPI surface density increased. Adsorption of a cationic dye, rhodamine 6G (36), at the interface of octadecylsilyl (C18)-modified silica gel and acetonitrile was studied by Zhong et al. [115] using singlemolecule spectroscopy and fluorescence correlation imaging, which provided direct evidence of strong adsorption of cationic

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Fig. 8. Concentration dependence of absorbance isotherm of DP and DMPI at silica/CH3CN interface. (Ref. [114]).

molecule onto C18 silica gel where the strong adsorption sites are randomly distributed throughout the silica beads.

to the absorbance of charge–charge interaction between adsorbate and adsorbent leading to S-type isotherm (Fig. 2). The adsorption of cationic surfactants may be driven energetically as well as thermodynamically. On adsorption, entropy decreases due to the ordered arrangement. On the other hand, replacement of water molecules from silica surface results in increase in entropy. The later entropy change can substantially exceed the former in the case of solid surface, which exerts a pronounced long-range effect on the arrangement and motion of the surface water molecules. According to two-plateau adsorption isotherm of cationic surfactants on silica gel, an empirical approach [123] and a mass action model [128,129] were developed. In general, the adsorption of non-ionic surfactant from aqueous solution onto polar surface like silica gel exhibits S-type isotherm. Gu and Zhu [112] extended the mass action model to non-ionic surfactant system from which previously mentioned Eq. (3) was derived. Misra et al. [130] studied the adsorption of polyethoxylated alkyl phenols on silica surface using pyrene probe. It was observed that in a series of surfactants having the same hydrophobic chain length but with different hydrophilic chain length, the presence of smaller number of hydrophilic oxyethylene units led to a conspicuous increase in adsorption density in the premicellar region compared to surfactant having a large number of oxyethylene units. This was attributed to the formation of hemimicelles in the silica

3.3. Adsorption of surfactants on silica surface Generally, the adsorption of surfactant at solid/liquid interface involves surface micelle or hemimicelle formation through association or hydrophobic interaction between hydrocarbon chains of the adsorbed molecules (Fig. 9) [116–118]. During adsorption of ionic surfactants on silica surface, electrostatic interactions between ionic head groups and the surface result in neutralization of surface charge at low concentration of surfactants. With the increase in the surfactant concentration in the bulk, a little effect on the adsorption is felt initially, but beyond a certain concentration, the adsorption increases. This transition concentration is termed as hemimicellization concentration (hmc) [119]. The hemimicelle can be viewed as a hemispherical piece of Hartley's micelle adsorbed on silica surface and is a result of lateral interactions among the monomers. However, little is known about the phenomenon of hemimicelle formation at various solid surfaces. Gu et al. and others [120–127] studied the adsorption of various types of surfactants on silica gel and generalized the followings. The adsorption isotherm of cationic surfactants on silica exhibits two plateaus. The first plateau is due to adsorption of the surfactant, which is a charge–charge interaction. At hemimicelle concentration, the adsorption increases as hemimicelle is formed through the association of hydrocarbon chains at the adsorbed species. In non ionic surfactants, the existence of a single plateau is attributed

Fig. 9. Adsorption isotherms for BDDA+ (solid line) and BTMA+ (dotted line) ions onto precipitated silica from water at 298 K (pH free), From Ref. [129].

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Fig. 10. Schematic representation of hemimicellization during the adsorption of sodium dodecyl sulphate on alumina. (Ref. [116]).

surface in the former and it was also supplemented from the emission characteristics of pyrene (Fig. 10).

Adsorption of some quaternary ‘onium’ salts i.e. cetyltrimethyl ammonium bromide (37), benzyltrimethyl ammonium bromide (38) etc. was studied by Ghosh et al. [131]. They found the adsorption isotherm to be pH-dependant. Adsorptions of alkyl trimethyl ammonium bromide [132], some fluorocarbon (FC) surfactants and hydrocarbon (HC) surfactants [133] on silica gel exhibited S or LS type isotherms. Adsorption of ionic surfactants on molybdenum modified silica gel [134] and the influence of electrolyte and pH on silica powder modified with quaternary ammonium groups [135] were reported. The adsorption phenomena of cationic surfactants on silica/water interface were investigated by a number of workers with different experimental techniques [123,136–149]. The surfactant adsorption from water on mineral substrates is governed mainly by electrostatic and hydrophobic interactions [77,150–154]. At low surface coverage surfactant cations adsorb physically as individual ions on the negatively charged surface sites. In the first stage of the adsorption process, they displace water molecules and exchange with protons and with other counter ions present at the solid/water interface. The stoichiometry and energies of such an exchange depend on properties of both solid/water interface and type of surfactant. The equilibrium and kinetic aspects of the adsorption of alkyltrimethylammonium surfactants at the silica/water interface was studied using optical reflectometry. The added electrolyte, the length of the hydrocarbon chain and type of the counter-and coions were found to have much influence on the extent of adsorption [154]. Increase in the length of the surfactant hydrocarbon chain resulted in the adsorption isotherm being displaced to lower concentrations. The adsorption kinetics indicated that above the critical micelle concentration (CMC), micelles adsorbed directly

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to the surface and that as the chain length increased, the hydrophobicity of the surfactant experienced a greater influence on the adsorption kinetics. While the addition of 10 mM KBr increased the cetyltrimethylammonium bromide (CTAB) maximal surface excess, there was no corresponding increase for the addition of 10 mM KCl to the cetyltrimethylammonium chloride (CTAC) system due to the decreased binding efficiency of the chloride ion relative to the bromide ion. Variations in the co-ion species (Li, Na, K) had little effect on the adsorption rate and surface excess of CTAC up to a bulk electrolyte concentration of 10 mM. However, the rate of adsorption was increased in the presence of electrolyte. Slow secondary adsorption was observed over a range of concentrations for CTAC in the absence of electrolyte and importantly, in the presence of LiCl; the origin of this slow adsorption was attributed to a structural barrier to adsorption. A study of adsorption of short chain tatraalkylammonium (TAA) bromide showed that the driving force for adsorption of TAA ion on silica is being relayed to the hydrophobic character of the TAA ions, rather than to the attraction by charged silanol groups (though the later contributes to the adsorption energy) [155]. The hydrophobic hydration of TAA ions increases with increasing alkyl chain [156]. Investigations concerning viscosity and phase separation in solution containing TAA and silicate ions indicate a repulsive interaction between hydrophilically induced hydration region around silicate ions and hydrophobically induced hydration region around TAA ions. Since surface silanol groups are strongly hydrophillically hydrated [157], repulsion between TAA ions and silanol groups exists. It is likely that the hydration regions present around surface siloxane bridges are more compatible with hydrophobic hydration. The driving force for adsorption of TTA ions may be due to the overlap between hydrophobically induced hydration regions around TAA ions and siloxane bridges. The adsorption on silica of four alkanediyl-α-ω-bis(dodecyldimethylammonium bromide) dimeric surfactants (gemini surfactant) with the alkanediyl spacer groups C2H4, C4H8, C6H12, and C10H20 (the corresponding surfactants are referred to as 12-2-12, 12-4-12, 12-6-12, and 12-10-12, respectively) (39), was investigated by measuring the amount of surfactant adsorbed, the sodium and bromide ion concentrations in the supernatant solution along the binding isotherm and by the electrophoretic mesurment of the silica particles [158].

The maximum amount of adsorbed surfactant was found to increase in the sequence 12-10-12 b 12-6-12 b 12-4-12 b 12-2-12. At very low equilibrium concentration, the adsorption isotherms showed a rapid but small increase of the amount of adsorbed surfactant, Γ, followed by an adsorption plateau corresponding to

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a Γ value that was nearly independent of the spacer carbon number, n. In the first adsorption step, the residual sodium ions present at the silica surface were released upon binding of dimeric surfactant ions to the silica surface. The length of the adsorption plateau was found to depend on the nature of the surfactant, being the shorter for the most hydrophobic surfactant. At the end of the first adsorption plateau, the value of Γ showed a rapid increase when plotted against the concentration of free surfactant. This increase was accompanied by a decrease of the supernatant pH from 6.5 to about 4.0 and an increase of the concentration ratio [free bromide ion] / [free surfactant ion]. This second adsorption step was assigned to the formation of surfactant aggregates at the silica surface. Part of the bromide ions were shown to be bound to these aggregates which, however, might be more ionized than the free micelles formed by the dimeric surfactants. In a subsequent investigation on the adsorption of two surfactants i.e., DTAB (Dodecyltrimethylammonium bromide) and 12-2-12 on normal silica and HCl-treated silica it was found that the adsorption mechanism of the two types of surfactants on the two types of silica are similar [159]. There was only quantitative difference in adsorption between the two silica due to (i) large numbers of surface sites present in normal silica as compared to the other and, (ii) the larger ionic strength of the supernatant in silica/surfactant system as compared to the other. Thus, the amount of surfactant adsorbed at the point of zero charge and at the saturation of the silica particles, and the decrease in the CMC in the supernatant with respect to pure water were found to be larger for the normal silica than for the other. The adsorption isotherm was proposed to consist of two steps, the first step corresponds to the adsorption of the individual surfactant ions on the negative sites on the silca surface, which is driven by electrostatic attraction and depends on the number of sites and ionic strength and the second step is driven by hydrophobic interaction between the surfactant alkyl chains and the results in the formation of surface aggregates. The surfactant adsorption on the surface was found to continue even after the attainment of CMC. The adsorption isotherms of benzyl dimethyldodecylammonium bromide (BDDAB) (40) and benzyltrimethylammonium bromide (BTMAB) (41) on hydrophilic silica were used to calculate the amount of the surfactant adsorbed by using equation, ne2 ¼ mo1 ðC2o −C2b Þ=ð103 ms Þ

ð8Þ C2b

where m°1 is the initial mass of the solvent, C°2 and are the molalities of the initial solution (before adsorption) and the equilibrium bulk solution (after the attainment of adsorption equilibrium) respectively, ms is the mass of the solid sample [160].

The adsorption of BDDAB was found to be more than that of BTMAB (six times greater at plateau), which illustrates an important role of a polar portion of the surfactant ions in

Fig. 11. Schematic diagram of surfactant adsorption on silica surface as monomers, as hemimicelles, and as surfactant concentrations above CMC, where pyrene is bound to micelles. From Ref. [130].

promoting the adsorption process. The presence of alkyl chains increases the affinity of surfactant ions to the interfacial regions. This effect results from pushing out the hydrophobic moiety of surfactant molecules from the bulk aqueous phase and from the cohesive chain–chain interaction between the hydrophobic moieties of the adsorbed ions [77,150–154]. The isotherm obtained shows a more or less continuous increase in adsorption with increasing molality below the CMC (Fig. 11). Stiernstedt et al. attempted to measure the forces between silica surfaces with adsorbed surfactants by means of a bimorph surface force apparatus by using a cationic surfactant (tetradecyltrimethylammonium bromide:TTAB), a nonionic surfactant (hexakis (ethylene glycol) mono-n-tetradecyl ether: C(14)E(6)) and a mixture of both the surfactants [161]. An increase in the adsorption of TTAB was attributed to a highly charged glass surface at high pH. Despite the low adsorption generally seen for nonionic surfactants on silica at high pH, addition of C(14)E(6) had a considerable effect on the surface forces between two glass surfaces in a TTAB solution. The barrier force was found hardly affected, but the adhesion was reduced remarkably. Addition of salt decreased the adhesion, but increased the barrier force. In the presence of salt, addition of C(14)E(6) increased the thickness of the adsorbed layer. Adsorptions of non-ionic surfactants such as Triton X-100 and acid-chlorides were investigated by many workers [162,163]. In an adsorption study of non-ionic surfactant from aqueous solution onto silica and polystyrene latex particles it was observed that there is no significant difference in surface property between silica and the latex particle [164]. The solid surface characteristic depends on adsorption isotherm. The adsorption isotherm for the adsorption of ethoxylated isoamyl phenol from n-decane solution on silica gel was investigated [165] and a model was proposed where the end portion of the oxide ethylene chain of the molecule of ethoxylated isoamyl phenol (no of adsorbate, n = 4) interacts with the two silanol groups forming four hydrogen bonds. A study of bilayer formation by coadsorption of dipalmitoyl phosphatidylcholine (DPPC) with a non ionic surfactant, β-Ddodecyl maltoside, by neutron reflection revealed that the composition of bilayers is related with sequential dilution process, which enriches the adsorbed layer in phospholipids and leads to complete elimination of the surfactant at 25 °C [166]. The final supported bilayers have thickness of 51 ± 3 A° and are stable to heating to 37 °C. For the coadsorption of cholesterol and DPPC surfactant mixture, cholesterol is located in a 18 ± 1 A° thick layer

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below the lipid head group region and leads to an increased bilayer thickness of 58 ± 2 A° at 26 mol% of cholesterol. Interrelation between coagulation rate, adsorption and electrokinetic properties of silica polydisperse suspensions in the presence of cationic surfactant showed that the highest coagulation rate could be observed in a definite concentration range of the cationic surfactants [167]. When pH increased, an increasing amount of cationic surfactant was required to achieve maximal coagulation rate. For bisquaternary cationic surfactants, ethonium and decamethoxine, maximal coagulation rate was observed at concentration by an order of magnitude lower than for monoquaternary CTAB. The suspensions lost their stability due to both neutralization of particle surface charge and flocculating effect of the cationic surfactant. The adsorption of a cationic surfactant, cetyltrimethylammonium bromide (CTAB) on precipitated silica and the adsolubilization of organic solutes (toluene and acetophenone) into the adsorbed surfactant aggregates were examined as a function of surfactant adsorption at two pH values (5 and 8) [168]. The results showed that the adsorption of CTAB depends strongly on pH and for each pH the adsolubilization increases with increasing surface adsorption. The structural arrangement of the surfactant aggregates plays an important part in addition to the amount of adsorbed surfactant. This phenomenon was more noticeable at low surfactant adsorption than at higher surface adsorption. In mixed-solute systems, the presence of acetophenone had little effect on the toluene adsolubilization. In contrast, a synergetic effect was observed in the adsolubilization of acetophenone in the presence of toluene. Adsolubilization is the solubilization of organic compounds into adsorbed surfactant aggregates which has attracted much attention in the past few years because the phenomenon can be exploited for a variety of new commercial applications including the formation of engineered surfaces, pharmaceutical applications, and nanotechnology. Adsolubilization is strongly influenced by the amount of adsorbed surfactant, which in turn depends upon pH, ionic strength, and surfactant type and concentration. Anionic surfactants do not adsorb on silica surface. However, the presence of these surfactants affects the surface charge of silica particles. With increasing concentration of Aerosol OT, a sign reversal of the electrophoretic mobility occurs in decane [169]. As a synthetic application of adsorption of surfactants on silica, the synthesis of silica-templated microstructures was reported by Pirez-Arivalo et al. [170]. Using surfactants with different cross-linking densities i.e., CTAB and TX 100, microstructures have been synthesised with characteristics XRD patterns with periodicities 40–140 A°. 3.4. Adsorption of polymers on silica surface Polymers are attracted to the OH-groups of silica surface owing to their polyfunctional groups and hence can undergo adsorption. The extent of adsorption depends on various factors such as the structure of polymer, surface structure, pH of the medium, temperature, the geometry of adsorbate etc. and can be studied by

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modern techniques such as IR, reflectance fluorescence, nearBrewster reflectivity, PMR, relaxation techniques, etc. The adsorption and desorption of polystyrene on silica/CCl4 interface using radio labelled polymer were studied at three different temperature i.e. 4, 25 and 35 °C [171] which led to the proposition of hydrogen bonding between hydroxyl group and phenyl ring. At low surface coverage, each elementary segment of polystyrene is paired to a surface hydroxyl group. On a larger scale, as the silica surface bears only hydroxyl groups, the adsorbed macromolecules lie flat on the solid surface. The adsorption kinetics of hydroxyethylcellulose (HEC) was studied by reflectance fluorescence and near-Brewster reflectivity [172]. HEC was found to adsorb at the transparent limited rate. Relaxation occurred at nearly constant interfacial mass when HEC layers are exposed to aqueous solvent causing exchange of chains between the layers and free solution to become increasingly hindered. Actually the layers become immobilized and are unable to accommodate chains from free solution. The polydispersed HEC behaves similar to polyethylene oxide (PEO) on silica. Relaxation in PEO layers occurred at time scale of 10–20 h like those of HEC layers. Young layers of HEC exhibited self-exchange kinetics which is slower than the PEO layers of same age. This is attributed to the stiff backbone of HEC as compared to the PEO. It is known that most adsorbed homopolymer layers set up quickly on a surface after chains diffuse to the interface from the bulk solution. [173–175]. After the initial mass accumulation, continued interfacial rearrangements cause the layer to become more tightly bound to the surface [176–178]. The dynamics in adsorbed homopolymer layers were probed using self-exchange studies with the model system poly(ethylene oxide), PEO, adsorbed on silica from aqueous solution which constitutes a good solvent for PEO [179]. In matured PEO layers, higher molecular weight layers are less mobile than lower molecular weight layers, with a linear influence of molecular weight on the stretched exponential time constant and on the time constant describing the initial stages of exchange. The aging rate was found to be greatly dependent on molecular weight with longer chains immobilizing more quickly than short ones. It was also reported that chains aged in unsaturated or starved layers exhibited faster self-exchange rates compared with chains aged in saturated layers [180]. The exchange rate can be controlled by surface coverage to a better extent than molecular weight. The results of the study ruled out the number and strength of segment-surface contacts per chains as the determining kinetic factor in exchange dynamics. The schematic diagrams of potential differences between exchange of chains aged in unsaturated layers vs saturated layers is presented in Fig. 12. Chains aged in the unsaturated layers become flat, with greater numbers of segment-surface contacts per chain. This increases the likelihood of pinning by an adsorbed overlayer when unlabeled chains are added, perhaps increasing the difficulty of displacing the preadsorbed chains. Saturated layers (on the right) are expected to be more entangled, but the loops and tails may osmotically repel the subsequently added labeled chains. Huang and Santore [181] investigated the adsorption of hydrophobically modified PEO, focusing on the model system comprised of a 35,000 narrow molecular weight backbone, end-

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Fig. 12. Schematic diagram of potential differences between exchange of chains aged in unsaturated layers (on the left) vs saturated layers (on the right).

capped with C16 hydrophobes. Adsorption of hexadecyl urethane (NH2COOC16H33) onto planar silica, which serves as a model for many different substrates exhibiting attractions between the surface and the main polymer backbone was studied, which indicates that hydrophobe adsorption to the surface is not the main driving force for adsorption, though the hydrophobes may indeed adsorb at some coverage levels. It was thus proposed from the evidence based on the isotherm, adsorption, and desorption dynamics that the adsorbed layer contains two main parts: an under-layer which is tightly physisorbed to the surface and a loosely bound layer which makes up much of the interfacial mass at modest bulk solution concentrations. The latter forms a shear-responsive transient network of associated clusters. Adsorption of low charge density cationic polyacrylamide (CPAM) on to monodispersed silica was studied by taking a wide range of KCl concentration and parallel surface density of silica particles with or without adsorbed polyelectrolyte were determined [182]. The maximum adsorption takes place at the electrolyte concentration of 0.1 M. As the pH changes from 4 to 6, the maximum adsorbed amount increases and then reaches a plateau or decreases. This was explained in terms of interplay between the different factors that governs polyelectrolyte adsorption. The effect of polyelectrolyte adsorption on the surface charge density of silica is the largest at low concentration. It decreases with increasing the salt concentration and vanishes when the salt concentration is more than 0.1 M. This indicates that polyelectrolyte plays a major role in surface charge regulation. The ratio of pre-adsorbed charge is close to unity at high pH and increases drastically with decreasing pH and increasing salt. The behaviour of adsorption of polymers (mainly polystyrene and poly methyl methacrylate) on silica (Aerosil 130) was investigated by using IR spectrophotometeric method [85]. Both adsorption isotherms are of high affinity type by reflecting the narrow molecular weight distribution of both the polymers. The amount adsorbed at plateau regions showed a clear and

slight increase with increasing molecular weight. The adsorbed amount of polymethyl methacrylate at the plateau was found to be twice that of polystyrene for the same order of magnitude of molecular weight. Similar results were obtained by Herd et al. [183] who used unfractionated samples. NMR methods were utilized to measure coverage and interfacial conformation of a low-molecular weight of poly (dimethylamino ethyl methacrylate) (DMAEMA) oligomer (42) on colloidal silica, focusing on the effects of pH and ionic strength on the adsorbed conformation [184,185].

pH affects the surface density of potential adsorption sites, at the same time alters the underlying DMAEMA charge. Strong binding occurs at the pH 6 and at higher pH, greater coverage, though achieved, gradually becomes constant. Solvent relaxation suggests a more mobile interface, or a slightly smaller bound fraction. Shin et al. observed a charge overcompensation on the isotherm of polycation on silica and attributed the phenomenon to the denser positive charge on the adsorbing polycation. The ultimate coverage obtained corresponds to the adsorption of one oligomer onto each original negative silica charge, when the silica charge is most sparse, at pH 6 [186]. This limiting behavior breaks down at higher pHs where the greater silica charge density accommodates single chains adsorbing onto multiple negative sites (Fig. 13). As a result of the greater substrate charge density and reduced polycation charge at higher pHs, the extent of charge

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Fig. 13. Degree of DMAEMA protonation and silica surface charge density as a function of pH.

overcompensation diminishes while the coverage increases on the plateau of the isotherm. Ultimately at the high pHs, a regime is approached where the excluded surface area, not surface charge, limits the ultimate coverage. A model corresponding to the observations has been given in Fig. 14. The “Before” state corresponds to the bare silica surface while the “After” state corresponds to one where there is no net electrokinetic charge on the particle. The black negative charges in the figures correspond to those originally present on the bare silica surface. The white (or hollow) negative charges correspond to those which ionize locally as a result of oligomer adhesion. The plus signs in the circles illustrate hydrodynamically entrained potassium and sodium ions, which are present near the surface initially but are displaced as a result of adsorption. The positive charges on the oligomer represent the net oligomer charge after binding of phosphate counter ions to the backbone. Protons

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released as a result of the local surface ionization are shown explicitly in the “After” state. When ionic strength increases, a sharp adsorption cutoff for a polyelectrolyte like DMAEMA adsorbing on an oppositely charged surface takes place in the absence of other interactions [187]. The cutoff is sharper if the polymer is weakly charged. However, when the polymer charge is denser, a greater concentration of salt is needed for displacement and the adsorption cutoff is substantially more gradual. The failure of the model for dense polymer charges is due to the difficulty in predicting the behavior of concentrated salt solutions (above 1 M) where specific ion adsorption may occur, or where the size of the ions becomes important. The interaction of suspension of fumed silica and pyrocarbon silica (CS) in presence of dissolved PEG was studied using PMR spectroscopy [188]. The study showed that in the aqueous suspension of fumed silica or CS, PEG molecules are localized in the solid/liquid interface and do not form the bulk solution even at large concentration. The amount of bulk undisturbed water rises due to formation of the immobilized PEG layer. The adsorption behaviour of (polyethylene glycol) nonyl phenol with 10(NP 10) and 84(NP 100) oxyethylene groups on colloidal silica and hydrophobic polystyrene surface was investigated by using proton NMR relaxation technique [189]. NP 100, which may be considered to be polymer-like, seemed to adsorb with more contact onto silica surface when compared to polystyrene. 3.5. Adsorption of biomolecules on silica surface Various workers have studied adsorption behaviour of biomolecules like proteins, nucleotides, phospholipids and

Fig. 14. Schematic diagram of 25 nm2 unit cell on silica surface before and after DMAEMA adsorption at (A) pH 6, (B) pH 7, and (C) pH 8.

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pharmaceutical molecules on silica surface. Basiuk et al. [190] investigated the adsorption isotherm of adenosine (43), inosine (44) and uridine-5-triphosphate (45) on hydroxylated silica surface.

Fig. 16. Schematic illustration of layer structure before and after the isotherm transition based on the center-to-center distance between adsorbed molecules assuming monolayer coverage: (A) center-to-center distance of 9.9 nm; (B) center-to-center distance of 8 nm; (C) center-to-center distance of 5.6 nm; (D) center-to-center distance of 5.2 nm. The orientation preferred by unmodified lysozyme is shown for comparison.

They determined the equilibrium constant (K) and free energy change (ΔG) and compared the values with those of carboxylic acids. Adsorption of proteins, such as hemoglobin [191], fibrinogen [192], lactoalbumins, caesin [193] and lipase [194] on silica or modified silica surface were reported by various workers. In most of the reports more emphasis has been given to the adsorbate–adsorbent interaction to identify the hydrophobic and hydrophilic parts of the proteins. The adsorption of highly purified lipase from Humicola lanuginosa on silica and methylated silica surface was measured at different pH conditions [194]. The adsorption of lipase reaches the highest value at pH 5 and decreases with increasing pH. The amount adsorbed increases with increasing concentration at pH 7 and 9 while at pH 5, high value can be attained at the lowest concentration. The reversibility decreases with the concentration of the lipase. There was no effect of NaCl addition. On the other hand, the lipase adsorption increases with the increased CaCl2 concentration. The study of adsorption of glucoside on silica/water interface showed that dodecyl dimethylamine oxide (DDAO) renders assistance in the adsorption of dodecyl maltoside (C12G2) [195]. The adsorption was found to be a two-step process: first, DDAO adsorbs on silica surface through electrostatic interaction and then, C12G2 adsorbs at the hydrophobic site of DDAO tails through hydrophobic bonding (Fig. 15).

Fig. 15. Adsorption isotherms of C12G2 (open diamonds) and DDAO (open circles) and after Lunkenheimer purification (filled circles) on silica.

Bovine serum albumin was adsorbed on silica of three different types, i.e., aerosil, macroporous silica gel and ground natural quartz [196] and the adsorption of this protein on aerosil was measured and analyzed by IR spectra. The carbonyl groups of albumin macromolecules interact with vicinal hydroxyl groups, while imido groups with individual hydroxyl groups of silica surface. The geminal hydroxyl groups of the surface behave as single adsorption sites with respect to albumin. The IR spectral data indicated that the adsorption of albumin macromolecules are responsible for the dehydration of aerosil surface and the appearance of a small amount (∼10%) of unfold β-regions in the secondary structure of the adsorbed protein, while the αhelical macromolecular structure remains preserved as a whole. Protein macromolecules folded into globules remained tilted with respect to the adsorbent surface. The adsorption behaviour of lysozyme and bovine serum albumin on silica and AlOOH coated silica particles were studied by means of zeta potential analysis and UV/Visible spectroscopy [197]. The adsorption process depends on pH and at pH 7, a protein oppositely charged to the oxide surface adsorbs in significant amount. The adsorption process is chiefly dominated by electrostatic interaction between adsorbate and adsorbent. The electrostatic effect on the adsorption of proteins such as lysozyme, ribonuclease-A (RNase) and L-lectalbumin was investigated using differential scanning calorimetry (DSC) and adsorption isotherms [198]. The thermal denaturation and adsorption onto silica nanoparticles at three different concentrations: 10 M and 100 mM of sodium, and 100 mM of sodium added with 10 mM of calcium, were analyzed and the denaturation enthalpy (ΔH), the temperature at which the denaturation was half completed (Tm) and the temperature range of denaturation transition were calculated. The investigation revealed that the adsorption isotherm depends upon the ionic strengths in case of lysozyme and RNase. At low ionic strength, both the proteins have high affinity for silica surface and there is 15–25% reduction in ΔH and 3–6 °C decrease in the Tm, which indicates the instability of adsorbed state of protein. An increase in the width of denaturation of transition also was observed, which indicates a larger conformational heterogeneity of the surface bound protein. At higher ionic strength with and without the addition of calcium, no alteration in ΔH was observed for all proteins. The addition of calcium

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decreases the width of denaturation transition for lysozyme and RNase in the adsorbed state. The adsorption of unmodified and PEGylated chicken egg lysozyme onto silica was studied using optical reflectometry, total internal reflection fluorescence (TIRF) spectroscopy and atomic force microscopy (AFM) under varying condition of ionic strength and extent of PEG modification [199]. The result indicates that PEGylation of lysozyme changes the shape of the adsorption isotherm and alters the preferred orientation of lysozyme on the surface. There is an abrupt transition in the isotherm from low to high surface excess concentrations that correlates with a change in the orientation of mono-PEGylated conjugates lying with the long axis parallel to the silica surface to an orientation perpendicular to the surface (Fig. 16). The decrease in the number of protein molecules per unit area relative to the adsorption of unmodified lysozyme, even under the condition where the surface is densely packed with conjugates, is the net effect of PEGylation. PEGylation makes conjugate adsorption significantly less irreversible than unmodified lysozyme adsorption. The study of adsorption of lysozyme at the silica–water interface using neutron reflectometry was carried out to investigate the adsorbed structure from the adsorbed layer thickness [200]. The effect of ionic strength at pH 7 was also investigated. The adsorption was not affected if the ionic strength is below 0.05 M but above this, the addition of NaCl gradually reduces the lysozyme adsorption and complete removal is achieved at the ionic strength above 0.5 M. The adsorption was found to be reversible with respect to pH over a wide protein concentration range. At pH 7, a monolayer of 30 ± 2 A° thickness was found when the concentration was below 0.03 gm dm−3 showing that lysozyme was adsorbed with its axis parallel to the surface. A layer of thickness 60 ± 2 A° was formed at higher concentration which suggests the formation of bilayer in the sidewise configuration. At pH 4, the electrostatic repulsion between the adsorbed molecules is stronger than at pH 7 resulting in lower surface excess and a tilting away from the sidewise configuration at lower surface concentration. The interaction of silica with dipalmitoyl phosphatidyl choline (DPPC) liposomes was studied by 1H and 2D NOESY NMR spectroscopy [201]. The result showed that the silica particles mainly bind to the phosphate moiety of the polar head group of DPPC bilayer through hydrogen bond between Si–OH and O–P groups. The addition of silica particles slightly decreases the mobility of hydrocarbon chain. The adsorption of plasmid and chromosomal DNA on microcrystalline silica surface and the effect of ionic strength, temperature, pH, DNA size and conformation on the adsorption phenomenon were reported by Melzak et al. [202]. It was inferred from the isotherms that (i) shielded intermolecular electrostatic forces, (ii) dehydration of the DNA and silica surface and (iii) intermolecular hydrogen bond formation in the DNA silica contact layer are the major contributing driving force for adsorption. Adsorption of DNA on polypyrrole powder (PPy-powder), a colloidal silica sol, was investigated by Saoudi et al. [203]. The adsorption of DNA on polypyrrole (PPy)-silica monocomposite particles (untreated and amine or carboxylic

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acid functionalized) at neutral pH having sodium phosphate buffer was reported. The DNA adsorption was calculated to be 32 and 22 mg/g for the aminated silica sol and for the aminated PPy-silica particles respectively and 6.5 mg/g for the carboxylated particles. DNA was proposed to be adsorbed on to polypyrrole chloride bulk powder and also to the aminated PPysilica particles having cationic binding sites. Both the silica sols and unfunctionalized PPy-silica particles possess negative surface charge at this pH for which there is no adsorption of DNA on the substrate. The adsorption of DNA onto carboxylated PPy-silica particles is enhanced due to hydrogen bond formation. Hence the adsorption phenomenon is chiefly due to electrostatic and hydrophobic interaction. The behaviour of R-phycoerythrin (RPE) adsorbed on water/ fused silica interface was studied by fluorescence imaging at various pH and ionic strengths within the evanescent-field layer (EFL) [204]. Above the isoelectric point (pI), the individual protein molecules moved between exposures with random motion, but as the pH approached the pI of the protein, the RPE molecules were partially adsorbed onto the fused-silica surface. The residence time and the number of molecules within the EFL also increased near the pI. Below the pI, the protein molecules were completely and permanently adsorbed onto the surface. Glycine adsorption on silica surface was studied using middle IR and UV–Vis–NIR spectroscopy [205]. The amino acid was found to be present as zwitterion on silica surface, except at pH 2. At low concentration, glycine is specifically adsorbed on silica surface site through NH3+ moiety. At higher concentration, specific adsorption sites are saturated and surface induced precipitation of β-glycine is observed. The study of thermal reactivity of adsorbed glycine by in situ differential thermo-gravimetric analysis, in situ reflectance IR spectroscopy and thermo-programmed desorption coupled with mass spectrometry proposed that glycine molecules form peptide bonds at a temperature lower than that for bulk crystalline alpha-glycine. The secondary structure of adsorbed protein immunoglobulin (IgG) on hydrophilic, hydrophobic with preadsorbed triblock copolymer consisting of polypropylene oxide buoy and two polyethylene oxide dangling in the solution was investigated using ATR-FTIR spectroscopy [206]. The secondary structure of adsorbed IgG on hydrophilic silica resembles that of IgG in solution, which contains 60% β-sheet and almost no α-helix content. The presence of preadsorbed triblock copolymers reduces the adsorbed amount and causes an effect on the adsorbed protein similar to that exerted by ethylene glycol, a different type of β-sheet structure in IgG and more ordered αhelix structure in BSA are provoked. Acid pretreated antibodies (Ig) exhibits higher antigen binding capacity than native antibodies [207] because acid pretreatment leads to an increased exposure to hydrophobic regions in the constant fragment of the Ig. Selective adsorption of these regions produces a preferential adsorption in which the antigen binding fragments are accessible to bulk solution. Adsorption of protein on fumed silica surface was studied using thermally stimulated depolarization spectra [208]. The frozen aqueous suspension of fumed silica shows a dependence of relaxation process on the concentration on silica due to

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change in particle–particle interaction and the concentration of bulk water with increasing concentration of silica. The thickness of an interfacial water layer perturbed by silica surface or protein molecule was estimated from the dependence of 1H NMR signal intensity of unfrozen water on temperature below 273 K, separating the signal from water molecules weekly and strongly bound to the surface. Amino group of protein and SiOH or SiO−group of silica are hydrogen bonded. Aqueous suspension of mechanochemically activated fumed silica does not loose protein adsorption ability during a long period in which the particles remain as micro-scale agglomerates. Isotherms for adsorption of bilayer-forming synthetic amphiphiles or phospholipids from vesicles onto hydrophilic silica particles (Aerosil OX-50) were studied using phosphatidylcholine (PC), dipalmitoylphosphatidylcholine (DPPC), dihexadecylphosphate (DHP), and dioctadecyldimethylammonium bromide (DODAB) dispersed in 10 mM Tris, pH 7.4, as small unilamellar vesicles [209]. The affinities for silica follow the sequence, DODAB N DPPC N PC N DHP. Among these, only DHP adsorption is below that expected for one bilayer deposition. Interaction at 65 °C for 1 h between DPPC (or, at 25 °C, for PC) vesicles and silica efficiently leads to bilayer deposition at maximal adsorption. Preliminary centrifugation of the PC or DPPC vesicle dispersion is necessary to prevent overestimated adsorption. PC affinity for silica and its deposition as a bilayer depend on the nature of buffer used. Being much higher for Tris than for Hepes at pH 7.4, formation of ion pairs between protonated amino groups in Tris and silanol groups on silica may lead to Tris adsorption and an increase in density of –OH groups on the solid surface. Formation of cooperative hydrogen bridges between – P = O in the phosphatidyl part of the phospholipid bilayer and the –OH groups of Tris adsorbed on silica increases PC affinity for silica in the presence of Tris. For Hepes as buffer, PC affinity for silica is much lower and no plateau indicative of bilayer deposition is observed in the adsorption isotherm. Stabilization of supported phospholipid bilayers on solid surfaces requires several cooperative hydrogen bridges between the phospholipid and the solid surface. DODAB adsorption was unaffected by vesicle age and physical state of the bilayer vesicle. Adsorption isotherms for DODAB are of the high-affinity type with a maximum indicative of competition between intervesicle interactions and DODAB deposition on silica. Stabilization of DODAB bilayer deposition requires more than –1 μC/cm2 surface charge densities on silica. It was observed that deposition of phospolipid bilayer is favoured by the presence of Tris as buffer, pH ≤ 7.4 and the temperature at the phase transition temperature. If Tris is the buffer used, interaction between DPPC and silica at 65 °C and for 1 h leads to bilayer deposition at maximal adsorption [210]. The adsorption isotherms indicate that adsorption of DPPC makes the silica surface hydrophobic. Reduction of merocyanine absorbance at 565 nm was used as a marker for bilayer adsorption onto silica particles. The absorbance at 565 nm displays a decrease with time that corresponds to the percentage of dye sandwiched between the bilayers and the solid particle surface and thereby hidden from the incident light. The temperature dependence activity and conformational changes in α-amylase with different thermostability upon ad-

sorption on ultrafine silica particles [211] reveal that the extent of activity reduction upon adsorption of B. subtilis α-amylase on the ultrafine silica particles is correlated closely with that of conformational changes and both of them are significantly increased by raising the temperature from 4 to 40 °C. At low adsorption amount and at low pH a stronger temperature dependence was observed. The adsorption of riboflavin (RF: 46) on the surface of nonporous highly dispersed silica and the effect of copper (II) ions on the physicochemical parameters of RF adsorption were studied by Bidzilya et al. [212].

Based on the correlation between the RF adsorption versus pH dependences they concluded that riboflavin is mainly adsorbed on undissociated silanol groups of the surface through the formation of hydrogen bonds. It was shown that the riboflavin copper complex is adsorbed much more weakly than free riboflavin. Hemoglobin adsorption on silica/water interface was studied by using evanescent wave cavity ring-down spectroscopy [213]. The adsorption isotherm shows a binding constant of (18.23 ± 7.5) × 106 for hemoglobin-silica. The adsorption behaviour and carbodiimide coupling were examined for a carboxyl containing cyclic hexapeptide, Ferichrome-A (FeA) on amine modified silica beads [214]. Adsorption decreases with increased salt concentration, and at high NaCl concentration (10 mM) no adsorption takes place. FeA in water adsorbs to amine modified silica beads to a maximum surface concentration of 1.69 mg/m2. Chemically modified silica with attached aminopropyl, imidazolyl, and trimethylsilyl groups, with adsorptive and coordinative grafted hemin were synthesized (47–55) and adsorption of some bile acids on the surface of hydroxylated silica, synthesized siliceous adsorbents and cholestyramine were investigated by Belyakova et al. [215]. It was found that the main contribution to the total adsorption is caused by electrostatic attraction between anions of bile acids and positively charged sites of the surface of modified silica and also by dispersion interactions between steroid skeleton of bile acids and functional groups of modified silicon dioxides. It was established that the kinetic parameters of adsorption and adsorptive capacity for all investigated siliceous adsorbents exceed similar characteristics for cholestyramine. The best of synthesized adsorbents is hemincontaining adsorbent 55, and the sequence of increase in its adsorptive capacity in relation to bile acids corresponds to the following series: 47 b 49 b 48 b 50 b 52 b 51 b 54 b 53 b 55.

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3.6. Competitive adsorptions Competitive adsorption occurs where adsorption of a mixture of adsorbates is carried out on one surface. Some of the components in the mixture may induce the adsorption of others or may coadsorb along with another component. There may be competitive adsorption among polymers, polymer and surfactants or among surfactants depending on the component adsorbate. Examples of competitive adsorptions are many in literature [216–218]. The coadsorption of naphthalene derivative and cationic surfactants on porous silica [219] showed that cationic surfactants like cetyltrimethylammonium chloride or cetyltrimethylammonium bromide adsorption on silica surface can induce the coadsorption of neutral molecules like 2-naphthol (56) and 2-(2-naphthyl) ethanol (57).

In the coadsorption of 2-naphthol along with cetyl pyridinium chloride, the adsorption of 2-naphthol rises along with the adsorption of the surfactant. A plateau region was observed while empty micelles are formed, which conform to this result. Micellar solubilization is preferred to coadsorption and 2-naphthol desorption starts to occur which completes at higher temperature. At higher pH, the maximum solute adsorption was observed at a surfactant concentration below CMC. 3.6.1. Competition among polymers The competitive displacement adsorption of chemically different water soluble polymer has been least studied. Adam and Robb [220] studied the competitive adsorption between sodium polyacrylate (NaPA) and sodium polystyrene sulphonate (NaPSS) and the displacement adsorption of NaPSS by NaPA on the CaCO3 surface. In organic solvent, the studies of competitive and displacement adsorption between different polymer species, such as polymethyl methacrylate (PMMA)/polystyrene(PS) [221,222] and polyethylene oxide (PEO)/PS pair on silica surface led to some significant findings: (i) in competitive adsorption, polar molecules adsorb preferentially on silica surface and (ii) in displacement adsorption, large displacer chains have greater exchangeability and adsorbed polymer chains are not easily desorbed from the adsorbing surface [223]. The adsorption of homodispersed polystyrene, polydispersed polystyrene and binary mixture of homodispersed polystyrene at a porous silica surface showed that when a polystyrene chain is easily penetrated into the pores of silica surface, the time to attain the equilibrium is less then 10 h [224]. When the larger polystyrene is forced to penetrate in to the pores, with the much deformation, an adsorption equilibrium time of 35 h is required. For the adsorption of binary mixture of the homodispersed polystyrene at higher initial concentration, the small polystyrene was observed over the large polystyrene at the early adsorption stage and with an increase in adsorption time, the large poly-

styrene was found to adsorb more than small polystyrene and equilibrium was reached at 35 h. The adsorption of polystyrene and poly(methyl methacrylate) (PMMA) on to a non-porous silica (Aerosil 130) in trichloro acetic acid was studied by spectroscopic method [225]. The amount adsorbed, A, the fraction, θ, of the silanol groups occupied by polymers chains and the fraction, ρ, of the polymer repeating unit directly attached to the surface silanol groups were calculated. The isotherms are of high affinity type, the θ-value for all the polymers are independent of molecular weight, but the ρvalue decreases with increasing molecular weight. The θ-value for polystyrene varies from 0.20 to 0.28, whereas that for PMAA varies from 0.29 to 0.77. When the individual, competitive and displacement adsorption of PEO with a narrow molecular weight distribution and poly 4vinyl N-propyl pyridinium bromide (PVPP) with complete adsorption onto a nonporous silica (Aerosil 130) from an aqueous KBr solution were compared, the isotherm for individual adsorption were found to be of high affinity type [226]. The adsorbed amount steeply reaches a plateau at the lower equilibrium concentration in the supernatant solution. The plateau of adsorbed amount of PEO increases with increasing KBr concentration. This stems from the fact that concentrated aqueous KBr solution is a poorer solvent for PEO than dilute aqueous KBr solution. It was found that the amount adsorbed under poor solvent condition is larger than those adsorbed under good solvent condition [227–230]. The result of competitive adsorption between PVPP and PEO shows the preferential adsorption of PEO over PVPP due to stronger hydrogen bond [226]. Exchangeability of PVPP by PEO increases with doses of PEO. The slight difference between two PEO molecules for the competitive and displacement adsorption stems from the constant adsorbed amount of PEO irrespective of molecular weight. The effects of poly(acrylic acid)(PAA)–Poly(ethylene) (PEO) comb polymer dispersant on the rheological properties and interparticle forces in aqueous silica suspensions under varying pH conditions were investigated [231]. The comb polymer was found to adsorb more strongly under acidic condition than basic condition, indicating that the PAA backbone of the copolymer preferentially adsorbs onto silica surfaces with the PEO teeth extending out from the surface into the solution. When concentration of the copolymer was less, silica suspension were stable due to electrostatic repulsion between the silica surfaces, but at high copolymer concentration under neutral and basic conditions, where the copolymer interacts weakly with silica surface, the suspension showed a transition from a dispersed to weakly flocculated state. Under acidic conditions, the silica dispersion also destabilized at intermediate copolymer adsorbed density and then was destabilized at higher adsorbed coverage. Studies on adsorption of copolymer with two different monomers units were reported by several workers [183,232– 238]. A study was carried out on adsorption of ethylene vinyl acetate (EVA) copolymer on silica surface using IR spectroscopy [239]. The adsorption isotherm consists of an initial steep rise in absorbance, followed by a plateau where the absorbance is almost constant. The plateau somewhat increases with decreasing vinyl acetate content. This observation reveals that ethylene units in the

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EVA copolymer protrude into the bulk solution while vinyl acetate units are attached on the silica surface. Therefore, the EVA copolymers with low vinyl acetate content are expected to lead to the formation of larger loops than those for the high vinyl acetate content EVA copolymer. This result is consistent with data of Botham and Thies [233], who measured the adsorption of EVA copolymer on the silica from trichloroethylene solution. The adsorption of random copolymers of ethylene/vinyl acetate on modified silica i.e. (i) methylated and (ii) cyclohexyl modified silica showed that the adsorption isotherms (determined using FT-NMR spectroscopy) obtained for the adsorption of PVAC, on the two silica surfaces from CCl4 solution are of low affinity type because of the low molecular weight and polydispersity of the sample [240]. The adsorbed amount on the methyl surface was found to be greater than on the cyclohexyl surface and this difference increases with increasing equilibrium concentration of polymer. The desorption isotherms obtained for copolymer on methyl and cyclohexyl surface were of low affinity and this is due to polydispersity of the sample. For random copolymers there was polydispersity both in chain length and in composition. The adsorption of block polymers of ethylene oxide (EO), propylene oxide (PO) and the corresponding homopolymers on silica (Aerosil 1200) from CCl4 and CHCl3 solution were studied using IR spectroscopy by Killman et al. [241]. The adsorption isotherms were determined from concentration difference in the solution and the fraction of the adsorbed segments from the IR absorption of free and hydrogen bonded SiOH group. The adsorption isotherms show high affinity character with a more or less rounded transition to the plateau value. All the adsorption isotherms of the block polymers are located between isotherms of pure PEO and PPO homopolymers. With increasing EO fraction, the isotherms become similar to the PEO isotherms. At constant length of the PO middle block, the plateau value increases and the transition becomes more rounded with increasing length of the two EO and blocks. At constant EO fraction, the plateau value increases with increasing molecular mass. At constant molecular mass, the plateau value increases slightly and the isotherm becomes more rounded with increasing EO fraction. Competition and displacement studies demonstrate that the EO segments are preferentially adsorbed compared to the PEO segments. The fraction of adhered segments decreases with increasing amount adsorbed, molar mass, EO fraction and block length. 3.6.2. Competition between polymer and surfactant The adsorption of ethoxylated octyl phenols, triton-X-100 (TX100), triton-X-305 (TX305) and cetyltrimethylammonium bromide (CTAB) from their single and mixed aqueous solution on silica gel and methylated silica gel were studied at 25 °C [124]. The adsorption isotherm of CTA cation on silica shows a two-step behaviour. The first adsorption plateau due to the ionic group of a surfactant ion is directly opposed to a SiO−group on the surface by a charge interaction mechanism. At a particular concentration, the adsorption increases due to hemimicelle formation. As the concentration increases further, the isotherm reaches second plateau somewhat above the CMC of the surfactant. The general form of isotherm of TX-100 and TX-305 are similar to those

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obtained earlier [242] whereas that of TX-100 was found to be convex to concentration axis at low surface coverage, indicating a weak interaction between adsorbate and adsorbent. As the surface coverage increases, the interaction between hydrophobic chains becomes important and the isotherm turns up. That of TX-305 is of Langmuir type. It is suggested that the TX-100 and TX-305 molecules in the first layer are attached to the silica gel surface by their EO chains such that their hydrocarbon chains are exposed outwards. As the concentration increases, the second layer forms on the surface. The surfactant molecules on the second layer are presumed to adsorb on those of the first in the opposite orientation with the EO chains directed towards water phase. On methylated silica, there is no significant adsorption of CTA cation. This supports that SiO− group is no longer present on the silica surface. Moreover, in view of hydrophobic surface of methylated silica gel and the hydrophobic EO long chain of TX-305, it is not surprising that TX-305 has very low adsorption on methylated silica gel. The adsorption of TX-100 on silica and methylated silica gel has also been measured where the general scheme of isotherms are similar in both the cases, though at low concentration the adsorption on methylated silica gel is greater than that on silica gel and at high concentration reverse is true [243]. On the other hand, the adsorption on methylated silica is greater than that on silica at low concentration for the same nonionic surfactant, but adsorption isotherms at very low concentrations are different in shape. When mixed solution of CTAB and poly vinyl alcohol (PVA) is adsorbed on silica gel, the adsorption of PVA is increased significantly in the presence of pre-adsorbed CTA+ ions at high pH and CTA+ ions are adsorbed to a great extent in presence of pre-adsorbed PVA at low pH [244]. In this case, CTA+ ion acts as “anchor” between the dissociated SiO-sites on the surface at high pH, and PVA as the “anchor” between the undissociated SiOH groups and CTA+ ions at low pH. In a mixed isotherm study it was observed that TX-100 and CTA+ mutually increase adsorption of each other at low concentration, but decrease the adsorption of each other at high concentration [124]. This may be due to the fact that CTA+ ion can be adsorbed through electric interaction at low concentration and this in turn, will attract TX-100 molecules through hydrophobic interaction. Similarly, the adsorbed TX-100 molecules can also promote the adsorption of CTA+ ions through hydrophobic interaction and as the concentration increases, the mixed micelle is formed. The adsorption properties of poly(vinylimidazole) (PVI) and sodium dodecyl sulphate (SDS) from PVI/SDS mixed solution on negatively charged silica surface were studied at pH 9 using reflectometry [245]. Both the PVI/SDS uptake and the kinetic of adsorption decreased with the amount of SDS bound to PVI. At low PVI/SDS binding the complex, adsorption is transport limited while at high binding, the incoming complex experiences a blocking barrier of an electrostatic nature. The interaction between dodecyltrimethylammonium chloride (DTAC) and poly (vinyl pyrrolidone) (PVP) on silica in aqueous solution shows that for individual adsorption, PVP exhibits a strong affinity towards silica due to sharp increase in the adsorption at very low PVP concentration and shows a nearly horizontal plateau in the concentration range of 0.7–1.2 g dm− 3 [246]. It is known that at low PVP concentration, the typical conformation of

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PVP chain molecules consists of a sequence of many short loops and trains and at higher coverage, the bound fraction decreases due to the formation of a dense loop layer [247]. The interaction between PVP and silica is due to hydrogen bonding. In addition, since PVP molecules are slightly positively charged and electrostatic attraction force operates between the negatively charged silica and positively charged PVP, two-step adsorption isotherm is noticed. As the concentration increases, a bilayer is formed by hydrophobic interaction between the hydrocarbon chains of the first and second adsorbed layers. When the mixed adsorption is studied, it is found that at the highest concentration of PVP, its adsorption decreases gradually with increasing DTAC concentration. But, while the amount of DTAC increases, the magnitude of increased adsorption is lower than that in the absence of PVP. A similar result was obtained at low concentration of PVP, where the adsorption of PVP becomes smaller with decreasing feed concentration of PVP (Fig. 17). Since the interaction between PVP and cationic surfactant in aqueous solution was observed to be nonexistent or very weak [248–252], contrary to the case of nonionic surfactants, it is

Fig. 17. Competitive adsorption of PVP and DTAC on silica in the presence of PVP. PVP concentration: (a) 0.2; (b) 1.0; (c) 1.8 g dm−3.

understood that the competitive adsorption between PVP and DTAC on silica occurs. 3.6.3. Competition among surfactants A study of mixed adsorption on methylated silica gel showed that the adsorption of TX-100 from mixed solution at constant feed molar ratio of 1:1 TX100-CTAB is less than one tenth that of TX-100 solution [124]. The CTAB is scarcely adsorbed on methylated silica and the depression effect of CTAB on the adsorption of TX-100 is attributed to the mixed micelle formation in the bulk solution. The mixed adsorption study of TX-305 and CTAB on to methylated silica showed that there is no adsorption of either of the surfactant because neither TX-305 nor CTAB adsorbs individually on methylated silica surface. Another study of adsorption of mixed solution of TX-100 and sodium n-alkyl sulphates on silica gel showed that the adsorption amounts of pure sodium n-alkyl sulphates are very much smaller than that of TX-100 at the same molarity which may be attributed to the negative charge of the sodium n-alkyl sulphates [120]. When the mixed surfactant is adsorbed on silica, it is found that below the CMC of TX-100 in the mixed solution, addition of sodium n-alkyl sulphates (if its concentration is lower than the CMC of pure sodium n-alkyl sulphates) is not different from that of pure TX-100. This observation indicates that there is no influence of n-alkyl sulphates in this region. However, above CMC, the adsorption of TX-100 gradually tends to be limiting one. The limit of adsorption decreases with increasing sodium n-alkyl sulphate concentration. The adsorption of dodecyltrimethylammonium bromide (DTAB), dodecylpyridinium bromide (DPB), sodium dodecylbenzene sulphonate (SDBS) and sodium dodecyl sulphate (SDS) from their single aqueous solutions and DTAB-SDBS and DPBSDS binary mixed solution on silica gel was investigated [253]. The results showed that the individual cationic surfactants can be adsorbed strongly on to the silica gel, but no significant adsorption of anionic surfactants can be detected. However, in the mixed system, the adsorption amount of both the cationic and anionic surfactant ions are enhanced, and the excess adsorption of cationic surface active ions is exactly equal to the adsorption of anionic surface-active ions [254,255]. The adsorption isotherm onto hydrophilic silica mixtures of sodium dodecyl sulphate (SDS) and all the oligomers of polydispersed nonyl ethylene glycol n-dodecyl ether (C12E9) surfactants were determined using high performance liquid chromatography [256]. The incorporation of the anionic surfactant to silica surface was found to be favoured by non ionic surfactant. Comparison between the adsorption isotherm of mixture of SDS with monodispersed C12E9 and poly dispersed C12E9 shows a stronger adsorption of SDS at silica/water interface with the later material than with the former with a large surface coverage domain. There are extensive studies [257–260] on mixed aqueous surfactant solutions because they show unique properties, which are not expected from single surfactants. In particular, mixed systems of hydrocarbon and fluorocarbon surfactants were studied by various methods such as conductivity [261–264], surface tension [265], fluorescence probes [266,267] and 19F

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NMR [268]. It was observed that the system contains mixed micelles or two kinds of micelles, each rich in one of the surfactants depending on the nature of the surfactants, the composition of the system and the total surfactant concentration. These solution properties affect the adsorption behavior at the solid–liquid interface [124,269–271]. The adsorption of binary mixtures of diethanol heptadecafluoro-2-undecanol methyl ammonium chloride (DEFUMAC) and hexadecylpyridinium chloride (HDPYC) from their aqueous solution on silica shows that the adsorbed amount of HDPYC is larger than that of DEFUMAC and the total adsorbed amount of the mixture decreases with increased feed mole fraction of DEFUMAC [272]. The increase in adsorption of both DEFUMAC and HDPYC increases with equilibrium concentration of their respective surfactants. Total adsorption of these mixture are found to be intermediate between those of respective single surfactants. 4. Thermodynamics of adsorption The standard free energy of adsorption of one component from a dilute solution may be estimated from the adsorption data by using Langmuir equation and from the adsorption study at different temperatures, the enthalpy parameters can be calculated by using the Clausius Clapyeron equation [273,274]. Thermodynamic properties can be studied and estimated by using modern techniques like calorimetry, microcalorimetry, mass spectrophotometry, photon correlation spectroscopy (PCS), kinetic study using temperature jump relaxation techniques and many others. The adsorption characteristic of polymer is expressed in thermodynamic parameters such as ΔG, ΔH, and ΔS. Evaluation of these parameters gives an insight into the possible mechanism of polymer adsorption. A negative ΔG value stands for the adsorption to take place. Change in enthalpy (ΔH) gives an indication of the bonding strength. The higher the value of heat of adsorption, the weaker is the bond between adsorbate and adsorbent since adsorption of polymer molecules on to solid from dilute solution is believed to be entropically driven. The sign of ΔS would indicate the direction, for adsorption, (+ ΔS) and for desorption, (− ΔS).

Fig. 18. Plot of ΔHdil for DTAB at 28 °C showing the method of calculating ΔHmic.

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The free energy of adsorption (ΔGads°) was calculated for the adsorption of EHEC (ethyl (hydroxy ethyl) cellulose) polymer on to colloidal silica surface from aqueous solution [275], which depends on silica surface chemistry but does not correlate directly with advancing water contact angle of silica and suggests that EHEC adsorption is not directly controlled by hydrophobic interaction. On increasing the temperature from 18–37 °C, the plateau coverage of EHEC increases while the layer thickness decreases due to reduction in solvation. For hydrophilic and dehydrated silica particles, ΔGads° decreased in magnitude with increasing temperature, whereas for chemically treated silica that increases with temperature. From the study of thermodynamic properties of micellization as well as enthalpy change of adsorption by isothermal titration calorimetry using cationic surfactant such as dodecyl trimethylammonium bromide, tetradecyl trimethylammonium bromide and hexadecyl trimethylammonium bromide with Hisil 233 as substrate at different temperature, the values of ΔHdil, ΔHmic., CMC, ΔGmic. and ΔSmic. were determined by Stodghill et al. [276] (Fig. 18). They proposed an adsorption mechanism from the plot of ΔHdisp. versus concentration. Calorimetric data at 27 °C for the adsorption of n-fatty acid from benzene solution on silica indicates the adsorption to be independent of surface coverage and chain length [277]. It was further postulated that the expected decrease in heat of adsorption due to adsorbate– adsorbent interaction is offset by a decreasing enthalpy of mixing of the adsorbed film. Heats of adsorption of benzene, pyridine, and furan on to silica were measured calorimetrically [276–280]. Much work has been devoted to the development of infra red spectroscopy as an indirect method of determining heats of adsorption on silica including aromatics from both the liquid [281–283] and gas [279,280,284–286]. The heats of immersion of a well-characterized mesophorous silica gel (Davisil 62) in the solution of six organic compounds 2-methyl naphthalene (58), benzothiophene (59), 2-methyl benzothiophene (60), pyridine, isoquinoline (61), and quinaldine (62) in isooctane (2,2,4,-trimethyl pentane)

were measured as a function of surface coverage by batch solution microcalorimetry [286]. The contribution of wetting to the heat of adsorption of pyridine at the silica/isooctane interface and the effect of structure and function on the heats of adsorption of aromatic compounds and heterocycles at the silica/isooctane interface was ascertained. The heat of immersion of silica in pure isooctane was found to be — 4.2 ± 0.2 cal/ gm and in pure anhydrous pyridine — 16.3 ± 0.5 cal/gm. 2Methyl naphthalene molecule becomes too large to be adsorbed parallel to this surface up to the observed maximum coverage. Thus half the maximum is reached with — 1.6 kcal/mol heat and than substituent adsorption takes place either by formation of an alternant bilayer or by tilting of the rings to accommodate more molecules. This type of change in adsorption mode does

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not occur with benzene. Nitrogen bases like pyridine, isoquinoline, quinaldine adsorb much more strongly than the aromatic hydrocarbons due to hydrogen bonding between basic nitrogen and acidic silanol protons. The study of heats of adsorption of polyethylene oxide (PEO) of 5 million molecular weight by microcalorimeter on dolomite, alumina and silica surface at various temperature and concentration of the polymer show positive ΔH value i.e., the process is endothermic [287]. This indicates that PEO adsorption on the various substrates is governed mainly by the entropy changes in the system and ΔH is a function of temperature and also polymer preparation time. The heats of adsorption of series of n-fatty acid methyl esters at silica/benzene and silica/CCl4 interfaces were determined by calorimetric method [288]. The heats of solution of the esters were also determined in benzene and CCl4. In all cases, the heat of immersion increases with the solution concentration, until the limiting adsorption value is attained, thereafter, the heat of immersion remains constant. The preferential molar heats of adsorption from both solutions are independent of the surface coverage of the adsorbate. The heats of adsorption at 25 °C from the benzene solutions show no detectable dependence on the chain length of the ester. A similar lack of dependence on chain length is observed at 50 °C, although there is a decrease in the value of the heats of immersions. The latter is in agreement with the hypothesis that there is a net decrease in order of adsorbed layer as the adsorption temperature is increased. Similar results were found with the CCl4 solutions at 25 °C, although the value of the preferential molar heats of adsorption were higher from this particular solvent. Experimental results indicate that the isolated surface silanol groups of the adsorbent are the center of adsorption. An elegant way to describe the interaction between silica particle with water and organic solvent is to combine adsorption isotherm and immersion calorimetry. The equation correlating the different surface thermodynamic parameters were discussed in detail, even in the case of three component solutions [289– 291]. Adsorption calorimetric studies of solution and water and benzene on silica were also reported [292]. Interaction of water and organic solvent with silica shows that although silica is poorly hydrophilic, water is the most energetic solvent for adsorption [293]. All the energetic terms related to the adhesion between the water and the XOB-015 silica indicate that the number of layers necessary to obtain a perfect wetting of silica is less than two, as in case of other silicates. The decrease in surface tension due to water vapour adsorption is around 100 dyn cm− 1. The enthalpy of immersion is responsible for adhesion for hydrocarbons. The thermodynamics of adsorption involved two aspects: the tendency and extent of the process, which have theoretical and practical importance to understand the inherent characteristics of adsorption. Although there are complexities of solid/liquid interface system and experimental difficulties, a few investigation on the thermodynamics of adsorption from the solution have been reported [294–298]. The adsorption thermodynamics of benzene-n-heptane system [299], organic compounds like n-fatty alcohols, cyclohexanone, toluene and the oxy-aromatic compounds at the silica gel/

carbon tetrachloride and silica gel/cyclohexane interfaces were used to calculate the standard free energy of the adsorption process (ΔG°) by the Langmuir parameters [98]. The results suggested that (i) For n-fatty alcohol homologs, the value of ΔG° follows an empirical formula, −DG- ¼ 23:5−3:80lnnðkJ mol−1 Þ where n is the number of carbon atoms in the alcohol chain, and the hydroxyl groups on the silica gel show properties to those water or alcohol molecules in forming a hydrogen bond with the fatty alcohol molecules. (ii) For the aromatic compounds, the values of ΔG° decrease in the order of toluene, anisole, benzaldehyde, benzoic alcohol, and benzoic acid due to the different strength of hydrogen bonds between their molecules and surface silanol groups. (iii) The value of ΔG° for the pretreated silica gel to adsorb cyclohexanone has some relation to the concentration of the surface silanol groups. Calorimetric investigation on the effects of position of functional group on surfactant adsorption indicates that at low adsorption densities, enthalpy is the driving force for adsorption while at higher adsorption density, entropy is the driving force [300]. It was observed that the adsorption enthalpy for the paraxylene sulphonates at low adsorption densities is greater than that of meta-xylene sulphonate (63). However, at high adsorption densities, the adsorption enthalpy is similar for surfactant, but the adsorption entropy is higher for the para-xylene sulphonates (64), indicating tighter packing of the molecules in the hemimicelles which, leads to higher adsorption of the para-xylene sulphonates.

Hence, the position of the sulphonate and the methyl groups on the aromatic ring of alkyl sulphonates is found to be having a major effect on the energetics of their adsorption and the nature of their aggregation at the interface leading to a marked effect on the extent of adsorption. The kinetics of adsorption of cationic surfactants like CTAB, MTAB and DTAB at different pH, ionic strength and temperature in presence of various electrolyte and urea shows two steps, each of first order kinetics with two kinetic constants, k1 and k2 [301]. The values of energy of activation and entropies of activation indicate the involvement of enthalpy controlled activation reaction. The thermodynamics of adsorption of three different proteins, lysozymes, β-lactoglobulin, and hemoglobin on silica powder also follow two kinetic steps, each of first order with two kinetic constants i.e. k1 and k2 [104] (Table 4).

S.K. Parida et al. / Advances in Colloid and Interface Science 121 (2006) 77–110 Table 4 Kinetic parameters for dynamic adsorption of Lysozyme at silica–water interface Protein concentration C0p(g%)

pH

Temperature (K)

k1 (h−1)

k2 (h−1)

0.01 0.01 0.01 0.02 0.02 0.02 0.10 0.10 0.10 0.01 0.01 0.01 0.15 0.15

11.0 11.0 11.0 11.0 11.0 11.0 11.0 11.0 11.0 7.0 7.0 7.0 7.0 7.0

291 301 310 291 301 310 291 301 310 291 301 310 301 310

20.0 34.0 56.0 24.0 37.0 60.0 – – – 23.0 50.8 83.8 – –

– 18.3 22.4 12.3 18.8 24.8 2.3 2.4 2.6 16.0 20.4 24.0 1.1 1.2

In case of all the proteins, ΔH1 N TΔS1 for k1 and TΔS2 N ΔH2 for k2. The values of k1 follow the order, lysozyme N βlactoglobulin N hemoglobin, but the activation energy, enthalpy of activation, entropy of activation, are found to be of same order for the adsorption of each of the three biopolymers. Adsorption–desorption kinetics at methylated silica/solution interface were investigated using the temperature jump relaxation technique and thermodynamic parameters for the adsorption of carbonyl compounds [302,303]. Some methods were reported to estimate the thermodynamic parameters [304– 306]. There is no direct method to measure ΔG and ΔS. But ΔH can be experimentally measured. ΔG is generally estimated from the equilibrium adsorption data under the assumption that the adsorption of a long chain organic molecule is reversible and that an equilibrium condition is established in the system. ΔH and ΔS are estimated by determining the adsorption isotherm at different temperatures assuming these parameters to be independent of temperature. However, in case of polymer adsorption these assumption may not be valid because more than one type of adsorption force may be acting at any instant. Mass spectrophotometry and differential scanning calorimetric study of adsorption of lysozyme from white of hen egg to small colloidal silica particles at various surface concentrations using H–D exchange, presented the lysozyme structure in terms of enthalpy and Gibb's free energy [307]. At high surface coverage, the structural ability of lysozyme was only marginally effected by adsorption to silica particles whereas the unfolding enthalpy decreased by 10% meaning that the entropy of lysozyme increased with a similar value upon adsorption. At lower surface concentration, the structural heterogeneity increased further where as the enthalpy of unfolding decreased. Folding/unfolding of lysozyme occurs through two domain process and there is a difference in stabilization energy of 8 kJ/ mol between two domains. The interaction of fumed silica A-300 (SBET = 297 m2g− 1) with bovine serum albumin (prepared by different methods), ovalbumin, human hemoglobin, and gelatin in aqueous medium was studied using photon correlation spectroscopy (PCS) methods [308]. Comparison of equilibrium adsorption of

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proteins on A-300 (with approximately 1 h of incubation time), minute flocculation rate (approximately 1 min) and the particle size distributions measured by the PCS method shows different rearrangement of particle swarms depending on pH, salinity, and concentration of proteins, especially at pH close to isoelectric potential (IEP) of silica or proteins. The electrokinetic mobility of protein/silica swarms is greater than that of individual components at pH far from the IEP of proteins. Changes in the Gibbs free energy (ΔG) on protein adsorption depend on pH (− ΔG is minimal at pH 2, close to the IEP of silica, and maximal at pH between the IEP of protein and silica), concentration, type of proteins, and their preparation technique. 5. Adsorbate–adsorbent interactions The interaction of adsorbent and adsorbate leads to the formation of adsorbed surface. To know the molecular interaction of these two species, the analysis of the structure of adsorbed layer is prima facie. The cruxes of the analysis are: (a) (b) (c) (d)

the chemical nature of the materials involved the solution concentration the interaction between adsorbate and adsorbent lateral interaction between adsorbed species

For a majority of system involving adsorption from dilute solution, a monolayer is postulated. The analysis of adsorption isotherm also suggests either a change in orientation or the formation of a multilayer. The measurements of macroscopic properties like adsorption density, zeta potential, floatation efficiency etc. [77,309] are used for the characterization of the adsorbed surface. However, such studies, which are helpful in developing an insight in to the adsorption mechanism, are unable to provide any direct information on the microscopic character of the adsorbed species. Spectroscopic techniques are mostly in use to answer many questions regarding atomic and molecular level concerning the bonding of adsorbates into surface sites of solid particles. The elemental nature of surface can be characterized by Xray photoelectron spectroscopy [310–312] and augur spectroscopy [313], and its chemical state can be viewed through valence level photoemission spectra and molecular vibrational spectra. Similarly vibrational spectroscopy can be used to study local ordering while diffraction method can be used to study long range ordering. However, these techniques require ultra high vacuum. Some vibrational spectroscopic methods such as IR [314] and Raman [315] are found to be useful for the structural study of the adsorbed layer under equilibrium condition. Luminescence and electron spin resonance spectroscopic techniques [316,317] are also found useful in the analysis of adsorbed surfaces. Use of the above techniques to the adsorbed layer of surfactant on solids can provide information on the interior of the adsorbed surfactant layers on solid at solid/liquid interface under in situ condition. By this technique one can get information regarding the micropolarity, microfluidity and aggregation numbers of adsorbed surfactant layer. Generally a probe is employed, whose emission property

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depends upon the environment of the probe. The commonly employed probes are pyrene (65), 1,3-dinaphthyl propane (66), tris(2,2-bipyridyl) ruthenium (II) chloride (67) etc. Berquier et al. [318] used atomic force microscopy images for the study of polymer layer adsorbed on two similar substrates such as silica and glass and proposed that the structure of the adsorbed layers are drastically different.

Hemimicelles, which result from aggregation of surfactant molecules on solid–water interface and which form two dimensional surfactant structures can be studied by fluorescence [319] and esr [320,321] spectroscopy methods. Hemimicellar micropolarity is found to be comparable to that of a micellar system, but the hemimicellar microviscosity shows marked difference (about ten times more) from the bulk micellar viscosity. Somasundaran et al. [322] studied the orientation of the sodium dodecyl sulphate molecules on alumina surface by excited state Resonance Raman Spectra of Ru (bpy)32+in the adsorbed layer of surfactant. The study clearly shows several transitions sensitive to the evolution and structure of hemimicelles. The nature of sorption of nonpolar sorbates at polar solvent/ oxide interface and polar sorbates at nonpolar solvent/oxide interface were studied using Raman spectroscopy [323–325]. The investigation of adsorption behaviour of benzene, pyridine and azobenzene on CCl4 /silica surface provided the evidence of strong hydrogen bonding involving benzene π-bonding electrons and silanols. The study indicated that at high surface coverage, azobenzene interacts with silica as it would react in polar media and therefore, interaction at such high coverage is explained in terms of hydrogen bonding involving azo non bonding-electron and weakly acidic silanol. Whereas, at low surface coverage azobenzene interacts with silica strongly at the proton donating sites on silica. The adsorption of non ionic surfactants, C12(EO)6 and C12 (EO)25 on hydrophilic surface was studied experimentally and theoretically [326]. Neutron reflection study reveals that the plateau of the adsorption isotherm is due to bilayer formation. NMR measurement shows that the EO segments in the adsorbed layer of C12(EO)6 and C12(EO)25 have more conformational restriction than those in PEO layers because of the association of aliphatic tail. By using self consistent field lattice theory for C12 (EO)6 adsorption and/or association, stepwise adsorption and bilayer formation were proposed. 13 C NMR and 15N NMR [284,327–332] spectral techniques were also used to analyze the adsorbed layer. The chain conformation of adsorbed surfactant on both alumina and silica was analyzed by 13C chemical shift [333]. From 13C CPMAS NMR analysis, Ebener et al. [334] showed that larger nonvolatile benzenoid aromatics (MW N 120) can also be incorporated into

silica or alumina surface by mixing or mechanically grinding the components or by solvent assisted mixing. The adsorption of pyrazole (68) and 3,5-dimethyl pyrazole (69) on alumina and silica surface studied by 13C

and 15N NMR spectroscopy shows that (i) pyrazole and 3,5dimethyl pyrazole adsorb as neutral species and (ii) there are different adsorption sites where pyrazole and 3,5-dimethyl pyrazole experience not only different chemical shifts but also different rate constants of proton exchange [335]. Since the proton exchange in the cyclic pyrazole self association is much slower in the adsorbed state, it follows that it is catalyzed by –OH groups on the alumina and silica surface. Optical reflectometry and atomic force microscopy techniques were used to study the adsorption behaviour of CTAB [336] and cetylpyridinium bromide at silica–aqueous interface [337]. Before a critical surface aggregation concentration (CSAC) of surfactants, adsorption proceeds slowly and at CSAC, adsorption occurs immediately. Lysozyme adsorption study was carried out using dual polarization interferometry [338] at a range of concentration from 0.03–4.0 g/dm3 and at pH 4 and 7. Adsorbed layer ranging from 14 to 43 ± 1 A° in thickness and 0.21 to 2.36 ± 0.05 mg/m2 in mass coverage was observed at pH 4 with increasing lysozyme concentration, indicating a strong deformation of the monolayer over the low concentration range and the formation of an almost complete sideways—on bilayer occurred towards the high concentration of 4 g/dm3. At pH 7, the thickness of adsorbed layer varied from 16 to 54 ± 1 A° with significantly high surface coverage (0.74 to 3.29 ± 0.05 mg/m 2 ), again indicating structural deformation during the initial monolayer formation, followed by gradual transition to bilayer adsorption over high concentration end. The conformation of cationic alpha-helical peptide (DDDDAAAA-RRRRR) adsorbed to anionic colloidal silica was investigated by using circular dichroism spectroscopy as a function of temperature and pH [339]. It was found that increased temperature destabilizes the helicity of peptide in solution, while changes in pH alter the substrate surface charge and the corresponding strength of interaction with the peptide. Near neutral pH, the helicity of adsorbed peptide decreases with increased temperature. The adsorption and formation of DNA and cationic surfactant complexes at the silica–aqueous interface were studied by ellipsometry technique [340]. It was observed that there was no adsorption of DNA. However, adsorption occurred when there was an excess of cationic surfactant just below the point of phase separation (Fig. 19). The adsorbed layer thickness was large at low surface coverage. A long range repulsive force was observed between adsorbed layer DNA-cationic surfactant complex, which was suggested to be both of electrostatic and steric origin.

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Fig. 19. Adsorption isotherm for C12TAB (filled circles) and DNA-C12TAB mixtures (open circles) as measured by ellipsometry. The 0.06 mg/mL DNA was used for the mixed system.

Intermittent contact atomic force microscopy (ICAFM) was used to investigate the structure of human plasma fibronectin adsorbed on to silica and methylated silica surface [341]. Fibronectin was found to remain as elongated structure on silica surface with partial intermolecular chain interaction, compared to molecule adsorbed on hydrophobic, methylated surface where a compact structure predominates. Hydrogen–Deuterium (H–D) exchange of amide hydrogen was employed to study the conformational stability of various parts of a protein physically adsorbed on to a nanometer sized particle [342]. This process is extremely sensitive to structural features of proteins. The resulting mass increase after adsorption was analyzed with Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometry. Higher structural specificity was obtained by enzymatically cleaving the adsorbed proteins prior to mass spectrometric analysis. The mass increase of the four peptic fragments of myoglobin is followed as a function of the H–D exchange time. The four peptic fragments cover 90% of the myoglobin structure. Two of the peptic fragments, located in the middle of the myoglobin sequence and close to the heme group, do not show any adsorption-induced changes in their structural stability, whereas the more stable Cand N-terminal fragments are destabilized. Interestingly, for the N-terminal fragment, comprising first 29 residues, two distinct and equally large conformational populations were observed. One of these populations has a stability similar to that in solution (− 23 kJ/mol), whereas the other population can be highly destabilized upon adsorption (− 11 kJ/mol). Evanescent wave spectroscopy was used to study the spatial distribution of charged group on an adsorbed electrolyte on the silica/water interface [343]. To illustrate this technique, the adsorption of a model polyelectrolyte polymer (polyacrylamide/ diacetoneacrylamide copolymer) grafted with an ionizable

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acridine chromophore, EPI-26, on to silica/water interface was studied. This technique was used to determine surface excess and the mean separation distance of charged and uncharged segments attached to the adsorbed polyelectrolyte from the interface. This technique was also used to measure the kinetics of adsorption. Generally alkyl modified silica are prepared by reacting either alkyl chloro or alkyl alkoxy silanes with silica. The interfacial properties of these surfaces were investigated by different methods such as IR [344–346], NMR [347–349] and electron spin resonance [350,351]. Deuterated dodecyl trichloro silane (Cl3 Si(CH2)10CD2CH3) was synthesised and treated with porous chromatographic grade silica to obtain a labelled alkyl modified surface [352]. The 2H NMR quadruple was recorded and the relaxation time for the material in presence of organic and aqueo-organic solvents, aqueous surfactant solutions and in the nematic phase of thermotropic liquid crystal were calculated. The results suggest that the motional averaging process for the C–2H bond is a combination of fast and intermediate molecular fluctuation similar to those observed in other restricted system such as lipid bilayers and micellar system. When the IR spectrum obtained from silica immersed in nheptane and toluene mixture was studied, the absorbance values show the perturbation of the proportion of surface hydroxyl groups by contact with each component of a liquid mixture [353]. Hence spectroscopic isotherms were deduced for the adsorption of each component on to specific (hydroxyl group) sites in oxide surface. Adsorption of pentane-3-one and diketones adsorption on silica surface from CCl4 solutions indicate the interaction of molecules containing a single carbonyl group with silanol group in two ways [354]. An isolated silanol, which shows IR bands at 3750 cm−1, can form a hydrogen bond with the ketone. Alternatively, a pair of adjacent surface silanol groups responsible for the shoulder at 3650 cm−1 in the spectra of preheated silica (at 723 K) can form two hydrogen bonds with a carbonyl group. For pentane-3-one at silica/CCl4 interface, the two modes of adsorption were characterized from the ΔνCO values (17 and 33 cm−1) in the infrared band at 1727 cm−1 in CCl4 solution. The adsorption of the molecules is thermodynamically more favourable when adsorption takes place on isolated silanols. The IR band at 1694 cm−1 has been found to be prominent in spectra for low coverage of available sites. The modes of adsorption of pentane2,4 dione on silica has been found to be of complex nature due to the enol-form of diketone. However, the study suggests that both keto-and enol-forms exist at solid/vapor interface. The adsorption of dioxane on porous silica and the adsorption behaviour of some silane derivatives on silica surface by using IR spectroscopic technique were also studied in similar line by the same workers [355–358]. The IR spectroscopic study of adsorption of cyclohexanone on silica surface immersed from 2,2,4 trimethyl pentane [359] medium reveals that cyclohexanone is adsorbed onto isolated silanol via the formation of hydrogen bond. The isolated silanols present close enough to each other form pair of hydrogen bonds with single adsorbate molecule. Surface hydroxyl groups in triplicate which interact laterally also provide sites for adsorption

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of cyclohexanone. At low surface coverage, the adsorption of cyclohexanone onto pair of surface hydroxyl group predominates over the adsorption onto single isolated silanol. From the study of adsorption of ethyl acetate on silica/CCl4 interface, Cross and Rochester [360] showed that the carbonyl group of ethyl acetate is responsible for forming hydrogen bond with silanol. The existence of hydrogen bonding interaction between adsorbate and adsorbent was also shown by Neagle and Rochester [361]. 1,2-Dihydroxy ethane and its methyl ether adsorb on silica/CCl4 interface through hydrogen bonding. No change in spectra of hydroxy compound after adsorption was recorded which suggest that one of the functional groups is linked to the surface by a hydrogen bond. However, hydroxy compounds are found to be involved in multilayer adsorption with high surface coverage at isolated silanol group adsorption sites. The adsorption behaviour of butane 2,3-diol, butane 1,3-diol and butane 1,4-diol on silica/ CCl4 interface suggests the involvement of associative adsorption in the formation of hydrogen bonds between isolated silanol groups and either one or both hydroxy groups of adsorbates [362]. Increase in the separation of two hydroxy groups in the series of three diols enhances the proportion of adsorbed molecules for which both groups interact with the surface. The adsorption study of 1,2-dimethoxy ethane (DME) on silica reveals the involvement of hydrogen bond formation with single methoxy group in the interaction of isolated silanol group [363]. A stronger mode of adsorption involving both methoxy groups in each DME molecules bonded to the surface has been proposed. This mode of adsorption is found to be predominant at low surface coverages during adsorption or partial adsorption induced by evacuation. Some theoretical works were undertaken on the polymer adsorption to analyze the adsorbate–adsorbent interactions [364– 367]. Copolymer can be divided in to two groups depending on whether only one component of the copolymer interacts with solid surface [368]. The adsorption of dimethyl acrylamide glycidyl acrylate copolymer on non-porous silica particle was studied by IR technique [369]. The large variation of the fraction of links was attributed to the preferential adsorption of the dimethyl acrylamide units over the glycidyl acrylate ones. The maximum amount adsorbed at plateau was found at 25 mol% glycidyl acrylate content. The FTIR investigation of CTAB and cetylpyridinium chloride on silica surface shows that micelle like surfactant aggregate cluster are formed on the silica surface in presence of water, even at surfactant surface coverage as low as 12% [370]. Increase in the surface density results in the increase of size and/or the number of surfactant aggregate cluster. The state of wetness changes the structure of aggregates. At low surface coverage, adsorbed surfactant molecules change from aggregate cluster state to a monomer state upon drying. During the drying process the methylene chain changes from aggregated state to flat and parallel orientation on the silica surface, preferring an association with silica surface instead of with other methylene chains. On wetting, the surfactant molecules revert back to original form. For high coverage no change is observed for wetting and drying process, which may be attributed to insufficiency of space over silica surface for rearrangement.

6. Future prospective Silica has a well-characterized surface and can be modified to a wide range of functionality owing to the presence of active hydroxyl groups. The surface can be tuned to high acidic system by impregnating carboxylic functions as well as can be made strongly alkaline. Thus it can promise for a wide range of surface catalytic systems for various types of chemical reactions for industrial, biological, and pharmacological processes. The industrial processes like flotation, flocculation, and ceramic processing etc., which chiefly depend on the nature of the interface, can be controlled by manipulating surface characteristics. The surface charge of the silica can also be tuned by the adsorption of the surfactants, which can be beneficial for controlling dispersion and flocculation in various industrial processes. Silica surface can act as a template for the synthesis of high molecular weight polymers and biomolecules of medical importance. Using this template drug delivery technique can be developed whereby specific drugs can be applied directly to localized area thus resulting in an enhanced remedy without affecting other parts of the body. Photosensitive compounds adsorbed on silica surface can be subjected to external irradiations and their reaction mechanisms can be studied with more ease and with better profoundness. Such study can be utilized in developing photo-sensitized chemical machines which can replace electronic chips and can produce pollution-free micro machineries. Acknowledgement Generous funding from the I.U.C., Calcutta, DST (FIST) and the U.G.C. (DRS), Research award of IX plan period to BKM and Minor research project to SKP, New Delhi are acknowledged. References [1] Bose A, Gilpin RK, Jaroniec M. J Colloid Interface Sci 2001;240:224. [2] Bolis V, Fubini B, Marchese L, Martra G, Costa D. J Chem Soc Faraday Trans 1991;87:497. [3] Daou K, Wang RZ, Xia ZZ. Appl Therm Eng 2006;26:56. [4] Davydov V Ya, Zhuravlev LT, Kiselev AV. Russ J Phys Chem 1964;38:1108. [5] Hockey JA. Chem Ind (Lond) 1965;2:57. [6] Peri JB. J Phys Chem 1966;70:70. [7] Hair ML, Hertl W. J Phys Chem 1969;73:4269. [8] Van Cauwelaert FH, Jacobs PA, Uylterhoeven B. J Phys Chem 1972;76: 1434. [9] Morrow BA, Cody LA, Lee LSM. J Phys Chem 1978;82:2761. [10] Van Roosmalen AJ, Moe JC. J Phys Chem 1978;82:2748. [11] Van Roosmalen AJ, Moe JC. J Phys Chem 1979;83:2485. [12] Ghiotti G, Garrone E, Moterra C, Boccuzzi F. J Phys Chem 1979;83: 2863. [13] Hoffmann P, Knozinger F. Surf Sci 1987;188:181. [14] Zhdano VSP, Kosheleva LS, Titova TI. Langmuir 1987;3:960. [15] Peri JB, Hansley AL. J Phys Chem 1968;72:2926. [16] Rochester CH, Trebilco DA. J Chem Soc Faraday Trans I 1979;75:2211. [17] Jal PK, Sudarshan M, Saha A, Patel S, Mishra BK. Colloids Surf A Physicochem Eng Asp 2004;240:173. [18] Bell RJ, Bird NF, Dean P. J Phys Chem 1968;72:299. [19] Morrow BA, McFarlan AJ. J Phys Chem 1992;96:1395. [20] Hino M, Sato T. Bull Chem Soc Jpn 1971;44:33.

S.K. Parida et al. / Advances in Colloid and Interface Science 121 (2006) 77–110 [21] Boccuzzi F, Coluccia S, Ghiotti G, Morterra C, Zecchina A. J Phys Chem 1978;82:1298. [22] Lang SJ, Marrow BA. J Phys Chem 1994;98:13314. [23] Benesi HA, Jones AC. J Phys Chem 1959;63:179. [24] Low MJD, Severda AG, Arnold TH. Spectrosc Lett 1983;16:207. [25] Fukui K, Miyauchi H, Iwasawa Y. Chem Phys Lett 1997;274:133. [26] Ferraro J, Manghari M. J Appl Phys 1972;43:4595. [27] Civalleri B, Garrone E, Ugliengo P. Chem Phys Lett 1998;294:103. [28] Zarzycki J, Naudin F. J Chim Phys Phys Chim Biol 1961;58:830. [29] Ugliengo P, Garrone E. J Mol Catal 1989;54:439. [30] Smirnov KS, Smirnov EP, Tsyganenko AA. J Electron Spectrosc Relat Phenom 1990;54–55:815. [31] Senchenya IN, Garrone E, Ugliengo P. J Mol Struct Theochem 1996;368: 93. [32] Ermoshin VA, Smirnov KS, Bougeard D. Surf Sci 1996;368:147. [33] Orcel G, Phalippou J, Hench LL. J Non-Cryst Solids 1986;88:114. [34] Delia Valle RG, Venuti E. J Chem Phys 1994;179:411. [35] Wilson M, Madden PA, Hemmati M, Angell CA. Phys Rev Lett 1996;77:4023. [36] Sarnthein J, Pasquarello A, Car R. Science 1997;275:1925. [37] Taraskin SN, Elliot SR. Phys Rev B 1997;56:8605. [38] Brinker J, Sherer W. Sol–Gel Science, The Physics and Chemistry of Sol–Gel Processing. New York: Academic Press; 1990. Chapter 9. [39] McCool Ben, Murphy Lawrence, Tripp Carl P. J Colloid Interface Sci 2006;295:294. [40] Carteret C. Spectrochim Acta Part A 2006;64:670. [41] Feng A, McCoy BJ, Munir ZA, Cagliostro DE. J Colloid Interface Sci 1996;180:276. [42] Shen JH, Zeltlemoyer AC, Klier K. J Phys Chem 1980;84:1453. [43] Devydov V Ya, Kiselev AV, Lokutsievskii VA, Ligin VI. Russ J Phys Chem 1974;48:1342. [44] Kiselev AV, Lokutsievskii VA, Ligin VI. Russ J Phys Chem 1975;49: 1053. [45] Shen JH, Klier K. J Colloid Interface Sci 1980;74:75. [46] Yamauchi H, Hondo S. Colloid Polym Sci 1988;266:855. [47] Fripiat JJ, Uytterhoeven J. J Phys Chem 1962;66:800. [48] Burneau A, Barres O, Vidal A, Ballard H, Ligner G, Papiren E. Langmuir 1990;6:1389. [49] Burneau A, Barres O, Gallas JP, Lavalley JC. Langmuir 1990;6:1364. [50] Gallas JP, Lavalley JC, Burneau A, Barrers O. Langmuir 1991;7:1235. [51] Caillerie J, Aimeur AR, Kortobi YE, Legrand AP. J Colloid Interface Sci 1997;194:434. [52] Tonks L. J Chem Soc Faraday Trans I 1989;85:585. [53] Baker BG. J Chem Phys 1966;45:2694. [54] Garrone E, Ugliergo P. J Chem Soc Faraday Trans I 1989;85:585. [55] Beebe TP, Galin P, Jates JT. Surf Sci 1984;184:526. [56] Zechina A, Ghiotti G, Cerruti L, Morterra C. J Chim Phys 1971;68:1479. [57] Knozinger H. In: Schuster P, Zundel G, Sanderfi C, editors. The Hydrogen bond, Recent developments in theory and experiments, vol. III. Amsterdam: North Holland; 1976. p. 1263. [58] Cant NW, Little LH. Can J Chem 1964;42:802. [59] Garrone E, Ugliergo P. Langmuir 1991;7:1409. [60] Lavalley JC, Gallas JP, Burneau A, Barres O. Langmuir 1991;7:1235. [61] Yoon R, Vivek S. J Colloid Interface Sci 1998;204:179. [62] Drost-Hansen W. Ind Eng Chem 1969;10:61. [63] Drost-Hansen W. In: Brown DH, editor. Chemistry of the Cell Interface, Part B. New York: Academic Press; 1971. [64] Drost-Hansen W. In: Franks F, editor. Biophysics of Water. New York: Wiley; 1982. [65] Etzler FM, Drost-Hansen W. Croat Chem Acta 1983;56:563. [66] Etzler FM, Drost-Hansen W. In: Drost-Hensen W, Clegg S, editors. Cell Associated Water. New York: Academic Press; 1979. p. 125. [67] Bogdan A, Kulmala M. J Colloid Interface Sci 1996;177:79. [68] Braun CV, Drost-Hansen W. In: Kerker M, editor. Colloid and Interface Science, vol. III. New York: Academic Press; 1976. p. 533. [69] Frank M, Etzier J, White Pamela. J Colloid Interface Sci 1987;1:4320. [70] Zhang J, Grischkowsky D. Opt Lett 2004;29(9):1031. [71] Dinh LN, Balooch M, LeMay JD. J Colloid Interface Sci 2000;230:432.

107

[72] Fedeev AY, Eroshenko VAE. J Colloid Interface Sci 1997;187:275. [73] Gun'ko VM, Turov VV, Bogatyrev VM, Zarko VI, Leboda R, Goncharuk EV, et al. Adv Colloid Interface Sci 2005;118:125. [74] Puibasset J, Pellenq RJM. J Phys Condens Matter 2004;16:S5329. [75] Puibasseta J, Pellenq RJM. J Chem Phys 2005;122:094704. [76] Mischler C, Horbach J, Kob W, Binder K. J Phys Condens Matter 2005;17:4005. [77] Parfitt D, Rochester H. In: Parfitt GD, Rochester CH, editors. Adsorption from solution at the solid/liquid Interface. New York: Academic Press; 1983. p. 3. [78] Giles CH, Mac Ewan TH, Nakhwa SN, Smith D. J Chem Soc 1960:397; Giles CH, Smith D, Whitson A. J Colloid Interface Sci 1960;47:755. [79] Cosgrove T, Fergie Woods JW. Colloids Surf 1987;25:91. [80] Pefferkorn E, Caroy A, Varoqui R. J Polym Sci Polym Phys Ed 1987;23: 1997. [81] Cosgrove T, Vincent B, Stuart MC, Barnett KG, Sissons DS. Macromolecules 1981;14:1018. [82] Pefferkorn E, Haouam A, Varoqui R. Macromolecules 1989;22:2677. [83] Kawaguchi M, Takahasi A. Macromolecules 1983;16:1465. [84] Kawaguchi M, Hayakawa K, Takahasi A. Polym J 1980;12:265. [85] Kawaguchi M, Hayakawa K, Takahasi A. Macromolecules 1983;16:631. [86] Dejardi Ph, Varoqui R. J Chem Phys 1981;75:4115. [87] Gebhard H, Killman E. Angew Makromol Chem 1976;53:171. [88] Bandosz TJ. J Colloid Interface Sci 1997;193:127. [89] Magnacca G, Morterra C. Langmuir 2005;21(9):3933. [90] Miotto R, Ferraz AC, Srivastava GP. Phys Rev 2002;1365:075401. [91] Tao F, Qiao MH, Li ZH, Yang L, Dai YJ, Huang HG, et al. Phys Rev 2003;1367:115334. [92] McMahon A, Rhodes N. J Colloid Interface Sci 2005;287(2):379. [93] Mizukarni M, Moteki M, Kurihara K. Aust J Chem 2005;56(10):1071. [94] Mizukarni M, Moteki M, Kurihara K. J Am Chem Soc 2002;124:12889. [95] Liu D, Ma G, Allen HC. Environ Sci Technol 2005;39(7):2025. [96] Schenk M, Smit B, Maesen TL, Vlugt TJ. Phys Chem Chem Phys 2005;7:2622. [97] Fox JP, Bates SP. J Phys Chem 2004;108:17136. [98] Zhao ZG, Zhang LH, Lin Y. J Colloid Interface Sci 1994;166:23. [99] Yakimova TI, Matchenko AV, Koganovskii AM, Botsan V Ya. Zh Fiz Khim 1980;54:476. [100] Kagioa T, Sumida V, Tachi T. Bull Chem Soc Jpn 1971;44:1219. [101] Ahmad I, Dines TJ, Anderson JA, Rochester CH. J Colloid Interface Sci 1997;195:216. [102] Rochester CH, Strachan A. J Colloid Interface Sci 1996;177:456. [103] Andrieux D, Jestin J, Kervarec N, Pichon R, Privat M, Olier R. Langmuir 2004;20(24):10591. [104] Mishra A, Patel S, Behera RK, Mishra BK, Behera GB. Bull Chem Soc Jpn 1997;70:2913. [105] Mishra A, Behera PK, Behera RK, Mishra BK, Behera GB. J Photochem Photobiol A Chem 1998;116:79. [106] Mishra A, Behera RK, Mishra BK, Behera GB. J Photochem Photobiol A Chem 1999;121:63. [107] Mishra A, Behera RK, Behera PK, Mishra BK, Behera GB. Chem Rev 2000;100:1973. [108] Parida SK, Mishra BK. J Colloid Interface Sci 1996;182:473. [109] Parida SK, Mishra BK. Indian J Chem 1998;37A:618. [110] Parida SK, Mishra PK, Mishra BK. Indian J Chem 1999;38A:639. [111] Parida SK, Mishra BK. Colloids Surf A Physicochem Eng Asp 1998;134:249. [112] Gu T, Zhu BY. Colloids Surf 1990;44:81. [113] Wang J, Huang CP, Allen HE, Cha DK, Kim D. J Colloid Interface Sci 1998;208:518. [114] H.F. Fan, C.Y. Hung, K.C. Lin, Anal Chem, Published on Web 04/26/ 2006. [115] Zhong Z, Lowry M, Wang G, Geng L. Anal Chem 2005;77:2303. [116] Gaudin AM, Fuerstenau DW. Trans AMIE 1955;202:958. [117] Fuerstenau DW. J Phys Chem 1956;60:981. [118] Scamehorn JF. In: Scamehorn JF, editor. Phenomena in Mixed Surfactant System, ACS Sympo. Series, vol. 113. Washington DC: American Chemical Society; 1986.

108 [119] [120] [121] [122] [123] [124] [125] [126] [127] [128] [129] [130] [131] [132] [133] [134] [135] [136] [137] [138] [139] [140] [141] [142] [143] [144] [145] [146] [147] [148] [149] [150] [151] [152] [153] [154] [155] [156] [157] [158] [159] [160] [161] [162] [163] [164] [165] [166] [167]

S.K. Parida et al. / Advances in Colloid and Interface Science 121 (2006) 77–110 Weshie MM, Harwell JH. J Phys Chem 1988;92:2346. Gao Y, Ye C, Lu S, Gu W, Gu T. J Colloid Interface Sci 1984;100:581. Li W, Gao Y, Li X, Gu T. Acta Chim Sin (Engl Ed) 1985:306. Gao Y, Gu W, Lu S, Yue C, Gu T. Sci Sin Ser B (Engl Ed) 1986;29:919. Gao Y, Du J, Gu T. J Chem Soc Faraday Trans I 1987;83:2671. Huang Z, Gu T. Colloids Surf 1987;28:159. Zhu BY, Gong H, Zhao X. Acta Chim Sin (Chin Ed) 1988;46:928. Zhu BY, Zhao X. J Colloid Interface Sci 1988;125:727. Zhu BY, Zhao X, Gu T. J Chem Soc Faraday Trans I 1988;84:3951. Shirahama K. Colloid Polym Sci 1974;252:978. Smith ML, Muller N. J Colloid Interface Sci 1975;52:507. Misra PK, Mishra BK, Somasundaran P. J Colloid Interface Sci 2003;265:1. Ghosh AC, Satyanarayana K, Srivastava RC, Dutta NN. Colloids Surf A Physicochem Eng Asp 1995;96(3):219. Wangnerud P, Berling, Olofsson G. J Colloid Interface Sci 1995;169 (2):365. Zhu B, Yang B. Wuli Huaxue Xuebao 1995;11(1):15. Mohamed MM. Colloids Surf A Physicochem Eng Asp 1996;108:39. Suhara T, Fukui T, Yamaguchi M. Colloids Surf A Physicochem Eng Asp 1995;101:29. Lier RK. The Chemistry of Silica. New York: Wiley Interscience Publishers; 1979. Ter Minassian Saraga L. Adv Chem Ser 1964;43:232; J Chem Phys 1964;57:10. Rupprech H. Kolloid Z Z Polym 1971;2491:1127. Bijsterbosch BH. J Colloid Interface Sci 1974;47:186. Aronson MP, Princen HM. Colloid Polym Sci 1978;256:140. Elton GA. 2nd Int. Congr. Surf Activ. III. Electrical phenomena of solid/ liquid Interface. London: Butterworth; 1957. Jorin ZM, Churaev NV, Esipova NE, Sergeeva IP, Sobolev VD, Gasanov EK. J Colloid Interface Sci 1992;152:170. Rennie AR, Lee BM, Simister EA, Thomas RK. Langmuir 1990;6:1031. Menezes JL, Yan J, Shar MTVL. Colloids Surf 1989;38:365. Wong K, Cabane B, Duplessix R, Somasundaran P. Langmuir 1989;5: 1346. Woodburry GW, Noll LA. Colloids Surf 1988;33:301. Huang Z, Ma JM, Gu TR. Acta Chim Sin (Engl Ed) 1989;2:105. Partyka S, Lindheimer M. Proc. Symposium AECAT, Clermont Ferrand, May; 1990. p. 121. Bouzerda M, Zajac J, Trompette JL, Parlyka S. Proc. Symposium AFCAT, Compiegne, May; 1992. p. 249. Zhu BY, Gu TR. J Chem Soc Faraday Trans I 1989;85:3813. Cases JM, Villieras F. Langmuir 1992;8:125. Cases JM, Mutaftschiev B. Surf Sci 1968;9:57. Ingram BT, Ottewill RH. In: Rubingh DN, Holland PM, editors. Cationic Surfactants, vol. 37. New York, Bessel: Mercel Dekker Inc; 1991. p. 51. Atkin R, Craig VS, Wanless EJ, Biggs S. J Colloid Interface Sci 2003;266 (2):236. Van der Donck CJ, Vaessen GEJ, Stein HN. Langmuir 1993;9:3535. Ju W, Heuvelsland M, Devisser C, Somsen G. J Phys Chem 1978;82:29. Vandieman AJG, Stein HN. J Colloid Interface Sci 1978;67:213. Chorro C, Chorro M, Dolladile O, Partyka S, Zana R. J Colloid Interface Sci 1998;199:169. Chorro C, Chorro M, Dolladile O, Partyka S, Zana R. J Colloid Interface Sci 1999;210:134. Trompette JL, Zajac J, Partyka S. Langmuir 1994;10:812. Stiernstedt J, Froberg JC, Tiberg F, Rutland MW. Langmuir 2005;21 (5):1875. Nevskaia DM, Rojas Carventas ML, Guerrero Ruiz A, Lopez, Gonzalez, Juan de Dios. J Chem Technol Biotechnol 1995;63:249. Gorlov YI, Nesterenko AM, Chuiko AA. Colloids Surf A Physicochem Eng Asp 1996;106:83. Boomgand D, Tadros ThF, Lyklema J. J Colloid Interface Sci 1987;116:8. Larin AV, Frolova EA. Colloid J 1995;57(3):413. Vacklin HP, Tiberg F, Fragneto G, Thomas RK. Langmuir 2005;21 (7):2827. Mikhailova IV, Gerashchenko II. Colloid J 2002;64(5):583.

[168] Asvapathanagul Potjanee, Malakul Pomthong, O'Haver John. J Colloid Interface Sci 2005;292:305. [169] Kiez Suparno RI, Thomal JC. Langmuir 2002;18:1463. [170] Pirez-Arivalo JF, Doninguez JM, Terres E, Hernandez AR, Mini M. Langmuir 2002;18:962. [171] Houam, A. Pefferkorn, E. Colloids Surf, 34 (1988/89) 371. [172] Mubarekyan E, Santore MM. J Colloid Interface Sci 2000;227:334. [173] Dijt J, Cohen Stuart MA, Hoffman JE, Fleer GJ. Colloids Surf 1990;51: 141. [174] Fu Z, Santore MM. Colloids Surf A Physicochem Eng Asp 1998;63:135. [175] Dijt J, Cohen Stuart MA, Fleer GJ. Macromolecules 1994;27:3207. [176] deGennes PG. Adv Colloid Interface Sci 1987;27:189. [177] Fu Z, Santore MM. Macromolecules 1999;32:1939. [178] Pefferkorn E, Elaissari A. J Colloid Interface Sci 1990;138:187. [179] Mubarekyan E, Santore MM. Macromolecules 2001;34:4978. [180] Mubarekyan E, Santore MM. Macromolecules 2001;34:7504. [181] Huang Y, Santore MM. Langmuir 2002;18:2158. [182] Shubin V. J Colloid Interface Sci 1997;191:372. [183] Herd JM, Hopkins AJ, Howerd GJ. Polym Sci Part C 1971;34:211. [184] Shin YW, Roberts JE, Santore M. J Colloid Interface Sci 2001;244:190. [185] Shin Y, Roberts JE, Santore MM. Macromolecules 2002;35:4090. [186] Shin Y, Roberts JE, Santore MM. J Colloid Interface Sci 2002;247:220. [187] Hansupalak N, Santore MM. Langmuir 2003;19:7423. [188] Laboda R, Turov VV, Gun'ko VM, Skubiszewska-Jieba J. J Colloid Interface Sci 2001;237:120. [189] Boissier C, Lofrith JE, Nyden M. Langmuir 2002;18:7313. [190] Basiuk VA, Gromovoy T Yu, Khilchevskaya EG. Pol J Chem 1995;69 (1):127. [191] Sarkar D, Chattoraj DK. J Colloid Interface Sci 1993;157:219. [192] Schaaf P, Dejarden Ph, Johnes A, Schmitt A. Langmuir 1992;8:514. [193] Vinaraphong P, Krisdhasima V, Mc. Guirej J. J Colloid Interface Sci 1995;174(2):351. [194] Wannerberger K, Arnebrant T. J Colloid Interface Sci 1996;177:316. [195] Matsson MK, Kronberg B, Claesson PM. Langmuir 2005;21(7):2766. [196] Tarasevich YI, Monakhova LI. Colloid J 2002;64(4):482. [197] Rezwan K, Meier LP, Gauckper LJ. Biomaterials 2005;26(21):4351. [198] Larsericsdotter H, Oscarsson S, Buijs J. J Colloid Interface Sci 2001;237:98. [199] Daly CM, Przybycien TM, Tilton RD. Langmuir 2005;21(4):1828. [200] Su TJ, Lu JR, Thomas RK, Cui ZF, Penfold J. J Colloid Interface Sci 1998;203:419. [201] Chunbo Y, Daqing Z, Aizhuo L, Jiazuan N. J Colloid Interface Sci 1995;172:536. [202] Melzak KA, Sherwood CS, Turner RFB, Haynes CA. J Colloid Interface Sci 1996;181:635. [203] Soudi B, Jammul N, Chehimi M, Mc Carthy GP, Armes SP. J Colloid Interface Sci 1997;192:269. [204] Kang SH, Yeung FS. Anal Chem 2002;74(24):6334. [205] Meng M, Stievano L, Lambert JF. Langmuir 2004;20(3):914. [206] Giacomelli CE, Bremer MGEG, Norde W. J Colloid Interface Sci 1999;220:13. [207] Chang I, Lin J, Andrade JD, Herron JN. J Colloid Interface Sci 1995;174 (1):191. [208] Gun'ko VM, Turov VV, Zarko VI, Dudnik VV, Tischenko VA, Kazakova OA, et al. J Colloid Interface Sci 1997;192:166. [209] Rapuano R, Cormona-Ribeiro AM. J Colloid Interface Sci 1997;193:104. [210] Rapuano R, Cormona-Ribeiro AM. J Colloid Interface Sci 2000;226:299. [211] Kondo A, Urabe T. J Colloid Interface Sci 1995;174(1):191. [212] Bidzilya VA, Golovkova LP, Vlasova NN, Bogomaz VI. Russ J Phys Chem 1998;72(3):436. [213] Martin WB, Mirov S, Martyshkin D, Venugopalan R, Shaw AM. J Biomed 2005;10(2):24025. [214] Meizek KA, Janzen J, Brooks DE. J Colloid Interface Sci 1995;174 (2):480. [215] Belyakova LA, Besarab LN, Roik NV, Lyashenko DY, Vlasova NN, Golovkova LP, et al. J Colloid Interface Sci 2006;294:11. [216] Okisheva NA, Rakhlevskaya MN, Rodñvilova IS. Zh Hz Khim 1994;68 (10):1912.

S.K. Parida et al. / Advances in Colloid and Interface Science 121 (2006) 77–110 [217] Goworek J, Nieradka A. Colloids Surf A Physicochem Eng Asp 1995;97 (1):27. [218] Goworek J, Nieradka A. Polym J Chem 1995;69(6):966. [219] Monticone V, Triener C. Langmuir 1995;11(5):1753. [220] Adam US, Robb ID. J Chem Soc Faraday Trans I 1983;79:2745. [221] Thies C. J Phys Chem 1966;70:3783. [222] Lipatov Yu S, Sergeeva LM, Todosiichuk TT, Chomaya VS. J Colloid Interface Sci 1983;86:437. [223] Kawaguchi M, Sakai A, Takahashi A. Macromolecules 1986;19:2952. [224] Kawaguchi M, Sakata Y, Anada S, Kato T, Takahasi A. Langmuir 1994;10:538. [225] Kawaguchi M, Yamagiwa S, Takahashi A, Kato T. J Chem Soc Faraday Trans 1990;89(6):1383. [226] Kawaguchi M, Kawaguchi H, Takahashi A. J Colloid Interface Sci 1988;124:57. [227] Vincent B, Whittington S. In: Metijevic F, editor. Surface and Colloid Science, vol. 12; 1981. p. 1. [228] Takahashi A, Kawaguchi M. Adv Polym Sci 1982;46:1. [229] Fleer GJ, Lyklema J. In: Perfitt GD, Rochester CH, editors. Adsorption from solution at the solid/liquid Interface. London/New York: Academic press; 1983. p. 153. [230] Howard GJ, McConnel P. J Phys Chem 1967;71:2981. [231] Whitby CP, Scales PJ, Grieser F, Healy JW, Kirby G, Lewis JA, et al. J Colloid Interface Sci 2003;282:274. [232] Thies C. Macromolecules 1968;1(4):335. [233] Botham R, Thies C. J Polym Sci Part C 1979;33:369. [234] Hara K, Imoto T. Kolloid Z Z Polym 1970;237:297. [235] Hopkins A, Howard GJ. J Polym Sci Part A Gen Pap 1971;29:841. [236] Barron MJ, Howard GJ. J Polym Sci Polym Chem Ed 1974;12:1269. [237] Kawaguchi M, Aoki M, Takahashi A. Macromolecules 1983;16:635. [238] Kawaguchi M, Inoue A, Takahashi A. Polym J 1983;15:537. [239] Kawaguchi M, Funayama A, Yamaguchi SI, Takahasiti A, Kato T. J Colloid Interface Sci 1988;121:130. [240] Cosgrove T, Finch N, Vincent B, Webster I. Colloids Surf 1988;31:33. [241] Killman E, Fulka C, Reiner M. Faraday Trans 1990;86:1389. [242] Partyka S, Zaini S, Lindheimer M, Burn B. Colloids Surf 1984;12:255. [243] Levitz P, Miri AE, Keravis D, Damme HV. J Colloid Interface Sci 1984;99:484. [244] Tadros Th F. J Colloid Interface Sci 1974;46:528. [245] Roques-Carmes T, Aouadj A, Filiatre C, Membrey F, Foissy A. J Colloid Interface Sci 2004;274(2):421. [246] Esumi K, Oyama M. Langmuir 1993;9:2020. [247] M.A. Coherstuart, C.J. Beer, B.H. Bijsterbosch, J Colloid Interface Sci, 90 (1982) 310, 321 [248] Robb ID. In: LucasenReynders ID, editor. Anionic Surfactants, Surfáctant Science Ser., vol. 37. New York: Marcoel Dekker; 1981. p. 109–12. [249] Hayakawa KC. In: Holland PM, editor. Cationc Suifactant, Surfactant Ser, vol. 37. New York: Marcell Dekker; 1981. p. 109–42. [250] Tadros Th F. J Colloid Interface Sci 1974;46:528. [251] Schwuger MJ. J Colloid Interface Sci 1974;43:491. [252] Saito S. J Colloid Interface Sci 1967;24:227. [253] Huang Z, Yan Z, Hu T. Colloids Surf 1989;36:353. [254] Muller H, Kremp E. Tenside 1968;5:333. [255] Schwuger MJ. Kolloid Z Z Polym 1971;243:129. [256] Koltalo-Portet F, Desbene PL, Treiner C. J Colloid Interface Sci 2003;261 (1):40. [257] Lange H, Beck HH. Kolloid Z Z Polym 1973;251:424. [258] Akasu H, Ueno M, Meguro K. J Am Oil Chem Soc 1974;51:519. [259] Rubing DN, Jones T. Ind Eng Chem Prod Res Dev 1982;21:176. [260] Mukerjee P, Yang AYS. J Phys Chem 1976;80:1388. [261] Ueno M, Shioya K, Nakamura T, Meguro K. In: Kerker M, editor. Aerosols Emulsions and Surfactants, New York, vol. 2; 1976. p. 441. [262] Meguro K, Ueno M, Suzuki T. J Jpn Oil Chem Soc 1982;31:909. [263] Funasaki F, Hara S. J Phys Chem 1980;84:736. [264] Muto Y, Esumi K, Meguro K, Jane R. J Colloid Interface Sci 1962;120:162. [265] Asakawa T, Mouri M, Miyagishi S, Nishida M. Langmuir 1989;5:343. [266] Carifors J, Stilbs P. J Phys Chem 1984;488:4410.

109

[267] Schwuger MJ, Smolka HG. Colloid Polym Sci 1977;255:589. [268] Scamehorn JF, Schechter RS, Eade WH. J Colloid Interface Sci 1982;85: 479. [269] Hey MJ, Mactagart JW, Rochester CH. J Chem Soc Faraday Trans I 1986;82:1805. [270] Esumi K, Otsuka H, Meguro K. J Colloid Interface Sci 1991;142:142. [271] Asakawa T, Johten K, Miyagishi S, Nishida M. Langmuir 1988;4:136. [272] Esumi K, Tokui Y, Nagahama I. J Colloid Interface Sci 1991;146:313. [273] Crisp DJ. J Colloid Interface Sci 1956;11:356. [274] Groszek AJ. Faraday Discuss Chem Soc 1975;59:109. [275] Kapsabelis S, Prestidge CA. J Colloid Interface Sci 2000;228:297. [276] Stodghill SP, Smith AE, O'haver JH. Langmuir 2004;20:11387. [277] Armistead, CG Tyler, AJ Hockey, JA. Trans Faraday Soc, 1971;67: 493, 500. [278] Fowkes FM, Mc.Carthy DC, Tisehler DO. Polym Sci Technol 1985;27: 401. [279] Davydov V Ya, Kiselev AV, Kuzhetsov BV. Russ J Phys Chem 1970;44: 1. [280] Curthoys G, Cavydov VYa, Kiselev AV, Kiselcv SA, Kuznetsov BV. J Colloid Interface Sci 1974;48:58. [281] Elkingtom PA, Curthoys G. J Colloid Interface Sci 1968;28:371. [282] Rouxhet PG, Sempels RE. J Chem Soc Faraday Trans I 1974;70:2021. [283] Griffiths DM, Marshall K, Rochester CH. J Chem Soc Faraday Trans I 1974;70:400. [284] Parry EP. J Catal 1963;21:371. [285] Herd SV, Hair ML. J Phys Chem 1968;72:4676. [286] Sanders ND, Keweshan CF. J Colloid Interface Sci 1988;124:606. [287] Moudgil P, Behl S, Kulkarni NS. J Colloid Interface Sci 1992;548:337. [288] Mills AK, Hockey JA. J Chem Soc Faraday Trans I 1975;71:2392. [289] Woodbury CW, Noll LA. Colloids Surf 1989;9:40. [290] Tyler AJ, Taylor JAG, Pethica BA, Hockey JA. Trans Faraday Soc 1971;67:483. [291] Whalen IW. J Phys Chem 1962;66:511. [292] Noll LA. Colloids Surf 1987;28:327. [293] Douillard M, Elwafir M, Partyka S. J Colloid Interface Sci 1994;164:238. [294] Everette DH, Fletcher AJP. J Chem Soc Faraday Trans I 1986;82:2605. [295] Ahmad H. Sci Int (Lahore) 1990;2:101. [296] Norde WJ. Dispersion Sci Technol 1992;13:361. [297] Pal TK, Majumdar A, Raha TK, Fetting F. Chem Eng Technol 1990;13: 298. [298] Lopatkin AA. Pure Appl Chem 1989;61:1981. [299] Dekany I, Nagy I. J Colloid Interface Sci 1991;147:119. [300] Sivkumar A, Somasurdaran P, Thach S. J Colloid Interface Sci 1993;159: 481. [301] Biswas SC, Chattoraj DK. J Colloid Interface Sci 1998;205:12. [302] Ren FY, Waite SW, Harris JM. Anal Chem 1995;67(19):3441. [303] Allian M, Ugliengo P, Spano G. Langmuir 1995;11:4811. [304] Prcedip Y, Attia A, Fuerstenau DW. Colloid Polym Sci 1980;258:1343. [305] Koral J, Uumann R, Eirich FR. J Phys Chem 1958;62:541. [306] Somasundaran P, Furestenau DW. AIME Trans 1972;252:275. [307] Larsericsdotter H, Oscarsson S, Buijs J. J Colloid Interface Sci 2004;276 (2):261. [308] Gun'ko VM, Mikhailova IV, Zarko VI, Gerashchenko II, Guzenko NV, Janusz W, et al. J Colloid Interface Sci 2003;260(1):56. [309] Somasundaran P, Healy TW, Fuerstenau DW. J Phys Chem 1964;68: 3562. [310] Roberts WM. Chem Br 1981;17:510. [311] Horr TJ, Arora PS, Ralston J, Smart RStC. Colloids Surf A Physicochem Eng Asp 1995;102:181. [312] Horr TJ, Ralston J, Smart RStC. Colloids Surf A Physicochem Eng Asp 1994;92(3):277. [313] Cznadema AW. Methods of surface analysis. Amsterdam: Elsevier; 1975. [314] Hayden BE. In: Yates JT, Madey TE, editors. Vibrational Spectroscopy of Molecules on Surface. New York: Plenum Press; 1987. [315] Takenaka T. Adv Colloid Interface Sci 1979;11:219. [316] Weber G. Annu Rev Biophys Bioeng 1972;1:553. [317] Ohnishi S, McConnel H. J Am Chem Soc 1965;87:2293. [318] Berquir H, Creuzet F, Grimal JM. Langmuir 1996;12:597.

110

S.K. Parida et al. / Advances in Colloid and Interface Science 121 (2006) 77–110

[319] Somasundaran P, Turro NJ, Chandar P. Colloids Surf 1986;20:145. [320] Waterman KC, Turro NJ, Chandar P, Somasundaran P. J Phys Chem 1986;90:6830. [321] Chandar P, Somasundaran P, Waterman KC, Turro NJ. J Phys Chem 1987;91:150. [322] Somasundaran P, Kunjappu JI, Kumar CV, Turro NJ, Barton JK. Langmuir 1989;5:215. [323] Sayed MB, Cooney RP. J Colloid Interface Sci 1983;91:552. [324] Sayed MB, Cooney RP. J Colloid Interface Sci 1983;96:381. [325] Sayed MB, Cooney RP. J Colloid Interface Sci 1987;111:118. [326] Bohmer MR, Koopal MR, Jenssen R, Lee EM, Thomas RK, Rennie AR. Langmuir 1992;8:2228. [327] Michel D, Gennanus A, Pfeifer H. J Chem Soc Faraday Trans I 1982;78:237. [328] Pfeifer H, Meiler W, Deininger D. Annu. Rep. NMR Spectrosc., vol. 15; 1983. p. 291. [329] Ripmeester JA. J Am Chem Soc 1983;105:2925. [330] Major PD, Ellis PPD. J Am Chem Soc 1987;109:1648. [331] Maciel GE, Haw TF, Chuang S, Hawkins BL, Early JA, Mc kay DR, et al. J Am Chem Soc 1983;105:5529. [332] Earl WL, Fritz WL, Gibson AAV, Lunsford JH. J Phys Chem 1987;91:2091. [333] Soderlind E, Stubs P. Langmuir 1993;9:1678. [334] Ebener M, Von Firks G, Gunther H. Helv Chim Acta 1991;74:1296. [335] Parrilla FA, Claramunt RM, Lopez C, Sanz D, Limbach HH, Elguero J. J Phys Chem 1994;98:8752. [336] Atkins R, Craig VSJ, Biggis S. Langmuir 2000;16:9374. [337] Atkins R, Craig VSJ, Biggis S. Langmuir 2001;16:6155. [338] Lu JR, Swann MJ, Peel LL, Freeman NJ. Langmuir 2004;20(5):1827. [339] Read MJ, Mayes AM, Burkett SL. Colloids Surf B Biointerfaces 2004;37 (3–4):113. [340] Cardenas M, Campos-Teran J, Nylander T, Lindman B. Langmuir 2004;20(20):8597. [341] Bergkvist M, Carlsson J, Oscarsson S. J Biomed Mater Res A 2003;64 (2):349.

[342] Buijs J, Ramstrom M, Danfelter M, Larsericsdotter H, Hakansson P, Oscarsson S. J Colloid Interface Sci 2003;263(2):441. [343] Trau M, Grleser F, Healy TW, White LR. J Chem Soc Faraday Trans 1994;90(9):1251. [344] Sander LC, Callis JB. Anal Chem 1983;55:1068. [345] Surffolk BR, Gilpin RK. Anal Chem 1985;57:596. [346] Suffolk BR, Gilpin RK. Anal Chem Acta 1987;181:259. [347] Sindorf DW, Maciel GE. J Am Chem Soc 1983;105:1848. [348] Gulpin RK, Gangoda ME. J Magn Res 1985;64:408. [349] Gilpin RK. Anal Chem 1984;56:1470. [350] Gangoda ME, Gilpin RK, Fung BM. J Magn Res 1987;74:134. [351] Gilpin RK, Kasturi A, Gelerinter E. Anal Chem 1987;59:1177. [352] Gangoda ME, Gilpin RK. Langmuir 1990;6:941. [353] Rochester CH, Trebilco DA. J Chem Soc Faraday Trans I 1977;73:883. [354] Cross SNW, Rochester CH. J Chem Soc Faraday Trans I 1978;74:2130. [355] Rochester C. J Colloid Interface Sci 1996;177:456. [356] Piers AS, Rochester CH. J Chem Soc Faraday Trans 1995;91(8):1253. [357] Piers AS, Rochester CH. J Colloid Interface Sci 1995;174(1):97. [358] Piers A, Rochester CH. J Chem Soc Faraday Trans 1995;91(2):359. [359] Rochester CH, Trebilco DA. J Chem Soc Faraday Trans I 1979;75:2211. [360] Cross SNW, Rochester CH. J Chem Soc Faraday Trans I 1979;75:2865. [361] Neagle W, Rochester CH. J Chem Soc Faraday Trans I 1982;78:3081. [362] Neagle W, Rochester CH. J Chem Soc Faraday Trans I 1983;79:263. [363] Anderson JA, Rochester CH. J Chem Soc Faraday Trans I 1989;85 (10):3505. [364] Cohen Stuart MA, Cosgrove T, Vicent B. Adv Colloid Interface Sci 1986;24:143. [365] Ploehn WB, Russel WB. Macromolecules 1989;22:266. [366] De Hennes PG. Macromolecules 1981;14:1637. [367] Des Cloizeaux J. J Phys (France) 1989;50:845. [368] Yamagiwa S, Kawaguchi M, Kato T, Takahasi A. Macromolecules 1989;22:2199. [369] Amiel C, Sebille B. J Colloid Interface Sci 1992;149:481. [370] Kung KS, Hayes KF. Langmuir 1993;9:263.