Adsorption of polyethyleneimine (PEI) on hematite. Influence of magnetic field on adsorption of PEI on hematite

Materials Chemistry and Physics 144 (2014) 451e461

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Adsorption of polyethyleneimine (PEI) on hematite. Influence of magnetic field on adsorption of PEI on hematite J. Patkowski*, D. Mysliwiec, S. Chibowski Department of Radiochemistry and Chemistry of Colloids, Faculty of Chemistry, University of Maria Curie Sklodowska, Pl. Marii Curie-Sklodowskiej 3, 20031 Lublin, Poland

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


We studied an influence of magnetic field on adsorption of PEI on hematite.
A mechanism of adsorption of PEI on hematite is proposed as well as its changes under the influence of magnetic field.
A multilayer character of adsorption of PEI on hematite in geomagnetic field is revealed.
A complex statistical analysis of obtained results is presented.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 March 2013 Received in revised form 20 December 2013 Accepted 12 January 2014

A research on influence of magnetic field of various inductions on adsorption of polyethyleneimine (PEI) onto hematite is presented. Properties of the adsorbent surface (Magnetic Force Microscopy images and nitrogen adsorption data) and its behaviour in a water solution (particle size distribution) are also studied. Moreover, new analytical approach is proposed to compare the shape of the isotherms (e.g. presence of plateaus) by calculating a reciprocal derivative. A proposal of adsorption mechanism that explains a multilayer character of the adsorption itself as well as shapes of adsorption isotherms is presented. Changes in the before-mentioned mechanism due to exposure to magnetic field are also explained. Study revealed that in most cases presence of magnetic field decreases an amount of PEI adsorbed and this decrease is not a linear function of magnetic field strength e the tendency is reversed for the strongest field investigated (what is consistent with literature data). Ó 2014 Elsevier B.V. All rights reserved.

Keywords: Interfaces Polymers Atomic force microscopy (AFM) Visible and ultraviolet spectrometers Adsorption Magnetic properties

1. Introduction In recent years an influence of magnetic field (MF) on chemical systems is being studied increasingly by numerous authors. The way the MF affects behaviour and physicochemical properties of liquid water is very important for obvious reasons. There is also a

* Corresponding author. Tel.: þ48 81 5375622; fax: þ48 81 5375610. E-mail address: [email protected] (J. Patkowski). 0254-0584/$ e see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2014.01.019

need to resolve which physical parameters of MF are responsible for changes that have been reported by many authors [1e26]. The influence of MF on contact angles of water on a platinum surface was investigated by Ozeki and Otsuka [1]. A significant decrease in contact angle values was observed as long as water was in contact with oxygen or air during magnetic exposure e the decrease was stronger if more oxygen was dissolved in liquid. Reported effect was also more significant when water was shaken during exposure (about 3% and 14% decrease of contact angle in case of “static” and “dynamic” magnetization respectively). Kinetic studies confirmed presence of phenomenon often called “magnetic

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memory of water” e the effect of magnetization remained for about twenty e thirty minutes after an exposure was ended. However, water returns to its “nonmagnetic” state after heating to 320 K, sonification or addition of ethanol. Same authors investigated influence of MF on water spectra, mostly via Raman and IR spectroscopy [2]. Data suggest, that structure of hydrogen bonds is changed, most likely as a result of formation of specific structures involving oxygen molecules or atoms (transient oxygen clathratelike hydrate). Conclusions published in this paper are consistent with the previous ones e water exhibits “magnetic memory effect” and all changes observed due to magnetic exposure are stronger if more oxygen is dissolved in water. Thus it is possible that liquid/gas interface is disturbed by MF. A study done by Szczes et al. [3] about rate of evaporation of water in a presence of MF supports this hypothesis. Other authors [4,5] also suggest that MF may affect a heat of evaporation in such a way that the data obtained by Szczes et al. shows. Higashitani et al. have confirmed presence of “magnetic memory effect” of water during their studies with fluorescent probes [6]. Also in this case an effect disappears after heating, sonification or an addition of alcohol. Hosoda et al. have studied influence of MF on refractive indices of water and aqueous solutions of some simple inorganic salts [7]. For pure water and electrolytes of concentration lower than 0.5 mol L1 applying MF slightly increases a refractive index of a solution. In case of concentrations greater than 2 mol L1 an exposure to MF reduces the refractive index value whereas in case of hexane no change was observed. Thus presence of ions or dipoles is necessary for magnetic effect to occur in this case. In presented situation a scale of observed effect is proportional to magnetic field induction vector (its length). A calorimetric study of influence of MF on phase transition process (melting) of water of different isotopic composition was done by Inaba et al. [8]. Studies (most of them theoretical) also involved particularly an effect of exposure of MF on hydrogen bonds [9e11]. A research on the influence of MF on systems containing polymer molecules in most cases involve orientating liquid crystals and carbon nanotubes by strong MF [12e18]. Kimura points out, that key factor that is responsible for an interaction of polymer molecule with MF is anisotropic magnetic susceptibility [19]. This parameter can be estimated with following equation:

ca ¼ cjj  ct

(1)

where cjj and ct stand for magnetic susceptibilities parallel and perpendicular to arbitrary direction in space respectively. In most cases this direction is coaxial with a chemical bond. For example in case of CeC bond an anisotropic magnetic susceptibility is negative, which means, that bonds of this kind take up a position which is perpendicular to local magnetic induction vector. Anisotropic magnetic susceptibility of the aromatic ring is positive and as a result these rings favour such a setup where a ring plane is parallel to magnetic induction vector. It has to be pointed out that magnetic ordering will not be observed until thermal movement is overcome by force of interaction with MF. For obvious reasons a presence of charged groups in molecules will strongly affect this interaction. Apart from the composition of a molecule a strength of MF, anisotropic magnetic susceptibility, a viscosity of a solvent (medium) and effective molecular volume affect the behaviour of polymer molecules exposed to MF [19]. Petrova et al. have investigated influence of MF strength (induction up to 0.6 T) on mechanical properties of latex films [11]. Obtained results show that mechanical properties of such films (degree of crystallinity, density, relative elongation) decrease with increasing strength of MF up to 0.3e0.4 T, where the minimal value is reached. However, they start to increase in the range of 0.4e0.6 T

Fig. 1. Geometry of magnetic field in analysed system.

compared to a minimal value e for some properties initial “nonmagnetic” values are exceeded. Van Ewijk and Vroege proved that it is possible to change dipole dispersive interactions between colloidal particles by exposing them to MF [21]. They also point out conditions at which MF stabilises dispersions and at which phase separation is favoured. Adsorption in magnetic field was also studied by some authors in recent years [22e26]. However, none of this study involved polymers. In the presented study influence of MF on the amount of polyethyleneimine (PEI) adsorbed on hematite surface is investigated. MF was in this case generated by permanent neodymium magnets e maximal inductions of MF applied were (declared by the producer): 0.19 T, 0.38 T, 0.40 T, 0.43 T and 0.53 T. However, it has to be said that MF perceived by the studied system was smaller than those values and non-uniform in space (further discussed in Chapter 2.2 of this paper). Initial kinetic study was also performed to ensure a time that is necessary for the system to equilibrate. 2. Experimental 2.1. Chemicals and apparatus PEI of molecular weight 25,000 and hematite Fe2O3 were produced by Sigma Aldrich. NaCl and CuCl2 (both of analytical grade) were produced by POCH S.A. (Polskie Odczynniki Chemiczne).

Table 1 Description of magnetic fields generated by magnets used. Producer name

MW 20  2/N38

MW 29  10/N38

MW 22  MW 25  MW 33  10/N38 12/N38 30/N42

Diameter [mm] Height [mm] MF at point A [mT] MF at point B [mT] MF at point C [mT] MF at point D [mT] Reference in the text

20 2 94.1  3.2 190  4.6 75.9  0.83 12.9  0.58 120 mT

29 10 336  6.1 385  8.6 252  5.9 65.8  0.39 320 mT

22 10 371  11 404  4.4 261  15 48.8  1.8 345 mT

25 12 398  20 428  11 283  19 64.5  5.1 370 mT

33 30 526 529 369 126 475

 9.9  5.5  10  9.4 mT

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Fig. 2. Fishbone diagram for estimation of measurement uncertainty.

hematite. Properties of MFs (magnetic induction) were measured with a Gauss/Tesla Meter 5070 by F.W. Bell. Neodymium magnets were produced by ENES Magnesy Pawe1 i Tomasz Zientek Sp. k. Properties of MFs generated are discussed further in the text. 2.2. Magnetic field description

Doubly distilled water of conductivity lower than 1 mS cm1 was used in all the experiments. An UVeVIS spectrophotometer Cary 100 by Varian Instruments was used for spectrophotometric determination of concentration of PEI. Atomic Force Microscopy (AFM) images of hematite surface were made with a NanoScope V by Veeco. Specific surface and pore size distribution of an adsorbent was determined via lowtemperature nitrogen adsorption isotherm measured with ASAP 2405 by a Micromeritics Inc. A Mastersizer 2000 mP by Malvern Instruments was used to determine particles size distribution of

MF generated by permanent magnets used in the experiment is relatively non-homogenous, thus its strength was measured in different points of space. The most important points are shown in Fig. 1. These are: (A) geometric centre of the surface of a magnet, (B) edge of the surface of a magnet, (C) bottom of the Erlenmeyer flask above the geometric centre of a magnet, (D) level of solution surface above the geometric centre of a magnet. Parameters of MF are collected in Table 1. Each value is a mean of ten measurements for a given magnet. Uncertainty was estimated with statistical approach [27]. Studied system was exposed to MF generated by the same pole of the magnet every time (magnetisation vector had always the same direction). However, it has to be said that due to magnetic properties of hematite [28e32] a strength of MF perceived by ingredients of the system near the interface (in the bulk solution) is greater than the value mentioned in Table 1, as long as an external magnet is present. To be more precise hematite is attracted to the region of the strongest MF, which was observed by the authors. Thanks to that the real strength of MF that affects interphase phenomena is close to the one mentioned in the last row of Table 1. Accurate measurement of this value was not possible due to presence of the solution e thus magnetic induction values used in this paper are not fully empirical.

Fig. 4. Adsorption-desorption isotherm of nitrogen on analysed hematite (77 K).

Fig. 5. Pore size distribution of analysed hematite (data from adsorption and desorption of nitrogen).

Fig. 3. Uncertainty budget for presented data.

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Table 2 A characteristic of the surface of used hematite, based on data from nitrogen adsorption (77 K). Specific surface (BET model) [m2 g1] Specific surface (Langmuir model) [m2 g1] Average pore diameter (BET model) [ A] Average pore diameter (BJH adsorption model) [ A] Average pore diameter (BJH desorption model [ A]

6.265  0.142 7.985  0.153 94.5 106.5 110.3

2.3. Procedure Intralaboratory reference solution for measurements of PEI concentration was prepared by dissolving approximately 0.25 g of PEI in 500 mL of redistilled water. Concentration of this solution

was measured gravimetrically. 21 watch glasses were dried to a constant mass at 70  C. After that 10 mL of the solution was pipetted on each one of them and they were dried again at 70  C until constant mass was reached. The difference between the final and initial mass of watch glass was the amount of polymer in 10 mL. The uncertainty of its concentration was estimated using both: bottom-up approach [33,34] and statistical approach [27]. As a result, a reference solution concentration was (468  5.4) ppm (U ¼ 95%, k ¼ 2). All PEI concentrations mentioned further in this paper are traceable to this intralaboratory reference solution. Adsorption experiment was done as follows. 1.6 g of Fe2O3 (what gave approximately 10 m2 of a surface) was added to the 25 mL of PEI solution in 0.01 mol L1 NaCl. A pH was natural for polymer solution (in range 9.3e9.6). Suspension was shaken during an entire process (for 22 h). After that 10 mL of sample was centrifuged twice, and 4 mL of clear solution was taken for determination of concentration of PEI. 40 mL of 0.5 mol L1 CuCl2 solution was added and after 10 min absorbance at 285 nm was measured against water as a blank solution. Averaging time and spectral beam width were set to 5 s and 1 nm respectively. Initial (pre-adsorption) concentration of PEI was also measured. Each point of the isotherm is an average from at least 6 (up to 14) independent replicates. For the analysis of specific surface via nitrogen adsorption the sample was degassed at 200  C for 1 h before the experiment. Particle size distribution was measured in two ways. Firstly, only stirring was applied (1750 rpm). Secondly both stirring and sonification were applied (1750 rpm and 50% of maximal power respectively). Hematite was washed with doubly distilled water until conductivity of the supernatant became lower than 2 mS cm1 before it was used. 2.4. Estimation of uncertainty and uncertainty budget Uncertainty of the final result was estimated in two ways. Firstly, with the bottom-up approach and uncertainty propagation law [27,33,34]. Secondly with statistical approach based on results obtained from at least six independent replicates of each measurement [27]:

SD MU ¼ pffiffiffi$t0:05;n1 n

(2)

where MU is measurement uncertainty, SD stands for standard deviation, n is a number of replicates, and t0.05;n1 is a parameter of a t-Student distribution. A Grubbs test for outliers was performed before the uncertainty was calculated [27]. In case of a bottom-up approach some basic sources of uncertainty are presented in a fishbone diagram (Fig. 2.). Dashed lines denote the sources that were not quantitatively determined. Fig. 3 presents uncertainty budget of the experiment e one can clearly see that the main source of uncertainty is related to process of pipetting during a preparation of solutions and an analytical procedure. A total uncertainty obtained with a bottom-up model at a confidence interval of 95% was equal to 6.4%. An average uncertainty calculated with a statistical approach was equal to 8.1%. In experimental part of this paper we use the second value, because some sources of uncertainty were not estimated (and therefore included) in a bottom-up approach. Moreover, statistical uncertainty includes possible random errors. 2.5. Physical properties of the adsorbent Fig. 6. Microscopic images of the surface of analysed hematite: (a)e(c) AFM images, (d) MFM image (exactly the same fragment of the surface as presented in (c)).

Nitrogen adsorption data is presented in Fig. 4. According to IUPAC classification of isotherms [35] this is an example of type II

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Fig. 7. Particle size distribution of analysed hematite: (a) volume fraction diagram, (b) quantities characteristic for particle size distribution.

isotherm e mostly because of a wide linear range. It is characteristic for a non-porous or macroporous adsorbents. In case of hematite this is confirmed by pore size distribution obtained with BJH model [36] (see Fig. 5.). Other properties of the hematite obtained from nitrogen adsorption isotherm are collected in Table 2. The most important parameter for adsorption experiment is the specific surface of an adsorbent e in further study we use the value obtained from BET model [35] (because our data suggest multilayer character of adsorption, what will be discussed with more details further in a text). Images obtained with AFM show that the surface of hematite is homogenous in case of a topography (see Fig. 6aec, where different areas of the same sample are presented). Moreover, magnetic force microscopy pictures of the area shown in Fig. 6c were taken. One can clearly notice that without applying MF there is a strong correlation between geometry and magnetic properties of the surface (see Fig. 6ced). Particle size distribution data are presented in Fig. 7. An obvious conclusion is that distribution obtained when sonification and stirring were applied does not match the one obtained when only stirring was applied. It is most likely a consequence of a strong aggregation of particles. Our sample of hematite is polydisperse and when only stirred four fractions are visible, which modes are: 0.28 mm, 3.3 mm, 33 mm, 400 mm. The second and the third of these fractions are the dominant ones. A median of particle size without sonification is equal to 11.9 mm. In measurements done with sonification only two fractions are detected: a distinct one in a range 0.1e3.4 mm (mode equal to 0.53 mm) and a small one in a range 4e 20 mm (mode equal to 9.8 mm). Median of particle size is with sonification is equal to 0.668 mm. Data obtained may suggest that most of single hematite particles are smaller than 3 mm, however, they exhibit a strong tendency to aggregate. Important parameters concerning particle distribution of hematite are collected in Table 3. During our experiment we did not apply sonification. Therefore it is very likely that surface available for polymers to adsorb was lower than the value obtained from nitrogen adsorption experiment (6.265  0.142 m2 g1). However, it was impossible to estimate how

strongly aggregation influenced specific surface of the sample and whether polymer molecules were able to diffuse into aggregates (between aggregated particles). Thus, as it was mentioned before, we were still using a value of the surface area obtained from ASAP analysis further in the text. 3. Results and discussion 3.1. Adsorption kinetics Before proper adsorption kinetics measurements were done, initial measurements on adsorption kinetics of PEI onto Fe2O3 were made for three magnetic fields: geomagnetic field (GMF), 120 mT and 345 mT (initial concentration of a polymer solution was 275 ppm) e Fig. 8. Adsorption measurements were always compared to the one made in geomagnetic field (GMF). Conclusions of conducted experiments are as follows: (i) adsorption equilibrium is reached after about 22 h; (ii) MF does not significantly affect equilibration time; (iii) 120 mT MF does not significantly change the amount of PEI adsorbed on hematite, while in case of 345 mT MF adsorbed amount is lowered by 17% of geomagnetic value (confirmed by a two-tailed t-Student test); (iv) in case of GMF and 345 mT MF a short plateau is observed in a range 20e30 min, while for 120 mT MF no plateau is present;

Table 3 Results from size analysis of the used hematite.

d(0.1) [mm] d(0.5) ¼ median [mm] d(0.9) [mm] Mode [mm] Number of fractions Most distinct fraction [mm]

Stirring

Sonification and stirring

2.09  0.14 11.9  4.5 54.3  5.4 37.7 4 1.0e10 and 12e100

0.258  0.06 0.668  0.04 5.26  1.7 0.53 2 0.1e3.2

Fig. 8. Adsorption kinetics of PEI onto hematite in GMF, 120 mT and 345 mT. Adsorption quantities are normalized against equilibrium adsorption in GMF.

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molecules e less trains and more tails and loops according to Scheutjens and Fleer theory [37,38]) is possible when some critical value of a concentration is exceeded; (iii) it is an effect of changing of the pH of a solution (9.3e9.6);

Fig. 9. Adsorption isotherm of PEI onto hematite in GMF showing three distinct plateaus.

(v) all data are presented as a fraction of adsorption in GMF (Fig. 8), therefore initial slopes of all presented curves are different. This is a consequence of the fact that equilibria adsorptions are different (increasing equilibrium adsorption amount increases the slope of the kinetic curve).

Hypothesis (iii) is less possible because previous study suggests that such small changes in pH should not result in such a distinct change in amount of adsorbed polymer [39]. We were not able to confirm or deny hypotheses (i) and (ii). However, data obtained from UVeVIS spectroscopy show that PEI demonstrates a tendency to association e most likely through hydrogen bonds or electrostatic interactions between lone electron pairs and ionized amine groups. This can be seen in Fig. 10a. A shift of the wavelength of maximal absorbance towards longer wavelengths (bathochromic effect) is often caused by an association of molecules, what have been reported in case of amines [40]. Another argument was mentioned before e the short plateau was observed during the kinetics study. All arguments mentioned are sufficient to conclude, that the shape of an adsorption isotherm is a consequence of a multilayer character of the process. An adsorption of second (and

3.2. The analysis of adsorption isotherms A starting point for investigating an influence of MF on adsorption of PEI was a measurement of adsorption isotherm in GMF. Obtained results are shown in Fig. 9. What is clearly visible is the presence of three plateaus on the isotherm in the investigated range. The first and most obvious conclusion would be a multilayer character of the adsorption of PEI on hematite. However, it could not be true and some other hypotheses were also proposed: (i) A conformation of a polymer molecule in a solution depends on its concentration e increasing concentration induces a more compact conformation, therefore one polymer molecule needs less space on the surface of an adsorbent for adsorption to occur in higher concentrations; (ii) conformation of a polymer molecule on a surface depends on its concentration in a solution, therefore rapid reconformation (creating more available space on a surface for other

Fig. 10. (a) spectra of PEI solutions of various concentrations; (b) spectra of CuePEI complexes of various concentration after subtracting from them spectrum of 5 mmol L1 CuCl2 solution.

Fig. 11. Classification of isotherms measured in various MFs.

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(iv) there is no statistically significant difference between an amount of PEI adsorbed in following MF: 320 mT, 345 mT, 370 mT, 475 mT. Three groups of isotherms measured in MFs were distinguished according to the effect denoted as ‘B’ above (see Fig. 11): (i) isotherms obtained in MFs diffusing the step-like shape: 120 mT and 475 mT (Fig. 11a) e referred to as Type 1 further in text; (ii) isotherms obtained in MFs conserving the step-like shape: 320 mT and 345 mT (Fig. 11b) e referred to as Type 2 further in text; (iii) isotherm obtained in a MF partially conserving and partially diffusing step-like shape: 370 mT (Fig. 11c) e referred to as Type 3 further in text.

Fig. 12. Confirmation of presence of plateaus by plotting reciprocal derivative of adsorption isotherm.

further) layer is most likely driven by hydrogen bonding and/or electrostatic interactions between lone electron pairs on nitrogen and protonated amine groups. Moreover, PEI can form stable complexes with various ions [41,42]. Thus, another mechanism that may lead to association of PEI molecules is possible e it is a creation of intermolecular complex with Naþ. However, bathochromic shift was observed even when no background electrolyte was present (Fig. 10a) and it was not observed when PEI amine groups are involved in a complex (Fig. 10b). Therefore, the first mechanism mentioned is a dominant one and responsible for a multilayer character of an adsorption in a GMF. The same mechanism could be also used to explain a presence of other adsorption plateaus. MF affects isotherms in two ways by:

In order to improve our argumentation of this classification of isotherms we have introduced a reciprocal derivative of adsorption isotherm for analytical plateau localisation:



vc vG



 ¼ f ðcÞ

or

T

vc vG

 T

¼ f ðGÞ

(3)

where: c is an equilibrium concentration, G stands for an adsorbed amount and T for temperature. In case of our experiment we did not propose an exact analytical equation for the adsorption isotherm, therefore a reciprocal derivative was calculated as follows:



vc vG



 ¼ f

T

c2  c1 2

 ¼

c2  c1

G2  G1

(4)

or: A. decreasing an amount of adsorbed polymer; B. conserving or diffusing a multilayer shape (step-like shape). In case of the first of these effects the following conclusions have been statistically proven via paired t-Student test at confidence interval of 95% [27]: (i) there is a statistically significant difference between an amount of PEI adsorbed in GMF and each of following MFs: 320 mT, 345 mT, 370 mT, 475 mT; (ii) there is a statistically significant difference between an amount of PEI adsorbed in 120 mT MF and each of following MFs: 320 mT, 345 mT, 370 mT, 475 mT; (iii) there is no statistically significant difference between an amount of PEI adsorbed in GMF and 120 mT MF;



vc vG



 ¼ f

T

G2  G1 2

 ¼

c2  c1

G2  G1

(5)

where: Gn is an amount of adsorbed PEI when equilibrium concentration is equal to cn and c2 > c1. When reciprocal derivative is plotted as a function of an equilibrium concentration or adsorbed amount, an appearance of distinct peaks indicates a presence of a plateau. The peak is higher if adsorption isotherm has lower gradient. An example of reciprocal derivative plot is shown in Fig. 12. It is clear that plots of this kind are useful as tools for comparison of isotherms and for mathematical (analytical) proving of existence of plateaus. Reciprocal derivative curve also shows that a plateau, if present on the isotherm, is related to amount of adsorbed PEI, not the equilibrium

Fig. 13. Reciprocal derivatives of adsorption isotherm as a function of: (a) equilibrium concentration; (b) adsorption.

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Fig. 16. Adsorption amount as a function of MF induction for three initial polymer concentrations (225 ppm, 275 ppm, 325 ppm).

Fig. 14. Explanation of proposed classification of adsorption isotherms by plotting a reciprocal derivative of adsorption isotherm as a function of adsorption.

Fig. 15. Linear fit of adsorption isotherms for three different MFs (120 mT, 370 mT, 470 mT).

concentration e see Fig. 13. An idea of classification of adsorption isotherms obtained in MFs is also justified, when reciprocal derivatives are plotted (see Fig. 14.). For type 1 and type 3 isotherms a linear fitting can be successfully done in a rather wide range of concentrations (see Fig. 15.). The R2 coefficients of this fitting are greater than 0.97 in all cases. A greater slope corresponds to greater values of adsorption at given equilibrium concentration. In case of type 2 isotherm a t-Student test confirmed that there is no statistically significant difference between the first and a second layer of adsorption capacities at confidence interval of 95%, when they are compared to corresponding GMF values (approximately 0.175 mg m2 for the first and 0.115 mg m2 for the second one). Moreover, a total capacity of bilayer (approximately 0.29 mg m2) remains unchanged for type 2 and type 3 isotherms at a confidence interval of 99% in comparison to a GMF value. For type 1 isotherms we cannot consider any capacities. However, it has to be said, that amounts of adsorbed polymer of those isotherms exceed capacities of monolayer of a GMF isotherm. The most possible scenario is that MF in those cases change the conformation of the first layer, thus enabling a creation of a second layer before the first one is totally filled. Energies of adsorption may also be lowered in case of 475 mT isotherm e that would explain a decrease in an amount of the adsorbed polymer. A more detailed discussion will be presented further in the text, when the mechanism of the adsorption will be introduced and described (see Chapter 3.3). A very interesting fact is that the magnetic effect seems to be reversed when induction of MF exceeds some critical value. Fig. 16 presents the dependence of an adsorption on the MF induction for a few initial concentrations of PEI solution. An outcome of this kind is highly consistent with the data reported by Petrova et al. [20]. We have mentioned before that there is no significant difference between amounts of adsorbed polymer between 475 mT and 370 mT at a confidence interval of 95%. However, when a confidence interval is lowered to 90% we can prove a difference of this kind between values obtained in 475 mT MF and any of following fields: 320 mT, 345 mT, 370 mT (for initial concentration shown in Fig. 16 which were equal to 225  3.1 ppm, 275  3.4 ppm, 325  4.4 ppm). This fact is very important for the proposed mechanism of adsorption in MF (discussed in Chapter 3.3).

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Fig. 17. A proposed mechanism of adsorption of PEI molecules onto hematite.

3.3. Mechanism of the adsorption and the MF influencing adsorption Firstly we will propose and justify the mechanism of the adsorption of PEI in absence of MF, however this mechanism is also

Fig. 18. A proposed mechanism of changes induced by MF (blurring) on adsorption of PEI molecules onto hematite; (a)e(c) referred in the text.

valid for type 2 isotherms. A simple scheme illustrating this mechanism is presented in Fig. 17. In order to simplify the system we have assumed that at the beginning the surface was empty (no adsorption of background electrolyte ions or water molecules). Moreover, the influence of a surface charge and interactions of ions of a background electrolyte with polymer chains was not taken into consideration because of the same reason. And finally we assume that adsorption is irreversible. In the first stage polymer chains are attracted to the surface of hematite (adsorption). During the second stage other molecules adsorb on the free sites of the adsorbent, while previously adsorbed ones undergo reconformation. After a monolayer is entirely filled, no adsorption occurs until all molecules already attached to the surface reach the structure of the lowest energy (reconformation of all molecules in the first layer is ended). We can postulate this based on the shape of kinetics curve in GMF where a plateau is observed (approximately after 20e35 min). An amount of polymer adsorbed at that time is equal to the capacity of a monolayer obtained from isotherm. Therefore for some period of time (at least 15 min) an adsorption of second layer almost does not occur. Most likely this time corresponds to the reconformation of molecules in the first adsorption layer. It also suggests, that energetically it is more favourable for the system to “find a place” for a molecule on the surface or in the bulk solution than in another adsorption layer (this is a reason why a plateau is observed). When the reconformation of an entirely filled monolayer is over, adsorption of the second layer begins (stage 4). A capacity of the second layer is smaller than the first one by approximately 30%. Such a decrease in capacity of a monolayer may be explained in terms of porosity e when surface is empty molecules can adsorb in the pores, while when a monolayer is entirely filled a surface area of a polymer film available for the second layer of molecules is smaller. Despite the fact that our study revealed that hematite is macroporous, some mesopores are also present (Fig 5). This relatively small specific volume of mesopores results in approximately 2.32 m2 g1 of specific surface (BJH adsorption data). This value is almost equal to 30% of total specific surface of an adsorbent. Moreover, we postulate that molecules in the second layer reconformate to more chainlike (unfolded) structures. This is because of a tendency for association shown via UVeVIS spectroscopic study (bathochromic shift). In the chain-like conformation PEI molecules of the second

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layer can maximize number of interactions with the molecules already adsorbed on the surface (stage 5 and 6). After filling the second layer an existing structure still enables adsorption of another layers. Now we may advance and discuss the way the MF affects proposed mechanism to give an outcome that was observed. Firstly, we propose a set of mechanisms that may cause diffusing of the multilayer character of a GMF isotherm. They are presented schematically in Fig. 18. The fact that we do not observe plateaus on the isotherms (Type 1) does not necessarily mean a monolayer character of an adsorption. The fact, that an amount of adsorbed polymer exceeds the value of monolayer capacity in GMF strongly suggests that more than one adsorption layer is formed. Therefore filling of the second layer has to take place at the same time when the first one is formed. It may be so because: (a) A MF disturbs interactions of a PEI molecule with the surface and as a result a reconformation does not occur (or takes place in a lower extent and faster). Thus a second layer can adsorb almost immediately after a molecule of the first layer is attached to the surface. (b) A MF disturbs interactions of a PEI molecule with the surface and as a result reconformation takes more time. Thus molecules already adsorbed appear more static compared to the ones in a solution and an adsorption of the second layer takes place during reconformation. A similar process for molecules in the first layer may later proceed. (c) A MF disturbs interactions of a PEI molecule with the surface and as a result a different structure is energetically favourable (with longer tails or/and loops). Thus reconformation takes as much time as it does in a GMF and molecules of the second layer may attach to those long tails or/and loops, despite other PEI chains in the first layer still undergoing reconformation. It has to be said that UVeVIS spectra of polymer solution remain unchanged under mentioned conditions. Constant scale of bathochromic effect under action of MF suggests, that polymerepolymer interactions are not significantly affected by action of MF. We did not propose any mechanism based on the change in interaction energy between PEI molecules, though. Based on data obtained it is impossible to point out which of proposed mechanisms is the most possible. What is more, we cannot rule out that the nature of changes observed is caused by different, previously not mentioned factors. An observed decrease of an amount of adsorbed polymers by the MF can be explained in several ways as well: (a) A MF attracts hematite particles, which exhibit a tendency to aggregation. Therefore the possibility of such a behaviour is increased along with the size of the aggregate. As a result some closed volumes may be created. This leads to a decrease of the effective surface available for polymer particles to adsorb onto. (b) A MF attracts hematite particles, however the scale of an aggregation and size of the aggregates are not significantly increased. Locally a ratio of the adsorbent to solution is increased. Lipatov et al. [43] have shown that this may lead to a decrease in the amount of an adsorbed polymer depending on its thermodynamic flexibility. We also postulate that in such a case adsorptive mechanism of flocculation may become the main motor causing a decrease of adsorption. This is because PEI chains are attached to adsorbate particles instead of creating loops in the bulk solution.

(c) Most stable conformation of the molecule “consumes” more volume at the interface and more active sites on the surface of the hematite. (d) An energy of adsorption of a polymer chain is decreased. This may be caused by a change of a surface charge (MF may affect equilibrium of dissociation of surface groups) or increased energy of adsorption (and thus energy of desorption) of other components of the investigated system (water molecules and ions of background electrolyte), etc. Therefore the affinity of polymer molecule to the surface is decreased. (e) A MF modifies a behaviour of the PEI molecules in solution. Conformation, hydratation and dissociation equilibria are all affected by the MF. Thus the presence of the molecule in the bulk solution is more thermodynamically favourable in comparison to the system with the same concentration of PEI but no magnetic exposure. In this case we can deduce that mechanism (a) is the least possible. As it was shown before in case of type 2 isotherms a presence of a plateau is strictly combined with the adsorbed amount, not the equilibrium concentration. Moreover, adsorption value of monolayer capacity is not changed due to the magnetic exposure. This is not consistent with the hypothesis of decreasing surface. If this mechanism would be the proper one a decrease in monolayer capacity would have been observed as a result of using overestimated specific surface (comparing to “real” value accessible for polymer). And finally reversing of the decreasing tendency showed in Fig. 16 would not occur, because stronger fields should lead to lower available surface. Using the same reasoning one can state that mechanism (c) is also less possible than the others. As it was mentioned many times before no change in UVe VIS spectra of PEI exposed to MF was observed. This fact may indicate that phenomena associated with mechanism (e) do not occur or occur very rarely. Following the above elimination we have to say, that the most possible scenario is that the mechanism responsible for decrease in amount of polymer adsorbed is a mixture of (b) and (d).

4. Conclusions An influence of MF on the amount of PEI adsorbed on hematite was investigated. To improve the quality of conclusions, properties of the adsorbent surface (MFM images and ASAP data) and its behaviour in a water solution (particle size distribution) were also studied. Moreover, a new analytical approach was proposed to compare the shape of the isotherms (e.g. presence of plateaus) by calculating a reciprocal derivative. Obtained results show that there is a strong correlation between topography of the surface of hematite and its magnetic properties. Thus no magnetic ordering is present without external MF. Hematite used in the presented study is a macroporous material with a relatively low specific surface. When suspended in water its particles exhibit strong tendency to aggregate. An UVeVIS spectroscopy study revealed a presence of the bathochromic shift for polymer solution. It is most likely a result of an association of molecules. In case of adsorption study both: the shape of an isotherm and the amount of an adsorbed polymer were precisely studied and analysed. To sum up all presented data we will point out the most important conclusions about influence of MF on investigated system: (i) in most cases a presence of MF decreases an amount of PEI adsorbed;

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(ii) isotherms obtained in MFs do not always conserve a shape characteristic for multilayer adsorption e sometimes plateaus (originally observed in GMF) are diffused; (iii) a decrease in the amount of adsorbed polymer is not a linear function of MF strength e the tendency is reversed for the strongest field investigated (what is consistent with literature data); (iv) plateaus on isotherms (if present) are strongly correlated with amount of adsorbed PEI, not the equilibrium concentration; (v) a mechanism of changes observed is most likely based on a local increase of adsorbent to solution ratio and a decrease in adsorption energies (however other factors may also participate in the observed outcome). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

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