The adsorption of a polysaccharide at the talc–aqueous solution interface

Colloids and Surfaces A: Physicochemical and Engineering Aspects 139 (1998) 27–40

The adsorption of a polysaccharide at the talc–aqueous solution interface Paul Jenkins *, John Ralston Ian Wark Research Institute, University of South Australia, The Levels, Adelaide, SA 5095, Australia Received 24 June 1997; received in revised form 15 December 1997; accepted 15 December 1997

Abstract An investigation of the mechanism of adsorption of guar, a natural nonionic polysaccharide, at the talc–aqueous solution interface has been performed. Hydrophobic interactions were found to dominate the adsorption process, leading to adsorption of the guar onto hydrophobic sites, i.e. the talc ‘‘face’’. Hydrogen bonding made only a small contribution to the adsorption, whilst chemical interactions were completely absent. In the case of ionically modified guars, the electrostatic contribution to the overall thermodynamics of adsorption was weak. Both unmodified and modified guars were found to adsorb in a very flat conformation with in excess of 75% of the constituent segments present in the form of ‘‘trains’’ at the talc–aqueous solution interface. Determinations of adsorbed layer thickness, calculated from microelectrophoresis measurements, supported this finding. The mannose backbone of the guar chain adsorbed to the talc surface, leaving the pendant galactose groups protruding into the bulk solution. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Guar; Hydrophobic surfaces; Polymer adsorption; Polysaccharides; Talc

1. Introduction A perennial problem, which bedevils the selective recovery of valuable metal sulphide minerals during the froth flotation process, is the rejection of layer silicate minerals, particularly those containing MgO [1]. The presence of these minerals in the flotation concentrate causes difficulties during subsequent smelting and, if present above set levels (‘‘trigger points’’) cause financial penalties to be imposed upon the concentrate seller. Some silicate MgO minerals are naturally hydrophobic and, hence, floatable. Talc, with an ideal composition of Mg (Si O )3 4 10 * Corresponding author. Fax +61 8 8302 3683; e-mail: [email protected] 0927-7757/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0 9 2 7 -7 7 5 7 ( 9 8 ) 0 0 21 7 - 9

(OH ) , is an example of a layer silicate whose 2 structure is well known [2]. Upon breakage, e.g. during grinding, two different surfaces are formed. One surface results from the easy cleavage of one layer from its neighbour (forming ‘‘faces’’), whilst the other arises due to the rupture of the ionic/covalent bonds within the layers (to form ‘‘edges’’). The faces consist of siloxane groups, which yield a neutral and hydrophobic surface. Typically, low levels of isomorphous substitution of Si by AI or Ti and Mg by Fe or Al occur in the talc lattice, leaving the face with a pH independent negative charge. This contrasts with the hydrophilic edges, consisting of -SiOH and -MgOH groups, whose amphoteric behaviour means that their charge is pH dependent. However, since the hydrophobic faces are the dominant surface, talc floats readily.

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In practice, macromolecular reagents are added during the flotation process to depress or retard the flotation of the talc [3]. Guar is a widely used depressant, the monomeric structure of which may be seen in Fig. 1. The hydrocarbon backbone is formed from a chain of mannose units. Pendant from this backbone, at regular intervals, are galactose groups. There are two mannose units for each one of galactose. Both the mannose and galactose contain hydroxyl groups—nine are accommodated into each guar monomer unit. Guar does not naturally contain any other polar groups, but these may be chemically introduced if desired. Healy [4], and later Pugh [3] and Mackenzie [5], proposed that the mechanisms which play important roles in governing the adsorption of polymers onto mineral surfaces are chemical, electrostatic, hydrogen bonding and hydrophobic interactions. The hydrophobic effect has been the subject of much research over the past few decades. Essentially, it arises due to the desire of hydrophobic moieties, which may form part of a polymer chain, to ‘‘escape’’ the aqueous environment and attach to less polar surroundings, e.g. a hydrophobic mineral surface. The standard free energy of adsorption (DG° ) of a polymer at the mineralads –solution interface may be represented as the sum of the standard free energies of these four separate contributions: DG°

ads

=DG°

chem

+DG° +DG° +DG° . el h−bond phobic (1)

The surface characteristics of the mineral and the properties of the polymer will determine the sign and magnitude of each of these terms for a particular mineral–polymer pair, but it is the sum of all four terms that determines whether or not a particular polymer will adsorb. The specific contributions from each individual free energy change have been dealt with in other studies [6–9] and are adequately reviewed elsewhere [3,5]. Although a substantial body of published work on the adsorption of polymers onto talc exists, the nature of the actual adsorption mechanism(s) in the case of guar is uncertain. For example, Steenberg and Harris [10] proposed that adsorption of guar onto talc occurs predominantly onto the faces via a hydrophobic mechanism, with hydrogen bonding playing a minor role. Conversely, Rath et al. [11] proposed that the adsorption of guar occurs through hydrogen bonding onto the talc edge. There is little conclusive evidence in the literature which supports the occurrence of chelation/chemical bonding in ‘‘pure’’ talc systems, although many studies have shown that the presence of metal ions, e.g. calcium [12] and iron [13], can enhance the adsorption of guar. Electrostatic interactions appear to be important only in those cases where the guar has been modified to contain ionic functional groups [5]. Certainly, the mechanism of adsorption of guar at the talc–aqueous solution interface is poorly understood and is the focus of this paper.

2. Materials and experimental 2.1. Minerals characterisation

Fig. 1. The monomeric structure of guar.

Two talc samples were used in the course of this work. One talc ( TALC1) was obtained from BDH Chemicals, whilst the other talc sample ( TALC2) was a natural mineral sample obtained from Wards Natural Science Establishment Laboratories. The particle size distributions of both talc samples were measured by laser diffraction using a Malvern Master Sizer X—both samples were nominally less than 75 mm in size. The N BET surface area for 2 the two talcs was measured using a Coulter Omnisorb 100. No evidence for porosity was

P. Jenkins, J. Ralston / Colloids Surfaces A: Physicochem. Eng. Aspects 139 (1998) 27–40

observed in the N isotherms determined. Hence, 2 the surface area measured by BET was assumed to be totally available for the adsorption of the reagents used in this study. The specific surface areas obtained were 2.8 m2 g−1 and 3.0 m2 g−1 for TALC1 and TALC2, respectively. These values indicated that both talc samples contained a considerable quantity of very small (<5 mm) particles. This was confirmed by examination of the measured particle size distributions, shown in Fig. 2. X-ray diffraction ( XRD) and bulk analysis indicated that TALC1 was a very pure sample with no detectable impurities. TALC2 was found to be less pure (approximately 85% talc) with quartz as the major contaminant. The XRD measurements were made with a Phillips X-Pert MPD with a Cavex PSi detector operated at 40 kV and 50 mA. Mineral phase detection was conducted by Reitvelt pattern matching using computer software. X-ray photoelectron spectroscopy ( XPS) detected oxygen, magnesium, silicon and adventitious carbon on the surface of TALC1. The same four elements were seen in the XPS of TALC2 along with a small amounts of iron and aluminium. The iron and aluminium probably arise from low levels of isomorphic substitution of Fe for Mg and Al for Si in the talc lattice. XPS measurements

29

were carried out with a Perkin-Elmer Physical Electronics Division 5100 X-ray photoelectron spectrometer, using a Mg Ka X-ray source at 300 W. 2.2. Polymers and other reagents Five reagents were employed in this study. Four of them were polysaccharides, based on guar gum, and the fifth was a hexametaphosphate (polyphosphate). The first guar sample (GUAR), supplied by Polypro International, was an unmodified guar gum, with a molecular weight of approximately 2500 kg mol−1. The other guar samples had been modified. GUAR(+), kindly donated by Rhone-Poulenc, had a molecular weight of 1700 kg mol−1 and cationic alkyl ammonium (C H ON+Me ) groups had been substituted 3 6 3 along the chain to yield a degree of substitution of 0.15. GUAR1(−) and GUAR2(−) were supplied by Henkel and had been reduced in size through a chain degradation process, which yielded final molecular weights of 110 and 39 kg mol−1, respectively. Anionic phosphate (PO2− ) groups 3 had also been introduced into the guar chain, leaving GUAR1(−) and GUAR2(−) with a degree of substitution of 0.19. All molecular weight and

Fig. 2. Particle size distributions for the talc samples used.

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P. Jenkins, J. Ralston / Colloids Surfaces A: Physicochem. Eng. Aspects 139 (1998) 27–40

degree of substitution data was provided by the respective manufacturers. All guar stock solutions were prepared by the following method. 100 cm3 of water was vigorously stirred and 0.25 g of powdered guar was quickly added. The solution was stirred, at a lower speed, for a further 30 min and then refrigerated overnight at 4°C. This process permitted the guar to dissolve and solvate fully. The next morning the guar stock solution was filtered through Whatman 541 filter paper and cellulose nitrate filters (0.8 mm pore size) to remove any undissolved material. Fresh guar solutions were made up every day, as microbiological degradation can occur. The hexametaphosphate (HMP) sample was obtained from Albright and Wilson with a molecular weight of approximately 0.6 kg mol−1 (Na P O ). It was used without further 6 6 18 purification. Trimethylchlorosilane ( TMCS) was obtained from Aldrich Chemicals. Galactose and mannose were supplied by Sigma. Potassium chloride, cyclohexane, phenol, sulphuric acid and spectroscopic grade potassium bromide (for use in infra-red studies) were obtained from BDH Chemicals. All reagents were analytical grade unless stated otherwise and used without further purification. All aqueous solutions were prepared using high-purity water (with a surface tension of 72.8 mN m−1 and a conductivity of 0.5 mS m−1) from an Elga UHQ system and pH adjustment was made by the addition of small quantities of concentrated HCl and KOH. 2.3. Adsorption isotherm measurements Adsorption isotherms were conducted in a 500 cm3 conditioning vessel thermostated to 25°C except where stated otherwise. A sample of talc (10 g) was conditioned in 500 cm3 of the required strength KCl solution for 30 min. The pH was then adjusted to the desired value using HCl and KOH. After a further 15 min, a known concentration of reagent, i.e. hexametaphosphate or guar, was added and left to condition for 1 h (a time during which adsorption equilibrium was achieved). A 10 cm3 aliquot was then removed and centrifuged twice to remove the talc. A further

aliquot of polymer was then added to the conditioning vessel and the process repeated until a total added reagent concentration of between 100 and 200 ppm had been attained. High purity nitrogen (99.9%), scrubbed through an aqueous suspension of silica, was bubbled through the talc dispersion for the duration of the experiment, in order to minimise solution adsorption of carbon dioxide. Various methods were used to assess the amount of polymer left in the solution. For guar, the UV-visible complexation method of Dubois et al. [15] was used, whilst inductively coupled plasma (ICP) emission measurements enabled the determination of phosphorus and, hence, hexametaphosphate concentrations. It was assumed that the amount of polymer depleted from solution had been adsorbed at the talc–solution interface. The UV measurements were performed using a Cary 5 double-beam recording spectrophotometer with 1 cm pathlength quartz cells. The ICP measurements were made with a Spectroflame M Spectro ICP emission spectrophotometer. This sequential addition method was verified using single additions of polymer to individual talc samples. The results obtained were the same, within experimental error. The sequential addition method was preferred for this work for two reasons. Possible surface area variations of the talc samples are circumvented and, more importantly, the control of solution pH is more precise. The authors are aware that sequential adsorption is generally avoided, since rearrangement of the already adsorbed polymer needs to take place before adsorption of a second aliquot of polymer can begin [14]. Given the agreement between the sequential and ‘‘single-shot’’ methods in this work, it appears that this rearrangement readily occurs during the 1 h equilibration periods of the sequential addition method.

3. Results and discussion

3.1. The mechanism of adsorption All the adsorption isotherms for guar onto talc measured in this study exhibited pseudo-

P. Jenkins, J. Ralston / Colloids Surfaces A: Physicochem. Eng. Aspects 139 (1998) 27–40

Langmuirian behaviour. The classical Langmuir monolayer adsorption isotherm expression [16 ] is given in Eq. (2): C 1 C eq = eq , + (2) x/m K(x/m) (x/m) max max where C is the equilibrium solution concentration eq of polymer, x is the amount of polymer adsorbed, m is the mass of solid substrate, K is the Langmuir adsorption equilibrium constant and (x/m) is max the maximum amount of polymer adsorbed per mass of solid. A plot of C /(x/m) against C eq max eq should yield a linear relationship. The values of K and (x/m) can be determined from the intercept max and slope of such a plot. The Langmuir adsorption equilibrium constant, K, can be considered to represent the affinity of a polymer for a particular surface. It can be related to the standard free energy of adsorption, DG° , ads from the expression given in Eq. (3): =−RT ln K, (3) ads where R is the general gas constant and T is the temperature. DG° may be dissected into the four ads terms already discussed in Section 2. Eqs. (2) and (3) have been applied to all of the adsorption isotherms determined in this work. The authors acknowledge that more sophisticated models of polymer adsorption may be used (see, for example, Refs [14,17]), but they are not sufficiently tractable to permit the individual contributions of the terms in Eq. (1) to be revealed. It is pertinent to note that adherence to Langmuirian behaviour for polymers is undoubtedly owing to a mutual compensation of non-idealities [6,18], e.g. the two assumptions that the solute and solvent have equal molecular cross-sectional surface areas and that there is no net solute–solvent interaction in the surface or bulk phases. DG°

3.1.1. Preliminary studies Adsorption studies carried out in both 0.01 and 0.1 M KCl at pH values of 5 and 9, detected no adsorption at the talc–solution interface of either of the simple saccharides, galactose or mannose (the two constituent sub-units of guar). This observation supports the premise that there is no chemi-

31

cal basis for the specific adsorption of guar onto talc. 3.1.2. The influence of charge groups in the guar chain Fig. 3 shows the isotherms determined for the adsorption of GUAR, GUAR(+) and GUAR1(−) onto TALC1, dispersed in 0.1 M KCl at pH 9. Shown in the inset of Fig. 3 is the linearised Langmuir plot of the adsorption data. The values calculated for (x/m) and D°G , are max ads given in Table 1. The values of (x/m) , observed max for all three guars were high (>1 mg m−2). Within experimental error, the calculated (x/m) values max for GUAR and GUAR1(−) were identical, whilst that for GUAR(+) was just slightly higher. Since the conformation that the guar adopts at the talc–solution interface is the major factor determining the value of (x/m) , it is reasonable to max assert that the segment density profiles of the adsorbed guar macromolecules are similar and independent of any charge on the polymer chain. However, there is a complicating issue—namely that the three guar samples vary in molecular weight. In an attempt to delineate the effect of molecular weight, adsorption isotherms for GUAR1(−) and GUAR2(−), which differ only in their molecular weight, were performed. Fig. 4 shows the isotherms for the adsorption of GUAR1(−) and GUAR2(−) from 0.1 M KCl solution (pH 9) at the TALC1–aqueous solution interface, with the calculated values of (x/m) max and DG° listed in Table 1. (x/m) is greater for ads max the smaller guar, whilst DG° , for the larger ads GUAR1(−) was determined to be slightly greater than for GUAR2(−). This is fairly typical for polymers differing only in size—as the polymer molecular weight increases the thermodynamics of the adsorption process become increasingly more favourable [14]. From Table 1, it can be seen that the values of DG° , calculated for all four guar samples indiads cate that adsorption of the guar onto talc is highly favourable, i.e. DG° <0. In fact, the values of ads DG° , obtained for GUAR, GUAR1(−) and ads GUAR(+) were the same within experimental error. This indicates that electrostatic interactions make no contribution to the overall thermodynam-

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P. Jenkins, J. Ralston / Colloids Surfaces A: Physicochem. Eng. Aspects 139 (1998) 27–40

Fig. 3. Adsorption isotherms for GUAR, GUAR1(−) and GUAR(+) onto TALC1 (−75 mm, 10 g) from 0.1 M potassium chloride solution (500 cm3) at pH 9. The inset shows the linearised Langmuir plot of the raw data.

Fig. 4. Adsorption isotherms for GUAR1(−) and GUAR2(−) onto TALC1 (−75 mm, 10 g) from 0.1 M potassium chloride solution (500 cm3) at pH 9.

ics of the adsorption process of guar onto talc. It is possible to calculate the contribution to DG° ads by electrostatics using Eq. (4) below. DG° =zD Fy , (4) el S d where DG° is the free energy of adsorption due el to electrostatics, z is the dissociation constant of the charge group in the polymer chain, D the S degree of substitution of the polymer chain, F is Faraday’s constant and y is the potential at the d plane of adsorption. y is assumed to be equivalent d to f , which is the zeta potential of talc in the p presence of the adsorbed polymer. For all values of the zeta potential from −50 mV to +50 mV,

the magnitude of DG° is below 1 kJ mol−1, i.e. el less than 10% of the magnitude of DG° . ads These results indicate that an electrostatic mechanism is not the driving force which governs the thermodynamics of non-ionic guar adsorption at the talc–aqueous solution interface. Moreover, the presence of charged groups in the guar chain does not influence significantly the thermodynamics of the adsorption process. The adsorption of HMP onto TALC1, dispersed in 0.1 M KCl at pH 9, was measured. Essentially, very little adsorption of HMP was detected. Although the adsorption mechanism of polyphosphates is not fully understood, various authors have reported that the adsorption of these shortchain, hydrophilic polymers occur through hydrogen bonding [19], metal ion chelation or electrostatic mechanisms [20]. Given that the adsorption of HMP onto talc was minimal, it can Table 1 The calculated values of (x/m) and DG° , for the adsorption max ads of four different guar samples onto TALC1 (−75 mm, 10 g) from 0.1 M potassium chloride solution (500 cm3) at pH 9 Polymer

(x/m)

GUAR GUAR1(−) GUAR2(−) GUAR(+)

1.31±0.05 1.24±0.05 1.47±0.05 1.33±0.05

max

(mg m−2)

DG° (kJ mol−1) ads −14.9±0.2 −14.8±0.2 −14.3±0.2 −15.1±0.2

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P. Jenkins, J. Ralston / Colloids Surfaces A: Physicochem. Eng. Aspects 139 (1998) 27–40

Fig. 5. Adsorption isotherms for GUAR onto TALC1 (−75 mm, 10 g) from 0.1 M potassium chloride solution (500 cm3) at varying pH values.

be summarised that these three mechanisms play only a minor role in the adsorption of molecules at the talc–aqueous interface. It is probable that the very low levels of HMP adsorption observed in this study occurred via hydrogen bonding onto the talc edge.

Fig. 6. Adsorption isotherms for GUAR onto TALC2 (−75 mm, 10 g) from solutions (500 cm3) of varying potassium chloride concentration at pH 9.

3.1.4. The influence of ionic strength In Fig. 6 the adsorption isotherms of GUAR onto TALC2 at pH 9 in varying ionic strength solutions of KCl are shown. Table 3 gives the experimentally determined values of (x/m) and max DG° . ads Within experimental error, very little difference is seen in the calculated (x/m) and DG° values. max ads Fleer et al. [14] indicate that only an unchanged polymer adsorbing at an unchanged surface (e.g. the talc face but not the talc edge) will exhibit such a minor salt concentration effect. Clearly, the influence of ionic strength is extremely limited.

3.1.3. The influence of pH The isotherms determined for the adsorption of GUAR onto TALC1 from 0.1 M KCl solution and at pH values of 5, 7 and 9, are shown in Fig. 5. The values calculated for (x/m) and max DG° are given in Table 2. ads It can be seen that, over the range of pH values studied, (x/m) varies only slightly—the value max calculated for pH 9 is slightly lower than those for pH values 5 and 7. However, DG° becomes ads slightly more negative as pH increases, indicating that GUAR has a higher affinity for the surface at the higher pH values. This result is most probably a consequence of the variation with pH of the ionisation of the -SiOH and -MgOH groups present at the talc surface [21,22].

3.1.5. The influence of surface ‘‘character’’ The evidence presented thus far strongly suggests that the adsorption of guar at the talc– aqueous solution interface is strongly dominated by adsorption onto the hydrophobic talc face. Adsorption onto the hydrophilic talc edge appears to occur, but to a limited extent. In an effort to quantify the importance of edge compared with

Table 2 The calculated values of (x/m) and DG° for the adsorption max ads of GUAR onto TALC1 (−75 mm, 10 g) from 0.1 M potassium chloride solution (500 cm3) at various pH values

Table 3 The calculated values of (x/m) and DG° for the adsorption max ads of GUAR on TALC2 (−75 mm, 10 g) from potassium chloride solution (500 cm3) at pH 9

pH

(x/m)

5 7 9

1.40±0.05 1.44±0.05 1.31±0.05

max

(mg m−2)

DG°

ads

(kJ mol−1)

−13.8±0.2 −14.2±0.2 −14.9±0.2

KCl strength (M )

(x/m)

0.001 0.1 1.5

0.72±0.05 0.78±0.05 0.83±0.05

max

(mg m−2)

DG° (kJ mol−1) ads −13.1±0.2 −12.6±0.2 −12.3±0.2

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Fig. 7. Adsorption isotherms for GUAR onto TALC1 (−75 mm, 10 g) and silanated TALC1-TS from 0.1 M potassium chloride solution (500 cm3) at pH 9.

face adsorption, samples of TALC1 and spherical colloidal silica were treated with trimethylchlorosilane ( TMCS) following the procedure of Tripp and Hair [23]. According to Morris [22] reaction with talc occurs with the -SiOH and -MgOH groups on the talc edge, whilst reaction with silica occurs with the surface -SiOH groups. This effectively leaves the talc edge and colloidal silica coated with layers of hydrophobic trimethylsilane groups. The modified TALC1 (TALC1-TS ) and silica were both characterised using DRIFT and bulk analysis methods, which showed the presence of the silane groups. In preliminary measurements, no adsorption of GUAR onto the modified silica surface could be detected. In subsequent adsorption isotherm studies of GUAR onto TALC1-TS, it was, thus, assumed that the silanation of the talc prevented any adsorption of GUAR onto the talc edge. Fig. 7 shows the isotherms determined for the adsorption of GUAR from 0.1 M KCl solution onto TALC1 and TALC1-TS at pH 9. Table 4 gives the calculated values of (x/m) and DG° , max ads determined from these isotherms. Table 4 The calculated values of (x/m) and DG° for the adsorption max ads of GUAR onto TALC1 (−75 mm, 10 g) and silanated TALC1-TS (−75 mm, 10 g) from 0.1 M potassium chloride solution (500 cm3) at pH 9 Talc type

(x/m)

TALC1 TALC1-TS

1.31±0.05 1.17±0.05

max

(mg m−2)

DG°

ads

(kJ mol−1)

−14.9±0.2 −12.6±0.2

Silanation of the TALC1 has resulted in an 11% decrease in (x/m) . Clearly, ‘‘blocking’’ the talc max edge does reduce adsorption of guar onto talc. Morris [22] determined the relative face to edge area ratio for TALC1 by scanning electron microscope (SEM ) and bulk carbon analysis of the silanated talc (talc treated with dimethyloctadecylchlorosilane). The contribution of the edges to the total talc surface area was found to be 11%. On a mass of polymer per unit surface area basis, adsorption of guar onto the talc edge and face are the same. DG° was determined to be more negative for ads adsorption of GUAR onto TALC1 before it was modified with TMCS. If it assumed that the guar does not adsorb onto the edge of TALC1-TS then it is possible to estimate the contribution to DG° due to adsorption at the talc face, DG° . ads face Obviously, DG° is equal to −12.6 kJ mol−1, the face value of DG° calculated from the adsorption ads isotherms of GUAR onto TALC1-TS. Further, from the isotherm of GUAR onto untreated TALC1, it is possible to deduce the contribution to DG° , from adsorption at the talc edge, ads DG° , assuming a simple linear combination of edge the contributions of the edge and face areas to DG° : ads DG° =h DG° +h DG° , (5) ads edge edge face face where h is the fractional talc edge surface area edge (=0.11) and h is the fractional talc face surface face area (=0.89). The calculated value for DG° is edge −20.1 kJ mol−1. This is very similar to the free energy of formation for 1 mol of hydrogen bonds [5]. It seems feasible that the adsorption of guar onto the hydrophilic talc edge is facilitated by the formation of hydrogen bonds between the hydroxyl groups in the guar and the talc edge hydroxyls, in particular the -MgOH groups. 3.1.6. The influence of temperature The effect of temperature on the adsorption of GUAR from 0.01 M KCl (pH 9) at the talc– aqueous interface was performed, using a sample of TALC1 that had been wet sieved through a 38 mm stainless steel sieve. The −38 mm fraction was employed in the adsorption studies, owing to its larger surface area (4.2 m2 g−1).

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P. Jenkins, J. Ralston / Colloids Surfaces A: Physicochem. Eng. Aspects 139 (1998) 27–40

Fig. 8. A plot of DG° , against temperature (T ) for the adsorpads tion of GUAR onto TALC1 (−38 mm, 10 g from 0.01 M potassium chloride solution (500 cm3) at pH 9.

Very little difference was seen in the values of (x/m) determined from the adsorption isomax therms. However, DG° , became increasingly ads negative as the temperature was raised and it was possible to estimate the enthalpic (DH° ) and ads entropic (DS° ) contributions to DG° , from ads ads Eq. (6). A plot of DG° , against T will yield a ads straight line, with the values of DH° and ads DS° , determinable from the intercept and slope, ads respectively, assuming that both are independent of temperature over the range studied. =DH° −TDS° . (6) ads ads ads Fig. 8 shows the experimental plot obtained, whilst the calculated values of DH° and ads −TDS° are given in Table 5. It is apparent that ads the adsorption is dominated by entropic effects. Similar findings have been made for nonionic surfactants adsorbing at the silicon–aqueous solution interface [24] and for the formation of micelles by similar surfactants [25]. The result indicates that dehydration of both the hydrocarbon portions of the guar polymeric chain and the hydrophobic

Fig. 9. The zeta potential of talc as a function of pH in the presence and absence of adsorbed, non-ionic guar. Data taken from Rath et al. [11].

talc surface are the driving forces which control transport, and subsequent adsorption, of the guar from the bulk solution to the hydrophobic surface. Essentially, the adsorption does not arise due to a strong interaction between the guar segments and the talc surface, but rather because the number of unfavourable hydrocarbon polymer segments– water and hydrophobic talc–water contacts is minimised. 3.2. The conformation of adsorbed guar macromolecules at the talc–solution interface

DG°

3.2.1. Adsorbed layer thickness Fig. 9 reproduces electrophoretic data, originally reported by Rath et al. [11], for talc in the presence of various concentrations of guar. Clearly, the adsorption of guar reduces the observed zeta potential of the talc. Essentially, a polymer adsorbed on a charged particle will shift the plane of shear outwards with respect to its position in the absence of the polymer layer. The new distance of the plane of shear from the surface can be

Table 5 Thermodynamic parameters for the adsorption of GUAR onto TALC1 (−38 mm, 10 g) from 0.01 M potassium chloride solution (500 cm3) at pH 9 Temperature (°C )

DG°

25 46 60

−11.6 −12.5 −12.9

ads

(kJ mol−1)

DH°

ads

−0.4 −0.4 −0.4

(kJ mol−1)

−TDS° (kJ mol−1) ads −11.2 −12.1 −12.5

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equated to the thickness of the adsorbed polymer layer, assuming the polymer is unchanged. Hunter [26 ] has derived the relationship given in Eq. (7) to link the zeta potential of a bare surface to the zeta potential of the same surface carrying an adsorbed polymer layer. tanh

A B zef

4kT

=tanh

A

zefp 4kT

B

e−k(D−d) ,

(7)

where f is the zeta potential, situated at a distance d from the surface, in the absence of polymer, f p is the zeta potential at a distance D of the polymer covered surface, k is the reciprocal Debye length, k is the Boltzmann constant, T is temperature and z is the valency of the ions in the double layer. Eq. (7) assumes a symmetric electrolyte and that the ion distribution in the double layer does not alter upon the adsorption of polymer. Fleer et al. [27] has estimated a value of 0.4 nm for d at an ionic strength of 0.001 M. Since at low mobilities, the experimental error in the determination of the zeta potential is large, the values used in the calculations performed for this work were those obtained at the commencement of the plateau region of the zeta potential against pH curves. The results obtained for the adsorbed guar layer thickness with different concentrations of guar are given in Table 6. The adsorbed layer thickness is a function of polymer concentration. A guar concentration of 0.1 ppm is equivalent to an (x/m) value of 0.2 mg m−2 and is still on the very steep part of the isotherm. Under these conditions, the guar may adopt a very extended structure with a high Table 6 The adsorbed layer thicknesses calculated from electrophoretic data (pH 6, 0.001 M KNO ) for the system investigated by Rath 3 et al. [11] Guar

Adsorbed layer thickness (nm)

Concentration (ppm)

Coverage [(x/m)/(x/m) ] max

0 0.1 1 10

0 0.2 1.0 >=1.0

0 0.3 1.2 1.6

proportion of its segments in contact with the surface. The adsorbed layer thickness will become commensurate with the dimensions of a single adsorbed layer of saccharide units. Using commercially available molecular modelling software (Hyperchem 5, Hypercube Inc.), a guar monomer unit (see Fig. 1) was constructed, whilst simultaneous geometry optimisation was performed using the MM+ force field [28]. Using the facilities available in Hyperchem 5, the molecular size of the guar monomer was then assessed. The dimensions of the guar monomer were determined to be x=1.05 nm, y=0.64 nm and z=1.11 nm—the mannose backbone is assumed to be concomitant with the x-axis and lie in the xy plane, whilst the pendant galactose groups protrude along the zaxis. The value calculated for the adsorbed layer thickness was 0.3 nm—less than any of the three calculated dimensions and indicates that, at low concentrations, the guar adsorbs in a ‘‘patchwise’’ manner on the talc. Hence, the actual adsorbed layer thickness determined should be considered an average value. Increasing the guar concentration to 1 ppm, equivalent to a (x/m) value of around 1 mg m−2 ( just below monolayer coverage), results in an increase in calculated layer thickness. The layer thickness of 1.2 nm is now almost identical to that of a mannose chain adsorbed on the surface, with the galactose groups protruding perpendicularly into the bulk solution (a kind of comb arrangement). Garti and Reichman [29] have also proposed a similar model for the conformation of guar adsorbed at the liquid–liquid interface of oilin-water emulsions. Only a small increase in layer thickness is observed when the guar-concentration is increased to 10 ppm, an unsurprising finding since this level of guar addition still equates to monolayer coverage level of just over 1 mg m−2. The small increase in layer thickness observed, at this high guar concentration, almost certainly indicates a slightly enhanced contribution from unadsorbed tails, which occurs owing to the greater density of packing of the guar at the talc–solution interface. Clearly, guar adopts a flat conformation on adsorption onto talc, a finding consistent with the premise that the adsorption process occurs

P. Jenkins, J. Ralston / Colloids Surfaces A: Physicochem. Eng. Aspects 139 (1998) 27–40

mainly due to hydrophobic interactions between the talc face and the guar hydrocarbon backbone.

Eq. (11). %trains=

3.2.2. Effective surface area occupied per chain, s° The effective area occupied on the substrate surface per polymer chain, denoted by s°, may be calculated using the relationship [30] given in Eq. (8). s°=h

A ns N 2 A

,

(8)

where h is the fraction of the surface which is covered by polymer, A is the substrate surface area, ns is the number of moles of polymer 2 adsorbed and N is Avogadro’s number. The A number of moles of polymer adsorbed per unit area of talc, ns /A, is given by Eq. (9). 2 ns (x/m) 2= max , A M w

(9)

where M is the molecular weight of the polymer. w Hence, the effective substrate area occupied per polymer molecule, s°, can be calculated using Eq. (10). s°=h

M w . (x/m) N max A

(10)

For a particular polymer, the value of s° will be determined by the conformation that the polymer adopts upon adsorption. Clearly, s° will be much greater if the polymer is adsorbed mainly in ‘‘trains’’ (i.e. a conformation in which the majority of the polymer is in contact with the surface) than if it adsorbs in a conformation with a high degree of ‘‘loops’’ and ‘‘tails’’ (i.e. one in which a lower proportion of the polymer segments are in contact with the surface). Using the (x/m) values determax mined from the adsorption isotherms for GUAR onto TALC1 discussed in Section 3.1, values for s° have been calculated. They are given in exp t Table 7. The, percentage of guar segments in trains has also been calculated through the comparison of the experimental s° values to those obtained exp t by molecular modelling s° , as shown in mm

37

s°

exp t ×100. s° mm

(11)

It can be clearly seen from Table 7 that adsorption of guar onto solely the talc edge is extremely unlikely, since less than 10% of the guar segments would be in intimate contact with the talc surface. Fleer et al. [14] state that the fraction of segments adsorbed to a surface is reduced as the polymer chain length increases. However, even very large polymers (in excess of 10 000 segments) were still found to have at least 20% of their segments adsorbed as trains. If adsorption of guar is assumed to occur onto the whole talc surface (i.e. both faces and edges) then the calculated fraction of guar segments that take the form of trains was always in excess of 75%. The extremely high proportion of guar segments that adsorb as trains clearly indicate that the guar must adopt a very flat conformation at the talc–aqueous solution interface. This is in agreement with the layer thicknesses calculated in Section 3.2.1. Morris [22] investigated the adsorption of carboxymethylcellulose (CMC ) and modified polyacrylamide (PAM ), two alternative talc depressants, at the talc–aqueous interface and concluded that the CMC and PAM adsorbed predominantly onto the talc face with a high proportion of their segments in trains. From studies of dextrin on hydrophobic coal, Miller et al. [31] have calculated values for DG° ranging between −2.5 and phobic −3.3 kJ mol−1 for the transfer of one CH group 2 from the bulk aqueous solution to the coal surface. Each guar monomer unit contains 18 CH or CH 2 groups. If all of these groups participate in an hydrophobic interaction with the talc surface, a theoretical value ranging between −45 and 59 kJ mol−1 can be estimated for DG° . These ads values are much greater in magnitude than those, typically around −15 kJ mol−1, determined from the adsorption isotherms. However, guar appears to adsorb with the mannose chain as trains at the surface and the galactose groups protruding perpendicularly into the bulk solution. Hence, the six CH and CH groups in the galactose ring are 2

38

P. Jenkins, J. Ralston / Colloids Surfaces A: Physicochem. Eng. Aspects 139 (1998) 27–40

Table 7 Experimentally determined values for the effective talc surface area occupied per guar chain, s° , and the percentage of the guar exp t segments present as trains at the talc–aqueous solution interface Experimental system

TALC1, GUAR, 0.1 M KCl, pH 9 TALC1-TS, GUAR, 0.1 M KCl, pH 9 TALC1, GUAR1(−), 0.1 M KCl, pH 9 TALC1, GUAR(+), 0.1 M KCl, pH 9

% of guar segments in trains

s° (nm2) exp t Total surface h=1

Face only h=0.89

Edge only h=0.11

Total surface h=1

Face only h=0.89

Edge only h=0.11

3160 3560 150 2120

2810 3160 130 1890

350 390 16 235

85 96 90 84

76 85 81 75

9 11 10 9

unlikely to interact with the talc. Similarly, the CH group that links the galactose ring to the 2 mannose chain is likely to be sterically hindered in its interaction with the talc surface. This leaves 11 remaining CH or CH groups which may be 2 available for interaction (10 in the two mannose rings and one CH group present on C-2 of the 2 mannose ring not attached to the galactose ring). If the six-membered saccharide rings adopt either the ‘‘chair’’ or ‘‘boat’’ conformations upon adsorption then it is unlikely that all 11 of these groups will strongly interact with the talc. In fact, inspection of Fig. 10 reveals that the maximum number of C atoms that can lie in a single plane is five to

six, depending on whether the boat or chain form of the mannose rings is adopted. If it assumed that the boat conformation is the favoured adsorbed form, since in this arrangement the saccharide ring can maximise the number of CH/CH group–talc 2 contacts, then a realistic minimum number of CH/CH groups that will interact fully with the 2 talc surface is six. Further, only a proportion (found to be around 80%) of the guar segments are adsorbed as trains. Hence, a theoretical value for DG° of between −12 and −16 kJ mol−1 can ads be estimated. The experimentally determined values of DG° fall within this range and lend ads further support to the mechanism of adsorption

Fig. 10. A schematic representation of the adsorbed boat and chair forms of the guar monomer. The atoms in ‘‘contact’’ with the surface are denoted using outlined letters (i.e. C, O).

P. Jenkins, J. Ralston / Colloids Surfaces A: Physicochem. Eng. Aspects 139 (1998) 27–40

and adsorbed layer conformation proposed in this work. It is possible that some of the other CH/CH groups in the mannose rings may interact 2 less strongly with the talc, owing to the greater separations between the carbon centre and talc surface. This will serve to marginally increase the magnitude of theoretical value estimated for DG° . The exact extent of these ‘‘partial’’ interads actions are difficult to quantify and outside the scope of this study.

4. Conclusions This work shows that the vast majority of the adsorption of guar at the talc–aqueous solution interface occurs mainly at the talc faces by hydrophobic interactions, in agreement with Steenberg and Harris [10]. Similar to those authors, we too believe that Hydrogen bonding, between the guar and the talc edge, plays a cameo role in determining the total adsorption. In this study, evidence was seen to support the limited action of an electrostatic mechanism, provided ionically substituted guars were in use—a condition stated previously by Mackenzie [5]. No evidence for a chemical/ chelation mechanism was observed in the current work. However, the talc used was very pure. In systems where the talc contains a high degree of isomorphous, lattice substitution (e.g. Si by Al and Mg by Fe) chelation may play a role. This is also true in systems which contain polyvalent metal ions in solution. These metal ions may precipitate onto the talc surface and act as the sites at which chelation occurs. Nevertheless, in all situations hydrophobic interactions almost certainly are the major controlling force which drives the adsorption of guar at the talc–aqueous solution interface. Guar was found to adsorb in a very flat conformation, with a high degree of segments (>75%) present at the talc–aqueous solution interface in the form of trains. Calculated adsorbed layer thickness, determined by microelectrophoresis, support the hypothesis that the mannose backbone of the guar chain adsorbs to the talc surface, leaving the pendant galactose groups protruding into the bulk solution. Guar can be considered to adopt a comb-

39

like conformation upon adsorption, as shown schematically in Fig. 11. At this juncture, it is appropriate to consider how the guar actually promotes depression of the talc. Pugh [3] proposes that the adsorption of a depressant onto a mineral surface can give rise to a strong repulsive hydration force owing to the adsorbed layer. This may impose a structure on the localised aqueous environment which Pugh believes may be of sufficient magnitude to repel an approaching bubble. Of course, hydrophilic surfaces, such as the talc edge, will give rise to this phenomenon naturally. Thus, it is hard to conceive how the adsorption of depressant onto solely the talc edge can improve the depression of talc via increases in ‘‘water structuring’’. This is particularly true when one considers that the talc edge generally accounts for significantly less than 25% of the total talc surface area. Hence, the substantially greater area of the hydrophobic faces will always lead to the natural notability of talc. Altering the water molecule ordering in the vicinity of the talc faces should result in decreased talc flotation and adsorption of a depressant onto the talc face would be expected to achieve this desirable outcome.

Acknowledgment It is a pleasure for the authors to thank the members of the Colloid Science and Minerals Processing group for useful technical and theoretical discussions, the Surface and Materials Processing group for collecting and analysing the XRD and XPS data, Trevor Muggleton for ICP analysis and Andrew Robinson for BET surface area measurements.

Fig. 11. An illustration of the proposed comb-like nature of guar adsorbed onto talc.

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