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Adsorption of Cationic Surfactants on Silica. Comparison of Experiment and Theory Tatiana P. Goloub† and Luuk K. Koopal* Department of Physical and Colloid Chemistry, Wageningen Agricultural University, Dreyenplein 6, 6703 HB Wageningen, The Netherlands Received July 15, 1996. In Final Form: November 27, 1996X The adsorption of alkylpyridinium surfactants on Aerosil OX50 silica has been studied as a function of surfactant concentration, aliphatic chain length, pH, and ionic strength in solution. The adsorption isotherms are complemented by electrophoretic mobility measurements. The experimental results have been compared with calculations based on the SCFA theory for adsorption on a charge-regulating surface. The experimental results and SCFA calculations both show that the isotherms of the cationic surfactant on silica, obtained at different salt concentrations, have a common intersection point (cip). As was shown before by surface charge adaptation experiments and is now confirmed by the theoretical calculations, aliphatic segments have a moderate affinity to the surface due to hydrophobic interactions. These interactions result in a flat orientation of the surfactant molecules in the adsorbed layer. The shape of the adsorption isotherms depends on the salt concentration. The first pseudoplateau observed at low salt concentrations vanishes at high salt concentration. At low salt concentrations, electrostatic repulsions between “head-on” adsorbed headgroups and local crowding prevent the aggregation process in the adsorbed layer and inhibit further adsorption in the region of the first pseudoplateau. At high salt concentrations, the adsorption starts later due to the screening of the surface charge by the salt ions, and it increases steeper due to the diminished lateral repulsions between surfactant molecules. The common intersection point in the isotherms measured at different salt concentrations marks the transition from a flat monolayer to a flat bilayer.

Introduction Cationic surfactant adsorption from aqueous solutions on silica has been extensively studied during the last few decades.1-10 Interpretation of the adsorption mechanism is often based on the analysis of the shape of the adsorption isotherms, which will depend on the type of representation. Ordinary lin-lin isotherms of cationic surfactants on silica at low ionic strengths often show two steps.2-7 A pseudoplateau is observed at low concentrations, and a higher adsorption plateau is reached at the critical micelle concentration (cmc). In view of this, Gu et al.4,7 proposed a simple two-step adsorption model for surfactant adsorption on silica. In this model, the first step is due to adsorption of isolated molecules adsorbing with their headgroup on the surface due to electrostatic interactions. When all the charged sites at the surface are about occupied by surfactants ions, the already adsorbed molecules begin to act as nuclei for the formation of small aggregates of the surfactants ions. Surfactant tail-tail association is favored by hydrophobic interactions, and the headgroups of most of the newly adsorbed molecules will be directed toward the solution. Although this model is fairly “flexible” and simple, it cannot explain the effects of salt concentration and pH on the adsorption, and most † Present address: Department of Colloid Chemistry, St. Petersburg University, 198904 St. Petersburg-Petergoph, Russia. * Corresponding author. X Abstract published in Advance ACS Abstracts, January 15, 1997.

(1) Ball, B.; Fuerstenau, D. W. Discuss. Faraday Soc. 1971, 52, 361. (2) Rupprecht, H.; Ullmann, E.; Thoma, K. Fortsch. Kolloid. Polym. 1971, 55, 45. (3) Rupprecht, H. J. Pharm. Sci. 1972, 61, 700. (4) Rupprecht, H.; Gu, T. Colloid Polym. Sci. 1991, 269, 506. (5) Bijsterbosch, B. H. J. Colloid Interface Sci. 1974, 47, 186. (6) Gao,Y.; Du, J.; Gu, T. J. Chem. Soc., Faraday Trans. 1 1987, 83, 2671. (7) Gu, T.; Huang, Z. Colloids Surf. 1989, 40, 71. (8) Zhu, B.; Gu, T. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3813. (9) Zhu, B.; Gu, T.; Zhao, X. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3819. (10) Rennie, A. R.; Lee, E. M.; Simister, E. A.; Thomas, R. K. Langmuir 1990, 6, 1031.

S0743-7463(96)00690-7 CCC: $14.00

information on the structure of the adsorbed layer is introduced a priori. More rigorous but necessarily more complex models, such as the self-consistent field theory for adsorption, SCFA,11-13 are better suited to increase our insight into the adsorption mechanism. Surfactant adsorption on other oxides, such as rutile12-16 and alumina,16 has also been studied in detail. On these minerals, the adsorption isotherms of both cationic and anionic surfactants on an oppositely charged surface show that adsorption increases continuously until the cmc is reached and no pseudoplateau exists at low surfactant and low salt concentrations. Koopal and Bo¨hmer13,18 and Koopal and Goloub19 have discussed the differences between the isotherms observed for surfactant adsorption on silica and on rutile in terms of the different charging behavior of these oxides. The discussion is partly based on the results obtained with the SCFA theory.12,13,18 This theory shows quite different shapes of isotherms for constant-charge and constantpotential surfaces. For rutile-type metal oxide surfaces that, at first approximation, can be considered as constant potential surfaces (variable-charge surfaces), the calculated lin-lin isotherms do not show two-step behavior. The adsorption increases continuously at all salt concentrations, and the surface charge follows the surfactant adsorption up to the charge compensation point. New charges are created in the direct vicinity of the charge sites already occupied with a surfactant molecule, and (11) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at interfaces; Chapman and Hall: London, 1993. (12) Bo¨hmer, M. R.; Koopal, L. K. Langmuir 1992, 8, 1594. (13) Bo¨hmer, M. R.; Koopal, L. K. Langmuir 1992, 8, 2649. (14) Fuerstenau, D. W.; Jang, H. Langmuir 1991, 7, 3138. (15) Koopal, L. K.; Lee, E. M; Bo¨hmer, M. R. J. Colloid Interface Sci. 1995, 170, 85. (16) Lee, E. M.; Koopal, L. K. J. Colloid Interface Sci. 1996, 177, 478. (17) Somasundaran, P.; Fuerstenau, D. W. J. Phys. Chem. 1966, 70, 90. (18) Bo¨hmer, M. R.; Koopal, L. K. Langmuir 1992, 8, 2660. (19) Koopal, L. K.; Goloub, T. P. In Surfactant Adsorption and Surface Solubilization; Sharma, R., Ed.; ACS Symposium Series 615; American Chemical Society: Washington, DC, 1995; Vol. 78.

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the formation hemimicelles (“head-on” adsorbed surfactant aggregates) are promoted. For constant-charge (variable-potential) surfaces, the calculated lin-lin isotherms at low salt concentrations show two steps. A comparison of the calculated isotherms with the experimental isotherms for silica led Bo¨hmer and Koopal13 to the conclusion that silica behaves approximately as a constant-charge surface. But according to recent data20 and our own observations,21,22 silica can hardly be referred to as a constant-charge surface because its surface charge at low salt concentrations increases significantly upon surfactant adsorption. Apparently, the new charges are not formed at the silica surface in the close vicinity of the charged sites already occupied with the head-on adsorbed surfactant molecules, and the hemimicelle formation is not promoted. A second difference for silica as compared with other mineral oxides is that the silica surface is partially hydrophobic due to the presence of siloxane groups. Also, this is a reason that the basic adsorption mechanism on silica is different from that on other metal oxides. On silica, the aliphatic surfactant tails can interact favorably with the surface, whereas on the more hydrophilic oxides, this is not the case. Strong evidence for the presence of surface-surfactant tail interactions is derived from the surface charge measurements of silica in various electrolyte solutions in the absence and presence of cationic surfactants; see ref 22 for details. Further understanding of the adsorption process can be reached also by comparing the isotherms measured at different pH’s and salt concentrations with the isotherms calculated with the SCFA theory for adsorption. In the calculations that resemble surfactant adsorption on silica, neither the surface charge nor the surface potential should be a constant. Instead, a “regulating” surface model with variable charge and potential has to be chosen. Recent advances in the SCFA theory make it possible to calculate surfactant adsorption on such a surface. Within the SCFA theory, there is no need to make assumptions about the structure of the adsorbed layer. The equilibrium distribution of the surfactant segments in the direction perpendicular to the surface is calculated and gives a clear insight into the surfactant orientation in the adsorbed layer. Moreover, the surface charge regulation follows from the calculations. In the present paper, this type of SCFA calculation will be compared with the experimentally determined adsorption isotherms of cationic surfactants on silica at different pH’s and salt concentrations and the surface charge adaptation measured previously. The calculations are used to aid the interpretation of the effects observed in the experimental data; no attempts are made to fit the data. Materials and Techniques Chemicals. Dodecylpyridinium chloride (DPC) was synthesized using the method of Colichman23 and purified by recrystallization from acetone. Cetylpyridinium chloride (CPC) was obtained from BDH and was used without further purification. For both surfactants, the surface tension-concentration curves indicated the absence of chemical impurities. The cmc’s for both DPC and CPC, measured at room temperature and various salt concentrations, are given in our previous paper.22 (20) Wa¨ngnerud, P.; Olofson, G. J. Colloid Interface Sci. 1992, 153, 392. (21) Goloub, T. P.; Sidorova, M. P. Kolloid Zh. 1992, 34, 17. (22) Goloub, T. P.; Koopal, L. K.; Bijsterbosch, B. H.; Sidorova, M. P. Langmuir 1996, 12, 3188. (23) Colichman, E. L. J. Am. Chem. Soc. 1950, 72, 1834.

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Figure 1. Isotherms of DPC and CPC adsorption on Aerosil OX50 at 0.001 M KCl and pH 9.

Pro-analyze-quality KCl was received from Merck. The water was deionized using an Elgastat UHP unit. Adsorbent. A nonporous pyrogenic silica, Aerosil OX50 (Degussa) with a BET surface area of 50 m2/g and an average particle size of 60 nm, was used. The surface charge of this silica in the absence and presence of surfactant has been measured potentiometrically; see ref 22. Adsorption Isotherms. Adsorption isotherms were measured at 20 ( 1 °C using the depletion method. To prepare the silica suspension, an appropriate amount of Aerosil was weighed into a 10-mL polycarbonate tube and 4 mL of a KCl solution of appropriate concentration was added. The suspension was sonified for 5 min. The pH was adjusted to the desired value using 0.01 or 0.1 M KOH and 0.01 M HCl. After the pH adjustment of the suspension, the desired amounts of KCl solution and surfactant solution of the required pH were added to give a total volume of 8 mL. The thus obtained samples were rotated end-over-end for several hours to reach an equilibrium. To ensure that the initial and final pH’s were the same, the pH was adjusted several times during the equilibrium process. Aerosil was separated from the solution at 20.000 rpm in a Beckman J2-21 centrifuge. The UV adsorption of the pyridinium ion in the supernatant was measured at 260 nm. Electrophoretic Mobilities. The electrophoretic mobilities were measured at room temperature using a Malveren Zetasizer MK III. The measurements were made with suspensions of 0.01 g Aerosil in 50 mL of 0.001 M KCl solution with known equilibrium concentrations of surfactant. Experimental Results General Features of the Isotherms. The adsorption isotherms for both DPC and CPC, measured at pH 9 and 0.001 M KCl, are shown in Figure 1. Two scales of

Adsorption of Cationic Surfactants on Silica

Figure 2. Isotherms of DPC and CPC adsorption on Aerosil OX50 at 0.1 M KCl and pH 9.

presentation have been used: log-log in panel a and linlog in panel b. Both for DPC and CPC, the isotherms have the same shape. In a lin-lin plot (not presented here), the isotherms reflect the well-known two-step adsorption isotherms. In the log-log plot, the pseudoplateau at low surfactant concentrations is represented by the relatively low slope of region II. An advantage of the log-log type of representation is that it “magnifies” the lower part of the isotherms. In general, in these isotherms, four linear regions can be distinguished. The changes in the slope of different regions of the isotherms upon adsorption of surfactant indicate “turning points” of the adsorbed layer formation. Initially, the slope of the plot in region I, both for DPC and CPC, is close to unity, pointing toward a constant affinity. In region II, the slope is less than unity, indicating a continuing decrease in the overall affinity. It means that the further adsorption in this region is progressively more difficult either due to the local “crowding” of the hydrophobic surface sites or due to the poor screening of headgroup repulsion. The effect appears to be rather strong, and the surfactant concentration has to be increased significantly before progressive adsorption starts in region III. At the II/III transition, the slope increases as compared with region II, but it is still less than unity. At the cmc, region IV is reached, where the chemical potential of the surfactant monomer remains constant upon an increase in surfactant concentration, resulting in constant adsorption values. In region II, the surfactant and surface charge nearly balance. In region III, superequivalent surfactant adsorption occurs and new surfactant molecules will be predominantly adsorbed with the headgroups pointing toward the solution (head-out). The adsorption isotherms, both for DPC and CPC, measured at 0.1 M KCl and pH 9 are shown in Figure 2. As before, in panel a, the log-log isotherms are presented and, in panel b, the lin-log isotherms. At high salt concentrations, S-type isotherms are observed and the

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four regions can be distinguished in the log-log plots. The shape of both isotherms at high salt concentrations, however, differs from that at low salt concentrations (see Figures 1 and 2). The main difference is observed in region II where the slope is much larger then unity, indicating a strong increase in overall affinity. Due to the effective screening of the headgroup repulsion, headgroups can adsorb relatively close together, tail-tail attraction will be relatively strong and hemimicelle formation occurs. Meanwhile, the slope of region III, both for the DPC and the CPC isotherm, hardly depends on the salt concentration. Similar S-type isotherms were also observed for cationic surfactant adsorption on rutile but in this case at both low and high salt concentrations.12 Chain Length. Both for DPC and CPC, the general shape of the isotherms measured at a given salt concentration is the same, but differences in the position of the isotherms exist, indicating the effect of aliphatic chain length on adsorption behavior. As shown in Figures 1 and 2, for both salt concentrations, the isotherms for DPC are located at higher surfactant concentrations and the adsorption values of DPC at the cmc are lower than that of CPC. The difference in position is due to the fact that both the hydrophobic attraction with the surface and the lateral attraction between the hydrocarbon chains of surfactant increase with chain length. Similar results were obtained for alkylpyridinium surfactants on rutile.15 Due to lateral interactions between the adsorbed surfactant molecules, the slopes of regions II and III are dependent on the tail length. The addition of CH2 groups in a surfactant molecule causes a steeper slope. This is most clearly shown in the lin-log plots, where the rate of adsorption for CPC is higher than for DPC. The increase of hydrophobic attraction between surfactant chains with increasing the chain length leads also to a larger adsorption level at the cmc for CPC than for DPC. pH of the Solution. The adsorption isotherms for DPC and CPC on Aerosil at three pH values are shown in Figures 3 and 4, respectively. The salt concentration in both cases is 0.001 M. The results are again presented in log-log and lin-log plots. By increasing the pH, the system is moved away from the pzc and both the surface charge and the surface potential of silica increase. Consequently, the Coulombic attraction increases, and this results in a gradual rise of the adsorbed amounts of DPC and CPC in all regions of adsorption isotherms. It should be noticed that the four-region isotherms have disappeared at pH values close to the pzc. For CPC, this effect is most pronounced and region I vanishes even at pH 7. The experimental results obtained for CPC at pH 5 and 7 for high salt concentrations (not presented here) also show that for the long-chain molecules of CPC, it is difficult to observe region I because of the strong hydrophobic effect. Ionic Strength and Common Intersection Point (cip). In Figures 5 and 6, the adsorption isotherms for, respectively, DPC and CPC measured at pH 7 are presented at several concentrations of KCl both in loglog and lin-log scales. The similar isotherms for DPC and CPC measured at pH 9 are shown in Figures 7 and 8. The isotherms of CPC show a nearly common intersection point. For CPC, the coverage in the cip at pH 7 is very low; therefore, the reverse of the salt effect is shown clearly only at pH 9. For DPC, only two concentrations are considered. The intersection point of the isotherms at both pH values is in region II, similarly as with CPC at pH 9. According to De Keizer et al.,24 the cip (24) De Keizer, A.; Bo¨hmer, M. R.; Mehrian, T.; Koopal, L. K. Colloids Surf. 1990, 51, 33.

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Figure 3. Isotherms of DPC adsorption on Aerosil OX50 at 0.001 M KCl and three pH values.

Figure 5. Isotherms of DPC adsorption on Aerosil OX50 at pH 7 and two salt concentrations.

Figure 4. Isotherms of CPC adsorption on Aerosil OX50 at 0.001 M KCl and three pH values.

Figure 6. Isotherms of CPC adsorption on Aerosil OX50 at pH 7 and three salt concentrations.

corresponds, in the absence of specific adsorption of the background electrolyte, with surface charge neutralization or the isoelectric point (iep). Below the cip, adsorption decreases with increasing salt concentration because the Coulombic attraction between the surfactant headgroup and charged surface sites is progressively screened by the

salt ions. Adsorption in this region occurs predominantly head-on, and the surfactant charge is compensated by the surface charge. In the present case, the head-on adsorption, although due to Coulombic attraction, is promoted by the hydrophobic attraction between the surface and surfactant tail, and as soon as there is a specific attraction,


Figure 7. Isotherms of DPC adsorption on Aerosil OX50 at pH 9 and two salt concentrations.

Figure 8. Isotherms of CPC adsorption on Aerosil OX50 at pH 9 and three salt concentrations.

the surfactant ion will compete favorably with the indifferent salt ions. Adsorption above the cip is promoted with increasing the salt concentration due to a reduction in mutual headgroup repulsion. The combination of these two effects leads to the cip of isotherms measured at different salt concentrations. At the cip, the electrostatic

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Figure 9. Electrophoretic mobilities of Aerosil particles as a function of DPC concentration at two salt concentrations and two pH values.

contribution to the free energy of adsorption vanishes, and the salt effect disappears. The adsorption both of DPC and CPC in the cip is dependent on the pH or the surface charge and surface potential of silica; see Figures 5-8. The farther away the system is from the pzc, the higher the surface coverage is at which the cip occurs. The position of the cip with respect to the concentration axis depends only slightly on the pH value (Figures 5 and 6). The concentration of DPC at which the cip occurs is 2 mmol/dm3 at pH 7 and 1.5 mmol/dm3 at pH 9. The position of the cip for CPC at pH 7 can hardly be observed precisely because of the low coverage around the cip. For a comparison of the position of the cip and that of the iep, the electrophoretic mobilities of the Aerosil particles with adsorbed surfactant have been measured. The electrokinetic results for DPC measured at pH 7 and 9 are presented in Figure 9. A similar set of results for CPC is shown in Figure 10. The iep’s for DPC measured at pH 7 and 9 are located at about the same equilibrium concentration of 1 or 1.5 mmol/dm3, and this corresponds well with the position of the (c)ip in Figures 5 and 7. For CPC, the position of the iep is at both pH values about 0.01 mmol/dm3. Also, this value corresponds well with the measured cip’s. Comparison of the adsorption isotherms with the surface charge isotherms (not presented here) shows that the cip corresponds also with the charge compensation point (ccp). The correspondence of the cip, iep, and ccp indicates that the specific adsorption of the background electrolyte is relatively weak or absent in the present case. The cip is therefore the turning point for the Coulombic interactions. Below the cip, the Coulombic attractions between the headgroup and the surface are important. Above the cip, these attractive interactions have vanished and Coulombic repulsions occur. Therefore, additionally adsorbing mol-

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Figure 10. Electrophoretic mobilities of Aerosil particles as a function of CPC concentration at two salt concentrations and two pH values.

ecules are predominantly oriented with their headgroups toward the solution, and a bilayer is formed. Theoretical Calculations To model both the association of ionic surfactants in solution and the adsorption of cationic surfactants on silica, the one-dimensional self-consistent mean field lattice theory has been used.19,25 With this theory, the equilibrium distribution of segments in the radial direction for spherical micelle in solution and that perpendicular to the surface in the case of adsorption can be calculated by means of the minimization of the free energy of the system. For a detailed account of the theory, reference is made to Fleer et al.11 A difference with the previous calculations is that now a “regulating” surface is modeled, i.e., a surface with a variable charge and a variable potential. This can be done by considering the proton association-dissociation equilibrium of the surface groups. The parameter characterizing this equilibrium is the proton affinity constant, KH. To a reasonable approximation, the charging of a silica surface can be described with just one KH value describing the association of a proton with a negative site to form a neutral site.26-29 The density of the surface segments that can become charged is regulated by composing the surface layer of two types of segments that are equal except for the fact that part of them can become charged and the other part does not have this possibility. The surface charge density can be simply obtained as the fraction of the all surface sites that is actually dissociated. Since the charges in the SCFA model are located at the (25) Bo¨hmer, M. R.; Koopal, L. K.; Lyklema, J. J. Phys. Chem. 1991, 95, 9569. (26) Hiemstra, T.; De Wit, J. C. M.; Van Reimsdijk, W. H. J. Colloid Interface Sci. 1989, 133, 105. (27) Hiemstra, T.; Van Riemsdijk, W. H. Colloids Surf. 1991, 59, 7. (28) Koopal, L. K.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 1989, 128, 188. (29) Koopal, L. K. Electrochim. Acta 1996, 41, 2293.

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midplane of each lattice layer, the electrostatic surface potential also depends on the relative dielectric constant of the surface layer. By adjusting this value, the slope of the charge-pH curves can be modified somewhat to improve the resemblance with the experimental chargepH curves of silica. In the calculations, the behavior of a linear surfactant molecule with 12 aliphatic segments, A12, and three headgroups segments, P3, is discussed. The headgroup segments each have one-third of a unit of charge. Water and salt ions (C+ and D-) are included as monomeric species. The segments S of the solid form a flat surface. The distance between the lattice layers is 0.31 nm, which gives 55.5 mol of lattice sites/dm3. A face-centered cubic lattice is used to minimize lattice artifacts.11,25 The dielectric constants are taken as 20 for the A segments and 800 for all other segments in solution. For the surface segments, a value of 400 is used unless stated otherwise. The total number of active surface sites per squared nanometer is taken as 5.2 and the pKH value of these sites as 7.8. These values are in good agreement with the literature data for silica.26,29,30 To reduce the flexibility of the surfactant chain, the chain conformations are calculated using the rotational isomeric state (RIS) scheme of Leermakers et al.,31,32 which forbids the backfolding of segments and introduces an energy difference between the gauche and trans configurations (1 kT). The Flory-Huggins χ parameters are used to calculate the nonelectrostatic contact interactions between the segments in solution and between the segments and the surface. To promote the phase separation between A segments and the other segments, the χ parameter between an aliphatic A segment and all other segment types is chosen as 2. Water is assumed to be a perfect solvent for P, C, and D, and therefore, the remaining χ parameters in solution are set to 0. Using these parameters, a good agreement between the calculated and measured cmc values as a function of chain length and salt concentration has been reached.25 The interaction parameter between a surface segment and W, C, D or P is zero. This means that the electrolyte is indifferent with respect to the surface and that the decay of the electrostatic potential with distance is very similar to that predicted by the Gouy-Chapman theory.12 The surface-aliphatic segment parameter χAS is taken as -1 kT. For a facecentered cubic lattice, this value corresponds to an exchange free energy upon adsorption of 1 kT/segment, which indicates that the aliphatic tail has a specific interaction with the surface. As shown in our previous work, specific adsorption effects leading to the surface charge adaptation are controlled by the aliphatic tail rather than by the headgroup.22 To check qualitatively the effect of χAS and χPS on the adsorption of cationic surfactants on silica, some calculations have been done with other values of the interaction parameters between surfactant segments and the surface. Theoretical Results CmO’s and Pure Silica. First, the critical micelle volume fractions or cmφ’s of the surfactant have been calculated at three salt concentrations. The results are shown in Table 1; they compare well with those obtained by Bo¨hmer et al.13,25 The differences are due to the fact (30) Sidorova, M. P.; Goloub, T. P.; Musabekov K. B. Adv. Colloid Interface Sci. 1993, 43, 1. (31) Leermakers, F. A. M.; Scheutjens, J. M. H. M. J. Phys. Chem. 1988, 89, 3264. (32) Leermakers, F. A. M.; Scheutjens, J. M. H. M. J. Phys. Chem. 1989, 93, 7417.


Figure 11. Experimental and calculated surface charges of silica (panel a) and calculated surface potential of silica (panel b) as a function of pH at three salt concentrations. Table 1. Calculated Critical Micelle Volume Fractions, cmO csalt, M

cmφ

0.001 0.01 0.1

3.9 × 10-3 3.1 × 10-3 1.0 × 10-3

that Bo¨hmer et al. used a hexagonal lattice and the present calculations are based on a more appropriate face-centered cubic lattice. In order to indicate how closely the SCFA theory predicts the mechanism of surface charge formation of silica in 1-1 electrolyte solutions, first some calculations have been done in the absence of surfactant. The calculated and the measured surface charge isotherms and the calculated surface potential isotherms at three salt concentrations are shown in Figure 11a and b, respectively. In view of simplicity of the present model, the agreement is good. The value of the dielectric constant of the surface (s) has been used as a fitting parameter. The above agreement between the calculated and the measured surface charge curves is achieved for s ) 400. The physical meaning of this value can be best expressed in terms of the Stern layer capacitance used in the classical electrical doublelayer models. For the given lattice spacing, this value of 0 corresponds with a Stern layer capacitance of 2.0 F/m2, which is quite reasonable.27,29 The data show that both the surface charge and the surface potential of silica are a function of pH and salt concentration. The surface potential as a function of pH is strongly non-Nerstian. This behavior is well-known and distinguishes silica from the other metal oxides.26,27,29 Adsorption and Surface Charge Isotherms. The calculated adsorption isotherms of A12P3 at three salt concentrations are shown in Figure 12 (panel a shows the log-log isotherms, panel b the lin-log isotherms). The isotherms are calculated up to the critical micelle volume fraction and plotted as a function of the equilibrium bulk

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Figure 12. Calculated adsorption isotherms for the surfactant A12P3 on the “regulating” surface at pH 7 and three salt concentrations: (a) log-log; (b) lin-log. The following χ parameters are used: χAW ) χAP ) χAC ) χAD ) 2. The other χ parameters in solution are 0. The χ parameters for the interaction with the surface are χAS ) -1 and χPS ) χCS ) χDS ) χWS ) 0.

volume fraction of surfactant φb. The cmφ’s are indicated by the kink of the isotherms. Qualitatively, the results are in good agreement with the measured isotherms. As will be shown below, this is largely due to the choice of the surface-segment interaction parameters χAS ) -1 and χPS ) 0. These values ensured specific binding of the aliphatic tails to the surface rather than specific headgroup binding. The log-log isotherms show the different regions very similar to the experimental results. Also for calculated isotherms, the slope of region II is most strongly affected by the salt concentration. The slope of the linlog isotherms increases with increasing salt concentration in correspondence with the experimental results. Similarly to the experimental results shown in Figures 5, 7, and 9, the initial adsorption decreases with an increase of the salt concentration, whereas at high adsorption values, this effect is reversed. The distinction between these two regions is indicated by the cip of the isotherms. In this point, the surface charge is neutralized by adsorbed surfactant molecules. Further adsorption occurs because the tail segments of the adsorbing molecules may decrease the number of contacts with water by association with already adsorbed surfactant molecules. In Figure 13, the calculated adsorption isotherms of A12P3 at two salt concentrations and pH 7 are combined with the calculated surface charge isotherms. Both the adsorbed amount and the surface charge are expressed as nexc, the excess number of surfactant molecules or charges per surface site. At low salt concentrations (Figure 13a), the adsorption isotherms show two regions. In the first region, the surface charge follows the surfactant adsorption. From φb values of about 5 × 10-5 to 2 × 10-3, nearly every headgroup adsorbed creates an opposite charge on the surface. This means that in this region, the

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Figure 13. Adsorption of A12B6 and the surface charge adjustment upon surfactant adsorption at pH 7 and two salt concentrations. Adsorption and the surface charge are both expressed in the same units. See Figure 12 for χ parameters.

surface charge is almost completely compensated by adsorbed surfactant molecules. Further adsorption only causes a weak surface charge adaptation. At the cmφ, the surfactant adsorption is only slightly larger than the surface charge due to the electrostatic repulsion between the headgroups. At high salt concentrations, the surface charge is hardly adapted. Surfactant ions have to compete with salt ions in the first part of the isotherm. As a result, the surface charge adaptation vanishes. The effective screening of the headgroup repulsion leads to a much higher final adsorption than at low salt concentrations. In order to illustrate how the values of the interaction parameters between the surfactant segments and the surface affect the shape of adsorption and surface charge isotherms, some typical examples for two salt concentrations are presented in Figures 14 and 15. Decreasing χAS to -2 kT shifts the beginning of the adsorption isotherms to lower volume fractions; see Figure 14. At low salt concentrations, the surface charge adjusts itself to the surfactant adsorption, but the affinity of the tail segments to the surface is so strong that near the cmφ, the aliphatic segments replace the headgroup segments from the surface. As a result, the surface charge decreases at high coverages. At high salt concentrations, the adjustment of the surface charge is rather weak, but also here, the tail segments replace the head segments in the last part of the isotherm. If χAS is reduced to 0, resulting in an exchange energy of 0.67 kT, and χPS is set to -5 (strong affinity of the headgroups to the surface) (Figure 15), a strong phase separation in the adsorbed layer occurs near the cmc indicated by the instability loops. Molecules form a condensed bilayer on the surface just before the concentration where they associate in solution. Also, the process of the surface charge adaptation is strongly exaggerated. Structure of the Adsorbed Layer. The profiles of the segment volume fractions give information on the

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Figure 14. Calculated adsorption and surface charge isotherms at pH 7 and two salt concentrations. The χ parameters for the interactions with the surface are χAS ) -2 and χPS ) 0. For the other χ parameters, see Figure 12.

Figure 15. Calculated adsorption and surface charge isotherms at pH 7 and two salt concentrations. The χ parameters for the interactions with the surface are χAS ) 0 and χPS ) -5. For the other parameters, see Figure 12.

structure of the adsorbed layer. The volume fractions of the surfactant segments and salt ions as a function of the layer number z are shown in Figures 16 and 17 for two adsorbed amounts of surfactant in Figure 16 for a salt


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coverage, this ratio equals 7 to 8 in the first layer, and at high coverage, it is 10 to 12 as compared to the bulk ratio of 4. At the low surface coverage (nexc ) 0.01), at both salt concentrations, the headgroup segments are almost all located in the first three layers in order to be able to compensate the surface charge. The ions C and D hardly contribute to the charge compensation. Further adsorption leads to an increase of the number of surfactant segments in all layers near the surface and to an increase of layer thickness. A noteworthy point is that at high surface coverage, a weak maximum in φP occurs in layer 2 (low salt) or layer 3 (high salt). This is an indication of the presence of a bilayer. Headgroups at the surface compensate the surface charge; the other headgroups are located at the solution side of the adsorbed layer. Hence, close to the cmc, the adsorbed layer has a bilayer structure. The overcompensation of the surface charge leads to a reversal of the salt ion adsorption: D- is excluded and C+ is adsorbed. Conclusions Figure 16. Volume fraction profiles of the A and P segments and the ions C+ and D- near the “regulating” surface in a 0.001 M CD solution at two values of nexc (panels a and b). See Figure 12 for the χ parameters.

Figure 17. Volume fraction profiles of the A and P segments and the ions C+ and D- near the “regulating” surface in a 0.1 M CD solution at two values of nexc (panels a and b). See Figure 12 for the χ parameters.

concentration of 10-3 M, and in Figure 17 for 10-1 M salt. To see the profiles of simple salt ions more clearly, the results are also plotted using a logarithmic volume fraction scale. The most interesting feature of these results is that the orientation of the adsorbed molecules on the surface is always relatively flat. The headgroup and aliphatic tail segments are distributed only over 4-5 lattice layers. Due to the high affinity of the surfactant tail for the surface, the volume fraction of A segments on the surface is considerably higher than the volume fraction of headgroup segments. For the entire tail, the hydrophobic attraction competes favorably with the Coulombic attractions between headgroups and surface. The enrichment is most clearly observed in the ratio φA/φP at the surface. At low

The experimental results obtained for cationic surfactant adsorption on silica and the calculations based on the SCFA theory for adsorption on a charge-regulating surface are in qualitative agreement. This indicates that the SCFA theory can give the realistic information about the main driving forces for the adsorption behavior of cationic surfactants on silica and about the structure of the adsorbed layer. The experimental results and SCFA calculations both show that the shape of adsorption isotherms of the cationic surfactant on silica depends on the salt concentration. At low salt concentrations, the isotherms show two domains. In the first domain, the surface charge increases due to surfactant adsorption, but this surface charge adaptation does not promote the aggregation process in the adsorbed layer. The low slope of region II of the logarithmic isotherms is the most clear evidence for this behavior. Molecules are adsorbed on the surface with their headgroups and tails close to the surface. The electrostatic repulsions between the headgroups and local crowding inhibit the further adsorption in the region of the first pseudoplateau. At the concentration close to the cmc, the adsorption increases again and reaches a final constant value at the cmc. The experimental fact that the process of surface charge adaptation is affected by the aliphatic tail, as revealed in our previous work, has been confirmed by the theory. Due to a moderate hydrophobic affinity of the aliphatic segments to the surface, the molecular orientation in the adsorbed layer is fairly flat. At high salt concentrations, the initial surface charge is relatively large but the Coulombic attraction is low due to the screening by the salts ions. Consequently, the adsorption starts later than at low salt concentrations, but it increases steeper due to the aggregation between aliphatic tails. This process does not promote the formation of new charged sites on the surface. Beyond the cip, the formation of the head-out aggregates (admicelles) starts at low and high salt concentrations. Acknowledgment. This work was supported by EUINTAS Project 93-3372 and by a fellowship that T.P.G. obtained from the Wageningen Agricultural University. We are very grateful to Frans Leermakers for the discussions related to the SCFA theory for the surfactant adsorption. LA960690D

Adsorption of Cationic Surfactants on Silica. Comparison of

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