Adsorption of Cationic Surfactants on a Hydrophilic Silica Surface at

Mar 6, 1996 - Individual surfactant adsorption at free pH causes the pH of the equilibrium bulk .... Surface and Coatings Technology 2010 204, 1445-14...
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Adsorption of Cationic Surfactants on a Hydrophilic Silica Surface at Low Surface Coverages: Effects of the Surfactant Alkyl Chain and Exchangeable Sodium Cations at the Silica Surface J. Zajac, J. L. Trompette, and S. Partyka* Laboratoire des Agre´ gats Mole´ culaires et Mate´ riaux Inorganiques, Universite´ des Sciences et Techniques du Languedoc, place E. Bataillon, 34095 Montpellier Cedex 05, France Received July 31, 1995. In Final Form: November 27, 1995X The influence of the alkyl chain length and the concentration of exchangeable sodium cations at the solid/water interface on the energetics of cationic surfactant adsorption on a hydrophilic silica surface at free pH has been studied. Adsorption isotherms, electrophoretic mobilities of the silica particles, and differential molar enthalpies of displacement for benzyldimethyldodecylammonium bromide (BDDAB) on the original silica sample (SilNa) at different ionic strengths (pure deionized water and 0.1 M NaBr solution) were compared, as well as those on a washed silica sample (SilH). The stability of silica suspensions, as characterized by the changes in turbidity, was analyzed with the intention of shedding light on the possible orientation of the adsorbed surfactant ions. Adsorption of BDDAB was also compared with that of benzyldimethyloctylammonium bromide (BDOAB) on SilNa. Individual surfactant adsorption occurs by ion pairing or cation exchange mechanisms, depending on the purity of silica sample and composition of the bulk phase. A significant endothermic contribution to the total enthalpy of displacement upon individual surfactant adsorption derives from desorption of the very structured interfacial water due to the specific adsorption of a hydrophobic surfactant moiety. In the presence of sodium cations at the silica surface, this endothermic contribution dominates over others because the enthalpy of adsorption for surfactant ionic heads and the enthalpy of desorption for sodium cations cancel each other out to a great extent. For moderate interfacial concentrations of sodium, the overall enthalpy change is endothermic in a certain adsorption interval, whereas for high concentrations it is always exothermic. Individual surfactant adsorption at free pH causes the pH of the equilibrium bulk solution to decrease, indicating an increase in the surface charge of silica.

Introduction The adsorption of ionic surfactants at the solid/water interface is strongly affected by so many factors that very systematic studies are needed to quantify the effect of each of them on the degree of adsorption and the energetics involved. In order to do effective experimental work at a fundamental level, it is highly desirable that the adsorption systems are reproducible and free of any impurities. In everyday laboratory practice, much attention is paid to the control and subsequent removal of certain troublesome impurities, like surface-active agents, inorganic ions, or polymeric materials, from adsorption systems. The presence of quite small quantities of such substances may greatly modify the phenomenon. In particular, special care is required in the purification of the surfactant solutes, as well as in the preparation of distilled and deionized water. By far the most difficult problem is that of surface contamination derived from the preparation or storage of solid supports. Under technological conditions it sometimes happens that a given “impurity” is introduced with the raw materials and its presence is necessary to ensure the formation and stability of the final product. Even a small amount of impurity, when located on the surface, may play an unrecognized role in adsorption. Silica powders obtained by precipitation from sodium (or potassium) silicate solutions are good examples of the problem. Such materials exhibit relatively high bulk concentrations of sodium, because it is very difficult to remove this element after particles have been formed. Some sodium atoms are located directly on the silica * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, February 15, 1996.

surface as impurities; some others may come from the solid interior.1 The silica surface even in pure water develops a negative charge and an ionic double layer, but the overall charge density will be low or moderate at most. Certain sodium atoms may be thus released to the water surrounding the negatively charged solid particles where they constitute “natural” counter cations located in either the diffuse or Stern layer. Such counterions may be further exchanged in the process of surfactant adsorption. They will affect the adsorption energetics, giving two additional contributions to the total enthalpy change: enthalpy of their desorption (endothermic) and enthalpy of their rehydration (probably exothermic). The hydroxylated surface of silica has a point of zero charge at about pH 2 in the presence of a background electrolyte. The density of negative charges remains low until pH 6 and greatly increases between pH 6 and 11.2 When the adsorption of cationic surfactants is carried out from aqueous solutions containing an extra salt at high pH values, the degree of ionization of the surface silanol groups reaches its maximum value. Such a highly charged hydrophilic silica behaves as a constant-charge surface during the adsorption process. The phenomenon has a marked two-step character.3-5 In the case of mineral supports with a lower density of negative charges, the surface charge can adjust itself upon adsorption and its neutralization does not yield a pseudoplateau on the adsorption isotherm.5-7 For constant-potential surfaces (1) Iler, R. K. The Chemistry of Silica; Wiley-Interscience Publishers: New York, 1979. (2) Bolt, G. H. J. Phys. Chem. 1957, 61, 1166. (3) Bijsterbosch, B. H. J. Colloid Interface Sci. 1974, 47, 186. (4) Bo¨hmer, M. R.; Koopal, L. K. Langmuir 1992, 8, 1594. (5) Koopal, L. K. In Coagulation and Flocculation; Dobias, B., Ed.; Marcel Dekker: New York, 1993; Chapter IV. (6) Bo¨hmer, M. R.; Koopal, L. K. Langmuir 1992, 8, 2649, 2660.

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(surface potential fixed by the pH of the aqueous phase), a typical adsorption isotherm of a monoisomeric ionic surfactant shows four characteristic regions when plotted on a double logarithmic scale.6,8,9 However, in the absence of added electrolyte at a low surfactant concentration, neither a Henry region I nor a discernible region I to region II transition can be distinguished on the curve.9 Therefore, the complete analysis of a given adsorption system should not be limited only to the interpretation of adsorption isotherms. When the surface charge density is relatively high, electrostatic interactions and hydrophobic effect are the principle driving forces of adsorption. Other specific (nonelectrical) interactions between the surfactant adsorbate and the adsorbent are thought to be of little importance.5,6,10,20 In such adsorption systems, the role of surface heterogeneity may be essentially reduced. Individual electrostatic adsorption of surfactant ions occurs at very low adsorption densities. Then the surfactant aggregation starts at the solid surface. The bilayer coverage represents complete saturation of the surface. Adsorption of ionic surfactants on low-charged and moderately charged mineral surfaces is additionally influenced by other types of interaction, like the dispersion forces between the hydrophobic surfactant tails and the support or short-range structural forces due to changes in the solvent structure in the vicinity of a surface. For example, the importance of the adsorbed water molecules and the orientation of the hydrophobic moiety at the solid surface has been confirmed by their impact on the stability of mineral dispersions in aqueous solutions. The behavior of such a dispersion, being dependent on the action of different forces between the particles, is commonly determined by the total outcome of the van der WaalsHamaker, electrostatic, and hydrophobic interactions. In the case of silica, particle-to-particle attraction by dispersion forces is substantially reduced in water (small Hamaker constant), presumably because of the hydration layer.1 To destabilize an aqueous silica dispersion, it is thus necessary to reduce the degree of hydration. The adsorption of a cationic surfactant on a low-charged silica surface may quickly lead to a destabilization of the mineral dispersion owing to the aggregation of partially hydrophobized mineral particles.11 In previous works,12,13 the cationic surfactant adsorption on a negatively charged silica surface at low surface coverages, up to about 0.5 µmol m-2, was compared to an ion exchange between the individual surfactant cations (7) Zajac, J.; Lindheimer, M.; Partyka, S. Colloids Surf. 1995, 98, 197. (8) Somasundaran, P.; Fuerstenau, D. W. J. Phys. Chem. 1966, 70, 90. (9) Harwell, J. H.; Schechter, R.; Wade, W. H. In Solid-Liquid Interactions in Porous Media; Cases, J. M., Ed.; Technip Publisher: Paris, 1985; p 371. (10) Wa¨ngnerud, P.; Berling, D.; Olofsson, G. J. Colloid Interface Sci. 1995, 169, 365. (11) Somasundaran, P.; Ramachandran, R. In Coagulation and Flocculation; Dobias, B., Ed.; Marcel Dekker: New York, 1993; Chapter XIII. (12) Trompette, J. L.; Zajac, J.; Keh, E.; Partyka, S. Langmuir 1994, 10, 812. (13) Zajac, J.; Trompette, J. L.; Partyka, S. J. Therm. Anal. 1994, 41, 1277. (14) Trompette, J. L. Ph.D. Thesis, Montpellier, 1995. (15) Partyka, S.; Lindheimer, M.; Zaini, S.; Keh, E.; Brun, B. Langmuir 1986, 2, 101. (16) Zajac, J.; Chorro, M.; Chorro, C.; Partyka, S. J. Therm. Anal. 1995, 45, 781. (17) McGuiggan, P. M.; Pashley, R. M. J. Colloid Interface Sci. 1988, 124, 560. (18) Kunjappu, J. T.; Somasundaran, P. J. Phys. Chem. 1989, 93, 7745. (19) Iler, R. K. J. Colloid Interface Sci. 1975, 53, 476. (20) Rosen, M. J. Surfactants and Interfacial Phenomena; WileyInterscience: New York, 1978.

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and the preadsorbed sodium ions. The process appeared to have a pronounced competitive character. Calorimetric measurements of the differential molar enthalpy of displacement revealed the existence of an endothermic contribution to the overall enthalpy change. The contribution was ascribed to the local disruption of the structure of the interfacial water molecules and the release of some of them to the bulk phase. It was proposed that the enthalpy changes were accompanied by a significant increase in the entropy of the system in order that the driving free energy be principally entropic. The influence of temperature on the effectiveness and energetics of adsorption was analyzed in order to confirm this hypothesis. The experimental results reported previously indicate that these effects are probably induced by the adsorption of the hydrophobic surfactant moiety at the solid surface. The alkyl chain length and the sodium concentration at the silica surface may play a crucial role in the phenomenon. The influence of both parameters is studied more thoroughly in this paper. Although the main emphasis is placed on the region of low surface coverages, evolution of the adsorption mechanism with the amount adsorbed is followed as well. Adsorption of a cationic surfactant, benzyldimethyldodecylammonium bromide, will be compared with that of a molecule having the same polar headgroup and a shorter alkyl chain, benzyldimethyloctylammonium bromide. The interfacial concentration of sodium cations can be modified either by adding an electrolyte, such as NaBr, to the aqueous phase or by washing the silica sample with HCl. The latter procedure aims at replacing all the sodium ions trapped on the silica surface by protons. The comparative study includes calorimetric measurements of the differential molar enthalpies of displacement, supplemented by adsorption and electrophoretic measurements. Observations of the silica suspension stability, as characterized by the changes in turbidity, provide some additional information about the adsorbate arrangement in the adsorbed layer. All experimental data concerning the adsorption of benzyldimethyldodecylammonium bromide and benzyltrimethylammonium bromide (the headgroup molecule) onto the original silica sample, used for comparative purposes, have been taken from the previous work.12 Materials and Experimental Methods Materials. The surface-hydroxylated amorphous silica, RP 63-876, was manufactured by Rhoˆne-Poulenc (Aubervilliers, France).12 The specific surface area, measured by nitrogen gas adsorption at 77 K (BET method; am(N2) ) 16.2 Å2), was found to be 40 m2 g-1. It is a nonporous adsorbent of high purity except that it contains some traces of sodium as impurities. The mean particle size, observed in a scanning electron micrograph, was 0.13 µm. This solid sample was used without further treatment. The water used throughout all experiments was deionized and purified with a Millipore Super Q system. It had a pH value of 6 and a conductivity which varied between 0.05 and 0.1 µS cm-1. The increase in ionic strength was effected with NaBr. All inorganic chemicals were of analyzed reagent grade. Benzyldimethyldodecylammonium bromide, C6H5CH2N+(CH3)2C12H25Br- (BDDAB), from Fluka (France), was used as an example of commercially available cationic surfactant. It was successively purified by recrystallization from ethyl acetate and water. Its Krafft temperature was 283 K, and the cmc (at 298 K) was 5.6 mmol kg-1. The cross-sectional area at the air/water interface at 298 K was assessed at 0.71 nm2.12 In a second series of measurements another cationic surfactant, belonging to the same homologous series of benzyldimethylalkylammonium bromides, was used. It was synthesized in the laboratory by the alkylation of a tertiary amine.14 The quaternization reaction was carried out by heating an anhydrous ethanol solution fo benzyldimethylamine with 1-bromooctane at

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Adsorption of Cationic Surfactants 333 K under nitrogen. The agitation was stopped after 24 h, and the solution was allowed to cool under nitrogen. The precipitation in diethyl ether yielded a white solid product which was subsequently filtered and recrystallized twice from pure acetone. The ultimate analysis and nuclear magnetic resonance studies of the product structure based on the recorded 1H NMR spectra were perfectly in accordance with the stoichiometry and molecular structure of benzyldimethyloctylammonium bromide, C6H5CH2N+(CH3)2C8H17Br- (BDOAB). The maximum solubility of BDOAB at 298 K was 5.25 g in 100 mL of water (0.16 mol kg-1). Methods. Adsorption isotherms were usually obtained using the solution depletion method, which consisted of comparing the solute concentrations before and after the attainment of adsorption equilibrium.12 The mixtures of known amounts of silica with a known mass of calibrated surfactant solution were equilibrated at a constant temperature for 12 h by slow rotation in glass joint stoppered tubes of capacity 30 mL. Each mixture was then centrifugated (13 000 rpm during 20 min) and the final concentration of the supernatant determined spectroscopically (UV, 262.8 nm). The amount adsorbed was calculated from the difference between the initial and the final molality. Simultaneously, samples of solid suspensions were collected for electrophoresis measurements. A Rank Brothers microelectrophoresis apparatus with a rectangular cell was applied to measure the average velocity at which charged silica particles moved under the action of a steady and weak electric field between platinum black electrodes. From the average velocity at both stationary levels, the electrophoretic mobilities of solid particles were calculated.12 The adsorption and electrophoretic measurements were supplemented by the qualitative observation of the stability of silica particles in the surfactant solutions. The turbidity of the content of each adsorption tube after adsorption, recorded at various time intervals, was compared with that of a reference tube containing the same solid concentration but in pure solvent. The latter suspension was always prepared under the same conditions as the others. A marked increase in the turbidity was taken as evidence of flocculation. The enthalpy changes accompanying the adsorption of the benzyldimethylalkylammonium bromides onto silica were measured with a Montcal microcalorimeter. The method and equipment are described in detail elsewhere.15,16 The stock solution of molality 0.05 mol kg-1 was injected into the calorimetric cell containing 16 g of solvent and 1 g of silica by small steps. The differential molar enthalpy of displacement corresponding to a given adsorption step was calculated from the experimentally measured enthalpy changes, effects of dilution, and adsorption isotherm. The dilution experiments were carried out with the same apparatus. Since the experimental results obtained in adsorption and calorimetric measurements are to be correlated with one another, the same experimental conditions have to be maintained. All experiments were carried out at free pH. However, the variations of pH of the supernatant liquid were monitored along the isotherm using the Tacussel electrode.

Results Adsorption of BDOAB onto the Original Silica Sample. In order to test certain properties resulting from the hypothetical amphiphilic structure of BDOAB, the surface tension measurements at the air/solution interface were carried out with an electrobalance type tensiometer (Prolabo TD 2000, France). The results are shown in Figure 1a. The presence of BDOAB in the system causes a reduction in the surface tension of the solvent. The solute molecules are thus able to concentrate at the interface and to assume some particular conformations, consistent with the relative orientation of their hydrophilic and hydrophobic groups with respect to the interface. From this point of view, BDOAB exhibits a behavior representative of all the surface-active agents. However, the characteristic break in the curve at the critical micelle concentration has never been observed. This substance does not aggregate to form micelles in the bulk solution. Nevertheless, the possibility of formation of small mo-

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Figure 1. The behavior of benzyldimethyloctylammonium bromide (BDOAB) in the bulk aqueous solution at 298 K: (a) adsorption at the air/solution interface (surface tension of solution against the logarithm of molality); (b) dilution of a micellar stock solution (differential molar enthalpy of dilution against the equilibrium solution molality).

lecular associations cannot be excluded. The same conclusions have been drawn from the analysis of the calibration curve obtained by measuring the UV absorbancy of the blank aqueous solutions containing known concentrations of the solute at constant temperature (298 K). The calibration line had the same slope in the whole concentration range (from 0.1 mmol kg-1 to 0.1 mol kg-1). If any surfactant clusters exist in a sufficiently concentrated stock solution, the enthalpy of their formation may be evaluated in the calorimetric dilution experiment. Such an experiment was carried out at 298 K for two different stock solutions; their molalities were equal to 0.05 and 0.1 mol kg-1. Figure 1b shows the differential molar enthalpies of dilution plotted against the molality of the equilibrium solution in the calorimetric cell. The enthalpic effect of the process is exothermic and constant throughout the whole concentration range. The same enthalpic value was obtained for both stock solutions. It seems clear that the unassociated molecules are quite stable in aqueous solution. Each BDOAB molecule contains a benzyl group and a short alkyl chain attached at both sides of the quaternary ammonium group. Perhaps, the mutual conformations of these structural units, including their interweaving impose steric restrictions on the aggregation process. Only in very concen-

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Figure 2. Adsorption of benzyldimethyloctylammonium bromide (BDOAB) onto precipitated silica from aqueous solution at 298 K and free pH: (a) adsorption isotherm (the numbers represent the corresponding amounts of adsorption at two plateaus); (b) evolution of the electrophoretic mobility of solid particles (iep specifies the isoelectric point concentration); (c) the differential molar enthalpy of displacement against the amount adsorbed in a limited adsorption range (the enthalpic values were taken with the opposite sign).

trated solutions is the existence of small monodisperse associations probable. The enthalpy change accompanying the formation of such aggregates would be endothermic and equal to about 1 kJ mol-1. The adsorption isotherm of BDOAB onto the original silica sample at 298 K is shown in Figure 2a. Figure 2b illustrates the evolution of the electrophoretic mobility of silica particles upon the BDOAB adsorption. The adsorption isotherm has a marked two-step character. The first plateau of about 37.5 µmol g-1 corresponds well to the isoelectric point on the electrophoretic curve. The first step is thus consistent with the compensation of the surface charge by BDOA+ ions adsorbed with the ionic heads at the surface. A great increase in the chemical potential of the solute in the bulk phase is necessary (bulk molality changes from 18 to 35 mmol kg-1) to induce a new mechanism of adsorption. It results in a marked rise in the adsorption amount accompanied by a reversal in the sign of the electrophoretic mobility of silica particles. The adsorption density is about twice as great at the second plateau (88.7 µmol g-1). An excess of the adsorbed BDOAB cations over the negatively charged surface sites can be only explained by the possibility of forming surface aggregates. Interactions of the benzyl group or the short alkyl chain with the surface may reduce the possibility of their interweaving and thus facilitate chain-chain associations with incoming molecules. The latter would

leave their ionic heads in the bulk solution. At high concentrations of the bulk phase, their short tails would interpenetrate into outwardly disposed alkyl chains of the already adsorbed surfactants. The formation of a statistical monolayer on the silica surface can be deduced from the average limiting area of about 0.75 nm2 obtained per adsorbed BDOAB molecule at saturation. The resultant structure would be comparable to the hemimicellar reverse orientation model of the adsorbed layer.17,18 The differential molar enthalpy of displacement is plotted in Figure 2c as a function of the amount adsorbed. Only the initial part of the curve (below the first adsorption plateau) is presented. Firstly, the titration calorimetric technique leads to an important uncertainty in the differential enthalpy evaluation in the neighborhood of an adsorption plateau12 so the curve is less reliable in the subsequent regions. Secondly, BDOAB was chosen in order to elucidate some additional information on the surfactant orientation with respect to the surface only in the early adsorption stages. In the next section, the enthalpy curve for this adsorbate will be compared with the curves for two other benzyldimethylalkylammonium bromides at low adsorption densities. The exothermic effect related to the adsorption of BDOAB on the silica surface decreases with increasing quantity of adsorption. Since the formation of polydisperse surface aggregates is less probable in this region, the downward tendency in

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Figure 3. Adsorption of benzyldimethyldodecylammonium bromide (BDDAB) onto precipitated silica from 0.1 M NaBr solution at 298 K and free pH: (a) adsorption isotherm (the number represents the corresponding amount of adsorption at surface saturation); (b) the differential molar enthalpy of displacement against the amount adsorbed (the enthalpic values were taken with the opposite sign).

the enthalpy has to be ascribed to the direct solidadsorbate interaction. However, there is an endothermic minimum on the curve at about 27 µmol g-1, and it corresponds to the maximum enthalpy of displacement (about 2 kJ mol-1). Only a great endothermic contribution to the total enthalpy change, different from the enthalpies of adsorption and dominating over them, can cause the whole process to be endothermic in a certain adsorption range. Adsorption of BDDAB from NaBr Solutions. The objective of the further studies was to quantify the influence of interfacial sodium cations on the energetics of cationic surfactant adsorption onto silica. In order to increase the sodium concentration at the silica surface, BDDAB adsorption experiments were carried out from 0.1 M NaBr solutions. The idea was based on the fact that small univalent Na+ ions could readily follow the negative charge into the surface and adsorb from aqueous salt solutions near the outer silanol layer, especially at higher pH values.19 Of course, the addition of electrolyte to an aqueous solution containing surfactant ions induces several new effects which will influence adsorption. Since some of them are competing, it is difficult to extract information on the net effect, especially in regard to the quantity of adsorption. The problem has been already discussed in detail.5,9,10,20 In this paper, the main attention is focused on the interpretation of calorimetric data. The results are summarized in Figure 3. Figure 3a shows the adsorption isotherm for BDDAB from 0.1 M NaBr solution. An increase in the ionic strength of the aqueous phase causes a rapid decrease in the solubility of the surfactant monomers which must now compete with the electrolyte ions for hydration. The micellization is thermodynamically favored, and the cmc is reduced from 5.6 mmol kg-1 in the pure deionized water to 1.33 mmol kg-1 in 0.1 M NaBr solution. The molar enthalpy of micellization decreases from -5.3 kJ mol-1 in water to -6.6 kJ mol-1 in the presence of salt; the formation of micelles becomes a little more exothermic. A drop in the surfactant solubility should also enhance a general affinity of monomers for an interface. However, the initial adsorption of BDDAB decreases in the presence of salt; the isotherm is concave with respect to the molality axis in the region of low molalities. This effect is presumably due to competition between organic and salt ions for ionic sites on the substrate.6,10,20 The adding of electrolyte gives rise to the effectiveness of surfactant adsorption; the amount adsorbed at the

plateau reaches a value of 210 µmol g-1 (cf. 173 µmol g-1 in the absence of salt12). The effective size of the ionic head is smaller at higher ionic strengths because the principal effect of the salt is to partially screen the electrostatic repulsion between the headgroups.20 If adsorption of surfactants is predominantly perpendicular to the silica surface at surface saturation, the effectiveness of adsorption becomes greater as a result of closer packing of the adsorbed ions. On the basis of the nitrogen specific surface area of silica particles and the cross-sectional area of the surfactant head at the water/air interface, the formation of a close-packed bilayer can be postulated; the average density of bilayer adsorption is equal to 0.76 nm2 (without salt) and 0.63 nm2 (in 0.1 M NaBr solution). Ionic atmosphere imposed by the electrolyte ions should moderate the electrostatic surface-ion interaction. In spite of the decreased attraction between ionic surfactant heads and oppositely charged surface sites, the enthalpy change upon adsorption in Figure 3b is exothermic in the whole region. At very low surface coverages, it decreases continuously with an increasing amount adsorbed as far as 7 µmol g-1. The minimum on the salt curve is about -3.5 kJ mol-1 (cf. 5 kJ mol-1 for the system without salt12). This unexpected increase in the exothermicity of the process has to be rather ascribed to a smaller endothermic contribution, which is greatly reduced in the presence of salt. In the case of adsorption from salt surfactant solution, flocculation of silica particles was not distinctly observed. This means that the silica surface remains fairly hydrophilic over the whole adsorption range. Since even the presence of sodium cations at the surface can promote flocculation, the lack of this effect suggests that the formation of a second layer starts quite early. This conclusion is supported by a constant enthalpy of adsorption. In Figure 3b the adsorption enthalpy begins to level off at relatively low adsorption densities. In the adsorption range between 43 and 160 µmol g-1, it oscillates around an averaged value of -5.9 kJ mol-1, which is not far from the molar enthalpy of micellization in the bulk phase. These findings may even be consistent with the admicelle hypothesis; the possibility of forming local bilayered aggregates has been predicted for high ionic strengths.6,9,21 Adsorption of BDDAB onto the Washed Silica Sample. The next logical step was to investigate the effect of a decrease in the number of exchangeable sodium cations (21) Yeskie, M. A.; Harwell, J. H. J. Phys. Chem. 1988, 92, 2346.

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Figure 4. Adsorption of benzyldimethyldodecylammonium bromide (BDDAB) onto washed silica sample from aqueous solution at 298 K and free pH: (a) adsorption isotherm (the numbers represent the corresponding amounts of adsorption at two plateaus); (b) the differential molar enthalpy of displacement against the amount adsorbed (the enthalpic values were taken with the opposite sign); (c) variation of pH of the supernatant liquid; (d) evolution of the electrophoretic mobility of solid particles (iep specifies the isoelectric point concentration).

at the silica surface. Washing of a solid sample in HCl would cause the quantity of interfacial sodium to be reduced to a minimum. The overall exchange capacity of the silica surface depends on many factors, and different mechanisms exist at different pH values. Some Na+ cations are so strongly and even irreversibly held by surface forces that it is not possible to release them to solution during addition of acid. There is thus a poor chance that they will leave the surface and contribute to the ionic double layer surrounding the negatively charged silica particles when immersed in aqueous surfactant solution. The silica sample was washed repeatedly in 3 N HCl and deionized water until the filtrate showed no trace of chloride ions (test with silver nitrate). The time of solid-acid contact was short enough to modify the surface without dissolving the silica. It was subsequently dried at 423 K in a vacuum dryer for a day. The mean particle size and the nitrogen specific surface area did not change at all in comparison with the original sample. Both silica samples will be described as SilNa (original silica) and SilH (washed silica) for further convenience. The pH value of a natural suspension of silica particles in pure deionized water was equal to 8.3 for SilNa and 6.6 for SilH. It may be that numerous sodium atoms, which are released from the SilNa surface, disturb the subtle electric equilibrium between the interface and the bulk phase, giving rise to an increase in the pH. It is difficult

to predict exactly the evolution of surface charge in the above samples, since the point of zero charge is usually determined at a constant ionic strength in the aqueous phase. For the same pH, the adsorption of counterions within the Stern layer can promote further release or uptake of protons in the surface plane. This increases the total number of charged sites, though from the standpoint of electrophoresis the charge outside the slipping plane is reduced. Meanwhile, the electrophoretic mobility of silica particles suspended in pure deionized water was measured to be smaller for SilH (-1.36 × 10-4 cm2 V-1 s-1) than for SilNa (-4.7 × 10-4 cm2 V-1 s-1). This discrepancy can be attributed to different pH values of both suspensions and permits one to suppose that the initial negative charge (i.e., available before adsorption of BDDAB) of silica is much smaller in the case of SilH. The experimental results of BDDAB adsorption from pure deionized water onto SilH are presented in Figure 4: adsorption isotherm (Fig. 4a), adsorption enthalpy curve (Fig. 4b), variations of pH of the supernatant liquid along the isotherm (Fig. 4c), and evolution of the electrophoretic mobility of silica particles upon adsorption (Fig. 4d). The hypothesis of a low charge density on SilH seems to be supported by the experimental adsorption data. The initial surfactant adsorption is markedly reduced in Figure 4a. There is even a pseudoplateau on the isotherm at low adsorption densities. It does not occur exactly at the

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isoelectric point concentration found in the electrophoretic measurement (Figure 4d). Quantitative information obtained from the analysis of electrophoretic curve is probably not precise on account of a low density of surface charge. This can result, for example, in a nonuniform distribution of adsorption densities on different particles. Moreover, the interpretation of the electrophoretic behavior is additionally complicated by the adjustment of surface charge to changes in the pH of the aqueous phase. Commonly, a marked decrease or increase in the pH is considered as evidence of a change in the surface charge during adsorption of ionic surfactants on the oppositely charged mineral surfaces.5,6,22,23 For the BDDAB adsorption onto SilH, a drop in the pH of the supernatant liquid was observed (Figure 4c) in a range of low bulk molalities up to about 0.8 mmol kg-1 (the related amount adsorbed is 2.5 µmol g-1). The total enthalpy change is exothermic in the whole adsorption range (Figure 4b). At the beginning, it quickly decreases with an increasing amount adsorbed up to about 8 µmol g-1. It seems clear that the endothermic contribution is either small or neutralized by much stronger exothermic effects. Then, the enthalpy becomes a constant function of the adsorption density; this constant value is equal to -11 kJ mol-1. The effectiveness of BDDAB adsorption onto SilH decreases twice compared with SilNa and corresponds to the area of 0.8 nm2 per one adsorbed molecule. A comparison with the value obtained at the water/air interface argues for only a monolayer or a noncomplete bilayer formed on the SilH surface. Flocculation of particles was detected in a quite narrow interval: beyond the isoelectric point until 7 µmol g-1. Since the flocculate is redispersed at higher quantities of adsorption, most of the surfactant ionic heads at the interface have to be oriented toward the aqueous solution. It is hardly conceivable that the hydrophobic tails could adsorb on a hydrophilic surface at this stage of the process. Hence, further adsorption can be seen as a deposition of new ions from the solution (with ionic heads outwardly disposed) on the previously “anchored” surfactants.24 The surface aggregates will not be able to fuse with each other and yield a compact bilayer at saturation because of a low charge density on the SilH surface. If the structure of these aggregates resembled that of bulk micelles, the enthalpies of their formation would be similar. Indeed, for quantities of adsorption greater than 42 µmol g-1, the enthalpy effect of adsorption decreases steadily and arrives at a value of -6.6 kJ mol-1 just before the plateau adsorption region. On the other hand, Figure 4d illustrates the changes in the net surface charge because the electrophoretic mobility, as related directly to the ζ potential, can be considered as an approximate measure of the net charge. The electrophoretic mobility of the SilH particles monotonously increases at high molalities. This indicates that the positive net charge is compensated by the surfactant counterions, Br-, located in the diffuse layer rather than between the headgroups at the solution side, and thus contrary to the bulk micelles. Perhaps, the resulting adsorbate structures are less compact. Discussion It is usually concluded that, at low or even very low surface coverages, ionic surfactants adsorb on oxide surfaces in much the same way as simple ions. Alkyl chains adopt a vertical orientation with respect to the surface, and the phenomenon is ascribed only to the (22) Bitting, D.; Harwell, J. H. Langmuir 1987, 3, 500. (23) Siracusa, P. A.; Somasundaran, P. J. Colloid Interface Sci. 1986, 114, 184. (24) Gao, Y.; Du, J.; Gu, T. J. Chem. Soc., Faraday Trans. 1 1987, 83, 2671.

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electrostatic interaction between the ionic heads of surfactant molecules and oppositely charged ionic sites on the substrate surface.24-28 Since the distances between the adsorbed ions are great enough for lateral chainchain attractions to be neglected, this early adsorbed stage is regarded as being chain length independent (the only difference is due to the various solubilities of the solutes in water). However, parallel orientation of the adsorbate at the adsorbent/solution interface cannot be ultimately excluded, and this encourages some authors to suggest that one part of the hydrophobic surfactant tail lies near enough to the surface to experience interactions with it.6,9,29-31 Scamehorn et al.30 have provided an alternative analysis of the initial stage of adsorption in which at least the terminal 10th through 12th carbons in the alkyl chain of isomerically pure alkyl benzenesulfonates interact strongly with the surface, in addition to the ionic headsurface site electrostatic attraction. The possibility of a more horizontal arrangement of the individually adsorbed surfactant ions has been previously confirmed by comparing calorimetric measurements for BDDAB and its polar headgroup, BTMAB.12,13 The hydrophobic association of the chain and the support would lead to a partial dehydration of the surface and induce some changes in the energetic state of solvent remaining in the adsorbed phase. Water molecules tend to adopt a special structure in the vicinity of a charged hydrophilic surface.32-35 Such a structured layer of water is energetically different from bulk water. Adsorption of a hydrophobic surfactant moiety will destroy this particular arrangement. The magnitude of enthalpic and entropic changes, accompanying the desorption of the very structured interfacial water, will depend on the temperature and the alkyl chain length. The effect of the former parameter has been already investigated in previous work.12 A more precise conclusion concerning the role of an alkyl chain at an early stage may be formulated by comparing the enthalpy effects of adsorption for BTMAB, BDDAB, and BDOAB on the same silica surface and under the same initial conditions. The displacement enthalpy curves are presented in Figure 5. The adsorption domain was limited to 50 µmol g-1. For quantities of adsorption ranging between 1 and 20 µmol g-1, the BDOAB curve superimposes on that of the polar headgroup, BTMAB. This means that the short alkyl chain (C8) adopts an orientation perpendicular to the silica surface. On the other hand, the enthalpic values for BDDAB are positive in this adsorption interval. A longer alkyl chain (C12) of BDDAB exhibits numerous conformations which allow an important flexibility and a spatial expansion. As a result, the orientation is predominantly parallel with respect to the surface. Dewetting of silica and local disruption of the interfacial water structure makes an (25) Hough, D. B.; Rendall, H. M. In Adsorption from Solution at the Solid/Liquid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: London, 1983; Chapter VI. (26) Menezes, J. L.; Yan, J.; Sharma, M. M. Colloids Surf. 1989, 38, 365. (27) Chen, Y. L.; Chen, S.; Frank, C.; Israelachvili, J. J. Colloid Interface Sci. 1992, 153, 244. (28) Ginn, M. E. In Cationic Surfactants; Jungermann, E., Ed.; Marcel Dekker: New York, 1970; Chapter X. (29) Ter-Minassian-Saraga, L. J. Chim. Phys. 1960, 57, 10. (30) Scamehorn, J. F.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1982, 85, 463. (31) Narkiewicz-Michalek, J. Langmuir 1992, 8, 7. (32) Packer, K. J. Philos. Trans. R. Soc. London, Ser. B 1977, 278, 59. (33) Brun, M.; Lallemand, A.; Quinson, J. F.; Eyraud, C. Thermochim. Acta 1977, 21, 59. (34) Israelachvili, J. N. Chem. Scr. 1985, 25, 7. (35) Kavanau, J. L. Water and Solute-Water Interactions; HoldenDay: San Francisco, 1964.

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Figure 5. Influence of the hydrophobic tail length on the adsorption energetics for benzyldimethylalkylammonium bromides onto precipitated silica from aqueous solutions at 298 K and initial pH 8.3: the differential molar enthalpies of displacement against the quantity of adsorption in a limited adsorption range (the enthalpic values were taken with the opposite sign).

important endothermic contribution to the total enthalpy change. It is interesting to note that the silica dispersion is destabilized well below the isoelectric point (beyond 20 µmol g-1 12), where the net surface charge is still negative. This indicates the extent to which the mineral surface has been dewetted and hydrophobized by the alkyl chains lying near the surface. The net energy resulting from dispersion and hydrophobic (hydrophobized particles are forced out of the solvent structure) interactions exceeds that of the electrostatic repulsion. Of course, it is scarcely possible to imagine that the above-mentioned endothermic contribution could neutralize all exothermic effects, like the adsorption enthalpies of the ionic head (Coulombic interaction with the charged surface sites) and the hydrophobic tail. However, BDDA+ and BTMA+ ions adsorb through ion exchange. They enter the Stern layer by replacing Na+ cations, preadsorbed at the silica/water interface.12 The presence of a quasihorizontal segment on the electrophoretic curve of BDOAB at small bulk molalities (Figure 2b) supports the same adsorption mechanism for this molecule. Thus, the enthalpies of adsorption and dehydration of the adsorbing ion are cancelled out to a great extent by the enthalpies of desorption and rehydration of the desorbing Na+ ions. The overall enthalpy of displacement will be dominated by the non-Coulombic interactions between the adsorbate, solvent, and support. The ability of organic cations, like BTMA+, BDOA+, and BDDA+, to displace equally charged sodium cations may be interpreted in terms of their greater size. They are also partially hydrophobic (benzyl group, alkyl chain) and so induce important changes in the structure of the interfacial water molecules, by comparison with sodium cations. The resulting endothermic effect is the greatest for BDDAB, the long alkyl chain of which is in direct contact with the solid surface, but has to exist also in the case of BTMAB, since its enthalpy values tend to zero. The enthalpy change during displacement, being a function of the current state of both the adsorbed and the bulk phase, initially decreases with an increasing amount adsorbed. Sodium cations and interfacial water molecules are freed from more and more strongly held positions. After having reaching its minimum value, the displacement enthalpy for surfactants monotonously increases.

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Figure 6. Effect of the ionic strength and washing of silica surface on the energetics of the BDDAB adsorption onto precipitated silica at 298 K and free pH: the differential molar enthalpies of displacement against the amount adsorbed in a limited adsorption range (the enthalpic values were taken with the opposite sign).

The effect can be attributed to reduction in the rate of ion exchange (e.g. one BDDA+ ion is exchanged for six sodium cations12). Moreover, the alkyl chain conformation may change from the quasi-parallel to more perpendicular as the adsorption continues, owing to strong lateral attractions between the adjacent surfactant tails. Changes in the energetic state of interfacial water molecules upon adsorption are less drastic, and the overall enthalpy of displacement becomes less endothermic. Since the changes progress steadily, there is a minimum on the enthalpic curve. The BTMAB curve does not show this tendency. The enthalpic values are zero at higher adsorption densities. One can speculate that the next BTMA+ cations are adsorbed at a greater distance from the surface. The hypothesis is supported by the fact that adsorption of the headgroup ions is not able to reverse the surface charge of silica.12 The influence which a given solid surface exerts on the arrangement and motion of the adjacent water molecules depends on the nature of this surface. The formation of an ionic double layer around the silica particles in water liberates certain sodium atoms from the surface. Some of them remain specifically adsorbed at the silica/water interface and induce a local ordering of interfacial water by binding free water molecules. The net structuremaking effect of relatively small ions, such as Na+, on water structure is well-known in the literature.35 One can imagine an analogous phenomenon at the interface.36 Therefore, the magnitude of endothermic contributions should depend on the sodium concentration both near the silica surface and in the bulk phase. This conclusion is illustrated in Figure 6, which shows three enthalpic curves corresponding to adsorption of BDDAB onto (a) SilNa from aqueous solution, (b) SilNa from 0.1 M NaBr solution, and (c) SilH from aqueous solution, in a limited adsorption interval (up to 110 µmol g-1). At low surface coverages, there is the same decreasing tendency in the enthalpy of displacement for all curves. The initial adsorption of BDDAB is more exothermic for SilH than it is for SilNa. The adding of NaBr also results in a more exothermic displacement. There are two reasons which may be responsible for a more exothermic adsorption in the presence of salt. The (36) Kihira, H.; Matijevic, E. Langmuir 1992, 8, 2855.

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excess of sodium counterions adsorbed at the silica/water interface can destroy the particular ordering of interfacial water molecules (it essentially reduces the amount of hydration36) and decrease the endothermic displacement of water. Moreover, the energetic difference between interfacial and bulk water is substantially reduced on account of the presence of sodium cations in the bulk phase. Another cause of the phenomenon is related to a change in the adsorbate orientation at the adsorbent/solution interface. Hydrophobic association of the alkyl chain with the support is less probable owing to the difficulty of displacing a high concentration of sodium cations in the vicinity of the surface. Large hydrated sodium ions remain near the charged sites and prevent attachment of the hydrophobic tails. As a consequence, the orientation of the adsorbate at the interface is more perpendicular in comparison with the system without salt. The endothermic contribution owing to the displacement of interfacial water has to diminish. In the case of SilH, there is no reason to believe that surfactant tails adopt an orientation different from that on the SilNa surface; the lack of sodium counterions and low charge density make the attachment of an alkyl chain to the silica surface easier. It is even conceivable that a kind of flat surfactant layer with many contacts with the silica surface forms in the first place. The exothermicity of adsorption may be considered as evidence for a poor structuring of interfacial water in the absence of Na+ ions at the silica surface. It should be noted that here the total enthalpy change does not include the desorption enthalpy of sodium cations. Even if the endothermic contribution due to desorption of the very structured interfacial water were the same as for SilNa, the exothermic contributions would control the total enthalpy change. The above-presented interpretation of the enthalpic curves at low adsorption densities is not the only one possible. Recently, Wa¨ngnerud et al. measured the differential molar enthalpies of adsorption for alkyltrimethylammonium bromides onto Spherosil X015 porous silica at initial pH 9 in the presence of sodium, tetramethylammonium, and calcium bromides.10 In order to explain the anomalously endothermic enthalpy effects at low surface coverages, they made use of the concept of surface nucleation, according to which discrete adsorbate associations formed in solution adjacent to the surface are subsequently adsorbed on it. The formation of such nuclei for future surface aggregates would be characterized by an endothermic enthalpy. A rise in temperature and a longer alkyl chain cause the enthalpy value to be less endothermic, but the general shape of enthalpy curves remains unchanged. As the adsorption density increases, the primary aggregates grow into more extended structures and the enthalpy of aggregation becomes more and more exothermic. It should be noted that this model is able to explain correctly the shape of the enthalpy curve for surfactant adsorption onto SilNa in the whole domain of surface coverage and at different temperatures. Calorimetric studies of the BDOAB association in the bulk phase has showed that the formation of small aggregates may be endothermic. By analogy with the bulk solution, the first surface aggregates are expected to appear at adsorption densities, which are much greater for BDOAB than they are for BDDAB. As a result, the enthalpy curve for BDOAB is identical with that for the headgroup molecule over the initial adsorption region. However, the effect of the alkyl chain length on the energetics of adsorption, predicted by this model, is at variance with the experimentally observed tendency (Figure 5). Although surface aggregation is an universal mechanism of surfactant adsorption at higher surface coverages, and the possibility

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Figure 7. Effect of the ionic strength and washing of silica surface on the BDDAB adsorption onto precipitated silica at 298 K and free pH: adsorption isotherms on a double logarithmic scale (slp specifies the average slope of the initial isotherm part).

of forming aggregation nuclei should be characteristic of each adsorption system, the existence of an endothermic minimum on the enthalpy curve is an exception rather than a rule. For the same surfactant molecule, it disappears at higher salt concentrations (note 0.01 M NaBr solution used in ref. 10) and for other silica surfaces (e.g. SilH, quartz,37 or precipitated silica rich in potassium37). It is appropriate at this point to consider the problem of individual surfactant adsorption, since the region in which this phenomenon occurs is the subject of some debate in the literature. Experimental adsorption isotherms are usually plotted on a double logarithmic scale, and four characteristic regions of adsorption are distinguished. In region I, the adsorption is believed to obey Henry’s law because isotherms generally are linear and have a slope of unity.6,8,9 Sometimes the tangent of the log-log plots is less than unit at very small surface coverages, and this is viewed as the result of adsorption on a strongly heterogeneous solid surfaces (the Freundlich isotherm equation).38 Region II is characterized by a sharply increasing slope indicating that surfactant adsorption becomes cooperative, i.e., strong tail-tail attraction between the adsorbed ions enhances the affinity of the surfactant for the surface. In Figure 7 three adsorption isotherms for the systems presented in Figure 6 are given as log-log plots. It is evident that only the isotherm for BDDAB onto SilNa from 0.1 M NaBr solution matches up to the abovementioned pattern. The slope of the initial part, the domain of which extends up to about 4 µmol g-1, is equal to 1.3. The individual surfactant adsorption has to exhibit a pronounced competitive character because of a high electrolyte concentration in the system. The behavior of the displacement enthalpy, which falls off very quickly in this adsorption range (Figure 3b), can be interpreted in terms of heterogeneity effects interfering in ion exchange (the exchange rate and energy may vary with increasing surface coverage). Beyond 4 µmol g-1, aggregates begin to appear locally on the surface. The most likely mechanism involves the coverage of the more highly charged regions by admicelles, combined with further adsorption (37) Zajac, J.; Lindheimer, M.; Partyka, S. Prog. Colloid Polym. Sci. 1995, 98, 303. Trompette, J. L.; El Ghazaoui, A.; Zajac, J.; Lindheimer, M.; Partyka, S. Cal. Anal. Therm. 1995, 26, 139. (38) Lajtar, L.; Narkiewicz-Michalek, J.; Rudzinski, W.; Partyka, S. Langmuir 1993, 9, 3174.

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of individual monomers. Since admicelles are envisaged as structures with ionic headgroups at both sides of the adsorbed layer, the related average enthalpy change upon displacement is smaller than that corresponding to adsorption of the same number of monomers with all the headgroups in contact with the surface. Moreover, different charged zones are filled in the sequence of their decreasing charge densities. Consequently, the molar enthalpy of displacement continues to decrease at the beginning of region II. When the adsorption of monomers stops, the displacement enthalpy becomes a constant function of the amount adsorbed, as can be seen in Figure 3b. The presentation of isotherms on the double logarithmic scale is not adequate to identify the domain of individual adsorption for BDDAB onto SilH and SilNa from pure aqueous solution. In the absence of background electrolyte, the relationship between amount adsorbed and equilibrium molality is not perfectly linear and the slope of isotherms decreases with increasing bulk concentration throughout region I. One should recall, in this connection, the conclusion, drawn from the theoretical analysis of Harwell et al.,9 that the linearity of isotherms in region I is a consequence of high electrolyte concentrations in the bulk phase and ought to be considered as a limiting condition. It can reasonably be argued that the lack of a steep transition between region I and region II is a consequence of the horizontal orientation of the surfactant tail with respect to the surface at an early stage of adsorption and its further evolution with the adsorption density. Both changes in the tail orientation and formation of surface aggregates are induced by the same driving force, namely the tail-tail attraction. In addition to this, primary aggregates have to be formed of surfactant ions having their heads in contact with the surface in order to compensate the negative surface charge.4,5,39,40 All these effects cause the subsequent modes of adsorption to change gradually. Thus, the individual adsorption range has been equated with a region, where the rate of exchange between BDDA+ and Na+ ions is constant and almost equal to unity (up to 20 µmol g-1 12). Taking into account the fact that an amorphous form of silica involves a random distribution of charged sites (topography “random”38), the preadsorbed sodium cations can be seen as quite uniformly distributed over the surface. The monovalent-monovalent cation exchange preserves a large surface area corresponding to one adsorbed ion, and lateral interactions are negligible at least for small adsorption densities. In the case of SilH, the main difference lies in a much smaller density of surface charge. Consequently, continued adsorption even well above the isoelectric point seems to have little influence on the tail orientation. This observation, in connection with a linear dependence of the surface charge and ζ potential upon the amount adsorbed (Figure 8), permits one to predict a region of individual adsorption up to 2.5 µmol g-1 (first pseudoplateau). Beyond this limit, the exothermic enthalpy continues to decrease in the whole flocculation region (Figure 4b). On the one hand, changes in the adsorbate arrangement are still very slow, and on the other hand, the strength of surface-ionic head interaction in this region diminishes in comparison with the individual adsorption range. According to Bo¨hmer and Koopal,5,6 the ideal four-region behavior is characteristic of adsorption isotherms of ionic surfactants on constant-potential surfaces at different salt concentrations. It would be thus interesting to investigate (39) Gaudin, A. M.; Fuerstenau, D. W. Trans. AIME 1955, 202, 66, 958. (40) Somasundaran, P.; Healy, T. W.; Fuerstenau, D. W. J. Phys. Chem. 1964, 68, 3562.

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Figure 8. Evolution of the pH of the supernatant and of the electrophoretic mobility of silica particles upon the BDDAB adsorption onto (a) original and (b) washed silica samples from aqueous solutions at 298 K and free pH. The individual adsorption ranges are separated by vertical dashed lines.

the evolution of surface charge upon adsorption of BDDAB on both silica surfaces. The electrical nature of the silica/ water interface is strongly determined by the pH of the aqueous phase because its negative charge is caused by the dissociation of amphoteric surface hydroxyl groups.2,5,41 This can be represented in the following way

dSiOH a dSiO- + H+

(1)

The protons are assumed to lie in the surface plane (location of the potential-determining ions) so that the surface activity of H+ is estimated from the Boltzmann equation5,41

[H+]s ) [H+]b exp(-eψ0/kT)

(2)

where ψ0 is the electric potential of mean force experienced in the surface plane and the superscripts “b” and “s” refer to bulk and surface concentration, respectively. Individual BDDA+ cations are adsorbed onto SilNa through ion exchange. In the absence of exchangeable sodium cations, an ion-pairing mechanism has to be postulated. In both cases, individual adsorption involves electrostatic compensation between surfactant ions with their heads adhered to the surface and negatively charged groups at (41) Davis, J. A.; James, R. O.; Leckie, J. O. J. Colloid Interface Sci. 1978, 63, 480.

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the surface. It is usually visualized as follows

dSiO- + BDDA+ a dSiO-‚‚‚BDDA+

(3)

The surfactant counterions reside in the Stern layer where the potential of mean force is ψi. In such a case, ion pairing cannot be further seen as a direct exchange between the surfactant cations and protons preadsorbed on the surface (see the previous work12). It is more reasonable to assume that the surface sites for the adsorbate ions are independent of those for potential-determining ions.42-44 The counterions will be retained onto adsorption sites, defined as the local minima in the potential energy of the surfaceadsorbate interaction. However, surface ionization and specific adsorption can influence each other by modifying the electrical double-layer properties (both potentials ψ0 and ψi are strictly related by the electroneutrality condition). Partial neutralization of surface charge leads to a drop in the negative surface potential. This may affect the amphoteric dissociation process by shifting the chemical equilibrium 1 toward formation of additional negatively charged sites in order to counteract the diminution in the surface potential. For a given physical regime, an equilibrium between both effects is established. If the charge reproduction is completely stoichiometric, the surface potential remains constant; otherwise, it will decrease. Changes in the surface charge are accompanied by a release of protons from the surface and a consequent decrease in the pH of the aqueous phase. Ultimately, evolution of pH of the supernatant liquid and ζ potential (electrophoretic mobility) of solid particles may illustrate changes in both surface charge and surface potential.4,7 The experimental results are presented in parts a (for SilNa) and b (for SilH) of Figure 8. For SilH, a marked drop in the pH from the initial value of 6.6 to 4.6 can be observed in the range of individual adsorption (up to 2.5 µmol g-1). On average there is thus one proton released to the bulk phase for every two adsorbing surfactant ions. The negative surface charge substantially increases, and this is accompanied by a linear decrease in the net surface charge (the negative surface potential is reduced). During individual adsorption of BDDAB on the SilNa surface, monovalent sodium cations are replaced in the Stern layer by similarly charged surfactant ions. Since the exchange rate is slightly smaller than unity and the pH of the bulk phase is not maintained constant, the mechanism of surface charge regulation is activated (pH decreases from the initial value of 8.3 to 7.5 at 20 µmol g-1). The net surface charge does not vary in this adsorption interval, and the SilNa surface behaves as if it were the constant-potential surface. Of course, the mechanism of surface charge regulation induces a thermal effect, which contributes to the total enthalpy change upon individual adsorption. Neverthe(42) Bowden, J. W.; Posner, A. M.; Quirk, J. P. Aust. J. Soil Res. 1977, 15, 121. (43) Barrow, N. J.; Bowden, J. W.; Posner, A. M.; Quirk, J. P. Aust. J. Soil Res. 1981, 118, 454. (44) Fokkink, L. G. J.; De Keizer, A.; Lyklema, J. J. Colloid Interface Sci. 1990, 135, 118.

less, the task of evaluating the enthalpy of deprotonation of amphoteric surface sites is very difficult.45 When the absolute amount of protons released to the bulk phase is small (SilNa), this effect can be neglected. Conclusions Depending on the purity of a silica sample, the individual adsorption of surfactant cations occurs by ion-pairing or cation exchange mechanisms. Since the pH of the bulk phase is not readjusted to the initial value during adsorption, the system satisfies the condition of surface charge regulation. The negative charge of silica surface increases upon adsorption, and a decrease in the pH of the supernatant is monitored in this region. The desorption of a few interfacial water molecules and local changes in the structure of the remaining water owing to the specific adsorption of a part of the hydrophobic surfactant moiety give an endothermic contribution to the overall enthalpy of the process. Comparative calorimetric studies of surfactants having different lengths of an alkyl chain permit experimental confirmation of the hypothesis that the orientation of short tails (shorter than C9) is substantially perpendicular to the silica surface. The presence of small and moderate concentrations of Na+ cations at the surface has a net structure-making effect on interfacial water; much energy is needed to displace it from the surface. Since individual adsorption involves replacement of sodium counterions by similarly charged surfactant cations, the enthalpic contributions due to adsorption of one cation and desorption of the other cancel each other out to a great extent. In such a case, the overall enthalpy is substantially determined by the adsorption enthalpy of an alkyl chain and changes in the energetic state of interfacial water. When the latter is sufficiently large, the resulting displacement process becomes endothermic in a certain adsorption interval. At high surface concentrations of sodium (the addition of NaBr to the aqueous phase), this effect disappears and the competitive surfactant adsorption is a completely exothermic process. If the adsorption density is sufficiently high, lateral interactions between adsorbed ions induce a change in the surfactant tail conformation toward a more perpendicular orientation. This favors the hydrophobic bonding mechanism of adsorption and leads to the formation of various adsorbate structures on the silica surface. For high densities of charged sites, many experimental arguments support the existence of a compact bilayered structure at surface saturation. For low charge densities, a great increase in the chemical potential of the solute in the bulk phase (much above the isoelectric point) is necessary to initiate surface aggregation. As a result, the final structures are less compact. The adding of NaBr to the system prevents the surfactant tails from adopting the orientation parallel to the surface. The growth of the adsorbed layer through local bilayered admicelles results in a greater effectiveness of adsorption. LA950645Q (45) Kallay, N.; Zalac, S.; Stefanic, G. Langmuir 1993, 9, 3457.