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Surface Interactions on Silica Particles between a Cationic Surfactant and Sodium Salicylate P. Favoriti, M. H. Mannebach, and C. Treiner* Laboratoire d’Electrochimie, URA CNRS 430, Universite´ Pierre et Marie Curie, 4 Place Jussieu, Bat. 74, Paris 75005, France Received February 27, 1996. In Final Form: May 21, 1996X Cetylpyridinium chloride, a cationic surfactant, adsorbs at a silica/water interface, forming double-layer structures which coadsorb salicylate ions below the equilibrium critical micelle concentration (cmc) as the result of strong interactions between the surfactant head-group and the aromatic anion. Above the cmc the salicylate ions are distributed between adsorbed surfactant aggregates, free micelles, and water. Using a simple pseudophase formalism, ion partition coefficients are calculated in dilute solutions as ratios of bound to free salicylate concentrations for the various adsorbed or free surfactant structures. It is shown that the salicylate coadsorption ion partition coefficient decreases as the pH increases above the silica isoelectric point. The partition coefficient is larger for the adsorbed aggregates than for the free micelles at low (acidic) pH but is equal to the latter values at higher (alkaline) pH values. On the basis of a previous investigation on neutral solutes in the same silica/surfactant system, the following interpretation of these observations is suggested. At low pH values, salicylate ions form complexes with the cationic head-groups of both surfactant layers: the inner layer with the head-groups facing the silica surface and the outer-layer facing the bulk of the solution. At higher pH values, salicylate ions are repelled from the inner layer by the high negative surface charge density on the silica particles. As a consequence only the surfactant outer layer complexes the aromatic anions. This configuration is closer to that of free micelles, hence, the decrease of the salicylate partition coefficients and the similar partion coefficient values for the aromatic anion for both types of adsorbed and free surfactant structures. Additional evidence is provided by the behavior of sodium naphthoate and 2-4-6-trichlorophenol in the same chemical systems.
Introduction Monomer surfactants adsorb at solid/water interfaces, forming various types of aggregates which may incorporate molecular species below the equilibrium surfactant critical micelle concentration (cmc). Above the cmc, as free micelles are formed, the solutes are partitioned between the surfactant-adsorbed aggregates and the micelles. At higher concentrations, the solutes are eventually completely desorbed from the solid/water interface. The former phenomenon has been coined adsolubilization1-5 as a reminder of the classical parent micellar solubilization effect which occurs in micellar solution, i.e. above the cmc. The term coadsorption has also been used.6-9 This effect may be described by a partition coefficient, defined as the ratio of the solute concentration in the adsorbed surfactant aggregates to that in the bulk solution, with the same basic assumptions as for micellar solubilization constants. Most of the studies concerned with the incorporation of solutes into surfactant aggregates, whether as free micelles or as adsorbed structures, have been dealing with neutral species, as shown by the various compilations of data which have been published in recent years.10-14 The case of ionic species has been much less investigated. The * To whom all correspondence should be addressed. X Abstract published in Advance ACS Abstracts, August 1, 1996. (1) Harwell, J. H.; Hoskins, J. C.; Schechter, R. S.; Wade, W. H. Langmuir 1985, 1, 251. (2) Lee, C.; Yeskie, M. A.; Harwell, J. H.; O’Rear, E. A. Langmuir 1990, 6, 1758. (3) Esumi, K.; Mizuno, K.; Yamanaka, Y. Langmuir 1995, 11, 1571. (4) Esumi, K.; Shibayama, M.; Meguro, K. Langmuir 1990, 6, 826. (5) Funkouser, G. P.; Are´vako, M. P.; Glatzhofer, D. T.; O’Rear, E. A. Langmuir 1995, 11, 1443. (6) Klumpp, E.; Heitmann, H.; Lewandowski, H.; Schwuger, M. J. Prog. Colloid Polym. Sci. 1992, 89, 181. (7) Schieder, D.; Dobias, B.; Klumpp, E.; Schwuger, M. J. Colloids Surf., A 1994, 88, 103. (8) Monticone, V.; Mannebach, M. H.; Treiner, C. Langmuir 1994, 10, 2395. (9) (a) Monticone, V.; Treiner, C. Langmuir 1995, 11, 1753. (b) Monticone, V. Ph.D. Thesis, University of Paris, 1995. (10) Foley, J. P. Anal. Chim. Acta 1990, 231, 237.
S0743-7463(96)00180-1 CCC: $12.00
effect of ions on micellar solubilization and related phenomena using ionic surfactants is usually concerned with electrolytes bearing a common ion with the surfactant. One important exception concerns the behavior of weak organic acids or bases in micellar solutions. These compound solutes may dissociate, depending upon the pK of the solute and the pH of the solution. However most micellar solubilization studies have been concerned with such solutes in their undissociated form.11,12 One of the most studied of this class of molecular species is the phenols in association with cationic surfactants.15-22 Many reasons may explain this choice besides the importance of these compounds in many industrial areas such as pesticides and environment problems. Phenol derivatives and the corresponding phenoxide ions interact in various ways with cationic surfactants. They may form compleses, as demonstrated by studies of phenols and cetyltrimethylammonium bromide,15,17-19,21 of naphtholates with cetylpyridinium chloride (CPC)16 or naphthoates (11) Valsaraj, K. T.; Thibodeaux, L. J. Sep. Sci. Technol. 1990, 25, 369. (12) Sepulveda, L.; Lissa, E.; Quina, F. Adv. Colloid Interface Sci. 1986, 25, 1. (13) (a) Treiner, C.; Mannebach, M. H. J. Colloid Interface Sci. 1987, 118, 243. (b) Treiner, C. In Solubilization in Surfactant Aggregates; Christian, S. D., Scamehron, J. F., Eds.; Surfactant Science Series, Vol. 55; Marcel Dekker: New York, 1995; p 383. (c) Abraham, M. H.; Chadha, H. S.; Dixon, J. P.; Rafols, C.; Treiner, C. J. Chem. Soc., Perkin Trans. 2 1995, 887. (14) Pramauro, E.; Minero, C.; Saini, G.; Graglia, R.; Pelizzetti, E. Anal. Chim. Acta 1988, 212, 171. (15) Bunton, C. A.; Sepulveda, L. J. Phys. Chem. 1979, 83, 680. (16) Amire, S. A.; Burrows, H. D. J. Chem. Soc., Faraday Trans. 2 1982, 78, 2040. (17) Bertolotti, S. G.; Garcia, N. A.; Gsponer, H. E. J. Colloid Interface Sci. 1989, 129, 406. (18) Bunton, C. A.; Cowell, C. P. J. Colloid Interface Sci. -1988, 122, 154. (19) Sepulveda, L. J. Colloid Interface Sci. 1974, 46, 372. (20) Thalody, B. P.; Warr, G. G. J. Colloid Interface Sci. 1995, 175, 297. (21) Senz, A.; Gsponer, H. E. J. Colloid Interface Sci. 1994, 165, 60. (22) Bachofer, S. J.; Simons, V.; Nowicki, T. A. J. Phys. Chem. 1991, 95, 480.
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with tetradecyltrimethylammonium bromide.22 Moreover, depending upon the position of the substituent group, phenols, but also aromatic amines, may form with cationic micelles viscoelastic solutions, as with the much documented case of sodium salicylate.23-27 If the interaction of cationic micelles and phenols seems complicated because specific and electrostatic effects are intrinsically related, the situation looks even worse when the interaction takes place at a solid/water interface. This situation is encountered on many occasions where pesticides, for example, are concerned. Mineral oxides are one of the most important types of dispersions. Their surface-charge-determining ions are the proton and the hydroxyl ions. Therefore, in the case of compounds such as phenol derivatives, the pH may influence simultaneously the adsorption of the surfactants on the particle surface and the fraction of dissociated phenoxide ions. The first effect depends upon the charge on the mineral oxide surface, and the second effect is related to the pK of the phenol. In this preliminary study it was decided to investigate the case of a chemical system which has been studied recently in the case of nondissociating molecular species.8,9 A nonporous silica was chosen as the solid substrate. The isoelectric point of this mineral oxide being of the order of 3, the particles are negatively charged above this pH value and the dispersion easily adsorbs cationic surfactants. CPC was used as the cationic surfactant. The representative phenolic derivative is sodium salicylate. It is the most studied of the different surfactant/solute binaries which form viscoelastic solutions above a critical concentration in bulk water. This phenomenon should not occur at a two-dimensional solid/water interface. Sodium salicylate adsorbs on a positively charged solid surface such as C-alumina in the absence of added surfactant.28 The aromatic anion is not expected to adsorb on a negatively charged surface such as silica above its isoelectric point. In the present investigation the interaction of sodium salicylate with CPC adsorbed on silica will be determined as a function of the pH of the system. In recent investigations on the coadsorption of various neutral species, it was shown that the partition coefficient of coadsorption Pads was independent of the pH of the system for undissociating species such as aromatic alcohols but varied with pH for 2-naphthol in its molecular form.8,9 This point is one of the fundamental questions which will be addressed in this work on sodium salicylate. It is believed that the answer to this question concerns many more of this important class of chemical compounds. Materials and Methods CPC was from Sigma. Sodium salicylate was from Fluka (puriss). Sodium naphthoate was prepared from naphthoic acid (Aldrich, 99% pure). The acid was first dissolved in a mixture of ethanol and water. An equimolar concentration of sodium hydroxide in water was then added, and the resulting solution was evaporated to dryness. The purity of the salt was checked by potentiometric measurements using a sodium ion-selective electrode. NaCl was from Prolabo, France (normapur). 2,4,6Trichlorophenol was 98% pure (Aldrich). The silica used (Aerosil 200) was a nonporous material kindly provided by DegussaFrance with a specific surface area of 200 ( 25 m2/g, as determined (23) Grasvholt, S. J. Colloid Interface Sci. 1976, 57, 575. (24) Bayer, O.; Hoffmann, H.; Ulbricht, W.; Thurn, H. Adv. Colloid Interface Sci. 1986, 26, 177. (25) Hirata, H.; Sato, M.; Sakaiguchi, Y.; Katsube, Y. Colloid Polym. Sci. 1988, 266, 862. (26) Underwood, A. L.; Anacker, E. W. J. Colloid Interface Sci. 1985, 106, 86. (27) Imae, T.; Kohsaka, T. J. Phys. Chem. 1992, 96, 10030. (28) Thomas, F.; Bottero, J. Y.; Cases, J. M. Colloids Surf., A 1989, 37, 281.
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Figure 1. (O) Adsorption isotherm of CPC (left scale) on Aerosil 200 at pH ) 4.2 (0.01 mol/L NaCl). (b) Coadsorption of sodium salicylate (right scale). The arrow designates the cmc in Figures 1-3, 6, and 7. by the manufacturer. The cmc of CPC in the presence of salt was determined by surface tension measurements at 25 ( 0.1 °C (Kruss K10T) in the usual way. The adsorption of CPC and the uptake of salicylate ions by the adsorbed surfactant aggregates onto the silica particles were determined simultaneously as a function of surfactant concentration. The technique used has been described previously in detail for neutral solutes:8 4 × 10-4 mol/L of sodium salicylate, 1.0 × 10-2 mol/L of NaCl, and increasing quantities of CPC were added to 0.2 g of silica in 20 mL of solution (1% of solid dispersion). The resulting dispersion was equilibrated for 24 h. The pH of the solution was adjusted with sodium hydroxide. Three pH values were investigated: 4.2, 6.5, and 8.5. The pH value was controlled regularly during the equilibration process using a combined glass electrode and a pH meter from Tacussel (France). After ultracentrifugation (Sigma 2K 15) at 25 °C and 10 000 rpm, the supernatant was analyzed using a double-beam Cary 1E UV-visible spectrophotometer. The wavelengths employed were λ(CPC) ) 259 nm and λ(salicylate) ) 297 nm. The mixing of CPC and sodium salicylate induced a very small change of the optical density of the latter compound (but not of the former) at concentrations above 2 × 10-4 mol/L. This effect could be ignored in the present investigation. The cmc of CPC in the presence of 0.01 mol/L of NaCl was 2.0 × 10-4 mol/L; in the presence of a concentration of sodium salicylate of 4.0 × 10-4 mol/L (the coadsorption conditions) the cmc was found equal to 5.0 × 10-5 mol/L. The pK values of salicylic acid, naphthoic acid, and 2,4,6-trichlorophenol are respectively equal to 3.0, 4.2, and 6.0.
Results and Discussion 1. General Observations. Figures 1-3 present the CPC adsorption isotherms at the three pH values investigated as the amount of adsorbed surfactant (left scale) as a function of the logarithm of the free concentration, log Ceq. On the same graph is plotted the concentration of salicylate ions coadsorbed with the surfactant (right scale) as a function of log Ceq. The total initial salicylate concentration is maintained constant and equal to 4 × 10-4 mol/L in all experiments. The profile of the CPC adsorption isotherms is classical and could be interpreted as commonly accepted for this type of system: Monomer surfactant ions adsorb onto negatively charged sites imposed on the silica surface by the pH conditions (region 1). Then aggregates begin to form, as shown by the more rapid increase of adsorption (region 2). This region begins at a surfactant coverage of the order of 20%, as deduced from neutron reflection
Interactions of a Surfactant and Sodium Salicylate
Figure 2. (O) Adsorption isotherm of CPC (left scale) on Aerosil 200 at pH ) 6.5 (0.01 mol/L NaCl). (b) Coadsorption of sodium salicylate (right scale).
Figure 3. (O) Adsorption isotherm of CPC (left scale) on Aerosil 200 at pH ) 8.5 (0.01 mol/L NaCl). (b) Coadsorption of sodium salicylate (right scale).
studies29 and 2H NMR spectral analysis,30 or perhaps as low as 12%, as inferred by Fourier transform infrared spectroscopy.31 As the cmc is approached, a plateau adsorption is obtained (region 3). Free micelles are formed in the plateau region (region 4). Note on Figures 1-3 that the higher the pH, the more negative sites are created on the silica particles and the larger the extent of surfactant adsorption in region 1. At pH ) 8.5 the 20% threshold lies within what would be defined as region 1 of the isotherm according to Figure 3. The surfactant coverage values reported above were deduced from studies either on silica plates29,31 or on silica dispersions at high pH values.30 No effect of added electrolyte was reported in these investigations. The following preliminary remarks may be put forward. Firstly, under the present experimental conditions (i.e. with 0.01 mol/L of added NaCl) the CPC adsorption (29) Rennie, A. R.; Lee, E. M.; Simister, E. A.; Thomas, R. K. Langmuir 1990, 6, 1031. (30) Kung, K. H. S.; Hayes, K. F. Langmuir 1993, 9, 263. (31) Soderlind, E.; Stilbs, P. Langmuir 1993, 9, 2024. (32) Leimbach, J.; Jigg, J.; Rupprecht, H. Colloids Surf., A 1995, 94, 1.
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isotherm is marginally changed by the presence of sodium salicylate (not shown). This is due to the presence of NaCl. Experiments conducted in the absence of added salt showed that the adsorption of CPC is greatly enhanced especially at lower surfactant concentrations by the addition of sodium salicylate. One forms in situ the surfactant, cetylpyridinium salicylate (CPSa), which adsorbs much more strongly on silica than CPC.32 Thus, the added NaCl imposes a chloride ion environment to the solid/water double layer with the adsorbed cetylpyridinium ions. As noted above, the addition of sodium salicylate decreases considerably the cmc of CPC, considering the small concentration of added organic salt. This effect does not show on the surfactant adsorption isotherm. The surfactant concentration at which the adsorption plateau occurs is hardly changed in Figures 1-3 and corresponds to the cmc of CPC in the presence of added NaCl as measured in the bulk. The reason for this apparent contradiction with the preceding observation is that, as will be discussed below, all salicylate ions are coadsorbed below the cmc; thus, it is the chloride ion concentration which imposes the occurrence of the plateau. This point deserves some further comments. The salicylate ions are coadsorbed well below the cmc, in the region of the surfactant isotherm where only monomer surfactants are adsorbed through Coulombic interactions with the negative sites on the silica surface. In fact, as indicated in Figures 1-3, more than 90% of the salicylate ions are already coadsorbed in the monomer region. At the cmc, close to 100% of the initial salicylate concentration is incorporated to the solid/liquid interface. It may be recalled that salicylate ions are not adsorbed to any measurable degree in the absence of surfactant at the imposed pH. Thus, the salicylate ion condensation occurs directly with the adsorbed monomer CP+ ions at the silica/ water interface. As a number of studies have shown, at least in the case of micelles in bulk aqueous solutions, that aromatic anions form complexes with cationic surfactants, one may assume that the same sort of specific interaction occurs at the silica/water interface as the result of a competition between the negative surface sites and the salicylate ions for the CP+ monomers. It would seem therefore that the negative sites play a minor role (if at all) in the formation of the CP+/salicylate complex. This point will be refined below. Finally as free micelles are formed above the cmc, the salicylate ions are partitioned between the adsorbed surfactant aggregates and the free micelles, hence the decrease of coadsorption noted in Figures 1-3. Before any attempt to quantify these observations, the effect of the ionic sites on the CP+/salicylate interaction may be demonstrated. Figure 4 shows the effect of pH at constant concentration of surfactant and aromatic anions on the ratio of coadsorbed to free ion concentration for three aromatic ions: salicylate, naphthoate, and 2,4,6trichlorophenoxide. The total anion concentration is equal to 4.0 × 10-4 mol/L in each case. The variation of surfactant concentration adsorption with pH (from 6.0 × 10-4 to 6.4 × 10-4 to 6.4 × 10-4 mol/L) was small enough so that in all cases only region 1 of the surfactant isotherms was concerned. In the case of the salicylate and the naphthoate ions, the coadsorption decreases rapidly with pH to close to zero at pH ) 9. As the quantity of adsorbed CP+ ions is essentially constant, the decrease of coadsorption is clearly due to the increase of negative sites on the silica surface. The following interpretation may be suggested. One recalls that, in region 1, the surfactant adsorbs through its monomers. Thus, as the salicylate ions are added to the solution, they form complexes with CP+ at the expense of the interaction between CP+ and
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selectivity constant20,33-36 or by complex formation with a binding constant.8,9,16-18,21 In the present work the latter model was preferred, which corresponds in fact to a partitioning formalism much as that used in the case of neutral molecules. As the sodium salicylate concentration is small and the sodium ion plays no specific role in the presence of a swamping NaCl concentration, only the salicylate ion is considered as distributed between adsorbed surfactant aggregates or free micelles and the aqueous solution. Below the cmc, one can write the following coadsorption partition coefficient in the molality scale:
Pads ) Figure 4. Variation of the concentration ratios of coadsorbed to free aromatic ions or molecules at the same constant CPC concentration below the equilibrium cmc (region 1) as a function of pH: (O) naphthoate; (x) salicylate; (b) 2,4,6-trichlorophenol.
the negative surface sites. As more negative sites are created, the repulsion of the salicylate ions becomes larger than the favorable interaction with the adsorbed CP+ ions. The fact that the coadsorption of the naphthoate ion is larger than that of the salicylate ion is qualitatively in line with the respective complex constants in cationic micellar solutions. Note however that as the concentration of adsorbed CP+ ions increases at a constant salicylate ion concentration of 4 × 10-4 mol/L along the adsorption isotherm, the decrease of Cads/Ceq becomes smaller with increasing pH (not shown). This is due to the fact that, close to the equilibrium cmc, all salicylate ions are eventually complexed by the CPC aggregates, as shown in Figures 1-3. The case of 2,4,6-trichlorophenol as a weak acid is also of interest. Its pK is equal to 6.0; i.e., it is predominantly present as undissociated species below this pH value and as dissociated species above this value. As the pH is increased above pH ) 3, the coadsorption increases because more anions are dissociated which interact more strongly with the adsorbed CP+ ions than do the undissociated species. This result is in agreement with previous data on phenols and the corresponding phenoxide ions in cetyltrimethylammonium bromide micellar solutions:15 the interaction of the dissociated form with the cationic micelles is stabilized by Coulombic forces. However, above pH ) 6, the repulsion interaction on the dissociated aromatic species due to the increasingly negative charge on the silica particles induces a decrease of coadsorption as with the salicylate and naphthoate ions. This effect is clearly dominant at higher pH values. The fact that the maximum is observed at the pK value may be partly coincidental. It may be concluded from this part of the discussion that at low surfactant coverage, unlike the case of undissociated molecular species, the concentration of coadsorbed ionic species varies with pH even as the adsorbed surfactant concentration remains constant. This observation may be looked upon as a pH destabilizing effect of surface complexes between surfactant and added counterions. Furthermore the effect of pH on the dissociation of the solid surface sites is predominant over the effect on the dissociation of the weak acids. The attempt of the evaluation of partition coefficients may enable us to stress these points on more standard grounds. 2. Partition Coefficients. The interaction between ions and ionic aggregates with unlike counterions is often described either by an ion-exchange formalism with a
Cads CwΓads
(1)
where the salicylate concentrations are expressed in mol/L of solution. Γads is the adsorbed surfactant concentration. Cads and Cw are respectively the adsorbed and the aqueous solute concentrations. For the micellar solubilization (in the absence of solid) the classical relationship may be used in the same concentration scale, where Cmic is the salicylate concentration in the micellar phase:
Pmic )
Cmic Cw(C - cmc)
(2)
Above the cmc, but in the presence of the solid phase, the salicylate ions are distributed between adsorbed aggregates, free micelles, and the aqueous solution. Starting from the definitions of the partition coefficient for coadsorption and micellar solubilization (eqs 1 and 2), the concentration of adsorbed salicylate ions may be calculated from the equation37
Cads )
Ct (1 + Pmic(Ceq - cmc)) 1+ PadsΓads,ms
(3)
Ct is the total salicylate concentration, Ceq is the surfactant equilibrium concentration, Pmic is the classical solubilization constant, and Γads,mx is the maximum adsorbed surfactant concentration at the adsorption plateau, which is a constant in the present situation. The term adsorbed concentration has been used throughout although bound concentration would be more appropriate. The former term was used here in order to keep the same conventions that in the parallel studies on the interaction of neutral species which is clearly then a coadsorption (adsolubilization) effect. Pads is calculated from eq 1, as will be shown below; thus, all variables and constants are known in eq 3, except for Pmic, which is obtained through a fitting procedure. It has been demonstrated previously for neutral species that, using this approach, the value of Pmic obtained is exactly the solute micellar partition coefficient, as determined in micellar solution in the absence of solid dispersion.9 Thus, the interaction between the coadsorbed solute (or ion) and free micelles or adsorbed aggregates can be quantified. Figure 5 presents an illustration of eq 1 for pH ) 4.2. A (33) Morgan, J. D.; Napper, D. H.; Warr, G. G.; Nicol, S. K. Langmuir 1994, 10, 797. (34) Gamboa, C.; Sepulveda, L.; Soto, R. J. Phys. Chem. 1981, 85, 1429. (35) Hafiane, A.; Issid, I.; Lemordant, D. J. Colloid Interface Sci. 1991, 142, 167. (36) Bunton, C. A.; Carrasco, N.; Huang, S. K.; Paik, C. A.; Romsted, L. S. J. Am. Chem. Soc. 1978, 100, 5420.
Interactions of a Surfactant and Sodium Salicylate
Figure 5. Illustration of the determination of the coadsorption partition coefficient of the salicylate ion: pH ) 8.5 (see text).
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Figure 7. Variation of the coadsorption of sodium salicylate as a function of the free CPC concentration at pH ) 8.5. The fitting of eq 2 to the data enables the calculation of the micellar partition coefficient Pmic: full line. Table 1. Partition Coefficients for the Coadsorption and for the Micellar Condensation of Salicylate Ions on Silica/CPC Dispersions and on CPC Micelles at Various pH values in the Presence of NaCl at 0.01 mol/L (Molality Scalea) Pads Pmic Γads,mxb
pH ) 4.2
pH ) 6.5
pH ) 8.5
15500 4500 4.3
6300 3500 8.0
2700 2500 9.5
a In order to transform these P values to the molarity scale (as displayed in Figure 8), they should be divided by an estimated head-group partial molar volume of 0.15 L/mol. b Maximum adsorption value for CPC at the plateau in units of 10-4 mol/g.
Figure 6. Variation of the coadsorption of sodium salicylate as a function of free CPC concentration at pH ) 4.2. The fitting of eq 3 to the data enables the calculation of the micellar partition coefficient Pmic: full line.
straight line is observed from which the value of Pads is obtained. A slight correction was applied in order to take into account the departure from a linear variation at small CPC values. In this concentration domain, the ratio of salicylate ions to CPC monomers is relatively high (of the order of 0.4), with, as a result, a deviation from linearity because of strong nonideality effects due to the high salicylate surface concentration. Activity coefficient corrections due to pairwise and higher order salicylate interactions cannot be performed in such complicated situations. However, as more CP+ ions are added to the system, the ratio of salicylate ions to CP+ monomers at the solid/liquid interface decreases, and below a value of 0.1, a straight line is observed at all pH values from which a single partition coefficient may be deduced. The calculation of Pmic is illustrated in Figures 6 and 7 at the extreme pH values investigated of 4.2 and 8.5 by the full lines using the Pads values obtained as described before, with τ ) Cads/Ct the ratio of bound to total salicylate concentration. Table 1 summarizes the results obtained. The Γads,mx values refer to the CPC plateau values of eq 2. The partition coefficients were calculated in the molality scale.
In order to present these data in the more conventional (nondimensional) molarity scale, the values of Table 1 should be divided by the partial molar volume V° of the surfactant head-group. In the case of neutral species, this approximation is generally accepted. In the present case it is more controversial and the use of the partial molar volume of the surfactant head-group only seems more appropriate. A value of V° ) 0.15 L/mol may be used.36 This correction is useful if the present data are to be compared for example to the results obtained with the same substrate/surfactant system with neutral molecules. Otherwise, the comparison between Pads and Pmic does not depend upon the concentration scale used. The following observations may be put forward. One notes that Pads decreases as the pH increases. This is another way of looking at the results of Figure 4. The reasons for this behavior have been suggested before: at a higher pH value (e.g. pH ) 8.5), the salicylate ions may not complex any more the first layer of CP+ ions with their head-groups facing the silica surface. Only the second layer may be concerned with the salicylate complexation with the cationic head-groups facing the bulk of the solution. Note that, according to the various spectroscopic investigations stated above,29-31 bilayers would form within the region noted as 1 on the surfactant isotherm. The comparison between the relative values of Pads and Pmic helps to strengthen this point. It was shown recently9 that, for nondissociating species such as aromatic alcohols or substituted quinones, Pads ) Pmic. The solubilization capabilities of the surfactant double layer were identical to those of free micelles. However with weak acids or bases, such as 2-naphthol, 1-naphthylamine, paranitro-
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Figure 8. Correlation of Pads with Pmic in the presence of cationic surfactants and silica at pH ) 4.2. Names of solutes in increasing partitioning order: 2-phenoxyethanol;37 1-phenoxy3-propanol;37 1,4-benzoquinone;9b 1,4-naphthoquinone;9b paranitroaniline;9b 1-naphthylamine;38 2-(2-naphthyl)ethanol;38 2-naphthol;9a salicylate (pH ) 8.5) (this work); salicylate (pH ) 6.5) (this work); salicylate (pH ) 4.2) (this work).
aniline, and 2,4,6-trichlorophenol, Pads was systematically larger than Pmic by a factor of 3 for 2-naphthol and the chlorinated phenol and by a factor of 2 for the weak bases. Among the reasons which were suggested for the interpretation of the difference observed, one ad hoc hypothesis is that, close to the solid/surface interface, the concentration of indifferent electrolyte is such that the local pH is different from that measured in the bulk of the solution. Note, however, that the micellar solubilization constant of aromatic hydrocarbons in anionic micelles was found to be larger than the corresponding coadsorption coefficients obtained with the surfactant adsorbed on the positively charged alumina. Although the calculation procedure was different from that used in the present study, this point deserves some further investigation. The present results for the salicylate ion look qualitatively similar to those obtained with the weak acids and bases coadsorbed on the CPC/silica dispersions. Pads is larger than Pmic by a factor of 3 at pH ) 4.2. The factor is 2 at pH ) 6.5 and 1 at pH ) 8.5 (Pads ) Pmic). Thus, plotting log Pads as a function of log Pmic, the equipartition straight line passes through the undissociated molecules, alcohols and quinones, and the salicylate ion at pH ) 8.5 (Figure 8), leaving the neutral weak acids and bases above this line together with the salicylate ion at the lower pH values. Here all P values were transformed into the same molar concentration scale. It may be recalled that the reason for the higher Pads value over Pmic for the weak acids or bases in their molecular form was suggested to be related to a possible dissociation under the influence of a local pH. The present results suggest a possible reason for the strengthening of such an effect. It has been noted before that surfactant bilayers may be formed by cationic surfactants on silica in the absence of added salt at a surfactant coverage as low as 0.12-0.20.29-31 This would (37) Favoriti, P.; Monticone, V.; Treiner, C. J. Colloid Interface Sci. 1996, 179, 173.
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mean that bilayers would begin to form in the present situation even in region 1 at pH ) 8.5. We have suggested above that the decrease of salicylate coadsorption with pH is most probably due to the repulsion of those anions which are complexed by the CP+ cations close to the silica surface as more negative charges are created. The equality of the salicylate P values obtained at pH ) 8.5 for the coadsorbed and for the micellar processes would then mean that the partitioning corresponds then to the interaction of the salicylate ion with the outer surfactant layer only, that which faces the bulk solution. In the case of free micelles, there is evidently only one layer of surfactant, hence the equality of partition coefficients. When the two surfactant layers adsorb the salicylate ions, then the partition coefficient should increase grossly by a factor of 2 when compared to micelles. This would mean that for phenols or aromatic amines the reason for the higher Pads than Pmic values is related to the same phenomenon: specific interactions with the cationic head-groups of both surfactant layers. In the case of aromatic alcohols, the driving force for the coadsorption or the micellar solubilization is essentially the hydrophobic effect which may, understandably, be of the same order of magnitude for both adsorbed and free surfactant structures, as it concerns not specifically the surfactant head-groups as with the salicylate ion but also the hydrocarbon core of the surfactant aggregates. There are some grounds to this hypothesis besides the cases just mentioned. For example, weak bases such as toluidine derivatives are known to form viscoelastic solutions with cationic surfactants,25 just as phenols do. Extension of studies such as the present one to anionic micelles adsorbed on alumina or titanium dioxide would be interesting for further testing of the ideas put forward in this investigation. A last point must be addressed concerning the results of Table 1. If the coadsorption change with pH may be reasonably interpreted as suggested above, the change of Pmic with pH may be somewhat disturbing. At the pH of the experiments the condensation of the salicylate ions onto the pure micelles of CPC should be independent of the pH of the solution. This result is what would be expected in the absence of the silica particles. In fact the variation of Pmic with pH is much less than the change of Pads. It may be noted at this point that the variation of Pmic with pH as deduced from eq 2 is not specific to its application to ions. The same result had been obtained before with 2-naphthol in its undissociated state when coadsorbed on the same CPC/silica system.9 However, the important difference is that Pads increased by a factor of 2.5 as the pH was varied from 4.2 to 6.5. This is in line with the previous argument, as the pK of 2-naphthol is equal to 9.5. Thus, it seems that, unlike nondissociating molecular species, the interaction of weak acids such as phenols with cationic micelles coadsorbed on pH-sensitive dispersions is somewhat dependent upon the pH of the system even at values remote from their pK. The fact that the behavior of these substances in their undissociated form looks qualitatively similar to that of the salicylate ions stresses the possibility of a partial dissociation of weak acids or bases at the silica/water interface in a pH domain remote from their pK values by more than 2 orders of magnitude from the equilibrium pH of the bulk solution. This effect is enhanced by complex formation of phenol derivatives with cationic surfactants. LA960180J
