J. Phys. Chem. 1996, 100, 3237-3240
3237
Surface Potentials and Ion Binding in Tetradecyltrimethylammonium Bromide/Sodium Salicylate Micellar Solutions Marta A. Cassidy and Gregory G. Warr* School of Chemistry, UniVersity of Sydney, NSW 2006, Australia ReceiVed: October 10, 1995X
Surface potentials of mixtures of tetradecyltrimethylammonium bromide and sodium salicylate have been determined by titration of a micelle-bound indicator, 4-heptadecyl-7-hydroxycoumarin. We find that the strong binding of salicylate ion is effective at lowering the surface potential of the micelles but see no evidence for charge reversal as recently reported on the basis of electrokinetic measurements (Imae and Kohsaka. J. Phys. Chem. 1992, 96, 10030). Results are compared with models and previous results for salicylate binding to micelles.
Introduction Cationic surfactant systems of the alkyltrimethylammonium type (RTA+) show marked viscoelastic behavior when the salicylate (Sal-) counterion is present.1-9 The work of Hoffmann and others on RTA+/Sal- systems has shown conclusively that these species form wormlike micelles above a critical salicylate concentration.7,8 More recently it has also been demonstrated that further addition of salicylate will lead to the transformation of rods back into spherical micelles.6,9,10 There have been many efforts to explain this phenomenon, and the manner in which salicylate binds to the micelles appears to be the key to this problem. The general consensus in the literature is that the salicylate does not bind exclusively to the exterior surface of the micelles as do simple ions, such as halides. Due to its amphiphilic character, the salicylate inserts its benzene ring some way into the hydrophobic part of the micelle. NMR studies have provided the main evidence in support of this conclusion.2,3,5,11-13 This penetration is universally thought of as an intercalation of the salicylate between RTA+ molecules. The aromatic ring of the salicylate is thought to be more or less in line with the plane of the trimethylammonium head groups, and there has been some speculation that there is an electrostatic attraction between the electron-rich salicylate ring and the positively charged ammonium head group.14,15 In this orientation the hydroxyl and carboxyl functional groups are effectively hydrated by the bulk water. It has been postulated by some authors that there is a unique 1:1 complex formed between RTA+ and salicylate.5,9,11 The supporting evidence largely hinges upon the maximum in viscosity of these systems observed at, or about, a 1:1 RTA+: Sal- mole ratio and on an NMR study of the binding of salicylate to cetyltrimethylammonium (CTA+) micelles at this viscosity maximum.11 More recent work however casts some doubt on this idea, as we discuss below. Imae and Kohsaka10 have recently extended studies of the effect of added salicylate to Sal-:RTA+ ratios much greater than 1. Their dynamic light-scattering measurements on tetradecyltrimethylammonium salicylate (TTSal: no other counterion present) have shown that the length of even such purified TTASal rodlike micelles initially increases with increasing concentration of added salicylate. In 25 mM surfactant solutions, the micellar length reaches a maximum at [Sal-] ∼ 0.1 X
Abstract published in AdVance ACS Abstracts, January 15, 1996.
0022-3654/96/20100-3237$12.00/0
Figure 1. Schematic diagram of a cationic micelle surface, showing the positions of the bound salicylate counterions in the Imae and Kohsaka model10 and of the acid-base indicator. The positions where the ζ-potential and surface potential, ψ0, are measured are also shown.
M and the system is composed of entangled wormlike micelles. At this point the viscosity of the system is also a maximum. As the concentration of salicylate is further increased above 0.1 M, the micelles decrease in size until at 1 M salicylate they are again spherical and the viscosity is comparable to that of water. In conjunction with the light-scattering results Imae and Kohsaka also determined the electrophoretic mobility of TTASal micelles as sodium salicylate was added. They observed that the mobility and hence the ζ-potential of the micelles decreased from positive at [Sal-] ) 0.0 M to neutral at [Sal-] ) 0.1 M and then became negative at [Sal-] > 0.1 M. This behavior is explained by invoking the existence of two sites for salicylate binding to micelles. One is the conventional penetration of salicylate ions between the surfactant head groups, and adsorption of salicylate onto the micelle surface is the other. Figure 1 shows a schematic diagram of a cationic micelle/water interface, indicating the positions of both intercalated and surface-bound salicylate in the Imae and Kohsaka mechanism. As salicylate becomes embedded in the micelles, Imae and Kohsaka propose that the charge on the micelle is first neutralized and then reversed at sufficiently high concentration of salicylate. Thus, binding of the salicylate to the micelle continues to increase above [Sal-] ) 0.1 M, creating ‘anionic’ micelles which revert to spherical due to electrostatic repulsion. Imae and Kohsaka postulate that salicylate ions are both © 1996 American Chemical Society
3238 J. Phys. Chem., Vol. 100, No. 8, 1996 adsorbed on the micelle surface and embedded in the micelles at all concentrations.10 In the present study, solutions containing tetradecyltrimethylammonium bromide (TTAB) have been examined at various concentrations of added salicylate by reliable acid-base indicator absorbance techniques and the surface potential, ψ0, of these micelles has been determined.16-19 This method detects the change in potential at the plane of the head groups where the prototrophic moiety of the indicator resides, rather than at the plane of shear, as occurs with ζ-potential measurements. Hence, this method probes only those salicylate ions intimately associated with the surfactant head groups. The relative positions where the electrokinetic and thermodynamic potentials, ζ and ψ0, are measured are shown schematically in Figure 1. Also shown is the position of the indicator in the micelle. By comparing the potential of the micelle at the plane of shear and at the plane of the head groups, we will infer the location of the excess salicylate giving rise to the reported charge reversal. In addition, we review the evidence for a 1:1 complex between salicylate and trimethylammonium head groups in light of our results for surface and ζ-potentials of tetradecyltrimethylammonium micelles. Materials and Methods Tetradecyltrimethylammonium bromide (TTAB) was purchased from Aldrich, and n-dodecyloctaoxyethylene glycol monoether (C12E8), from Nikkol Chemical Company. 4-Heptadecyl-7-hydroxycoumarin (HHC) was obtained from Molecular Probe Inc. All were used without further purification. The remaining reagents, NaOH, HCl, and sodium salicylate (NaSal), and the THF were of analytical grade and used as received. Aqueous solutions were prepared with Millipore Milli-Q purity water. Solutions of TTAB at 25 mM were prepared, and the required amount of NaSal was then added to give solutions with salicylate concentrations of 0 M, 25 mM, 0.10 M, and 1.0 M. Solutions were allowed to stand for 2 days to ensure that the systems had reached equilibrium. Extensive work by Hoffmann and others7 has shown that salicylate binds so strongly to the micelles compared to bromide that the system of interest, TTA+ micelles and Sal- counterions, is effectively the same in Imae’s study and the present work except with no added salicylate. The electrophoretic mobilities of the TTAB micelles with added salicylate were measured to verify the equivalence of these solutions and those used by Imae.10 The electrophoresis was carried out on a Coulter Electronics DELSA 440 electrophoretic light-scattering analyzer at the UNESCO center for membrane technology, University of New South Wales. Micellar surface potentials were measured by the technique of Fernandez and Fromherz20 and Drummond17,18 in which the absorbance of the micelle-bound acid-base indicator 4-heptadecyl-7-hydroxycoumarin (HHC) was monitored as a function of solution pH. All absorption spectra were recorded on a Varian Cary 1 spectrophotometer, with an identical micellar solution, except for the presence of HHC, used as a reference. Sufficient HHC dye was dissolved in 1 mL of THF to give a final concentration of 3 × 10-4 M when dissolved in the micellar solution. This yields an average of approximately one dye molecule per micelle, which ensures that the micelles are not significantly distorted by the inclusion of many dye molecules per micelle. The dye/THF solution was added to the sample solution and stirred for 3 h to ensure that the dye was fully dispersed into the micelles before the titration commenced. The apparent pKa of a micelle-bound dye is altered by its microenvironment due to the change in the pH near the micelle
Cassidy and Warr surface. The surface potential of a micelle, ψ0, has been shown to obey the relationship20
ψ0 ) 2.303
RT (pKai - pKaobs) F
(1)
Where pKai is the pKa of a reference system to zero surface potential and pKaobs is the observed pKa of the system of interest. R is the gas constant (8.314 J K-1 mol-1), T the temperature (K), and F the Faraday constant (96486 C mol-1). It has been shown by Drummond and Grieser16 that the pKai value may be obtained from pH titrations in a nonionic micellar system such as C12E8, which acts as a zero potential surface. Other surfactants possessing the poly(ethylene oxide) head group may also be used as a zero potential reference surface. C12E8 has been well characterized by Drummond and Grieser16 and was therefore selected as the zero potential surface for this study. The important considerations when choosing such a reference are that the interaction between the surfactant and the dye molecule be relatively insignificant and that the interfacial dielectric constants of the reference and the system under examination be close or equivalent. Drummond and Grieser16 examined the validity of using C12E8 as the reference zero potential surface for TTA+ micelles and determined that, although the interfacial dielectric constants are not equal, they are sufficiently close that the use of C12E8 as a reference surface for this system is justified. Drummond and Grieser16 also found that any specific molecular interactions between the TTA+ head group and the HHC anion do not markedly affect the acidbase dissociation of the HHC. Also, the deviation of the referenced surface potential determined by this technique from the surface potential determined by other techniques was less than +12mV in most cases.20 The fraction of the indicator in its acidic form, R, varies with pH according to the Henderson-Hasselbach equation16
(1 -R R)
pH ) pKa - log
(2)
The pKaobs value of the dye may thus be obtained from a plot of log{R/(1 - R)} vs pH, by noting that the x-intercept is the pKaobs of the sample.20 In this study R was determined by measuring the absorbance of the basic form of the dye, at 380 nm (I380). The absorbance at each pH was compared with the absorbance at high pH (I380,base), when the dye is fully deprotonated, and the fraction of acid R was determined according to
R)
(I380,base - I380) I380,base
(3)
Results Absorbance spectra of a 25 mM TTAB/0.1 M sodium salicylate solution containing HHC are shown in Figure 2 for a range of pH values. The noise in these spectra at low pH is due to both the turbidity of the solutions and to the absorbance of the salicylate ion, which has a maximum around 280 nm. The absorbance of salicylate prevented monitoring of the absorbance of the acidic form of HHC (λmax ) 324 nm16) and observation of the isosbestic point of the indicator. However, the absorbance of the conjugate base of HHC was still accessible, and results based on TTAB only, where both acid and base peaks are visible, suggest that adequate data can be obtained from the base results alone. Figure 3 shows HHC titration data in TTAB solutions with various concentrations of added salicylate presented according
Surface Potentials of Salicylate Micelles
J. Phys. Chem., Vol. 100, No. 8, 1996 3239
Figure 2. Absorbance spectra of HHC in 25 mM TTAB with 0.1 M added NaSal. The pH of each spectrum is shown adjacent.
Figure 4. Surface potential, ψ0 (9), and ζ-potential (b) data for 25 mM TTAB (cmc ) 3.6 mM) solutions for the addition of sodium salicylate as a function of total ionic strength. At the lowest concentration, no salicylate has been added to the solution. Also shown is ζ-potential (2)10 data for 25 mM TTASal (cmc ) 0.36 mM) solutions for the addition of sodium salicylate as a function of total ionic strength.
The surface potential, ψ0, decreases with increasing salicylate concentration, eventually approaching zero at very high concentrations. This potential is identified with uptake of salicylate intimately associated with the micelle surface, presumably intercalated between TTA+ molecules. The measured ψ0 values differ substantially from the ζ-potentials, which display a charge reversal above 0.1 M salicylate.10 The marked difference between the potential at the plane of the head groups (ψ0) and the plane of shear (ζ-potential) suggests that there is a second population of adsorbed salicylate ions beyond the head-group region. These are responsible for the observed reversal of the ζ-potential. Figure 3. HHC titration data, plotted as log{R/(1 - R)} vs pH for 25 mM TTAB solutions with (b) no added NaSal; (0) 0.025 M NaSal; ([) 0.10 M NaSal; and (2) 1.0 M NaSal. Also shown is the titration curve for the reference solution C12E8 (9).
TABLE 1: pKaobs Values of Tetradecyltrimethylammonium Micelles with Varying Concentrations of Added Sodium Salicylate surfactant
[Sal-]/M
pKaobs
ψ0/mV
TTAB
0 0.01 0.025 0.05 0.1 0.5 1.0 0
6.11
165.5
7.46
87.8
8.33
37.8
8.85 8.97
7.7 0
C12E8
ζ-potential/mV 67 ( 15 42 ( 12 5.4 ( 0.5 -3.1 ( 0.1 -6.0 ( 0.8
to eq 2. Note that the pH remains well above the pKa of salicylic acid in aqueous solution, 3.03 ( 0.07.21 Thus, it is not expected that there will be significant quantities of protonated salicylate in solution either in the micelle or in the bulk of any of the solutions studied. Table 1 lists the pKaobs values obtained in accordance with eq 2. All titration curves are linear as expected, and Figure 2 clearly shows that pKaobs increases with increasing concentration of sodium salicylate, corresponding to a decrease in ψ0. Figure 4 shows the surface and ζ potentials of TTAB/Sal micelles derived from the data of Table 1 and Figure 3 as a function of ionic strength. Also shown are ζ-potentials of the TTASal systems derived from Imae and Kohsaka’s electrophoretic mobility results10 using the Schmolukowski equation.22 It can be clearly seen that the ζ-potential data is equivalent for micelles with and without bromide, consistent with effective displacement of adsorbed bromide by salicylate.23
Discussion These results are therefore consistent with the two-site model for salicylate binding to cationic micelles proposed by Imae and Kohsaka, in which both intercalated and surface-bound salicylate ions arise.10 It is not, however, consistent with the suggestion that these surface-adsorbed counterions are responsible for both charge reversal and the reversion of the micelles from cylinders to spheres. If we adopt the premise that counterions which do not affect ψ0 are surface adsorbed, then they can only minimize their mutual interactions by arranging themselves on the micelle surface at the maximum distance from one another. This arrangement results from the lateral interactions between adsorbate ions and is independent of their position relative to the electrokinetic shear plane. For such ions the micelles are an adsorbent whose morphology they can scarcely affect. Micelle shape is determined by surfactant geometry and packing. Adopting the Israelachvili, Mitchell, and Ninham convention, the progression from spherical TTAB to cylindrical TTAB/Sal micelles necessitates an increase in the surfactant packing parameter, V/a0lC, above 1/3 with addition of salicylate.24 Addition of salt often induces a sphere to rod transformation in micelles due to the reduction in the area per molecule, a0, consequent upon the screening of electrostatic repulsions between surfactant head groups.25 A lower bound on V/a0lC is set by the physical size of the trimethylammonium head groups. The strong binding of salicylate to TTA+ micelles is thus sufficient to explain the observed shape transformation, although the volume of the intercalated phenyl ring and the physical area of the cosurfactant-like salicylate ion should be included in V/a0lC for TTAB/Sal mixtures, modulating area reduction somewhat.
3240 J. Phys. Chem., Vol. 100, No. 8, 1996 In order to cause a reversion of the micelle structure into spheres, it is necessary that V/a0lC be reduced back below 1/3 by some mechanism. Repulsions between purely surface adsorbed salicylate cannot achieve any change in the head group packing, as they are not specifically bound to head groups. Our results demonstrate that the fully intercalated salicylate ions never bring about charge reversal, so there is also no possibility of electrostatic repulsion between these ions increasing the mean a0 in the plane of the head groups. Two alternatives remain. Firstly, we can propose a refinement of Imae and Kohsaka’s model.10 We need only propose that the volume and steric area of the intercalated salicylate ions come into play at very high surface coverages. This increases both V and a0, leaving the chain length constant and decreasing V/a0lC overall. In this model the intercalated salicylate ions are responsible for the shape transformation to cylinders and back to spheres, whereas the surface adsorbed ions cause charge reversal. The second model does not make such a distinction between surface-adsorbed and intercalated salicylate. Instead we propose that the counterions associate with the micelle surface to a continuously varying degree, of which fully intercalated and surface-adsorbed represent two extremes. In this scenario there is a population of partly intercalated counterions whose charge centers lie above the plane of the head groups and are not sensed by the measurement of ψ0. As with fully intercalated ions, these may affect surfactant packing through their contribution to the volume of the micelle core. These putative partly intercalated counterions may change the micellar area through electrostatic repulsions if sufficiently locally bound, although this is not likely to be a major factor for any ions not contributing to the measured ψ0. This model also returns to a rod f sphere transition driven by intercalated counterions independent of the charge reversal at the shear plane. These two descriptions try to recognize that ψ0 and ζ simply represent two positions in a potential distribution ψ(r) whose exact shape is unknown, as is the concentration profile of bound salicylate. Our primary conclusion supports that of Imae and Kohsaka,10 that a single counterion adsorption site is insufficient to describe the behavior of TTASal micellar solutions. However we do not believe it is possible to discriminate between a specific two-site model and one including a continuum of adsorption or association positions. Specific Complexation. The results of the present work are at odds with the model of a 1:1 complex between Sal- and TTA+. Imae and Kohsaka10 and this work have both shown that the adsorbed salicylate exists in (at least) two states, even at a 1:1 mole ratio. The difference between ψ0 and ζ at all added salicylate concentrations, and particularly the charge reversal in the shear plane, is strong evidence for more than one type of adsorption site. The existence of a unique 1:1 complex at this composition is inconsistent with the observation of multiple adsorbed states. The microscopic evidence for a 1:1 complex rests largely on an NMR study of chemical shifts in CTAB/Sal- solutions.11 This work certainly demonstrated that salicylate associates strongly with CTAB micelles. However the argument in favor of a 1:1 complex seems to rely mainly on the fact that, above a 1:1 ratio, the excess added salicylate is not incorporated into the hydrophobic part of the micelle, as sensed by aromatic proton chemical shifts, and that the amount of free salicylate changes abruptly at this ratio. If such a complex was formed, then one would expect a neutralization of the ψ0 due to the presence of stoichiometric amounts of Sal- and TTA+, resulting in an overall micellar
Cassidy and Warr charge of zero; i.e., ψ0 ) 0 at [NaSal] ) [TTAB] ) 25 mM. This is not observed in either ψ0 or ζ. Indeed the surface potential remains slightly positive up to 1 M added salicylate. The NMR data thus shows nothing more than strong binding of salicylate to CTA+ micelles. Of course the behavior can be explained by invoking a 1:1 complex, as a specific instance of strong binding with a particular stoichiometry. However this does not constitute evidence for its existence. Conclusion Even at a 100-fold excess of salicylate over TTAB no reversal in the sign of ψ0 of the TTA+ micelles was observed. When this is compared to ζ-potentials, it suggests that there are salicylate ions intercalated between the surfactant molecules and also bound to the exterior surface of the micelles. This agrees with the two-site model of salicylate binding proposed by Imae10 but is also consistent with a continuous variability in the adsorption site. These results do not support the existence of a stoichiometric 1:1 complex between salicylate and TTA+. Acknowledgment. This work was supported by the Australian Research Council. M.A.C. acknowledges the receipt of a Sydney University Postgraduate Scholarship. References and Notes (1) Ulmius, J.; Wennerstrom, H.; Johansson, B.-A.; Lindblom, G.; Gravsholt, S. J. Phys. Chem. 1979, 83, 2232. (2) Olsson, U.; Soderman, O.; Guering, P. J. Phys. Chem. 1986, 90, 5223. (3) Manohar, C.; Rao, U. R. K.; Valaulikar, B. S.; Iyer, R. M. J. Chem. Soc., Chem. Commun. 1986, 379. (4) Gravsholt, S. J. Colloid Interface Sci. 1976, 57, 575. (5) Bachofer, R.; Turbitt, R. M. J. Colloid Interface Sci. 1990, 135, 325. (6) Claussen, T. M.; Vinson, P. K.; Minter, J. R.; Davis, H. T.; Talmon, Y.; Miller, W. G. J. Phys. Chem. 1992, 96, 474. (7) Rehage, H.; Hoffmann, H. Faraday Discuss. Chem. Soc. 1983, 76, 363. (8) Lobl, M.; Thurn, H.; Hoffmann, H. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 1102. (9) Nemoto, N.; Kuwahara, M. Langmuir 1993, 9, 419. (10) Imae, T.; Kohsaka, T. J. Phys. Chem. 1992, 96, 10030. (11) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1988, 4, 354. (12) Rao, U. K. R.; Manohar, C.; Valaulikar, B. S.; Iyer, R. M. J. Phys. Chem. 1987, 91, 3286. (13) Anet, F. A. L. J. Am. Chem. Soc. 1986, 108, 7102. (14) Bachofer, S. J.; Simmonis, U.; Nowicki, T. A. J. Phys. Chem. 1991, 95, 480. (15) Underwood, A. L.; Anacker, E. W. J. Phys. Chem. 1984, 88, 2390. (16) Drummond, C. J.; Grieser, F. Photochem. Photobiol. 1987, 45, 19. (17) Drummond, C. J.; Warr, G. G.; Grieser, F.; Ninham, B. W.; Evans, D. F. J. Phys. Chem. 1985, 89, 2103. (18) Drummond, C. J. PhD Thesis, University of Melbourne, Australia, 1987. (19) Zachariasse, K. A.; Van Phuc, N.; Kozankiewicz, B. J. Phys. Chem. 1981, 85, 2676. (20) Fernandez, M. S.; Fromherz, P. J. Phys. Chem. 1977, 81, 1755. (21) Shikata, T.; Hirata, H.; Kotaka, T. J. Phys. Chem. 1990, 94, 3702. (22) Hunter, R. J. Zeta Potential in Colloid Science; Academic Press: London, 1981; Chapter 3. (23) Thalody, B.; Warr, G. G. J. Colloid Interface Sci. 1995, 175, 297. (24) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (25) Gamboa, C.; Rı´os, H.; Sepu´lveda, L. J. Phys. Chem. 1989, 93, 5540.
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