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Synthesis of a New Zwitterionic Surfactant Containing an Imidazolium Ring. Evaluating the Chameleon-like Behavior of Zwitterionic Micelles Daniel W. Tondo,† Elder C. Leopoldino,† Bruno S. Souza,† Gustavo A. Micke,† Ana C. O. Costa,† Haidi D. Fiedler,† Clifford A. Bunton,‡ and Faruk Nome*,† †
Departamento de Quı´mica, Universidade Federal de Santa Catarina, Florian opolis, Santa Catarina 88040-900, Brazil, and ‡Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106 Received June 11, 2010. Revised Manuscript Received August 13, 2010
Synthesis of a new zwitterionic surfactant containing the imidazolium ring 3-(1-tetradecyl-3-imidazolio)propanesulfonate (ImS3-14) is described. The solubility of ImS3-14 is very low but increases on addition of a salt which helps to stabilize the micellized surfactant. Fluorescence quenching and electrophoretic evidence for ImS3-14 shows that the micellar aggregation number is only slightly sensitive to added salts, as is the critical micelle concentration, but NaClO4 markedly increases zeta potentials of ImS3-14 in a similar way as in N-tetradecyl-N,N-dimethylammonio-1-propanesulfonate (SB3-14) micelles. The rate of specific hydrogen ion catalyzed hydrolysis of 2-(p-heptoxyphenyl)-1,3dioxolane and equilibrium protonation of 1-hydroxy-2-naphthoate ion in zwitterionic micelles of ImS3-14 and SB3-14 are increased markedly by NaClO4 which induces anionoid character and uptake of H3Oþ, but NaCl is much less effective in this respect. Comparison of ImS3-14 with SB3-14 is based on experimental evidence, and computational calculations indicate similarities and differences in structures of both compounds.
Introduction Although well-known, and naturally important, zwitterionic surfactants have been investigated less than other classes of surfactants. Their micelles have no net charge, but unlike other nonionic micelles they bind anions, becoming modestly anionic with nonzero electrostatic potentials1-4 and, in suitable conditions, they also interact with cations.1,5 Anions interact specifically with zwitterionic micelles, and ionic competition follows the Hofmeister series and Pearson’s hard-soft classification.3,6-13 Anions with low charge densities, such as PF6- and ClO4-, bind much more strongly to sulfobetaine micelles than the strongly hydrated anions, OH- and F-, and although the driving forces for ion binding are not fully understood quantitatively, specific anion binding to sulfobetaine micelles is related to hydration free energies.2 *To whom correspondence should be addressed. E-mail:
[email protected]. (1) Priebe, J. P.; Souza, B. S.; Micke, G. A.; Costa, A. C. O.; Fiedler, H. D.; Bunton, C. A.; Nome, F. Langmuir 2010, 26, 1008–1012. (2) Priebe, J. P.; Satnami, M. L.; Tondo, D. W.; Souza, B. S.; Priebe, J. M.; Micke, G. A.; Costa, A. C. O.; Fiedler, H. D.; Bunton, C. A.; Nome, F. J. Phys. Chem. B 2008, 112, 14373–14378. (3) Iso, K.; Okada, T. Langmuir 2000, 16, 9199–9204. (4) Marte, L.; Beber, R. C.; Farrukh, M. A.; Micke, G. A.; Costa, A. C. O.; Gillitt, N. D.; Bunton, C. A.; Profio, D. P.; Savelli, G.; Nome, F. J. Phys. Chem. B 2007, 111, 9762–9769. (5) Tondo, D. W.; Priebe, J. M.; Souza, B. S.; Priebe, J. P.; Bunton, C. A.; Nome, F. J. Phys. Chem. B 2007, 111, 11867–11869. (6) Chevalier, Y.; Kamenka, K.; Chorro, M.; Zana, R. Langmuir 1996, 12, 3225– 3232. (7) Kamenka, K.; Chorro, M.; Chevalier, Y.; Levy, H.; Zana, R. Langmuir 1995, 11, 4234–4240. (8) Kamenka, K.; Chevalier, Y.; Zana, R. Langmuir 1995, 11, 3351–3355. (9) Cuccovia, M. C.; Romsted, L. S.; H., C. J. Colloid Interface Sci. 1999, 220, 96–102. (10) Profio, D. P.; Germani, R.; Savelli, G.; Cerichelli, G.; Chiarini, M.; Mancini, G.; Bunton, C. A.; Gillit, N. D. Langmuir 1998, 14, 2662–2669. (11) Brinchi, L.; Di Profio, P.; Germani, R.; Marte, L.; Savelli, G.; Bunton, C. A.; Spreti, N. J. Chem. Soc, Perkin Trans. 2 1998, 2, 361–364. (12) Bongiovanni, R.; Ottewill, R. H.; Rennie., A. R; Laughlin, R. G. Langmuir 1996, 12, 4681–4690. (13) Okada, T.; Patil, J. M. Langmuir 1998, 14, 6241–6248.
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Such methods as NMR spectroscopy, ionic conductance, diazo trapping, kinetics, and the use of ion selective electrodes establish micellar preferences for anions.10,14,15 Capillary electrophoresis is effective in studying electrokinetic natures of colloidal particles, measuring local ionic concentrations, and by giving evidence related to surface potentials it complements the other methods.3,16-20 Interactions between ions and sulfobetaine micelles were treated by Okada and co-workers by using partition and ion-pair models.3,13 In the partition model, the ionic chemical potential term is included in the Poisson-Boltzmann equation, while in the ion-pair model inner and outer potentials of the zwitterionic micelle are considered. The anion order ClO4- > SCN- > I- > Br- >Cl- follows the Hofmeister series. Binding of inorganic anions to sulfobetaine micelles, studied by capillary electrophoresis21,22 and other physical methods, was related to kinetics of nucleophilic reactions and displacement of such reactive anions as Br-, I-, and OH- by ClO4-.4 Zwitterionic micelles with consequent anionoid character incorporate H3Oþ, promoting acid-catalyzed reactions and protonation of hydrophobic organic bases. This chameleon-like behavior of sulfobetaine micelles, controlled by salt addition,1,2,4,5 indicates that ion incorporation by zwitterionic surfaces is relevant to understanding behaviors of biological membranes.1,2,4,5,23 (14) Masudo, T.; Okada, T. Phys. Chem. Chem. Phys. 1999, 1, 3577–3582. (15) Bertoncini, C. R. A.; Nome, F.; Cerichelli, G.; Bunton, C. A. J. Phys. Chem. 1990, 94, 5875–5878. (16) Cai, J.; Rassi, Z. E. J. Chromatogr., A 1992, 608, 31–45. (17) Lee, C. S.; McManigill, D.; Wu, C. T.; Patel, B. Anal. Chem. 1991, 63, 1519– 1523. (18) Hayes, M. A.; Kheterpal, I.; Ewing, A. G. Anal. Chem. 1993, 65, 27–31. (19) Yoon, R.-H.; Yordan, J. L. J. Colloid Interface Sci. 1986, 113, 430–438. (20) Morini, M. A.; Schulz, P. C. Colloid Polym. Sci. 1997, 275, 802–805. (21) Yokoyama, T.; Macka, M.; Haddad, P. R. Anal. Chim. Acta 2001, 442, 221–230. (22) Yokohama, T.; Macka, M.; Haddad, P. R. Fresenius’ J. Anal. Chem. 2004, 371, 502–506. (23) Ruzza, A. A.; Nome, F.; Zanette, D.; Romsted, L. S. Langmuir 1995, 11, 2393–2398.
Published on Web 09/17/2010
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Anion incorporation in zwitterionic micelles has been examined largely with simple sulfobetaines. We synthesized a new zwitterionic surfactant, ImS3-14, with new physical-chemical properties. It differs from other sulfobetaines in that it has very low water solubility in the absence of salts and the cationic imidazolium ring is rigid and unsymmetrical. We probed anion effects by electrophoresis and spectrophotometric analysis of acid dissociation of 1-hydroxy-2-naphthoic acid (HNA) and by following acid hydrolysis of 2-(p-heptoxyphenyl)-1,3-dioxolane (HPD), and compared these effects to those in simple sulfobetaine micelles.
Experimental Section Materials. We used 1-hydroxy-2-naphthoic acid (HNA) (Sigma) as the UV-vis micellar acidity probe and pyrene (Aldrich) as the fluorescent probe in determination of aggregation numbers. The preparation and purification of 2-(p-heptoxyphenyl)-1,3-dioxolane (HPD) are described.23-25 4-Carboxy-1n-dodecylpyridinium salt (DPC) was synthesized as described26 (mp 189 C, dec.). 1H NMR spectrum (CDCl3, 400 MHz), δ, ppm: 8.61 d (2H), 8.42 d (2H), 4.64 t (2H), 2.04 m (2H), 1.36 m (2H), 1.25 m (16H), 0.88, t (3H). Sodium hydride, imidazole, 1-bromotetradecane, and other reagents were of analytical grade and were used without further purification. The salts (NaCl and NaClO4) and solvents were dried by standard methods. Synthesis of 1-Tetradecylimidazole. A solution of imidazole (16.3 g, 0.24 mol) in dry 1,4-dioxane (100 mL) was added to 150 mL of a suspension of oil-free sodium hydride (5.8 g, 0.24 mol) with stirring for 1 h at 90 C. A solution of 1-bromotetradecane (33.3 g, 0.12 mol) in 1,4-dioxane (100 mL) was then added dropwise to the reaction solution, and the mixture was stirred for 48 h at 90 C. The solvent was removed on a rotary evaporator giving a yellow residue that was suspended in 500 mL of water, followed by extraction with CH2Cl2 (4 70 mL) and brine washing, and the organic layer was dried with anhydrous Na2SO4. After removal of CH2Cl2, the resulting yellow oil was purified by column chromatography (silica gel) with ethyl acetate as eluent, giving 17.8 g (55.7%) of 1-tetradecylimidazole as a pale yellow oil. νmax/cm-1 IR (film): 3421, 3106, 2924, 2849. 1H NMR spectrum (CDCl3, 400 MHz), δ, ppm: 7.46 s (1H), 7.05 s (1H), 6.90 s (1H), 3.92 t (2H), 1.77 m (2H), 1.25 m (22H), 0.88, t (3H). Synthesis of 3-(1-Tetradecyl-3-imidazolio)propanesulfonate (ImS3-14). A solution of 1,3-propanesultone (9.0 g, 0.074 mol) in acetone (80 mL) was slowly added in a round-bottom flask to 1-tetradecylimidazole (17.6 g, 0.067 mol) and acetone (80 mL) at 0 C. The reaction mixture was then warmed to room temperature and stirred for 5 days. Filtration gave a white powder that was washed four times with fresh acetone, filtered, and dried under vacuum at 50 C for 6 h giving 22.7 g (88.2%) of the zwitterion ImS3-14. νmax/cm-1 IR (film): 3471, 3428, 3133, 3079, 2926, 2846, 1189, 1047. 1H NMR spectrum (CDCl3, 400 MHz), δ, (24) Ruzza, A. A.; Walter, M. R. K.; Nome, F.; Zanette, D. J. Phys. Chem. 1992, 96, 1463–1467. (25) Fife, T. H.; Jao, L. K. J. Org. Chem. 1965, 30, 1492–1495. (26) Amhar, J.; Monnet, C.; Perchec, L. P.; Chevalier, Y. New J. Chem. 1993, 17, 237–247.
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ppm: 10.26 s (1H), 8.03 s (1H), 7.68 s (1H), 4.87 t (2H), 4.53 t (2H), 3.04 t (2H), 2.55 t (2H), 1.99 m (2 H), 1.33 m (22H), 0.94 t (3H). Surface Tension Measurements. Surface tensions of surfactant-water mixtures containing 0.08 M NaCl or NaClO4 were measured by the du No€ uy ring method on a K8 tensiometer (KRUSS) at 25 C. Before each measurement, the ring was briefly heated above a Bunsen burner until glowing. The vessel was cleaned with chromic-sulfuric acid and boiling distilled water and then flamed in a Bunsen burner. Surface tension measurements were repeated twice with 1 mN 3 m-1 precision. Fluorescence Measurements. Fluorescence quenching of pyrene by DPC was monitored in a Varian Cary Eclipse spectrofluorimeter. The low probe concentration (2 10-6 M) avoided excimer formation, and the quencher concentration was 2.44 10-5-2.31 10-4 M. The [pyrene]/[micelles] and [quencher]/ [micelles] ratios were low enough to ensure Poisson distributions.27-30 An excitation wavelength of 337 nm and an emission wavelength of 394 nm were used. Kinetics and Spectrophotometric Titrations. All experiments were made with a diode-array spectrophotometer, with a thermostatted cell holder, in aqueous ImS3-14, and with 0.08 M NaCl or NaClO4, under conditions such that the spectroscopic micellar acidity probe 1-hydroxy-2-naphtoic acid (HNA) and the organic substrate 2-(p-heptoxyphenyl)-1,3-dioxolane (HPD) are almost wholly micellar-bound and the surfactant concentration is much higher than the critical micelle concentration (cmc). All pH measurements were made with a Metrohm model 713 pH meter calibrated with standard buffers, pH 7.00 and 4.00 (Carlo Erba), at 25.0 C. A solution of HNA (8.0 10-5 M) was titrated with HCl and NaOH, and the equilibrium dissociation was followed spectrophotometrically at 358 nm. Hydrolysis of 2-(pheptoxyphenyl)-1,3-dioxolane (HPD) was followed at 286 nm, and reactions were started by adding 30 μL of a stock solution of the substrate (1.1 mM) in water to 3 mL of reaction solution, giving 1.1 10-5 M substrate. Absorbance versus time data were stored directly on a microcomputer, and first order rate constants, kobs, were estimated from linear plots of ln(A¥ - At) against time for at least 90% of the reaction by using an iterative least-squares program; correlation coefficients were >0.999 for all kinetic runs. Capillary Electrophoresis. Experiments were conducted with an Agilent CE3D capillary electrophoresis system, with oncolumn diode-array detection at 25 C, as previously described,1,2,22 and electropherograms were monitored at 272 nm. Samples were introduced by hydrodynamic injection at 50 mbar/ 5 s. Fused-silica capillaries (Polymicro Technologies) of total length 60.0 cm, effective length 51.5 cm, and 50 μm i.d. were used. The electrophoresis system was operated under normal polarity and constant 30 kV. The capillary was conditioned by flushes of 1 M NaOH (5 min), deionized water (5 min), and electrolyte solution (10 min). Between experiments, the capillary was reconditioned by a pressure flush with the electrolyte containing 3 mM sodium borate (2 min). The mobility of the micelles was monitored by following the migration of micellar-bound pyrene (1 μM), and acetone (0.1%) was used as the electroosmotic flow marker.
Computational Methods: Charges and Geometry Search. All computational calculations were made with the Gaussian 03 program.31 Structures of zwitterionic SB3-14 and ImS3-14 were obtained by full geometry optimization at the HF/6-31þG(d) level with the polarizable continuum model (PCM)32 adding an aqueous-like environment to the system with the molecular cavity (27) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289–292. (28) Infelta, P. P.; Gratzel, M. J. Chem. Phys. 1979, 70, 179–190. (29) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951–5952. (30) Rodriguez Prieto, M. F.; Rios Rodriguez, M. C.; Gonzalez, M. M.; Rios Rodriguez, A. M.; Mejuto Fernandez, J. C. J. Chem. Educ. 1995, 72, 662–663. (31) Frisch, M. J. et al. Gaussian 03, revision D.01; Gaussian, Inc.: Pittsburgh, PA, 2004. (32) Cossi, M.; Barone, V.; Cammi, R.; Tomasi, J. J. Chem. Phys. Lett. 1996, 255, 327–335.
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Figure 1. Absorbance at 480 nm versus T (C) of ImS3-14 0.002 M. The solubility temperature is taken from the break point.
obtained with the simple united atom topological model (UAO). The nature of the stationary points was determined by harmonic frequency calculations. Natural atomic charges were determined by single-point calculations on the optimized geometries with the NBO program33 in Gaussian 03. Solubility Measurements. Solubilities of ImS3-14 were determined spectrophotometrically at 480 nm from turbidities due to low surfactant solubilities. The ImS3-14 solutions were heated slowly until complete dissolution, and the temperature was held for 1 h and then decreased gradually (0.1 C.min-1) by circulating water with a temperature control of (0.1 C. The solubility temperature was given by the break in the absorbance versus temperature plot (see Figure 1). Dissolution and precipitation temperatures were generally similar (within 1.0 C) as for bolaform surfactants.34 Plots of ImS3-14 concentration against solubility temperature are in Figure 2.
Results and Discussion Critical Micelle Concentration, Aggregation Number, and Solubility. Solubilities and Krafft temperatures (KT) of surfactants are sensitive to alkyl chain length and are strongly dependent on headgroups and counterions. For ionic surfactants, the counterion is important, but for nonionic and zwitterionic surfactants the headgroup plays a major role. For anionic surfactants, for example, alkanoates, KT increases with decreasing atomic number of the counterion and the effect is opposite for sulfonates and sulfates. KT values for cationic surfactants follow the counterion order I- >Br- >Cl- and are much higher with divalent anions.35,36 However, zwitterionic surfactants, including phosphocholines and betaines, are very soluble with a few exceptions. Carboxybetaines, for example, are more soluble than sulfobetaines because carboxylates are more hydrophilic than sulfonate groups. Despite expected high sulfobetaine solubilities, replacement of ammonium by an imidazolium group, as in ImS314, sharply decreases solubility. Figure 2 shows temperature dependence of ImS3-14 solubility in pure water with a break at 56 C and an abrupt change in solubility increment. A small temperature increase then sharply increases solubility and ImS3-14 is (33) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO, version 3.1. Theoretical Chemistry Institute, University of Wisconsin: Madison, WI, 2001. (34) Davey, T. W.; Ducker, W. A.; Hayman, A. R.; Simpson, J. Langmuir 1998, 14, 3210–3213. (35) Holmberg, K.; Jonsson, B.; Kronberg, K.; Lindman, B. Surfactant and Polymers in Aqueous Solution; John Wiley & Sons: Chichester, UK, 2003; pp 49-54. (36) Walstra, P. In Physical Chemistry of Foods; Marcel Dekker: New York, 2003.
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Figure 2. Temperature dependence of ImS3-14 solubility with a temperature break at 56 C, corresponding to a change in solubility increment.
freely soluble. This break temperature is much higher than that for a betaine surfactant with the same alkyl chain. Concentrated sulfobetaine surfactants are completely soluble over a large temperature range (a 10% solution of SB3-14 is completely homogeneous for T>0 C). The low solubility of ImS3-14 certainly involves stronger interactions in the crystal due to the rigid and bulky cyclic system, which increases headgroup rigidity and enhances crystal packing. There may be an interaction in the crystal between ImHþ of one surfactant and sulfonate in another, equivalent to hydrogen bonding as observed for 3-(1-methyl3-imidazolio)propanesulfonate where the C-N;C-C torsion angle of 100.05 allows the positively charged imidazolium headgroup and the negatively charged sulfonate group to interact with neighboring zwitterions, forming a C-H 3 3 3 O hydrogen-bonding network.37 In aqueous micelles with added electrolyte, where ions limit interactions of SO3- with ImH+, salts strongly increase the solubility of ImS3-14, and a 0.01 M sample of the surfactant is readily soluble in 0.08 M NaCl at 25 C. Similarly, adddition of NaClO4 increases ImS3-14 solubility, indicating that salts affect micellar structure, and therefore, the cmc, measured because of necessity in aqueous electrolyte, is found to be similar to those of other sulfobetaines surfactants. Added electrolytes generally reduce the cmc by interacting with the micellar interface35,36,38,39 and nonionic and zwitterionic surfactants have lower cmc’s than otherwise similar ionic surfactants because ionic dipoles interact without charged headgroup repulsions. Increased concentration of NaClO4 resulted in a noticeable increase in viscosity, and at 1 M NaClO4 a gel is formed, a result which indicates micellar growth at high salt concentration. At 0.08 M electrolyte, substitution of NaCl by NaClO4 modestly decreases the cmc of ImS3-14 from 1.02 10-4 to 5.75 10-5, respectively, indicating the stronger binding of ClO4- than Cl- and consequent micellar stabilization (see Supporting Information, Figure S1). Increases in zeta potentials of sulfobetaine micelles generally follow the extent of anion incorporation,4,40 provided that micellar size is insensitive to added ions. We therefore examined effects of 0.08 M NaCl and NaClO4 on aggregation numbers of ImS3-14 micelles before studying electrophoretic effects. Figure S2 (37) Reichert, W. M.; Trulove, P. C.; De Long, H. C. Acta Crystallogr., Sect. E: Struct. Rep. Online 2010, 66, O591–U4799. (38) Mukerjee, P. J. Phys. Chem. 1965, 69, 4038–4040. (39) Corrin, M. L.; Harkins, W. D. J. Am. Chem. Soc. 1947, 69, 683–688. (40) Stigter, D.; Mysels, K. J. J. Phys. Chem. 1955, 59, 45–51.
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Figure 3. PCM/HF/6-31þG(d) geometries of the surfactant molecules (left) and natural atomic charges of the headgroups (right).
(Supporting Information) shows salt effects upon the relationship between fluorescence intensity and quencher concentration, eq 1.27-30 ½QNagg I0 ¼ ð1Þ ln I ½ImS3-14 - cmc where Q is the DPC quencher, Nagg is the aggregation number, and cmc values of ImS3-14 are shown in Supporting Information Figure S1. The expected anion specificity on Nagg and the effect of increasing [NaClO4] on micellar size is shown in Supporting Information Figure S2. The slopes are identical within error, and the aggregation number in the presence of NaClO4 is about 58 ( 2, which is within limits of experimental error and very similar to that in the presence of 0.08 M NaCl (Nagg=65 ( 5); both values are similar to those found for SB3-14 micelles (Nagg =67, ref 4). Interaction of Perchlorate Ion with ImS3-14. Perchlorate ion has a strong affinity for zwitterionic micelles.1,2,4,5 As noted earlier, anionic distribution between aqueous and micellar pseudophases can be monitored by capillary electrophoresis, and the micellar electrophoretic mobility (μ, m2 V-1 s-1) is given by Leff 1 1 μ ¼ E tapp teo
! ð2Þ
where E is the applied electric field strength (V m-1), Leff is the effective capillary length, and tapp and teo are migration times of the micelle and the electroosmotic flow, respectively. The capillary electrophoresis separations were carried out in 0.01 M ImS314 or SB3-14 and varying concentrations of NaClO4 (0.000.08 M) at constant ionic strength, 0.08 M, maintained with NaCl to avoid precipitation of ImS3-14. When the zeta potential (ζ) is not very high, Henry’s equation (eq 3) relates that of the micelle (ζm) to its mobility: ζm ¼
μη εε0 ðKRm Þ
ð3Þ
where η is the medium viscosity, f(κRm) corresponds to Henry’s function, κ is the Debye-Huckel shielding parameter (m-1), Rm is the radius of the spherical SB3-14 or ImS3-14 micelle, and ε0 and ε correspond to the vacuum and relative solvent permittivities, Langmuir 2010, 26(20), 15754–15760
respectively. As previously described,4 we used values of Rm = 26 A˚ for micellar SB3-143,4,14 and f(κRm) is 0.73.4,41 Geometrical optimization gave a length of 22.96 A˚ for an ImS3-14 molecule, and we take this value as Rm for the corresponding micelle. Figure 3 shows optimized geometries of ImS314 and SB3-14 monomers and the charge distributions of the respective headgroups. Although both surfactants have similar structures, their dynamics in solution may be very complex because they can have different conformations depending on the energy barriers for rotation of each headgroup and other conformations are possible in the micelle. Calculations of natural atomic charges indicate that zwitterionic headgroups have similar charge distributions, although there are subtle differences in the cationic portions. In both cationic ammonium and imidazolium groups, the positive charge is not located on nitrogen atoms, which are negatively charged, as are some surrounding carbons, and the charge balance involves positively charged hydrogen atoms. Therefore, there is an important difference between ImS3-14 and SB3-14 in that the carbon atom located between the two imidazolium nitrogens carries a considerable positive charge while the other carbons are negatively charged, as in SB3-14 (Figure 3), consistent with the acidity of the hydrogen bonded to the negative carbon in the imidazolium ring.42 The dependence of the micellar zeta potential ζm on the concentration of added anion fits eq 4: ζm ¼
ζmax KL ½anion 1 þ KL ½anion
ð4Þ
where ζm, the zeta potential, is a function of the concentration of the added anion, ζmax represents the maximum zeta potential, and KL, M-1, corresponds to the Langmuir association constant, which adequately describes the change in potential as a function of added anion. Because the cmc’s of SB3-14 sulfobetaine and ImS3-14 are low (2.2 10-4 M and 1.02 10-4 M, respectively), electrolyte effects on the cmc should be unimportant. Values of ζm become significantly negative with added NaClO4, reflecting the zwitterionic micelle becoming progressively anionoid, up to a maximum given by ζm (Figure 4). Because in our experiment we (41) Ohshima, H. J. Colloid Interface Sci. 1994, 168, 269–271. (42) Jeon, Y.; Sung, J.; Seo, C.; Lim, H.; Cheong, H.; Kang, M.; Moon, B.; Ouchi, Y.; Kim, D. J. J. Phys. Chem. B 2008, 112, 4735–4740.
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Tondo et al. Table 1. Perchlorate Ion Effects on Zeta Potentials and Langmuir Constants Calculated from Capillary Electrophoresis surfactant
ζmax (mV)
SB3-14 -61.5 ImS3-14 -71.2 a Nagg = 67 for SB3-14; ref 4.
KL (M-1)
Q
Q/Nagga
0.174 0.212
-14.8 -14.0
0.22 0.24
Scheme 1. Acid Dissociation of 1-Hydroxy-2-naphthoic Acid (HNA)
Figure 4. Effect of NaClO4 on zeta potentials of micelles of 0.01 M SB3-14 (9) and ImS3-14 (b) at 25 C, sodium tetraborate, pH 9.0. For all the experimental points, [NaCl] þ [NaClO4] = 0.08 M.
kept the ionic strength constant at 0.08 M with NaCl, we modified eq 4 to give ζm - ζCl - ¼
ðζClΟ4 - - ζCl - ÞKL ½anion 1 þ KL ½anion
ð5Þ
where ζCl- and ζClO4- refer to the zeta potentials with 0.08 M chloride and perchlorate ion, respectively, and ζmax is given by the sum of ζCl- and ζClO4-. On the assumption that the micellar shear plane corresponds to the physical headgroup surface, ζmax = ψ0, as discussed by Mysels and Stigter,40 the zeta potential corresponds approximately to the electrostatic potential, and we calculate the charge excess per micelle, Q, by using eq 6, which describes the mobility of spherical ionic colloids and was applied to describe mobilities. Q ¼ 0:1401 108 Rm ð1 þ KRm ÞΦ0
ð6Þ
In eq 6, Φ0 is a nondimensional surface potential, Φ0 = (eζmax)/(kBT), where kB, T, and e are Boltzmann’s constant, temperature (K), electrostatic potential (V), and elementary charge, respectively (Φ = 1 for ζmax = 25.7 mV at 25 C). All other parameters are identical to those described above, and Rm is expressed in centimeters.40 Equation 6 involves the assumption that the surface electric charge density can be estimated from zeta potentials39 and, therefore, the surface excess per micelle includes all the ions up to the shear plane and should be a maximum value for the anion adsorption in the sulfobetaine micelle. Values of Q and Q/Nagg (from aggregation numbers, Nagg, 56 and 67 for SB312 and SB3-14, respectively)10,43 follow those of θmax (Table 1), showing that these independent methods lead to similar conclusions on anion transfer between water and sulfobetaine micelles, but Q/Nagg values are always slightly larger than θmax because of approximations in the treatments, and aggregation numbers10,43 for sulfobetaines were estimated without salt, but the added salts NaCl, NaClO4, and NaNO3 have minor effects on Nagg,2,10 as for ImS3-14 (see Supporting Information Figure S2). (43) Graciani, M. M.; Rodrı´ guez, A.; Mu~noz, M.; Moya, M. Langmuir 2005, 21, 7161–7169.
15758 DOI: 10.1021/la102391e
Interactions between SB3-14 micelles and several monovalent anions were recently described1,2,4,5 by using capillary electrophoresis (CE) with saturation reached between 0.04 and 0.08 M anion and the extent of binding depending on the concentration and nature of the added anion. The results obtained for the binding of ClO4- to both SB3-14 and ImS3-14 are shown in Figure 4, and KL and ζm are larger for ImS3-14. Because, for all the experimental points [NaCl] þ [NaClO4] = 0.08 M, the first point in Figure 4, at zero NaClO4, corresponds to a ImS3-14 micelle saturated with NaCl. Although hydrophilic anions are weakly incorporated in the zwitterionic micelles, larger and lower charge density, less hydrated anions such as ClO4- markedly increase negative zeta potentials. The increasingly negative zeta potential favors protonation by H3Oþ as shown by changes in dissociation equilibria of HNA in zwitterionic micelles (Scheme 1). Spectrophotometric titration of fully micellar-bound HNA is shown in Figure 4, where the open points show absorbance at 358 nm increasing on protonation of NA- in 0.08 M NaCl and no surfactant, and acid dissociation constant pKa of 2.80 ( 0.03, similar to that of 2.88 ( 0.02 at 25.0 C in pure water from a similar titration (data not shown). The titration was repeated in 0.01 M ImS3-14 with 0.08 M NaCl and with 0.08 M NaClO4 (filled points). With added NaClO4, ImS3-14 micelles become strongly anionoid, with lower values of apparent pH compared to that of the solution, and therefore, we observe a different profile in the spectrophotometric titration. However, NaCl has little effect on micellar surface acidity, and the plot in Figure 5 is similar to that for titration in water. The apparent pH in the interfacial region of ImS3-14 can be estimated from data in Figure 5. Comparisons between pHapp and pH of the solution are in Figure 6 for ImS3-14 with NaCl and NaClO4. For NaClO4, but not NaCl, ion induced differences are considerable. Hydrolysis of 1.1 10-5 M HPD (Scheme 2), in 0.001 M HCl with micellized ImS3-14 (Figure 7), is markedly accelerated by NaClO4 with krel values relative to that in 0.08 M NaCl. These results are very similar to those with normal sulfobetaines,2,5 and, as discussed elsewhere,5 dilute Naþ does not affect our general conclusions. With fully micellar-bound substrate and sulfobetaine micelles, kinetic salt effects follow hydronium ion concentrations in the micellar pseudophase. Salt effects on these HPD hydrolyses are largely due to increased hydronium ion concentration in the micellar pseudophase, and not to changes in the micellar association of the very hydrophobic HPD. With fully micellar-bound substrate, and neglecting the cmc, the first-order rate constant, with respect to substrate, is given by eq 7: kobs ¼ kM;H ½HM þ =½ImS3-14
ð7Þ
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Figure 5. Plot of absorbance of HNA (8.0 10-5 M) at 358 nm versus pH with addition of HCl to 0.08 M NaCl solution and no surfactant (O) and 0.01 M ImS3-14 with 0.08 M NaCl (1) and NaClO4 (2).
Figure 7. Perchorate ion effects on log krel for hydrolysis of HPD with 0.001 M HCl and 0.01 M ImS3-14 at 25.0 C. Below 0.08 M NaClO4, the ionic strength was maintained at 0.08 M with NaCl. Without added perchlorate, kobs=7.45 10-4 s-1. Scheme 3. Cartoon Model of Anion Binding to ImS3-14 Micelles
Figure 6. Plot of apparent against observed pH with 0.01 M ImS314 and 0.08 M NaCl (1) or NaClO4 (2). Scheme 2. Acid Hydrolysis of 2-(p-Heptoxyphenyl)-1,3-dioxolane (HPD)
those in water, given as M-1 s-1, by estimating local molarities in the micellar reaction regions in terms of assumed molar volumes of these regions. This approximate comparison has been made for reactions in ionic micelles, but we do not know the molar volume of the reaction region in ImS3-14 micelles. For other zwitterionic micelles, with assumed molar volumes as for anionic micelles, second-order rate constants are generally slightly lower than those in water, showing the importance of reactant concentration at the micelle-water interface. Without perchlorate ion, Hþ rate constants (eq 7) are similar in sulfobetaine and ImS3-14 micelles and factors that control micellar rate effects on acid hydrolyses are apparently similar for both types of zwitterionic surfactants.
Conclusions where the second-order rate constant, kM,H, is written with local concentration as a mole ratio of [HMþ] to [ImS3-14] rather than as a local molarity; that is, [HMþ] indicates the amount of hydronium ion in the micellar pseudophase. At constant surfactant concentration and ionic strength, with kM,H unaffected by specific salt effects, krel, the ratio of observed rate constants with and without added perchlorate, follows changes in hydronium ion concentrations in the micellar pseudophase (eq 8). krel ¼ ½HM þ salt =½HM þ 0
ð8Þ
The approximately 10-fold rate increase (Figure 7) is similar to that observed for this reaction in micellized SB3-14 with added NaClO4.5 The second-order rate constants can be compared with Langmuir 2010, 26(20), 15754–15760
The synthesis of the new surfactant ImS3-14 tests the importance of the nature and positioning of charged headgroups in zwitterionic surfactants, and the imidazolium moiety was introduced to reinforce the packing ability of chameleon-like zwitterionic micelles. The water solubility of ImS3-14 is low due to strong interactions in the crystal promoted by the imidazolium ring which enhances crystal packing. In aqueous micelles, salts strongly increase the solubility of ImS3-14 and the surfactant becomes readily soluble in 0.08 M NaCl (or NaClO4) at 25 C. The observed solubility increases indicate that salts affect micellar structure, and the results are consistent with the induced anionoid character in ImS3-14. A simplified model for the incorporation of chloride and perchlorate anions into ImS3-14 micelles is shown in Scheme 3, with the anions preferentially located next to the DOI: 10.1021/la102391e
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cationic center (anions represented by the green circle in Scheme 3). As a consequence, ImS3-14 micelles promote uptake of H3Oþ into the micellar pseudophase, accelerating acid hydrolysis of HPD and protonation of NA-, as with sulfobetaine micelles. Even very dilute ClO4- is much more effective than Cl- in increasing local concentrations of H3Oþ, and capillary electrophoresis experiments show that ImS3-14 has a greater ζmax than that of SB3-14. As noted, addition of 1.0 M NaClO4 results in gel formation, a result which indicates micellar growth at high salt concentration. As for sulfobetaines, ImS3-14 micelles show chameleon-like behavior controlled by the choice of added salt anion. The molecular structure and the hydrophobic effect of the
15760 DOI: 10.1021/la102391e
Tondo et al.
ImS3-14 headgroup play important roles in anion binding and control of micellar properties. Acknowledgment. We are grateful to PRONEX, INCTCatalise, to the Brazilian Foundations, CAPES, FAPESC and CNPq from Brazil, and to NSF (USA) for support of this work. Supporting Information Available: Cartesian coordinates and natural charges of SB3-14 and ImS3-14, experimental data for both critical micelle concentration and aggregation number determinations, and complete ref 31. This material is available free of charge via the Internet at http://pubs. acs.org.
Langmuir 2010, 26(20), 15754–15760
