Reactivity and Models for Anion Distribution: Specific Iodide Binding to

Oct 24, 2008 - Departamento de Química, Universidade Federal de Santa Catarina, Florianópolis, Santa Catarina 88040-900, Brazil, and Department of ...
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Langmuir 2008, 24, 12995-13000

12995

Reactivity and Models for Anion Distribution: Specific Iodide Binding to Sulfobetaine Micelles M. Akhyar Farrukh,† Rosane C. Beber,† Jacks P. Priebe,† Manmohan Lal Satnami,† 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, Floriano´polis, Santa Catarina 88040-900, Brazil, and Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California 93106 ReceiVed July 9, 2008. ReVised Manuscript ReceiVed September 11, 2008 The reaction of I- with methyl naphthalene-2-sulfonate (MeONs) is accelerated by the micellized sulfobetaine surfactants N-decyl, N-dodecyl, N-tetradecyl, and N-hexadecyl-N,N-dimethylammonio-1-propanesulfonate. Concentrations of micellar-bound I- were determined by using ion-selective electrodes (ISE), and capillary electrophoresis. At low concentrations, I- incorporation fits Langmuir isotherms and is related to changes in micellar surface potentials. Rate effects of dilute KI are fitted quantitatively by a pseudophase model that describes I- binding in terms of a sorption isotherm, but at higher [KI], where the simple model predicts saturation, rates increase due to electrolyte invasion. This model considers transfer equilibria of both reactants between water and micelles and second-order rate constants in each pseudophase. Estimated second-order rate constants for reaction of MeONs with I- in the micellar pseudophase are 3.2- to 3.5-fold higher than the second-order rate constant, k2w, in water, depending on surfactant structure and assumptions in the treatment.

1. Introduction Ionic distributions between bulk aqueous solution and micellar surfaces determine the behaviors of ionic micelles that bind counterions specifically, and they can be displaced from the surface by like-charged ions; coion concentrations are initially low, but increase in high electrolyte.1-4 Theoretical and experimental approaches are used to describe ion distributions and competition. The pseudophase, ion-exchange formalism treats competition between counterions in terms of nonspecific charge-charge interactions and ion-specific interactions, which are related to the extents of ion hydration, following the Hofmeister series, and in the simplest form, assume that the extent of fractional ionic micellar coverage, β, is insensitive to ionic properties or concentration, and is typically 0.6-0.8. These approximations allowed treatment of extensive kinetic data, and estimation of the extent of reactant transfer from water to micelles, consistent with independent physical measurements. Much of the kinetic work, with emphasis on interactions of ionic micelles with counterions, involved dilute solutions of surfactant and electrolyte, and this simple model has limitations in other than dilute solutions.1-4 For example, in the absence of inert counterions, rate constants tend toward limiting values as the substrate becomes fully micellar bound, but with increasing ionic concentration constants increase monotonically beyond the * Corresponding author. E-mail: [email protected]. † Universidade Federal de Santa Catarina. ‡ University of California. (1) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357–363. (2) Brinchi, L.; Profio, P. D.; Germani, R.; Marte, L.; Savelli, G.; Bunton, C. A. J. Colloid Interface Sci. 2001, 243, 469–475. (3) (a) Frescura, V. L. A.; Marconi, D. M. O.; Zanette, D.; Nome, F.; Blasko, A.; Bunton, C. A. J. Phys. Chem. 1995, 99, 11494–11500. (b) Nome, F.; Rubira, A. F.; Franco, C.; Ionescu, L. G. J. Phys. Chem. 1982, 86, 1881–1885. (c) Chaimovich, H.; Aleixo,R. M. V.; Cuccovia, I. M.; Zanette, D.; Quina, F. H. In Solution BehaVior of Surfactants; Mittal, K. L., Fendler, E. J., Eds.; Plenum Press: New York, 1982; Vol. 2, p 949. (4) Savelli, G.; Germani, R.; Brinchi, L. In Reactions and Synthesis in Surfactant Systems; Texter, J., Ed.; Marcel Dekker: New York, 2001; Chapter 8.

expected limit. This electrolyte invasion behavior is observed with high concentrations of the reactive ion and basically corresponds to the correction by the background electrolyte concentration.1,3b Qualitatively, rate enhancements of bimolecular reactions are due largely to the concentration of reactants in the micellar reaction region,1-4 with overall rate effects depending on reactant concentrations at the micelle-water interface. Nonionic micelles generally have little effect on ionic reactions with moderately hydrophilic substrates, but inhibit reactions of very hydrophobic substrates, which bind in an apolar region from which ions are partially excluded.1,4 Spontaneous bimolecular (water-catalyzed) hydrolyses are typically inhibited by ionic micelles, but inhibition is usually larger for anionic micelles, than it is for cationic and sulfobetaine ones.5-8 Zwitterionic micelles are formally uncharged, but for an approximately spherical sulfobetaine micelle with an extended trimethylene sulfonate headgroup, the charge density at the positive surface encompassing the quaternary ammonium groups should be slightly higher than that encompassing the sulfonate anions.5,9 Early evidence for interaction of sulfo- and carboxybetaines with anions2,6-9 included the observation that betaine micelles accelerate spontaneous anionic dephosphorylations and decarboxylations and behave like cationic micelles in incorporating the substrate into a region that is less polar than water.4,6,7,10-12 (5) (a) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. J. Org. Chem. 1987, 52, 3832–3835. (b) Buurma, N. J.; Serena, P.; Blandamer, M. J.; Engberts, J. B. F. N. J. Org. Chem. 2004, 69, 3899–3906. (c) Buurma, N. J.; Herranz, A. M.; Engberts, J. B. F. N. J. Chem. Soc., Perkin Trans 2 1999, 113–120. (6) Brinchi, L.; Profio, P. D.; Germani, R.; Savelli, G.; Spreti, N.; Bunton, C. A. J. Chem. Soc., Perkin Trans 2 1998, 361–364. (7) Brinchi, L.; Profio, P. D.; Germani, R.; Savelli, G.; Bunton, C. Colloids Surf. A: Physchem. Eng. Asp. 1998, 132, 303–314. (8) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. J. Phys. Chem. 1989, 93, 854–858. (9) Kamenka, N.; Chorro, M.; Chevalier, Y.; Levy, H.; Zana, R. Langmuir 1995, 11, 4234–4240. (10) Cerichelli, G.; Chiarini, M.; Di Profio, P.; Germani, R.; Savelli, G.; Mancini, G.; Bunton, C. A.; Gillitt, N. D. Langmuir 1998, 14, 2662–2669.

10.1021/la802179m CCC: $40.75  2008 American Chemical Society Published on Web 10/25/2008

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Kinetic and physical evidence show that betaine micelles incorporate anions, and the salt-order of binding follows the Hofmeister series, or the Pearson Hard-Soft relationship, as with cationic micelles. There is evidence, some of it qualitative, for micellar binding of anions from dediazonization trapping,13a chemical kinetics,13b,c chromatography,14a conductance,10,13c potentiometry and capillary electrophoresis,,15 and NMR spectroscopy.16 Some methods allow estimation of local anionic concentrations in the interfacial region of sulfobetaines, but generally apply only to dilute solutions.1,3b With high concentrations of surfactant, or electrolyte, bimolecular ionic reactions occur in coionic micelles, although they are very slow but are increased by added ions due to electrolyte invasion of the micellar region.3 The interaction of dilute hydrophilic ions with zwitterionic micelles has been treated quantitatively in terms of electrostatic interactions,17 but Iso and Okada included ion-specific interactions in their treatment.,15 Potentiometry, with use of ion specific electrodes (ISEs), fits Langmuir isotherms in describing the binding of dilute ClO4- and Br- to sulfobetaine micelles, and the results fit kinetic evidence and that from NMR spectroscopy and dediazonization trapping with anion-specific interactions, in showing that ionic interactions follow the Hofmeister series, and cannot be treated solely in terms of electrostatic effects, which are important in the dilute limit.,15,17 There are specific anion interactions with sulfobetaı´nes, as well as ionic, micelles, i.e., ion affinity is not governed solely by nonspecific electrostatic interactions.16,18 We examined the reaction of methyl naphthalene-2-sulfonate (MeONs) with I- in the presence of sulfobetaine micelles (Scheme 1). The reaction is mechanistically simple, and the hydrophobic substrate is extensively micellar bound. This reaction is convenient for quantitatively examining anionic incorporation in zwitterionic micelles. Scheme 1

Anion transfer to sulfobetaine micelles, monitored by use of a specific ion electrode, tends to apparent plateau values, similar to those from capillary electrophoresis, with the anion order being the same as that for binding to cationic micelles, although the maximum ionic coverage, θmax, is much lower (ca. 20%) for ClO4-, and significantly lower for higher charge density anions.16,18 At the simplest level, rate constants for reaction of micellar-bound MeONs with I- should become constant at this limiting value, θmax. However, rate constants with other nucleophilic anions do not become constant at the predicted ionic (11) Profio, P. D.; Brinchi, L.; Germani, R.; Savelli, G.; Cerichelli, G.; Bunton, C. A. J. Chem. Soc., Perkin Trans 2 2000, 2162–2167. (12) Brinchi, L.; Profio, P. D.; Germani, R.; Savelli, G.; Gillitt, N. D.; Bunton, C. A. J. Colloid Interface Sci. 2001, 236, 85–95. (13) (a) Cuccovia, I. M.; Romsted, L. S.; Chaimovich, H. J. Colloid Interface Sci. 1999, 220, 96–102. (b) Graciani, M. M.; Rodrı´guez, A.; Mu´n˜oz, M.; Moya´, M. L. Langmuir 2002, 18, 3476–3481. (c) Graciani, M. M.; Rodrı´guez, A.; Mu´n˜oz, M.; Moya´, M. L. Langmuir 2005, 21, 7161–7169. (14) (a) Hu, W. Langmuir 1999, 15, 7168–7171. (b) Iso, K.; Okada, T. Langmuir 2000, 16, 9199–9204. (c) Stigter, D.; Mysels, K. J. J. Phys. Chem. 1955, 59, 45–51. (15) Masudo, T.; Okada, T. Phys. Chem. Chem. Phys. 1999, 1, 3577–3582. (16) Marte, L.; Beber, R. C.; Farrukh, M. A.; Micke, G. A.; Costa, A. C. O.; Gillitt, N. D.; Bunton, C. A.; Profio, P. D.; Savelli, G.; Nome, F. J. Phys. Chem. B 2007, 111, 9762–9769. (17) Baptista, M. S.; Cuccovia, I.; Chaimovich, H.; Politi, M. J.; Reed, W. F. J. Phys. Chem. 1992, 96, 6442–6449. (18) (a) Beber, R. C.; Bunton, C. A.; Savelli, G.; Nome, F. Prog. Colloid Polym. Sci. 2004, 128, 249–254. (b) 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.

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Figure 1. Langmuir plot of bound I- in 0.05 M SB3-12 (b), SB3-14 (3), and SB3-16 (2) as a function of total [KI]. Table 1. Iodide Ion Effects on Langmuir Constants and Zeta Potentials from ISE and Capillary Electrophoresis surfactant

ζmax, mV

SB3-10 SB3-12 SB3-14 SB3-16

-47.1 -52.4 -54.5 -56.7

KLa, M-1

Q

84 78 (102)a 97 (104)a 122 (117)a

-9.3 -11.4 -13.1 -14.2

Q/Nb

θmax

b

0.19 0.20b 0.20b 0.20b

0.15 0.17 0.18

a The KL value (in parentheses) and θmax values were obtained with ISE in terms of total [anion] (Figure 2). b Values of N ) 56 and 67 for SB3-12 and SB3-14 are from refs 10 and 13c. Values of N ) 50 for SB3-10 and N ) 70 for SB3-16 were extrapolated from data in refs ,10, 13c and 14b.

coverage. The present work involved physical methods for estimation of θ for I- and monitoring reactions with [KI] greater than that required for this micellar coverage. In terms of the Hofmeister series, I- should interact readily with micelles and other association colloids, although low solubility prevents its study with cationic micelles at the low temperatures convenient for kinetic work in aqueous media.16,18

2. Experimental Section 2.1. Materials. We used sulfobetaines N-decyl, N-dodecyl, N-tetradecyl, and N-hexadecyl-N,N-dimethylammonio-1-propanesulfonate (SB3-10, SB3-12, SB3-14, and SB3-16, respectively), from Sigma. Preparation and purification of MeONs have been described.19 The pH of the solutions was maintained with pH 9.0 borate (0.01 M), and solutions in deionized water were prepared immediately before use. All other reagents and solvents were analytical grade and were used without further purification. 2.2. Kinetics. Reactions were monitored spectrophotometrically in aqueous sulfobetaine solutions with 0 to 1.0 M [KI] at 25.0 ( 0.1 °C by the decreasing absorbance at 326 nm and pH 9.0. Reactions were started by adding 20 µL of MeONs (in MeCN) to 3 mL of reaction solution so that [MeONs] ) 10-4 M. Temperatures in quartz cuvettes were controlled with a thermostatted water-jacketed cell holder. Absorbance versus time data were stored 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 with an iterative least-squares program; correlation coefficients were >0.999 for all kinetic runs. 2.3. Potentiometric Measurements. A halide ISE16,18a was used for potentiometric measurements with an Ag/AgCl reference electrode, pH 9.0, borate (0.01 M), and a Metrohm, model 713, pH-meter, calibrated with standard buffers, pH 7.00 and 9.00 (Carlo Erba). Iodide ion standards were KI (in 0.01 M borate), and the (19) Bertoncini, C.; Nome, F.; Cerichelli, G.; Bunton, C. A. J. Phys. Chem. 1990, 94, 5875–5878.

Ion Binding and ReactiVity in Sulfobetaine Micelles.

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response varied linearly with log anion activity in 10-1 to 10-4 M KI, with slope ) 58 ( 3 mV. Sulfobetaine solutions (0.05 M) contained 0.01 M borate, and [KI] was in the range of the standards.16,18 Our conclusions depend on comparison of potentiometric, electrophoretic, and kinetic evidence and conditions were the same in all experiments. 2.4. Capillary Electrophoresis. Capillary electrophoresis was monitored on an Agilent CE3D system with on-column diode-array detection at 25 °C.16 Borate, pH 9.0, was used to avoid pH effects on electroosmotic flow, because, at higher pH, the charge on the Si-O groups affects the zeta potential and flow. Samples were introduced by hydrodynamic injection at 50 mbar/5 s. Fused-silica capillaries (Polymicro Technologies), total length 60.0 cm, effective length 51.5 cm, and 50 µm i.d., were used. The system was operated under normal polarity and 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 electrolyte containing 3 mM sodium borate (2 min). Micellar mobility was monitored by following migration of micellar bound pyrene (1 µM), and acetone (0.1%) was used as an electroosmotic flow marker.16

3. Results and Discussion ISE Measurements. For the sulfobetaine systems, ISE distinguishes between ions in the aqueous and micellar pseudophases, and we studied the equilibrium transfer of Ibetween water and micelles in dilute KI16,18 and SB3-12, SB314, and SB3-16 (Figure 1). The partitioning of I- between water and sulfobetaine micelles fits a Langmuir isotherm (eq 1), -

([I ]M ⁄ [SB3-n]) )

θmaxKL[I-]total 1 + KL[I-]total

(1)

where SB3-n is the surfactant, θmax is the maximum occupancy of head groups by I-, and KL (M-1) is the Langmuir association constant. Quantities in squared brackets are molarities in terms of total solution volumes and, because the critical micelle concentration (cmc) of most of the sulfobetaines are much lower than surfactant concentration, monomeric surfactant can be neglected for SB3-12, SB3-14, and SB3-16.20-22 Within the concentration limits, anion concentrations in the micellar pseudophase tend toward ion-specific constant values. As discussed,16 we use the ratio of bound to total ions for convenient treatment of kinetic data that does not affect θmax. Values (Table 1) are KL ) 117 M-1 and θmax ) 0.177 for the interaction of Iwith SB3-16 micelles, and for SB3-14 and SB3-12 micelles, θmax is 0.165 and 0.152, with KL ) 104 and 102 M-1, respectively. This procedure is useful only for dilute KI. Binding of I- to sulfobetaine micelles is slightly sensitive to alkyl chain length (C12 to C16), but capacities for anions are much lower than those of cationic micelles where moderately hydrophilic counterions typically neutralize 70-80% of the micellar charge.1,23 Capillary Electrophoresis. Anionic distributions between aqueous and micellar pseudophases were monitored by capillary electrophoresis, which also gives evidence on surface potentials. The electrophoretic mobility of a zwitterionic micelle (µ, m2 V-1 s-1) is given by (20) Sapelli, E.; Branda˜o, T. A. S.; Fiedler, H. D.; Nome, F. J. Colloid Interface Sci. 2007, 314, 214–222. (21) Marconi, D. M. O.; Frescura, V. L. A.; Zanette, D.; Nome, F.; Bunton, C. A. J. Phys. Chem. 1994, 98, 12415–12419. (22) (a) Kamenka, N.; Chorro, M.; Chevalier, Y.; Levy, H.; Zana, R. Langmuir 1995, 11, 4234–4240. (b) Chevalier, J.; Kamenka, N.; Chorro, M.; Zana, R. Langmuir 1996, 12, 3225–3232. (23) (a) Neves, M. D. S.; Zanette, D.; Quina, F.; Moretti, M. T.; Nome, F. J. Phys. Chem. 1989, 93, 1502–1505. (b) Nascimento, M. D.; Miranda, S. A. F.; Nome, F. J. Phys. Chem. 1986, 90, 3366–3368.

µ)

(

Leff 1 1 E tapp teo

)

(2)

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. When the zeta potential (ζm) is not very high, the following Henry equation (eq 3), relates the zeta potential of the micelle to its mobility:

ξm ) µη ⁄ εoεf(κRm)

(3)

where η is the viscosity of the medium, f(κRm) corresponds to Henry’s function, κ is the Debye-Hu¨ckel shielding parameter (m-1), Rm is the radius of a spherical zwitterionic micelle, and εo and ε correspond to the vacuum permittivity and the relative permittivity of the solvent. Reported radii, Rm, of the decyl-, dodecyl-, tetradecyl-, and hexadecyl- zwitterionic SB3-n micelles16,24-26 are 21, 23.4, 26, and 26.5 Å, respectively.24 Ionic strengths of 1:1 electrolyte (KI) were 0.002-0.04 M, with κ ranging between 1.5 × 108 and 6.6 × 108 m-1, respectively. Thus, κRm, for SB3-12 micelles is between 0.35 and 1.54, and Henry’s function f(κRm), following Ohshima’s approximation,26 is between 0.67 and 0.71.16 Values of the corresponding Henry function f(κRm) for the SB3-10, SB3-14, and SB3-16 sulfobetaine micelles, in 0.04 M KI, are 0.70 to 0.74.16,24 The assumption that micellar radii are insensitive to added KI is reasonable because micellar aggregation numbers, from fluorescence quenching, are insensitive to salts less than 0.10 M.16,18 The dependence of mobility or zeta potential of the micelle on salt concentrations allowed estimation of anion binding parameters by fitting experimental data (Figure 2) to a typical isotherm (eq 4):

ζm )

ζmaxKL[Iodide] 1 + KL[Iodide]

(4)

where ζmax and KL correspond to the maximum zeta potential with added KI and the binding constant of I- to the sulfobetaine micelle, respectively. The zeta potential increases from near zero, in borate buffer, to a significantly negative value with the addition of anions that interact with the zwitterionic micelle. The binding of I- to sulfobetaine micelles, and comparison with evidence with other ions,16 show that specific ion effects control binding and Soft anions, which can readily dehydrate and pair with the quaternary ammonium moiety of the surfactant, and more hydrophilic ions are less incorporated. This binding isotherm (eq 4) is formally equivalent to equation 1, and the occupancy limit is expressed as the maximum zeta potential, depending on micellar coverage by I-. Values of Langmuir constants, KL, from ISE and electrophoresis, and maximum zeta potentials (ζmax) from electrophoresis are in Table 1. Provided that the shear plane of micelles approximates the physical headgroup surface, i.e., that the zeta potential is approximately the electrostatic potential, we estimate the charge excess per micelle, Q, with eq 5, which relates mobilities of spherical sodium dodecyl sulfate micelles to zeta potential and charge.14c

Q ) 0.1401 × 108Rm(1 + κRm)Φo

(5)

In eq 5, Φo is a nondimensional surface potential, Φo ) (eζmax)/ (kBT), where kB, T, and e are Boltzmann’s constant, temperature (24) Masudo, T.; Okada, T. Phys. Chem. Chem. Phys. 1999, 1, 3577–3582. (25) (a) Kamenka, N.; Chorro, M.; Chevalier, Y.; Levy, H.; Zana, R. Langmuir 1995, 11, 4234–4240. (b) Chevalier, J.; Kamenka, N.; Chorro, M.; Zana, R. Langmuir 1996, 12, 3225–3232. (26) Ohshima, H. J. Colloid Interface Sci. 1994, 168, 269–271.

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Figure 2. Iodide ion effects on zeta potentials of 0.05 M SB3-10 (0), SB3-12 (b), SB3-14 (3), and SB3-16 (2) micelles, at 25.0 °C, pH 9.0.

(K), and elementary charge, respectively (Φo ) 1 for ζmax ) 25.7 mV at 25 °C). Other parameters are as described above with Rm in centimeters.14c Because the surface electric charge density, Q, was estimated from zeta potentials, the surface excess per micelle includes all ions up to the shear plane and should be a maximum value for anion adsorption in the micelle. Values of Langmuir constants, KL, and of Q/N follow the ISE results and are included in Table 1 for comparison. These independent experiments lead to similar conclusions regarding anionic transfer between water and sulfobetaine micelles, but Q/N values are slightly larger than θmax because of approximations in the treatment and the fact that reported aggregation numbers6,14b were obtained without salt, but added salts, such as NaCl, NaClO4, NaNO3, have little effect on N,10,13c,14b,18 and dilute I- should behave similarly. Values of KL,ζmax and θmax for I- in Table 1 are between those found earlier for Br- and ClO4- in sulfobetaine micelles, as from the Hofmeister series.16 Incorporation of dilute I- is limited by the balance of (i) the increase in micellar charge, which restricts anion uptake, and (ii) interaction with the cationic moiety of the headgroup.16,18 The less hydrated anions are more readily incorporated, so that sulfobetaine and anionic micelles behave similarly in regards to mobility.18 Reaction of Methyl Naphthalene-2-sulfonate and Iodide Ion. For the SN2 reaction of I- with MeONs in water, pH ) 9.0 (Scheme 1), the observed first-order rate constants, kobs, increase linearly with [KI] (Figures 2, 3, and 4), and the second-order rate constant, k2I, is (6.14 ( 0.13) × 10-4 M-1 s-1, i.e., I- is approximately 8- and 40-fold more reactive than Br- and Cl-, respectively (k2Br ) 0.76 × 10-4 M-1 s-1and k2Cl ) 0.15 × 10-4 M-1 s-1).10,11,16 Effect of Sulfobetaines on the Reaction of MeONs and Iodide Ion. For the reaction of 0.2 M KI, with MeONs, kobs increases monotonically with [SB3-n] in the sequence SB3-10 < SB3-12 < SB3-14 < SB3-16 (Figure 3). Rate constants tend toward constant values as the substrate becomes micellar bound, as expected from examination of anion binding in dilute KI (Figures 1 and 2) and the simple pseudophase treatment of micellar rate effects, but this generalization breaks down with increasing [KI]. The reaction of MeONs and I- at a given [SB3-16], for example, shows a complex dependence of kobs on increasing [KI] (Figure 4). Results are qualitatively similar for all the sulfobetaines,

Farrukh et al.

Figure 3. Effect of increasing [SB3-n] on kobs for reaction of 0.2 M Iwith MeONs, in the presence of (0) SB3-10, (b) SB3-12, (3) SB3-14, and (2) SB3-16, 25.0 °C, borate buffer 0.01 M.

Figure 4. Effect of [KI] on kobs for reaction of I- and MeONs, in water (9) and with (1) 0.0063 M, (O) 0.01 M, and (2) 0.05 M SB3-16, at 25.0 °C, borate buffer 0.01 M.

except SB3-10 with its high cmc,3a in that there is an initial steep increase in kobs due to the incorporation of MeONs (Figure 3) and I-, followed by an approximately linear increase with [KI] > 0.2 M (Figure 4), where, in terms of the Langmuir isotherms (eqs 1 and 4 and Figures 1 and 2), the micellar region should be saturated with I-. The overall rate enhancements decrease with decreasing surfactant chain length, as shown for SB3-10, SB3-12, SB3-14 and SB3-16 (Figures 3 and 5). However, instead of the expected saturation in terms of the Langmuir isotherm and the simple pseudophase model, represented by the dashed line (Figure 5), rate constants increase approximately linearly with [KI] > 0.2 M. In low [KI], rate constants apparently level off (Figure 3), but with increasing [KI] up to 1 M, additional KI is not excluded from the micellar pseudophase (Figure 4). Treatment of the Kinetic Data. Analysis of the micellar rate effects is based on the pseudophase model, which for bimolecular, nonsolvolytic reactions involves incorporation of I- and MeONs, in the micellar pseudophase and rate constants in the pseudophases. The reaction of MeONs with I- in water is slower than in micelles (Figure 5), and values of KS in different surfactants are ca. 1000 M-1,3a,21 so that MeONs should be extensively micellarbound in our conditions. The cmc values of sulfobetaines are

Ion Binding and ReactiVity in Sulfobetaine Micelles.

Langmuir, Vol. 24, No. 22, 2008 12999 Table 2. Rate and Binding Constants of MeONs and cmc for the Reaction of I- and MeONs in Sulfobetaine Micellesa surfactants c

SB3-10 SB3-12 SB3-14 SB3-16

103 k2M, M-1 s-1

Ks, M-1

cmcb, M

2.0 2.2 2.1 2.2

350 500 750 1000

1.3 × 10-2 2.0 × 10-3 2.0 × 10-4 2.0 × 10-5

a VM ) 0.14. b Values of cmc from ref 3a. c Values of KL ) 84 M-1 from capillary electrophoresis, and θmax ) 0.14 extrapolated from data in Table 1.

Figure 5. Effect of [KI] on kobs for reaction of I- and MeONs, in water (9) and in the presence of 0.05 M (0) SB3-10, (b) SB3-12, (3) SB3-14 and (2) SB3-16, at 25.0 °C, borate buffer 0.01 M. Solid lines represent fits including electrolyte invasion, and the dashed line is that without adding the background electrolyte concentration term for SB3-10.

such2,3a that, except for SB3-10, the surfactant is largely micellized. With these simplifications, kobs is given by eq 6: M W kobs ) k2M[IM]χMeONs + k2W[I ]SχMeONs M χMeONs

(6)

W χMeONs

where and are mole fractions of organic substrate M in micelles and water, respectively, i.e., χMeONs ) KS[SB]/(1 + W M KS[SB])and χMeONs ) 1 - χMeONs. The terms k2M and k2W are second-order rate constants, expressed in units of M-1 s-1, in the micellar and aqueous pseudophases, respectively, [I-]S is the total concentration of I- and [IM ]is the local molarity of bound I in terms of the volume of the micellar reaction region and is related to that in the total solution, [ I-]M, by eq 7. [IM] ) [I ]M ⁄ (VM[SB])

(7)

where VM is the molar volume of the micellar reaction region, and [I-]M is the concentration of bound I- in terms of the total solution volume, which, for dilute KI, is described by Langmuirtype isotherms (eqs 1 and 4), with saturation limits, θmax< 0.2 (Table 1). The form of eq 7 indicates that plots of kobs against [KI] should level off when eq 1 and ISE or capillary electrophoresis indicate micellar saturation by I-, e.g., at values greater than ca. 0.2 M KI (Figure 3), but the increasing kobs with [KI] (Figures 4 and 5) shows that concentrations of I- in the micellar pseudophase are greater than given by eqs 1 and 4. The increase of kobs is fitted by eqs 8 and 9, which include total [KI]-, as considered later. [IM] ) [I ]M ⁄ ([SB]VM) + [I ]S

(8)

M W + k2W[I-]WχMeONs kobs ) k2M([I-]M ⁄ ([SB]VM) + [I-]S)χMeONs (9)

The contributions of reactions with and in water (eq 9) can be neglected, except in very dilute surfactant, and the kinetic data cover a larger range of [KI] than can be examined by capillary electrophoresis or ISE, (Table 1 and Figures 1-4). The electrolyte invasion of I- in the region of high [KI] electrolyte (where screening permits ions to enter the interfacial region regardless of charge) should approximately follow [KI], (Figures 4 and 5), and other ions, e.g., K+, should behave similarly, although the extents of ion entry may differ, and this cation is kinetically invisible. We note that H3O+ can be induced to enter the micellar

pseudophase of SB3-14 by increasing electrolyte.18b The solid lines in Figures 4 and 5 show data fits with electrolyte invasion. Since the entry of ions into the micellar pseudophase in the electrolyte invasion region basically corresponds to the increase in background electrolyte, it is not sensitive to electrostatic interactions as shown by the similar slopes of plots of kobs against [KI] in this region (Figure 5). It appears that this entry of I- is relatively insensitive to ion-ion interactions, and, at relatively high [KI], especially in the micellar pseudophase, anions, more than cations, can become less hydrated.4,9-12 The kinetic fits to eq 9 are reasonable, because MeONs is fully micellar-bound, and, in dilute KI, kobs follows the concentration of I- in the micellar pseudophase (eq 1); coverage by I- is given by θmax, i.e., as the first term in the right-hand side of eq 9, and, at higher [KI], kobs increases approximately linearly with increasing [KI] (Figures 3 and 4 and eqs 8 and 9). Equation 9 relates second-order rate constants to local molarities in the micellar pseudophase, and k2M can be compared to second-order rate constants in water or other media (Table 2). The partitioning of I- between water and SB3-10 micelles follows a pattern similar to that for the other sulfobetaine micelles (Figure 2), but the relatively high cmc, uncertainties regarding salt effects upon its value (Table 2), and reaction with and in water in dilute SB3-10 (Figure 3) make the value of Ks for SB310 uncertain, but k2M should be reliable with entry of I- in high KI and 0.05 M SB3-10 (Figure 5). The fractional coverages by I-, [I-]M/[SB3-n], of 0.18 to 0.20 in dilute [KI], are much lower than coverage, β, for ionic micelles. With 0.19 ) [I-]M/[SB3-n] and VM ) 0.14, the concentration of bound I-, at ion binding saturation, is ca. 1.36 M and with the electrolyte invasion contribution (eq 8) becomes ca. 2.36 M with 1 M KI, and similar to values for Cl- (1.96 M) and Br(2.28 M) from chemical trapping13 with 0.95 M sodium halide. The dependences of “bound” halide ion on added salt, are consistent with eq 8, with the decrease in ion selectivity at high [salt]13 because ion specificity should disappear with high [salt].3,27 The second-order rate constants, k2M, are 3.2- to 3.5-fold higher than k2I ) 6.14 × 10-4 M-1 s-1 for reaction in water, a result somewhat surprising since estimated second-order rate constants in the micellar pseudophase are typically lower than in water.1 The reported values of k2M represent minimum values since depending on the assumed value of VM ) 0.14 M-1, which is lower than those usually applied to ionic micelles,1,4 and higher values would increase k2M (eq 9). The fit to eq 9 involves the product, k2M [I-]S, and we assume uniform second-order rate constants in the overall interfacial regions, a general limitation of pseudophase treatments. In the kinetic treatment we neglect cation interactions with the micelles, (27) (a) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. J. Phys. Chem. 1989, 93, 7851–7856. (b) Blasko, A.; Bunton, C. A.; Armstrong, C.; Gotham, W.; He, Z.-M.; Nickles, J.; Romsted, L. S. J. Phys. Chem. 1991, 95, 6747–6750. (c) Bertoncini, C. R. A.; Neves, M. D. S.; Nome, F.; Bunton, C. A. Langmuir 1993, 9, 1274–1279.

13000 Langmuir, Vol. 24, No. 22, 2008

although anions affect local concentrations of H+.18b,28 Inorganic cations are chemically invisible, but effects of Li+, Na+, and K+ on rate constants for reaction of methyl 4-nitrobenzenesulfonate with Br- are very small,13b and any incursion of K+ into the micellar interfacial region should not affect reactions of I-. High charge density ions are probably not strongly dehydrated in ionic micelles, and this generalization may apply to sulfobetaine micelles.29,30 For acid catalyzed hydrolysis and equilibria in sulfobetaine micelles, added salts increase local hydrogen ion concentration in the micellar pseudophase, depending specifically on the anion.18b The kinetic data in Figures 4 and 5 indicate that, as in ionic micelles at high concentrations of reactive ion,1 the “approximately linear” part with high [KI] and fully bound MeONs corresponds to an electrolyte invasion contribution, of [KI] > 0.2 M with a second-order rate constant, k2M,, similar to k2W, as for many bimolecular ionic reactions in micelles.1,4 Two region treatments, with sharp boundaries, have limitations in fitting rate data at high electrolyte concentrations due to problems with describing, as in this system, partitioning of both MeONs and I- between possible regions and thr definition of concentration, so that treatment of bimolecular reactions in zwitterionic micelles is more complex than in otherwise similar ionic micelles.

4. Conclusions In dilute electrolyte, anions, more than cations, bind to sulfobetaine micelles, and the extents of specific anion transfers estimated by ISE and capillary electrophoresis follow the O+

(28) The entry of H3 into the interfacial region may be assisted by hydrogen bonding with the sulfonate oxygens, and this observation may not be a good indicator of the extents of entry of inorganic monocations into sulfobetaine micelles. (29) Aoki, T.; Harada, M.; Okada, T. Langmuir 2007, 23, 8820–8826. (30) Harada, M.; Satou, H.; Okada, T. J. Phys. Chem. 2007, 111, 12136– 12140.

Farrukh et al.

Hofmeister series and fit Langmuir isotherms with limits, θmax, increasing from 0.1 to 0.2 with increasing alkyl group length, and are much lower than fractional coverage, β, for ionic micelles. With fully micellar bound MeONs, kobs increases with increasing [KI], up to ca. 0.2 M, following the Langmuir isotherm, but then increases approximately linearly. This behavior is also evident with some bimolecular ionic reactions in ionic micelles, at higher [electrolyte]. There is a problem in treating all the kinetic data in this complex region, where a simple pseudophase two-step function with limited ion transfer is inadequate. Kinetics in sulfobetaine micelles depend on the charge gradient between micellar and aqueous pseudophases decreasing with increasing [KI] and ions entering the accessible region. In high electrolyte, electrostatic and ion-specific interactions should become less important, allowing this entry, especially with zwitterionic association colloids. The estimated ratio ([I-]M/[SB3-n]) is consistent with interfacial halide ion concentrations from chemical trapping13a and reaction kinetics,13b and the model is consistent with the anion selectivity decrease at high salt.13a With any ionic or zwitterionic association colloid surface charge is screened by added electrolyte, and the interfacial region then behaves somewhat as a concentrated salt in water. For many anionmolecule reactions, with small kinetic salt effects, rate constants with micellar bound substrate are similar to those in the aqueous pseudophase, and there should be low ionic specificities with high [electrolyte]. Acknowledgment. We are grateful to PRONEX, CNPq (Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico, Brazil), TWAS (The Academy of Sciences for the Developing World, Italy), and the National Science Foundation, for support of this work. LA802179M