2662
Langmuir 1998, 14, 2662-2669
Effects of Headgroup Structure on the Incorporation of Anions into Sulfobetaine Micelles. Kinetic and Physical Evidence Pietro Di Profio,† Raimondo Germani,† Gianfranco Savelli,*,† Giorgio Cerichelli,‡ Marco Chiarini,‡ Giovanna Mancini,§ Clifford A. Bunton,| and Nicholas D. Gillitt| Dipartimento di Chimica, Universita` di Perugia, I-06123 Perugia, Italy, Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universita` di L’Aquila, I-67010 L’Aquila, Italy, Centro CNR di Studio sui Meccanismi di Reazione, Dipartimento di Chimica, Universita` “La Sapienza”, P. le Aldo Moro 5, I-00185 Roma, Italy, and Department of Chemistry, University of California, Santa Barbara, California 93106 Received October 9, 1997. In Final Form: February 9, 1998
Zwitterionic micelles of (tetradecyldialkylammonio) propane-, butane- and pentanesulfonate increase the rate of reaction of Br- with methyl naphthalene-2-sulfonate, MeONs, but this reaction is largely suppressed by addition of NaClO4. The NMR line width of 35Cl in ClO4- increases very sharply on incorporation into micelles of tetradecyldimethylammonio propanesulfonate, SB3-14. The 14N line width of SB3-14 also increases significantly under these conditions, but there is no evidence for strong interactions with 23Na+. Aggregation numbers, Nagg, of SB3-14 increase modestly with increasing [SB3-14], but not with the other surfactants. The increase is greater with added NaClO4, but NaOH, NaCl, and NaNO3 have little effect on Nagg. Interactions of the anions with the micelles are treated with equations of the form of the Langmuir isotherm, and binding parameters estimated kinetically and conductometrically agree. Second-order rate constants of the reaction of Br- with MeONs at micellar surfaces are similar in sulfobetaine and cationic micelles. The reaction of I- with MeONs is also accelerated by micelles of SB3-14.
Introduction Aqueous micelles can control reaction rates in water by taking up reactants and providing a reaction medium distinct from bulk solvent.1-3 For example, reactions of anionic nucleophiles with nonionic substrates are enhanced by cationic micelles that concentrate both reactants at the micelle-water interface. Anionic micelles inhibit these reactions by taking up the substrate and leaving anionic nucleophiles largely in water. These rate effects are treated quantitatively in terms of pseudophase models, which include transfer equilibria between water and micelles and rate constants in these two regions. These treatments are satisfactory for reactions that are slower than transfer of the reactants. The situation is simpler for spontaneous reactions where only transfer of the substrate has to be considered.1,3a Zwitterionic micelles derived from carboxy- and sulfobetaine surfactants (1 and 2) are formally neutral, but they incorporate anions, although less strongly than cationic micelles, and they have less affinity for metal cations.4,5
R′N+R2(CH2)nSO32
R′N+R2(CH2)nCO21
In this paper we focus attention on the sulfobetaines (2), although there is extensive physical evidence on the betaines (1) and other zwitterionic surfactants.4a,5b,c †
Universita` di Perugia. Universita` di L’Aquila. § Universita ` “La Sapienza”. | University of California. ‡
(1) Fendler, J. H. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982. (2) Romsted, L. S. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 2, p 1015.
Anions should bind to these zwitterionic micelles electrostatically, because the charge density due to the cationic ammonium centers is higher than that at the anionic sulfonate or carboxylate centers.4b,5 This interaction has been analyzed quantitatively for sulfobetaine micelles5a and on a simpler, more qualitative basis for zwitterionic micelles in general.5b,c However, anion specificity cannot be neglected, and kinetic evidence for SN2 reactions in sulfobetaine micelles shows that binding increases as ionic charge density decreases, following the Hofmeister series,6 with Br- > Cl- > F- ∼ OH-.4b,c The kinetic evidence also indicates that the relatively weak cationic binding is not very ion-specific.4b We focus attention on reactions in solutions of sulfobetaine surfactants, and in this work we use tetradecyldimethylammonio propanesulfonate, SB3-14, n ) 3, R ) Me, and the homologues with bulkier headgroups (R ) Et, n-Pr, n-Bu; SBEt3-14, SBPr3-14, SBBu3-14, respectively), or longer tethers tetradecyldialkylammonio butanesulfonate (SBR4-14) and tetradecyldialkylammonio pentanesulfonate (SBR5-14) (alkyl ) Me, n-Pr, respectively). The hexadecyl surfactant SB3-16 had been used earlier but its low water solubility is a disadvantage.4b,c These, or similar, zwitterionic micelles increase rates of (3) (a) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. (b) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (4) (a) Pillersdorf, A.; Katzhendler, J. Isr. J. Chem. 1979, 18, 330. (b) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. J. Org. Chem. 1987, 52, 3832. (c) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. J. Phys. Chem. 1989, 93, 854. (5) (a) Da Silva Baptista, M.; Cuccovia, I.; Chaimovich, H.; Politi, M. J. J. Phys. Chem. 1992, 96, 6442. (b) Kamenka, N.; Chorro, M.; Chevalier, Y.; Levy, H.; Zana, R. Langmuir 1995, 11, 4234. (c) Chevalier, Y.; Kamenka, N.; Chorro, M.; Zana, R. Langmuir 1996, 12, 3225. (6) Hofmeister, F. Arch. Exp. Pathol. Pharmacol. 1888, 24, 247.
S0743-7463(97)01106-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/24/1998
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Langmuir, Vol. 14, No. 10, 1998 2663
spontaneous anionic decarboxylations and cyclizations,7 of dephosphorylations,8 and of some reactions of nucleophilic anions with p-nitrophenyl diphenyl phosphate and methyl naphthalene-2-sulfonate (MeONs).4b,c These reactions, mediated by cationic micelles, are well studied and rate-surfactant profiles are fitted quantitatively by various treatments based on pseudophase models.1-3,9 Anion affinity orders are similar for cationic and betainesulfonate micelles.4b,c Nucleophilic attack on MeONs is mechanistically simple and is followed spectrophotometrically.4c
molarity in terms of the molar volume of the reaction region, VM, at the micelle-water interface.1-3 Therefore,
k′M ) kM
[NuM] [SBn]
) km 2
[NuM] VM[SBn]
(4)
and:
kobs )
kW[NuW] + kMKS[SBn][NuM] 1 + KS[SBn]
(5)
These second-order rate constants kM and km 2 differ in dimensions (eq 4) but km 2 can be compared directly with kW and km 2 ) kMVM. Distributions of anions between water and sulfobetaine micelles are written in terms of a Langmuir isotherm:5b,10-12 Analysis of micellar rate effects is based on a pseudophase treatment of micellar binding of organic substrates, e.g., MeONs1-3 (eq 1), where subscripts W and M denote aqueous and micellar pseudophases respectively, KS is a binding constant written in terms of micellized sulfobetaine, [SBn] ) [SB] - cmc, where cmc, the critical micelle concentration, is taken as the concentration of monomeric surfactant (eq 2).9 First-order rate constants, k′Wand k′M, with respect to MeONs, depend on nucleophile concentrations in the two pseudophases, and kobs is the overall first-order rate constant with respect to substrate. Quantities in squared brackets are molarities in terms of total solution volume. Provided that MeONs is in low concentration, its partitioning is given by eq 11-3,9 (subscript T denotes total
[MeONsM] [MeONsT]
)
KS[SBn] 1 + KS[SBn]
(1)
concentration), and the observed first-order rate constant with respect to MeONs is given by eq 2:
kobs )
k′W + k′MKS[SBn] 1 + KS[SBn]
(2)
Values of KS have been estimated kinetically for MeONs in cationic micelles,10 and provided that KS[Dn] g 1, fits are relatively insensitive to KS. Treatments of nonsolvolytic nucleophilic reactions based on eq 2 depend on estimation of local nucleophile concentrations in the aqueous and micellar pseudophases;1-3 with dilute surfactant the concentration of nucleophile in the aqueous pseudophase is approximately the total concentration, so that
k′W ) kW[NuW] ≈ kW[NuT]
(3)
where kW is a second-order rate constant, M-1 s-1. Concentration of a nucleophile in the micellar pseudophase can be written in various ways, e.g., as a mole ratio of bound nucleophile to surfactant or as a local (7) (a) Cerichelli, G.; Mancini, G.; Luchetti, L.; Savelli, G.; Bunton, C. A. J. Phys. Org. Chem. 1991, 4, 71. (b) Di Profio, P.; Germani, R.; Savelli, G.; Cerichelli, G.; Spreti, N.; Bunton, C. A. J. Chem. Soc., Perkin 2 1996, 1505. (8) Del Rosso, F.; Bartoletti, A.; Di Profio, P.; Germani, R.; Savelli, G.; Blasko´, A.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1995, 673. (9) Menger, F. M.; Portnoy, C. E. J. Am. Chem. Soc. 1967, 89, 4698. (10) (a) Bacaloglu, R.; Bunton, C. A.; Ortega, F. J. Phys. Chem. 1989, 93, 1497. (b) Bonan, C.; Germani, R.; Ponti, P. P.; Savelli, G.; Cerichelli, G.; Bacaloglu, R.; Bunton, C. A. J. Phys. Chem. 1990, 94, 5331.
K′X )
[X-M] [X-W]([SBn] - [X-M])
(6)
Equation 6 is reasonably satisfactory in fitting transfer equilibria of anions between water and cationic micelles, and it predicts that zwitterionic micelles will gain negative charge as they take up anions. This treatment may therefore be useful only with dilute anions. It fits rate data for the spontaneous hydrolysis of 2,4-dinitrophenyl phosphate dianion8 and conductivity data for NaCl in solutions of zwitterionic micelles.5b We planned to complement our kinetic work with physical studies of sulfobetaine micelles with added salts. The effect of salts on the aggregation number, Nagg, of SB3-14 was determined by Turro’s fluorescence quenching method,13a and in favorable cases, NMR spectrometry provides evidence on the transfer of ions from water to micelles.14,15 The effect of sulfobetaines upon conductivities of added electrolytes provides information on micelleion interactions,5 although it is not useful for OH- because the very high mobility of this ion and its low binding to cationic moieties of micellar surfaces make the method insensitive. Reaction of Br- with MeONs has been studied in a variety of aqueous association colloids,4c,10,16 and rate effects are treated quantitatively by using various pseudophase treatments,3a,16 including use of eq 6.10 Experimental Section Materials. SB3-14 was from Fluka and was purified by recrystallization from acetone. SBEt3-14 and SBPr3-14 were prepared from propane sultone and the corresponding tertiary amine as described.17 SBBu3-14 was prepared as follows: a solution containing 0.33 mol of propane sultone and 0.24 mol of tetradecyldibutylamine in 1,2-dichloroethane was refluxed for approximately 2 days. After cooling to room temperature, the solvent was evaporated under vacuum. The semisolid product (11) Bunton, C. A.; Gan, L. H.; Moffatt, J. R.; Romsted, L. S.; Savelli, G. J. Phys. Chem. 1981, 85, 4118. (12) (a) Gan, L.-H. Can. J. Chem. 1985, 63, 598. (b) Rodenas, E.; Vera, S. J. Phys. Chem. 1985, 89, 513. (13) (a) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951. (b) Jones, D. B. J. Am. Chem. Soc. 1915, 37, 589. (14) Fabre, E.; Kamenka, N.; Khan, A.; Lindblom, G.; Lindman, B.; Tiddy, J. T. G. J. Phys. Chem. 1980, 84, 3428. (15) (a) Bacaloglu, R.; Bunton, C. A.; Cerichelli, G.; Ortega, F. J. Phys. Chem. 1989, 93, 1490. (b) Blasko´, A.; Bunton, C. A.; Cerichelli, G.; McKenzie, D. C. J. Phys. Chem. 1993, 97, 11324. (16) (a) Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1986, 90, 538. (b) Bunton, C. A.; Moffatt, J. R. J. Phys, Chem. 1988, 92, 2896. (17) Spreti, N.; Bartoletti, A.; Di Profio, P.; Germani, R.; Savelli, G. Biotechnol. Prog. 1995, 11, 107.
2664 Langmuir, Vol. 14, No. 10, 1998 was recrystallized from ethyl acetate/acetone and then from acetone/ethyl ether until all impurities had been removed, followed by drying under vacuum (10-2 Torr) at 40 °C. An aqueous solution of the crystalline solid was dialyzed (porosity of membrane: 10000 Da) and its specific conductivity monitored with a platinized electrode and an Orion Research conductivity meter. The dialysis was ended when conductance was a few microsiemens. Water was then evaporated and the resulting solid crystallized from acetone, filtered, and dried in vacuo over P2O5. SBBu3-14: mp 124-126 °C; cmc ) 1.03 × 10-4 M; 1H NMR (300 MHz, D2O) (in D2O values of chemical shifts are relative to internal 1,4-dioxane: δ ) 3.75 ppm at 25 °C) δ 0.89 (t, 3H, CH3), 0.99 (t, 6H, 2CH3), 1.29-1.36 (m, 22H, (CH2)11CH3), 1.42 (m, 4H, CH3CH2(CH2)2N+), 1.69 (m, 6H, C2-CH2CH2N+ and C12CH2CH2N+), 2.16 (m, 2H, -CH2-C-SO3-), 2.93 (t, 2H, -CH2SO3-), 3.22 (m, 6H, -N+(CH2)3), 3.47 (t, 2H, N+-CH2-CH2CH2-SO3-). SB4-14 and SBPr4-14 were prepared as follows: a solution containing 0.174 mol of 1,4-butanesultone and 0.124 mol of the appropriate dialkyltetradecylamine in toluene (dried over Na) was refluxed for 1-4 days. The solution was concentrated, the crude solid was taken up with diethyl ether, and the solvents were evaporated. Recrystallization from acetone and methyl alcohol yielded a solid that was dried in vacuo over P2O5. SB4-14: mp 286-289 °C; cmc ) 3.70 × 10-4 M; 1H NMR (200 MHz, CDCl3; values of chemical shifts are relative to TMS) δ 3.6 (m, 2H, N+CH2RSO3-), 3.1-3.3 (m, 8H, N+(CH3)2 + N+CH2C13), 2.9 (t, 2H, CH2SO3-), 1.9 (m, 4H, CH2CH2CH2SO3-), 1.65 (m, 2H, N+CH2CH2C12), 1.3 (m, 22H, (CH2)11CH3), 0.9 (t, 3H, (CH2)13CH3). SBPr4-14: mp 189-193 °C; cmc ) 3.09 × 10-4 M; 1H NMR (200 MHz, CDCl3) δ 3.45 (m, 2H, N+CH2RSO3-), 3.2 (m, 6H, N+(CH2CH2CH3)2 + N+CH2C13), 2.85 (m, 2H, CH2SO3-), 1.9 (m, 4H, CH2CH2CH2SO3-), 1.6-1.8 (m, 6H, N+CH2CH2C12 + N+(CH2CH2CH3)2), 1.3 (m, 22H, (CH2)11CH3), 0.85 (t, 3H, (CH2)13CH3). SB5-14 and SBPr5-14 were prepared as follows: 0.307 mol of Na2SO3 and 45 g of N,N-dialkyltetradecyl-N-(5-chloropentyl)ammonio chloride (obtained as a crude oil from the appropriate dialkyltetradecylamine and 1,5-dichloropentane in refluxing MeCN over 4-7 days) were dissolved in water and heated under reflux for 3 days and then concentrated in vacuo. The crude concentrate was first treated with ethanol to precipitate salts, the ethanol was evaporated, and the crude solid was crystallized from ethyl acetate and then from ethyl ether. Dialysis was conducted as described above for SBBu3-14. SB5-14: mp 253-257 °C; cmc ) 2.75 × 10-4 M; 1H NMR (200 MHz, CDCl3) δ 3.5 (m, 2H, N+CH2RSO3-), 3.2-3.4 (m, 8H, N+(CH3)2 + N+CH2C13), 2.8 (m, 2H, CH2SO3-), 1.3-1.7 (m, 8H, (CH2)3CH2SO3- + N+CH2CH2C12), 1.1 (m, 22H, (CH2)11CH3), 0.9 (t, 3H, (CH2)13CH3). SBPr5-14: mp 163-167 °C; cmc ) 2.63 × 10-4 M; 1H NMR (200 MHz, CDCl3) δ 3.4-3.1 (m, 8H, N+CH2RSO3- + N+(CH2CH2CH3)2 + N+CH2C13), 2.8 (t, 2H, CH2SO3-), 2-1.5 (m, 12H, (CH2)3CH2SO3- + N+CH2CH2C12 + N+(CH2CH2CH3)2), 1.41.2 (m, 22H, (CH2)11CH3), 1.05 (t, 6H, N+(CH2CH2CH3)2), 0.9 (t, 3H, (CH2)13CH3). Melting points were determined on a Bu¨chi 510 melting point apparatus, and values are uncorrected. The preparation of MeONs has been described.10 Kinetics. Reactions were followed at 25.0 ( 0.1 °C in either HP diode-array or Shimadzu double-beam spectrometers with 10-4 M MeONs at 326 nm.4c,10 There is a minor contribution from the reaction of MeONs with water, with kH2O going from 1.2 × 10-5 s-1 in water to ca. 7 × 10-6 s-1 in 0.1 M sulfobetaine. The various sulfobetaines behaved similarly, and our reported values of kobs are corrected for kH2O. Electrolytic Conductance. Specific conductances of surfactant-salt solutions were monitored at 25 °C in an Orion Research conductivity meter with a 1 cm platinized electrode. Fluorescence Quenching. The fluorescence of pyrene was monitored as a function of quencher concentration on a SPEX Flurolog DM3000 fluorescence spectrometer. The general procedure is that described,13a except that the fluorescence of pyrene was generally quenched by 2-iodohexadecanoic acid (2-IHDA), which was prepared as described13b from the bromo acid and KI
Cerichelli et al.
Figure 1. Headgroup effects on first-order rate constants of reaction of MeONs with Br- (0.1 M NaBr) at 25.0 °C in sulfobetaine surfactants: SB3-14 (9); SBEt3-14 (b); SBPr3-14 (2); SBBu3-14 (1). Solid lines are theoretical.
Figure 2. Effects of headgroups and tether lengths on firstorder rate constants of reaction of MeONs with Br- (0.1 M NaBr) at 25.0 °C in sulfobetaine surfactants: SB3-14 (9); SB414 (b); SB5-14 (2); SBPr3-14 (1); SBPr4-14 (×); SBPr5-14 ([). Solid lines are theoretical. in refluxing MeOH (mp: 59-60 °C, lit. 60-61 °C). All solutions were O2-free by irrigation with dry N2. ln(I0/I) plots were linear with quencher concentration. NMR. 23Na, 35Cl, 14N, and 81Br NMR spectra were recorded on a Bruker AC 300 P spectrometer operating at 79.391, 29.407, 21.688, and 81.053 MHz, respectively. Typical acquisition conditions were as follows: 23Na, 90° pulse, 0.2-0.6 s acquisition, 2000 Hz sweep width; 35Cl, 90° pulse, 0.02-0.07 s acquisition, 1000 Hz sweep width; 14N, 90° pulse, 0.02-0.06 s acquisition, 2000 Hz sweep width; 81Br, 90° pulse, 0.005-0.01 s acquisition, 80 000 Hz sweep width. T1 measurements for 23Na have been carried out by the standard inversion recovery pulse sequence. All measurements were made in 20% D2O in water and at 25 °C. Critical Micelle Concentrations. Values of the critical micelle concentration, cmc, were determined from plots of surface tension vs -log[surfactant] with no minima. Surface tensions were measured on a Fisher, du Nou¨y type, tensiometer at room temperature in deionized, bidistilled water.
Results KineticssMicellar Reactions. Micellized sulfobetaines accelerate the reaction of Br- with MeONs (Figures 1 and 2), and the first-order rate constants for reaction of Br-, kobs, are corrected for the minor contribution of reaction with H2O (Experimental Section). Values of kobs increase monotonically with increasing [sulfobetaine] and follow the sequence SB3-14 < SBEt3-
Anion Incorporation into Sulfobetaine Micelles
Langmuir, Vol. 14, No. 10, 1998 2665
Table 1. First-Order Rate Constants of Reaction of MeONs with Br- in Sulfobetaine Surfactantsa 103 [SB3-14], M
NaBr
CsBr
(Me)4NBr
(n-Bu)4NBr
0 1 3 5 8 10 30 50 100
0.082 0.926 1.56 1.77 1.96 1.98 2.03 1.96 2.07
0.056 0.884 1.56 1.80 1.84 1.92 1.82 2.05 1.99
0.10 0.843 1.26 1.44 1.95
0.19 0.949 1.62 1.90 1.98 2.13 2.29 2.36 2.68
a
2.04 2.21 2.06
104kobs, s-1 at 25.0 °C with 0.1 M Br-.
K′ClO4 )
[Br-w] ([Br-M] - [ClO4-M]) [ClO4-M]
[ClO4-W] ([SBn] - [Br-M] - [ClO4-M])
0 0.005 0.02 0.05 0.1 0.2 0.5
4.6 3.9 2.7 1.5 0.9 0.4 0.2
6.5 5.3 3.9 2.7 1.5 0.8 0.4
3.6 2.5 0.8 0.3 0.2 0.011 0.0057
2.8 2.0 0.9 0.4 0.2 0.2 0.089
2.6 1.9 0.8 0.3 0.2 0.0066
2.0 1.6 0.8 0.4 0.2 0.2 0.075
Values of 104kobs, s-1 with 0.05 M sulfobetaine and 0.1 M NaBr unless specified. b [NaBr] ) 0.5 M. c [NaBr] ) 1 M.
14 < SBPr3-14 < SBBu3-14 for the propane series, SB414 < SBPr4-14 for the butane series, SB5-14 < SBPr5-14 for the pentane series and SB3-14 < SB4-14 < SB5-14 for changes in the tether length (Figures 1 and 2). A change of cation from Na+ to Cs+, Me4N+ did not significantly affect kobs, althoug reactions are about 30% faster when using tetrabutylammonium bromide (Table 1). Reaction of MeONs with Br- in sulfobetaine micelles is strongly inhibited by added NaClO4, indicating that ClO4has a strong affinity for the micelles (Figure 3 and Table 2). Alkylammonium perchlorates are typically insoluble in water, so we cannot examine inhibition by ClO4- of reactions of Br- in cationic micelles. The observation that the reaction of Br- is largely suppressed by ClO4- shows that it effectively excludes Br- from the micelles. This competition can be treated quantitatively by modifying eq 6:12
K′Br )
[NaClO4], M SB3-14b SB3-14c SBBu3-14 SB5-14 SB4-14 SB3-14
a
Figure 3. First-order rate constants of reaction of MeONs with 0.5 M Br- at 25.0 °C in SB3-14 with no NaClO4 (9), 0.005 M NaClO4 (b), 0.05 M NaClO4 (2). Solid lines are theoretical.
[Br-M]
Table 2. First-Order Rate Constants of Reaction of MeONs with Br- in Sulfobetaine Surfactants with Added NaClO4a
(7a)
(7b)
Equations of these general forms fit salt effects on reactions of anions in cationic micelles. Conductance. Micelles can incorporate counterions and reduce their ionic conductance. This method has been used extensively with ionic and, to a limited extent, with zwitterionic micelles.1-3,5 Addition of SB3-14 to 0.005 and 0.01 M NaBr, NaI, and NaClO4 sharply decreases electrolytic conductance (Figure 4) in the sequence Br- < I< ClO4-, indicating the relative anion affinities for the
Figure 4. Salt effects on the specific conductance of solutions of SB3-14 at 25.0 °C: 0.005 M NaBr (9); 0.005 M NaI (0); 0.005 M NaClO4 (b). Solid lines are theoretical.
Figure 5. Salt effects on the specific conductance of solutions of sulfobetaines at 25.0 °C: SB3-14 + 0.01 M NaClO4 (9); SB414 + 0.01 M NaClO4 (0); SB3-14 + 0.01 M NaBr (∆); SB3-14 + 0.01 M NaI (∇); SBBu3-14 + 0.01 M NaClO4 (b). Solid lines are theoretical.
micelle. Similar behavior was observed on addition of SB4-14 and SBBu3-14 (Figure 5). Aggregation Numbers. Aggregation numbers, Nagg, of the sulfobetaine micelles and effects of added electrolytes are given in Tables 3 and 4. The static quenching method is complicated by Br- quenching, but we examined effects of other anions. There is modest growth with increasing [SB3-14], but not with the other surfactants, and as expected,18 an increase in headgroup bulk significantly decreases Nagg, and it decreases with increasing tether length. Values of Nagg increase modestly with increasing
2666 Langmuir, Vol. 14, No. 10, 1998
Cerichelli et al.
Table 3. Aggregation Numbers of Micelles of SB3-14a
Table 6. Line Widths of
[SB3-14], M 0.01 no salt NaOH, 0.1 M NaOH, 0.5 M NaCl, 0.1 M NaCl, 0.5 M NaNO3, 0.1 M NaClO4, 0.1 M NaClO4, 0.5 M
0.03
60 (62) 59
0.05
69 (75)
0.1
67 (67)
86 91 90 84 104 108
a Values of N agg, with 2-iodohexadecanoic acid (Na salt) as quencher. Values in parentheses were obtained by using cetylpyridinium chloride as quencher.
Table 4. Aggregation Numbers of Micelles of SBR3-14, SB4-14, and SB5-14a surfactant SBEt3-14 SBPr3-14 SBBu3-14 SB4-14 SB5-14 no salt NaOH, 0.1 M NaCl, 0.1 M NaNO3, 0.1 M NaClO4, 0.1 M NaClO4, 0.5 M
54 (60)b
(64)
48 (55) 59 66 64 66 (57)
48c
61
50d 49 52 53 56
a
LW 81Br-, Hz
LW 23Na+, Hz
0 0.033 0.067 0.1
336 1850 3475 5170
8.9 9.1 9.2 9.1
[NaBr] ) 0.02 M.
[surfactant]. Added electrolytes have little effect upon Nagg except that ClO4- appears to increase Nagg at a surfactant concentration, 0.1 M, where there is already some growth (Tables 3 and 4). NMR Spectrometry. Evidence from the present, and earlier, work shows that small inorganic anions are not excluded from sulfobetaine micelles.4,5 There is interanionic competition with an anion order analogous to that seen with cationic micelles.4b (The salt effect of ClO4cannot be examined in cationic micelles, but other lowcharge density anions are effective inhibitors1-3). The NMR line widths of 35Cl-, 79Br-, and 81Br- increase when these ions are incorporated in cationic micelles,14,15 and we also saw line broadening of the NMR signal of 81Brin solutions of SB3-14 (Table 5). We used variations in the line width of the 35Cl- signal of ClO4- in solutions of SB3-14 to examine the transfer equilibrium qualitatively. The data in Tables 6 and 7 show that ClO4- is strongly micellar-bound with SB3-14 in significant excess over NaClO4. However, binding is complete up to only a [surfactant]/[perchlorate] ratio of approximately 10, which gives a maximum line width; further addition of NaClO4 decreases the line width, as a higher fraction of ClO4- remains in the aqueous pseudophase. We expected ClO4- to bind close to the quaternary ammonium center of SB3-14, and it should therefore decrease the mobility of nitrogen within the (18) (a) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1991. (b) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 1976, 72, 1525.
LW 35Cl, Hz
19.2 20.6 20.7 23.1 24.5 25.8 27.6 29.6 33.7 37.9 39.3 49.2 56.0 66.7 64.2 70.7
42.1 41.3 41.8 38.5 37.35 30.8 28.4 21.1 14.7 11.0 8.2 7.1 6.7 5.8 5.4
23Na
in 0.055 M
102[NaClO4], M
LW 14N, Hz
LW 35Cl, Hz
LW 23Na, Hz
0b
13.1 21.9 22.2 24.8 26.4
38.6 36.7 32.2 26.5
7.0 5.7
30.6 33.5 37.8 48.6 51.9
17.1 12.9 8.1 5.5 4.2
56.7
3.7
0.20 0.50 1.00 2.00 4.00 5.00 10.0 20.0 50.0 70.0 80.0 100.0
in Aqueous
[SB3-14], M
LW 14N, Hz
0b
Table 7. Line Widths of 14N, 35Cl, and SB3-14 + NaClO4a
With 0.01 M surfactant; values in parentheses are for 0.1 M surfactant. b Nagg ) 60 with 0.03 M SBEt3-14. c Nagg ) 48 with 0.05 M SBBu3-14. d With cetylpyridinium chloride as quencher. 23Na+
in 0.095 M SB3-14 +
a Solvent D O:H O ) 1:4. b Line widths of 23Na and 35Cl in 0.05 2 2 M aqueous NaClO4 are 6.5 and 1.44 Hz, respectively.
a
Table 5. Line Widths of 81Br- and SB3-14a
14N
102[NaClO4], M 0.18 0.45 0.90 1.35 1.80 3.00 4.50 9.00 18.0 30.0 45.0 55.0 63.0 75.0 90.0
66 64 66
35Cl and NaClO4a
a
6.0 5.9
6.1 6.2
Solvent D2O:H2O ) 1:4. b See note b of Table 6.
micelle. Consistently, line widths of the 14N signal increase on addition of NaClO4 (Tables 6 and 7). Discussion Kinetic data provide clear evidence for interactions of Br- and ClO4- with sulfobetaine micelles and show that ion affinity is not governed solely by electrostatic interactions, cf. refs 4 and 5. We fit the data by using eqs 5 and 6 with the binding parameters, K′X, in Table 8, which are considerably lower than those used to fit kinetic data in cationic micelles.3a,10 Values of K′Br decrease with increasing bulk of the N-dialkyl groups, as found with cationic micelles, but an increase in tether length has little effect. Similar headgroup effects had been observed upon the kinetically estimated parameters for the binding of 2,4-dinitrophenyl phosphate dianion.8 Changes in the cation have little effect on the binding of Br- (Table 8), as expected in view of the relatively weak, unspecific, interaction of cations with sulfobetaine micelles.4,5 Despite the approximations involved in describing ion binding by eq 6,11,12 the treatment is surprisingly robust, because it fits data for reactions of MeONs with 0.1 and 0.5 M Br- without major changes in rate or equilibrium constants (Table 8). In addition, we use the same values of K′Br to fit conductivity data with dilute electrolyte (Table 9) and kinetic data with 0.1-0.5 M NaBr. The kinetic fits are shown in Figures 1 and 2, and fits of the conductance data are in Figures 4 and 5. There are deviations in the kinetic data from the predictions of eq 6 with high [NaBr], e.g., ∼1 M. This phenomenon is frequently seen for counterionic reactions
Anion Incorporation into Sulfobetaine Micelles
Langmuir, Vol. 14, No. 10, 1998 2667
Table 8. Fitting Parameters for the Reactions of MeONs with Halide Ionsa 104 kM, s-1 SB3-14 SBEt3-14 SBPr3-14 SBBu3-14 SB4-14 SBPr4-14 SB5-14 SBPr5-14
K′X, M-1
NaBr
CsBr
(Me)4NBr
(n-Bu)4NBr
NaI
NaBr
CsBr
(Me)4NBr
(n-Bu)4NBr
NaI
7.0 (6.5) 12 18 (16.5) 29 9.5 (8.6) 22 (17) 10 15
6.0
6.5
6.0
35
4.3 (4.0) 3.2 2.6 (2.0) 1.8 4.3 (4.3) 2.6 (2.6) 4.3 4.5
4.3
4.3
4.3
21
18 9.6
9.5
9.6 22 10
2.6 4.5
4.5
4.5 2.7 4.3
a Second-order rate constant in the micellar pseudophase (k ) and binding parameters of anions (K′ ) estimated by applying eq 2 to kinetic M X data; values in parentheses are calculated for NaBr 0.5 M. kW ) 0.82 × 10-4 and 9.1 × 10-4 M-1 s-1 for Br- and I-, respectively.
Table 9. Binding parameters of sodium ion (K′Na) and anions (K′X) obtained from conductometric dataa K′X, M-1 surfactant SB3-14 SB4-14
K′Na,
M-1
1.5 1.5
Br- b
I- c
ClO4- d
4.3 4.3
21
40
a Obtained by fitting eq 13 to specific conductance data. b With 0.005 and 0.01 M NaBr. c With 0.005 and 0.01 M NaI. d With 0.005, 0.01, and 0.02 M NaClO4.
Table 10. Second-Order Rate Constants for Reactions of Br- in Cationic and Sulfobetaine Micellesa alkyl group cationic sulfobetaine
Me
Et
Pr
Bu
9.6 6.8
14 12
18 17
25 29
a Values of 104k , s-1 at 25.0 °C in cetyltrialkylammonium M bromide and SBR3-14 micelles.10
in solutions of ionic micelles and can be explained by the entropically driven invasion of ions into the interfacial micellar region.19 In addition, the high concentration of aqueous cations should screen electrostatic repulsions due to Br-, as it builds up in the zwitterionic micelles giving them anionic character. Sulfobetaine and especially cationic micelles have strong effects upon rates of many reactions and our present values of kM for reaction with Br- (Table 10) in ammonio propanesulfonate micelles (SBR3-14) are similar to those for the corresponding reaction in cetyltrialkylammonium bromide micelles, despite differences in the lengths of the alkyl tails (C14 and C16).18 These observations confirm the speculation made earlier that kM values for reaction of Br- with MeONs should be very similar in cationic micelles of cetyltrimethylammonium bromide, CTABr, and zwitterionic micelles of SB3-16, which provided a basis for estimation of local concentrations of Br- in a sulfobetaine micelle.4c We note that some reactions, e.g., decarboxylation of the 6-nitrobenzisoxazole-3-carboxylate ion, are faster in betaine than in otherwise similar cationic micelles.7a However, cyclization of o-[(3-halopropyl)oxy]phenoxide ions, which is an intramolecular analogue of an intermolecular SN2 reaction,20 gives very similar values of kM in the two sets of micelles with similar effects of bulky headgroups.7 It is important to note that questions regarding the transfer equilibrium of a second reagent do not arise in such spontaneous reactions as cyclizations or decarboxylations. (19) (a) Nome, F.; Rubira, A. F.; Franco, C.; Ionescu, L. G. J. Phys. Chem. 1982, 86, 1881. (b) Stadler, E.; Zanette, D.; Rezende, M. C.; Nome, F. J. Phys. Chem. 1984, 88, 1892. (c) Ferreira, L. C. M.; Zucco, C.; Zanette, D.; Nome, F. J. Phys. Chem. 1992, 96, 9058. (20) Mandolini, L. Adv. Phys. Org. Chem. 1986, 22, 1.
Inhibition by ClO4-. Physical evidence indicates that ClO4- binds readily to sulfobetaine micelles, and it should strongly inhibit reaction of Br- by expelling it from the micelle. Inhibition is very effective even with NaBr in large excess over NaClO4 and is evident with several sulfobetaines (Figure 3 and Table 2). Interionic competition in cationic micelles can, in favorable cases, be treated experimentally by estimating local concentrations, e.g., by using dediazonization trapping21 or by using NMR spectroscopy,15b and several theoretical treatments have been developed.1-3 The pseudophase ion-exchange model treats the micelles as ion exchangers with constant fractional micellar charge.2,22 Another model applies eqs 7a,b to binding of both the reactive and inert ion without placing restrictions on the fractional micellar charge,12,23 and a third involves solution of the Poisson-Boltzmann equation, PBE, with a parameter for the specificity of ion-micelle interactions.16,24 The PBE has been used to treat nonspecific interactions of ions, e.g., OH-, with zwitterionic micelles,5a but in view of the strong specificity of interactions of anions with sulfobetaine micelles it is reasonable to use eqs 6 and 7, at least for dilute electrolyte. Treatments based on application of eqs 7a,b underpredict the inhibition by ClO4-, except at very low concentrations, probably because they do not adequately account for the build up of anionic character in the micelle due to incorporation of ClO4- (Figure 3). In addition, ClO4induces micellar growth, at least at high [surfactant]. We conclude that eq 6 and its extended form for two ions (eqs 7a,b) is adequate only for minor extents of ionic incorporation, at least for zwitterionic micelles where charge development due to anion incorporation can be important, and we could not fit data for 0.05 M NaClO4 in Figure 3. Physical Evidence. Incorporation of mobile ions into a sulfobetaine micelle decreases conductivity, but there is a small compensation due to charge formation in the micelle. We write the distribution of anions into the micelles in terms of eq 6 and apply a similar equation to the binding of cations. On the basis of extensive kinetic data and other evidence, we assume that anion binding will be the more important.4,5 In fitting the conductivity data, we neglect ion-ion or ion-micelle interaction and also excluded volume effects, which become more serious with [sulfobetaine] > 0.1 M. (21) (a) Chaudhuri, A.; Romsted, L. S. J. Am. Chem. Soc. 1991, 113, 5052. (b) Chaudhuri, A.; Loughlin, J. A.; Romsted, L. S.; Yao, J. J. Am. Chem. Soc. 1993, 115, 8351. (22) (a) Quina, F.; Chaimovich, H. J. Phys, Chem. 1979, 83, 1844. (b) Romsted, L. S. J. Phys. Chem. 1985, 89, 5107, 5113. (23) Bunton, C. A.; Gan, L.-H.; Hamed, F. H.; Moffatt, J. R. J. Phys. Chem. 1983, 87, 513. (24) (a) Ortega, F.; Rodenas, E. J. Phys. Chem. 1987, 91, 837. (b) Dolcet, C.; Rodenas, E. Can. J. Chem. 1990, 68, 932. (c) Al-Lohedan, H. J. Chem. Soc., Perkin Trans. 2 1995, 1707.
2668 Langmuir, Vol. 14, No. 10, 1998
Cerichelli et al.
The contribution of micelles to overall conductivity is based on their charge, esu,
Q ) qRNagg
(8)
where q is the elementary charge, 4.8 × 10-10 (esu), and R, the fractional charge, depends on relative concentrations of the anion, Xs, and Na+ in the micelle,
R ) ([X-M] - [Na+M])/[Dn]
(9)
and [X-M] is given by eq 6 and [Na+M] is given by the corresponding relation for Na+ in terms of K′Na. The ionic mobility of the micelle is given by:
u ) Q/F
(10)
and its contribution to the specific conductivity can be written as25
κ ) [Dn]uF/Nagg
(11)
where F is the Faraday constant, esu mol-1, and F is the friction coefficient (for stick-boundary conditions) as given by Stokes’ approximation:
F ) 6πηRh
(12)
In eq 12 η is the macroscopic viscosity of the dilute electrolyte solution, which can be approximated to that of water for the electrolyte concentrations used herein (0.89 cP at 25 °C),26 and Rh is the micellar hydrodynamic radius, which is 27.0 Å for SB3-14 based on diffusion coefficients estimated by dynamic light scattering and extrapolated to the cmc.27 This term, however, makes only a small contribution to the overall conductance. The specific conductance of mixtures of SB3-14 and salt is given by straightforward combinations of and substitutions in eqs 8-12 to give
κ ) (qFR[Dn])/(9.11 × 1014F) + 10-3 [X-W]ΛX + 10-3[Na+W]ΛNa (13) where [X-W] and [Na+W] are given by mass balance, Λ is the equivalent conductance at infinite dilution, and F ) 9.65 × 104 C mol-1. The numerical factors in eq 13 are introduced to allow for expression of κ in the CGS system. Fits of κ against [SB3-14] are reasonably good on the basis of K′Br ) 4.3 M-1, as used in fitting the kinetic data (Table 8), and K′I) 21 M-1 and K′ClO4 ) 40 M-1 (Figure 4, Table 9). Binding of Na+ does not strongly affect the specific conductivity in these conditions, but in order to fit the data with [SB3-14] > 0.1 M, we had to include this binding term with K′Na ) 1.5 M-1, cf. ref 5b,c, to allow for conductance by Na+. However, in these conditions, neglect of excluded volume effects may not be justified, and the use of the equivalent conductance at infinite dilution for Na+, which resides mostly in the water, is questionable in that a decrease in its equivalent conductance with concentration (by ca. 10% in going from infinite dilution to 0.02 M for a number of inorganic salts)26 is equivalent to Na+ binding by the micelle as given by K′Na. The fits are reasonably good for the other surfactants and dilute (25) (a) Evans, H. C. J. Chem. Soc. 1956, 579. (b) Eicke, H. F.; Denss, A. In Solution Chemistry of Surfactants, Mittal, K. L., Ed.; Plenum Press: New York, 1979; p 699. (26) CRC Handbook of Chemistry and Physics, 2nd ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1987. (27) Di Profio, P. Thesis, University of Perugia, 1996.
Figure 6. 23Na NMR T1 of aqueous NaClO4 (9) and NaClO4 with 0.095 M SB3-14 (0). Solid lines are meant as a guide for the eye only.
salts (Figures 4 and 5) except for SBBu3-14 + NaClO4 (Figure 5) where we could not fit the data with reasonable parameters. The simple pseudophase treatments (eqs 6 and 7) do not allow for possible changes in headgroup conformations on addition of electrolyte. In this simplified treatment we assume independent binding of X- and M+ to the sulfobetaine micelles. This simplification is supported by kinetic data showing that the reactivity of Br- toward MeONs is unaffected by a change of cation from Na+ to Cs+, Me4N+, and slightly increased (ca. 30%) with Bu4N+ (Table 8). Insofar as these ions have different affinities for anionic micelles, e.g., dodecyl sulfate,1-3,28 we conclude that electrostatic interactions are not of major importance in the binding of Br-, and similar “soft” anions29 to sulfobetaine micelles, in accord with kinetic data on reactions of anions with low and high charge densities.1-3,16,24 The NMR evidence on the line widths of 35Cl- (Tables 6 and 7) and of 81Br- (Table 5) shows that ClO4- and Brbind to micelles of SB3-14. In 0.055 or 0.095 M SB3-14 35Cl- line widths of dilute ClO - are very high relative to 4 aqueous NaClO4, and are initially independent of [NaClO4] due to extensive micellar incorporation (Tables 6 and 7), but they decrease toward that in water as the concentration of NaClO4 is increased. There are no large changes in the 23Na line width of NaClO4 or NaBr (Tables 5 and 7), and values of 23Na T1 of NaClO4 with 0.095 M SB3-14 are approximately the same as those with no sulfobetaines (Figure 6), indicating that Na+ is not tightly bound to the micelles despite their development of anionic character. Evidence for changes in the structure of the headgroups and possibly of the micelles18 of SB3-14 on addition of NaClO4 is given by an increase in the 14N line width (Tables 6 and 7). The increase is initially steep, probably because location of ClO4- close to the quaternary ammonium center affects the environment and symmetry of the nitrogen. There is then a more gradual increase in line width as the micelle becomes saturated with ClO4- due to a change in micellar structure consistent with the increase in Nagg at high [SB3-14] and high [NaClO4] (Table 3). We could not fit the 35Cl- line widths by applying eq 6, probably because of structural changes in the micelle, as indicated by changes in Nagg (Table 3) and the 14N line width (Tables 6 and 7), although the fit is reasonable for the kinetic and conductance data. However, the latter depend largely on (28) Cordes, E. H.; Gitler, C. Prog. Bioorg. Chem. 1973, 2, 1. (29) (a) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533. (b) Pearson, R. G. Acc. Chem. Res. 1993, 26, 250.
Anion Incorporation into Sulfobetaine Micelles
Langmuir, Vol. 14, No. 10, 1998 2669
the behavior of the free ions, and anionic reactivity in the interfacial region is probably insensitive to changes in the overall micellar structure.1-3 Ion Binding. Relation between Kinetic and Physical Evidence. The application of different methods to the solution of a common problem is desirable because each has its own approximations and assumptions and illustrates different facets of the problem. The kinetic approach to micelle-ion interactions is useful because, even without a theoretical treatment such as that based on eqs 2-7, the effect of sulfobetaine micelles on the rate of reaction of Br- and MeONs (ref 4 and Figures 1 and 2) shows that Br- is incorporated in the micelles, which strongly bind MeONs. At high [surfactant], values of kobs become approximately constant with all the sulfobetaines, demonstrating that reactant concentrations in the micellar pseudophase become approximately constant. The striking inhibition by dilute ClO4- (Figure 3 and Table 2) indicates that it binds much more strongly to sulfobetaine micelles than Br-, consistent with the NMR and conductivity evidence (Tables 5-8). However, aggregation numbers with dilute surfactant are not very sensitive to dilute NaClO4, or other salts (Table 3), even though the 14N NMR data indicate that dilute ClO4- is interacting with the cationic center (Tables 6 and 7). These observations are consistent with the sulfobetaine micelles having a relatively open interfacial region, which can tolerate added anions,5b,c provided that they are polarizable and not very hydrophilic; i.e., that they are “soft” within the Pearson classification.29 A change of headgroup in the sequence Me2N+, Et2N+, Pr2N+, Bu2N+ decreases ion binding, as with cationic micelles,10 but increases the local second-order rate constant (Table 8 and Figures 1 and 2). This result is as expected for an SN2 reaction of Br-, which in nonamphiphilic systems is accelerated by a decrease in polarity and water content of the medium.30 These observations accord with evidence on spontaneous decarboxylation, cyclization, and dephosphorylation in micelles.7,8,31 All these reactions are accelerated by nonpolar solvents in the sequence decarboxylation32 > dephosphorylation33 > cyclization.20
The agreement between values of K′Br in SB3-14 and SB4-14 used to fit conductometric and kinetic data (Tables 8 and 9) is reassuring and the values of K′I and K′ClO4 indicate the importance of specific anion-micelle interactions. The relatively high value of K′ClO4 is consistent with the strong displacement of Br- from the sulfobetaine micelles by ClO4- (Tables 6 and 7 and Figure 3). The affinity of I- for micelles of SB3-14 is intermediate between that of Br- and ClO4- (Table 8) and I- is a much more effective nucleophile than Br-; this order of reactivity, which is the same as in bulk water, indicates that reaction in the micellar pseudophase occurs in a relatively waterrich region.30 The reaction with I- cannot be examined in micelles of CTABr, for example, because the solutions are too viscous. Interionic competition for micelles can be treated by extending eq 6 to mixtures of dilute ions, e.g., for Br- + ClO4- (eqs 7a,b). With higher concentrations of these salts, eqs 7a and 7b significantly underpredict the inhibition by ClO4-, probably due to a perturbation of the micellar structure, especially by ClO4- and to “invasion” by concentrated ions.19 The fact that eqs 7a,b fit conductometric data and partially fail with kinetic data is understandable because changes in electrolytic conductance depend largely upon concentrations of free ions in the aqueous pseudophase and only to a lesser extent upon the charge, or other properties, of the micelle, whereas kinetics depend on both the properties of the micellewater interface and the local concentrations of micellarbound ions. The kinetic and physical evidence on interactions of ClO4- with sulfobetaine micelles is reinforced by measurement of diffusivities by dynamic light scattering, which shows that addition of NaClO4 to micelles of SB3-14 gives them anionic character leading to strong intermicellar repulsions.27
(30) (a) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 2nd ed.; VCH Verlagsgesellschaft mbH: Weinheim, 1988; p 132. (b) Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry. Part B: Reactions and Synthesis, 2nd ed.; Plenum Press: New York, 1985.
(31) (a) Bunton, C. A.; Fendler, E. J.; Sepulveda, L.; Yang, K.-U. J. Am. Chem. Soc. 1968, 90, 5512. (b) Bunton, C. A.; Minch, M. J.; Hidalgo, J.; Sepulveda, L. J. Am. Chem. Soc. 1973, 95, 3262. (32) Kemp, D. S.; Paul, K. G. J. Am. Chem. Soc. 1975, 97, 7305. (33) Kirby, A. J.; Varvoglis, A. G. J. Am. Chem. Soc. 1967, 89, 415.
Acknowledgment. Support of this work by CNR, Progetto Finalizzato Chimica Fine II, Rome, MURST, Rome, and the U.S. Army Office of Research is gratefully acknowledged. LA971106J
