5936
J . Phys. Chem. 1991, 95, 5936-5942
corresponds to D,O = 2.5 X lo-" m2 s-I, in excellent agreement with the 0,"value measured in the L, a t the same surfactant Concentration. From this we may conclude that D: is concentration-dependent. With increasing asin the cubic phase there is a strong decrease in D,,which within the ICR model is due mainly to a decrease in 0:. (The change in geometrical obstruction is only about 10% in this a, range.) The data point on the binary axis corresponds m2 s-l. The concentration dependence of D: to D: = 8 X becomes stronger as 9, increases. We note that similar strong concentration dependence of the surfactant diffusion constant was recently observed in the Vi phase of the water-dodecyltrimethylammonium chloride (DOTAC) system.61 In that system the surfactant diffusion constant decreases by a factor of 2 when the surfactant volume fraction increases from 0.84 to 0.88. In the L3 phase, D, is less concentration dependent. The fact that 02 is concentration dependent complicates a quantitative analysis by the introduction of additional parameters, and we refrain from a quantitative fit. However, some general remarks can be made. A general result of ref 36 is that the geometrical obstruction on the surfactant diffusion has only a weak dependence on as(cf. eq 6b). In Figure 3 we show the D, vs ascurve for the "P" family : = 4.2 X m2 s-l. The surfactant diffusion in the with 0 case of the "D" family was not calculated; however, the concentration dependence here is expected to be even weaker than for (61) Siiderman, 0.;Olsson, U.; Wong, T. C. J . Phys. Chem. 1989, 93, 7474.
the "P" family-this however is not yet firmly established. (b" appears to increase with increasing coordination number.) Hence, assuming a weak concentration dependence of the geometrical obstruction, the concentration dependence of D, in the L3.phase is to a large extent due to a concentration dependence in D:. 6. Conclusion We have demonstrated that self-diffusion data from the L3 phase in the AOT-water-NaCl system are inconsistent with a model based on a structure of discrete micellar aggregates. On the other hand, the data are consistent with a multiply connected bilayer structure. Data imply that the L3 phase microstructure consists of a multiply connected bilayer of low average coordination number (possibly 3 or 4). This is implied by the smooth variation of D, and D, when going from the L3 phase to the cubic phase (coordination number 3). Also, the concentration dependence D, in the L3 phase is consistent with a low average coordination number if the range and the magnitude of the perturbation of water molecules induced by the polampolar interface are similar in the cases of molecular reorientation and lateral diffusion. Furthermore, contrary to what has been claimed by Miller et al.,23we show that self-diffusion data indeed may discriminate between continuous and discontinuous bilayer structures.
Acknowledgment. We acknowledge David Anderson for stimulating discussions. This work was supported by the Swedish Natural Sciences Research Council (NFR). B.B. acknowledges a grant from the Swedish Institute. Registry No. AOT,577-1 1-7; NaCI, 7647-14-5; water, 7732-18-5.
Dipole Oriented Anion Binding and Exchange in Zwitterionic Micelles Mauricio S. Baptists and Mario J. Politi* Departamento de Bioqujmica e Laboratbrio Interdepartamental de Cinetica Rbpida, Instituto de Quimica, Universidade de Siio Paulo, Caixa Postal 20780, SiS.0 Paulo, S.P., CEP 01498, Brazil (Received: April 25, 1990: In Final Form: January 28, 1991)
The binding of 8-hydroxy-l,3,6-pyrenetrisulfonateanion (POH)to zwitterionic interfaces of 3-(N-hexadecyl-N,N-dimethy1ammonium)propanesulfonate (HPS),lysolecithin (lysPC) micelles, or lecithin sonicated vesicles was measured by steady-state fluorescence emission of POH. The addition of inert salts displaced bound FQH in the three systems, showing the electrostatic nature of the binding. IH NMR spectra, degree of polarization of fluorescence (P), and emission intensities due to the excited-state prototropic dissociation of POH together pointed to a dipolspotential-drivenanion binding and exchange in these formally neutral surfaces.
Introduction
The binding of ions to zwitterionic interfaces is well kn0wn.l The driving forces for ion binding, however, are not clear. Recent studies report surface enrichment of anions in sulfobetaine aqueous micelles and exchange with bulk aniom2 An accepted description of the driving force for binding in the case of betaines, and analogous monomer-forming aggregates, is the larger surface positive charge density. This distinct radial charge separation, although elcctroneutral, generates a dipole moment that, in turn, can attract charged species. This reasoning, in the case of aqueous (1) (a) Hauser, H.;Hinchley, C. C.; Krebs, J.; Levinc, B. A,; Philip, M. C.; Williams, R. J. P. Blochem. Biophys. Acra 1977, 468, 364. (b) Chrzeszczyk, A.; Wishia, A. Blochem. Biophys. A d a 1981, 618,28. (2) (a) Bunton, C.A.; Mhala, M. M.;Moffatt, J. R. J. Phys. Chem. 1989, 93, 854. (b) /bid. 1987, 52, 3836. (c) Pillersdrof, A.; Katzhcndler, J. Isr. J . Chem. 1979, 18. 330. (d) Pottel, R.; Kaatzc, U.; Muller, St. Eer. Eunsen-Ges. Phys. Chem. 1978, 82, 1086. (e) Clunie, J. S.; Corbill, J. M.; Goodman, J. F.; Ogden, C. P. Trans. Faraduy Soc. 1%7, 63, 505. (0 Brochsztain, S.;Berci, P. F.; Toscano, V. G.; Chaimovich, H.; Politi. M.J. J . Phys. Chcm. 1990, 94,6781.
0022-3654191 12095-5936$02.50/0
micelles, presumes a radial distribution of the monomers in a spherical aggregate. Thus in the case of betaine headgroups the positive charge should remain closer to the micelle core. Support for this rationale can be found in the work of Chevallier on phosphobetaine micelles, which pictures the individual dipoles pointing out radially in a spherical micelle, at least for monomers having from one to four methylene spacers between the charged groupsS3 In the present work we investigated the binding and exchange of a triply negatively charged probe in sulfobetaine and lysolecithin micelles and in lecithin vesicles. The extent of probe binding could be monitored due to the decreased excited-state acid dissociation rate constant upon probe binding. Probe binding to the sulfobetaine micelle interface clearly demonstrated a dipolar electrostatic nature of the binding. Furthermore, the binding of the probe to lysolecithin (lysPC) micelles will be discussed in terms of either a specific binding of the probe or a dipole configuration inversion (3) Chevalier, Y . ;Germanauk, L.; Perchec, P. Colloid Polym. Sci. 1988, 266, 44 1.
1991 American Chemical Society
Dipole Oriented Anion Binding in Zwitterionic Micelles
The Journal of Physical Chemistry, Vol. 95, No. 15, 1991 5937
TABLE I: Spectral and Emisaive Propertiea of POH in A q u "
SCHEME I. Excited-State Prototropic Proteas + (H20)"
POH*
kL
Solution" PO-*
1
+ (H7H20)")
kk,
k'n+ k',,,
POH
-
PO-
+
UV-visb fluorescene abs max, c, em max, nm M-I cm-l nm q+ q,ns s-I POH 403 19900 445 -1 5.2 1.OX 1O'O PO455 21600 510 -0.1 6.0
(H+(H*O),)
at the IysPC micelle surface. As earlier suggested by Clement and Could,' we have demonstrated that 8-hydroxy-l,3,6-pyrenetrisulfonate (POH) binds to the internal surface of lecithin vesicles by the same type of mechanism.
Materials and Metbods The surfactants employed were synthesized according to known 3-(N-Hexadecyl-N,N-dimethylammonium)procedures. propanesulfonate (HPS)was triply recrystallized from acetone/methanol (90/ 10, v/v) and shown to be pure by elemental analysis and IH NMR mea~urements.~Lysolecithin (IysPC) was obtained from pure egg lecithin (PC) (prepared in this laboratory) by treatment with phospholipase The prototropic probe 8-hydroxy-l,3,6-pyrenetrisulfonate(POH) (trisodium salt, Eastman Kodak) was triply recrystallized from acetone/H20 (%/lo, v/v) and showed no impurities on TLC plates8 Sephadex G-50 (Pharmacia) was used for vesicle preparation (see below). Water doubly distilled in glass was used throughout. All other reagents were of analytical grade. Absorbance and fluorescence spectra were obtained by using Beckman DU-7and Perkin-Elmer LS-5instruments, respectively. For the polarization data a pair of polarizers (Perkin-Elmer) were used. 'H and I3C NMR spectra were obtained by using a Bruker AC 200 operating at 200.18 and 50.32 MHz, respectively. pH readings were obtained from an Incibras (Bras) instrument using a combined glass electrode. Lecithin vesicles were prepared by soni~ation.~Typically, phosphate buffer (pH = 6.20, p = 10 mM), sodium chloride (appropriate aliquot for the desired ionic strength ( p ) ) , and POH (adequate aliquot from stock solution for a final entrapped concentration of N 2.0 X IOd M) were added to a lipid film in a test tube. The mixture was vortexed and ultrasonified for 10 min (Sonifier Cell Disrupter, Model W1850,nominal output 100 W). Large particles and residual titanium were removed by ultracentrifugation (L3-50 Beckman, 50 Ti rotor). Nonencapsulated POH was separated by gel exclusion chromatography (Sephadex G-50).
Results Aromatic alcohols, lite POH, having an excited-state dissociation rate constant (kOff)higher than, or of the order of, the excited-state lifetime, can undergo an H+dissociation reaction (process depicted schematically in Scheme I). For a cogent review, see ref 10. The fluorescence and nonradiative decay rate constants ?f the p:otonated and conjugated base pair respectively are, kn, kn, k,,, k,,, and hu stands for the light excitation. The process represented by Scheme I is purposely simplified in order to present only the features relevant to this work; that is, the prototropic reaction scheme was designed to hold the solution pH between two limiting values: pK:+I
