Langmuir 1992,8, 452-455
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Incorporation of n-Alkanols in Reverse Micelles in the AOT/n-Heptane/Water System E. A. Lissi* and D.Engel Departamento de Quimica, Facultad de Ciencia, Universidad de Santiago de Chile, Casilla 307, Correo 2, Santiago, Chile Received July 25, 1991.I n Final Form: October 24, 1991 Partitioning of alkanols (from ethanol to n-decanol) between the bulk (n-heptane) solvent and the micellar pseudophase is obtained from their effect upon the fluorescenceintensity of indoleacetate anions incorporated into AOT reverse micelles and partially quenched by carbon tetrachloride as a function of the surfactant concentration (at fixed water/surfactant ratios). For all the alkanols considered, the extent of incorporation into the micelles decreases when the water/AOT ratio increases from 4 to 20. At a given water/AOT ratio, the incorporationof the n-alkanol increases when the length of the alkyl chain increases. However, this dependence is small, the partition constant changing by less than a factor of 6 between ethanol and n-decanol. These results are interpreted in terms of an interfacial localization of the polar head of the alkanols in the micellar interface, with the alkyl chains extended toward the bulk solvent. 1. Introduction
Increased solubility in the presence of supermolecular aggregates (solubilization) is a phenomenon of importance in a variety of scientific and technological areas, and there exist extensive data regarding the solubilization of small guest molecules in biologicalmembranes,' liposomes,2and normal micelle^.^ In these systems, the solubilization of the guest molecule is mainly determined by ita hydrophobicity and correlates with octanol/water partition c o e f f i ~ i e n t s .Solubilization ~~~ in reverse micelles has been considerably less investigated, and the only systematic investigation of solubilization in reversed micelles has been carried out by Leodidis and Hatton5v6 by measuring interfacial partition coefficients for a series of amino acids in AOT using the phase-transfer method. As a result of a comprehensive study employing amino acids of widely different characteristics, these researchers concluded that the free energy of transfer from water to the surfactant interfaces of AOT/isooctane and DTAC/ heptane/hexanol water/oil microemulsions correlates well with existing empirical hydrophobicity scales, indicating the importance of the hydrophobic effect as a driving force for interfacial solubilization. One of the main shortcomings of the phase-transfer method is that it can only be applied to water-saturated reverse micelles in equilibrium with an aqueous solution of high ionic strength. In the present work, we report fluorescence quenching data that allow an evaluation of the partition of a series of alkanols of widely different hydrophobicity between the micellar pseudophase and the surrounding solvent in the AOT/n-heptanelwater system over a wide range of water/AOT ratios (W). Steady-state fluorescence measurements have been widely employed to determine the distribution of both fluorescent compounds and fluorescent quenchers in m i ~ e l l e s ,reverse ~?~ micelles,8and vesicle^.^ Abuin and Lissi have described
a procedure that, in micelles'O and vesicles? allows the evaluation of partition constants of compounds that do not fluoresceor act as quenchen, provided that they modify a bimolecular quenching rate between a microphaseincorporated fluorophore and an added quencher. In the present work, we extend this approach to evaluate the degree of incorporation of n-alkanols to reverse micelles, employing as fluorophore ionic indole derivatives and carbon tetrachloride as quencher. These data, besides providing information regarding the contribution of hydrophobicity to solute association with reverse micelles, are considered to be relevant for the quantitative description of processes taking place in reverse micelles involving alkanols as reactants, such as their enzymic oxidation by micelle-incorporated enzymes." 2. Experimental Section Experimental conditions were as previously described.'*J3 AOT (Sigma, St. Louis, MO) was purified by the standard procedure. Indole derivatives (Sigma) were purified by recrystallization. n-Alkanols (AldrichChemical Co.) were ofthehighest purity available, and were used without further purification. Fluorescence quenching experimentswere carried out in a LS-5 Perkin-Elmer luminescence spectrometer. Water content of the micellar solutionswas varied by addition of triple-distilledwater. The pH was adjusted,in both the waterand probe-containingsolutions, by HC1 or NaOH addition. All measurements were carried out at 25 "C in air-equilibrated solutions at pH 2.7 (tryptamine as probe) or 10 (indoleacetate as probe).
(1)Gargas, M. L.; Burgess, R. J.;Voisard, D. E.; Cason, G. H.; Anderson,
3. Results and Discussion The photophysical properties of indolic compounds incorporated to AOT reverse micelles have been thoroughly investigated.12J3 In particular, comparison of the influence of the fluorophore characteristics (i.e., charge and size of the alkyl chain) and water content of the micellar pseudophase upon the quenching efficiency of watersoluble (acrylamide) and lipid-soluble (carbon tetrachlo-
M. E. Toxicol. Appl. Pharmacol. 1989, 98, 87. (2)Gobas, F. A. P. C.; Lahittete, J. M.; Garofalo, G.; Shiu, W. Y.; J. Pharm. Sci. 1988, 77, 265. (3)Sepulveda, L.; Lissi, E.; Quina, F. Adu. Colloid Interface Sci. 1986, 25, 1. (4)Treiner, C. J. Colloid Interface Sci. 1983, 93, 33. (5) Leodidis, E. B.; Hatton, T. A. J. Phys. Chem. 1990, 94, 6400. (6)Leodidis, E. B.; Hatton, T. A. J. Phys. Chem. 1990, 94, 6411. (7) Encinas, M. V.; Lissi, E. A. Chem. Phys. Lett. 1982, 91, 55. (8) Encinas, M. V.; Lissi, E. A. Chem. Phys. Lett. 1986, 132, 545.
(9)Abuin, E. B.; Lissi, E. A.; Aravena, D.; Zanocco, A,; Macuer, M. J. Colloid Interface Sci. 1988, 122, 201. (10)Abuin, E. B.; Lissi, E. A. J. Colloid Interface Sci. 1983, 95,198. (11)Martinek, K.; Klyachko, N. L.; Kabanov, A. V.; Khmelnitsky, Y. L.; Levashov, A. V. Biochim. Biophys. Acta 1989, 981, 161. (12)Lissi, E. A.; Ecinas, M. V.; Bertolotti, S. G.; Cosa, J. J.; Previtali, C. M. Photochem. Photobiol. 1990,51, 53. (13)Encinas, M. V.; Lissi, E. A.; Bertolotti, S. G.; Cosa, J. J.;Previtali, C. M. Photochem. Photobiol. 1990, 52, 981.
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0743-7463/92/2408-0452$03.00/0
0 1992 American Chemical Society
Incorporation of n-Alkanols in Reverse Micelles
Langmuir, Vol. 8, No. 2, 1992 453
1. I
. 25
15
0.1
CCLbImM)
Figure 1. Effect of n-alkanol (0.85%, v/v) addition upon the quenching of tryptamineby carbon tetrachloridein AOT reverse micelles: [AOT] = 0.1 M, W = 8;pH of the aqueous pseudophase 2.7; ( 0 )without alkanol; (a) n-octanol; (+) ethanol; (A)n-pentanol; (0)n-butanol.
Figure 3. Effect of n-alkanol (0.85%, v/v) addition upon the quenching of tryptamineby acrylamide in AOT reverse micelles. Acrylamide concentrations given correspond to thosein the added water. Conditions are as in Figure 1: (A)without alkanol; (0) ethanol; (0) n-odanol; (A)n-pentanol; (+) n-butanol.
100
100
300 CCLb(mM1
Figure 2. Effect of n-alkanol (0.85%, v/v) addition upon the quenching of indoleacetate anions by carbon tetrachloride in AOT reverse micelles: [AOT] = 0.1 M; W = 8;pH of the aqueous pseudophase 10; ( 0 )without alkanol; (A)n-octanol;(0) n-pentanol; (A)n-butanol; ( 0 )ethanol. ride) quenchers allows an estimation of the probe localization and the polarity of ita microenvironment. Indoleacetate (basic pH) and tryptamine (acidic pH) are compounds that are totally incorporated to the micellar pseudophase.13 Tryptamine cations remain, a t any W value, bound to the micellar interface by electrostatic interactions. On the other hand, indoleacetate anions could, particularly at large W ratios, be partitioned between the interface and the water p00ls.l~ Over a wide range of W values, the location of tryptamine is more exposed to the organic solvent than that of indoleacetate anions, and the desactivation experiments show that, even a t low W values, the excited indole moiety of the later compound is distributed among different mi~roenvironmenta.~~ From these considerations, it can be expected that any change in the interface characteristics will modify the interaction rate between these fluorophores and both water-soluble (acrylamide) and lipid-soluble (carbon tetrachloride) quenchers. An increase in the polarity of the interface will favor quenching by acrylamide and reduce the quenching rate by carbon tetrachloride, while opposite effects will be produced by a decrease in the interface polarity. Figures 1-4 show that n-alkanol addition to the micellar solutions modifiesthe quenching efficiency of both quenchers, pointing to significant changes in the probe microenvironment characteristics. The changes introduced by the alkanols are evidenced irrespective of the charge of the probe and the micelle wateicontent. For both-cationic and anionic fluorophores, n-alkanol addition increases the quenching efficiency Of and reduces the quenching efficiency of arylamide, pointing to a less
0.2 Acrylamide (MI
200 CCLblmM)
Figure 4. Effect of n-alkanol (0.85%,v/v) addition upon the quenching of indoleacetate anions by carbon tetrachloride in AOT reverse micelles: [AOT] = 0.1 M W = 20; pH of the aqueous pseudophase 10; ( 0 )without alkanol; (m) n-octanol;(A)ethanol; (0) n-butanol; ( 0 )n-pentanol. polar localization of the probes. Furthermore, the results given in Figures 1and 2 show that indoleacetate quenching is more affected than tryptamine quenching, a result compatible with the higher dependence of negatively charged probes to the micellar interface characteristic~.~~J3 The observed effect is due to changes in the rate of the probe/quencher (either acrylamide or carbon tetrachloride) interaction, since addition of n-alkanols in the absence of quenchers does not produce any noticeable change in the probe fluorescenceintensity. Furthermore, it is important to note that, in the absence of added alkanols, quenching by carbon tetrachloride is independent of the surfactant concentration (at fixed W values), making this compound suitable for ita use as quencher in the n-alkanol partition measurement~~J~ At low (
