The Use of the n-(9-Anthroyloxy) Stearic Acids to Probe the Water Content of Sodium Dodecyl Sulfate, Dodecyltrimethylammonium Chloride, and Triton X-100 Micelles EURICO C. C. MELO, 1,2 SILVIA M. B. COSTA, ANT()NIO L. MA(~ANITA, 1 AND HELENA SANTOS 1 Centro de Qu[mica Estrutural, Complexo lnterdisciplinar, IST, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal Received January 29, 1990; accepted July 11, 1990 Quenching of the n-(9-anthroyloxy) stearic acids (n = 2, 6, 9, and 12) fluorescence by water is used to evaluate the water content in the vicinity of the anthroate chromophore in micelles of sodium dodecyl sulfate (SDS), dodecyltrimethylammonium chloride (DTAC), and Triton X-100. The mean locations of the probes in the three micelles are evaluated from the ring current effects of the anthracene ring on the ~H-NMR resonances of the surfactant protons. It is found that there is practically no water in the hydrocarbon core of the Triton X-100 and the DTAC micelles while in SDS micelles water seems to penetrate further down to the/3 carbon of the surfactant molecule. Water concentrations obtained from fluorescence decays are compared with those obtained from the response of the emission spectra of the same probes to changes in the solvent polarity. © 1991 Academic Press, Inc. 1. INTRODUCTION The esters of the 9-anthracene carboxylic acid have been widely used to probe the prop- erties of micelles (1, 2), vesicles (3, 4), and membranes ( 5, 6). Among these properties, the fluidity of mo- lecular organizates was investigated from measurements of the fluorescence depolariza- tion of the n-(9-anthroyloxy) stearic acids, n = 2, 6, 9, and 12, incorporated in CTAB (7) and Triton X-100 (8) micelles, in bilayers (5) or black lipid membranes (9). Fluidity was also investigated from intermolecular quenching of methyl 9-anthroate by amines in Triton X- 100 (10) and of the n- (9-an- throyloxy) stearic acids in CTAB, SDS, Triton X- 100, and phospholipid bilayers ( 1 1, 12). The polarity of the solubilization site of the 9-anthroate chromophore in micelles of SDS ~Present address: Centro de Tecnologia Qulmica e Biolrgica, Esta~o Agronrmica Nacional, Quinta do Marquis, 2780 Oeiras, Portugal. 2 To whom correspondence should be addressed. has been evaluated using the large sensitivity of its fluorescence spectrum to the dielectric properties of the solvent (1). While this ap- proach may give some insight into the prop- erties of the medium surrounding the probe, specific quantitative information on the amount of water in the probe vicinity is not easy to derive from polarity data. In fact, in order to distinguish between the relative con- tributions of the dipole moment, the polariz- ability, and the hydrogen bonding ability of the solvent, a multiparametric polarity scale should be used, e.g., the Kamlet et al. scale (13). But, to obtain quantitative results from this procedure one needs a set of chromo- phores to which the same location inside the micelle could be assigned, or else a set of lin- early independent properties for a single probe (14). Any of these prerequisites is hard to sat- isfy with the presently available probes, which makes the method impractical. Recently we have found that the fluores- cence lifetime of methyl 9-anthroate is strongly decreased in the presence of water or alcohols, 439 Journal of Colloid and lnlerfaceScience, Vol. 141, No. 2, February 1991 0021-9797/91 $3.00 Copyright © 1991 by Academic Press, Inc. All rights of reproduction in any form reserved.