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Some ultraviolet spectroscopic characteristics sensitive to the polarity of the medium have been used to determine the average polarities of the microenvironments of benzene, several of its derivatives, Triton X-100, and naphthalene when solubilized in micelles at low solubilizate to surfactant ratios. Concordant results are obtained using several parameters and, for benzene, a micelle-water distribution coefficient is obtained in good agreement with literature values. The polarity parameters show little dependence on the charge of the micelles. The microenvironment of benzene is polar and roughly comparable with that of Triton X-100, used as a solubilizate, for which the chromophore is located at the micelle-water interface. The polarity of the microenvironment decreases systematically on progressive alkyl substitution. A two-state model for the solubilized species involving a distribution between a nonpolar "dissolved state", associated with the hydrocarbon core, and a polar "adsorbed state", associated with the micelle-water interface, provides a satisfactory description. The adsorption can be ascribed to the surface activity of the aromatic species using an "oil-drop" model for the micelle. Although interfacial tension data indicate that benzene is only mildly surface active at hydrocarbon-water interfaces, the adsorption effects are greatly magnified by the extremely high surface to volume ratios of micelles. The interfacial tensions of alkyl benzenes against water are higher than that of benzene; the reduced adsorption expected is in agreement with the observed lowering of the microenvironmental polarity. The fractional adsorption is also expected to be lower at high mole fractions of the solubilizates in the micelles. Some apparently contradictory results in the literature are rationalized on this basis. The implications of the surface activity of solubilized species for biological membranes, for understanding the solubilizing capacity of micelles, as also for the use of surface active species as microenvironmental "probes" are discussed.

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