Pressure effect on solubilization by surfactants: II. Influence of laplace pressure upon incorporation of octane or decane into micelles of octaoxyethylene dodecyl ethers

Abstract

In order to determine the pressure dependence of the maximum additive concentration (MAC) of octane or decane in the aqueous solution of octaoxyethylene dodecyl ether (C12E8, 1% w/w), the pressure dependence of the cloud points and that of the temperatures of the solubilization limit for the solutions containing various concentrations of the alkane were determined by the change in transmittance of the solution. The MAC monotonously decreased with increasing pressure up to 300 MPa at a constant temperature below the cloud point. This relation is different from that obtained for the solubilized systems in which β-phenylethyl alcohols are solubilized in aqueous solutions of cetyltrimethylammonium bromide: the MAC versus pressure relation with a maximum near 100 MPa [Part I, J. Colloid Interface Sci.94, 348 (1983)]. The difference may be associated with the differences of the incorporation sites of solubilizates in micelles and of their solubility in the bulk (water); i.e., alkanes are incorporated into the micelle core and dissolve very little in water, while β-phenylethyl alcohols penetrate into the palisade layer close to the micelle surface and dissolve slightly in water. The pressure dependence of the MAC was analyzed by means of the pseudophase model for micelles. The conventional pseudophase model in which the Laplace pressure inside a micelle is not allowed failed to explain the MAC-pressure relation; however, the corpuscular pseudophase model, taking into account the Laplace pressure, could quantitatively explain the relation well. It can therefore be concluded that the Laplace pressure obviously contributes to the micellar solubilization of the alkanes.