Pressure effect on solubilization of β-phenylethyl alcohol in aqueous solutions of cetyltrimethylammonium bromide

Abstract

The maximum additive concentrations (MAC) of β-phenylethyl alcohol (PEA) in aqueous solutions of cetyltrimethylammonium bromide (CTAB, constant concentration) were determined at 308.15°K as a function of pressure up to 210 MPa. The pressure dependence of the MAC gave a curve exhibiting a maximum around 100 MPa, irrespective of the CTAB concentrations investigated. By analyzing the obtained MAC—pressure relation in terms of the pseudophase model, we have inferred the following facts valid for the solubilized system in which the solubilizate is a slightly soluble liquid in water such as a hydrocarbon or alcohol. (1) The relation of the critical micelle concentration (CMC) versus pressure for the solubilized system at the MAC may exhibit a maximum around 100 MPa in a same way as is the case of the system of surfactant alone. (2) The MAC maximum against pressure reflects the fact that the solubility of the solubilizate in water exhibits a maximum around 100 MPa against pressure. (3) The solubility maximum may be attributed to the fact that the solubilizate in liquid state is more compressible than in the solute state. In addition, we have proposed the idea that iceberg formation (the hydrophobic hydration) increases the solubility of a hydrocarbon. This idea conflicts with the current view.