Solubilization sites in micellar sodium octylsulphate solutions by ultrasonic spectroscopy

Abstract

Relaxation frequencies f c and amplitudes A were measured ultrasonically over the 3.5–52.5 MHz range at 25 °C for the fast exchange of monomer between micelles and bulk solution in micellar sodium octylsulfate solutions with solubihzates present in small amounts. The additives were cyclohexane, pyridine-2-azo-p-dimethylaniline (PADA), benzene, toluene, and p-xylene. Only cyclohexane produced no changes in f c or A. The variations in the relaxational parameters suggested that cyclohexane was situated within the micellar core and PADA, primarily in the surface region of the micelle. Benzene and toluene were adsorbed in minute quantities into the micelle surface but, with additive concentrations beyond a few molecules per micelle, were found principally in the micellar core. Toluene, along with p-xylene, showed an initial dip in f c that may indicate that the first few molecules of these additives were incorporated into the palisade layer of the micelle.