Potentiometric studies on solubilisation in non-ionic micellar solutions. Part II. Solubilisation of benzoic acid and the hydroxybenzoic acids and location of the solubilisates

Abstract

Previous studies on the solubilisation of benzoic acid by aqueous solutions of the polyoxyethylene glycol monoalkyl ether (C_nH_{2n+ 1}·[OCH₂·CH₂]_m·OH) Cetomacrogol (n= 16, m= 24) have been extended to the hydroxybenzoic acids in the same system, and to benzoic acid in glycol monoethers of the ranges: n= 12, m= 9, 16, and 20; n= 16, m= 20, 24, and 60. The results all fit the form of the Langmuir isotherm, indicating that solubilisation involves binding to definite sites in the systems; this is interpreted in terms of a “core-and-capsule” model of the micelle, with the alkyl chains restricted to the core and the polyoxethylene chains to the capsule. The results are consistent with the core–capsule interface's being the location of the solubilisate in the cases of benzoic and salicylic acids, and with the polyoxyethylene chains in the capsule being this location in the cases of the meta- and para-hydroxy acids.