Adsorption and micellisation of a surface-active dye in aqueous methanol solutions

Abstract

The surface tension of aqueous methanol solutions of a surface-active dye, p-t-octylphenol yellow amine poly(ethylene oxide), has been measured by means of the drop weight method. At a given methanol content, < 20%, the surface tension of methanol + water solutions of the dye exhibits two break points when plotted against the logarithm of the dye concentration, which indicates two-step micellisation of the dye molecules. The adsorption of the dye on aqueous surfaces is cooperative and multimolecular. An analysis of the surface tension data leads to the following results: the primary micelles are formed when the concentration is higher than ca. 10^–5 mol dm^–3, and their aggregation number, at most, 15, suggesting that the micellar structure consists of a stack of molecules; the secondary micelles are formed when the concentration exceeds ca. 10^–4 mol dm^–3, and their formation is induced by the hydrophobic mechanism, as for the common surfactant. In methanol + 0.1 mol dm^–3 HCl solutions the dye is protonated and is adsorbed monomolecularly on aqueous surfaces: the micellisation occurs as for the common surfactant.