Solubilisation of dyes by surfactant micelles. Part 2; Molecular interactions of azo dyes with cationic and zwitterionic surfactants

A UV–vis spectroscopic investigation has been made of the interactions of a specially synthesised series of o - and p -substituted, model arylazonaphthol dyes with the cationic and zwitterionic surfactants above and below their critical micelle concentrations at pH 10. Spectra of dyes incorporated in micelles of zwitterionic surfactant or cationic surfactant at pHs < 8 are similar to those found in nonionic micelles, i.e. dye substituents control its location similarly for all the micelle types. However, the common anion is selectively favoured in cationic micelle solutions at pH 10, due to electrostatic interactions within the micellar surface. Introduction of polar groups at either end of the molecule confines the dyes to the surface of either zwitterionic or cationic micelles and are characterised by atypical p K _A shifts. Electrostatic complexes between dyes and cationic or zwitterionic surfactants were formed in sub-micellar regions, those with cationic surfactant being sparingly soluble.