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 at the interactions at a specially synthesised at o- and rho-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 at polar groups at either end of the molecule confines the dyes to the surface at either zwitterionic or Cationic micelles and are characterised by atypical pK(A) shifts. Electrostatic complexes between dyes and cationic or zwitterionic surfactants were formed in sub-micellar regions, those with cationic surfactant being sparingly soluble.