Solubilisation of dyes by surfactant micelles. Part 1: Molecular interactions of azo dyes with nonionic and anionic surfactants

An ultraviolet-visible spectroscopic investigation has been made of the interactions of a specially synthesised series of substituted, model arylazonaphthol dyes with nonionic and anionic surfactants. Changes in spectral features were recorded above the critical micelle concentrations, suggesting specific interactions of dyes with micelles of the respective surfactants. The affinity of the dye for the surfactant micelles increased when various p-substituent were incorporated in to the dyes. Similarly, there was a shift in azo-hydrazone tautomeric equilibria and an increase in measured dye pK(a) values. Models are proposed for the location of dyes in nonionic or anionic micelles. Unlike earlier studies, it is concluded that the solubilised dye experiences only one environment in nonionic micelles but the specific location, i.e. whether preferentially incorporated in the hydrophobic micellar interior or in the more hydrophilic, outer polyoxyethylene layer, depends upon the nature of the substituent.