Interaction of aqueous sulfonephthalein dyes, viz., phenol red, cresol red and cresol purple with alkyltrimethylammonium and alkylpyridinium halide surfactants in submicellar concentration ranges of the surfactants has been studied by UV-VIS spectroscopy. The spectral changes of the dyes observed on addition of the surfactants at concentrations far below critical micelle concentration (c.m.c.) indicate the formation of doubly deprotonated dye anions, which otherwise are formed only at much higher pH in the absence of the surfactant. The equilibrium constants for the interactions were determined. The deprotonation has been found to increase considerably with increase in hydrophobicity of the surfactant. It has been suggested that combined electrostatic-hydrophobic interaction increases acidity of the sulfonephthalein dyes in their closely packed dyesurfactant ion-pairs, formed at concentrations of surfactant far below c.m.c., causing deprotonation of the dye.

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