Effect of association of sulfonephthalein dyes with sodium dodecyl sulfate micelles on their acid–base equilibria

Abstract

The association of some sulfonephthalein indicator dyes with sodium dodecyl sulfate micelles has been investigated by visible absorption spectroscopy. The equilibrium constants of the associations have been determined and found to increase considerably with the increase in the hydrophobicity of the dyes. The effect of such association on the pK_a2 of the dyes at various concentrations of the surfactant has been studied for Bromothymol Blue and Thymol Blue. The pK_{a2, app} values of the dyes at various concentrations of the surfactant were predicted from the equilibrium constants and were also determined experimentally. An attempt has been made to account for the discrepancies observed between the experimental and the predicted values.