Surface potential measurements in pentanol–sodium dodecyl sulphate micelles

Abstract

The surface potential of sodium dodecyl sulphate (SDS) micelles has been measured by the apparent pK_a of a micelle solubilized acid–base indicator. The indicator used was 4-heptadecyl-7-hydroxycoumarin. The surface potentials were measured under conditions of changing surface charge density and bulk ionic strength and the results analysed by a Gouy–Chapman–Stern model for the double layer of the micelle. The results indicate that the sodium counterions in the micelle's Stern layer bind between the sulphate head groups. When pentanol is solubilized by the micelle the experimental potentials are smaller than predicted. The difference is attributed to an increase in the number of sites for counterion binding at the micelle surface.