Predicting the critical micelle concentration in a pH-mediated ternary-surfactant mixture using the regular solution approximation

The regular solution approximation was used to model the critical micelle c oncentration (CMC) of a mixed-surfactant system containing dimethylhexadecy l betaine (cetyl betaine) and dimethyltetradecylamine oxide (myristylamine oxide) in the pH range of 4-6. With a pK(a) of 4.95, myristylamine oxide ex ists in both the protonated and unprotonated forms while cetyl betaine with a pK(a) of 1.83 does not have a significant fraction of protonated form un der the experimental conditions. Therefore, the result is a ternary-surfact ant system with the relative amounts of the amine oxide forms dictated by t he pH. Individual- and binary-surfactant CMC values were determined by Wilh emy plate mel:hod and the regular solution approximation used to determine the binary beta values. When these beta values are used, the micellar mole fractions and the component micellar activities were obtained and a priori CMC predictions were made. Predictions agreed well with experimental CMC me asurements conducted at various mole fractions of the three components. The regular solution approximation provided accurate results, While data colle ction is initially time consuming, multiple-surfactant CMC values can be re adily predicted once:the binary beta values have been determined. (C) 1999 Academic Press.