Examining surfactant micelles via acid-base indicators: Revisiting the pioneering Hartley–Roe 1940 study by molecular dynamics modeling

Abstract

Examining micellar pseudophases and related systems by using acid-base indicators is presently a very popular method. Normally, the acid-base couple of an appropriate indicator was expected to be located in a definite place, i.e., the microenvironment of both acidic and basic dye species is equal. This assumption allowed modeling the solvation properties of the pseudophase by water–organic mixed solvents or by some other media. This vision of the indicator locus should be checked using some independent approaches. In the present paper, the molecular dynamics modeling was used for such purpose. As indicator, an azo dye was used, which was first utilized for estimating electrical potential of micelles by Hartley and Roe in their pioneering study (1940). The modeling was processed in micelles of the same surfactants which were used by Hartley and Roe, namely cetylpyridinium bromide and cetyl sulfonate salt. The results support the idea of indicator location within the interfacial region (Stern layer). However, the orientation of the dye molecule and the hydration of the ionizing group, OH → O−, are quite different for the acidic (HA) and basic (A−) forms in both cationic and anionic micelles. This is a precaution against the simple concept of uniform location of the equilibrium forms of indicators. Hence, further molecular dynamics study of other indicators, often used nowadays for examining micelles and other molecular assemblies, seems to be pertinent and timely.