Surface Properties of Alkylpyridinium Chlorides and the Applicability of the Pendant Drop Technique

Abstract

Measurement of interfacial tension isotherms and critical micelle concentrations (CMCs) of surfactant solutions is of major concern in surface science. In this study we investigate the group of cationic alkylpyridinium surfactants. Despite widespread applications, their physicochemical properties (especially tetra- and octadecylpyridinium chloride) are not so well characterized. The surface tension isotherms of 12 different surfactant/water/KCl systems are systematically determined by means of a new laser tensiometer (LASDA) with the pendant drop technique. The surfactants are purified using the Lunkenheimer method, which is capable of providing surfactants of high purity. From surface tension isotherms, values for excess concentrations, cross-sectional areas, and CMCs are obtained. The long-chain surfactants, especially octadecylpyridinium chloride, show some strange behavior, which is probably due to slow transport processes. We discuss in some detail diffusion limitations of the pendant drop technique that can drastically increase the time until equilibrium surface tension is reached, especially in the case of highly surface-active substances.