Surface properties of alkylpyridinium chlorides and the applicability of the pendant drop technique

Measurement of interfacial tension isotherms and critical micelle concentra tions (CMCs) of surfactant solutions is of major concern in surface science . In this study we investigate the group of cationic alkylpyridinium surfac tants. Despite widespread applications, their physicochemical properties (e specially tetra- and octadecylpyridinium chloride) are not so well characte rized. The surface tension isotherms of 12 different surfactant/water/KCl s ystems are systematically determined by means of a new laser tensiometer (L ASDA) with the pendant drop technique. The surfactants are purified using t he Lunkenheimer method, which is capable of providing surfactants of high p urity. From surface tension isotherms, values for excess concentrations, cr oss-sectional areas, and CMCs are obtained. The long-chain surfactants, esp ecially octadecylpyridinium chloride, show some strange behavior, which is probably due to slow transport processes. We discuss in some detail diffusi on limitations of the pendant drop technique that can drastically increase the time until equilibrium surface tension is reached, especially in the ca se of highly surface-active substances. (C) 1999 Academic Press.