Interfacial Tensions for System of <i>n</i>-Heptane + Water with Quaternary Ammonium Surfactants and Additives of NaCl or C₂–C₄ Alcohols

Abstract

Interfacial tensions of the oil–water interface versus surfactant concentration for n-heptane + water system with each of six single chain quaternary ammonium surfactants, and six Gemini quaternary ammonium surfactants around their critical micelle concentrations (CMC) have been measured by using the spinning drop method. The influences of additives (NaCl, ethanol, n-propanol, and n-butanol) on the interfacial tensions have also been investigated. It is observed that all of the surfactants can significantly lower the interfacial tensions of the n-heptane + water system. The required concentrations to reach the lowest interfacial tensions for Gemini surfactants with C12 and C14 alkyl chains are less than 10 % of those values for the corresponding single chain surfactants. For the n-heptane + water + Gemini surfactant systems, the addition of NaCl can lower the required surfactant concentrations to obtain the lowest interfacial tensions; however, the addition of C2–C4 alcohols has no evident effects but leads to a slight decrease of the values of these interfacial tensions.