Prediction of the effect of branching of molecular chain on the structure of interface and interfacial tension in systems containing water, alkane and nonionic surfactant

In this work, we extend the multilayer quasichemical model (MQuM) of a nonuniform fluid mixture that contains linear chainlike molecules and associated species to mixtures that may also contain branched chainlike molecules. The extended model is tested by predicting the interfacial tension of the planar interfaces between the equilibrium aqueous and hydrocarbon phases for a series of mixtures of water and normal alkanes with 6-12 carbon atoms or isomeric alkanes with 6-8 carbon atoms. The MQuM interaction parameters for the functional groups have been taken from the previous work on modeling vapor-liquid equilibria in water + <i>n</i>-alkane mixtures. MQuM predicts correctly the experimental trends (the calculated interfacial tension increases with the carbon number of <i>n</i>-alkane, branching of alkane chain leads to a decrease of interfacial tension) and serves to interpret these trends in terms of the details of the local structure of the interface between the phases. The effect of branching of surfactant tail on the interfacial properties is demonstrated using a model mixture water + surfactant (7-mer) + oil (3-mer).