Statistical thermodynamics of adsorption, micellisation and solubilisation in water–oil–surfactant systems
We applied a self-consistent field lattice theory for chain molecules in inhomogeneous systems to a model for water–oil–nonionic surfactant. Assuming planar symmetry, the interface between macroscopic aqueous and oil phases in the presence of surfactant is examined. Our calculations reproduce oil–water phase separation, an increasing adsorption of surfactant at the planar oil/water interface upon increasing surfactant concentration and an accompanying decrease in the interfacial tension. If no micelle formation is accounted for, the tension would eventually change sign and become negative. These results are complemented by calculations on micelle formation in aqueous phases with low concentrations of dissolved oil. Upon increasing oil concentration the solubilised amount of oil per micelle and the surfactant aggregation number increase and the critical micelle concentration (c.m.c.) decreases. We pay special attention to the point where the solubility boundary of oil and the micellisation boundary intersect since this point is indicative of the properties beyond the c.m.c. and beyond the solubility boundary. Accounting for micellisation in this way, the value of the interfacial tension remains positive. For all these phenomena the effect of the variation of the surfactant tail length is considered.