SIMPLE THEORETICAL FRAMEWORK FOR A SYSTEMATIC MOLECULAR-DYNAMICS STUDY OF THE INTERFACIAL PROPERTIES OF OIL WATER SYSTEMS IN THE PRESENCE OF SURFACTANT MOLECULES/

This paper presents a simple theoretical methodology for a systematic molecular dynamics study of the interfacial properties of oil/water/su rfactant systems. This formalism is based on the appropriate definitio n and classification of the different energy terms which contribute to the free energy of the ternary system, and thus, to the chemical pote ntials of the surfactants in oil, water, and at the oil/water interfac e. Using molecular dynamics techniques over interfacial and bulk model s atomistically described, series expansions fittings of those energy terms as a function of surfactant concentration can be obtained, and a simple thermodynamic expression for the free energy of the whole syst em results. This report is solely related with the development of that free-energy expression along with analytical formulae for the surface excess, the interfacial tension, and the chemical potentials of a sur factant in oil, water, and at the oil/water interface. The application of this theoretical approach to the study of the interfacial properti es produced by chemically different surfactants, and a systematic stud y of the effect of the formulation variables on the interfacial behavi our of an oil/water system, will be forwarded in future reports.