CALCULATION OF INTERFACIAL PROPERTIES OF DEMIXED FLUIDS USING DENSITYGRADIENT THEORY

This paper deals with theoretical investigation of interfacial propert ies between two coexisting liquid or fluid phases in thermodynamical e quilibrium. The Cahn-Hilliard gradient theory was combined with an act ivity coefficient model (Koningsveld-Kleintjens model) or with an equa tion of state (Sanchez-Lacombe lattice fluid model). Using an activity coefficient model, only one variable (concentration) changed passing the interface. The interfacial tension between demixed liquids was cal culated for two systems (water + ethylene glycol isobutyl ether (EIB) and water + diethylene glycol diethyl ether(DDE)). Adjusting one param eter, the theory yields satisfactory estimates of interfacial tensions as a function of temperature. Applying an equation of state, two vari ables (density and concentration) change in the interface. In this cas e the theory gives the density profiles of both components. The calcul ated density profiles demonstrate the ability of the concept to predic t preferential adsorption phenomena in binary systems. The theoretical approach was applied to three systems (aniline + cyclohexane, acetone + carbon disulfide, and stearic acid + propane). If the systems show a closed miscibility gap, the interfacial tension passes a maximum. A comparison of theoretical and experimental interfacial tensions of the system aniline + cyclohexane indicates that the theoretical concept i s able to describe the experiment if two parameters were fitted.