Calculation of Interfacial Properties of Demixed Fluids Using Density Gradient Theory

Abstract

This paper deals with theoretical investigation of interfacial properties between two coexisting liquid or fluid phases in thermodynamical equilibrium. The Cahn-Hilliard gradient theory was combined with an activity coefficient model (Koningsveld-Kleintjens model) or with an equation 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 calculated for two systems (water + ethylene glycol isobutyl ether (EIB) and water + diethylene glycol diethyl ether (DDE)). Adjusting one parameter, the theory yields satisfactory estimates of interfacial tensions as a function of temperature. Applying an equation of state, two variables (density and concentration) change in the interface. In this case the theory gives the density profiles of both components. The calculated density profiles demonstrate the ability of the concept to predict 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 is able to describe the experiment if two parameters were fitted.

Generalized van der Waals Theory of Interfaces in Simple Fluid Mixtures

“…By using a similar analysis, i.e., an expansion in the spatial variation of the particle densities, but in conjunction with the Peng-Robinson equation of state, Carey and co-workers (32) obtained good agreement with experiment for mixtures of complex molecules including alkanes and alkyl alcohols. Later work by Cornelisse et al (33) on hydrocarbon mixtures with carbon dioxide has confirmed the efficiency of this approach, at least where the temperature is not too low, and applications to other complex fluids continue to appear (34). Lee et al (35,36) have applied both a mean-field theory, similar to our GvdW treatment except for the lack of nonlocal entropy, and molecular dynamics (MD) simulation to the same type of LJ models of binary mixtures to obtain density profiles and surface tension.…”

Phase- and interfacial behavior of hyperbranched polymer solutions

“…First the interfacial properties will be considered at atmospheric pressure. Keeping the pressure constant, the density gradient theory will be blended with an activity coefficient model [28]. The Cahn-Hilliard theory [29] describes the thermodynamic properties of a system where an interface exists between two equilibrium phases.…”

“…Additionally, the interfacial properties (interfacial tension and interfacial profiles) were studied theoretically using a special version [28] of the density gradient theory, introduced by Cahn and Hilliard [29]. The combination of the LCT or the LCT with the Wertheim lattice-perturbation theory allows the investigation of the influence of association forces on interfacial properties.…”

Using a New Proposed Influence Parameter of Gradient Theory for CH4/n-alkane Binary Systems: What Advances Can Be Achieved?

“…According to the previous investigations, the preferential adsorption in the vapor-liquid interface depends on the size and interactions of the molecules; therefore, the models combining density (or volume) with the surface tension are more successful than the models utilizing only surface tension. The gradient theory of fluid interfaces [3][4][5] is one of the prediction models combining density (or volume) with the surface tension. The gradient theory has two inputs containing the Helmholtz free energy density and the influence parameter for the homogeneous and the inhomogeneous fluids, respectively.…”