Thermodynamic properties and structure of the liquid-vapor interface: A neoclassical Redlich-Kwong model

It is well known that the classical mean field theory of van der Waals for liquid-vapor interfacial region properties deviates from real fluid behavior in several important ways. In particular, the variations of the surface tension and interfacial region thickness with temperature near the critical point are not consistent with those for real fluids. This paper presents a modified version of the classical mean field model that incorporates Redlich-Kwong fluid properties. It is shown here that this neoclassical Redlich-Kwong fluid model predicts property variations with temperature that agree better with measured data for real fluids. Predictions of the critical exponents associated with the temperature variation of surface tension and interfacial region thickness are developed from the Redlich-Kwong model. This new model predicts that surface tension varies about proportional to (1-T/T-c)(1.33) and that the interfacial region thickness varies about proportional to (1-T/T-c)(-0.67), with T and T-c being the system temperature and critical temperature, respectively. Compared to the critical exponents predicted by classical van der Waals theory, the Redlich-Kwong model predictions are found to be in better agreement with values inferred from experimental measurements. Predictions of the Redlich-Kwong model regarding interfacial region structure and property fluctuations are also examined. (C) 2003 American Institute of Physics.