Non-classical interfacial tension and fluid phase behaviour

Abstract

Gradient theory, as originally introduced by van der Waals [1], was used to predict interfacial tensions at temperatures close to the critical region of the pure components nitrogen, carbon dioxide, methane, propane, octane, decane, tetradecane, ethanol, butanol, and water, and of the binary system carbon dioxide + butane. A modified van der Waals model was introduced using an expression that incorporates the proper temperature dependence for the influence parameter. The Helmholtz free energy density of the homogeneous fluid of pure components was modelled using the modified Peng-Robinson equation of state as suggested by Chou and Prausnitz [2]. This model improves the description of the behaviour of equilibrium densities near critical points up to a reduced temperature of 0.99, and yields for the critical exponent β a value of 0.34, which is close to the experimental value of 0.325. Along the coexistence curve β is defined by ( ϱ l− ϱ v)∼(T c−T) β . The interfacial tension behaviour could be described with deviations less then 3%, for reduced temperatures lower than 0.99. However, the calculation of interfacial tensions at reduced temperatures above 0.99, shows a critical scaling exponent μ equal to 1.38, while experiments give μ=1.26. The exponent μ is defined by γ∼(T c−T) μ .