Calculation of interfacial tensions with gradient theory

In this research work, the gradient theory (GT) of inhomogeneous fluids was used to calculate interfacial tensions (IFTs). The correlations of the influence parameter are presented for pure hydrocarbons, which can improve the scaling behavior of pure fluids under near-critical conditions. The overall average absolute deviations (AADs) of the calculated IFTs from the GT model with the SRK, PR and PT equations of state (EOSʹs) for 86 non-polar and weakly polar pure substances are 2.34%, 2.10% and 2.29%, respectively. At low pressure, the lumping method proposed by Leibovici [Leibovici, C.F, 1993. A consistent procedure for the estimation of properties associated to lumped systems. Fluid Phase Equilibria, 87: 89–197] was used to lump a mixture into one pseudocomponent, and its IFTs were calculated by means of the method of pure fluids. On the basis of the SRK EOS, the overall AAD of mixtures was 3.26% at low pressure, and that of naphtha reformate cuts was 3.6%. In addition, the gradient theory was used to predict interfacial tensions for binary systems in the near-critical region. The results show excellent agreement between the predicted and experimental IFTs at high and moderate levels, while the agreement is reasonably accurate in the near-critical region as the models reveal classical scaling behavior. To predict low IFTs accurately (σ < 0.1 mN m−1), an equation of state with proper scaling behavior in the vicinity of the critical point is at least required.