Gradient theory modeling of surface tension for pure fluids and binary mixtures

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This work uses the gradient theory (GT) to represent the surface tension of pure fluids and binary mixtures. The only inputs to the theory are the Helmholtz free energy density of the homogeneous fluid and the influence parameter of the inhomogeneous fluid. The volume-translated Peng–Robinson (VTPR) and Soave Redlich–Kwong (VTSRK) equations of state were applied to determine the Helmholtz free energy density and the bulk properties. The influence parameters for pure non-polar and polar fluids were obtained from a correlation which was generalized as a function of the critical compressibility factor, the acentric factor, the reduced dipole moment and the reduced temperature. The only adjustable coefficient in the gradient theory was set to zero which made the theory predictive. The surface tension predicted by the gradient theory show good agreement with experimental data for pure fluids and binary mixtures.