Test-area simulation method for the direct determination of the interfacial tension of systems with continuous or discontinuous potentials

Abstract

A novel test-area TA technique for the direct simulation of the interfacial tension of systems interacting through arbitrary intermolecular potentials is presented in this paper. The most commonly used method invokes the mechanical relation for the interfacial tension in terms of the tangential and normal components of the pressure tensor relative to the interface the relation of Kirkwood and Buff J. Chem. Phys. 17, 338 1949 . For particles interacting through discontinuous intermolecular potentials e.g., hard-core fluids this involves the determination of functions which are impractical to evaluate, particularly in the case of nonspherical molecules. By contrast we employ a thermodynamic route to determine the surface tension from a free-energy perturbation due to a test change in the surface area. There are important distinctions between our test-area approach and the computation of a free-energy difference of two or more systems with different interfacial areas the method of Bennett J. Comput. Phys. 22, 245 1976 , which can also be used to determine the surface tension. In order to demonstrate the adequacy of the method, the surface tension computed from test-area Monte Carlo TAMC simulations are compared with the data obtained with other techniques e.g., mechanical and free-energy differences for the vapor-liquid interface of Lennard-Jones and square-well fluids; the latter corresponds to a discontinuous potential which is difficult to treat with standard methods. Our thermodynamic test-area approach offers advantages over existing techniques of computational efficiency, ease of implementation, and generality. The TA method can easily be implemented within either Monte Carlo TAMC or molecular-dynamics TAMD algorithms for different types of interfaces vapor-liquid, liquid-liquid, fluid-solid, etc. of pure systems and mixtures consisting of complex polyatomic molecules.