Surface Tension of Chain Molecules: Molecular Simulations and Theory

Abstract

Surface tension is an important property of liquids due to its fundamental influence on interfacial properties in materials and consequent applications to areas ranging from commercial industrial processes to enhanced oil recovery. The creation of transferable models for surface tension is especially important in the face of the numerous experimental challenges associated with measuring the surface tension of high-molecular-weight molecules. Fully flexible Lennard-Jones (LJ) chains (i.e., chains of beads with intra- and intermolecular forces represented by the standard Lennard- Jones potential) are a simple model for polymer chains and have been successfully applied to model coexistence properties and the surface tension of low-molecular-weight alkanes using parameters taken from equations of state, such as soft-SAFT. Additionally, Galliero (J. Chem. Phys. 2010, 133, 074705) and Blas et al. (J. Chem. Phys. 2012, 137, 024702) have developed a relation between reduced surface tension and the reduced difference in coexistence densities and demonstrated its universality for short LJ chains. It is unclear, though, if it holds for high-molecular-weight LJ chains. In this work, the vapor-liquid equilibrium of LJ chains ranging in length from 2 to 80 beads is simulated. Coexistence properties, critical properties, interfacial properties, and surface tension are calculated. The thermodynamic data is used to verify the proposed universal relation and demonstrate the limitations of LJ chains as a coarse-grain model for alkanes. Field-theoretical equations of state coupled with Cahn-Hilliard theory are applied to shed light on the underlying phenomena.