Short chains at surfaces and interfaces:: A quantitative comparison between density-functional theories and Monte Carlo simulations

The surface and interfacial properties of a molecular liquid composed of short linear chains are investigated using molecular density-functional theories. The molecules are modeled as spherical sites connected by springs, and each site interacts with other sites and the surfaces with a modified Lennard-Jones interaction. In the density-functional theories, the ideal gas free energy functional is treated exactly (using a partial enumeration scheme) and the excess (over ideal gas) free energy functional is treated using a weighted density approximation (WDA). The latter requires the thermodynamic properties of the homogeneous fluid and a prescription for the weighting function. The thermodynamics of the homogeneous system is described via Wertheim's perturbation theory, and various approximations for the weighting function in the WDA are tested. We find that for the theory to be accurate, it is important to decompose the excess free energy function into a repulsive and an attractive part, with different approximations for the two parts. Results from several approximations are in good agreement with Monte Carlo simulations for the chain conformations, density oscillations (packing) in the vicinity of surfaces, and the surface tension, for both liquid-vapor interfaces and attractive surfaces. (C) 2003 American Institute of Physics.