Bulk and interfacial properties of polar and molecular fluids

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Abstract
Based on a suitable density-functional theory for inhomogeneous fluids we investigate both the liquid-vapor coexistence curve of the bulk phase diagram and interfacial structures of molecular fluids in general and Stockmayer fluids in particular. We derive a scheme which allows us to analyze one-component fluids composed of molecules with strong permanent dipole moments which are shown to affect significantly the bulk phase diagram. The critical temperature increases as a function of the strength of the dipole moments, whereas the critical density remains unchanged if the repulsive interaction of the molecules at short distances is modeled by a constant hard-core potential. A temperature-dependent diameter of the hard-core potential that mimics the actual soft repulsive potential is also considered. It results in an additional enhancement of the critical temperature and a slight increase of the critical density at larger dipole strengths and higher temperatures. Our analytic results turn out to be in good agreement with published Monte Carlo data. The density profile and the orientational ordering at interfaces are shown to be determined by a system of coupled integral equations that are applicable for dipole moments of arbitrary strength. Its corresponding numerical solution will be presented in a subsequent paper.