Self-consistent-field description of n-alkanes in bulk and at the liquid-vapor interface

A self-consistent-field (SCF) lattice theory for chain molecules in inhomogeneous systems is used to investigate the physical and thermodynamic properties of n-alkane liquid-vapor (LV) interfaces. The bulk vapor and liquid phase, where the SCF theory reduces to the Flory-Huggins theory, are studied to obtain a reliable set of parameters. The model fluid is considered as a mixture of (chain) molecules and monomeric vacancies. Intermolecular interactions are described in terms of Flory-Huggins (FH) chi parameters. The most simple set of parameters is attained when each carbon atom of the linear alkane is considered as a segment with a volume of 0.027 nm(3) and the value of chi(AO) is taken inversely proportional to the temperature T in kelvin according to 580/T. On the basis of these two parameters, the theory predicts both n-alkane bulk properties such as vapor pressures, density, and heat of vaporization, and properties of the liquid-vapor interface quite reasonably. Volume fraction profiles of the LV interfacial region reveal that chain ends are the major constituent of the vapor side of the interface. For longer chains and lower temperatures the relative preference of the chain ends to protrude into the vapor phase is more pronounced. The scaling of the LV interfacial tension of n-alkanes with increasing molecular weight is correctly predicted. The calculated variation of the n-alkane LV interfacial tension (gamma) with temperature agrees quantitatively with experimental data. At temperatures close to 0 K, the theory predicts the occurrence of positive d gamma/dT values (surface freezing).