A comparative study of confined fluid films composed of three different alkanes has been carried out using molecular dynamic simulation techniques. The films were confined in thin slit pores, only a few molecular diameters thick, and the substances studied were n-butane, n-decane, and 5-butyl-nonane. The properties of the film were obtained in equilibrium conditions and under shear. All the studied films show a strong layering of the distribution of methylene subunits. Chains at the solid boundaries align with the walls and show a tendency to stretch. The diffusion parallel to the solid walls is found to be higher in the proximity of the walls than in the inner part of the pore. The molecular motion normal to the confining walls can be described as noncorrelated molecular transitions between the contact layer and the inner part of the pore. Shear flow was induced in the film by moving the solid walls. The resulting velocity profiles across the pore were computed as well as the viscosity of the films. The viscosities of the confined fluids in the three cases appear to be the same as those of the bulk, within the uncertainty of the results. No significant influence of the shear flow on the inter- or intramolecular was found.

Recently, Barker and Henderson have introduced a semimacroscopic approximation to the second order in the expansion of the configuration integral by considering the pair potential as the sum of a strong (repulsive) part and a weaker (long-range) part. We analyze this approximation and show that the essential part of it is to reduce the higher-order distribution functions to a second-order nonuniform distribution function, the nonuniformity coming from fixing a particle at the origin. The approximation can be done to all orders, and the series can be summed to give an expression for the free energy and pressure. The expression involves the grand partition function for the nonuniform system taken at the chemical potential for the reference system minus the perturbing potential. The functions of the nonuniform system are approximated by taking the functional form they have at uniformity and by using the product of the density and the radial distribution function of the reference system for the nonuniform density. The summed series can then be computed. For the three-dimensional square-well fluid it is confirmed that the convergence is very rapid down to reduced temperatures about 0.5. Comparison with the one-dimensional square-well fluid shows that below this temperature the convergence is slow and it is necessary to use the whole seri~

Articles you may be interested inFluid properties from equations of state compared with direct molecular simulations for the Lennard-Jones system AIP Conf. Proc. 1501, 954 (2012); 10.1063/1.4769645 Excluded volume in the generic van der Waals equation of state and the self-diffusion coefficient of the Lennard-Jones fluidThe influence of higher-order terms in the Barker-Henderson approximation to the Zwanzig perturbation expansion is examined for a Lennard-Jones fluid. We find that the error introduced by ignoring these higher-order terms is negligible compared with the error due to the replacement of the reference system by a hard-sphere system in which the diameter of the spheres ensures coincidence to only first order between the free energy of the reference system and the free energy of the hard-sphere system. In calculating the contribution to the pressure due to the perturbation, one often uses the Percus-Yevick solution for the hard-sphere reference system. The error introduced by this approximation is found to be considerable.