Structure of confined alkane liquids

Alkane and polymer fluids near surfaces and in confined geometries arepresently of intense interest but poorly understood.‘” Direct experimental probing of confined liquid structure is difficult and indirect due to the small amount of material present. Computer simulations on coarse-grained’ and atomistic’ model chain molecule fluids have provided considerable insight, but are expensive and limited with regards to accessible time scales, molecular size, and abiity to compute the small solvation forces characteristic of .the confinement distances of common experimental interest. The goal of this communication is to present, to the best of our knowledge, the first theoretical study of a chemically realistic model of n-alkane and polymer liquids confined between parallel plates of variable adsorptive strength. Comparison is made to experiments using both high and low energy surfaces. ,The basis of calculation is the “wall-PRISM” integral equation theory of Yethiraj and Hall6 for chain molecule fluids coniined between flat walls or “slit pores.” The reader is referred elsewhere for the technical aspects.6*7 Briefly, the approach entails applying the bulk polymer RISM (reference interaction sites model*) theory ,of Curro. and Schweizerg in .conjunction with the growing adsorbent procedure and its extension to slit pores by Zhou and Stell.” Comparisons of the predictions of this theory with simulations of fluids composed of short tangent hard core chains have shown that wall-PRISM is quite accurate.6 We adopt a hard core interaction between the united atom carbon sites (diameter d) of the CNHm+z chains and the site-site Percus-Yevick closure approximation for the bulk PRISM integral equation.“’ There are two noteworthy technical aspects of this work. (1) A chemically realistic description of hydrocarbon molecules, the rotational isomeric state (RIS) model,” is employed.‘! This enters the theory via the single chain intramolecular structure factor in the bulk ,liquid.6.g We employ the accurate method of McCoy et al. l3 to calculate this function for n-alkanes using standard RIS parameters. (2) A pair decomposable interaction potential, u,(z), between the alkane carbon sites and the flat surface is introduced via the.closure approximation for the fluidwall direct correlation function, C,(z), where z denotes the perpendicular distance of a carbon site from a wall. The site-wall Percus-Yevick closure approximation is employed: C,(z)=[l-exp(&,(z))] g,(z) for z inside the pore, where g,(z) is the carbon site-wall pair correlation function, temperature T = l/( k/3), and k is Boltzmann’s constant. For a slit pore z=O or H corresponds to the position of a sites’s center when at its closest distance of d/2 from a wall. The wall-fluid interaction potential consists of a hard core part which defines a total slit width, W=H+d, plus a “9-3 tail” in the pore. The latter potential is characterized by a jinite spatial range, pd, and a strength parameter E which defines the potential upon contact of a carbon site with the wall: