High-resolution ellipsometry and neutron diffraction measurements have been used to investigate the structure, growth, and wetting behavior of fluid pentane (n-C(5)H(12)) films adsorbed on graphite substrates. We present isotherms of the thickness of pentane films adsorbed on the basal-plane surfaces of a pyrolytic graphite substrate as a function of the vapor pressure. These isotherms are measured ellipsometrically for temperatures between 130 and 190 K. We also describe neutron diffraction measurements in the temperature range 11-140 K on a deuterated pentane (n-C(5)D(12)) monolayer adsorbed on an exfoliated graphite substrate. Below a temperature of 99 K, the diffraction patterns are consistent with a rectangular centered structure. Above the pentane triple point at 143.5 K, the ellipsometric measurements indicate layer-by-layer adsorption of at least seven fluid pentane layers, each having the same optical thickness. Analysis of the neutron diffraction pattern of a pentane monolayer at a temperature of 130 K is consistent with small clusters having a rectangular-centered structure and an area per molecule of approximately 37 A(2) in coexistence with a fluid monolayer phase. Assuming values of the polarizability tensor from the literature and that the monolayer fluid has the same areal density as that inferred for the coexisting clusters, we calculate an optical thickness of the fluid pentane layers in reasonable agreement with that measured by ellipsometry. We discuss how these results support the previously proposed "footprint reduction" mechanism of alkane monolayer melting. In the hypercritical regime, we show that the layering behavior is consistent with the two-dimensional Ising model and determine the critical temperatures for layers n = 2-5.

Adsorption/desorption isotherms of acetone on highly oriented pyrolytic graphite have been measured by ellipsometry for temperatures above the bulk triple point. The behavior in the monolayer and submonolayer regime is conventional, with 2D gas-liquid and 2D liquid-solid coexistence regions. Further liquid monolayers grow on top of the completed monolayer. The growth is basically layer-by-layer. For temperatures between 190 K and the triple point a prewetting-type transition occurs with a thin-thick jump of the layer thickness on adsorption but a layer-wise removal of the film on desorption. In this temperature regime the first monolayer is solid and its molecules are oriented perpendicular to the substrate whereas the higher layers are orientationally disordered polar liquid.