Heat-capacity study of butane on graphite

Abstract

Heat-capacity measurements were performed on butane films adsorbed on graphite for coverages between nϭ0.50 and the equivalent to nϭ6.0 layers. The evolution of the melting of the first layer was studied as a function of coverage. Below nϭ0.83 melting occurs at a triple line at 112.6 K. This transition moves to higher temperature as the coverage is increased; above nϭ1.20 the melting of the first layer produces a sharp heat-capacity peak near 134 K. Beginning at nϭ2.34 the height of this sharp peak starts to decrease with increasing coverage; the peak disappears near nϭ3.0. The decrease and disappearance of the sharp first-layer melting peak coincide with a compression of the first layer into full commensuration observed in a recent neutron-scattering study. A broad heat-capacity peak at 127 K appears in the data starting near nϭ2.65. This broad peak is identified as resulting from two phase transitions occurring at close-by temperatures: melting of the fully commensurate first layer, and a transition involving the bulk. The heat-capacity peak from the melting of the first layer remains broad for all coverages above nϭ2.65; this melting transition is continuous. The results for butane and those for other systems are compared to the Janke and Kleinert theory of twodimensional melting. We suggest that the extended monolayer regime is the region where this theory is applicable. Evidence of the presence of a second solid layer is presented. Second-layer melting occurs at 114 K. No heat-capacity peaks corresponding to any additional layers beyond the second are found, indicating that solid butane incompletely wets graphite. The heat-capacity results are extensively compared, and found to be in very good agreement with results from a recent neutron-scattering study.

Quasielastic neutron scattering and molecular dynamics simulation studies of the melting transition in butane and hexane monolayers adsorbed on graphite

“…These results are consistent with the abrupt, first order melting transition observed at monolayer coverage in both neutron diffraction 4,6,7 and specific heat experiments. 29 Turning to the hexane monolayer, the analysis of the neutron quasielastic spectra below T m is in qualitative agreement with the simulation, but differs in quantitative details. Generally, the amount of molecular motion inferred from the experiment is less than that predicted by the simulation.…”

Heat Capacity Study of Monolayer Propane on Graphite

“…The submonolayer melting temperature for propane is from this work and from ref . For butane, we used ref , and for hexane we use the melting temperature from ref . The submonolayer melting temperature zigzags between odd and even chains in a manner analog to that for bulk. …”

“…Hydrocarbon films provide sharp contrasts in 2D melting behavior: ethane and ethylene , have broad and weak heat capacity peaks at melting, both are identified as melting continuously; methane has some of the strongest first-order melting transitions observed in any physisorbed system; acetylene has first-order melting; butane has first-order melting at submonolayer coverages but melts continuously in the extended monolayer regime …”

High-Resolution Volumetric Adsorption and Molecular Dynamics of then-Alkanes on the Surface of Hexagonal Boron Nitride

“…Extensive literature exists on atomic and molecular adsorption on the surface of graphite. A subset of this work has focused on the interaction of alkanes on the basal plane of graphite using heat capacity and diffraction measurements. − In contrast, studies of adsorption on hBN are less substantial. Nearly four decades ago, Bockel et al published a limited set of adsorption isotherms of methane on hBN between 77 and 90 K. On the basis of these measurements, they proposed a 2D critical temperature of ∼77 K. Several years later, Tessier and Larher extended the adsorption studies of Bockel et al to lower temperatures and to include deuterated-methane on hBN.…”