Investigation of the Adsorption of Alkanes on Hexagonal Boron Nitride from Their Liquids and Binary Mixtures

Abstract

We report a differential scanning calorimetry investigation of the adsorption of normal alkanes on the surface of hexagonal boron nitride. The alkanes investigated (pentane–hexadecane) all show small transitions above the bulk melting point. On the basis of similar results on graphite, we assign these transitions as the melting of solid monolayers adsorbed at the interface between the boron nitride and bulk-like fluid layers. However, in this case, the induced stabilization of the monolayer is significantly less than that for the same alkanes on graphite. The alkane monolayers melt approximately 5–8% above the bulk melting point (compared with about 10–14% on graphite). In addition, we have performed an extensive investigation of the behavior of this transition for binary mixtures of alkanes. These show similar behavior to that seen on graphite, which suggests that the rules developed for those systems are also applicable for boron nitride. A regular solution model has been used to determine parameters related to the excess enthalpy of mixing and preferential adsorption. These suggest that, although the preferential adsorption is almost exactly the same on both h-BN and graphite, there is a small, but significant, difference in the miscibility of alkanes when adsorbed on the two substrates. We suggest that there may be a structural reason for this behavior that is related to the small change in the substrate cell parameters and surface potential.