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

Abstract

High-resolution, volumetric adsorption isotherm measurements of methane through octane on hexagonal boron nitride (hBN) were performed in the vicinity of their bulk triple-point temperatures. It is observed that the adsorption behavior can be classified into three regimes depending on the molecular geometry and chain length; i.e., quasispherical, cigarlike, and rodlike. The film growth is observed to change from six discrete steps, i.e., layer-by-layer like, to two discrete steps, i.e., incomplete wetting as the molecular length increases. The heat of adsorption and differential entropy and enthalpy were calculated from these isotherm data for monolayer, bilayer, and where possible, thicker films. It is found that the monolayer heat of adsorption exhibits a smooth increase with increasing chain length. Molecular dynamics simulations were performed to provide complementary microscopic insight into the adsorption properties of these alkanes on hBN. Concentration profiles (C(z)) normal to the hBN surface plane calculated using these modeling studies reveal that with increasing chain length, there is less stratification of individual layers. The simulations also clearly show that at the highest temperatures, interlayer traffic increases. The resulting fluidlike upper layers are found to stabilize the solid layer closest to the surface such that it melts near or above the bulk triple point.

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