Evidence of bimolecular layer of ethanol on graphite at 190K - Experimental and simulation studies

Abstract

Adsorption mechanism of ethanol on graphite is comprehensively investigated with a combined experimental measurements and computer simulation. The unique feature of the interplay of clustering and molecular layering of ethanol is highlighted, and ethanol serves as an ideal adsorbate because it behaves as a species intermediate between simple gases and water. For simple gases such as argon and krypton, molecular layering is a dominant mechanism for temperatures above their bulk triple point temperatures, while for water the mechanism of clustering is the dominant one. The intermediate feature of ethanol lies in the ethyl group that interacts with graphite while the hydroxyl group induces intermolecular interactions which are stronger than the adsorbate-graphite interaction. It is this strong intermolecular interaction that leads to two-stage mechanism of monolayer formation, with discrete cluster formation and growth in the form of oligomers (dominantly in tetramers and pentamers), followed by the coalescence of growing clusters. The interplay of clustering and coalescence of growing clusters depends on temperature, and for low temperature at 190 K, a bilayer formation was observed with network of hydrogen bonding holding two adsorbate layers, exposing the ethyl groups to the gas phase. As the temperature is increased to 298 K, the hydroxyl groups are no longer limited to the interlayer region between the first and second adsorbate layers but have the freedom to expose themselves to the gas phase, which results in multilayering of the third and higher layers.