Characterization of CF4/CF3Br Binary Mixture Adsorption on Hydrophobic/Hydrophilic Surfaces via Atomistic MD Simulation

GM Leuty and M Tsige, JOURNAL OF PHYSICAL CHEMISTRY B, 115, 12694-12708 (2011).

DOI: 10.1021/jp2057139

Molecular dynamics simulations of multilayer adsorption of binary mixtures of two tetrasubstituted halomethanes (CF4 and CF3Br) on two very different substrates (graphite vs hydroxylated SiO2) were performed for three different bulk compositions (40%, 50%, and 60% CF4) and over a range of temperatures from 80 to 200 K. The goal of these simulations was to investigate in depth how these factors affect film structure, layer composition, lateral arrangement, and molecular orientation in the first adsorbed layer on each substrate. In line with a previous study of single-component adsorption on these surfaces, mixtures adsorbed on the hydroxylated SiO2 surface show stable number density profiles that are largely independent of temperature, up to 160 K. This level of stability is essentially absent in the case of adsorption on graphite, which show densities and surface populations that are largely dependent on overall film composition, molecular orientation, and adsorbate substrate interactions, in addition to system temperature. Further, the composition of the first adsorbed layer at each solid surface appears to be influenced by the choice of substrate, with CF3Br the majority component at the graphite surface for all compositions and temperatures, while the first adsorbed layer on hydroxylated SiO2 more clearly mirrors the overall film composition at temperatures below 160 K.

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