Alcohol and Thiol Adsorption on (Oxy)hydroxide and Carbon Surfaces: Molecular Dynamics Simulation and Desorption Experiments
JA Greathouse and DB Hart and ME Ochs, JOURNAL OF PHYSICAL CHEMISTRY C, 116, 26756-26764 (2012).
Classical molecular dynamics simulations were used to investigate the interaction of methyl, ethyl, and n-propyl alcohols and thiols with the hydroxylated basal surfaces of aluminum hydroxide and iron oxyhydroxide, as well as a model graphite surface. Comparisons were made to concurrently run desorption experiments investigating the interaction of methyl, ethyl, and n-propyl alcohols with aluminum hydroxide and activated carbon. The metal (oxy)hydroxide surfaces represent the basal surfaces of the neutral end-member layered double hydroxides gibbsite and lepidocrocite, respectively, while the graphite surface is a simplified model of the pore walls in activated carbon used in the chemisorption experiment. Adsorption enthalpies obtained from simulations at infinite adsorbate dilution show that adsorption is greatly favored on the hydroxylated surfaces compared to the graphite surface, with the ethyl molecules adsorbing most favorably. Heats of desorption calculated from chemisorption experiments show the same increased interaction strength for the alcohols on the aluminum hydroxide surface compared with activated carbon, with the most favorable interaction being ethanol with the aluminum hydroxide surface. In general, simulations show that alcohols adsorb more strongly than thiols on the hydroxylated surfaces, while the reverse is true on the graphite surface. The structure of adsorbed monolayers was obtained from simulations of a liquidlike layer above each surface. As expected, monolayer surface densities decreased with increasing molecule size. The hydroxylated surfaces were found to be amphoteric with respect to both alcohol and thiol adsorption, and primary adsorption sites facilitate hydrogen bonding between the adsorbate and several surface hydroxyl groups. Alcohols and thiols adsorb at much larger distances to the graphite surface, resulting in the smaller adsorption enthalpies.
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