In supercritical fluid chromatography, molecules from the mobile phase adsorb on the stationary phase. Stationary-phase alkylsilane-terminated silica surfaces might adsorb molecules at the silica, among the silanes, on a silane layer, or in pore space between surfaces. Mobile phases of carbon dioxide, pure and modified with methanol, and stationary phases were simulated at the molecular scale. Classical atomistic force fields were used in Gibbs-ensemble hybrid Monte Carlo calculations. Excess adsorption of pure carbon dioxide mobile phase peaked at fluid densities of 0.002–0.003 Å−3. Mobile phase adsorption from 7% methanol in carbon dioxide peaked at lower fluid density. Methanol was preferentially adsorbed from the mixed fluid. Surface silanes prevented direct interaction of fluid-phase molecules with silica. Some adsorbed molecules mixed with tails of bonded silanes; some formed layers above the silanes. Much adsorption occurred by filling the space between surfaces in the stationary-phase model. The distribution in the stationary phase of methanol molecules from a modified fluid phase varied with pressure.
Bibliographical noteFunding Information:
Thanks to Donald P. Poe for helpful discussions. Computing resources for this work were provided by the Open Science Grid [project SFCphases, with help especially from Mats Rynge] which is supported by the National Science Foundation and the Department of Energy's Office of Science ; the Extreme Science and Engineering Development Environment [grant CHE140098 ]; the Minnesota Supercomputer Institute and the University of Minnesota Duluth .
© 2017 Elsevier B.V.
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- Carbon dioxide adsorption
- Excess adsorption
- Gibbs ensemble
- Methanol modifier
- Monte carlo simulation
- Supercritical fluid chromatography