(524e) Exploring the Retention Mechanism of Aromatic Compounds in Supercritical Fluid Chromatography By Molecular Simulations

In this presentation, Gibbs ensemble Monte Carlo simulations and experimental measurements were carried out to provide molecular-level insights on the retention mechanism of aromatic analytes in supercritical fluid chromatography (SFC). The stationary phases investigated were 9 nm silica pores with grafted dimethyl octadecyl silane (ODS, or C18) ligands at a coverage of 3.7 μmol/m² with and without trimethyl silane endcaps at an additional coverage of 0.5 μmol/m². The mobile phases studied were CO₂/methanol mixtures (neat CO₂, 5/95, and 10/90 v/v methanol in CO₂) at temperatures of 40 and 60°C, and a pressure of 130 bar. The analytes consisted of three kinds of alkylbenzenes (ethyl-, butyl-, and hexylbenzene), their end-functionalized alcohol derivatives (i.e., phenylalkyl alcohols), and several bioactive compounds such as p-cymene, thymol, and carvacrol. The simulations and experiments indicate that the incremental retention of a methylene group (separating an alkylbenzene from another alkylbenzene with a shorter alkyl chain) positively correlates with temperature because the solvent becomes less dense (i.e., weaker solvent) at higher temperature. The resolution of a hydroxyl group (separating a phenylalkyl alcohol from the alkylbenzene with the same number of carbon atoms) is larger for neat CO₂ solvent and dramatically diminished for the methanol-containing systems. This is attributed to the “deactivation” of surface silanols by the adsorbed methanol and the greater tendency to form a hydrogen bond with methanol for an alcohol (i.e., stronger solvent). The presence of endcaps further mitigates the effect due to the fewer hydrogen-bonding sites at the surface. The simulations and experiments reveal weak position-dependence of a hydroxyl group (separating carvacrol or thymol from p-cymene) in SFC with C18 phases, indicating the comparable steric hinderance effect of the methyl and isopropyl substituents on methanol-thymol/carvacrol hydrogen bond formation. The simulation trajectories were further analyzed to provide information on the structures of bonded chains, the orientations of aromatic compounds, the spatial distributions of the mobile phase and analyte molecules in the retentive phase, and the propensities for silanol-solvent, silanol-solute, and solute-solvent hydrogen bond formation in the stationary and the mobile phases.

This work is supported by the Chemical Measurement and Imaging program of the National Science Foundation Division of Chemistry under Grant No. CHE-2003246 (with partial co-funding from the Division of Chemical, Bioengineering, Environmental, and Transport Systems).