A Molecular Dynamics Study of a Reversed-Phase Liquid Chromatography Model

We describe a molecular dynamics simulation study of a model of the reversed-phase chromatographic system. The model consists of a slab of aqueous solvent sandwiched between two walls having attached C 8 hydrocarbon chains at a surface coverage of 5.09 μmol/m 2 or 32.6 A° 2 /chain. Long-ranged Coulombic interactions are taken into account using the Ewald sum method of Rhee et al. [ Phys. Rev. B 1989, 40, 36]. The density and solvent orientation profiles are computed as a function of distance from the walls. The density profiles are found to be sensitive to the treatment of the long-ranged electrostatic interactions. The presence of an organic cosolvent (methanol or acetonitrile) at 30.8 mol % has little effect on the chain structure, which is largely collapsed against the walls. We also estimate the change in residual Helmholtz free energy along the pore width for a methane solute in the acetonitrile/water system, which indicates that a substantial portion of the free energy driving force for retention occurs in an organic-rich layer of solvent adsorbed to the hydrocarbon phase.