Molecular simulation studies of reversed-phase liquid chromatography

Determination of the solvent density profiles across mesopores of silica-C18 bonded phases in contact with acetonitrile/water mixtures: A semi-empirical approach

“…3-The fraction of the pore volume occupied by the tethered C 18 chains is mathematically represented by an extreme value cumulative distribution function. This assumption is qualitatively supported by the MD visuals reported in [23,24,34,35] for C 18 chains in contact with methanol-water and acetonitrile-water binary liquid mixtures. Accordingly,…”

“…As shown in [23,24,34], the silica-bonded C 18 chains extend to longer distances within the centre of the pores at high than at low contents of organic modifier. Therefore, CH 2 should increase with increasing the amount of organic modifier in the mobile phase.…”

“…Roughly, r I corresponds to the radial position where ϕ CH 2 (r) is about half it s value at r = 0. From MD, r I is expected to be nearly independent on the bulk mobile phase composition with r I 15Å [23,24,34]. 4-The volume fraction of water molecules in the pores increases from 0 to the bulk volume fraction, ϕ W,bulk .…”

“…In this work, the silica-C 18 bonded phase is in thermodynamic equilibrium with binary mixtures of acetonitrile and water. The mathematical formulation of the model is based on previous results of MD for planar surface silica-C 18 in contact with either methanol/water or acetonitrile/water liquid mixtures [23,24,26,35,34].…”

“…Aqueous organic (methanol or acetonitrile) mixtures were used as the bulk mobile phases. Monte-Carlo molecular dynamics (MD) calculations have provided unprecedented data for the local density, solute distribution coefficients, and preferential orientations of analytes, solvent components, and C 18 materials [23][24][25][26][27][28][29][30][31][32][33][34][35]. These calculations also revealed that water is mostly excluded from the C 18 layer, the organic modifier penetrates deeper into it, polar analytes (n-propanol) concentrate in the interface region between the C 18 layer and the bulk phase, and that apolar analytes (n-butane) accumulate in two distinct regions deeper into the C 18 layer.…”