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

The local volume fractions of water, acetonitrile, and C-18-bonded chains across the 96 Amesopores of 5 mu m Symmetry particles were determined semi-empirically. The semi-empirical approach was based on previous molecular dynamics studies, which provided relevant mathematical expressions for the density profiles of C-18 chains and water molecules, and on minor disturbance experiments, which measured the excess amount of acetonitrile adsorbed in the pores of Symmetry-C-18 particles. The pore walls of the Symmetry-Cis material were in thermodynamic equilibrium with a series of binary mixtures of water and acetonitrile. The results show that C-18 chains are mostly solvated by acetonitrile molecules, water is excluded from the C-18-bonded layer, and acetonitrile concentrates across a 15-25 Athick interface region between the C-18 layer and the bulk phase. These actual density profiles are expected to have a direct impact on the retention behaviour of charged, polar, and neutral analytes in RPLC. They also provide clues to predict the local mobility of analytes inside the pores and a sound physico-chemical description of the phenomenon of surface diffusion observed in RPLC. (C) 2015 Elsevier B.V. All rights reserved.