Influence of Residual Silanol Groups on Solvent and Ion Distribution at a Chemically Modified Silica Surface

We report on molecular dynamics simulations of solvent and ion distribution at a prototypic alkyl-modified silica surface under explicit consideration of residual silanol group activity. The model contains two beta-cristobalite silica walls with dimethyloctylsilyl (C-8) ligands as the main modification and trimethylsityl groups for end-capping, grafted at surface densities of 2.95 mu mol/m(2) and 0.85 mu mol/m(2), respectively. Residual silanol groups are present at a surface density of 3.8 mu mol/m(2). The mobile phase consists of water/acetonitrile mixtures over the whole range of volumetric compositions. We have studied the two limiting cases of residual silanol group activity: (i) undissociated silanol groups, and (ii) dissociated silanol groups with sodium ions as counterions. Solvent and ion distribution in the system as well as the orientational arrangement of solvent molecules and the conformation of the bonded phase are presented for both cases of residual silanol activity. We analyze the influence of mobile phase composition an system conformation to illustrate that the presence of residual silanol groups in their ionized, sodium salt form induces larger changes than a variation of the water/acetonitrile mixture.