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

Abstract

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 β-cristobalite silica walls with dimethyloctylsilyl (C₈) ligands as the main modification and trimethylsilyl groups for end-capping, grafted at surface densities of 2.95 μmol/m² and 0.85 μmol/m², respectively. Residual silanol groups are present at a surface density of 3.8 μmol/m². 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 on 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. Copyright © 2009 American Chemical Society [accessed July 24, 2009

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

“…By simple analogy, the C 18 layer (r< 15Å) in RPLC plays a role similar to that of the diffuse water-rich layer in HILIC [45][46][47][48][49]. The degree of freedom of the methylene groups progressively increases with increasing the distance from the silica wall.…”

Molecular Dynamics Study of the Relation between Analyte Retention and Surface Diffusion in Reversed-Phase Liquid Chromatography

“…Surface diffusion describes the lateral mobility within a narrow region inside the mesopore, and this spatially dependent mobility is difficult to access experimentally. Molecular dynamics (MD) simulations, on the other hand, can provide spatially resolved diffusion coefficients for solvent and solute molecules as well as bonded-phase groups in an RPLC mesopore. − We therefore chose this approach to study surface diffusion in RPLC. − Our RPLC mesopore model consisted of a 10 nm slit pore formed by two planar silica surfaces modified with C 18 chains and trimethylsilyl endcapping groups (Figure ). Surface modification, ligand density, endcapping, and pore size were chosen to represent a typical mesopore in an RPLC column .…”

Stationary-Phase Contributions to Surface Diffusion in Reversed-Phase Liquid Chromatography: Chain Length versus Ligand Density

“…) Higher ligand density increases the shape selectivity of a column, which is important for the separation of isomeric/isobaric compounds . C 8 phases have been included in molecular simulation studies that investigated the conformation of the bonded phase, solvent penetration into the bonded phase, and the retention of n -alkanes and primary alcohols. ,− These earlier works established some key differences between a C 8 and a C 18 phase (at identical ligand density) relevant to this study: (i) C 8 chains have a higher preference to align orthogonally to the silica surface than C 18 chains, which have a tendency to fold back toward the surface; (ii) organic solvent is present along the length of the C 8 chains, whereas the middle segment of the C 18 chains is nearly solvent-depleted. Judging by the results of former molecular simulation studies, which compared silica-based, monomeric, trimethylsilane (C 1 ), C 8 , C 18 , and dimethyl triacontylsilane (C 30 ) phases, some details of the chromatographic interface can be highly specific to a particular chain length and organic solvent (MeOH or ACN) in the mobile phase. − It is thus impossible to predict a general influence of chain length on surface diffusion that applies to all RPLC phases without studying them first.…”