Recent Advances

Influence of bonded-phase coverage in reversed-phase liquid chromatography via molecular simulation. II. Effects on solute retention

J.L. Rafferty, J.I. Siepmann and M.R. Schure, 'Influence of bonded-phase coverage in reversed-phase liquid chromatography via molecular simulation. II. Effects on solute retention,' J. Chromatogr. A, 1204, 20-27 (2008).

Particle-based Monte Carlo simulations were employed to examine the molecular-level effects of bonding density on the retention of alkane and alcohol solutes in reversed-phase liquid chromatography. The simulations utilized octadecylsilane stationary phases with various bonding densities (1.6, 2.3, 2.9, 3.5, and 4.2 μmol/m²) in contact with a water/methanol mobile phase. In agreement with experiment, the distribution coefficient for solute transfer from mobile to stationary phase initially increases then reaches a maximum with increasing bonding density. A molecular-level analysis of the solute positional and orientational distributions shows that the stationary phase contains heterogeneous regions and the heterogeneity increases with increasing bonding density. Above, the distribution of n-butane in the x-y plane (the plane parallel to the silica surface) is shown for molecules within 10 Å of the silica surface (top row) and for molecules more than 10 Å from this surface but within the Gibbs dividing surface (bottom row). Black circles represent the location of the residual silanol groups and white circles indicate where the dimethyl octadecylsilane chains are tethered to the substrate.

Influence of bonded-phase coverage in reversed-phase liquid chromatography via molecular simulation. I. Effects on chain conformation and interfacial properties

J.L. Rafferty, J.I. Siepmann and M.R. Schure, 'Influence of bonded-phase coverage in reversed-phase liquid chromatography via molecular simulation. I. Effects on chain conformation and interfacial properties,' J. Chromatogr. A, 1204, 11-19 (2008).

Particle-based Monte Carlo simulations were employed to examine the effects of bonding density on molecular structure in reversed-phase liquid chromatography. Octadecylsilane stationary phases with five different bonding densities (1.6, 2.3, 2.9, 3.5, and 4.2 μmol/m²) in contact with a water/methanol (50/50 mol%) mobile phase were simulated at a temperature of 323 K. The simulations indicate that the alkyl chains become more aligned and form a more uniform alkyl layer as coverage is increased. However, this does not imply that the chains are highly ordered (e.g., all-trans conformation or uniform tilt angle), but rather exhibit a broad distribution of conformations and tilt angles at all bonding densities. At lower densities, significant amounts of the silica surface are exposed leading to an enhanced wetting of the stationary phase. At high densities, the solvent is nearly excluded from the bonded phase and persists only near the residual silanols. An enrichment in the methanol concentration and a disruption in the mobile phase’s hydrogen bond network are observed at the interface as bonding density is increased. Below, snapshots from simulations at each grafting density are shown. Atom types are as follows: solvent molecules shown in ball and stick form with oxygen in red, CH₃ groups in blue, and hydrogen in white. The stationary phase is depicted in tube form with CH_x in gray, hydrogen in white, oxygen in orange, and silicon in yellow.

Molecular-level comparison of alkylsilane and polar-embedded reversed-phase liquid chromatography systems

J.L. Rafferty, J.I. Siepmann, and M.R. Schure 'Molecular-level comparison of alkylsilane and polar-embedded reversed-phase liquid chromatography systems,' Anal. Chem., 80, 6214-6221 (2008).

The presence of certain anions at the Stationary phases with embedded polar groups possess several advantages over conventional alkylsilane phases, such as reduced peak tailing, enhanced selectivity for specific functional groups, and the ability to use a highly aqueous mobile phase. To gain a deeper understanding of the retentive properties of these reversed-phase packings, molecular simulations were carried out for three different stationary phases in contact with mobile phases of various water/methanol ratios. Two polar-embedded phases were modeled, namely, amide and ether containing, and compared to a conventional octadecylsilane phase. The simulations show that, due to specific hydrogen bond interactions, the polar-embedded phases take up significantly more solvent and are more ordered than their alkyl counterparts. Alkane and alcohol probe solutes indicate that the polar-embedded phases are less retentive than alkyl phases for nonpolar species, whereas polar species are more retained by them due to hydrogen bonding with the embedded groups and the increased amount of solvent within the stationary phase. This leads to a significant reduction of the free-energy barrier for the transfer of polar species from the mobile phase to residual silanols, and this reduced barrier provides a possible explanation for reduced peak tailing. The figure above shows the bonded phase (black), water (red), methanol (blue), combined solvent (cyan), and total (violet) density profiles. The Gibbs dividing surface is indicated by the vertical orange line and the surrounding shaded region represents the 10-90 interfacial region as defined by the overall solvent density.