Molecular Dynamics Study On The Retention Mechanism In Reversed-Phase Liquid Chromatography

This paper dealt with all-atom modeling of RPLC systems including mobile phase (MP) molecules, solute molecules and bonded stationary phase (BP) molecules. The effects of MP composition, ligand density, alkyl-ligand length, and temperature on BP solvation were investigated. This simulation also examined the effects of the polarity and the number of functional groups on the solute's retention behavior. The results were concerned with the chain structure and the dynamical properties of the alkyl chains and solvent in order to probe the influence of BP solvation on the retention behavior.With increasing organic modifier content, the C18 ligands become more extended with greater solvation, leading to an increase in the overall mobility of ligand atoms,the retention mode tends to be a transition from partition-like to adsorption-like; The ordering of BP and the extraction from the binary MP into BP increases in the series: methanol, acetonitrile, tetrohydrofuran; With increasing ligand density, the thickness of BP increased, the solvation of the ligands decreases leading to a decrease in the overall mobility of ligand atoms,the retention mode changes between partition-like and adsorption-like; With increasing alkyl-ligand length, the solvation of the ligands also decreases with a transition of the retention mode from adsorption-like to partition-like; As temperature increases from 298K to 313K, the ligand chains become more bent with higher mobility, the state of BP may change from solid-like to liquid-like, the retention modes tends to be adsorption-like, however, BP solvation doesn't seem to be influenced by temperature; Partition becomes dominant along with the increase of the molecule volume of non-polar solute, however, adsorption becomes predominant with the increase of the polarity of polar solute.Consequently, neither the partition nor the adsorption separation mechanism could fully explain the underlying mechanism for separation in the model system we have chosen. It is doubtful that one of these theories can offer a comprehensive picture of RPLC retention mechanism.