A molecular simulation study of the effects of stationary phase and solute chain length in reversed-phase liquid chromatography

The effects of stationary phase and solute chain length are probed by carrying out Monte Carlo simulations of dimethyl triacontyl (C30), dimethyl octadecyl (C18), dimethyl octyl (C8), and trimethyl (C1) silane grafted, and bare silica stationary phases in contact with a water/methanol mobile phase and by examining the retention of solutes from 1 to 14 carbons in length. Fairly small differences in structure are observed when comparing the C30, C18, C8 systems and the retention mechanism of nonpolar alkane solutes shows contribution from both partitioning and adsorption on all three of these stationary phases. Unlike in the other systems, the mobile phase solvent is highly structured at its interface with the C1 and bare silica phases, the former being enriched in methanol and the latter in water. Alkane solutes are unretained at the bare silica surface while alcohol solutes are only slightly enriched at the silica surface due to hydrogen bonding with surface silanols and surface bound solvent. With regard to solute size, it appears that the retention mechanism is not affected by the chain length of the solute.