Effects of Chain Grafting Positions and Surface Coverage on Conformations of Model Reversed-Phase Liquid Chromatography Stationary Phases
ER Mansfield and DS Mansfield and JE Patterson and TA Knotts, JOURNAL OF PHYSICAL CHEMISTRY C, 116, 8456-8464 (2012).
Chemical separations in reversed-phase liquid chromatography (RPLC) are made possible by the detailed molecular-level interactions that take place between analyte molecules and the interface between the stationary and mobile phases. Changes in operational conditions lead to different retention times, suggesting that the structures of the stationary phase and the stationary/mobile phase interface are altered. However, the details of such alterations, and their relationship to separation performance, are not well understood. In this study, the conformations of model RPLC stationary phases were investigated with all-atom molecular dynamics simulations to elucidate the detailed structural response of the stationary phase to different conditions. The model system consists of a quartz surface functionalized with dimethyloctadecylsilane immersed in H2O. Pressurization of the mobile phase did not affect the structure of the stationary phase, in agreement with previous studies. The choice of grafting positions on the surface, however, had a significant impact on the stationary phase structure. Specifically, randomization of the grafting positions led to increased local density, steric crowding, and stationary phase swelling compared with uniform chain placement with the same surface coverage. This finding, which has not been previously established, demonstrates the importance of characterizing the system-specific, chain-placement effects on the behavior of the RPLC stationary phase when attempting to study the molecular mechanisms controlling chromatographic retention.
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