A covalently modified surface represents an ambiguous state of matter that is not well represented as a liquid or a solid. Mounting evidence supports models for alkyl stationary phases as ordered systems. By way of comparison, liquid alkanes are composed of unconstrained, randomly oriented molecules. Chromatographic surfaces are more ordered than bulk liquids since the tethered chains are restricted from translational motion and the linkages orient the ligands relative to the surface. Intrachain order can be expressed in terms of gauche and trans conformations that produce bends and kinks in the alkyl ligands. These conformational defects are influenced by interchain spacing (surface coverage), alkyl chain length, and environmental parameters (temperature, and potentially, the mobile phase environment). The molecular order inherent to chromatographic surfaces directly affects the chromatographic properties of the stationary phase. Variations in parameters that influence alkyl chain conformational order also affect chromatographic performance, with predictable results.<br/><br/>Insight into the origins of shape selectivity has been gained through chromatographic, spectroscopic, and simulation-based studies of alkyl-modified surfaces. A compelling model of surface architecture has emerged that offers an explanation of the role played by the stationary phase in promoting shape recognition. This presentation will summarize findings related to stationary phase conformational order, obtained from spectroscopic and computational studies, for correlation with chromatographic performance.

Structure and function of chromatographic surfaces

ACS Spring Meeting 2021

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