One current challenge in chromatography is separating compounds of similar molecular weights and hydrophobicity. Polyaromatic hydrocarbons (PAHs) are one example, where several compounds may exhibit similar retention under traditional reversed phase chromatographic conditions. Planarity is a characteristic which has been previously employed to enhance separation of chemically-similar PAHs. Here, high density C18 stationary phases, where the acyl chains adopt a more ordered conformation, have been reported to more strongly retain planar vs. non-planar compounds. It has been suggested that this shape selectivity is due to the partitioning of planar compounds into ‘slots’ between adjacent trans-conforming acyl chains in the C18 stationary phase. Recently, hybrid bilayers have been formed within C18-modified silica particles for use in biomimetic separations. Characterization of these materials has revealed that the acyl chains from the upper phospholipid leaflet interdigitate into the substrate C18 chains, leading to a highly-trans acyl chain structure, and the first observation of an ordered brush-like phase which may be optimal for planar aromatic separations. Upon heating, these highly ordered bilayers exhibit a melting transition, eliminating trans character in the bilayer acyl chains. This melting behavior makes temperature-dependent shape selectivity a possibility. Here we report on confocal Raman microscopy-monitored temperature dependent pyrene partitioning performed in C18–modified silica particles and in interdigitated hybrid-bilayer particles. As temperature increased, the trans character of the acyl chain in the hybrid bilayer becomes disordered, and the concentration of pyrene within the particle decreased. This suggests that temperature-dependent acyl-chain structure directly impacts pyrene partitioning, consistent with the above hypothesis. These results support the use of hybrid bilayers as temperature-controlled stationary phases for chromatographic separation of planar PAHs where the retention of planar molecules could be controlled by adjusting the proportion of trans-conformers with temperature.

Confocal-Raman microscopy determination of temperature-dependent partitioning of PAHs in C18-modified silica particles and interdigitated hybrid-bilayer particles

255th National Meeting (2018)

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