Separation of compounds based on their affiliation with phospholipid membranes is critical for predicting bioavailability. A model membrane for these types of separations has been developed employing an upper leaflet of phospholipid self-assembled on lower leaflet of n-alkyl chains covalently bound to the surfaces of C18-chromatographic silica. In chromatographic applications, this stable hybrid-supported bilayer (HSB) has provided retention and separations of compounds based on their based on lipid-bilayer affinities. Recently, the structure of HSBs in chromatographic silica has been characterized using confocal Raman microscopy. The results show that the acyl-chains of phosphatidylcholine-hybrid bilayers are interdigitated structures that undergo a temperature-dependent order-disorder transition that is characteristic of lipid bilayers, and the ordering of the acyl chains governs the membrane affiliation of non-polar solutes. Despite the attributes of this model-membrane, one disadvantage of HSBs material is the high cost of the large quantity of phospholipid required to modify the C18-silica in a chromatographic column. In the present work, we have sought a less expensive alternative to modify C18-silica surfaces while mimicking the interfacial properties of phosphatidylcholine lipids including zwitter-ionic head-group and two acyl chains for each pair of charges. To address this challenge, we have adsorbed a pair of oppositely-charged long-chain surfactants on C18-modified silica, resulting in a stable dense monolayer. The two surfactants, cetylpyridinium chloride and hexadecylphosphonic acid, have charged-head groups (quaternary amine, phosphonic acid) and equivalent 16-carbon hydrocarbon tails comparable to 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). Raman microscopy reveals a stable, high density layer of mixed surfactants, which does not desorb upon rinsing. The acyl chains appear to be interdigitated and exhibit a temperature-dependent transition from highly ordered to disordered structure, similar to a DMPC-HSB. The composition and structure of this mixed surfactant layer are investigated in this work, along with the prospects for its application in assessing lipid-bilayer affinity of molecules based on their retention on these surfaces.

Confocal Raman microscopy investigation of mixed-surfactant monolayers at n-alkyl-chain functionalized surfaces in chromatographic particles

255th National Meeting (2018)

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