Hybrid-Supported Bilayers Formed with Mixed-Charge Surfactants on C₁₈-Functionalized Silica Surfaces

Mixtures of cationic-anionic surfactants have been shown to spontaneously form ordered monolayers at hydrophobic-hydrophilic boundaries, including air-water and oil-water interfaces. In this work, confocal Raman microscopy is used to investigate the structure of hybrid-supported surfactant bilayers (HSSBs) formed by deposition of a distal leaflet of mixed cationic-anionic surfactants onto a proximal leaflet of n-alkane (C₁₈) chains on the interior surfaces of chromatographic silica particles. The surface coverage of the two surfactants in a hybrid bilayer was determined from carbon analysis and the relative Raman scattering of their respective head-groups. Within the measurement uncertainty, the stoichiometric ratio of the two surfactants is one-to-one, equivalent to mixed-charge-surfactant monolayers at air-water and oil-water interfaces and consistent with the role of the head-group electrostatic interactions in their formation. When self-assembled on the hydrophobic surface, pairs of oppositely charged n-alkyl chain surfactants resemble a phospholipid (phosphatidylcholine) molecule, with its zwitterionic head-group and two hydrophobic acyl chain tails. Indeed, the structure of these hybrid-supported surfactant bilayers on C₁₈-modified silica surfaces is similar to that of hybrid-supported lipid bilayers (HSLBs) on the same supports, but with denser and more-ordered n-alkyl chains. Hybrid-supported surfactant bilayers exhibit a melting phase transition (gel to liquid-crystalline phase) with structural and energetic characteristics similar to those of hybrid-supported bilayers prepared from a zwitterionic phospholipid of the same alkyl chain length. These mixed-charge surfactants on n-alkane-modified silica are stable in water over time (months), results that suggest the potential use of these hybrid bilayers for generating supported lipid-bilayer-like surfaces or for separation applications.