Hybrid Bilayer Interfaces within Reversed-Phase Chromatographic Silica Formed by Self-Assembly of Long-Chain Primary...

Modification of silica interfaces by covalent attachment of functional ligands is a primary means of controlling the interfacial chemistry of porous silicas used in separations, environmental cleanup, and biosensing. Recently, modification of hydrophobic, n-alkyl-silane-functionalized interfaces has been achieved through self-assembly of zwitterionic phospholipids or mixed-charged surfactants to form "hybrid bilayers", producing interfaces that mimic lipid-bilayer partitioning and provide shape-selective partitioning of aromatic hydrocarbons. Charged headgroups, however, introduce electrostatic interactions that strongly influence the retention of ionizable solutes and require careful control over pH and ionic strength in the solution phase. In this work, we propose modification of C₁₈-functionalized chromatographic silica surfaces through self-assembly of long-chain primary alcohols to form uncharged hybrid-bilayer surfaces. Hybrid bilayers formed from alcohols ranging from C₁₂OH to C₂₂OH are investigated with in situ confocal-Raman microscopy, and the spectra indicate that they form highly ordered n-alkane structures, with order increasing as a function of alcohol chain length. Temperature-dependent Raman spectra of C₁₂OH-C₂₂OH hybrid bilayers were collected to investigate their melting transitions. Multivariate curve resolution of these spectra show broad, two-component melting transitions, indicating alcohol and C₁₈ alkyl chains melt simultaneously. These results suggest an interdigitated interfacial structure, where the hydrocarbon chains of the adsorbed alcohol extend into the underlying C₁₈ chains, ordering both layers. Interdigitation is confirmed by a temperature-dependent study of a deuterated C₁₆-OH bilayer, where spectrally resolved Raman bands from deuterated and protiated hydrocarbons melt together. Finally, n-alkyl alcohol bilayers were tested for protein repellency, where no protein adsorption was observed when equilibrated with ∼1 mg/mL bovine serum albumin. Bilayers C₁₆OH in chain length are shelf stable at refrigerated temperatures for months. These results demonstrate long-chain alcohol bilayers can be utilized to control the interfacial hydrocarbon structure of C₁₈-modified silica and have potential for use in separations, biosensing, and anti-biofouling applications.