Imidazolium ionic liquid bonded silica stationary phases: Part I: Hexadecylimidazolium stationary phase

In this first research report, the design of an ionic liquid (IL) silica based stationary phase is described and characterized chromatographically using standard solute probes and via physical measurements including Fourier transform infrared (FTIR) and thermogravimetric analysis (TGA). The stationary phase is hexadecylimidazolium bonded to silica microparticles for the retention of various small and large solutes primarily by reversed-phase retention mechanism and to some different extent via π-π interactions and electrostatic interactions. The hexadecylimidazolium functionalized silica stationary phase was obtained by first grafting the surface of 5 µm silica particles with 3-chloropropyltrimethoxysilane forming the so-called a primary sublayer followed by attaching a top layer of hexadecylimidazole, thus resulting in the formation of IL based reversed-phase column. The hexadecylimidazolium column thus obtained exhibited relatively symmetrical peak shapes and high separation efficiencies for the studied analytes with a wide array of functional groups including ionizable analytes. The observed unique retention characteristics and selectivity of the hexadecylimidazolium column can be attributed to its multi interactions with various solutes including hydrophobic, electrostatic, π–π and hydrogen bonding interactions. Abbreviations: ABs: alkylbenzenes; APKs:n-alkyl phenyl ketones; CPS: chloropropylsilyl-silica; CPTMS: chloropropyltrimethoxysilane; HILIC: hydrophilic interaction chromatography; ILs: ionic liquids; IPA: isopropyl alcohol; TFA: trifluoroacetic acid; TGA: thermogravimetric analysis; THF: tetrahydrofuran; PAHs: polyaromatic hydrocarbons