Molecular Selectivity of a pH-Tunable Lecithin-Modified Zirconia Stationary Phase

We synthesized lecithin-modified zirconia (LMZ) as an adsorbent, solid-phase extractant, and stationary phase for organic compounds such as organic acids or amino acids. The molecular selectivity of LMZ was evaluated through high-performance liquid chromatography (HPLC). LMZ exhibits electrostatic and hydrophobic interactions with organic compounds that depend on the pH of the mobile phase. Zirconia was chosen because of its strong affinity for phosphate-containing compounds. Soybean lecithin, a model phospholipid, was directly and stably grafted onto zirconia particles through a Lewis acid-base reaction to form LMZ. The presence of lecithin on the zirconia surface was confirmed through solid-state analytical techniques such as confocal laser scanning microscopy and thermogravimetric analysis. The molecular selectivity of LMZ was assessed by measuring the retention behavior of aromatic compounds and amino acids on an LMZ column using HPLC. LMZ recognized analyte organic compounds through both electrostatic and hydrophobic effects that depended on the pH of the mobile phase. Interestingly, LMZ could clearly discriminate between electrostatic interactions with hydrophilic compounds such as amino acids and hydrophobic interactions with hydrophobic compounds such as aromatics. The specific molecular selectivity achieved using LMZ was attributed to not only the zirconia substrate but also the ionic and alkyl functional groups of lecithin.