Protonation of polyaniline-coated silica stationary phase affects the retention behavior of neutral hydrophobic solutes in reversed-phase capillary liquid chromatography.

Because of its high conductivity when acid doped, polyaniline is known as a synthetic metal and is used in a wide range of applications, such as supercapacitors, biosensors, electrochromic devices, or solar and fuel cells. Emeraldine is the partly oxidized, stable form of polyaniline, consisting of alternating diaminobenzenoid and iminoquinoid segments. When acidified, the nitrogen atoms of emeraldine become protonated. Due to electrostatic repulsion between positive charges, the polarity and morphology of emeraldine chains presumably change; however, the protonation effects on emeraldine have not yet been clarified. Thus, we investigated these changes by reversed-phase capillary liquid chromatography using a linear solvation energy relationship approach to assess differences in dominant retention interactions under a significantly varied mobile phase pH. We observed that hydrophobicity dominates the intermolecular interactions under both acidic and alkaline eluent conditions, albeit to different extents. Therefore, by tuning the mobile phase pH, we can even modulate the retention of neutral hydrophobic solutes, such as aromatic hydrocarbons, because the pH-dependent charge and structure of polymer chains of the emeraldine-coated silica stationary phase show a mixed-mode separation mechanism.