Dodecyl/imidazole dual-functional silica with polyacrylamide coating: A multi-mode HPLC stationary phase toward modulated separation selectivity

Mixed-mode chromatography (MMC) stationary phases integrate multiple retention mechanisms, enabling the efficient separation of complex samples containing components with varying polarities. A typical MMC stationary phase is imidazolium ionic liquid-modified silica gel, which can provide π-π and electrostatic interactions that are not available on C18. However, the imidazolium ionic liquid-modified silica stationary phase suffers from notable drawbacks, including peak tailing for basic analytes caused by exposed silanol groups and an insufficiently broad separation scope. Polyacrylamide (PAM) can effectively shield residual silanol groups and introduce additional interaction sites. However, PAM tends to swell in aqueous mobile phases and is generally restricted to the separation of highly polar analytes, rendering it unsuitable for hydrophobic components. The introduction of long-chain alkyl groups provides an effective solution by not only enhancing hydrophobic retention but also suppressing the swelling of PAM in aqueous media, thereby improving column stability. In this work, we rationally designed and synthesized a novel PAM-coated mixed-mode stationary phase via a stepwise functionalization strategy. Long-chain dodecyl (DD) and thiol groups were first immobilized onto silica through bis-silylation, followed by covalent attachment of 1-allyl-3-vinylimidazolium (AVI) bromide via thiol-ene click chemistry and free-radical polymerization of acrylamide to form a hydrophilic polymer coating. The resulting PAM@AVI-DD-Sil shows excellent separation performance for compounds with diverse polarities, including hydrophilic nucleosides/bases as well as hydrophobic steroid hormones, alkylbenzenes and alkylphenols. The synthesized PAM@AVI-DD-Sil outperforms the bare silica, commercial XAmide and C18 columns in terms of retention and separation selectivity. This work establishes PAM-coated long-chain alkyl/ionic liquid-functionalized silica as a transformative mixed-mode separation material for the efficient handling of complex samples across a wide polarity range, while simultaneously introducing a novel synthetic method for multifunctional polymer-based materials intended for HPLC.