Mixed-mode acrylamide-based continuous beds bearing tert-butyl groups for capillary electrochromatography synthesized via complexation of N-tert-butylacrylamide with a water-soluble cyclodextrin. Part I: Retention properties

- Synthesis of a new amphiphilic mixed-mode acrylamide-based monolithic stationary phase (CEC).
- Hydrophobic monomer N-tert-butylacrylamide is solubilized by complexation with methylated β-cyclodextrin.
- Investigation of stoichiometry, complex formation constant and spatial arrangement of involved host-guest inclusion complex.
- Significant influence of complex stability (formation constant) on specific permeability (domain size) of produced monolith.
- High versatility of formed amphiphilic stationary phase: NP, RP, mixed-mode, and HILIC retention mode.

With the aim to improve the understanding of morphology and efficiency properties, we investigate in this series the impact of the complex formation constant of the hydrophobic monomer with respect to statistically methylated-β-cyclodextrin (Me-β-CD) on the electrochromatographic properties of highly crosslinked amphiphilic mixed-mode acrylamide-based monolithic stationary phases. Based on our previous work on amphiphilic mixed-mode monolithic stationary phases for capillary electrochromatography (CEC) using N-(1-adamantyl)acrylamide (Ad-AAm) as hydrophobic monomer that forms an extremely strong water-soluble inclusion complex with Me-β-CD, we now selected N-tert-butylacrylamide (NTBA) as hydrophobic monomer forming an inclusion complex with Me-β-CD with a much lower value of the formation constant. Mixed-mode monolithic stationary phases are synthesized by in-situ free radical copolymerization of cyclodextrin-solubilized N-tert-butylacrylamide, a water soluble crosslinker (piperazinediacrylamide), a hydrophilic neutral monomer (methacrylamide), and a negatively charged monomer (vinylsulfonic acid) in aqueous medium in bind silane pre-treated fused silica capillaries. The synthesized monolithic stationary phases have both hydrophobic and hydrophilic moieties and can be employed in the reversed-phase mode, in the normal-phase mode, in a mixed-mode or in the hydrophilic interaction liquid chromatography (HILIC) mode (depending on the composition of the mobile phase and on the properties of the solute). Morphology and retention properties of this new type of stationary phase are compared to those reported in our previous series. With a homologues series of alkylphenones it is confirmed that the hydrophobicity (methylene selectivity αmeth) of the stationary phase is strongly dependent on the type of hydrophobic monomer employed. The studies reveal a significant influence of the formation constant of the involved host-guest inclusion complex on the morphology (i.e., the domain size) of the produced monoliths.