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

With the aim to improve the understanding of morphology and efficiency properties, we investi-gate in this series the impact of the complex formation constant of the hydrophobic monomer withrespect to statistically methylated--cyclodextrin (Me--CD) on the electrochromatographic propertiesof highly crosslinked amphiphilic mixed-mode acrylamide-based monolithic stationary phases. Basedon our previous work on amphiphilic mixed-mode monolithic stationary phases for capillary elec-trochromatography (CEC) using N-(1-adamantyl)acrylamide (Ad-AAm) as hydrophobic monomer thatforms an extremely strong water-soluble inclusion complex with Me--CD, we now selected N-tert-butylacrylamide (NTBA) as hydrophobic monomer forming an inclusion complex with Me--CD witha much lower value of the formation constant. Mixed-mode monolithic stationary phases are synthe-sized by in-situ free radical copolymerization of cyclodextrin-solubilized N-tert-butylacrylamide, a watersoluble crosslinker (piperazinediacrylamide), a hydrophilic neutral monomer (methacrylamide), and anegatively charged monomer (vinylsulfonic acid) in aqueous medium in bind silane pre-treated fusedsilica capillaries. The synthesized monolithic stationary phases have both hydrophobic and hydrophilicmoieties and can be employed in the reversed-phase mode, in the normal-phase mode, in a mixed-modeor in the hydrophilic interaction liquid chromatography (HILIC) mode (depending on the compositionof the mobile phase and on the properties of the solute). Morphology and retention properties of thisnew type of stationary phase are compared to those reported in our previous series. With a homologuesseries of alkylphenones it is confirmed that the hydrophobicity (methylene selectivity meth) of the sta-tionary phase is strongly dependent on the type of hydrophobic monomer employed. The studies reveala significant influence of the formation constant of the involved host-guest inclusion complex on themorphology (i.e., the domain size) of the produced monoliths.