Controlling the surface chemistry and chromatographic properties of methacrylate-ester-based monolithic capillary columns via photografting

Preparation of monolithic capillary columns for separations in the CEC mode using UV-initiated polymerization of the plain monolith followed by functionalization of its pore surface by photografting has been studied. The first step enabled the preparation of generic poly(butyl methacrylate-co-ethylene dimethacrylate) monoliths with optimized porous properties, controlled by the percentages of porogens 1-decanol and cyclohexanol in the polymerization mixture, irradiation time, and UV light intensity. Ionizable monomers [2-(methacryloyloxy)ethyl]trimethylammonium chloride or 2-acryloamido-2-methyl-1-propanesulfonic acid were then photografted onto the monolithic matrix, allowing us to control the direction of the EOF in CEC. Different strategies were applied to control the grafting density and, thereby, the magnitude of the EOF. To control the hydrophobic properties, two approaches were tested: (i) cografting of a mixture of the ionizable and hydrophobic monomers and (ii) sequential grafting of the ionizable and hydrophobic monomers. Cografting resulted in similar retention but higher EOF. With sequential grafting, more than 50% increase in retention factors was obtained and a slight decrease in EOF was observed due to shielding of the ionizable moieties.