Effect of ethoxylated sorbitan ester surfactants on the chromatographic efficiency of poly(ethylene glycol)-based monoliths

The effect of adding ethoxylated sorbitan ester surfactants (Tweens®) to poly(ethylene glycol) diacrylate-based monolithic recipes was investigated. Five different Tweens® have been evaluated to investigate the exact role of non-ionic surfactants in poly(ethylene glycol) diacrylate-based monolith preparations. These monoliths were characterized by scanning electron microscopy, infrared spectroscopy, and nitrogen physisorption analysis. Different morphological features, and surface areas were observed when different types of Tween® were included in the recipe; Tween® 20 and 85 showed small globules, while Tween® 40, 60 and 80 exhibited larger globular structures with different sizes and degrees of coalescence. The different Tween®-based monoliths were investigated for the chromatographic separation of mixtures consisting of hydroxybenzoic acids and alkylbenzenes. These columns were mechanically stable, except for Tween® 80. The highest methylene selectivity and the best overall performance were achieved by Tween® 60. The efficiency was increased by increasing the concentration of the Tween® 60 and the amount of poly(ethylene glycol) diacrylate M_n 700 in the recipes up to 30 wt%, each. Further increases in either Tween® 60 or poly(ethylene glycol) diacrylate M_n 700 led to formation of non-permeable columns. The optimized column was successfully used for separation of mixtures of nonsteroidal anti-inflammatory and sulfa drugs, with a maximum efficiency of 60,000 plates/m.