Facile chemical hydrophobization of thin-layer plates by vapor deposition of methyltrimethoxysilane for reversed-phase chromatography

Reversed-phase chromatography is based on a polar mobile phase and an apolar stationary phase. This separation mode is regularly used in planar (thin-layer) chromatography, and the necessary plates are commercially available. We investigated the preparation of hydrophobic plates for thin-layer chromatography by chemical vapor sorption of methyltrimethoxysilane directly onto a normal-phase plate. For this, a commercial normal-phase plate is simply exposed to the vapors of the reagent in a closed vessel. The obtained plates were characterized by infrared spectroscopy and dynamic water vapor sorption, which reported an extensive conversion of free hydroxyl groups by the reagent. The obtained plates were hydrophobic with a water contact angle close to 135 degrees. The extent of hydrophobization precluded the use of pure water as an eluent while mixtures with organic solvents were perfectly adequate. The plates' chromatographic performance was compared with that of C18 and paraffin-coated plates. For this, a set of parabens was separated with mixtures of acetone and water. The height of a theoretical plate was similar for the hydrophobized and the C18 plates (50-90 mu m) and larger for the paraffin-coated ones. In contrast to the C18 plate, the hydrophobized and the paraffin-coated plates showed some selectivity for the analyte pair n-butylparaben and iso-butylparaben, which indicates a separation mechanism with the potential for regioselectivity.