Liquid Chromatography - Bonded Phase Synthesis by Reaction in a Solvent 3
The amount of material bonded per square meter, which indicates the efficacy of the synthetic process, will be directly proportional to the carbon content of the product and inversely proportional to the surface area of the original silica gel.
The method of synthesis is very similar for the alkoxysilane reagents. The same solvents can be used but, as no hydrochloric acid is generated, there is no need to have pyridine present as a scavenger. The most reactive alkoxy reagents are the methoxy and ethoxysilanes and their reaction with a hydroxyl group is accompanied by the release of methanol or ethanol.
The reaction is best carried out in a distillation flask and the methanol removed as it is formed, the heating rate is adjusted to ensure that the aromatic solvent is not removed as well. The reaction is allowed to proceed for about 5 hours and the product then filtered through a sintered glass filter and washed with the same sequence of solvents as those used in the chlorosilane synthesis. The product is then refluxed with the THF/water mixture, filtered and again washed with the appropriate solvents and dried. The final capping process is also the same as that employed in the method using the chlorosilanes reagents, utilizing hexamethyldisilazane as the capping reagent. The alkoxy-silanes are almost as readily available as the chlorosilanes and are easier and more pleasant to handle. They are, however, just as hygroscopic as the chlorosilanes and must be kept under the same anhydrous conditions. Approximately the same yields can be expected and the same degree of bonding.