Bonded Phases - Bonded Phase Synthesis by Reaction in a Solvent
Bonded Phase Synthesis by Reaction in a Solvent
The solvents commonly used in bonded phase synthesis are the aromatic hydrocarbons e.g. toluene that boils at 110oC or the mixed xylenes that boil at 138-140oC. The details of the procedure vary a little depending on the batch size and the type of silanizing reagent that that has been chosen for use. A typical example of a laboratory scale bonded phase synthesis employing a chlorsilane is as follows.
10 g of the chosen silica is dried at 250oC for about 2 hours and dispersed in a flask containing 100 ml of toluene dried over sodium. If a mono-chlorsilane reagent is used a small trace of water in the toluene can be tolerated and can be eventually eliminated by the use of excess chlorsilane reagent. Under such circumstances the toluene need not be dried over sodium. If, a dichlorsilane is used however, for example in the initial step in the synthesis of an oligomeric phase, the presence of water may cause linear polymerization. Consequently, stringent precautions must be taken to eliminate all traces of water. Returning to the procedure, a slight excess of the chlorsilane is then added to the silica dispersion together with 5 ml of pyridine. The pyridine acts as scavenger for the hydrochloric acid released during the reaction. The mixture is refluxed for about 5 hours and the product is then filtered on a sintered glass filter, washed sequentially with toluene, tetrahydrofuran (THF), methanol, methanol water (50:50 v/v) and finally with methanol and dried under reduced pressure (by suction). The last traces of any reagent and any chlorinated material, can be removed from the product by refluxing it for an hour with a THF water mixture (50%v/v) if required. It is then again filtered on a sintered glass filter, washed with THF and methanol and dried under vacuum. The basic bonded phase has now been prepared and requires end capping ; that is, any unreacted silanol groups are treated with a small molecular weight silanizing reagent to react with those hydroxyl groups that were sterically unavailable to the larger reagent due to exclusion. To end cap the product, the bonded phase is refluxed for two hours in a mixture of 100 ml of toluene and 25 ml of hexamethyldisilazane. The product is again filtered free of the reaction liquid mixture and washed sequentially with, toluene, tetrahydrofuran, methanol, methanol water (50:50 v/v) and finally with methanol and then dried under vacuum. It should be pointed out that end capping cannot eliminate the hydroxyl groups that are sterically hindered by the bonded moiety, or at best, only a small proportion of them will be removed. It can, however, react with any readily available hydroxyl groups, particularly those contained in pores that the original reagent could not enter but to which the smaller capping reagent has access. Capping will also eliminate any hydroxyl groups attached to the bonded moiety resulting from the presence of dichlor or trichlor impurities in the silanizing reagent. After capping, the carbon content of the product should then be determined to estimate the extent of reaction .