Brush Phases

There are three basic types of bonded phases which are produced by the use of the mono-, di- and tri-substituted silanes: brush phases, oligomeric phases and bulk phases. For example, the monochlorsilanes (e.g. octyldimethylchlorsilane) react with the hy-droxyl groups on the silica surface to produce dimethyloctylsilyl chains attached to the silica. The alkyl chains are thought to stand out from the surface like the bristles of a brush, hence the term brush phase. After reaction, the material is usually treated with trimethylchlorsilane or hexamethyldisilazane to eliminate any remaining unreacted hydroxyl groups. This procedure is called capping the bonded phase.

The extent to which the silyl groups are reacted is still a subject of some debate. It is thought that the two methyl groups next to the silicon atom of the silyl reagent hinder reaction with adjacent hydroxyl groups on the silica gel surface. Consequently, a considerable amount of unreacted hydroxyl groups will remain between the bonded moieties even after capping. It has also been suggested that one hydroxyl group may remain situated between each bonded chain. There is certainly evidence of some polar interactions with reversed phases which, if completely covered with hydrocarbon chains, should only exhibit dispersive interactions. However, all reversed phases are predominantly dispersive in character, and it would appear that if there are any hydroxyl groups still present on the surface, their influence on retention is relatively small compared with that of the bonded moieties. However, the residual hydroxyl groups, being strongly polar, can cause separation problems under certain circumstances and thus methods that reduce their effect have been developed and will be discussed later.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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