Bulk Phases

The trichlorosilanes (e.g. octyltrichlorsilane) bond dichlorosilyl moieties to the surface and, in the presence of water, or if water is added, cross-linking occurs and a polymeric hydrocarbon stationary phase is produced. The same procedure can be used as that in the synthesis of oligomeric phases. The silica can be alternately treated with water and the trichlorosilane reagent. Layers of bonded phase are built up on the surface with extensive cross-linking, which produces a multilayer polymeric phase that has been termed a bulk phase. The bulk phases are almost as popular as the brush phases as they tend to have a higher carbon content (more organic material bonded to the surface) and thus provide a greater retention and selectivity. Bulk phases have about the same stability to aqueous solvents and pH as the brush phases.

The residual polarity of the bonded phase arising from the unreacted hydroxyl groups can cause peak tailing under certain circumstances. Consequently, it would be highly desirable to remove them or, at least reduce their effect on solute retention, particularly when present in brush phases. This was partly achieved by Kirkland who used reagents with larger base side chains (e.g. dipropyloctylchlorsilane as opposed to dimethyloctylchlorsilane) which linked a dipropyloctylsilyl group to the surface silicon atom. The larger propyl groups close to the silica surface tended to screen the adjacent hydroxyl groups steri-cally and thus significantly reduced their interactive availability. This, in turn, reduced their effect on retention. All three types of reversed phase are fairly stable, at least between pH 4 and pH 8 and at ambient temperature. The phase stability decreases as the temperature is raised: to some extent this can be overcome by introducing a phenyl group in the hydrocarbon chain.

Both brush and bulk reversed phases are commonly used in liquid chromatography, but the brush phase, although providing less retention for a given stationary phase loading, appears to be easier to manufacture in a reproducible form. Reversed phases are produced commercially with a range of chain lengths. Commonly available brush reversed-phase chain lengths range from a single carbon (using trimethyl-chlorsilane or ester as the reagent) to C30.

In general, the retention of a solute increases linearly with the carbon content of the reversed phase, as shown in Figure 1. It is seen, however, that the brush phases and the bulk phases fall on different lines. The bulk phases both contain C18 chains but with different degrees of reaction; the ODS has a significant number of the hydroxyl groups unreacted. It appears that the hydrocarbon chains in the bulk

Figure 1 Graph of retention volume (V) against carbon (%w/w). ODS and ODS2, C18 bulk-reversed phases. RP2, RP8 and RP18, C2, C8 and C18 brush-reversed phases.

material are more available to the solute than in the brush material, indicating that in the bulk phases, the polymeric material might have a fairly open structure. The brush phases were fully reacted (although, as already discussed, some hydroxyl groups will still be present) and it is seen that retention increases linearly with the chain length.

Reversed phases with long chains and high retention properties, although apparently desirable due to their high selectivity, are not always appropriate. For example, the strong dispersive interactions exhibited by these phases can cause irreversible retention of proteins and other biopolymers, usually accompanied by denaturization. As a result, the C2 and C4 phases are very popular in the biotechnology field. In con trast, the separation of complex mixtures of stable compounds such as essential oils, fatty acids, pesticides, herbicides and the host of other mixtures produced and used in industry today are best achieved using the longer chain materials.

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Solar Panel Basics

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