Basicity, silanol group activation

The pentyl-bonded silica gel used in this experiment did not show silanol group activity." Therefore, a model butyl-bonded phase previously used for phenolic compounds was used for analyzing the chromatographic behavior of basic drugs. The surface is flat, and docking on the surface is simple. 

The highly covered phases Zorbax ODS and Hypersil BDS show similar selectivity behavior for a lot of separations. The consequence of a deactivation of the phase surface (reduction of the silanol group activity) in the case of Hypersil BDS is clearly evident for the separation of basic compounds (see Fig. 25). 

Silica gel and aluminium oxide layers are highly active stationary phases with large surface areas which can, for example, — on heating — directly dehydrate, degrade and, in the presence of oxygen, oxidize substances in the layer This effect is brought about by acidic silanol groups [93] or is based on the adsorption forces (proton acceptor or donor effects, dipole interactions etc) The traces of iron in the adsorbent can also catalyze some reactions In the case of testosterone and other d -3-ketosteroids stable and quantifiable fluorescent products are formed on layers of basic aluminium oxide [176,195]... 

This assay is altogether more difficult since three active ingredients are involved and several excipients interfere in the analysis, including one major excipient (methylparaben), which is not removed in the extraction process. In addition the active ingredients are bases which have a tendency to interact with any uncapped silanol groups in the stationary phase and it is essential to use a column which is deactivated with respect to the analysis of basic compounds. The three active ingredients are all at different concentrations in the formulation so that attention has to be paid to selection of a detection wavelength at which each component can be detected. In this particular assay a DAD would be useful. 

A purpose designed CEC stationary phase that gives excellent EOF character yet allows rapid simultaneous acidic, basic and neutral separations under isocratic conditions without tailing is yet to be discovered. Nevertheless, the Hypersil BDS unbonded silica used in this work was taken forward to explore such complex mixture separations as it combines features of both pure and traditional media. As a phase, it possesses a reasonable EOF (as it is based on traditional silica) with a lower number of activated silanol groups on the silica surface (due to the pre-treatment procedure used to remove surface metal contamination). Figure 3.3 shows the separation of benzylamine, caffeine and p-hydroxybenzoic acid in a single chromatographic analysis. 

The basic skeleton o7 tEe modified sorbents consists of a surface-active silica gel. Modification is in the form of a surface reaction with specially active silanes at the silanol groups of the silica gel. Following elimination of these silanol groups, new siloxane groupings are formed, on which aliphatic hydrocarbon groups are chemically bound by silicon-carbon bonds to the silica gel skeleton. Pre-coated plates with these modified silica gels are designated ... 

Mechanical activation ofkaolinite was investigated in [14-19], It was stated that at the initial stages of activation kaolinite undergoes a substantial increase of the number of active surface sites of acidic and basic character. Their concentration increases from 0.9 mmol/g in the initial sample to 1.3 mmol/g in the samples activated for 0.5 h then it decreases with increasing activation time. These sites are of different nature, e.g., silanole groups Si-OH, exhibiting weakly acidic character (with pK, 6.5-9.5), or strongly acidic sites of Lewis type arose due to coordination-unsaturated metal cations (mainly aluminium) on the surface [17]. 

However, more interesting from an application point of view are silylation reactions which introduce new functions into the materials. These can be created either directly or in subsequent further steps after silylation. Most simple is the direct conversion of the silica to a basic material by reaction, for instance, with 3-aminopropyltriethoxysilane [17]. Also two-step processes have been employed to synthesize basic materials, where first chloropropyl groups are anchored to the surface with subsequent conversion of the chloro group into an amine. In order to remove the residual, unreacted silanol groups, a second silylation with hexamethyldisilazane can be used. Such materials were found to be reasonably active in different base-catalyzed reactions, such as Knoevenagel condensations and Michael additions. A survey of the catalyzed reactions and the types of modification used can be found in Ref. [5]. 

Fused silica columns have obvious advantages, but do not have these possibilities. The fused silica capillary columns do not solve all the problems. The activity of silicium based capillary columns (glass or fused silica) is due to the trace of metal ions, silanol groups and siloxane bridges. Whereas a leached soda-lime column is always basic, fused silica columns are always acidic. 

The surfaces of several silica gels are partly contaminated with metal ions, depending on the different manufacturing procedures. On the other hand, there are big differences between materials according to the concentration of active silanol groups. The situation is quite complicated, but the above-mentioned two factors are the reasons in simplified form. For the acidic or basic pH of silica gels see Table 3.1... 

Nolice Endcapped C, phases are suilable for ihc separaiion of basic soluies. The C j phase has been treated in a second step with a small silane molecule. Because it is small and therefore sierically compatible, ii can reach the deeperlying, less reactive silanol groups. As a result you will get a surface ihar contains less active silanol groups. Fewer problems therefore arise during the separation of polar groups, because no additional interactions of a polar nature are possible. Endcapping can be found sometimes within the manufacturer s information about the column marked as e or E , see also Tip No. 1. 

This is the so-called silanophilic activity of the reversed phase. The compounds affine to silanol-groups are usually bases that interact with the acidic silanol groups via ionic interactions. These interactions are energetically much stronger than the usual hydrophobic interactions of reversed phases, which usually lead to wider elution bands for basic analytes on classical reversed phases. 

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