Silica groups, deactivation

One of the first steps in modifying the performance of capillary electrophoresis was the deactivation of silica groups of the capillary column by physically coating the capillary wall with methylcellulose (58,59), as well as via silane derivatization (10,44,60). Presently, many other changes have been carried out either to the capillary surface or addition of chemical agents to the separation buffer (see Table II), including manipulation of... 

We have developed a more efficient method of the silica surface deactivation, based on the liquid phase modification by organosilicon olygomers [35]. The main advantage of this method is that a layer of chemisorbed and sewed together molecules of the hydrophobizer is formed on the surface. This layer not only neutralizes the active OH groups of silica surface, but also covers the surface with the continuous film, making other active ad-sorption-catalytic sites (like coordinately unsaturated Al + or Fe ions) to be sterically inaccessible. The thickness of this layer on silica surface was estimated in [36] to be of about 2 nm. 

Alcohols, as well as water and acetonitrile, bind to the silanol groups, deactivate silica, and prevent strongly tailing peaks. Due to the bound polar molecules, silica cannot be used for gradient elution because equilibration to starting conditions takes too long time. 

Exhaustive silanization with the most active of silanizing reagents cannot eliminate completely all silanol groups on the i silica surface. Encapsulating the deactivated surface by, ... 

Figure 2.6 Reagents used for the deactivation of silanol groups on glass surfaces. A - disilazanes, B > cyclic siloxanes, and C -silicon hydride polysiloxanes in which R is usually methyl, phenyl, 3,3,3-trifluoropropyl, 3-cyanopropyl, or some combination of these groups. The lover portion of the figure provides a view of the surface of fused silica with adsorbed water (D), fused silica surface after deactivation with a trimethylsilylating reagent (E), and fused silica surface after treatment with a silicon hydride polysiloxane (F).

Several groups have reported deactivation of silica-supported cobalt catalysts. Holmen and coworkers19 30 have reported increased deactivation due to added (external) water in the feed to silica-supported Co catalysts. Kogelbauer et al 1 reported the formation of silicates. Catalysts recovered from FTS as well as catalysts deactivated by steam-treatment both showed fractions of non-reducible cobalt in TPR. The presence of metallic cobalt was a prerequisite for the silicate formation. 

Silica gel is a polar material. The presence of silanol groups is responsible for the acidic catalytic effect of this material (the pK of Si OH is comparable to that of phenol). The mode of action of silica gel is based on adsorption (Fig. 3.9), a phenomenon that leads to the accumulation of a compound at the interface between the stationary and mobile phases. In the simplest case, a monolayer is formed (known as a Langmuir isotherm) but there is also some attraction and interaction between molecules that are already adsorbed and those still in solution. This contributes to the asymmetry of the elution profile. Although it demonstrates good resolution and a high adsorption capacity, bare silica gel is seldom used for analytical purposes. For most applications, it must be deactivated by partial rehydration (in 3-8% water). 

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