Silica activation with organosilanes

This strategy consists in the initial modification of the silica surface with organosilanes having suitable anchoring groups, which are either reactive themselves or can be additionally activated for the final attachment of the chiral selector. The choice of the proper silane will depend on the presence of suitable functional groups on the chiral entity to be fixed to the matrix. As macrocyclic antibiotics contain hydroxyl, amine, and carboxylic acid functionalities, they can be linked to the silica surface in a variety of different ways [7, 55]. The obvious drawback of the stepwise assemblage of chiral selectors on the silica surface is the eventual formation of additional polar or ionizable sites on the matrix, which may cause unselective retention of chiral analytes. 

RPLC with chemically bonded C18, C8, or other alkyl-bonded phases. The reason is the activity of polar residual silanol groups that remain on the support surface after incomplete reaction of silica gel with organosilanes. Thus, such a stationary phase may behave as a deactivated polar adsorbent in nonpolar or weakly polar organic solvents. 

DPG is the most commonly used member of the guanidine class of rubber accelerators. DPG is commonly used as a secondary accelerator (or kicker ) with primary sulfenamide accelerators and sometimes with thiazole accelerators as well. DPG is also a silanization accelerator. It can interact with the silica surface, which then reacts with organosilanes in tread stocks to help reduce rolling resistance. DPG is not as chemically active as the thiuram or dithiocarbamate accelerators. 

Organosilanes are particularly effective as positive ultraviolet photoresists (Section 3.5.1). The oxide layer exposed where the resist has been removed may then be etched with an appropriate solution (buffered aqueous HF in the case of a silica layer) to expose the silicon underneath. Application of the desired metal over the whole surface is then followed by removal of the remaining photoresist, taking the unwanted parts of the metal layer with it and leaving the metal contact on the Si that had been exposed by etching. Variations on this approach can be used to attach shaped deposits of other electronically active materials. 

Control of pore sizes of known catalysts like zeolites has been known for some time although the use of chemical vapor deposition (CVD) of organosilanes to control pore sizes has been the focus of recent research.7 Other catalysts like silica have been treated with methods like CVD and sol-gel in order to deposit thin films. Monolayer coatings of titanium oxide prepared by sol-gel methods have been recently used to coat silica and such films are active in alcohol dehydrogenation reactions.8... 

Apart from the use of organosilanes that form covalent bonds with silica surfaces, chemical modification can also be performed through physicochemical adsorption of appropriate molecules (or macromolecules) active in the polymerization process. For instance, Reculusa et al. reported the synthesis of silica/PS raspberry-like... 

As with the amino-bonded phase, the organosilane reagent used for bonding the CN phase has a trimethylene (-(CH2)3-) between the silicon atom (with its three leaving groups for bonding) and a cyano group. This thin layer can be used in the RP mode with polar mobile phases or in the NP mode with nonpolar mobile phases. As mentioned, it has less activity compared to a silica gel layer and also a less nonpolar character than C18- or C8-bonded phases. 

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