Surface-derivatized silica

Another type of derivatized silica surface that has received considerable attention is Cis-derivatized silica gel that is, silica gel that has been silylated with reagents such as Cl(CH3)2Si(CH2)i7CH3 (45-51). This system was studied via 13C NMR (47) and 2H NMR spectroscopy (45) as a function of surface loading of the Cis chains and of certain added liquids. The ability of cross polarization to select static components of a surface derivative relative to more mobile components—which would be emphasized by non-CP (single-pulse) techniques that rely on direct 13C spin-lattice relaxation—can be seen in Figure 8, which provides a comparison between the single-pulse and cross-polarization 13C MAS NMR spectroscopy results on four Cis-silica samples. Dramatic differences in line shapes and intensities, which can be interpreted at least qualitatively in terms of... 

Scholten, A.B. Claessens, H.A. de Haan, J.W. Cramers, C.A. Chromatographic activity of residual silanols of alkylsilane derivatized silica surface. J. Chromatogr. A, 1997, 759,31-46. 

Synthetic chiral adsorbents are usually prepared by tethering a chiral molecule to a silica surface. The attachment to the silica is through alkylsiloxy bonds. A study which demonstrates the technique reports the resolution of a number of aromatic compoimds on a 1- to 8-g scale. The adsorbent is a silica that has been derivatized with a chiral reagent. Specifically, hydroxyl groups on the silica surface are covalently boimd to a derivative of f -phenylglycine. A medium-pressure chromatography apparatus is used. The racemic mixture is passed through the column, and, when resolution is successful, the separated enantiomers are isolated as completely resolved fiactions. Scheme 2.5 shows some other examples of chiral stationary phases. 

In 1975, the fabrication of a chiral electrode by permanent attachment of amino acid residues to pendant groups on a graphite surface was reported At the same time, stimulated by the development of bonded phases on silica and aluminia surfaces the first example of derivatized metal surfaces for use as chemically modified electrodes was presented. A silanization technique was used for covalently binding redox species to hydroxy groups of SnOj or Pt surfaces. Before that time, some successful attemps to create electrode surfaces with deliberate chemical properties made use of specific adsorption techniques... 

The use of silica particles in bioapplications began with the publication by Stober et al. in 1968 on the preparation of monodisperse nanoparticles and microparticles from a silica alkoxide monomer (e.g., tetraethyl orthosilicate or TEOS). Subsequently, in the 1970s, silane modification techniques provided silica surface treatments that eliminated the nonspecific binding potential of raw silica for biomolecules (Regnier and Noel, 1976). Derivatization of silica with hydrophilic, hydroxylic silane compounds thoroughly passivated the surface and made possible the use of both porous and nonporous silica particles in all areas of bioapplications (Schiel et al., 2006). 

Fujimoto et al. [77] synthesized a novel phase by coupling a dodecylamino-substituted P-cyclodextrin (P-CD) to 3-glycidoxypropyl-derivatized silica gel. The surface coverage of this phase was reported as 0.37 xmol/m, which amounts to a surface coverage of 2.6 xmol/m for C12 chains (seven chains per fi-CD). An increase in shape selectivity was observed when compared with a conventional Cis monomeric phase as determined by selectivity differences between j9/m-terphenyl, j9/o-terphenyl, and coronene/phenanthro[3,4-c]phenanthrene solute pairs and was attributed to the localized high ligand density as constrained by the fi-CD platform structure. 

In most of these applications, silica has the role of a support material. The popularity of silicon-based supports for multiple modification applications is well summarized by Mottola.1 Chemical modification requires seemingly paradoxal support properties (a) supports need to have a surface hydrophilic in nature but also to be insoluble in aqueous solutions and polar solvents (b) supports are required, in many instances, to be porous but retain mechanical stability and (c) they must be chemically stable but easily derivatized. Silica gel meets all of these requirements. 

This more recent design has evolved from earlier supported columns prepared with propylthiol, propylnitrile, or propyldiphenylphosphine units linked to the silica surface,101 which separate ketones, thioethers, and ethers.102 Ethylenediamine-like binding sites were prepared in early developments by linking N-(2-aminoethyl)-3-aminopropyltrimethoxysilane,103 which derivatized as Cu11 or Cr111 complexes are able to separate hydrocarbons, esters, ethers, and thioethers.104... 

Another type of metal-support site may be created by the reaction of acetylacetone with the mono-amine-bound derivatized silica gels. This type has imino-acac ligands attached to the surface. There are Cu11 species with only one iminoacac chelate in its coordination sphere, and other sites coordinated in a square-planar array by two iminoacac ligands from adjacent surface locations.105,106 These types of supported IMCOS sites also have been used to separate hydrocarbons, ethers, and thioethers. 

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