Silica gels surface silanol group, chemical

Reversed-phase PLC precoated plates are based on silica gel matrices with chemical modifications in such a manner that the accessible polar, hydrophilic silanol groups at the silica gel surface are replaced by nonpolar, hydrophobic alkyl chains via silicon-carbon bonds. For preparative purposes, up to now only PLC precoated RP plates with C-18 modification are available. This abbreviation is often also designated as RP-18, meaning that an octadecyl alkyl chain is chemically bonded to the silica gel surface. 

Spherical porous silica gel is the easiest stationary phase material to handle however, although it is physically strong it is chemically unstable. Surface modification can expand its capability for different modes of chromatography, such as normal-phase, reversed-phase, size-exclusion, and ion-exchange liquid chromatography. These stable modifications are performed by chemical deriva-tization of the surface silanol groups. 

For steric reasons bifunctional or trifunctional silanes can react with either one only or, at most, two silanol groups on the silica gel surface (second reaction in Fig. 1.8A). Thus, some of the functional (Cl or alkoxy) groups remain unreacted and easily hydrolyse to form new silanol groups. If the reaction mixture contains even traces of water, the hydrolysis occurs during chemical modification of silica and the new silanol groups react with excess molecules of reagents to form a polymerised surface layer (Fig. 1.8B). These bonded phases may be more stable and usually show stronger retention than monomeric phases at low pH. However, the reaction is difficult to reproduce and various batches of the same material may have different properties, so that the reproducibility of separation is poorer than with monomeric phases. Polymeric phases are more resistant to penetration of analytes and may show increased mass-transfer resistance and decreased efficiency (plate number) of separation [- 91. 

Fig. 8.3. Silica gel surface can consist of siloxane (Si-O-Si) bonds or silanol (Si-OH) groups, which are hydrophobic or hydrophilic respectively. Depending on material treatment, either chemical or thermal, the surface can have hydrophobic (left) or hydrophilic (right) properties.

The products made by the above synthetic processes still have large numbers of residual silanols, which lead to poor peak shapes or irreversible adsorption, because chemically bonded groups on the silica gel surface have large, bulky molecular sizes and, after the bonding, the functionalized silane cannot react with the silanols around the bonded ligands. Because such alkyl-bonded phases are used for reversed-phase separations, especially for chromatography of polar molecules, any silanol groups that remain accessible to sol-... 

Silica gel-based materials for RP chromatography with nonpolar (most often Cg or Cjg alkyls) or moderately polar stationary phases covalently bonded via Si-O-Si-C bonds are prepared by chemical modification of the silanol (Si-OH) groups on the silica gel surface by chloro-silane or alkoxy-silane reagents, and are relatively stable to hydrolysis. The retention in RP increases with increasing surface coverage and length of the bonded alkyl chains, so that Cjg phases show greater retention than Cg-bonded phases. 

C. Reversed-phase chemically bonded plates Whatman KC,g plates consist of a layer of sorbent produced by reacting octadecyl monochlorosilane with the surface silanol groups of a special silica gel [30]. This presents a nonpolar hydrocarbon surface to the solutes for separations by reversed-phase TLC. The advantage of this chemically bonded medium compared to conventional impregnated nonpolar phases... 

Chemically bonded layers are prepared by reacting silica gel with various functionalized organosilane reagents forming siloxane Ssonds with some of the silanol groups originally present on the silica surface (10,23,71-77). Chemically bonded siloxane layers... 

Packed columns contain an inert and stable porous support on which the stationary phase can be impregnated or bound (varying between 3 to 25%). The solid support is made of spheres of approximately 0.2 mm in diameter, obtained from diatomites, silicate fossils (such as kieselguhr, tripoli) whose skeleton is chemically comparable to amorphous silica. These materials, which have a specific surface area ranging from 2 to 8 m2/g, have been commercialised by several companies such as Johns Manville, under the name of Chromosorb , and are used universally. Other synthetic materials have been developed such as Spherosil , made of small silica beads. All of these supports have a chemical reactivity comparable to silica gel because of the presence of silanol groups. 

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 ... 

Chemically, silica gel is a polymer composed of tetrahedral silicon atoms connected through oxygen atoms (siloxane, Si-O-Si) with silanol (Si-OH) groups present at the surface (Figure 4.2). 

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