Surface silanol groups

Condensation occurs most readily at a pH value equal to the piC of the participating silanol group. This representation becomes less vaUd at pH values above 10, where the rate constant of the depolymerization reaction k 2 ) becomes significant and at very low pH values where acids exert a catalytic influence on polymerization. The piC of monosilicic acid is 9.91 0.04 (51). The piC value of Si—OH decreases to 6.5 in higher order sihcate polymers (52), which is consistent with piC values of 6.8 0.2 reported for the surface silanol groups of sihca gel (53). Thus, the acidity of silanol functionahties increases as the degree of polymerization of the anion increases. However, the exact relationship between the connectivity of the silanol sihcon and SiOH acidity is not known. 

The hydrogel is allowed to stand for a few days during which time a process called sinerisis takes place. During sinerisis the condensation of the primary particles, one with another, continues and the gel shrinks further, accompanied by the elimination of more saline solution that exudes from the gel. After three or four days, sinerisis is complete and the gel becomes firm and can now be washed free of residual electrolytes with water. The washed product is finally heated to 120°C to complete the condensation of the surface silanol groups between the particles, and a hard xerogel is formed. It is this xerogel that is used as the LC stationary phase and for bonded phase synthesis. It is not intended to discuss the production of silica gel in detail and those interested are referred to "Silica Gel and Bonded Phases", published by Wiley (1). 

The method most commonly used to prepare bonded phases from silica involves reaction of the silica with a substituted dimethylchlorosilane. Fig. 3.2a shows the reaction, in which HC1 is eliminated between a surface silanol group and the silylating agent. 

Many other methods have been used to prepare bonded phases these include esterification of the surface silanol groups with alco-Tiols, or conversion of the silanol groups to Si—Cl using thionyl chloride, followed by reaction with an organometallic compound. If you are interested, there are details in the textbooks by Knox or by Hamilton and Sewell. 

Jt is not possible to bond all of the surface silanol groups. Unreacted silanols are capable of adsorbing polar molecules, and will thus affect the chromatographic properties of the bonded phase. Usually, the unreacted silanols produce undesirable effects, such as tailing and excessive retention in reverse phase separations, although there have been cases reported where the unreacted silanols improve such... 

Silica-supported metal (e.g., Pd/Si02) catalysts also have surface silanol groups that can react with the alkoxysilane groups of the complexes. These combination catalysts consist of a tethered complex on a supported metal. A Rh complex was tethered to the surface of a Pd/Si02 catalyst, and the tethered catalyst was more active for the hydrogenation of aromatic compounds than the free complex or the supported catalyst separately.33 It is possible that the H2 is activated on the supported metal and the hydrogen atoms migrate to the silica, where they react with the reactant molecules coordinated by the tethered complex. 

The reaction of alkyllithium with surface OH groups produces reactive lithiated surface. For example, neopentyllithium reacts with the surface silanol groups of silica (Scheme 7.12).240... 

Renewed interest in this method came recently from its adaptation to the immobilization of water/ organic solvent biphasic catalysts, resulting in the so-called supported aqueous phase catalysts (SAPCs).117 The molecular catalyst is immobilized via water, which is hydrogen bonded to the surface silanol groups reactants and products are in the organic phase (Figure 11)... 

The adverse effect of the hydrophilic silica was attributed to the condensation reaction of surface silanol groups on the silica and phenylsilane moieties on the polymer backbone. This results in increased cross-linking via formation of siloxane bonds between the polymer and silica. 

Griot, O. and Kitchener, J. A. Trans. Faraday Soc. 61 (1965) 1026. Role of surface silanol groups in the flocculation of silica by polycrylamide. 

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. 

Another consequence of this surface sol-gel process on SBA-15 is the reduction of the surface silanol groups induced by the condensation reaction with titaniumbutoxide. The reaction, after hydrolysis, converts surface silanol functionality, (SiO)3SiOH, into Ti-OH functionality. Given the similar electronegativity of silicon (1.8) and titanium (1.7), the expected consequence of this modification in the solid state Si NMR is a decrease in the band and a corresponding increase in the Q band that is, the OTi(OH)3 and OSi(OH)3 ligands will cause equivalent NMR shift perturbations on the substituted silicon site. 

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