Silanol deprotonation

If the reaction takes place under basic conditions then the silica species are present as anions, that is, deprotonated silanol groups (Si-O-) in this case the surfactants have to be charged positively to ensure interactions between both components commonly cationic quaternary ammonium surfactants are used as the SDA this synthesis pathway is termed S+I (Fig. 3.6a). 

Experiments generating sulfate radicals, SO/", by UV photolysis of S2O82 in aqueous suspensions of silica nanoparticles showed a fast disappearance of the aqueous sulfate radicals yielding two transient species with absorption maxima around 320 and 600 nm, respectively [20]. The results indicated that at pH 3-9 S04 radicals build up an adduct on the surface with maximum absorption at 320 nm. This adduct shows similar reactivity to that observed for the sulfate radical in aqueous solution. The transients with absorption maximum at 600 nm were identified as SiO surface defects formed from the reaction between the adduct and deprotonated geminal and single silanols. Other less oxidative radicals lead to different radical-silica interactions. For example, thiocyanate radicals react with deprotonated silanols, not involving silanol oxidation. 

An important question to be explained by the chemical mechanism is why base catalysis often leads to condensed structures. In 1950, Aelion et al. (13) pointed out that the condensation reaction in base-catalyzed systems was faster than with acid catalysis and that the microstructure of the final product was different. Many of these facts are due to the opposite effects of, for example, substituents, on silicon on the stabilization of the transition state in base- and acid-catalyzed reactions (15). For the base-catalyzed condensation reaction to take place, a silicon atom has to be attacked by a deprotonated silanol oxygen the nucleophile. The acidity of the silanol proton increases as the basicity of the other groups bonded to the silicon decreases. This feature implies that poly silicic acid is a stronger acid than Si(OH)4 (20). Therefore, monomers react preferentially with higher polymerized species. 

Alcohol- or water-producing condensation generates a three-dimensional network. Pohl and Osterholtz (6) showed that condensation of alkylsilane-triols is specific acid and base catalyzed. Above the isoelectric point of silica (about pH 2.5) condensation proceeds by nucleophilic attack of deprotonated silanols on neutral silicates (6, 23). Below the isoelectric point the reaction proceeds by protonation of silanols followed by electrophilic attack (2, 6). These reactions favor less highly condensed sites, because these are the most electron rich, and lead to more extended, ramified structures. Above pH 2.5, reactions favor more highly condensed sites. 

The best solution found thus far for the preventing peptide and protein samples from adsorbing to the capillary wall in UHVCE is electrostatic repulsion. This can be accomplished in bare silica capillaries by using very basic pH buffers where the deprotonated silanol groups and the majority of peptides and proteins are negatively charged. This method is not directly compatible with positive mode ESl-MS, but may work for UV absorbance detection or LIF. ... 

The right-hand side of Eq. (53) is a function of as can be seen after retrieving the concentrations of protonated amino groups and deprotonated silanols from Eq. (35). By taking into account the evident relations,... 

Electro-osmosis motion of a buffer solution in contact with an electrically charged surface such as fused silica at pH > 3 (deprotonated silanol groups) under the influence of an external electric field. The resulting movement of the bulk liquid is called electro-osmotic flow (EOF). 

The most widely accepted mechanism for the base-catalyzed condensation reaction involves the attack of a nucleophilic deprotonated silanol on a neutral siH-cic acid. 

The most widely accepted condensation mechanism involves the attack of a nucleophilic deprotonated silanol on a neutral silicate species, as outlined earlier for the condensation in aqueous silicates [Eq. (5.4)]. This condensation mechanism pertains above the PZC (or lEP) of silica (pH > 2) because the surface silanols are deprotonated (i.e., they are negatively charged) and the mechanism changes with the charge on the silanol. Condensation between larger, more highly condensed species, which contain more acidic silanols, and smaller, less weakly branched species is favored. The condensation rate is maximized near neutral pH where significant concentrations of both protonated and deprotonated silanols exist. A minimum rate is observed near the PZC (or lEP). 

Indeed, polysaccharide-silica composites are frequently used for the formation of electrochemical films, because there is a large affinity between cellulose and the silicate oligomers in addition to the compatibility in the nature of solvents that can be used. The opposite is also true, and silicates (particularly sodium silicate) are useful additives in the paper industry. Chitosan is quite appealing, at least from the interpenetration point of view, because in addition to all these benefits, it is also positively charged and thus undergoes electrostatic interactions with the deprotonated silanols and can also assist in the attachment of biomolecules that are often negatively charged. 

As discussed in Section 2.4.4, above pH 2.0 the condensation reaction occurs preferentially between a nucleophilic deprotonated silanol species and a neutral species. The most highly condensed species are the most acidic and therefore the most likely to be deprotonated. Conversely the least highly condensed species, monomers, are the least likely to be deprotonated. Thus the base-catalyzed condensation mechanism (pH > 2.0) biases the growth process toward monomer-cluster growth. 

BASE DEPROTONATES SILANOL. CHARGES CAUSE FLOCCULATION... 

Gelation mechanism of silica dispersion in chloroform. Base deprotonates silanol. Charges cause flocculation. From Scherer and Luong [153]. 

The most widely accepted mechanism for silica condensation reactions involves the attack of a nucleophilic deprotonated silanol Si—0 on a neutral silicate species as proposed by Her [43] to explain condensation in aqueous silicate systems. This pertains to reaction (Eq. (17.3)) above the isoelectric point of silica where surface silanols are deprotonated to a significant extent. Note that not all silanol groups are identical, as it depends on the electron density on the... 

As the hydrolysis of the silane is completed, the hydrolyzed silane is added at a controlled rate to concentrated aqueous coUoidal silica dispersion. Since aUcahne-sodium-stabilized coUoidal silica dispersions cffe used, the condensation of the hydrolyzed silane with the surface of the coUoidal silica particle is favored. However, addition rate, mixing and temperature are critical pcffcuneters cuid side reactions, e.g., self-condensation of sUcUie, may take place. Further, reaction with charged silica surface groups, deprotonized silanol groups, will lead to ui increase in pH as indicated by Reaction 9.4. 

---