Silanols acidity

Silanol Acid Free O-H stretch0 (cm 1) Ap (cm"1) AH (kJmol-1) Refs. 

Surface area Pore volume Pore diameter Pore size distribution Surface properties Number of silanols Types of silanols Acidity of surface... 

There is also the possibility that intermediate sized silicate species are capable of forming ligand complexes with metal ions due to their increased Silanol acidity and structure. The... 

In their work on synthesis and characterization of Beta molecular sieves, Borade and Clearfield [534] identified in H-[Fe,Al]Beta three types of OH groups giving rise to IR bands at 3740,3670 and 3610 cm, ascribed to silanol,acid [=Si-(OH)-Fe=] and [=Si-(OH)-Al=] groups, respectively. This assigmnent was supported by pyridine adsorption e eriments. The strength of the Bronsted acidity of bridging OH groups in H-[Fe]Beta was shown by TPD of ammonia to be lower than in H-[Al]Beta. 

Only a few attempts have been made to use computational methods for the silanol acidities. Sverjensky and Sahai have developed a method for estimating surface protonation equilibrium constants from the surface dielectric constant and an average Pauling bond strength. Rustad and coworkers [319] have used molecular dynamics methods to estimate the pK, for the reaction >SiOH — >SiO + H+ at 8.5. Tossell [320] used several quantum chemical levels of theory to establish correlations between calculated gas-phase AE and AH values and experimental aqueous solution pK s. Liu et al. [308] have employed both experimental and computation methods to study the gas-pha.se properties of organic silanols. ... 

Kiel et al. [1450], studied the effect of the presence and absence of an amine buffer and solvent pH on the retention and tailing of nortriptyline, desmethylnor-triptyline, and amitriptyline. A Cg column and a 50/50 acetonitrile/water (25 mM TEA) solvent gave baseline resolution, excellent peak shape, and elution within 4 rnin. Removal of the TEA led to significant peak tailing and an elution time of nearly 7 min. With no TEA, the mobile phase was buffered to different pH values (2.5-8) with 0.1M phosphate. A U-shaped plot of k versus pH resulted. This phenomenon was not because of a protonated-to-deprotonated form of the amine compounds, their piif, values are >8. Rather, it is due to surface silanol acidity functions and the complex interaction of the solutes with these sites. Therefore, it is important to consider the use of a basic mobile phase modifier when analyzing basic compounds. 

Amorphous sihca exists also ia a variety of forms that are composed of small particles, possibly aggregated. Commonly encountered products iaclude sihca sols, sihca gels, precipitated sihca, and pyrogenic sihca (9,73). These products differ ia their modes of manufacture and the way ia which the primary particles aggregate (Fig. 8). Amorphous sihcas are characterhed by small ultimate particle si2e and high specific surface area. Their surfaces may be substantially anhydrous or may contain silanol, —SiOH, groups. These sihcas are frequentiy viewed as condensation polymers of sihcic acid, Si(OH)4. 

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. 

For binder preparation, dilute hydrochloric or acetic acids are preferred, because these faciUtate formation of stable silanol condensation products. When more complete condensation or gelation is preferred, a wider range of catalysts, including moderately basic ones, is employed. These materials, which are often called hardeners or accelerators, include aqueous ammonia, ammonium carbonate, triethanolamine, calcium hydroxide, magnesium oxide, dicyclohexylamine, alcohoHc ammonium acetate, and tributyltin oxide (11,12). 

The first mechanistic studies of silanol polycondensation on the monomer level were performed in the 1950s (73—75). The condensation of dimethyl sil oxanediol in dioxane exhibits second-order kinetics with respect to diol and first-order kinetics with respect to acid. The proposed mechanism involves the protonation of the silanol group and subsequent nucleophilic substitution at the siHcone (eqs. 10 and 11). 

The condensation catalyzed by a strong base is first order with respect to substrate and catalyst (74,75). Because of the high acidity of silanol, all the alkah metal base (MtOH) is usually transformed into the silanolate anion. In the rate-determining step, the sdanolate anion attacks the siHcon atom in the silanol end group (eq. 12 and 13). 

The relative contributions from these processes strongly depend on the reaction conditions, such as type of solvent, substrate and water concentration, and acidity of catalyst (78,79). It was also discovered that in acid—base inert solvents, such as methylene chloride, the basic assistance requited for the condensation process is provided by another silanol group. This phenomena, called intra—inter catalysis, controls the linear-to-cyclic products ratio, which is constant at a wide range of substrate concentrations. 

Lateral interactions between the adsorbed molecules can affect dramatically the strength of surface sites. Coadsorption of weak acids with basic test molecules reveal the effect of induced Bronsted acidity, when in the presence of SO, or NO, protonation of such bases as NH, pyridine or 2,6-dimethylpyridine occurs on silanol groups that never manifest any Bronsted acidity. This suggests explanation of promotive action of gaseous acids in the reactions catalyzed by Bronsted sites. Just the same, presence of adsorbed bases leads to the increase of surface basicity, which can be detected by adsorption of CHF. ... 

Interesting products may also be produced by introducing boron atoms into the chain. The amount of boron used is usualy small (B Si 1 500 to 1 200) but its presence increases the self-adhesive tack of the rubber, which is desirable where hand-building operations are involved. The products may be obtained by condensing dialkylpolysiloxanes end-blocked with silanol groups with boric acid, or by reacting ethoxyl end-blocked polymers with boron triacetate. 

The resulting silanol group of a polymer chain condenses with acetoxy siloxy group of another polymer chain to form a siloxane (Si-O-Si) linkage (Scheme 8). Further similar reactions finally result in a crosslinked elastomer. Acetic acid is... 

Silica gel and aluminium oxide layers are highly active stationary phases with large surface areas which can, for example, — on heating — directly dehydrate, degrade and, in the presence of oxygen, oxidize substances in the layer This effect is brought about by acidic silanol groups [93] or is based on the adsorption forces (proton acceptor or donor effects, dipole interactions etc) The traces of iron in the adsorbent can also catalyze some reactions In the case of testosterone and other d -3-ketosteroids stable and quantifiable fluorescent products are formed on layers of basic aluminium oxide [176,195]... 

Protein-Pak packings are designed for the size exclusion chromatography of proteins and related compounds. They are based on silica, which is deactivated with glycidylpropylsilane. The diol function prevents the interaction of the target analytes with the silica surface. However, because coverage of the silica surface is always incomplete, residual acidic silanols can interact with the analytes. For this reason, most applications are carried out with a salt concentration above 0.2 mol/liter, which eliminates the interaction of analytes with surface silanols. Protein-Pak packings are stable from pH 2 to pH 8. 

Alkoxysilanes undergo hydrolysis, condensation (catalysts for alkoxysilane hydrolysis are usually catalysts for condensation), and a bond formation stage under base as well as under acid catalyzed mechanisms. In addition to this reaction of silanols with hydroxyls of the fiber surface, the formation of polysiloxane structures also can take place. 

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