Acidity, silanol group

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]... 

The pH of the buffer has complex effects on separation of analytes since it affects the ionization of the chargeable groups at the surface of the stationary phase. This is particularly important for stationary phases in which the weakly acidic silanol groups are the only driving... 

The surface of the silica is covered with weakly acidic silanol groups and it is assumed that the pK, value is in the range of 5 to 7. The maximum surface concentration of silanol functions is about 8 /imol/m. As the binding of ihe organic funclioitK lakes place via Ihe accessible silanol groups it is desirable to maximize their surface concentralion and to pre... 

The initial adsorption of the oxime in zeolites was studied through a combination of solid-state NMR spectroscopy and theoretical calculations ". The calculated adsorption complexes formed over silanol groups and complexes over Brpnsted acid sites in zeolites are depicted. This study suggests that the A-protonated oxime is formed over Brpnsted acid centers, but not over weakly acidic silanol groups. It has been also suggested that weakly acidic or neutral silanol groups or silanol nests are active catalysts of the rearrangement reaction ... 

Electroosmosis occurs in fused silica capillaries because acidic silanol groups at the surface of the capillary dissociate when in contact with an electrolyte solution6,7 (buffer), according to... 

A. Effect of pH Acidic silanol groups at the surface of the capillary wall will dissociate when in contact with an electrolyte solution, as illustrated by Eq. (4.5). At high pH, the silanol groups are fully ionized, generating a dense compact layer and a high zeta potential. As a result, the magnitude of the EOF in untreated fused silica capillaries increases with increasing pH. 

The proton sponge, l,8-bis(dimethylaminonaphthalene) (DMAN), has been anchored onto amorphous and pure silica MCM-41.[182] DMAN supported on MCM-41 is an excellent base catalyst for the Knoevenagel condensation between benzaldehyde and different active methylene compounds, as well as for the Claisen-Schmidt condensation of benzaldehyde and 2 -hydroxyacetophenone to produce chalcones and flavanones. It was found that the activity of the supported catalyst is directly related to the polarity of the inorganic support. Moreover, the support can also preactivate the reagents by interaction of the carbonyl groups with the weakly acidic silanol groups of MCM-41. This preactivation step enables DMAN, anchored onto MCM-41, to abstract protons with a higher pK than that of the DMAN. 

Quartz is a crystalline form of SiC>2 where silicon is tetrahedrally bound to four oxygen atoms. At the surface of quartz and other siliceous substances, acidic silanol groups formed from hydrolysis are present. Surface silanols are titratable in aqueous solution and are a source of pH-dependent surface charge. Fully hydroxylated silicas are considered to have a silanol population of 5/nm2 with an intrinsic pKa from 5.8 to 7.2 [18]. 

Under aqueous conditions, the inner wall of the capillary becomes negatively charged as a result of the presence of acidic silanol groups. In an electric field, this negative charge results in a bulk flow of fluid in the capillary toward... 

The presence of residual silanol groups can be detected most readily by using Methyl Red indicator [1], which turns red in the presence of acidic silanol groups, but a more sensitive test is to chromatograph a polar solute on the reversed-phase material. 

Since silica is such a common support for immobilization, the effect of the weakly acidic silanol groups has been studied. A cooperative silanol effect is thought to improve the catalytic activity of mesoporous silica-supported amines in base-catalyzed reactions such as the nitroaldol (Henry) condensation [6, 7], Knoevenagel condensation [6, 8,9], and Michael addition [6]. Thus immobilizing amines onto supports with stronger acid groups could be expected to further increase the catalytic activity. 

To overcome the problem of tailing in reversed-phase HPLC, salts can also be added to the mobile phase - so-called ion-suppression. Ammonium carbonate, sodium acetate and sodium phosphate have been used for this purpose in the analysis of alkaloids. Also, ion-pairing has proved to be successful in alkaloid analysis. It allows analysis of alkaloids under acidic conditions, thus avoiding the problem of chemisorption of the basic compounds on the acidic silanol groups. 

The 269 mp band in Fig. 4 shifting to the lower frequencies, cannot thus be assigned to an n-u transition, but to a tt-tt one displaced to the red by interaction with the acidic silanol groups, as is usually the case for transitions of this type. 

The spectrum of aniline, adsorbed on silica gel either from the gas phase (27), or from a cyclohexane solution (46) is situated at higher frequencies with respect to that of the vapor, owing to the expected H-bond formation with the acidic silanol groups of the surface. The shift is larger than that observed in an aqueous solution but smaller than that characteristic of the anilinium ion. For aniline vapor, adsorbed on silica gel in vacuo at low coverages (6 = 0.05-0.1) the first band is situated at 280 mp, whereas on the silica-alumina gels of different compositions, it is shifted to 260 mjal Fig. 11 (27). Within the precision limits the latter corresponds to the spectrum of the C H5NH2-H+ ion in an acidified aqueous solution. A similar behavior was shown by a-and 8-napthylamine vapor adsorption on silica-alumina gel and bentonite (27). 

Dugger, D.L. et al.. The exchange of twenty metal ions with the weakly acidic silanol group of silica gel, J. Phys. Chem., 68, 757, 1964. 

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