Silanol groups, interpretation

An understanding of the surface chemistry of silica is required to interpret its chromatographic properties. The silica surface consists of a network of silanol groups, some of which may. be hydrogen bonded to water, and siloxane groups, as shown in Figure 4.2. A fully hydroxylat silica surface contains about 8... 

The above interpretation can also explain the IR spectra of the HYUS sanples. Indeed, the HYUS-8 sample shows two bands at 3555 and 3635 cm, a considerable amount of silanol groups, and hydroxyl vibrating at 3675 and--3695 cm (Fijg. 4a). After pyridine is adsorbed the bands at 3555 and 3635 cm disappear, while the other two bands remain and a new band at 3615 cm is now clearly visible (Fig. 4b. If the difference spectrum is obtained a band at 3600-3610 cm (Fig. 4c) again indicates the presence of strongly acid hydroxyl groups. ... 

The most plausible interpretation of the above results is as follows Al in aluminate solutions is present in the form of AKOH) anions irrespective of the Al concentration or the Na/Al ratio (150,181,182). When solution A, containing low molecular weight silicates and thus a high proportion of reactive SiOH groups, is mixed with the aluminate solution, a predominantly aluminous intermediate results. When, by contrast, solution B, richer in silicon and with fewer silanol groups, is used, a silca-rich intermediate is formed. But the composition of zeolite A (with Si/Al = 1.00) is achieved via structural rearrangements in the amorphous precursors and is dictated by the final structure itself. 

NH3 desorption on porous glass. The addition of NH3 had no effect on the IR band of surface silanol groups until enough NH3 had been added to cover 1% of the surface. Thus, the Mossbauer and IR results provide additional evidence for the complexity of surface acid site measurement and interpretation. 

The reversal of the direction of the electro-osmotic flow by the adsorption onto the capillary wall of alky-lammonium surfactants and polymeric ion-pair agents incorporated into the electrolyte solution is widely employed in capillary zone electrophoresis (CZE) of organic acids, amino acids, and metal ions. The dependence of the electro-osmotic mobility on the concentration of these additives has been interpreted on the basis of the model proposed by Fuerstenau [6] to explain the adsorption of alkylammonium salts on quartz. According to this model, the adsorption in the Stern layer as individual ions of surfactant molecules in dilute solution results from the electrostatic attraction between the head groups of the surfactant and the ionized silanol groups at the surface of the capillary wall. As the concentration of the surfactant in the solution is increased, the concentration of the adsorbed alkylammonium ions increases too and reaches a critical concentration at which the van der Waals attraction forces between the hydrocarbon chains of adsorbed and free-surfactant molecules in solution cause their association into hemimicelles (i.e., pairs of surfactant molecules with one cationic group directed toward the capillary wall and the other directed out into the solution). 

Additionally, with high pH values a negative absorption band occurs at 980 cm The band is assigned to the bending vibration of protonated silanol groups (Si—OH). It can be interpreted as deprotonation of Si—OH at the surfaces of the silica abrasive and/ or the silica film resulting in Si—0 . Similar behaviour has been found with different types of silica abrasives, for example Stober-sols. 

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