Silica silanol type

The simplest possible way to estimate the concentration of the different silanol types on the silica surface consists of an integration of the corresponding infrared bands. 

These results are presented in figure 5.27, showing the absolute distribution of the three silanol types on the silica surface as a function of temperature. The absolute concentration of the free silanols increases in the low temperature region, due to dehydroxylation processes of bridged silanols (reaction 5.1). 

The book is divided into three major parts. The first part (Part I) reviews the characterization of the silica surface, discussing the preparation and properties of pure silica (Chapter 1), the physical characterization of the silica surface (Chapter 2), the chemistry of silica (Chapter 3), the quantification of the silanol number (Chapter 4), the distribution of the silanol types and their desorption energies (Chapter 5), the effect of surface morphology on the dehydroxilation behaviour (Chapter 6) and the related silicate materials (Chapter 7). Part II discusses the chemical modification of... 

These phases are used without further chemical modification, normally in the reversed phase mode with moderately polar mobile phases. Apart from the fact that silanol groups are absent, these packing materials differ in several ways from silica based types. One of the most useful differences is that they are stable over a wide pH range, commonly quoted as from 1 to 13. This allows a much greater choice of mobile phases than would be advisable with silica and, in the case of bases permits the ionisation to be suppressed by operating at high pH values. 

The Colloid Chemistry of Silica included results for a study of the desorption kinetics of pyridine as it was removed from the surface of an amorphous, porous silica gel [30], Included in this chapter are data, results, and discussion regarding pyridine desorption experiments that have been carried out on two different silica surface types which were exposed to thermal pretreatment in the 200-400°C range. Pyridine interactions have been studied to achieve an enhanced understanding of the types, configuration, distribution and relative acidity and concentrations of silanol sites present. The research completed includes studies of a general-purpose, type A, porous silica gel, and a type B, low acidity, chromatographic grade silica. 

The second type of interaction, displacement interaction, is depicted in Figure 10. This type of interaction occurs when a strongly polar solute, such as an alcohol, can interact directly with the strongly polar silanol group and displaces the adsorbed solvent layer. Depending on the strength of the interaction between the solute molecules and the silica gel, it may displace the more weakly adsorbed solvent and interact directly with the silica gel but interact with the other solvent layer by sorption. Alternatively, if solute-stationary phase interactions are sufficiently strong, then the solute may displace both solvents and interact directly with the stationary phase surface. 

Apart from the mentioned advantages, the polymeric reagents covalently adsorbed by silica also diminish its inherent non-specific adsorptivity. One of the ways to synthesize a polymeric modifier of this type is a copolymerization of a vinylsilane with a compound of the desired functionality. The segments carrying silyl groups will condense with the surface silanols forming anchors or trains . 

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