The Surface Chemistry of Silica

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 triol 16 can be deprotonated according to Scheme 6. The initial step involves met-allation and this is followed by a coproportionation on addition of two further equivalents of 16. Finally, addition of an ammonium or phosphonium salt gives 36. The stability of the ion has implications for the surface chemistry of silica where similar multiply hydrogen bonded anion sites are likely to be more acidic than isolated silanol groups144. The related cubic polyanion [SigOigtOHji]6- can be prepared directly from aqueous solutions of silica and a quaternary ammonium hydroxide derivative such as [NPhMe3]OH145 146. 

Silica materials are thermally stable and exhibit ample chemical stability. They dissolve in strong basic solutions and hydrofluoric acid. The surface chemistry of silica... 

An account of the surface properties of silica powders has been given by Hockey (H7). Basila (B3) has studied the surface chemistry of silicas. In the diatomaceous earths, it is generally considered that the main forces responsible for adsorption of solutes are the weak van der Waals forces and the stronger forces due to hydrogen bonding. The van der Waals forces are neutralized by all liquid phases and so need not be considered here. In the diatomaceous support there will be both... 

The work of Her and his co-workers led to the creation of new materials and industries. His studies of the surface chemistry of silica led to the invention of unique organophilic and hydrophobic silica products termed estersils . Virtually a new industry was created when he rendered titania pigment particles in organic films and finishes photochemically inert by coating them with silica. 

The surface chemistry of silica was a subject of intensive study in the period between 1960 and 1970 as a consequence of the widespread industrial use of colloidal, pyrogenic, and precipitated silicas, as well as silica hydrogels and xerogels. Chemical surface reactions and IR spectroscopy were the most-applied methods in surface structure elucidation. Significant contributions to the understanding of the silica surface were made by Fripiat (I), Kiselev and co-workers (2), Hair (3), Little (4), Peri... 

Vibrational spectroscopy has had a profound effect on our understanding of the surface chemistry of silica. Indeed, it has been the model system for the use of IR spectroscopy for probing the surface chemistry of oxides, the first studies having been carried out in late 1950. The books written by Hair [1] and Little [2] in the sixties have become classics with respect to the early use of IR spectroscopy for studying the surface properties of silica and adsorbed species thereon. 

This paper will only be concerned with the surface chemistry of silica in vacuum and/or in the presence of an adsorbate. The IR spectrum of a typical self-supporting disc of a pyrogenic or fumed sifica after heating under vacuum for 1 h at 150 C is shown in Figure 25.1 A. Pyrogenic or fumed silicas [some trade names are AerosU and Cab-O-Sil] are made by the flame hydrolysis of SiCLt at 1000°C. These non-porous silicas have a low bulk density and adsorbed water can be removed by evacuation at 20°C. However, the spectral properties are identical when evacuation is carried out at 150 C evacuation at the latter temperature is preferred in order to remove any trace impurities which may be present. The spectrum is characterized by a sharp absorption band at 3747 cm with a broad tail to low wavenumber having a maximum near 3550 cm . The sharp peak is due to the OH... 

Chapter 22 The Surface Chemistry of Silica — The 22iuravlev Model. 261... 

The NMR techniques were also used to analyze different structural, surface reactions, reaction mechanisms, and other aspects related to the surface chemistry of silicas. For instance. Brunet et al. (2008) studied the electron irradiation effects on controlled-pore borosilicate glasses (CPGs at 8 and 50 nm in pore size, 96% Si02 and 3% B2O3) using multinuclear solid-state NMR technique. H MAS NMR was used to study the surface proton sites. They showed that the irradiation leads to a dehydration of the material. The observed variation of the Q4, Q3, and Q2 species from H- Si CPM AS spectra showed an increase of the surface polymerization under irradiation, implying in majority a Q2 to Q3/Q4 conversion mechanism, and CPMAS measurements exhibited an increase in... 

Silica gels have extremely good mechanical properties. They can be packed with minimum attrition and can withstand considerable pressures. They resist many chemicals (especially oxygen) and easily withstand temperatures up to 600 C without any structural changes. However, the surface chemistry of silica gel changes on heating. 

From the foregoing discussion on the bulk structure, the pore structure, and the surface chemistry of silica, one can derive some important conclusions and practical guidelines that are hi ly relevant for the separation of biopo miers by HPLC. 

The surface chemistry of silicas is dominated by the surface hydroxyl groups, or silanols (Si-O-H). The silanol groups participate in adsorption (for water and other compounds, including organic compounds) as well as the chemical modification of silica surfaces. The subject of surface silanols has been studied extensively over the past seven decades and has been reviewed (e.g., Her, 1979 Unger, 1979 Zhuravlev, 1993 Bergna, 1994 Vansant et al 1995). 

Surface treatments for fillers have been extensively reviewed [124]. Ernstsson and Larsson studied a range of mineral fillers in terms of the Lewis acid-base character [125, 126]. It was found that each one had a different surface chemistry and they were all amphoteric (possessing both Lewis acid and basic sites). Another point was that trace impurities of iron on the filler surface, presumably picked up during processing, significantly changed the surface chemistry of silica. 

Oxides.—The adsorbent properties of silica gel are known to be highly dependent on the degree of hydroxylation of the surface, and this dependency has been widely investigated in relation to gas adsorption.Khopina and Eltekov have continued earlier work of Kiselev and his collaborators on the influence of the surface chemistry of silica on adsorption from solution. In the present paper a comparison is made of the adsorption of a series of aromatic hydrocarbons (benzene, naphthalene, biphenyl, phenanthrene, o- and m-terphenyl) from n-heptane solution by hydroxylated silica, dehydroxylated silica, and graphitized carbon black, using data obtained in the present work together with earlier published data. The results are analysed in terms of the separation factor(partition coefficient)... 

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