Pyrogenic silica, surface groups

Activation parameters of the organosilicon compounds reactions with OH groups on the pyrogenic silica surface (A, pre-exponential factor and observed activation energy on free surface) [79-81], the estimated heat of adsorption complex formation (Qads) and activation energy of proton transfer (E j ) from [49]... 

Reactions of these compounds with OH groups of pyrogenic silica surface proceed via the SeI mechanism [4,57,108,109,133,140]. Nevertheless, it has been disclosed that, apart from the occurrence of chemisorption by the SeI mechanism, the dissociation of silica surface Si-0 bonds occurs by the Aji mechanism in the interaction of such compounds as (CH3)3SiX (X = N3, NCS and NCO) [4,58-61]. 

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

We conclude that in the precipitation process, the trapping of water has resulted in the formation of a more open intraparticle microstructure than in the pyrogenic silicas. The internal surface remains hydroxylated, but is not easily accessible to most adsorptive molecules. Water molecules are able to undergo specific interactions with the internal OH groups which accounts for the abnormally high uptake of water vapour. 

In fig. 1.26 the effect of sample pretreatment is illustrated. The original sample is "Cab-0-Sir, a pyrogenic silica. It has a fairly low affinity for water. The isotherm type is between II and III (fig. 1.13). No hysteresis is observed. Stronger outgasslng (fig. (b)), further reduces the affinity for water the curve is now definitely of type II but also shows considerable hysteresis which was attributed to incomplete hydroxylation. In case (c) the surface is made hydro-phobic by methylatlon. The water adsorption isotherm (not shown) remains of type II but as Nj adsorption is not determined by hydrophilic groups, the corresponding Isotherm is of type III. Again, it is hysteresis-free. By application of the theories outlined before, information can be extracted from these isotherms in terms of available areas and enthalpies of adsorption. The authors extended this work with infrared studies. 

This is a simple second order equation which usually applies to description of dissociative chemisorption from the gas phase on the homogeneous surface. However, Eq. (43a) describes well only an initial part of kinetic chemisorption isotherm for interaction of trimethylchlorosilane on dehydrated pyrogenic silica [113] and mixed alumina-silica and titania-silica [114] surface. At the same time, whole kinetic chemisorption isotherms are described using equations derived for the heterogeneous surface. Thus, high fractional reaction orders in respect to the silica surface OH groups obtained by Hertl and Hair for chemisorption of silanes and siloxanes [106,107] and in other studies [113,114] may be explained by heterogeneity of the oxides surface. 

Besides their comparatively large specific surface area, their well-defined spherical primary particles, and their high chemical purity, the surface chemistry of pyrogenic oxides is another reason for their use in catalytic applications. Siloxane (Si-O-Si) and silanol groups (Si-OH) are the main functional groups (Figure 1.4) on the surface of pyrogenic silica. 

Boehm and Schneider (185) caused alumina to be adsorbed on the surface of a pyrogenic silica by suspending it in a dilute solution of aluminum chloride and adding enough NaOH to neutralize two-thirds of the chloride, thus forming basic aluminum chloride. The silica surface adsorbed one Al atom per SiOH group and the solubility of the silica dropped from 123 to 6 ppm. 

The simplest example of the former is the effect of treating a pyrogenic silica with trimethylchlorosilane, which coats the surface with a chemisorbed monolayer of SiOSi(CH3)3 groups. In undisturbed suspensions of treated and untreated powders, the rate of dissolution of the treated material in neutral 1% NaHC03 solution at 25 C is only 15% as fast as the untreated. When. shaken, however, the coated silica aggregates are apparently broken apart, leaving bare spots on the ultimate particles which then dissolve much faster. The difference is noted mainly below pH 7 (195). 

As shown also by Yates and Healy, if attempts are made to obtain nonporous silica particles by heating to 800°C, much of the surface is dehydroxylated to siloxane groups. When this surface is then rehydroxylated there is no evidence that a gel layer is formed again. However, on such heat-treated silica and on pyrogenic silica particles, after hydration, the surface charge density in relation to pH is still much higher than observed, for example, at the classical Agl-solution interface. It was concluded that even on a nonporous surface of amorphous silica the charging ions must be able to penetrate the surface to some extent. 

There appears to be a significant difference between silica particles that have been made in an aqueous medium and those made at high temperature, that is, pyrogenic silica, when initially dispersed in water. Part of the surface of pyrogenic silica can remain as a nonhydroxylated siloxane surface for some time so that only part of the surface is covered with ionizable SiOH groups. The difference has been clearly demonstrated by Tschapek and Torres Sanchez (287), who showed that the dehydrated silica acted as though it were hydrophobic. At low pH where there is very little surface charge, the silica is flocculated by traces of salt. 

The dielectric constant of a given type of silica powder is related to the SiOH surface groups and therefore to specific surface area. Pyrogenic silicas, low in silanol groups, have a lower constant (104). 

A siloxane surface which consists mainly of oxygen atoms, each bonded to adjacent silicon atoms. Usually a low fraction of isolated or paired SiOH groups is also present. Pyrogenic silicas condensed from the vapor state are of this type. Also, hydroxylated silicas which have been dehydrated at around lOOO C develop a siloxane surface by removal of water from adjacent silanol groups. 

An enzyme can also be attached to the surface of a voluminous pyrogenic silica powder, by reaction with aminopropyl silyl groups previously chemisorbed on the surface. An activator may simultaneously be applied. Reynolds and Miller have patented a recoverable trypsin of this type (278). 

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