Silanol surface concentration

Silanol Surface Concentration Surface Area Primary Particle... 

The above silica-mediated syntheses are limited by the number of silanol groups on the surface, which control the maximum surface concentration of the intermediate [HOs3(CO)io(OSi j [89]. Only loadings up to 4wt% Os/Si02 can be used [90]. Despite this limitahon, circa 300 mg of cluster can be obtained in a single reaction using circa 10 g of silica, which can be recycled after completion of the reaction [13]. 

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... 

Factors that influence the retentive powers and selectivity of such bonded phases include the surface concentrations of hydrodartenaceous ligates and free silanol groups. The thermodynamic aspectitm solute interactions with the hydrocarbonaceous ligates at the surface, which are hydrophobic interactions in the case of aqueous eluents, are discussed later in this chapter within the framework of the solvophobic theory. In practice, however, solute interactions with surface silanol which may be termed silanophilic interactions can also contribute ]to retention (71, 75, 93), particularly in the case of amino compounds. Consequently the retention mechanism may be different from that which would be ol served with an ideal nonpolar phase. Therefore, increasing attention is paid to the estimation of the concentration of accessible sianols and to their elimination from the surface of bonded phases. 

Consistent with the lower degree of association between the silanol and amino groups, an increase in the intensities of the ions at +45, +62 and +79 amu is observed. These are Si(OH) type ions. However, the Si+ and the SiO+ peak intensities, which are a measure of the total amount of silicon, have decreased. This is to be expected in view of the higher amino group surface concentration. 

Heat-cleaned E-glass slide surface. Table 1 shows that the heat-cleaned glass surface is confirmed to be silica-rich because of higher Si and O surface concentrations compared to the bulk analysis obtained by inductively coupled plasma (ICP). This is accommodated by a lower surface calcium concentration. The SIMS results given below demonstrate that a significant proportion of the oxygen is probably present as silanol. 

Results similar to Figure 7 have been obtained with triethylamine adsorption from cyclohexane onto Hi-Sil, a silica gel with a high surface density of silanol groups (34). This is in contrast with Cab-0-Sil, a silica gel known to have a lower surface concentration of silanol groups, some of which are stronger acids... 

In the temperature range above 873 K, the surface is mainly covered with free silanols. Their concentration decreases as a function of increasing temperature, due to a condensation reaction forming siloxane bridges. Some of these condensation reactions occur as interparticle condensations, explaining the collapse of surface area and porosity above 873 K. 

Recent surface force measurements revealed a similar trend (20). Comparing steam-treated to flame-treated silica sheets using site-dissociation/site-binding model, a decrease in silanol surface sites and apparent decrease in average pKa was observed upon heat treatment. Furthermore, a repulsive force other than double-layer and van der Waals forces was observed 15 A from the surface. This repulsion was attributed to hydration of the surface and was found to be independent of surface treatment and electrolyte concentration. In Bums treatment, an arbitrary plane of shear was introduced to provide a best model fit (l 3). A value of 9 A from the surface for the plane of shear was determined from electro-osmosis measurements. 

Many attempts have been made to measure silanol surface density (aon)-Her [7] estimated Ooh to be equal to 8 groups/nm on the basis of the [100] face of P-cristobalite. However, most porous amorphous silicas show surface silanol concentration on the level of 4.6 to 5 groups/nm [6]. 

All monofunctional modifiers decrease the surface concentration of silanols by 50% (depending on the alkyl ligand) compared to original silica (7.6pmol/m ), while difunctional modifiers only slightly reduces the surface concentration of silanols by 12% and trifunctional modifiers apparently increases the amount of silanols. This reflects actual hydrolysis of nonreacted chlorine groups of the alkyl di- and trifunctional modifiers and their conversion into silanols. 

All these effects are strongly dependent on the surface concentration of bonded ligands (bonding density), and despite the presence of a significant amount of unreacted silanols on the sihca surface, only the silanols accessible for analytes should be considered. 

An understanding of the mechanisms by which organosilicon compounds become attached to mineral substrates requires a knowledge of the crystal structure of the substrates examined. There is no doubting the presence of copious functional silanol groups (Fig. 10) on silicate material (quartz, feldspar, mica, clay) [2, 19-21], whose surface concentration on quartz fracture planes is... 

HCl treatment Removal of inorganic impurities, increase of surface silanol group concentration... 

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