Silicates surface silanol

Sears 189) and Heston et al. 190) used the adsorption of sodium hydroxide for the determination of the surface area of colloidal silica. An empirical factor was used for the conversion of alkali consumption into surface area. This is permissible provided the packing density of surface silanols is constant. The determination was performed in concentrated sodium chloride solution in order to keep down the dissolution of silica. Using the same technique, it was found in my laboratory that all surface silanol groups as determined by other methods are neutralized at pH 9.0. At higher pH, siloxane bonds in the surface were opened. A maximum in the sorption of Na+ ions occurred usually at pH 10.5-10.6 which corresponded to a packing density of ca. 5 OH/100 A. On further addition of alkali, silicate ions H3Si04 went into solution. 

The siliceous surface provides various reactive sites that have been assigned to distinct silanol groups (isolated, perturbed isolated, vicinal, geminal) and strained... 

S-SBR, and organoclay it can be assumed that the surface silanol groups of the layered silicates react with the carboxyl groups of the XNBR and, thus, direct rubber-filler bonds are formed. In this way, the high elongation properties can be explained. 

Metal ions that diffuse to the surface react with atmospheric gases to form hydroxides in a thin layer of high pH [36]. Fowkes improved adhesion of a basic polymer to glass with an acid wash [6, 7] a similar treatment also improved adhesion to epoxy [37], an acidic polymer. The former effect was attributed to replacement of sodium silicate with silanols by ion exchange the latter may result from removing a corrosive layer of metal hydroxides from the interface. IGC and PTD measurements in this study demonstrate a measurable basicity on bare fibers, which other investigators also noted [6,14,17],... 

The specific activity of Ti-sites in oxidation with aqueous H202 was shown to decrease from microporous to mesoporous molecular sieves such as MCM-41 and HMS. The surface hydrophilicity of the latter mesoporous silicates is likely one of the main reasons for the low activity of Ti sites.[47] Water molecules adsorbed on the surface silanol groups would limit the access of organophilic reagents to the Ti sites.1751 This can be limited, to a certain extent, by grafting hydrophobic groups on the surface silanols.1761... 

Quartz is a crystalline form of SiC>2 where silicon is tetrahedrally bound to four oxygen atoms. At the surface of quartz and other siliceous substances, acidic silanol groups formed from hydrolysis are present. Surface silanols are titratable in aqueous solution and are a source of pH-dependent surface charge. Fully hydroxylated silicas are considered to have a silanol population of 5/nm2 with an intrinsic pKa from 5.8 to 7.2 [18]. 

Condensation occurs most readily at a pH value equal to the pKa of the participating silanol group. This representation becomes less valid at pH values above 10, where the rate constant of the depolymerization reaction (Ar 2 ) becomes significant and at very low pH values where acids exert a catalytic influence on polymerization. The pKa of monosilicic acid is 9.91 d= 0.04 (51). The pKa value of Si—OH decreases to 6.5 in higher order silicate polymers (52), which is consistent with pKa values of 6.8 d= 0.2 reported for the surface silanol groups of silica gel (53). Thus, the acidity of silanol functionalities increases as the degree of polymerization of the anion increases. However, the exact relationship between the connectivity of the silanol silicon and SiOH acidity is not known. 

It would appear that the most probable explanation of the results is that in higher ratio solutions the presence of colloidal size silicate anions with more acidic surface silanol groups readily adsorb nucleating metal ion hydroxides thus enhancing the reduction of pMe. This adsorption might be enhanced by the presence of 0H , or increased5Si-0 sites on the colloidal sized silica surfaces at higher pH values. 

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