Silanols calculations

If we take the number of surface silanols calculated and divide by the area of the particle surface we can calculate the surface density of silanol groups. Such calculations are plotted in Figure 12.19 for several particle sizes in the range of 5 to 100 nm. For larger particles the calculated values asymptotically approach 7.9. In the normal range of colloidal silica sizes sold (5-25 nm) the calculated values range from about 7-7.6. 

Prepare a solution of the silanation agent. For trimethylchlorosilane, use toluene as the solvent. The minimum requirement of the silanation agent is calculated by assuming the surface density of silanol to be one functionality per 0.1 nm. ... 

Bleiber, A., Sauer, J., 1995, The Vibrational Frequency of the Donor OH Group in die H-Bonded Dimers of Water, Methanol, and Silanol. Ah Initio Calculations Including Anharmonicities , Chem. Phys. Lett., 238, 243. 

Resins 2 and 3 are treated with dichloromethane containg 3% and 1.5% trifluoroacetic acid (lOmL/g resin), respectively, for 18 h. The resin is filtered off and washed twice with dichloromethane (10 mL / g of resin). The filtrate is washed with saturated NaHCCL (5 mL) and brine (5 mL), and the organic phase is separated and filtered through a short path silica gel column to obtain a colourless solution. In the case of polymer-bound allyl esters giving rise to cleavage products of type 5f, the aqueous workup is omitted. The products obtained after removal of solvent under reduced pressure contain small amounts of silanol by-products (note 5), which is to be accounted for in the calculation of cleavage yields. 

A. Bleiber and J. Sauer, The vibrational frequency of the donor OH group in the H bonded dimers of water, methanol and silanol. Ab initio calculations including anharmonicities. Chem. Phys. Lett. 238, 243 252 (1995). 

The removal of free silanol groups is important for correct calculation of the ratio of the two monomers in the subsequent coequilibration-step. This step is required in order to generate a copolymer of appropriate viscosity and to separate the functional groups in. the polysiloxane by at least five dialkylsiloxy units. The reason for this necessity is discussed below. 

De Boer and Vleeskens (181) calculated the packing density of surface silanol groups from the crystal structures of cristobalite and tridymite. The results varied between 4.55 and 4.85/100 A . Similar estimates by Schneider (182) are presented in Table XV. The silanol groups on... 

Esterification of Silanol Groups on Aerosil Surfaces [Calculated from Measurements by... 

Very few direct measurements of the reaction of surface silanol groups on quartz have been reported. This is apparently caused by the small effects due to the limited surface areas available. Adsorption of sodium ions on quartz was measured by radioactive tracer techniques by Gaudin et al. (293). Saturation was achieved at high pH (>10) and sodium ion concentrations above 0.07 Jlf. The calculated packing density of silanol groups was 4.25/100 A. Goates and Anderson (294) titrated quartz with aqueous sodium hydroxide and alcoholic sodium ethylate. The occurrence of two types of acidic groups was reported. 

Similarly, [(Bu CH2)2Mo(=NAr)(=CHCMe2R)] reacts with the silanols group of a silica or a molecular silanol to yield [(=SiO)Mo(=NAr)(=CHCMe2R)(CH2Bu )] via direct electrophilic cleavage of the Mo-C bond, according to mass balance analysis, NMR and calculations (Scheme 2.27) [72, 73]. 

The initial adsorption of the oxime in zeolites was studied through a combination of solid-state NMR spectroscopy and theoretical calculations ". The calculated adsorption complexes formed over silanol groups and complexes over Brpnsted acid sites in zeolites are depicted. This study suggests that the A-protonated oxime is formed over Brpnsted acid centers, but not over weakly acidic silanol groups. It has been also suggested that weakly acidic or neutral silanol groups or silanol nests are active catalysts of the rearrangement reaction ... 

As appears from the examination of the equations (giving the best fit to the rate data) in Table 21, no relation between the form of the kinetic equation and the type of catalyst can be found. It seems likely that the equations are really semi-empirical expressions and it is risky to draw any conclusion about the actual reaction mechanism from the kinetic model. In spite of the formalism of the reported studies, two observations should be mentioned. Maatman et al. [410] calculated from the rate coefficients for the esterification of acetic acid with 1-propanol on silica gel, the site density of the catalyst using a method reported previously [418]. They found a relatively high site density, which justifies the identification of active sites of silica gel with the surface silanol groups made by Fricke and Alpeter [411]. The same authors [411] also estimated the values of the standard enthalpy and entropy changes on adsorption of propanol from kinetic data from the relatively low values they presume that propanol is weakly adsorbed on the surface, retaining much of the character of the liquid alcohol. 

The proposed mechanism of the effect of water can be supported by two other findings (i) the calculations of Maatman et al. [410] revealed that the active sites could be identified with surface silanol groups [Sect. 4.1.2.(a)] and(ii) independent studies of other authors [424—426] showed that silica gel could actually adsorb two layers of water the first layer is strongly chemisorbed whereas the second is less strongly adsorbed and retains much of the character of free water. The standard enthalpy and entropy changes on adsorption determined from kinetic adsorption coefficients, Kr and Kr, for the first and second layer, respectively [411], are consistent with this observation. 

The number of -OH groups present on the PCH was determined to be 1.89 mmol/g or 1.03 OH/nm2. For the different concentrations of Al(acac)3 with respect to the silanol number (25%, 50%, 75% and 200%), the parameter R can be calculated. This R-value is defined as follows and gives us further confirmation about the bonding mechanism ... 

Further calculations with respect to the loading capacity on the surface can be performed knowing the surface area of the support (SA= 999 m2/g) and the mean cross-sectional area of the Al(acac)3 complex (0.60 nm ) [8] A full monolayer would therefore correspond to an Al-loading of 2.7 mmol/g. However, the actual maximal Al-loading obtained here is 0.672 mmol/g, which means only 25% of the monolayer capacity. When comparing this value to the silanol number of 1.89 OH mmol/g, it can be concluded that 36% of the available OH-groups on the surface have reacted with the Al-complex. 

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