Factors affecting the rate of hydrolysis

At relative low pH, condensation rates are proportional to the [H ] concentration but because of low solubility rates of silica ( pH 2] (isoelectric point of silica pH 2.2), precipitation occurs. Formation and aggregation of primaiy silica particles occur together and contributes little to the growth of network. Thus developing gel network consists of extremely small primary particles, not exceeding 2 nm in diameter [37]. 

As the solubility of silica particles is greatly affected by the temperature of the reaction mixture, the particle size shows dependence on temperature as well. At a higher temperature, the solubility of silica is more therefore, larger particles are formed before they drop out of the solution. 

The R values greatly affect the potysilicate product formed. R values ranging from less than 1 to over 50 can be performed to carry out hydrolysis reaction. Under acidic condition, hydrotysis of TEOS is found to be first-order in water concentration, which is apparent different from base-catal3 ed hydrolysis showing zero-order dependence on water concentration [39]. 

The downstreaming of sol-gel processes routes to the production of different nanostructures with distinct and unique surface properties. For example xerogels and aerogels are produced 

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A. R. Butler, K. A. Freeman, D. E. Wright, A Study of Factors Affecting the Rates of Hydrolysis of Some /V-Arylazetidin-2-oncs ,./. Chem. Soc., Perkin Trans. 2 1977, 765-769. 

It should be noted that the inductive effect is not the only factor affecting the rate of hydrolysis. The substituent may also have a steric effect. For example, a bulky substituent may shield the ester from attack and lower the rate of hydrolysis. It is therefore necessary to separate out these two effects. This can be done by measuring hydrolysis rates under basic conditions and also under acidic conditions. Under basic conditions, steric and electronic factors are important, whereas under acidic conditions only steric factors are important. By comparing the rates, values for the electronic effect (ar), and for the steric effect (Es) (see below) can be determined. 

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