Hydrolysis and condensation of silanes in aqueous solutions

Aminosilanes hydrolyze rapidly in aqueous acidic solutions and exhibit good stability; however 1H NMR measurements indicate that γ-MPS (γ-methacryloxypropyltrimethoxy silane) hydrolyzes slowly and condenses rapidly under similar conditions. The rate constant of these two reactions depends upon the pH of the solution, and the rate constant of the condensation reaction also depends upon the initial γ-MPS concentration. The hydrolysis is faster and the condensation slower at low pH's. These factors explain why an induction period is observed before the adsorption of γ-MPS onto mica takes place and why it does not occur if the induction period is too long. Using the Lentz technique, it is also shown that the main species resulting from the condensation of a concentrated γ-MPS solution is a cyclic trimer or tetramer, in agreement with literature results.