The effect of HF in a two-step sol-gel process of TEOS

The addition of HF to a TEOS-ethanol-water mixture results in faster gelation compared to a system without HF. When HF is added in the basic step the effect is most pronounced. The addition of HF in the acid step only results in an enhancement of the condensation reactions. The effect of HF in the acid step is largely determined by the H+ concentration due to the low acidity of HE At low HF concentrations a maximum in gelation time is found as a function of the H+ concentration. At low H+ concentrations the F- enhances gelation; at high H+ concentrations the influence of F- is negligible but gelation is enhanced by the protons.