Hydrolysis and condensation of transition metal alkoxide: Experiments and simulations

Abstract

Abstract. We present here an experimental and numerical study of the chemical and aggregative mechanisms for titanium alkoxide in a reverse micellar medium. NMR experiments were done to study the first step of the sol-gel process: hydrolysis of titanium isopropoxide. Light and X-ray scattering experiments were then conducted to characterize the clustering of aggregates and aggregation kinetics. Fractal dimension, measured by U.S.A.X.S., varies with the hydrolysis ratio and the hydration of the surfactant. This evolution was explained by polydispersity of the hydrolysis which depends on the surfactant concentration. This hypothesis was tested through a numerical simulation of irreversible aggregation using a chemically limited aggregation model. The hydrolyzed sites are distributed among the monomers by a random procedure governed by a polydispersity parameter p, varying from 0 to 1. The total number of hydrolyzed sites is governed by a parameter q belonging to the interval [0, 1]. The p-q evolution of the fractal dimension coincides rather well with the experimental data in all the region of the two dimensional phase space.

Theoretical study of the mechanisms of the hydrolysis and condensation reactions of silicon and titanium alkoxides: similarities and differences

“…The equilibrium structure of the titanium tetramethoxide dimer is shown in Fig. 6 (21). The expansion of titanium coordination up to the five-fold one is provided by sharing Ti(OMe) 4 tetrahedra edges.…”

“…In contrast to silicon alkoxides, where catalysts are usually used to promote hydrolysis, the high reactivity of titanium alkoxides towards water must be controlled and moderated by chemical additives to obtain well-defined gels. [19][20][21][22][23] The other manifestation of the coordination unsaturation of titanium is that alkoxides readily form oligomers in which titanium expands its coordination up to a sixfold one. In the solid phase, both the titanium tetraethoxide 24 and tetramethoxide 25 exist in the tetrameric form, as revealed by X-ray diffraction analyses of single crystals.…”

In Situ High‐Energy Synchrotron Radiation Study of Sol–Gel Nanoparticle Formation in Supercritical Fluids

“…The study used supercritical, seed‐enhanced crystallization of anatase TiO 2 nanoparticles 23. TiO 2 is a preferred system for experimentalists because it is well suited for many experimental techniques 24. The combined real‐time use of SAXS and WAXS provides fundamental insight into the mechanism of supercritical nanoparticle formation: nanoparticle formation follows the same progression seen in traditional solvothermal synthesis but on a much shorter timescale and at lower temperatures.…”

“…Nanoparticle formation is shown in Figure 1, where the WAXS (b) and SAXS (c) curves are plotted as a function of time. In the first 20 min of the experiment, the SAXS curves are those of a “wet gel” 24. 25 After the gelation period a transformation period is observed from 20 to 80 min, which is followed by slow particle growth.…”