Hydrolysis and Condensation of Transition Metal Alkoxide: Experiments and Simulations

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 parameterp, varying from 0 to 1. The total number of hydrolyzed sites is governed by a parameterq belonging to the interval [0, 1]. Thep-q evolution of the fractal dimension coincides rather well with the experimental data in all the region of the two dimensional phase space.     

Extreme swelling of a lyotropic lamellar liquid crystal

The evolution of the OBS/water/decane/pentanol lyotropic lamellar crystal is followed from 0% to more than 90% decane content. The lamellar spacing (d) varies then from 35 Å to 11000 Å. The swelling is followed with small angle x-ray scattering and the Bragg divergence in the intensity is found to disappear in moderately swollen lamellar crystal (d from 200 Å up to 800 Å) while the central scattering increases. More striking is the reappearance of the Bragg divergences observed by SAXS (d from 800 Å to 1100 Å) and for extremely swollen lamellar crystals in the angular distribution of scattered light (d from 2 000 Å to 10 000 Å). We discuss these observations along the lines of the recent models of swollen lamellar liquid crystals and in particular the apparent evolution of the dilute lamellae.     

Gelation of tenuous titania gels in a reverse micellar medium

Tenuous gels of TiO₂ are synthesized from titanium isopropoxide in a reverse micellar medium. The volume fraction of TiO₂ in these gels is equal to 0.2%. The chemical and aggregative mechanisms are studied using ¹H NMR, small-angle X-ray scattering, dynamic and static light scattering. The alkoxide hydrolysis occurs at the first stage of the gelation. Small clusters (70–80 Å) are produced. The aggregation induces mass fractal structures with fractal dimensions depending on the solvent used in the synthesis: 1.4<D _f<1.9. The interpretation of these results is based on interactions between the growing inorganic chains and the surfactant molecules.     

Inorganic Polymerization in a Non-Ionic Lyotropic Lamellar Phase

Silicon tetramethoxide was polymerized within a nonionic lyotropic lamellar phase. After gelation, the birefringence of the sample is preserved and polarized light optical microscope observations reveal no segregation. High Resolution Small Angle X-ray Scattering patterns of the gel shows Bragg peaks characteristic of a layered structure. Freeze fracture experiments on the lamellar gel show stacks of parallel bilayers. The experimental observations indicate that the silica is located in the lamellar structure.