Fluorescence Probing Investigation of the Mechanism of Formation of MSU-type Mesoporous Silica Prepared in Fluoride Medium

Abstract

The mechanism of formation of a MSU-type siliceous material from tetraethyl orthosilicate (TEOS) in the presence of the nonionic surfactant tergitol T-15-S-12, sulfuric acid, and sodium fluoride has been investigated using mainly fluorescence probing techniques and, to a lesser extent, dynamic light scattering (DLS) and 29Si NMR spectroscopy. The tergitol micelles present in the systems obtained by progressively generating the reaction mixture giving rise to the mesostructured material by adding to an appropriate tergitol solution sulfuric acid, TEOS, and NaF were characterized by fluorescence probing (micelle aggregation number, micropolarity, and microviscosity) and also by dynamic light scattering (apparent micelle diameter). 29Si NMR experiments were also performed on selected systems after hydrolysis of the TEOS. The fluorescence probing techniques were also used to follow the changes of micelle characteristics with time during the evolution of the full reaction mixture from a limpid solution to a system containing a minor amount of condensed siliceous material. The synthesized solid material was characterized by X-ray diffraction and nitrogen adsorption-desorption analyses. The micelle aggregation number N was found to change only little, and the micropolarity remained constant when going from the tergitol solution to the full reaction mixture. The results of DLS measurements agree with this finding. Besides, while the condensation of silica took place after addition of NaF, the N value increased only very little with time up to the point where a small amount of mesostructured material precipitated out. These results indicate that the interaction between tergitol micelles and the siliceous species formed in the system by the hydrolysis of TEOS and also between micelles and the growing siliceous species must be very weak. As in our previous studies of the mechanism of formation of MCM41-type material from sodium silicate in the presence of cetyltrimethylammonium bromide, it appears that the locus of formation of the mesostructured material is not the micelle surface but the bulk phase. Micelles only act as reservoirs of surfactant providing surfactant monomer that binds to the growing siliceous species.

Particle Growth of Hybrid Materials Followed by Dynamic Light Scattering

“…These results are in good agreement with those recorded in the case of the hydrolysis of tetraethoxysilane, a precursor totally different from the perorganotins used in this study, in the presence of a nonionic surfactant. In that case, fluorescence probing investigations showed that the surfactant micelles do not interact with the inorganic precursor of the material, the siliceous species do not bind to the micelles, and the condensation of silica occurs in the aqueous phase.…”

“…The polar surface of hybrid particles should interact with the poly-(ethylene oxide) chains of the surfactant, as it was already reported for silica surfaces. 6,22 This complexation could hinder interparticular condensation and lead to a more divided hybrid, which would result in a higher specific surface.…”

“…Over the past few years hybrid materials have been the subject of considerable attention. These new materials, where the organic and the inorganic components interact through strong bonds and are assembled for the purpose of generating desirable properties and functionalities, show promising applications in optics, electronics, membranes, coatings, etc. − From a single type of precursor it is easy to modulate the properties of the final material, as hydrolysis and condensation reactions are under kinetic control, which has opened up a wide and diversified field of fascinating research. To get a better insight in the early stage of the preparative process, studies have been conducted on the size of the generated particles, mainly by dynamic liquid scattering (DLS), which appeared as a useful tool to follow the growth of hybrid particules and determine the rates of the reactions …”

A Mechanistic Study of the Formation of Mesoporous Structures from in Situ AC Conductivity Measurements

“…A rapid but small increase in micellar size is observed within the first 3000 s and is consistent, considering the hydrocarbon chain length and amino acid headgroup, with the formation of spherical micelles. In the mechanistically very different fluoride-assisted formation of MSU-type mesoporous silica, a similar behavior was found …”

Time-Resolved in Situ Raman and Small-Angle X-ray Diffraction Experiments: From Silica-Precursor Hydrolysis to Development of Mesoscopic Order in SBA-3 Surfactant-Templated Silica

“…They combined very nicely in situ EDXRD as well as multinuclear NMR ( 29 Si,17 O, 14 N) techniques; however, reactant molar ratios needed to be adapted to the employed techniques in order to optimize the experiments, and ended up to be quite far from the SBA-1 synthetic conditions. Lesaint et al 31 investigated the formation of the 2D-hexagonal SBA-We will show that it can be elegantly combined with SAXS experiments that will track the appearance and development of long-range order in various systems. A whole picture of the material synthesis can thus be obtained, starting from time t=0 when the inorganic precursor is added to the surfactant solution, until the recover from solution of the precipitated silica-surfactant mesophase powder (Figure 1).…”