Correlations between Silica Chemistry and Structural Changes in Hydrothermally Treated Hexagonal Silica/Surfactant Composites Examined by in Situ X-ray Diffraction

Abstract

In this paper, the effect of silica chemistry on hydrothermal restructuring of silica/ surfactant composites is investigated. The materials were studied using real time X-ray diffraction to follow structural changes in p6mm hexagonal samples as they were hydrothermally treated in buffers ranging from pH 7 to pH 11. Changes in pore shape, repeat distance, and peak area were found to depend on the treatment conditions. Treatment at pH 11 caused expansion of the lattice, a small amount of pore shape restructuring, and a small increase in diffraction peak area. Treatment in a pH 7 hydrothermal solution, by contrast, resulted in contraction of the lattice, significant pore shape restructuring, and large increases in diffraction peak areas. These changes were correlated with 29 Si MAS NMR, which was used to examine changes in framework polymerization, and with liquid 1 H NMR, which was used to follow loss of surfactant from the composite. It was found that lattice expansion is facilitated by low framework polymerization and little or no surfactant loss while the opposite conditions were necessary for lattice contraction. The maximum amount of pore restructuring occurred under pH 9 conditions. At this pH, both silica condensation and silica hydrolysis have appreciable rates, suggesting that both processes are necessary for optimum restructuring. Finally, correlations between surfactant loss and changes in overall diffraction intensity indicated that these changes resulted primarily from increased electron density contrast in the material caused by loss of surfactant from the organic domains. The conclusions of this work help explain the molecular basis for the modifications that are commonly observed in X-ray diffraction patterns after hydrothermal treatment of periodic silica/surfactant composites.

Postsynthesis Stabilization of Free-standing Mesoporous Silica Films

“…This thermally conditioned disorder is suggested to be contributing to the expansive force that drives the swelling process of the mesostructure, shown in Figure 1. Under hydrothermal conditions, similar effects have been observed by Gross et al and Tolbert et al [37][38][39] Additionally, the creation of basic conditions (mixed ammonia-water vapor and to a lesser extent plain water vapor) changes the charge balance at the surfactant headgroup-silica interface. This leads to a net surfactant removal.…”

Controlled Release and Absorption Resonance of Fluorescent Silica‐Coated Platinum Nanoparticles

“…However, in the absence of excessive alkoxysilane, silica can be slowly hydrolyzed and dissolved by breaking up siloxane bonds under the similar condition. [12,13] The slow dissolution of silica can be accelerated by increasing reaction temperature. [14] At an elevated temperature of 50°C, the Pt@SiO 2 LPNs underwent a drastic change in a pH 11 aqueous solution after heat treatment for a longer time.…”

Hydrothermal synthesis of mesoporous silica spheres: effect of the cooling process

“…Hydrothermal methods have been extensively adopted for the fabrication of silica-related micro-/nanomaterials. 4,[24][25][26][27][28][29][30][31][32][33][34][35] Especially, hydrothermal methods are very effective for the synthesis and post-synthesis treatment of mesoporous silicas, which can lead to products with higher hydrothermal stability, improved mesoscopic regularity and extended pore size. 29,[30][31][32] However, systematic studies on the silica chemistry under hydrothermal conditions are relatively lacking.…”

“…29,[30][31][32] However, systematic studies on the silica chemistry under hydrothermal conditions are relatively lacking. 30,33 Especially, the effect of ''post-synthesis'' sample cooling process is barely involved in related studies. In 2002, Di Renzo and coworkers developed a unique ''pseudomorphic transformation'' method to introduce mesoporosity into pre-formed silicas, which involves fine control over the silica dissolution/regrowth kinetics under alkaline hydrothermal conditions.…”