T.M.H. Costa

Infrared and thermogravimetric study of high pressure consolidation in alkoxide silica gel powders

High density, transparent, crack-free and hard compacts of silica gel were produced by compaction under nearly hydrostatic environment at 4.5 GPa, at room temperature. The starting material was used three days after synthesis by hydrolysis of alkoxides without additional treatment. Fourier transform infrared spectroscopy (FTIR), using the KBr technique and a high vacuum cell at temperatures up to 450°C, and thermogravimetric analysis (TGA) up to 900°C was conducted. A reduction up to 60% in the adsorbed water content of the compacted silica gel was observed. Changes in the 3000 to 3800 cm−1 region indicate that the surface silanol groups became bridged, which promotes condensation reactions of dehydroxylation. Those results suggest that the mechanism for consolidation under high-pressure, is ‘cold sintering’ process, where silanol groups at the surface of the nanoparticles condense to form siloxane bonds between the particles and water, resulting in a stiff body with closed nanopores containing trapped water.

High pressure compaction of nanosize ceramic powders

High-density ceramic materials from nanosize ceramic powders were produced by high pressure under nearly hydrostatic environment up to 5.6 GPa, on a special configuration in a toroidal-type apparatus, at room temperature. Attempts to use a common solid pressure transmitting medium, as NaCl, resulted in cracked samples. Lead and indium, which have an extremely low shear strength, proved to be the suitable choices as a pressure-transmitting medium to compact these ceramic materials, in order to obtain high-density samples. Transparent amorphous SiO 2 -gel and translucent γ−Al 2 O 3 samples, in bulk, with volumes about 40 mm 3 , hard and crack-free were obtained. Densities over 90% of full density for the γ −Al 2 O 3 samples and over 80% for the compacted SiO 2 -gel samples were obtained. In addition, from the density-pressure curve, the yield strength ( σ ) for γ −Al 2 O 3 was estimated, for the first time, as 2.6 GPa. Vickers microhardness values were in the range of 5.7 GPa for the γ −Al 2 O 3 samples, and 4.0 GPa for the SiO 2 -gel samples, under loads of 50 g. An important and practical application of these results is the possibility of producing bulk γ −Al 2 O 3 , a new alumina material, which was not possible to prepare before due to the conversion to a phase during the normal sintering process. Additionally, specially for SiO 2 -gel, a very important application of this study is the possibility of incorporation of organic substances in an inorganic matrix, using high pressure at room temperature.

FTIR thermal analysis on anilinepropylsilica xerogel

FTIR thermal analysis was used for a hybrid xerogel, anilinepropylsilica, obtained from three different organic precursor amounts, using HF and NaF as catalysts in the sol–gelprocess. The aniline ring vibrational mode at 1500 cm−1 of attached aniline groups was used to obtain the relative aniline content in the xerogel materials after being submitted to thermal treatment in the temperature range from 100 to 400°C. This technique allowed to evaluate the thermal stability of organic phase. The organic coverage on the surface and the fraction of trapped organic groups in closed pores can also be evaluated.

High pressure loading of organic dyes into a silica matrix

Abstract High density, transparent and crack-free compacts of silica gel doped with naphthazarin, quinizarin and rhodamine 6G, were produced by sol–gel synthesis with high pressure at room temperature. Compacts are very stable, being resistent to polishing and leaching by ethanol and acetone. They show optical properties, as absorption and fluorescence, similar to those at dilute solutions, indicating that the dyes are dispersed in the silica matrix as individual molecules. Heat treatment to 150°C for 1 h led to almost no degradation.

Nanocapsule@xerogel microparticles containing sodium diclofenac: A new strategy to control the release of drugs

The aim of this work was to evaluate the potentiality to control the drug release of a new architecture of microparticles organized at the nanoscopic scale by assembling polymeric nanocapsules at the surface of drug-loaded xerogels. Xerogel was prepared by sol-gel method using sodium diclofenac, as hydrophilic drug model, and coated by spray-drying. After coating, the surface areas decreased from 82 to 28 m(2)/g, the encapsulation efficiency was 71% and SEM analysis showed irregular microparticles coated by the nanocapsules. Formulation showed satisfactory gastro-resistance presenting drug release lower than 3% (60 min) in acid medium. In water, the pure drug dissolved 92% after 5 min, uncoated drug-loaded xerogel released 60% and nanocapsule coated drug-loaded xerogel 36%. After 60 min, uncoated drug-loaded xerogel released 82% and nanocapsule coated drug-loaded xerogel 62%. In conclusion, the new system was able to control the release of the hydrophilic drug model.

Chitosan-stabilized gold nanoparticles supported on silica/titania magnetic xerogel applied as antibacterial system

AbstractThe sol-gel method is an excellent choice to produce composite materials with enhanced performance by efficiently combining the individual features of their components. In this work, chitosan-stabilized gold nanoparticles (ChAuNPs) were immobilized onto a SiO2/TiO2 magnetic xerogel, which was synthesized through hetero-condensation of silica and titania precursors in the presence of magnetite particles covered with a silica shell. This system allies the antimicrobial capacity of ChAuNP, the surface reactivity of titania, porous structure of silica, and magnetic response of the magnetite particles. The magnetite phase was characterized by X-ray diffraction and the shape and size of the particles were observed by scanning and transmission electron microscopy. ChAuNPs were obtained in spherical shape with size below 10 nm, as characterized by UV–Vis spectroscopy and transmission electron microscopy. SiO2/TiO2 magnetic xerogel containing the ChAuNP was also characterized by thermogravimetric and textural analysis, transmission electron microscopy, and magnetism. The ChAuNP-SiO2/TiO2 magnetic xerogel is mesoporous with facile magnetic recovering and its performance as antimicrobial agent was assessed against the pathogen E. coli. The ChAuNP-SiO2/TiO2 magnetic xerogel presented inhibitory effect against the tested bacteria, even with such low gold content. After the magnetic recovering, the material was reused and maintained its antibacterial activity. HighlightsMagnetic composite embedding magnetite particles in silica/titania network.Adhesion of chitosan-stabilized gold nanoparticles to silica/titania surface.Porous and high surface area material containing gold nanoparticles as antimicrobial agent.Efficient and reusable antimicrobial system against E. coli bacteria.