Structure Of Silica Xerogels Research Articles

The Role of Polydimethylsiloxane in the Molecular Structure of Silica Xerogels Intended for Drug Carriers.

The aim of this study was to prepare and examine polymer/oxide xerogels with metronidazole (MT) as delivery systems for the local application of a drug to a bone. The nanoporous SiO2-CaO and PDMS-modified SiO2-CaO xerogel materials with different amounts of the polymer, polydimethylsiloxane (PDMS), were prepared by the sol-gel method.Characterization assays comprised the analysis of the composite materials by using Fourier transform infrared spectroscopy (FTIR), determining the specific surface area of solids (BET), using X-ray powder diffraction (XRD) and scanning electron microscope (SEM) techniques, and further monitoring in the ultraviolet and visible light regions (UV-Vis) of the in vitro release of the drug (metronidazole) over time. According to these results, the bioactive character and chemical stability of PDMS-modified silica xerogels have been proven.The release of MT from xerogels was strongly correlated with the composition of the matrix. In comparison with the pure oxide matrix, PDMS-modified matrices accelerated the release of the drug through its bigger pores, and additionally, on account of weaker interactions with the drug.The obtained results for the xerogel composites suggest that the metronidazole-loaded xerogels could be promising candidates for formulations in local delivery systems particularly to bone.

Effect of Aging in the Structure of Silica Xerogels with Incorporation of Extract’s Leaves

The spectral fluorescence characteristics of extracts leaves embedded in silica xerogel matrix promoted by aged time was studied. We obtain a higher PSII stability for extract of leaves embedded in xerogel matrix, with concentration of 8 g / 20 ml and water to TEOS molar ratio of 11, which remain bioactive over a very long period of time, whereas for a water to TEOS molar ratio of 5 a blue-shift fluorescence is observed indicating the PSII denaturizing and formation of fluorescing aggregates associated with the activation of carotenoids radicals.

Effects of poly(vinyl alcohol) additions on the structure of silica xerogels

Poly(vinyl alcohol)/silica hybrid xerogels were prepared from sonohydrolysis of tetraethoxysilane (TEOS) and additions of water-solution of poly(vinyl alcohol) (PVA). The samples were studied by small-angle X-ray scattering (SAXS), nitrogen adsorption, and differential scanning calorimetry (DSC). On drying at room temperature the resulting xerogels exhibit a fairly bimodal porous structure composed by small mesopores and micropores. The pore size distribution of the mesopores was found to follow approximately a power-law with the pore size. The micropore structure was associated to an evolution at a high resolution level of the mass fractal structure of the original wet gels. The role of the PVA addition on the pore structure of the xerogels is to diminish the specific surface area and the pore volume without to change substantially the pore mean size.

Correlation between physical properties and structure of silica xerogels

An effort is made to establish quantitative correlations between the molecular structure and relevant physical properties of silica xerogels, ranging from dense to highly porous materials. The samples were synthesized by tuned hydrolytic polycondensation of tetraethoxysilane (TEOS), following a two-step acid/basic catalyzed sol–gel process. They were differently modified after ageing, and dried at various temperatures. The molecular structures were characterized by Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy, and the physical properties assessed by envelope density measurements and nitrogen sorption isotherms. It is shown that an increase in the proportion of cyclohexasiloxane units in the silica network is associated with an increase in the total porosity (lower density) and average pore size, and with a decrease in the proportion of residual silanol groups (higher hydrophobicity). The fitted empirical correlations eventually allow estimating those physical properties of silica xerogels, based only on a DRIFT analysis.

X-ray diffraction and Raman scattering measurements on silica xerogels

The dynamics and the structure of silica xerogels at different degrees of densification have been studied by small angle X-ray scattering and Raman scattering, with the aim to investigate the effects of the inhomogeneous structure on the excitations that give rise to the excess in the density of states (Boson peak). The results are compared with those of specific heat measurements on the same samples. The present results show that the existence of the pores, whose medium size is in the 30–60 Å range, does not appreciably affect the density of states in the Boson peak frequency range. On the contrary, at lower frequencies ( ω<20 cm −1), a strong pore-size dependence is observed in the Raman spectra.

Hydrothermal modification of silica gels (xerogels) 2. Effect of the duration of treatment on their porous structure

Abstract The effect of the duration of hydrothermal treatment (in the range 1–20 h) on the parameters of the pore structure of silica xerogels was investigated. Silica gels with different initial porous structure parameters were used. The specific surface area (S) of the adsorbents changed from 830 to 38 m2 g−1. A simple mathematical equation describing the dependence of the specific surface area S on the autoclaving treatment time is proposed. The main changes in the geometrical structure of silica gel occur during the first 6 h of treatment. A long-term (over 6 h) hydrothermal treatment can be recommended to improve the homogeneity of the pore structure of silica gels.