Abstract
In this work, a multi-analytical approach involving nitrogen porosimetry, small angle neutron and X-ray scattering, Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopies, X-ray diffraction, thermal analysis and electron microscopy was applied to organically modified silica-based xerogels obtained through the sol–gel process. Starting from a tetraethoxysilane (TEOS) precursor, methyltriethoxysilane (MTES) was added to the reaction mixture at two different pH values (2.0 and 4.5) producing hybrid xerogels with different TEOS/MTES molar ratios. Significant differences in the structure were revealed in terms of the chemical composition of the silica network, hydrophilic/hydrophobic profile, particle dimension, pore shape/size and surface characteristics. The combined use of structural characterization methods allowed us to reveal a relation between the cavity dimensions, the synthesis pH value and the grade of methyl substitution. The effect of the structural properties on the controlled Captopril release efficiency has also been tested. This knowledge facilitates tailoring the pore network for specific usage in biological/medical applications. Knowledge on structural aspects, as reported in this work, represents a key starting point for the production of high-performance silica-based hybrid materials showing enhanced efficacy compared to bare silica prepared using only TEOS.
Highlights
To meet the challenges of medical, pharmaceutical, cosmetic industry, renewable energy and environmental science, the application of interdisciplinary knowledge has become vital [1,2,3,4,5,6]
The results obtained on the methyl-substituted silica gels are presented
The A and B labels represent the gels obtained at pH = 2.0 and pH = 4.5, respectively
Summary
To meet the challenges of medical, pharmaceutical, cosmetic industry, renewable energy and environmental science, the application of interdisciplinary knowledge has become vital [1,2,3,4,5,6]. The recent developments on hybrid silica materials have proven the utility and applicability of MTES and MTMS precursors in the following various fields: gas storage and separation [15,16], biocatalysis [17], corrosion protection [18], adsorption of organic solvents [19]. At pH < 7, it is well-known that hydrolysis and condensation occur successively, the polycondensation process mechanism consists of cluster–cluster aggregation and primary particles are represented by linear or weakly branched polymer chains. Under basic conditions (pH > 7), hydrolysis and condensation occur simultaneously, the silica polycondensation follows cluster–particle mechanism and primary silica particles look similar to highly branched clusters. Fluoride anions play an important role as sol–gel process catalysts by significantly increasing the rate of condensation reactions
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