Structural modifications caused by the increasing MTES amount in hybrid MTES/TEOS-based silica xerogels

In this work, MTES/TEOS-based silica xerogels have been synthesized by sol-gel method from tetraethoxysilane (tetraethylorthosilicate, TEOS) and methyltriethoxysilane (MTES) using ethanol/water mixture, different MTES/TEOS molar ratios and two different pH levels. Starting with the TEOS silica precursor, the structural and dynamical variations - as a consequence of the introduction of an increasing amount of MTES from 5% to 80% of substitution - have been monitored by nitrogen porosimetry, small angle neutron scattering (SANS), electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR) and thermal analysis, with the aim to provide a clear picture about the relationship between the MTES/TEOS molar ratio and the induced structural variations. Knowledge on this aspect can furnish an experimental evidence of the hydrophilic/hydrophobic behavior of the whole matrix, which is strongly related to the size of the particles/pores within the investigated network. The obtained results revealed significant skeletal variations with the increase of the MTES amount, suggesting a strong influence of MTES in the inorganic silica matrix and the presence of three different structural regimes in the explored MTES substitution range. The possibility to "tune" the structural parameters varying the MTES content, represents a key point for the design of organic hybrid materials showing enhanced performances with respect to those prepared with TEOS only, particularly useful in various potential applications such as medicine and nanotechnology.