Chemical Tailoring of Porous Silica Xerogels: Local Structure by Vibrational Spectroscopy

Abstract

Monolithic porous silica xerogels were synthesized by the sol-gel process, and their local structure was analysed by vibrational spectroscopy. The silica alcogels were prepared by a two-step hydrolytic polycondensation of tetraethoxysilane (TEOS) in isopropanol, with a water/TEOS molar ratio of 4. The hydrolysis step was catalysed by hydrochloric acid (HCl), with different HCl/TEOS molar ratios (ranging from 0.0005 to 0.009), and the condensation step was catalysed by ammonia (NH(3)), with different NH(3)/HCl molar ratios (ranging from 0.7 to 1.7). After appropriate ageing, the alcogels were washed with isopropanol and subcritically dried at atmospheric pressure. The diffuse reflectance infrared Fourier transform (DRIFT) spectra were analysed in terms of the main siloxane rings that form the silica particles, taking into account the splitting of the nu(as)Sibond;Obond;Si mode into pairs of longitudinal and transverse optic components, by long-range Coulomb interactions. It was proven that the proportion of residual silanol groups (which correlates with hydrophilicity), and the fraction of siloxane 6-rings (which correlates with porosity) may be tailored by adequate catalytic conditions, mostly by the hydrolysis pH. This was explained in terms of the reactions' mechanisms taking place in the two-step sol-gel process followed.

Fourier Transform Infrared and Raman Characterization of Silica-Based Materials

“…The FTIR technique can also be employed to evaluate silica network characteristics such as hydrophilicity/hydrophobicity degree and siloxane ring structure regarding thin film porosity [34,35]. In terms of sensors, these features may impact the analyte interaction and access to the encapsulated molecules, known as receptor elements, within the silica matrix.…”

“…Literature reports that the maximum centers and relative intensities of the longitudinal optic (LO) and transversal optic (TO) modes of this bond are shifted with the introduction of chemical groups or organic molecules in the silica network [35]. So, a complete analysis of its components can be conducted, including the deconvolution of the LO and TO modes in their relative main contributions: the four-membered (SiO) 4 and six-membered (SiO) 6 siloxane rings (Figure 3A), resulting in a total of four components (LO 6 , LO 4 , TO 6 and TO 4 ) ( Figure 3B).…”

Reversible Switching of a Micelle‐to‐Vesicle Transition by Compressed CO2

“…At the same time, tetraethoxysilane (TEOS) can hydrolyze to form silica under acidic conditions. [47] To obtain more information about the MVT, the surfactant solution with and without CO 2 was used as a template to prepare silica particles. CO 2 acted both as the catalyst and reagent to control the morphology of the template.…”

Natural and Modified Zeolite—Alginate Composites. Application for Removal of Heavy Metal Cations from Contaminated Water Solutions

“…On the other side, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy provides information about the outer 6-8 µm layer of a sample, meaning that it has a potential for the investigation of surface of sample [30]. From that point, the detection and identification of the hydrophobic and hydrophilic functional groups on the surface of sample and finding their ratio can be alternative way for the quantification of sample hydrophobicity degree [31]. However, to the best of our knowledge, the information about hydrophobicity degree obtained for a given set of materials from adsorption isotherms or DRIFT experiments were not yet studied.…”

“…The deconvolution fittings for all the samples are given in Figure 4, while the absorption maxima and relative areas of each group are given in Table 3. Finally, the hydrophilicity degree for samples was calculated using Equation (1) [31], and results are given in Table 4. Due to presence of the hydrophilic [32] goethite and iron oxy-hydroxides on the surface of FeZ [7], after modification of the NZ with Fe(III) ions, its degree of the hydrophilicity slightly increased from 9.3 to 10.5.…”