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.