Surface characterization of silica aerogels with different proportions of hydrophobic groups, dried by the CO2 supercritical method

Silica aerogels, dried by the CO2 supercritical method, were made from tetramethoxysilane (TMOS) and methyltrimethoxylsilane (MTMS) in different proportions to change the proportions of hydrophilic and hydrophobic surface groups. The aerogel hydrophobic or hydrophilic properties were related to their textural and surface structural characteristics. In particular, water adsorption isotherms were recorded by successive equilibrium with saturated aqueous metal salt solutions. They were compared to nitrogen adsorption isotherms. The relative proportion of Si-OH, Si-OCH3, or Si-CH3 end groups present on the gel surface was determined by nuclear magnetic resonance (NMR) of H-1. The results obtained were correlated with measurements of the contact angle with water, after equilibration of the gels at various water activity. From these data, a possible description of the mixed hydrophilic-hydrophobic nature of the surface of these materials is finally proposed.