Surface modification of silica aerogels by hexamethyldisilazane–carbon dioxide mixtures and their phase behavior

The surfaces of monolithic silica aerogels were rendered hydrophobic using hexamethyldisilazane (HMDS) as surface modification agent and scCO2 as solvent. The treatment led to hydrophobic silica aerogels which are as transparent as untreated aerogels. The effects of HMDS concentration in the fluid phase and the reaction time were investigated and the contact angles were found to be 130° at different conditions. FTIR spectra indicated a reduction in hydrophilic surface silanol groups and the emergence of hydrophobic CH3 groups. The bubble point pressures of the HMDS–CO2 system were obtained at temperatures 298.2 K, 313.2 K, 327.7 K and 342 K at various concentrations. At a fixed temperature, the bubble point pressure decreased as the concentration of HMDS increased. At a fixed composition, bubble point pressure increased as the temperature increased. The bubble point pressures were modeled using the Peng–Robinson Stryjek–Vera Equation of State (PRSVEOS) and compared well with the experimental data.