Surface modification of porous silica using supercritical carbon dioxide

Surface modification of porous silica in supercritical carbon dioxide was studied using various alcohols as hydrophobizing reagents. The reaction was carried out using methanol, ethanol and 2-propanol at temperatures from 40 to 100°C, pressures from 8 to 20MPa and reaction time from 0.5 to 6 h. The experimental results indicated that the surface modification occurred under all conditions, and methanol could provide the highest alkyl group density after modification among the three alcohols. Pressure and concentration dependence showed that the maximum density of alkyl group was obtained near the phase boundary condition for the carbon dioxide and methanol system. It was also suggested that hydrophobicity of the surface after modification was determined by density and size effects of alkyl group, which implied the existence of a threshold value of inversion from hydrophilicity to hydrophobicity.