Formation of hierarchical pore structure in silica gel

By inducing a phase separation parallel to the sol-gel transition of alkoxy -derived silicate systems, gels with well-defined macroporous structure can be prepared. Depending on the post-gelation treatment such as aging and so lvent exchange, the final pore structure in the nanometer range of dried an d heat-treated gels exhibits a considerable variation. With an aim of compl etely controlling the hierarchical pore structure in the discrete size rang es of nanometers and micrometers, systematic experimental studies have been performed. The macroporous nature of the wet gels allows an efficient solv ent exchange process compared with conventional gels only with mesopores. I n addition, the surface chemistry of the wet gel skeleton affects the mesop ore formation process by the solvent exchange to a great extent. The median size of mesopores larger than 5 nm can be controlled by adjusting the basi c solvent exchange conditions such as pH value, temperature and bath ratio for any kind of macroporous silica gel. On the other hand, the control of p ore volume independent of the mesopore size is possible only in the system incorporated with the micelle-forming surfactant. Some examples of the effe cts of controlled mesopores on the analytical performance of monolithic-typ e chromatographic columns are also presented.