The chemistry of and physics with porous sol-gel glasses

Porous media with well-defined pore diameters can be employed as hosts within which one may study a wealth of physical phenomena in well-characterized confining geometries. In this paper we describe the procedures for forming variable pore size glasses via the sol-gel process. Two general methods were used to form the gels from which the porous glasses were formed; the hydrolysis of the alkoxides and the gelling of colloidal silica, e.g., DuPont LUDOX. By controlling the pH of the gelling solution, the water/silica ratio and the firing temperature, glasses with pore diameters ranging from less than 2–12 nm were formed from the alkoxide hydrolysis. Larger pore diameters were obtained from colloidal silica gels, the largest being in the 45-nm range in glasses made from leached alkali silicate gels. Extensive high resolution transmission electron microscopy analysis showed both ‘‘colloidal’’ and ‘‘polymeric’’ type glasses, in addition to an intraparticle structure in the 1–3 nm range. An example is given where subpicosecond optical birefringence techniques have been used to directly probe the rotational dynamics of different molecular liquids in several glasses having different pore sizes. Thus, a systematic evaluation of the liquid behavior as a function of its geometrical confinement is obtained and the results discussed.