Processes of silica network structure formation in reverse micellar systems

The processes by which the silica-network structure is formed in reverse micelles were elucidated. The reverse micellar system was composed of didodecyldimethylammonium bromide (DDAB), cyclohexane, and HCl- aqueous solution, and the silica source was tetraethoxysilane (TEOS). The hydrolysis reactions for TEOS were completely different from those of usual homogeneous sol-gel processes. The rate of hydrolysis was affected by the curvature of the reverse micelles. The reverse micelles also promoted the formation of network bonds of silica even under acidic conditions. Primary silica particles spherical in shape were generated first. DDAB and partially hydrolyzed TEOS cooperatively formed the primary spherical particles, which were stable against coagulation and did not grow to large particles. The primary particles formed clusters, which were converted to linear silica rods with some branches upon neutralization of HCl with NaOH, and the system became a transparent gel. The diameter of the silica rods was similar to that of rodlike micelles of DDAB in the absence of TEOS. Thus, the DDAB micelles seemed to direct the formation of the silica microstructure as if the rodlike micelles functioned as a template. However, the formation of silica rods and a network structure arises from the connection of primary particles through condensation reactions.