Processes of silica network structure formation in reverse micellar systems

The processes by which the silica-network structure is formed in reverse mi celles were elucidated. The reverse micellar system was composed of didodec yldimethylammonium bromide (DDAB), cyclohexane, and HCl- aqueous solution, and the silica source was tetraethoxysilane (TEOS). The hydrolysis reaction s for TEOS were completely different from those of usual homogeneous sol-ge l processes. The rate of hydrolysis was affected by the curvature of the re verse micelles. The reverse micelles also promoted the formation of network bonds of silica even under acidic conditions. Primary silica particles sph erical in shape were generated first. DDAB and partially hydrolyzed TEOS co operatively formed the primary spherical particles, which were stable again st coagulation and did not grow to large particles. The primary particles f ormed clusters, which were converted to linear silica rods with some branch es upon neutralization of HCl with NaOH, and the system became a transparen t gel. The diameter of the silica rods was similar to that of rodlike micel les of DDAB in the absence of TEOS. Thus, the DDAB micelles seemed to direc t the formation of the silica microstructure as if the rodlike micelles fun ctioned as a template. However, the formation of silica rods and a network structure arises from the connection of primary particles through condensat ion reactions.