Macropore Formation of Silica Gel in the Presence of Sodium Dodecyl Sulfate by Inducing Phase Separation in Alcohol-free Aqueous Solution

Bimodal porous silica gel with both macropores and mesopores was prepared from water glass (WG) in the presence of low-molecular-weight surfactant, sodium dodecyl sulfate (SDS), by inducing phase separation during sol-gel reaction. The macroporous morphology was controlled by changing starting composition and gelation temperature: the change in the concentration of SDS was effective to control connectivity of macropores and silica gel skeleton in micrometer size, and the gelation temperature significantly affected macropore size. From the change in morphology with the concentration of SDS, it is considered that a homogeneous reacting solution separated into a phase of SDS and that of polymerized silica in contrast to other silica-surfactant systems, where phase separation proceeds between solvent and surfactant associated with silica gel surface. It is also suggested that SDS forms associated complex or micelle in aqueous acidic solution, and the formation of such structure plays an important role in the phase stability. We also controlled mesopore structures in the macroporous silica gel by aging as-geled samples under basic conditions.