Preparation of mesoporous materials from the flow-induced microstructure in aqueous surfactant solutions

In this study, we synthesized the mesoporous materials under a shear flow at relatively low temperatures using the wormlike micellar solution of a cationic surfactant cetyltrimethylammonium bromide (CTAB). The additive sodium salicylate (NaSal) enhanced the microstructure evolution by forming the micron-order flexible chains, which were stable at low temperatures. The mesoporous structures were prepared from the flow-induced template of wormy CTAB micelles by controlling the composition, flow intensity, reaction time, and temperature. One of the most significant features of the present study is that the shear flow was generated in a Couette cell by a rotating inner cylinder with a coaxial outer cylinder kept stationary. This typical flow configuration produces spring-coil streamlines which eventually stretch and twist a bundle of the long wormy micelles, and thereby form the helical mesoporous silica structures. The X-ray diffraction and transmission electron microscopy analyses showed that the ordered hexagonal phase was formed by the shear flow; otherwise, an amorphous or disordered hexagonal phase prevailed in an equilibrium state. In addition, the presence of cosurfactants or swelling agents also affected the formation of mesoporous material. For example, the primary heptanol enhanced the formation of a stable structure with a well-defined orientation toward the flow direction.