Insight into the role of anions in the formation of mesoporous silicates

Cationic surfactants can form micellar structures in solution and the condensation of silicate around these micellar structures results in the inorganic/organic mesostructured composites. Anions other than silicates in the solution were believed to be located closely to the headgroup of the cationic surfactant micelle and thus affected the effective headgroup area, a component of the surfactant packing parameter g (g = V/a0l, where V is the chain volume, a0 is the effective headgroup area, and l is the chain length). In the presence of H2SO4, HCl, HBr, and HNO3, the surfactant cetyltriethylammonium bromide (CTEABr) directed the well-ordered mesophases of three-dimensional (3D) hexagonal P63/mmc, cubic Pm 3 n, 2D hexagonal p6mm, and cubic Ia 3 d, respectively. The diversity of the mesophases from CTEABr was attributed to the anion tuning of the effective headgroup area of CTEA+ in surfactant packing parameter g by closely located anions of SO42-, Cl-, Br-, and NO3- with various radii and hydratability. In this work, we discovered that part of Cl-, Br-, and NO3- can penetrate into the micellar system of CTEA+, which may increase the chain volume V of CTEA+ to some degree. The distance of anions to the center of the micelle, determined by the liquid 1H NMR analyses, was in the order of SO42-> Cl- > Br- > NO3-, which is consistent with the Hofmeister series. The deeper penetration of anions into the micelle may lead to the larger chain volume V and surfactant packing parameter g. Such insight into the location of anions in micellar system provided us a new picture on the formation mechanism of mesoporous silica and other mesostructured materials in solution.