The influence of the alcohol concentration on the structural ordering of mesoporous silica: Cosurfactant versus cosolvent

Increasing the ethanol concentration in the tetraethoxysilane (TEOS)-hexadecyltrimethylammmonium bromide (C(16)TMABr)-ammonia-water system at room temperature permits one to obtain a succession of different mesophases in the order MCM-41 --> MCM-48 --> lamellar phase --> radial hexagonally ordered phase. First, the original hexagonal (MCM-41) phase is replaced by a cubic phase (MCM-48) and later, upon ethanol addition, by a lamellar phase. Such phase succession is the result of the cosurfactant behavior of the ethanol. At lower alcohol concentration, scanning electron microscopy (SEM) shows only undefined or barely spherical structures, indicating that the ethanol has only a limited effect on the external morphology. When the alcohol concentration is further increased, it will mainly act as a cosolvent producing spherical particles. A TEM study reveals the radial arrangement of the pores within the spherical particles. A hexagonal closed pore packing can be considered on a local scale around the center of the spherical particle. This hexagonal pore arrangement is the result of a combination of a very slow equilibrium toward the hydrolysis of the TEOS, its good homogenization in the synthesis solution due to the solvating effect of the alcohol, and the interference of the alcohol on the cooperative process of the micelle formation. To complete the study, parallels have been drawn with other alcohols such as methanol and propanol.