Nucleation, growth and form of mesoporous silica: Role of defects and a language of shape

Topological defects in a silicate liquid crystal seed are considered to play a central part in directing the growth of hexagonal mesoporous silica towards specific morphologies. Below the silica isoelectric range, pH = 1-3, silicate is protonated and the seed carries a sheath of positive surface charge, that can be tuned by adjusting the acidity of the growth medium. Surface anchoring interactions at the polar interface between the seed and surroundings are believed to favor certain types of defects and patterns of director fields in the seed, while van der Waals attractive and electrical double layer repulsive forces between colloidal dimension seeds and rod micelles promote particular kinds of accretive growth processes between them. A survey of the relation between synthesis composition and shape reveals that high acidity and large electrical double layer repulsions favor end-to-end (homeotropic anchoring) seed-rod growth, leading to fiber-based morphologies bearing different extents of curvature. Lowering the acidity facilitates side-to-side (planar anchoring) seed-rod growth resulting in higher curvature gyroid and spiral shapes. As the isoelectric point is approached, a further reduction in electrical double layer repulsive interactions induces a switch in the growth process to one involving seed-seed flocculation, culminating in sphere shapes. A defect initiated growth model is able to rationalize the origin of curved mesoporous silica shapes.