Change in microstructure of an aqueous citric acid solution under crystallization

Abstract

The micro morphology changes of an aqueous citric acid solution with increasing supersaturation ratio S and in the presence of a seed crystal were observed by using a scanning electron microscope (SEM). With increasing S, the number density of particles of ∼ 60 nm in diameter increases to form a network structure. With further increasing S, particles of ∼ 60 nm aggregate to several microns in size and then are rearranged into an ordered crystal structure. When a seed crystal is placed in a supersaturated solution, ∼ 60 nm particles gather and migrate to the crystal surface and are rearranged in a regular structure on it. From these results, it is supposed that the crystallization of citric acid monohydrate proceeds not through a molecular-by-molecular process but by an aggregation process of solute cluster units of ∼ 60 nm in size.