Heterogeneity in aqueous solutions: electron microscopy of citric acid solutions

Abstract

The structures of aqueous citric acid solutions of various concentrations, including supersaturated solutions, were examined directly by scanning electron microscopy (SEM). Electron microscopic samples were prepared by solidifying a droplet of solution in a refrigerant at very high quenching speed, then replicating it with electron-beam-gunned platinum and carbon. Solute clusters and their aggregates were observed in all the samples irrespective of the solute concentration. The solutions showed a double-zone structure in scanning electron microphotographs, consisting of a smooth zone and a compartment of many solute clusters. With the increase in solute concentration, the number density of compartments increased, while their volume decreased. Electron microscopy thus strongly suggested that the solutions examined were heterogeneous, namely, the so-called colloidal mixtures. X-ray diffraction analysis revealed the very important fact that the crystal of citric acid monohydrate contains fine particles which seem to be the remnants of solute clusters in solution. This is supported by the observation in the boundary region between the crystal and solution of fine particles which stretch like elastic to form a network structure.