Nucleation and crystallization in aqueous systems during drying: Theory and practice

Abstract

The actual phase behaviour of alkali metal phosphates during the freeze and air drying of aqueous solutions has been studied and compared to the predictions based on the equilibrium phase diagrams. For the ternary sodium phosphate + water system, the ternary eutecticconcentration ratio is far removed from the concentration ratio corresponding to pH 7. It has been found that under conditions of freezing, two of the four phosphate salts studied, precipitate rapidly, but not completely, at the eutectic temperature, one salt is subject to a low rate of nucleation and NaH2PO4 does not precipitate at all. In every case thedegree of precipitation increases with the initial salt concentration. In ternary mixtures, containing sucrose, salt and a low residual moisture content, the salts form a solid solution with sucrose and can be made to crystallise slowly from the amorphous phase as the temperature is raised slightly above the glass transition temperature of the mixture. © 1992, IUPAC.