Interactions between polyelectrolytes and sparingly soluble salts

Effects of polyelectrolytes on crystallization processes of sparingly soluble salts are classified on the basis of (i) the nature of the polyelectrolyte: soluble synthetic polymers, polypeptides, proteins, polysaccharides and (ii) their role as impurities in crystallization systems, namely their effect on nucleation, growth, aggregation, aging and polymorphic form. Studies are presented in which investigators looked into the effects of polyelectrolytes and tried to derive the general principles of host crystal/impurity interactions. The methods include (i) follow-up of kinetic changes in processes induced by the additives, (ii) characterization of the nascent solid phase and (iii) studies of crystal/additive interaction on a molecular level. The emphasized basic principle of interaction is the dimensional fit between the active functional groups of the polyelectrolyte and the intercationic or interanionic distances on the crystal faces of the host crystal. The second principle is the fulfilment of stereochemical requirements in the affected crystals, by the polyelectrolytes. The idea that any macromolecule is not exclusively a retardant or a promotor, but may function in either role, depending on conditions and host crystal/impurity ratio, is illustrated by a number of studies. The same approach is used in recent publications in explaining the multiple role of a polyelectrolyte in a crystallizing system. The effect of polyelectrolytes on aging processes, aggregation and phase transformation, is also illustrated by numerous studies. Copyright (C) 1996 Elsevier Science Ltd