Cluster formation in aqueous electrolyte solutions observed by dynamic light scattering

Using dynamic light scattering we show that aqueous super- to undersaturated solutions of NaCl, (NH4)(2)SO4, and Na-citrate contain submicrometer size clusters at room temperature. The particle size distributions deduced by Laplace inversion of the spectra contain two predominant components. The smaller components with radii below 1 nm are attributed to mixtures of solvated ions and the larger with radii between 50-500 nm to ion clusters. Knowledge of the mesoscale structure of concentrated electrolyte solutions may be necessary to describe effective protein-protein interactions which are of importance in biochemical applications such as protein crystallization.