Metastable cluster intermediates in the condensation of charged macromolecule solutions

The authors examine the possibility of a two-step nucleation to the bulk condensation transition that proceeds via a metastable liquid cluster intermediate having some preferred size. The metastable intermediate is stabilized by electrostatic repulsion, which becomes screened by small mobile ions at sufficiently large cluster sizes, thus allowing the eventual condensation to a bulk phase. Our calculation employs a capillary model for the cluster and the electrostatic interactions are treated using the Poisson-Boltzmann approach. Condensation via this metastable intermediate may be a very general phenomenon which applies not only to solutions of charged particles (e.g., proteins, colloidal particles, and polyelectrolytes) but to any system involving short-range attraction and long-range repulsion undergoing macrophase separation in which a metastable microphase separation is also possible.