Supersaturated electrolyte solutions: Theory and experiment

Highly supersaturated electrolyte solutions can be prepared and studied employing an electrodynamic levitator trap (ELT) technique. The ELT technique involves containerless suspension of a microdroplet thus eliminating dust, dirt, and container walls which normally cause heterogeneous nucleation. This allows very high supersaturations to be achieved. A theoretical study of the experimental results obtained for the water activity in microdroplets of various electrolyte solutions is based on the development of the Cahn-Hilliard formalism for electrolyte solutions. In the approach suggested the metastable state for electrolyte solutions is described in terms of the conserved order parameter ω(r,t) associated with fluctuations of the mean solute concentration ${\mathit{n}}_0$. Parameters of the corresponding Ginzburg-Landau free energy functional which defines the dynamics of metastable state relaxation are determined and expressed through the experimentally measured quantities. A correspondence of 96–99 % between theory and experiment for all solutions studied was achieved and allowed the determination of an analytical expression for the spinodal concentration ${\mathit{n}}_{\mathrm{spin}}$ and its calculation for various electrolyte solutions at 298 K. The assumption that subcritical solute clusters consist of the electrically neutral Bjerrum pairs has allowed both analytical and numerical investigation of the number-size ${\mathit{N}}_{\mathit{c}}$ of nucleation monomers (aggregates of the Bjerrum pairs) which are elementary units of the solute critical clusters. This has also allowed estimations for the surface tension α, and equilibrium bulk energy β per solute molecule in the nucleation monomers. The dependence of these properties on the temperature T and on the solute concentration ${\mathit{n}}_0$ through the entire metastable zone (from saturation concentration ${\mathit{n}}_{\mathrm{sat}}$ to spinodal ${\mathit{n}}_{\mathrm{spin}}$) is examined. It has been demonstrated that there are the following asymptotics: ${\mathit{N}}_{\mathit{c}}$=1 at spinodal concentration and ${\mathit{N}}_{\mathit{c}}$=∞ at saturation.