Novel Approach to Analyze Metastable Zone Width Determined by the Polythermal Method: Physical Interpretation of Various Parameters

Experimental data on the maximum supercooling ΔTmax, a measure of metastable zone width, for solutions saturated at a temperature T0, as a function of cooling rate R are analyzed, for some solute−solvent systems chosen as examples, using Nývlt’s semiempirical approach and a new approach based on the classical theory of three-dimensional nucleation combined with the formation of n-sized embryos from monomers according to the law of mass action. Instead of a linear relation between ln(ΔTmax) and lnR of the Nývlt’s approach, the new approach predicts a linear dependence of (T0/ΔTmax)2 on lnR with slope F1 and intercept F. The quantity F1/F is independent of saturation temperature T0, characteristic of a solute−solvent and is associated with the growth of the stable three-dimensional nuclei to visible entities. The value of F1 is determined by thermodynamic and solvation processes, while that of F is governed by thermodynamic and kinetic parameters as well as processes associated with solvation of solute ions/molecules and their transport in the solution. Limitations of Nývlt’s approach and advantages of the new approach in terms of its physical basis are exposed. It is pointed out that the new approach can also be extended to explain the value of metastable zone width by the isothermal method and to explain the effect of saturation temperature and impurities on metastable zone width.