Effect of impurities on the metastable zone width of solute–solvent systems

The novel approach to interpret the metastable zone width obtained by the polythermal method using the classical theory of three-dimensional nucleation proposed recently [K. Sangwal, Cryst. Growth Des. 9 (2009) 942] is extended to describe the metastable zone width of solute–solvent systems in the presence of impurities. It is considered that impurity particles present in the solution can change the nucleation rate J by affecting both the kinetic factor A and the term B related with the solute–solvent interfacial energy γ. An expression relating metastable zone width, as defined by the maximum supercooling ΔTmax of a solution saturated at temperature T0, with cooling rate R is proposed in the form: (T0/ΔTmax)2=F(1−Z ln R), where F and Z are constants. The above relation can also be applied to describe the experimental data on maximum supercooling ΔTmax obtained at a given constant R as a function of impurity concentration ci by the polythermal method and on maximum supersaturation σmax as a function of impurity concentration ci by the isothermal method. Experimental data on ΔTmax obtained as a function of cooling rate R for solutions containing various concentrations ci of different impurities and as a function of concentration ci of impurities at constant R by the polythermal method and on σmax as a function of impurity concentration ci by the isothermal method are analyzed satisfactorily using the above approach. The experimental data are also analyzed using the expression of the self-consistent Nývlt-like approach [K. Sangwal, Cryst. Res. Technol. 44 (2009) 231]: ln(ΔTmax/T0)=Φ+β ln R, where Φ and β are constants. It was found that the trends of the dependences of Φ and β on impurity concentration ci are similar to those observed in the trends of the dependences of constants F and Z on ci predicted by the approach based on the classical nucleation theory.