Some features of metastable zone width of various systems determined by polythermal method

Abstract

The experimental data on the metastable zone width, as determined by the maximum supercooling ΔTmax using the conventional polythermal method, of the following six compounds: keto-1,2,3,4-tetrahydro-6-methylcarbazole (KTMC), L(+)-ascorbic acid (LAA), 3-nitro-1,2,4-triazol-5-one (NTO), potassium tetraborate tetrahydrate (KTB), 1,3-dipalmitoyl-2-oleoylglycerol (POP), and tripalmitoylglycerol (PPP) published in the literature are analyzed using the relations: ln(ΔTmax/T0) = Φ + βlnR (K. Sangwal, Cryst. Res. Technol., 2009, 44, 231) and (T0/ΔTmax)2 = F(1 − ZlnR) (K. Sangwal, Cryst. Growth Des., 2009, 9, 942). Here Φ, β, F and Z are constants, and T0 is the saturation temperature of a solute–solvent system. It was found that: (1) for all solute–solvent systems the values of −Φ and ln(F1/2) depend on saturation temperature T0 by an Arrhenius-type equation with an activation energy Esat, (2) the activation energy Esat for different solute–solvent systems does not show any well-defined relationship with their corresponding enthalpy of dissolution ΔHs, and (3) the parameters β and Z are relatively insensitive to changes in saturation temperature T0. Analysis of the results suggests that: (1) the absence of intimate relationship between Esat and ΔHs for a system is a consequence of the nature of the processes involved; the value of ΔHs is associated with solute–solvent interactions, whereas the value of Esat is connected with the diffusion of species involved in the formation of three-dimensional nuclei, and (2) the values of β and Z for a solute–solvent system are determined by the nature of species diffusing in the solution during its cooling.