Influence of additives on crystallization kinetics: Comparison between theory and measurements in aqueous solutions

Differential scanning calorimetry (DSC) was applied to study the crystallization kinetics induced by homogeneous nucleation mechanisms. Aqueous glycerol solutions were emulsified to create a large number of droplets free of heterogeneous nucleation sites. The nucleation rates were calculated from scanning experiments according to an established method. The results, which are in good accordance with the classical nucleation theory, were used to determine the system-dependent nucleation parameters. The thus determined dependencies were then applied to calculate nucleation rates in the vicinity of the glass transition temperatures, neglecting the influence of the increasing viscosity on the nucleation rates. In the second part, a set of experiments was performed with microemulsions in the isothermal mode of the DSC by recording the release of latent heat after rapidly lowering the temperature into the undercooled range. An Avrami-type growth law was adapted to the thermograms, treating the case of spherical diffusion controlled growth from small dimensions implementing the estimates for the nucleation rates. Applying this theory to calculate time-temperature-transformation (TTT) curves, good agreement with the measurements was achieved. The theoretical description of the crystallization process has the advantage of not being restricted to microemulsions, which generally exhibit a different kinetic behavior due to growth limiting surfaces. TTT curves for bulk samples with any given crystalline fraction can thus be calculated. This puts the estimates for ‘‘critical’’ cooling rates required for vitrification on a more reliable basis.