CRYOMICROSCOPY AS A SUPPORT TECHNIQUE FOR CALORIMETRIC MEASUREMENTS BY DSC FOR THE STUDY OF THE KINETIC-PARAMETERS OF CRYSTALLIZATION IN AQUEOUS-SOLUTIONS .1. NUCLEATION IN THE WATER-1,2-PROPANEDIOL SYSTEM

Cryomicroscopy is shown to be complementary to calorimetry for qualitative and quantitative studies of crystallization kinetic parameters of aqueous solutions. The binary system water-1,2-propanediol is investigated as an example, using the Johnson-Avrami theoretical model under isothermal conditions. The effect of nucleation on crystallization parameters is discussed, comparing calorimetric measurements with direct observations by cryomicroscopy. The experimental results depend strongly upon the thermal history of the sample prior to the isothermal experiment. The density of nuclei formed during warming after vitrification in 42.5% w/w 1,2-propanediol is inversely proportional to the warming rate, i.e. proportional to the time spent in the homogeneous nucleation thermal domain. The results support the hypothesis of a kinetic component for liquid-crystal phase separation during homogeneous nucleation. Assuming an Arrhenius dependence for the crystallization constant, K = K0 exp(- E/RT), K0 is shown to vary as the square root of the nucleus density, in agreement with the discoidal form of the crystals; the activation energy E is shown to be independent of nucleus density.