Investigation of binary solid phases by calorimetry and kinetic modelling

Abstract

The traditional methods for the determination of liquid-solid phase diagrams are based on the assumption that the overall equilibrium is established between the phases. However, the result of the crystallization of a liquid mixture will typically be a non-equilibrium or metastable state of the solid. For a proper description of the crystallization process the equilibrium approach is insufficient and a kinetic approach is actually required. In this work, we show that during slow crystallization of binary mixtures of 1,4-dichlorobenzene and 1,4-dibromobenzene, performed in an adiabatic calorimeter, it can be assumed that the liquid phase is in equilibrium only with the growing solid phase, i.e., the solid phase at the surface. The proposed kinetic model, based on the previous assumption, is extended to a method for the determination of the excess properties when the cooling path of the mixture is at disposal. Using these excess properties, the phase diagram is achieved, without adopting the approach of complete equilibrium between totally homogeneous phases. However, this method is appropriate only for slow crystallization and it cannot be applied for conditions away from near-equilibrium. To investigate the kinetic segregation, the crystallization of the mentioned binary mixture is performed at non-equilibrium conditions, i.e., at a certain degree of undercooling. The measured compositions of the grown solid phases are compared to that calculated from the equilibrium and kinetic segregation model. Moreover, the kinetic segregation model is coupled with mass and heat transport limitations. The experimental results are in a good agreement with the solid compositions obtained from the proposed kinetic model, while the solid compositions predicted by equilibrium show too strong segregation. These results demonstrate the effect of interfacial undercooling on the segregation during growth in mixed molecular systems. One part of the thesis concerns the thermal analysis of different polymorphs of the three pure TAGs, being tristearin (SSS), tripalmitin (PPP) and trielaidin (EEE), and their binary mixtures. The reported thermodynamic properties of the pure TAGs are measured by adiabatic and differential scanning calorimetry. From the adiabatic data of melting the beta-polymorphs of SSS, EEE and PPP, the purities are calculated assuming that the compound and impurity form a eutectic mixture. Finally, the phase diagrams of the beta-polymorph of three binary TAG mixtures (EEE-SSS, EEE-PPP, PPP-SSS) were constructed using the DSC data obtained from slow cooling of the melts and subsequent melting of the formed solid phases. All three binary mixtures exhibit limited miscibility of the components in the most of the composition range. For the EEE-SSS system, the SSS-rich mixtures form solid solutions. The components EEE and PPP do not co-crystallize in a solid solution over the whole composition range. Regarding the PPP-SSS mixture, the beta-polymorph yields a typical eutectic phase diagram with some mixing in the regions close to the pure components. Additionally, the mixing in the alpha-polymorph, formed by fast cooling of the melts in the DSC, is not ideal for the EEE containing samples, due to the significant structural differences in the molecular shapes of the saturated and the unsaturated TAG component.