Kinetic
approach to the determination of the phase diagram of a solid solution
Marija Matovic, J.C. van Miltenburg, H.A.J. Oonk and Jan H. Los
Computer Coupling of Phase Diagrams and Thermochemistry 30 (2006)
209-215
Abstract
A binary mixture of 1,4-dichlorobenzene and 1,4-dibromobenzene, as a
representative of a system that forms a solid solution, was measured in
an adiabatic calorimeter. Several mixtures of different compositions
were heated to 380 K, kept in the liquid phase at this temperature
overnight, and then cooled slowly down to 250 K. The enthalpy path of
the mixture measured during cooling carries the information that
enables us to describe the crystallization process. In the first stage,
crystallization develops rapidly and clearly deviates from equilibrium,
while in the second stage it proceeds significantly slower, whereby we
assume that the surface of the solid is in equilibrium with the
existing liquid phase at the given temperature during cooling. Such an
approach opposes the one that assumes equilibrium between the liquid
and solid bulks, known as the equilibrium model. We show that by
employing the kinetic model, the experimental enthalpy path of the
mixture during cooling in the adiabatic calorimeter can be very
successfully reproduced, while the equilibrium model fails in this
aspect. Furthermore, we propose a procedure where the kinetic model is
used to obtain the excess thermodynamic properties of the solid phase.
These quantities enable the calculation of the liquid–solid phase
diagram using the excess parameters obtained from the approach that
does