Improving Estimates of the Crystallization Driving Force: Investigation into the Dependence on Temperature and Composition of Activity Coefficients in Solution

In this work the influence of temperature and composition on the activity coefficient in solution has been investigated, based on isothermal and isobaric vapor–liquid equilibrium data for 30 binary systems classified into four groups: water–organic, polar–polar, polar–nonpolar, and nonpolar–nonpolar systems. It is shown that under most conditions the temperature dependence of the activity coefficient is clearly weaker than the composition dependence. The analysis is extended to include solid–liquid solubility data of 15 binary systems of relatively large and complex organic molecules in organic solvents. Based on this, a novel approach to estimate the thermodynamic driving force of crystallization from solution is proposed. Rather than assuming that the activity coefficient ratio equals unity, it is shown that in most cases a more accurate assumption is to neglect only the temperature dependence of the activity coefficient. This allows the activity coefficient ratio to be estimated from solid–liquid equilibrium data.