Thermodynamic descriptors of sensible heat driven liquid-liquid phase separation

Abstract

• Solvent (Water/alcohol) provides the principal contribution in governing the solution thermoresponsive behavior. • Analytical derivation of temperature and concentration dependent gradient of the scaled Gibbs energy of mixing. • Scaled Gibbs energy gradient derived as a function of the Gibbs stability criterion and also in activity coefficient terms. • Scaled/Reduced activity coefficient can delineate LCST/UCST regimes without computing the Gibbs energy of mixing. • All the above parameters successfully characterize LCST/UCST behavior with literature/experimental corroboration. Separation of a homogeneous solution into its constituent liquids through sensible heating obviates the need for vaporization/distillation and the associated latent heat of phase change. Therefore, it could enable alternative more energy efficient cycles in many applications that involve the separation of miscible liquids. This can be achieved by utilizing Lower and Upper Critical Solution Temperature (LCST/UCST) driven phase separation. This study identifies the thermodynamic descriptors for predicting LCST/UCST exhibiting ionic liquid (IL) - water/alcohol (solvent) solutions, and therefore provides a fundamental insight into the underlying energetics responsible for temperature and concentration dependent miscibility of multicomponent solutions. Excess Gibbs energy analysis of 50 solutions revealed that the solvent (water/alcohol) played a pivotal role in phase separation and exhibited contrasting excess Gibbs energy contributions in the LCST and UCST regimes. Furthermore, analysis of scaled Gibbs energy was carried out to elucidate the energetics of phase separation and differentiate between LCST and UCST thermodynamically. Consequently, we introduce the solvent Reduced Activity Coefficient (RAC) which was used to successfully predict the phase separation behavior without evaluating the Gibbs energy function, thus saving significant computation time.