Thermodynamic properties of lower critical solution temperature (LCST) mixtures for application in energy–water systems

Abstract

Mixtures that possess a lower critical solution temperature (LCST) phase behavior are homogeneous at temperatures below the LCST and separate into two phases above the LCST. These materials have recently received interest for use in various applications, including refrigeration, dehumidification, desalination, and atmospheric water harvesting. However, proof-of-concept demonstrations of cycles employing these materials have shown poor performance, due to the thermodynamic properties of existing LCST mixtures. This work develops a theoretical framework to evaluate the thermodynamic properties (chemical potential, partial molar enthalpy, and partial molar entropy) required for a mixture to exhibit LCST behavior and meet the application-specific property targets. Our analysis reveals that a hypothetical LCST mixture that would outperform existing ones would need a more negative partial molar enthalpy (to achieve lower chemical potentials), but it would also need a more negative partial molar entropy (to preserve LCST behavior). Specifically, LCST refrigeration and dehumidification would require a partial molar enthalpy and entropy of water that are an order of magnitude greater than existing LCST mixtures, while LCST-based desalination requires properties 2.5× greater than existing mixtures. We show that improved LCST mixtures with lower chemical potentials would necessarily need more heat to induce phase separation, and we derive an expression to predict the enthalpy of separation using water activity data. Finally, we demonstrate that the addition of a hygroscopic additive (e.g., LiCl) to an LCST mixture does not increase the chemical potential difference between the two phases. Overall, this work lays out the thermodynamic framework and target properties that new LCST mixtures must possess and discusses implications for system performance.