A Structural Underpinning of the Lower Critical Solution Temperature (LCST) Behavior Behind Temperature-Switchable Liquids

Abstract

In this study we use state of the art x-ray scattering and molecular dynamics structural probes to carry out a careful analysis of amine-water mixtures that show the unusual lower critical solution temperature (LCST) behavior. The goal is to provide direct experimental evidence for the entropy-lowering molecular cluster formation originally hypothesized as necessary to LCST behavior. Combined differential wide-angle x-ray scattering and pair distribution analysis, and small angle x-ray scattering measurements were combined with molecular modeling and liquid-liquid equilibrium measurements, revealing direct supporting experimental evidence for the hypothesis. However, the response of the amine phase to accommodating water is even more subtle and interesting than the simple hypothesis suggests, with the formation of robust, nanoscale reverse micelles. The techniques developed in this paper can be expected to yield important insights in the use of temperature-switchable liquids in solvent extraction and other separations. It can also help to explain the stabilization of organelles in living cells that do not have physical membranes but do require compositional gradients to operate.

Keywords: Lower critical solution temperature, LCST, pair distribution function, PDF, solvent-water mixtures, liquid structure, switchable solvent, chemical separations, desalination