Abstract
The thermodynamics, structures, and applications of thermoresponsive systems, consisting primarily of water solutions of organic salts, are reviewed. The focus is on organic salts of low melting temperatures, belonging to the ionic liquid (IL) family. The thermo-responsiveness is represented by a temperature driven transition between a homogeneous liquid state and a biphasic state, comprising an IL-rich phase and a solvent-rich phase, divided by a relatively sharp interface. Demixing occurs either with decreasing temperatures, developing from an upper critical solution temperature (UCST), or, less often, with increasing temperatures, arising from a lower critical solution temperature (LCST). In the former case, the enthalpy and entropy of mixing are both positive, and enthalpy prevails at low T. In the latter case, the enthalpy and entropy of mixing are both negative, and entropy drives the demixing with increasing T. Experiments and computer simulations highlight the contiguity of these phase separations with the nanoscale inhomogeneity (nanostructuring), displayed by several ILs and IL solutions. Current applications in extraction, separation, and catalysis are briefly reviewed. Moreover, future applications in forward osmosis desalination, low-enthalpy thermal storage, and water harvesting from the atmosphere are discussed in more detail.
Highlights
Thermoresponsive systems are “a special case of” responsive materials [1,2], whose properties crucially depend on external stimuli, which, in the thermoresponsive case [3], primarily consist of a change in temperature
A closely related system was investigated in reference [124], considering water solutions of a [P4444]+-based ionic liquid (IL) made with 5-phenyl tetrazolate ([Ph-tet]−]) anion, which should make it more hydrophobic than benzoate, and having a lower critical solution temperature (LCST)
Thermoresponsive IL/water solutions consist of systems with a mixing/demixing transition located in the liquid–water range ([0 − 100] ◦C) of temperature, taking place with decreasing T in systems with a upper critical solution temperature (UCST), or with increasing T in systems having a LCST
Summary
Thermoresponsive systems are “a special case of” responsive materials [1,2], whose properties crucially depend on external stimuli, which, in the thermoresponsive case [3], primarily consist of a change in temperature. This, in turn, represents an alternative route to determine phase equilibria, since a biphasic state requires a stable interface between the two phases, whose destabilisation with changing thermodynamic conditions marks the proximity of the mixing/demixing transition These simple considerations outline a seldom discussed relation between different research subjects, whose exploration could benefit both the thermodynamic modelling of phase equilibria and classical density functional theory. Besides thermodynamic modelling, the major computational activity on thermoresponsive IL solutions is computer simulation, represented primarily by MD [44] In most cases, MD simulations of molecular fluids are based on empirical force field, describing the potential energy of the system as a function of the atomic positions. In the implicit solvent picture, high short-range coordination and low volume imply de-hydration, while the opposite corresponds to hydrated ions of larger effective size
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