Yuki Kohno

Thermoresponsive polyelectrolytes derived from ionic liquids

Ionic liquid (IL)-based polyelectrolytes (PILs), referred to as polymeric ILs, polymerised ILs, or poly(IL)s are a new subclass of polymer materials. They are distinct from conventional polyelectrolytes due to their unique physico-chemical properties originated from a dense packing of ILs in the macromolecular architecture. Mixtures of PILs and solvents, in particular, water have attracted a great deal of interest especially in terms of their compatibilities depending on temperature, namely, thermoresponsiveness of PIL/solvent mixtures. Apart from static compatibility, such as the compatibility of PILs with solvents, which do not change largely by a temperature change, there are mainly two types of dynamic phase changes, an upper critical solution temperature (UCST)- and a lower critical solution temperature (LCST)-type phase behaviour. Some PILs dissolved in solvents homogenise upon heating; this behaviour is classified as UCST behaviour. On the other hand, only in the last two years have PIL/water mixtures with LCST been discovered. This article summarises rapidly growing studies on the design of thermoresponsive PIL systems with water or organic solvents. The hydrophobicity/hydrophilicity balance of the starting IL monomers features the phase behaviour of the resulting polyelectrolytes, and some IL monomers that show thermoresponsive phase behaviour in solvents were found to maintain their thermoresponsiveness even after the polymerisation. Based on their unique combination of properties derived from an ionic and thermoresponsive nature, these thermoresponsive PILs will attract considerable interest, and their wide applications are expected in the fields of separation, sensing and desalination.

Key Factors to Prepare Polyelectrolytes Showing Temperature-Sensitive Lower Critical Solution Temperature-type Phase Transitions in Water

Tetrabutylphosphonium styrenesulfonate and its homopolymer showed a lower critical solution temperature-type phase transition in water. As the hydrophobicity of these monomeric and polymeric salts affects phase behaviour, the phase transition temperature of the polyelectrolyte was changed by the introduction of monomers having different alkyl chain length on the phosphonium cations.

Material design of ionic liquids to show temperature-sensitive LCST-type phase transition after mixing with water

Phosphonium cations bearing different alkyl chains were coupled with several common anions so as to prepare ionic liquids (ILs) with diverse hydrophobicity. A temperature-driven phase behaviour of the mixture of various ILs and water has been examined. A few ILs were found to exhibit temperature-sensitive lower critical solution temperature (LCST)-type phase transition after mixing with water. The phase separation temperature (Tc) of the IL/water mixtures depended strongly on the hydrophobicity of the component ions as well as mixing ratio. The number of water molecules per ion pair in the IL phase (mwater) increased dramatically upon cooling. The temperature dependence of this parameter was found to be useful to predict the possibility of the ILs to show the LCST-type phase behaviour after mixing with water. Since the value of mwater depended on the ion structure, especially on the hydrophobicity, the Tc was accurately set out by suitably mixing two ILs with different hydrophobicity.

Extraction of proteins with temperature sensitive and reversible phase change of ionic liquid/water mixture

Tetrabutylphosphonium N-trifluoromethanesulfonyl leucine ([P4444][Tf-Leu], IL) shows phase separation with water at 25 °C, but is miscible at 20 °C. Such a reversible phase transition, called lower critical solution temperature (LCST) behaviour, has been utilised to extract proteins. An aqueous solution of cytochrome c (Cyt. c) was mixed with an equal volume of the IL and the solution was cooled down to 20 °C to obtain a homogeneous solution. After phase separation was induced by heating it at 25 °C, all the Cyt. c was found in the IL phase. The IL, containing at least 21 wt% of water, was effective to extract Cyt. c. The distribution ratio (D) of several proteins in the IL/water mixture depended strongly on the isoelectric point (pI) of the corresponding proteins. Based on the difference in the D value, separation of a target protein from their mixtures was successfully carried out using the LCST behaviour of the IL/water mixture.

Dual stimuli-responsive phase transition of an ionic liquid/water mixture

A mixture of tetrabutylphosphonium N-trifluoromethanesulfonyl leucine and water showed a reversible change between a homogeneous phase and separated liquid-liquid phase by bubbling CO(2) gas or N(2) gas at atmospheric pressure as well as a small temperature change.

Ionic liquids showing phase separation with water prepared by mixing hydrophilic and polar amino acid ionic liquids

Both water soluble lysine- and aspartic acid-based amino acid ionic liquids (AAILs) were mixed to prepare polar AAILs showing phase separation with water.

Addition of suitably-designed zwitterions improves the saturated water content of hydrophobic ionic liquids

The saturated water content in a hydrophobic ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)-imide, was improved from 0.4 wt% to 17.8 wt% by adding a 3-(1-butyl-3-imidazolio)propanesulfonate-type zwitterion in appropriate amounts. The mixture containing 17.8 wt% water successfully dissolved horse heart cytochrome c without significant change of the higher ordered structure.

Design of phosphonium-type zwitterion as an additive to improve saturated water content of phase-separated ionic liquid from aqueous phase toward reversible extraction of proteins.

We designed phosphonium-type zwitterion (ZI) to control the saturated water content of separated ionic liquid (IL) phase in the hydrophobic IL/water biphasic systems. The saturated water content of separated IL phase, 1-butyl-3-methyimidazolium bis(trifluoromethanesulfonyl)imide, was considerably improved from 0.4 wt% to 62.8 wt% by adding N,N,N-tripentyl-4-sulfonyl-1-butanephosphonium-type ZI (P555C4S). In addition, the maximum water content decreased from 62.8 wt% to 34.1 wt% by increasing KH2PO4/K2HPO4 salt content in upper aqueous phosphate buffer phase. Horse heart cytochrome c (cyt.c) was dissolved selectively in IL phase by improving the water content of IL phase, and spectroscopic analysis revealed that the dissolved cyt.c retained its higher ordered structure. Furthermore, cyt. c dissolved in IL phase was re-extracted again from IL phase to aqueous phase by increasing the concentration of inorganic salts of the buffer solution.

Temperature-Driven and Reversible Assembly of Homopolyelectrolytes Derived from Suitably Designed Ionic Liquids in Water

Polycationic-type homopolymers derived from polymerisable ionic liquids with adequate hydrophobicity were prepared to form spherical nanoparticles in water that changed their diameter as a function of temperature. The diameter change was attributed to a temperature-sensitive and reversible lower critical solution temperature-type phase change of the polymers in water.

A cobalt(II) bis(salicylate)-based ionic liquid that shows thermoresponsive and selective water coordination

A metal-containing ionic liquid (MCIL) has been prepared in which the [Co(II)(salicylate)2](2-) anion is able to selectively coordinate two water molecules with a visible colour change, even in the presence of alcohols. Upon moderate heating or placement in vacuo, the hydrated MCIL undergoes reversible thermochromism by releasing the bound water molecules.