Using the concept of biostere substitution taken from pharmacy, ionic liquids containing anions derived from functionalised 5-tetrazoles were investigated ans analogues of ionic liquids with carboxylate anions.
Ionic liquids with small carboxylate anions, such as acetate, are water miscible, but lead to interesting characteristics including the ability to dissolve cellulose. Ionic liquids with longer chain alkyl-carboxylate anions have been shown to be capable of dissolving sulfur, and the series of tetrabutylphosphonium benzoate, salicylate and phthalate ionic liquids, with aromatic carboxylate anions show a progression from completely water miscible (benzoate) through to partial miscibility with LCST behaviour.
The key work performed during the project covered five major activities:
1. Synthesis of the tetrazole anion precursors using [2+3] cycloaddition reactions of azides and nitriles.
2. Synthesis of ionic liquid combining tetrazolate anions with a matrix of representative cations (1-alkyl-3-methylimidazolium, N,N-dialkylpyrrolidinium, tetraalkylammonium and tetraalkylphosphonium) and thermophysical characterisation, including rapid identification of chemically unstable materials allowing research pathways to be defined.
3. Behaviour of each ionic liquid with water, including LLE and SLE phase behaviour was determined and critical point phase diagrams were constructed. For the key prototype ionic liquid target, tetrabutylphosphonium 5-phenyltetrazolate, the full LLE/SLE phase behaviour was determined using optical cloud-point and melting point observation, differential scanning calorimetry and single crystal X-ray diffraction identifying LCST phase behaviour and SLE with novel eutectic and peritectic points that have not previously been reported in ionic liquids. This work was published in ChemPhysChem DOI: 10.1002/cphc.201700942.
4. The effects of phenyl-substituent groups on the tetrazolate anions has been assessed, examining ionic liquid/water compositions using ionic liquids prepared from commercially available and synthesised functionalised phenyltetrazoles to derive structure-property correlations that identify how the critical temperatures for miscibility and composition limits can be controlled through selective functionalisation. This will allow, for example polymerisable ionic liquid hydrogel membranes to be produced with defined and predictable gellation and separation characteristics.
5. Phase behaviour of these ionic liquids in contact with electrolyte solutions was evaluated. Lower critical temperatures were observed from brines, compared to contacting with pure water demonstrating potential for the removal and recovery of water from aqueous electrolyte solutions using a temperature-swing mechanism.
