The Direct Piezoelectric Effect in Deep Eutectic Solvents

We report the direct piezoelectric response of four deep eutectic solvents (DESs): choline chloride:ethylene glycol (ChCl:EG), choline chloride:glycerol (ChCl:Gly), choline chloride:1,3-propanediol (ChCl:PD), and choline chloride:urea (ChCl:urea). Measurement of current as a function of applied force produces a linear relationship from which the piezoelectric coefficient (<i>d</i>₃₃) was determined. The piezoelectric effect has previously been observed in room-temperature ionic liquids (RTILs), attributable to a pressure-induced liquid-to-crystalline solid phase transition. The observation of this phenomenon in DESs is unprecedented and underscores its generality. The magnitude of <i>d</i>₃₃ in these DESs is similar to that for RTILs, suggesting the potential to tune the piezoelectric response through careful selection of the DES constituents and constituent ratios.