Probing the influence of Zn and water on solvation and dynamics in ethaline and reline deep eutectic solvents by 1H nuclear magnetic resonance

Abstract

A range of ethaline and reline deep eutectic solvents (DESs) have been investigated in the absence and presence of Zn (0–0.3 M) and water (0–29 wt%) by one-dimensional ¹H NMR spectroscopy, two-dimensional ¹H–¹H nuclear Overhauser effect and exchange spectroscopy, ¹H T₁ NMR relaxation times and ¹H NMR diffusion. The role of zinc and water in controlling solvation and microstructure in reline and ethaline were investigated. We show that in ethaline there is proton exchange between hydroxyl groups in ethaline glycol and choline chloride. The rate of exchange between these protons is found to significantly increase in the presence of Zn, but decreases with increasing water content. In the case of reline, no proton exchange is observed between the amide protons in urea and hydroxyl protons in choline chloride. However, the addition of water decreases the viscosity of the system, as well as changes the distance between amide and hydroxyl protons in urea and choline chloride, respectively. The addition of Zn does not appear to change the interactions between urea and choline chloride species, but does reduce the rate of exchange between water and hydroxyl protons in reline formulations containing water.