Structure and properties of ‘Type IV’ lanthanide nitrate hydrate:urea deep eutectic solvents

A series of lanthanide nitrate hydrate:urea ‘Type IV’ deep eutectic solvents (DES; Ln=Ce,Pr,Nd) were prepared and their physical properties measured, showing very high surface tension and density, with low viscosity and glass transition temperatures. Calculated Gordon parameters were similar to water, with lower molecular volumes than ‘Type III’ DES. The LnDES were used as reaction media for efficient combustion synthesis of lanthanide oxides. The nanostructure of the Ce(NO3)3.6H2O:urea DES was measured using neutron and X-Ray scattering and resolved with empirical potential structure refinement (EPSR) atomistic modelling. The models showed the existence of strongly-bonded yet fluxional oligomeric [ Ce-NO3-] polyanions and polycations. Because of the excess of the molecular component in the mixture, an intercalating H-bonded nanostructure containing mainly water and urea was observed, which can be considered as a lubricating molecular pseudophase. This dichotomous structural observation helps to explain some of the unusual physical properties such as low viscosity and high surface tension, while also challenging the fundamental definitions of DES.