Nanostructure of an ionic liquid-glycerol mixture.

Abstract

The nanostructure of a 50 : 50 vol% mixture of glycerol and ethylammonium formate (EAF), a protic ionic liquid (IL), has been investigated using neutron diffraction and empirical potential structure refinement (EPSR) fits. EPSR fits reveal that the mixture is nanostructured. Electrostatic interactions between IL charge groups leads to the formation of ionic regions. These solvophobically repel cation alkyl groups which cluster together to form apolar domains. The polar glycerol molecules are preferentially incorporated into the charged domains, and form hydrogen bonds with EAF groups rather than with other glycerol molecules. However, radial distribution functions reveal that glycerol molecules pack around each other in a fashion similar to that found in pure glycerol. This suggests that a glycerol channel runs through the ionic domain of EAF. The absence of significant glycerol-glycerol hydrogen bonding indicates that glycerol molecules are able to span the polar domain, bridging EAF charge groups. Glycerol can adopt six distinct conformations. The distribution of conformers in the EAF mixture is very different to that found in the pure liquid because hydrogen bonds form with EAF rather than with other glycerol molecules, which imparts different packing constraints.