Effect of cation alkyl chain length and anion type on protic ionic liquid nanostructure

The local and long-range bulk liquid structures of four primary alkylammonium protic ionic liquids (PILs)-ethylammonium hydrogen sulfate, ethylammonium formate, ethylammonium thiocyanate and butylammonium thiocyanate-are determined using neutron diffraction and computer simulations and compared with those determined previously for ethylammonium nitrate and propylammonium nitrate. All these PILs arrange into a sponge-like bicontinuous nanostructure consisting of polar and apolar domains. Lengthening the cation alkyl chain leads to nanostructures in which the polar and apolar domains are better segregated, with the ions in more precisely defined positions relative to one another. Changing the anion (for the same cation) has comparatively little effect on structure. The reason all these PILs adopt low-curvature sponge-like morphologies, despite the marked changes in the cation and anion structures, is that the preferred areas of the nonpolar and polar fragments within the nanostructure are similar.