X-Ray diffraction and resonance shear measurement of nano-confined ionic liquids

Abstract

X-ray diffraction measurement at the SPring-8 synchrotron was employed to investigate the structures of two types of imidazolium-based ionic liquids (ILs), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C₄mim][NTF₂]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([C₄mim][BF₄]), confined between silica surfaces by varying the surface separation distances of ca. 500 nm (bulk liquid), ca. 10 nm, and ca. 2 nm (hard wall thickness). The obtained diffraction profiles and intensities were discussed by considering the structures and properties of the nano-confined ILs between the silica surfaces investigated by resonance shear measurement (RSM) and molecular dynamics simulation (MD) in our previous reports. [C₄mim][NTf₂] showed two diffraction peaks at q = 8.8 nm⁻¹ (spacing d = 0.71 nm) and at q = 14.0 nm⁻¹ (spacing d = 0.45 nm) at the greatest distance (D = ca. 500 nm), which were assigned to the interval between the same ions (anion–anion or cation–cation) within the polar network of [C₄mim][NTf₂] and the interval between the neighboring anion–cation, respectively. The positions of these two peaks remained the same at D = ca. 10 nm and at the hard wall (D = ca. 2 nm) and their intensity factor increased, indicating that both the cation and anion existed in the same layer. This result was consistent with the checkerboard structure of [C₄mim][NTf₂] on the silica surface computer simulated in our previous studies. On the other hand, [C₄mim][BF₄] showed a peak at q = 15.4 nm⁻¹ (spacing d = 0.41 nm) corresponding to the anion–cation interval at the greatest distance (D = ca. 500 nm). This peak became broader and weaker at D = ca. 12 nm and at D = ca. 2 nm.