Resonance shear measurement of nanoconfined ionic liquids

Two types of imidazolium-based ionic liquid (IL), 1-butyl-3- methylimidazolium bis(trifluoromethanesulfonyl)amide ([C₄mim] [NTF₂]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([C ₄mim][BF₄]), confined between silica surfaces were investigated by surface force apparatus (SFA)-based resonance shear measurements together with surface force measurements. The surface force profiles in the ILs showed oscillatory solvation forces below the characteristic surface separations: 10.0 nm for [C₄mim][NTf₂] and 6.9 nm for [C₄mim][BF₄]. The more pronounced solvation force found in [C₄mim][NTf₂] suggests that the crystal-forming ability of the IL contributes to the stronger layering of the ILs adjacent to the surface. The resonance shear measurement and the physical model analysis revealed that the viscosities of the confined ILs were 1-3 orders of magnitude higher than that of the bulk IL. This paper also focused on the correlation between the resonance shear behaviour and the lubrication property of the ILs, and the suspension rheology in the ILs. An understanding of the solid-IL interface and of ILs confined in nanospace will facilitate the further development of novel applications employing ILs.