Pressure-dependent cationic associations of ionic liquids with bentonite nanoclay

Abstract

Pressure-dependent IR spectroscopy is used to investigate the possibility of stabilizing ionic liquid (IL)–hydrophilic bentonite (Bent) composites with the application of high pressures. Ambient-pressure experiments suggest IL hydrophilicity/hydrophobicity impacts cation and anion interactions with Bent. For example, both the cation and anion of hydrophilic 1-butyl-3-methylimidazolium dicyanamide ([BMIM][DCA]) experience composition-dependent vibrational mode wavenumber shifts. However, anion–Bent interactions are suppressed at ambient pressure when the nanoclay is paired with hydrophobic 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][NTf2]). Compression to high pressures triggers significant blue shifts for ring-bound C–H stretching bands. The rates of these changes upon pressurization indicate the two ILs experience different interaction motifs with the Bent surface. Large wavenumber shifts for C4–H and C5–H stretching modes of [BMIM][NTf2] imply preferential coordination to Bent surface sites through these groups. This presumably leaves the C2–H group available for hydrogen bonding with the NTf2 anion. The situation is different for [BMIM][DCA], where all three C–H groups on the imidazolium ring appear to interact with Bent surface sites. Depressurization reveals further differences between the two systems. Spectroscopic features are reversible for [BMIM][NTf2], whereas the C–H stretching mode blue shifts for [BMIM][DCA] are either irreversible or the relaxation is kinetically hindered upon return to ambient pressure. Taken together, the spectroscopic data strongly suggests that IL–Bent interactions are pressure sensitive, and IL assemblages along the interface may be manipulated through the application of high pressure.