Ion-Specific Interfacial Hydration Beyond Classical Chaotropicity

Ion specificity at charged aqueous interfaces plays important roles in many natural and technological phenomena. While ion-specific effects have been frequently investigated for halides and other molecular anions, the bis(trifluoromethanesulfonyl)imide (TFSI) anion, a common ionic liquid anion, has been rarely investigated in aqueous interfaces. However, TFSI-water interactions are highly important in rare earth element separations. Here, using sum-frequency generation (SFG) spectroscopy, we demonstrate that TFSI restructures water at a positively charged imidazolium Langmuir monolayer interface through a mechanism qualitatively different from those of the conventional chaotropes thiocyanate and nitrate. Specifically, we study TFSI-, SCN-, and NO3- adsorption at a 1-octadecyl-3-methylimidazolium (C18mim) Langmuir monolayer over a 250 nM to 10 mM concentration range. Direct SO2 and CN vibrational signatures quantified TFSI and SCN adsorption, respectively. The OH stretch response was used to characterize ion-induced changes in the interfacial water structure and diffuse-layer screening. TFSI- adsorbs exceptionally strongly with an apparent adsorption free energy of -36.8 kJ/mol. Its OH band response is fundamentally different from that of SCN- and NO3-. After TFSI- SO2 stretch signal saturates, the hydrogen-bonded OH signal continues to decrease and a distinct high frequency band at ∼3620 cm-1 persists. These results identify an asymmetric hydration shell of TFSI as an additional structural factor in ion specificity.