Hydrophilicity\nDependent\nDistribution of Water at\nIonic Liquids/Metal Interface Monitored by Electrochemical SERS

The\nunderstanding of the interfacial processes is critically\nimportant\nfor extending the practical application of ionic liquids, particularly\nfor the role of interfacial water. In the electrochemical system based\non ionic liquid electrolytes, small amounts of water at the interface\ngenerate a significant change in the electrochemical behaviors of\nionic liquids. Therefore, the investigation on the interfacial behavior\nof water is highly desired in ionic liquids with different anions,\nwater content, and hydrophilicity. Herein, based on the probe strategy,\nin situ surface enhanced Raman spectroscopy (SERS) combined with electrochemical\ncontrol (EC-SERS) was developed to investigate the influence of hydrophilicity/hydrophobicity\nof ionic liquids on the interfacial water. The water-sensitive transformation\nreaction of 4,4′-dimercaptoazobenzene (DMAB) to <i>para</i>-aminothiophenol (PATP) was employed as a probe reaction for investigating\nthe behavior of interfacial water. The changes of relative SERS intensities\nof DMAB to PATP served as an indication of the quantity variation\nof interfacial water. The results show that the transformation reaction\nefficiencies were critically dependent on the additional water contents,\npotential, and hydrophilicity of ionic liquids. With a very low molar\nfraction of additional water (X<sub>w</sub> = 0.01), transformation\nefficiency of DMAB (the amount of interfacial water) followed the\nsequence of [BMIm]BF<sub>4</sub> < [BMIm]PF<sub>6</sub> < [BMIm]Tf<sub>2</sub>N. It was in agreement with the hydrophobicity order of the\nionic liquids. With the increase in additional water content, the\npotential for the full transformation was positively moved, and the\nefficiency increased significantly. The stronger hydrophobicity allowed\nmore water molecules to migrate to the interface, which was attributed\nto the difference in interactions between water and the anions of\nionic liquids. It demonstrated that the small amount of water tended\nto gather at the interface in hydrophobic ionic liquids. Compared\nto traditional cyclic voltammetry, the EC-SERS technique combined\nwith probe reactions is more sensitive to interfacial water. It is\nanticipated to develop as a promising tool for the investigating water-related\nissues at interfaces and to provide guidance to screen ionic liquids\nfor practical application.