Many applications of ionic liquids, especially those involving electrochemistry, require understanding and controlling local electric fields at the interfaces of ionic liquids with solids. However, the conventional understanding of ions near an interface in electrochemistry is based on dilute electrolytes and often does not apply to ionic liquids, not even approximately. Non-negligible ion sizes and hindered motion of molecules near the surface lead to non-trivial and unexpected behavior. Our approach to understanding the electric field at the interface is to measure the vibrational Stark shift of chromophores tethered at the interface using Sum Frequency Generation (SFG) spectroscopy. Since the Stark tuning properties of the probe molecule are reasonably well-known, the observed shifts can be related to the local electric fields. We will report results on vibrational Stark shift of benzonitriles tethered at the junction of gold and ionic liquids such as 1-Ethyl-3-methylimidazolium tetrafluoroborate. We have previously developed an interfacial solvation theory as an extension of the Onsager solvation model that predicts the local electric fields influencing dipolar molecules at the interface. While the theory is successful in predicting the shifts for well-behaved dielectric solvents, our experimental Stark shifts for ionic liquids do not conform to this continuum theory. This work highlights the importance of discreteness and finite ion size in determining the local electric fields near the interface, and calls for more advanced theoretical descriptions of interfacial solvation by ionic liquids.

Local electric fields at the interfaces of ionic liquids and metals measured by Stark shift spectroscopy

255th National Meeting (2018)

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