Cracking Ion Pairs in the Electrical Double Layer of Ionic Liquids

TL;DR

This paper presents an analytical theory for ion pairing in the electrical double layer of ionic liquids, revealing how ion pairs influence capacitance and electrostatic properties, with implications for understanding ionic liquid behavior.

Contribution

It introduces a simplified analytical model focusing on ion pairs in the EDL, contrasting with previous free ion approaches, and explains the competition between association energy and electrostatic potential.

Findings

Ion pairs dominate under certain conditions in the EDL.

Differential capacitance exhibits a double hump 'camel' shape.

Theory aligns with and extends previous free ion models.

Abstract

Here we investigate a limiting case of the theory for aggregation and gelation in the electrical double layer (EDL) of ionic liquids (ILs). The limiting case investigated only accounts for ion pairs, ignoring the possibility of larger clusters and a percolating ionic network. This simplification, however, permits analytical solutions for the properties of the EDL. The resulting equations demonstrate the competition between the free energy of an association and the electrostatic potential in the EDL. For small electrostatic potentials and large negative free energies of associations, the ion pairs dominate in the EDL. Whereas, for electrostatic potential energies larger than the free energy of an association, electric-field-induced cracking of ion pairs occurs. The differential capacitance for this consistent ion pairing theory has a propensity to have a ``double hump camel'' shape. We compare this theory against previous free ion approaches, which do not consistently treat the reversible associations in the EDL.