# When Dielectric Constants Deceive: Interrogating Solvation in Ionic Liquids with Cyclic Voltammetry

## TL;DR

This paper shows that ionic liquids provide higher solvation energies than expected based on their dielectric constants, due to their high ionic strength.

## Contribution

The study experimentally demonstrates that ionic liquids deviate from predictions based on the Born model due to enhanced electrostatic screening.

## Key findings

- Ionic liquids provide solvation energies comparable to strongly polar solvents like acetonitrile.
- The solvation energies of ionic liquids exceed those of dipolar solvents with the same dielectric constant.
- High ionic strength in ionic liquids enhances electrostatic screening and increases solvation energies.

## Abstract

Ionic liquids are
increasingly discussed as alternatives to conventional
organic solvents for applications based on photoinduced electron transfer.
For the rational design of such applications, reliable estimates of
electron-transfer driving forces are essential. Based on the Born
model of solvation, the moderate dielectric constants of ionic liquids
(εr ≈ 8 – 15) suggest that they should
resemble medium-polarity solvents such as dichloromethane or pyridine
in photoinduced electron transfer and exhibit comparable solvation
energies. Here, we test this assumption by experimentally comparing
the solvation energies of three small organic solutes relevant to
photochemistry in several imidazolium-based ionic liquids and in conventional
dipolar solvents. Solvation energies were inferred from shifts of
half-wave reduction potentials obtained from cyclic voltammetry. We
find that, for the investigated solutes, ionic liquids provide solvation
energies comparable to those of strongly polar solvents such as acetonitrile
or dimethyl sulfoxide. While the organic solvents follow the qualitative
trend predicted by the Born equation, ionic liquids deviate from it
and yield much larger solvation energies compared to dipolar solvents
of the same dielectric constant. This behavior is attributed to the
intrinsically high ionic strength of ionic liquids, which enhances
electrostatic screening and results in substantially larger solvation
energies and consequently much larger driving forces for photoinduced
electron transfer than would be expected based on their dielectric
constants alone.