Electron Transfer Kinetics in Ethaline/Water Mixtures: An Apparent Non-Marcus Behavior in a Deep Eutectic Solvent

Abstract

Effects of water on the charge-transfer (CT) kinetics in deep eutectics solvents (DES) have been investigated in ethaline (1 : 2 choline chloride+ethylene glycol), taken as a prototypical example of a hydrophilic DES. Standard heterogeneous CT rate constants ks were measured for two redox couples: 1,1’-ferrocene-dimethanol/1,1’-ferrocenium-dimethanol and ferro-/ferricyanide on a glassy carbon electrode in ethaline as function of the water amount in the DES. Contrarily to the behavior reported in classical solvents and in apparent contradiction with the Marcus Theory, ks values show very little or no variation with the amount of water in the DES or the changes of viscosities or diffusion coefficients that are observed. This unexpected phenomenon is discussed as function of the known physical-chemistry parameters of ethaline.