This study examined how hydrotropes influence the electrochemical and mass transport properties of ferrocene in water. Hydrotropes are small amphiphilic organic compounds that are used to increase the aqueous solubilities of sparingly soluble organic compounds. They differ from surfactants in that they do not form micelles. We used tertiary butyl alcohol (TBA) and sodium xylenesulfonate (NaXS) as two model hydrotropes. Both hydrotropes increased the solubility of ferrocene, with larger solubility increases observed at higher hydrotrope concentrations. Cyclic voltammetry experiments were conducted to quantify the diffusion coefficients of ferrocene and ferrocenium, half-wave potentials, and standard reaction rate constants as a function of ferrocene concentration, hydrotrope concentration, and salt concentration. We compared our observations to those made with surfactants. The hydrotropes increased the diffusivities of both ferrocene and ferrocenium. This observation differed from what has previously been observed for surfactants, where surfactants decreased the diffusivity of ferrocene preferentially over ferrocenium by binding the neutrally charged ferrocene in the micelle system. This difference suggested that hydrotrope did not solubilize ferrocene by forming aggregates, and the presence of hydrotrope facilitated the movement of both ferrocene and ferrocenium in solution. The diffusivity of ferrocene depended on the salt concentration, but not on the ferrocene concentration, suggesting the presence of the hydrotrope did not alter the solvent effect of salt nor the Poisson distribution relationship between the ferrocene concentration and diffusivity. The positive and negative dependence of ferrocene half-wave potential and standard reaction rate on the concentration of hydrotrope suggested that the oxidation of ferrocene to ferrocenium was more favorable with hydrotrope, but the presence of hydrotrope slowed down the electron transfer process. These findings may help in understanding how hydrotropes can be used in the electrochemical systems, such as flow batteries, in the future.

Solubilizing ferrocene in water using hydrotropes or surfactants: Differences in mass transport and electrochemical properties

ACS Spring 2022

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