Effect of Chaotropes on the Transfer of Ions and Dyes across the Liquid–Liquid Interface

Chaotropes such as urea can break the structure of water, weakening the hydrophobic effect and reducing aggregation. Here, we investigated how the addition of urea affects the transfer of ions and cationic dyes across the interface between immiscible electrolyte solutions, both water-1,2-dichloroethane and water-trifluorotoluene. For most cations, their half-wave potential of transfer shifted toward more negative values, indicating that it is easier to transfer these ions from the aqueous phase with urea to the organic phase, showing that the addition of urea decreases the solvation of ions in the aqueous phase. However, the half-wave potentials for a series of cationic phenothiazine dyes shifted toward more positive potentials, indicating improved solvation in urea solution. The effect of urea was investigated also by differential capacitance and electrocapillary curves, as well as by molecular dynamics simulations. Finally, electrochemistry at liquid–liquid interfaces allows determination of the transfer energies of ions between water and aqueous solutions of urea via a thermodynamic cycle.