Specific adsorption of Cl−, Br−, and I− ions on liquid Ga electrode from N-methylformamide solutions

An ac bridge is used to measure the differential capacitance curves in the systems of Ga/[N-MF + 0.1m M KCl + 0.1(1 − m) M KClO4], Ga/[N-MF + 0.1m M KBr + 0.1(1 − m) M KClO4], and Ga/[N-MF + 0.1m MKI + 0.1(1 − m) M KClO4] for the following values of fraction m of the surface-active anion: 0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1. As compared to other solvents, N-methylformamide (N-MF) allows achieving much higher positive charges of the Ga electrode, at which it remains ideally polarizable (up to 20 and in some cases 30 μC/cm2). The values of fraction m of the surface-active anion, 0.05, 0.1, 0.2, 0.5, and 1, are used to calculate adsorption parameters for model based on the Frumkin isotherm, in which free adsorption energy ΔGads is a function of the electrode charge. It is found that the ΔGads value at the Ga/N-MF interface changes inversely, Br− ≈ I− < Cl−, to the sequence observed on the Hg/N-MF interface. The obtained results can be used for the unique assessment of an increase in the metal–Hal− interaction energy in the series of ΔGM–Cl- > ΔGM–Br- > ΔGM–I- and of difference (ΔGGa–Hal1--ΔGGa–Hal2-) exceeding the difference of solvation energies of Hal− (ΔGS–Hal1--ΔGS–Hal2-).

► Adsorption of Cl–, Br– and I– ions was found to follow Frumkin adsorption isotherm.
► As opposed to the Hg/N-MF interface, ΔGads on the Ga/N-MF interface change in the following series: Cl–> Br– ≈ I–.
► The free metal-Hal- interaction energy grows in the series of ΔGGa-Cl– > ΔGGa-Br– > ΔGGa-I–.
► The difference of (ΔGGa-Hal2– > ΔGGa-Hal1–) exceed the difference of solvation energies (ΔGNMF–Hal2– > ΔGNMF–Hal1–).