Comparative study of Cl−, Br−, and I− ion adsorption from N-methylformamide at Ga-, (In-Ga)-, and (Tl-Ga)-electrodes

Adsorption of Cl−, Br−, and I− (Hal−) ions from their 0.1 M solutions in N-methylformamide, a solvent with the highest permittivity (182.4 at 25°C), at liquid Ga-, (In-Ga)-, and (Tl-Ga)-electrodes with refreshable surface was studied by the measuring of differential capacitance and by using an open-circuit jet electrode. It is shown that the adsorption parameters and the surface activity series depend significantly on the metal nature. For the (In-Ga)- and (Tl-Ga)-electrodes, like for Hg-electrode, the halogenide-ion surface activity increases in the following series: Cl− < Br− < I−; for the Ga-electrode, it varies in the opposite sequence: I− < Br− < Cl−. By example of the Ga-electrode in N-methylformamide, it is shown for the first time that the phenomenon of the surface activity series reversal can be observed not only in aprotic solvents but also in protonic ones. The data obtained in N-methylformamide is compared with that obtained in dimethyl formamide and acetonitrile. The halogenide-ion adsorption at the Ga-, (In-Ga)-, and (Tl-Ga)-electrodes decreases in the series N-methylformamide < dimethyl formamide < acetonitrile. The results in aggregate are explained in terms of the Andersen-Bockris model. The data obtained in N-methylformamide, dimethyl formamide, and acetonitrile evidence the increase of ΔGM-Hal−in the series (Tl-Ga) < (In-Ga) < Ga, that is, with the increasing of the metal work function. This points out the donor-acceptor nature of the metal-halogenide-ion interaction, in which the halogenide-ions are donors of electron pair with respect to the metals.