Comparative study of the electrical double layer on liquid electrodes of mercury, gallium, and an In-Ga alloy in hexamethylphosphortriamide

Abstract

The structure of the electrical double layer (EDL) on sp metals is studied by exploring it on liquid renewable electrodes of mercury, gallium, and an indium-gallium alloy containing 16.4 at % In. The study is performed in a solvent with a high donor number (DN), specifically, in hexamethylphosphoramide (HMPA, DN = 38.8). A very strong chemisorption interaction between the metal and HMPA is fixed on the Ga and In-Ga electrodes. It is shown that the energy of the metal-HMPA chemisorption interaction increases in the series Hg < In-Ga < Ga. The pattern revealed by the study is exactly the opposite to that previously observed on these very electrodes. The strong chemisorption interaction between the metal and HMPA does not lead to an increase in the capacitance of the inner part of the EDL and is at the same time characterized by a very large chemisorption jump of the solvent potential. The data obtained in HMPA show that, for sp metals in contact with a solvent whose DN is high enough, the effects of the metal-solvent chemisorption interaction may be commensurate with the effects previously observed on catalytically active metals. Such a result is an invincible proof, which requires no additional modeling notions, of the existence of a correlation between energies of the chemisorption interaction between a metal and a solvent and the solvent’s DN. This in turn is a convincing evidence that the specific interaction between a metal and a solvent has a donor-acceptor origin. The data obtained in HMPA make it possible to unite all the available results yielded by research into the EDL structure on the catalytically active and sp metals.