Electric double layer on a renewable liquid (Cd[sbnd]Ga) electrode in dimethyl sulfoxide solutions

Abstract

The electric double layer (EDL) structure on a renewable liquid (CdGa) (0.3at.% Cd) electrode in solutions of surface-inactive electrolyte is studied in a solvent with a high donor number, namely, dimethyl sulfoxide (DMSO). The following parameters: the potential of zero charge (PZC) unaffected by specific adsorption of ions, the value of “corrected electrochemical work function”, and the potential drop due to chemisorption of solvent are obtained on (CdGa)/DMSO interface and are compared with analogous results on Hg/DMSO, (TlGa)/DMSO, (InGa)/DMSO, Ga/DMSO, (CdGa)/acetonitrile (AN), (CdGa)/gamma-butyrolactone (GBL), and (CdGa)/dimethylformamide (DMF) interfaces. The chemisorption of DMSO molecules in the Helmholtz layer shifts the potential of the (CdGa) electrode in the negative direction, which suggests that chemisorbed DMSO dipoles are oriented with their negative (oxygen) ends to the metal surface. It is found that the solvent chemisorption potential drop on the (CdGa) electrode increases in the series AN<GBL<DMF<DMSO, i.e. with increasing donor number of the solvent. The energy of metal–DMSO chemisorption interaction increases in the series (TlGa)<(CdGa)≈(InGa)<Ga, i.e. with an increase in the electron work function. These results indicate the donor-acceptor nature of the metal-solvent chemisorption interaction in the Helmholtz layer, where the solvent molecules are donors of the electron pair relative to the metal. In spite of a decrease in the electron work function when passing from (InGa) to (CdGa), the energies of their chemisorption interaction with DMSO are close. This result can be explained by a closer approach of DMSO dipoles to the surface of the (CdGa) electrode as compared with the (InGa) electrode. It contributes to a more efficient overlapping of the donor-acceptor levels of the solvent and metal during their chemisorption interaction. Based on the PZC shifts observed in 0.1 М LiCl, LiBr and LiI solutions with respect to the PZC in the solution of surface inactive electrolyte, the adsorbability of halide ions on the (CdGa)/DMSO interface is studied. It is shown that on the (CdGa)/DMSO interface, the adsorption of halide ions increases in the sequence Cl−<Br−<I−, whereas at the Hg/DMSO and Bi/DMSO interfaces, an opposite row of surface activity for these ions is observed.