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The interpretation of solution electrolyte vibrational bands in potential-difference infrared spectroscopy

1988; Elsevier BV; Volume: 239; Issue: 1-2 Linguagem: Inglês

10.1016/0022-0728(88)80269-9

2590-2954

Dennis Corrigan, Michael J. Weaver,

Spectroscopy and Quantum Chemical Studies

The influence of ionic migration to and from the surrounding solution reservoir upon potential-difference infrared (PDIR) spectra is examined for some cases involving anionic adsorption in order to elucidate its consequences upon the net potential-induced compositional changes in the thin-layer solution. Representative PDIR spectra for the adsorption of azide anions on gold are compared in the absence and presence of excess alkali perchlorate supporting electrolyte. In the latter, the loss of solution azide in the spectral thin layer upon stepping to a more positive potential, resulting from increased azide adsorption, is accompanied by extensive migration of perchlorate into the thin layer. The form of the spectra induced by potential-dependent azide specific adsorption differs in these two circumstances since in the former the ionic migration between the thin-layer cavity and the solution reservoir necessary for charge compensation is provided by the azide electrolyte itself, whereas in the latter case migration of the supporting electrolyte yields a fixed quantity of azide in the thin layer. The intensities and sign of the PDIR bands arising from solution-phase azide and perchlorate enable the extent of the potential-dependent anionic redistribution in the thin-layer cavity to be quantified. In the absence of added perchlorate, the magnitude of the solution azide band is diminished substantially, inferring that replenishment of the thin-layer solution concentration occurs predominantly via N−3 migration from the surrounding solution reservoir. Similar results were also obtained for cyanate adsorption on gold. The influence of cation as well as anion migration on this thin-layer charge redistribution was examined by employing an infrared-active cation, NH+4, as well as from the addition of H3O+. While the results indicate that cation migration can contribute substantially to this charge redistribution, anion migration typically appears to predominate when specific anion adsorption is encountered. Some general consequences of such ion migration effects to the interpretation of PDIR spectra are noted.