Double layer structure at cathodic potentials in N-methylformamide solutions of the alkali metal perchlorates
✍ Scribed by W.R. Fawcett; R.O. Loutfy
- Publisher
- Elsevier Science
- Year
- 1972
- Weight
- 634 KB
- Volume
- 39
- Category
- Article
- ISSN
- 0022-0728
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✦ Synopsis
In spite of the fact that N-methylformamide (NMF) has attracted considerable attention as a solvent for electrolyte solutions because of its unusually high dielectric constant 1, very few studies of its interracial properties have been made 2. Damaskin and Povarov 3 measured the differential capacity of the mercury/solution interface for solutions of various alkali metal halides in NMF. They observed that the capacitypotential curves were qualitatively similar to those obtained with the corresponding formamide systems 4,s, specifically, that a capacity "hump" was found at potentials cathodic of the point of zero charge, and that the capacity at more negative potentials depended on the nature of the alkali metal cation. The sharp increase in capacity at far cathodic potentials in Cs + solutions was ascribed to specific cationic adsorption. In an extension of this study, Damaskin and Ivanova 6 calculated the surface excesses of Cs + and C1-for 0.1 M CsC1 in NMF from interfacial thermodynamic data and compared these values with those predicted by the Gouy-Chapman theory. The disagreement obtained at potentials close to the point of zero charge was attributed to dielectric saturation in the diffuse layer which resulted in the dielectric constant dropping from 182 to 84; at more cathodic potentials, specific adsorption of Cs ÷ was cited.
It is apparent from these and related studies 2 that Cs ÷ is specifically adsorbed on mercury from solutions in NMF and other amide solvents. The purpose of the present work was to determine the potential range over which cation adsorption is significant, and to re-examine the question of dielectric saturation in the diffuse layer in this highly structured solvent. EXPERIMENTAL Differential capacity-potential data were obtained by measuring the in-and out of-phase components of the a.c. admittance of the mercury electrode/solution interface using the PAR model 170 electrochemistry system. A 5 mV amplitude a.c. voltage with a frequency of 200 Hz was superimposed on the d.c. voltage applied between the dropping mercury electrode and platinum counter electrode, the rate of d.c. voltage scan being 1 mV s-1. The drop time was mechanically controlled at 5s, * Author to whom correspondence should be addressed.