Properties of the Electrical Double Layer in Solutions of Electrolytes in Some Organic Solvents

Abstract

Differential capacities at the dropping mercury electrode in solutions of some inorganic halides in formamide, dimethylformamide, and acetonitrile have been measured. For cesium iodide in dimethylformamide and acetonitrile a rather large increase of the differential capacity, depending on the concentration of electrolyte in the region of high cathodic polarization, has been observed. For lithium chloride and bromide in the same solvents both effects were very small. It has been shown for a number of solvents that the minimum values of the differential capacity for 0.1M electrolyte solutions are roughly proportional to the dielectric constant of the solvent. The hump usually appearing on the curves of differential capacity for aqueous solutions of electrolytes has also been observed for formamide solutions, but was absent in the case of solutions in organic solvents of low dielectric constant.

The Electrical Double Layer in Dimethyl Sulfoxide Solutions

“…The curves bear a striking resemblance to typical aqueous solutions curves.4 DMSO is the fourth solvent system in which a capacity hump has been observed. 6 The other systems are water,4 formamide,6-9 and N-methylformamide.10 In the formamides, however, the hump occurs at potentials of ~0.8 v negative of the ecm, whereas in DMSO and aqueous solutions it occurs on the anodic side. This important point is discussed below.…”

“…The capacity is shown as a function of potential in Figure 6 for five solutions of 0.1 M KPF6 in mixtures of DMSO and water covering the complete range from pure DMSO to pure water. A number of 6 Data of D. C. Grahame and co-workers tabulated in "Electrochemical Data," B. E. Conway, Ed., Elsevier Publishing Co., Amsterdam, 1952. = KC104.…”

“…According to Table II Hg|Hg2Cl2¡l M aq KCljO.l M KPFs¡x M KPFJHg in DMSO in DMSO 6 Feor is value corrected for LJP2 using the data in ref 25 shifts in the positive direction with increasing salt concentration. This, however, is due to the liquidjunction potential deliberately introduced between the 0.1 M KPF6 solution and the solution in which the measurements are being made, which is done in order to avoid disturbing the aqueous-nonaqueous liquid junction at the reference electrode.…”

The Structure of the Electrical Double Layer at the Metal-Solution Interface

“…A. ORGANIC SOLVENTS (BULK DIELECTRIC CONSTANT < 80) Thus in the case of methanol (134, 223-229, 234, 235), butanol (181, 182), and acetonitrile (230) the minimum capacity is 9-10 /if./cm.2. But for ethanol (186,227,230) and dimethylformamide (230) the corresponding value is 6-7 µ ./cm.2. This pattern can be understood from the following model which serves for a semi quantitative calculation of the solvent capacity Ks-If it is assumed that the solvent molecules are adsorbed with the oxygen end on the mercury surface and lie flat, then with the aid of Catalin models, it is seen that the distance of closest approach of an ion is approximately 4 A.…”

Capacitance Measurements in a Series of Room-Temperature Ionic Liquids at Glassy Carbon and Gold Electrode Interfaces

“…One of the general aspects of the capacitance−potential curves is the appearance of a hump. Several causes, such as orientation of the solvent molecule at the interface, − change in dipole moment of the solvent, adsorption of ions or ion aggregates, structural change of the electrolytes, and ion association have already been put forward for explaining the appearance of a hump. Mott and Watts-Tobin, in an attempt to explain the hump observed in aqueous media, have proposed that at the electrode surface there are two types of water molecules; one with the oxygen end toward the electrode and the other with the hydrogen end, both at an angle of cos −l (1/√3) to the normal of the surface.…”