The equilibrium of heterogeneous systems including electrolytes. Part III. The effect of an electric field on the adsorption of organic molecules, and the interpretation of electro­-capillary curves

Abstract

In an exhaustive series of researches, Gouy examined the effect of the addition of organic substances on the electro-capillary curve of mercury in a solution of an inactive salt, such as sodium sulphate. He found that, with rare exceptions, these substances caused a depression of the curve which was greatest in the neighbourhood of its maximum, falling off on each side and, in many cases, ultimately becoming zero. He ascribed this behaviour to the effect of the electric field on the adsorption of the added substance, the amount adsorbed being greatest when the electric field at the interface is small and diminishing as the field increases in either direction. A quantitative calculation of the effect was attempted by Frumkin. He obtained the work done in adsorption on account of the electric forces at the interface as the sum of two terms : (1) a general term, due to the smaller dielectric constant and greater length of the organic molecule, than water, which he evaluated as ½ ( c — c' ) ψ 2 , where c is the capacity of the double layer per unit surface in the pure solvent, c' that when the surface is saturated with the adsorbed substance, and ψ the potential difference between the interface and the interior of the solution; (2) a term dependent on the dipole moment of the adsorbed molecule, given as c' ψ N ψ, where ψ N is the potential difference due to the dipoles, between uncharged mercury and the solution when the surface of the former is completely covered with adsorbed molecules. He thus obtained the total work done by the electric forces in adsorption, which he wrote ϕs , and took their effect as equivalent to a dilution of the solution in the ratio e ϕs/RT . The effect of this apparent dilution on the surface tension was obtained by the use of somewhat com­plicated adsorption equations, but by their use Frumkin was able to account for the effect of amyl alcohol on the electrocapillary curve.