The Nature of the Electrical Double Layer' DAVID C. GRAHAME, Amherst College, Amherst, Mossaehwetts
THE MOST prominent theories of the electrical double layer a t a polarized electrode were reviewed and their distinctive characteristics pointed out. The original Helmholtz theory was an oversimplification because i t ignored the thermal agitation of the ions forming the double layer. The Gouy -theory which followed it took account of the effects of thermal agitation but led to too large values of the capacity of the double layer. Neither Helmholtz nor Gouy considered the effects of possible specific adsorption of ions. Stern attempted to combine the theories of Helmholtz and of Gouy by postulating the existence of both kinds of double layers simultaneously. Stem also introduced the concept of specific adsorption, although his fundamental postulate of a constant value for the specific ad-
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1 Abstract of an address presented at the 217th Meeting of the N.E.A.C.T.,Springfield, Mass., October 10. 1942.
sorption potential turns out to have been incorrect. According to the present author, the electrical double layer is composed of a compact layer of specifically adsorbed ions and a diffuse layer of non-adsorbed ions. Since the adsorbed ions are not fully hydrated, they may be expected to be closer to the metallic surface than other non-adsorbed ions. This fact gives rise to a large discrepancy between the results predicted by the Stern theory and those actually found. At any particular concentration, the diffuse double layer shows a very pronounced saturation effect, as a result of which there is a limit to the amount of charge which can be attributed to the d i u s e layer regardless of the applied potential. This limit occurs a t about 15,9,and 6 microcoulombs per square centimeter for solutions which are normal, tenth-normal, and hundredth-normal, respectively.
