“Effective” Dielectric Constant for Short-range Ionic Interactions at High Salt Concentrations

Abstract

AT any highly charged interface with aqueous media, for example those of surfactant micelles, membranes, ion-exchange resins or proteins, the “effective” dielectric constant D controlling coulombic interactions is likely to be different from the value for water^1,2. At least three identifiable factors, not necessarily independent, may be involved²: (a) the proximity of a low-dielectric region, for example the micelle core, (b) dielectric saturation in the electric field present³, and (c) the presence of high local concentrations of ions^1,4. Factors (b) and (c) have often been used in theories of electrical double layers^1,4. The contribution from factor (c) is usually calculated on the basis of microwave measurements of static D values of salt solutions⁵, which can be substantially lower than that of water.