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

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 involved 2 : ( a ) the proximity of a low-dielectric region, for example the micelle core, ( b ) dielectric saturation in the electric field present 3 , 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 solutions 5 , which can be substantially lower than that of water.