Diffusivities of organic electrolytes in water

The diffusivities in water of large organic ions such as biomolecules are not readily available. There is a need for adequate measurement and estimation methods. In this work, a rapid method is presented for the determination of diffusivities of ionic species in aqueous systems, based on steady state conductivity measurements. As transient composition gradients are essentially absent in our proposed method, an important source of errors in the form of poorly controlled gradients in thermodynamic non-ideality is eliminated. Ionic transport is described with the generalized Maxwell-Stefan model, taking friction between solvent and ionic solutes, and between the moving ionic species into account. Ion-water diffusivities could be correlated satisfactorily for a wide range of organic ions, zwitterions and neutral species with the Wilke-Chang relation, and to a lesser extent with a modified Stokes-Einstein relation. Furthermore, the model describes ionic fluxes over a large range of concentrations when corrected for concentration and temperature dependent viscosity effects, by using a modified Stokes-Einstein relation.