A unified description of macroion diffusiophoresis, salt osmotic diffusion and collective diffusion coefficient

Abstract

Diffusiophoresis describes the net migration of a macroion within an aqueous fluid due to a concentration gradient of salt, while salt osmotic diffusion describes the net motion of small ions toward Donnan equilibrium due to a macroion concentration gradient. A new theoretical description of macroion diffusiophoresis, salt osmotic diffusion and the light-scattering collective diffusion coefficient of macromolecules conforming to Onsager Reciprocal Relation (ORR) is developed. Specifically, symmetric transport equations for macroion-salt coupled transport in isothermal conditions are derived. These equations, which can be regarded as a generalization of Nernst–Planck equations, incorporate an electrokinetic factor for diffusiophoresis, salt thermodynamic non-ideality and concentration-dependent ionic transport coefficients. Due to the electrokinetic factor, thermodynamic driving forces must be modified to ensure that ORR is not violated. A mathematical expression for the collective diffusion coefficient compatible with ORR was then extracted. Diffusiophoresis and salt osmotic diffusion equations are discussed in relation to experimental transport data on lysozyme in aqueous NaCl. These transport equations represent a more appropriate theoretical reference model, compared to multicomponent-diffusion limiting laws for describing protein-salt coupled diffusion in aqueous mixtures.