Ionic diffusivity, electrical conductivity, membrane and thermoelectric potentials in colloids and granular porous media: A unified model

Ionic diffusivity, electrical conductivity, membrane and thermoelectric pot entials in isotropic and homogeneous colloidal suspensions, and granular po rous media saturated by a binary symmetric 1:1 electrolyte are four interre lated phenomena. The microstructure and the surface properties of the solid grains-water interface influence directly these properties, The ionic diff usivities (and the electrical conductivity, respectively) in colloids and p orous media have contributions from diffusion (and electromigration, respec tively) through the bulk solution occupying the pores, together with electr omigration occurring at the grains-water interface in the electrical double layer. Surface diffusion in porous materials has no contribution from conc entration gradients along the grains-water interface. Instead, surface diff usion is envisioned as a purely electromigration process due to the membran e potential. The tortuosities of the transport of anions and cations are eq ual to the bulk tortuosity of the pore space only at high ionic strength. A s the ionic strength decreases, the dominant paths for transport of the ion corresponding to the counterion of the electrical double layer shift from the pore space to the solid grains-water interface. Because anions and cati ons do not move independently, the membrane potential created by the charge polarization alters the velocity of the anions and influences the mutual d iffusivity coefficient of the salt in the porous material. An electric pote ntial of thermal origin is also produced in nonisothermal conditions. The i onic contributions to the electrical conductivity are based on a differenti al effective medium approach. These ionic contributions to the electrical c onductivity are used to derive the ionic diffusivities and the membrane and thermoelectric potentials. The influence of the temperature and the presen ce, in the pore space, of a second immiscible and nonwetting phase is also considered in this model. Porosity is shown to affect the membrane potentia l. Several predictions of the model are checked with success by comparing t he model to a set of experimental data previously published. (C) 1999 Acade mic Press.