Frictional and Electrical Effects Involved in the Diffusive Transport through a Nanoporous Alumina Membrane

Abstract

Diffusive transport of neutral and charged solutes across a nanoporous alumina membrane (NPAM) with a pore size of 24 nm and interpore distance of 65 nm obtained by the two-step anodization method in sulphuric acid solution (Sf/NPAM sample) is studied. Diffusion experiments were performed with tritiated water (neutral solute) to demonstrate frictional effects, while charged solute measurements (NaCl solutions at different concentration gradients and 22Na radiotracer) allow us to estimate also the contribution of electrical interactions in the movement of ionic species in the charged membrane nanopores as well as electrolyte/membrane interface effects. Values for water, NaCl, and Na+ diffusion coefficients in the membrane were determined from these measurements as well as the electropositive character of the Sf/NPAM sample and its fixed charge concentration by analyzing the diffusion–concentration curve. According to these results, ion-pore wall frictional and electrical effects are both involved in the reduction of diffusion coefficient in the membrane, but electrical interactions with the membrane surface also affect the ion transport. The increase of electrolyte concentration to values higher than 0.01 M seems to be necessary in order to minimize this latter effect.