Question

How does rate of diffusion depend on ionic composition?

Answer 1

The presence of salts can change significantly the membrane process performances because of the resulting modification of the neutral solutes transfer through the membrane. The influence of the ionic composition on the transfer of neutral solutes through membranes could be explained by a modification of the membrane properties, due to electrostatic effects, by a modification of the solute radius, likely due to its dehydration induced by the electrolyte, or more probably by a combination of both. By increasing electrolyte concentration results in a higher transfer and that this increase depends on the nature of the added electrolyte. On one hand, the addition of electrolyte can lead to an increase of the membrane charge density and to a higher concentration of counter-i ons in the electrical double layer at the pore surface. It is assumed that these electrost atic interactions result  in a lower hydration shell at the pore surface, whereas it is also proposed that the stronger repulsion forces between the pore walls are likely to lead to an increase of the pore radius, commonly called ‘‘pore swelling’’, assuming a certain elasticity of the membrane. On the other hand, the influence of electrolyte on the overall transfer of neutral solutes can be explained by a change in the solute propertie s, more particularly by a modification of the solute hydration, since it is expected to vary with the ionic composition . In conclusion the influence of the ionic composition on the transfer of neutral solute through membranes could be explained by a modification of the membrane properties, and/or by a solute dehydrati on induced by the electrolyte. The contributions of those two coupled phenomena can change with respect to the electrolyte composition and to the structural properties of the membranes.