The Gibbs-Donnan effect is also known as the Donnan Law and the Donnan Equilibrium. It refers to the observation
that charged molecule
s starting on one side of a semipermeable membrane
sometimes will not evenly distribute
themselves by diffusion
on both sides of the membrane. This effect is probably because there are other charged substances already present which cannot move through the membrane themselves and which are creating an electric field
that influences the movement of the incoming charged molecules.
The Gibbs-Donnan effect is very important in understanding how living cells function and for understanding medical procedures like dialysis. For instance, animal cells swell with fluid when they are diseased and their membranes' sodium ion pump stops working properly; this is directly due to the Gibbs-Donnan effect.
Some of this writeup is based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/