The Gibbs-Donnan equilibrium refers to the behavior that charged particles exhibit when they are near a semipermeable membrane. Specifically, it explains how the electrostatic and osmotic forces balance out to restore equilibrium. To help with these relationships, the Gibbs-Donnan equilibrium assigns factors to different molecules. These values vary depending on whether they are assigned to a negatively charged anion or positive cation. The values are slightly higher for the anions.
Also referred to as the Gibbs-Donnan effect, this equilibrium is named after Josiah Willard Gibbs, a physicist, and Frederick Donnan, a chemist. This equation refers to a concentration on both sides of a semipermeable membrane. Since these are charged particles, there is a concentration gradient and an electrostatic gradient.
For this equilibrium to take effect, there needs to be an unequal charge distribution of impermeable ions across the membrane. An example of the Gibbs-Donnan equilibrium is when an anionic fluid is introduced to a molecule containing an anion and a cation. Both gradients would react to balance out the new molecules and restore equilibrium. One method for observing this reaction would be to place a mixture into intravascular and interstitial compartments. Within the human body, this equilibrium is responsible for the negative resting membrane potential.