Donnan equilibrium (which can
also be referred to as the Gibbs-
Donnan equilibrium) describes
the equilibrium that exists
between two solutions that are
separated by a membrane. The
membrane is constructed such
that it allows the passage of
certain charged components
(ions) of the solutions. The
membrane, however, does not
allow the passage of all the ions
present in the solutions and is
thus a selectively permeable
membrane.
Donnan equilibrium is named
after Frederick George Donnan ,
who proved its existence in
biological cells. J. Willard Gibbs
had predicted the effect some 30
years before.
The impermeability of the
membrane is typically related to
the size of the particular ion. An
ion can be too large to pass
through the pores of the
membrane to the other side. The
concentration of those ions that
can pass freely though the
membrane is the same on both
sides of the membrane. As well,
the total number of charged
molecules on either side of the
membrane is equal.
A consequence of the selective
permeability of the membrane
barrier is the development of an
electrical potential between the
two sides of the membrane. The
two solutions vary in osmotic
pressure, with one solution
having more of a certain type
(species) or types of ion that
does the other solution.
As a result, the passage of some
ions across the membrane will
be promoted. In bacteria , for
example, the passage of
potassium across the outer
membrane of Gram-negative
bacteria occurs as a result of an
established Donnan equilibrium
between the external
environment and the periplasm
of the bacterium. The potassium
enters in an attempt to balance
the large amount of negative ion
inside the cell. Since potassium is
freely permeable, it will tend to
diffuse out again. The inward
movement of sodium corrects
the imbalance. In the absence of
a Donnan equilibrium, the bulky
sodium molecule would not
normally tend to move across the
membrane and an electrical
potential would be created.