How does an ion in aqueous soln induce the same charge on a membrane?

[Urmi Roy]

So if we have an ion, say Cl- in solution, and we have a membrane immersed into the solution.

Apparently there is a method in which any ion, of any charge is inhibited from crossing the membrane because it induces the same charge as itself on the membrane.

This is called dielectric exclusion.

However I'm not sure how the ion induces the same charge as itself onto the membrane.

The Cl- will attract polarized water molecules toward itself. So the negative side of the water dipole will be facing outward. This negative side would induce a positive charge on the membrane. So its still the opposite polarity as that of Cl-.

The paper in which I was reading about this says its supposed to be a very obvious application of electrostatics...I guess I'm just missing it.

 

[jbsteele]

The concept of dielectric exclusion is based on the fact that when a charged particle, such as Cl-, is placed in an electric field, it will cause a displacement of the electrons in the medium (in this case, the membrane) so that the electric field is reduced. This is because the negatively charged Cl- ion will attract the positive side of the water dipole, thus inducing a positive charge on the membrane. The positive charge induced on the membrane will then repel the Cl- ion, preventing it from crossing the membrane.