The word isotonic is used to refer to the solutions (two solutions) which have the same osmotic pressure in both sides of the semipermiable membrane. The condition allows the free movement of water across the membrane. However, this does not change the concentration of the solution across the membrane.
Well if you're talking about the distance from one charged particle to another then the change from the particle stays the same unless there is polarization, but the electric field/charge stays the same
