Consider a fictitious membrane permeable to Cl- but not to K+:
Assuming only the negative ions can permeate the membrane, and then the negatively charged chloride ions will leak. The concentration of ions inside the membrane is higher than the outside, c2 > c1, therefore the ions will move outwards. This will increase the negative charge just outside the membrane and decrease it just inside. This is the reverse of the situation shown in Figure 11.2 a.
The electrostatic potential across the membrane will also be opposite. Visualise a positively charged test object. As the object moves from the outside of the membrane in its potential energy will increase; as it moves from a complementary-charged negative region to a repulsive positively-charged region. Thus the potential curve decreases moving from the inside to the outside of the membrane. This again is the reverse of the situation shown in the text, Figure 11.2 b.
Don't forget that even if the system is only permeable to one of the ions, once those ions have passed through, we won't see a dispersion of them throughout their new container. Instead, what happens is that they somewhat cling to the container wall due to the electrostatic interactions of their counter ions. The overall amount that actually pass through in the end is rather small, due to the attraction between counterions, and the electrostatic interactions restricting the ion ability to dissociate.
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