These are the water molecules. Now, when the sodium ion moves into this region, what happens is, because its radius is so much smaller than the radius of this potassium, these distances are so much greater, these interactions will be much weaker. In fact, these interactions that are formed are not as stable as the interactions that we have here. And so what that means is, on this side, to break this, we have to input a certain amount of energy. But when this is formed, we release a certain amount of energy, but the amount of energy that we release is less than the amount of energy that we need to break these bonds between the water and that sodium. And so, ultimately, we see that because of the small size of the sodium ions, they cannot form stabilizing interactions with amino acids of that selectivity filter.