Misconception

As I read through your introductions I am seeing a common misconception, that the reasons ions can't get across the cell membrane is due to the charged phosphate group.  This might be a small factor, but the main barrier to ions are the hydrophilic tails.

I think the problem may be due to the way the phospholipid bilayer is drawn, like below.

From this diagram it looks like the hydrophilic portions are large and act as a barrier.  But in reality the phosphate group is pretty small compared to the hydrocarbon chains.  Below is a space-filling model (closer to the real structure).

Here you can see that that portion is fairly small.  

So, to summarize, if you are going to discuss why ions can't cross the cell membrane your primary reason should be that they can't get across the lipid portion.