From data found on the web,

the water molecule diameter is 280 pm;

the chlorine ion is 184 pm;

the sodium ion is 102 pm.

 

If the water molecule is bigger, how can an osmotic membrane work to filter seawater ions out in desalination?

 

Miguel

That's a very good question.

I think the answer is that the ions in solution are hydrated and these hydrated ions are too big to go through the membrane pores.