I was wondering whether there is a material that can pass ions (such as the ions existing in seawater) but not water. I am aware that water molecules are smaller, so they can pass through some materials but ions don’t. However, is there a material that is the opposite? Or is there a hydrophobic material that can be used as a coating for a porous material, and doing the same job by preventing water from passing but allow ions to pass through it?

Thanks in advance

  • 5
    $\begingroup$ Some potassium channels are water free, DOI: 10.1126/sciadv.aaw6756 $\endgroup$
    – Karsten
    Jun 8, 2022 at 15:03
  • $\begingroup$ @KarstenTheis, I think he is asking about non-living materials as used in water purification. $\endgroup$
    – AChem
    Jun 8, 2022 at 15:13
  • 7
    $\begingroup$ @AChem It's all atoms, whether the context is non-living or living, so you can take one domain and learn from it to understand the other. For water purification, the opposite is used (let water molecules pass while retaining ions) in reverse osmosis. $\endgroup$
    – Karsten
    Jun 8, 2022 at 15:21
  • $\begingroup$ Your H3O+ and OH- ions might form a problem. $\endgroup$
    – MSalters
    Jun 9, 2022 at 14:06

1 Answer 1


There are so many materials that allow the passage of ions but not water. They are called ion-exchange membranes. I believe you are interested in water purification based on your previous posts. The trick is to attract the ions on one side. Student always tend to forget that a solution must remain electrically neutral by all means. If you want a certain ion to leave water, you must provide another ion of the same charge that has left the solution.

An example is given here Image below

enter image description here

  • $\begingroup$ Why do other ions need to pass in the opposite direction? Couldn't electrons pass between the two? For the sake of argument, an electrolysis cell with an external battery and an ion-exchange membrane would appear to send ions in one direction across the membrane? $\endgroup$
    – abligh
    Jun 9, 2022 at 4:44
  • $\begingroup$ @abligh, Good question: There are no free electrons in ordinary aqueous solutions. The charge in solution is carried by ions. If you are interested, ask a separate question with a diagram in your mind. $\endgroup$
    – AChem
    Jun 9, 2022 at 5:06
  • 1
    $\begingroup$ done: chemistry.stackexchange.com/questions/165688/… $\endgroup$
    – abligh
    Jun 9, 2022 at 6:11

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