In osmosis, there is flow of solvent particles from a region of lower solute concentration to a region of higher solute concentration, until such time that the concentration on each sides are equal.
But does there exist a membrane in which the equilibrium concentration is not necessarily equal (without applying unbalanced pressure)?

  • $\begingroup$ What research have you done into this? $\endgroup$
    – auden
    Commented Aug 2, 2016 at 14:31
  • $\begingroup$ @heather Not so much, just looked at osmosis in wikipedia. I also think this is physically impossible because you have to apply some pressure to cause the solvent to move from higher to lower concentration.. I was just thinking perhaps there's a way to do this by perhaps changing the geometry of the membrane, or the molecules composing it? $\endgroup$ Commented Aug 2, 2016 at 14:40
  • $\begingroup$ Oh, I see. Now you've made me curious! $\endgroup$
    – auden
    Commented Aug 2, 2016 at 14:41
  • 1
    $\begingroup$ Depends exactly what you mean by 'equilibrium'. Active transport (e.g. protein pumps) can provide this function, but the entire system would be in a continuous state of thermodynamical non-equilibrium. $\endgroup$
    – lemon
    Commented Aug 2, 2016 at 15:16

1 Answer 1


1) Osmosis is more commonly defined as the diffusion of water across a membrane. Unless specifically talking about water its best to just say diffusion. (just a small clarification)

2) The solutions on either side of the selectively permeable membrane will attempt to reach the same concentration (in units of molarity = moles of solute/liter of solution) of solutes by the movement of species across the membrane. The goal is for both sides to reach the same osmotic pressure.

The membrane cannot affect the osmotic pressure of the solutions. It merely decides which species are allowed to move for the system to reach equilibrium.

3) At equillibrium, the concentration of each species in the solutions may be different, as long as both systems have the overall same concentration of solutes.

An example of this is if you have a semi-permeable membrane which allows the movement of ions but not large proteins. If the initial conditions are the same concentration of ions on both sides of the membrane, but an uneven concentration of protein across the membrane, the ions will move to balance out the uneven concentration of proteins. At equilibrium the osmotic pressure of both sides will be equal, but the individual concentration of species on each side will not be equal.

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    $\begingroup$ A wall may provide a completely non equal conditions on both sides (I am joking). Yes, such a membrane may work as a filter. $\endgroup$ Commented Aug 3, 2016 at 16:05

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