How to Study Osmosis If the Membrane Water Moves Across Is Nonideal

Abstract

In our past works, we introduced two fundamental osmosis systems (a simple S-m-H 2 O and a composite S 1 -m-S 2 ) to study osmosis 1,2 , where m is a selectively permeable membrane that separates two fluid compartments. A composite S 1 -m-S 2 can be deconstructed into two simple osmosis systems (S 1 -m-H 2 O and H 2 O-m-S 2 ) 1,2 . When applying these two fundamental osmosis systems, a given m can either be permeable to water only (an ideal m) or its permeability to one species of non-charged solute particles (SP, e.g., urea) can be ignored. If m' represents a nonideal membrane whose permeability to urea cannot be ignored, then a new research question emerges: how can osmosis be studied in S-m'-H 2 O and S 1 -m'-S 2 , respectively? This abstract provides the answers by logical reasoning. First, S-m'-H 2 O can be viewed as the overlap of a pure osmosis system with an ideal m (S-m-H 2 O) and a permeant SP (pSP), urea diffusion system: S-m'-H 2 O = S-m-H 2 O + S urea -m'-H 2 O, so that osmosis and urea diffusion can be studied separately. The former is governed by van ’t Hoff’s law and the latter by Fick’s law. Second, similarly, S 1 -m'-S 2 = S 1 -m-S 2 + S 1urea -m'-S 2urea . Third, the effect of urea diffusion on osmosis depends on several conditions: 1) In S-m'-H 2 O, urea diffusion and osmosis are always in opposite directions, so urea diffusion reduces the resulting osmotic pressure (π). 2) In S 1 -m'-S 2 , if S 1 is hypertonic to S 2 and [urea] S1 > [urea] S2 , then the diffusion and osmosis are in opposite directions, so the diffusion reduces the resulting osmotic pressure gradient (Δ π). 3) If S 1 is hypotonic to S 2 and [urea] S1 > [urea] S2 , then osmosis and diffusion are in the same direction, so the diffusion augments the resulting Δ π. In conclusion: 1) Deconstruction of an osmosis system that has a nonideal m' allows separate studies of osmosis and pSP diffusion, respectively. 2) The effect of pSP diffusion on osmosis needs to be differentiated according to the type of osmosis systems and the direction of osmosis and pSP diffusion. 3) While the simple S-m-H 2 O and composite S 1 -m-S 2 are fundamental osmosis systems, S-m'-H 2 O and S 1 -m'-S 2 are complex passive membrane transport systems that transport both water and pSP. 4) If a given nonideal m' is permeable to two types of pSP (e.g., urea and glucose), then the diffusion system needs to be further deconstructed into a urea diffusion system and a glucose diffusion system.