Osmosis: Two Complementary Views

Well, we all learned at one time or another that systems tend towards equilibrium, and since only water can cross the selectively permeable membrane in these simulations, this tendency towards equilibrium "causes" osmosis. Continuing this line of reasoning, water molecules, like solute molecules, seem to diffuse down their own "concentration gradient", producing osmosis. If we consider a "concentration gradient" to be equivalent to a "gradient of mole fractions" or an "activity gradient", this equilibrium statement is accurate and, moreover, correctly indicates the direction of water movement. Why isn't such an explanation completely satisfactory?

For one thing, it's unwieldy: the "concentration of water" is difficult to measure and hard to relate in any meaningful way to osmosis. Also, something significant is missing from the explanation!

The equilibrium approach is unsatisfactory for two more fundamental reasons. First it brings us no closer to understanding what causes osmosis in a molecular sense, that is, how random water movement produces the difference in volume. Why? Because equilibrium statements provide no information about mechanisms. (In more technical jargon, answers couched in "equilibrium" terms deal with the initial and final states of a system and are independent of the path taken between the states. Scientists interested in mechanisms, however, investigate and characterize the various paths involved!) Secondly, and as you perhaps have already guessed, the equilibrium approach tells us the direction osmosis takes when two compartments approach equilibrium, but it doesn't provide any means for predicting the magnitude of osmosis or for measuring such important parameters as the velocity of solvent diffusion.

Fortunately, these issues are interrelated: understanding the underlying mechanism will help us forge a quantitative description of osmosis, and vice-versa.