So You Think You Know About Osmosis

A persistent problem as science continues to grow is the difficulty with which knowledge can percolate from one field to another. This problem, which I discuss in my book The Half-Life of Facts, can be found in many situations. Well, it can even be found in the study of osmosis.

After reading my book, Eric Kramer, a physics professor at Bard College at Simon's Rock, pointed me to an article he co-authored about osmosis. For many of us, osmosis revolves primarily around half-remembered concepts related to "semi-permeable membranes." But it's a well-developed concept. While there is a sophisticated theory of the nature of osmosis—"the flow of solvent across a semipermeable membrane from a region of lower to higher solute concentration"—derived in the world of physics and biophysics, osmosis is sometimes understood in chemistry and biology in a more simplistic, and not always correct, fashion.

According to Kramer and his co-author David Myers, there are "a range of surprising misconceptions about osmosis continue to appear in papers, web sites, and textbooks. Erroneous ideas about osmosis are especially common in educational materials aimed at students of chemistry and biology."

Kramer and Myers lay out the misconceptions (not all of which are necessarily included in each text that discusses osmosis):

(1) The phenomenology of osmosis is the same for gases, liquids, and supercritical fluids. The misconception is that osmosis is limited to liquids.

(2) Osmosis does not depend on an attractive force between solute and solvent. The misconception is that osmosis requires an attractive force.

(3) Osmosis can drive solvent from a lower to a higher solvent concentration compartment. The misconception is that osmosis always happens down a concentration gradient.

(4) The osmotic pressure cannot be interpreted as the partial pressure of the solute. The misconception is that it can.

(5) The semipermeable membrane exerts the force that drives solvent flow. The misconception is that no force is required to explain the flow.

In addition to deriving the thermodynamics related to osmosis, the authors also discuss a number of textbooks from the chemistry and biology literature that perpetuate of these misconceptions. And I think I myself was subject to at least one of these errors. I can recall being taught (or at least misunderstanding my teachers) that osmosis was a type of diffusion, related the misconception #3, as opposed to osmosis being more related to thermodynamics and an increase in entropy.

And these misconceptions seem to be adhered to quite widely:

These misconceptions are surprisingly robust. Nearly all have been discussed by other authors during the long history of osmotic research, and yet they continue to find adherents in each generation of professionals. The main reasons for the continued popularity of various misconceptions are their simplicity, their pedagogic utility, and the appeal of an analogy with more easily explained phenomena (in particular, solvation and simple diffusion). Part of the challenge facing those of us interested in improving the situation lies with disciplinary insularity. While authors in physics and biophysics have generally settled on the correct understanding of osmosis, these ideas have not penetrated into the fields of chemistry and biology. We suggest that improved communication between the disciplines on this topic would be beneficial. [emphasis added]

This last sentence is exactly one of the problems we increasingly have to contend with as science becomes larger and more ramified. Whether it's related to teaching osmosis or avoiding the recapitulation of research in different fields, this is a big problem. While we encourage interdisciplinary research and collaboration, all too often concepts still take too long to spread from one field to another. We must actively promote the diffusion (pardon the word choice) of ideas.

Top image:Program Executive Office, Assembled Chemical Weapons Alternatives/Flickr/CC