What drives osmosis?

Abstract

This presentation claims that a fall in free energy, which is the rigorous thermodynamic criterion for a spontaneous change, cannot be the drive in osmotic processes. Our models and theories require the introduction of a parameter that explicitly represents structure in liquids, which until now has had no place in the thermodynamic description of solutions. This lack is surprising, when one remembers that experimental results from the broad range of fields of colloid, clay and biological sciences have clearly established the marked effect of solutes on the structural properties of water, globally called ‘hydration phenomena’.

The introduction of such a parameter can help explain the direction in which energy flows during osmosis, which has been so puzzling to those of us interested in mechanism since the time of Pfeffer, more than a century ago. Further, elementary work cycles show, that changes in this parameter correspond to changes in the energy associated with solvent structure which can be used to produce useful work. The ability of osmotic systems to do work is familiar to all of us (indeed, a nuisance to many!), and is the basis of cytomechanics, i.e., the physical processes observed in the living cell. The fact that it still has no satisfactory explanation is clearly an urgent problem for us all.