John Grant Watterson claims in his paper "What drives osmosis?" 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.
In his article "Quantum Worlds", James Watson reviews osmotic theory of Watterson:
Osmosis is viewed as a direct result of the wave
structure of water or, more specifically, of the structural aggregates
of solvent molecules known as wave units or clusters. It is the
structure wave itself, and not the solutes, that governs the molecular
motions underlying osmosis. Since solvent can move through the
semipermeable membrane, it can be considered as a single continuous
medium pervading both phases. This means that the structure wave can
pass unhindered from one phase to the other transferring structural
energy in the process. Addition of solute breaks down the extent of
solvent-solvent cooperative interactions because the molecules in
contact with the solute can no longer rotate as freely as before. As a
consequence, the wave length is shortened in the solution resulting in
clusters smaller in size and energy but increased in number
(concentration). In other words, the solute causes a decrease in the
size of the pressure pixel. The increase in concentration of clusters
in the solution phase is equal to the concentration of solute
molecules.
At the membrane, there is a net energy flow from the energy-rich
clusters of solvent into the smaller clusters of solution. This
increases the tensile strength of the intermolecular bonds, so that
the smaller clusters can pull solvent across the membrane increasing
the pressure on the solute side. At equilibrium, the pressure in the
solution has become high enough to counteract the pull of the smaller
clusters and flow equalizes. At this point, the energy of the smaller
clusters equals that of the pure solvent clusters.
Finally, Stephen Lower agrees with a quantum explanation in his textbook section "Some Applications of Entropy and Free Energy":
Dilution of a liquid creates uncountable numbers of new microstates,
increasing the density of quantum states in the solution compared to
that in the pure liquid. To the extent that these new states are
thermally accessible, they will become populated at the expense of
some of the microstates of the other phase.
