Dear Colleagues,
If chemical work does not depend on the concentrations, a transmembrane
concentration difference should be "forceless", i.e. we cannot expect any
work from it. However this is seemingly in contradiction with a number of
cases: the difference "pushes" an osmotic membrane, or it "pushes" ions against
a transmembrane electrical gradient, or it synthesizes ATP. Let me present
a simple model allowing to see some of these issues in a different light.
Please consider the following model:
_ +
_ +
_membrane +
_permeable+
pH 1 _to H+ + pH 7
_only +
_ +
_ +
At equilibrium, a capacitor-like double layer is established at the
membrane. In order that the H+ electrochemical potential is uniform
throughout the system, there must be a potential difference even between
points symmetrical with respect to but distant from the membrane. This is
certainly not the case - away from any finite charged body, capacitors
included, the electrical potential approaches zero (otherwise we will have
to doubt infinity as a reference point in electrostatics). However, for
capacitors, the potential decreases in magnitude with distance very slowly,
and this has created the experimental illusion that it is constant outside
the double layer. I claim that, for any Donnan system placed in a container,
say, 2 m long and 0.5 cm wide, there will be a concentration difference
of some ion between the two ends of the container, but zero potential
difference, i.e. the electrochemical potentials will be different. This
means that the concentration difference is no force in general. What happens
at a piston,at an osmotic membrane, in galvanic systems, is just a result
of functioning of a special device converting the thermal energy of a
LARGE NUMBER of particles into work, always against a MACROforce. No such
process can take place in a elementary chemical act - there the concentration
difference is "forceless" - the work done is entirely a function of
intramolecular energy - at the macrolevel this energy is expressed as the
enthalpy of the reaction.
I do not know whether you are interested in electrochemical systems - if
not, please just ignore this message. What is much more interesting is how
the concentration difference alone synthesizes ATP - there is something
incredible there. Would you like to discuss this?
Best regards,
Pentcho
