CCL: translational entropy and solvation

 Sent to CCL by: moura.|a|.ufscar.br
 Dear all,
 I would just like to point out that it is experimentally found out that
 molecules
 in solution have a concentration dependent partial volume, meaning that in a
 real solution, as opposed to an ideal one, there should be a different volume
 available for each molecule, a volume that depends on the intermolecular
 interactions but does not depend on the constant pressure condition. but I do
 agree with Andreas Klamt when he says that we shall probably get to a point
 in this discussion when it will become clear that we cannot compute
 quantities
 like translational entropies and/or free energies.
 best regards,
 Andre
 &*&*&*&*&*&*&*&*&*&*&*&*&*&*&*&*&*&
 Prof. Dr. André Farias de Moura
 Departamento de Química
 Universidade Federal de São Carlos
 São Carlos - SP - Brasil
 tel. 16-3351-8090
 &*&*&*&*&*&*&*&*&*&*&*&*&*&*&*&*&*&
 >
 > Sent to CCL by: Andreas Klamt [klamt\a/cosmologic.de]
 > Just a short reply:
 > - I personly am not that happy that the cell method always is applied to
 aqueous systems. Here we have strong contributions to enthalpy and
 entropy for the formation of hydrogen bonds, ... Would your method also
 apply for a cylohexane? If yes, what are the results?
 > - I am confused by your statement that the solute reduces the entropy of
 the solvent by the excluded volume: As far as I can see solvation is
 usually considered at constant pressure, not at constant volume. It is
 assumed that the solvent can get the missing volume elsewhere.
 > - And I am not happy that you agree with my regarding the 3/2 vs. 5/2 RT
 for the translational entropy of a molecule in the gasphase: Meanwhile
 Frank Jensen told me that his erratum was wrong and that is should be
 5/2, and he sent me a plausible derivation of that. The Atkins book also
 says 5/2. When you look to the internet you find 3/2 and 5/2 about
 equally often, and you find nice derivations for both. I am completely
 confused now and have to clarify this for myself over Christmas. Is
 there a difference in the ensembles considered? I do not find that in
 the premisses of the literature derivations.
 > - I admit that I did not take into account that in a classical ensemble
 the reduction of the kinetic energy would correspond to a temperature
 decrease. I myself am not sure about the degree of quantum effects here.
 Anyway, in a quantum system we cannot do the integrals for position and
 momentum separately and the discussion becomes useless.
 >
 > I am afraid that at the end of the discussion we will have to admit that
 there is no way to define or to measure the translational entropy of a
 solute in solution. I only can say that empirically we find the
 > described significant free energy change of ~3 kcal/mol in the
 > previously described A + A --> AA reaction, where all surface
 > proportional, electrostatic, and hydrogen bonding interactions of AA are
 just twice those of A.
 >
 > Best regards
 >
 > Andreas
 >
 >
 >> Sent to CCL by: "Richard  Henchman"
 [henchman,manchester.ac.uk] It is
 possible to regard a solute as having the ideal-gas entropy in
 solution, but it is not the only possibility because there is no unique
 way to allocate entropy to each molecule in the solution, as Raphael
 Ribeiro (9 Dec) and Mike Gilson (11 Dec) have already pointed out.
 >> This can be made clear by considering the example of water dissolved in
 water (use D2O if you prefer to have labels and ignoring the fact that
 it would form HOD). If you assign this solute water the ideal-gas
 entropy (~129 J/K/mol at 298 K for water at the density of liquid
 water), then the entropy of the surrounding water molecules is reduced
 due to excluded volume by the solute, as is the common practice. This
 ensures that the total entropy of the solution equals that of bulk
 water. Another possibility is to give the solute water molecule the
 same entropy of all the other water molecules i.e. less than the gas
 phase (~70 J/K/mol at 298 K). How one could calculate such an entropy I
 have shown in some recent publications on water:
 >> http://link.aip.org/link/?JCP/126/064504 and a quantised
 version
 http://pubs.acs.org/cgi-bin/abstract.cgi/jpcbfk/2008/112/i32/abs/jp0737303.html
 These same ideas would apply to any solute, not just a solute water
 molecule. The solute would have lower entropy than the ideas-gas value
 due to confinement by the solvent. The solute takes all possible
 positions and orientations but its freedom is still constrained by
 where the other solvent molecules are. If this seems
 >> counter-intuitive, it is only because the dimensionality of 3N-space is
 so large. One could even consider a case intermediate between the
 confined solute and the ideal-gas solute. In any case, whichever
 solution model one uses, the solute and solvent entropy must add up to
 give the total entropy of the solution. I think a number of
 >> contributors to this discussion are right to question whether current
 solvation models do this.
 >> A few other comments: Andreas Klamt is correct to emphasise that the
 ideal-gas entropy should be 3k/2 and not 5k/2 because the entropy
 relates to a single molecule. However, I am not convinced by the
 statement that "the solvent definitely reduces the motional (kinetic)
 phase space". That amounts to saying that the solvent is at a lower
 temperature. Ignoring quantum effects, which I believe to be small
 (based on my second publication given above), the momentum partition
 functions should be very similar.
 >> Richard Henchman
 >> __
 >> Dr Richard H Henchman
 >> The University of Manchester
 >> http://personalpages.manchester.ac.uk/staff/henchman/
 >
 >
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