From owner-chemistry $#at#$ ccl.net Wed Dec 17 13:03:00 2008 From: "moura]^[ufscar.br" To: CCL Subject: CCL: translational entropy and solvation Message-Id: <-38326-081217114513-28265-0vQ6yv7Ss0c1s1uTl02vTQ^^server.ccl.net> X-Original-From: moura%ufscar.br Content-Transfer-Encoding: 8bit Content-Type: text/plain;charset=utf-8 Date: Wed, 17 Dec 2008 14:08:30 -0200 (BRST) MIME-Version: 1.0 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/ > > > -- > -------------------------------------------------------------------------- Dr. habil. Andreas Klamt > COSMOlogic GmbH&CoKG > Burscheider Str. 515 > 51381 Leverkusen, Germany > > Tel.: +49-2171-73168-1 Fax: +49-2171-73168-9 > e-mail: klamt . cosmologic.de > web: www.cosmologic.de > -------------------------------------------------------------------------- COSMOlogic > Your Competent Partner for > Computational Chemistry and Fluid Thermodynamics > -------------------------------------------------------------------------- Please note our COSMO-RS Symposium in 2009 > (For details see: http://www.cosmologic.de/Symposium/symposium.html)> > >