CCL: dielectric constant of a binary mixture

[John McKelvey <jmmckel**gmail.com>]

 Sent to CCL by: John McKelvey [jmmckel(!)gmail.com]
 I would comment that I believe that Klamt's "D-COSMO-RS" handles mixed
 solvents correctly, properly taking into account "hot-spot"
 interactions" between solute and solvent in solvent mixtures.
 John McKelvey
 On Tue, Aug 21, 2012 at 7:38 AM, Pierre Archirel
 pierre.archirel!=!u-psud.fr <owner-chemistry _ ccl.net> wrote:
 >
 > Sent to CCL by: "Pierre  Archirel" [pierre.archirel.:.u-psud.fr]
 > This is an answer to Jean-Pierre Djukic.
 > Dear colleague,
 > Your system is a difficult one for PCM methods, because it can be expected
 that in the vicinity of your solute the composition of the solvent is not that
 of the bulk. Consequently the dielectric constant to be used is simply unknown.
 > This statement is actually true for any pure solvent: in the vicinity of a
 solute the (pure) solvent is more rigid than in the bulk, and its dielectric
 constant is smaller. This is called 'electrostriction' in textbooks. In most
 PCM methods this is globally taken into account via the famous alpha=1.2 factor:
 you give the bulk value of epsilon in your data and the program multiplies all
 the atomic radii by alpha. The error due to this simplification is corrected
 (along with several other errors !) by the optimisation of the atomic radii.
 > I once performed PCM calculation of anionic solutes in water/alcohol
 mixtures. Obviously I got the better results with the epsilon of PURE WATER.
 This is due to the fact that - due to its larger dipole moment - water is more
 present in the vicinity of the solutes than in the bulk solvent.
 >
 > In summary, any formula giving the epsilon of a mixture may be poorly
 suited to the PCM calculation of a solvation DG in this mixture. If you expect
 that some component of the mixture concentrates in the vicinity of the solute,
 then take the epsilon of this pure component.
 >
 > Pierre Archirel, LCP
 > Orsay France>
 >
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 John McKelvey
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