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Re: WSN: h-bonds/cavities (fwd)
From: bruce_bush@merck.com
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To -- David Rhodes, re density of h-bonds in water
The two questions you ask are completely different, as far as I understand.
A non-polar solute in water "disrupts" (or leads to the breaking of)
very few hydrogen bonds -- nothing at all like the number of hydrogen bonds
per solute volume in undisturbed water.
The "number of hydrogen bonds" is somewhat model-dependent in any case
(obtained from an atomic configuration by e.g. classifying geometries of
near-neighbor O-H....O pairs, with cutoffs on distance and angle.)
If what you're after is THERMODYNAMIC reality, I suggest you consider
ENTHALPY (not free energy) of pure water per molecule (or per volume)
and, more important, the enthalpy of transfer of a solute molecule from gas
into water.
The former (enthalpy of vaporization) is strongly negative; it includes
packing forces and hydrogen bonds. You could subtract the enthalpy of
some reference nonpolar liquid to get a notion of the hydrogen-bonding
contribution. It's not really obtainable by "counting h-bonds" unless
we neglect the large three-body (non-additive, cooperative) effects of
2 or more h-bonds to a single molecule.
The latter (enthalpy of transfer) is often negative (favorable); assuming no
great changes in the packing forces, this means that the hydrogen bond
network is getting stronger as a result of introducing a solute.
(Free energy will be unfavorable because entropy of transfer, delta_S,
is negative, leading to positive -T*delta_S.)
-- Bruce
bruce_bush@merck.com (908)594-6758
>
> What is the density of hydrogen bonds in water?
> That is, to a _rough_ approximation, how many hydrogen bonds will be disrupted
> per A^3 of non-polar solute molecule?
> I'm looking through my library, but if someone on the water net knows without
> looking in his/her library, I'd appreciate a number.
> Thanks 10^6. }:)
> | David G. Rhodes | Internet: RHODES@VAX.PHR.UTEXAS.EDU |
> | The University of Texas at Austin | Fax : (512)471-7474 |