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Re: hbonds/cavities (corrected) (fwd)
Sender: bruce_bush@merck.com (Bruce Bush)
Subject: WSN: Re: hbonds/cavities (corrected)
== (I made an error in the first posting of the message below
== by substituting "vaporization" for "condensation".
== Corrected and modified version follows (lines begin "==". -- Bruce
> From: bruce_bush@merck.com
>
> -------------------------------------------------------------------------------
> 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.)
== Each water molecule participates in roughly 4 h-bonds, so one might
== estimate (1/2)*4 per 30 A^3 or (1/15) (h_bond)/(A^3). But that's
== not a thermodynamically important number for introducing the solute.
> If what you're after is THERMODYNAMIC reality, I suggest you consider
> ENTHALPY (not free energy) of pure water per molecule (or per volume),
== that is, enthalpy of transfer from water gas to water liquid; and, more
> important, the enthalpy of transfer of a SOLUTE molecule from gas into water.
>
== The former (enthalpy of condensation) 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 also often negative (favorable) or
== at worst not very positive; assuming no
> great changes in the packing forces, this means that the hydrogen bond
== network is getting stronger or at worst remaining equally strong as
== result of introducing a solute. In other words, the "number of
== hydrogen bonds" remains almost the same, though the water near
== the solute may have to reorient so as to make these h-bonds.
> (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
>