Xref: utzoo sci.chem:2296 sci.bio:3742
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From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.chem,sci.bio
Subject: Re: osmosis - how is it caused?
Summary: "Salting-out" effects on osmotic pressure
Keywords: osmosis
Message-ID: <4148@kitty.UUCP>
Date: 28 Oct 90 16:09:19 GMT
References: <1990Oct22.110253.2277@newcastle.ac.uk> <4137@kitty.UUCP> <1990Oct27.035951.27558@midway.uchicago.edu>
Organization: Recognition Research Corp., Clarence, NY
Lines: 44

In article <1990Oct27.035951.27558@midway.uchicago.edu>, chi9@quads.uchicago.edu (Lucius Chiaraviglio) writes:
> >	No.  It is important to understand that osmosis is a colligative
> >property of solutions in that the determining factors pertain solely to
> >the number of molecules of solute in solvent (and thermodyamic factors),
> >and are *independent* of actual chemical composition.
> 
> 	I know what you're saying -- that hydration of ions is not required to
> achieve osmotic effects -- but wouldn't charge-dipole interactions between
> ions and water molecules enhance the osmotic effects by lowering the
> concentration of free water to an extent beyond what they could accomplish by
> simple displacement?

	I"m a little confused at the wording of your question, especially
the use of the word "enhance", but I believe you might be alluding to what
is usually called the "salting-out" effect, as often seen in biochemical
applications.

	While osmosis per se is colligative in nature and not dependent upon
the presence or concentration of ions, the resultant ionic strength of ions
that are present may *modify* solubility and osmotic pressure.

	Let us consider a situation where a neutral ionic solute, like a
salt, is dissolved in water.  Since each salt anion and cation is surrounded
by water dipoles in a hydration shell, the overall structure of the water
is changed, therefore diminishing its solvent properties for a *second*
solute, such as a protein.  There is, in effect, less water (solvent)
available for the second solute.

	In the case of osmosis in biological systems involving proteins
as the solute, the presence of salt ions as mentioned above will have an
effect upon osmotic pressure.  A quantitative determination of the effect
of such salt ions may be made through use of the Gibbs-Donnan equilibrium.

	It follows from the above that in the case of proteins the pH of
the solution will also be a factor.  The effect of salt ion concentration
upon osmotic pressure will be maximum at the isoelectric point of the
protein (where the net charge of the protein molecule is zero).

	While the effect of salting-out is commonly encountered with
proteins in biological systems, it is by no means limited to proteins.

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
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