Xref: utzoo sci.chem:2274 sci.bio:3726
Path: utzoo!attcan!uunet!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.chem,sci.bio
Subject: Re: osmosis - how is it caused?
Summary: A thermodynamic explanation...
Keywords: osmosis
Message-ID: <4137@kitty.UUCP>
Date: 27 Oct 90 00:55:07 GMT
References: <1990Oct22.110253.2277@newcastle.ac.uk>
Organization: Recognition Research Corp., Clarence, NY
Lines: 53

In article <1990Oct22.110253.2277@newcastle.ac.uk>, william@lorien.newcastle.ac.uk (William Coyne) writes:
> Osmosis & concentration gradients
> I have read that osmosis is caused
> by a concentration gradient with the number of water molecules building
> up on the side of the membrane which has the highest concentration of the
> salt.  Why is this?

	My personal preference is to explain osmosis using a thermodynamic
approach.  If one considers "chemical potential", as defined by the Gibbs
equilibrium theory, then it is simple to remember that in diffusional
transport (and chemical reactions in general, for that matter) chemical
substances move from higher to lower chemical potential.

	Osmosis represents a case where a solvent is common to both sides
of a semipermiable membrane.  The chemical potential of such a pure solvent
would therefore be equal across such a membrane, and no transport would
occur.  However, the chemical potential of a solvent containing a solute
is *less* than that of the solvent alone due to entropy (the solute being
dispersed in a random fashion within the solvent).

	Therefore, from a purely thermodynamic standpoint, the pure solvent
has a tendency to flow across the membrane into the side containing the
solvent and solute.  Unless, of course, a pressure is exerted on the side
containing the solvent and solute which opposes the osmotic pressure
developed in that side.

> Does it have something to do with the salt forming temporary weak bonds
> with the water molecules, so on the side of the membrane with the higher
> concentration fewer water molecules will be free to cross the membrane.

	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.

	Therefore, osmosis can occur with alcohol/water solutions just
as well as salt/water solutions.

	It is interesting to note that osmosis was discovered by Nollet
in the mid 1700's.  Nollet filled animal bladders (*real* bladders, btw)
with brandy and placed them in containers of water.  Osmosis transport
occurs across the membrane of the bladder resulting in water flowing
into the bladder containing the brandy, in many instances causing it to
burst.

	I won't speculate as to what possessed Nollet to begin filling
animal bladders with brandy and then placing them in conainers of water...

	:-)

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