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From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.bio,sci.chem
Subject: Re: Textbook errors - OSMOSIS
Summary: Amen!
Keywords: osmosis
Message-ID: <4171@kitty.UUCP>
Date: 14 Nov 90 05:14:44 GMT
References: <1748@ruunsa.fys.ruu.nl>
Organization: Recognition Research Corp., Clarence, NY
Lines: 46

In article <1748@ruunsa.fys.ruu.nl>, hooft@ruunsa.fys.ruu.nl (Rob Hooft) writes:
> I've got enough osmosis-nonsense today.
> Please stop using all this empirical nonsense and start using plain 
> thermodynamics. The effect is so simple to understand once you know the
> fundamental laws of thermodynamics.

	Amen!  I was one of the first to respond to the question two
weeks or so ago, and I gave a (somewhat) simple explanation based upon
a thermodynamic approach.  I briefly repeat:

$$> 	My personal preference is to explain osmosis using a thermodynamic
$$> 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.

	It is also important to realize that as a concentration gradient
across a membrane becomes "large", the relation of osmotic pressure becomes
increasingly difficult to predict (i.e., "non-linear").  At least one
reader already pointed this out.

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