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Re: WSN: activity of deionized water (fwd)

Sender: jw@newt.phys.unsw.edu.au (Joe Wolfe)
Subject: Re:  WSN: activity of deionized water

To Soaring Bear at Univ Arizona,

regarding your 
"nice practical question for a theoretical group.
you might want to consider the area of 'salting out'
where raising ion level causes proteins to precipitate out
of solution."

- I hope that what I am about to write is not to
naive, but your question sounds very much like a
standard effect in colloid science.

The usual situation is this: identical colloidal particles
have similar charge of several or more units which
provides a repulsive interaction. van der Waals forces
provide the attraction, though others may apply for
proteins. At low salt concentrations, the Debye length
is large and so the separation between particles is large.
Adding ions to the solution reduces the Debye length
and allows the particles to approach more closely to
a range at which thermal fluctuations may bring them
within the range of the attractive forces. The short
range attractive forces are stronger at close range and
so a "primary (energy) minimum" is attained, the 
particles stick together and precipitate. Divalent and
trivalent ions (of the opposite sign to that of the particles, 
usually therefore divalent or trivalent cations) have
a greater effect at the same concentration because the
Debye length goes inversely as the valence but only
inversely as the root of the concentration.

This was analysed in the forties by Verwey and Overbeeck
and by Lifschitz. If it seems relevant to your problem, 
then consult any standard text on colloid science and look 
up DLVO theory.

If I have missed some subtlety in your question, please
accept my apologies for giving such an obvious answer!

Best wishes
Joe Wolfe, Physics, University of New South Wales
Sydney 2052 Australia

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