Dear Patrick,
this issue of matching relative humidity values to crystallisation
solutions had us scratching our heads for quite some time - we ended up
measuring a whole series of concentrations of PEGs and salts - when
discussing this with a particle physicist of my acquaintance he pointed
out Raoult's Law. He showed in 1887 (coincidentally in Grenoble) that
the relationship between the RH above a solution is dependent on the mol
fraction of water. This leads to a counter-intuitive observation - it
is only the number of molecules in solution that affect the RH and not
the type of molecule/ion - therefore one molecule of glycerol has the
same contribution as a chloride ion or anything else. This means that
there is no effect for charge etc. What does matter is how many species
the salt dissociates into - this means that a given concentration of
sodium malonate (3 species) will have a lower RH than ammonium sulphate
(2 species (NH4+ and (NH4SO4)-) and not 3 as might be expected).
Raoult's law breaks down for polymers over 1000Da but this can be
corrected using the Flory-Huggins model for the entropy of mixing of
polymers - this has the interesting effect that all PEGs over 1000Da
will have almost the same RH for a given concentration and that for the
concentrations typical in crystallisation, the difference in RH between
the well and reservoir is so small the experiment is essentially
"microbatch out of oil".
MgSO4 does seem to be an outlier in your measurements - but not in
Raoult's law - I wonder if it has odd behavior in solution - I will look
into it.
Cheers, Matt.
On 22/11/2012 18:32, Patrick Shaw Stewart wrote:
Matt
My old "Rubber Book" (Handbook of Physics and Chemistry, 1976) has a
table (attached) showing the "lowering of vapor pressure by salts in
aqueous solution", taken from the Smithsonian Tables.
I've never been able to make any sense of it in terms of cations,
anions, valency, or charge density (position in Hofmeister series),
whether concentrations are expressed as M or N solutions.
For example, of the salts mentioned on your website, MgSO4 seems to be
anomalous.
My spreadsheet also has a little converter that my colleague wrote
many years ago to convert vapor diffusion conditions to batch. This
might be of interest to people who have to work in batch, e.g. people
making crystals for X-FEL data collection. You can download it as a
program from http://www.douglas.co.uk/vdtomb/vdtomb.htm - it seems to
work pretty well.
Best wishes
Patrick
On 22 November 2012 12:11, Matthew Bowler <mbow...@embl.fr
<mailto:mbow...@embl.fr>> wrote:
Dear All,
after a few requests, I have now added equations to the online
calculator that uses Raoult's law to calculate the relative
humidity equilibria for precipitant solutions (see
http://go.esrf.eu/RH). The new equations (4 and 5) allow the
calculation of salt concentrations that will be in equilibrium
with a certain PEG or other molecule solution - this will allow
slow and controlled dehydration experiments to be designed in
vapour diffusion plates, by slowly increasing the salt
concentration in the reservoir above the equilibrium and thereby
reducing the amount of water in the crystallisation drop by a
controlled amount. Hope it is useful, cheers, Matt.
--
Matthew Bowler
Synchrotron Science Group
European Molecular Biology Laboratory
BP 181, 6 rue Jules Horowitz
38042 Grenoble Cedex 9
France
===================================================
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Fax: +33 (0) 4.76.88.29.04 <tel:%2B33%20%280%29%204.76.88.29.04>
http://www.embl.fr/
===================================================
--
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Instruments Ltd.
Douglas House, East Garston, Hungerford, Berkshire, RG17 7HD, UK
Directors: Peter Baldock, Patrick Shaw Stewart
http://www.douglas.co.uk
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--
Matthew Bowler
Synchrotron Science Group
European Molecular Biology Laboratory
BP 181, 6 rue Jules Horowitz
38042 Grenoble Cedex 9
France
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Fax: +33 (0) 4.76.88.29.04
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