Re: [Phys-l] thermo question

[John Denker <jsd@av8n.com>]

On 03/03/2008 01:32 PM, fizix29@aol.com wrote:

> Next question:? Silica works as a good buffer, presumably, because it
> has such a large surface area to volume ratio and can adsorb a lot of
> water.? 

Yes, large surface area.

But there's also the issue of binding energy.  
 -- There's no point in having a large surface area if the 
  H2O doesn't stick to the surface.
 -- On the other edge of the same sword, if you're trying
  to /regulate/ the humidity as opposed to just zeroize
  the humidity, it is unhelpful to have a too-large binding
  energy, because then all the binding sites would be
  fully occupied at any nonzero humidity, and the buffer
  capacity would be zero.  It would be all used up.

This is where you can do think about some actual thermo if 
you want.  You can get a good fit to the saturated vapor
density of H2O from simple thermo:
  -- liquid = favored due to low energy
  -- vapor = favored due to high entropy
... so the occupation probability PER MICROSTATE is lower in
the vapor by a Boltzmann factor involving the latent heat.
This is a huge factor.  The only reason there is any vapor at
all is that the number of states is larger (which is connected
to the higher entropy).

The Boltzmann factor tells you the vapor density is an exponential
function of inverse temperature.  Throw in another factor of T
to get the SVP (as opposed to density);  that's just an algebraic
correction to the exponential behavior, but the fit to the data
is noticeably better if you include it.

> How does the propylene glycol/water mixture work?? The
> retailers claim it will maintain 70% RH humidity regardless of
> temperature and the *exact* ratio of water to PG.? Is this true?

Not true.  Not a chance.

It's analogous to the aforementioned SVP situation.  The PG just
shifts the vapor pressure by changing the Boltzmann factor, in 
the sense that the water is bound to the PG as opposed to being 
bound to other liquid-state water molecules.  The vapor pressure
will be sensitive to first order to PG concentration and exponentially
sensitive to inverse temperature.

This is a solvation effect ... which is *not* to be confused with
osmotic effects.
 -- Solvation effects depend on solvent/solute interaction energy,
  which varies wildly depending on the choice of solute and solvent.
 -- Osmosis is a colligative effect.  That means it is independent
  of the choice of solute.
     http://www.av8n.com/physics/gas-laws.htm#sec-osmosis


Why PG?  Why not glycerin, which has a higher boiling point, lower
vapor pressure, and lower evaporation rate?  It is plenty hygroscopic
enough.  Kellogg's puts it in the raisins in raisin bran; otherwise
during storage, the flakes would get soggy and/or the raisins would
get hard as rocks.  It's easy to buy food-and-drug-grade glycerin.  
It's not restricted.  They just keep it behind the counter, to mollify 
idiot customers who don't know the difference between glycerin and 
nitroglycerin.  Also it's cheap, since it's a byproduct of making soap. 
On the order of a million tons annually.