From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.engr.chem,sci.chem
Subject: Re: Analog of Capillary condensation
Date: 18 Feb 1996 06:12:04 GMT

In article <31269CD4.72E4@gold.tc.umn.edu>, "Patrick K. Notz"
<notz0001@gold.tc.umn.edu> says:

>JTW8150 wrote:
>> 
>> I am curious to know if anyone has ever heard of "capillary
>> solidification".  That is the increase in the liquid boiling point when a
>> liquid is confined in a small pore.  I think this would be the analog of
>> capillary condensation ie the increase the boiling point of a vapor that
>> is confined in a small pore such that the vapor condenses to the liquid
>> state.
>
>I am not particularily familiar with "capillary solidification" but it
>could be the result of the pressure differential (that is, capillary
>pressure) across the meniscus (assuming one exists).  Depending on the
>liquid-solid pair, the pressure drop could be positive or negative
>giving rise to a boiling point elevation or declination... this idea
>doesn't seem to mesh with you capillary condensation idea tho.

I am not sure I understand what is being asked, but let me make a
stab at what I think may be some pertinent information.

Imagine water in a capillary which it does not wet, i.e., in which
it has a convex meniscus. The surface tension of the curved water-
air interface will create a surface pressure which increases in
inverse proportion to the diameter of the capillary. 

At modest diameters in the micron range the surface pressure is
of the order of a few psi or thereabouts and results in a very slight
increase in the vapor pressure of the water and a modest increase
in the boiling point. As the capillary size is reduced to, near
atomic dimensions, say of the order of 10exp-7 cm, the surface
pressure may rise to a few thousand psi. Although the vapor 
pressure is significantly higher than the normal vapor pressure
under these conditions, the net effect is to very greatly increase
the initial boiling point. However, if two segments of liquid were 
separated by either an air or vacuum space, I would expect them to 
be propelled apart against external atmospheric pressure well below 
the normal boiling point. One could say that the liquid then had TWO 
boiling points.

Suppose, however that the water does wet the capillary. Then,
we can imagine that the water column is under tension of as
much as several thousand psi. It cannot, however fly apart
because its tensile strength is still greater than the net
surface tension effects. Will the initial boiling point be
reduced? In small, smooth capillaries using very pure water,
I would guess that the initial boiling point, even under tension
would be increased rather than decreased in most cases because of 
the absence of nucleation sites and the difficulty of forming the 
initial bubble. Once a bubble was in place between two segments,
though, the pressure between the two should be reduced slightly
because of column tension, and, from the standpoint of the two 
opposing menisci, the boiling point could be said to be increased
over that of the normal liquid
 
It may be that the phenomena I have proposed above were at least
in PART responsible for the erroneous discovery of "polywater." 
This is a personal theory and welcomes criticism.

If there were phases present in the capillary near their freezing
points, the capillary pressure or tension could well be sufficient
to cause solidification. Water, for instance should freeze a little 
above zero degrees C under tension whereas most materials would
freeze at slightly higher temperatures under pressure.

Jerry (Ico)