X-Message-Number: 33239
Date: Sat, 15 Jan 2011 12:11:00 -0800
Subject: Re: Glass vs. Crystal transitions
From: Brian Wowk <>

> Message #33230
> Date: Fri, 14 Jan 2011 16:51:49 -0500
> From: "Perry E. Metzger" <>
>
>> From: 
> [...]
>> vitrify. A potential problem arises, especially when ice growth
>> inhibiting  molecules are also present, in that such tissues will
>> tend to supercool far below their true freezing point,
>
> One side note here. (I'm sure Mike is aware of this, but others may
> not be.) A uniform idealized substance (and sometimes a uniform
> idealized mixture) can have a single definable freezing point, but
> non-uniform substances do not. Biological tissues and individual cells
> are nearly the definition of a non-uniform substance -- if they were
> uniform, they would not function.

     In the above, Mike's usage of "freezing point" actually referred
to the thermodynamic melting point: the highest temperature at which
ice can stably exist in the solution, and the highest temperature at
which freezing can theoretically begin.  In common speech, freezing
point point and melting point are often used interchangeably, even
though, as you point out, the temperature a solution begins freezing
at can be below its melting point.

>
> I know little about the physical chemistry of glycerol undergoing this
> transition. Are the crystals formed pure water?

      Yes.
>
> The "sentinel" ice balls you describe may fail to grow faster
> precisely because their formation alters nearby concentration of
> glycerol, a self limiting process especially since the formation of
> pure water ice requires energy since it is entropically disfavored. It
> would be interesting to learn if this is true. However, with time, as
> the liquids diffuse, they should grow until the overall concentration
> is sufficient to impede further change. The phenomenon, as you
> describe it, is self limiting even in a slowly cooled solution, which
> is slightly puzzling.

      That's a good guess, but it's not really what's going on.  You
are correct that freezing of water increases the surrounding
cryoprotectant concentration.  However this does not cause ice to grow
as a solid sphere that eventually stops growing.  Freeze-concentration
of cryoprotectants actually causes ice to grow as dendrites, like
branches of a tree.  Freeze-concentration impedes lateral growth of
the dendrites, while the tip continues growing, and occasionally
branches due to instabilities.  So the big sentinel ice balls
describes are not solid crystals, but opaque white fluff balls
consisting of millions of microscopic ice dendrites.  The reason that
the ice balls (their microscopic dendrite tips, really) stop growing
with continued cooling is because of increasing solution viscosity.

>
> That would seem on its surface to be correct, but what limits them to
> a few millimeters?

     The reason that ice balls (specifically, their microscopic
dendrite tips) stop growing with continued cooling is because of
increasing solution viscosity causing ice growth to proceed slower and
slower, and eventually stop.  The effect of increasingly glycerol
concentration on growth rate is indirect by forcing ice growth to be
dendritic.  As a result of dendritic growth, the
increased-concentration glycerol becomes located between dendrite
branches within the ice ball.  The exterior of the ice ball where
dendrite tips continue to grow is virgin solution.  At that exterior,
it is viscosity that eventually stops ice growth.

>
> In particular, can you be sure these are not formed of some sort of
> organized glycerol-h2o crystals or small particles of glassy solid and
> are instead pure water?

       Glycerol and water are not known to form a hydrate.  Crystals
would be water ice.

> I would tend to agree that pure water seems
> most plausible, but mere plausibility isn't enough, and this actually
> may be important. Also, can you be sure the crystals grow no larger
> because they have no time to do so, or could it be because the overall
> concentration of glycerol has gone up enough to impede growth, or for
> some other reason? It seems to me that this would require some
> experimentation.

      This is fairly well understood in Cryobiology.  You can find
references for further reading among the references listed in the
article I'll point to at the end of this message, especially the
papers by Mehl.

>> You
>> can bet whatever "blasted areas" or ice holes were seen  in your
>> 7.5 Molar 1995 canine brains were areas hit by ice that started
>> growing  in a very supercooled state." What I take away from this
>> is that ice will still form and grow extracellularly under
>> conditions where the colligative CPA concentration is very  high
>> and the solution is very viscous. Where nucleation and ice growth
>> occur close to the melting point of the solution (Tm), the ice
>> formed will be 'large mass' ice, and likely mechanically
>> disruptive. Where ice forms well below Tm, it will likely be in
>> microscopic domains that still begin forming (nucleate)  outside of
>> cells and consequently do little damage.

      To make solutions cloudy, the millions of tiny ice crystals
observed to form in supercooled 7.5 glycerol solutions would have to
be bigger than a wavelength of light (at least one micron wide) which
means possibly big enough to damage cells.  I merely observe that if
such crystals do form in some areas after perfusion with vitrification
solutions, they would also have been forming in patients perfused with
high molarity glycerol in the 1990s.

>
> Now, going beyond all that we've discussed up to here, this whole
> topic brings to mind a very basic question: has anyone in the cryonics
> world or cryobiology world done a deep exploration of the physical
> chemistry literature on the formation of glasses at low temperatures?

      I was recently invited to write a review article on this subject.

http://www.21cm.com/pdfs/2010-Thermodynamics.pdf

>
> I'm unsure as to whether we would learn anything valuable by exploring
> that topic in depth, but there is a good chance we would. Knowing what
> things are like in most of science, I suspect that the pchem guys and
> even conventional cryobiology types have never really talked, though
> of course I could be entirely wrong on this.

       There has been collaboration.  A leading expert on the physical
chemistry of glasses, Charles Austen Angell, was coauthor on the paper
that invented the modern approach to cryopreservation by
vitrification.  Another coauthor, Angell's student Doug MacFarlane,
went on to become a leading researcher in the physical chemistry of
vitrification in cryobiology.

---BW

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