Why freeze-thaw cycles generate a gradient

Rosie Redfield rosie.redfield at zoology.oxford.ac.uk
Fri Sep 8 09:36:56 EST 2000

The presence of a solute such as sucrose interferes with formation of ice
crystals and so lowers the freezing point of the solution.  But solutions
usually become less homogeneous as they freeze, and that's why melting
causes a gradient to form.

When a concentrated sucrose solution freezes, the tiny ice crystals that
form first don't contain any sucrose, so the not-yet-frozen part of the
solution is enriched in sucrose.  Eventually all the sucrose is trapped in
between ice crystals and the solution is solid.

When it slowly thaws, the first parts to melt are the parts that froze
last, because they have the highest sucrose concentration and thus the
lowest melting point.  As they thaw, the liquid that forms falls to the
bottom of the tube because its high sucrose concentration makes it more
dense than the low-sucrose ice that hasn't thawed yet.  The last parts to
melt are the crystals that formed first, which contain no sucrose at all.
This pure water stays at the top because it's lightest.

So you can get a sucrose gradient in a single freeze-thaw step.  Several
cycles are probably needed to give a very steep gradient.

The formation of gradients is also why stock solutions that are stored
frozen must be vortexed before being used.

Rosie Redfield


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