Figure 1: The measured sodium sulfate concentration as a function
of the temperature.
As can be seen up on cooling the sample starts to supersaturate
first with repect to mirabilite and later with respect to heptahydrate.
As coon as the concentration reaches the spontaneous crystallization
line, heptahydrate is formed and the concentration drops fast. In one
experiment after reaching 0 oC the
temperature is increased again and the concentration is
following the equilibrium heptahydrate line. Whereas in the other
experiment the sample was further cooled and upon reaching -4 oC a
transformation from heptahydrate into mirabilte is observed and upon
heating up the equilibrium line of mirabilite is followed.
These experiments were also reproduced in an XRD experiment. These experiments were conducted on the beamline 16.4 at the UK Synchrotron Radiation Source, Daresbury. The resulting XRD diffraction patterns are given in figure 2.
Figure 2. X-ray diffraction patterns as obtained during coolingg.
As can be seen there again a very fast formation of heptahydrate is observed, whereas also the transformation into mirabilite can clearly be seen. Hence these independent measurements clearly confirm the formation of heptahydrate in a porous material.
Andrea Hamilton, Christopher Hall and Leo Pel, Sodium sulfate
heptahydrate: direct observatio of the crystallization in a porous
material, J. Phys. D: Appl. Phys.
41 212002 (2008).