P - Technische Universiteit Eindhoven

3. Crystallization under wetting/non-wetting conditions 38
It can be deduced from figure 3.5 that for the present drying experiment diffusion
is dominant, indicating P e ≪ 1 and a slowly increasing concentration. Indeed still no
crystallization is observed as the concentration reaches the heptahydrate supersolubility
line (point B) and the concentration gradually increases up to the heptahydrate supersolubility
line (t ≈ 30000 s, point C). From that moment on the sodium content drops and
the concentration decreases until the heptahydrate solubility, indicating the formation
of sodium sulfate heptahydrate crystals (C–E in Fig. 3.3). Figure 3.4-C shows that the
growth of these heptahydrate crystals starts from the top of the moon-shape like droplet,
after which the growth proceeds to the bottom, see Fig. 3.4-D.
After approximately 40000 s, the water and the sodium content of the solution have
decreased to almost zero, indicating there is no solution left in the holder. That is, all
water has evaporated or has been incorporated in crystals. From that moment on no
information can be obtained anymore from the NMR experiments. However, the timelapse
microscopy shows dehydration of the heptahydrate crystals to anhydrous sodium
sulfate (thenardite), due the low RH. Would the initial crystals have been mirabilite, this
transformation would have occurred instantly [62, 69], with a thernadite growth rate of
about 2 cm/s [59]. As indicated in figure 3.4-E, the thenardite crystals start to spread
out. After some time the heptahydrate crystal is completely covered by thenardite and
the two phases cannot be distinguished any more from the images, as is illustrated in Fig.
3.4-F.
At the end of the experiment the crystalline structure was taken out and broken
in two parts. Visual inspection showed that inside there was still a (wet) heptahydrate
crystal present covered by thenardite at the outside. The cracked surface immediately
started to dehydrate again, indicating that the formed thenardite layer either dramatically
slows down or completely stops water escaping from the heptahydrate. Obviously, the
final crystalline system represents a mix of crystalline phases, i.e., two forms of sodium
sulfate (heptahydrate and thenardite) are coexisting at the same time, where a cover layer
of thenardite crystals isolates the heptahydrate from the environment, prevention further
dehydration.
Wetting:
A wetting condition was created by using glass-quartz as a sample holder. In this
case the experiment was performed at of 6 ◦ C. In figure 3.6 the measured water content,
sodium content, and the concentration of sodium sulfate solution in the droplet are shown
as a function of time. The corresponding images of the droplet at various times are shown
in figure 3.7. In this case it is observed that the droplet spreads out completely over the
insert sample holder.
It can be seen from Fig. 3.6 that when the drying starts water evaporates, i.e.,
the water content decreases and the salt concentration increases. Because in this case
the contact line is pinned near the edge of the sample holder, no receding drying front
is observed. After about 2 hours, the sodium signal NMR starts to decrease slightly,
indicating the formation of crystals. Fig. 3.7-B reveals that these crystals are formed
near the edge of the droplet. The corresponding ADD is given in figure 3.8. Despite the
noise in the data, this diagram shows that near the start of the drying P e > 1. Hence
advection is dominant and a large concentration gradient is present in the droplet, as was