Chemical composition: hygroscopicity
On the surface, hydrogen atoms are provided by hydroxyl, amine and imine radicals. However, carbonyl groups are hydrogen acceptors.
Consequently, we observe that the water molecules are attracted:
- - in the first instance, by their oxygen atom that bonds to the surface hydrogen atoms;
- - in the second instance, by their hydrogen atoms that bond to the carbonyl groups.
Therefore, soda crystals are highly hygroscopic due to their hydroxyl radicals.
More generally, a crystalline surface can be polarized positively or negatively and thereby attract polarized molecules, one of which could be water for instance.
A series of steps can occur successively on a given face F.
Figure 1.2. Macrostep and vicinal face
The rate of advance for successive simple steps is roughly the same globally due to their height being comparable to the size of a single molecule. They can reach the height of a macrostep, which is considerably greater in height, and consequently, slower in advance rate. There are also additives that slow and even stop macrostep progression [VAN 86]. The state of the surface resulting from this grouping of steps presents striations that are clearly visible, corresponding to these macrosteps. However, if the microsteps are not visible, the surface will still present a slope that is very low in gradient relative to the ideal F face. Such a surface is known as a vicinal face, since its orientation is close to that of face F (from the Latin vicinus: neighbor).