Attachment energy and layer energy
Crystallization energy is the total of the two terms:
Indeed, let us “peal” a crystal face by removing a layer of molecules of minimum thickness dhkl, so that the layer obtained allows the crystal to be reproduced by simple orthogonal translation of itself. In order to detach the unit of surface of such a layer, we would need to expend the energy equivalent to the attachment energy Efix, which is calculated by considering the molecules situated in a hemisphere centered on the molecule in question, with a radius of 3-4 mm.
On the other hand, each molecule of the layer is bonded to the other molecules of the same layer. The corresponding energy calculation Ecou is done by considering the molecules situated in a radius of 3 or 4 mm.
Within a crystal, each layer is in contact with the rest of the layers in the crystal on both faces, hence the coefficient 2 in equation [1.1].
Equation [1.1] shows that when considering the faces directed in different ways, energies Ecou and Efix vary in opposite ways due to their total remaining constant and equal to Ecris.
Energies Efix are negative and in the order of 100 MJ.kmol-1.