Form modifier additives
Additives slow the growth of certain faces. They strongly attach to the face in question, and effective additives must have significant bonding energy with the crystal.
In order to attach themselves to the crystal, additives must be “tailor made”, which essentially means being a crystal molecule that has been modified so that the modified part emerges from the surface so as to prevent other molecules from attaching, thereby reducing the growth of the face in question.
Additives act differently according to whether they are cutting or blocking.
Cutting additives alter the sequence of bonds or, more precisely, cut the bond between the face on which they are attached and the next layer, as the additive is unable to bond with molecules of this layer. Let us examine the two modes of action of cutting additives. Some use an oxygen atom that takes the place of a hydrogen atom. The hydrogen atom bonds with the oxygen atom of molecules attaching to it. With this oxygen atom, bonding is no longer possible, being replaced by repulsion between the two oxygen atoms. The H - O bond is cut.
Similarly, if the normal molecule has an oxygen atom susceptible to bond to the hydrogen atom of a molecule fixing on it, we replace the oxygen atom with the hydrogen atom (brought by CH or NH), and the O - H bond is cut.
The emerging part of a blocking additive has a greater volume than that of normal molecules, which prevents the next layer from attaching. This emerged part can be a large chlorine atom, a methyl radical, a benzene ring, a naphthalene radical or even anthracene.
Paraffin typically crystallizes as plates, with the additive attaching to the edges of these plates that are then forced to increase in thickness. We then obtain crystals that are strongly equidimensional in character, that is, their three dimensions are of the same order. The specific surface of these crystals is lower, which means that they can be more readily obtained by filtration.
The insertion energy of a single additive molecule is the total:
- - of the additive’s bonding energy with the crystal surfaces below;
- - of the bonding energy with the molecules of the layer to which the additive belongs.
Here, all energy is counted positively (despite being a potential well in this case).
Once the insertion energy EAs for an additive has been calculated, together with that for a normal molecule EcNrist , we can calculate:
The additive receptor face(s) are those for which:
These calculations are performed by assigning a potential to each atom (see section 1.1.1). AE increases with the molecular mass of the additive.
Both the size of the metastable zone and the crystallization induction time increase with the additive’s insertion energy.