The many roles of mellitic acid during barium sulfate crystallization

What is it about?

It is often found that organic molecules can modify the way crystals grow, the question always remains how does it work? Crystal modifiers might change the nucleation rate, the growth rate, the agglomeration behaviour or even the solution activity of one of the species crystallising. In this paper we looked at a small molecule mellitic acid, which had quite a profound effect on barium sulfate crystallisation. We looked at how the mellitate ion interacted with barium, what this did to the number of particles nucleating and how the final structure evolves. We found that the mellitate ion is intimately involved during nucleation and forms an amorphous solid that crystallises over time. Also, 2-dimensional nucleation appears to be promoted in the presence of mellitate with lots of water incorporated into the structure. Finally, the final shape does not appear to be due to growth but due to self assembly of the nanoparticles formed. So we concluded that a non-classical crystallisation mechanism (amorphous to crystalline transition) occurs and this process does not inhibit primary nucleation. What is happening is that primary nucleation is the same but the particles are not growing to a size detectable by our initial method. These nanoparticles then form mesocrystals that minimise their surface energy (forming crystals that mimic the shape of a regular crystal). The surface charge of the solids after adsorption of the mellitic acid can help in the assembly process as it limits coagulation if the concentration is right. In fact, it is shown that the surface charge is much more negative when mellitic acid is present, particularly at short times. This is important as this allows the self assembly to be non-random. So mellitic acid impacts on many aspects of the crystallisation process

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Why is it important?

Crystallisation is a simple and important process we use to purify materials and separate important products from waste. But sometimes, crystallisation can cause problems too, such as blocking pipes or forming kidney stones. We use crystallisation modifiers to control crystal processes all the time but in order to use them well we need to understand how they work and why certain modifiers behave the way they do. This work shows that there is always more going on than we first think and that to understand the mechanism we need to determine all the ways in which an additive will impact. An important result from this work is that this additive doesn't actually lower the primary nucleation rate but it does have a profound effect on size. Also, the surface charge will be important for the oriented attachment that occurs during the self assembly process and that this surface charge can change over time as well as with concentration of the additive.

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