Origin and Type of Impurities

In solution crystallization the solvent is an inevitable impurity. Additives play a role in the improvement of product quality (e.g. shape of the crystals, crystal size distibution) or process performance (e.g. filterability). One source of impurities in the crystallization process is the synthesis of the crystal compound, as synthesis by-products greatly affect crystallization.

Impurities can also be classified based on their character. Apart from organic and inorganic compounds, a special group of impurities is the polyelec-trolytes, long carbon chain molecules with many functional groups. Because of the large number of functional groups the interaction between polyelec-trolytes and crystal surfaces is strong. The large number of functional groups also makes the molecule kinetically very hard to remove, as all the bonded functional groups have to be detached from the surface. One of the detached functional groups remaining bonded to the polyelectrolyte and is thus positioned very near the crystal surface. The attachment of that one functional group onto the surface is faster than the detachment from the surface of the other functional groups of the polyelectrolyte molecule. The strong interaction and the large number of interaction points make polyelectrolytes highly effective additives at very low concentrations. Another special group of impurities is the crystallizing compounds themselves. Long chain hydrocarbons such as fats may cause self-poisoning on certain surfaces if the fat molecules in the melt near the surface have a different conformation from the fat molecules in the surface.

Figure 1 Additive adsorption at various positions on a crystal surface: 1, flat surface; 2, step; 3, kink site.
Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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