Crystalline-pigment-surface structure

Pigment-related aspects, which involve the chemical constitution, crystalline modification, particle size distribution, particle shape, surface structure, preparation, and processing of the pigment powder, especially in terms of drying and milling. 

Considering the crystal imperfections that are typically found in all crystals, the crystal quality of organic pigments is a major concern. The external surface of any crystal exhibits a number of defects, which expose portions of the crystal surface to the surrounding molecules. Impurities and voids permeate the entire interior structure of the crystal. Stress, brought about by factors such as applied shear, may change the cell constants (distances between atoms, crystalline angles). It is also possible for the three dimensional order to be incomplete or limited to one or two dimensions only (dislocations, inclusions). 

At higher cooling rates the crystallization conditions for formation of the f form are less favorable. On the surface of the phthalocyanine pigment only a nudei are formed. The growth of nuclei leads to formation of the high crystalline structure built only from a crystals. 

The semiconductor most frequently used is undoubtedly titanium dioxide, produced in large amounts as a low cost pigment. The photocatalytic activity of anatase, rutile and brookite polymorphic modifications of Ti02 is affected by several factors, such as the crystalline structure, the surface area, the particle size distribution and the density of surface hydroxyl groups. Although the positions of valence and conduction bands of both anatase and rutile are positive enough to allow the oxidation of many organic molecules, anatase... 

Powder X-ray diffraction is an excellent analytical tool for identification and physical characterization of crystalline samples. It can be used for chemical identification (by matching unknowns with X-ray patterns of known reference samples), for crystal structure information, and for crystallite size determination. Such information is extremely valuable for pigment characterization. Many pigments exhibit polymorphism and each morph can have dramatically different physical properties (shade, opacity, heat stability, surface area, particle size and shape, melting point). These properties define to a large extent the performance characteristics of the pigment and therefore its applications. 

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