Dispersed-crystal organic substances

VIII. Investigations of Dispersed-Crystal Organic Substances. . 261... 

VIII. INVESTIGATIONS OF DISPERSED-CRYSTAL ORGANIC SUBSTANCES... 

Figure 18-82 illustrates the relationship between solids concentration, iuterparticle cohesiveuess, and the type of sedimentation that may exist. Totally discrete particles include many mineral particles (usually greater in diameter than 20 Im), salt crystals, and similar substances that have httle tendency to cohere. Floccnleut particles generally will include those smaller than 20 [Lm (unless present in a dispersed state owing to surface charges), metal hydroxides, many chemical precipitates, and most organic substances other than true colloids. 

The crystals of such inorganic substances as carbon dioxide andjthe hydrogen halides and of the majority of organic substances are composed of molecules bound together by van der Waal s forces. If the molecule has a relatively simple structure and only a small polarity, the heat of sublimation is found to be small and for such molecules, London s has calculated the heats of sublimation assuming that only dispersion forces are responsible for the inter-molecular attraction. The attraction energy is considered to be given by =. — (7/r where r is the distance between the molecules and C = J aH (see equation 12.3). In addition to non-polar or weakly polar molecules,... 

Sustained release from disperse systems such as emulsions and suspensions can be achieved by the adsorption of appropriate mesogenic molecules at the interface. The drug substance, which forms the inner phase or is included in the dispersed phase, cannot pass the liquid ciystals at the interface easily and thus diffuses slowly into the continuous phase and from there into the organism via the site of application. This sustained drug release is especially pronounced in the case of multilamellar liquid crystals at the interface. 

X-ray diffraction of sulfur particles excreted by Thiobacillus sp. showed the presence of orthorhombic sulfur crystals. The solubility of crystalline orthorhombic sulfur in water is known to be only 5 /tg 1 [42]. In the solubility test shown in Fig. 7 it was seen that the biologically produced sulfur particles can be dispersed in water but not in hexadecane, whereas crystalline orthorhombic sulfur is soluble in hexadecane but not in water. The reason for the observed hydrophilicity of the biologically produced sulfur particles has to be attributed to the hydrophilic properties of the surface of the sulfur particles. Because of the relatively high stability of the biologically produced sulfur particles at high salt concentrations, it is concluded that the colloidal stability is not merely based on electrostatic repulsion. It is known that hydrophobic sulfur can be wetted by Thiobacillus thiooxidans bacteria due to formation of organic surface-active substances [43, 44]. 

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