Tests of dispersion quality

A simple test for wetting efficiency is to make up suspensions using the same concentration of powder but different wetting agents and allow the suspension to settle out. About a gram of powder dispersed in 10 ml of liquid is suitable. Slow settling, a clear interface between the clear liquid and the turbid lower layers and a small depth of sediment indicate the best 

Another test for the degree of flocculation of a paste is to measure the difference between the smear and flow points. The test is made by adding known quantities of dispersing medium to a known weight of powder and working it in with a spatula. The difference is noted between the amount required to smear and to flow the better the dispersion, the smaller the difference [107]. 

It has been found that for pigments in solvents, a high dielectric constant leads to a more dispersed system. In general, polar liquids disperse polar solids and non-polar liquids disperse non-polar solids. For polar solids suspended in non-polar liquids, it is possible to use the difference in polarity to anchor a stabilizing molecule to the powder surface. The effectiveness is characterized by the heat of wetting, which can be determined by calorimetry. 

For sub-micron metallic powders, precoating with gelatin aids dispersion in aqueous systems. Coating of commercial powders in order to stabilize them is widely used e.g. aluminum oxide coated titanium dioxide. In such cases the interfacial properties will be those of the coating rather than those of the bulk powder. 

Many weird and wonderful methods for obtaining stable, well-dispersed systems have been proposed. Some of the more valuable are incorporated into National Standards for particle systems. A more general one is to be found in BS 3406 [107]. 

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