Crystal changes in suspensions

The cationic surfactant, trimethyldodecylammonium chloride (TMDAC) was twice as effective in hindering the growth of the 001 face as that of the 110 and 

The crystallographic faces whose growth rates were depressed most were those upon which surfactant adsorption was the greatest. Cationic additives adsorb on the face composed of carboxylic acid groups (001), anionic additives on the (110) and (200) faces which are hydrophobic. A coulombic interaction of the cationic head groups and the —COO groups on the (001) faces has been suggested. The adsorption of the anionic surfactant, repelled from the anionic (001) faces, takes place amphipathically on the hydrophilic (110) faces and (100) faces (Fig. 9.27b). 

Adipic acid has also been used as a model crystal by Fairbrother and Grant [66] in studies of the effect of alkanols and alkanoic acids on habit modification, during crystallization procedures. Crystallization of drugs in aqueous surfactant solutions can lead to enhanced dissolution rates of poorly soluble drugs such as prednisone, sulphathiazole and chloramphenicol [67, 68]. 

Crystal growth can be considered to be a reverse dissolution process. The diffusion theories of Noyes and Whitney and Nernst consider that matter is deposited continuously on a crystal face at a rate proportional to the difference of concentration between the surface and the bulk solution. So an equation for crystallization can be proposed in the form 

Kg being the overall crystal growth coefficient and n the order of the crystal growth process. Solubilization, by increasing the saturation solubility of the solute, will increase crystal growth in a supersaturated medium such as a concentrated suspension. 

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