The colloidal behaviour of dyes

Abstract

The colloidal behaviour of four acid dyes has been studied by light-scattering. It was shown that none of these dyes was aggregated in the absence of electrolyte and that Orange II does not form stable micelles, even in high concentrations, but the process of flocculation in the absence of dust nuclei can be followed and is similar to addition polymerization. Benzopurpurin forms stable micelles, and the aggregation process is similar to a diffusion-controlled condensation polymerization. In low concentrations of electrolyte, micelles with molecular weights between 10000 and 50000 are formed between 20 and 40° C. In higher concentrations of salt large asymmetric micelles with molecular weights about 10 6 are formed. At 60° C no aggregates were detected in similar electrolyte concentrations. In general, the behaviour of Coomassie Milling Scarlet resembled that of Benzopurpurin except that it was more salt sensitive and did not form giant micelles. Polar Yellow formed a very stable micelle the molecular weight of which, 1·1 x 10 5 , was unchanged over the electrolyte range 0·05 to 0·2 M - NaCl at temperatures between 20 and 60° C, although the micelle dissociates above this temperature. The addition of urea and phenol was found to be effective in preventing the aggregation of Benzopurpurin and Coomassie Milling Scarlet, but not of Polar Yellow. The factors involved in the formation and stability of the colloidal micelles of dyes are discussed.