Electro-kinetic and diffusion measurements of association of dyes in solution and adsorbed at solid surfaces

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Abstract
Electro-kinetic effects have been investigated with both anionic and cationic dye solutions and alumina, carbon, and silica surfaces. With increase in concentration of adsorbed dye, the streaming potential E of alumina, carbon (anthracite), and silica at first steadily approaches zero and then continues with opposite sign. At a point at, or near, zero the plot (E, log dye concn) shows a sharp change of slope, which is identified with the completion of a monolayer containing monomeric dye ions Dm adsorbed by ion exchange on the surface, and the onset of micellar adsorption. It is suggested that the monomeric dye ions then act as nuclei for the further adsorption, by dispersion forces, of dye ions, with formation of ionic dye micelles, which have aggregation numbers similar to those for the respective dyes in solution, and eventually cover the surface in a condensed monolayer. The number Ds of dye ions in this monolayer is determined from normal adsorption isotherms; the ratio Ds/Dm is close to the adsorption aggregation number of the respective dyes determined in a previous investigation. Carbon from acid-extracted bone char gives no streaming potential with, or without, the presence of adsorbed dye, and apparently the non-specific adsorption of both anionic and cationic dyes which takes place on its surface, is due entirely to dispersion forces rather than to ion exchange. The adsorption aggregation numbers, however, are similar for the respective dyes on the two types of surface. Infra-red spectroscopy shows that cationic dye ions are adsorbed on silanol groups on the silica surface. From the variation with dye ionic weight of dye concentration at zero streaming potential, using Fuerstenau's method, the heats of association of the pairs of dye ions in the adsorbed micelles are ca. –0.36 and –0.69 kJ mole–1 for anionic and cationic dyes respectively, consistent with the formation of the micelles by dispersion forces. Molecular sieve effects are shown by the porous powders towards dyes of different micellar sizes.
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