The effects of aggregation, counterion binding, and added NaCl on diffusion of aqueous methylene blue

Abstract

A simplified version of the Hamed conductimetric technique is used to measure binary diffusion of the self-associating dye methylene blue chloride in water at 25 °C. As the dye concentration is increased from 0.0006 to 0.02 mol dm−3, aggregation of the dye monomers and binding of the chloride counterions cause the dye's binary diffusion coefficient to drop from 0.83 × 10−9 to 0.63 × 10−9 m2 s−1. Ternary diffusion coefficients for aqueous methylene blue chloride +NaCl solutions are also reported. These results show that small amounts of added NaCl (0.005 or 0.01 mol dm−3) sharply reduce the diffusivity of the dye component, and that each mole of diffusing dye cotransports 0.2–0.7 mol NaCl. Diffusion of trace amounts of the dye in the presence of a large excess of an inert supporting electrolyte is discussed briefly.