Diffusion coefficients for direct dyes in aqueous and polar aprotic solvents by the NMR pulsed-field gradient technique

The nuclear magnetic resonance pulsed-field gradient (NMR-PFG) technique was applied to determine the diffusion coefficients of selected direct dyes, including Congo Red, in D2O and a deuterated polar aprotic solvent (DMSO-d6). In addition, the selfdiffusion coefficients of the solvents were determined in the presence and absence of dye, to reveal solvent¯dye interactions. Diffusion coefficients ranged from about 1.7×10-10 to 4.4×10-8 depending on the dye, temperature and solvent employed. It is proposed that dye aggregation exists in DMSO-d6 and D2O, with the level of aggregation depending on the temperature and dye.