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

Abstract

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 D 2O and a deuterated polar aprotic solvent (DMSO- d 6). In addition, the self-diffusion 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- d 6 and D 2O, with the level of aggregation depending on the temperature and dye.