The interaction between cationic dye neutral red and anionic surfactant sodium dodecyl sulfate in aqueous
solution was investigated using conductance measurements and spectrophotometry. Changes in the measured
specific conductances of dye-surfactant solution were observed at eleven temperatures which is considered as
the result of the formation of dye-surfactant ion pair. The evaluation of thermodynamic parameters was carried
out using a theoretical model. The variation in the concentration of surfactant was done to monitor the spectral
behaviour of the dye-surfactant system from pre-micellar to post-micellar region. The dissolution of dye-surfactant
aggregates was only seen beyond this concentration. The addition of oppositely charged surfactant was made to
investigate the association of the added surfactant with the surfactant in the mixture or to recover dye from the
mixture of dye-surfactant. The strength of the association was further confirmed by the addition of a strong
electrolyte to the dye-surfactant mixture.