The absorbance measurements of CV (4,4',4''-tris(dimethylamino)triphenylmethyl chloride or crystal violet) in W/O microemulsions with surfactant sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol OT, AOT) at various temperatures have been carried out by the UV-Vis spectrophotometer. An association model has been used to analyze the experimental data to obtain the association constant of CV and AOT at various temperatures. It was estimated that about up to 73% CV was associated by AOT, which resulted in a significant reduction of the effective concentration of CV and retarded the reaction of alkaline fading of CV in the microemulsion. The values of thermodynamics functions of association Δ_rG_m, Δ_rH_m and Δ_rS_m have also been calculated from the association constants at various temperatures.

Key words: microemulsion, Aerosol OT, isooctane, crystal violet, association constant, Gibbs free energy, enthalpy, entropy