Toluidine blue: aggregation properties and distribution behavior in surfactin micelle solution.

Aggregation properties and distribution behavior of toluidine blue in surfactin solution have been investigated by UV, infrared spectrum and fluorescence measurements. The UV measurement indicates that the location of toluidine blue can facilitate the formation of toluidine blue aggregates in contrast to that in the absence of surfactin. The fluorescence results show that the micropolarity of pyrene initially increases and then decreases with the addition of toluidine blue molecules. The binding constants of toluidine blue with surfactin micelles and the distribution coefficients between the micelles phase and the aqueous phase have been calculated by Benesi-Hildebrand method and pseudo-phase model, respectively. The values of DeltaH degrees < 0 (-22.11 kJ/mol) and DeltaS degrees > 0 (12.58+/-0.07 J/(mol K)) show that electrostatic attractive interaction plays an important role in the location of toluidine blue in surfactin micelles.