Acid alizarin violet interactions with surfactants: ionization and thermodynamic parameters in buffered cationic, anionic and nonionic surfactant solutions

Proton ionization constants for Acid alizarin violet N (AVN) were obtained in micellar solutions by spectrophotometry. The effect of addition of three surfactants, CTAB, TX-100 and SDS, was investigated in buffered solutions in the pH range 2.0-13.0. Enthalpies and entropies for the second proton ionization were evaluated by the thermochromic method. At 25 degreesC, in the absence of surfactant, pK(2)(') and pK(3)' for AVN were 7.00 and 12.80, respectively. At concentrations above CMC, CTAB caused a decrease in these values to 6.35 and 12.56, whereas TX-100 led to an increase to 8.09 and 13.04. With SDS, changes were not pronounced. Results were explained on the basis of hydrophobic and electrostatic interactions between AVN and the micelle, taking into account possibilities of short or deep penetration mechanisms of AVN into micelle. In addition to their effect on pK, these micelles significantly altered enthalpy and entropy of ionization values. Finally, association constants for the acidic and basic forms of AVN with each surfactant were obtained and the results interpreted in terms of dye-micelle interactions. (C) 2004 Elsevier Ltd. All rights reserved.