Binding of sulfonephthalein anions to the micelles of an anionic surfactant

The applicability of the concept of 'binding constants' of ions to micelles for the description of acid-base behaviour of the dye bromothymol blue ia sodium dodecylsulfate solutions is examined at ionic strength 0.2 mol dm(-3) (NaCl). The values of 'apparent' ionization constants (K-a(a)) are determined. The values of binding constants (K-b) and partition constants (P-i = a(i)(m)/a(i)(w)) of the dye anions HR- and R2- are obtained from the dependencies of pK(a)(a) vs. surfactant concentration C-surf. It is revealed that these values vary along with the interval of C-surf chosen for calculations. In the region of 'sphere double left right arrow rod' transitions of SDS micelles, the K-b value of the R2- ion increases markedly. The influence of Brij 35 as well as of pentanol additions upon the K-b value is studied. (C) 2000 Elsevier Science B.V. All rights reserved.