A partition equilibrium study of sulfonephthalein dyes in anionic surfactant systems: determination of CMC in buffered medium

A spectrophotometric method has been reported which can be used to simultaneously determine partition equilibrium constant of anionic sulfonephthalein (acid–base indicator) dyes between micellar pseudophase and aqueous phase, and critical micelle concentration (CMC) of the surfactants in buffered aqueous anionic surfactant systems. The method is based on two assumptions: (1) only one form of the dye goes into the micellar pseudophase, which holds for similarly charged dye–surfactant systems and (2) the activity terms for the conjugate acid and base forms of the dye can be combined in buffered solution. An absorption band, which decreases with addition of surfactant, is used for the analysis. The method has been found suitable to the systems of sulphonephthalein (anionic) dyes, viz. bromophenol blue (BPB), bromothymol blue (BTB) and thymol blue (TB) with anionic surfactants, viz. sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfonate (SDSN). The partition equilibrium constants have been found to be dependent on hydrophobicity of the dye and pH of the solutions. The CMCʹs have been found to increase with increase in pH for a particular buffer system. The method can conveniently be used for study of interactions in such dye–surfactant systems and to determine CMC of the anionic surfactants in buffered solutions.