Partition equilibrium of phenol red in aqueous polymer-surfactant system: Determination of critical aggregation concentration

Acid-base equilibrium of phenol red (PR), a sulphonephthalein dye, was studied spectrophotometrically in aqueous media containing water soluble nonionic polymers viz., polyvinyl alcohol (PVA) and polyethylene glycol (PEG) in the presence of an anionic surfactant, sodium dodecyl sulfate (SDS). A partition equilibrium method was utilized to determine the equilibrium constant of partition of the PR between micellar pseudo phase and aqueous phase and the critical aggregation concentration (CAC) of SDS in buffered aqueous systems containing the neutral polymers. The pH dependent association constants, Kass of PR with SDS-PVA, and SDS-PEG system increase in the order PEG 200 < PEG 400 < PEG 600 < PVA. The value of Kass was found to be greater in Tris buffer system than in phosphate buffer system of same pH. Interaction of the dye with the aqueous SDS-PVA and SDS-PEG systems is found to be endothermic and entropy oriented.