Acid-base equilibrium of neutral red in aqueous nonionic surfactant-polymer systems

► Aqueous neutral red (NR) has been studied in nonionic polymer-surfactant systems. ► Partition equilibrium (PE) method has been applied in the study. ► Acidity of NR depends on the chain length of the surfactants. ► The acidity of NR also depends on the polymer molecular weight. ► Observed pKa shifts of NR agreed well with the values predicted by the PE method.