Spectrophotometric Study of the Effect of Micellar Medium on the Dissociation Equilibrium of Sulfonephthalein pH Indicator Dye

The experimental determination of the mono-dianionic dissociation equilibrium constant (pKa2m) of sulfonephthalein pH-indicator dye bromocresol green (BCG) in cetyltrimethylammonium bromide (CTAB) micellar media revealed a deviation from its value in pure water (pKa2w). This discrepancy indicates that the presence of CTAB micellar media alters the dissociation behaviour of BCG compared to its behaviour in pure water. Such alterations suggest that the micellar environment influences the protonation and deprotonation processes of the dye molecule, likely through interactions between the dye and the micellar environment. To further understand this phenomenon, theoretical models depicting the dissociation equilibrium of BCG in aqueous micellar solutions were employed. These models utilized non-linear regression analysis methods integrated into microsoft excel solver. The calculations involved the use of spectrophotometrically determined binding constants (kb) and partition coefficients (Kx) of the dye between the micellar pseudo phase and water. The obtained pKa2m values for BCG in different pH media were found to be varied. This variability was attributed to the flexible extent of solubilization of the dianionic form of BCG in the CTAB micellar medium, which changes with variations in aqueous pH. The observed relative intensity of absorption bands of mono and dianionic forms of the dye in the micellar medium correlated with these differences in pKa2m magnitudes.