The pH dependent adsorption of Coumarin 343 at the water/dichloroethane interface

Abstract

The surface sensitive technique of optical second harmonic generation (SHG) and quasi-elastic laser scattering (QELS) have been applied to study the effect of pH on the adsorption of Coumarin 343 (C343) dye molecules at the water/1, 2-dichloroethane (DCE) interface. A pH dependent aggregation is observed for C343 molecules adsorbed at the interface. The surface spectrum shows that at pH 8 and lower, C343 adsorb at the interface in J-aggregated protonated form. At pH 9 and pH 10, C343 adsorb in both protonated and deprotonated forms at the interface, whereas, at pH 11, C343 is in H-aggregated-deprotonated form at the interface. The observed large shift in pK_a value of C343 at the interface is attributed to the intramolecular hydrogen bonding along with the aggregation of dye molecules. From a light polarization analysis of the SHG signal, the average angle of orientation of C343 at pH 5 was determined to be 30°, and at pH 11, the angle was determined to be 42°. The QELS data show a weak adsorption of C343 at the interface for pH 11 and pH 3 and a strong adsorption at the intermediate pH values, reaching maximum for pH 10, and are consistent with SHG data. The calculated ratio of β values for protonated and deprotonated forms of C343, is in agreement with the experimentally measured ratio of β values. Both, experimental and calculated β values for the deprotonated form is lower than the β values for protonated one and is attributed to the electron-donating effect of the –COO⁻ substituent.