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 pKa 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.