Interaction of a Novel Dihydroxy Dibenzoazacrown with Different Surfactants: A Physicochemical and Spectroscopic Investigation

Interaction of a novel dihydroxy dibenzoazacrown (H₂DTC) with various surfactants of different charges, for example, anionic (sodium dodecylsulfate, SDS), cationic (dodecyl trimethylammonium bromide, DTAB), cationic gemini (butanediyl-1,4-bis(dimethylcetylammonium bromide), 16-4-16), ionic liquid (1-hexadecyl-3-methylimidazolium chloride, C₁₆MImCl), and nonionic (polyoxyethylene sorbitan monostearate, Tween-60), has been investigated at a widespread range of surfactant concentrations (including premicellar, micellar, and postmicellar regime) in 15% (v/v) EtOH medium at room temperature. Several experimental techniques, viz., tensiometry, UV-vis spectroscopy, and steady-state fluorimetry, are implemented to explicate these interactions. Incorporation of H₂DTC with surfactants having several inflections in surface tension, steady-state fluorimetry, and absorption profiles at different [surfactant] varying from low to high is found, apart from the conventional critical micelle concentration (cmc) of surfactants itself. This present study assesses equilibrium binding constant (K_b), Gibbs free energies of binding ($\Delta G_b^0$) of H₂DTC with micelles of different surfactants, partition coefficient for H₂DTC (K_x) in the micellar phase to solvent, molar absorptivity of H₂DTC in the absence (ε₀) and presence (ε) of micelles, and Gibbs free energy of partition of H₂DTC ($\Delta G_p^0$) in the micellar to solvent phase. It has been observed that H₂DTC strongly binds with the micelles of Tween-60 than the micelles of other surfactants investigated here and the corresponding K_x values are also found higher in micellar medium of Tween-60. On the other hand, the ε value of H₂DTC is found lower in micellar Tween-60 medium, whereas it shows little rise in other cationic and anionic micellar medium, especially in cationic, compared with the H₂DTC free solvent. Negative values of $\Delta G_b^0$ and $\Delta G_p^0$ strongly suggest that the binding and partitioning of H₂DTC with micelles are feasible in solution medium. DFT calculations have been used to predict the geometries of the assemblies of the surfactants in monomeric form with H₂DTC and the interaction energies.