Chiang Mai Journal of Science

     The present study investigates the spectrophotometric and thermodynamic evaluation of a hydrophobic drug-surfactant complex below and above the CMC using Benesi–Hildebrand, Scott, and Kawamura models. Significant changes in the drug's absorption spectra are observed when interacting with polysorbate 80 (PS 80) and octylphenol ethoxylate (OPE). PS 80 exhibited the highest partition coefficients for SDX (K_x = 2.1 × 10⁷) compared to OPE (K_x = 3.42 × 10⁶) in the micellar system, indicating strong interactions and micelle stabilization. The maximum partition coefficient value (K_x = 2.83 × 10⁷ with 36.8µM PS 80) was observed upon adding 2850µM OPE. The binding constant derived from the Benesi-Hildebrand equation for the PS 80 micellar system (K_b = 1.690 × 10⁷) exceeds that of OPE (K_b = 8.1 × 10⁴). Binding constants calculated using the Scott equation are higher than those from Benesi-Hildebrand—specifically, (K_b = 2.50 × 10⁷) for PS 80 and (K_b = 0.020 × 10⁷ with OPE), indicating multi-site binding. The more negative ΔG values suggest that hydrophobic forces predominantly stabilize the drug-micelle complexes and also presents spontaneity of the system. At higher surfactant concentrations, a decrease in drug adsorption occurs due to shifts in equilibrium and disruption of pre-formed SDX-PS 80/OPE assemblies. The partition coefficient (K_x), derived from the Kawamura model, shows a high affinity for drug incorporation into the micellar core. Notably, there is more than a two-fold increase in solubilizing power within the mixed micellar system. Overall, the findings indicate that non-ionic mixed micellar systems can serve as efficient and environmentally friendly carriers for pyrimidine-based sulfonamides.