Buffer-Dependent Micellization of Sodium N-Dodecanoyl Taurinate: Combined Experimental and Theoretical Insights

Understanding surfactant-buffer interactions is essential for controlling micellization in applications such as metal-ion extraction, drug delivery, and wastewater treatment. In this study, the micellization behavior of sodium N-dodecanoyl taurinate (C12Tau), an anionic amino acid-based surfactant, was systematically investigated in three representative Good's buffers-2-morpholinoethanesulfonic acid (MES), 2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid (HEPES), and N-cyclohexyl-2-aminoethanesulfonic acid (CHES)-selected because they possess distinct molecular structures and buffering ranges, enabling the independent evaluation of buffer-specific and pH-dependent effects on micellization, using fluorescence spectroscopy, isothermal titration calorimetry (ITC), ζ-potential measurements, fluorescence quenching, dynamic light scattering (DLS), and density functional theory (DFT). Fluorescence measurements showed that the critical micelle concentration (CMC) increased in the order MES < HEPES < CHES under the optimal buffering conditions of each buffer. Constant-pH experiments further demonstrated that buffer identity influences micellization independently of pH through specific buffer-surfactant interactions. Within individual buffer systems, the CMC increased with pH in MES but decreased in HEPES and CHES, revealing that the pH dependence of micellization is strongly buffer-dependent. ITC confirmed that micellization is spontaneous in all systems, with exothermic aggregation in MES and slightly endothermic but entropy-driven micellization in HEPES and CHES. ζ-potential, aggregation number (Nagg), and DLS measurements demonstrated that buffer composition significantly modifies the surface charge, aggregation behavior, and hydrodynamic diameter of C12Tau micelles. DFT calculations predicted stronger hydrogen-bonding and charge-transfer interactions for HEPES than for CHES and MES, providing qualitative molecular insight into the experimental observations. Overall, these findings highlight the decisive roles of buffer chemistry and pH in regulating the self-assembly of C12Tau, providing mechanistic insight into surfactant-buffer interactions relevant to colloidal and interfacial applications.