Thermodynamic Evaluation of Micellization of an Azepanium Ionic Liquid-Based Surfactant in Water and Organic Solvent-Water Media.

Abstract

A novel azepanium ionic liquid-based surfactant, 1-(2-hexadecyloxy-2-oxoethyl)-1-methylazepanium bromide (Azp-ILBS; C25H50NO2Br), was successfully obtained and structurally characterized using 1H, 13C NMR, FT-IR, HRMS, and CHNS elemental analysis. The critical micelle concentration (cmc) was evaluated via electrical conductivity measurements. Thermodynamic investigations of micellization were conducted in various water-organic solvent mixtures (ethylene glycol (EG), ethanol (Et-OH), 1,4-dioxane (DO), and N,N-dimethylformamide (DMF)) at temperatures of 303, 313, and 323 K. The influence of solvent composition and temperature on micellization behavior was systematically examined. Results indicated that cmc values increased from 0.328 to 0.614 mM with rising temperature from 303 to 323 K and from 0.328 to 0.761 mM with increasing solvent content of EG from 10% to 30%. All solvents showed similar trends: higher temperature and lower polarity delayed micellization, requiring higher surfactant concentrations. Degree of ionization (α) showed a similar trend, although a U-shaped pattern was observed specifically in the water-ethanol system. The Gibbs free energy of surfactant tail transfer (ΔG°m trans) values revealed the hydrophobic surfactant tail transfer into the micelle core from the surrounding medium as the ΔG°m trans value shifts from positive to negative on increasing temperature, affirming the role of hydrophobic interactions. Overall, the micelle formation phenomenon was observed to be spontaneous and exothermic, with entropy and enthalpy contributions varying depending on temperature and solvent environment. Notably, (ΔH-ΔS) compensation analysis shows that Azp-ILBS micellization becomes more enthalpy-driven as temperature increases. The negative ΔH°*m values reflects the micelle formation is an exothermic process at zero entropy. The calculated compensation temperatures (Tc) at experimental temperatures 303, 313, and 323 K are 295.9, 296.15, and 301.75 K, respectively. These values are thermodynamically in a plausible range, showing a mild transition near 313 K, likely attributed to subtle variations in solute-solvent interactions or reorganization of surfactant structure.