Interfacial And Structural Insights Into Cetylpyridinium Chloride-Deep Eutectic Solvent Systems: A Surface Tension, Viscosity And FTIR Study

Abstract

This study investigates the interfacial and structural behavior of cetylpyridinium chloride (CPC) in the presence of three amino acid-based deep eutectic solvents (DESs) synthesized from choline chloride with L-leucine (DES1), L-cysteine (DES2), and L-alanine (DES3). Surface tension analysis revealed that the addition of DESs significantly reduced the critical micelle concentration (CMC) of CPC, accompanied by variations in interfacial parameters such as surface tension at CMC (γCMC), surface pressure at CMC (πCMC), maximum surface excess concentration (Γmax), minimum surface area per molecule (Amin), and adsorption efficiency (pC20). Among the systems, DES1 at 1.0 wt% exhibited the strongest reduction in CMC and enhanced πCMC, attributed to hydrophobic interactions from leucine side chains. DES2 displayed higher Γmax and pC20 values, indicating efficient adsorption and compact interfacial packing through thiol-mediated interactions, whereas DES3 exerted comparatively weaker effects. Viscosity measurements confirmed micellar growth, with DES2 promoting elongated or worm-like aggregates that enhanced flow resistance. FTIR spectroscopy further supported these findings, showing hydrogen bonding, electrostatic interactions, and shifts in vibrational bands that validate structural modifications. Overall, the results demonstrate that amino acid-based DESs can effectively modulate CPC micellization, offering promising insights into surfactant-DES interactions for sustainable formulations and green chemistry applications.