Cetyl pyridinium chloride-amino acid-based ionic liquids: Synthesis, characterization, and physicochemical properties by FT-IR, UV–visible, density, conductivity, viscosity, surface tension, and contact angle studies

Abstract

The ionic liquids (ILs) are gaining much attention because of their many special properties, including highly solvating, tunable, non-flammable, and reusable extractants. In the current study, a novel set of room-temperature ILs based on the active compound Cetyl Pyridinium Chloride - Amino Acid ([CetPyl] [AA]) where ionic liquids have been successfully synthesized and characterized by both spectral (FT-IR and UV-visible), and physiochemical (density, surface tension, contact angle, molar free energy, conductivity, viscosity, and pH) properties. The structure of both organic and inorganic compounds is analyzed by FT-IR and UV-visible spectra of pure Al-ILs, Me-ILs, and Val-ILs showed strong absorbance peaks at 215 nm, 270 nm, and 270 nm, respectively. The alanine anion (Al^-) has the highest hydrogen bond-accepting character due to its unhindered carboxylate group, making it more accessible for hydrogen bonding. The conductivity of Al-ILs increases from 17.46 mS/cm at 298.15 K to 50.90 mS/cm at 343.15 K, representing an almost threefold increase. Similarly, Me-ILs and Val-ILs show significant increases in conductivity over the same temperature range. The conductivity order (Al-ILs > Val-ILs > Me-ILs) is consistent with the structural differences in the amino acids. Val-ILs have the highest viscosity (9.976 Ns/m²) among the three. The trend in viscosity is: Al-ILs < Me-ILs < Val-ILs. The pH of Al-ILs, Me-ILs, and Val-ILs decreases with increasing temperature, reflecting differences in their structural and chemical behavior. Al-ILs exhibited the most stable pH. Me-ILs showed significant pH sensitivity, while Val-ILs showed a unique non-monotonic trend of variation, suggesting complex interactions at higher temperatures.