Physicochemical Properties and Structural Effects of 1-Ethyl-4-Alkyl-1,2,4-Triazolium Acetylalaninate Ionic Liquids

Abstract Two 1-ethyl-4-alkyl-1,2,4-triazolium acetylalaninate ionic liquids, [Taz(2,5)][Acala] and [Taz(2,6)][Acala], were synthesized through sequential N-alkylation, anion exchange, and acid–base neutralization steps. The standard addition method was applied to correct for trace moisture interference, and anhydrous density, surface tension, and refractive index were measured across the temperature range of 288.15–323.15 K. Volumetric properties, surface thermodynamic parameters, molar refraction, and polarity scales were derived from experimental data using classical thermodynamic relationships. Comparison with acetylglycinate homologues and non-acetylated analogues indicates that elongation of the cationic alkyl chain generally decreases density, surface tension, refractive index, and polarity while increasing molecular volume and isobaric thermal expansion coefficient under the tested conditions. The additional methyl group on the alaninate anion correlates with reduced interionic cohesion and overall polarity whereas acetylation of the anion corresponds to a strengthened hydrogen-bonding network relative to unmodified alaninate ionic liquids. This study addresses the lack of systematic cationic chain length investigations for triazolium acetyl-amino acid ionic liquids, offering new dual cationic–anionic structure–property insights into acetylalaninate homologues beyond existing short-chain reports.