Alkyl chain length effect on experimental density of three iodide anion-based ionic liquid at high-pressure and application of four group contribution estimation methods for ionic liquids density

Abstract

Ionic liquids (ILs) constitute a captivating class of materials that has garnered significant attention from research and industrial applications, being recognized as promising substitutes for organic solvents in industrial processes. Consequently, comprehensive experimental physicochemical characterization, explicitly focusing on density data complemented by computational tools, is imperative. In pursuit of this objective, the present study explores the density behavior, both experimentally and computationally, of three iodide-based ionic liquids under high-pressure and high-temperature (HPHT) conditions. Here, it was studied the alkyl chain impact on density for 1-propyl-3-methylimidazolium iodide ([C₃C₁Im][I]), 1-butyl-3-methylimidazolium iodide ([C₄C₁Im][I]), and 1-hexyl-3-methylimidazolium iodide ([C₆C₁Im][I]) ILs. The vibrating tube method was utilized to determine the density over a range of pressure (P = 0.20 to 100.00 MPa) and temperature (T = 298.15 to 398.15 K). The density of the ionic liquids (ILs) was found to be influenced by the length of the cation alkyl chain, exhibiting the following sequence of increasing density: [C₆C₁Im][I] < [C₄C₁Im][I] < [C₃C₁Im][I]. Tammann-Tait equation was used to correlate experimental density data, resulting in an average absolute relative deviation ($\%AARD$) less than 0.008 % for [C₆C₁Im][I] and [C₄C₁Im][I], and less than 0.025 % for [C₃C₁Im][I]. The ILs density values were used to determine the isothermal compressibility (${\kappa }_T$), the isobaric expansivity (${\alpha }_p$), the thermal pressure coefficient (${\gamma }_v$)_, and the internal pressure ($P_i$). These results revealed the structural impact of the alkyl chain on packing efficiency and its relationship with its physicochemical properties. Furthermore, ionic liquid density was estimated by using four group contribution estimation methods. Among these methods, the one proposed by Paduszynski and Domanska provided the best density estimative with a maximum $\%AARD$ of 1.09 %.