Amino acid ionic liquids based on imidazolium-hydroxyl functionalized cation: New insight from molecular dynamics simulations

Abstract

Various thermodynamic and structural properties of amino acid ionic liquids (AAILs), comprising 1-(2-Hydroxyethyl)-3-methyl imidazolium ([C2OHmim]+) cation mixed with Glycinate [Gly], Serinate [Ser], Alaninate [Ala], and Prolinate [Pro] AA anions are explored using molecular dynamic (MD) simulations and quantum theory of atoms in molecules (QTAIM) analysis. In general, the simulated thermodynamic results are in good agreement with the reported experimental data. Structural dependence of vdW- and electrostatic energies of AAILs is [Pro] > [Ala] > [Ser] > [Gly] and [Gly] > [Ala] > [Pro] > [Ser], respectively. The similar trend of electrostatic energies is found for their interaction energies. Molecular level information on organization of ions are attained by computing site-site cation-cation, cation-anion, and anion-anion radial- and spatial distribution functions (RDF and SDF). According to the cation-cation RDFs and SDFs results, the weakest cation-cation interactions belongs to [C2OHmim][Gly] IL and strongest of corresponding interactions is attributed to AAILs composed of [Pro] anions. On the contrary, the reverse trend of cation-cation RDFs and SDFs is observed for cation-anion interactions. These results are also confirmed by reduced density gradients (RDG). Site-site anion-anion RDFs between acidic hydrogen and interactive site of AA anions (carrying high negative charge) become sharper and more intensive in the following order: [Ser] < [Gly] < [Ala] < [Pro]. This trend arises from more association of the smaller anions such as [Gly] and [Ala] with their cations, which leads to weakening cation-cation and anion-anion interactions. Even though, [Ser] has more polar group than [Ala], more intramolecular hydrogen bonds in [Ser] anion are believed to be responsible for aforementioned trends. These results are in excellent consistency with the calculated thermodynamic properties, RDG, and interaction energies. Amazingly, intramolecular hydrogen bond between O atom of –OH group and H atoms of imidazolium-ring can be clearly understood from the average structures of cations.