Foreseeing ionic liquid properties in methyl phosphate / propylamine system: some ab-initio considerations

Opportunely chosen alkyl phosphates and alkylamines can be mixed together to give structured fluids with a lot of emerging properties, where phosphate-to-amine proton transfer may yield ionic liquids. Using Density Functional Theory at the 6–311 + G(2d,p) level, QTAIM and RDG-NCI theories we investigated methyl phos phate (MP) association with n-propylamine (nPA) and isopropylamine (iPA). These species were selected for their minimal alkyl content, serving as models for high polar-volume fraction systems where this mechanism is pronounced. Interestingly, despite their protic nature, proton transfer is hindered, as polar interactions outweigh electrostatic stabilization from ion formation. This highlights the critical role of alkyl chain size and shape, often overlooked in such systems. Lower level (6–31 + G*) calculations were also made to allow comparison with literature data, and confirmed the coexistence of neutral and ionic associates imparting “partial” ionic liquid character. The small energy difference (~k B T) among these species suggests that external stimuli could induce transitions from one species to another, enabling stimuli-responsive smart materials. Experiments for validation have been indicated.