The different structure and properties of protic ionic liquid in alcohol from acid and base: butylammonium butyrate mixing with butanol, butyric acid, and butylamine respectively

Solvents are commonly added into protic ionic liquids (PILs) to reduce viscosity in practical applications. Understanding the relationship between the structure and properties of PILs mixed with solvents is also essential for tailoring specific applications, however, such research is limited. In this study, we measured and compared the density, viscosity, and conductivity of three mixed systems: n-butylammonium butyrate ionic liquid (PIL) mixing with N-butyric acid (PrCOOH), PIL-N-butylamine (BuNH₂), and PIL-N-butanol (BuOH). Small- and wide-angle X-ray scattering (S/WAXS), molecular dynamics (MD) simulation, and electron paramagnetic resonance (EPR) techniques were used to explore their inherent structural differences. The results indicate that the properties of the PIL-BuOH and PIL-PrCOOH systems exhibit more overall similarity in trends compared to the PIL-BuNH₂ system. However, when the molar fraction of alcohol or acid exceeds 0.8, structural differences between the two systems lead to the differences in properties. The hydrogen bond network between the BuOH molecules outside the ion cluster leads to higher viscosity and conductivity than the PIL-PrCOOH system. However, the strong hydrogen bond between PrCOOH and anions will replace the position of cations and form spherical clusters. This research highlights how distinct structures influence diverse properties, providing deeper insights into the structure-property relationship.