Effect of water content on physicochemical properties and nanostructure of amino acid protic ionic liquid comprising with 2-ethylhexylethylenediaminium cation and butanoylalaninate anion

Abstract

The [HEtHexen][Butala] type amino acid protic ionic liquid (AA-PIL) composed of protic 2-ethylhexylethylenediaminium ([HEtHexen]⁺) cation and butanoylalaninate (Butala) anion exhibits strong hydrophilicity, therefore, the physicochemical properties of [HEtHexen][Butala] were significantly influenced by the water content. Thus, this study primarily investigated the relationship between density, viscosity, and conductivity of [HEtHexen][Butala] as a function of temperature (T = 303–343 K) and varying water content (w₀ = [H₂O] / [AA-PIL] = 0, 2, 4, 5, 10, 15, 20). Results indicate that at the same temperature, increasing water content decreases both density and viscosity (η) values while increasing conductivity (σ). Under constant water content, temperature elevation linearly decreases density, exponentially decreases viscosity, and exponentially increases conductivity. The lnσ and lnη show a linear relationship with 1000/(T-T₀), consistent with the Vogel–Fulcher–Tammann equation. In the w₀ range of 2–20, the aqueous [HEtHexen][Butala] exhibits Walden plot behavior with ΔW values ranging from 0 to 0.5, indicating better ionization. The result showed that the ionic character increased sharply with the addition of a small amount of water. Small angle X-ray scattering (SAXS) techniques were employed to characterize the nanostructure of hydrated [HEtHexen][Butala] systems, which were further utilized for the preparation of Cu(0) metal nanoparticles, revealing that higher water content promotes molecule aggregation in [HEtHexen][Butala]/H₂O mixed systems. At T = 443 K, smaller spherical Cu(0) nanoparticles with a size of approximately 5 nm were successfully synthesized.