Water Sorption in Amino Acid Ionic Liquids: Kinetic, Mechanism, and Correlations between Hygroscopicity and Solvatochromic Parameters

Amino acid ionic liquids (ILs) are biocompatible, biodegradable, and easy to synthesis, thus resulting in many potentially sustainable applications (e.g., sour gases capture and biomass dissolution). If mixed or contaminated with water, their properties would be altered and degradation may be induced, hence influencing their applications. Therefore, the hygroscopicity of nine amino acid ILs was investigated in this study. Furthermore, an arrow-shooting ball mechanism was proposed to simulate the interaction mechanism of water sorption. Namely, a fragile melon ILs ball would easily be split by the water arrow, i.e., a weaker cation anion interaction induces a stronger ion water interaction, hence producing greater hygroscopicity. Instead, a hard golden ILs ball with a higher ILs ILs interaction would disfavor the ILs water interaction, hence producing lower hygroscopicity. Finally, the correlations between the hygroscopicity of amino acid ILs and the solvatochromic parameters were investigated. The results showed that hygroscopicity had no direct correlation with the solvatochromic parameters, whereas it did have a close relationship with the polarity depending on the region of hydrogen-bonding basicity. Therefore, hygroscopicity could be designed by three procedures: (1) estimating the hydrogen-bonding basicity, (2) determining the region of hydrogen-bonding basicity, and (3) decreasing (increasing) the polarity, which would lead to more hydrophobic ILs in a + (-) region, where "+" and "-" indicate a positive and negative correlation between polarity and hygroscopicity parameters in a specific region of hydrogen-bonding basicity, respectively.