Phase behavior study of LCST type ionic liquid water mixture system containing amino acids
Liquid liquid extraction is a commonly used technique for separating organic compounds, pharmaceutical components, amino acids, and other substances by utilizing the solubility differences of solutes in different phases of the liquid. However, traditional extraction systems often contain flammable, volatile, toxic, and high energy consuming organic reagents such as ethylene glycol, N-methylpyrrolidone, and N-methylmorpholine. Therefore, building an environmentally friendly liquid-liquid extraction system to replace organic solvents has become a development trend in the field of extraction research.

Ionic liquid (IL) is a low-temperature liquid salt composed of organic cations and organic or inorganic anions. In recent years, ionic liquids have shown broad application prospects in natural product extraction due to their high solubility, high thermal and chemical stability, low vapor pressure, and structural tunability. Lee et al. added 10mg of 1-ethyl-3-methylimidazolium bromide as an extraction aid to 40mL of acetone, which increased the extraction amount of astaxanthin from Eriocheir sinensis by nearly 9 times. Ionic liquids as adjuvants for organic solvents can significantly improve the extraction efficiency of biomolecules, but these studies still have limitations and rely heavily on the use of organic solvents. On the other hand, ionic liquids have high viscosity and their use alone is not conducive to mass transfer, greatly limiting their application in the field of extraction. Research has shown that adding a small amount of water to ionic liquids can effectively reduce their viscosity and enhance their physicochemical properties. More importantly, adding a certain amount of water to the ionic liquid can effectively reduce the amount of ionic liquid used and lower costs.

However, the ionic liquids commonly used in extraction research have strong hydrophilicity and often form homogeneous systems with water, which still has limitations in selectively extracting biomolecules. In 2017, Shi et al. achieved effective separation of phosphatidylserine and phosphatidylcholine using a two-phase system prepared from 1-ethyl-3-methylimidazolium bromide/methanol n-hexane. The ionic liquid water mixed system produces liquid-liquid phase separation under specific conditions, which not only significantly improves the selective extraction of biomolecules, but also helps to maintain the activity of biomolecules. Therefore, constructing a new liquid-liquid equilibrium system with hydrophobic ionic liquids and water as the main components has important guiding significance for the selective extraction of biomolecules. Research has shown that the types of ionic liquids that can phase the ionic liquid water mixture system include 1-butyl-3-methylimidazolium hexafluoroborate, tetrabutylphosphine trifluoroacetate, tetrabutylphosphonium acetate, etc. Most of these ionic liquids contain hydrophobic anions such as tetrafluoroborate ions, hexafluorophosphate ions, bis (trifluoromethanesulfonyl) sulfone ions, and trifluoroacetate ions.

From the food, chemical, and pharmaceutical fields, solvent systems containing ionic liquids are suitable for extracting an increasing number of target substances, involving a wider range of research areas. Among them, temperature sensitive liquid-liquid extraction systems have attracted much attention. Research reports that some ionic liquid water mixed systems exhibit temperature sensitive characteristics, with phase transitions exhibiting two types: high critical melting temperature (UCST) and low critical melting temperature (LCST). Wang et al. found that the binary liquid-liquid extraction system of 1-hexyl-3-methylimidazolium tetrafluoroborate and water can selectively separate lipophilic and hydrophilic substances in chrysanthemums. However, previous reports have overlooked the impact of extracting target products as participants in ionic liquid systems on phase equilibrium.

This article uses amino acids as a biomolecule model to systematically investigate the effects of glycine, alanine, lysine, arginine, and proline on the phase transition temperature of [P4444] CF3COO water and [P4448] Br water mixed systems. The distribution characteristics of amino acids in the two phases were studied as a function of temperature, aiming to provide basic data for the design of liquid-liquid systems containing ionic liquids and new ideas for the application of ionic liquids in the extraction and separation of natural products.

This article investigates the influence of amino acids on the phase behavior of LCST type [P4444] CF3COO water and [P4448] Br water mixed systems. The interaction between amino acids and water is strong, forming a competitive relationship with ionic liquids for water molecules. The “salting out effect” enhances the phase formation ability of LCST type ionic liquid water system, and its promoting trend is: glycine>alanine>lysine>proline>arginine. In addition, amino acids are mainly distributed in the aqueous phase. As the amount of amino acids added increases or the temperature rises, the spatial position of the enriched phase in the ionic liquid and the enriched phase in the water undergoes a transition. The displacement of the upper and lower phase components is reversible and is a spontaneous reaction process driven by enthalpy and entropy. Amino acids are excellent phase forming agents in ionic liquid water systems. In the future, the hydrophilicity differences of amino acids can be utilized to design ideal liquid-liquid extraction systems for selective extraction of biomolecules.