Abstract
The use of water as a component of deep eutectic systems (DES) has raised some questions regarding its influence on the nature of the mixture. Does it form a DES or an aqueous solution and what is the role of water? In this work, the nature of citric acid:l-arginine:water mixtures was explored through phase equilibria studies and spectroscopic analysis. In a first step, PC-SAFT was validated as a predictive tool to model the water influence on the solid liquid equilibria (SLE) of the DES reline using the individual-component approach. Hence, activity coefficients in the ternary systems citric acid:l-arginine:water and respective binary combinations were studied and compared using ePC-SAFT. It was observed that the water-free mixtures citric acid:l-arginine showed positive deviation from Raoult's law, while upon addition of water strong negative deviation from Raoult's law was found, yielding melting depressions around 100 K. Besides these strong interactions, pH was found to become acidic (pH = 3.5) upon water addition, which yields the formation of charged species ([H2Cit]- and [l-arg]+). Thus, the increased interactions between the molecules upon water addition might be caused by several mechanisms such as hydrogen bonding or ionic forces, both being induced by water. For further investigation, the liquid mixtures citric acid:l-arginine:water were studied by FTIR and NMR spectroscopy. FTIR spectra disproved a possible solubility enhancement caused by salt formation between citric acid and l-arginine, while NMR spectra supported the formation of a hydrogen bonding network different from the binary systems citric acid:water and l-arginine:water. Either being a DES or other type of non-ideal solution, the liquefaction of the studied systems is certainly caused by a water-mediator effect based on the formation of charged species and cross interactions between the mixture constituents.
Highlights
Water is recognized as a universal solvent due to its ability in establishing hydrogen-bonding interactions with the solute
Water has a bifunctional role as it causes acidic environment and it acts as hydrogen-bonding mediator for the L-arginine and citric acid
As the phase behaviour depends on the ratio of the mixture’s constituents, the thermodynamic model ePC-SAFT was used as a qualitative predictive model to predict the influence of water on the solid liquid equilibria (SLE)
Summary
Water is recognized as a universal solvent due to its ability in establishing hydrogen-bonding interactions with the solute. ‘‘no other molecule has the hydrogen-bonding potential of water’’.7. Due to this strong capacity in forming hydrogen bonds, either as hydrogen-bond acceptor or donor, the influence of water on the development of new solvent classes, as the DES, has been explored. Ackhar and co-workers presented a review summarizing some of the studies involving the influence of water in DES, up to 2018. Most of the works reported similar conclusions. The addition of water to DES up to 50 wt% maintained their original intermolecular network, even after incorporation of water hydrogen-bonds.[8] above certain amounts, water can solvate the isolated compounds, disrupting
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