Differences on the impact of water on the deep eutectic solvents betaine/urea and choline/urea

The differences on the impact of water on the intermolecular interactions present in the deep eutectic solvents betaine/urea and choline/urea are investigated in this work by measuring the solid–liquid phase diagrams of these mixtures with different amounts of added water. These data are analyzed in terms of ternary systems, rather than the usual pseudo-binary approach, and are used to calculate activity coefficients, which provide precious insight into how water affects the interactions of these systems. It is found that the addition of water greatly enhances the intermolecular interactions of betaine/urea near its eutectic composition, hinting at the formation of a 1:1:1 betaine/urea/water aggregate. On the other hand and contrary to what is commonly believed, water has an asymmetric impact on the interactions present in the choline/urea system. The addition of water to choline-rich mixtures leads to weaker interactions, whereas its addition to urea-rich mixtures leads to stronger interactions. This shows that the decrease in the melting temperature of choline/urea mixtures due to the presence of water does not necessarily mean that intermolecular interactions are strengthened. Finally, a minimum in the activity coefficient of urea in the choline/urea system with 2 wt. % of water was found, which coincides with several anomalies in the properties of this system previously reported in the literature.