Large Decrease in Melting Point of Benzoquinones via High-n Eutectic Mixing Predicted by a Regular Solution Model

Decreasing the melting point (Tm) of a mixture is of interest in cryopreservatives, molten salts, and battery electrolytes. One general strategy to decrease Tm, exemplified by deep eutectic solvents, is to mix components with favorable (negative) enthalpic interactions. We demonstrate a complementary strategy to decrease Tm by mixing many components with neutral or slightly positive enthalpic interactions, using the number of components (n) to increase the entropy of mixing and decrease Tm. In theory, under certain conditions this approach could achieve an arbitrarily low Tm. Furthermore, if the components are small redox-active molecules, such as the benzoquinones studied here, this approach could lead to high energy density flow batteries. Finding the eutectic composition of a high-n mixture, however, is challenging due to the large compositional space. We demonstrate with differential scanning calorimetry measurements that 1,4-benzoquinone derivatives exhibit eutectic mixing that decreases their Tm, despite slightly positive enthalpies of mixing (0-5 kJ/mol). We show how the enthalpies of mixing can be used in a regular solution model (assuming immiscible solids and only binary interactions) to predict the eutectic composition and temperature. By investigating all 21 binary mixtures of a set of seven 1,4-benzoquinone derivatives with alkyl substituents (Tm’s between 44-120 °C), we demonstrate that the eutectic melting point of a mixture of all seven achieves a large decrease in Tm to -6 °C, and that the regular solution model shows improvement over an ideal solution model in predicting the eutectic properties.