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

Decreasing the melting point (T (m)) ofa mixture is of interest in cryopreservatives, molten salts, and batteryelectrolytes. One general strategy to decrease T (m), exemplified by deep eutectic solvents, is to mix componentswith favorable (negative) enthalpic interactions. We demonstrate acomplementary strategy to decrease T (m) bymixing many components with neutral or slightly positive enthalpicinteractions, using the number of components (n)to increase the entropy of mixing and decrease T (m). In theory, under certain conditions this approach couldachieve an arbitrarily low T (m). Furthermore,if the components are small redox-active molecules, such as the benzoquinonesstudied here, this approach could lead to high energy density flowbattery electrolytes. Finding the eutectic composition of a high-n mixture can be challenging due to the large compositionalspace yet is essential for ensuring the existence of a purely liquidphase. We reformulate and apply fundamental thermodynamic equationsto describe high-n eutectic mixtures of small redox-activemolecules (benzoquinones and hydroquinones). We illustrate a novelapplication of this theory by tuning the entropy of melting, ratherthan the enthalpy, in systems highly relevant to energy storage. Wedemonstrate with differential scanning calorimetry measurements that1,4-benzoquinone derivatives exhibit eutectic mixing that decreasestheir T (m), despite slightly positive enthalpiesof mixing (0-5 kJ/mol). By rigorously investigating all 21binary mixtures of a set of seven 1,4-benzoquinone derivatives withalkyl substituents (T (m)'s between44 and 120 & DEG;C), we find that the eutectic melting point of amixture of all seven achieves a large decrease in T (m) to -6 & DEG;C. We further determine that theregular solution model shows improvement over an ideal solution modelin predicting the eutectic properties for this newly investigatedtype of mixture composed of many small redox-active organic molecules.

Automated summary

Decreasing the melting point of mixtures is important in various applications. The traditional approach is to mix components with negative enthalpic interactions, while a complementary strategy is to mix components with neutral or slightly positive enthalpic interactions, using the number of components to increase the entropy of mixing. This approach can achieve a low melting point and has potential applications in high energy density flow battery electrolytes. The study investigates the eutectic composition of benzoquinone derivatives and demonstrates a large decrease in melting point by tuning the entropy of melting. The regular solution model shows better prediction accuracy for this type of mixture compared to the ideal solution model.