Department of Materials Science & Engineering, Stanford University, Stanford, California 94305, United States.
Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States.
J Phys Chem B. 2023 Jul 13;127(27):6102-6112. doi: 10.1021/acs.jpcb.3c01125. Epub 2023 Jun 29.
Decreasing the melting point () of a mixture is of interest in cryopreservatives, molten salts, and battery electrolytes. One general strategy to decrease , exemplified by deep eutectic solvents, is to mix components with favorable (negative) enthalpic interactions. We demonstrate a complementary strategy to decrease by mixing many components with neutral or slightly positive enthalpic interactions, using the number of components () to increase the entropy of mixing and decrease . In theory, under certain conditions this approach could achieve an arbitrarily low . Furthermore, if the components are small redox-active molecules, such as the benzoquinones studied here, this approach could lead to high energy density flow battery electrolytes. Finding the eutectic composition of a high- mixture can be challenging due to the large compositional space yet is essential for ensuring the existence of a purely liquid phase. We reformulate and apply fundamental thermodynamic equations to describe high- eutectic mixtures of small redox-active molecules (benzoquinones and hydroquinones). We illustrate a novel application of this theory by tuning the entropy of melting, rather than the enthalpy, in systems highly relevant to energy storage. We demonstrate with differential scanning calorimetry measurements that 1,4-benzoquinone derivatives exhibit eutectic mixing that decreases their , despite slightly positive enthalpies of mixing (0-5 kJ/mol). By rigorously investigating all 21 binary mixtures of a set of seven 1,4-benzoquinone derivatives with alkyl substituents ('s between 44 and 120 °C), we find that the eutectic melting point of a mixture of all seven achieves a large decrease in to -6 °C. We further determine that the regular solution model shows improvement over an ideal solution model in predicting the eutectic properties for this newly investigated type of mixture composed of many small redox-active organic molecules.
降低混合物的熔点 () 在冷冻保护剂、熔融盐和电池电解液中很重要。一种降低 () 的一般策略是混合具有有利（负）焓相互作用的成分，例如深共晶溶剂。我们展示了一种通过混合许多具有中性或略微正焓相互作用的成分来降低 () 的互补策略，使用成分的数量 () 来增加混合熵并降低 ()。从理论上讲，在某些条件下，这种方法可以实现任意低的 ()。此外，如果成分是小的氧化还原活性分子，如这里研究的苯醌，则这种方法可以导致高能量密度的流电池电解液。由于组成空间大，找到高 () 的共晶组成可能具有挑战性，但对于确保存在纯液相是必不可少的。我们重新制定并应用基本热力学方程来描述小氧化还原活性分子（苯醌和对苯二酚）的高 () 共晶混合物。我们通过调整与能量存储高度相关的系统中的熔化熵，而不是焓，来展示该理论的新颖应用。我们通过差示扫描量热法测量证明，1,4-苯醌衍生物表现出共晶混合，尽管混合焓略有正（0-5 kJ/mol），但其熔点降低。通过严格研究一组 7 种带有烷基取代基的 1,4-苯醌衍生物的 21 种二元混合物 ('s 在 44 至 120 °C 之间），我们发现七种 1,4-苯醌衍生物的混合物的共晶熔点在很大程度上降低到 -6 °C。我们进一步确定，正则溶液模型在预测由许多小氧化还原活性有机分子组成的这种新研究类型的混合物的共晶性质方面优于理想溶液模型。
