Universität Rostock, Institut für Chemie, Abteilung für Physikalische Chemie, Dr-Lorenz-Weg 2, 18059, Rostock, Germany.
Phys Chem Chem Phys. 2019 Sep 18;21(36):20308-20314. doi: 10.1039/c9cp04337c.
The enthalpy of vaporization is mainly the amount of the energy needed for transferring quantities from the liquid into the gas phase. It simply describes the energy required to overcome the interaction energy between quantities if those evaporate as monomers as is the case for molecular liquids. The situation for ionic liquids (ILs) is more complex. We do not know the delicate composition of different types of interaction, neither for the liquid nor for the gas phase. Additionally, we have to consider that ILs evaporate as ion pairs which carry substantial interaction energy of all kind into the vapor phase. In this study, we measured the vaporization enthalpies of well-selected hydroxyl-functionalized and non-hydroxyfunctionalized ILs. In particular, we focussed on the case of hydroxyl-functionalized ILs providing possible hydrogen bonding between cation and anion in the liquid as well as in the gas phase. With infrared spectroscopy, we showed that all the hydroxyl groups are involved in hydrogen bonding in the liquid state of the ILs. However, molecular dynamics simulations showed that the evaporating ion pairs also include this hydrogen bond. A detailed analysis of the potential energies for all IL constituents showed that the hydrogen bond hinders favourable interaction between the polarizable ring of the cations and the anions leading to higher vaporization enthalpies for the hydroxyl-functionalized ILs.
蒸发焓主要是将液体中的物质转移到气相中所需的能量。它简单地描述了克服物质蒸发成单体时的相互作用能所需的能量，这是分子液体的情况。对于离子液体（ILs）来说，情况更加复杂。我们既不知道液体和气相中不同类型相互作用的微妙组成，也不知道离子对蒸发时所携带的各种相互作用能。在这项研究中，我们测量了经过精心选择的具有羟基官能团和非羟基官能团的 ILs 的蒸发焓。特别地，我们关注的是具有羟基官能团的 ILs 的情况，因为它们在液体和气相中可能存在阳离子和阴离子之间的氢键。通过红外光谱，我们表明所有的羟基都参与了 ILs 的液体状态下的氢键形成。然而，分子动力学模拟表明，蒸发的离子对也包含这种氢键。对所有 IL 成分的势能的详细分析表明，氢键阻碍了阳离子的可极化环与阴离子之间的有利相互作用，导致具有羟基官能团的 ILs 的蒸发焓升高。
