Department of Chemistry and Biochemistry, Rowan University, 201 Mullica Hill Road, Glassboro, New Jersey, USA.
J Phys Chem B. 2013 Jun 13;117(23):7057-64. doi: 10.1021/jp402582r. Epub 2013 May 30.
The hydrogen ion is one of the most important species in aqueous solutions, as well as in protic ionic liquids (PILs). PILs are important potential alternatives to H2O for swelling the proton exchange membranes (PEMs) and improving the high-temperature performance of fuel cells. The hydrogen ion (H(+)) or hydronium (H3O(+)) solvation mechanism is not only a fundamental principle of acid/base chemistry in ionic liquids but also key to understanding the charge- and proton-transport properties of the PIL solutions. In this paper, a PIL system was prepared by mixing 1-butyl-3-methyl-imidazolium tetrafluoroborate (BMIBF4) IL with an aqueous solution of a strong acid, HBF4. Water can be partially evaporated from the solution under a vacuum at room temperature. Conductivity and vibrational spectroscopy (IR and Raman) measurements were used in combination with density functional theory (DFT) calculations to characterize the molecular-level solvation of H(+) and H2O in the IL solution. When water is present at high molar fraction, the cations (BMI(+) and H(+)) and anions (BF4(-)) are both solvated by water and the solutions have high conductivity. After water evaporation, the PIL solution has excess H(+) and reduced conductivity, which is still significantly higher than that of pure BMIBF4. Vibrational spectroscopy suggests that the BMI(+) and BF4(-) ions are desolvated from water during the water evaporation. DFT calculations assist the interpretation of the vibrational spectroscopy and show that the remaining water is in the form of H3O(+) solvated by the IL molecular ions. Hence, the species remaining after evaporation is a ternary PIL consisting of BMI(+) cation, BF4(-) anion, and H3O(+) cation. The H3O(+) may be the principle charge carrier in the PIL solution and responsible for the high solution conductivity.
氢离子是水相溶液以及质子离子液体（PILs）中最重要的物种之一。PILs 是替代 H2O 用于溶胀质子交换膜（PEM）并提高燃料电池高温性能的重要潜在选择。氢离子（H(+)）或水合氢离子（H3O(+)）的溶剂化机制不仅是离子液体酸碱化学的基本原理，也是理解 PIL 溶液中电荷和质子传输性质的关键。在本文中，通过将 1-丁基-3-甲基-咪唑四氟硼酸盐（BMIBF4）离子液体与强酸 HBF4 的水溶液混合制备了 PIL 体系。在室温下，真空可使溶液部分蒸发水。使用电导率和振动光谱（IR 和 Raman）测量结合密度泛函理论（DFT）计算来表征 H(+)和 H2O 在 IL 溶液中的分子水平溶剂化。当水处于高摩尔分数时，阳离子（BMI(+)和 H(+)）和阴离子（BF4(-)）都被水溶剂化，溶液具有高电导率。水蒸发后，PIL 溶液具有过量的 H(+)和降低的电导率，但仍明显高于纯 BMIBF4。振动光谱表明，在水蒸发过程中，BMI(+)和 BF4(-)离子从水中解溶剂化。DFT 计算有助于对振动光谱的解释，并表明剩余的水以 H3O(+)的形式存在，被 IL 分子离子溶剂化。因此，蒸发后剩余的物质是由 BMI(+)阳离子、BF4(-)阴离子和 H3O(+)阳离子组成的三元 PIL。H3O(+)可能是 PIL 溶液中的主要电荷载流子，负责溶液的高电导率。
