Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
Johannes Gutenberg University Mainz, Staudingerweg 7, 55099 Mainz, Germany.
J Chem Phys. 2018 May 21;148(19):193811. doi: 10.1063/1.5005143.
Room temperature ionic liquids (RTILs) have been shown to exhibit spatial heterogeneity or structural heterogeneity in the sense that they form hydrophobic and ionic domains. Yet studies of the relationship between this structural heterogeneity and the ∼picosecond motion of the molecular constituents remain limited. In order to obtain insight into the time scales relevant to this structural heterogeneity, we perform molecular dynamics simulations of a series of RTILs. To investigate the relationship between the structures, i.e., the presence of hydrophobic and ionic domains, and the dynamics, we gradually increase the size of the hydrophobic part of the cation from ethylammonium nitrate (EAN), via propylammonium nitrate (PAN), to butylammonium nitrate (BAN). The two ends of the organic cation, namely, the charged N-H group and the hydrophobic C-H group, exhibit rotational dynamics on different time scales, evidencing dynamical heterogeneity. The dynamics of the N-H group is slower because of the strong coulombic interaction with the nitrate counter-ionic anions, while the dynamics of the C-H group is faster because of the weaker van der Waals interaction with the surrounding atoms. In particular, the rotation of the N-H group slows down with increasing cationic chain length, while the rotation of the C-H group shows little dependence on the cationic chain length, manifesting that the dynamical heterogeneity is enhanced with a longer cationic chain. The slowdown of the N-H group with increasing cationic chain length is associated with a lower number of nitrate anions near the N-H group, which presumably results in the increase of the energy barrier for the rotation. The sensitivity of the N-H rotation to the number of surrounding nitrate anions, in conjunction with the varying number of nitrate anions, gives rise to a broad distribution of N-H reorientation times. Our results suggest that the asymmetry of the cations and the larger excluded volume for longer cationic chain are important for both the structural heterogeneity and the dynamical heterogeneities. The observed dynamical heterogeneities may affect the rates of chemical reactions depending on where the reactants are solvated in ionic liquids and provide an additional guideline for the design of RTILs as solvents.
室温离子液体 (RTILs) 表现出空间异质性或结构异质性，即在它们形成疏水性和离子性域的意义上。然而，关于这种结构异质性与分子组成部分的 ∼皮秒运动之间的关系的研究仍然有限。为了深入了解与这种结构异质性相关的时间尺度，我们对一系列 RTILs 进行了分子动力学模拟。为了研究结构，即疏水性和离子性域的存在，与动力学之间的关系，我们逐渐增加阳离子中疏水部分的大小，从乙基硝酸铵 (EAN) 经过丙基硝酸铵 (PAN) 到丁基硝酸铵 (BAN)。有机阳离子的两端，即带电的 N-H 基团和疏水性的 C-H 基团，表现出不同时间尺度上的旋转动力学，表现出动力学异质性。由于与硝酸根反离子的强库仑相互作用，N-H 基团的动力学较慢，而由于与周围原子的较弱范德华相互作用，C-H 基团的动力学较快。特别是，N-H 基团的旋转随着阳离子链长的增加而减慢，而 C-H 基团的旋转几乎不依赖于阳离子链长，表明随着阳离子链长的增加，动力学异质性增强。N-H 基团随阳离子链长增加而减慢与 N-H 基团附近的硝酸根阴离子数量减少有关，这可能导致旋转的能量势垒增加。N-H 旋转对周围硝酸根阴离子数量的敏感性，以及周围硝酸根阴离子数量的变化，导致 N-H 重新取向时间的广泛分布。我们的结果表明，阳离子的不对称性和较长阳离子链的较大排除体积对于结构异质性和动力学异质性都很重要。观察到的动力学异质性可能会影响化学反应的速率，具体取决于反应物在离子液体中的溶剂化位置，并为作为溶剂的 RTILs 的设计提供了另一个指导原则。
