Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA.
Department of Applied Chemistry and Biochemistry, National Institute of Technology, Wakayama College, Wakayama 644-0023, Japan.
J Chem Phys. 2018 May 21;148(19):193815. doi: 10.1063/1.5009765.
Mesoscopic aggregation in aprotic ionic liquids due to the microphase separation of polar and non-polar components is expected to correlate strongly with the physicochemical properties of ionic liquids and therefore their potential applications. The most commonly cited experimental evidence of such aggregation is the observation of a low-q pre-peak in the x-ray and neutron scattering profiles, attributed to the polarity alternation of polar and apolar phases. In this work, a homologous series of phosphonium ionic liquids with the bis(trifluoromethylsulfonyl)imide anion and systematically varying alkyl chain lengths on the phosphonium cation are investigated by small and wide-angle x-ray scattering, dynamic-mechanical spectroscopy, and broadband dielectric spectroscopy. A comparison of the real space correlation distance corresponding to the pre-peak and the presence or absence of the slow sub-α dielectric relaxation previously associated with the motion of mesoscale aggregates reveals a disruption of mesoscale aggregates with increasing symmetry of the quaternary phosphonium cation. These findings contribute to the broader understanding of the interplay of molecular structures, mesoscale aggregation, and physicochemical properties in aprotic ionic liquids.
介观聚集在质子惰性离子液体中由于极性和非极性组分的微相分离预计将与离子液体的物理化学性质密切相关，因此也与它们的潜在应用相关。这种聚集最常被引用的实验证据是在 X 射线和中子散射谱中观察到低-q 前峰，归因于极性和非极性相的极性交替。在这项工作中，通过小角和广角 X 射线散射、动态力学光谱和宽带介电光谱研究了具有双（三氟甲基磺酰基）亚胺阴离子和在磷鎓阳离子上系统变化的烷基链长度的一系列同系物磷鎓离子液体。对前峰对应的实空间相关距离与先前与介观聚集体运动相关的慢亚-α介电弛豫的存在或不存在进行比较，揭示了随着四元磷鎓阳离子对称性的增加介观聚集体的破坏。这些发现有助于更广泛地理解质子惰性离子液体中分子结构、介观聚集和物理化学性质的相互作用。
