Department of Mechanical Engineering, Virginia Tech, Blacksburg, Virginia 24061, USA.
Center for Nanophase Materials Sciences and Computational Sciences and Engineering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
J Chem Phys. 2017 Oct 7;147(13):134704. doi: 10.1063/1.5005541.
Surface forces mediated by room-temperature ionic liquids (RTILs) play an essential role in diverse applications including self-assembly, lubrication, and electrochemical energy storage. Therefore, their fundamental understanding is critical. Using molecular simulations, we study the interactions between two nanorods immersed in model RTILs at rod-rod separations where both structural and double layer forces are important. The interaction force between neutral rods oscillates as the two rods approach each other, similar to the classical structural forces. Such oscillatory force originates from the density oscillation of RTILs near each rod and is affected by the packing constraints imposed by the neighboring rods. The oscillation period and decay length of the oscillatory force are mainly dictated by the ion density distribution near isolated nanorods. When charges are introduced on the rods, the interaction force remains short-range and oscillatory, similar to the interactions between planar walls mediated by some protic RTILs reported earlier. Nevertheless, introducing net charges to the rods greatly changes the rod-rod interactions, e.g., by delaying the appearance of the first force trough and increasing the oscillation period and decay length of the interaction force. The oscillation period and decay length of the oscillatory force and free energy are commensurate with those of the space charge density near an isolated, charged rod. The free energy of rod-rod interactions reaches local minima (maxima) at rod-rod separations when the space charges near the two rods interfere constructively (destructively). The insight on the short-range interactions between nanorods in RTILs helps guide the design of novel materials, e.g., ionic composites based on rigid-rod polyanions and RTILs.
室温离子液体（RTILs）介导的表面力在各种应用中起着至关重要的作用，包括自组装、润滑和电化学储能。因此，对其基本原理的理解至关重要。我们使用分子模拟研究了两种纳米棒在棒-棒分离时的相互作用，此时结构力和双层力都很重要。当两根棒相互靠近时，中性棒之间的相互作用力会发生振荡，类似于经典的结构力。这种振荡力源于 RTIL 中靠近每根棒的密度振荡，并且受到相邻棒的堆积约束的影响。振荡力的振荡周期和衰减长度主要由孤立纳米棒附近的离子密度分布决定。当在棒上引入电荷时，相互作用力保持短程和振荡，类似于先前报道的一些质子 RTIL 介导的平面壁之间的相互作用。然而，向棒引入净电荷会极大地改变棒-棒相互作用，例如，通过延迟第一力谷的出现并增加相互作用力的振荡周期和衰减长度。振荡力和自由能的振荡周期和衰减长度与孤立带电棒附近的空间电荷密度一致。当两根棒附近的空间电荷相互增强（相互抵消）时，棒-棒相互作用的自由能在棒-棒分离处达到局部最小值（最大值）。对 RTIL 中纳米棒之间短程相互作用的深入了解有助于指导新型材料的设计，例如基于刚性棒状聚阴离子和 RTIL 的离子复合材料。
