Department of Chemistry, University of Iowa , Iowa City, Iowa 52242, United States.
Langmuir. 2016 May 24;32(20):5147-54. doi: 10.1021/acs.langmuir.6b00304. Epub 2016 May 10.
We report the transformation of ionic liquid films from isotropic bulk to a fluid-ordered state over micrometer length scales. Data from infrared and nonlinear spectroscopy measurements show clear transitions that, for varying ionic liquids, occur over time frames of 10 min to 2 h. These maturation times depend linearly on the chosen ionic liquids' bulk viscosities. Interestingly, the ionic liquids do not form solids upon ordering but do exhibit strong preferential alignments of molecules that persist throughout the fluid films' thicknesses. Our measurements characterize this ordering process and show that it is largely insensitive to substrate surface chemistry or small amounts of absorbed water. Additional experiments show the transition is observed across several of the most common ionic liquid cations and that the process is completely reversible. The driving force for this organization is attributed to electrostatic and steric forces combined with a slow shearing of the viscous ionic liquid. These interactions work together to slowly bring the molecules within the film to a preferred, global orientation. The physical length and time scales of this transformation are unexpected and intriguing and invite additional studies to develop an understanding and control of ionic liquid materials' behavior, particularly near surfaces, to benefit their uses in lubrication, capacitive energy storage, and heterogeneous catalysis.
我们报告了离子液体薄膜从各向同性体相到微米尺度上的有序流体相的转变。来自红外和非线性光谱测量的数据显示了明显的转变，对于不同的离子液体，这些转变在 10 分钟到 2 小时的时间范围内发生。这些成熟时间与所选离子液体的体粘度线性相关。有趣的是，离子液体在有序化时不会形成固体，但确实表现出分子的强烈优先取向，这种取向在整个流体膜厚度中都存在。我们的测量结果表征了这个有序化过程，并表明它对基底表面化学或少量吸收水基本不敏感。进一步的实验表明，这种转变在几种最常见的离子液体阳离子中都观察到，而且这个过程是完全可逆的。这种组织的驱动力归因于静电和空间位阻的合力以及粘性离子液体的缓慢剪切。这些相互作用共同作用，使薄膜内的分子逐渐达到一个优选的整体取向。这种转变的物理长度和时间尺度是出乎意料的，令人着迷的，这促使我们进行更多的研究，以了解和控制离子液体材料的行为，特别是在表面附近的行为，从而有利于它们在润滑、电容储能和多相催化中的应用。
