Probing Phase transitions in orientationally ordered monolayers on solid surfaces with liquid crystals

界面上高度有序的两亲分子单层为理解准二维纳米体系的热力学和流变性能提供了一个很好的模型。它们在各种应用中也很有前景，如润湿、润滑、分子电子学、传感器和非线性光学。其中一些应用依赖于单分子层中两亲分子的取向顺序。本研究的目的是利用液晶作为光学放大探针，研究固体衬底上两亲分子的长距离定向有序单层中仅由分子倾斜方位角变化引起的相变。与分子倾斜方位角的规律变化相关的具有较好特征的条形和星形纹理将被选择作为我们研究的原型。实现这一目标的具体目标是：1)在玻璃衬底上组装具有星形和条纹纹理的远距离定向有序单层；2)用液晶成像星形和条纹纹理；3)测量液晶在星形和条纹纹理上的方位锚定能；4)观察玻璃衬底上星形纹理的绽放过渡和条纹纹理的之字形过渡。5)研究固体衬底的表面性质对这些相变的影响。知识价值：在固体底物上转移的单层的保真度是单层研究中一个长期存在的问题。据预测，在固体基底上转移的单分子膜的结构和相应与在水面上观察到的结构和相一致，但可能在不同的表面压力、温度、亚相组成条件下。然而，这些条件的细节仍然缺失。该项目旨在开发一个基于液晶光学放大的简单快速的平台，以成像转移单层中的长距离定向有序相及其之间的转变，以提高我们对这些条件的理解并测试理论预测。此外，在衬底上组装长程定向有序单层膜的能力对纳米摩擦学具有相当大的技术潜力。所提出的工作也将促进我们对液晶与分子工程表面相互作用机制的理解，这在显示应用中是至关重要的。广泛影响：细胞膜中的脂筏被认为参与细胞过程的一个成员，如膜运输和信号传导。它们被认为是富含胆固醇和鞘脂的脂质有序结构域，与富含磷脂的液态无序相共存。本课题提出的液晶光学放大的长期实际应用之一是对膜中的脂筏进行原位成像。从更广泛的角度来看，除了推进对固体基板上两亲单层的长程定向有序相和相变的发现和理解外，该计划还要求PI教育和培养界面工程、分子组装和液晶方面的研究生和本科生。PI积极参与NSF REU项目和UCF外展项目，这些项目鼓励未被充分代表的群体、高中教师和老年人参与。拟议研究的整合将使研究生、本科生和少数民族参与科学研究和发现，这将使广泛的社区受益。

Highly ordered monolayers of amphiphilic molecules at interfaces provide an excellent model for the understanding of thermodynamic and rheological properties of quasi two dimensional nano scale systems. They are also promise in a variety of applications, such as wetting, lubrication, molecular electronics, sensors, and nonlinear optics. Some of these applications rely on the orientational order of amphiphilic molecules in monolayers. The objective of this proposed research is to study the phase transitions which arise solely from the change of the molecular tilt azimuth in long range orientationally ordered monolayers of amphiphilic molecules on solid substrates using liquid crystals as an optical amplification probe. Well characterized splay stripe and star textures, which are associated with the regular variation in the molecular tilt azimuth, will be chosen as prototypes in our studies. The specific aims for achieving the objective are to 1) assemble the long range orientationally ordered monolayers with star and stripe textures on glass substrates, 2) image the star and stripe textures with liquid crystals, 3) measure the azimuthal anchoring energy of liquid crystals on the star and stripe textures, 4) observe the blooming transition of star textures and the zigzag transition of stripe textures on glass substrates, and 5) investigate the effect of the surface properties of solid substrates on these phase transitions. Intellectual Merit: The fidelity of transferred monolayers on solid substrates is a long standing issue in monolayer studies. It has been predicted that the structure and phase of a transferred monolayer on a solid substrate should correspond to one which has been observed on water surface, but possibly under different conditions of surface pressure, temperature, subphase compositions. However, details of these conditions are still missing. The project aims to develop a simple and fast platform based on liquid crystal-optical amplification to image the long range orientationally ordered phases in transferred monolayers and the transitions between them to advance our understanding of these conditions and test the theoretical prediction. In addition, the ability of assembling the long range orientationally ordered monolayers onto substrates has a considerable technological potential for nanotribology. The proposed work will also advance our understanding of the interaction mechanism of liquid crystals with the molecularly engineered surfaces, which is critical in display applications.Broad Impacts: Lipid rafts in cell membranes are believed to participate in a member of cellular processes, such as membrane trafficking and signaling. They are considered as lipid ordered domains enriched in cholesterol and sphingolipids, which coexist with a liquid disordered phase enriched in phospholipids. One of long term practical applications of the liquid crystal optical amplification, which is proposed in this research project, is to in situ image lipid rafts in membranes. From a broader perspective, aside from advancing the discovery and understanding of the long range orientationally ordered phases and phase transitions of amphiphilic monolayers on solid substrates, the proposed program obligates the PI to educate and train graduate and undergraduate students in interface engineering, molecular assembly, and liquid crystals. The PI actively participates in NSF REU programs and UCF outreach programs, which encourage the participation of underrepresented groups and high school teachers and seniors. The integration of the proposed research will involve graduate students, undergraduates, and minorities in scientific research and discovery, which will benefit the broad community.