NER: Single File Diffusion: Application to Transport of Encapsulated Atoms and Molecules Inside Nanotubes

Khantha，Mahadevan宾夕法尼亚大学"NER：单列扩散：应用于纳米管内封装原子和分子的运输"这一纳米级探索性研究提案考察了纳米管内封装原子和分子的单列扩散。在一维通道中的“粒子”链的扩散，其中粒子始终保持其排列的顺序，称为“单”文件扩散。（SFD）。SFD显示异常（非马尔可夫）的扩散特性，即使不考虑粒子通道的相互作用。这项探索性研究的目的是了解"颗粒-通道"相互作用和"颗粒-颗粒"相互作用在通道（纳米管）内如何影响SFD。选择了具有代表性和技术重要性的系统，C60（富勒烯）分子和Li原子，扩散到单壁碳纳米管内。碳纳米管内部的富勒烯分子和富勒烯与管壁的相互作用都是货车德瓦尔斯型的。在目前NSF资助的研究项目下，PI正在研究纳米管内Li原子的相互作用。到目前为止获得的结果表明，这种相互作用类似于屏蔽的Yukawa势，而这与纳米管内富勒烯的货车范德华型相互作用截然不同。PI将首先使用这两种势对有限开管内的传输进行分子动力学模拟，以了解SFD的特性如何随粒子相互作用而变化。这些结果将作为一个基础，建立分析随机模型的SFD回答以下基本问题：（i）如何一个原子进入一个开放的纳米管的概率取决于它与管的相互作用？ (ii)在纳米管内部，被封装的分子链平均花费的时间是多少？ 以前的工作SFD在纳米管中的重点是液体或气体的传输，其中流体和纳米管壁之间的相互作用往往导致润湿和分层过渡。已知C60分子和Li原子都与碳纳米管壁具有排斥相互作用，并且有强有力的实验证据表明它们在管内形成一维链。此外，由于管内产生的电荷转移和屏蔽程度不同，C60分子和Li原子与纳米管的相互作用也有不同的类型。这种探索性的研究涉及数值模拟和分析模型，将导致预测模型的纳米管通道内的颗粒传输在短期，中期和长期的时间尺度。通过使用真实的相互作用势，分析的结果可以推广到包含内富勒烯的填充纳米管"豆荚"的更复杂的系统（即，包封外来原子的富勒烯），金属卤化物，金属氧化物等，其是在技术应用中具有相当大潜力的纳米结构。更广泛的影响：这里解决的纳米管中SFD的基本问题对涉及通过膜或通道的纳米流体运输的环境过程、工业过程（例如形状选择性催化、基于同位素的气体分离）和生物学进展（例如开发具有允许直接递送到细胞通道的封装药物的纳米注射器）具有影响。这项研究的结果将为一个长期的研究项目奠定基础，该项目涉及复杂的有机分子在一个单一的文件中通过纳米管的运输，其中分子的非球形形状和构象转变可以影响运动。

Khantha, MahadevanUniversity of Pennsylvania "NER: Single File Diffusion: Application to Transport of Encapsulated Atoms and Molecules Inside Nanotubes"This Nanoscale Exploratory Research proposal examines the single file diffusion of encapsulated atoms and molecules inside nanotubes. The diffusion of a chain of "particles" in one-dimensional channels with the particles maintaining the order of their arrangement at all times is called "single" file diffusion. (SFD). The SFD displays anomalous (non-Markovian) diffusion characteristics even when particle-channel interactions are not taken into account. The objective of this exploratory research is to understand how "particle-channel" interactions and "particle-particle" interactions within the channel (nanotube) influence SFD. Representative and technologically important systems, C60 (fullerene) molecules and Li atoms, diffusing inside single wall carbon nanotubes were chosen. The interactions of fullerene molecules inside carbon nanotubes and fullerenes with nanotube walls are both of the van der Waals type. Under a current NSF funded research project, the PI's are studying the interactions of Li atoms inside nanotubes. The results obtained so far show that the interaction is similar to that of a screened Yukawa potential which is quite different from the van der Waals type interactions of fullerenes inside nanotubes. The PI's will first carry out a molecular dynamics simulation of transport within finite open tubes using these two potentials to understand how the characteristics of SFD varies with particle interactions. These results will serve as a foundation to build analytical stochastic models of SFD to answer the following basic questions: (i) How does the probability of an atom entering an open nanotube depend on its interaction with the tube? (ii) What is the average duration of time spent by the chain of encapsulated molecules inside the nanotube? Prior work on SFD in nanotubes has focused on transport of liquids or gases where theattractive interaction between fluids and the nanotube walls often lead to wetting and layering transitions. Both C60 molecules and Li atoms are known to have repulsive interactions with the carbon nanotube walls and there is strong experimental evidence to show they form one-dimensional chains inside the tubes. In addition, the interactions of C60 molecules and Li atoms with nanotubes are of different types due to the different degrees of charge transfer and screening produced within the tubes. This exploratory study involving numerical simulation and analytical models will lead to predictive models of particle transport within nanotube channels at short, intermediate and long time scales. By using realistic interaction potentials, the results of the analysis can be generalized to more complex systems of filled nanotube "peapods" containing endofullerenes (i.e., fullerenes encapsulating foreign atoms), metallic halides, metal oxides, etc., which are nanostructures of considerable potential in technological applications. Broader impacts: The fundamental problem of SFD in nanotubes addressed here has implications for environmental processes that involve nanofluidic transport through membranes or channels, industrial processes such as shape selective catalysis, isotope based separation of gases and biological advances such as development of nano-syringes with encapsulated drugs allowing direct delivery to cell channels. The results of this study will lay the groundwork for a longer-term research project which involves the transport of complex organic molecules in a single file through nanotubes where the non-spherical shape and confirmational transitions of the molecules can influence the motion.