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

University Of Pennsylvania, Philadelphia PA

Investigators

Abstract

Khantha, Mahadevan University 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 the attractive 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.