Molecular shape dictates the dynamic course in narrow channels

Significance 

Both fundamental and application physics demand the complete comprehension of fluid behavior under confinement in porous material. This can be attributed to the fact that confining a fluid, leads to immense alteration of its dynamic and structural attributes when compared to those of bulk fluids. Furthermore, the orientational structure and dynamics of molecules is known to be affected by confinement in space comparable in size to the molecule itself. Such intriguing attributes have attracted much research attention, where a number of already published studies have focused particularly on investigating hydrocarbons confined in zeolites. Diffusive properties of hydrocarbons in zeolites are dependent on several factors thereby making them a popular choice of study. Zeolite Socony Mobil–5 (ZSM-5) has already been identified as a good candidate in studying the confinement effect on the rotational motion of different hydrocarbons. An important aspect that affects the behavior of confined fluids is the molecular shape of the confined component. Several studies highlighting the importance of molecular shape have been reported in literature but none considers the effects of confinement especially on orientational structure and dynamics. This can be studied by using ZSM-5 zeolite as the confining medium as with its narrow pores, it can put a severe restriction on motion and thus greatly influence the orientational structure and dynamics of the confined molecules.

Ohio State University (OSU) researchers in collaboration with scientists at Oak Ridge National Laboratory (ORNL) assessed the effect of molecular shape on the orientational structure and dynamics of molecules under strict geometrical confinement imposed by ZSM-5 zeolite. They focused on the influence of molecular shape on the rotational dynamics of confined molecules. Dr. Siddharth Gautam from OSU observed “Normally under confinement, the translational motion of molecules gets affected while their rotational motion remains relatively unaffected. ZSM-5 is a unique and interesting confining medium with channels so narrow that they can influence even the rotational properties of the confined molecules. For this I have been exploring molecular rotation of several molecules in this medium”. In the current work, he along with his colleagues extended this exploration1,2 to include the effect of molecular shape on rotation under confinement in ZSM-5. Their work is currently published in the research journal, Chemical Engineering Science3. A follow up work on the translational behavior has been recently accepted in Chemical Physics and has been published online4.

The research technique employed the application of TraPPE-UA force field formalism for simulating hydrocarbon molecules in a simpler united atom approach, which was computationally less expensive compared to an all atom simulation. Dr. Debsindhu Bhowmik who ran those simulations with Dr. Gautam, said “these types of intelligent computer simulations are effective in answering many of those questions that generally are difficult to tackle by other available techniques.” They started by selecting four molecules: propane, acetaldehyde, acetonitrile and acetone, of comparable kinetic diameters and confined in ZSM-5, and had their orientational structure and dynamics assessed. The studied attributes included probing the differences in the orientational distribution of molecules in the ZSM-5 channels, and extracting time scales of the decay of correlation functions related to rotational motion. “Detecting the effect of molecular shape in the confined alleys and subsequently quantifying them have been our goals and we believe our work has shed light on that fundamental question.” Bhowmik continued.

The authors observed that the orientational correlation functions of all the four molecules exhibited two regimes of rotational motion. In addition, they also noted that the short time regime represented free rotation of the molecules before they collided with the pore walls, while as the long time orientational jumps driven by inter-channel migrations give rise to a very slow varying second regime. Moreover, the longtime inter-channel driven rotational motion was noted to be highly restricted in the case of acetone and acetaldehyde. “Our work provides the molecular level picture of the behavior of these molecules under severe confinement, which is difficult to visualize from the experimental techniques. We have also calculated the observables that can be assessed from experiments such as neutron scattering for a direct comparison.” Dr. Shrestha emphasized the significance of the work.

In a nutshell, Dr. Gautam and his colleagues Prof. David R. Cole from OSU and Dr. Indu Dhiman, Dr. Shrestha and Dr. Bhowmik from ORNL have successfully presented an in-depth assessment of the effects of confinement on the orientational structure and dynamics of four guest molecules with similar kinetic diameters but different shapes in the channels of all silica analogue of ZSM-5 zeolite. In general, orientational structure and dynamics of propane was found to be least affected by confinement under ZSM-5, whereas charge and shape asymmetry of other molecules made their inter-channel migration-driven rotation slow. Altogether, charge asymmetry plays a dominant role in the rotational dynamics of molecules under confinement.

Molecular shape dictates the dynamic course in narrow channels-Advances in Engineering

About the author

Dr. Indu Dhiman is a postdoctoral research associate in the neutron scattering division since Dec 2015. She received her PhD from Mumbai University in the field of condensed matter physics and neutron diffraction under the Bhabha Atomic Research Centre and Mumbai University Collaborative Program.

Her current research interests are in the field of granular matter using neutron imaging and investigation of magnetic materials and superconductor using polarized neutron imaging. In addition, she has also been involved in providing user support at neutron imaging facility at CG-1D beamline, working in collaboration with the scientific community on research topic such as plant/root remediation systems, materials science, Li ion batteries etc. Further, she has contributed towards the development of polarized neutron imaging technique at the Oak Ridge National Laboratory High Flux Isotope Reactor at CG-1D imaging beamline. Before coming here, she was working at Helmholtz Zentrum Berlin, Germany as a postdoctoral research scientist in the field of polarized neutron tomography and ultra-small angle neutron scattering.

About the author

Dr. Utsab R. Shrestha is a biophysicist, working as a postdoctoral research associate at UT/ORNL Center for Molecular Biophysics, Biosciences Division, Oak Ridge National Laboratory since May 2017. He earned Ph. D. degree in Physics from Wayne State University, Detroit, Michigan in 2017. His work includes the use of neutron/x-ray scattering techniques combined with molecular dynamics simulations. Currently, he focuses on modeling the conformational ensembles of flexible and intrinsically disordered proteins, and plant cell wall structure.

About the author

Dr. Debsindhu Bhowmik is Computational Scientist in Computational Sciences & Engineering Division and Health Data Sciences Institute at Oak Ridge National Laboratory. He received his Bachelor’s and Master’s in Physics from Jadavpur University and Ph.D. from Université Pierre et Marie Curie with CFR grant from CEA, France. Later he worked as Postdoctoral Researcher at Donostia International Physics Center and at Wayne State University.

His current focus is in understanding of complex biological and genetic phenomenon and studying disordered systems by implementing new generation large scale simulation blended with Deep learning and scattering techniques.

About the author

David Cole is Professor and Ohio Research Scholar Endowed Chair in the School of Earth Sciences at The Ohio State University. Prior to coming to OSU in 2010 he was a Distinguished Scientist and head of the Geochemistry and Interfacial Sciences Group in the Chemistry Division of Oak Ridge National Laboratory in TN. He earned his PhD from The Pennsylvania State University.

His current research focuses on the use of advanced electron microscopy, nuclear magnetic resonance spectroscopy, neutron scattering, X-ray computed tomography and molecular dynamics to quantify the physical and chemical properties of subsurface energy materials and associated fluid-solid interactions relevant to gas shale, seal rocks, conventional oil and gas reservoirs, and active geothermal systems.

He is Director of the OSU Subsurface Energy Materials Characterization and Analysis Laboratory (SEMCAL), Director of the OSU Center for Energy Research, Training and Innovation (CERTAIN), and Executive Committee member of the A.P. Sloan Foundation funded Deep Carbon Observatory (DCO).

About the author

Dr. Siddharth Gautam is currently a research associate at the School of Earth Sciences of the Ohio State University. Prior to this he received his PhD degree in Physics from Mumbai University in 2009 while working in the Solid State Physics Division of Bhabha Atomic Research Centre in India. After graduating he moved to Spain, working at the Donostia International Physics Center in the Guipuzcoan city of San Sebastian as a postdoctoral fellow. He has over a decade long experience in the complementary use of neutron scattering and molecular dynamics simulations to understand material properties. Using this combination, he has studied a variety of systems including molecules confined in zeolites, liquid crystals, water in clay minerals and polymers.

His current research interest is in the development of computational protocols to directly calculate experimental observables from MD simulations and studying the interaction of geofluids with minerals in the subsurface environment.

References

1. S. Gautam, V. K. Sharma, S. Mitra, R. Mukhopadhyay, Rotational Dynamics of Propylene in ZSM-5 zeolite frameworks.. Chemical Physics Letters, volume 501 (2010) page 345-350.

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2. S. Gautam, T. Liu, S. Patankar, D. Tomasko, D. R. Cole. Location dependent orientational structure and dynamics of ethane in ZSM-5.. Chemical Physics Letters, volume 6.<8 (2016) 130-136.

Go To Chemical Physics Letters

 

3. I. Dhiman, D. Bhowmik, Utsab R. Shrestha, D. R. Cole, S. Gautam. Effect of molecular shape on rotation under severe confinement.. Chemical Engineering Science, volume 180 (2018) page 33–41.

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4. I. Dhiman, U. R. Shrestha, D. Bhowmik, D. R. Cole, S. Gautam. Influence of molecular shape on self-diffusion under severe confinement: A molecular dynamics study. . Chemical Physics, volume 516 (2019) 92-102.

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