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Structure and Dynamics of Ethane Confined in Silica Nanopores in the Presence of CO2



Tingting Liu, Siddharth Guatam, David R. Cole, Sumant Patankar, David Tomasko, Wei Zhou, Gernot Rother


Fundamental understanding of subcritical/supercritical behavior of key hydrocarbon species inside nanoporous matrices at elevated pressure and temperature is less developed compared to bulk fluids, but this knowledge is needed for efficient utilization of the hydrocarbon resources in subsurface energy systems. This study explores in detail the structure and dynamics of ethane (C2H6) fluid confined in silica nanopores, with a focus on the effects of pressure and different ratios of C2H6 and CO2 at non-ambient temperature. Quasi-elastic neutron scattering (QENS) experiments were carried out for the pure C2H6, C2H6: CO2 =3:1, and 1:3 mixed fluids confined in 4-nm cylindrical silica pores at three different pressures (30, 65, and 100 bar) at 323 K. Two Lorentzian functions were required to fit the spectra, corresponding to fast and slow translational motions. No localized motions (rotations, vibrations) were detected. Higher pressures resulted in hindrances of the diffusivity of C2H6 molecules in all systems investigated. Pore size was found to be an important factor, i.e., dynamics of confined C2H6 is more restricted in smaller pores. Molecular dynamics (MD) simulations were performed to complement the QENS experiment at 65 bar. The simulations indicate CO2 molecules are more strongly attracted to the pore surface compared to Cd2^H6. The C2H6 molecules adsorbing to the pore surface (for pure C2H6 system) or the CO2 molecules in the mixtures form a dense first layer (L1) close to the pore surface and a second less dense layer extending into the pore center (L2). Both the experiments and simulations revealed the role that COd2 molecules play in enhancing C2H6 diffusion ('molecular lubrication') at high CO2: Cd2^H6 ratios. The energy scales of the two dynamic components, fast and slow, are in good agreement between experiments and simulations. The simulations also identified the fast component as the main contributor to the dynamics. Molecule motions in the L2 region are mostly responsible for the dynamics (fast and slow) that can be detected by the instrument. The simulation data shows that although the energies involved in the rotational motions of C2H6 fall into the energy window accessible to the instrument, the contribution of the rotational motion to the experimental spectra is negligible.
Journal of Physical Chemistry


Nanoporous Material, Quasi-elastic neutron scattering, Molecular dynamics


Liu, T. , Guatam, S. , Cole, D. , Patankar, S. , Tomasko, D. , Zhou, W. and Rother, G. (2020), Structure and Dynamics of Ethane Confined in Silica Nanopores in the Presence of CO<sub>2</sub>, Journal of Physical Chemistry, [online], (Accessed July 22, 2024)


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Created February 27, 2020, Updated October 12, 2021