Abstract
Fluids confined in nanopores exhibit significant deviations in their structure and dynamics from the bulk behavior. Although phase, structural, and diffusive behaviors of confined fluids have been investigated and reported extensively, confinement effects on the vibrational properties are less understood. We study the vibrational behavior of propane confined in 1.5 nm nanopores of MCM-41-S using inelastic neutron scattering (INS) and molecular dynamics (MD) simulations. Vibrational spectra have been obtained from INS data as functions of temperature and pressure. At ambient pressure, a strong quasielastic signal observed in the INS spectrum at 80 K suggests that confined propane remains liquid below the bulk phase melting point of 85 K. The quasielastic signal is heavily suppressed when either the pressure is increased to 1 kbar or the temperature is lowered to 30 K, indicating solidification of pore-confined propane. Confinement in MCM-41-S pores results in a glass-like state of propane that exhibits a relatively featureless low-energy vibrational spectrum compared to that of the bulk crystalline propane. Increasing the pressure to 3 kbar results in hardening of the intermolecular and methyl torsional modes. The INS data are used for estimating the isochoric specific heat of confined propane, which is compared with the specific heat of bulk propane reported in literature. Data from MD simulations are used to calculate the vibrational power spectra that agree qualitatively with the experimental data. Simulation data also suggest a reduction of the structural ordering (positional, orientational, and intramolecular) of propane under confinement.
Original language | English |
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Pages (from-to) | 6736-6745 |
Number of pages | 10 |
Journal | Journal of Physical Chemistry A |
Volume | 122 |
Issue number | 33 |
DOIs | |
State | Published - Aug 23 2018 |
Funding
This research at Oak Ridge National Laboratory’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Research at OSU was sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division for S.G. and D.C. (Contract No. DE-SC0006878). Work by G.R. and S.D. was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. DL_POLY_4 is a molecular dynamics simulation package written by I. T. Todorov and W. Smith and has been obtained from STFC’s Daresbury laboratory via the website http://www.ccp5.ac.uk/ DL_POLY. We also acknowledge computational support from the Deep Carbon Observatory cluster hosted by Rensselaer Polytechnic Institute (Peter Fox and Patrick West).