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Title: Translational diffusion of water inside hydrophobic carbon micropores studied by neutron spectroscopy and molecular dynamics simulation

Abstract

When water molecules are confined to nanoscale spacings, such as in the nanometer-size pores of activated carbon fiber (ACF), their freezing point gets suppressed down to very low temperatures (~150 K), leading to a metastable liquid state with remarkable physical properties. Here we have investigated the ambient pressure diffusive dynamics of water in microporous Kynol ACF-10 (average pore size ~11.6 Å, with primarily slit-like pores) from temperature T = 280 K in its stable liquid state down to T = 230 K into the metastable supercooled phase. The observed characteristic relaxation times and diffusion coefficients are found to be, respectively, higher and lower than those in bulk water, indicating a slowing down of the water mobility with decreasing temperature. The observed temperature-dependent average relaxation time ($${{\tau}}$$) when compared to previous findings indicate that it is the width of the slit pores-not their curvature-that primarily affects the dynamics of water for pore sizes larger than 10 Å. The experimental observations are compared to complementary molecular dynamics simulations of a model system, in which we studied the diffusion of water within the 11.6 Å gap of two parallel graphene sheets. We find generally a reasonable agreement between the observed and calculated relaxation times at the low momentum transfer Q (Q ≤ 0.9 Å -1). At high Q, however, where localized dynamics becomes relevant, this ideal system does not satisfactorily reproduce the measurements. Consequently, the simulations are compared to the experiments at low Q, where the two can be best reconciled. The best agreement is obtained for the diffusion parameter D associated with the hydrogen-site when a representative stretched exponential function, rather than the standard bimodal exponential model, is used to parametrize the self-correlation function I (Q,t).

Authors:
 [1];  [2];  [3];  [4];  [5];  [6];  [2]
  1. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Quantum Condensed Matter Division
  2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Chemical Sciences Division
  3. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Chemical and Engineering Science Division
  4. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Instrument and Source Division
  5. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Center for Nanophase Materials Science (CNMS)
  6. American Technical Trading, Inc., Pleasantville, NY (United States)
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Spallation Neutron Source (SNS)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1261423
Alternate Identifier(s):
OSTI ID: 1181535; OSTI ID: 1265416
Grant/Contract Number:  
AC05-00OR22725
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics
Additional Journal Information:
Journal Volume: 91; Journal Issue: 2; Journal ID: ISSN 1539-3755
Publisher:
American Physical Society (APS)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; Water Diffusion; Confined Fluids; Supercooled Water; Molecular Dynamics; Quasi-Elastic Neutron Scattering

Citation Formats

Diallo, S. O., Vlcek, L., Mamontov, E., Keum, J. K., Chen, Jihua, Hayes, J. S., and Chialvo, A. A.. Translational diffusion of water inside hydrophobic carbon micropores studied by neutron spectroscopy and molecular dynamics simulation. United States: N. p., 2015. Web. doi:10.1103/PhysRevE.91.022124.
Diallo, S. O., Vlcek, L., Mamontov, E., Keum, J. K., Chen, Jihua, Hayes, J. S., & Chialvo, A. A.. Translational diffusion of water inside hydrophobic carbon micropores studied by neutron spectroscopy and molecular dynamics simulation. United States. doi:10.1103/PhysRevE.91.022124.
Diallo, S. O., Vlcek, L., Mamontov, E., Keum, J. K., Chen, Jihua, Hayes, J. S., and Chialvo, A. A.. Tue . "Translational diffusion of water inside hydrophobic carbon micropores studied by neutron spectroscopy and molecular dynamics simulation". United States. doi:10.1103/PhysRevE.91.022124. https://www.osti.gov/servlets/purl/1261423.
@article{osti_1261423,
title = {Translational diffusion of water inside hydrophobic carbon micropores studied by neutron spectroscopy and molecular dynamics simulation},
author = {Diallo, S. O. and Vlcek, L. and Mamontov, E. and Keum, J. K. and Chen, Jihua and Hayes, J. S. and Chialvo, A. A.},
abstractNote = {When water molecules are confined to nanoscale spacings, such as in the nanometer-size pores of activated carbon fiber (ACF), their freezing point gets suppressed down to very low temperatures (~150 K), leading to a metastable liquid state with remarkable physical properties. Here we have investigated the ambient pressure diffusive dynamics of water in microporous Kynol ACF-10 (average pore size ~11.6 Å, with primarily slit-like pores) from temperature T = 280 K in its stable liquid state down to T = 230 K into the metastable supercooled phase. The observed characteristic relaxation times and diffusion coefficients are found to be, respectively, higher and lower than those in bulk water, indicating a slowing down of the water mobility with decreasing temperature. The observed temperature-dependent average relaxation time (${{\tau}}$) when compared to previous findings indicate that it is the width of the slit pores-not their curvature-that primarily affects the dynamics of water for pore sizes larger than 10 Å. The experimental observations are compared to complementary molecular dynamics simulations of a model system, in which we studied the diffusion of water within the 11.6 Å gap of two parallel graphene sheets. We find generally a reasonable agreement between the observed and calculated relaxation times at the low momentum transfer Q (Q ≤ 0.9 Å-1). At high Q, however, where localized dynamics becomes relevant, this ideal system does not satisfactorily reproduce the measurements. Consequently, the simulations are compared to the experiments at low Q, where the two can be best reconciled. The best agreement is obtained for the diffusion parameter D associated with the hydrogen-site when a representative stretched exponential function, rather than the standard bimodal exponential model, is used to parametrize the self-correlation function I (Q,t).},
doi = {10.1103/PhysRevE.91.022124},
journal = {Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics},
number = 2,
volume = 91,
place = {United States},
year = {Tue Feb 17 00:00:00 EST 2015},
month = {Tue Feb 17 00:00:00 EST 2015}
}

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Works referenced in this record:

Elucidating the porous structure of activated carbon fibers using direct and indirect methods
journal, January 1996