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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. 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 < τ > 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 reasonablemore » 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 ) .« less

Authors:
; ; ; ; ; ;
Publication Date:
Research Org.:
Energy Frontier Research Centers (EFRC) (United States). Fluid Interface Reactions, Structures and Transport Center (FIRST)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1386365
DOE Contract Number:  
ERKCC61
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics; Journal Volume: 91; Journal Issue: 2; Related Information: FIRST partners with Oak Ridge National Laboratory (lead); Argonne National Laboratory; Drexel University; Georgia State University; Northwestern University; Pennsylvania State University; Suffolk University; Vanderbilt University; University of Virginia
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 36 MATERIALS SCIENCE; catalysis (heterogeneous), solar (fuels), energy storage (including batteries and capacitors), hydrogen and fuel cells, electrodes - solar, mechanical behavior, charge transport, materials and chemistry by design, synthesis (novel materials)

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. Sun . "Translational diffusion of water inside hydrophobic carbon micropores studied by neutron spectroscopy and molecular dynamics simulation". United States. doi:10.1103/PhysRevE.91.022124.
@article{osti_1386365,
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. 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 < τ > 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 = {Sun Feb 01 00:00:00 EST 2015},
month = {Sun Feb 01 00:00:00 EST 2015}
}