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 TM ACF-10 (average pore size ∼11.6Å, 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 10Å. The experimental observations are compared to complementary molecular dynamics simulations of a model system, in which we studied the diffusion of water within the 11.6Å 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 ≤ 0.9Å −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 bi-modal exponential model, is used to parameterize the self-correlation function I(Q, t) .

Self-diffusion coefficient of bulk and confined water: a critical review of classical molecular simulation studies

“…They observed that the self-diffusion coefficient increases with the temperature and is higher for lower adsorption loadings. Diallo et al [212] also simulated water confined by activated carbon fibers. They evaluated the diffusion coefficient of supercooled water (220 ≤ T ≤ 280 K) and compared the results with experimental data from quasi-elastic neutron scattering.…”

Molecular dynamics simulation study of meso-confined propane in TiO2

“…The interaction of a fluid with either a 2D or 3D substrate gives rise to interesting properties at the interface which are absent in the bulk phase [36]. Pore size, shape, connectivity and surface roughness (fractality) may produce somewhat different results compared to a simple 2D surface as pore dimensionality is reduced to the point where the sorption layers start to overlap [37][38][39].…”

Water under extreme confinement in graphene: Oscillatory dynamics, structure, and hydration pressure explained as a function of the confinement width

“…Experiments in slit pores show that water forms layers parallel to the walls [72]. Computer simulations of water between two nanoscopic hydrophobic plates reveal that these layers correspond to local minima in the free-energy profile as a function of the plate-plate separation w [73] and that the water mobility in a hydrophobic slit pore monotonically increases as w becomes larger [74,75,76,77,78,79,80,81,82,83]. Similar results hold for water confined in graphite [84] and quartz [45,46,76], with freezing of the dynamics at sub-nm confinement.…”