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 inves- tigated the ambient pressure diffusive dynamics of water in microporous KynolTMACF-10 (average pore size ∼11.6 ̊A, 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 re- laxation 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 size of the confining pores - not their shape - that primarily affects the dy- namics of water for pore sizes larger than 10 ̊A. The experimental observations are compared to complementary molecular dynamics simulations of a model system, in which we studied the diffu- sion of water within the 11.6 ̊A 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 ̊A−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).
