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When water molecules are confined to nanoscale spacings, such as in thenanometer size pores of activated carbon fiber (ACF), their freezing point getssuppressed down to very low temperatures ($\sim$ 150 K), leading to ametastable liquid state with remarkable physical properties. We haveinvestigated the ambient pressure diffusive dynamics of water in microporousKynol\texttrademark ACF-10 (average pore size $\sim$11.6 {\AA}, with primarilyslit-like pores) from temperature $T=$ 280 K in its stable liquid state down to$T=$ 230 K into the metastable supercooled phase. The observed characteristicrelaxation times and diffusion coefficients are found to be respectively higherand lower than those in bulk water, indicating a slowing down of the watermobility with decreasing temperature. The observed temperature-dependentaverage relaxation time $<\tau>$ when compared to previous findings indicatethat it is the size of the confining pores - not their shape - that primarilyaffects the dynamics of water for pore sizes larger than 10 {\AA}. Theexperimental observations are compared to complementary molecular dynamicssimulations of a model system, in which we studied the diffusion of waterwithin the 11.6 {\AA} gap of two parallel graphene sheets. We find generally areasonable agreement between the observed and calculated relaxation times atthe low momentum transfer $Q$ ($Q\le 0.9$ \AA${^{-1}}$). At high $Q$ however,where localized dynamics becomes relevant, this ideal system does notsatisfactorily reproduce the measurements. The best agreement is obtained forthe diffusion parameter $D$ associated with the hydrogen-site when arepresentative stretched exponential function, rather than the standardbi-modal exponential model, is used to parameterize the self-correlationfunction $I(Q,t)$.

Diffusive Dynamics of Water inside Hydrophobic Carbon Micropores Studied by Neutron Spectroscopy and Molecular Dynamics Simulation