Low-Temperature Dynamics of Water Confined in Unidirectional Hydrophilic Zeolite Nanopores

The dynamical properties of water molecules confinedin the unidirectionalhydrophilic nanopores of AlPO4-54 are investigated withquasi-elastic neutron scattering as a function of temperature downto 118 K. AlPO4-54 has among the largest pores known foraluminophosphates and zeolites (about 1.3 nm), though they are smallenough to prevent water crystallization due to the high degree ofconfinement. Water molecular diffusion into the pore is here measureddown to 258 K. Diffusion is slower than in bulk water and has an activationenergy of E (a) = (20.8 +/- 2.8) kJ/mol,in agreement with previous studies on similar confining media. Surprisingly,local hydrogen dynamics associated with water reorientation is measureddown to temperatures (118 K), i.e., well below the expected glasstransition temperature of bulk water. The reorientational time scaleshows the well-known non-Arrhenius behavior down to the freezing ofwater mass diffusion, while it shows a feeble temperature dependencebelow. This fast local dynamics, of the order of fractions of nanoseconds,is believed to take place in the dense, highly disordered amorphouswater occupying the pore center, indicating its possible plastic nature.