THERMAL-PROPERTY, STRUCTURE, AND DYNAMICS OF SUPERCOOLED WATER IN POROUS SILICA BY CALORIMETRY, NEUTRON-SCATTERING, AND NMR RELAXATION

Thermal properties, structure, and dynamics of supercooled water in po rous silica of two different pore sizes (30 and 100 Angstrom) have bee n investigated over a temperature range from 298 down to 193 K by diff erential scanning calorimetry (DSC), neutron diffraction, neutron quas i-elastic scattering, and proton NMR relaxation methods. Cooling curve s by DSC showed that water in the 30 Angstrom pores freezes at around 237 K, whereas water in the 100 Angstrom, pores does at 252 K. Neutron diffraction data for water in the 30 Angstrom pores revealed that wit h lowering temperatures below 237 K hydrogen bond networks are gradual ly strengthened, the structure correlation being extended to 10 Angstr om at 193 K. It has also been found that crystalline ice is not formed in the 30 Angstrom pores in the temperature range investigated, where as cubic ice (I-c) crystallizes in the 100 Angstrom pores at 238 K. Th e self-diffusion coefficients of water protons in both pores determine d from the quasi-elastic neutron scattering measurements showed that t he translational motion of water molecules is slower by a factor of tw o in the 30 Angstrom pores than in the 100 Angstrom pores, the motion of water molecules in the 100 Angstrom pores being comparable with tha t of bulk water. The self-diffusion coefficients of water in both pore s at different temperatures showed that the translational motion of wa ter molecules is gradually restricted with decreasing temperature. The spin-lattice relaxation time (T-1) and the spin-spin relaxation time (T-2) data obtained by the proton NMR relaxation experiments suggested that the motions of water molecules in the 100 Angstrom pores are fas ter by a factor of 2-3 than those of water molecules in the 30 Angstro m pores. The peak area, the half-width at half maximum, the relaxation rates (T-1(-1) and T-2(-1)) of water molecules at the various tempera tures all showed an inflection point at 238 and 253 K for the 30 and 1 00 Angstrom pores, respectively, suggesting the freezing of water in t he pores.