Thermal Property, Structure, and Dynamics of Supercooled Water in Porous Silica by Calorimetry, Neutron Scattering, and NMR Relaxation

Abstract

Thermal properties, structure, and dynamics of supercooled water in porous silica of two different pore sizes (30 and 100 Å) have been investigated over a temperature range from 298 down to 193 K by differential scanning calorimetry (DSC), neutron diffraction, neutron quasi-elastic scattering, and proton NMR relaxation methods. Cooling curves by DSC showed that water in the 30 Å pores freezes at around 237 K, whereas water in the 100 Å pores does at 252 K. Neutron diffraction data for water in the 30 Å pores revealed that with lowering temperatures below 237 K hydrogen bond networks are gradually strengthened, the structure correlation being extended to 10 Å at 193 K. It has also been found that crystalline ice is not formed in the 30 Å pores in the temperature range investigated, whereas cubic ice ( I c ) crystallizes in the 100 Å pores at 238 K. The self-diffusion coefficients of water protons in both pores determined from the quasi-elastic neutron scattering measurements showed that the translational motion of water molecules is slower by a factor of two in the 30 Å pores than in the 100 Å pores, the motion of water molecules in the 100 Å pores being comparable with that of bulk water. The self-diffusion coefficients of water in both pores at different temperatures showed that the translational motion of water 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 Å pores are faster by a factor of 2−3 than those of water molecules in the 30 Å 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 temperatures all showed an inflection point at 238 and 253 K for the 30 and 100 Å pores, respectively, suggesting the freezing of water in the pores.