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.

Powder X-ray diffraction observations of ice crystals formed from disaccharide solutions

“…This anomalous stability has been reported in other solution systems (including glycerol, 20-21 glucose, 22-23 salts, 24 or aqueous aerosol 28 ), or in the confined space systems (in porous silica 35 or in emulsion 39 ). Then the ice Ic phase is considered to be stabilized by foreign molecules.…”

“…24 In a pure water system, ice Ic can be obtained by the following methods: 1) from the high-pressure phase of ice (ice III or VII) by a decrease of the pressure during an increase of temperature, 25 2) from the temperature increase of amorphous ice condensed on a cold surface (below 110 K), 19 Other earlier studies had revealed that the ice Ic formed in confined space. 35-39 Takamuku et al 35 reported that ice Ic was formed from water existed in a porous silica of 3 nm and 10 nm in diameter. It was found that the ice Ic formed in the confined space was also more stable than that in the pure water system.…”

Temperature and Size Effects on Structural and Dynamical Properties of Water Confined in 1 – 10 nm-scale Pores Using Proton NMR Spectroscopy

“…Neutron scattering, X-ray diffraction, optical Kerr effect, nuclear magnetic resonance (NMR), and differential scanning calorimetry (DSC) measurements have shown that the freezing point of water confined in about 3 nm pores is about 230 -240 K, which then approaches the homogeneous nucleation temperature of bulk water, and shifts to a higher temperature with increasing sizes. [16][17][18][19][20] Recent spectroscopic studies have reported that water molecules confined in pores smaller than 2 nm do not freeze even around 200 K and the transition from high-density to lowdensity hydrogen bonding structures occurs. [21][22][23][24][25] The slowing down of both rotational and/or translational motions of confined water molecules has been also verified to be promoted with decreasing pore sizes and temperatures.…”

Dielectric study on two dynamic phases of 1-propanol confined in mesopores of MCM-41

“…This subject has long been studied from various viewpoints [1][2][3][4][5][6] and carry accelerating progress since the development of synthetic porous silica having well-defined pore structures starting from FSM-16, MCM-41, SBA-15 etc. [7][8][9][10][11][12][13][14][15][16][17] Among many fluids, water has been one of the most important targets to study.…”