An NMR study on the mechanisms of freezing and melting of water confined in spherically mesoporous silicas SBA-16

Abstract

Thermodynamic and dynamic properties of water confined in mesoporous silica glass SBA-16 were investigated by DSC, and (1,2)H NMR spectroscopy and (2)H NMR spin-lattice relaxation time (T1) as a function of pore size. SBA-16 possesses the main spherical pores, interconnecting channels and micropores (corona). Water in the characteristic spherical pores of SBA-16 freezes at the homogeneous nucleation temperature of water. Between room and freezing temperatures, the correlation time of the isotropic rotation of water in the pores of SBA-16 followed the Vogel-Fulcher-Tammann (VFT) relation, which reflects the formation and growth of clusters of fragile water for changing to the strong water. The vitrification of water in micropores around 200 K was observed by (2)H NMR. Above 200 K, the correlation time of the rotation of water in micropores exhibited non-Arrhenius behavior, which is correlated with the gradual decrease in the mobility of water due to the growth of hydrogen bonding, forming low density water before vitrification. After vitrification, the activation energy of the rotation of water in micropores was 25-33 kJ mol(-1), which was similar to that in ice Ih for all samples. The freedom of cluster formation and water rotation increased with the increasing the pore size.

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

“…Our finding that the freezing point of the confined water shifted to lower temperatures with a decrease in pore size is consistent with that determined by previous measurements. [16][17][18][19][20][21][22] In addition, previous studies including ours have shown that the confined water molecules are composed of at least two phases, such as a bulk-like water phase having hydration structure and an adsorbed water phase near the surfaces at an ambient temperature, and two-state or core/shell models have been applied. 14,15,30,31 These results suggest that a bulk-like hydration water phase and an adsorbed water phase are ascribed to water that freezes and does not freeze below freezing point in both 1 nm-and 10 nm-scale environments, respectively.…”

“…[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. [26][27][28][29] These results have suggested that the specific hydrogen bonding network of water layer on surfaces differing from bulk water, i.e., the adsorbed water phase, is responsible for causing the freezing behavior of confined water in the supercooled state.…”

Small Molecules, Non-Covalent Interactions, and Confinement

“…To tackle this problem, 2 H NMR was used to investigate reorientation dynamics of unfreezable water (D 2 O) in MCM-41 and SBA-15 pores over wide temperature ranges towards the glass transition [ 45 , 176 , 181 , 183 , 213 , 214 , 215 , 216 , 217 , 218 , 219 ]. In particular, spin-lattice relaxation, line-shape analysis, and stimulated-echo experiments were combined to ensure broad dynamic ranges and the pore-size was systematically varied to study possible finite-size effects.…”

Specific surface area and neutron scattering analysis of water’s glass transition and micropore collapse in amorphous solid water

“…4 We noticed that Miyatou et al recently observed the vitrification of water in micropores around 200 K via their 2 H NMR measurement. 5 The microscopic origin of the above is: The rotation of water in micropores is strongly affected by the growth of a hydrogen bond before forming low density non-crystalline ice in this temperature range. 5 Similarly Roussenova et al based on the positron annihilation lifetime spectroscopy measurements observed the glass transition temperature of water in confinement around 6 190 K. The latter is a little higher than that (glass transition temperature of water: 185 K) observed by Sattig et al 7 Although a glass transition temperature of (bulk supercooled) water around 160 K or above (due to the development of an energetically more stable hydrogen-bonding network of water molecules) was proposed after using the adiabatic calorimetry.…”

“…5 The microscopic origin of the above is: The rotation of water in micropores is strongly affected by the growth of a hydrogen bond before forming low density non-crystalline ice in this temperature range. 5 Similarly Roussenova et al based on the positron annihilation lifetime spectroscopy measurements observed the glass transition temperature of water in confinement around 6 190 K. The latter is a little higher than that (glass transition temperature of water: 185 K) observed by Sattig et al 7 Although a glass transition temperature of (bulk supercooled) water around 160 K or above (due to the development of an energetically more stable hydrogen-bonding network of water molecules) was proposed after using the adiabatic calorimetry. We remind the readers that using data obtained for neutron diffraction from ice nucleation Seyed-Yazdi et al demonstrated that several defective ice states contribute to the measured diffraction pattern for the nucleated solid phase of water in confined geometry (their results suggested that the phase relationship of ices in confined geometry is more complex than has been previously realized).…”