A neutron spin echo study of low-temperature water confined in the spherical silica pores of SBA-16

Abstract

The dynamic properties of heavy water (D₂O) and light water (H₂O) confined in porous silica SBA-16 were studied over a temperature range of 210–290 K by neutron spin echo measurements. SBA-16 has predominant spherical pores (7.1 nm in pore size), channels interconnecting the spherical pores, and micropores (corona). The coherent intermediate scattering function on D₂O filled SBA-16 showed the rotational dynamics of confined water without significant translational motion over the temperature range measured. This finding is due probably to collective entities of water due to cooperativity of hydrogen-bonds among water molecules in SBA-16 pores. The relaxation time of the collective entities followed the Vogel–Fulcher–Tammann relation at temperatures down to the freezing temperature of 235 K, suggesting a behavior of fragile water in the spherical pore. A comparison with previous NSE measurements of D₂O in MCM-41 showed that the collective entities of water in the SBA-16 spherical pores have higher rotational mobility than those in the MCM-41 cylindrical pores. On the other hand, the incoherent intermediate scattering function on H₂O filled SBA-16 revealed the translational motion of individual water molecules in the collective entities. It has been found that water in micropores is not frozen and is mobile down to 210 K from data of both D₂O and H₂O in SBA-16. Phase changes of various water confined in SBA-16 with decreasing and increasing temperatures are discussed based on the obtained dynamic properties.