Low-Temperature Phase Transition of Water Confined in Mesopores Probed by NMR. Influence on Pore Size Distribution

The NMR signal intensity vs temperature (IT curve) of water confined in mesoporous materials (pore radius lager than 10 Å) reveals one or more "high-temperature" transitions above 222 K, which are dependent on pore size, and an additional transition temperature below 209 K, which is independent of pore dimension. This latter transition shows no hysteresis effect or discontinuity and contributes to more than 65% of the total water content of the porous materials investigated and is explained as interfacial water in contact with the surface of the matrix and the solid ice phase. The thickness of this interface water is estimated to be 5.4 ± 1.0 Å (cylindrical pores). It is shown that the observed NMR intensity of water associated with the "high-temperature" transition phases has to be corrected in order to present the actual amount of water within these phases. It is further demonstrated that these intensity corrections must be implemented in the pore size distribution functions to give quantitative results. The significance of the correction factors increases with decreasing pore radius.