The Nature of the Low-Temperature Crossover of Water in Hard Confinement

The dynamics of water confined in mesoporous MIP (2-3nmpores in size) with silica gel (secondary silica; further, the abbreviationSG will be used) and MAP (10-35 nm pores in size) without SGborosilicate glasses have been studied by broadband dielectric spectroscopy(BDS), nuclear magnetic resonance (NMR), and differential scanningcalorimetry (DSC). MIP samples contain secondary silica inside thepores and provide a confinement size of about 2-3 nm, whereasMAP samples are free of secondary silica and provide a confinementsize of about 10-35 nm. It is shown by BDS and NMR techniquesthat water exhibits a dynamic crossover of around 180 K when it isconfined in MIP samples. By contrast, water confined in larger pores(MAP) does not exhibit any changes in its relaxation behavior. Itis also shown that the crossover temperature depends on the hydrationlevel (the higher the hydration level, the lower the crossover temperature).Below the crossover temperature, we find that water reorientationis isotropic (NMR) and that the temperature-dependent dielectric relaxationstrength (BDS) follows the tendency expected for a solid-like material.In contrast, water reorientation is related to long-range diffusionabove the crossover temperature, and the dielectric relaxation strengthfollows the tendency expected for a liquid-like material. Furthermore,the calorimetric results are compatible with crossing a glass transitionnear 180 K. Finally, the results are discussed within the Gibbs-Thomsonmodel. In this framework, the crossover could be related to ice crystalsmelting.

Automated summary

The dynamics of water confined in mesoporous materials were studied using a combination of techniques including broadband dielectric spectroscopy, nuclear magnetic resonance, and differential scanning calorimetry. The results show that water confined in MIP samples exhibits a dynamic crossover at around 180 K, while water confined in MAP samples does not show any changes in relaxation behavior. The crossover temperature is found to depend on the hydration level. Below the crossover temperature, water reorientation is isotropic and the dielectric relaxation strength follows a solid-like behavior, while above the crossover temperature, water reorientation is related to long-range diffusion and the dielectric relaxation strength follows a liquid-like behavior. The results are also consistent with a glass transition near 180 K.