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

Abstract

The dynamics of water confined in mesoporous MIP (2–3 nm pores in size) with silica gel (secondary silica; further, the abbreviation SG will be used) and MAP (10–35 nm pores in size) without SG borosilicate glasses have been studied by broadband dielectric spectroscopy (BDS), nuclear magnetic resonance (NMR), and differential scanning calorimetry (DSC). MIP samples contain secondary silica inside the pores and provide a confinement size of about 2–3 nm, whereas MAP samples are free of secondary silica and provide a confinement size of about 10–35 nm. It is shown by BDS and NMR techniques that water exhibits a dynamic crossover of around 180 K when it is confined in MIP samples. By contrast, water confined in larger pores (MAP) does not exhibit any changes in its relaxation behavior. It is also shown that the crossover temperature depends on the hydration level (the higher the hydration level, the lower the crossover temperature). Below the crossover temperature, we find that water reorientation is isotropic (NMR) and that the temperature-dependent dielectric relaxation strength (BDS) follows the tendency expected for a solid-like material. In contrast, water reorientation is related to long-range diffusion above the crossover temperature, and the dielectric relaxation strength follows the tendency expected for a liquid-like material. Furthermore, the calorimetric results are compatible with crossing a glass transition near 180 K. Finally, the results are discussed within the Gibbs–Thomson model. In this framework, the crossover could be related to ice crystals melting.

Hydration effects on thermal transitions and molecular mobility in Xanthan gum polysaccharides

“…Therefore, the properties of this so-called interfacial or hydration water may reveal specific features of water–polymer interactions. 15,21–25 Among other issues, the non-freezing fraction of adsorbed water, 18,26,27 the confinement of water molecules 28,29 and the process of ice formation at sub-zero temperatures and the dynamics in partially crystallized water mixtures 30–37 are topics that provide insight into the properties of hydration water. Especially for polysaccharides, it is known that a small amount of water significantly affects the T g of the hydrated polysaccharide by disrupting inter-molecular hydrogen bonds.…”

Impact of Interface Modification on the Behavior of Phenyl Alcohols within Silica Templates

“…Previous investigations have revealed that materials adjacent to a solid and near-surface can exhibit interfacial regions of reduced density, − interfacial freezing, − interfacial melting, − molecular layering, − molecular orientation, or specific lateral molecular arrangements . Consequently, significant deviations in their properties, such as flow and intermolecular entanglements, local chain mobility, glass transition temperature, diffusion, tautomerization, or crystallization, are noted when compared to bulk samples. − Due to these facts, confined systems hold promise for diverse industrial applications, including nanolithography, lubrication, paints, surface treatments, and elastic membranes.…”