At solid-liquid interfaces, water molecules interact with the surface to present a non-uniform three-dimensional distribution (i.e. hydration structure). Such hydration phenomena play important roles in various interfacial functions and phenomena, yet the detailed mechanisms have largely remained elusive. To address this issue, a hydration structure measurement technique using atomic force microscopy has recently attracted much attention. This method allows us to directly visualize subnanometer-scale hydration structures and hence can significantly advance our understanding on the hydration phenomena. In this review, here we summarize the technological development and applications of this emerging technology.
Hydration behavior is essential information for a discussion on physicochemical characteristics of polymeric materials including polysaccharides in aqueous solution. In addition to a classical acoustic method, dielectric spectroscopic measurements over an extremely high frequency range beyond the relaxation frequency of water molecules are useful techniques to determine hydration numbers of solute molecules. Since this technique also provides information about how long hydrated water molecules stay in their hydration sites, i.e. a hydration lifetime, more detailed consideration of hydration behavior in aqueous systems has been dramatically developed in these years.
To answer the simple question, i.e., does water have a specific role for the self-assembly of amphiphilic molecules such as lipids and surfactants?, we have investigated the precise hydration states of the amphiphilic molecules and the relation between the state and the formation of the aggregated structures. From the results using THz spectroscopy to measure the hydration states including slightly bound water as the hydration water, it has been found that the long-range hydration states strongly relate to the structural phase transitions (such as micelle to lamellar) and to the interactions between bilayers.
The objective of the present report is to review literature concerning the different insights of the water structure obtained by vibrational spectroscopic and calorimetric methods. Water sorbed into the solid polymers is categorized into "free", "intermediate", and "non-freezing" waters based on its freezing/melting behaviors observing by differential scanning calorimetry (DSC). Crystallization behavior of water sorbed into various non-water soluble polymers was examined by DSC and temperature-variable infrared spectroscopy (TV-IR). In general understanding of the sorbed water at low water content on the basis of DSC, all of the sorbed water has been considered to be non-freezing water. However, their TV-IR spectra clearly demonstrated the existence of the change in state (condensation, crystallization, vapor-deposition, sublimation, fusion, and vaporization) including recrystallization of water by revapor deposition, which was vapor-deposition (sublimation) during heating. Recrystallization water being categorized into intermediate water has been proposed to be devitrification of amorphous ice. These results require to reconsider the definition of water structure based on DSC.