Elucidating the State of Water in Saccharide Solutions by a New Spectroscopic Approach Using Infrared and Far-Infrared Spectra

Abstract This study pioneers the concept of partial molar properties using infrared (IR) and far-infrared (FIR) spectra, establishing a novel framework for evaluating hydration structures in solutions. We acquired the IR and FIR spectra of a series of aqueous saccharide solutions to derive spectroscopic parameters sensitive to both the local and the collective hydrogen-bonding environments of water. The linear dependence of these parameters on the saccharide mole fraction facilitated the definition of IR- and FIR-derived partial molar properties, which quantify the contribution of a single mole of the solute to the overall hydration structure of the aqueous system. The IR-derived properties exhibited a positive correlation with nuclear magnetic resonance hydration parameters and the viscosity B coefficient, whereas the FIR-derived counterparts exhibited a negative correlation with the viscosity C coefficient. These relationships imply that the IR parameter is predominantly sensitive to hydrogen bond structuring within the immediate hydration shell of the solute. Conversely, the FIR parameter reflects modifications in the cooperative dynamics and fluidity of the hydrogen-bonding network extending toward bulk water. Collectively, our findings provide robust spectroscopic evidence for two distinct, hierarchical hydration domains in aqueous saccharide solutions: a locally structured hydration shell immediately surrounding the solute and a more bulk-like hydrogen-bonding network characterized by enhanced mobility. Consequently, this study establishes a new spectroscopic method based on partial molar properties, offering a universally applicable strategy for investigating hydration phenomena in complex aqueous systems.