Complementary techniques for investigating the molecular dynamics of uncrystallized water

Information from the abstract

Uncrystallized water (UCW) refers to the fraction of water in hydrated materials that remains uncrystallized upon cooling. Rather than forming bulk ice, this water retains molecular mobility even below the nominal freezing point. In polymers and proteins, UCW includes strongly bound water associated with specific functional groups and more weakly confined water residing in nanoscale environments. As a result, it plays an important role in mechanical behavior, structural stability, and biological function. Because UCW is heterogeneous, no single technique can fully capture its properties, and multiple complementary methods are required. This review highlights how dielectric spectroscopy (DS), nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), neutron scattering, particularly quasielastic neutron scattering (QENS), and terahertz and infrared spectroscopies each probe different aspects of UCW across distinct time and different sample sizes. We outline what each technique measures, the models commonly used to interpret the data, and how combining these approaches helps distinguish between “free,” “bound,” and “non-freezable” water. Studies that apply multiple methods to the same samples, with consistent control of hydration and temperature, are essential for developing a coherent and quantitative picture of UCW. This integrated approach is also key to translating UCW behavior into practical design principles for cryopreservation, pharmaceutical freezing, and biomaterials. Importantly, complementarity arises from differences in timescale and the distinct correlation functions each method probes. Computational spectroscopy–ranging from classical molecular dynamics to emerging machine learning approaches–provides an atomistic link between dielectric, NMR, neutron, and vibrational measurements and shared descriptors of hydration dynamics.

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