Characterization and quantitative analysis of unfrozen water at ultra-low temperatures: Insights from NMR and MD study

Unfrozen water, integral to numerous processes such as heat transfer, frost heave, and hydro-thermo-mechanical simulations, has been traditionally studied at 0 ∼ −30 °C, but remains under-investigated at ultra-low temperatures. Understanding this component at ultra-low temperatures and its categorization requires more explorative, quantitative research, particularly considering the ubiquity of frozen soils. The nuclear magnetic resonance (NMR) and molecular dynamics (MD) methods were employed to characterize and quantify unfrozen water during 0 ∼ −80 °C. Our results indicated that bulk and capillary water could completely freeze at −3°C and −5°C, respectively, but only bound water exists below −10 °C. The evolution of unfrozen water with temperature in MD agreed with our NMR results, where the existence of unfrozen water molecules was due to the breakage of hydrogen bonds in ice molecules and the surface effect of clay. This mechanism elucidated the water–ice-clay atomic system’s role in quantifying experimentally measured unfrozen water content. These findings have important implications for frozen soil engineering, polar region development, artificial freezing technology, and lunar soil exploration.