We investigate the intricate relationship among temperature, pH, and Brownian velocity in a range of differently sized upconversion nanoparticles (UCNPs) dispersed in water. These UCNPs, acting as nanorulers, offer insights into assessing the relative proportion of high-density and low-density liquid in the surrounding hydration water. The study reveals a size-dependent reduction in the onset temperature of liquid-water fluctuations, indicating an augmented presence of high-density liquid domains at the nanoparticle surfaces. The observed upper-temperature threshold is consistent with a hypothetical phase diagram of water, validating the two-state model. Moreover, an increase in pH disrupts the organization of water molecules, similar to external pressure effects, allowing simulation of the effects of temperature and pressure on hydrogen bonding networks. The findings underscore the significance of the surface of suspended nanoparticles for understanding high- to low-density liquid fluctuations and water behavior at charged interfaces.

Liquid water is the main constituent of the human body and covers a majority of the Earth’s surface. It plays a vital role in a myriad of biological, chemical, physical, geological, industrial, and environmental processes. (1−7) In addition to its chemical properties as a solvent, proton transfer medium, and active component of reactions, the physical properties of water are also fundamentally relevant. (8) Although water is the most commonly used liquid, its complex behavior under varying pressure and temperature conditions leads to numerous anomalies in its properties that differ significantly from those of other commonly used liquids. To date, more than 60 anomalous properties have been reported for water, (9−12) including a minimum specific heat at 308 K, a negative thermal expansion coefficient below 277 K, and a minimum isothermal compressibility at 319 K.

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