The origin of water’s anomalous properties remains to date elusive and a subject of debate. Among its anomalous behaviors is the deviation of dynamic quantities, such as diffusion and viscosity, from an Arrhenius temperature-dependence as it is supercooled below its freezing point. Moreover, upon cooling, the molecular diffusion of water and its bulk viscosity increasingly decouple, causing violation of the Stokes-Einstein (SE) relation. These anomalies have been attributed to transient spatial heterogeneities on the molecular scale which emerge in the supercooled regime.<br/><br/>Here we investigate the structural dynamics of water from ambient to supercooled temperatures from the perspective of diffusing spherical tracer particles. We employ dynamic light scattering to measure the temperature-dependence of the particles’ diffusion and compare the result with the SE relation and the self-diffusion of water. In addition, the effect of particle size on the diffusion dynamics is studied over two orders of magnitude (1-100 nm), with the smallest particle being polyhydroxylated fullerenes (PHF) and the larger silica nanoparticles.<br/><br/>We find that the SE relation holds for all particle sizes, i.e. down to the molecular size of hydrated PHF in the order of ~1 nm. Furthermore, the activation energies for diffusion obtained from Arrhenius analysis are similar (~23.1 kJ/mol) for all particles but is significantly higher than that for water molecules (~20.5 kJ/mol) in the investigated temperature range. This observation indicates that all nanoparticles studied effectively experience similar local viscosity independent of their size. In addition, we extract information about clustering effects, which in the case of PHF manifest dynamics distinct from that observed for single PHF particles.

Diffusive dynamics of nanoparticles in supercooled water

ACS Spring Meeting 2021

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