Distributions of single-molecule properties as tools for the study of dynamical heterogeneities in nanoconfined water

The explicit trend of the distribution functions of single-molecules rotational relaxation constants and atomic Mean-Square Displacement are used to study the dynamical heterogeneities in nanoconfined water. The trend of the single-molecule properties distributions is related to the dynamic heterogeneities and to the dynamic crossovers, found in water clusters of different shape and size and confined in a variety of zeolites. In all the cases that were considered, in spite of the different shape and size of the clusters. It is confirmed that the high temperature dynamical crossover (HTDC) occurring in the temperature range 200-230 K can be interpreted at a molecular level as the formation of almost translationally rigid clusters, characterized by some rotational freedom, hydrogen bond (HB) exchange and translational jumps as cage-to-cage processes. We also suggest a mechanism for the low temperature dynamical crossover (LTDC), falling in the temperature range 150-185 K, through which the adsorbed water clusters are made of nearly rigid sub-clusters, slightly mismatched, and thus permitting a relatively free librational motion at their borders. It appears that the condition for LTDC to occur is the presence of highly heterogeneous environments for the adsorbed molecules, letting some hydrogen bonds dangling or weaker than water-water hydrogen bonds. Under these conditions some dynamics is permitted also at very low temperature, even if most rotational motion is frozen. Therefore, it is unlikely, but a priori not excluded to find LTDC in supercooled bulk water, where no heterogeneous interface is present.