Dynamic Crossovers and Stepwise Solidification of Confined Water: A ²H NMR Study

Abstract

(2)H NMR reveals two dynamic crossovers of supercooled water in nanoscopic (∼2 nm) confinement. At ∼225 K, a dynamic crossover of liquid water is accompanied by formation of a fraction of solid water. Therefore, we do not attribute the effect to a liquid-liquid phase transition but rather to a change from bulk-like to interface-dominated dynamics. Moreover, we argue that the α process and β process are observed in experiments above and below this temperature, respectively. Upon cooling through a dynamic crossover at ∼175 K, the dynamics of the liquid fraction becomes anisotropic and localized, implying solidification of the corresponding water network, most probably, during a confinement-affected glass transition.

2H NMR Studies on the Dynamics of Pure and Mixed Hydrogen-Bonded Liquids in Confinement

“…Figure 2a compares this NMR data with dielectric spectroscopy (DS) and neutron scattering (NS) results for water dynamics in similar confinements from the literature [46,47]. At high temperatures, NMR and NS results well agree and follow Vogel-Fulcher-Tammann (VFT) relations [43,47], τ 0 exp[B/(T − T 0 )]. At ~225 K, the NS data exhibit a sharp kink, which was taken as evidence for a FS transition related to a LL transition [47], whereas the NMR data show only a mild crossover.…”

“…Thus, liquid and solid fractions of water coexist inside the silica pores below this temperature, although calorimetric studies did not yield evidence for ice formation in such narrow confinement [4,36]. 2 H STE experiments allow us to follow the rotational dynamics of the liquid fraction to lower temperatures [43]. In Figure 1b, we see that the correlation functions cc 2 m ( ) F t obtained from such studies shift to longer times upon cooling, reflecting the slowdown of water reorientation, and decay in a strongly nonexponential manner (β K ≈ 0.3) [45], implying an existence of a broad distribution of correlation times G(log τ), i.e.…”

“…( ) F t with Kohlrausch fits. In panel (a), dark blue and light blue symbols indicate that the SLR relaxation step associated with the liquid fraction is exponential and nonexponential above and below ~185 K, respectively [43].…”

“…Detailed analysis of the 2 H SLR behavior led to the conclusion that the intermediate step can be assigned to a solid water fraction, e.g. distorted ice nuclei, emerging in the pore volume at ~223 K [10,43,44]. Thus, liquid and solid fractions of water coexist inside the silica pores below this temperature, although calorimetric studies did not yield evidence for ice formation in such narrow confinement [4,36].…”

“…Correlation times of water reorientation in the liquid fraction were obtained from analysis of the 〈T 1,l 〉 data using a CC spectral density and from fits of the cc 2 m ( ) F t decays with a Kohlrausch function [10,43,44]. Due to the existence of dynamical heterogeneities, it is useful to characterize water reorientation by an appropriate mean correlation time.…”