NMR studies on the temperature-dependent dynamics of confined water

Abstract

We use (2)H NMR to study the rotational motion of supercooled water in silica pores of various diameters, specifically, in the MCM-41 materials C10, C12, and C14. Combination of spin-lattice relaxation, line-shape, and stimulated-echo analyses allows us to determine correlation times in very broad time and temperature ranges. For the studied pore diameters, 2.1-2.9 nm, we find two crossovers in the temperature-dependent correlation times of liquid water upon cooling. At 220-230 K, a first kink in the temperature dependence is accompanied by a solidification of a fraction of the confined water, implying that the observed crossover is due to a change from bulk-like to interface-dominated water dynamics, rather than to a liquid-liquid phase transition. Moreover, the results provide evidence that α process-like dynamics is probed above the crossover temperature, whereas β process-like dynamics is observed below. At 180-190 K, we find a second change of the temperature dependence, which resembles that reported for the β process of supercooled liquids during the glass transition, suggesting a value of Tg ≈ 185 K for interface-affected liquid water. In the high-temperature range, T > 225 K, the temperature dependence of water reorientation is weaker in the smaller C10 pores than in the larger C12 and C14 pores, where it is more bulk-like, indicating a significant effect of the silica confinement on the α process of water in the former 2.1 nm confinement. By contrast, the temperature dependence of water reorientation is largely independent of the confinement size and described by an Arrhenius law with an activation energy of Ea ≈ 0.5 eV in the low-temperature range, T < 180 K, revealing that the confinement size plays a minor role for the β process of water.

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

“…To address the size aspect, we compare correlation times of water reorientation in MCM-41 C10 (d = 2.1 nm) and C14 (d = 2.8 nm). In Figure 2b, we observe that the temperature dependence is higher and, hence, more bulk-like in the wider pores at high temperatures, while the correlation times have comparable temperature dependence at low temperatures, where they follow an Arrhenius law with an activation energy E a ≈ 0.5 eV [43,45], in harmony with DS results for water at various types of inner surfaces [10,49,50]. As a consequence, the dynamic crossover is sharper in MCM-41 C14 than in MCM-41 C10.…”

“…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. pronounced dynamical heterogeneities.…”

Structure and dynamics of water in nanoscopic spheres and tubes

“…[3][4][5][6][7][8][9][10][11][12][13][14][15] The properties of water in nanoconfinement have been studied with model systems like reverse micelles (water-in-oil microemulsions) [3][4][5][6][7][8][9][10][11] and nanoporous materials. [12][13][14][15] Reverse micelles in particular have proven to be convenient model systems because of the ease of sample preparation and the possibility of controlling the water-pool size. In addition, the chemical nature of the interface of the reverse micelle can be conveniently modified using cationic, anionic, or nonionic surfactants.…”

“…Studies on reverse micelles have focused mainly on the effect of size in the case of spherical confinement, [3][4][5][6][7][8][9][10][11] and investigations on nanoporous materials are restricted to cylindrical geometries (channels). [12][13][14][15][20][21][22][23] Reverse micelles can adopt spherical or cylindrical shapes, depending on the combination of apolar solvent and surfactant. [24][25][26][27][28] Hence, reverse micelles are excellent systems to investigate the effect of the shape of nanoconfined water volume on its dynamics.…”

Thermal perturbation of NMR properties in small polar and non-polar molecules

“…The ability to understand and interpret the signatures of water under reactive environments has become increasingly important as in situ and operando NMR studies become more prominent. The non-destructive insight provided for applications such as geochemistry or catalysis reveals much about the chemical systems, where the structure of water in porous materials and on surfaces play an important role in reactivity and modifying the active centers [4][5][6][7][8] . In protonated MFI zeolite, for instance, water has been shown to interact with Brønsted acid sites to form hydrated hydronium ions which can serve as the catalytically active center 4 .…”