(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.

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.

We use differential scanning calorimetry, broadband dielectric spectroscopy, and deuteron nuclear magnetic resonance to investigate water dynamics in MCM-41 pores with a diameter of d = 2.5 nm. At high pore fillings, partial crystallization at T m = 221 K leads to a dynamic crossover. The reorientation of all water molecules shows non-Arrhenius temperature dependence above 221 K, while two Arrhenius processes associated with liquid and crystalline water species can be distinguished below this temperature. Thus, the confined liquid water fraction exhibits an apparent fragile-to-strong transition as a consequence of a reduction of the accessible pore volume to a narrow interfacial layer during fractional freezing. The confined crystalline water fraction shows considerable dynamics with water reorientation in the milliseconds regime near 200 K. At low pore fillings, we observe neither partial crystallization nor a dynamic crossover.

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Abstract:2 H NMR is used to ascertain dynamical behaviors of pure and mixed hydrogen-bonded liquids in bulk and in confinement. Detailed comparisons of previous and new results in broad dynamic and temperature ranges reveal that confinement effects differ for various liquids and confinements. For water, molecular reorientation strongly depends on the confinement size, with much slower and less fragile structural relaxation under more severe geometrical restriction. Moreover, a dynamical crossover occurs when a fraction of solid water forms so that the dynamics of the fraction of liquid water becomes even more restricted and, as a consequence, changes from bulk-like to interface-dominated. For glycerol, by contrast, confinement has weak effects on the reorientation dynamics. Mixed hydrogen-bonded liquids show even more complex dynamical behaviors. For aqueous solutions, the temperature dependence of the structural relaxation becomes discontinuous when the concentration changes due to a freezing of water fractions. This tendency for partial crystallization is enhanced rather than reduced by confinement, because different liquid-matrix interactions of the molecular species induce micro-phase segregation, which facilitates ice formation in water-rich regions. In addition, dynamical couplings at solvent-protein interfaces are discussed. It is shown that, on the one hand, solvent dynamics are substantially slowed down at protein surfaces and, on the other hand, protein dynamics significantly depend on the composition and, thus, the viscosity of the solvent. Furthermore, a protein dynamical transition occurs when the amplitude of water-coupled restricted backbone dynamics vanishes upon cooling.

Mesoporous silica MCM-41 is prepared, for which the inner surfaces are modified by 3-(aminopropyl)triethoxysilane (APTES) in a controlled manner. Nitrogen gas adsorpition yields a pore diameter of 2.2 nm for the APTES functionalized MCM-41.2H nuclear magnetic resonance (NMR) and broadband dielectric spectroscopy (BDS) provide detailed and consistent insights into the temperature-dependent reorientation dynamics of water in this confinement. We find that a liquid water species becomes accompanied by a solid water species when cooling through ~210 K, as indicated by an onset of bimodal2H spin-lattice relaxation. The reorientation of the liquid water species is governed by pronounced dynamical heterogeneity in the whole temperature range. Its temperature dependence shows a mild dynamic crossover when the solid water species emerges and, hence, the volume accessible to the liquid water species further shrinks. Therefore, we attribute this variation in the temperature dependence to a change from bulk-like behavior towards interface-dominated dynamics. Below this dynamic crossover,2H line-shape and stimulted-echo studies show that water reorientation becomes anisotropic upon cooling, suggesting that these NMR approaches, but also BDS measurements do no longer probe the structural (α) relaxation, but rather a secondary (β) relaxation of water at sufficiently low temperatures. Then, another dynamic crossover at ~180 K can be rationalized in terms of a change of the temperature dependence of theβrelaxation in response to a glassy freezing of theαrelaxation of confined water. Comparing these results for APTES modied MCM-41 with previous findings for mesoporous silica with various pore diameters, we obtain valuable information about the dependence of water dynamics in restricted geometries on the size of the nanoscopic confinements and the properties of the inner surfaces.

In this study, we have combined dielectric spectroscopy and 2H NMR to elucidate the molecular dynamics of aqueous solutions of dipropylene glycol monomethyl ether (DiPGME) confined into 2.8 nm pores of MCM-41. The results show that the concentration dependence of the dynamics is completely different compared to the corresponding bulk solutions, where a pronounced nonmonotonic concentration dependence was observed for the glass transition and its related α-relaxation. In the confinement, both the cooperative α-relaxation and the more local β-relaxation are almost unaffected by the water concentration. The main reasons for this seem to be that there is a preferential hydration of the inner pore surfaces, leading to a strong concentration gradient in the pores, as well as ice formation at higher water concentrations (45 wt % and above during heating), also leading to less water and a weaker concentration dependence in DiPGME-rich regions. The β-process is observed in the DS measurements even for confined DiPGME, without any water. This implies that the β-relaxation is strongly enhanced, compared to the α-relaxation, in the confinement, since it could not be clearly observed in the bulk liquid. A β-relaxation due to water was observed in the bulk solutions, but this process was rapidly speeding up with increasing water concentration, while it is basically concentration independent in the confinement. From the NMR measurements, it was also possible to conclude that the α-relaxation of the confined solutions is composed of a number of consecutive small-angle elementary rotational jumps, and that the β-process is related to a spatially restricted motion.

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