Effects of Partial Crystallization on the Dynamics of Water in Mesoporous Silica

Abstract

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.

Confinement Effects on Glass-Forming Aqueous Dimethyl Sulfoxide Solutions

“…More precisely, the relaxation processes of confined liquids showed a broadening on the low-frequency flank, which did not occur for the processes of bulk liquids. This broadening was mainly caused by slow molecules near the confining walls [ 32 , 44 , 76 ], as was anticipated in core-shell models and confirmed in systematic filling-level and solvation-level dependent studies on neat confined liquids [ 65 , 77 , 78 , 79 ]. Here, the CC distribution is used to phenomenologically consider this effect for the confined solutions, while the Cole-Davidson distribution proved to be useful to describe the process of glass-forming bulk liquids.…”

“…Two relaxation processes with similar properties were observed for neat hydrogen-bonded liquids in silica confinement [ 33 , 86 , 88 , 97 , 98 ]. While they were interpreted in terms of core-shell models for confined alcohols [ 33 , 97 ], they were attributed to coexisting liquid and solid phases or high-density and low-density liquid phases for confined water [ 78 , 86 , 98 ]. In our case of binary mixtures, another possible explanation resulted from the finding that preferential interactions with the pore walls could lead to microphase separation and, thus, to bimodal dynamics [ 28 , 29 , 40 , 43 , 44 , 99 , 100 ].…”

Layer-by-Layer Freezing of Nanoconfined Water

“…strong) behavior upon cooling 11,12 . As recently noted by Lederle et al 34 , with progressive freezing of nanoconfined water from the center towards the pore wall, the remaining liquid fraction becomes increasingly confined by the surrounding. This confinement is expected to be particularly severe in smaller pores.…”

Small Molecules, Non-Covalent Interactions, and Confinement

“…Figure 4 shows temperature-dependent correlation times τ obtained from 2 H NMR approaches to water reorientation in mesoporous silica. Clearly, there is a dynamic crossover near 220 K. Combining this NMR analyses with DSC and BDS studies [ 215 ], it turned out that, for a pore diameter of 2.8 nm, the dynamic crossover occurs near the melting temperature T m so that liquid and crystalline water fractions with, respectively, faster and slower rotational dynamics coexist inside the pores below this temperature (see Figure 4 a). It was concluded that partial crystallization causes the effect, explicitly, that the dynamics of water changes when ice forms and further restricts the accessible pore volume.…”

“…To tackle this problem, 2 H NMR was used to investigate reorientation dynamics of unfreezable water (D 2 O) in MCM-41 and SBA-15 pores over wide temperature ranges towards the glass transition [ 45 , 176 , 181 , 183 , 213 , 214 , 215 , 216 , 217 , 218 , 219 ]. In particular, spin-lattice relaxation, line-shape analysis, and stimulated-echo experiments were combined to ensure broad dynamic ranges and the pore-size was systematically varied to study possible finite-size effects.…”