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

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.