Abstract
We review our simulation results on properties of supercooled confined water. We consider two situations: water confined in a hydrophilic pore that mimics an MCM-41 environment and water at interface with a protein. The behavior upon cooling of the α relaxation of water in both environments is well interpreted in terms of the Mode Coupling Theory of glassy dynamics. Moreover, we find a crossover from a fragile to a strong regime. We relate this crossover to the crossing of the Widom line emanating from the liquid-liquid critical point, and in confinement we connect this crossover also to a crossover of the two body excess entropy of water upon cooling. Hydration water exhibits a second, distinctly slower relaxation caused by its dynamical coupling with the protein. The crossover upon cooling of this long relaxation is related to the protein dynamics.
Highlights
The properties of water affect almost every phenomena occurring in nature
Water shows a number of anomalies in all its thermodynamic space [6], the most well-known among them is the presence of a temperature of maximum density (TMD), corresponding to 4 ◦ C at ambient pressure
A very accurate study of confined water has been performed in a series of computer simulation of water in a silica pore that mimics the properties of MCM-41 [51,58,59]
Summary
The properties of water affect almost every phenomena occurring in nature. Water in all its states, ice, liquid and vapor, influences many chemical and biological processes [1,2]. Water shows a number of anomalies in all its thermodynamic space [6], the most well-known among them is the presence of a temperature of maximum density (TMD), corresponding to 4 ◦ C at ambient pressure. How the anomalies of water affect its properties in solution or in confinement is still not well understood. The anomalous behavior becomes more relevant in metastable supercooled liquid state [6,7,8]. With special techniques it is possible to avoid crystallization and keep water in a liquid metastable condition. There is an experimental limit to supercooling determined by the homogeneous nucleation temperature TH. New experimental techniques using nanodroplets, nanoconfinement and association with biomolecules have successfully shifted this limit down to ≈ −46 ◦ C [9]
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