Freezing, melting and structure of ice in a hydrophilic nanopore

EMILY B. MOORE; EZEQUIEL DE LA LLAVE; KAI WELKE; DAMIAN A SCHERLIS; VALERIA MOLINERO

PHYSICAL CHEMISTRY CHEMICAL PHYSICS

ROYAL SOC CHEMISTRY

Año: 2010 vol. 12 p. 4124 - 4134

1463-9076

The nucleation, growth, structure and melting of ice in 3 nm diameter hydrophilic nanopores arestudied through molecular dynamics simulations with the mW water model. The meltingtemperature of water in the pore was Tporem = 223 K, 51 K lower than the melting point of bulkwater in the model and in excellent agreement with experimental determinations for 3 nm silicapores. Liquid and ice coexist in equilibrium at the melting point and down to temperatures as lowas 180 K. Liquid water is located at the interface of the pore wall, increasing from one monolayerat the freezing temperature, Tporef = 195 K, to two monolayers a few degrees below Tporem .Crystallization of ice in the pore occurs through homogeneous nucleation. At the freezingtemperature, the critical nucleus contains B75 to 100 molecules, with a radius of gyration similarto the radius of the pore. The critical nuclei contain features of both cubic and hexagonal ice,although stacking of hexagonal and cubic layers is not defined until the nuclei reachB150 molecules. The structure of the confined ice is rich in stacking faults, in agreement with theinterpretation of X-ray and neutron diffraction experiments. Though the presence of cubic layersis twice as prevalent as hexagonal ones, the crystals should not be considered defective Ic assequences with more than three adjacent cubic (or hexagonal) layers are extremely rare in theconfined ice.