J. Phys. Colloques
Volume 48, Numéro C1, Mars 1987VIIth Symposium on the Physics and Chemistry of Ice
|Page(s)||C1-681 - C1-683|
J. Phys. Colloques 48 (1987) C1-681-C1-683
MONTE CARLO SIMULATION OF ICE Ih : COMPARISON OF BULK MELTING AT CONSTANT PRESSURE AND STRUCTURE OF ICE LAYERS ON AN ICE NUCLEATING SUBSTRATEB.N. HALE et K.K. HAN
Department of Physics and Graduate Center of Cloud Physics Research, University of Missouri-Rolla, Rolla, MO 65401, U.S.A.
Recently effective pair potentials (1) and Metropolis Monte Carlo methods have been used to study the melting of a periodic rigid molecule model ice system at constant volume (2, 3) near 290 K. We have extended these studies to examination of the system at constant pressure and present the results for approximately 1 atm pressure. In this approach the constant number, pressure and temperature (NPT) ensemble is approximated by treating the volume as an additional variable in the Metropolis Monte Carlo procedure (4-7). The unit cell for these calculations contains 192 rigid central force (1) water molecules and the initial configurations are taken from the constant number, volume and temperature (NVT) equilibrated system (3) at 260 K. This initial ice Ih unit cell has approximately zero dipole and quadrupole moments and was shown to remain in the ice Ih structure over a range of temperature from 20 K to about 290 K in constant NVT ensemble studies. The pressure is found to be extremely sensitive to the intermolecular interactions and to the instantaneous density of molecules in the unit cell. The unit cell properties (dipole moment, ice structure factors, specific heat, and pair correlation functions) will be presented. A comparison will be made with simulations of two water layers on a model ice nucleating substrate (8) at 200 K and 265 K near zero pressure. At 200 K the structure of the top layer of this system shows considerable disorder and liquid - like properties, while the water layer adjacent to the substrate has a solid ice - like hexagonal ring structure. There appears to be no preference for water dipole orientation in the exposed layer of water molecules. The liquid-like properties of the exposed layers in this system at 200 K and the melting of the bulk near 290 K are consistent with recent observations that surface layers have noticably reduced solid-liquid transition temperatures (9). When these absorbed water layers are subjected to a large external electric field or constant presure (~ 200 atm) there appears to no significant change in the exposed layer structure and liquid-like states. Unit cell properties for the layer systems will be discussed. The motivation for these studies has been to examine the effect of pressure, temperature and other external perturbations (such as substrate structure and substrate defects) on atmospheric ice nucleation.