A MOLECULAR-DYNAMICS SIMULATION OF WATER CONFINED IN A CYLINDRICAL SIO2 PORE

A molecular dynamics;simulation of water confined in a silica pore is performed in order to compare it with recent experimental results on w ater confined in porous Vycor glass at room temperature. A cylindrical pore of 40 Angstrom is created inside a vitreous SiO2 cell, obtained by computer simulation. The resulting cavity offers water a rough hydr ophilic surface and its geometry and size are similar to those of a ty pical pare in porous Vycor glass. The site-site distribution functions of water inside the pore are evaluated:and compared with bulk water r esults. We find that the modifications of the site-site distribution f unctions, induced by confinement, are in qualitative agreement with th e recent neutron diffraction experiment, confirming that the disturban ce to the microscopic structure of water mainly concerns orientational arrangement of neighboring molecules. A layer:analysis of MD results indicates that, while the geometrical constraint gives an almost const ant density profile up to the layers closest to the interface, with a uniform average number of hydrogen bonds (HB), the hydrophilic interac tion produces the wetting of the pore surface at: the expenses of the adjacent water layers. Moreover, the orientational disorder together w ith a reduction of;he average number of HB persists in the layers clos e to the interface, while water molecules cluster in the middle of the pore at a density and with a coordination similar to bulk water. (C) 1998 American Institute of Physics.