  A molecular dynamics simulation of water confined in a silica pore is
performed in order to compare it with recent experimental results on water
confined in porous Vycor glass at room temperature. A cylindrical pore of 40 A
is created inside a vitreous SiO2 cell, obtained by computer simulation. The
resulting cavity offers to water a rough hydrophilic surface and its geometry
and size are similar to those of a typical pore in porous Vycor glass. The
site-site distribution functions of water inside the pore are evaluated and
compared with bulk water results. We find that the modifications of the
site-site distribution functions, induced by confinement, are in qualitative
agreement with the recent neutron diffraction experiment, confirming that the
disturbance to the microscopic structure of water mainly concerns orientational
arrangement of neighbouring molecules. A layer analysis of MD results indicates
that, while the geometrical constraint gives an almost constant density profile
up to the layers closest to the interface, with an uniform average number of
hydrogen bonds (HB), the hydrophilic interaction produces the wetting of the
pore surface at the expenses of the adjacent water layers. Moreover the
orientational disorder togheter with a reduction of the average number of HB
persists in the layers close to the interface, while water molecules cluster in
the middle of the pore at a density and with a coordination similar to bulk
water.
