We present results of molecular dynamics simulations of water confined in a silica pore. A cylindrical cavity is created inside a vitreous silica cell with geometry and size similar to the pores of real Vycor glass. The simulations are performed at different hydration levels. At all hydration levels water adsorbs strongly on the Vycor surface; a double layer structure is evident at higher hydrations. At almost full hydration the modifications of the confinement-induced site-site pair distribution functions are in qualitative agreement with neutron diffraction experiment. A decrease in the number of hydrogen bonds between water molecules is observed along the pore radius, due to the tendency of the molecules close to the substrate to form hydrogen-bonds with the hydrophilic pore surface. As a consequence we observe a substrate induced distortion of the H-bond tetrahedral network of water molecules in the regions close to the surface.
The structure and dynamics of confined water in cylindrical pores
have been investigated by molecular dynamics
simulations. Both rigid (TIP4P) and flexible (BJH) models have
been used. Pore radii between 4.2 and 20
Å have been studied; the pore walls are modeled either as a smooth
(10−4) Lennard-Jones wall or as a
structured wall consisting of (12−6) Lennard-Jones particles.
Polar functional groups on the pore surface
are modeled by arrays of point charges. We present results on
density and orientational distribution functions
and on the water mobility. We observe that water transport through
nonpolar pores is fast and dominated by
the surface layer, whereas transport in polar pores is slowed down
relative to bulk liquid water and occurs
preferentially through the center of the pore.
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