  We study the wetting properties of water on silica surfaces using molecular
dynamics (MD) simulations. To describe the intermolecular interaction between
water and silica atoms, two types of interaction potential models are used: the
standard Br\'odka and Zerda (BZ) model, and the Gulmen and Thompson (GT) model.
We perform an in-depth analysis of the influence of the choice of the potential
on the arrangement of the water molecules in partially filled pores and on top
of silica slabs. We find that at moderate pore filling ratios, the GT silica
surface is completely wetted by water molecules, which agrees well with
experimental findings, while the commonly used BZ surface is less hydrophilic
and is only partially wetted. We interpret our simulation results using an
analytical calculation of the phase diagram of water in partially filled pores.
Moreover, an evaluation of the contact angle of the water droplet on top of the
silica slab reveals that the interaction becomes more hydrophilic with
increasing slab thickness and saturates around 2.5-3 nm, in agreement with the
experimentally found value. Our analysis also shows that the hydroaffinity of
the surface is mainly determined by the electrostatic interaction, but that the
van der Waals interaction nevertheless is strong enough that it can turn a
hydrophobic surface into a hydrophilic surface.
