Molecular Dynamics Study of Water Interacting with Siloxane Surface Modified by Poly(ethylene oxide) Chains

In this work, static and dynamic properties of interfacial water molecules on the amorphous siloxane surface covered by poly(ethylene oxide) (PEO) chains grafted at different coverage densities sigma were studied using atomistic molecular dynamics simulations. It is shown that water molecules compete with the ethylene oxide (EO) monomers for interaction sites on the siloxane surface and for the space above it. Two types of orientations of the water molecules are identified as the most populated: one with the dipole moment vector pointing toward the siloxane surface and another with the dipole moment vector pointing about 5 degrees away from the surface. In both the hydrogen pair is amenable to rotate around the dipolar axis. The higher the coverage density the larger the disorder of water molecules, but the dipole moment-down orientation still dominates. The number of water-water hydrogen bonds monotonically decreases with sigma, whereas the number of water-PEO hydrogen bonds displays a maximum around the crossover to the brush regime (stretched chains). The interfacial water molecules seem to be anchored to the siloxane surface by the water oxygen atom rather than by hydrogen bonding. The confinement due to the solid substrate does not affect the first solvation layer of PEO chains. The mobility of water molecules is attenuated by the siloxane surface and also by the PEO chains more considerably in the direction perpendicular to the siloxane substrate.