Hydrogen bond structure and vibrational spectrum of water at a passivated metal nanoparticle

Molecular Dynamics computer simulations of gold passivated nanoparticles in water are presented. We show that butanethiol passivated gold nanoparticles of similar to 3 nm diameter exhibit weak de-wetting, similar to that observed in extended hydrophobic surfaces. The nanoparticle water interface is characterised by a water density depletion region, where the water molecules show a dramatic reduction in the number of hydrogen bonds with respect to bulk water. This weakening of the hydrogen bond structure is accompanied by the orientation of the dipole water vector towards the nanoparticle surface. The investigation of the vibrational spectrum of interfacial water at the nanoparticle surface supports the existence of free OH bonds. Overall our results indicate that small passivated metal nanoparticles ( similar to 3 nm diameter) behave like hydrophobic surfaces.