UC San Diego

The thermodynamic stability of water next to graphitic surfaces is of fundamental interest, as it underlies several natural phenomena and important industrial processes. It is commonly assumed that water wets graphite more than graphene due to increased, favorable van der Waals interactions between the interfacial water molecules with multiple carbon sheets. Here, we employed extensive computer simulations and analysis of the molecular correlation functions to show that the interfacial water thermodynamics is in fact dominated by surface entropy. We show that on graphite, destabilization of the interfacial hydrogen bond network leads to an overcompensating increase in population of low frequency translational and librational modes, which is ultimately responsible for the increased interfacial stability compared to graphene. The spectroscopic signature of this effect is an enhancement of the modes near 100 and 300 cm^-1. This subtle interplay between entropy and surface binding may have important consideration for interpretations of various phenomena, including the hydrophobic effect.