Combining ab-initio and classical molecular dynamics simulations to unravel the structure of the 2D-HB-network at the air-water interface

Abstract : An extensive structural characterization of the 2D-HB-Network formed by water molecules at the air-water interface has been carried out by means of DFT-based and classical molecular dynamics simulations. SPC/E and SPC/Fw force fields commonly used for modelling liquid water are shown to correctly reproduce the 2D-HB-Network structure obtained from the reference ab-initio simulation. For both DFT-MD and classical MD representations, identical results have been obtained by increasing the size and time-scale of the simulations, starting from a simulation box of 256 water molecules in the liquid phase, simulated for 20 ps up to 25 ns. One pivotal result is that ∼90% of the water molecules in the interfacial layer are connected by a collective and extended net of HBs oriented parallel to the surface, and this 2D-HB-Network is built upon adjacent water rings mostly formed by 4, 5 or 6 water molecules. This finding in particular rationalizes previous non linear SFG spectroscopic results suggesting that liquid water at the interface with hydrophobic media could arrange in rings.