Hydrogen-Bond Dynamics in the Air−Water Interface

Abstract

Hydrogen-bond (H-bond) dynamics in the air-water interface is studied by molecular dynamics simulations. The analysis reveals that the dynamics of breaking and forming hydrogen bonds in the air-water interface is faster than that in bulk water for the polarizable water models. This is in contrast to the results found on a protein surface. We show that the difference stems from more rapid translational diffusion in the interface. When the effect of pair diffusion is eliminated, the hydrogen-bond dynamics in the interface is observed to be slower than that in the bulk. This occurs because the number of water molecules adjacent to a hydrogenbonded pair and available to accept or donate a hydrogen bond is smaller in the interface than in the bulk. The comparison between polarizable water models and fixed-charge models highlights the potential importance of the polarization effect in the water-vapor interface.

Polarizable contributions to the surface tension of liquid water

“…͗ e ͑z͒͘ shows nine oscillations in the bulklike liquid region, with variations in the peaks value of Ϸ0.08. These peaks reflect the mean molecular positions and likely give rise to a mobility dependence with the position in the slab, in agreement with a previous report of low diffusion coefficients found in the bulk water compared to the values found at the interface, reported by Liu et al 13 If we consider that the peaks extend across Ϸ29 Å, each peak has an average width of Ϸ3.2 Å, which corresponds to the approximate molecular diameter of water, which has been estimated from adsorption studies of water molecules in micropores. 28 To obtain information about the orientation of the permanent and induced dipoles, we calculate the angle between the dipole direction and the parallel to the interface.…”

“…A previous work with a polarizable model of water found that the relaxation times for water at the bulk and at the interface are 4.93 and 4.07 ps, respectively. 13 To equilibrate, the system was slowly heated, starting from a configuration at 0 K, in order to avoid translation of the "slab" of water through the simulation cell. An illustrative snapshot of the system is shown in Fig.…”

“…More recently, the hydrogen-bond dynamics at the airliquid interface of water was studied by Liu et al 13 using the polarizable TIP4P/FQ model at 1 atm and 298.15 K. They found that hydrogen bonds at the interface form and break faster than hydrogen bonds in the bulk phase and that this behavior produces an increase in the diffusion coefficient in the interfacial region relative to the bulk. They explained this behavior as a result of a decrease in the number of hydrogen bonded molecules at the interface.…”

Untitled

“…The diffusion within the surface is increased for some molecules (in the surface) but decreased for others and depends on the number of hydrogen bonds and size of the water clusters (Liu et al 2005). The refractive index of the surface of water at 22°C has been shown to be higher than that of the bulk and opposite in behavior to other normal and hydrogen-bonding liquids, ethanol for example (Greef and Frey, 2008).…”

Reactivity and Dynamics at Liquid Interfaces

“…Berne and coworkers examined the hydrogen bond dynamics at the water liquid/vapor interface. 252 They found faster hydrogen bond dynamics at the interface than in bulk water for the polarizable water models, but slower dynamics if non-polarizable models are used. Even with the polarizable models, however, the shorter lifetime at the interface was attributed to more rapid translational diffusion at the interface rather than to faster kinetics of hydrogen bond formation.…”