Molecular dynamics studies of the liquid–vapor interface of water

Abstract

We report the results of molecular dynamics simulations of the liquid–vapor interface of water. Two different water–water interactions were studied. The density profile of the interfacial transition region between liquid and vapor is monotone with a 10%–90% width of 3.45 Å, in good agreement with the value determined from x-ray reflectance measurements, namely 3.30 Å. The water molecules in the interface tend to lie with the HOH bisector in the plane of the surface and one OH bond pointing out of the surface. As the density falls in the transition region the tetrahedral structure of the bulk liquid breaks up and there is a tendency towards dimerization. The diffusion constant in the interfacial region is 58% larger than that in the bulk region.

Ab initio molecular dynamics simulation of liquid water and water–vapor interface

“…The maximum probability is at an angle of 120°with the outward surface normal, which qualitatively agrees with the experimental second-harmonic generation results. 19 Most of the ''classical'' simulations studies also agree with our result, [21][22][23]25 although two studies reported a water dipole orientation parallel to the surface. 20,24 The intramolecular O-H bond orientation can be described by an angle variable between the bond and the surface normal in analogy to the dipole moment orientation angle .…”

“…2, which shows the computed xy (Nϭ2) and z (Nϭ1) components of the MSD of the O atoms, only ''cage movement'' effects are observed, resulting in the small slope of the curve at longer times. This picture contradicts the findings in earlier ͑classical͒ MD simulations, 22,25 where an increase of the self-diffusion coefficient in the surface transition zone compared to the bulk region was reported. These simulations were performed using much bigger systems containing 350 or more water molecules.…”

“…The structure of liquid water-vapor interface has been studied quite intensively using experimental 19 or theoretical [20][21][22][23][24][25][26] methods. These studies, however, lead to different conclusions regarding the orientational distributions of the surface water dipole moments.…”

Polarization and experimental configuration analyses of sum frequency generation vibrational spectra, structure, and orientational motion of the air/water interface

“…12,38,39 Besides SHG and SFG-VS experimental studies, 33,40 Structure and dynamics of water molecules at the air/water interface have also been intensively discussed with theoretical simulations. 5,6,7,8,9,10,11,12,13 Even though with so much efforts and progresses both by experimentalists and theoreticians, our detailed understanding of air/water interface is still limited. Just as indicated by B. C. Garrett recently, 41 '...(direct) experiments are difficult to perform because the liquid interface is disordered, dynamic, and small (typically only a few molecules wide) relative to the bulk'.…”

“…1,2,3,4 Among them, air/water interface has been intensively investigated theoretically or experimentally over the last decades. Spectroscopy, molecular structure and dynamics at air/water interface is studied with theoretical analysis such as ab initio calculation or molecular dynamics simulation, 5,6,7,8,9,10,11,12,13 or experimental techniques such as X-ray reflection, 14,15 Stimulated Raman Scattering (SRS), 16 Near-edge X-Ray Adsorption Fine Structure (NEXAFS), 17 Second Harmonic Generation (SHG), 18,19 as well as Sum Frequency Generation, etc. 20,21,22,23,24,25,26,27,28 Among these experimental techniques, Second Harmonic Generation and Sum Frequency Generation are the most important methods for molecular interface studies because of their surface sensitivity and specificity.…”