Single Particle Dynamics at the Intrinsic Surface of Various Apolar, Aprotic Dipolar, and Hydrogen Bonding Liquids As Seen from Computer Simulations

We investigate the single molecule dynamics at the intrinsic liquid/vapor interface of five different molecular liquids (carbon tetrachloride, acetone, acetonitrile, methanol, and water). After assessing that the characteristic residence times in the surface layer are long enough for a meaningful definition of several transport properties within the layer itself, we characterize the dynamics of the individual molecules at the liquid surface by analyzing their normal and lateral mean-square displacements and lateral velocity autocorrelation functions and, in the case of the hydrogen bonding liquids (i.e., water and methanol), also the properties of the hydrogen bonds. Further, dynamical properties as well as the clustering of the molecules residing unusually long in the surface layer are also investigated. The global picture emerging from this analysis is that of a noticeably enhanced dynamics of the molecules at the liquid surface, with diffusion coefficients up to 4 times larger than in the bulk, and the disappearance of the caging effect at the surface of all liquids but water. The dynamics of water is dominated by the strong hydrogen bonding structure also at the liquid surface.