Molecular diffusion at liquid surfaces
Abstract Book of the XXIII IUPAP International Conference on Statistical Physics, Genova, Italy, (9-13 July 2007)Abstract
The Intrinsic Sampling Method (ISM), for computer simulations of
free liquid surfaces 1-3, provides the sharpest molecular view for the structure of free liquid surfaces. The blurring of the density profiles produced by the capillary waves is eliminated in the intrinisic density profiles, which uncover a rather generic surface layering structure 4-5, which was previously thought to be characteristics of some liquid metal surfaces. We have extended the ISM analysis to the kinetics of the molecules at the surface, along Molecular Dynamics (MD) simulations of the Lennard-Jones
and other simple liquid models. The method gives a precise information on the turn-over rate for the molecules at the first liquid layer, and for their diffusion within that layer. The surface transverse diffusion
constant is found to be several times larger than the bulk value, in
agreement with previous coarser estimates based on samplings in a fixed
surface slab. The molecular diffusion constant in the direction normal to
the surface is coupled to the collective capillary wave modes, and the
MD results are very well represented by a Smoluchowski equation with an
effective surface potential. The diffusion constant extracted from that
analysis shows characteristics finite size effects, associated to the
long wavelength spectrum of the capillary waves. Moreover, the analysis
of the turn-over rates for the molecules at the first liquid layer
provides a deeper insight into the ISM, and gives unambiguous values
for the optimal parameters in the method.
1) E. Chacon, and P. Tarazona, Phys. Rev. Lett. 91, 166103 (2003)\\
2) P. Tarazona, and E. Chacon, Phy. Rev. B 70, 235407 (2003)\\
3) E. Chacon, and P. Tarazona, J. Phys.:Condens. Matter 17, S3493 (2005)\\
4) E. Chacon et al., Phys. Rev. Lett 87, 166101 (2001)\\
5) E. Velasco et al., J. Chem. Phys. 117, 10777 (2002)