Handbook of Solvents - George Wypych - ChemTech - Ventech!

498 Jacopo Tomasi, Benedetta Mennucci, Chiara Cappelli
The van der Waals regime locally corresponds to the large scale energy profile, with a
smooth shape.
The behavior of the density profile largely depends on the chemical nature of the composite
system. Some phenomenological parameters, like mutual solubility, may give a good
guess about the probable energy profile, but “surprises” are not rare events. It may be added
that the density profile is sensitive to the quality of the potential and the method using the
potentials. It is not clear, in our opinion, if some claims of the occurrence of a thin void (i.e.,
at very low density) layer separating two liquid surfaces is a real finding or an artifact due to
the potentials. Old results must be considered with caution, because computations done
with old and less powerful computers tend to reduce the size of the specimen, thus reducing
the possibility of finding capillary waves. The dimension of the simulation box, or other
similar limiting constraints, is a critical parameter for all phenomena with a large length
scale (this is true in all condensed systems, not only for surfaces).
The density profile for liquid/solid surfaces has been firstly studied for hard spheres on
hard walls, and then using similar but a bit more detailed models. It is easy to modify the potential
parameters to have an increment of the liquid density near the wall (adhesion) or a
decrement. Both cases are physically possible, and this effect plays an important role in
capillarity studies.
A corrugation at the solid surface can be achieved by using potentials for the solid
phase based on the atomic and molecular constitution. This is almost compulsory for studies
on systems having a large internal mobility (biomolecules are the outstanding example), but
it is also used for more compact solid surfaces, such as metals. In the last quoted example
the discreteness of the potential is not exploited to address corrugation effects but rather to
introduce mobility (i.e., vibrations. or phonons) in the solid part of the system, in parallel to
the mobility explicitly considered by the models for the liquid phase.
Another type of mobility is that of electrons in the solid phase: this is partly described
by the continuum electrostatic approach for dielectrics and conductors, but for metals (the
most important conductors) it is better to resort to the jellium model. A QM treatment of
jellium models permits us to describe electric polarization waves (polarons, solitons) and
their mutual interplay with the liquid across the surface.
Jellium, and the other continuum descriptions of the solid, have the problem of exactly
defining the boundary surface. This is the analog of the problem of the definition of the cavity
boundary for solutes in bulk solvents, occurring in continuum solvation methods (see
Section 8.7.3). The only difference is that there are more experimental data for solutes than
for liquid/solid surfaces to have hints about the most convenient modeling. The few accurate
ab-initio calculations on liquid/metal systems are of little help, because in order to reach
an acceptable accuracy, one is compelled to reduce the solid to a small cluster, too small to
describe effects with a large length scale.
The disturbances to the bulk density profile are often limited to a few solvent diameters
(typically 2-3), but the effect may be larger, especially when external electric fields are
applied.
The second information derived from the application of our methods to surfaces regards
the preferred orientation of liquid molecules near the surface. To treat this point there
is the need of a short digression.
In bulk homogeneous liquids at the equilibrium orientation, the location of a specific
molecule is immaterial. We need to know the orientational modes (often reduced to libra-