Handbook of Solvents - George Wypych - ChemTech - Ventech!

8.9 Liquid surfaces 493
to study the temperature dependence of the Helmholtz free energy in a bulk liquid or in the
same liquid under the form of a fog near condensation.
Things become more complex when a solute is added to the liquid, and more and more
complex when the system contains three or more components.
We are not sure whether any case occurring in this field has been subjected to theoretical
analysis and modeling; however, the number of problems considered by theoretical research
is very large, and there are models, generally based on the use of molecular
potentials, that can give hints, and often useful computational recipes.
These remarks, which are trivial for every reader, with even a minimal experience in
working with solvents, have been added here to justify our choice of not trying to give a
schematic overview, and also to justify the extremely schematic presentation of the unique
specific more advanced subject we shall here consider, the interfacial properties of liquids.
8.9.1 THE BASIC TYPES OF LIQUID SURFACES
The surface of a liquid is often represented as a very small fraction of the liquid specimen,
and it may be neglected. This statement is not of general validity, however, and often the
surface plays a role; in some cases, the dominant role. To be more specific, it is convenient
to consider a classification of liquid surfaces, starting from a crude classification, and introducing
more details later.
The basic classification can be expressed as follows:
• Liquid/gas surfaces;
• Liquid/liquid surfaces;
• Liquid/solid surfaces.
Liquid/gas surfaces may regard a single substance present in both phases, liquid and
vapor at equilibrium or out of equilibrium, but they may also regard a liquid in the presence
of a different gas, for example, air (in principle, there is always a given amount of solvent in
the gas phase). In the presence of a different gas, one has to consider it in the description of
the liquid, both in the bulk and in the interface. In many cases, however, the role of the gas is
marginal, and this is the reason why in many theoretical studies on this interface the gas is
replaced by the vacuum. This is a very unrealistic description for liquids at equilibrium, but
it simply reflects the fact that in simplifying the model, solvent-solute interactions may be
sacrificed without losing too much in the description of the interfacial region.
The potentials used to describe these interfaces are those used to describe bulk liquids.
The differences stay in the choice of the thermodynamical ensemble (grand canonical ensembles
are often necessary), in the boundary conditions to be used in calculations, and in
the explicit introduction in the model of some properties and concepts not used for bulk liquids,
like the surface tension. Much could be said in this preliminary presentation of liquid/gas
interfaces, but we postpone the few aspects we have decided to mention, because
they may be treated in comparison with the other kind of surfaces.
Liquid/liquid surfaces regard, in general, a couple of liquids with a low miscibility coefficient,
but there are also cases of one-component systems presenting two distinct liquid
phases, for which there could be interest in examining the interfacial region at the coexistence
point, or for situations out of the thermodynamical equilibrium.
If the second component of the liquid system has a low miscibility with the first, and
its molar ratio is low, it is easy to find situations in which this second component is arranged
as a thin layer on the surface of the main component with a depth reducing, in the opportune
conditions, to the level of a single molecule. This is a situation of great practical importance