Handbook of Solvents - George Wypych - ChemTech - Ventech!

496 Jacopo Tomasi, Benedetta Mennucci, Chiara Cappelli
introducing important changes in the standard theoretical approach with respect to that used
for other liquid systems. Computer simulations are the methods more largely used to describe
pores and micro-pores in molecular sieves of various nature.
A different type of confined liquid is given by a thin layer spread on a surface. The difference
with respect to the micro-pore is that this is a three-component system: the surface
may be a solid or a liquid, while the second boundary of the layer may regard another liquid
or a gas. An example is a thin layer liquid on a metal, in the presence of air (the prodrome of
many phenomena of corrosion); a second example is a layer on a second liquid (for example,
oil on water). The standard approach may be used again, but other phenomena occurring
in these systems require the introduction of other concepts and tools: we quote, for
example, the wetting phenomena, which require a more accurate study of surface tension.
The thin layer in some case exhibits a considerable self-organization, giving rise to organized
films, like the Blodgett-Langmuir films, which may be composed by several layers,
and a variety of more complex structures ending with cellular membranes. We are here at
the borderline between liquid films and covalently held laminar structures. According to the
degree of rigidity, and to the inter-unit permeation interaction, the computational model
may tune different potentials and different ways of performing the simulation.
Another type of confined liquid regards drops or micro-drops dispersed in a medium.
We are here passing from confined liquids to the large realm of dispersed systems, some
among which do not regard liquids.
For people interested in liquids, the next important cases are those of a liquid dispersed
into a second liquid (emulsions), or a liquid dispersed in a gas (fogs), and the opposite cases
of a solid dispersed in a liquid (colloids) or of a gas dispersed in a liquid (bubbles).
Surface effects on those dispersed systems can be described, using again the standard
approach developed in the preceding sections: intermolecular potentials, computer simulations
(accompanied where convenient by integral equation or continuum approaches).
There is not much difference with respect to the other cases.
A remark of general validity but particular for these more complex liquid systems
must be expressed, and strongly underlined.
We have examined here a given line of research on liquids and of calculations of pertinent
physico-chemical properties. This line starts from QM formal description of the systems,
then specializes the approach by defining intermolecular potentials (two- or
many-body), and then it uses such potentials to describe the system (we have paid more attention
to systems at equilibrium, with a moderate extension to non-equilibrium problems).
This is not the only research line conceivable and used, and, in addition, it is not sufficient
to describe some processes, among them, those regarding the dispersed systems we
have just considered.
We dedicate here a limited space to these aspects of theoretical and computational description
of liquids because this chapter specifically addresses interaction potentials and because
other approaches will be used and described in other chapters of the Handbook.
Several other approaches have the QM formulation more in the background, often never
mentioned. Such models are of a more classical nature, with a larger phenomenological
character. We quote as examples the models to describe light diffraction in disordered systems,
the classical models for evaporation, condensation and dissolution, the transport of
the matter in the liquid. The number is fairly large, especially in passing to dynamical and