Handbook of Solvents - George Wypych - ChemTech - Ventech!

8.9 Liquid surfaces 495
The potentials in use for crystals and those used for covalently bound systems are cognate
with those used for liquids, so it is not compulsory to open here a digression on this
theme. We have already quoted the MM methods generally used for solids of molecular
type; for more general classes of compounds, a variety of ‘atom-atom’ potentials have been
given. 126 The difference with respect to the bulk is mainly due, again, to different ways of
handling the opportune boundary conditions. It is evident that the techniques used to describe
a water-protein interface greatly differ, in microscopic detail, time scale, etc., to those
used to describe the surfaces between a liquid and a regular crystal surface.
For many problems, the solid surface in contact with the liquid may be held fixed: this
characteristic has been amply exploited in simulation studies. We quote here two problems
for which this simplification is not possible: problems in which the structure of the solid (or
solid-like) component is subjected to conformational changes, and problems in which a progressive
expansion (or reduction) of the solid phase plays the main role.
To the first case belong problems addressing the structure of the interfacial zone in the
case of biopolymers (e.g., proteins) or similar covalently bound massive molecules, and ordered
assemblies of molecules held together by strong non-covalent interactions (as membranes
or similar structures). In both cases, the liquid and the solid component of the
systems are generally treated on the same footing: interaction force fields are used for both
components, introducing, where appropriate, different time scales in the molecular dynamics
calculations.
The second case mainly regards the problem of crystal growth or dissolution in a liquid
system (pure liquid or solution). A mobile surface of this type is not easy to treat with
standard methods: it is better to pass to more specialistic approaches, for which there is the
need of introducing a different mathematical treatment of the (mobile) boundary surface
conditions. This is a subject that we cannot properly treat here.
8.9.2 SYSTEMS WITH A LARGE SURFACE/BULK RATIO
To complete this quick presentation of the three main types of liquid surfaces, it is convenient
to consider systems in which the interfacial portion of the liquid is noticeably larger
than in normal liquids.
Some among them are denoted in literature as constrained liquids, others as dispersed
systems. This classification is not universal, and it does not take in account other cases.
A typical example of constrained liquid is that present in pores or capillaries. The surface
is of liquid/solid type, and the same remarks about interaction potentials and computational
methods we have already offered may be applied here. More attention must be paid in
computations of interactions among separate portions of the liquid interface. In the slit pore
case (i.e., in a system composed of two parallel plane solid surfaces, with a thin amount of
liquid between them), there may be an interference between the two distinct surfaces, more
evident when the thickness of the liquid is small, or when there is an electrical potential between
the two faces. The slit pore model is extensively used to describe capillarity problems:
in the most usual cylindrical capillary types there is a radial interaction similar in
some sense to the interaction with a single solid surface.
Pores of different shape are present in many solid materials: zeolites and other
aluminosilicates are outstanding examples. When the pore is very small there is no more
distinction between superficial and bulk molecules of the liquid. Nature (and human ingenuity)
offers examples over the complete range of situations. For all the systems we have
here introduced (quite important in principle and for practical reasons), there is no need of