ABSTRACT
Membranes can be defined as a sheet of solid or semisolid material, insoluble
in its surrounding medium, which separates phases that are usually (but not
necessarily) fluid [1]. Based on mechanisms ruling mass transport, typically,
membranes can be divided into two main classes: biological and synthetic
membranes [2]. While in synthetic membranes, mass transport is usually due
only to a chemical potential gradient, in biological membranes mass transport
can be due also to other special mechanisms. These mechanisms can act
cooperatively with chemical potential or they can act against it. In this last
case, they need energy from the surrounding, and a typical energy source is
that coming from the hydrolysis of high energy compounds such as ATP
(adenosine triphosphate) that transforms into ADP (adenosine diphosphate)
plus P (inorganic phosphate)þ energy (ATP pump mechanism) [3]. While biological membranes permeability can be highly influenced by these non-
chemical potential driven mechanisms, synthetic membranes permeability is
essentially dependent on chemical-physical properties of the solute and
membrane microscopic structure. As membranes are nothing more than
special matrices where one-dimension is considerably smaller than the other
two, they are characterized by the same structures of matrices discussed in
Chapter 7. Nevertheless, it is important to recall some important structural
aspects. In the most general case, they are composed of three different phases:
(a) continuous, (b) shunt, and (c) dispersed [1]. In turn, these phases can be
classified as primary, secondary, tertiary, etc., on the basis of their relative
spatial relationships. A primary continuous phase is an uninterrupted phase
between membrane surfaces as well as laterally or in the plane perpendicular
to the flux vector. From the membrane permeability point of view, it can
represent an uninterrupted diffusional path for a solute or it represents an
inaccessible zone acting as a mere supporting structure. A primary shunt
phase can be seen as an ensemble of pores or channel that completely cross
membrane thickness (for example, in the direction perpendicular to mem-
brane surfaces) but it is laterally discontinuous. Accordingly, a primary shunt
phase cannot influence permeability or can affect it providing parallel diffu-
sional pathways or can represent the sole diffusional pathway. Dispersed
phases, embedded in continuous or shunt phases, are discontinuous along the
flux vector and do not provide an uninterrupted pathway through the mem-
brane or any of its subphases.
