Chemistry Reference
In-Depth Information
more than a single repeating unit, the secondary structure is important for its
properties: the different repeating units might be statistically distributed or
arranged in, more or less uniform, blocks. The polymer can be a linear structure
or a branching one. It might have a certain shape in space (tertiary structure) that
can depend on the surrounding solution. All these parameters influence the proper-
ties of an aqueous solution of polymer, but there are more parameters when the
polymer has electrolyte groups. These electrolytes can be weak or strong electro-
lytes, resulting in different degrees of dissociation/protonation. The electrolyte
groups of the backbone might be all cationic or all anionic, but they might also be
partially cationic and partially anionic. Such polyelectrolytes are called polyam-
pholytes. There is another parameter that has an important influence on the proper-
ties of polyelectrolytes in aqueous solutions: the distance between the electrolyte
groups in the polymer backbone. When that distance is small, the attractive electro-
static forces between the ionic groups in the backbone and their counterions in the
aqueous solutions become so strong that, even if the repeating unit is a strong
electrolyte, one observes an ion pairing, i.e., some of the counterions condensate
(at least partially) with the ions of the backbone. Therefore, even at high dilution in
water such polyelectrolytes are not completely dissociated and the degree of disso-
ciation might depend on the composition of the surrounding aqueous phase. The
large number of parameters that influence the properties of aqueous solutions of
polyelectrolytes is reflected in the variety of areas where such solutions are found
and applied. Table
1
gives some typical examples of applications. These applica-
tions take advantage of the particular thermodynamic properties of aqueous solu-
tions of polyelectrolytes. Therefore, there is a need for methods to describe such
properties. In applied thermodynamics, the properties of solutions are described by
expressions for the Gibbs energy as a function of temperature, pressure, and
composition. From such equations all other thermodynamic state functions can
be derived.
There are many well-established models for the Gibbs energy of nonelectrolyte
solutions and also several methods to describe conventional polymer solutions.
However, the state of the art for modeling thermodynamic properties of aqueous
solutions of polyelectrolytes is far less elaborated. This is partly due to the particu-
lar features of such solutions but is also caused by insufficiencies in the knowledge
of the parameters that characterize a polyelectrolyte, for example, the polydisper-
sity and the different structures (primary, secondary etc.) of the polyelectrolytes.
The development and testing of thermodynamic models has always been based on
reliable experimental data for solutions for which all components are well char-
acterized. Such characterization is particularly scarce for biopolymers and biopo-
lyelectrolytes. Furthermore, such polymers are generally more complex than
synthetic polymers. Therefore, the present contribution is restricted to a discussion
of the thermodynamic properties of aqueous solutions of synthetic polyelectrolytes
that consist of only two different repeating units that are statistically distributed.
Furthermore, it is restricted to systems where sufficient information on the poly-
electrolyte's polydispersity is available.
