Handbook of Solvents - George Wypych - ChemTech - Ventech!

12.2 Chain conformations of polysaccharides 727
phenomenon is justified through a ‘condensation process’ of counterions and it has also
been interpreted theoretically.
On the polymer side, among the dramatic changes that the presence of charged groups
imparts to solution properties, there are the enhanced chain dimensions, the increased hydrodynamic
volume (i.e., viscosity), and, in general, a strong influence on all
conformational properties. Subject to the constraints imposed by the chemical structure of
the chain, the distribution of charged groups and their degree of ionization contribute to determining
the equilibrium chain conformation; both the Coulombic interaction among the
charged groups and the distribution and concentration of the screening counterions are important.
Most of the physico-chemical properties of the system result from a non-linear
combination of these parameters.
However, one has not to forget that the variability of conformation alters the distances
between charged groups on the polymeric chain and that the equilibrium is statistically defined
by the Gibbs energy minimum of the system. As an important consequence of this energy
balance, changes in temperature, ionic strength, pH, etc., can provoke changes in
polyelectrolyte conformation, often cooperatively in the case of biopolymers, between
states with different values of the charge density. These states may be characterized by different
structural orders (e.g. helix →extended chain transition), by different degrees of flexibility
of the chain (globular coil → expanded chain) or by different extent of aggregation
(monomeric → dimeric or multimeric chains).
Theoretical calculations based on molecular grounds are still extremely complicated
and incomplete 72 and other routes must be more empirically used in order to interpret the experimental
data and to understand the correlation between conformational properties and
structure. The central problem is to quantify the interactions among charges on the polymer
and among these same charges and their respective counterions.
As far as it concerns the short-range interactions, the introduction of charged groups
modifies the equilibrium geometry of the monomeric units and the contribution of the electrostatic
nature on the nearest-neighbor conformational energy. These conclusions also derive
from the already demonstrated effect of the solvent interactions on the unperturbed
dimensions of amylose and cellulose, 53 and from the evidence of the perturbation on the
conformational energy surface of several charged saccharidic units. 73
There at least two approaches that may be relevant for this review; one is that described
by Haug and Smidsrød 74 for the rationalization of the dimensional properties of
polyelectrolytes as a function of salt concentration, the other is the formulation of a statistical
thermodynamic theory for the “physical” framing of the ion-polyelectrolyte interactions.
Both these theoretical formulations deal with the conformation of the polymer and
predict that the conformational features must be function of ionic strength (see for example
refs. 75 and 76).
12.2.6.2 The Haug and Smidsrød parameter: description of the salt effect on
the chain dimension
A peculiarity of the correlation between the viscometric parameters and the dimensions of
the macromolecular chain has long been recognized and theoretical approaches have been
developed for several chain models. 77,78 The behavior of polyelectrolytes adds some complications
especially in the low ionic strength regime. It has however been understood that
the intrinsic viscosity, [η], of a polyion (i.e., its hydrodynamic volume) decreases with increasing
ionic strength, I, as a consequence of the screening of the fixed charges on the