Physical Chemistry of Foods

Polyelectrolytes, i.e., polymers containing charged groups (negative or
positive or both), show some specific relations. Due to the repulsive effect of
the charges if of the same sign, the polymer chain is relatively stiff. This
implies a very expanded conformation, whereby polyelectrolytes produce
very high viscosities. This greatly depends on the charge density, which in
turn may depend on pH, and on ionic strength. At a high ionic strength,
shielding by counterions causes the distance over which the repulsive effect
is sensed to be far smaller, leading to a less expanded conformation. At high
ionic strength, say above 0.2 molar, the difference with neutral polymers
mostly tends to be small.
Polyelectrolytes also show theDonnan effect. Charged macromole-
cules always are accompanied by counterions, i.e., small ions of opposite
charge, in order to make the solution electroneutral. In the presence of salt,
also the distribution of coions (small ions of the same charge) is affected,
and relatively more so for a lower salt concentration. These phenomena are
most readily observed when the polymer solution is contained within a
semipermeable membrane, where the osmotic pressure is greatly affected by
the Donnan effect. However, the effect also occurs for a polyelectrolyte
in solution; this implies, for instance, that it will be difficult to remove all
ions of a certain species, even if exchanged for other ions of the same charge
sign.
Concentratedpolymer solutions show strong nonideality. This is, for
instance, observed in the osmotic pressure being very much higher than
would follow from the molar concentration. The main variables are theb
value and the volume fraction of polymer, and for polyelectrolytes also
charge and ionic strength.
Thesolubilityof neutral polymers depends primarily on molecular size
andb. For long polymers, abvalue just below zero leads already to very
poor solubility. Generally, the polymer does not precipitate, but phase
separation occurs into a highly concentrated solution (a coacervate) and a
very dilute one. Quite in general, a number of ‘‘regimes’’ can be
distinguished for polymer–solvent mixtures, depending on b value and
concentration. Besides the dilute, there are semidilute and concentrated
regimes.
Outside the dilute regime,chain overlapoccurs, which implies that
polymer molecules are mutually entangled. This greatly increases viscosity,
as well as the dependence of viscosity on concentration and the extent of
strain rate thinning. Moreover, the solution shows elastic besides viscous
behavior, and if intermolecular cross-links are formed, a gel is obtained. The
chain overlap concentration decreases with increasing molecular size, b
value, and stiffness. For overlapping chains, the solution is characterized by
a correlation length, which does not depend on molecular size, and which is