suggested to consist ideally of only one or a few polycation/polyanion pairs held
together by long range electrostatic interactions. Since the whole PEC formation
process is claimed to be athermal [10, 72], the driving force of the evidently
occurring polycation/polyanion pairing is claimed to be the gain of entropy when
the respective counterions are released (“escaping tendency of the counterions”
[10]) from their parent PEL backbone. By contrast, secondary particles, the final
particles found in a freshly prepared raw PEC dispersion, consist of some 100
primary PEC particles held together by short range dispersive interactions. It might
be speculated that this second process is slightly enthalpic (i.e. exothermic) because
no entropy gain is expected during this process. We see this aggregation process of
primary to secondary PEC particles in the line of the classical concept of Ostwald
ripening [73]. According to Voorhees [74], Ostwald ripening denotes “a first-order
transformation process resulting in a two-phase mixture composed of a dispersed
second phase in a matrix. However, as a result of the large surface area present, the
mixture is not initially in thermodynamic equilibrium. The total energy of the twophase system can be decreased via an increase in the size scale of the second phase
and thus a decrease in total interfacial area. Such a process is termed Ostwald
ripening or coarsening”. Transferred to colloidal systems, this means that a dispersion
of small primary colloid particles below a critical size tends to become unstable and
thus aggregates to larger secondary particles until theoretically reaching one final
macroparticle to decrease surface area. Since Ostwald assigned the final aggregated
product state to a lower energy than the initial unaggregated educt state, it might be
questioned whether the process should be of enthalpic nature. However, this
dynamic (see Sect. 3.1.4) and occasionally long-term irreversible process is
influenced by the two classic types of colloid interaction forces: the short range
(<5 nm) dispersive attractive force and the long range ()5 nm) electrostatic
repulsive force, which was summarized in classical DLVO theory [75, 76].
Salt Addition After Complexation
Examining the salt effect on already-formed PEC particles is a completely different
task. The response or stability of the PEC system can be studied on the colloidal
level as well as on the (molecular) ionic binding level. Generally, the salt tolerance
of the PEC system concerning ionic binding is quite large. Dautzenberg [68]
claimed such ionic binding stability up to c S ¼ 4 M. The salt tolerance concerning
colloidal stability is lower because flocculation and aggregation sets in with
increasing c S , beginning at c S % 0.6 M. This can be illustrated by the observation
that the structural density remains constant, even for c S > 0.6 M. However, the
latter salt-mediated colloidal stability is also crucially dependent on the mixing
ratio because the more nonstoichiometric the mixing ratio (i.e., the greater the
deviation from neutrality: n
À /n
+
) 1 or n
À /n
+
( 1), the thicker is the surrounding
excess PEL shell. In other words, PEC particles with thicker PEL shells have higher
salt tolerance. The influence of ionic strength on size parameters of already-formed
PEC particles was also shown in a narrower c S range with higher resolution by
Sizing, Shaping and Pharmaceutical Applications of Polyelectrolyte Complex. . .
221
together by long range electrostatic interactions. Since the whole PEC formation
process is claimed to be athermal [10, 72], the driving force of the evidently
occurring polycation/polyanion pairing is claimed to be the gain of entropy when
the respective counterions are released (“escaping tendency of the counterions”
[10]) from their parent PEL backbone. By contrast, secondary particles, the final
particles found in a freshly prepared raw PEC dispersion, consist of some 100
primary PEC particles held together by short range dispersive interactions. It might
be speculated that this second process is slightly enthalpic (i.e. exothermic) because
no entropy gain is expected during this process. We see this aggregation process of
primary to secondary PEC particles in the line of the classical concept of Ostwald
ripening [73]. According to Voorhees [74], Ostwald ripening denotes “a first-order
transformation process resulting in a two-phase mixture composed of a dispersed
second phase in a matrix. However, as a result of the large surface area present, the
mixture is not initially in thermodynamic equilibrium. The total energy of the twophase system can be decreased via an increase in the size scale of the second phase
and thus a decrease in total interfacial area. Such a process is termed Ostwald
ripening or coarsening”. Transferred to colloidal systems, this means that a dispersion
of small primary colloid particles below a critical size tends to become unstable and
thus aggregates to larger secondary particles until theoretically reaching one final
macroparticle to decrease surface area. Since Ostwald assigned the final aggregated
product state to a lower energy than the initial unaggregated educt state, it might be
questioned whether the process should be of enthalpic nature. However, this
dynamic (see Sect. 3.1.4) and occasionally long-term irreversible process is
influenced by the two classic types of colloid interaction forces: the short range
(<5 nm) dispersive attractive force and the long range ()5 nm) electrostatic
repulsive force, which was summarized in classical DLVO theory [75, 76].
Salt Addition After Complexation
Examining the salt effect on already-formed PEC particles is a completely different
task. The response or stability of the PEC system can be studied on the colloidal
level as well as on the (molecular) ionic binding level. Generally, the salt tolerance
of the PEC system concerning ionic binding is quite large. Dautzenberg [68]
claimed such ionic binding stability up to c S ¼ 4 M. The salt tolerance concerning
colloidal stability is lower because flocculation and aggregation sets in with
increasing c S , beginning at c S % 0.6 M. This can be illustrated by the observation
that the structural density remains constant, even for c S > 0.6 M. However, the
latter salt-mediated colloidal stability is also crucially dependent on the mixing
ratio because the more nonstoichiometric the mixing ratio (i.e., the greater the
deviation from neutrality: n
À /n
+
) 1 or n
À /n
+
( 1), the thicker is the surrounding
excess PEL shell. In other words, PEC particles with thicker PEL shells have higher
salt tolerance. The influence of ionic strength on size parameters of already-formed
PEC particles was also shown in a narrower c S range with higher resolution by
Sizing, Shaping and Pharmaceutical Applications of Polyelectrolyte Complex. . .
221