solution the differences in kinetics show up as differences in reversibility of the
polyelectrolyte complex formation. The lysozyme-containing micelles (system A)
fall into the second kinetics category (De % 1) where relaxation phenomena are
observed during the experiment. Therefore, the maximum intensity is dependent on
the direction of the titration (see Fig. 10). By increasing the waiting time after each
addition, the position of the maximum intensity becomes independent of the
direction of the titration.
The a-lactalbumin-containing system (system B) belongs to the kinetically
quenched category where De ) 1. In this system, the polyelectrolytes do not
rearrange or there is only little rearrangement of the polyelectrolytes within the
micelles. If two oppositely charged polymers come into contact they stick. It is
plausible that micelles that were formed starting at F
À
¼ 0 are different from
micelles that are formed starting at F
À
¼ 1. The micelle formation is not reversible,
therefore micelles are found at a much wider composition interval than for system A.
The reason why these two systems show different kinetics is probably that the
lysozyme-containing system consists of two weakly charged polyelectrolytes (PAA
and PDMAEMA), whereas the a-lactalbumin-containing system consists of the
weakly charged homopolymer (PAA) and a strongly charged (quartenised) cationic
polymer (P2MVP).
Differences between these two systems also are found when studying their saltinduced disintegration [51, 66]. This salt-induced disintegration can again be
studied using light scattering titrations. In this case, a micellar solution is prepared
into which a salt solution is titrated. The intensity and hydrodynamic radius can
then be studied as function of the salt concentration.
In Fig. 12, the light scattering titrations with salt for system A, system B and
micelles without incorporated lysozyme molecules (i.e., containing only the
diblock copolymer and homopolymer) are presented. The I(C salt ) of system A
first shows a gradual decrease in light scattering intensity (see Fig. 12), then the
intensity levels off and a small plateau is found. The hydrodynamic radius
decreases in the ionic strength interval at which I(C salt ) decreases. An increase in
hydrodynamic radius is observed at the salt interval where the plateau is observed.
For micelles without lysozyme, the hydrodynamic radius becomes too inaccurate to
measure at this salt range. Above a certain salt concentration, the intensity starts
decreasing again until the polyelectrolyte complexes disintegrate [66]. An explanation why this plateau is observed in the I(C salt ) plot may be that the critical salt
concentration of polyelectrolyte complex formation is approached [67]. This
behaviour has been observed not only with increasing salt concentration, but also
with decreasing salt concentration [68].
Similar behaviour was also observed for PEMs. For two weakly charged
polyelectrolytes at pH ¼
1
2 ðpK anion þ pK cation Þ , the growth regime switches from
linear to exponential upon a small increase in salt concentration; larger increases in
the ionic strength may result in dissolution of the PEM and formation of whole
polyelectrolyte complexes in solution [31]. In the pH regime where one of the
polyelectrolytes is almost fully charged and the other partly charged, an increased
158
S. Lindhoud and M.A. Cohen Stuart
polyelectrolyte complex formation. The lysozyme-containing micelles (system A)
fall into the second kinetics category (De % 1) where relaxation phenomena are
observed during the experiment. Therefore, the maximum intensity is dependent on
the direction of the titration (see Fig. 10). By increasing the waiting time after each
addition, the position of the maximum intensity becomes independent of the
direction of the titration.
The a-lactalbumin-containing system (system B) belongs to the kinetically
quenched category where De ) 1. In this system, the polyelectrolytes do not
rearrange or there is only little rearrangement of the polyelectrolytes within the
micelles. If two oppositely charged polymers come into contact they stick. It is
plausible that micelles that were formed starting at F
À
¼ 0 are different from
micelles that are formed starting at F
À
¼ 1. The micelle formation is not reversible,
therefore micelles are found at a much wider composition interval than for system A.
The reason why these two systems show different kinetics is probably that the
lysozyme-containing system consists of two weakly charged polyelectrolytes (PAA
and PDMAEMA), whereas the a-lactalbumin-containing system consists of the
weakly charged homopolymer (PAA) and a strongly charged (quartenised) cationic
polymer (P2MVP).
Differences between these two systems also are found when studying their saltinduced disintegration [51, 66]. This salt-induced disintegration can again be
studied using light scattering titrations. In this case, a micellar solution is prepared
into which a salt solution is titrated. The intensity and hydrodynamic radius can
then be studied as function of the salt concentration.
In Fig. 12, the light scattering titrations with salt for system A, system B and
micelles without incorporated lysozyme molecules (i.e., containing only the
diblock copolymer and homopolymer) are presented. The I(C salt ) of system A
first shows a gradual decrease in light scattering intensity (see Fig. 12), then the
intensity levels off and a small plateau is found. The hydrodynamic radius
decreases in the ionic strength interval at which I(C salt ) decreases. An increase in
hydrodynamic radius is observed at the salt interval where the plateau is observed.
For micelles without lysozyme, the hydrodynamic radius becomes too inaccurate to
measure at this salt range. Above a certain salt concentration, the intensity starts
decreasing again until the polyelectrolyte complexes disintegrate [66]. An explanation why this plateau is observed in the I(C salt ) plot may be that the critical salt
concentration of polyelectrolyte complex formation is approached [67]. This
behaviour has been observed not only with increasing salt concentration, but also
with decreasing salt concentration [68].
Similar behaviour was also observed for PEMs. For two weakly charged
polyelectrolytes at pH ¼
1
2 ðpK anion þ pK cation Þ , the growth regime switches from
linear to exponential upon a small increase in salt concentration; larger increases in
the ionic strength may result in dissolution of the PEM and formation of whole
polyelectrolyte complexes in solution [31]. In the pH regime where one of the
polyelectrolytes is almost fully charged and the other partly charged, an increased
158
S. Lindhoud and M.A. Cohen Stuart