of 0.1 and 5 M, indicating that the complexes are still intact at 5 M NaCl. This
indicates that the interaction strength between the polyelectrolytes is important for
their behaviour in solution.
2.2 Force Measurements
Direct interactions between polyelectrolyte complexes can be studied using atomic
force microscopy (AFM). When a colloidal probe coated with one type of polyelectrolyte is glued to a cantilever, the force measured as function of the distance to
a surface coated with the oppositely charged polyelectrolytes can be measured.
These force–distance curves give insight into the interactions between the oppositely charged molecules. This method has been used by Spruijt and coworkers.
A colloidal probe was coated with an negatively charged polymer brush and its
interactions with a positively charged brush attached to a flat silica surface were
studied [72]. To study the forces between these two surfaces it is necessary to start
at a high salt concentration (2–3 M) and gradually reduce the ionic strength. At low
salt, the attractive force is too strong and separation of the two surfaces is impossible, i.e., the glue joint between the colloidal probe and the cantilever breaks when
one tries to pull the surfaces apart. At 2–3 M salt, no attractive force is measured
upon approach, and a weak adhesion force (0.1–1 nN) is measured upon separating
the surfaces. Apart from an attractive force, a repulsive force due to compression of
the polymers brushes is first measured at distances smaller than 50 nm [72]. An
attractive force in both the approach and separation curve is measured at a salt
concentration of 1.4 M. At this ionic strength no repulsive force is measured.
Some hysteresis in the force–distance curves is found due to slow processes
occurring when the two surfaces are brought into contact (see Fig. 14). When the
oppositely charged brushes come into contact, a thin polyelectrolyte complex layer
is formed. Initially the brushes are compressed on both sides of this layer, but at a
certain stage of the approach interpenetration of the polymer brushes takes place,
resulting in growth of the polyelectrolyte complex phase. To separate the surfaces
again, the chains have to be stretched until enough energy is stored to disrupt the
complex phase. Once a few ion pairs are disrupted, this force is transferred to the
remaining pairs until the whole complex has disintegrated and the brushes are
completely separated. Now relaxation of the individual polymer chains occurs.
Because relaxation takes place at a finite rate, the force–distance curve measured is
dependent on the scan rate. An illustration of this process can be found in Fig. 14.
At very low scan rates the approach and separation curve become almost identical,
indicating a near-equilibrium process. From the hysteresis in the free
force–distance curves, the true free energy of ion pairing can be derived using a
kinetic model [72]. This model shows that ion pair formation and disintegration of
the polyelectrolyte complex take place in a zipper-like fashion.
Johansson et al. performed AFM and surface force apparatus (SFA) measurements
on PEMs. For the force measurements, the colloidal probe and the surface were both
162
S. Lindhoud and M.A. Cohen Stuart
indicates that the interaction strength between the polyelectrolytes is important for
their behaviour in solution.
2.2 Force Measurements
Direct interactions between polyelectrolyte complexes can be studied using atomic
force microscopy (AFM). When a colloidal probe coated with one type of polyelectrolyte is glued to a cantilever, the force measured as function of the distance to
a surface coated with the oppositely charged polyelectrolytes can be measured.
These force–distance curves give insight into the interactions between the oppositely charged molecules. This method has been used by Spruijt and coworkers.
A colloidal probe was coated with an negatively charged polymer brush and its
interactions with a positively charged brush attached to a flat silica surface were
studied [72]. To study the forces between these two surfaces it is necessary to start
at a high salt concentration (2–3 M) and gradually reduce the ionic strength. At low
salt, the attractive force is too strong and separation of the two surfaces is impossible, i.e., the glue joint between the colloidal probe and the cantilever breaks when
one tries to pull the surfaces apart. At 2–3 M salt, no attractive force is measured
upon approach, and a weak adhesion force (0.1–1 nN) is measured upon separating
the surfaces. Apart from an attractive force, a repulsive force due to compression of
the polymers brushes is first measured at distances smaller than 50 nm [72]. An
attractive force in both the approach and separation curve is measured at a salt
concentration of 1.4 M. At this ionic strength no repulsive force is measured.
Some hysteresis in the force–distance curves is found due to slow processes
occurring when the two surfaces are brought into contact (see Fig. 14). When the
oppositely charged brushes come into contact, a thin polyelectrolyte complex layer
is formed. Initially the brushes are compressed on both sides of this layer, but at a
certain stage of the approach interpenetration of the polymer brushes takes place,
resulting in growth of the polyelectrolyte complex phase. To separate the surfaces
again, the chains have to be stretched until enough energy is stored to disrupt the
complex phase. Once a few ion pairs are disrupted, this force is transferred to the
remaining pairs until the whole complex has disintegrated and the brushes are
completely separated. Now relaxation of the individual polymer chains occurs.
Because relaxation takes place at a finite rate, the force–distance curve measured is
dependent on the scan rate. An illustration of this process can be found in Fig. 14.
At very low scan rates the approach and separation curve become almost identical,
indicating a near-equilibrium process. From the hysteresis in the free
force–distance curves, the true free energy of ion pairing can be derived using a
kinetic model [72]. This model shows that ion pair formation and disintegration of
the polyelectrolyte complex take place in a zipper-like fashion.
Johansson et al. performed AFM and surface force apparatus (SFA) measurements
on PEMs. For the force measurements, the colloidal probe and the surface were both
162
S. Lindhoud and M.A. Cohen Stuart