2.2 Preparation
2.2.1 Determination of True Charge
Before mixing oppositely charged PEL in defined stoichiometric ratios, the molar
concentration related to the charged monomer groups of polycation and polyanion
must be determined because the molar concentration of all repeating units is
definitely not identical to the molar concentration of the charges. This mismatch
is predominantly the case for weak PEL because their charge density (fraction
charged/all repeating units) is pH-dependent. This step is mandatory, even for
strong PEL, because highly charged PEL show counterion binding.
Colloid titration [41] is a powerful analytical technique for determining the
number or molar concentration of charges in a sample. Commonly, this can be
realized by a particle charge detector (Mutek, Herrsching, Germany) and involves
titrating a given PEL solution by a titrator solution of an oppositely charged low
molecular weight PEL (PDADMAC or poly(vinylsulfate) (PVS)) until a zetapotential of zero is reached. Zeta-potential is related to the voltage [mV] needed
to compensate the sheared ion cloud when a PTFE piston is periodically moved
within a PTFE tube filled with the sample PEL solution. Usually, a volume of 1 mL
of a 0.001 M sample PEL solution is further diluted to 10 mL and the titrator
solution is dosed in. From the volume of the consumed titrator solution, the factor F
of the PEL solution is determined using F ¼ consumed volume/probe volume.
Repeating units bearing one potential charged group (e.g., monobasic acid) can
ideally have a factor of F ¼ 1; those bearing two to three charges can ideally have
values of F ¼ 2–3. To control the stoichiometry of the complexation of polyanion/
polycation mixtures, which is an often-used parameter for PEC dispersions, the
mixing ratio n
À /n
+ has to be directly related to these obtained factors of the used
PEL solutions. Unfortunately, in the literature the reported n
À /n
+ values are based
on different concentration expressions so that no consistent picture prevails when
parameters like turbidity, size or polydispersity are plotted versus n
À /n
+ .
In this review, we use the abbreviation e.g. “PEC-0.66” for PEC systems with a
mixing ratio n
À /n
+
¼ 0.66, (i.e., a cationic PEC system) and e.g. “PEC-1.50” for
those with n
À /n
+ ¼ 1.50 (i.e., an anionic PEC system).
Different modes of realizing nonstoichiometric mixing ratios are possible. Some
authors always use equally concentrated solutions with respect to charge or monomer
concentration and control the mixing ratio by the volumes of the PC and PA solutions
(e.g., PEC-0.66 indicates 0.66 mL PA in 1 mL PC) [21, 42]. Others use differently
concentrated PC and PA solutions and use equal volumes or even different volumes.
2.2.2 Mixing Procedure
Mixing polycation and polyanion solutions to form PEL complexes is expected to
be dependent on the mixing type, protocol, and device because the irreversible
process is said to be kinetically controlled and local effects may play a role [12].
204
M. M€ uller
2.2.1 Determination of True Charge
Before mixing oppositely charged PEL in defined stoichiometric ratios, the molar
concentration related to the charged monomer groups of polycation and polyanion
must be determined because the molar concentration of all repeating units is
definitely not identical to the molar concentration of the charges. This mismatch
is predominantly the case for weak PEL because their charge density (fraction
charged/all repeating units) is pH-dependent. This step is mandatory, even for
strong PEL, because highly charged PEL show counterion binding.
Colloid titration [41] is a powerful analytical technique for determining the
number or molar concentration of charges in a sample. Commonly, this can be
realized by a particle charge detector (Mutek, Herrsching, Germany) and involves
titrating a given PEL solution by a titrator solution of an oppositely charged low
molecular weight PEL (PDADMAC or poly(vinylsulfate) (PVS)) until a zetapotential of zero is reached. Zeta-potential is related to the voltage [mV] needed
to compensate the sheared ion cloud when a PTFE piston is periodically moved
within a PTFE tube filled with the sample PEL solution. Usually, a volume of 1 mL
of a 0.001 M sample PEL solution is further diluted to 10 mL and the titrator
solution is dosed in. From the volume of the consumed titrator solution, the factor F
of the PEL solution is determined using F ¼ consumed volume/probe volume.
Repeating units bearing one potential charged group (e.g., monobasic acid) can
ideally have a factor of F ¼ 1; those bearing two to three charges can ideally have
values of F ¼ 2–3. To control the stoichiometry of the complexation of polyanion/
polycation mixtures, which is an often-used parameter for PEC dispersions, the
mixing ratio n
À /n
+ has to be directly related to these obtained factors of the used
PEL solutions. Unfortunately, in the literature the reported n
À /n
+ values are based
on different concentration expressions so that no consistent picture prevails when
parameters like turbidity, size or polydispersity are plotted versus n
À /n
+ .
In this review, we use the abbreviation e.g. “PEC-0.66” for PEC systems with a
mixing ratio n
À /n
+
¼ 0.66, (i.e., a cationic PEC system) and e.g. “PEC-1.50” for
those with n
À /n
+ ¼ 1.50 (i.e., an anionic PEC system).
Different modes of realizing nonstoichiometric mixing ratios are possible. Some
authors always use equally concentrated solutions with respect to charge or monomer
concentration and control the mixing ratio by the volumes of the PC and PA solutions
(e.g., PEC-0.66 indicates 0.66 mL PA in 1 mL PC) [21, 42]. Others use differently
concentrated PC and PA solutions and use equal volumes or even different volumes.
2.2.2 Mixing Procedure
Mixing polycation and polyanion solutions to form PEL complexes is expected to
be dependent on the mixing type, protocol, and device because the irreversible
process is said to be kinetically controlled and local effects may play a role [12].
204
M. M€ uller