Download Measuring polyelectrolyte charges using a colloidal charge sensor

Measuring polyelectrolyte charges using a colloidal charge sensor
Supervisors: Mark Vis & Jos van Rijssel
Project description
For many problems in physical chemistry, the charge of colloids (e.g. particle dispersions, polyelectrolytes) is highly important. Yet, experimentally it is very difficult to accurately and directly
measure the absolute value of the charge. Rather, related quantities are measured from which an
estimate of the charge can be made. An example of such a, relatively easy, measurement is the
electrophoretic mobility. Electrophoretic mobilities can be converted to zeta-potentials, assuming a
certain particle geometry (this is what commercial equipment such as the Malvern Zetasizer does).
However, obtaining the absolute number of charges on a particle from these measurements is highly
problematic and theoretically challenging.
Our lab has developed a colloidal charge sensor, which measures the absolute charge on colloids
based on a thermodynamic principle called the Donnan equilibrium. Simply put, this method works
by determining the concentration of counterions of the charged colloids. If the number density of
the colloids is known, the charge is straightforward to calculate.
In this project, you will be systematically investigating the charge of gelatin, which is a biopolymer that can have both positive and negative charges, depending on the pH. In other projects, we
are studying the phase separation in aqueous mixtures of dextran (an uncharged polysaccharide)
and gelatin, as shown in Figure 1. For the phase behavior of this system, the charge of the gelatin
is a very important parameter, which is why we want to study the charge of gelatin in great detail
with our colloidal charge sensor. Since the charge of gelatin comes from acidic and basic amino
acids, we can compare our findings with for instance an acid/base titration.
+
-
3.0%/3.0% 3.5%/3.5% 4.0%/4.0% 4.5%/4.5% 5.0%/5.0%
+
dextran-rich phase:
+
- - - - - + + + + + +
+ + +
+ +- +
+ + +- +
- uncharged dextran
- salt
- small amount of gelatin
water/water interface:
- permeable to water & ions
- electric double layer
- potential difference
gelatin-rich phase:
- charged gelatin + counterions
- salt
- small amount of dextran
Figure 1: Aqueous mixtures of dextran and gelatin phase separate above a certain concentration into
dextran- and gelatin-rich phases. The critical point of this system is very sensitive to the charge of the
gelatin, which is determined by the pH of the mixture.
This project is intended for bachelor students who have a strong interest in physical chemical
characterizations and are interested in doing both practical and theoretical work. Your daily supervision will be by PhD-student Mark Vis (who investigates the aforementioned phase separation in
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aqueous dextran/gelatin mixture) and postdoc Jos van Rijssel (who currently works on the colloidal
charge sensor). Ben Erné will act as a weekly supervisor.
If you have any questions regarding this project, feel free to contact one of us or our student
coordinator.
Contact information
Mark Vis
H.R. Kruytgebouw, room N709
E: [email protected]
T: +31 30 253 3981
Jos van Rijssel
H.R. Kruytgebouw, room N708
E: [email protected]
T: +31 30 253 2390
Student coordinator: Roel Baars
E: [email protected]
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