Polyelectrolytes and Measuring their Zeta Potential

The zeta potential of polyethylenimine (PEI) is measured as a function of polymer concentration with the SZ-100 Nanoparticle Analyzer. There is a negative relationship between Zeta potential and polyelectrolyte concentration, as the former decreases when the latter is decreased. This is due to the increasing ionic strength of the suspension and intermolecular interactions. Charged polymers, such as polyelectrolytes, could be analyzed with the SZ-100.

Introduction

Polyelectrolytes are polymer chains with an electrolyte group on every repeat unit. Due to the dissociation of small counterions that leave behind a charged macro ion, these polymers are charged when dissolved in a polar solvent. While this article will focus on a discussion of synthetic polymers and polyelectrolytes that include biologically important molecules such as polypeptides and DNA.

The charge found on a polyelectrolyte is a function of the conditions of its solutions. Unlike simple electrolytes, a select portion of counterions will not dissociate away from a polyelectrolyte. As the charge magnitude on the chain increases, it becomes progressively more difficult to remove the next ion. This process is known as counterion condensation.

The behavior of polyelectrolytes is different from that of uncharged polymers. For instance, a polyelectrolyte chain will tend to be more rod-like than a typical Gaussian chain. This is due to electrostatic repulsion between different segments on the same chain. Additionally, the long range electrostatic interactions occurring within the system enable polyelectrolyte solution properties to differ from those of neutral polymers. An instance of this is when the second virial coefficient (a measure of solution thermodynamics) could be an order of magnitude higher for a polyelectrolyte than a neutral polymer, significantly altering solution viscosity.

These solution effects enable polyelectrolytes to be used in a number of applications, such as viscosity modifiers or surface-charge modifiers in nanoparticles. An expected effect of such is either (a) the stabilization of suspensions through the increase of particle surface charge or (b) the bridging of particles.

Polyethylenimine (PEI) is a cationic polyelectrolyte and is used to attach negatively charged cells in biological processes. It is also utilized for DNA transfection, a method of introducing new DNA into cells. Furthermore, polyethylenimine can be used to capture carbon dioxide for various processes. These useful PEI behaviors are obtained through the particle’s charged state and its manipulability using pH.

Illustration of structure of branched PEI. The NH groups give the polymer a charge that is manipulated via, among other things, solution pH.

Because of their large molecule size, polyelectrolytes scatter light similar to particles. Their response to an applied electric field may also be likened to particles. As such, polyelectrolyte mobility is measurable and the extraction of a zeta potential value to characterize these materials is possible.

Materials and Methods

Branched PEI was obtained from Sigma Aldrich as a 50-weight percent solution in water. This was diluted further with 1 mM aqueous KCl to prepare solutions with appropriate polymer concentration. The small amount of KCl is used as a background electrolyte to ensure that the effects of small confounders or impurities do not dramatically affect the results of zeta potentials. The polymer molecular weight was given by the manufacturer as 750000 g/mol by light scattering. After these preparations, the zeta potential was measured using the SZ-100. The measurements were repeated six times.

SZ-100 Nanoparticle Analyzer

Results and Discussion

The obtained zeta potential values are shown in the table below. Meanwhile, the graph presents the effect of concentration on zeta potential. Expectedly, the zeta potential decreases with increasing ionic strength. The cause of this is associated with the increasing concentration of macro-ions and an increasing chain overlap.

Table 1. Obtained zeta potential values

Zeta potential of PEI as a function of polymer concentration. Error bars correspond to one standard deviation.

Conclusions

The SZ-100 could be effectively used in characterizing the charge of polymeric species. As seen in the study, zeta potential decreases as polyelectrolyte concentration is increased, primarily due to the increasing ionic strength of the suspension and intermolecular interactions. The zeta potential of charged polymers, such as polyelectrolytes, could easily be analyzed with the SZ-100.

This information has been sourced, reviewed and adapted from materials provided by HORIBA.

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