4 ways that zeta-potential measurements make a difference

The measurements drive research and product development in areas as diverse as cosmetics, water treatment, and biomedicine

By Andrew Williams, C&EN BrandLab Contributing Writer

Zeta (ζ) potentials predict the shelf life of numerous materials and products, such as cosmetics, membranes, and biomaterials, so scientists must often have an in-depth knowledge of the measurement when they're developing these products. Put simply, the ζ potential is the electrical potential between a solid surface, such as the surface of a particle, and a solvent. The magnitude of the ζ potential indicates the degree of electrostatic repulsion or attraction between particles in a dispersion. A higher magnitude means particles are more likely to stay in suspension and remain stable.

Many companies and research centers routinely measure ζ potential as part of fundamental research, technology creation, and product quality control. Here are four ways that ζ potentials help science and technology move forward.

1) More than skin deep

The ζ potential is an important parameter to understand attractive and repulsive forces in particulate systems and is considered to be essential in many product sectors. Carina Santner, product specialist for particle characterization at Anton Paar, says the ζ-potential measurements are often a fundamental quality-control parameter in the formulation of cosmetics, paints, inks, syrups, or infusions because of their direct relation to product stability.

In cosmetics, the ζ potential can be determined for any product that contains particulate structures, such as liposomes. These hydrophobic particles increase the efficacy of skin care products by delivering ingredients into the deepest layer of skin. “Usually you aim for low ζ-potential values in order to increase the uptake of your liposomes and membrane interactions. The cell membrane is negatively charged; therefore, low values result in a better interaction between cell membrane and liposomes,” Santner says.

Santner says the dosing system feature of the Anton Paar Litesizer 500 is useful for achieving optimal ζ-potentials because it can measure the ζ potential in a range of different pH values, and pH is known to have a big influence on the ζ potential.

“Understanding the electrophoretic properties of a skin care product is a crucial factor in formulation design. It is necessary to select the right formulation of pH value, buffers, and emulsifier to maximize the ζ potential and hence predict the product stability,” Santner says.

“Using the dosing system, users can avoid adjusting the pH manually,” she notes. “Automating this process not only saves time and effort but, most importantly, also considerably reduces the possibility of human error in terms of better accuracy and reproducibility.”

2) Self-assembly required

The Litesizer has also proved useful for carrying out fundamental research on nanoparticles. One interesting example is at Rensselaer Polytechnic Institute, where a team led by R. Helen Zha uses the device for work on bioinspired self-assembled materials. These materials may have applications in health care, including coatings, drug delivery vessels, and nanomedicine. Here, ζ-potential measurements have emerged as a valuable tool for measuring the charge of colloidal particles. Zha says the measurement is a helpful indicator in assessing the ability of these particles to interact with other components, such as polyelectrolytes or cells.

“Charge-charge interaction is a powerful mechanism for self-assembly of different components, like polyelectrolytes and nanoparticles, into nanostructured soft matter,” she says. “Frequently, the charge of components will dictate the structure of the resulting composite material.”

Scott McPhee and his team at the City University of New York take advantage of the Litesizer's multiple functions to characterize self-assembling peptide nanoparticles and functionalized gold nanoparticles. “We use it to measure the surface potential, which is then compared with the proposed model of self-assembly to help verify structure,” he says. He explains that many scientific journal editors now expect ζ-potential measurements to be included in manuscripts because the measurements guide the development and formulation of nanoparticles by providing key information about stability and surface charge.

3) Water treatment

An in-depth knowledge of the surface ζ potential of materials proves invaluable in the development of membranes for the water treatment sector, where instruments like Anton Paar's SurPASS 3 help describe the interaction of solutes with a solid surface.

According to Thomas Luxbacher, product manager at Anton Paar, the surface ζ potential is directly related to the performance of membranes used in ultrafiltration and nanofiltration, where electrostatic forces help capture solutes in water and aqueous solutions.

“Tuning the surface charge of ultrafiltration and nanofiltration membranes is therefore crucial to optimize their performance,” he says. “Either the bulk composition of these membranes or the membrane surfaces are modified by a membrane posttreatment process to adjust the surface charge. The surface ζ potential reveals the success or failure of such attempts for membrane development.”

4) Infection prevention

ζ-Potential measurements are also important in the creation of innovative coatings. For example, Kenneth J. Wynne at Virginia Commonwealth University says his team acquired an Anton Paar SurPASS instrument to measure ζ potential for developing modified polyurethane coatings. These applications could help prevent infections associated with urinary catheters.

The team has made a mixed soft-block polyurethane to blend with a conventional polyurethane coating. One of these soft blocks has polyethylene glycol and quaternary side chains. Designed to be concentrated at the surface of the coating, polyethylene glycol is biocompatible, while the quaternary side chains are antimicrobial. According to Wynne, ζ potentials provided a method for analyzing surface concentration of charge from the quaternary side chains, which increases steadily with the weight percentage of mixed soft-block polyurethane. At about a 5 weight percentage of the mixed soft-block polyurethane results in a charge density adequate to kill bacteria on contact by disrupting their outer membranes, a mechanism known as contact kill. "Analyzing charge density by ζ potentials gives insight into the mechanism of contact kill," Wynne says.

Measuring the ability of a coating to prevent hazardous infections is just one more fascinating way the ζ-potential measurement is propelling advances in science.