• Apr 17, 2024
  • Physical Measurement
  • By Cesar Estrada

Realizing the Potential in Surface Zeta Potential: An Introduction to the SurPASS 3

The Surface Potential Analyzer of Solid Surfaces, or more aptly named SurPASS, is an instrument that performs zeta potential analyses on macroscopic materials. PTL has long conducted analyses with an instrument capable of Zeta Potential. So, is adding the SurPASS to PTL’s growing list of instruments a redundancy? Quite the opposite; PTL is expanding its repertoire with the addition of this instrument.

Why the New Instrument?

Zeta Potential is a useful metric for many different real-world uses, including insight into colloidal stability in pharmaceutical products (if charge stabilization is the main stabilization mechanism used) or particle flocculation in water reclamation, and, as it has previously exclusively been measured at PTL, it is measured based on electrophoretic mobility. To measure Zeta Potential through measuring the electrophoretic mobility of particles, particles must be freely suspended in Brownian motion and suspended in an aqueous medium for analysis.

This meant that PTL was typically only capable of measuring Zeta Potential on samples containing particles in the submicron region (i.e. < 1 µm), although certain exceptions could be made as particle density also plays a role in suspensions. With the introduction of the SurPASS, PTL is able to expand not only its size range but also the sample types feasible for Zeta Potential analysis; no longer does PTL exclusively measure liquid suspensions in the submicron region!

With the SurPASS, solid surfaces, powders, and fibers above 25 µm can be analyzed, though it should be noted that powders should not be water-soluble; powders in the 1 to 25 µm region have the potential to be analyzed but would require a pre-submission discussion with lab staff to assess testing feasibility. The SurPASS still utilizes the Zeta Potential technique but in a completely different manner.

This instrument expands PTL’s Zeta Potential capabilities by, as its name implies, measuring the Zeta Potential of solid surfaces upon interacting with an aqueous electrolyte solution. The resulting Zeta Potential measurement, differentiated by the usage of the term Solid Surface Zeta Potential, gives insight into the interfacial behaviors of solid surfaces upon contact with an aqueous electrolytic solution and can provide insight into the surface chemistry of samples.

How Does the SurPASS Work?

The SurPASS measures streaming current or streaming potential, and the value that is obtained from that measurement is used to further calculate the Zeta Potential. In short, a capillary is formed between two solid surfaces, which creates a pressure difference between the inlet and outlet of the capillary. Using atmospheric air, the pressure is then equalized in the capillary, which causes the electrolyte solution to flow.

Upon contact with the solution, a charge on the surface of the sample is induced, which gives further rise to an electric double layer (see PTL’s blog post on Demystifying Zeta Potential for further information on the electric double layer). When streaming the electrolytic solution through the capillary, shear forces act on the counterions that are relative to the surface charge and move them in the direction of the liquid flow. From here, two different methods are applied to acquire the streaming potential or streaming current. In short, two different types of circuits are used to assess the electrostatic interactions between the electrolyte solution streaming through the capillary and the solid surface; further along, two different applications of the same equation (i.e. the Helmholtz-Smoluchowski equation) are used.

Depending on several factors, such as conductance sourced from the electrolytic solution used, conductance intrinsic to the material, or surface conductance, either of these measurements can be used. Typically, for planar samples, such as flat sheets or membranes, streaming current is the preferred approach as there are fewer measurable parameters to input into the equation used to calculate Zeta Potential using the parameter of streaming potential thus reducing measurement error. However, for samples that are not planar, such as powders and fibers, where the area of the streaming channel (i.e. capillary) is not known, an approximation of the Helmholtz-Smoluchowski equation can be used for either streaming current or streaming potential. If the sample contributes to the conductance in the streaming channel, then an approximation using the streaming potential may be most appropriate.

Aside from the wide range of sample types that can be analyzed, which will be covered briefly, one of the most beneficial uses of this instrument comes from its titration capabilities. Using the titration unit that is built into the instrument, the SurPASS can titrate across a range from pH 3 to 11 and measure the Zeta Potential at different preset pH increments. One of the biggest benefits of titration is the ability to approximate the Isoelectric point (referred to as IEP) of a sample as well as the influence of pH on Zeta Potential, which can help to mimic/visualize different sample environments.

It is common to assume the IEP is the point at which the sample surface charge disappears, but the IEP is best interpreted as the pH of an aqueous solution where the net electrokinetic charge density is 0 and the zeta potential reverses signs. The IEP is important in biological processes as well as providing general info on the contribution of surface groups to surface charge. In addition to the titration capabilities of the instrument, other beneficial traits of this instrument are the types of cells that can be equipped. PTL currently has three attachments for this instrument: the cylindrical cell, which can analyze powders samples larger than 25 µm or fiber samples, as well as the clamping cell and adjustable gap cell, which can analyze planar materials such as wafers, membranes, and textile sheets.

Why Does It Matter?

The SurPASS has many practical applications in a number of industries. For example, wet chemical processes are used in the manufacturing of semiconductor wafer chips, which introduce an interfacial charge on the surface. An example of these processes and the electrostatic behaviors on the surface can be observed in the process of chemical-mechanical planarization: slurries of ceramic particles are used to polish the surface of these wafers. Having information on the zeta potential of the wafer can provide insight into the charging behavior of the wafer and thus the successful adhesion or lack thereof for the slurry.

Another example can be seen in the SurPASS’s applications in the field of filtration. At times, treatment of a membrane filter may be necessary to optimize the filtration process, and analyzing the membrane’s Zeta Potential across different pH values can provide knowledge on the surface chemistry of the membranes and how it may change as pH changes; additionally, analyzing membranes before and after treatment can provide a reasonable prediction on the efficiency and efficacy of said treatment.

The SurPASS has the potential to have practical applications in industries such as the cosmetic industry by measuring the efficiency of the washing process with different shampoos and conditions on hair, the textile industry by measuring the effects dyeing or bleaching fibers or other textile material has on the Zeta Potential. The number of real-world applications and interpretations of Zeta Potential results acquired on the SurPASS truly accentuate the potential in Zeta Potential!

Reach out to a sales representative at PTL if you believe your sample might be a fit for Solid Surface Zeta Potential analysis!

By Cesar Estrada, Particle Characterization Chemist III.