Vinod Radhakrishnan, Senior Product Specialist for the Particle and Material Characterization group at Anton Paar USA, talks to AZoM about the new SurPASS 3 and how it enables users to generate surface charge and zeta potential information with real life samples.
What parameters have an impact on the surface properties of a material?
There are three main parameters which describe a solid surface and, as such, have an impact on its surface properties. Firstly, roughness; the most established method to characterize surface roughness is Atomic Force Microscopy (AFM). Secondly, wettability; this describes how the surface behaves upon contact with water, whether it is hydrophobic (repels water) or hydrophilic (easily covered by a water film). Contact angle measurements are typically used to gain such information. Thirdly, functionality; functional groups, which show acidic or basic behaviour, describe the chemistry of a surface. This is an extremely important factor when it comes to describing and understanding the solid surface behaviour and its suitability to any application. Streaming potential measurements are an excellent means to determine and validate surface functionality.
What impact do these parameters have on the overall application of these materials?
All three parameters described above determine how well a particular material interacts with its environment, which, certainly, is the end goal of the entire process. Roughness greatly affects the accessible surface area of a material and, thus, has a direct impact on the reactivity of the material. Wettability, of course, determines how well the material interacts with water; a solvent that we come across every single day in our lives and that is also used extensively in industry. Functionality or surface charge determines how well the surface of a particular material can be optimized in order to make it more favourable for an application or environment.
Keeping in mind these materials are fabricated for real life applications, it is important to analyse these surfaces under real-life conditions in order to better predict their applicability and use. Unfortunately, indirect methods to gain such information may result in misleading conclusions.
How can surface charge information help researchers in creating better surfaces?
The charge of a surface may be positive, negative, or neutral and reveals the material’s affinity to other surfaces. The surface charges of both a substrate and solute play a very important role in determining how a dissolved or suspended molecule is adsorbed or binds on the substrate surface. For example, it has been shown that proteins prefer binding to a neutral or positively charged PVA or silicon substrate [1]. Being aware of this crucial information enables researchers to modify their substrate surfaces to make them more conducive for protein binding. Similar arguments can be extended to many other application areas such as membranes, semiconductors, textiles, polymers, and more.
What is the role of zeta potential?
While surface charge is an important piece of information, it cannot be determined directly. The zeta potential gives information on the charge density at the solid/liquid interface, and is vital in understanding the behaviour of solid materials in many technical and biological processes. In addition to surface charge itself, zeta potential measurements provide valuable information such as surface functionality, surface treatment validation, material isoelectric point, adsorption kinetics, swelling, hydrophobicity, and many more. For example, zeta potential measurements on membranes can be used to understand and optimize surface modifications that in turn control membrane performance and filter efficiency.
How does the SurPASS 3 enable users to generate surface charge and zeta potential information using real life samples?
The SurPASS 3 provides insight into the surface charge and zeta potential of real life materials by employing the streaming potential technique. While electrophoretic light scattering is used to determine zeta potential of colloids and suspensions, the streaming potential technique offers the most reliable approach when working with larger solid materials. The SurPASS 3 uses real life materials and enables investigation of both small (micro size) and large (macro size) solid surfaces.
SurPASS™ 3: Introduction
In the SurPASS 3, an aqueous electrolyte solution flows through the measuring cell that contains the solid sample. The electrolyte flow shears the surface-charge compensating ions off their equilibrium position within the electrical double layer. This causes an electrical charge separation in the flow direction along the measuring cell. This charge separation generates an electrical potential difference linearly dependent on the differential pressure: the streaming potential. This streaming potential, or alternatively the streaming current, is used to determine the zeta potential of the materials.
What areas of research and development have seen benefits of using direct routine surface analysis and why?
Many areas of research and development, including but not limited to membrane technology, semiconductors, biopolymers, textiles etc. have seen benefits from using routine surface analysis. Surface analysis allows the determination of surface chemistry and surface charge of materials in these areas, which is crucial for their performance.
What specific benefit does the SurPASS™ 3 offer when studying membranes?
The SurPASS 3 permits zeta potential and surface charge investigation of membranes in both tangential and trans-membrane modes. As such, it reveals the zeta potential of the membrane surface and also that of the pores within the membrane, which may be different depending on requirements of the application. In addition, the adsorption mode of the SurPASS 3 monitors and records the effect of adsorption of proteins, surfactants, or other chemistries on the surface charge of the membrane, and permits the study of the interaction of these chemistries with the membrane surface. Furthermore, the SurPASS 3 allows the investigation of large pieces of materials using real solution under real conditions. A model or ideal solution or test conditions is not required. As such, it offers a very versatile solution to the membrane research community.
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Why is zeta potential and surface charge important when developing membranes?
It is important to know how the membrane behaves or performs when in contact with actual solutions under real life conditions. The membrane surface behaviour can change quite significantly depending on the pH or ionic strength of the feed solution it is in contact with. As such, it is important to know the zeta potential and surface charge under such pH conditions beforehand in order to modify the membrane chemistry to tune its performance for the environment it is introduced to.
How do you plan on developing the SurPASS 3 further?
The SurPASS 3 is a state-of-the-art equipment that was released in late 2015. We continuously work on incorporating new capabilities and applications. As such, we are looking to introduce measuring cells capable of investigating contact lenses, tubing used in medical equipment, single hollow fiber membranes, core samples and many more.
Where can our readers learn more?
The Anton Paar website provides more information on the SurPASS 3 instrument and its capabilities. Furthermore, a free copy of the ZETA guide, a great resource with very useful information about streaming potential technique and applications, may be requested from our website.
Download the Full ZETA Guide
About Vinod Radhakrishnan
Dr. Vinod Radhakrishnan is the Senior Product Specialist for the Particle and Material Characterization group at Anton Paar USA and has been with the company since 2010. Dr. Radhakrishnan has a degree in Chemical Engineering from Auburn University, USA.
References
[1] Biomater Sci. 2015 Feb;3(2):265-78.
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