Using Inverse Gas Chromatography to Understand Surface Energy

The quantification of attractive intermolecular forces within a solid provides its surface energy - an analogous value to a liquid’s surface tension. Surface energy is a key property of a particulate or powdered material by researchers.

AZoM talks with Anett Kondor, PhD, from Surface Measurement systems about the iGC SEA benefits anyone who wishes to measure surface energy.

What is specific in iGC as compared to other chromatographic techniques?

Chromatography is a process which involves distribution of solute component between two phases, a mobile phase and a stationary phase. In inverse Gas Chromatography (iGC), the mobile phase is a gas and the stationary phase is a solid, while the solute is the adsorbate.

The term inverse indicates that the material of interest is placed into a chromatographic column and the behaviour of carefully selected test solutes is studied. iGC provides the easiest of all means of studying the thermodynamics of the interaction of a volatile solute with a non-volatile material. In the past 60 years, this technique has become a well-established and accepted source of physicochemical data of different solid materials.

The Inverse Gas Chromatography Surface Energy Analyzer (iGC-SEA) by SMS is the first commercial iGC on the market since 2009.

shutterstock.com | Rueangwit

What are the advantages of iGC-SEA?

iGC-SEA is called Surface Energy Analyzer because the main application is the determination of surface energy (SE), nevertheless, it is suitable to determine other physicochemical properties such as Heat of Sorption, BET Specific Surface Area, Glass Transition Temperature and so on.

iGC-SEA is specially designed for inverse gas chromatographic measurements, therefore, the temperature, pressure and flow rate in the column oven are more stable as compared to the home-built iGC systems, and it is required for the determination of thermodynamic parameters. The operation of home-built iGC systems require a person in full time, so one has to live with it in symbiosis, so to speak.

The iGC-SEA system is fully automatic with up to 250 injections preprogramed in each method, and up to 10 methods can be preprogramed to run in a sequence. The column packing is quite easy due to the straight column design and the helpful tools that we provide along with the system. To top it all, it has an excellent reproducibility (~1%RSD). And finally, the user friendly analysis software provides directly and automatically all the physicochemical properties of solid materials. In short, iGC-SEA is a versatile and powerful technique in characterization of solid materials.

How are the samples examined by iGC-SEA?

The sample of interest has to be packed into a straight silanized glass column which is 30cm long and the inner diameter of which can be 2mm, 3mm or 4mm depending on the morphology of the solid material.

There are two column positions in the oven of iGC-SEA that can be measured sequentially.  The packed column has to be placed into the oven then known vapour probes have to be injected into the column via an inert carrier gas. These probe molecules interact with the solid sample and are eluted by the carrier gas whereupon their retention behaviour is recorded by the Flame Ionization detector (FID).

This allows for the explicitly determination of retention volume and partition coefficient for the solid-vapour interaction.

In this demonstration, hair fibre is used as an example

Please click here if you would like more information on the instrument used in this article or a quote

What extra options does Surface Measurement Systems (SMS Ltd.) offer in the field of iGC?

Surface Measurement Systems provides some unique options to iGC-SEA systems such as background humidity controller, external film cell oven and large column oven design.

One of the greatest benefits is the background humidity option which allows running experiments under relative humidity, providing an extremely authentic analysis of solid materials. The impact of relative humidity can be determined on the different surface properties, which could affect the applications of solid material.

There are hardly any analytical instruments, if any, that can determine this; therefore, this option makes iGC-SEA even more unique and powerful in its field. With respect to practical examples, the external film cell oven can be used for characterization of polymer films, composite plates and coated metal or glass sheets etc.

The iGC-SEA

What different properties influence the surface energy (SE)?

There are some chemical and structural properties that could affect the surface energy of a solid material, especially its surface energy heterogeneity. Such properties are, for example, the crystallinity degree, crystal imperfections, surface irregularities, the particle size distribution, the impurities, chemical degradants and so on.

On the other hand, if we are interested in milling, grinding or micronization, we must consider the effect of these processes on the surface properties, more specifically the surface energy of the solid material.

What attributes of solid materials significant in Food Industry deducted from measurement of SE?

Surface energy is used to estimate bulk powder properties such as flow, wetting, coating, mixing and micronization. It has been found to be very effective to determine wetting phenomena, shelf life and processing (like moulding) of solid materials significant in the food industry.

Besides surface energy, the BET surface area (or effective surface area) of solid materials also has influence on bulk powder properties, which can be measured by iGC-SEA as well.

Recently you have published an article about relationships between surface energy analysis and functional characteristics of dairy products. Could you summarize it in a few sentences?

The study was completed in collaboration with Teagasc, Ireland, and we co-authored it with Sean Hogan. We determined the surface energetics of demineralised whey (DMW), skimmed milk (SMP), phosphocasein (PCN) and infant milk formula (IMF) powders using iGC-SEA.

All four milk powders were amphoteric in nature with the dispersive (apolar) component of surface energy dominating the specific (polar) contribution. PCN and IMF has the highest and lowest extent of surface heterogeneity, respectively. PCN also demonstrated the poorest functional properties of the powders examined. In contrast, IMF has excellent flow and rehydration properties. Thermodynamic work of cohesion was measured highest in PCN, which may have contributed to its inadequate rehydration behaviour.

Using iGC-SEA, we also measured the Glass Transition Temperature (Tg) of IMF powder, and the resulting value suggested a surface dominated by lactose. Surface heterogeneity provided a better indicator of functional behaviour than total surface energy. We also found that IGC is a useful complementary technique for chemical and structural analysis of milk powders, and it allows improved insight into the contribution of surface and bulk factors to functionality.

shutterstock.com | science photo

How does Surface Measurement Systems help its customers tailor measurements to their applications?

Surface Measurement Systems (SMS) is dedicated to sorption science, and several scientists who received their PhD in this field are working for the company. They are all happy to be of help in tailoring measurements to customers’ needs and providing world-class scientific and technical support to our international customers.

Based on our experience, it is especially important during the initial steps our customers take in this field when they conduct their first few measurements or research studies using our instruments.  

Where can our readers find out more about iGC-SEA and Surface Measurement Systems?

Surface Measurement Systems is based in London, UK, but also have an office in Allentown, PA, USA, as well as distributors world-wide.

They can contact us directly through our website or through our local distributors. The contact list is available on our website at surfacemeasuremnetsystems.com.

About Dr Anett Kondor

Anett Kondor, PhD is the IGC-SEA Product Manager and Product Specialist of Surface Measurement Systems, and has 13 years of experience in gas chromatographic (GC and iGC) techniques.

She studied chemical engineering at the University of Pannonia, Hungary where she received her MSc in Chemical Engineering, accredited by British Institution of Chemical Engineers, and her PhD in Chemistry and Environmental Science. She published several research work in peer-reviewed journals collaborating with research groups around the world (Deakin University – Australia, Composite Innovation Centre – Canada, Teagasc – Ireland, Boku University – Austria, Liverpool John Moores University – UK, Givaudan –Switzerland and so on).

Her research interest lies in the area of surface characterization of different solid materials and improvements in inverse gas chromatographic technique.