Neutron beams: key to a deeper understanding of surface properties

While several probes allow for the characterization of formulations in great detail, only neutron beams can generate a truly detailed pictures of surface composition. Richard A. Campbell explains why, when it comes to optimising the performance of complex formulations in the chemical industry, neutron beams are rather special.


We demonstrated using a set of air / liquid troughs the link between the peak and aggregation in the bulk solution

There are lots of probes that can be used to characterise formulations both in the bulk solution and their performance at surfaces. Well known laboratory tools include UV-vis spectroscopy to monitor sample turbidity and mass spectrometry or nuclear magnetic resonance, to identify reaction products and their distribution.

A more exotic probe is neutrons. There are only a handful of places in the world which generate neutron beams that can be used to zap samples, in a bid to understand their detailed structure and composition. As such, time on neutron beam lines is at a premium. However, their capabilities are rather special.

In comparison with visible light, neutrons have a wavelength around a thousand times shorter, which is on the order molecular dimensions. Therefore, when neutrons interact with nanostructures in solution or self assembled at interfaces, they produce an interference pattern that can be used to deduce precisely the molecular structure.

Another advantage of neutrons is that they interact differently with different isotopes of the same sample. Hence isotopic substitution can be employed to produce several different scattering patterns of the same sample which can then be used to deduce the composition of a mixture.

FIGARO: answering hard questions about soft matter

"There are only a handful of places in the world which generate neutron beams that can be used to zap samples."

A typical question that can be answered with neutrons is to determine the functionality of a particular active ingredient. For example, if it were demonstrated that an expensive new component improves the performance of a given formulation, neutrons could be used to determine just how much of it is present in an aggregate in solution or coated on a surface.

Also, the mechanism of its functionality can be resolved by determining the mode of binding with other components of the same sample.

As a physical chemist by training, I am responsible for a neutron reflectometer called FIGARO (Fluid Interfaces Grazing Angles ReflectOmeter) at the Institut Laue-Langevin (ILL) in Grenoble, France.

The ILL is a publically-funded research facility by a consortium of different countries providing a resource for scientists who publish their results; although some beam time is sold to private companies. On my instrument, neutrons are skimmed off surfaces at an angle of around one degree to understand the interfacial properties.

"A typical question that can be answered with neutrons is to determine the functionality of a particular active ingredient."

FIGARO is used to solve all sorts of problems in soft matter. For example, researchers from Spain and Mexico are interested in why films of sugar based molecules with surfactants are viscoelastic, researchers from Sweden and Australia are trying to understand the structure of films of proteins with nanoparticles concerning nanotoxicology, and researchers from the UK have looked at the activity of biosurfactants in mixtures with conventional surfactants.

FIGARO is also a platform that allows me to run my own research programme into the surface organization of polymer / surfactant mixtures in collaboration with Dr. Imre Varga of Eötvös-Loránd University in Budapest, as well as colleagues from Sweden and the UK.

What interests us is how aggregation in strongly interacting mixtures can often be seen as a problem by industry, yet this is where non-equilibrium effects can be experienced, hence there is potential to tune the behavior of a sample by changing its pathway of interaction and thus overcome the problem. The industrial relevance of three of our recent papers is described below.

Industrial applications for neutron beam technology

First we investigated a peculiar peak in the surface tension of a model strongly interacting polymer / surfactant mixture (ref. 1). In formulations such an anomaly would reduce the performance of the additive, often requiring the introduction of further surfactant at extra cost.

"First we investigated a peculiar peak in the surface tension of a model strongly interacting polymer / surfactant mixture."

We demonstrated using a set of air / liquid troughs the link between the peak and aggregation in the bulk solution, yet simply by controlling the way the materials are handled we showed that we could reload the interface with as much surface-active material as we wanted.

Then we looked at the mechanism of formation of interfacial multilayers. We used a novel set of sandwich-like solid / liquid / solid interface cells to decouple of effects of surface self assembly from the transport to interfaces under gravity of nanostructured particles that had formed in solution (ref. 2). The distinction of such processes in a range of mixtures can give a better understanding of how formulations are achieving optimum properties at surfaces.

Earlier this year we exploited a piece of equipment called an overflowing cylinder in a bid to create conditions more relevant to processing and applications of formulations (ref. 3). The surface is continually perturbed to allow the study of interfacial properties under controlled flow. We found that a polymer / surfactant mixture behaved very differently just by changing the pH: in one case bulk aggregation reduced the surface activity, but in another case it enriched the surface excess due to a convection / spreading mechanism. Clearly the basis of our ability to predict surface properties of formulations under dynamic conditions is missing, and this work highlighted the need for more detailed studies under industrially relevant conditions.

It strikes me that several ingredients were required to make possible these findings: not only a world class neutron reflectometer at a world class neutron source, but also the various pieces of equipment that we used: the air / liquid troughs, the solid / liquid / solid interface cells and the overflowing cylinder.

With these resources, I am optimistic for the future about our potential to bridge the gap between the fundamental understanding required by academic scientists and the financial benefit to the chemical industry of optimising the performance of complex formulations at surfaces under practically relevant conditions.

References

(1) Campbell, Yanez Arteta, Angus-Smyth, Nylander & Varga, J. Phys. Chem. B, 2011, 115, 15202.
(2) Campbell, Yanez Arteta, Angus-Smyth, Nylander & Varga, J. Phys. Chem. B, 2012, 116, 7981.
(3) Angus-Smyth, Bain, Varga & Campbell, Soft Matter, 2013, 9, 6103.

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