The neutron effect

Paul Boughton

Richard A Campbell explains the impact of neutrons on optimising the engineering of formulations in the chemical industry.

Neutrons are ideal probes to characterise soft matter formulations. They have a wavelength on the order molecular dimensions at about a thousand times shorter than visible light. An interference pattern is produced that can be used to resolve molecular structure when neutrons interact with nanostructures in solution or at interfaces. Furthermore, neutrons interact differently with different isotopes of the same sample, which means that isotopic substitution can be employed to deduce the bulk or surface composition of a mixture.

As a result of neutrons being selective to different species in the same mixture, they can be exploited to determine the activity of a particular component in a mixture. For example, if a new ingredient improves the performance of a given formulation, neutrons can be used to determine just how much of it is present in a bulk particle or at an interface.

The Institut Laue-Langevin (ILL) in Grenoble, France, is a research facility funded publically by several different countries and provides a resource for scientists who publish freely their results.

Two techniques we routinely use to characterise liquid samples are neutron reflectometry (NR) for the properties and behaviour of surfaces and small-angle neutron scattering (SANS) for corresponding information in bulk solutions. In NR a collimated beam of neutrons is reflected off samples at a grazing angle of only a degree or so, and free air/liquid interfaces as well as buried solid/liquid interfaces can be probed.

Applications are quite diverse, from protein/nanoparticle interactions concerning nanotoxicity to solvent-drying in paints to rheometry of polymer blends. In SANS a ray of neutrons transmits directly through the bulk sample and is slightly deflected. Areas of interest include the interactions of macromolecules with lipid vesicles, the organisation of proteins on nanoparticles and the self-assembly of surfactants in emulsion droplets.

Many of the scientific domains studied with neutrons are of fundamental interest to scientific researchers. However, ILL is developing equipment to study materials under industrially relevant conditions.

Concerning NR, we have recently supported a PhD project to develop an overflowing cylinder on the beamline where the surface is continually perturbed and samples are under steady state flow to recreate more realistic non-equilibrium conditions relevant to the processing and applications of commercial products. For SANS, we have developed a suite of devices for the study of complex fluids under shear flow in order to correlate the macroscopic rheological response of a material with its internal structure. With these resources we aim to bridge the gap between the fundamental understanding required by academic scientists and financial benefits to the chemical industry of optimising formulations under practically relevant conditions. The industrial significance of some very recent studies, two at surfaces characterised by NR and two in the bulk solution characterised by SANS.

The first NR study concerns the mechanism of formation of interfacial multilayers in strongly interacting polymer/surfactant mixtures. We invented a set of solid/liquid/solid interface cells to decouple of effects of surface self-assembly from the transport to interfaces under gravity of nanostructured particles1. One single crystal sits above and another sits below a liquid reservoir. The comparison of results at different interfaces of the same sample revealed that the surface properties were being influenced by the position of the interface, ie, gravity determined the surface properties due to phase separation. Such measurements at a single interface would have missed this point. We showed that the distinction of different interfacial mechanisms in a range of mixtures can lead to a better understanding of how formulations can be used to achieve optimum surface properties.

Another recent NR study exploited an overflowing cylinder which recreates practical conditions relevant to the processing and applications of formulations2. Here neutrons are skimmed off the flowing liquid surface to reveal the composition of surface layers under non-equilibrium conditions. We showed that a polymer/surfactant mixture behaved differently under dynamic conditions when the pH changed: in one case bulk aggregation reduced the surface activity but in another case it enriched the surface excess due to a supplementary convection/spreading mechanism. This behaviour was almost exactly the opposite with what had been determined previously at static surfaces. It seems our ability to predict surface properties of formulations under dynamic conditions is missing.

Recently, small-angle neutron scattering experiments were used to study a new class of surfactants with magnetically-active sections containing iron3. It was demonstrated that surfactant micelles interact cooperatively in a magnetic field allowing the solutions to be manipulated with even with small magnet. The possibility to remove a soap after it has been added to a system widens potential large scale applications to areas such as oil spill clean-ups.

The reason for the magnetic properties was elucidated by a SANS experiment that provided evidence that within the micelles the iron ions were closely arranged. Now that this mechanism is known, the production of such soap on a large scale can be considered. Magnetic soap has also a potential application for drug encapsulation and targeted drug delivery to deliver medication where it is required without loss of the valuable pharmaceutical compounds to other areas of the body.

Lastly, another industrially-relevant area starting to be addressed by SANS is the application of soft materials in industry involving unusual complex flow behaviour4. A range of rheology and flow devices currently being developed at the ILL can benefit the engineering of soft material products and the optimisation of processing conditions.

Dr Richard A Campbell is with Institut Laue-Langevin, Grenoble, France.