Good hair day: how neutrons help understand shampoo-hair interactions

Research is constantly ongoing to optimise the properties of shampoos. This study could help design better, more sustainable hair-care products with improved conditioning effects. It was developed at ILL in collaboration with L'Oréal.

Hair is a highly complex and interesting structure. On a molecular level, it consists of a protein network covered by fatty molecules (lipids). Shampoos and other haircare products can adsorb on this structure, which is the basis for their beneficial effects such as cleansing and conditioning.

Different treatments, e.g. bleaching, can alter the lipid layer of hair, thereby also changing how shampoo adsorbs. Most studies are based on models of either fully damaged or fully undamaged hair, while it is actually crucial to take into account the high prevalence of hair that has been only partly modified or damaged by treatments.

A recent study focused on this particular challenge by designing novel, mixed-composition surfaces which mimic partially damaged hair lacking part of its lipid layer. The team then investigated the adsorption of two molecules on these surfaces: sodium dodecyl sulfate (SDS, a model cleansing agent present in most soaps) and chitosan, a complex sugar molecule which is a promising sustainable alternative to some presently used synthetic shampoo ingredients.

The study was part of lead author Serena Cozzolino's PhD project, in the framework of InnovaXN, a EU-funded doctoral training programme bringing together large-scale research infrastructures and industry. The project was developed at the ILL, in collaboration with researchers from the cosmetics company L'Oréal and KTH Royal Institute of Technology in Sweden.

The researchers used surface-sensitive methods to conduct their studies, with a particular focus on neutron reflectometry (NR) experiments performed on ILL's instrument D17 and on INTER at the ISIS Neutron and Muon Source. "NR is able to detect even small changes on surfaces and also allows for tracking of adsorption processes in real-time", explains Philipp Gutfreund, local contact on D17.

First results showed that SDS adsorbed on the hair-mimetic surface by interacting with its hydrophobic parts. In the case of chitosan, both a long and a short version of the molecule adsorbed to the surface irreversibly; more molecules of the shorter version adsorbed.

Subsequent experiments focused on SDS adsorption to hair after some chitosan had already adsorbed. "Here, SDS adsorption strongly depended on whether we used a longer or a shorter form of chitosan", explains Serena Cozzolino. The longer form of chitosan lead to more SDS adsorption than the monomer. In addition, SDS was shown to leave the surface more easily during rinsing, leaving behind a chitosan layer.

In a last experiment, both chitosan and SDS were allowed to adsorb simultaneously. Here, more SDS was present in the film formed on the biomimetic hair structure. The film extended to larger distances from the surface, containing a smaller fraction of chitosan.

"These successful experiments inspired us to conduct a further study on hair-shampoo interactions which is currently ongoing", says Serena Cozzolino. "Such studies are extremely important for the cosmetic and haircare industry since they help us understand our formulations in great detail. This allows us to optimise them on a molecular level", adds Gustavo S. Luengo research fellow at L'Oreal Research and Innovation, France - the corresponding industrial partner of the study.

Overall, the experiments performed by the team demonstrate how neutron science can bridge fundamental research and industrial needs, thereby driving sustainable innovations.