Hierarchical adsorption at hair-mimetic interfaces : A neutron reflectivity study

Formulating a shampoo is a complex process that has to consider not only the diverse physicochemical properties of the hair fibre but also customers’ needs. For this reason, shampoos normally contain surfactants as cleansing base, polyelectrolytes for a conditioning effect and several additives. The existing products had years of optimization, but the current environmental issues require the cosmetic industry to switch to more sustainable formulations. To replace traditional ingredients with eco-friendly, bio-sourced ones, a detailed knowledge of the interactions happening at the hair surface is essential. This PhD project aimed at contributing to this knowledge by using neutron reflectometry (NR) to study the adsorption of model compounds to hair-mimetic surfaces. The advantage of NR over other surface techniques is its ability to characterize buried interfaces and define a hierarchy of adsorption from mixtures. The biomimetic models can be tuned to reproduce the hair surface in different conditions. A healthy fibre is hydrophobic, as it is covered by a layer of lipids, the main one being 18-methyleicosanoic acid (18-MEA), which has a characteristic methyl branch and is the subject of several studies due to its interesting properties. Due to weathering, ageing, or treatments like bleaching, the lipid layer can be damaged, and a hydrophilic surface is exposed. This modifies the interaction of the hair fibre with components of hair-care products. Complementing NR with other surface techniques, specific adsorption behaviours have been identified, addressing factors such as surface hydrophobicity, surfactant charge or polyelectrolyte size. Results indicate that, for example, the presence of the methyl branch of 18-MEA modifies the surface properties compared to a layer of straight chain lipids, or that a fully damaged hair model surface unexpectedly adsorbs a bilayer of anionic surfactant, thanks to the balancing of several factors playing a role in the interaction.