Improving the sustainability of cosmetic products while maintaining a good performance requires a deeper understanding on the way that new eco-respectful ingredients interact with hair or skin. In the case of shampoos, the surface science is dominated by the diverse changes on the hair fiber due to both chemical and physical damages that particularly affect physicochemical properties such as hydrophobicity. A native, undamaged fiber is covered with a monolayer of lipids, mainly 18-methyleicosanoic acid (18-MEA), while a highly damaged hair surface, having completely lost the protective lipids, is hydrophilic and negatively charged. Intermediate states exist, where there is a partial loss of 18-MEA (“partially damaged hair”). Here, four model surfaces have been produced, to mimic different types of hair surfaces. Their interaction with selected surfactants and polyelectrolytes (natural and synthetic) has been studied by neutron reflectometry (NR). NR can reveal hierarchical adsorption from mixtures thanks to the scattering contrast between deuterated and hydrogenous molecules. Atomic force microscopy (AFM) measurements complement the study by adding information about the in-plane structure of adsorbed species. The presence of the methyl branch of 18-MEA is found to affect the interaction of the surface with adsorbates. For surfactant/polyelectrolyte mixtures, for example, the adsorption of polymer is enhanced. Of particular interest are the results on the partially damaged hair model, as it manifests patches of hydrophobic and hydrophilic moieties; it is possible to separately observe the different adsorption behaviors to the different sites in a single experiment.

Long, straight chain saturated fatty acids form homogeneous, featureless monolayers on a supramolecular length scale at the water–air interface. In contrast, a naturally occurring saturated branched fatty acid, 18-methyl eicosanoic acid (18-MEA) has been observed to form three-dimensional domains of size 20–80 nm, using a combination of Langmuir trough, Atomic Force Microscopy (AFM) images of the deposited monolayers, and Neutron reflectometry (NR) and X-Ray reflectometry (XRR). It is hypothesized that these domains result from the curvature of the water surface induced by the steric constraints of the methyl branch. Accordingly, in this work, we investigate in situ the structure of such films using Grazing Incidence Small Angle X-ray Scattering and Diffraction (GISAXS and GIXD). The branched fatty acids indeed form curved nanodomains as revealed by their two-dimensional scattering pattern whereas straight chain fatty acids form the expected featureless film, with no GISAXS scattering peaks. Mixed monolayers consisting of 18-MEA and eicosanoic acid (EA) display a phase transition in the structure from hexagonally packed at high 18-MEA ratio to structures with one-dimensional translational ordering (aligned stripes) for 50:50 mol% and lower ratios. Moreover, the GIXD patterns of monolayers containing 18-MEA display a peak with curved distribution of intensity, indicating a continuous distribution of collective molecular orientations, consistent with the local curvature of the water surface. Finally, we report on an unusual double peak phenomenon in the GISAXS data that is interpreted as being due to a hexagonal packing of elliptical domains – i.e. with two characteristic dimensions. Synchrotron X-Ray scattering experiments have thus unambiguously confirmed the self-assembly, out of plane, “cobbling” of the water interface by these branched structures.

A comprehensive understanding of chemical interactions at the surface of hair represents an important area of research within the cosmetic industry and is essential to obtain new products that exhibit both performance and sustainability. This paper aims at contributing to this research by applying a combination of surface techniques (neutron reflectometry, quartz-crystal microbalance and atomic force microscopy) to study adsorption of surface active ingredients onto hair-mimetic surfaces. The surface of hair is not homogeneous due to chemical and physical damage, and this work focuses on partly damaged hair models, in which both hydrophobic and charged moieties are present. Examples of such mixed-surface models are rare in the literature, despite the interest in the topic. The studied actives were an anionic surfactant (sodium dodecyl sulphate, SDS) and a natural polysaccharide (chitosan) of two different molecular weights, to represent soluble polymer-surfactant associations of cosmetic interest in hair-care rinsing applications. The effect of the concentration of SDS, the molecular weight of chitosan, and the order in which SDS and chitosan are introduced are studied, and compared to totally hydrophobic and totally hydrophilic surfaces. Results show that SDS can interact with the hydrophobic portions of the mixed surface, and its adsorption increases if associated with chitosan. Interestingly, differences have been found in the adsorption behaviour of chitosan depending on its chain size. Both types can deposit onto the surface, but when SDS is added, the lower molecular weight chitosan keeps its extended conformation in a ca. 70 Å thick layer, while the higher molecular weight chitosan collapses to form a layer of about 30 Å. This knowledge opens the door to developing hair-care formulations with improved performance and sustainability.

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.  

Att formulera ett schampo är en komplex process som inte bara måste ta hänsyn till hårfiberns olika fysikalisk-kemiska egenskaper utan också till kundernas behov. Därför innehåller schampon normalt tensider som rengörande bas, polyelektrolyter för en vårdande effekt och flera tillsatser. De befintliga produkterna har optimerats under många år, men de aktuella miljöfrågorna kräver att kosmetikaindustrin byter till mer hållbara formuleringar. För att kunna ersätta traditionella med miljövänliga, biologiskt framställda ingredienser krävs detaljerad kunskap om de interaktioner som sker på hårets yta. Detta doktorandprojekt syftade till att bidra till denna kunskap genom att använda neutronreflektometri (NR) för att studera adsorptionen av modellföreningar till hårmimetiska ytor. Fördelen med NR jämfört med andra yttekniker är dess förmåga att karakterisera begravda gränssnitt och definiera en hierarki av adsorption från blandningar. De biomimetiska modellerna kan ställas in för att återge hårytan under olika förhållanden. En frisk fiber är hydrofob, eftersom den är täckt av ett lager lipider, varav den viktigaste är 18-metyleicosanoic acid (18-MEA), som har en karakteristisk metylgren och är föremål för flera studier på grund av sina intressanta egenskaper. På grund av vittring, åldrande eller behandlingar som blekning kan lipidlagret skadas och en hydrofil yta exponeras. Detta förändrar hårfiberns interaktion med komponenter i hårvårdsprodukter. Genom att komplettera NR med andra yttekniker har specifika adsorptionsbeteenden identifierats, med hänsyn till faktorer som ytans hydrofobicitet, ytaktiva ämnenas laddning eller polyelektrolyternas storlek. Resultaten visar t.ex. att närvaron av metylgrenen i 18-MEA förändrar ytegenskaperna jämfört med ett lager av rakkedjiga lipider, eller att en helt skadad hårmodellyta oväntat adsorberar ett tvåskikt av anjoniskt ytaktivt ämne, tack vare en balansering av flera faktorer som spelar en roll i interaktionen.