Surface forces and frictional properties of adsorbed bio-based cationic polysaccharide thin films in salted aqueous medium

Inter-surface forces mediated by polymer films are important in a range of technological and industrial situations. In cosmetics, applications such as hair conditioning typically rely on the adsorption of polyelectrolyte films onto the charged surface of hair fibers, whose contact mechanics and tribological properties are central in determining the final sensorial perceptions associated with the cosmetic treatment. A major current challenge to be tackled by the cosmetic industry is to design high-performance products employing bio-sourced polyelectrolytes, with the aim of achieving eco-sustainable processes and products. In this context, the present study focuses on the mechanical properties of thin films obtained by adsorption from solution of fungal chitosan onto negatively charged mica surfaces. We use a Surface Forces Apparatus allowing for the simultaneous measurement of film thickness and friction force as a function of the applied normal load and shear velocity. We show that, in aqueous medium at an ionic strength of 40 mM, adsorbed films of chitosan give rise to repulsive inter-surface forces whose range, comparable to the Flory radius of the macromolecules, increases with the polymer molecular weight. In addition, the adsorbed layers are found to behave, under compressive forces, as pseudo-brushes of neutral polymers. Finally, we show that under shear forces, chitosan layers exhibit a transition from a low to a high friction regime under increasing confinement.