Boundary lubrication performance of polyelectrolyte-surfactant complexes on biomimetic surfaces

Aqueous mixtures of oppositely charged polyelectrolytes and surfactants are useful in many industrial applications, including shampoos and hair conditioners. In this work, we investigate the friction between biomimetic hair surfaces in the presence of adsorbed complexes formed from cationic polyelectrolytes and anionic surfactants in aqueous solution. For this purpose, we apply non-equilibrium molecular dynamics (NEMD) simulations using the coarse-grained MARTINI model. We develop new MARTINI parameters for cationic guar gum (CGG), a functionalized, plant-derived polysaccharide. The surface complexation of CGG and the anionic surfactant sodium dodecyl sulfate (SDS) on virgin and bleached hair is studied using a sequential adsorption approach, whereby CGG is deposited first, followed by SDS. The stability of the adsorbed complexes with respect to compression were then tested using squeeze-out simulations, where some of the water and SDS molecules are removed from the contact. Finally, we carry out sliding NEMD simulations to assess the boundary lubrication performance of the polyelectrolyte-surfactant complex compressed between virgin and chemically damaged biomimetic hair surfaces. We observe synergistic friction behaviour for the CGG-SDS complex, which gives lower shear stress than either pure CGG or SDS. We observe large reductions in the shear stress by the CGG-SDS complex at relatively low normal stress (≤ 20 MPa) compared to water. At higher normal stresses, which are probably beyond those usually experienced during hair manipulation, SDS and water are squeezed out and the shear stress with the complex increases to levels higher than those observed for pure water due to increased interdigitation. Thus, the CGG-SDS complex generally increases the friction coefficient, but decreases the adhesive Derjaguin offset compared to pure water. The outcomes of this work are expected to be beneficial to fine-tune and screen sustainable hair care formulations to provide low friction and therefore a smooth feel and superior entanglement behaviour.