Modifying polyelectrolyte charge density and surface coverage for enhanced polyelectrolyte-surfactant lubrication of biomimetic interfaces.

Mixtures of polyelectrolytes and surfactants are useful components of many fluid formulations, including shampoos and conditioners. These mixtures form polyelectrolyte-surfactant complexes in solution that can adsorb on solid surfaces and significantly reduce friction. Experiments have shown that the charge density of polyelectrolytes affects their adsorption and conformation on the surface, which are expected to strongly influence their lubrication performance. Here, we use coarse-grained, non-equilibrium molecular dynamics simulations to study how different charge densities and surface coverages of a polyelectrolyte, cationic guar gum, in the presence of an anionic surfactant, sodium dodecyl sulfate, influence hair friction. We observe improved lubrication for polyelectrolytes with higher charge densities on biomimetic surfaces representative of both virgin and bleached hair. This is attributed to increased surface separation, which is facilitated by higher retention of interfacial surfactants and water. Higher polyelectrolyte surface coverages also result in lower friction through the same mechanism. At sufficiently high polyelectrolyte surface coverages, friction becomes insensitive to the level of chemical damage on the hair surface. Most polyelectrolytes have flat, train-like conformations, particularly at high charge density and low surface coverage. The number of tail and loop conformations increases at lower charge density and higher surface coverage, but this has a minor effect on friction compared to the increased film thickness. These simulations show considerable promise for virtually screening polyelectrolytes with optimal lubricity performance in liquid formulations.