Effect of molecular structure of eco-friendly glycolipid biosurfactants on the adsorption of hair-care conditioning polymers

Pseudo-binary mixtures of different glycolipids, four different rhamnolipids (RL) and an alkyl polyglucoside (APG), with poly(diallyl-dimethylammonium chloride) (PDADMAC) have been studied in relation to their adsorption onto negatively charged surfaces to shed light on the impact of the molecular structure of surfactants from natural sources (instead of synthetic surfactant, such as sodium laureth sulfate) on the adsorption of hair-conditioning polymers. For this purpose, the self-assembly of such mixtures in aqueous solution and their adsorption onto negatively charged surfaces mimicking the negative charge of damaged hair fibres have been studied combining experiments and self-consistent field (SCF) calculations. The results show that the specific physico-chemical properties of the surfactants (charge, number of sugar rings present in surfactant structure and length of the hydrocarbon length) play a main role in the control of the adsorption process, with the adsorption efficiency and hydration being improved in relation to conventional sulfate-based systems for mixtures of PDADMAC and glycolipids with the shortest alkyl chains. SCF calculations and Energy Dispersive X-Ray Spectroscopy (EDS) analysis on real hair confirmed such observations. The results allow one to assume that the characteristic of the surfactants, especially rhamnolipids, conditions positively the adsorption potential of polyelectrolytes in these model systems. This study provides important insights on the mechanisms underlying the performance of more complex but eco-friendly washing formulations.