  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 biodegradable surfactants from natural sources (instead of
synthetic surfactant, such as sodium laureth sulfate) on the adsorption of
hair-care formulations. 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 using
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 studied surfactants, especially
rhamnolipids, conditions positively the adsorption potential of
polyelectrolytes in our model systems. This study provides important insights
on the mechanisms underlying the performance of more complex but natural and
eco-friendly washing formulations.
