Effect of a natural amphoteric surfactant in the bulk and adsorption behavior of polyelectrolyte-surfactant mixtures

The addition of a natural and biodegradable zwitterionic surfactant to mixtures formed by a polycation and an oppositely charged surfactant is expected to modify the self-assembly in solution, and the adsorption of the formed complexes onto solid surfaces holding negative charges. This is the result of a change of the physicochemical properties of the mixtures, which may affect to its performance in technological applications. This has been explored by studying the self-assembly in solution of pseudo- binary and pseudo-ternary mixtures formed by a cationic homopolymer, poly(diallyldimethylammonium chloride) (PDADMAC) and two different surfactants, the negatively charged sodium laurethsulfate (SLES) and the sulfate-free zwitterionic coco betaine (CB), and the adsorption of the formed complexes onto solid surfaces holding similar negative surface charge than damaged hair fibers. It was found that the mixing protocol used for preparing the polymer-surfactant mixturess leads to the formation of kinetically trapped polyelectrolyte-surfactant aggregates far from the isoelectric point due to local Marangoni gradients, with the adsorption of such aggregates onto solid surfaces being enhanced with the increase of the surfactant concentration, i.e. as they approach to the onset of the phase separation region, which leads to the densification of the adsorbed layers. A strong synergetic effect on the adsorption associated with the addition of the zwitterionic surfactant to the mixture polycation-anionic surfactant was found was found; which helps, in part, to provide an explanation for the observed conditioning performance of shampoos containing both surfactants.