Two Different Scenarios for the Equilibration of Polycation—Anionic Solutions at Water–Vapor Interfaces

Abstract

The assembly in solution of the cationic polymer poly(diallyldimethylammonium chloride) (PDADMAC) and two different anionic surfactants, sodium lauryl ether sulfate (SLES) and sodium N-lauroyl-N-methyltaurate (SLMT), has been studied. Additionally, the adsorption of the formed complexes at the water–vapor interface have been measured to try to shed light on the complex physico-chemical behavior of these systems under conditions close to that used in commercial products. The results show that, independently of the type of surfactant, polyelectrolyte-surfactant interactions lead to the formation of kinetically trapped aggregates in solution. Such aggregates drive the solution to phase separation, even though the complexes should remain undercharged along the whole range of explored compositions. Despite the similarities in the bulk behavior, the equilibration of the interfacial layers formed upon adsorption of kinetically trapped aggregates at the water–vapor interface follows different mechanisms. This was pointed out by surface tension and interfacial dilational rheology measurements, which showed different equilibration mechanisms of the interfacial layer depending on the nature of the surfactant: (i) formation layers with intact aggregates in the PDADMAC-SLMT system, and (ii) dissociation and spreading of kinetically trapped aggregates after their incorporation at the fluid interface for the PDADMAC-SLES one. This evidences the critical impact of the chemical nature of the surfactant in the interfacial properties of these systems. It is expected that this work may contribute to the understanding of the complex interactions involved in this type of system to exploit its behavior for technological purposes.

Surfactant-Like Behavior for the Adsorption of Mixtures of a Polycation and Two Different Zwitterionic Surfactants at the Water/Vapor Interface

“…For the latter, even in the one-phase region of the phase diagram, the onset in the two-phase region of the phase diagram was found for surfactant concentrations more than one order of magnitude lower than those corresponding to the equilibrium composition. This phase separation has its origin in the Marangoni stresses arising during the mixing process of polymer and surfactant solutions, which leads to the formation of persistent kinetically trapped aggregates far from the isoelectric point [25,27,34,41,42]. These aggregates remain upon dilution of the samples, yielding a turbidity increase [43].…”

Physicochemical Aspects of the Performance of Hair-Conditioning Formulations

“…These surfactants are considered to be the most efficient in the cleaning of sebum fats and dirt in general. However, an excessive deposition of this surfactant onto the surface of the hair fibers during the washing process may increase the number of negative charges, which would be detrimental for the conditioning process as result of the increase in the friction [7,[41][42][43][44][45][46]. • Cationic surfactants.…”

“…The interactions involved in the assembly of polyelectrolyte-surfactant complexes may be exploited by varying the characteristics of the polyelectrolytes (such as charge density, molecular weight or functional groups) and surfactants (nature of the hydrophilic head, hydrophobic chain length and concentration) [80,81] or the solution properties (pH, ionic strength and temperature). Such interactions decisively impact the rich phase behavior of polyelectrolyte-surfactant mixtures, in which one-or multiphase mixtures are obtained depending on the composition and the nature of the components [6,44,83]. The existence of multiphase regions, the so-called phase-separation regions, has a central role in the hair-conditioning process.…”

“…Such interactions decisively impact the rich phase behavior of polyelectrolyte-surfactant mixtures, in which one-or multiphase mixtures are obtained depending on the composition and the nature of the components [6,44,83]. The existence of multiphase regions, the so-called phase-separation regions, has a central role in the hair-conditioning process.…”

Colloids at Fluid Interfaces

“…This can lead to a change in the material size (dilation) or in its shape (shear). The understanding of the relaxation processes as a result of mechanical deformation is of central importance for describing the mechanical behavior of materials, and their potential use in the fabrication of systems with technological interest [51,[115][116][117][118]. This has stimulated extensive research activity aimed to shed light on the behavior of colloidal particles confined in 2D, which is of particular interest to the understanding of biological processes involving self-assembled systems operating under continuous mechanical perturbations.…”