Equilibration of a Polycation–Anionic Surfactant Mixture at the Water/Vapor Interface

Abstract

The adsorption of concentrated poly(diallyldimethylammonium chloride) (PDADMAC)-sodium lauryl ether sulfate (SLES) mixtures at the water/vapor interface has been studied by different surface tension techniques and dilational viscoelasticity measurements. This work tries to shed light on the way in which the formation of polyelectrolyte-surfactant complexes in the bulk affects the interfacial properties of mixtures formed by a polycation and an oppositely charged surfactant. The results are discussed in terms of a two-step adsorption-equilibration of PDADMAC-SLES complexes at the interface, with the initial stages involving the diffusion of kinetically trapped aggregates formed in the bulk to the interface followed by the dissociation and spreading of such aggregates at the interface. This latter process becomes the main contribution to the surface tension decrease. This work aids our understanding of the most fundamental basis of the physicochemical behavior of concentrated polyelectrolyte-surfactant mixtures which present complex bulk and interfacial interactions with interest in both basic and applied sciences.

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

“…Furthermore, the absence of effective screening of the average charge of the polyelectrolyte chains due to surfactant binding may determine the colloidal stability of the complexes (quasi-null values of turbidity close to pure PDADMAC solutions as shown Figure 2b). These results contrast with those corresponding to mixtures of PDADMAC and different anionic surfactants prepared using similar concentrations and mixing protocol [25,34]. 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.…”

“…), which implies that the number of positively charged monomers available was more than 30-fold the number of surfactant molecules available for surfactant concentrations around 1 mM. Hence, assuming the binding of the major part of surfactant molecules to the polymer chain (which was found to be true for anionic surfactants [25,34]), many monomers remain free without interacting with surfactant molecules, thus leading to the formation of undercompensated complexes. Furthermore, the zwitterionic character of CB and CAPB leads to a situation in which the neutralization of the charges of the monomers due to the binding of surfactant molecules is counteracted as result of the presence of a positive charge in the hydrophilic head of the surfactant.…”

“…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].…”

“…The PDADMAC concentration used for all the solutions was 0.5 wt%, and the surfactant concentration, c s , was varied in the range from 10 −6 to 80 mM. The methodology used for solution preparation was adapted from our previous works [25,27,34]. All experiments were carried out at 25.0 ± 0.1 °C.…”

Colloids at Fluid Interfaces

“…This lateral pressure is opposed to the tendency of the interface to contract and minimize its area. Therefore, it is possible to assume that the increase of the interfacial packing leads to a measurable decrease of the interfacial tension [40][41][42][43][44][45][46][47][48][49]. Thus, the different origins found for the surface tensions of conventional surfactants and particles at fluid interfaces have limited the development of accurate thermodynamics models describing the behavior of particle-laden interfaces.…”