Insights into Extended Structures and Their Driving Force: Influence of Salt on Polyelectrolyte/Surfactant Mixtures at the Air/Water Interface

This paper addresses the effect of polyelectrolyte stiffness on the surface structure of polyelectrolyte (P)/surfactant (S) mixtures. Therefore, two different anionic Ps with different intrinsic persistence length lP are studied while varying the salt concentration (0-10(-2) M). Either monosulfonated polyphenylene sulfone (sPSO(2)-220, l(P) similar to 20 nm) or sodium poly(styrenesulfonate) (PSS, l(P) similar to 1 nm) is mixed with the cationic surfactant tetradecyltrimethylammonium bromide (C(14)TAB) well below its critical micelle concentration and studied with tensiometry and neutron reflectivity experiments. We kept the S concentration (10(-4) M) constant, while we varied the P concentration (10(-5)-10(-3) M of the monomer, denoted as monoM). P and S adsorb at the air/water interface for all studied mixtures. Around the bulk stoichiometric mixing point (BSMP), PSS/C(14)TAB mixtures lose their surface activity, whereas sPSO(2)-220/C(14)TAB mixtures form extended structures perpendicular to the surface (meaning a layer of S with attached P and additional layers of P and S underneath instead of only a monolayer of S with P). Considering the different P monomer structures as well as the impact of salt, we identified the driving force for the formation of these extended structures: compensation of all interfacial charges (P/S ratio similar to 1) to maximize the gain of entropy. By increasing the flexibility of P, we can tune the interfacial structures from extended structures to monolayers. These findings may help improve applications based on the adsorption of P/S mixtures in the fields of cosmetic or oil recovery.

Automated summary

This paper investigates the effect of polyelectrolyte stiffness on the surface structure of polyelectrolyte/surfactant mixtures and finds that the interfacial structures can be tuned by adjusting the flexibility of the polyelectrolyte molecules. These findings are significant for improving applications based on the adsorption of polyelectrolyte/surfactant mixtures in fields such as cosmetics or oil recovery.