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

info:eu-repo/classification/ddc/600 600 02 engineering and technology 0210 nano-technology 01 natural sciences 0104 chemical sciences
DOI: 10.1021/acsami.2c04421 Publication Date: 2022-05-31T15:51:56Z
ABSTRACT
ACS applied materials & interfaces 14(23), 27347 - 27359 (2022). doi:10.1021/acsami.2c04421<br/>Published by Soc., Washington, DC<br/>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 l$_P$ are studied while varying the salt concentration (0–10$^{–2}$ M). Either monosulfonated polyphenylene sulfone (sPSO$_2$-220, l$_P$ ∼20 nm) or sodium poly(styrenesulfonate) (PSS, l$_P$ ∼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 ∼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.<br/>
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