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

Abstract

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 (sPSO2-220, l P ∼20 nm) or sodium poly­(styrenesulfonate) (PSS, l P ∼1 nm) is mixed with the cationic surfactant tetradecyltrimethylammonium bromide (C14TAB) 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/C14TAB mixtures lose their surface activity, whereas sPSO2-220/C14TAB 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.

Untangling effects of proteins as stabilizers for foam films

“…This behavior can be compared to the behavior of polyelectrolyte/surfactant (P/S) mixtures, where around the nominal bulk stoichiometric mixing point (comparable to the IEP) surface active complexes form. The stability of the P/S stabilized foam films is independent of the net charge within the films but highly dependent on the amount of material that is adsorbed to the foam film's interfaces [Kristen et al (2009); Braun et al (2022)]. But at the BSMP they did not observe a NBF formation and the maximum pressure before film rupture of the CBFs was low in contrast to the high stability of the CBFs and especially the NBFs in BLG solutions observed in this study.…”

Polyelectrolyte/surfactant films: from 2D to 3D structural control

“…The amount of SDS present in the extended structure is 29 AE 1%, consistent with the 27 AE 1% calculated above on the assumption that the surface monolayer does not change in coverage beyond the collapse. Although extended structures in adsorbed P/S layers have been recently the focus of an experimental study, 26 here we have resolved for the first time the extended structures of a P/S film formed by the aggregate spreading approach.…”

When Bulk Matters: Disentanglement of the Role of Polyelectrolyte/Surfactant Complexes at Surfaces and in the Bulk of Foam Films

“…For mixtures of the anionic sPSO 2 -220 with the cationic C 14 TAB, the surface excess of both, S and P, was measured individually by neutron reflectometry 43 and it either stays constant or even decreases with added LiBr, depending on the sPSO 2 -220 concentration. 42 Thus, there is a need to systematically study the effect of salt on foam films formed by P/S mixtures. In this study, we focus on sPSO 2 -220/C 14 TAB mixtures, which form CBF in the absence of added salt.…”

“…Recent neutron reflectometry measurements resolve the individual surface excess of sPSO 2 -220 and C 14 TAB. 42 (Figure S10 shows the surface excess of sPSO 2 -220 and C 14 TAB, and Figure S11 shows the ratio of the adsorbed amounts.) Above the BSMP, sPSO 2 -220 and C 14 TAB adsorb to the same amount (P/S ratio of ∼1) at the air/water interface, irrespective of c sPSOd 2 -220 and c LiBr .…”

“…37 In a recent publication, we found that the formation of extended structures vanishes with an increase in LiBr concentration, resulting in similar monolayer structures irrespective of c sPSOd 2 -220 . 42 Thus, the interfacial structures alone cannot explain the increased foam film stability, and there must be another explanation for it. A closer look at the disjoining pressure isotherms reveals a small but systematic deviation for the highest LiBr concentration [c LiBr = 10 −2 M (Figure 4c)] from the PB calculation: with an increase in Π, the foam film thickness stays constant at a value of ∼17 nm.…”