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

Foam films display exciting systems as on one hand they dictate the performance of macroscopic foams and on the other hand they allow studies of surface forces. With regard to surface forces, we attempt to disentangle the effect of the foam film surfaces and the foam film bulk. For that, we study the influence of salt (LiBr) on foam films formed by mixtures of oppositely charged polyelectrolyte and surfactant: anionic monosulfonated polyphenylene sulfone (sPSO₂-220) and cationic tetradecyltrimethylammonium bromide (C₁₄TAB). Adding a small amount of salt (≤10^-3 M) decreases the foam film stability due to a weakened electrostatic net repulsion. In contrast, a large amount of salt (10^-2 M) unexpectedly increases the foam film stability. Disjoining pressure isotherms reveal that the increased stability is due to an additional steric stabilization, which is attributed to sPSO₂-220/C₁₄TAB complexes in the film bulk. These bulk complexes also contribute to the measured apparent surface potential between the two air/water interfaces. We find, for the first time, the formation of Newton black films for mixtures of anionic polyelectrolytes and cationic surfactants.

A new model to describe small-angle neutron scattering from foams

The modelling of scattering data from foams is very challenging due to the complex structure of foams and is therefore often reduced to the fitting of single peak positions or feature mimicking. This article presents a more elaborate model to describe the small-angle neutron scattering (SANS) data from foams. The model takes into account the geometry of the foam bubbles and is based on an incoherent superposition of the reflectivity curves arising from the foam films and the small-angle scattering (SAS) contribution from the plateau borders. The model is capable of describing the complete scattering curve of a foam stabilized by the standard cationic surfactant tetradecyltrimethylammonium bromide (C14TAB) with different water contents, i.e. different drainage states, and provides information on the thickness distribution of liquid films inside the foam. The mean film thickness decreases with decreasing water content because of drainage, from 28 to 22 nm, while the polydispersity increases. These results are in good agreement with the film thicknesses of individual horizontal foam films studied with a thin-film pressure balance.

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 (sPSO2-220, lP ∼20 nm) or sodium poly(styrenesulfonate) (PSS, lP ∼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.

Different strategies of foam stabilization in the use of foam as a fracturing fluid

An attractive alternative to mitigate the adverse effects of conventional water-based fluids on the efficiency of hydraulic fracturing is to inject foam-based fracking fluids into reservoirs. The efficiency of foaming fluids in subsurface applications largely depends on the stability and transportation of foam bubbles in harsh environments with high temperature, pressure and salinity, all of which inevitably lead to poor foam properties and thus limit fracturing efficiency. The aim of this paper is to elaborate popular strategies of foam stabilization under reservoir conditions. Specifically, this review first discusses three major mechanisms governing foam decay and summarizes recent progress in research on these phenomena. Since surfactants, polymers, nanoparticles and their composites are popular options for foam stabilization, their stabilizing effects, especially the synergies in composites, are also reviewed. In addition to reporting experimental results, the paper also reports recent advances in interfacial properties via molecular dynamical simulation, which provide new insights into gas/liquid interfacial properties under the influence of surfactants at molecular scale. The results of both experiments and simulations indicate that foam additives play an essential role in foam stability and the synergic effects of surfactants and nanoparticles exhibit more favorable performance.

New structural approach to rationalize the foam film stability of oppositely charged polyelectrolyte/surfactant mixtures

The foam film stability of polyelectrolyte/surfactant mixtures is rationalized using structural data from neutron reflectometry for the first time.

CnTAB/polystyrene sulfonate mixtures at air-water interfaces: effects of alkyl chain length on surface activity and charging state

Binding and phase behavior of oppositely charged polyelectrolytes and surfactants with different chain lengths were studied in aqueous bulk solutions and at air-water interfaces. In particular, we have investigated the polyanion poly(sodium 4-styrenesulfonate) (NaPSS) and the cationic surfactants dodecyltrimethylammonium bromide (C₁₂TAB), tetradecyltrimethylammonium bromide (C₁₄TAB) and cetyltrimethylammonium bromide (C₁₆TAB). In order to reveal the surfactant/polyelectrolyte binding, aggregation and phase separation of the mixtures, we have varied the NaPSS concentration systematically and have kept the surfactant concentration fixed at 1/6 of the respective critical micelle concentration. Information on the behavior in the bulk solution was gained by electrophoretic mobility and turbidity measurements, while the surface properties were studied using surface tension measurements and vibrational sum-frequency generation (SFG). This has enabled us to relate bulk to interfacial properties with respect to the charging state and the surfactants' binding efficiency. We found that the latter two are strongly dependent on the alkyl chain length of the surfactant and that binding is much more efficient as the alkyl chain length of the surfactant increases. This also results in a different phase behavior as shown by turbidity measurements of the bulk solutions. Charge neutral aggregates that are forming in the bulk adsorb onto the air-water interface - an effect that is likely caused by the increased hydrophobicity of C_nTAB/PSS complexes. This conclusion is corroborated by SFG spectroscopy, where we observe a decrease in the intensity of O-H stretching bands, which is indicative of a decrease in surface charging and the formation of interfaces with negligible net charge. Particularly at mixing ratios that are in the equilibrium two-phase region, we observe weak O-H intensities and thus surface charging.

Polymers and surfactants at fluid interfaces studied with specular neutron reflectometry

This review addresses the advances made with specular neutron reflectometry in studies of aqueous mixtures of polymers and surfactants at fluid interfaces during the last decade (or so). The increase in neutron flux due to improvements in instrumentation has led to routine measurements at the air/water interface that are faster and involve samples with lower isotopic contrast than in previous experiments. One can now resolve the surface excess of a single deuterated component on the second time scale and the composition of a mixture on the minute time scale, and information about adsorption processes and dynamic rheology can also be accessed. Research areas addressed include the types of formed equilibrium surface structures, the link to foam film stability and the range of non-equilibrium effects that dominate the behavior of oppositely charged polyelectrolyte/surfactant mixtures, macroscopic film formation in like-charged polymer/surfactant mixtures, and the properties of mixtures of bio-polymers with surfactants and lipids.

Interfacial tension of reactive, liquid interfaces and its consequences

Dispersions of immiscible liquids, such as emulsions and polymer blends, are at the core of many industrial applications which makes the understanding of their properties (morphology, stability, etc.) of great interest. A wide range of these properties depend on interfacial phenomena, whose understanding is therefore of particular importance. The behaviour of interfacial tension in emulsions and polymer blends is well-understood - both theoretically and experimentally - in the case of non-reactive stabilization processes using pre-made surfactants. However, this description of the interfacial tension behaviour in reactive systems, where the stabilizing agents are created in-situ (and which is more efficient as a stabilization route for many systems), does not yet find a consensus among the community. In this review, we compare the different theories which have been developed for non-reactive and for reactive systems, and we discuss their ability to capture the behaviour found experimentally. Finally, we address the consequences of the reactive stabilization process both on the global emulsions or polymer blend morphologies and at the interfacial scale.