Hydrophobins are amphiphilic proteins produced by fungi. Cerato-ulmin (CU) is a hydrophobin that has been associated with Dutch elm disease. Like other hydrophobins, CU stabilizes air bubbles and oil droplets through the formation of a persistent protein film at the interface. The behavior of hydrophobins at surfaces has raised interest in their potential applications, including use in surface coatings, food foams, and emulsions and as dispersants. The practical use of hydrophobins requires an improved understanding of the interfacial behavior of these proteins, alone and in the presence of added surfactants. In this study, the adsorption behavior of CU at air/water interfaces is characterized by measuring the surface tension and interfacial rheology as a function of adsorption time. CU is found to adsorb irreversibly at air/water interfaces. The magnitude of the dilatational modulus increases with adsorption time and surface pressure until CU eventually forms a rigid film. The persistence of this film is tested through the sequential addition of strong surfactant sodium dodecyl sulfate (SDS) to the bulk liquid adjacent to the interface. SDS is found to coadsorb to interfaces precoated with a CU film. At high concentrations, the addition of SDS significantly decreases the dilatational modulus, indicating disruption and displacement of CU by SDS. Sequential adsorption results in mixed layers with properties not observed in interfaces generated from complexes formed in the bulk. These results lend insight to the complex interfacial interactions between hydrophobins and surfactants.

Aerosol-OT (AOT) and Tween 80 are two of the main surfactants in commercial dispersants used in response to oil spills. Understanding how multicomponent surfactant systems interact at oil/aqueous interfaces is crucial for improving both dispersant design and application efficacy. This is true of many multicomponent formulations; a lack of understanding of competition for the oil/water interface hinders formulation optimization. In this study, we have characterized the sequential adsorption behavior of AOT on squalane/aqueous interfaces that have been precoated with Tween 80. A microtensiometer is used to measure the dynamic interfacial tension of the system. Tween 80 either partially or completely irreversibly adsorbs to squalane/aqueous interfaces when rinsed with deionized water. These Tween 80 coated interfaces are then exposed to AOT. AOT adsorption increases with AOT concentration for all Tween 80 coverages, and the resulting steady-state interfacial tension values are interpreted using a Langmuir isotherm model. In the presence of 0.5 M NaCl, AOT adsorption significantly increases due to counterion charge screening of the negatively charged head groups. The presence of Tween 80 on the interface inhibits AOT adsorption, reducing the maximum surface coverage as compared to a clean interface. Tween 80 persists on the interface even after exposure to high concentrations of AOT.

•Guidelines for design of interfacial rheology measurements that incorporate curvature and surfactant isotherm.•Interfacial mechanics using a pendant drop/bubble.•Scaling to design dynamic interfacial tension experiments on curved interfaces.•Demonstration of scaling using experiments and simulations.Pendant bubble and drop devices are invaluable tools in understanding surfactant behavior at fluid–fluid interfaces. The simple instrumentation and analysis are used widely to determine adsorption isotherms, transport parameters, and interfacial rheology. However, much of the analysis performed is developed for planar interfaces. The application of a planar analysis to drops and bubbles (curved interfaces) can lead to erroneous and unphysical results. We revisit this analysis for a well-studied surfactant system at air–water interfaces over a wide range of curvatures as applied to both expansion/contraction experiments and interfacial elasticity measurements. The impact of curvature and transport on measured properties is quantified and compared to other scaling relationships in the literature. The results provide tools to design interfacial experiments for accurate determination of isotherm, transport and elastic properties.

In the 2010 Deepwater Horizon rig explosion and subsequent oil spill, five million barrels of oil were released into the Gulf over the course of several months. Part of the resulting emergency response was the unprecedented use of nearly two million gallons of surfactant dispersant at both the sea surface and well head, giving rise to previously untested conditions of high temperature gradients, high pressures, and flow conditions. To better understand the complex interfacial transport mechanisms that this dispersant poses, we develop a model surfactant-oil-aqueous system of Tween 80 (a primary component in the Corexit dispersant used in the Gulf), squalane, and both simulated seawater as well as deionized water. We measure surfactant adsorption dynamics to the oil–aqueous interface for a range of surfactant concentrations. Using techniques developed in our laboratory, we investigate the impact of convection, step changes in bulk concentration, and interfacial mechanics. We observe dynamic interfacial behavior that is consistent with a reorganization of surfactant at the interface. We demonstrate irreversible adsorption behavior of Tween 80 near a critical interfacial tension value, as well as measure the dilatational elasticity of equilibrium and irreversibly adsorbed layers of surfactant on the oil–aqueous interface. We report high values of the surface dilatational elasticity and surface dilatational viscosity, and discuss these results in terms of their impact regarding oil spill response measures.

Aqueous solutions of polyelectrolyte and oppositely charged ionic surfactant are proposed to have a string-of-pearls structure under strongly acidic conditions. This differs from the cylindrical structure presented by this same system at neutral pH conditions. Experimental data are consistent with the conclusion that there is a cylinder-to-sphere transition of the adsorbed surfactant when pH is dropped below a critical value. At intermediate concentrations, this transition is manifested as a pH-induced gelation. At dilute conditions, the aggregate structure is characterized through light scattering, potentiometry, 1H NMR, and solubility measurements. Protonation of the carboxylate groups on the polyelectrolyte and resulting restructuring of the micellar structure around the aggregate are argued to be the primary causes of the transition. Since these are completely reversible, the addition of NaOH, a strong base, is observed to reverse the structure from string-of-pearls back to cylindrical. This reversible pH-induced transition, which is caused by a shift in intermolecular forces within the aggregate, is likely to be common among self-assembled aggregates. These aggregates represent a region of the polyelectrolyte−surfactant aggregate formation (that of hydrophobic polymer and varying electrostatic attraction of opposite charged moieties) that exhibits distinctive aggregate structural behavior.

Polymer-like micelles are analogs to polymer solutions and provide an exciting class of materials for both applications and fundamental understanding of polyelectrolyte systems. Small angle neutron and X-ray scattering have been key to the characterization of these materials from the first observations of linear micelle growth. As new materials are developed, these techniques continue to be utilized and combined with other analytical tools to characterize the length and time scales of polymer-like micelle behavior. Recent reports on the use of small-angle scattering to characterize polymer-like and wormlike micelles are reviewed, with focus on new materials, improvements in analytical approaches and anisotropic structures.

Self-assembled polymer−surfactant aggregates offer an inexpensive and robust approach to controlling the nanoscale structure of materials in solution. However, complete understanding of the partitioning of different species is required for design and engineering of these structures. In this work, the surfactant partitioning of an oppositely charged polyelectrolyte−surfactant (PES) aggregate system in aqueous solution is characterized using a surfactant-specific coated-wire electrode. Previous work has shown that the structure of these aggregates is sensitive to the surfactant partitioning behavior, so characterizing this behavior is vital in controlling the aggregate structure. Due to our synthesis procedure, no small counterions (salts) are present in the system; therefore, the electrolyte concentration can be controlled very accurately and the effect of background electrolyte on surfactant partitioning behavior characterized. A simple model describing the surfactant partitioning in the system is presented and compared to the observed results. Results provide the tools to control the length and surface charge of these rodlike nanoparticles through overall composition.

A lyotropic phase transition is observed in a water-soluble polyelectrolyte–surfactant aggregate system (polymerized cetyltrimethylammonium 4-vinylbenzoate, or pC16TVB). Unlike other oppositely-charged polyelectrolyte–surfactant aggregates at the stoichiometrically-matched charge point, these aggregates do not precipitate, and instead form isotropic–nematic biphasic solutions in water. The aggregates maintain amphiphilic behavior through the phase transition and appear to maintain the structure observed in dilute solution – that of rod-like aggregates (L/d ∼ 35 with d = 4 nm). Rheology, microscopy and small-angle neutron scattering are used to verify the nature of the phase transition and structure of the mesogen. The phase transition occurs at a concentration higher than that predicted by simple theory; however, flexibility, charge repulsion and polydispersity must be considered in this system.

Development of an electrostatic stabilization mechanism for colloidal suspensions in nonpolar fluids requires an improved understanding of the interactions between the inverse micelles and particles as well as the roles that steric and electrostatic effects play. A droplet-based millifluidic device is designed and used to investigate the stabilization effects of surfactants on colloidal suspensions. A system containing carbon black and the surfactant OLOA 11000 suspended in dodecane is chosen as a well-characterized system to study sedimentation quantitatively. This device takes advantage of sub-millimeter optical path lengths to characterize sedimentation at concentrations at which sedimentation is not observable in the bulk and to achieve higher resolution in composition. A simple image analysis algorithm has been developed and applied to quantify sedimentation. Conductivity measurements using electrochemical impedance spectroscopy (EIS) are coupled with the sedimentation experiments to identify the concentration ranges in which steric and electrostatic effects are dominant. A more gradual transition is observed than previously reported.