David Carriere

On the origin of the stability of foams made from catanionic surfactant mixtures

Using mixtures of the anionic myristic acid (C13COOH) and the cationic cetyl trimethylammonium chloride (C16TA+Cl−) in aqueous solutions at a 2:1 ratio, we show that the outstanding stability of foams generated from sufficiently concentrated “catanionic” surfactant mixtures can be explained by a synergy effect between two fundamentally different mechanisms. Applying a multi-scale approach, in which we link static and dynamic properties of the bulk solutions, isolated gas/liquid interfaces, thin liquid films and foams, we identify these two mechanisms to be as follows: firstly, cationic mixtures create tightly packed surfactant layers at gas/liquid interfaces, which are strongly viscoelastic and also confer high disjoining pressures when two interfaces are approaching each other to form a thin liquid film. Foams created with such kind of interfaces tend to be extremely stable against coalescence (film rupture) and coarsening (gas exchange). However, typical time scales to cover the interfaces are much longer than typical foaming times. This is why a second mechanism plays a key role, which is due to the presence of micron-sized catanionic vesicles in the foaming solution. The bilayers of these vesicles are in a gel-like state, therefore leading to nearly indestructible objects which act like elastic micro-spheres. At sufficiently high concentrations, these vesicles jam in the presence of the confinement between bubbles, slowing down the drainage of liquid during the initial foaming process and therefore providing time for the interfaces to be covered. Furthermore, the tightly packed vesicles strongly reduce bubble coalescence and gas transfer between bubbles.

Interfacial Behavior of Catanionic Surfactants

We report a dramatic increase in foam stability for catanionic mixtures (myristic acid and cetyl trimethylammonium bromide, CTABr) with respect to that of CTABr solutions. This increase was related to the low surface tension, high surface concentration, and high viscoelastic compression moduli, as measured with rising bubble experiments and ellipsometry. Dialysis of the catanionic mixtures has been used to decrease the concentration of free surfactant ions (CTA(+)). The equilibrium surface tension is reached faster for nondialyzed samples because of the presence of these free ions. As a consequence, the foamability of the dialyzed solutions is lower. Foam coarsening has been studied using multiple light scattering: it is similar for dialyzed and nondialyzed samples and much slower than for pure CTABr foams.

In-plane distribution in mixtures of cationic and anionic surfactants

Mixtures of cationic and anionic surfactants, also called “catanionic” mixtures, self-assemble into aggregates which show a rich variety of morphologies in the nanometre to micron range controlled by the molar ratio between surfactants. At the molecular scale, little is known about the local distribution of surfactants in the bilayer. Here, we determine the in-plane distribution of the cationic and anionic surfactants in catanionic bilayers using neutron scattering and the contrast matching technique. The coexistence of two different in-plane orders is experimentally demonstrated: both surfactants share a common two-dimensional hexagonal lattice with a long-range order, but the distribution of alternate (+) and (−) charged head groups shows only a two-dimensional liquid-like local order. Comparing experiments and Monte-Carlo simulations, we establish that this lateral liquid order is attenuated by the counterions of the bilayers, but is less than expected in a mean-field approach. This demonstrates that electrostatic interactions participate in, but do not completely determine, the local distribution of surfactants of opposite charge in the bilayer.

Osmotically induced deformation of capsid-like icosahedral vesicles

We report the osmotic deformation of micron-size catanionic vesicles with icosahedral symmetry (20 faces, 12 vertices) upon incubation in small solutes (NaCl, glucose). The vesicles remain icosahedral at low osmotic pressure gradients across the bilayer, or spherical for outwards gradients. Above a threshold value of inwards pressure, the icosahedra develop a buckling instability: a depression is initiated at one or two ridges, grows until one vertex snaps into the icosahedra, leading to full collapse of one half of the vesicle into the other. Despite large local inversions in curvature, no release of encapsulated solutes is observed before the residual volume reaches negligible small values. Thin shell models correctly capture the buckling patterns of icosahedra in the low deformation limit. Comparison of experimental results with Monte Carlo simulations provides a first estimate for the conditions of shell disruption, and suggests it is predominantly driven by curvature rather than two-dimensional stretching or compression. The relevance of these results for the mechanics of viral capsids and controlled release applications is discussed.

Adsorption, organization, and rheology of catanionic layers at the air/water interface.

We have investigated the adsorption and organization at the air/water interface of catanionic molecules released from a dispersion of solid-like catanionic vesicles composed of myristic acid and cetyl trimethylammonium chloride at the 2:1 ratio. These vesicles were shown recently to be promising foam stabilizers. Using Brewster angle microscopy, we observed the formation of a catanionic monolayer at the air/water interface composed of liquid-condensed domains in a liquid-expanded matrix. Further adsorption of catanionic molecules forced them to pack, thereby forming a very dense monolayer that prevented further vesicle rupture by avoiding contact of the vesicles with air. Moreover, confocal fluorescence microscopy revealed the presence of layers of intact vesicles that were progressively creaming toward this catanionic monolayer; the surface tension of the vesicle dispersion remained constant upon creaming. The catanionic monolayer behaved as a soft glassy material, an amorphous solid with time- and temperature-dependent properties. Using interfacial oscillatory rheology, we found that the monolayer relaxed mechanical stresses in seconds and melted at a temperature very close to the melting transition temperature of the vesicle bilayers. These results have potential application in the design of smart foams that have temperature-tunable stability.

Ripening of catanionic aggregates upon dialysis.

We have studied the dialysis of surfactant mixtures of two oppositely charged surfactants (catanionic mixture) by combining HPLC, neutron activation, confocal microscopy, and NMR. In mixtures of n-alkyl trimethylammonium halides and n-fatty acids, we have demonstrated the existence of a specific ratio between both surfactant contents (anionic/cationic almost equal to 2:1) that determines the morphology, the elimination of ions, and the elimination of the soluble cationic surfactant upon dialysis. In mixtures prepared with lower anionic surfactant contents, ill-defined aggregates are formed, and dialysis quickly eliminates the ion pairs (H+X-) formed upon surfactant association and also the cationic surfactant until a limiting 2:1 ratio is reached. By contrast, mixtures prepared above the anionic/cationic 2:1 ratio form micrometer-sized vesicles resistant to dialysis. These closed aggregates retain a significant number of ions (30%) over 1000 hours, and dialysis is unable to eliminate the soluble surfactant. The interactions between surfactants have been estimated by measuring the partitioning of the CTA molecules between the catanionic bilayer, the bulk solution, and mixed micelles when they exist. The mean extraction free energy per CTA in the membrane has been found to increase by 1 kBT to 2 kBT as the soluble surfactant is depleted from the bilayer, which is enough to stop the dialysis. The vesicles produced above the anionic/cationic 2:1 ratio are formed by frozen bilayers and are resistant to extensive dialysis and therefore show an interesting potential for encapsulation as far as durability is concerned.

Fatty acid-cationic surfactant vesicles: counter-ion self-encapsulation

Vesicles of fatty acids in the fluid state show interesting biomimetic properties and are potentially versatile substitutes to phospholipid vesicles in materials science. However, their use is hindered by a poor stability against variations in pH, ionic strength, temperature etc., which can be improved by using, for instance, mixtures of fatty acids and other amphiphilic molecules. Here we report an original property of fatty acid vesicles with aliphatic chains in the gel state prepared from mixtures of fatty acids and cationic surfactants. Apart from encapsulating added solutes in high yields and showing a high durability, even against drastic dialysis, dilution, concentration etc., these vesicles also spontaneously encapsulate the counter-ions (H+ and halides) released upon surfactant association. This “self-encapsulation” leads to large and sustainable pH gradients across the bilayer (ΔpH ≈ 3 over months), and is mediated by the formation of gel-phase bilayers with little defects. Both the formation of the vesicles and their ability for self-encapsulation of counter-ions have a broad generality and can be exploited with other surfactants to generate pH gradients ranging from acid to base.

Counter-ion activity and microstructure in polyelectrolyte complexes as determined by osmotic pressure measurements

We have investigated the activity of counter-ions at 60 degrees C through the osmotic coefficient K in solutions of anionic and cationic polyelectrolyte complexes of variable compositions. For excess of polyanion in the complexes (molar fraction of polycation f < 0.5), K increases as the polyanion is neutralized by the polycation (f getting closer to 0.5). By contrast, for an excess of polycation (f > 0.5), K stays constant or even slightly decreases as the polycation is getting neutralized by the polyanion. This asymmetric behavior depending on the charge of the complexes indicates that the globally negatively charged complexes are homogeneous and can be treated as a single polyelectrolyte of reduced linear charge density. On the other hand, the positively charged complexes show a micro-phase separation between neutral fully compensated microdomains and domains where the excess polycation is locally segregated. These two different microstructures are reminiscent of the coacervation and segregation regimes observed at higher concentrations and salinities, and also of polyelectrolyte complexes with oppositely charged surfactants. This interpretation is supported by two simple predictive models.

Surface decoration of catanionic vesicles with superparamagnetic iron oxide nanoparticles: a model system for triggered release under moderate temperature conditions

We report the design of new catanionic vesicles decorated with iron oxide nanoparticles, which could be used as a model system to illustrate controlled delivery of small solutes under mild hyperthermia. Efficient release of fluorescent dye rhodamine 6G was observed when samples were exposed to an oscillating magnetic field. Our system provides direct evidence for reversible permeability upon magnetic stimulation.