Anne-Laure Fameau

Smart Foams: Switching Reversibly between Ultrastable and Unstable Foams†

Ultrastable foams with an optimal foamability have been obtained using hydroxyl fatty acids tubes. The stabilization results from the adsorption of monomers at the air-water interface preventing coalescence and coarsening and from the presence of tubes in the Plateau borders limiting the drainage. Upon heating, tubes transit to micelles, which induces foam destabilization. Such foams are thus the first to have a temperature tunable stability.

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.

Responsive Aqueous Foams

Remarkable properties have emerged recently for aqueous foams, including ultrastability and responsiveness. Responsive aqueous foams refer to foams for which the stability can be switched between stable and unstable states with a change in environment or with external stimuli. Responsive foams have been obtained from various foam stabilizers, such as surfactants, proteins, polymers, and particles, and with various stimuli. Different strategies have been developed to design this type of soft material. We briefly review the two main approaches used to obtain responsive foams. The first approach is based on the responsiveness of the interfacial layer surrounding the gas bubbles, which leads to responsive foams. The second approach is based on modifications that occur in the aqueous phase inside the foam liquid channels to tune the foam stability. We will highlight the most sophisticated approaches, which use light, temperature, and magnetic fields and lead to switchable foam stability.

12-Hydroxystearic acid lipid tubes under various experimental conditions

There is a growing interest for constructing supramolecular hollow tubes from amphiphilic molecules. Aqueous solutions of the ethanolamine salt of 12-hydroxystearic acid are known to form tubes of several tens of micrometers in length with a temperature-tunable diameter. However, the phase behavior of this system has not been fully studied. Herein, we report the variation of various physico-chemical parameters on the self-assembling properties of this system. The effects of the ionic strength, ethanol, doping with other lipids, pH, concentration, and the fatty acid/ethanolamine molar ratio R were investigated by both phase-contrast microscopy and DSC. We observed the formation of tubes in a wide range of parameters. For instance, the molar ratio R can be modified from 2/3 to 5/2 without altering the formation of tubes. In some but not all cases, the tube diameter still varied with temperature. These findings show that tubes form under various experimental conditions. This should increase the interest in producing such self-assemblies from low-cost fatty acids.

Self-assembly of fatty acids in the presence of amines and cationic components.

Fatty acids can self-assemble under various shapes in the presence of amines or cationic components. We assemble and compare these types of self-assembly leading toward a catanionic system either with a cationic surfactant or with an amine component playing the role of counter-ion. First, we focus on the molar ratio as a key driving parameter. Known and yet un-known values from other quantities governing the colloidal properties of these systems such as structural surface charge, osmotic pressure, molecular segregation, rigidity, in plane colloidal interactions and melting transition are discussed. We include also recent results obtained on the interfacial and foaming properties of these systems. We will highlight the specificity of these self-assemblies leading to unusual macroscopic properties rich of robust applications.

Self-assembly, foaming, and emulsifying properties of sodium alkyl carboxylate/guanidine hydrochloride aqueous mixtures.

Unsaturated fatty acids may be extracted from various agricultural resources and are widely used as soaps in the industry. However, there also exist a large variety of saturated and hydroxy fatty acids in nature, but their metal salts crystallize at room temperature in water, hampering their use in biological and chemical studies or for industrial applications. Addition of guanidine hydrochloride (GuHCl) to sodium salt of myristic acid has been shown to prevent its crystallization in water, forming stable flat bilayers at room temperature. Herein, we extend this finding to two other saturated fatty acids (palmitic and stearic acids) and two hydroxyl fatty acids (juniperic and 12 hydroxy stearic acids) and study more deeply (by using small angle neutron scattering) the supramolecular assemblies formed in both saturated and hydroxyl fatty acid systems. In addition, we take the advantage that crystallization no longer occurs at room temperature in the presence of GuHCl to study the foaming and emulsifying properties of those fatty acid dispersions. Briefly, our results show that all fatty acids, even juniperic acid, which is a bola lipid, are arranged in a bilayer structure that may be interdigitated. Depending on the nature of the fatty acid, the systems exhibit good foamability and foam stability (except for juniperic acid), and emulsion stability was good. Those findings should be of interest for using saturated long chain (and hydroxyl) fatty acids as surfactants for detergency or even materials chemistry.

Multi-stimuli responsive foams combining particles and self-assembling fatty acids

Foams that respond to external stimuli are a new type of highly functional soft matter and an emerging research subject in the field of materials and colloid science. Herein, we report a new way to control foam stability using light irradiation. This unconventional approach involves a combination of the thermal response of a fatty acid and the ability of carbon black to convert light stimulus to heat. This simple system features both high foamability and high foam stability, but can easily be destroyed by light. Upon exposure to UV or solar illumination, the carbon black particles absorb light causing an increase in temperature inside the foam lamella. This results in a transition of the fatty acid assemblies from tubes, which stabilize the foam films, to micelles, leading to rapid foam destabilization. The foam destabilization rate under UV irradiation was correlated to foam liquid fraction, fatty acid concentration, as well as carbon black concentration. Our approach for the formulation of photo-thermo-responsive foams could be easily generalized and extended to magnetic particles, giving rise to the first foams exhibiting thermo-photo-magneto-tuneable stability. Such multi-stimuli responsive systems can find applications in diverse industries requiring highly stable systems which can be destroyed on-demand.

Multiscale structural characterizations of fatty acid multilayered tubes with a temperature-tunable diameter.

It is known that 12-hydroxystearic acid coupled via ion-pairing with a counterion, such as ethanolamine, self-assembles into supramolecular multilayer tubes that exhibit a peculiar temperature-tunable diameter variation. At a given temperature threshold (T(Ø↑)), there is a large increase of the tube diameter by almost a factor of 10. We investigate here the nature of the counterion on the self-assembly of such tubes and the variation of their diameter with the temperature. Eight different counterions are used to produce tubes via ion-pairing with 12-hydroxystearic acid. We systematically measure structural parameters at different scales coupling phase contrast microscopy and small-angle neutron scattering (SANS), in combination with thermodynamic studies using differential scanning calorimetry (DSC). The temperature T(Ø↑) is found to depend sensitively on the nature of the counterion. One could not identify any DSC peak associated with the diameter variation. However, a detailed line-shape analysis of the SANS spectra reveals that a significant softening of the elastic properties of the multilayer tube walls occurs at T(Ø↑), for all counterions investigated. Depending on both the temperature and the nature of the counterion, the interlayer spacing in the multilayered structure (respectively, the bilayer thickness) varies in a range from 240 to 440 Å (respectively, from 22 to 42 Å), but those variations appear not to be related to T(Ø↑). Altogether, our results show that the variation of the diameter is correlated to the nature of the counterion and to the elastic properties of the bilayer stack.

Adsorption of multilamellar tubes with a temperature tunable diameter at the air/water interface.

The ethanolamine salt of 12-hydroxy stearic acid is known to form tubes having a temperature tunable diameter. Here, we study the behavior of those tubes at the air/water interface by using Neutron Reflectivity. We observed that tubes indeed adsorbed at this interface below a fatty acid monolayer and exhibit the same temperature behavior as in bulk. There is however a peculiar behavior at around 50 °C for which the increase of the diameter of the tubes at the interface yields an unfolding of those tubes into a multilamellar layer. Upon further heating, the tubes re-fold and their diameter re-decreases after which they melt into micelles as observed in the bulk. All structural transitions at the interface are nevertheless reversible. This provides to the system a high interest for its interfacial properties because the structure at the air/water interface can be tuned easily by the temperature.

Self-assembly of myristic acid in the presence of choline hydroxide: effect of molar ratio and temperature.

Salt-free catanionic systems based on fatty acids exhibit a broad polymorphism by simply tuning the molar ratio between the two components. For fatty acid combined with organic amino counter-ions, very few data are available on the phase behavior obtained as a function of the molar ratio between the counter-ion and the fatty acid. We investigated the choline hydroxide/myristic acid system by varying the molar ratio, R=n(choline hydroxide)/n(myristic acid), and the temperature. Myristic acid ionization state was determined by coupling pH, conductivity and infra-red spectroscopy measurements. Self-assemblies were characterized by small angle neutron scattering and microscopy experiments. Self-assembly thermal behavior was investigated by differential scanning calorimetry, wide angle X-ray scattering and nuclear magnetic resonance. For R<1, ionized and protonated myristic acid molecules coexisted leading to the formation of facetted self-assemblies and lamellar phases. The melting process between the gel and the fluid state of these bilayers induced a structural change from facetted or lamellar objects to spherical vesicles. For R>1, myristic acid molecules were ionized and formed spherical micelles. Our study highlights that both R and temperature are two key parameters to finely control the self-assembly structure formed by myristic acid in the presence of choline hydroxide.