Anniina Salonen

Unusually stable liquid foams.

Obtaining stable liquid foams is an important issue in view of their numerous applications. In some of these, the liquid foam in itself is of interest, in others, the liquid foam acts as a precursor for the generation of solid foam. In this short review, we will make a survey of the existing results in the area. This will include foams stabilised by surfactants, proteins and particles. The origin of the stability is related to the slowing down of coarsening, drainage or coalescence, and eventually to their arrest. The three effects are frequently coupled and in many cases, they act simultaneously and enhance one another. Drainage can be arrested if the liquid of the foam either gels or solidifies. Coalescence is slowed down by gelified foam films, and it can be arrested if the films become very thick and/or rigid. These mechanisms are thus qualitatively easy to identify, but they are less easy to model in order to obtain quantitative predictions. The slowing down of coarsening requests either very thick or small films, and its arrest was observed in cases where the surface compression modulus was large. The detail of the mechanisms at play remains unclear.

Surfactant foams doped with laponite: unusual behaviors induced by aging and confinement

We report results on foams stabilized by surfactant (sodium dodecyl sulfate) and containing clay particles (laponite). We have studied how these foams age with time (drainage and coarsening) and their rheological properties. Due to the doping with laponite, which provides an additional time evolution of the foaming fluid itself, unusual behaviors are observed: especially, drainage arrest and re-start and enhanced elasticity are observed as a function of time. These results can be interpreted in terms of both confinement of the laponite inside the foam liquid channels, and competition between the laponite aging and the one of the foam (controlled by its own physical parameters). By playing with these foam parameters and those of the bulk solution containing laponite, we can control the time evolution and these non-monotonous features. Qualitatively, it is found that time, laponite concentration and confinement have all the same effect, enhancing the jamming of the interstitial fluid inside the foam.

Phase behavior of Phytantriol/water bicontinuous cubic Pn3m cubosomes stabilized by Laponite disc-like particles.

The present article reports on the specific effects of temperature on Phytantriol-based cubosomes stabilized by inorganic stabilizers as opposed to organic stabilizers. The ability of Laponite to stabilize Phytantriol-based parent bulk phase is first demonstrated. The sub-micron-sized Laponite-stabilized particles were found to be both physically and chemically stable over time. The temperature-induced behavior, both in heating and cooling directions, of these lipid-based cubosomes has been investigated and compared with their polymer-stabilized counterparts (Pluronic F127). This allows us to extract the particular influence of each stabilizer. Whereas an increased hydration of the cubic structure was evidenced at high pH values, this effect was eliminated to compare the specific influence of both stabilizers on these Phytantriol-based cubosomes. Evidence of differences in the relaxation rates of the internal structures with temperature was found for the two stabilizers, in particular in the cooling direction whilst in the heating direction the two stabilizers could be considered as undisruptive. The origin of this difference is discussed.

On the long-term stability of foams stabilised by mixtures of nano-particles and oppositely charged short chain surfactants

We have studied the foaming properties of aqueous dispersions containing mixtures of silica nano-particles (Ludox TMA) and a short-chain amphiphile (n-amylamine). By combining standard hand shaking methods and microfluidic techniques we show that stable foams can be obtained at amine concentrations above approximately 0.5 wt%, which appears to be a critical concentration for cooperative association between particles and amine. In contrast to foams stabilised solely by nano-particles, these foams suffer from slow coarsening due to gas exchange between bubbles. “Superstable” foams for which coarsening is inhibited can only be produced at sufficiently high particle and amine concentrations (typically 10 and 3 wt%, respectively) for which the dispersions also gel in the continuous phase of the foam. We combine investigations of the static and dynamic properties of the particle-laden air–water interfaces in an attempt to elucidate some of the key mechanisms which control the observed behaviour.

Dual gas and oil dispersions in water: production and stability of foamulsion

In this study we have investigated mixtures of oil droplets and gas bubbles and show that the oil can have two very different roles, either suppressing foaming or stabilising the foam. We have foamed emulsions made from two different oils (rapeseed and dodecane). For both oils the requirement for the creation of foamulsions is the presence of surfactant above a certain critical threshold, independent of the concentration of oil present. Although the foamability is comparable, the stability of the foamed emulsions is very different for the two oils studied. Varying a few simple parameters gives access to a wide range of behaviours, indeed three different stability regimes are observed: a regime with rapid collapse (within a few minutes), a regime where the oil has no impact, and a regime of high stability. This last regime occurs at high oil fraction in the emulsion, and the strong slowing down of ageing processes is due to the confinement of packed oil droplets between bubbles. We thus show that a simple system consisting of surfactant, water, oil and gas is very versatile and can be controlled by choosing the appropriate physical chemical parameters.

Dispersions of Internally Liquid Crystalline Systems Stabilized by Charged Disklike Particles as Pickering Emulsions : Basic Properties and Time-Resolved Behavior

The present paper reports on dispersions of internally liquid crystalline particles, formed from monoglyceride and oil mixtures, stabilized with discrete disklike particles of Laponite clay. Small-angle X-ray scattering (SAXS) was used to probe the presence of dispersed particles as well as their internal liquid crystalline structure. The data were compared to scattering results of reference systems, namely, from the bulk as well as from well-defined particles formed with a polymer as the emulsifier. The submicrometer sizes of the various particles could be derived using dynamic light scattering (DLS). The possible mechanisms involved in the stabilization of each of the different phases by the Laponite platelets, including the role of the residual salt, are discussed. Time-resolved experiments were performed over 60 days in order to follow the evolution of both the internal structure and size of the particles. In particular, we discuss the peculiar behavior of the sample without added oil, where the cubosomes transform into hexosomes over time. The effect of the high pH induced by the Laponite platelets in water, which could result in a hydrolysis of the monoglycerides, was shown to be responsible for the observed cubosome-to-hexasome transition, as well as for the decrease in the lattice parameters.

Light and temperature bi-responsive emulsion foams

The possibility of using light and temperature as stimuli for controlling foamability is presented. The foamability of an emulsion, including both a light responsive polymer and a temperature sensitive surfactant, could be decreased upon heating, completely stopped under UV irradiation, and recovered upon cooling.

Adsorption-desorption kinetics of surfactants at liquid surfaces.

The paper discusses adsorption and desorption energy barriers for macroscopic interfaces of surfactant solutions. Literature data suggest that adsorption and desorption are not always fully diffusion controlled. Apart from electrostatic barriers that lead to strong deviations, other types of barriers are less easy to identify, because smaller deviations from diffusion controlled mechanisms are evidenced. Complete models involving both diffusion and sorption barriers are very complex and involve many adjustable parameters, making the data analysis frequently unreliable. Empirical equations of state are used in most cases, although they are inaccurate, especially close to the cmc. The variation of sorption energies with surface concentration is not accurately described in the models. Finally, convection can mask the effect of sorption energy barriers. Experiments are presented to illustrate the main difficulties encountered.

Strong improvement of interfacial properties can result from slight structural modifications of proteins: the case of native and dry-heated lysozyme.

Identification of the key physicochemical parameters of proteins that determine their interfacial properties is still incomplete and represents a real stake challenge, especially for food proteins. Many studies have thus consisted in comparing the interfacial behavior of different proteins, but it is difficult to draw clear conclusions when the molecules are completely different on several levels. Here the adsorption process of a model protein, the hen egg-white lysozyme, and the same protein that underwent a thermal treatment in the dry state, was characterized. The consequences of this treatment have been previously studied: net charge and hydrophobicity increase and lesser protein stability, but no secondary and tertiary structure modification (Desfougères, Y.; Jardin, J.; Lechevalier, V.; Pezennec, S.; Nau, F. Biomacromolecules 2011, 12, 156-166). The present study shows that these slight modifications dramatically increase the interfacial properties of the protein, since the adsorption to the air-water interface is much faster and more efficient (higher surface pressure). Moreover, a thick and strongly viscoelastic multilayer film is created, while native lysozyme adsorbs in a fragile monolayer film. Another striking result is that completely different behaviors were observed between two molecular species, i.e., native and native-like lysozyme, even though these species could not be distinguished by usual spectroscopic methods. This suggests that the air-water interface could be considered as a useful tool to reveal very subtle differences between protein molecules.

Synthesis of Macroporous Polystyrene by the Polymerization of Foamed Emulsions

Dependingontheapplication,afoammustmeetspecific requirements. Thus great effort has been invested inthe determination and manipulation of foam properties.Material composition and cellular structure constitute crucialparameters when it comes to the tailoring of foams. Asconventional manufacturing, where foams are produced frompolymer melts and blowing agents, is a very complex process,it is hard to control the product!s morphology and properties.In recent years alternative methods for the synthesis ofpolymer foams have been proposed which make use oftemplates: a template is generated first and the actualpolymer is subsequently synthesized. For example, emulsionshave been found to be suitable templates for the synthesis ofporous materials. In particular water-in-oil emulsions witha high concentration of the dispersed phase (high-internal-phase emulsions, HIPEs) have attracted a great deal ofattention.