Yielding and flow of solutions of thermoresponsive surfactant tubes: tuning macroscopic rheology by supramolecular assemblies

International audienceIn this article, we show that stimuli-induced microscopic transformations of self-assembled surfactant structures can be used to tune the macroscopic bulk and interfacial rheological properties. Previously, we had described the formation of micron-sized 12-hydroxystearic acid tubes having a temperaturetunable diameter in the bulk, and also adsorbing at the air-water interface. We report now a detailed study of the bulk and interfacial rheological properties of this solution of thermoresponsive tubes as a function of temperature. In the bulk, the structural modifications of tubes with temperature lead to sharp and non-monotonous changes of rheological behavior. As well, at the air-water interface, the interfacial layer is shifted several times from rigid-like to fluid-like as the temperature is increased, due to morphological changes of the adsorbed interfacial layer. The temperature-induced variations in the fatty acid supramolecular organization and the richness in structural transitions at this microscopic level lead to unique rheological responses in comparison with conventional surfactant systems. Also, this study provides new insights into the required packing conditions for the jamming of anisotropic soft objects and highlights the fact that this system becomes glassy under heating. Due to these unique macroscopic properties both in the bulk and at the interface, this simple system with stimuli-responsive viscoelasticity is of interest for their potential applications in pharmacology or cosmetic formulations

Adsorption of MultiLamellar tubes with a temperature tunable diameter at the air-water interface: a process driven by the bulk properties

The behavior at the air/water interface of multilamellar tubes made of the ethanolamine salt of the 12-hydroxy stearic acid as a function of the temperature has been investigated using Neutron Reflectivity. Those tubes are known to exhibit a temperature tunable diameter in the bulk. We have observed multilamellar tubes adsorbed at the air/water interface by specular neutron reflectivity. Interestingly, at the interface, the adsorbed tubes exhibit the same behavior than in the bulk upon heating. There is however a peculiar behavior at around 50\degree 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-decrease after what they melt as observed in the bulk. All structural transitions at the interface are nevertheless shown to be quasi-completely 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

Nanocapillarity-mediated magnetic assembly of nanoparticles into ultraflexible filaments and reconfigurable networks

The fabrication of multifunctional materials with tunable structure and properties requires programmed binding of their building blocks1,2. For example, particles organized in long-ranged structures by external fields3,4 can be bound permanently into stiff chains through electrostatic or van der Waals attraction4,5, or into flexible chains through soft molecular linkers such as surface-grafted DNA or polymers6–11. Here, we show that capillarity-mediated binding between magnetic nanoparticles coated with a liquid lipid shell can be used for the assembly of ultraflexible microfilaments and network structures. These filaments can be magnetically regenerated on mechanical damage, owing to the fluidity of the capillary bridges between nanoparticles and their reversible binding on contact. Nanocapillary forces offer opportunities for assembling dynamically reconfigurable multifunctional materials that could find applications as micromanipulators, microbots with ultrasoft joints, or magnetically self-repairing gels

Multi-scale structural characterizations of fatty acid tubes with temperature tuneable diameter in bulk and at the air/water interface

International audienceThe use of agricultural resources for industrial purposes will undoubtedly be one of the major challenges of the 21st century, either from the energetic point of view by the progressive replacement of fossil fuels or with respect to non-energy uses by making available new organic “biosynthons” to the chemicals industry. In such a context, we demonstrate here the strong potential of dispersions of saturated fatty acids and their hydroxylated derivatives, extracted from biological compounds of plant origin, as a new class of green surfactants

Assemblages d'acides gras : du volume aux interfaces

Soutenue le 20 avril 2011 Diplôme : Dr. d'UniversitéThis work is part of the general understanding of the impact of the structure of a supramolecular assembly in solution on the interfacial and foaming properties. In the current context of sustainable development, we decided to choose as a model system assemblies produced from long fatty acids chain and hydroxylated derivatives, which are biomolecules from plant origin rarely used. First, a study was conducted in bulk, at the air/water interface and in foam, using tubes made of 12-hydroxystearic acid which exhibit a temperature tunable diameter. The tubes properties in bulk vary with the physic-chemical parameters. In addition, they adsorb at the air/water interface, with a similar behavior than for tubes in bulk upon heating. Very stable foams are obtained from this assembly. It is possible to modulate the foam stability by adjusting the temperature by modifying the structure of the assembly in situ in the Plateau border. It has been shown that the structure in bulk governs the interfacial properties and thus the control of the foaming properties was possible. Then, to obtain other information about the link between the structure of assemblies in bulk with the emulsifying and foaming properties, a study involving other assemblies has been initiated. Thus, it has been shown that for a given assembly, the foaming and emulsifying properties depend on the nature of the fatty acid.Ce travail de thèse s'inscrit dans le cadre général de la compréhension de l'impact de la structure d'un assemblage supramoléculaire en solution sur les propriétés interfaciales et moussantes. Dans le contexte actuel de développement durable, nous avons décidé de choisir comme système modèle des assemblages obtenus à partir d'acides gras à longues chaînes et hydroxylés, qui sont des biomolécules végétales peu valorisées. Tout d'abord, une étude a été menée en solution, à l'interface air/eau et dans les mousses à partir de tubes d'acides gras 12-hydroxy stéarique possédant un diamètre variable avec la température. Les propriétés de ces tubes en solution varient en fonction des paramètres physico-chimiques du milieu. De plus, ils s'adsorbent à l'interface air/eau, avec un comportement similaire aux tubes en solution vis-à-vis de la température. Des mousses très stables sont obtenues à partir de cet assemblage. Il est possible en jouant sur la température de moduler la stabilité des mousses en modifiant la structure de l'assemblage in situ dans les bords de Plateau. Il a ainsi été montré que la structure en volume est conservée à l'interface et qu'un contrôle de la stabilité des mousses était possible. Ensuite, pour obtenir d'autres éléments d'informations sur le lien entre la structure des assemblages en volume avec leurs propriétés moussantes et émulsifiantes, une étude impliquant d'autres assemblages a été amorcée. Il a été mis en évidence que pour un assemblage donné les propriétés moussantes et émulsifiantes dépendent de la nature de l'acide gras

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

International audienceFatty 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

Responsive Self-assemblies based on Fatty acids

International audienceFatty acids are anionic surfactants under their deprotonated forms. They are surfactants with both biodegrability and low toxicity. Fatty acid molecules can self-assemble under various shapes in aqueous solution. These self-assembled structures can respond to stimuli such as pH, CO2 and temperature due to changes occurring at the molecular level. These specificities make them surfactants of special interest to tune the properties at the macroscopic scale. The aim of this article is to review the recent advances in the creation and in the understanding of responsive self-assemblies obtained from fatty acid molecules in aqueous solution. The links between the microscopic, mesoscopic and macroscopic scales are described. The alkyl chain melting phenomenon triggered by temperature at the molecular level leading to thermoresponsive interfaces and foams at the macroscopic scale is highlighted

Saturated long chain fatty acids as possible natural alternative antibacterial agents: opportunities and challenges

International audienceThe spread of new strains of antibiotic-resistant pathogenic microorganisms has led to the urgent need to discover and develop new antimicrobial systems. The antibacterial effects of fatty acids have been well-known and recognized since the first experiments of Robert Koch in 1881, and they are now used in diverse fields. Fatty acids can prevent the growth and directly kill bacteria by insertion into their membrane. For that, a sufficient amount of fatty acid molecules has to be solubilized in water to transfer from the aqueous phase to the cell membrane. Due to conflicting results in the literature and lack of standardization methods, it is very difficult to draw clear conclusions on the antibacterial effect of fatty acids. Most of the current studies link fatty acids' effectiveness against bacteria to their chemical structure, notably the alkyl chain length and the presence of double bonds in their chain. Furthermore, the solubility of fatty acids and their critical aggregation concentration is not only related to their structure, but also influenced by medium conditions (pH, temperature, ionic strength, etc.). There is a possibility that the antibacterial activity of saturated long chain fatty acids (LCFA) may be underestimated due to the lack of water solubility and the use of unsuitable methods to assess their antibacterial activity. Thus, enhancing the solubility of these long chain saturated fatty acids is the main goal before examining their antibacterial properties. To increase their water solubility and thereby improve their antibacterial efficacy, novel alternatives may be considered, including the use of organic positively charged counter-ions instead of the conventional sodium and potassium soaps, the formation of catanionic systems, the mixture with co-surfactants, and solubilization in emulsion systems. This review summarizes the latest findings on fatty acids as antibacterial agents, with a focus on long chain saturated fatty acids. Additionally, it highlights the different ways to improve their water solubility, which may be a crucial factor in increasing their antibacterial efficacy. We finish with a discussion on the challenges, strategies and opportunities for the formulation of LCFAs as antibacterial agents