Dynamic Surface Tension of Aqueous Solutions of Ionic Surfactants: Role of Electrostatics

The adsorption kinetics of the cationic surfactant dodecyltrimethylammonium bromide at the air-water interface has been studied by the maximum bubble pressure method at concentrations below the critical micellar concentration. At short times, the adsorption is diffusion-limited. At longer times, the surface tension shows an intermediate plateau and can no longer be accounted for by a diffusion limited process. Instead, adsorption appears kinetically controlled and slowed down by an adsorption barrier. A Poisson-Boltzmann theory for the electrostatic repulsion from the surface does not fully account for the observed potential barrier. The possibility of a surface phase transition is expected from the fitted isotherms but has not been observed by Brewster angle microscopy.Comment: 13 pages, 5 figure

Aqueous foams and foam films stabilised by surfactants. Gravity-free studies

AbstractThere are still many open questions and problems in both fundamental research and practical applications of foams. Despite the fact that foams have been extensively studied, many aspects of foam physics and chemistry still remain unclear. Experiments on foams performed under microgravity allow studying wet foams, such as those obtained early during the foaming process. On Earth, wet foams evolve too quickly due to gravity drainage and only dry foams can be studied. This paper reviews the foam and foam film studies that we have performed in gravity-free conditions. It highlights the importance of surface rheology as well as of confinement effects in foams and foam films behaviour

Interfacial behaviour of Catanionic surfactants

International audienceWe report a dramatic increase of foam stability for catanionic mixtures (myristic acid and cetyl trimethylammonium bromide, CTABr) with respect to pristine 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 non dialysed samples, due to the presence of these free ions. As a consequence, the foamability of the dialysed solutions is lower. Foam coarsening has been studied using multiple light scattering: it is similar for dialysed and non dialysed samples and much slower than for pure CTABr foams

Étude de la coalescence et du mûrissement dans les mousses liquides (des expériences modèles à différentes échelles.)

Les mousses liquides sont très utilisées dans l'industrie comme dans la vie quotidienne. Cependant, leur stabilité n'est ni complètement comprise ni complètement maîtrisée, la formulation étant essentiellement empirique. Il est donc difficile d'une part de prédire la capacité d'une solution de tensioactifs à générer des mousses et d'autre part de prédire et de contrôler leur déstabilisation. Ce travail de thèse propose des expériences conceptuellement simples qui s'appuient sur le caractère multi-échelle des mousses pour tenter d'établir des liens clairs entre physico-chimie et mécanismes de déstabilisation. Dans un premier temps, nous avons revisité une expérience d'entraînement de films liquides sur un cadre pour illustrer la stabilité des films lors de la génération des mousses ou pendant un réarrangement (processus T1). Nous avons mis en lumière l existence de deux régimes. Un régime dit non confiné, pour lequel la stabilité des films et réduite et dépend de la physico-chimie. Un régime dit confiné pour lequel le rôle de la physico-chimie est limité et la stabilité est majoritairement contrôlée par l hydrodynamique.Dans un second temps nous nous sommes intéressés à l'influence de la physico-chimie et de la fraction liquide sur la compétition entre coalescence et mûrissement grâce à l'étude d'une unique couche de bulles (mousse 2D). Nous proposons une méthode de mesure in situ de la perméabilité des films par l'étude simple et rapide du régime transitoire du mûrissement de mousses 2D. Nous avons observé une corrélation entre coefficient de diffusion du gaz contenu dans les bulles et perméabilité.Enfin nous décrivons les résultats d une expérience spatiale sur le vieillissement à temps long des mousses humides (fraction liquide supérieure à 30% en volume), extrêmement difficile sur Terre à cause du drainage gravitaire, et à laquelle nous avons participé.Liquid foams are widely used in industry and in everyday life. However, their stability is not fully understood or fully controlled, their formulation being essentially empirical. It is therefore difficult to predict the ability of a surfactant solution to generate foam and to control their destabilization. This thesis proposes simple experiments that rely on the multi-scale nature of foams in order to attempt to establish clear links between physical chemistry and stability.First of all, we revisited an experiment of liquid films entrainment to illustrate the stability of foam films during generation or rearrangement (T1 process). We have shown the existence of two regimes: (i) a unconfined regime, for which the stability of films is reduced and depends on physical chemistry and (ii) a confined regime for which the role of physical chemistry stability is limited and is mainly controlled by hydrodynamics.Secondly, we investigated the influence of physical chemical and liquid fraction on the competition between coalescence and coarsening thanks to the study of a single layer of bubbles (2D foam). We propose a method for in situ measurement of the permeability of films by simple and fast study of the transient regime of coarsening in 2D foam. We observed a correlation between diffusion coefficient of gas and permeability.Finally we describe the results of a space experiment on wet foam aging at large times (volume liquid fraction higher than 30%), extremely difficult on Earth because of gravity drainage, and to which we participated.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Particle-stabilised foams: structure and aging

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.We show that aqueous foams stabilised by nanoparticles can be easily imaged using an X-ray laboratory source. We have used hydrophobically modified silica nanoparticles that confer to the foam a remarkable stability. The X-ray tomography observations were compared with the information obtained using a multiple light scattering technique. Both techniques confirm that provided the concentration of particles in bulk water is high enough, the bubble size evolves little with time. X-Ray tomography revealed the presence of two populations of bubbles, small bubbles which size tends to decrease with time and large bubbles which number tends to increase with time. This behaviour could arise from an arrested coarsening process. The results demonstrate the great potential of the two techniques and of their combination for foam studies

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

International audienceIn 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

How antifoams act: a microgravity study

peer reviewe

Étude sur les propriétés interfaciales de tensioactifs et de leurs interactions avec l'ADN

Ayant une partie hydrophile et une partie hydrophobe, les tensioactifs peuvent s'adsorber sur des interfaces et d'abaisser la tension interfaciale (g), ce qui améliore les propriétés interfaciales. Tensioactifs chargés sont également utilisés dans des applications biologiques, par exemple dans la livraison de gènes. Dans cette thèse, nous avons étudié les propriétés d'adsorption des tensioactifs, à la fois aux interfaces air/eau et sur l'ADN pour former des complexes.La première partie de la thèse se concentre sur les études d'interface de tensioactifs. Pour comprendre comment ils fonctionnent dans ces applications, il est important de connaître les échelles de temps de l'adsorption et la désorption de surfactant. Ainsi, il est nécessaire d'étudier l'adsorption et la cinétique de désorption, qui sont déjà largement étudié. Cependant, les études traditionnelles ont tendance à faire de nombreuses hypothèses, par exemple, l'extension de l'applicabilité des relations d'équilibre à des cas de non-équilibre. Dans cette mémoire, l'adsorption des deux systèmes tensioactifs différents a été étudiée, C12E6 de tensioactif non ionique et d'agent tensio-actif ionique CTAB avec suffisamment de sel. Une mesure de la compression de la bulle unique combiné avec une tension superficielle d'équilibre connue (geq) de valeur permet de déterminer g( ), ce qui est plus précis que les résultats des méthodes traditionnelles. Les concentrations de surface en fonction du temps sont mesurés, ce qui montre que l'adsorption est contrôlée par la diffusion à temps courts.Après avoir montré que l'adsorption est contrôlée par diffusion, nous rapportons la désorption des tensioactifs à partir de l'interface air/eau pour différents systèmes. Les processus de désorption sont confirmées pas être purement limitée par diffusion, indiquant la présence d'une barrière d'énergie. La barrière d'énergie est influencée par la longueur de la chaîne alkyle, et non le type de contre-ion.Dans la deuxième partie de la thèse, nous nous concentrons sur les systèmes d'ADN/tensioactif. Bien que l'interaction entre les tensioactifs cationiques et anioniques polyélectrolyte a été largement étudiée, il reste nécessaire de mieux comprendre le système complexe, en particulier pour rationaliser le choix des tensioactifs pour atteindre une capacité de liaison de l'ADN contrôlable et une faible toxicité pour l'organisme. Dans cette thèse, nous avons lancé l'enquête systématique sur les interactions des deux tensioactifs cationiques avec l'ADN.Le premier tensioactif utilisé est un gemini tensioactifs cationiques 12-2-12 2Br. Avant de l'utiliser avec l'ADN d'une caractérisation approfondie a été effectuée. L'équilibrage du 12-2-12 2Br sur une interface air/eau en l'absence d'électrolyte est très lent. Ajout de NaBr affecte peu la cinétique d'adsorption à des temps courts, pendant lesquels l'adsorption de diffusion. Cependant, l'adsorption s'équilibre beaucoup plus rapide. La formation de micelles de tensioactif cationique gemini 12-3-12 2Br a été étudiée. La concentration micellaire critique (CMC) augmente légèrement avec la température et diminue avec la force ionique. 12-3-12 2Br interagit fortement avec l'ADN, en raison de l'attraction électrostatique entre les deux et les interactions hydrophobes entre les chaînes alkyles. Sel écrans l'attraction électrostatique, tout en augmentant la longueur d'écartement des Gémeaux tensioactif affaiblit son interaction avec l'ADN.Un autre agent a également été étudié pour sa capacité de liaison à l'ADN et nous présentons une étude systématique sur les interactions entre tensioactif cationique liquide ionique [C12mim]Br et de l'ADN par des techniques expérimentales et de dynamique moléculaire (MD) de simulation. En ajoutant [C12mim]Br, les chaînes d'ADN sont soumis à compactage, des changements conformationnels, avec le changement de charge nette portée par le complexe ADN/tensioactif. simulation de MD confirme les résultats expérimentaux.Bearing a hydrophilic part and a hydrophobic part, surfactants can adsorb onto interfaces and lower the interfacial tension (g), thereby enhancing the interfacial properties and leading to the applications in cleaning, surface functionalization, foaming and emulsification. Charged surfactants are also used in biological applications, in particular to extract and purify DNA, or for gene delivery. In this thesis we have studied the adsorption properties of surfactants, both to air/water interfaces and onto DNA to form complexes. The first part of the thesis concentrates on interfacial studies of surfactants. To understand how they work in these applications it is important to know the time-scales of the surfactant adsorption and desorption. Thus it is necessary to investigate the adsorption and desorption kinetics, which are already widely studied. However, traditional studies tend to make many assumptions, for example, extending the applicability of equilibrium relations to non-equilibrium cases. In this dissertation, the adsorption of two different surfactant systems has been investigated, non-ionic surfactant C12E6 and ionic surfactant CTAB with sufficient salt. A single bubble compression measurement combined with a known equilibrium surface tension (geq) value allows the determination of g( ) , which is more accurate than results from traditional methods. The time-dependent surface concentrations are measured, showing that the adsorption is diffusion controlled at short times.Having shown that adsorption is diffusion controlled, we report desorption of surfactants from the air/water interface for different systems. The desorption processes are confirmed not to be purely diffusion-limited, showing the presence of an energy barrier. The energy barrier is influenced by the alkyl chain length, but not the counterion type.In the second part of the thesis we concentrate on DNA/surfactant systems. Although the interaction between cationic surfactant and anionic polyelectrolyte has been extensively studied, there still remains need to further understand the complex system, especially to rationalize the choice of surfactants to reach controllable DNA binding ability and low toxicity to the organism. In this dissertation, we introduced the systematic investigation on the interactions of two cationic surfactants with DNA.The first surfactant used is a cationic gemini surfactant 12-2-12 2Br. Before using it with DNA a thorough characterization has been carried out. The equilibration of 12-2-12 2Br onto an air/water interfaces in the absence of electrolyte is very slow. Addition of NaBr hardly affects the adsorption kinetics at short times, during which the adsorption is diffusive. However, the adsorption equilibrates much faster. The micellization of cationic gemini surfactant 12-3-12·2Br has been investigated. The critical micelle concentration (CMC) increases slightly with temperature and decreases with ionic strength. 12-3-12·2Br interacts strongly with DNA, due to the electrostatic attraction between the two and the hydrophobic interactions between alkyl chains. Salt screens the electrostatic attraction, while increasing spacer length of gemini surfactant weakens its interaction with DNA.Another surfactant has also been studied for its DNA binding ability and we present a systematic study on interactions between cationic ionic liquid surfactant [C12mim]Br and DNA by experimental techniques and Molecular Dynamics (MD) simulation. By adding [C12mim]Br, DNA chains undergo compaction, conformational changes, with the change of net charges carried by the DNA/surfactant complex. MD simulation confirms the experimental results.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Molecular organization of the human serotonin transporter at the air/water interface

AbstractThe serotonin transporter (SERT) is the target of several important antidepressant and psychostimulant drugs. It has been shown that under defined conditions, the transporter spread at the air/water interface was able to bind its specific ligands. In this paper, the interfacial organization of the protein has been assessed from dynamic surface pressure and ellipsometric measurements. For areas comprising between 10 400 and 7100 Å2/molecule, ellipsometric measurements reveal an important change in the thickness of the SERT film. This change was attributed to the reorientation of the transporter molecules from a horizontal to their natural predictive transmembrane orientation. The thickness of the SERT film at 7100 Å2/molecule was found to be approximately equal to 84 Å and coincided well with the theoretical value estimated from the calculations based on the dimensions of α-helices containing membrane proteins. These data suggest that the three-dimensional arrangement of the SERT may be represented as a box with lengths dz=83–85 Å and dy or dx=41–47 Å