Self-Diffusion and Collective Diffusion of Charged colloids Studied by Dynamic Light Scattering

A microemulsion of decane droplets stabilized by a non-ionic surfactant film is progressively charged by substitution of a non-ionic surfactant molecule by a cationic surfactant. We check that the microemulsion droplets remain identical within the explored range of volume fraction (0.02 to 0.18) and of the number of charge per droplets (0 to 40) . We probe the dynamics of these microemulsions by dynamic light scattering. Despite the similar structure of the uncharged and charged microemulsions the dynamics are very different . In the neutral microemulsion the fluctuations of polarization relax, as is well known, via the collective diffusion of the droplets. In the charged microemulsions, two modes of relaxation are observed. The fast one is ascribed classically to the collective diffusion of the charged droplets coupled to the diffusion of the counterions. The slow one has, to our knowledge, not been observed previously neither in similar microemulsions nor in charged spherical colloids. We show that the slow mode is also diffusive and suggest that its possible origine is the relaxation of local charge fluctuations via local exchange of droplets bearing different number of charges . The diffusion coefficient associated with this mode is then the self diffusion coefficient of the droplets

Bending elasticity of a curved amphiphilic film decorated anchored copolymers: a small angle neutron scattering study

Microemulsion droplets (oil in water stabilized by a surfactant film) are progressively decorated with increasing amounts of poly ethylene- oxide (PEO) chains anchored in the film by the short aliphatic chain grafted at one end of the PEO chain . The evolution of the bending elasticity of the surfactant film with increasing decoration is deduced from the evolution in size and polydispersity of the droplets as reflected by small angle neutron scattering. The optimum curvature radius decreases while the bending rigidity modulus remains practically constant. The experimental results compare well with the predictions of a model developed for the bending properties of a curved film decorated by non-adsorbing polymer chains, which takes into account, the finite curvature of the film and the free diffusion of the chains on the film.Comment: 30 June 200

Unstable Flow and Non-Monotonic Constitutive Equation of Transient Networks

We have measured the nonlinear rheological response of a model transient network over a large range of steady shear rates. The system is built up from an oil in water droplet microemulsion into which a telechelic polymer is incorporated. The phase behaviour is characterized which comprises a liquid-gas phase separation and a percolation threshold. The rheological measurements are performed in the one phase region above the percolation line. Shear thinning is observed for all samples, leading in most cases to an unstable stress response at intermediate shear rates. We built up a very simple mean field model which involves the reduction of the residence time of the stickers in the droplets due to the chain tensions at high shear. The computed constitutive equation is non-monotonic with a range where the stress is a decreasing function of the rate, a feature that indeed makes homogeneous flows unstable. The computed the flow curves compare well to the experiments.Comment: mai 200

Dynamics of the undulation mode in swollen lamellar phases

We investigate the dynamics of the undulation mode (displacement of wave vector q parallel to the layers) in swollen lamellar phases. We calculate the dispersion equation of this mode for all wave vectors, from small q to high q (compared with the inverse of the layer spacing). We then calculate the static structure factor for scattering wave vectors Q parallel to the layers over the whole Q-range and also the dynamic structure factor at high Q.Nous étudions la dynamique du mode d'ondulation (vecteur d'onde de déplacement q parallèle aux couches) de phases lamellaires diluées. Nous calculons l'équation de dispersion de ce mode pour tous les vecteurs d'onde, i.e. des petits aux grands q (par rapport à l'inverse de la distance entre couches). Nous calculons ensuite le facteur de structure statique pour des vecteurs de diffusion Q parallèles aux couches sur toute la gamme de Q, ainsi que le facteur de structure dynamique aux grands Q

Etude d'un système mixte Thermotrope-Lyotrope

International audienceNous souhaitons réaliser des systèmes lyotropes-thermotropes dans lesquels une organisationlyotrope coexiste avec un solvant cristal liquide thermotrope.Dans un premier temps, nous nous sommes intéressés à un système mixte de typemicroémulsion/ nématique, composé d’un mélange de molécules amphiphiles (bromure dedidodecyldimethylammonium DDAB), d’eau et de 5CB. Ce mélange avait été étudié dansl’équipe de Tanaka[1] qui avait discuté de l’existence d’une phase intermédiaire baptisée« nématique transparent » résultant de la compétition entre l’apparition d’un ordre nématiquedans le solvant et les effets d’ancrages sur les micelles. Diverses études que nous avonsmenées (dont une étude SAXS) infirment cependant ce point.Une étude soignée des diagrammes de phases couplée à des expériences de diffusions delumière nous a permis de montrer :- que l’ajout d’un cosurfactant (bromure de cetyltrimethyammonium CTAB) permetde faire varier continûment la taille des micelles, sans toutefois conduire àl’existence d’une phase lamellaire mixte.- que le 5CB possède des propriétés de cosurfactant et modifie fortement la courburespontanée des micelles.- qu’une interaction attractive entre micelles est présente et est vraisemblablementresponsable de l’absence de transition microémulsion/ lamellaire dans ce système.Cette première étude physico-chimique devrait nous conduire, par la suite, à l’étudequantitative des interactions entre micelles dans un solvant nématique.[1] J. Yamamoto, H. Tanaka, Nature, 409, 321 (2001)