Etude et propriétés dynamiques de solutions micellaires de tensioactifs fluorés et hydrogènes et de leurs mélanges

Dans ce travail, nous avons étudié l'influence de l'énergie de scission sur les propriétés structurales et dynamiques de solutions de micelles cylindriques. Nous avons pour cela fait appel à la technique de diffusion de lumière pour les propriétés structurales et aux techniques combinées de relaxation après saut de température et de rhéologie (ou diffusion dynamique de lumière) pour les propriétés dynamiques.Dans une première approche nous avons procédé à l'étude de mélanges de composition variable de deux tensioactifs, qui, séparément, forment des agrégats de courbure différente (micelles et vésicules). Nous avons mis en évidence l'existence d'un maximum du temps de relaxation viscoélastique lorsqu'on fait varier la composition du mélange. Ce comportement a été attribué à une transition, provoquée par l'accroissement de l'énergie de bout, entre les trois structures suivantes : Micelles linéaires enchevêtrées ® Micelles branchées enchevêtrées ® Réseau micellaire saturé.Dans une seconde approche, nous avons abordé l'étude des effets rhéo-épaississants que l'on observe pour des solutions de tensioactifs faiblement concentrées, à faible force ionique et présentant une forte énergie de scission. Les expériences de diffusion de lumière ont montré la présence d'agrégats de grande taille ( đ 100-200 nm) dans le domaine de concentration, où les systèmes présentent un effet rhéo-épaississant important. Les expériences de relaxation après saut de température et de rhéologie ont montré que tous les processus dynamiques sont accélérés par addition de sel. Nous avons obtenu un résultat fondamental : à savoir que deux des temps caractéristiques de l'effet rhéo-épaississant peuvent être identifiés à deux temps de relaxation après saut de température. Cette observation nous a permis de proposer une explication possible de l'effet rhéo-épaississant fondée sur une transition entre des morceaux de lamelles perforées et des rubans enchevêtrés, structures révélées par des expériences de cryo-microscopie.We have studied the effect of the scission energy on the structural and dynamical properties of solutions of wormlike micelles. We have used dynamic and static light scattering experiments for the study of structural properties and combined T-Jump and rheology (or dynamic light scattering) techniques for the dynamical properties.In a first approach we have studied mixtures with variable composition of two surfactants which, separately form aggregates with different curvature (micelles and vesicles). We have shown the existence of a maximum of the viscoelastic relaxation time when the composition of the mixture is varied. This behavior has been assessed to a transition produced by the increased of the end-cap energy between the three following structures: Entangled linear micelles ® Entangled branched micelles ® Saturated network.In a second approach, we have studied the shear-thickening occurring for low ionic strength, for low concentration of surfactants with high scission energy. The light scattering experiments have revealed the presence of large aggregates ( đ 100-200 nm) in the concentration range where the systems exhibit a large shear-thickening. T-Jump and rheology experiments have shown that all the dynamical processes are accelerated by the addition of salt. We have obtained a fundamental result: i.e. that two characteristic times of the shear-thickening can be identified with two T-Jump relaxation times. This observation allowed us to suggest an explanation of the shear thickening effect based on a transition between patches of perforated lamellas and entangled ribbons, structures shown by Cryo-Tem experiments.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Towards a Rational Morphology Control of Frozen Copolymer Aggregates

Kinetically frozen copolymer micelles are commonly prepared by confining amphiphilic block copolymers in the evaporating dispersed phase of oil-in-water emulsions. We revisit the mechanisms of this process by examining its successive steps separately: the formation of the solvent/water interface, the emulsification, the solvent evaporation and the formation of aggregates. We bring into evidence that: (i) spontaneous water-in-solvent emulsification, i.e., the formation of a double emulsion, is a necessary step for the subsequent assembly of the copolymers into kinetically frozen aggregates with certain morphologies far from equilibrium. (ii) Equilibration of the copolymer conformation at the solvent–water interfaces is a relatively slow process that can be outpaced, or even quenched before completion, by fast solvent evaporation rates. (iii) Rather than being dictated by the packing parameter at equilibrium, the morphology of the aggregates is determined by the effective copolymer conformation at the solvent–water interface when they form. (iv) Ultra-long worm-like micelles do not form by a direct digitation of the dispersed oil phase into the water continuous phase but through the inversion of the double emulsion. From these findings, we design a simple setup that allows us to control the morphology of the frozen aggregates obtained from a given copolymer composition by simply tuning the solvent evaporation rate.ChemE/Advanced Soft Matte

Cytotoxicity and internalization of Pluronic micelles stabilized by core cross-linking

International audienc

Preparation of stabilized micellar carriers based on Pluronic F127 for targeted drug delivery

C

Effects of Perfluorocarbon Gases on the Size and Stability Characteristics of Phospholipid-Coated Microbubbles: Osmotic Effect versus Interfacial Film Stabilization

Micrometer-sized bubbles coated with phospholipids are used as contrast agents for ultrasound imaging and have potential for oxygen, drug, and gene delivery and as therapeutic devices. An internal perfluorocarbon (<i>F</i>C) gas is generally used to stabilize them osmotically. We report here on the effects of three relatively heavy <i>F</i>Cs, perfluorohexane (<i>F</i>-hexane), perfluorodiglyme (<i>F</i>-diglyme ), and perfluorotriglyme (<i>F</i>-triglyme), on the size and stability characteristics of microbubbles coated with a soft shell of dimyristoylphosphatidylcholine (DMPC) and on the surface tension and compressibility of DMPC monolayers. Monomodal populations of small bubbles (∼1.3 ± 0.2 μm in radius, polydispersivity index ∼8%) were prepared by sonication, followed by centrifugal fractionation. The mean microbubble size, size distribution, and stability were determined by acoustical attenuation measurements, static light scattering, and optical microscopy. The half-lives of <i>F</i>-hexane- and <i>F</i>-diglyme-stabilized bubbles (149 ± 8 and 134 ± 3 min, respectively) were about 2 times longer than with the heavier <i>F</i>-triglyme (76 ± 7 min) and 4–5 times longer than with air (34 ± 3 min). Remarkably, the bubbles are smaller than the minimal size values calculated assuming that the bubbles are stabilized osmotically by the insoluble <i>F</i>C gases. Particularly striking is that bubbles 2 orders of magnitude smaller than the calculated collapse radius can be prepared with <i>F</i>-triglyme, while its very low vapor pressure prohibits any osmotic effect. The interface between an aqueous DMPC dispersion and air, or air (or N₂) saturated with the <i>F</i>Cs, was investigated by tensiometry and by Langmuir monolayer compressions. Remarkably, after 3 h, the tensions at the interface between an aqueous DMPC dispersion (0.5 mmol L^–1) and air were lowered from ∼50 ± 1 to ∼37 ± 1 mN m^–1 when <i>F</i>-hexane and <i>F</i>-diglyme were present and to ∼40 ± 1 mN m^–1 for <i>F</i>-triglyme. Also noteworthy, the adsorption kinetics of DMPC at the interface, as obtained by dynamic tensiometry, were accelerated up to 3-fold when the <i>F</i>C gases were present. The compression isotherms show that all these <i>F</i>C gases significantly increase the surface pressure (from ∼0 to ∼10 mN m^–1) at large molecular areas (70 Ų), implying their incorporation into the DMPC monolayer. All three <i>F</i>C gases increase the monolayer’s collapse pressures significantly (∼61 ± 2 mN m^–1) as compared to air (∼54 ± 2 mN m^–1), providing for interfacial tensions as low as ∼11 mN m^–1 (vs ∼18 mN m^–1 in their absence). The <i>F</i>C gases increase the compressibility of the DMPC monolayer by 20–50%. These results establish that, besides their osmotic effect, <i>F</i>C gases contribute to bubble stabilization by decreasing the DMPC interfacial tension, hence reducing the Laplace pressure. This contribution, although significant, still does not suffice to explain the large discrepancy observed between calculated and experimental bubble half-lives. The case of <i>F</i>-triglyme, which has no osmotic effect, indicates that its effects on the DMPC shell (increased collapse pressure, decreased interfacial tension, and increased compressibility) contribute to bubble stabilization. <i>F</i>-hexane and <i>F</i>-diglyme provided both the smallest mean bubble sizes and the longest bubble half-lives

pH-Controlled Microbubble Shell Formation and Stabilization

We report on microbubbles with a shell self-assembled from an anionic perfluoroalkylated surfactant, perfluorooctyl­(ethyl)­phosphate (<i>F</i>8<i>H</i>2Phos). Microbubbles were formed and effectively stabilized from aqueous solutions of <i>F</i>8<i>H</i>2Phos at pH 5.6–8.5. This range overlaps the domains of existence of the monosodic and disodic salts. The shell morphology of microbubbles formed spontaneously by heating aqueous solutions of <i>F</i>8<i>H</i>2Phos was monitored during cooling, directly on the microscope’s stage. At pH 5.6, the shell collapses through nucleation of folds, as typical for insoluble surfactants. At pH 8.5, no folds were seen during shrinking. At higher pH, the microbubbles rapidly adsorb on the glass. The effect of pH (from 5.6 to 9.7) on adsorption kinetics of <i>F</i>8<i>H</i>2Phos at the air/water interface, and on the elasticity of its Gibbs films, was determined. At low pH, <i>F</i>8<i>H</i>2Phos is highly surface active. The interfacial film undergoes a dilute-to-condensed phase transition and a dramatic increase of elastic module, leading to extremely high values (up to 500 mN m^–1). At high pH, the surfactant’s adsorption is quasi-instantaneous, but interfacial tension lowering is limited, leading to very low elastic module (∼5 mN m^–1). At pH 5.6 and 8.5, the interfacial tension of <i>F</i>8<i>H</i>2Phos adsorbed on millimetric bubbles and compressed at a rate similar to that exerted on micrometric bubbles during deflation is lower than the equilibrium interfacial tension. Langmuir monolayers of <i>F</i>8<i>H</i>2Phos are highly stable at low pH and feature a liquid expanded/liquid condensed transition; at high pH, they do not withstand compression. Both mono- and disodic <i>F</i>8<i>H</i>2Phos salts are needed to effectively stabilize microbubbles: the rapidly adsorbed disodic salt stabilizes a newly created air/water interface; the more surface active monosodic salt then replaces the more water-soluble disodic salt at the interface. During deflation, the surfactant shell undergoes a transition toward a highly elastic phase, which further contributes to bubble stabilization