Didier Roux

Micelles, membranes, microemulsions, and monolayers

Contents: Statistical Thermodynamics of Amphiphile Self-Assembly (A. Ben-Shaul & W. Gelbart).- Micellar Growth, Flexibility and Polymorphism in Dilute Solutions (G. Porte).- Micellar Liquid Crystals (N. Boden).- Geometric Foundations of Mesomorphic Polymorphism (J. Charvolin & J.F. Sadoc).- Lamellar Phases: Effect of Fluctuations (Theory) (D. Sornette & N. Ostrowsky).- Lyotropic Lamellar Lx Phases (D. Roux, C.R. Safinya, & F. Nallet). Structure of Microemulsions: Experimenta (L. Auvray).- Lattice Theories of Microemulsions (G. Gompper & M. Schick).- Fluctuating Interfaces (S.A. Safran).- Interfacial Tension (D. Langevin & J. Meunier).- Critical Behavior of Surfactant Solutions (A-M. Bellocq).- Structures and Phase Transitions in Langmuir Monolayers (D. Andelman et al.).

Dilute lamellar and L3 phases in the binary water–C12E5 system

The binary phase diagram of water–C12E5 has been studied with emphasis on the L3 and dilute lamellar phases, which were found to swell to approximately 99.5 and 98.8 wt % of water, respectively, much further than has been reported previously. Focusing on these two phases, we have carried out static light and small-angle neutron scattering and electrical conductivity measurements. The repeat distance in the lamellar phase was found to exceed 3000 A. A small, but significant deviation from ideal one-dimensional swelling was observed. This deviation may be explained in terms of flexibility of the bilayers that are flat only on average. Electrical conductivity and small-angle neutron scattering data from the isotropic L3 phase are, over most of the stability range, consistent with a three-dimensional continuous bilayer structure. However, at large water contents an increase in the conductivity indicates a breakup of the structure into smaller fragments. Our results show that the simpler binary system exhibits the same characteristic features as the more complex multicomponent systems, involving brine, ionic surfactant and cosurfactant.

Rheology of lyotropic lamellar phases

The rheological behaviour of lyotropic lamellar phases is studied as a function of the membrane repeating distance. The steady-state rheology is described as a consequence of the so-called orientation diagram described previously. Three distinct regions of different orientations are described, which are separated by two out-of-equilibrium transitions. We show that these transitions can be either discontinuous (subcritical) or continuous. In one of these transitions, one can go continuously from one regime to the other through a bifurcation point.

Random Surface Model for the L3-Phase of Dilute Surfactant Solutions

We present a simple model for the anomalous (flow-birefringent) isotropic phase, known as L3, that is seen in certain surfactant solutions at volume fractions of a few percent. The proposed structure consists of locally sheetlike sections of semi-flexible surfactant bilayer, connected up at larger distances into a multiply connected random surface, having a preferred structural length scale of order the persistence length of the bilayer. A first-order transition between this isotropic sheetlike phase and the nearby swollen lamellar phase is described.

Experimental Evidence for Random Surface Structures in Dilute Surfactant Solutions

Anomalous flow birefringent phases, sometimes designated as L3, have been identified in both the water-rich and the oil-rich parts of the phase diagram of the water(NaCl)-dodecane-pentanol-SDS system. The presence of these phases appears to be associated with that of swollen lamellar phases. Conductivity and neutron scattering results provide evidence that the structure of these phases consists of a highly connected, spongelike, random bilayer-continuous surface. The surfactant surface separates, depending upont the system, either two water-continuous domains or two oil-continuous domains. The data are consistent with a recent theoretical models.

Spherulites: A new vesicular system with promising applications. An example: Enzyme microencapsulation

A new technology is tested for enzyme encapsulation. The capsules are small multilamellar vesicles of surfactant called spherulites which are produced by shearing a lamellar phase under well-controlled conditions. Encapsulation of alkaline phosphatase into spherulites is studied here as an example. Once encapsulated, the enzyme is shown to be unable to develop any enzymatic activity on its substrate, the p-nitrophenylphosphate. This is due to the absence of contact between the enzyme and the substrate. Interestingly, the whole enzymatic activity is recovered after destruction of the vesicles. Encapsulation efficiency ranges between 70% and 95% depending upon the enzyme over phospholipids ratio. Beyond the example of alkaline phosphatase, many applications of spherulites in the medical or in the biotechnology fields seem now at hand.

Interaction of short DNA fragments with the cationic polyelectrolyte poly(ethylene imine): a dynamic light scattering study

In genetic engineering and gene therapy, it is important to obtain high transfection efficiencies. This can be achieved by condensation or aggregation of the DNA using the attractive interaction of DNA with cationic polyelectrolytes or histones. In recent years, the use of poly(ethylene imine) has gained importance in this held. In our study, we characterize the hydrodynamic properties of the dendrimeric cationic polyelectrolyte poly(ethylene imine) (PEI; M-W = 25 kDa) and of short DNA fragments (approximate to 150 bp) in solution by dynamic light scattering. On this basis we determine structural parameters of the formed aggregates. On mixing the DNA fragments with the cationic polyelectrolyte, an instantaneous formation of large aggregates is found. This is different compared to condensation with small cations, which can not condense short DNA fragments. The experimental conditions (pH, etc.) during the measurements correspond to the situation in transfection experiments

Interaction of Cationic Colloids at the Surface of J774 Cells: A Kinetic Analysis

We have characterized the binding of multilamellar colloids to J774 cells. Cationic colloids were shown to bind much more efficiently than neutral ones. Particle uptake by cells was followed by flow cytometry and fluorescence microscopy. Analysis of the kinetics of uptake of cationic particles indicated that binding on the cell surface occurred with two characteristic times. Analysis of the dissociation properties allowed discriminating between several alternative models for adsorption and led us to propose a mechanism that involved two independent classes of binding sites on the cell surface. One class of sites appeared to be governed by a classic mass action law describing a binding equilibrium. The other sites were populated irreversibly by particles made of 10% cationic lipids. This was observed in the absence of endocytosis, under conditions where both the equilibrium and the irreversible binding occurred at the cell surface. We determined the rate constants for the different steps. We found that the reversible association occurred with a characteristic time of the order of tens of seconds, whereas the irreversible binding took a hundred times longer. The presence of serum proteins in the incubation medium did not drastically affect the final uptake of the particles. In contrast, the capture of the particles by cells significantly dropped when the fraction of positively charged lipids contained in the colloids was decreased from 10% to 5%. Finally, the results will be discussed within a comprehensive model where cationic particles find labile binding sites in the volume of the pericellular network (glycocalyx and extracellular matrix) and less-accessible irreversible binding sites at the cell membrane itself.

In vitro and in vivo stability of new multilamellar vesicles.

Factors affecting multilamellar vesicles transport to the blood compartment after oral administration to rats were evaluated first in vitro. A high entrapment of protein A was obtained when the vesicles were prepared by shearing a lyotropic lamellar phase composed of soybean phosphatidylcholine, cholesterol and polyoxyethylene alcohol (C12H25(OCH2CH2)4OH) as neutral detergent. In vitro tests showed that these vesicles (spherulites) were stabled in 50% of fetal calf serum, in acidic (pH 3) or basic (pH 10) buffers, in pancreatin but are partially lysed in 20mM bile salts. Oral administration of spherulites entrapping 111In-NTA in fasting rats showed a increase of radioacticivity in blood. This could be explained by passage of some spherulites in the enterocytes.

Dynamical behavior of a complex fluid near an out-of-equilibrium transition: approaching simple rheological chaos.

We report here an extensive study of sustained oscillations of the viscosity of a complex fluid near an out-of-equilibrium transition. Using well defined protocols, we perform rheological measurements of the onion texture near a layering transition in a Couette flow. This complex fluid exhibits sustained oscillations of the viscosity, on a large time scale (500 s) at controlled stress. These oscillations are directly correlated to an oscillating microstructural change of the texture of the fluid. We observe a great diversity of dynamical behavior and show that there is a coupling with spatial effects in the inverted Delta v direction. This is in agreement with a careful analysis of the temporal series of the viscosity with the dynamical system theory. This analysis indicates that the observed dynamical responses do not strictly correspond to three-dimensional chaotic states, probably because some spatiotemporal effects are present and are likely to play an important role.