Monique Dubois

Self-assembly of regular hollow icosahedra in salt-free catanionic solutions

Self-assembled structures having a regular hollow icosahedral form (such as those observed for proteins of virus capsids) can occur as a result of biomineralization processes, but are extremely rare in mineral crystallites. Compact icosahedra made from a boron oxide have been reported, but equivalent structures made of synthetic organic components such as surfactants have not hitherto been observed. It is, however, well known that lipids, as well as mixtures of anionic and cationic single chain surfactants, can readily form bilayers that can adopt a variety of distinct geometric forms: they can fold into soft vesicles or random bilayers (the so-called sponge phase) or form ordered stacks of flat or undulating membranes. Here we show that in salt-free mixtures of anionic and cationic surfactants, such bilayers can self-assemble into hollow aggregates with a regular icosahedral shape. These aggregates are stabilized by the presence of pores located at the vertices of the icosahedra. The resulting structures have a size of about one micrometre and mass of about 1010 daltons, making them larger than any known icosahedral protein assembly or virus capsid. We expect the combination of wall rigidity and holes at vertices of these icosahedral aggregates to be of practical value for controlled drug or DNA release.

Osmotic pressure and salt exclusion in electrostatically swollen lamellar phases

Measurements of osmotic pressure, mainly of electrostatic origin, are reported in the diluted regime of charged bilayers. The interlamellar distances (100–1000 A) are measured using small‐angle neutron scattering. The electrostatic repulsive pressure is evaluated within the framework of the Poisson–Boltzmann model, taking into account the conditions of ionic conservation and electrochemical equilibrium, and checked by Monte Carlo simulations in the grand canonical ensemble. The predicted phenomenon of salt ejection from the bilayers is evidenced by direct chemical analysis. Implications concerning the phase behavior of double‐tailed surfactants are discussed.

EQUATION OF STATE OF A CHARGED BILAYER SYSTEM : MEASURE OF THE ENTROPY OF THE LAMELLAR-LAMELLAR TRANSITION IN DDABR

The synthetic cationic double-chain surfactant didodecyldimethylammonium bromide shows two distinct thermodynamically stable lamellar phases; a dilute Lα phase stabilized predominantly by electrostatic forces, and a condensed Lα′ phase stabilized by “hydration” forces. Using six different experimental methods, applying osmotic stress from 102 to 109 Pa and varying temperature from 20 °C to 70 °C, we have measured the osmotic pressure vs interbilayer distance and thus mapped the phase diagram with an equation of state. In this binary system, the area per headgroup as well as bilayer thickness vary with concentration and temperature. Hence, lateral compressibility has to be taken into account in the free energy balance. The osmotic stress needed to effect the swollen-to-collapsed lamellar phase transition is determined as a function of temperature. From these data the entropy of the Lα–Lα′ transition is found to be a strong function of temperature. Below 40 °C, condensation from the dilute Lα phase, the cha...

Swelling limits for bilayer microstructures : the implosion of lamellar structure versus disordered lamellae

Swelling of charged bilayers can extend to well over 100 nm without loosing long range order. Attraction induced by solutes or screening induced by salt or buffers reduce the maximum periodicity. Microstructural evolutions near the maximum swelling are reviewed as well as associated equations of state, i.e. osmotic pressures versus interlayer distances. We describe the different mechanisms which have been identified as limiting the swelling of a lamellar phase, and the phase equilibria involved when maximum swelling limit is reached.

Swelling of a Lecithin Lamellar Phase Induced by Small Carbohydrate Solutes

In this paper, we consider the effect of adding small carbohydrate solutes (small sugars) to DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) L(alpha) dispersions and the consequences on the force balance at zero osmotic pressure (maximal swelling). We show the importance of long incubations required to obtain samples at thermodynamic equilibrium where molecular diffusion has been completed. The monotonic increase of maximal swelling versus sugar content occurs as a combined effect of the screening of the van der Waals contribution and fluctuations in the lamellar stacks. According to this new approach, it is shown that changes in dielectric properties result in a much less pronounced effect than entropic forces (undulations) generated by the softening of the membranes at high sugar content. However, this sugar-induced swelling cannot be explained quantitatively by adding an entropic contribution to molecular interactions. Quantitative disagreement between the proposed mechanism and our observations is due either to nonadditivity of molecular interactions with entropic forces or to the relation used to account for the entropic contribution.

Chain melting in swollen catanionic bilayers

The studies of temperature dependent transitions of bilayer structures are mainly focused on phospholipids systems. Since the appearance of catanionic surfactants it is possible to study another type of bilayer structure but as they were prepared until recently in the presence of initial counter-ions, the electrostatic interactions could not be taken in account. Our aim is to present some differential scanning calorimetry results of salt-free catanionic systems prepared with acid surfactant and hydroxide surfactant, and to compare them with literature data.

Mixed-Monolayer-Protected Gold Nanoparticles for Emulsion Stabilization

Nanometer-sized gold nanoparticles have been prepared and surface-modified in order to stabilize alkane-in-water emulsions. A mixed hexane-undecanol ligand layer at the surface of the nanoparticles allowed us to tune their wettability and thus the adsorption at the oil-water interface. Oil droplets of the stable emulsions have been evidenced by confocal fluorescence microscopy, freeze-fracture transmission electron microscopy, and dynamic light scattering. Prepared emulsions were stable during performed cooling-heating cycles, in which the temperature stability of the emulsions has been studied by means of dynamic light scattering. The interfacial structure of the oil droplets was investigated by small-angle X-ray scattering. The obtained area per nanoparticle at the oil droplet interface was 30 nm(2). The investigation of the nanoparticle adsorption at the curved interface of the emulsion droplets is in agreement with our previous study at a planar oil-water interface, in which the nanoparticles started to interact with each other at about the same area per particle.

In-plane distribution in mixtures of cationic and anionic surfactants

Mixtures of cationic and anionic surfactants, also called “catanionic” mixtures, self-assemble into aggregates which show a rich variety of morphologies in the nanometre to micron range controlled by the molar ratio between surfactants. At the molecular scale, little is known about the local distribution of surfactants in the bilayer. Here, we determine the in-plane distribution of the cationic and anionic surfactants in catanionic bilayers using neutron scattering and the contrast matching technique. The coexistence of two different in-plane orders is experimentally demonstrated: both surfactants share a common two-dimensional hexagonal lattice with a long-range order, but the distribution of alternate (+) and (−) charged head groups shows only a two-dimensional liquid-like local order. Comparing experiments and Monte-Carlo simulations, we establish that this lateral liquid order is attenuated by the counterions of the bilayers, but is less than expected in a mean-field approach. This demonstrates that electrostatic interactions participate in, but do not completely determine, the local distribution of surfactants of opposite charge in the bilayer.

Coexistence of two lyotropic lamellar phases induced by a polymer in a phospholipidwater system

Abstract The effect of a hydrophobically modified polysaccharide, cholesteryl-pullulan (CHP), on the swelling of the DMPC L α lamellar phase has been investigated by small angle neutron scattering. The CHP derivative can be introduced in the aqueous layers of the lamellar phase by anchoring lateral cholesterol groups into the bilayers. The resulting lamellar phase (L p ) is stabilized at large membrane separations by the introduction of a new repulsive and long range contribution in the force balance of the system. We emphasize here the temperature dependence of two coexisting lamellar phases (L α + L p ) differing in their polymer content and in their periodicities. At low polymer content (DMPC:CHP=99:1 by weight), the two lamellar phases at thermodynamic equilibrium change into a single phase on heating from room temperature to 50°C. The new phase (L p ′) is characterized by a very large correlation peak whose position is consistent with a lamellar structure following an ideal dilution law. The transition L α + L p → L p ′ is reversible on cooling, indicating that the observed coexistence of the two lamellar phases at room temperature is a true thermodynamic equilibrium. At higher polymer content (DMPC:CHP=95:5 by weight) the critical behaviour has not been observed. The periodicity of the L p phase slightly decreases on heating indicating a reduction in the miscibility gap and a possible critical point at temperatures higher than 50°C. However, in the investigated temperature range, the thermodynamic coexistence of the two lamellar phases is not affected in this case.

Formation of rigid nanodiscs: edge formation and molecular separation

We show that the control of size in catanionic nanodiscs relies on the balance between charge excess favoring the disc edge and frozen ion pair formation favoring rigid faces. In the absence of salt, average diameters in the range of 60 nm to 3 σm have been obtained by varying the molar ratio x (cationic/anionic + cationic) from x = 0.39 to 0.46. The cost in entropy of mixing associated with partial molecular separation inside each crystallized nanodisc is lower than the work associated with osmotic pressure in the region of the phase diagram where catanionic nanodiscs are produced.