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.

Osmotic pressure and phase boundary determination of multiphase systems by analytical ultracentrifugation.

We show that analytical ultracentrifugation can be applied to derive full equations of state of colloids in a single sedimentation equilibrium experiment, by determination of single-phase boundaries as well as of osmotic pressure versus concentration at fixed temperatures. A continuous dependence of the osmotic pressure, over orders of magnitude between at least approximately 10(1) and 10(4) Pa, and a wide concentration range, are determined in agreement with standard theoretical considerations. Two model experimental colloidal systems are investigated: For a well-known synthetic clay system (laponite), it is shown that two regimes-counter-ion ideal gas and interacting double layers-can easily be identified in the equation of state, whereas metastable glass- or microphase-separated gel states previously encountered in osmotic stress measurements of laponite are circumvented. For the case of rigid, crystallized catanionic bilayers, single phase domains can be identified. Osmotic pressure results in this case disagree with results obtained using the classical osmotic stress technique, as a result of sample adhesion to the ultracentrifuge cell windows and uncertainty due to possible micromolar ion contamination.

Soft X-ray microscopy to characterize polyelectrolyte assemblies.

Transmission microscopy with soft X-rays (TXM) is applied to image in-situ polyelectrolyte assemblies in aqueous environment. The method is element specific and at this stage exhibits a lateral resolution of 20 nm. With the specific examples of hollow capsules and full spheres made of PAH/PSS polyelectrolyte multilayers, it is shown quantitatively that heat treatment irreversibly reduces the water content in the membrane. These experiments complement those reported recently on the polyion system PDADMAC/PSS, which shows a different glass-transition behavior. Finally, the potential and present limitations of TXM are discussed.

Counter-ion activity and microstructure in polyelectrolyte complexes as determined by osmotic pressure measurements.

We have investigated the activity of counter-ions at 60 degrees C through the osmotic coefficient K in solutions of anionic and cationic polyelectrolyte complexes of variable compositions. For excess of polyanion in the complexes (molar fraction of polycation f < 0.5), K increases as the polyanion is neutralized by the polycation (f getting closer to 0.5). By contrast, for an excess of polycation (f > 0.5), K stays constant or even slightly decreases as the polycation is getting neutralized by the polyanion. This asymmetric behavior depending on the charge of the complexes indicates that the globally negatively charged complexes are homogeneous and can be treated as a single polyelectrolyte of reduced linear charge density. On the other hand, the positively charged complexes show a micro-phase separation between neutral fully compensated microdomains and domains where the excess polycation is locally segregated. These two different microstructures are reminiscent of the coacervation and segregation regimes observed at higher concentrations and salinities, and also of polyelectrolyte complexes with oppositely charged surfactants. This interpretation is supported by two simple predictive models.

Surface immobilization and mechanical properties of catanionic hollow faceted polyhedrons.

We report here for the first time on surface immobilization of hollow faceted polyhedrons formed from catanionic surfactant mixtures. We find that electrostatic interaction with the substrate dominates their adhesion behavior. Using polyelectrolyte coated surfaces with tailored charge densities, polyhedrons can thus be immobilized without complete spreading, which allows for further study of their mechanical properties using AFM force measurements. The elastic response of individual polyhedrons can be locally resolved, showing pronounced differences in stiffness between faces and vertexes of the structure, which makes these systems interesting as models for structurally similar colloidal scale objects such as viruses, where such effects are predicted but cannot be directly observed due to the smaller dimensions. Elastic constants of the wall material are estimated using shell and plate deformation models and are found to be a factor of 5 larger than those for neutral lipidic bilayers in the gel state. We discuss the molecular origins of this high stiffness.

Equation of state of colloids coated by polyelectrolyte multilayers.

We propose an adaptation of the osmotic stress technique for determining equations of state of colloids covered by polymer films. We describe here the case of silica particles covered by 6 or 7 layers of polyelectrolyte multilayers. The establishment of a pressure-distance curve allows the interpretation of colloidal interactions over more than four orders of magnitude in pressure. This large range allows a clear separation between three distinct regimes: layer compression, brush extension and at very low pressures a regime of weak repulsion. The physical origins of the regimes are discussed and described by models of colloidal interactions.

The role of counterions on the elasticity of highly charged lamellar phases: a small-angle x-ray and neutron-scattering determination.

The structure and fluctuations of the swollen L(alpha) lamellar phase of highly charged surfactant didodecyldimethylammonium halide fluid bilayers (DDA+X-) are studied using high-resolution small-angle x-ray scattering and medium-resolution, high-contrast small-angle neutron-scattering. The Caille parameter eta, as a function of the swelling (L(alpha) periodicity d), was determined from the full q-range fits of the measured scattering profiles for three different counterions (X- = Cl-, Br-, and NO3-). This parameter quantifies the amplitude of the membrane fluctuations within the Landau-de Gennes smectic-A linear elasticity theory. The different anions used gave strong specific effects at the maximum swelling of the L(alpha) phase, while at lower swellings a two-phase coexistence of swollen and collapsed lamellae (d approximately 30 and approximately 80 angstroms) was observed for bromide and nitrate ions. Over the intermediate dilution range for all three counterions, a single L(alpha) phase can be continuously swollen with pure water which is governed by an equation of state (i.e., osmotic pressure versus period) and thermally excited fluctuation amplitudes that can be well described by the same Poisson-Boltzmann calculation. The membranes were found to be slightly stiffer than predicted by purely electrostatic repulsions, and this is tentatively attributed to an extra bending rigidity contribution from the surfactant chains.

Shape control through molecular segregation in giant surfactant aggregates.

Mixtures of cationic and anionic surfactants crystallized at various ratios in the absence of added salt form micrometer-sized colloids. Here, we propose and test a general mechanism explaining how this ratio controls the shape of the resulting colloidal structure, which can vary from nanodiscs to punctured planes; during cocrystallization, excess (nonstoichiometric) surfactant accumulates on edges or pores rather than being incorporated into crystalline bilayers. Molecular segregation then produces a sequence of shapes controlled by the initial mole ratio only. Using freeze-fracture electron microscopy, we identified three of these states and their corresponding coexistence regimes. Fluorescence confocal microscopy directly showed the segregation of anionic and cationic components within the aggregate. The observed shapes are consistently reproduced upon thermal cycling, demonstrating that the icosahedral shape corresponds to the existence of a local minimum of bending energy for facetted icosahedra when the optimal amount of excess segregated material is present.

Determination of pore size of catanionic icosahedral aggregates.

We show that it is possible to measure the porosity of facetted micron-sized hollow icosahedra of catanionic solutions by performing fluorescence recovery after photobleaching measurements. The size of spontaneous permanent pores in bilayers formed via molecular segregation is compatible with what is observed by freeze-fracture electron microscopy and is discussed versus theoretical expressions of bending energy.

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.