François Muller

Phase behavior of Phytantriol/water bicontinuous cubic Pn3m cubosomes stabilized by Laponite disc-like particles.

The present article reports on the specific effects of temperature on Phytantriol-based cubosomes stabilized by inorganic stabilizers as opposed to organic stabilizers. The ability of Laponite to stabilize Phytantriol-based parent bulk phase is first demonstrated. The sub-micron-sized Laponite-stabilized particles were found to be both physically and chemically stable over time. The temperature-induced behavior, both in heating and cooling directions, of these lipid-based cubosomes has been investigated and compared with their polymer-stabilized counterparts (Pluronic F127). This allows us to extract the particular influence of each stabilizer. Whereas an increased hydration of the cubic structure was evidenced at high pH values, this effect was eliminated to compare the specific influence of both stabilizers on these Phytantriol-based cubosomes. Evidence of differences in the relaxation rates of the internal structures with temperature was found for the two stabilizers, in particular in the cooling direction whilst in the heating direction the two stabilizers could be considered as undisruptive. The origin of this difference is discussed.

Precipitating Sodium Dodecyl Sulfate to Create Ultrastable and Stimulable Foams

Ultrastable foams are made very simply by adding salt (NaCl or KCl) to sodium dodecyl sulfate. The addition of high concentrations of salt leads to the precipitation of the surfactant on the bubble surfaces and as crystals in the interstices between the bubbles. As a consequence, the ageing of the foams is stopped to make them stable indefinitely, or until they are heated above the melting temperature of the crystals. The use of KCl is shown to be much more effective than that of NaCl because potassium dodecyl sulfate has a higher melting temperature and faster rates of crystallization. The crystalline structures have been investigated inside the foam using small angle neutron scattering. The larger lattice spacing of the crystals formed with NaCl in comparison with KCl has been evidenced. These simple temperature stimulable foams could have many potential applications.

How clay colloids surround internally self-assembled phytantriol drops.

The stabilisation of emulsified microemulsions using different concentrations of colloidal disk-like particles (Laponite) is investigated. The resulting structures are characterised by cryo-TEM, small-angle and very small-angle neutron scattering methods. We show that the Laponite colloids are effectively attached onto the droplet interfaces, creating a protective layer around them, dense enough to be statistically observed by means of neutron scattering, although the mean coverage remains still rather low. The mean size of the internally organised droplets does not change with the colloidal concentration (up to 1 wt%), and a few free colloids are found in the continuous water solvent. However, the colloids are shown to be able to deform the soft interfaces of the microemulsion phase to create droplets that are not always spherical, but can have angled interfaces.

Probing structure in submicronic aqueous assemblies of emulsified microemulsions and charged spherical colloids using SANS and cryo-TEM

The spatial distribution of charged spherical colloids when used as stabilizers of phytantriol-based emulsified microemulsions (EME, L2 symmetry group) is investigated. The coverage of the lipid-based mesophases by the colloids is monitored using small-angle neutron scattering (SANS) in contrast matching conditions and visualized using cryogenic transmission electron microscopy (cryo-TEM) imaging. The results demonstrate that, despite the stability of the emulsion droplets, very few colloids are ever found on the droplets. The stability of the EMEs is suggested to arise from the very slow ripening rates combined with punctual repulsion against coalescence from the isolated charged colloids on the bigger droplet surfaces. We show the possibility of creating a dense cover around the droplets by partially hydrophobizing the colloids by adsorbing a cationic surfactant on their surface. This opens up the possibilities for further modulation of the colloidal coverage in these systems. This is an interesting route for the design of new Colloid-ISAsome assemblies in which dense protective armors could be advantageous such as controlled delivery.

Salt-Induced Behavior of Internally Self-Assembled Nanodrops: Understanding Stabilization by Charged Colloids

We explored the structural effects induced by the addition of salt on lipid-based liquid crystalline drops stabilized in aqueous media by charged sphere-like colloids. This allows us to distinguish two different stabilization regimes. In one case, the internal liquid crystalline phase has the ability to reorganize upon the coalescence of the drops and in the other not. This in turn depends mainly on the contact angle and the internal phase viscosity.

Varying the counter ion changes the kinetics, but not the final structure of colloidal gels

We show that, while the gelation of colloidal silica proceeds much faster in the presence of added KCl than NaCl, the final gels are very similar in structure and properties. We have studied the gelation process by visual inspection and by small angle X-ray scattering for a range of salt and silica particle concentrations. The characteristic times of the early aggregation process and the formation of a stress-bearing structure with both salts are shown to collapse onto master curves with single multiplicative constants, linked to the stability ratio of the colloidal suspensions. The influence of the salt type and concentration is confirmed to be mainly kinetic, as the static structure factors and viscoelastic moduli of the gels are shown to be equivalent at normalized times. While there is strong variation in the kinetics, the structure and properties of the gel at long-times are shown to be mainly controlled by the concentration of particles, and hardly influenced by the type or the concentration of salt. This suggests that the differences between gels generated by different salts are only transient in time.

Probing foam with neutrons.

Foams are multiscale materials that have an enormous number of uses. As the relevant structural length-scales span from a few nanometres up to millimetres a number of characterisation methods need to be combined to obtain the full material structure. In this review we explain how foams can be explored using Small Angle Neutron Scattering (SANS). We remind the reader of the basics of SANS and contrast variation before we describe the different types of experiments that have been carried out on foams emphasising the specific role of neutrons in learning about the systems. To date SANS has been used to measure different foam structural parameters, such as the film thickness and the bubble size. Several studies have also been carried out to elucidate the organisation of the stabilising objects in the bulk solution. Finally we show how SANS measurements can be used to measure foam composition. Some of the accessible information is unique to SANS experiments, but as the method is still not very widely used on foams the review is also aimed to act as an introduction on how to carry out such measurements on foams.

Chapter Eight – Stabilization of Lipid-Based Lyotropic Liquid Crystalline Phases by Nanoparticles: Perspectives for Colloid–Isasomes

Abstract We review the current understanding of the stabilization mechanisms of internally self-assembled domains dispersed in water. These hierarchical particles are potential carriers of active molecules. The use of solid nanoparticles as stabilizers is a novelty in the field. The properties and the possible gain that can be obtained by using such stabilizers as a part of a delivery system based on dispersed lyotropic liquid crystalline phases of lipids represent the core of the present contribution. Two different types of particles (spheres and disks) have been successfully used to stabilize such droplets. The specific lipid–particle interactions as well as the physical parameters (surface curvature and stiffness) play an important role in the stabilization mechanism. However, indirect effects such as a change of pH can have an impact on the droplet stability and the internal structure. Using small-angle neutron scattering (SANS) and cryo-TEM, it is possible to show the existence of a protective armor around the droplets. Such armor is formed with the disk-like particles, but the adsorption of spherical particles is much lower, and creation of armor is only possible through a hydrophobic modification of the sphere surface. The colloid-stabilized nanoemulsions are interesting as smart carriers due to the possibilities of hierarchical functionalization of both the internal phase and the stabilizing particles.We review the current understanding of the stabilization mechanisms of internally self-assembled domains dispersed in water. These hierarchical particles are potential carriers of active molecules. The use of solid nanoparticles as stabilizers is a novelty in the field. The properties and the possible gain that can be obtained by using such stabilizers as a part of a delivery system based on dispersed lyotropic liquid crystalline phases of lipids represent the core of the present contribution. Two different types of particles (spheres and disks) have been successfully used to stabilize such droplets. The specific lipid–particle interactions as well as the physical parameters (surface curvature and stiffness) play an important role in the stabilization mechanism. However, indirect effects such as a change of pH can have an impact on the droplet stability and the internal structure. Using small-angle neutron scattering (SANS) and cryo-TEM, it is possible to show the existence of a protective armor around the droplets. Such armor is formed with the disk-like particles, but the adsorption of spherical particles is much lower, and creation of armor is only possible through a hydrophobic modification of the sphere surface. The colloid-stabilized nanoemulsions are interesting as smart carriers due to the possibilities of hierarchical functionalization of both the internal phase and the stabilizing particles.