Fabrice Cousin

Smart Foams: Switching Reversibly between Ultrastable and Unstable Foams†

Ultrastable foams with an optimal foamability have been obtained using hydroxyl fatty acids tubes. The stabilization results from the adsorption of monomers at the air-water interface preventing coalescence and coarsening and from the presence of tubes in the Plateau borders limiting the drainage. Upon heating, tubes transit to micelles, which induces foam destabilization. Such foams are thus the first to have a temperature tunable stability.

Redispersible Hybrid Nanopowders: Cerium Oxide Nanoparticle Complexes with Phosphonated-PEG Oligomers

Rare earth cerium oxide (ceria) nanoparticles are stabilized using end-functional phosphonated-PEG oligomers. The complexation process and structure of the resulting hybrid core-shell singlet nanocolloids are described, characterized, and modeled using light and neutron scattering data. The adsorption mechanism is nonstoichiometric, yielding the number of adsorbed chains per particle N(ads) = 270 at saturation. Adsorption isotherms show a high affinity of the phosphonate head for the ceria surface (adsorption energy DeltaG(ads) approximately -16kT) suggesting an electrostatic driving force for the complexation. The ease, efficiency, and integrity of the complexation is highlighted by the formation of nanometric sized cerium oxide particles covered with a well anchored PEG layer, maintaining the characteristics of the original sol. This solvating brushlike layer is sufficient to solubilize the particles and greatly expand the stability range of the original sol (<pH 3) up to pH = 9. We underscore two key attributes of the tailored sol: (i) strong UV absorption capability after functionalization and (ii) ability to redisperse after freeze-drying as powder in aqueous or organic solvents in varying concentrations as singlet nanocolloids. This robust platform enables translation of intrinsic properties of mineral oxide nanoparticles to critical end use.

Structural Details of Cellulose Nanocrystals/Polyelectrolytes Multilayers Probed by Neutron Reflectivity and AFM

Neutron reflectivity measurements and AFM observations were used as complementary techniques to investigate multilayered films consisting of alternating sheets of rigid cellulose nanocrystals and flexible poly(allylamine hydrochloride) (PAH) prepared by the layer-by-layer assembly technique. Both techniques showed that smooth films with a high load of cellulose could be obtained. After deposition, the cellulose component occurred as a double layer with different densities: 50% and 25% for the lower and upper layer, respectively. A linear growth of the multilayer and the presence of a Bragg peak on neutron reflectivity curves indicated the formation of a well-ordered structure resulting from entropy-driven assembly and smoothening effect of the flexible PAH macromolecules. The possible alignment of the nanocrystals when anisotropic suspensions were used is also shown and opens the route to an improved control of the architecture of these multilayers.

Structural Description of the Interface of Pickering Emulsions Stabilized by Cellulose Nanocrystals.

The cotton cellulose nanocrystals (CNCs) used in this study are rod-like particles with dimensions in the nanoscale (195 nm long, 23 nm width and 6 nm thick) able to stabilize Pickering emulsions. The adsorption of CNCs at an oil-water interface has been investigated by small angle neutron scattering (SANS) with and without surface charge, and varying CNC concentration from 2 to 5 g/L. Average thicknesses of the interfacial CNC layer around the emulsion droplets of 7 and 18 nm were determined for charged and uncharged CNC, respectively, regardless of their concentration in suspension. This suggests that CNC particles lie as a monolayer varying in surface density. Using several phase contrast variations, the neutron wave vector (Q) dependence with the intensity showed that CNCs are in contact with the oil phase only via the surface of the CNC and not immersed in oil since the Porod behavior is observed over the whole Q-range revealing no deformation of the oil surface at a nanometer scale. This result promotes the postulate that the (2 0 0) crystalline plane of the CNC directly interacts with the interface.

Elaboration of spin-coated cellulose-xyloglucan multilayered thin films.

In the context of developing a biomimetic model of the primary cell wall, our aim was to produce multilayered thin films composed of cellulose nanocrystals (CN) and xyloglucan (XG). We investigated the effect of XG concentrations ranging from 0.5 g/L to 10 g/L. The choice of concentration was based on rheological investigation of the XG solutions which indicated that the two lower concentrations (0.5 and 1 g/L) correspond to a semidilute regime where the polymer chains are not entangled, whereas they are entangled at the highest concentrations (5 and 10 g/L). Several processes of film preparation were tested (dipping or spin-coating, with or without a rinsing step). The film growth profiles obtained for different XG concentrations by mechanical profilometry showed that spin-coating without rinsing was the most efficient process. Results showed that at high XG concentrations (XG = 5 g/L and XG = 10 g/L) plateau values were reached after the formation of 3 or 4 bilayers, whereas growth of the multilayer structure was linear at the lower XG concentrations (XG = 0.5 g/L and XG = 1 g/L). The thickness of one CN/XG bilayer corresponded to a single layer of CN covered by a thin XG layer, despite the absence of a rinsing step between successive coatings. The importance of the XG concentration was confirmed by determining by neutron reflectivity the film architecture obtained from four XG solutions after eight successive paired coatings. The results are discussed in relation to the role of XG in the plant cell wall.

Influence of Charge Density and Ionic Strength on the Aggregation Process of Cellulose Nanocrystals in Aqueous Suspension, as Revealed by Small-Angle Neutron Scattering

Aggregation of rodlike colloidal particles is investigated here through the aggregation process by either increasing ionic strength or decreasing surface charge density of cellulose nanocrystals (CNCs). The form factor of the nanoparticles is characterized up to the Guinier plateau using small-angle neutron scattering (SANS) extended to very small scattering vector Q. Ionic strength, above the threshold of screening charges, brings aggregative conditions that induced fractal organizations for both charged and uncharged CNCs. These two structures display respective fractal dimensions of 2.1 for charged CNCs at high ionic strength and 2.3 for desulfated CNCs over more than a decade of the scattering vector Q, irrespective of salinity, revealing a denser structuration for neutral particles. This is discussed in the framework of aggregation of rodlike particles with an aspect ratio higher than 8. Furthermore, dilution of the rod gel led to disentanglement of the network of fractal aggregates with a subsequent macroscopic sedimentation of the suspensions, with a characteristic time that depends upon the ionic strength and surface charge density. It revealed a threshold independent of salt content around 2.5 g/L and the metastable out-of-equilibrium character of CNC suspensions.

Design of crosslinked hybrid multilayer thin films from azido-functionalized polystyrenes and platinum nanoparticles

Crosslinked organic and hybrid multilayer thin films based on polystyrene-grafted platinum nanoparticles and azidomethyl-functionalized polystyrenes are built-up by sequential spin-coating and UV crosslinking processes. This approach allows to easily tune thickness, composition and periodicity of each layer, as highlighted by TEM and neutron reflectivity experiments. Room temperature UV crosslinking of hybrid layers allows to stabilize the layer prior to nanoparticles segregation.

Spatial structure and composition of polysaccharide-protein complexes from small angle neutron scattering.

We use small angle neutron scattering (SANS), with an original analysis method, to obtain both the characteristic sizes and the inner composition of lysozyme-pectin complexes depending on the charge density. Lysozyme is a globular protein and pectin a natural anionic semiflexible polysaccharide with a degree of methylation (DM) 0, 43, and 74. For our experimental conditions (buffer ionic strength I = 2.5 10(-2) mol/L and pH between 3 and 7), the electrostatic charge of lysozyme is always positive (from 8 to 17, depending on pH). The pectin charge per elementary chain segment is negative and can be varied from almost zero to one through the change of DM and pH. The weight molar ratio of lysozyme on pectin monomers is kept constant. The ratio of negative charge content per volume to positive charge content per volume, -/+, is varied between 10 and 0.007. On a local scale, for all charged pectins, a correlation peak appears at 0.2 A(-1) due to proteins clustering inside the complexes. On a large scale, the complexes appear as formed of spherical globules with a well-defined radius of 10 to 50 nm, containing a few thousands proteins. The volume fraction Phi of organic matter within the globules derived from SANS absolute cross sections is around 0.1. The protein stacking, which occurs inside the globules, is enhanced when pectin is more charged, due to pH or DM. The linear charge density of the pectin determines the size of the globules for pectin chains of comparable molecular weights whether it is controlled by the pH or the DM. The radius of the globules varies between 10 and 50 nm. In conclusion, the structure is driven by electrostatic interactions and not by hydrophobic interactions. The molecular weight also has a large influence on the structure of the complexes because long chains tend to form larger globules. This may be one reason why DM and pH are not completely equivalent in our system, because DM0 has a short mass, but this may not be the only one. For very low pectin charge (-/+ = 0.07), globules do not appear and the scattering signals a gel-like structure. We did not observe any beads-on-a-string structure.

Static and dynamic structural probing of swollen polyacrylamide ferrogels

The local structure of hybrid ferrogels resulting from the incorporation of γ-Fe2O3 nanoparticles into a polyacrylamide polymeric network is probed from a static and a dynamic point of view, under various synthesis conditions and at swelling equilibrium. Relaxation of magneto-optical birefringence and small-angle neutron scattering measurements show adsorption of the nanoparticles onto the polymer, which is ascribed to H-bonding. These measurements allow one to identify the conditions leading either to a homogeneous scaffold reinforced by the nanoparticles, or to a nano-structured composite with 2D nanoparticle decoration on the walls of percolating pockets, initially filled by the ferrofluid during the synthesis and leaking out during the swelling. Two parameters rule the structure and the properties of these ferrogels – the osmotic pressure ratio RΠ (ratio of the osmotic pressure of the initial ferrofluid to that of the hydrogel) and the steric parameter RS (ratio of the nanoparticle volume to the mesh volume of the polymeric hydrogel).

Coloured Semi‐reflective Thin Films for Biomass‐hydrolyzing Enzyme Detection

A new enzymatic activity detection assay based on colour change of the semi-reflective films is presented. The method is based on the preparation of multilayered thin films of controlled thickness obtained by sequential deposition of cellulose nanocrystals and xyloglucan. The hydrolysis of the films leads to a decrease in layer thickness that enables to detect enzyme activity, to the naked eye, from the resulting colour changes in a span of few minutes. The method allows direct, fast, highly sensitive, and easy-to-use characterization of enzymatic activities.