Jacques Jestin

Insight into Asphaltene Nanoaggregate Structure Inferred by Small Angle Neutron and X-ray Scattering

Complementary neutron and X-ray small angle scattering results give prominent information on the asphaltene nanostructure. Precise SANS and SAXS measurements on a large q-scale were performed on the same dilute asphaltene-toluene solution, and absolute intensity scaling was carried out. Direct comparison of neutron and X-ray spectra enables description of a fractal organization made from the aggregation of small entities of 16 kDa, exhibiting an internal fine structure. Neutron contrast variation experiments enhance the description of this nanoaggregate in terms of core-shell disk organization, giving insight into core and shell dimensions and chemical compositions. The nanoaggregates are best described by a disk of total radius 32 Å with 30% polydispersity and a height of 6.7 Å. Composition and density calculations show that the core is a dense and aromatic structure, contrary to the shell, which is highly aliphatic. These results show a good agreement with the general view of the Yen model (Yen, T. F.; et al. Anal. Chem.1961, 33, 1587-1594) and as for the modified Yen model (Mullins, O. C. Energy Fuels2010, 24, 2179-2207), provide characteristic dimensions of the asphaltene nanoaggregate in good solvent.

Tuning the mechanical properties in model nanocomposites: Influence of the polymer‐filler interfacial interactions

This paper presents a study of the polymer-filler interfacial effects on filler dispersion and mechanical reinforcement in Polystyrene (PS) / silica nanocomposites by direct comparison of two model systems: un-grafted and PS-grafted silica dispersed in PS matrix. The structure of nanoparticles has been investigated by combining Small Angle Neutron Scattering (SANS) measurements and Transmission Electronic Microscopic (TEM) images. The mechanical properties were studied over a wide range of deformation by plate/plate rheology and uni-axial stretching. At low silica volume fraction, the particles arrange, for both systems, in small finite size non-connected aggregates and the materials exhibit a solid-like behavior independent of the local polymer/fillers interactions suggesting that reinforcement is dominated by additional long range effects. At high silica volume fraction, a continuous connected network is created leading to a fast increase of reinforcement whose amplitude is then directly dependent on the strength of the local particle/particle interactions and lower with grafting likely due to deformation of grafted polymer.

Synergism by coassembly at the origin of ion selectivity in liquid-liquid extraction.

In liquid-liquid extraction, synergism emerges when for a defined formulation of the solvent phase, there is an increase of distribution coefficients for some cations in a mixture. To characterize the synergistic mechanisms, we determine the free energy of mixed coassembly in aggregates. Aggregation in any point of a phase diagram can be followed not only structurally by SANS, SAXS, and SLS, but also thermodynamically by determining the concentration of monomers coexisting with reverse aggregates. Using the industrially used couple HDEHP/TOPO forming mixed reverse aggregates, and the representative couple U/Fe, we show that there is no peculiarity in the aggregates microstructure at the maximum of synergism. Nevertheless, the free energy of aggregation necessary to form mixed aggregates containing extracted ions in their polar core is comparable to the transfer free energy difference between target and nontarget ions, as deduced from the synergistic selectivity peak.

Osmotically induced deformation of capsid-like icosahedral vesicles

We report the osmotic deformation of micron-size catanionic vesicles with icosahedral symmetry (20 faces, 12 vertices) upon incubation in small solutes (NaCl, glucose). The vesicles remain icosahedral at low osmotic pressure gradients across the bilayer, or spherical for outwards gradients. Above a threshold value of inwards pressure, the icosahedra develop a buckling instability: a depression is initiated at one or two ridges, grows until one vertex snaps into the icosahedra, leading to full collapse of one half of the vesicle into the other. Despite large local inversions in curvature, no release of encapsulated solutes is observed before the residual volume reaches negligible small values. Thin shell models correctly capture the buckling patterns of icosahedra in the low deformation limit. Comparison of experimental results with Monte Carlo simulations provides a first estimate for the conditions of shell disruption, and suggests it is predominantly driven by curvature rather than two-dimensional stretching or compression. The relevance of these results for the mechanics of viral capsids and controlled release applications is discussed.

Effect of aging and alkali activator on the porous structure of a geopolymer

Nitrogen sorption and small- and wide-angle X-ray and neutron scattering techniques were used to study the porous structure of geopolymers, inorganic polymers synthesized by reaction of a strongly alkaline solution and an aluminosilicate source (metakaolin). The effects of aging and the use of alkali activators (Na+, K+) of different sizes were investigated at room temperature. The influence of aging time on the microstructure of both geopolymer matrixes was verified in terms of pore volume and specific surface area. The results suggested a refinement of the porosity and therefore a reduction in the pore volume over time. Regardless of the age considered, some characteristics of the porous network such as pore size, shape and distribution depend on the alkali activator used. Whatever the technique considered, the potassium geopolymer has a greater specific surface area than the sodium geopolymer. According to the scattering results, the refinement of the porosity can be associated with, first, a densification of the solid network and, secondly, a partial closure of the porosity at the nanometre scale. The kinetics are much slower for the sodium geopolymer than for the potassium geopolymer in the six months of observation.

Controlled grafting of polystyrene on silica nanoparticles using NMP: a new route without free initiator to tune the grafted chain length

We synthesized well-defined polystyrene-grafted silica nanoparticles by adapting our previous synthesis process without using free initiator. We were able to obtain a more versatile system in which we can tune the masses of the grafted chains while controlling the polymerization, the colloidal stability and avoid the formation of free polymer chains. The final grafted objects were characterized in a refined way using SANS and the contrast matching method.

Coadsorption at the air/water interface likely explains some pollutants transfer to the atmosphere: benzene and lead case

This study attempts to evidence a physical interfacial mechanism for the passing of some non-volatile harmful molecules from water, where they are dissolved, to the atmosphere. The idea developed here is that an organic substance, at its solubility limit, forms a surface layer whose properties induce the coadsorption of another dissolved substance; both are then able to pass to the atmosphere by a bubbling mechanism. Experiments were made with benzene close of its solubility limit in an aqueous solution of lead nitrate, which is non-volatile and normally does not adsorb at water surface. Coadsorption really occurred. The impact of such a mechanism on the environment is discussed.

Controlled grafted brushes of polystyrene on magnetic γ-Fe2O3 nanoparticlesvianitroxide-mediated polymerization

We present a new multi-step efficient “grafting from” method to obtain well defined magnetic γ-Fe2O3 nanoparticles grafted by polystyrene (PS) chains, from the synthesis of the native nanoparticles up to the refined characterization of the final grafted objects in different organic solvents, using a combination of small angle scattering (SAXS and SANS) and deuteration methods. The polymerization technique, based on a two-step Nitroxide-Mediated Polymerization (NMP), has been adapted for maghemite particles from a protocol previously established for negatively charged silica nanoparticles in dimethylacetamide (DMAc), a polar organic solvent. Such a method requires an initial inversion of the charge surface of the maghemite particles after their synthesis in an aqueous solvent. The colloidal stability of the system has been kept and controlled at every stage of the process to avoid aggregation of the particles to optimize the synthesis parameters in terms of grafting density, conversion rates, and reproducibility. The grafted objects, redispersed in different solvents, have been studied by SAXS (to characterize their γ-Fe2O3 core) and SANS (to characterize their grafted PS corona, either hydrogenated or deuterated) at different concentrations to extract both the form factor of the object and the structure factor of the suspension. The experimental scattered curves were modelled by a Pedersen model. The parameters extracted from the model can be directly compared with the masses of the degrafted chains. The grafted objects, better dispersed in DMAc than in toluene, are organized as linear fractal aggregates of 3–4 native particles grafted with PS chains whose behavior is in agreement with scaling laws derived for brushes in a theta solvent.

Application of NMR Solvent Relaxation and SAXS to Asphaltenes Solutions Characterization

Abstract Asphaltenic crude oil is usually described as a colloidal dispersion of sub‐micronic asphaltene particle in maltene. The understanding of macroscopic properties (solubility, viscosity…) often requires asphaltene particle size measurements. Traditionally, methods like for instance small angle (x‐rays or neutrons) scattering are used. The objective of this work is to test an alternative and simple method through 1H NMR solvent measurements. Asphaltenes fractions were isolated by different methods (solvent precipitation, ultra‐centrifugation…) and dispersed in various solvents at different concentrations. Asphaltene particle size were obtained from small angle x‐ray scattering (SAXS) experiments. Specific solvent relaxation rate R sp (difference between relaxation rate of solution and pure solvent), determined from Carr‐Purcell‐Meiboom‐Gill (CPMG) pulses sequences, varies linearly with asphaltene content up to concentrations of few percents. Using the fast exchange model between bulk and surface molecules, we try to correlate specific relaxation rate with particle size considerations. We show that simple geometric models are not able to completely explain experimental data; other considerations and refined modelizations must be taking into account.

Influence of chain interdiffusion between immiscible polymers on dewetting dynamics

From dewetting and neutron reflectivity experiments, we demonstrate the major influence of chain interdiffusion on the friction properties for a couple of immiscible polymers. The interface between two immiscible polymers, polystyrene (PS) and polydimethylsiloxane (PDMS), was studied by neutron reflectivity and dewetting by using free PS chains and PDMS brushes. Unexpectedly, we found that the PS chains diffuse in the PDMS brushes at temperatures well below the glass transition temperatures of PS, the dynamics being largely determined by the grafting density of the brush. These observations strongly suggest that the puzzling ageing of PS thin films observed from dewetting experiments can be directly related to modifications of the PS/PDMS interface.