Counterion distribution in a spherical charged sparse brush

The counterion distribution within a spherical polyelectrolyte sparse brush was measured by small-angle X-ray scattering using contrast variation with different counterions by means of ion dialysis. The brush was made by self-association of charged diblock copolymers. Thanks to the contrast variation method, we were able to separate the signal due to the monomers and the signal due to the counterions. At a small length scale, it is demonstrated that the system behaves as independent charged rods whose counterion distribution follows the Poisson-Boltzmann model. Received 14 February 2001 and Received in final form 2 May 2001     

Detailed structure of diamond-type lipid cubic nanoparticles

Supramolecular three-dimensional self-assembly of nonlamellar lipids with fragments of the protein immunoglobulin results in a bicontinuous cubic phase fragmented into nanoparticles with open water channels (cubosomes). The structure of the diamond-type cubic nanoparticles is characterized experimentally by freeze-fracture electron microscopy, and it is mathematically modeled with nodal surfaces emphasizing the fluid-like undulations of the cubosomic interfaces. Based on scaling-up and scaling-down approaches, we present stable and intermediate-kind nanoparticles resulting from the cubosomic growth. Our results reveal the smallest stable diamond-type cubosomic entity that can serve as a building block of more complex nanostructured fluid drug delivery vehicles of therapeutic proteins. The evidence presented for lipid-bilayer undulations in the surface region of the protein/lipid cubosomes could have important consequences for possible applications of these hierarchically organized porous nanoparticles.     

Sur la mesure des triangles en géométrie affine plane

Sans résuméCe travail a fait l'objet d'une communication au Séminaire d'été sur la géométrie réuni à Frankfurt/Main en juillet 1964.     

Direct Oxidation of Csp3−H bonds using in Situ Generated Trifluoromethylated Dioxirane in Flow

A fast, scalable, and safer C_sp³−H oxidation of activated and un-activated aliphatic chains can be enabled by methyl(trifluoromethyl)dioxirane (TFDO). The continuous flow platform allows the in situ generation of TFDO gas and its rapid reactivity toward tertiary and benzylic Csp³−H bonds. The process exhibits a broad scope and good functional group compatibility (28 examples, 8–99 %). The scalability of this methodology is demonstrated on 2.5 g scale oxidation of adamantane.     

Erratum

A new method of acylation of 2,3-dihydrobenzoxazol-2-one 1 is proposed in a two-steps procedure involving an acyl migration.     

Small-angle neutron and X-ray scattering from amphiphilic stimuli-responsive diamond-type bicontinuous cubic phase

The structural evolution of a diamond-type bicontinuous lipid cubic phase upon application of thermal and chemical (hydration agent) stimuli is investigated by means of small-angle neutron (SANS) and X-ray scattering (SAXS). The soft-matter cubic architecture responds by dramatic swelling (DLarge cubic structure) upon incorporation of a hydration-enhancing guest component (octyl glucoside) at low and ambient temperatures, the aqueous channel diameter increasing twice to approximately 7 nm. DLarge appears to be built up from an assembly of cubosomic domains, which may coexist with an amphiphilic lamellae domain at low temperatures. The chemical stimulus concentration can be selected as to tune the hydration of the nanochannels in the DLarge phase and its transformation into a DNormal phase at temperatures above the body temperature. Two-dimensional SANS images recorded upon heating scan reveal growth of spontaneously oriented domains of single-crystal cubic nature. Phase separation and squeezing out the guest-hydrating agent from the higher-curvature regions of the amphiphilic bilayer suggest a possible mechanism for the established transformations. The order-order structural transition, cubic DLarge-cubic DNormal, is found to be reversible upon cooling. The obtained results put forward a structure-based concept for release of encapsulated guest molecules from stimuli-responsive and self-regulated cubosomic nanocarriers.     

Direct and large-eddy simulations of transition in the compressible boundary layer

The transition to turbulence in the three-dimensional compressible boundary layer over a semi-infinite insulated flat plate is studied by means of direct and large-eddy simulations. Results are presented in the quasi-incompressible (Mach number equal to 0.5) and high supersonic (Mach number equal to 5) cases, both in temporal and spatial configurations. Simulations of controlled transition, in which a two-dimensional wave corresponding to the primary instability is introduced at the initial stage, allows us to study the secondary instability of the flow. The latter is triggered with the aid of a three-dimensional white-noise perturbation of small amplitude superposed upon the wave. At a low Mach number, a direct-numerical simulation shows that the fundamental mode is selected, leading to the peak-valley structure found by Klebanoff et al. (1962). The complete transition process is then studied, with emphasis on vortex-filament dynamics. It is shown that the development to turbulence is well simulated, at least for the prediction of average quantities of the flow. In the high Mach number case, no direct-numerical simulation is possible, and we use a subgrid-scale model, the structure-function model, in order to perform a large-eddy simulation of the transition. In this case, the subharmonic mode appears, giving rise to a staggered pattern of vortices. These vortices, which affect the whole thickness of the boundary layer, are more elongated than in the incompressible case.This work was supported by CNES-Avions Marcel Dassault in the frame of the Hermès program (Contract No. RDMF3/86), by DRET (Contract Nos. 87/808/11 and 88/150), and by CNRS (GDR Mécanique des Fluides Numérique and GDR Hypersonique).Unité associée CNRS.