Adsorption de polyélectrolytes par une surface liquide chargée

We have studied the adsorption of weak polyelectrolytes by a soft surface (Langmuir film) of opposite charge, as a function of the surface charge density Sigma and of the polymers charge fraction f. Different regimes have been evidenced (ellipsometry, IR spectroscopy). For small values of Sigma, the adsorbed polyelectrolytes neutralize the surface : Gamma=Sigma/f (number of adsorbed monomers per unit surface). For high values of Sigma, it costs less to continue the surface neutralization with counter-ions : Gamma~SigmaION/f with SigmaION~f^0.35. For weak f (~0.12), Gamma is significant and the mixed film properties (amphiphile+polyelectrolytes) are dominated by polymers. The surface pressure due to the polymers is proportional to Sigma^2. A regime change was observed in elasticity and overcompensation of the surface charge. It corresponds, according to recent theories, to a change in the polymer conformation, from flat to 3D. For higher f, the film properties are governed by the amphiphiles.L'étude porte sur l'adsorption de polyélectrolytes faibles par une surface liquide (film de Langmuir) de charge opposée. Nous avons quantifié l'adsorption des polyélectrolytes et étudié les propriétés de la couche adsorbée en fonction de f le taux charge des polyélectrolytes et de Sigma le nombre de charges portées par la surface par unité de surface. Différents régimes d'adsorption ont été mis en évidence (ellipsométrie, spectrométrie IR). Aux faibles densités surfaciques

Adsorption de polyélectrolytes par une surface liquide chargée

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Behavioral/Systems/Cognitive A Temporal Bottleneck in the Language Comprehension Network

Humans can understand spoken or written sentences presented at extremely fast rates of �400 wpm, far exceeding the normal speech rate (�150 wpm). How does the brain cope with speeded language? And what processing bottlenecks eventually make language incomprehensible above a certain presentation rate? We used time-resolved fMRI to probe the brain responses to spoken and written sentences presented at five compression rates, ranging from intelligible (60–100 % of the natural duration) to challenging (40%) and unintelligible (20%). The results show that cortical areas differ sharply in their activation speed and amplitude. In modality-specific sensory areas, activation varies linearly with stimulus duration. However, a large modality-independent left-hemispheric language network, including the inferior frontal gyrus (pars orbitalis and triangularis) and the superior temporal sulcus, shows a remarkably time-invariant response, followed by a sudden collapse for unintelligible stimuli. Finally, linear and nonlinear responses, reflecting a greater effort as compression increases, are seen at various prefrontal and parietal sites. We show that these profiles fit with a simple model according to which the higher stages of language processing operate at a fixed speed and thus impose a temporal bottleneck on sentence comprehension. At presentation rates faster than this internal processing speed, incoming words must be buffered, and intelligibility vanishes when buffer storage and retrieval operations are saturated. Based on their temporal and amplitude profiles, buffer regions can be identified with th

Surface Complexation of DNA with Insoluble Monolayers. Influence of Divalent Counterions

DNA interacts with insoluble monolayers made of cationic amphiphiles as well as with monolayers of zwitterionic lipids in the presence of divalent ions. Binding to dioctadecyldimethylammonium bromide (DODAB) or distearoyl-sn-glycero-3-phosphocholine (DSPC) monolayers in the presence of calcium is accompanied by monolayer expansion. For the positively charged DODAB monolayer, this causes a decrease of surface potential, while an increase is observed for the DSPC monolayers. Binding to dipalmitoyl-sn-glycero-3-phosphocholine preserves most of the liquid expanded−liquid condensed coexistence region. The liquid condensed domains adopt an elongated morphology in the presence of DNA, especially in the presence of calcium. The interaction of DNA with phospholipid monolayers is ion specific: the presence of calcium leads to a stronger interaction than magnesium and barium. These results were confirmed by bulk complexation studies