Theory and simulations of rigid polyelectrolytes

We present theoretical and numerical studies on stiff, linear polyelectrolytes within the framework of the cell model. We first review analytical results obtained on a mean-field Poisson-Boltzmann level, and then use molecular dynamics simulations to show, under which circumstances these fail quantitatively and qualitatively. For the hexagonally packed nematic phase of the polyelectrolytes we compute the osmotic coefficient as a function of density. In the presence of multivalent counterions it can become negative, leading to effective attractions. We show that this results from a reduced contribution of the virial part to the pressure. We compute the osmotic coefficient and ionic distribution functions from Poisson-Boltzmann theory with and without a recently proposed correlation correction, and also simulation results for the case of poly(para-phenylene) and compare it to recently obtained experimental data on this stiff polyelectrolyte. We also investigate ion-ion correlations in the strong coupling regime, and compare them to predictions of the recently advocated Wigner crystal theories.Comment: 32 pages, 15 figures, proceedings of the ASTATPHYS-MEX-2001, to be published in Mol. Phy

The Role of the Gouy Phase in the Coherent Phase Control of the Photoionization and Photodissociation of Vinyl Chloride

We demonstrate theoretically and experimentally that the Gouy phase of a focused laser beam may be used to control the photo-induced reactions of a polyatomic molecule. Quantum mechanical interference between one- and three-photon excitation of vinyl chloride produces a small phase lag between the dissociation and ionization channels on the axis of the molecular beam. Away from the axis, the Gouy phase introduces a much larger phase lag that agrees quantitatively with theory without any adjustable parameters.Comment: 4 pages, 4 figure

Phylogenomic test of the hypotheses for the evolutionary origin of eukaryotes

International audienceThe evolutionary origin of eukaryotes is a question of great interest for which many different hypotheses have been proposed. These hypotheses predict distinct patterns of evolutionary relationships for individual genes of the ancestral eukaryotic genome. The availability of numerous completely sequenced genomes covering the three domains of life makes it possible to contrast these predictions with empirical data. We performed a systematic analysis of the phylogenetic relationships of ancestral eukaryotic genes with archaeal and bacterial genes. In contrast with previous studies, we emphasize the critical importance of methods accounting for statistical support, horizontal gene transfer, and gene loss, and we disentangle the processes underlying the phylogenomic pattern we observe. We first recover a clear signal indicating that a fraction of the bacteria-like eukaryotic genes are of alphaproteobacterial origin. Then, we show that the majority of bacteria-related eukaryotic genes actually do not point to a relationship with a specific bacterial taxonomic group. We also provide evidence that eukaryotes branch close to the last archaeal common ancestor. Our results demonstrate that there is no phylogenetic support for hypotheses involving a fusion with a bacterium other than the ancestor of mitochondria. Overall, they leave only two possible interpretations, respectively, based on the early-mitochondria hypotheses, which suppose an early endosymbiosis of an alphaproteobacterium in an archaeal host and on the slow-drip autogenous hypothesis, in which early eukaryotic ancestors were particularly prone to horizontal gene transfers

Ion-Size Effect at the Surface of a Silica Hydrosol

The author used synchrotron x-ray reflectivity to study the ion-size effect for alkali ions (Na$^+$, K$^+$, Rb$^+$, and Cs$^+$), with densities as high as $4 \times 10^{18}- 7 \times 10^{18}$ m$^{-2}$, suspended above the surface of a colloidal solution of silica nanoparticles in the field generated by the surface electric-double layer. According to the data, large alkali ions preferentially accumulate at the sol's surface replacing smaller ions, a finding that qualitatively agrees with the dependence of the Kharkats-Ulstrup single-ion electrostatic free energy on the ion's radius.Comment: 17 pages, 4 figure

Description beyond the mean field approximation of an electrolyte confined between two planar metallic electrodes

We study an electrolyte confined in a slab of width $W$ composed of two grounded metallic parallel electrodes. We develop a description of this system in a low coupling regime beyond the mean field (Poisson--Boltzmann) approximation. There are two ways to model the metallic boundaries: as ideal conductors in which the electric potential is zero and it does not fluctuate, or as good conductors in which the average electric potential is zero but the thermal fluctuations of the potential are not zero. This latter model is more realistic. For the ideal conductor model we find that the disjoining pressure is positive behaves as $1/W^3$ for large separations with a prefactor that is universal, i.e. independent of the microscopic constitution of the system. For the good conductor boundaries the disjoining pressure is negative and it has an exponential decay for large $W$. We also compute the density and electric potential profiles inside the electrolyte. These are the same in both models. If the electrolyte is charge asymmetric we find that the system is not locally neutral and that a non-zero potential difference builds up between any electrode and the interior of the system although both electrodes are grounded.Comment: 16 pages, 5 figures, added a new appendix B and a discussion on ideal conductors vs. good conductor

Radius of a Photon Beam with Orbital Angular Momentum

We analyze the transverse structure of the Gouy phase shift in light beams carrying orbital angular momentum and show that the Gouy radius $r_G$ characterizing the transverse structure grows as $\sqrt{2p+|\ell|+1}$ with the nodal number $p$ and photon angular momentum number $\ell$. The Gouy radius is shown to be closely related to the root-mean-square radius of the beam, and the divergence of the radius away from the focal plane is determined. Finally, we analyze the rotation of the Poynting vector in the context of the Gouy radius.Comment: 11 page

A novel vortex generator and mode converter for electrons

A mode converter for electron vortex beams is described. Numerical simulations, confirmed by experiment, show that the converter transforms a vortex beam with topological charge $m=\pm 1$ into beams closely resembling Hermite-Gaussian HG$_{10}$ and HG$_{01}$ modes. The converter can be used as a mode discriminator or filter for electron vortex beams. Combining the converter with a phase plate turns a plane wave into modes with topological charge $m=\pm 1$. This combination serves as a generator of electron vortex beams of high brilliance

Ion-ion correlations: an improved one-component plasma correction

Based on a Debye-Hueckel approach to the one-component plasma we propose a new free energy for incorporating ionic correlations into Poisson-Boltzmann like theories. Its derivation employs the exclusion of the charged background in the vicinity of the central ion, thereby yielding a thermodynamically stable free energy density, applicable within a local density approximation. This is an improvement over the existing Debye-Hueckel plus hole theory, which in this situation suffers from a "structuring catastrophe". For the simple example of a strongly charged stiff rod surrounded by its counterions we demonstrate that the Poisson-Boltzmann free energy functional augmented by our new correction accounts for the correlations present in this system when compared to molecular dynamics simulations.Comment: 5 pages, 2 figures, revtex styl

La chimiohyperthermie intrapéritonéale (CHIP) dans les cancers ovariens

RésuméLe cancer de l’ovaire reste, en France, la quatrième cause de décès par cancer chez la femme. Il s’agit d’une maladie souvent diagnostiquée à un stade évolué avec carcinose péritonéale (CP) et dont l’histoire naturelle est marquée par des récidives essentiellement péritonéales et l’acquisition d’un profil de chimiorésistance. Malgré les nombreuses lignes de chimiothérapie systémique et les chirurgies de cytoréduction (CCR), le pronostic de ces récidives reste sombre. Depuis plus de 20ans, plusieurs équipes spécialisées ont développé un traitement combiné des CP, associant une chirurgie de cytoréduction complète à une chimiohyperthermie intrapéritonéale (CHIP). Cette thérapeutique a une large place dans le traitement des CP d’origine non gynécologiques. Le rationnel pour une utilisation de la CHIP dans le traitement des CP d’origine ovarienne est important. D’une part, 3 études prospectives randomisées ont démontré la supériorité de l’utilisation de la chimiothérapie intrapéritonéale (sans hyperthermie) par rapport à la chimiothérapie systémique sur des patientes sélectionnées. D’autre part, des études rétrospectives et cas-témoins évaluant la CHIP font état de données de survie encourageantes, en particulier en cas de récidive chimiorésistante. Néanmoins, la morbidité et la mortalité associées doivent appeler à une sélection rigoureuse des patientes éligibles, et à une prise en charge multidisciplinaire dans des centres spécialisés. L’évaluation de la CHIP doit se faire par le moyen d’études randomisées à différents stades évolutifs : 1re ligne, consolidation, récidives qu’elles soient chimiorésistantes ou chimiosensibles. Plusieurs études européennes sont en cours.SummaryOvarian cancer remains the fourth leading cause of cancer death in women in France. It is all too often diagnosed at an advanced stage with peritoneal carcinomatosis (PC), but remains confined to the peritoneal cavity throughout much of its natural history. Because of cellular selection pressure over time, most tumor recurrences eventually develop resistance to systemic platinum. Options for salvage therapy include alternative systemic chemotherapies and further cytoreductive surgery (CRS), but the prognosis remains poor. Over the past two decades, a new therapeutic approach to PC has been developed that combines CRS with hyperthermic intraperitoneal chemotherapy (HIPEC). This treatment strategy has already been shown to be effective in non-gynecologic carcinomatosis in numerous reports. There is a strong rationale for the use of HIPEC for PC of ovarian origin. On the one hand, three prospective randomized trials have demonstrated the superiority of intraperitoneal chemotherapy (without hyperthermia) in selected patients compared to systemic chemotherapy. Moreover, retrospective studies and case-control studies of HIPEC have reported encouraging survival data, especially when used to treat chemoresistant recurrence. However, HIPEC has specific morbidity and mortality; this calls for very careful selection of eligible patients by a multidisciplinary team in specialized centers. HIPEC needs to be evaluated by means of randomized trials for ovarian cancer at different developmental stages: as first line therapy, as consolidation, and for chemoresistant recurrence. Several European Phase III studies are currently ongoing

X-ray study of the electric double layer at the n-hexane/nanocolloidal silica interface

The spatial structure of the transition region between an insulator and an electrolyte solution was studied with x-ray scattering.The electron density profile across the n-hexane/silica sol interface (solutions with 5-nm, 7-nm, and 12-nm colloidal particles) agrees with the theory of the electrical double layer and shows separation of positive and negative charges. The interface consists of three layers, i.e., a compact layer of Na+, a loose monolayer of nanocolloidal particles as part of a thick diffuse layer, and a low-density layer sandwiched between them. Its structure is described by a model in which the potential gradient at the interface reflects the difference in the potentials of "image forces" between the cationic Na+ and anionic nanoparticles and the specific adsorption of surface charge. The density of water in the large electric field (1-10 GV/m) of the transition region and the layering of silica in the diffuse layer is discussed.Comment: 9 pages, 9 figure