Colloidal charge reversal: Dependence on the ionic size and the electrolyte concentration

Extensive Monte Carlo simulations and scaling arguments are used to study the colloidal charge reversal. The critical colloidal surface charge density $\sigma_c$ at which the reversal first appears is found to depend strongly on the ionic size. We find that $\sigma_c$ has an inflection point as a function of the electrolyte concentration. The width of the plateau region in the vicinity of the inflection point depends on the temperature and the ionic radius $a$. In agreement with the theoretical predictions it is found that the critical colloidal charge above which the electrophoretic mobility becomes reversed diverges as $Z_c \sim 1/a^2$ in the limit $a \to 0$.Comment: 5 pages, 4 figure

Ions at the air-water interface: An end to one hundred year old mystery?

Availability of highly reactive halogen ions at the surface of aerosols has tremendous implications for the atmospheric chemistry. Yet neither simulations, experiments, nor existing theories are able to provide a fully consistent description of the electrolyte-air interface. In this paper a new theory is proposed which allows us to explicitly calculate the ionic density profiles, the surface tension, and the electrostatic potential difference across the solution-air interface. Predictions of the theory are compared to experiments and are found to be in excellent agreement. The theory also sheds new light on one of the oldest puzzles of physical chemistry -- the Hofmeister effect

Counterion correlations and attraction between like-charged macromolecules

A simple model is presented for the appearance of attraction between two like charged polyions inside a polyelectrolyte solution. The polyions are modeled as rigid cylinders in a continuum dielectric solvent. The strong electrostatic interactions between the polyions and the counterions results in counterion condensation. If the two polyions are sufficiently close to each other their layers of condensed counterions can become correlated resulting in attraction between the macromolecules. To explore the counterion induced attraction we calculate the correlation functions for the condensed counterions. It is found that the correlations are of very short range. For the parameters specific to the double stranded DNA, the correlations and the attraction appear only when the surface-to-surface separation is less than 7 /AA.Comment: 6 pages, 8 eps figures, RevTeX with epsfi

Introduction

La représentation politique ne s’épuise pas dans les actions des représentants politiques, dans leurs prises de décision ni dans la traduction des droits des représentés dans l’espace délibératif du gouvernement représentatif. Il suffit d’observer les bâtiments publics, les monuments commémoratifs, les symboles et rituels institutionnels – ou encore la communication politique des gouvernants – pour prendre la mesure des aspects proprement symboliques de la représentation politique. [Premier paragraphe]Titre du Cahier : Paula Diehl et Alexandre Escudier (dir.), La « Représentation » du politique : histoire, concepts, symbole

La « Représentation » du politique:Histoire, concepts, symboles

La question de la « représentation » politique ne se résume pas à l'espace délibératif du gouvernement représentatif. Il suffit d'observer les bâtiments publics, les monuments commémoratifs, les symboles et rituels institutionnels -- ou encore la communication politique des gouvernants -- pour prendre la mesure des aspects proprement « symboliques » de la représentation politique. Le présent numéro des /Cahiers du Cevipof/ propose d'explorer plus avant lesdits aspects (trop souvent négligés par la science politique) et tente ainsi d'ouvrir quelques pistes afin de re-problématiser le concept usuel de « représentation politique ». Se trouvent ainsi croisées et mises en discussion (Allemagne, France, recherches anglo-saxonnes) diverses théories et approches conceptuelles du problème de la « représentation politique », depuis le tournant du « gouvernement représentatif » à la fin du XVIIIe siècle. Nous reconsidérons pour ce faire différents aspects de la représentation proprement symbolique du politique (esthétisation, mise en scène, ritualisation, visualisation, etc.) et proposons de les articuler à une théorie de la démocratie

Phase behavior of the Lattice Restricted Primitive Model with nearest-neighbor exclusion

The global phase behavior of the lattice restricted primitive model with nearest neighbor exclusion has been studied by grand canonical Monte Carlo simulations. The phase diagram is dominated by a fluid (or charge-disordered solid) to charge-ordered solid transition that terminates at the maximum density, $\rho^*_{max}=\sqrt2$ and reduced temperature $T^*\approx0.29$. At that point, there is a first-order phase transition between two phases of the same density, one charge-ordered and the other charge-disordered. The liquid-vapor transition for the model is metastable, lying entirely within the fluid-solid phase envelope.Comment: 6 pages, color. submitted to J. Chem. Phy

Isothermal adsorption of polyampholytes on charged nanopatterned surfaces

We investigate the adsorption of neutral polyampholytes on charged nanopatterned surfaces. The surfaces have charged domains but are overall neutral. To perform efficient simulations, we use an approach which combines the explicit form of the interaction potential between the polyampholyte monomers and the surface with a 3d Ewald summation method. We observe that the amount of adsorption and the structure of the adsorbed polyampholytes depend strongly on the surface pattern, the relative size of the surface domains, and the charge distribution along the polyampholyte backbone

Phase diagrams in the lattice RPM model: from order-disorder to gas-liquid phase transition

The phase behavior of the lattice restricted primitive model (RPM) for ionic systems with additional short-range nearest neighbor (nn) repulsive interactions has been studied by grand canonical Monte Carlo simulations. We obtain a rich phase behavior as the nn strength is varied. In particular, the phase diagram is very similar to the continuum RPM model for high nn strength. Specifically, we have found both gas-liquid phase separation, with associated Ising critical point, and first-order liquid-solid transition. We discuss how the line of continuous order-disorder transitions present for the low nn strength changes into the continuum-space behavior as one increases the nn strength and compare our findings with recent theoretical results by Ciach and Stell [Phys. Rev. Lett. {\bf 91}, 060601 (2003)].Comment: 7 pages, 10 figure