Entropy driven key-lock assembly

The effective interaction between a sphere with an open cavity (lock) and a spherical macroparticle (key), both immersed in a hard sphere fluid, is studied by means of Monte Carlo simulations. As a result, a 2d map of the key-lock effective interaction potential is constructed, which leads to the proposal of a self-assembling mechanism: there exists trajectories through which the key-lock pair could assemble avoiding trespassing potential barriers. Hence, solely the entropic contribution can induce their self-assembling even in the absence of attractive forces. This study points out the solvent contribution within the underlying mechanisms of substrate-protein assembly/disassembly processes, which are important steps of the enzyme catalysis and protein mediated transport

Replica exchange Monte Carlo applied to Hard Spheres

In this work a replica exchange Monte Carlo scheme which considers an extended isobaric-isothermal ensemble with respect to pressure is applied to study hard spheres (HS). The idea behind the proposal is expanding volume instead of increasing temperature to let crowded systems characterized by dominant repulsive interactions to unblock, and so, to produce sampling from disjoint configurations. The method produces, in a single parallel run, the complete HS equation of state. Thus, the first order fluid-solid transition is captured. The obtained results well agree with previous calculations. This approach seems particularly useful to treat purely entropy-driven systems such as hard body and non-additive hard mixtures, where temperature plays a trivial role

Revisiting the phase diagram of hard ellipsoids

In this work the well-known Frenkel-Mulder phase diagram of hard ellipsoids of revolution [Mol. Phys. 55, 1171 (1985)] is revisited by means of replica exchange Monte Carlo simulations. The method provides good sampling of dense systems and so, solid phases can be accessed without the need of imposing a given structure. At high densities, we found plastic solids and fcc-like crystals for semi-spherical ellipsoids (prolates and oblates), and SM2 structures [Phys. Rev. E 75, 020402 (2007)] for x:1-prolates and 1:x-oblates with x>=3. The revised fluid-crystal and isotropic-nematic transitions reasonably agree with those presented in the Frenkel-Mulder diagram. An interesting result is that, for small system sizes (100 particles), we obtained 2:1 and 1.5:1-prolate equations of state without transitions, while some order is developed at large densities. Furthermore, the symmetric oblate cases are also reluctant to form ordered phases.Comment: 8 pages, 6 figure

Phase diagram of two-dimensional hard ellipses

We report the phase diagram of two-dimensional hard ellipses as obtained from replica exchange Monte Carlo simulations. The replica exchange is implemented by expanding the isobaric ensemble in pressure. The phase diagram shows four regions: isotropic, nematic, plastic, and solid (letting aside the hexatic phase at the isotropic-plastic two-step transition [PRL 107, 155704 (2011)]). At low anisotropies, the isotropic fluid turns into a plastic phase which in turn yields a solid for increasing pressure (area fraction). Intermediate anisotropies lead to a single first order transition (isotropic-solid). Finally, large anisotropies yield an isotropic-nematic transition at low pressures and a high-pressure nematic-solid transition. We obtain continuous isotropic-nematic transitions. For the transitions involving quasi-long-range positional ordering, i. e. isotropic-plastic, isotropic-solid, and nematic-solid, we observe bimodal probability density functions. This supports first order transition scenarios.Comment: 18 pages, 7 figure

La intervención anglo-francesa en el Río de la Plata entre 1847 y 1851. Un intento frustrado

En la quinta década del siglo XIX, Gran Bretaña y Francia intervinieron en el conflicto entre los bandos políticos hegemónicos en el Río de la Plata, asumiendo la defensa del gobierno de Montevideo, recluido en los límites de la ciudad. El enfrentamiento era con los federales, particularmente con el gobierno de la ciudad de Buenos Aires y los encargados de las Relaciones Exteriores de las provincias argentinas, encabezado por Juan Manuel de Rosas, quizá la figura más controvertida en la historia de la región. El artículo busca plantear que si bien de todas formas fue una resonante victoria del gobierno bonaerense, no deben olvidarse otras diferencias y motivos de fricción entre los interventores, lo cual debilitaba su accionar en la zona

El dictador Francia y la sociedad paraguaya

Este artículo intenta divulgar algunas características de ese peculiar líder paraguayo de la primera hora luego de la dominación española y de la forma en que su personalidad se correspondía con la mentalidad colectiva de la población paraguaya, en su enorme mayoría dedicada a las actividades agrícolas y ganadera