Understanding noble gas incorporation in mantle minerals: an atomistic study.

Ab initio calculations in forsterite (Mg 2 SiO 4 ) are used to gain insight into the formation of point defects and incorporation of noble gases. We calculate the enthalpies of incorporation both at pre-existing vacancies in symmetrically non-equivalent sites, and at interstitial positions. At high pressure, most structural changes affect the MgO 6 units and the enthalpies of point defects increase, with those involving Mg and Si vacancies increasing more than those involving O sites. At 15 GPa Si vacancies and Mg interstitials have become the predominant intrinsic defects. We use these calculated enthalpies to estimate the total uptake of noble gases into the bulk crystal as a function of temperature and pressure both in the presence and absence of other heterovalent trace elements. For He and Ne our calculated solubilities point to atoms occupying mainly interstitial sites in agreement with previous experimental work. In contrast, Ar most likely substitutes for Mg due to its larger size and the deformation it causes within the crystal. Incorporation energies, as well as atomic distances suggest that the incorporation mainly depend on the size mismatch between host and guest atoms. Polarization effects arising from the polarizability of the noble gas atom or the presence of charged defects are minimal and do not contribute significantly to the uptake. Finally, the discrepancies between our results and recent experiments suggest that there are other incorporation mechanisms such as adsorption at internal and external interfaces, voids and grain boundaries which must play a major role in noble gas storage and solubility.

Comparison of Implicit and Explicit Solvent Approaches in Ab Initio Evaluation of Thermochemistry in Solution: Application in Studying Boron Isotope Fractionation in Water.

Evaluation of thermochemistry in solution plays a key role in numerous fields. For this task, the solvent effects are commonly included in theoretical computations based on either implicit or explicit solvent approaches. In the present study, we evaluate and compare the performance of some of the most widely applied methods based on these two approaches. For studying the solvent effect on thermochemistry with an explicit solvent, we demonstrate that partial normal mode analysis with frozen geometry of solvent molecules for multiple solute-solvent configurations can yield quite accurate and reliable results for a drastically reduced computational cost. As a case study, we consider the evaluation of the equilibrium constant for the boron isotope exchange between boric acid and borate (k3-4) in pure and saline water which is of high geochemical importance. Employing three different rigorous and high-precision theoretical approaches, we provide a reliable estimation of k3-4 which is a value within 1.028 to 1.030 for both pure and saline water which is in excellent agreement with experimental data.

High energy density mixed polymeric phase from carbon monoxide and nitrogen.

Carbon monoxide and nitrogen are among the potentially interesting high-energy density materials. However, in spite of the physical similarities of the molecules, they behave very differently at high pressures. Using density functional theory and structural prediction methods, we examine the ability of these systems to combine their respective properties and form novel mixed crystalline phases under pressures of up to 100 GPa. Interestingly, we find that CO catalyzes the molecular dissociation of N2, which means mixed structures are favored at a relatively low pressure (below 18 GPa), and that a three-dimensional framework with Pbam symmetry becomes the most stable phase above 52 GPa, i.e., at much milder conditions than in pure solid nitrogen. This structure is dynamically stable at ambient pressure and has an energy density of approximately 2.2 kJ g(-1), making it a candidate for a high-energy density material, and one that could be achieved at less prohibitive experimental conditions.

Ab initio parameterization of an all-atom polarizable and dissociable force field for water.

A novel all-atom, dissociative, and polarizable force field for water is presented. The force field is parameterized based on forces, stresses, and energies obtained form ab initio calculations of liquid water at ambient conditions. The accuracy of the force field is tested by calculating structural and dynamical properties of liquid water and the energetics of small water clusters. The transferability of the force field to dissociated states is studied by considering the solvation of a proton and the ionization of water at extreme conditions of pressure and temperature. In the case of the solvated proton, the force field properly describes the presence of both Eigen and Zundel configurations. In the case of the pressure-induced ice VIII/ice X transition and the temperature-induced transition to a superionic phase, the force field is found to describe accurately the proton symmetrization and the melting of the proton sublattice, respectively.

Dispersion interactions in room-temperature ionic liquids: results from a non-empirical density functional.

The role of dispersion or van de Waals (VDW) interactions in imidazolium-based room-temperature ionic liquids is studied within the framework of density functional theory, using a recently developed non-empirical functional [M. Dion, H. Rydberg, E. Schröder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004)], as efficiently implemented in the SIESTA code [G. Román-Pérez and J. M. Soler, Phys. Rev. Lett. 103, 096102 (2009)]. We present results for the equilibrium structure and lattice parameters of several crystalline phases, finding a general improvement with respect to both the local density (LDA) and the generalized gradient approximations (GGA). Similar to other systems characterized by VDW bonding, such as rare gas and benzene dimers as well as solid argon, equilibrium distances and volumes are consistently overestimated by ≈7%, compared to -11% within LDA and 11% within GGA. The intramolecular geometries are retained, while the intermolecular distances and orientations are significantly improved relative to LDA and GGA. The quality is superior to that achieved with tailor-made empirical VDW corrections ad hoc [M. G. Del Pópolo, C. Pinilla, and P. Ballone, J. Chem. Phys. 126, 144705 (2007)]. We also analyse the performance of an optimized version of this non-empirical functional, where the screening properties of the exchange have been tuned to reproduce high-level quantum chemical calculations [J. Klimes, D. Bowler, and A. Michaelides, J. Phys.: Condens. Matter 22, 074203 (2010)]. The results for solids are even better with volumes and geometries reproduced within 2% of experimental data. We provide some insight into the issue of polymorphism of [bmim][Cl] crystals, and we present results for the geometry and energetics of [bmim][Tf] and [mmim][Cl] neutral and charged clusters, which validate the use of empirical force fields.

Clusters, liquids, and crystals of dialkyimidazolium salts. A combined perspective from ab initio and classical computer simulations.

We summarize results obtained by a combination of ab initio and classical computer simulations of dialkylimidazolium ionic liquids in different states of aggregation, from crystals to liquids and clusters. Unusual features arising from the competition between electrostatic, dispersion, and hydrogen-bonding interactions are identified at the origin of observed structural patterns. We also discuss the way Brønsted acids interact with ionic liquids leading to the formation of hydrogen-bonded anions.

Simulations of ionic liquids, solutions, and surfaces.

We have been using atomistic simulation for the last 10 years to study properties of imidazolium-based ionic liquids. Studies of dissolved molecules show the importance of electrostatic interactions in both aromatic and hydrogen-bonding solutes. However, the local structure strongly depends upon ion-ion and solute-solvent interactions. We find interesting local alignments of cations at the gas-liquid and solid-liquid interfaces, which give a potential drop through the surface. If the solid interface is charged, this charge is strongly screened over distances of a few nanometres and this screening decays on a fast time scale. We have studied the sensitivity of the liquid structure to force-field parameters and show that results from ab initio simulations can be used in the development of force fields.

Polarization relaxation in an ionic liquid confined between electrified walls.

The response of a room temperature molten salt to an external electric field when it is confined to a nanoslit is studied by molecular dynamics simulations. The fluid is confined between two parallel and oppositely charged walls, emulating two electrified solid-liquid interfaces. Attention is focused on structural, electrostatic, and dynamical properties, which are compared with those of the nonpolarized fluid. It is found that the relaxation of the electrostatic potential, after switching the electric field off, occurs in two stages. A first, subpicosecond process accounts for 80% of the decay and is followed by a second subdiffusive process with a time constant of 8 ps. Diffusion is not involved in the relaxation, which is mostly driven by small anion translations. The relaxation of the polarization in the confined system is discussed in terms of the spectrum of charge density fluctuations in the bulk.

Antibióticos profilácticos en apendicitis aguda: evaluación de un protocolo / Prophylactic antibiotics in acute appendicitis: evaluation of a protocol

Entre el 1º de abril de 1999 y el 31 de marzo del 2001 se realizaron en el Hospital Universitario Clínica San Rafael (HUCSR) de Bogotá, 732 apendicetomías por el diagnóstico de apendicitis aguda en pacientes mayores de 15 años. Hemos realizado un estudio retrospectivo de 100 de estos casos antes de la introducción de un protocolo de sulbactam/ampicilina y 100 casos después del protocolo en busca de los resultados del tratamiento con antibióticos profiláctico, comparando ambos grupos, buscando la incidencia de infección del sitio operatorio (ISO), estancia, antibióticos utilizados, dosis, combinaciones y costos de los mismos. Los resultados muestran que el comportamiento en ambos es similar en estancia (1.58 vs. 1.72 días) y aparición de ISO (3 por ciento vs. 4p por ciento), en el grupo con protocolo y sin protocolo respectivamente, y que el uso de antibióticos profilácticos con protocolo presenta una disminución en costos ($45.000 vs. $82.587), dosis (1 vs. 2.95), combinaciones y número de antibióticos utilizados (1 vs. 1.19).Lo anterior nos muestra que una utilización racional y adecuada de la profilaxis antibiótica redunda en beneficios para el paciente y la institución...

Structure and dynamics of a confined ionic liquid. Topics of relevance to dye-sensitized solar cells.

The behavior of a model ionic liquid (IL) confined between two flat parallel walls was studied at various interwall distances using computer simulations. The results focus both on structural and dynamical properties. Mass and charge density along the confinement axis reveal a structure of layers parallel to the walls that leads to an oscillatory profile in the electrostatic potential. Orientational correlation functions indicate that cations at the interface orient tilted with respect to the surface and that any other orientational order is lost thereafter. The diffusion coefficients of the ions exhibit a maximum as a function of the confinement distance, a behavior that results from a combination of the structure of the liquid as a whole and a faster molecular motion in the vicinity of the walls. We discuss the relevance of the present results and elaborate on topics that need further attention regarding the effects of ILs in the functioning of IL-based dye-sensitized solar cells.

Linfangioma esplenico / Splenic lymphagioma

El linfangioma esplénico es una rara neoplasia benigna caracterizada por dilataciones quisticas de vasos linfáticos en el parenquima del bazo. El linfangioma puede comprometer el bazo solamente, o múltiples organos. Describe el caso de una mujer de 49 años de edad con diagnóstico de linfangioma esplénico, quien presentó un cuadro de trombocitopenia de tres años de evolución; se realizó tratamiento médico con interferón durante seis meses sin mejoría y esplenectomía, con hallazgos macroscópicos de patología compatibles con linfangioma esplénico. Además, se realizó una evisión actualizada de la literatura, discutiendo su diagnóstico diferencial, pronóstico y opciones de tratamiento.

Carcinoma primario de la vesícula biliar / Cancer of the gallbladder

Objetivo. Determinar las características generales del carcinoma primario de la vesícula biliar (CVB), los procedimientos diagnósticos más frecuentes y la forma de manejo contemporánea así como analizar la experiencia en el Instituto Nacional de Cancerología (INCan). Introducción. El CVB es una neoplasia rara, frecuentemente letal ya que la mayoría de los casos se descubren en etapas avanzadas. En años recientes se ha demostrado que cirugías radicales incrementan la sobrevida. Material y métodos. Se realizó una revisión de artículos relevantes sobre características epidemiológicas, estudios de laboratorio y gabinete y formas de manejo de acuerdo a la etapa clínica. Se revisaron los expedientes clínicos de pacientes tratados en el INCan en los últimos 10 años y se analizaron los resultados. Resultados. El CVB es una neoplasia rara, pero es la más frecuente de las originadas en el árbol biliar. Su pronóstico depende de la etapa clínica cuando se diagnostique, neoplasias ubicadas a la pared vesicular (Nevin I-III) tienen mejor pronóstico que las que presentan invasión ganglionar o hepática. La cirugía radical más aceptada es la resección en cuña del lecho vesicular asociada a la linfadenectomía la cual en lesiones tempranas incrementa el periodo libre de enfermedad y la sobrevida. En el INCan se estudiaron 100 pacientes en los últimos 10 años, los que tuvieron neoplasias tempranas Nevin I se encuentran vivos con seguimiento promedio de 33 meses, en lesiones Nevin II de 13, cinco se encuentran vivos o colecistectomía más radioterapia al lecho vesicular, en lesiones Nevin III, cuatro que recibieron RT adyubante se encuentran vivos y todos los Nevin IV-V tuvieron pronóstico malo. Conclusiones. El CVB es altamente letal. El diagnóstico temprano permite ralizar un procedimiento oncológico adecuado lo que ha demostrado incrementar la sobrevida