Partition Constant of Binary Mixtures for the Equilibrium between a Bulk and a Confined Phase

It is well-known that the thermodynamic, kinetic and structural properties of fluids, and in particular of water and its solutions, can be drastically affected in nanospaces. A possible consequence of nanoscale confinement of a solution is the partial segregation of its components. Thereby, confinement in nanoporous materials (NPM) has been proposed as a means for the separation of mixtures. In fact, separation science can take great advantage of NPM due to the tunability of their properties as a function of nanostructure, morphology, pore size, and surface chemistry. Alcohol-water mixtures are in this context among the most relevant systems. However, a quantitative thermodynamic description allowing for the prediction of the segregation capabilities as a function of the material-solution characteristics is missing. In the present study we attempt to fill this vacancy, by contributing a thermodynamic treatment for the calculation of the partition coefficient in confinement. Combining the multilayer adsorption model for binary mixtures with the Young equation, we conclude that the liquid-vapor surface tension and the contact angle of the pure substances can be used to predict the separation ability of a particular material for a given mixture to a semiquantitative extent. Moreover, we develop a Kelvin-type equation that relates the partition coefficient to the radius of the pore, the contact angle, and the liquid-vapor surface tensions of the constituents. To assess the validity of our thermodynamic formulation, coarse grained molecular dynamics simulations were performed on models of alcohol-water mixtures confined in cylindrical pores. To this end, a coarse-grained amphiphilic molecule was parametrized to be used in conjunction with the mW potential for water. This amphiphilic model reproduces some of the properties of methanol such as enthalpy of vaporization and liquid-vapor surface tension, and the minimum of the excess enthalpy for the aqueous solution. The partition coefficient turns out to be highly dependent on the molar fraction, on the interaction between the components and the confining matrix, and on the radius of the pore. A remarkable agreement between the theory and the simulations is found for pores of radius larger than 15 Å

A cost-effective algae-based biosensor for water quality analysis: Development and testing in collaboration with peasant communities

New anthropic potentially harmful compounds are released into the environment everyday. In this context, broad range bioassays have emerged providing economically viable and widely applicable alternatives due to their ability to detect the cumulative toxicity of mixtures of both known and unknown chemicals in a sample, thus allowing direct information about water quality. Here we present a low-cost, wide-range algae-based biosensor that is easy to assemble and operate by untrained users and provides direct readings. It was developed as a request of a peasant social movement organization to assess the toxicity of drinking water in rural communities affected by pesticide spraying. Two fresh water algae strains, Scenedesmus acutus and Pseudokirchneriella subcapitata, were immobilized in alginate beads and tested as bioindicators. After incubation with different pollutants for five days, naked eye analysis by several observers proved to be a successful method to survey algae’s growth and establish the detection limits. Best detection limits were 10 ppm for technical-grade acid glyphosate, 15 ppm for glyphosate-based formulation, 50 ppb for atrazine formulation, 7.5 ppm for copper and 250 ppb for chromium. Absorbance measurements upon algae resuspension validated these results. The developed device was successfully tested in participatory workshops conducted at rural communities. Children, adults and elders with no scientific training were able to build the sensor and interpret the results, thus evaluating the quality of rain and well water used in their communities.Universidad Nacional de Santiago del EsteroConsejo Nacional de Investigaciones Científicas y Técnica

N-TiO 2: Chemical synthesis and photocatalysis

The chemical synthesis of nitrogen-doped titanium dioxide (N-TiO 2) is explored in an attempt to understand the mechanisms of doping. Urea is used as precursor in a sol gel synthesis of N-TiO 2. Chemical and structural changes during thermal treatment of the precursors were followed by several techniques. The effect of doping on band gap, morphology, and microstructure was also determined. The byproducts produced during firing correspond to those obtained during urea thermal decomposition. Polynitrogenated colored compounds produced at temperatures below 400°C may act as sensitizer. Incorporation of N in the TiO 2 structure is possible at higher temperatures. Degradation experiments of salicylic acid under UVA and visible light (γ≥400 nm) in the presence of TiO 2 or N-TiO 2 indicate that doping decreases the activity under UVA light, while stable byproducts are produced under visible light. © 2011 Matias Factorovich et al.Fil:Candal, R. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Ionic Transport and Speciation of Lithium Salts in Glymes: Experimental and Theoretical Results for Electrolytes of Interest for Lithium-Air Batteries.

Glycol ethers, or glymes, have been recognized as good candidates as solvents for lithium-air batteries because they exhibit relatively good stability in the presence of superoxide radicals. Diglyme (bis(2-methoxy-ethyl)ether), in spite of its low donor number, has been found to promote the solution mechanism for the formation of Li2O2 during the discharge reaction, leading to large deposits, that is, high capacities. It has been suggested that lithium salt association in these types of solvents could be responsible for this behavior. Thus, the knowledge of the speciation and transport behavior of lithium salts in these types of solvents is relevant for the optimization of the lithium-air battery performance. In this work, a comprehensive study of lithium trifluoromethanesulfonate (LiTf) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1,2-di-methoxyethane (DME) and diglyme, over a wide range of concentrations, have been performed. Consistent ion pairs and triplet ions formation constants have been obtained by resorting to well-known equations that describe the concentration dependence of the molar conductivities in highly associated electrolytes, and we found that the system LiTf/DME would be the best to promote bulky Li2O2 deposits. Unexpected differences are observed for the association constants of LiTf and, to a lesser extent, for LiTFSI, in DME and diglyme, whose dielectric constants are similar. Molecular dynamics (MD) simulations allowed us to rationalize these differences in terms of the competing interactions of the O-sites of the ethers and the SO x groups of the corresponding anions with Li+ ion. The limiting Li+ diffusivity derived from the fractional Walden rule agrees quite well with those obtained from MD simulations, when solvent viscosity is conveniently rescaled

Bacteriemia por enterobacterias en adultos en un hospital universitario: análisis de cinco años Bacteremia by enterobacteria in adults from a university hospital: a five year analysis

La bacteriemia sigue siendo una de las causas más importantes de morbilidad y mortalidad en pacientes adultos, a pesar de los numerosos antimicrobianos hoy disponibles y del aumento de las medidas de soporte. El objetivo del presente estudio fue analizar los episodios de bacteriemia por enterobacterias adquiridas en la comunidad y durante la hospitalización registrados durante un período de cinco años, estableciendo la prevalencia de especies, los factores de riesgo y los focos, así como la sensibilidad a los antimicrobianos de los microorganismos involucrados. Entre enero de 2000 y diciembre de 2004 se registraron en el Hospital Nacional de Clínicas de Córdoba 129 episodios de bacteriemias por enterobacterias: 45 correspondientes a pacientes ambulatorios (35%) y 84 a hospitalizados (65%). Los factores de riesgo más frecuentes fueron neoplasia (33,3%) y diabetes (12,4%); y los focos más habituales el urinario (29,5%) y el abdominal (13,9%). La enterobacteria aislada con mayor frecuencia en ambas poblaciones fue E. coli, con una incidencia media del 53,5%, seguida de Klebsiella spp. (21,7%) y Enterobacter spp. (12,4%). Las bacteriemias por Klebsiella spp. fueron más comunes en UTI. Esta especie junto con Enterobacter spp. fueron las bacterias más resistentes a los antimicrobianos ensayados.Bacteremia continues to be one of the main causes of morbidity and mortality in adult patients despite the existence of numerous antimicrobial agents and an increase in support measures. The aim of this study was to analyze the cases of community and hospital-acquired bacteremia, by evaluating the prevalence of species, risk factors, source of infection and antimicrobial susceptibility of the microorganisms involved. From January 2000 to December 2004, 129 cases of bacteremia due to enterobacteria were detected in 45 outpatients (35%) and 84 inpatients (65%). The most common risk factors were neoplasia (33.3%) and diabetes (12.4%); being urinary (29.5%) and abdominal (13.9%) the most frequently found sources of infection. E. coli was the most common enterobacteria isolated in both populations, followed by Klebsiella spp. (21.7%), and Enterobacter spp. (12.4%). Klebsiella spp. bacteremia was most common in ICU patients and, together with Enterobacter spp., constituted the most antibiotic-resistant microorganisms

Sorption Isotherms of Water in Nanopores: Relationship Between Hydropohobicity, Adsorption Pressure, and Hysteresis

The motivation of this study is to elucidate how the condensation and desorption pressures in water sorption isotherms depend on the contact angle. This question is investigated for cylindrical pores of 2.8 nm diameter by means of molecular dynamics simulations in the grand canonical ensemble, in combination with the mW coarse-grained model for water. The contact angle is characterized for different sets of water–surface interactions. First, we show that desorption in open-ended pores with moderate or low water affinity, with contact angles greater or equal than 24°, is a nonactivated process in which pressure is accurately described by the Kelvin equation. Then, we explore the influence of hydrophobicity on the capillary condensation and on the width of the hysteresis loop. We find that a small increase in the contact angle may have a significant impact on the surface density and consequently on the nucleation free energy barrier. This produces a separation of the adsorption and desorption branches, exacerbating the emerging hysteresis. These results suggest that the contact angle is not as relevant as the adsorption energy in determining condensation pressure and hysteresis. Finally, we consider nonequilibrium desorption in pores with no open ends and describe how homogeneous and heterogeneous cavitation mechanisms depend on hydrophilicity.Fil: Factorovich, Matias Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Gonzalez Solveyra, Estefania. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Molinero, Valeria. University of Utah; Estados UnidosFil: Scherlis Perel, Damian Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentin