Dioon spinulosum (Zamiaceae), a Notable Exotic Tree in Camagüey

The identity of a tree cultivated in the city of Camagüey for ornamental purposes was revealed. The tree had become popular due to the inexistence of similar specimens in other gardens of the country. The utilization of botanical methods, like collection management, the use of catalogs and keys, and scientific description and illustrations, facilitated its identification as Dioon spinulosum Dyer & Eichler. (Zamiaceae), a taxon only recorded in Cuba for scientific collections.  The species was described and a key was set up to differentiate the plant to another specimen that belongs to the genus observed on the island, and to determine the possible age of the plant was assessed

Gas separation with mixed matrix membranes obtained from MOF UiO-66-graphite oxide hybrids

UiO-66-GO hybrids were obtained by hydrothermal synthesis of MOF UiO-66 (a Zr terephthalate) on graphite oxide (GO). These hybrids with appropriate texture and presence of nanosized MOF particles (in the ca. 30–100 nm range) have been used as fillers to prepare mixed matrix membranes (MMMs) with two different polymers, polysulfone (PSF) and polyimide (PI), as the matrixes, with contents varying between 0 and 32 wt%. The MMMs were applied to the separation of H2/CH4 and CO2/CH4 mixtures at different temperatures (35, 60 and 90 °C). Besides finding a good filler-polymer interaction, in the particular case of the hybrid filler, the barrier effect of the GO and the microporosity of the MOF dominated the separation properties of the MMMs. In all cases (different MMMs and separation mixtures) the effect of the temperature was to increase the permeability with a simultaneous decrease in the corresponding selectivity. In terms of permselectivity, the best H2/CH4 separation results were obtained (at 35 °C) with a PI based MMM containing only UiO-66 as filler (H2 permeability of 73 Barrer and H2/CH4 selectivity of 151), while a hybrid UiO-66-GO filler produced the best CO2/CH4 performance (CO2/CH4 selectivity value of 51 at 21 Barrer of CO2), also using a PI polymer

NH2-MIL-53(Al) and NH2-MIL-101(Al) in sulfur-containing copolyimide mixed matrix membranes for gas separation

Amino functionalized MOFs (NH2-MIL-53(Al) or NH2-MIL-101(Al)) were used as dispersed phase in the fabrication of mixed matrix membranes (MMMs) with a polymer matrix of sulfur-containing copolyimides (6FDA:DSDA/4MPD:4,4’-SDA 1:1 (polymer P1) or 6FDA/4MPD:4,4’-SDA 1:1 (polymer P2)). The gas separation properties of the MMMs obtained were tested for permeation of H2, CH4 and CO2. Membranes comprising polymer P1 showed better interaction with the fillers used than polymer P2, and therefore better separation properties, especially for NH2-MIL-101(Al). Upon NH2-MIL-101(Al) loading the performance of pure polymer was improved approaching the Robeson 1991 H2/CH4 and CO2/CH4 upper bound limits with high permeabilities, e.g. 114, 71 and 1.7 Barrer for H2, CO2 and CH4, respectively, using 10 wt.% NH2-MIL-101(Al)@P1. These improvements are related to the pore size of the filler, the flexibility and functional groups of sulfone-containing DSDA, and polymer rigidification

Guest charges in an electrolyte: renormalized charge, long- and short-distance behavior of the electric potential and density profile

We complement a recent exact study by L. Samaj on the properties of a guest charge $Q$ immersed in a two-dimensional electrolyte with charges $+1/-1$. In particular, we are interested in the behavior of the density profiles and electric potential created by the charge and the electrolyte, and in the determination of the renormalized charge which is obtained from the long-distance asymptotics of the electric potential. In Samaj's previous work, exact results for arbitrary coulombic coupling $\beta$ were obtained for a system where all the charges are points, provided $\beta Q<2$ and $\beta < 2$. Here, we first focus on the mean field situation which we believe describes correctly the limit $\beta\to 0$ but $\beta Q$ large. In this limit we can study the case when the guest charge is a hard disk and its charge is above the collapse value $\beta Q>2$. We compare our results for the renormalized charge with the exact predictions and we test on a solid ground some conjectures of the previous study. Our study shows that the exact formulas obtained by Samaj for the renormalized charge are not valid for $\beta Q>2$, contrary to a hypothesis put forward by Samaj. We also determine the short-distance asymptotics of the density profiles of the coions and counterions near the guest charge, for arbitrary coulombic coupling. We show that the coion density profile exhibit a change of behavior if the guest charge becomes large enough ($\beta Q\geq 2-\beta$). This is interpreted as a first step of the counterion condensation (for large coulombic coupling), the second step taking place at the usual Manning--Oosawa threshold $\beta Q=2$

Onsager-Manning-Oosawa condensation phenomenon and the effect of salt

Making use of results pertaining to Painleve III type equations, we revisit the celebrated Onsager-Manning-Oosawa condensation phenomenon for charged stiff linear polymers, in the mean-field approximation with salt. We obtain analytically the associated critical line charge density, and show that it is severely affected by finite salt effects, whereas previous results focused on the no salt limit. In addition, we obtain explicit expressions for the condensate thickness and the electric potential. The case of asymmetric electrolytes is also briefly addressed.Comment: to appear in Phys. Rev. Let

Synthesis of layered titanosilicate JDF-L1 for fabrication of composite polyamide 6 film

In clay polymer nanocomposite technology, nowadays there is concern about the safety and environmental effects of the nanometric materials. In this work, sheets of layered titanosilicate JDF-L1 were synthesized with a size of 5.3 µmand thickness of 115 nm and used to fabricate composite polyamide 6 (PA6) films. The JDF-L1 synthesis was scaled in one pot and the rosette-like particles obtained were disaggregated using a simple process with NaOH solution. The composite with 2 wt.% of disaggregated JDF-L1, characterized by X-ray diffraction and electron microscopy, showed parallel orientation (with respect to the film itself) and good dispersion of the sheets. The composite had a similar barrier effect as the PA6 and its mechanical properties did not deteriorate. This opens up the use of this composite as a packing material providing other properties, such as a biocidal effect and synergy effect in combination with other additives

Insight into the reversible structural crystalline-state transformation from MIL-53(Al) to MIL-68(Al)

The reversible crystalline transformation between MIL-53(Al) and MIL-68(Al) is described. This followed a uniform conversion model with cleavage and formation of metal-ligand bonds after exchange or removal of guest molecules. MIL-68(Al) materials produced during the transformations had higher thermal stability and crystallinity than the as-synthesized MIL-68(Al)

Conversion of sugars to methyl lactate with exfoliated layered stannosilicate UZAR-S4

Biomass has been shown as an alternative to fossil fuels for obtaining chemicals. In this work, the transformation of sugars into methyl lactate (ML) at 160 °C was carried out using the layered stannosilicate UZAR-S3 (University of Zaragoza-solid number 3) and the delaminated material UZAR-S4 (University of Zaragoza-solid number 4) obtained from its exfoliation. The exfoliation of UZAR-S3 to UZAR-S4 increased the accessibility of the compounds to the catalytic sites and the medium-strength acidity. Thus, the yield to ML for sucrose transformation increased from 8% for UZAR-S3 to 49.9 % for UZAR-S4. In the reusability tests, the UZAR-S4 catalyst was characterized before and after reaction by several techniques such as X-ray diffraction, thermogravimetry analysis, scanning electronic microscopy, energy dispersive X-ray spectroscopy and nitrogen adsorption. A deactivation of the catalyst was observed, which was related to carbonaceous deposits that decreased the specific surface area and the pore volume of the catalyst

La educación primaria y secundaria en Colombia en el siglo XX

Este artículo analiza la evolución de la educación primaria y secundaria en Colombia durante el siglo XX desde una perspectiva de largo plazo. Se estudian las principales políticas educativas así como los cambios institucionales y organizacionales que ocurrieron. Se analiza la financiación de la educación, el comportamiento de las principales variables educativas y se examina la evolución de calidad de la educación en el país. Se encontró que la expansión de la educación, tanto primaria como secundaria a lo largo de la primera mitad el siglo XX fue muy lenta. Las transformaciones educativas sólo empezaron a ocurrir en la década de los cincuenta, cuando se presentó un rápido y sostenido crecimiento económico y un cambio signif icativo en la estructura económica y demográfica del país. Desde 1950 y hasta mediados de los setenta, los indicadores educativos crecieron a un ritmo nunca antes visto. Sin embargo, la expansión de los indicadores educativos se freno desde mediados de los setenta y hasta principios de los ochenta, cuando se dio una nueva expansión en los mismos que se mantuvo hasta finales de siglo. A pesar de estos grandes avances durante la segunda mitad del siglo XX, al finalizar los noventa el sector educativo Colombiano seguía presentando bajos niveles de cobertura, eficiencia y calidad así como vaguedad en las competencias y obligaciones en términos administrativos y financieros de los diferentes niveles gubernamentales.Educación, Colombia, Políticas Educativas