Simón Bolívar University

About: Simón Bolívar University is a education organization based out in Caracas, Venezuela. It is known for research contribution in the topics: Population & Crystallization. The organization has 5912 authors who have published 8294 publications receiving 126152 citations.

Patterns of spatial variation of assemblages associated with intertidal rocky shores: a global perspective

Abstract: Assemblages associated with intertidal rocky shores were examined for large scale distribution patterns with specific emphasis on identifying latitudinal trends of species richness and taxonomic distinctiveness. Seventy-two sites distributed around the globe were evaluated following the standardized sampling protocol of the Census of Marine Life NaGISA project (www.nagisa.coml.org). There were no clear patterns of standardized estimators of species richness along latitudinal gradients or among Large Marine Ecosystems (LMEs); however, a strong latitudinal gradient in taxonomic composition (i.e., proportion of different taxonomic groups in a given sample) was observed. Environmental variables related to natural influences were strongly related to the distribution patterns of the assemblages on the LME scale, particularly photoperiod, sea surface temperature (SST) and rainfall. In contrast, no environmental variables directly associated with human influences (with the exception of the inorganic pollution index) were related to assemblage patterns among LMEs. Correlations of the natural assemblages with either latitudinal gradients or environmental variables were equally strong suggesting that neither neutral models nor models based solely on environmental variables sufficiently explain spatial variation of these assemblages at a global scale. Despite the data shortcomings in this study (e.g., unbalanced sample distribution), we show the importance of generating biological global databases for the use in large-scale diversity comparisons of rocky intertidal assemblages to stimulate continued sampling and analyses.

Canonical quantization of constrained theories on discrete spacetime lattices

Abstract: We discuss the canonical quantization of systems formulated on discrete spacetimes. We start by analysing the quantization of simple mechanical systems with discrete time. The quantization becomes challenging when the systems have anholonomic constraints. We propose a new canonical formulation and quantization for such systems in terms of discrete canonical transformations. This allows us to construct, for the first time, a canonical formulation for general constrained mechanical systems with discrete time. We extend the analysis to gauge field theories on the lattice. We consider a complete canonical formulation, starting from a discrete action, for lattice Yang–Mills theory discretized in space and Maxwell theory discretized in space and time. After completing the treatment, the results can be shown to coincide with the results of the traditional transfer matrix method. We then apply the method to BF theory, yielding the first lattice treatment for such a theory ever. The framework presented deals directly with the Lorentzian signature without requiring a Euclidean rotation. The whole discussion is framed in such a way so as to provide a formalism that would allow a consistent, well-defined, canonical formulation and quantization of discrete general relativity, which we will discuss in a forthcoming paper.

Hydrogen bond interactions at the TiO2 surface : Their contribution to the pH dependent photo-catalytic degradation of p-nitrophenol

Abstract: We have obtained p K a values of p -nitrophenol–TiO 2 by measuring the adsorption equilibrium constants of p -nitrophenol (PNP) on the TiO 2 surface at different pH values These values have been obtained from Langmuir isotherms and from a plot of 1/rate vs 1/[PNP] o obtained during TiO 2 catalyzed solar light photo-degradation of PNP Two limit equilibrium constants are readily obtained depending on the solution pH: at pH 5 at which the TiO 2 surface is mainly positively charged and at pH 8 when it is negatively charged With these and other adsorption equilibrium constants and the PNP p K a value in solution, thermodynamic cycles are established in order to obtain the PNP p K a when it is adsorbed on positively charged, neutral and negatively charged TiO 2 surfaces From these p K a values useful information on the PNP–TiO 2 interaction is readily obtained For instance, the PNP nitro group interacts with the TiO 2 surface via a hydrogen bond, arising from the complex of water molecules with the Ti 4+ ions on its surface The weaker the hydrogen bond donor, the stronger the oxygen nitro group basicity Therefore, p K a changes on the phenolic hydroxyl group result from these interactions Linear free energy correlations, maximum PNP adsorption capacity values ( Q L ) and FTIR ATR, spectrum support this proposal A k obs vs pH degradation profile of p -nitrophenol is also provided