Chemical composition and nutrient concentrations of 39 relatively poorly known Patagonian lakes (38-501S and from 701 to 681W) are described and analysed using principal component analysis (PCA). The general relationships between nutrients (total phosphorus, bioavailable phosphorus and dissolved inorganic nitrogen) and plankton biomass are examined.We seek to demonstrate that the extreme oligotrophy characterising many lakes and reservoirs of the Argentine Patagonian region of South America owes more to nitrogen deficiency than to a shortage of available phosphorus. The data show a range of trophic conditions with variable water chemistry characteristics.The first two axes of the PCA ordination explain most of the variance (63%). The first component of the variance in the environmental data is a trophic gradient, with positive correlations with the concentrations of nutrients (TP, SRP, DIN) and electrical conductivity and a negative correlation with transparency. For all the reservoirs considered, the calculated annual, summer and winter chlorophyll-a carrying capacities of the available phosphorus were consistently and significantly (Po0.05) in excess of observations but maximum chlorophyll-a values correlate with DIN availability. Indeed the chlorophyll-a carrying capacities, as an index of the resource-sustainable maximum biomass, of the available nitrogen gives a better predictive yield relationship than does P. Our findings are remarkable in so far as the general expectation that dinitrogen-fixing phytoplankton should thrive in the absence of dissolved inorganic nitrogen, at least to the supportive limits of the available phosphorus, is unfulfilled.
A sample of asphaltenes (ACN) precipitated from Cerro Negro crude oil had been divided into 14 fractions using a dialysis procedure employing THF-acetone mixtures as the extracting solvent. Seven extracts (F 1 -F 7 ) and the corresponding residues R 1 -R 7 were obtained as the mixture composition changed from 40 to 100% THF. These materials were characterized by elemental analyses, VPO, solubility, C-13 NMR, and electron paramagnetic resonance (EPR). As the last fractions are aproached, the following trends were observed: H/C decreases, aromaticity increases, solubility descreases, and S d , the spin density, increases. Removal of the first fraction (F 1 , 15% of ACN, acetone-THF (40%)) afforded a residue (R 1 ) insoluble in toluene, indicating that 85% of ACN is present as colloidal particles (R 1 ) dispersed in this solvent by F 1 . More than 12% of ACN found in the last residues (R 6 and R 7 ) were found to be insoluble in organic solvents, suggesting that these fractions are formed by aggregates of molecules held in place by strong intermolecular forces. In an asphaltene micelle they would be at the core and its solubility in organic solvents is due to dispersion by the other components of the micelle. Althought other effects are not disregarded, it is suggested that transannular electron delocalization in free radicals could play an important role in aggregate formation.
We perform a representative series of semiclassical molecular dynamics simulations of aluminum nanocontact breakages, coupled to full quantum conductance calculations. This approach allows to obtain realistic conductance histograms of polyvalent species and understand the origin of their peaked structures. The results show that the conductance depends linearly on the contact minimum cross section for the geometrically favored nanocontact configurations. Valid in a broad range of conductance values, such relation suggests the definition of a transport parameter for the nanoscale, that represents the novel concept of ballistic resistivity. One of the major industrial challenges is to profit from some fascinating physical features present at the nanoscale. The production of dissipationless nanoswitches ͑or nanocontacts͒ is one of such attractive applications. 1 The inelastic electron mean free path is usually larger than nanocontact typical cross sections ͑of the order of few atoms in controlled experimets͒ even at room temperature, and, therefore, the electronic transport through these nanoconstrictions is expected to be ballistic. Nevertheless, the lack of knowledge of the real efficiency of this electronic ballistic/nondissipative transport limits future innovations.For contact sizes of the order of a few Fermi wavelengths F , well defined modes ͑channels͒ appear associated with the transversal confinement of electrons. For this situation, the conductance G is given by the Landauer formula G = G 0 ͚ n=1 N T n , where G 0 =2e 2 / h is the conductance quantum ͑e being the electron charge and h Planck's constant͒, T n is the transmission probability of the nth channel, and N is the number of propagating modes with energies below the Fermi energy. 2 It has been shown that the number of conducting channels is determined by the number of valence electrons of the respective chemical element. 3 For monovalent noble metals such as Cu, Ag and Au, the transmission probability T has been estimated to be approximately equal to 1 ͑i.e., each noble-metal atom contact contributes with G 0 to the conductance value 4,5 ͒. But for monovalent alkali metals or polyvalent chemical species, single-atom contact studies revealed that this channel transmittivity can have a result smaller than one. 3,6,7 Nowadays, there exist several experimental techniques to characterize the electronic transport through nanocontacts. Among them, the measurement of the conductance histogram during nanocontact breakages 8-10 is one of the most used. By putting in contact two opposite electrodes and then separating them, one observes a stepwise decrease in the electrical conductance ͑i.e., a conductance scan͒, until the breakpoint is reached. [11][12][13][14] It has been shown that each scan of the conductance dependence on the electrode retraction differs from one another, since the nanocontact structural evolutions during breakages are not identical. 15 Notwithstanding for fixed experimental parameter conditions ͑such as temperature and applied voltage...
Conductance histograms of aluminum and gold nanocontact rupture are studied experimentally and simulated using embedded atom potentials to assess the interplay between electronic and structural properties at room temperature. Our results reveal a crossover from quantized conductance structures to crystalline faceting or geometric shell/subshell structures at 300 K. The absence of electronic shell structure in gold and aluminum is in stark contrast with the behavior of alkaline metal nanowires which emulate their cluster counterparts. Semiclassical arguments suggest why rapid dominance of ionic structures takes place, and possible nanowire architectures are proposed in consistency with both the experimental and simulated nanocontact data.
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