131 research outputs found
Soil sodicity originating from marginal groundwater
Soil salinity and sodicity are among the oldest soil and groundwater pollution problems and are widespread across the globe. Where salinity affects crop water uptake and yield, sodicity may additionally cause poorly reversible soil structure degradation and a severely reduced hydraulic conductivity. We use the model HYDRUSâ1D to simulate sodicity development in soils with shallow, Naârich groundwater under a normal weather regime with distinct dry seasons. Attention is given to the impact of a sudden fresh water input on the formation of a sodic layer. The complex interplay between soil chemistry, soil physics, soil mechanics (as far as swellâshrink behavior is concerned), and fluctuating atmospheric conditions results in a remarkably regular relation between depth, location, and severity of a sodic layer that forms within the soil as a function of rainfall intensity. A threshold behavior is observed: sodic layer formation is absent at rainfall intensities below this threshold, whereas sodic layer thickness and hydraulic conductivity reduction increase rapidly with intensities exceeding this threshold. This is the case even for different soil types and groundwater depths. Field observations agree with our simulations: the properties of the layer with sodicityâinduced structure degradation are more strongly developed, as this layer is situated at a shallower depth. The implementation of hydraulic conductivity reduction as a function of exchangeable Na percentage and ionic strength in HYDRUSâ1D can be improved towards a smooth reduction function, changing soil physical parameters due to swelling and dispersion of clay and reconsideration of the reversibility of sodicity development.<br/
Flying-seed-like liquid crystals 5: Liquid crystals based on octakisphenylthiophthalocyanine and their optical properties
We have synthesized three novel flying-seed-like liquid crystals based on phthalocyaninato copper(II) (abbreviated as PcCu) substituted by bulky groups {(o-C-1)PhS (i), (m-C-1)PhS (j), [m, p(C-1)(2)]PhS (k)} instead of using long alkyl chains, in order to investigate their mesomorphism. Their phase transition behavior and the mesophase structures have been established by using a polarizing optical microscope, a differential scanning calorimeter, and a temperature-dependent small angle X-ray diffractometer. As the results, [(o-C-1)PhS](8)PcCu (8i), [(m-C-1)PhS](8)PcCu (8j) and {[m,p-(C-1)(2)]PhS}(8)PcCu (8k) show a Col tet. o mesophase at 314.9 similar to 362.9 degrees C, a Col(ro) (P2m) mesophase at 287.4 similar to 334.2 degrees C and a Col(ro) (P2m) mesophase at 331.8 similar to 386.8 degrees C, respectively. Very interestingly, each of the derivatives thus exhibits a columnar mesophase at very high temperatures. The mesomorphism is apparently originated from the novel bulky groups (i similar to k). It is also noteworthy that the Q-bands of the present PhScontaining Pc derivatives 8i similar to 8k in THF significantly red-shift by about 35 nm in comparison with those of the corresponding PhO-containing derivatives in THF
Exciton bimolecular annihilation dynamics in supramolecular nanostructures of conjugated oligomers
We present femtosecond transient absorption measurements on -conjugated
supramolecular assemblies in a high pump fluence regime.
Oligo(\emph{p}-phenylenevinylene) monofunctionalized with
ureido-\emph{s}-triazine (MOPV) self-assembles into chiral stacks in dodecane
solution below 75C at a concentration of M. We
observe exciton bimolecular annihilation in MOPV stacks at high excitation
fluence, indicated by the fluence-dependent decay of B-exciton
spectral signatures, and by the sub-linear fluence dependence of time- and
wavelength-integrated photoluminescence (PL) intensity. These two
characteristics are much less pronounced in MOPV solution where the phase
equilibrium is shifted significantly away from supramolecular assembly,
slightly below the transition temperature. A mesoscopic rate-equation model is
applied to extract the bimolecular annihilation rate constant from the
excitation fluence dependence of transient absorption and PL signals. The
results demonstrate that the bimolecular annihilation rate is very high with a
square-root dependence in time. The exciton annihilation results from a
combination of fast exciton diffusion and resonance energy transfer. The
supramolecular nanostructures studied here have electronic properties that are
intermediate between molecular aggregates and polymeric semiconductors
Core charge distribution and self assembly of columnar phases: the case of triphenylenes and azatriphenylenes
<p>Abstract</p> <p>Background</p> <p>The relation betweeen the structure of discotic molecules and columnar properties, a crucial point for the realization of new advanced materials, is still largely unknown. A paradigmatic case is that hexa-alkyl-thio substituted triphenylenes present mesogenic behavior while the corresponding azatriphenylenes, similar in shape and chemical structure, but with a different core charge distribution, do not form any liquid crystalline mesophase. This study is aimed at investigating, with the help of computer simulations techniques, the effects on phase behaviour of changes of the charge distribution in the discotic core.</p> <p>Results</p> <p>We described the shape and the pair, dispersive and electrostatic, interactions of hexa alkyl triphenylenes by uniaxial Gay-Berne discs with embedded point charges. Gay-Berne parameters were deduced by fitting the dispersive energies obtained from an atomistic molecular dynamics simulation of a small sample of hexa-octyl-thio triphenylene molecules in columnar phase, while a genetic algorithm was used to get a minimal set of point charges that properly reproduces the ab anitio electrostatic potential. We performed Monte Carlo simulations of three molecular models: the pure Gay-Berne disc, used as a reference, the Gay-Berne disc with hexa-thio triphenylene point charges, the Gay-Berne disc with hexa-thio azatriphenylene point charges. The phase diagram of the pure model evidences a rich polymorphism, with isotropic, columnar and crystalline phases at low pressure, and the appearance of nematic phase at higher pressure.</p> <p>Conclusion</p> <p>We found that the intermolecular electrostatic potential among the cores is fundamental in sta-bilizing/destabilizing columnar phases; in particular the triphenylene charge distribution stabilizes the columnar structure, while the azatriphenylene distribution suppresses its formation in favor of the nematic phase. We believe the present model could be successfully employed as the basis for coarse-grained level simulations of a wider class of triphenylene derivatives.</p
Anticipatory Drainage Base Management for Groundwater Level Optimization
Controlled drainage is normally operated on a seasonal timescale to increase availability of water and to limit nutrient loss to surface water. It is conceivable that with more frequent, or even daily, management of the drainage base, controlled drainage can be both more efficient and effective. With the analytical successive steady state model presented here, rapid simulations of groundwater level and discharge as a function of time for a field under either regular or controlled drainage are feasible. The model includes effects of sudden changes in drainage base as well as damping and attenuation of dynamics by the unsaturated zone. It is validated against a numerical model and shows good agreement with numerical results. We demonstrate how this analytical model may facilitate anticipatory drainage base management based on ensemble weather forecasts, such that groundwater levels can be maintained at a theoretical optimal level
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