A quantum molecular dynamics study of aqueous solvation dynamics

Ring polymer molecular dynamics experiments have been carried out to examine effects derived from nuclear quantum fluctuations at ambient conditions on equilibrium and non-equilibrium dynamical characteristics of charge solvation by a popular simple, rigid, water model, SPC/E , and for a more recent, and flexible, q-TIP4P/F model, to examine the generality of conclusions. In particular, we have recorded the relaxation of the solvent energy gap following instantaneous, ±e charge jumps in an initially uncharged Lennard-Jones-like solute. In both charge cases, quantum effects are reflected in sharper decays at the initial stages of the relaxation, which produce up to a ∼20% reduction in the characteristic timescales describing the solvation processes. For anionic solvation, the magnitude of polarization fluctuations controlling the extent of the water proton localization in the first solvation shell is somewhat more marked than for cations, bringing the quantum solvation process closer to the classical case. Effects on the solvation response from the explicit incorporation of flexibility in the water Hamiltonian are also examined. Predictions from linear response theories for the overall relaxation profile and for the corresponding characteristic timescales are reasonably accurate for the solvation of cations, whereas we find that they are much less satisfactory for the anionic case.Fil: Videla, Pablo Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires; ArgentinaFil: Rossky, Peter J.. University of Texas at Austin; Estados UnidosFil: Laria, Daniel Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires; Argentina. Comision Nacional de Energia Atomica. Gerencia Quimica. CAC; Argentin

Techno-economic assessment of a solar dish micro gas turbine system

In this thesis a thermo-economic assessment of a stand-alone solar dish MGT power plant is presented. The study aims to develop a fundamental understanding of the impact of the main cycle parameters on plant performances and costs and to evaluate the possibility to incorporate hybridisation and thermal storage, considering technical limitations and economic aspects. Objective have been achieved developing a flexible and modular tool for design and off-design performance evaluation of micro-turbine based solar power plants. This can then be adopted for different layouts and operational strategies. The model was validated against experimental data demonstrating the applicability and accuracy of the model for thermo-economic optimisation purposes. To guarantee modularity and flexibility an object-oriented programming approach, using the software C++, is adopted. Each component is represented using a class made of functions that perform a one dimensional physical design for both design point and off-design performance in addition to a cost function. Results in output from the above-mentioned tool were used to train machine learning algorithms, which were used to perform plant's optimisations considering dissimilar layouts, locations and operational strategies. A multi objective genetic algorithm was adopted as optimisation tool. The objective of the optimisation was to 1mmm1Se the levelised cost of energy of the plant while maxrnusrng the annual po,ver output. Both environmental impact and plant dispachability were considered. As a final outcome of the optimisation: for each considered layout and operationa.l strategy: a candidate point was selected and analysed. Parametric study on possible technological improvements of plants' components was also performed. The proposed optimum plant configurations are shmvn to be advantageous in term of levelised cost of energy, more dispatchable and with a reduced environmental impact

Excess protons in water-acetone mixtures

Using molecular dynamics experiments, we analyze equilibrium and dynamical characteristics related to the solvation of excess protons in water-acetone mixtures. Our approach is based on the implementation of an extended valence-bond Hamiltonian, which incorporates translocation of the excess charge between neighboring water molecules. Different mixtures have been analyzed, starting from the pure water case down to solutions with a water molar fraction x w 0.25. In all cases, we have verified that the structure of the first solvation shell of the H 3 O moiety remains practically unchanged, compared to the one observed in pure water. This shell is composed by three water molecules acting as hydrogen bond acceptors, with no evidence of hydrogen bond donor-like connectivity. Moreover, the increment in the acetone concentration leads to a gradual stabilization of Eigen-like [H 3O·(H 2O) 3] configurations, in detriment of Zundel-like [H·(H 2O) 2] ones. Rates of proton transfer and proton diffusion coefficients have been recorded at various water-acetone relative concentrations. In both cases, we have found a transition region, in the vicinity of x w ∼ 0.8, where the concentration dependences of the two magnitudes change at a quantitative level. A crude estimate shows that, at this tagged concentration, the volumes occupied by the two solvents become comparable. The origins of this transition separating water-rich from acetone-rich realms is rationalized in terms of modifications operated in the nearby, second solvation shell, which in the latter solutions, normally includes at least, one acetone molecule. Our results would suggest that one possible mechanism controlling the proton transfer in acetone-rich solutions is the exchange of one of these tagged acetone molecules, by nearby water ones. This exchange would give rise to Zundel-like structures, exhibiting a symmetric, first solvation shell composed exclusively by water molecules, and would facilitate the transfer between neighboring water molecules along the resonant complex. © 2012 American Institute of Physics.Fil:Semino, R. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Laria, D. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Solvation dynamics following electron photodetachment from l- in aqueous clusters

New insights into equilibrium and dynamical aspects of electron photodetachment reactions in small water clusters were given. It focuses on assessing the effects of thermal and polarization fluctuations provided by three cluster environments with different extents of spatial confinement, on the microscopic mechanisms that drive the reaction. These fluctuations, in turn, determine the characteristics of the electron localization and the subsequent detachment following photoexcitation of the probe.Fil:Elola, M.D. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Laria, D. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Solvatipn Of Excess Electrons In Supercritical Ammonia

Solvation of excess electrons in supercritical ammonia along the T = 450 K isotherm was investigated. Equilibrium aspects of solvation were analyzed using combined path integral-molecular dynamics techniques. Observations showed transition from localized to quasifree states at approximately one fourth of the triple point density.1191260446052Innovations in Supercritical Fluids, Science, and Technology (1995) ACS Symposium Series, 608. , edited by K. W. Hutchenson and N. R. Foster (American Chemical Society, Washington, D.C.)Me Hugh, M.A., Krukonis, V.J., (1986) Supercritical Fluid Extraction: Principles and Practice, , Butterworths, BostonKim, S., Johnston, K.P., (1987) ACS Symposium Series, 329. , Supercritical Fluids, Chemical, and Engineering Principles and Applications, edited by T. G. Squires and M. E. Paulatis (American Chemical Society, Washington, D.C.)Kim, S., Johnston, K.P., (1987) Ind. Eng. Chem. Res., 26, p. 1206Bennet, G.E., Johnston, K.P., (1994) J. Phys. 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Solvatipn of excess electrons in supercritical ammonia

Solvation of excess electrons in supercritical ammonia along the T = 450 K isotherm was investigated. Equilibrium aspects of solvation were analyzed using combined path integral-molecular dynamics techniques. Observations showed transition from localized to quasifree states at approximately one fourth of the triple point density.Fil:Rodriguez, J. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Laria, D. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Techno-economic assessment of a solar dish micro gas turbne system

In this thesis a thermo-economic assessment of a stand alone solar dish MGT power plant is presented. The study aims to develop a fundamental understanding of the impact of the main cycle parameters on plant performances and costs and to evaluate the possibility to incorporate hybridisation and thermal storage, considering technical limitations and economic aspects. Objective have been achieved developing a flexible and modular tool for design and off-design performance evaluation of micro-turbine based solar power plants. This can then be adopted for different layouts and operational strategies. The model was validated against experimental data demonstrating the applicability and accuracy of the model for thermo-economic optimisation purposes. To guarantee modularity and flexibility an object-oriented programming approach, using the software C++, is adopted. Each component is represented using a class made of functions that perform a one-dimensional physical design for both design point and off-design performance in addition to a cost function. Results in output from the above-mentioned tool were used to train machine learning algorithms, which were used to perform plant's optimisations considering dissimilar layouts, locations and operational strategies. A multi objective genetic algorithm was adopted as optimisation tool. The objective of the optimisation was to minimise the levelised cost of energy of the plant while maxmising the annual power output. Both environmental impact and plant dispachability were considered. As a final outcome of the optimisation, for each considered layout and operational strategy, a candidate point was selected and analysed. Parametric study on possible technological improvements of plants' components was also performed. The proposed optimum plant configurations are shown to be advantageous in term of levelised cost of energy, more dispatchable and with a reduced environmental impact.

Molecular dynamics simulations of AOT-water/formamide reverse micelles: Structural and dynamical properties

We present results from molecular dynamics simulations performed on reverse micelles immersed in cyclohexane. Three different inner polar phases are considered: water (W), formamide (FM), and an equimolar mixture of the two solvents. In all cases, the surfactant was sodium bis(2-ethylhexyl) sulfosuccinate (usually known as AOT). The initial radii of the micelles were R∼15 Å, while the corresponding polar solvent-to-surfactant molar ratios were intermediate between w0 =4.3 for FM and w0 =7 for W. The resulting overall shapes of the micelles resemble distorted ellipsoids, with average eccentricities of the order of ∼0.75. Moreover, the pattern of the surfactant layer separating the inner pool from the non-polar phase looks highly irregular, with a roughness characterized by length scales comparable to the micelle radii. Solvent dipole orientation polarization along radial directions exhibit steady growths as one moves from central positions toward head group locations. Local density correlations within the micelles indicate preferential solvation of sodium ionic species by water, in contrast to the behavior found in bulk equimolar mixtures. Still, a sizable fraction of ∼90% of Na+ remains associated with the head groups. Compared to bulk results, the translational and rotational modes of the confined solvents exhibit important retardations, most notably those operated in rotational motions where the characteristic time scales may be up to 50 times larger. Modifications of the intramolecular connectivity expressed in terms of the average number of hydrogen bonds and their lifetimes are also discussed. © 2008 American Institute of Physics.Fil:Pomata, M.H.H. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Laria, D. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Elola, M.D. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Computerized adaptive test and decision trees: A unifying approach

In the last few years, several articles have proposed decision trees (DTs) as an alternative to computerized adapted tests (CATs). These works have focused on showing the differences between the two methods with the aim of identifying the advantages of each of them and thus determining when it is preferable to use one method or another. In this article, Tree-CAT, a new technique for building CATs is presented. Unlike the existing work, Tree-CAT exploits the similarities between CATs and DTs. This technique allows the creation of CATs that minimise the mean square error in the estimation of the examinee’s ability level, and controls the item’s exposure rate. The decision tree is sequentially built by means of an innovative algorithmic procedure that selects the items associated with each of the tree branches by solving a linear program. In addition, our work presents further advantages over alternative item selection techniques with exposure control, such as instant item selection or simultaneous administration of the test to an unlimited number of participants. These advantages allow accurate on-line CATs to be implemented even when the item selection method is computationally costly