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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5136-5139 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this short note we describe a novel method, based upon the Kirkwood equation, for doing free energy simulations in asymmetric molecular systems typical of those encountered with supercritical fluid mixtures involving large organic species solvated in a lighter supercritical gas. The rationale behind this modified Kirkwood approach has been to develop simulation procedures that are both accurate and stable, by minimizing the perturbation to the system that is implicit to all perturbation-based simulation methods. A good example of an algorithm in this latter class of methods is the free energy perturbation method (FEP) which has been used in this work for comparative purposes with our proposed technique which we refer to as the modified Kirkwood coupling method (MKC). The MKC approach described here effectively makes the perturbation process in the simulation process vanishingly small and appears to result in a stable, accurate algorithm for free energy calculations in chemical mixtures. Numerical results supporting these conclusions are provided in two Lennard-Jones model systems at supercritical conditions.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The flux line lattice inside a single crystal of Bi2.15Sr1.95CaCu2O8+x has been observed using small-angle neutron diffraction. The diffracted intensity goes rapidly to zero at a magnetic-field-dependent flux lattice melting temperature; ...
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Journal of low temperature physics 105 (1996), S. 1047-1052 
    ISSN: 1573-7357
    Keywords: 74.60Ge
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The long growth defects often found in Bi2Sr2CaCu2O8, “single” crystals effectively weaken the geometrical barrier and lower the field of first flux penetration. This means that the intrinsic (bulk) magnetic properties can be more easily accessed using magnetic measurements. Thus, the onset of strong bulk flux pinning in the sample bulk is determined to lie at T ≈ 40 K, indepedent of whether the field strength is above or below the field of the second peak in the magnetisation.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 41 (1995), S. 2300-2305 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The relative merits of various approaches using computer simulation methods for estimating first-order correction terms to Henry's law in dilute supercritical mixtures are investigated. Three thermodynamic formulations are provided for the calculation of these properties. One method, termed the fluctuation integral approach, requires the explicit calculation of solute-solvent pair correlation functions at the infinite dilution limit, which is a difficult task for simulations given the conflicting demands of both small numbers of solute species (for approximating the infinite dilution limit) as well as the need for large numbers of solute species to improve simulation statistics. A second approach, called the pressure gradient method, does not require the explicit, determination of these solute-solute functions and, as a result, is not as sensitive to the choice of system size and other difficulties associated with establishing an adequate ensemble size and/or number of solute species to be used in the simulations. The third approach uses the exact formulation for the property of interest using Kirkwood-Buff theory. This approach, however, requires all the solute pair correlation functions to be established, making it the most sensitive to issues concerning number of solute species used in the simulations, and so on. An examples is given showing simulation results for these approaches, illustrating their respective strengths and drawbacks.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 41 (1995), S. 389-401 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: An asymptotic analysis based on Taylor series expansions is used for first-order correction terms to the Henry's law approximation to describe solvation phenomena in multiple solute-multiple solvent systems. The magnitude of these correction terms in solvent systems very near their critical points is of particular concern, as shown in model fluid calculations with the aid of integral equation theory. The results clearly demonstrate that close proximity to the critical point in pure and mixed solvent systems causes the Henry's law approximation to show large errors in predicting solubilities, especially near the critical azeotrope of a mixed solvent system. Theoretical results also show that cross solubility enhancements in a two solutesupercritical solvent system cause cooperative synergism (both solute solubilities are increased relative to the corresponding single solute situations) or reverse synergism (both depressed relative to the single solute situation). It appears to be consistent with the available data. In computer simulations, the solute's infinitely dilute reference state is often used as a basis for describing solute thermodynamic behavior. These simulations are best achieved in the canonical ensemble because of the weak composition dependence of free energies in terms of characteristic variables of this ensemble.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 39 (1993), S. 1985-1994 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Computer simulation results are presented for the residual chemical potential μ1r, local solvent density enhancement Δρloc, and average solute potential energy u1 of a solute molecule immersed in afluid very close to its critical point. The simulation algorithm involves the Monte Carlo method in conjunction with free energy perturbation ideas. A number of ideas related to the feasibility of computer simulations in near-critical solvents are investigated. Due to long-range correlations near the critical point, it is not clear that simulations in this regime can be expected to yield accurate results using ensemble sizes of the magnitude typically used in simulations. Examination of system size and number of sampling steps on the simulations shows that the accuracy of the results depend to a large extent on the nature of the thermodynamic property being investigated. The solute chemical potential is suited particularly to simulation since it depends primarily on short-range structure in the system. A comparison of the simulation results with integral equation theory calculations shows both approaches agree well with each other.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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