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  • 1
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Journal of chemical & engineering data 24 (1979), S. 208-210 
    ISSN: 1520-5134
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1573-1634
    Keywords: chemical models ; simulations ; calcite ; solubility ; carbon dioxide ; geothermal ; carbonate ; hydrogen sulfide ; thermodynamic.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract This paper describes chemical equilibrium models for predicting carbonate and silica scale formation, CO2 breakout and H2S gas exchange in geothermal brine systems to high concentration and temperature. The equilibrium description is based on a minimization of the free energy of the system with solute activities described by the semiempirical equations of Pitzer (1973; 1987). The carbonate model is parameterized by appropriate osmotic, electromotive force and solubility data (T ≤ 250°C) available in binary and ternary solutions in the seawater Na–K–H–Ca–Cl–SO4–H2O system. The silica model is parameterized by solubility data to 320°C in the Na–Mg–Cl–SO4–SiO2–H2O system. The H2S model is parameterized by solubility data in the H2S–NaCl–H2O system to 320°C. The predictive capabilities of the models are demonstrated by comparison to both laboratory and field data. Examples have been given to illustrate the use of the carbonate model to predict downhole brine compositions in contact with specified formation minerals, temperature and pressure effects on carbonate scaling, the effect of scale inhibitors and breakout characteristics. Application of the silica model demonstrates the effect of temperature on silica scale formation. These illustrations show that the models can be used to reliably predict important chemical behavior in geothermal operations.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    New York : Wiley-Blackwell
    Biopolymers 34 (1994), S. 565-597 
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A combination of detailed energy minimization and molecular dynamics studies of closed circular DNA offers here new information that may be relevant to the dynamics of short DNA chains and/or low superhelical densities. We find a complex dependence of supercoiled DNA energies and geometries on the linking number difference ΔLk as physiological superhelieal densities (|σ| ∼ 0.06) are approached. The energy minimization results confirm and extend predictions of classical elasticity theory for the equilibria of elastic rods. The molecular dynamics results suggest how these findings may affect the dynamics of super-coiled DNA.The minimization reveals sudden higher order configurational transitions in addition to the well-known catastrophic buckling from the circle to the figure-8. The competition among the bending, twisting, and self-contact forces leads to different families of supercoiled forms. Some of those families begin with configurations of near-zero twist. This offers the intriguing possibility that nicked DNA may relax to low-twist forms other than the circle, as generally assumed. Furthermore, for certain values of ΔLk, more than one interwound DNA minimum exists. The writhing number as a function of ΔLk is discontinuous in some ranges; it exhibits pronounced jumps as ΔLk is increased from zero, and it appears to level a characteristic slope only at higher values of ΔLk. These findings suggest that supercoiled DNA may undergo systematic rapid interconversions between different minima e both close in energy and geometry.Our molecular dynamics simulations reveal such transitional behavior. We observe the macroscopic bending and twisting fluctuations of interwound forms about the global helix axis as well as the end-over-end tumbling of the DNA as a rigid body. The overall mobility related to |σ| and to the bending, twisting, and van der Waals energy fluctuations. The general character of molecular motions is thus determined by the types of energy minima found at a given ΔLk. Different time scales may be attributed to each type of motion: The overall chain folding occurs on a time scale almost an order of magnitude faster than the end-over-end tumbling. The local bending and twisting of individual chain residues occur at an even faster rate, which in turn correspond to several cycles of local variations for each large-scale bending and straightening motion of the DNA. © 1994 John Wiley & Sons, Inc.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
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