182 research outputs found
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Critical Temperatures and Pressures for Hydrocarbon Mixtures Froman Equation of State With Renormalization-Group-Theory Corrections
A recently developed crossover equation of state incorporates contributions from long-wavelength density fluctuations by renormalization-group theory. This equation of state can satisfactorily describes the thermodynamic properties of chain fluids both far-from and near-to the critical region; it is used here to calculate the critical locus of a mixture. Because the calculations require much computation tim, especially for ternary (any higher) mixtures, an interpolation method is used as suggested by Redlich over 30 years ago. For a binary mixture, along the critical line that gives the critical temperature of critical pressure as a function of composition, the limiting slopes at the critical slopes at the critical points of the pure components are explicitly derived from the criteria for a critical point. Logarithmic-hyperbolic interpolation equations are selected to calculate the entire critical line of the binary mixtures; this procedure is then generalized to multicomponent mixtures. Upon comparison with experimental critical lines for binary and multicomponent Type I mixtures of n-alkanes
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Molecular thermodynamics for prevention of asphaltene precipitation
Crude petroleum is a complex mixture of compounds with different chemical structures and molecular weights. Asphaltenes, the heaviest and most polar fraction of crude oil, are insoluble in normal alkanes such as n-heptane, but they are soluble in aromatic solvents such as toluene. The molecular nature of asphaltenes and their role in production and processing of crude oils have been the topic of numerous studies. Under some conditions, asphaltenes precipitate from a petroleum fluid, causing severe problems in production and transportation Our research objective is to develop a theoretically based, but engineering-oriented, molecular-thermodynamic model which can describe the phase behavior of asphaltene precipitation in petroleum fluids, to provide guidance for petroleum-engineering design and production. In this progress report, particular attention is given to the potential of mean force between asphaltene molecules in a medium of asphaltene-free solvent. This potential of mean force is derived using the principles of colloid science. It depends on the properties of asphaltene and those of the solvent as well as on temperature and pressure. The effect of a solvent on interactions between asphaltenes is taken into account through its density and through its molecular dispersion properties
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Protein-salt binding data from potentiometric titrations of lysozyme in aqueous solutions containing KCl
An existing method for potentiometric titrations of proteins was improved, tested and applied to titrations of the enzyme hen-egg-white lysozyme in aqueous solutions containing KCl at ionic strengths from 0.1 M to 2.0 M at 25 C. Information about the protein`s net charge dependence on pH and ionic strength were obtained and salt binding numbers for the system were calculated using a linkage concept. For the pH range 2.5--11.5, the net charge slightly but distinctly increases with increasing ionic strength between 0.1 M and 2.0 M. The differences are most distinct in the pH region below 5. Above pH 11.35, the net charge decreases with increasing ionic strength. Preliminary calculation of binding numbers from titration curves at 0.1 M and 1.0 M showed selective association of chloride anions and expulsion of potassium ions at low pH. Ion-binding numbers from this work will be used to evaluate thermodynamic properties and to correlate crystallization or precipitation phase-equilibrium data in terms of a model based on the integral-equation theory of fluids which is currently under development
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Capillary electrophoretic separation of DNA restriction fragments using dilute polymer solutions
Because the mechanism of DNA separation in capillary electrophoresis is not well understood, selection of polymers is a {open_quotes}trial-and-error{close_quotes} procedure. We investigated dilute-solution DNA separations by capillary electrophoresis using solutions of four polymers that differ in size, shape and stiffness. Hydroxyethylcellulose of high molecular weight provides excellent separation of large DNA fragments (2027 bp - 23130 bp). Polyvinylpyrrolidone separates DNA from 72 bp to 23 kbp and star-(polyethylene oxide), like linear poly (ethylene oxide), provides separation of fragments up to 1353 bp
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Critical Properties of Polydisperse Fluid Mixtures From Anequation of State
Based on stability theory for plydisperse fluid mixtures, expressions have been developed for the spinodal criterion, critical criterion and various stability tests for systems containing one discrete component and one continuous homologue. Each criterion can be separated into two parts: the first part is the same in form as that for binary systems; when we assume particular mixing rules for parameters of the equation of state, that part is determined only by the average molar mass of the homologue. The second part is concerned with the distribution function that characterizes the continuous component. To illustrate results, the van der Waals equation of state is used to calculate critical properties; the composition dependences of parameters a and b of the van der Waals equation are assumed to be linear functions of molar mass. Numerical results for the critical loci are obtained. For a discrete component i and a continuous component j, systematic variations of parameters in the distribution function for j or of the interaction parameter show transitions between qualitatively different types of phase behavior
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Solubilities of nonvolatile solutes in polymers from molecular Thermodynamics
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Forces between aqueous non-uniformly charged colloids from molecular simulation
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