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  • 1
    ISSN: 1573-904X
    Keywords: transdermal drug delivery ; electroporation ; iontophoresis ; human skin ; percutaneous transport ; flow-through system
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 0570-0833
    Keywords: Thermodynamics ; Thermodynamics ; Industrial chemistry ; Chemical process design ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thermodynamic properties are essential for quantitative process design to produce chemical products. Caloric properties are required for heat balances, but these properties are usually available or estimated easily. More important - and often much more difficult to estimate - are the chemical potentials of components in mixtures; it is these potentials which determine phase equilibria, as required for separation operations, and chemical equilibria, as required for chemical reactors and for separation operations based on chemical reactions. Molecular thermodynamics is an engineering-oriented science for calculating the desired chemical potentials from a minimum of experimental data. This applied science, based on classical and statistical thermodynamics, yields chemical potentials through models that are based on molecular physics and physical chemistry. Selected examples are cited to illustrate the applicability of molecular thermodynamics: group-contribution methods for obtaining chemical potentials in highly nonideal mixtures as required for distillation-column and process-safety design; equation of state for precipitation of uniform-sized crystals from supercritical fluids; molecular-orbital calculations to guide process development for alternatives to environmentally dangerous chlorofluorohydrocarbons; molecular-simulation calculations for separation of gas mixtures with porous adsorbents; equilibria in two-phase aqueous systems for separation of protein mixtures; and, finally, extended polymer-solution thermodynamics to guide synthesis of hydrogels suitable for protein recovery from soybeans and for novel drug-delivery devices.
    Additional Material: 18 Ill.
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  • 3
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Previous work has shown that the simple point-charge (SPC) model can represent the experimental dielectric constant of water. In this work, we present results of Monte Carlo simulations of SPC water in the isothermal–isobaric (NPT) ensemble and in the Gibbs ensemble. Long-range intermolecular interactions are included in these simulations by use of the Ewald summation method. When Ewald sums are used, simulated, uniphase liquid potential energies are slightly lower (in absolute value) than those obtained for a simple spherical cutoff of the intermolecular potential. The coexistence curve of SPC water is obtained from 25 to 300°C. The critical constants of SPC water are estimated by adjusting the coefficients of a Wegner expansion to fit the difference between simulated liquid and vapor orthobaric densities; the estimated critical temperature is 314 °C and the estimated critical density is 0.27 g/cm3.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 8565-8572 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Simulation calculations are reported for single-chain polymer–wall conformation-averaged potentials of mean force and segment-density profiles for homopolymers of different structures as a function of the segment–wall attractive potential. When there is no attraction between the wall and the polymer segments, the presence of the wall generates a polymer-segment depletion layer whose thickness depends on polymer structure and on surface roughness. Segment-density profiles are characterized by three regions. In the proximal region, the segment density is determined by surface roughness and by polymer flexibility. In the distal region, the segment density approaches unity asymptotically. In the central region, the segment density depends on geometric characteristics of the polymer in the bulk solution. When the wall–segment attractive potential is sufficiently large, the depletion layer thickness is reduced and the polymers are adsorbed. When attraction is weak, compact polymers (e.g., dendrimers of high generation) are readily adsorbed. Due to their globular shape, high-generation dendrimers, at weak attractive interactions, are at contact with the surface with numerous segments; globular polymers experience a relatively small entropic penalty for adsorption. By contrast, linear polymers, due to their flexibility at good solvent conditions, pay a high entropic penalty for each segment at contact with the surface. Therefore, at weak attractive interactions, globular polymers are readily adsorbed, whereas linear polymers are more readily adsorbed at stronger attractive interactions. With rising surface roughness, flexible polymers tend to spread on the surface, whereas branched polymers are repelled at larger distances. © 2001 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 2927-2931 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Experimental osmotic second virial cross coefficients are reported for linear and 8-arm star polystyrenes in three solvents: toluene, cyclohexane, and methylcyclohexane. The osmotic second virial cross coefficient for 8-arm star and linear polystyrene is always positive and within the osmotic second virial coefficients measured for the single polymers. The positive cross coefficient indicates net repulsion between the two different polymers in dilute solution. The extent of repulsion is greatest in toluene and least in cyclohexane. To relate the macroscopic second virial coefficient to microscopic interactions, the potential of mean force between linear and 6-arm star polymers was computed by molecular simulation. The interaction between nonbonded polymer segments is given by a square-well potential. Well width was set equal to one half of the segment diameter. Different solvent conditions were investigated by using different well depths. Potentials of mean force were then used to compute the osmotic second virial cross coefficients. © 2000 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 3360-3365 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Statistical-thermodynamic theory for predicting the phase behavior of a colloidal solution requires the pair interaction potential between colloidal particles in solution. In practice, it is necessary to assume pairwise additivity for the potential of mean force between colloidal particles, but little is known concerning the validity of this assumption. This paper concerns interaction between small charged colloids, such as surfactant micelles or globular proteins, in electrolyte solutions and the multibody effect on phase behavior. Monte Carlo simulations for isolated colloidal triplets in equilateral configurations show that, while the three-body force is repulsive when the three particles are near contact, it becomes short-ranged attractive at further separations, contrary to a previous study where the triplet force is attractive at all separations. The three-body force arises mainly from hard-sphere collisions between colloids and small ions; it is most significant in solutions of monovalent salt at low concentration where charged colloids experience strong electrostatic interactions. To illustrate the effect of three-body forces on the phase behavior of charged colloids, we calculated the densities of coexisting phases using van der Waals-type theories for colloidal solutions and for crystals. For the conditions investigated in this work, even though the magnitude of the three-body force may be as large as 10% of the total force at small separations, three-body forces do not have a major effect on the densities of binary coexisting phases. However, coexisting densities calculated using Derjaguin–Landau–Verwey–Overbeek theory are much different from those calculated using our simulated potential of mean force. © 2000 American Institute of Physics.
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  • 7
    ISSN: 1572-8927
    Keywords: Potential of mean force ; proteins ; salts ; intermolecular interactions ; precipitation ; crystallization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Osmotic pressures have been measured to determine lysozyme—lysozyme,BSA—BSA, and lysosyme—BSA interactions for protein concentrations to 100 g-L−1in an aqueous solution of ammonium sulfate at ambient temperature, as a functionof ionic strength and pH. Osmotic second virial coefficients for lysozyme, forBSA, and for a mixture of BSA and lysozyme were calculated from theosmotic-pressure data for protein concentrations to 40 g-L−1. The osmotic second virialcoefficient of lysozyme is slightly negative and becomes more negative withrising ionic strength and pH. The osmotic second virial coefficient for BSA isslightly positive, increasing with ionic strength and pH. The osmotic second virialcross coefficient of the mixture lies between the coefficients for lysozyme andBSA, indicating that the attractive forces for a lysozyme—BSA pair areintermediate between those for the lysozyme—lysozyme and BSA—BSA pairs. For proteinconcentrations less than 100 g-L−1, experimental osmotic-pressure data comparefavorably with results from an adhesive hard-sphere model, which has previouslybeen shown to fit osmotic compressibilities of lysozyme solutions.
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  • 8
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Zeitschrift für die chemische Industrie 102 (1990), S. 1286-1295 
    ISSN: 0044-8249
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Das Wissen über thermodynamische Eigenschaften ist für die Planung quantitativer Prozesse zur Herstellung chemischer Produkte wesentlich. Kalorische Größen, die für Wärmebilanzen benötigt werden, sind gewöhnlich bekannt oder leicht abzuschätzen. Wichtiger - und oft viel schwerer zu schätzen - sind die chemischen Potentiale von Komponenten in Mischungen. Sie bestimmen die Phasengleichgewichte, die für Trennungen wesentlich sind, und die chemischen Gleichgewichte, deren Kenntnis für die Entwicklung von chemischen Reaktoren und von Trennungen, die auf chemischen Reaktionen beruhen, notwendig ist. Die Molekulare Thermodynamik ist eine technisch ausgerichtete Wissenschaft zur Berechnung der gewünschten chemischen Potentiale aus einem Minimum an experimentellen Daten. Sie beruht auf der klassischen und der statistischen Thermodynamik und ermittelt chemische Potentiale mit Modellen, die auf der Molekülphysik und der Physikalischen Chemie basieren. Dieser Beitrag stellt einige ausgewählte Anwendungen der Molekularen Thermodynamik vor: die Bestimmung chemischer Potentiale in stark nicht idealen Mischungen, wie sie für die Planung von Destillationskolonnen und die Prozeßsicherheit erforderlich sind; die Ermittlung der Bedingungen, unter denen Kristalle einheitlicher Größe aus überkritischen Gasen gefällt werden können, aus Zustandsgleichungen; Molekülorbitalberechnungen als Wegweiser bei der Entwicklung von Alternativen zu umweltgefährdenden Chlorfluorkohlenwasserstoffen; Moleküldynamik-Simulationsrechnungen für die Trennung von Gasgemischen mit porösen Adsorbentien; die Berechnung von Gleichgewichten in zweiphasigen wäßrigen Systemen für die Trennung von Proteinmischungen; die Erweiterung zur Polymerlösungsthermodynamik als Wegweiser bei der Synthese von Hydrogelen, die für die Proteingewinnung aus Sojabohnen und zur kontrollierten Arzneimittelfreisetzung geeignet sind.
    Additional Material: 18 Ill.
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  • 9
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 40 (1994), S. 676-691 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A semiempirical thermodynamic method is developed to establish a framework for calculating vapor-liquid and liquid-liquid equilibria in ternary systems containing water, an organic solvent, and a salt. Careful attention is given to precise definition of standard states. Short-range ion-solvent forces are taken into account primarily by a chemical-equilibrium method based on stepwise ion solvation; however, physical contributions also contribute. Water-cosolvent nonideality is described by an extended equation of the van Laar form. Long-range electrostatic forces between ions are taken into account by an extended Debye-Hückel equation with corrections for transferring from a McMillan-Mayer to a Lewis-Randall framework.The new method is illustrated with results for several systems including saturated aqueous mixtures of LiBr or LiCl with methanol where the salt concentration exceeds 20 molal. The method developed here is of particular interest for process calculations in extractive crystallization, a low-energy operation for producing salt from aqueous solution.
    Additional Material: 5 Ill.
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  • 10
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 40 (1992), S. 1155-1164 
    ISSN: 0006-3592
    Keywords: protein solubility ; precipitation ; salting-out ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The solubilities of lysozyme, α-chymotrypsin and bovine serum albumin (BSA) were studied in aqueous electrolyte solution as a function of ionic strength, pH, the chemical nature of salt, and initial protein concentration. Compositions were measured for both the supernatant phase and the precipitate phase at 25°C. Salts studied were sodium chloride, sodium sulfate, and sodium phosphate. For lysozyme, protein concentrations in supernatant and precipitate phases are independent of the initial protein concentration; solubility can be represented by the Cohn salting-out equation. Lysozyme has a minimum solubility around pH 10, close to its isoelectric point (pH 10.5). The effectiveness of the three salts studied for precipitation were in the sequence sulfate 〉 phosphate 〉 chloride, consistent with the Hofmeister series. However, for α-chymotrypsin and BSA, initial protein concentration affects the apparent equillibrium solubility. For these proteins, experimental results show that the compositions of the precipitate phase are also affected by the initial protein concentration. We define a distribution coefficient κe to represent the equilibrium ratio of the protein concentration in the supernatant phase to that in the precipitate phase. When the salt concentration is constant, the results show that, for lysozyme, the protein concentrations in both phases are independent of the initial protein concentrations, and thus κe is a constant. For α-chymotrypsin and BSA, their concentrations in both phases are nearly proportional to the initial protein concentrations, and therefore, for each protein, at constant salt concentration, the distribution coefficient κe is independent of the initial protein concentration. However, for both lysozyme and α-chymotrypsin, the distribution coefficient falls with increasing salt concentration. These results indicate that care must be used in the definition of solubility. Solubility is appropriate when the precipitate phase is pure, but when it is not, the distribution coefficient better describes the phase behavior. © 1992 John Wiley & Sons, Inc.
    Additional Material: 13 Ill.
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