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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 3868-3874 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We study the equilibrium and kinetic properties of a model for polydisperse mixture adsorption. The system consists of a bulk phase of hard disks with a given size distribution and overall concentration that adsorb and desorb on a continuous planar surface. The disks adsorb at a rate proportional to their bulk concentration and desorb at a rate that may depend on the particle size. The model is characterized by α, the dimensionless binding energy of a solute per unit area, and K which is proportional to the total bulk concentration. The properties of the model are determined with scaled particle theory (SPT) and with numerical simulation. If the desorption rate is independent of particle size, an equilibrium is rapidly established between the bulk and adsorbed phases. The resulting adsorption isotherms predicted by SPT agree well with the numerical simulations. If the desorption rate depends exponentially on the binding energy of the adsorbed particle, the approach to equilibrium is dramatically slowed. At high bulk concentrations and low values of α the adsorbed density increases monotonically with time, while the coverage displays an overshoot. At low K and high α, it is the coverage that increases monotonically, while the density passes through a maximim. For a given bulk phase distribution, one can construct an (α,K) kinetic phase diagram delineating this behavior. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 2109-2112 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Monte Carlo measurements of the chemical potential of hard diatomics and polyatomics dissolved in hard sphere fluids are reported. These are performed as a function of density, solute size, and diatomic bond length. Bond length derivatives are used to determine the mean force along the diatomic bond axis. The results are compared with analytical expressions derived from the hard fluid (HF) model, a model proposed by Boublik, and a spherical approximation to diatomic and polyatomic chemical potentials. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 9514-9514 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    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 101 (1994), S. 7064-7073 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effects of particle conformational changes on the kinetics and saturation coverage of irreversible macromolecular adsorption at liquid–solid interfaces are investigated by computer simulation of a modified random sequential adsorption model. In this model, macromolecules (modeled as disks of diameter σα) adsorb onto a surface at a rate ka. Once adsorbed, the particles spread symmetrically and discretely to a larger diameter σβ at a rate ks. Adsorption or spreading events which result in the overlap of particles on the surface are not allowed. We investigate the effects of changes in spreading magnitude Σ (=σβ/σα) and relative spreading rate Ks (=ks/ka). We observe that the saturation coverage of spread particles decreases while that of unspread particles increases with spreading magnitude. This dependence is most pronounced for small spreading: the derivative of the surface coverage of both spread and unspread particles with respect to Σ diverges logarithmically when Σ→1. An increase in the rate of spreading increases the saturation coverage of spread particles while decreasing that of unspread particles. The dependence of the coverage on spreading rate is weaker than its dependence on spreading magnitude: a four order of magnitude change in Ks results in a factor of 2 change in the partial coverages. The coverage of unspread particles may become nonmonotonic in time for certain values of Σ and Ks. The total density of particles on the surface decreases and the average particle size increases with Ks, in accordance with recent protein adsorption experiments.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 4696-4706 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We examine the thermodynamic properties of two-dimensional fluid mixtures of hard convex particles using scaled particle theory (SPT). Analytic expressions are obtained for the excess area, Gibbs free energy, and excess entropy of a binary mixture. For typical fluid densities and for a range of area and perimeter ratios of the two species the fluid mixtures exhibit small negative deviations from ideality. The excess quantities are smaller than the corresponding bulk (three dimensional) mixtures which offers some explanation for the success of the ideal adsorbed solution (IAS) theory. According to the SPT, binary mixtures of hard particles are stable for all compositions and no fluid-fluid demixing transition is possible. The SPT equations are used to examine the adsorption equilibrium between an ideal bulk phase and an adsorbed phase. Adsorption isotherms and selectivities are computed for a range of area and perimeter ratios, equilibrium constant ratio, and bulk mole fraction. Unlike the widely used multicomponent Langmuir equations, the selectivity computed from the SPT isotherms exhibits strong sensitivity to these parameters. The selectivity of the smaller species always increases with increasing bulk pressure which may lead to a selectivity reversal. Finally, we discuss systems where the adsorbed molecules can adopt various orientations with respect to the surface normal. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 97 (1993), S. 4256-4258 
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 6087-6097 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Transport properties of isotropic fluids composed of hard ellipsoids of revolution are studied using molecular dynamics simulation. The self-diffusion coefficient, the shear viscosity, and the thermal conductivity are evaluated for a range of densities and elongations and are compared with the results from an Enskog kinetic theory for nonspherical bodies. The full anisotropic pair correlation function, which is required input in an Enskog kinetic theory, can be obtained from simulation or can be approximated. If the pair correlation function is taken as isotropic on the contact surface, with a contact value derived from an accurate equation of state, the resulting kinetic theory transport properties agree to within a few percent of those calculated on the basis of the exact pair correlation function. The simulation and the kinetic theory values for the shear viscosity and the thermal conductivity show the same qualitative behavior, i.e., increasing with density and with particle nonsphericity. Quantitatively, there is good agreement at low densities (up to 30% of closest packing); at higher densities (60% of closest packing), deviations from Enskog theory are larger than and in the opposite direction to those seen for hard spheres. The Stokes–Einstein and Debye relations are tested and indicate a transition from a kinetic theory region towards the hydrodynamic limit as density increases.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 2232-2240 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Many real porous solids and possibly composite complex liquid systems such as microemulsions can be modeled as a random porous medium with given void fraction and specific interface area, thereby providing a means for estimation of many physical properties of the real systems. Another, even simpler model consists of a random array of mutually penetrable spheres [the random spheres model (RSM)], the void fraction and specific interface of which may be chosen. We have augmented this model to include penetrable spheres having a "random'' distribution of sizes. If the RSM and the random porous solid model exhibited similar behavior, the RSM could then be applied in studies of real porous solids and microemulsions, specifically in computation of the "entropy of mixing'' of oil and water domains in a microemulsion in the continuum rather than on a lattice. Recently it has been demonstrated that the entropy of mixing on a lattice may be appreciably less than the more accurate corresponding quantity in the continuum. We compare the RSM and the random solid model by means of their respective void–void correlation functions in one dimension where all results are exact.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 593-604 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have carried out molecular dynamic simulations over a range of densities for two-dimensional fluids consisting of hard, soft, and Lennard-Jones disks. For comparison we have also carried out simulations for the corresponding systems in which all but one particle are frozen in position. We have studied the velocity autocorrelation functions and the closely related velocity-sign autocorrelation functions, and have examined the probabilities per unit time that a particle will undergo a first velocity sign reversal after an elapsed time t measured alternately from the last velocity reversal or from a given arbitrary time. At all densities studied, the first of these probabilities per unit time is zero at t=0 and rises to a maximum at a later time, but as the hardness of the disks is increased, the maximum moves in toward t→0. This maximum can be correlated with the "negative'' dip observed in the velocity correlation functions when plotted versus time. Our conclusion is that all these phenomena can be explained qualitatively on the basis of a model where memory does not extend back beyond the last velocity reversal. However, at high density, the velocity-sign-autocorrelation function not only shows a negative dip (which is explained by the model) but also a second "oscillation'' which is not described, even qualitatively, by the model. We conclude that the first dip in the velocity and velocity-sign correlation functions can occur even if there are no correlated or coherent librations, but the existence of a "second'' oscillation is a better indication of such correlations.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    Adsorption 2 (1996), S. 89-94 
    ISSN: 1572-8757
    Keywords: protein adsorption ; desorption ; memory function
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Abstract A formalism for the adsorption kinetics of systems where the desorption rate is a function of the residence time of the adsorbed particle is presented. The adsorbed density at time t is expressed simply as a convolution of a memory kernel, Q(t), and the available surface function, φ(t). For completely irreversible adsorption, Q(t) = 1, while for a system which approaches an equilibrium state, Q tends to zero at sufficiently large times. When the desorption rate, k d , is constant, Q(t) = exp(−k d t). Two models for the memory kernel are considered. In the first, the molecule is assumed to interact with the surface via two ligands which bind and debind at rates λ and µ respectively. In the second model, the adsorption is assumed to be partially reversible: molecules transform to a permanently bound state at a rate λ and desorb at a rate µ. In both models, the adsorption kinetics and memory kernels are found analytically. Strategies for determining the memory function from experimental data are discussed.
    Type of Medium: Electronic Resource
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