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Ercan Çelik - One of the best experts on this subject based on the ideXlab platform.

Göran Frenning - One of the best experts on this subject based on the ideXlab platform.

  • Generalized Regular Singular-Point description of low-frequency dielectric responses
    Physical Review B, 2004
    Co-Authors: Göran Frenning, Martin Nilsson, Maria Strømme
    Abstract:

    Microcrystalline cellulose (MCC) is the most frequently used excipient for direct compaction of tablets within the pharmaceutical industry. It has earlier been indicated that the interactions between the hydration shell – surrounding the drug molecules in an MCC tablet – and the cellulose regulate the speed of the drug release process. These interactions, and the charge transport governed by moisture, are therefore important to analyze and understand to be able to tailor make new functional drug delivery systems.In this thesis the physical parameters affecting the water-induced ionic transport have been studied with impedance spectroscopy, transient current measurements, nitrogen adsorption and scanning electron microscopy. Dielectric relaxation processes, pertaining to other processes, have also been assessed and analysed, and a generalized Regular Singular Point model has been shown to be able to describe all features of the dielectric spectrum.It has been shown that the ionic charge transport mechanism in humid MCC most likely is governed by two parallel processes: One involving water constituent ions diffusing between adjacent lowest energy sites (free OH- groups) in disordered regions of the cellulose and the other caused by impurity ions, such as Na+, and protons or H3O+ ions, jumping between neighboring cellulose OH- groups to which primary water molecules are attached. At relative humidities of ~ 37 % (representing monolayer coverage) and higher, the latter process is totally dominating the charge transport.At a given moisture content, there are two parameters determining the magnitude of the water-induced ionic conductivity in MCC: The connectedness of the interparticulate bonds and the connectedness of pores with a diameter in the 5-20 nm size range. The presented findings emphasize the importance of analysing and being able to control the nanostructure of a pharmaceutical cellulose-based system in order to tailor the drug transport properties. The presented results should also be significant for other areas where cellulose-water interactions are of key issue; such as for paper and sanitary product research and for food industries using cellulose-based gels.

  • Dielectric-response function determined by Regular Singular-Point analysis
    Physical Review B, 2002
    Co-Authors: Göran Frenning
    Abstract:

    The fractional power laws frequently observed in dielectric measurements were interpreted in terms of Regular Singular Points of the underlying rate equation for the process giving rise to the dielectric response. Utilizing the fact that the experimentally determined power-law responses severely constrain the form of this rate equation, an explicit expression for the dielectric-response function was derived. This expression was found to be able to describe both relaxation and low-frequency dispersion, and it also incorporates the effects of a nonzero direct-current conductivity. The frequency-dependent dielectric susceptibility was calculated analytically and compared to experimental data obtained for the sputtered amorphous thin-film tantalum oxide, which exhibited low-frequency dispersion. The agreement between theory and experiment was good, and was, for the material studied, in fact slightly better than for the well-known Dissado-Hill model.

  • Characterization of solid-state ion-shuttling devices by isothermal transient ionic current measurements
    Physical Review B, 2002
    Co-Authors: Göran Frenning, A.k. Jonsson, Anna-lena Larsson, Maria Strømme
    Abstract:

    This thesis treats charged particle transport, mostly in solid materials but also, to some extent, in aqueous media. Three major types of materials have been investigated; dielectric materials, ion conductors, and drug-delivery systems.The frequency-dependent dielectric permittivity of sputtered amorphous thin film tantalum oxide (Ta2O5) has been determined by using impedance spectroscopy. A new interpolation formula has been derived, that interpolates between the two power-law regions at low and high frequencies usually observed in the dielectric spectrum. This formula is based on a Regular-Singular-Point (RSP) analysis of the conduction process, and the power-laws in the dielectric spectrum are interpreted in terms of RSPs of the underlying rate equation for the corresponding polarization-current response function.Lithium transport properties of Ta2O5 have been analyzed by using the galvanostatic intermittent titration technique and by isothermal transient ionic current measurements. Chemical and component diffusion coefficients for intercalated lithium have been extracted. Moreover, the ion conduction process has been analyzed theoretically, and expressions for transient ionic currents derived, both for single ion-conducting layers and for three-layered structures of ion conductors.Electrical measurement techniques have also been applied to pharmaceutical systems. The alternating ionic current technique has been developed as a tool for determining the release of electrically charged drug substances in aqueous media. Tablets made of agglomerated micronized cellulose have been investigated, and sodium chloride has been used as a model drug. An attempt has been made to describe the combined drug dissolution and drug release processes in mathematical terms.

Charles T. Fulton - One of the best experts on this subject based on the ideXlab platform.

  • Estimating Spectral Density Functions for Sturm-Liouville problems with two Singular endPoints
    arXiv: Numerical Analysis, 2013
    Co-Authors: Charles T. Fulton, David Pearson, Steven Pruess
    Abstract:

    In this paper we consider the Sturm-Liouville equation -y"+qy = lambda*y on the half line (0,infinity) under the assumptions that x=0 is a Regular Singular Point and nonoscillatory for all real lambda, and that either (i) q is L_1 near x=infinity, or (ii) q' is L_1 near infinity with q(x) --> 0 as x --> infinity, so that there is absolutely continuous spectrum in (0,infinity). Characterizations of the spectral density function for this doubly Singular problem, similar to those obtained in [12] and [13] (when the left endPoint is Regular) are established; corresponding approximants from the two algorithms in [12] and [13] are then utilized, along with the Frobenius recurrence relations and piecewise trigonometric - hyperbolic splines, to generate numerical approximations to the spectral density function associated with the doubly Singular problem on (0,infinity). In the case of the radial part of the separated hydrogen atom problem, the new algorithms are capable of achieving near machine precision accuracy over the range of lambda from 0.1 to 10000, accuracies which could not be achieved using the SLEDGE software package.

  • titchmarsh weyl m functions for second order sturm liouville problems with two Singular endPoints
    Mathematische Nachrichten, 2008
    Co-Authors: Charles T. Fulton
    Abstract:

    In this paper we consider some cases of Sturm–Liouville problems with two Singular endPoints at x = 0 and x = ∞ which have a simple spectrum, and show that the simplicity of the spectrum can be built into the definition of a Titchmarsh–Weyl m -function from which the eigenfunction expansion can be constructed. The use of initial conditions at a Point interior to the interval (0,∞) is avoided in favor of Frobenius solutions near the Regular Singular Point x = 0. In contrast to the classical theory associated with a Regular left endPoint, the growth behaviour of the associated spectral functions can be on the order of λβ for any β ∈ (0,∞). Application of the theory to the Bessel equation on (0,∞) and to the radial part of the separated hydrogen atom on (0,∞) is given. In the case of the hydrogen atom a single Titchmarsh–Weyl m -function is obtained which completely describes both the discrete negative spectrum and the continuous positive spectrum. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

  • Titchmarsh–Weyl m -functions for second-order Sturm–Liouville problems with two Singular endPoints
    Mathematische Nachrichten, 2008
    Co-Authors: Charles T. Fulton
    Abstract:

    In this paper we consider some cases of Sturm–Liouville problems with two Singular endPoints at x = 0 and x = ∞ which have a simple spectrum, and show that the simplicity of the spectrum can be built into the definition of a Titchmarsh–Weyl m -function from which the eigenfunction expansion can be constructed. The use of initial conditions at a Point interior to the interval (0,∞) is avoided in favor of Frobenius solutions near the Regular Singular Point x = 0. In contrast to the classical theory associated with a Regular left endPoint, the growth behaviour of the associated spectral functions can be on the order of λβ for any β ∈ (0,∞). Application of the theory to the Bessel equation on (0,∞) and to the radial part of the separated hydrogen atom on (0,∞) is given. In the case of the hydrogen atom a single Titchmarsh–Weyl m -function is obtained which completely describes both the discrete negative spectrum and the continuous positive spectrum. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Emrah Ünal - One of the best experts on this subject based on the ideXlab platform.

Ahmet Gökdoğan - One of the best experts on this subject based on the ideXlab platform.