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  • 1
    Electronic Resource
    Electronic Resource
    Spin relaxation by diffusion on biaxial rods (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 1460-1469 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theoretical framework is presented for the analysis of orientation and frequency dependent spin-relaxation rates from nuclei diffusing on biaxial rodlike aggregates in rectangular and other amphiphilic liquid crystals. With the aid of eigenfunction expansion and integral transform techniques, the problem of calculating the spectral density functions for surface diffusion on a biaxial rod can be reduced to quadrature, without the need to specify the rod geometry. The general results are applied to ribbonlike and elliptic rods, yielding analytical results. The calculations demonstrate that spin relaxation is a considerably more sensitive probe of interface curvature than the static line shape. The relaxation anisotropy, in particular, can discriminate between closely similar rod geometries with different curvature distributions. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474499
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  • 2
    Electronic Resource
    Electronic Resource
    Spin relaxation by collective director fluctuations and molecular diffusion in lamellar phases. Continuum theory of relaxation anisotropy and dispersion (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 9337-9352 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The orientation and frequency dependence of nuclear spin relaxation rates can provide detailed information about the amplitudes and rates of collective orientational fluctuations (director fluctuations) in liquid crystals. In particular, the low-frequency spin relaxation rates from a lamellar phase reflect the membrane bending rigidity and the intermembrane forces. This information is contained in three spectral density functions Jn(ω), n=0,1,2. We have recently presented a continuum-mechanical theory for the second-order spectral density J1(ω). Here we extend the theory to the fourth-order spectral densities J0(ω) and J2(ω), which dominate the transverse relaxation rate in the parallel and perpendicular configurations. These spectral densities have previously been considered in connection with director fluctuations in nematic phases, neglecting the elastic and hydrodynamic anisotropy of the phase. In lamellar phases, this anisotropy plays a crucial role and must be retained in the relaxation theory. Director fluctuations can be induced by elastic distortion modes as well as by molecular translational diffusion. In a lamellar phase, these independent processes give rise to qualitatively different spin relaxation behavior. In particular, J0(0) and J2(0) diverge in the limit of quenched disorder. The theoretical results presented here are directly applicable to spin relaxation data from a variety of lamellar systems, including phospholipid bilayers and sterically stabilized dilute lamellar phases. An analysis of published 2H and 31P relaxation data from phospholipid bilayer phases is presented, leading to a qualitatively different picture from what has previously been deduced in terms of a free membrane theory. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474003
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  • 3
    Electronic Resource
    Electronic Resource
    Diffusion on a flexible surface (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 1880-1887 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The self-diffusion of a particle on a flexible surface, such as a fluid bilayer membrane or a macroscopic interface, is analyzed theoretically in order to relate the macroscopic diffusion coefficient DM, describing displacements in a laboratory-fixed plane, to the intrinsic surface diffusion coefficient DS and to the configurational statistics of the surface. An exact result for DM is obtained for a rapidly fluctuating surface. For a static surface, rigorous bounds on DM are established and an effective medium approximation is derived that should remain accurate for strongly disordered surfaces. With the aid of these results, classical self-diffusion measurements can be used to study the configuration, bending rigidity, and interactions of flexible membranes and interfaces. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473326
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  • 4
    Electronic Resource
    Electronic Resource
    Solvent diffusion in ordered macrofluids: A stochastic simulation study of the obstruction effect (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 6807-6817 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An off-lattice random-flight simulation procedure is described which accurately predicts the obstruction factors for self-diffusion of small molecules in macrofluids. The simulation procedure, employing a continuous step length distribution that ensures rapid and accurate convergence, was validated by comparison with exact results for cylindrical and spherical obstructions on 2D and 3D lattices. The exact results were computed with Rayleigh's multipole method, which also was used to derive a new analytical formula for the obstruction factor of parallel cylinders on a hexagonal lattice, of much higher accuracy than the commonly used approximations. Random-flight simulations were used to assess the accuracy of existing mean-field approximations for the obstruction factors of orientationally ordered nonspherical objects. Due to a near-cancellation of errors, the mean-field result accurately describes the obstruction effect on the trace of the diffusion tensor, as measured in isotropic systems, up to moderately high volume fractions. In contrast, the diffusion anisotropy, a sensitive indicator of microstructure in anisotropic fluids, is accurately predicted by mean-field theory only at low volume fractions. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471347
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  • 5
    Electronic Resource
    Electronic Resource
    Microemulsions as macroelectrolytes (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 1655-1668 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Water-in-oil microemulsions, composed of discrete aqueous droplets dispersed in a continuous oil medium, constitute a special class of electrolyte solutions. Such macroelectrolytes are analogous to conventional electrolyte solutions in most respects, with the notable difference that, in a microemulsion, the ionic (droplet) charge is not fixed but depends on the droplet interactions. Describing the microemulsion as a primitive-model electrolyte mixture with ions of variable charge and evaluating the statistical mechanics within the mean-spherical approximation (MSA), we construct a self-consistent theory of charge fluctuations and droplet interactions in ionic microemulsions. The droplet charge distribution is calculated as a function of the size, shape, polydispersity, and volume fraction of the droplets. We argue that the net droplet charges can have a decisive influence on microemulsion structure, especially at the higher volume fractions where clustering and spinodal decomposition are observed. At lower volume fractions, where the MSA treatment should be quantitatively accurate, the Coulomb interaction between charged droplets has no effect on the structure factor deduced from scattering data. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469738
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  • 6
    Electronic Resource
    Electronic Resource
    Orientation-dependent electrical double-layer interactions. I. Rodlike macroions of finite length (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 7238-7250 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The orientation-dependent electrical double-layer interaction between two thin rodlike macroions of finite length is considered. Starting from an expression for the interaction free energy as a double integral of the Green's function for the linearized Poisson–Boltzmann equation, we obtain several useful analytical approximations. For certain rod configurations, we obtain simple analytical approximations to the double-layer interaction that are quantitatively accurate at physically relevant rod separations and electrolyte concentrations. For a general rod configuration, specified by the separation and three angles, we develop a generalized multipole expansion of the double-layer interaction energy, which converges rapidly if the rod length is not much larger than the center-of-mass rod separation or the Debye length. The orientation dependence of the double-layer repulsion can then be quantitatively accounted for in terms of the screened charge–quadrupole and quadrupole–quadrupole interactions. Furthermore, we show that the double-layer repulsion between finite rods depends on the rod orientations even at arbitrarily large separation. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469035
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  • 7
    Electronic Resource
    Electronic Resource
    Chloride ion binding to human plasma albumin from chlorine-35 quadrupole relaxation (1978)
    s.l. : American Chemical Society
    Biochemistry 17 (1978), S. 3774-3781 
    Details
    ISSN: 1520-4995
    Source: ACS Legacy Archives
    Topics: Biology , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/bi00611a016
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  • 8
    Electronic Resource
    Electronic Resource
    The effect of intermolecular interactions on the 2H and 17O quadrupole coupling constants in ice and liquid water (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 2002-2013 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using ab initio SCF molecular orbital techniques, the electric field gradients (efg's) at the oxygen and hydrogen nuclei were calculated for water clusters ranging from dimer to pentamer in an attempt to reproduce the shift in 17O and 2H nuclear quadrupole coupling constants (qcc's) that is observed on going from ice to vapor. For 2H, where the qcc shift is due mostly to the change in O–H bond length, excellent agreement with the experimental vapor → ice shift was obtained. For 17O, the change in the qcc is found to be mainly electronic in origin, effectively due to the polarization of the charge associated with the oxygen atom, and approximately 75% of the observed qcc shift was reproduced. On the basis of the calculations, estimates of the 17O and 2H qcc's in liquid water were made that are consistent with the values obtained from an analysis of the available NMR relaxation data, provided that librational motions are properly taken into account. We also present results of SCF calculations on water interacting with a Li+, Na+, or Cl− ion, indicating that the effect of a nearby ion on the 2H and 17O qcc's is similar to that produced by H bonding.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.448384
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  • 9
    Electronic Resource
    Electronic Resource
    A fluctuation approach to solvation in polar fluids (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 8056-8062 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A phenomenological approach to solvation in polar fluids is proposed that explicitly recognizes the role of thermal fluctuations in the solvent. The probability distributions for the fluctuating electric potential, field, field gradient, etc. in a polar fluid are shown to be Gaussian with a variance determined by the linear (dielectric) response function of the fluid. This Gaussian distribution is used to calculate the solvation free energy of a solute carrying a permanent multipole and/or a polarizability. Unlike the traditional reaction-field approach, the fluctuation approach can describe the solvation of a polarizability by the fluctuating field from the solvent. This contribution to the solvation free energy may be regarded as the zero-frequency part of the electrodynamic dispersion interaction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465632
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  • 10
    Electronic Resource
    Electronic Resource
    Nuclear spin relaxation in a hexagonal lyotropic liquid crystal (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 6945-6961 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The hexagonal (E) phase in the sodium dodecyl sulphate (SDS)/decanol/water system is investigated by 2H and 23Na nuclear magnetic resonance (NMR) of the selectively deuterated SDS and the sodium counterion. Using macroscopically oriented E phase samples, prepared from the magnetically aligned nematic (NC) phase, we measure the orientation-dependent relaxation rates R1Z and R1Q as well as the line shape of both nuclei. The orientation dependence of the lab-frame spectral densities, determined from the relaxation rates, allow us to separate contributions from different types of molecular motion. In particular, we find a dominant contribution from molecular diffusion around the cylindrical aggregate. From this contribution we determine the lateral diffusion coefficient of SDS to (1.4±0.2)×10−10 m2 s−1 at 25 °C (activation energy 26±2 kJ mol−1 ) and the counterion surface diffusion coefficient to (4.8±0.9)×10−10 m2 s−1 at 25 °C (a factor 2.8 smaller than in an infinitely dilute aqueous electrolyte solution). Furthermore, the flexibility of the cylindrical aggregates in the investigated E phase (aggregate volume fraction 0.27) is quantified in terms of an orientational order parameter SDC≈0.9.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461506
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