Michaela Zamponi

Oak Ridge National Laboratory, Oak Ridge, Florida, United States

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Publications (5)8.3 Total impact

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    ABSTRACT: The diffusive motions of covalently tethered 1, 3 diphenylpropane (DPP) via a silyl-aryl-ether linkage in the mesopores of MCM-41 were studied by quasielastic neutron scattering. The geometric effect of pore radius was investigated with samples having pores that ranged from 1.6 to 3.0 nm in diameter and highest achievable DPP grafting density. The effect of molecular crowding was investigated in 3.0 nm diameter pores for surface coverage ranging from 0.60 to 1.61 DPP/nm2. Temperature dependence was determined for large pore diameter samples from 240 K to 370 K. As the DPP molecules remain attached over this entire temperature range, data were analyzed in terms of a model of localized diffusion inside a sphere. Only the motions of the DPP hydrogen atoms were considered because of the high sensitivity of neutron scattering to the presence of hydrogen. As atoms far from the attachment point have a greater range of motion than those nearer the tether, the radius of the sphere limiting the motion of individual hydrogen atoms was allowed to increase based on the atom s distance from the tether point. Both smaller pore diameters and higher DPP surface coverage resulted in larger amplitude motion while the diffusion coefficient was greatest in the largest pores at highest surface coverage. These observations support a model where the DPP molecules prefer an orientation allowing close proximity to the MCM-41 pore surface and are forced into the pore interior by either the steric effect of small pore diameter or by increased competition for surface area at high molecule surface coverage.
    Journal of Physical Chemistry C - J PHYS CHEM C. 01/2012; 116(1):923-932.
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    ABSTRACT: A combined quasi-elastic neutron scattering (QENS) and high-resolution solution NMR spectroscopy study was conducted to investigate the internal dynamics of aqueous (D 2 O) G5 PAMAM dendrimer solutions as a function of molecular protonation at room temperature. Localized motion of the dendrimer segments was clearly exhibited in the QENS data analysis while the global, center-of-mass translational diffusion was measured by NMR. Our results unambiguously demonstrate an increased rapidity in local scale ($ 3 A) motion upon increasing the molecular protonation. This is contrary to an intuitive picture that increased charge stiffens the dendrimer segments thereby inhibiting local motion. These charge-induced changes may be a result of interactions with the surrounding counterions and water molecules as the segments explore additional intra-dendrimer volume made available by slight electrostatic swelling and redistribution of mass in the dendrimer interior. This observation is relevant to development of a microscopic picture of dendrimer-based packages as guest-molecule delivery vehicles because reorganization of the confining dendrimer segments must be a precursor to guest-molecule release.
    Soft Matter 01/2011; 7(2). · 4.15 Impact Factor
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    ABSTRACT: It seems to be increasingly accepted that the diversity and composition of lipids play an important role in the function of biological membranes. A prime example of this is the case of lipid rafts; regions enriched with certain types of lipids which are speculated to be relevant to the proper functioning of membrane embedded proteins. Although the dynamics of membrane systems have been studied for decades, the microscopic dynamics of lipid molecules, even in simple model systems, is still an active topic of debate. Neutron scattering has proven to be an important tool for accessing the relevant nanometre length scale and nano to picosecond time scales, thus providing complimentary information to macroscopic techniques. Despite their potential relevance for the development of functionalized surfaces and biosensors, the study of single supported membranes using neutron scattering poses the challenge of obtaining relevant dynamic information from a sample with minimal material. Using state of the art neutron instrumentation we were, for the first time, able to model lipid diffusion in single supported lipid bilayers. We find that the diffusion coefficient for the single bilayer system is comparable to the multi-lamellar lipid system. More importantly, the molecular mechanism for lipid motion in the single bilayer was found to be a continuous diffusion, rather than the flow-like ballistic motion reported in the stacked membrane system. We observed an enhanced diffusion at the nearest neighbour distance of the lipid molecules. The enhancement and change of character of the diffusion can most likely be attributed to the effect the supporting substrate has on the lipid organization.
    Soft Matter 11/2010; 6(23):5864-5867. · 4.15 Impact Factor
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    ABSTRACT: The diffusion of methane confined in nano-porous carbon aerogel with the average pore size 48 Å and porosity ∼60% was investigated as a function of pressure at T = 298 K using quasi-elastic neutron scattering (QENS). The diffusivity of methane shows a clear effect of confinement: it is about two orders of magnitude lower than in bulk at the same thermodynamic conditions and is close to the diffusivity of liquid methane at 100 K (i.e. ∼90 K below the liquid–gas critical temperature TC ≈ 191 K). The diffusion coefficient (D) of methane initially increases with pressure by a factor of ∼2.5 from 3.47 ± 0.41 × 10−10 m2 s−1 at 0.482 MPa to D = 8.55 ± 0.33 × 10−10 m2 s−1 at 2.75 MPa and starts to decrease at higher pressures. An explanation of the observed non-monotonic behavior of the diffusivity in the confined fluid is based on the results of small-angle neutron scattering experiments of the phase behavior of methane in a similar carbon aerogel sample. The initial increase of the diffusion coefficient with pressure is explained as due to progressive filling of bigger pores in which molecular mobility in the internal pore volume is less affected by the sluggish liquid-like molecular mobility in the adsorbed phase. Subsequent decrease of D, is associated with the effect of intermolecular collisions, which result in a lower total molecular mobility with pressure, as in the bulk state. The results are compared with the available QENS data on the methane diffusivity in zeolites, metal organic frameworks, and porous silica as well as with the molecular dynamics simulations of methane in nano-porous carbons and silica zeolites.
    Microporous and Mesoporous Materials. 01/2010;
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    ABSTRACT: A new spectrometer named BASIS has recently entered the general user program at the Spallation Neutron Source. BASIS is an acronym for Backscattering Silicon Spectrometer. While there are several operational reactor-based spectrometers that utilize backscattering reflection from silicon single crystals, such as IN10 and IN16 [1] at the ILL, France; HFBS [2] at the NCNR, USA; and SPHERES [3] at the FRM-II, JCNS, Germany, BASIS is the first silicon backscattering spectrometer built on a spallation neutron source. Conceptually, it is similar to previously built time-of-flight backscattering spectrometers that utilize reflections from pyrolytic graphite or mica, such as IRIS [4] and OSIRIS [5] at the ISIS, UK; LAM-80 [6] at the KENS, Japan; or MARS [7] at the SINQ, Switzerland.
    Neutron News 07/2008; 19(3):22-24.