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Browsing by Author "Topgaard, D."

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  • Homogeneous length scale of shear-induced multilamellar vesicles studied by diffusion NMR
    Publication . Aslund, I.; Medronho, B.; Topgaard, D.; Soderman, O.; Schmidt, C.
    A recently developed protocol for pulsed gradient spin echo (POSE) NMR is applied for the size determination of multilamellar vesicles (MLVs). By monitoring the self-diffusion behavior of water, the technique yields an estimate of the homogeneous length scale lambda(hom), i.e. the maximum length scale at which there is local structural heterogeneity in a globally homogeneous material. A cross-over between local non-Gaussian to global Gaussian diffusion is observed by varying the experimentally defined length- and time-scales. Occasional observation of a weak Bragg peak in the PGSE signal attenuation curves permits the direct estimation of the MLV radius in favorable cases, thus yielding the constant of proportionality between lambda(hom) and radius. The microstructural origin of the Bragg peak is verified through Brownian dynamics simulations and a theoretical analysis based on the center-of-mass diffusion propagator. lambda(hom) is decreasing with increasing shear rate in agreement with theoretical expectations and results from (2)H NMR lineshape analysis. (C) 2011 Elsevier Inc. All rights reserved.
    2011Journal article Restricted access Show more
  • Segmental order parameters in a nonionic surfactant lamellar phase studied with H-1-C-13 solid-state NMR
    Publication . Ferreira, T. M.; Medronho, B.; Martin, R. W.; Topgaard, D.
    A lyotropic nonionic lamellar system composed of pentaethyleneglycol mono n-dodecyl ether and D2O was studied using natural abundance C-13 NMR under magic-angle spinning. Applying a two-dimensional recoupling method proposed by Dvinskikh (R-PDLF), H-1-C-13 dipolar couplings were estimated over a range of temperatures (300-335 K), thus enabling analysis of structural changes in the liquid crystalline system. The results obtained are used to correlate the conformation and mobility of local sites in the surfactant molecule with overall changes in the lamellar structure.
    2008Journal article Open access Show more