Biomedical subjects

Valeri Petkov

Publications and source records attributed to Valeri Petkov.

4 recordsLinked to original sources

PubMed2005

Three-dimensional structure of nanocomposites from atomic pair distribution function analysis: study of polyaniline and (polyaniline)(0.5)V(2)O(5) x 1.0 H(2)O.

The three-dimensional structures of emeraldine base polyaniline (PANI) and (polyaniline)(0.5)V(2)O(5) x 1.0 H(2)O have been determined by total X-ray scattering experiments. Atomic pair distribution functions (PDF) were measured to obtain experimental observables against which structural models were tested and refined. The PDF approach is necessary because of the limited structural coherence in these nanostructured materials. Polyaniline possesses a well-defined local atomic arrangement that can be described in terms of an 84-atom orthorhombic unit cell. The nanocomposite (PANI)(0.5)V(2)O(5) x 1.0 H(2)O too is locally well ordered and may be described in terms of a small number of structure-sensible parameters. The PDF approach allows the construction of structure models of PANI and (PANI)(0.5)V(2)O(5) x 1.0 H(2)O on the basis of which important materials' properties can be explained predicted and possibly improved.

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PubMed2005

Mercury binding sites in thiol-functionalized mesostructured silica.

Thiol-functionalized mesostructured silica with anhydrous compositions of (SiO(2))(1)(-)(x)()(LSiO(1.5))(x)(), where L is a mercaptopropyl group and x is the fraction of functionalized framework silicon centers, are effective trapping agents for the removal of mercuric(II) ions from water. In the present work, we investigate the mercury-binding mechanism for representative thiol-functionalized mesostructures by atomic pair distribution function (PDF) analysis of synchrotron X-ray powder diffraction data and by Raman spectroscopy. The mesostructures with wormhole framework structures and compositions corresponding to x = 0.30 and 0.50 were prepared by direct assembly methods in the presence of a structure-directing amine porogen. PDF analyses of five mercury-loaded compositions with Hg/S ratios of 0.50-1.30 provided evidence for the bridging of thiolate sulfur atoms to two metal ion centers and the formation of chain structures on the pore surfaces. We find no evidence for Hg-O bonds and can rule out oxygen coordination of the mercury at greater than the 10% level. The relative intensities of the PDF peaks corresponding to Hg-S and Hg-Hg atomic pairs indicate that the mercury centers cluster on the functionalized surfaces by virtue of thiolate bridging, regardless of the overall mercury loading. However, the Raman results indicate that the complexation of mercury centers by thiolate depends on the mercury loading. At low mercury loadings (Hg/S < or = 0.5), the dominant species is an electrically neutral complex in which mercury most likely is tetrahedrally coordinated to bridging thiolate ligands, as in Hg(SBu(t))(2). At higher loadings (Hg/S 1.0-1.3), mercury complex cations predominate, as evidenced by the presence of charge-balancing anions (nitrate) on the surface. This cationic form of bound mercury is assigned a linear coordination to two bridging thiolate ligands.

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PubMed2002

Structure of V(2)O(5)*nH(2)O xerogel solved by the atomic pair distribution function technique.

A long-standing issue regarding the local and long-range structure of V(2)O(5)*nH(2)O xerogel has been successfully addressed. The full three-dimensional structure of the lamellar turbostratic V(2)O(5)*nH(2)O xerogel was determined by the atomic pair distribution function technique. We show that on the atomic scale the slabs of the xerogel can be described well as almost perfect pairs (i.e., bilayers) of single V(2)O(5) layers made of square pyramidal VO(5) units. These slabs are separated by water molecules and stack along the z-axis of a monoclinic unit cell (space group C2/m) with parameters a = 11.722(3) A, b = 3.570(3) A, c = 11.520(3) A, and beta = 88.65 degrees. The stacking sequence shows signatures of turbostratic disorder and a structural coherence limited to 50 A.

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PubMed2002

Role of framework sodium versus local framework structure in determining the hydrothermal stability of MCM-41 mesostructures.

Two mesostructured MCM-41 silicas that differ dramatically in hydrothermal stability have been examined by (29)Si MAS NMR spectroscopy and pair distribution function (PDF) analysis of synchrotron X-ray scattering data. The less stable mesostructure assembled from sodium silicate and the substantially more stable derivative made from fumed silica possess equivalent local framework wall structures, as judged by NMR and PDF methods. Approximately 80% of the SiO(4) tetrahedra are fully cross-linked as Q(4) (Si(OSi)(4)) units in both calcined samples. Additionally, the structural correlation distances for the two materials are nearly identical, having values of 1.62(1), approximately 2.60, and 3.09(1) A for the Si-O, O-O, and nearest neighbor Si-Si distances in the framework. Sodium ions in the framework play a crucial role in limiting the hydrothermal stability of the mesostructure. Residual sodium ( approximately 0.05-0.10% Na(2)O) is retained in the MCM-41 made from sodium silicate, even after two ion exchange reactions with ammonium ions in more than 300 - fold excess. The entrapped framework sodium ions catalyze the collapse of the mesopores upon exposure to 20% steam at 800 degrees C for 5 h. The sodium - free mesostructure assembled from fumed silica retains an open framework under the same hydrothermal conditions. The stability of the fumed silica derivative, however, is greatly compromised when doped with as little as 0.10% Na(2)O, thus confirming the deleterious effect of sodium on hydrothermal stability.

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