Scopus İndeksli Yayınlar Koleksiyonu / Scopus Indexed Publications Collection
Permanent URI for this collectionhttps://hdl.handle.net/11147/7148
Browse
5 results
Search Results
Article Citation - WoS: 2Citation - Scopus: 5Experimental Modeling of Antimony Sulfides-Rich Geothermal Deposits and Their Solubility in the Presence of Polymeric Antiscalants(Elsevier, 2022) Karaburun, Emre; Sözen, Yiğit; Çiftçi, Celal; Şahin, Hasan; Baba, Alper; Akbey, Ümit; Yeşilnacar, Mehmet İrfan; Erdim, Eray; Regenspurg, Simona; Demir, Mustafa MuammerAntimony (Sb)-rich geothermal deposits have been observed in many geothermal power plants worldwide. They occur as red-colored, sulfidic precipitates disturbing energy-harvesting by clogging the geothermal installations. In order to prevent the formation of this scale, information on its physicochemical features is needed. For this purpose, Sb-rich sulfide-based deposits were synthesized at controlled conditions in a pressurized glass reactor at geothermal conditions (135 °C and 3.5 bar). Various polymeric antiscalants with different functional groups, such as acrylic acid, sulphonic acid, and phosphonic acid groups were tested for their effect on Sb sulfide solubility. An additional computational study was performed to determine the binding energy of Sb and S atoms to these groups. The results suggest that sulfonic acid groups are the most affective. Therefore, it was concluded that these macromolecule containing sulfonic acid groups and poly (vinyl sulfonic acid) derivatives could potentially act as antiscalants for the formation of antimony sulfide.Article Citation - WoS: 56Citation - Scopus: 57Dynamic Nuclear Polarization of Spherical Nanoparticles(Royal Society of Chemistry, 2013) Akbey, Ümit; Altın, Burcu; Linden, Arne; Özçelik, Serdar; Gradzielski, Michael; Oschkinat, HartmutSpherical silica nanoparticles of various particle sizes (∼10 to 100 nm), produced by a modified Stoeber method employing amino acids as catalysts, are investigated using Dynamic Nuclear Polarization (DNP) enhanced Nuclear Magnetic Resonance (NMR) spectroscopy. This study includes ultra-sensitive detection of surface-bound amino acids and their supramolecular organization in trace amounts, exploiting the increase in NMR sensitivity of up to three orders of magnitude via DNP. Moreover, the nature of the silicon nuclei on the surface and the bulk silicon nuclei in the core (sub-surface) is characterized at atomic resolution. Thereby, we obtain unique insights into the surface chemistry of these nanoparticles, which might result in improving their rational design as required for promising applications, e.g. as catalysts or imaging contrast agents. The non-covalent binding of amino acids to surfaces was determined which shows that the amino acids not just function as catalysts but become incorporated into the nanoparticles during the formation process. As a result only three distinct Q-types of silica signals were observed from surface and core regions. We observed dramatic changes of DNP enhancements as a function of particle size, and very small particles (which suit in vivo applications better) were hyperpolarized with the best efficiency. Nearly one order of magnitude larger DNP enhancement was observed for nanoparticles with 13 nm size compared to particles with 100 nm size. We determined an approximate DNP penetration-depth (∼4.2 or ∼5.7 nm) for the polarization transfer from electrons to the nuclei of the spherical nanoparticles. Faster DNP polarization buildup was observed for larger nanoparticles. Efficient hyperpolarization of such nanoparticles, as achieved in this work, can be utilized in applications such as magnetic resonance imaging (MRI).Article Citation - WoS: 195Optical Properties of Composites of Pmma and Surface-Modified Zincite Nanoparticles(American Chemical Society, 2007) Demir, Mustafa Muammer; Koynov, Kaloian; Akbey, Ümit; Bubeck, Christoph; Park, Insun; Lieberwirth, Ingo; Wegner, GerhardLocate full-text(opens in a new window)|Full Text(opens in a new window)|View at Publisher| Export | Download | Add to List | More... Macromolecules Volume 40, Issue 4, 20 February 2007, Pages 1089-1100 Optical properties of composites of PMMA and surface-modified zincite nanoparticles (Article) Demir, M.M.a, Koynov, K.a, Akbey, Ü.a, Bubeck, C.a, Park, I.ab, Lieberwirth, I.a, Wegner, G.a a Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany b Seoul National University, School of Chemistry, Korea, South Korea View references (65) Abstract Composites that show visible light transmittance, UV absorption, and moderately high refractive index, based on poly(methyl methacrylate) (PMMA) and zinc oxide (zincite, ZnO) nanoparticles, were prepared in two steps. First, surface-modified ZnO nanoparticles with 22 nm average diameter were nucleated by controlled precipitation via acid-catalyzed esterification of zinc acetate dihydrate with pentan-1-ol. The surface of growing crystalline particles was modified with tert-butylphosphonic acid (tBuPO3H2) in situ by monolayer coverage. Particle size and graft density of -PO3H 2 on the particle surface were controlled by the amount of surfactant applied to the reaction solution. Second, the surface-modified particles were incorporated into PMMA by in-situ bulk polymerization. Free radical polymerization was carried out in the presence of these particles using AIBN as initiator. Volume fraction (0) of the particles was varied from 0.10 to 7.76% (0.5 to 30 wt %). Although the particles are homogeneously dispersed in monomer, segregation of the individual particles upon polymerization was observed. Optical constants of the films ca. 2.0 μm including absorption and scattering efficiencies, indices of refraction, and dispersion constants were determined. The absorption coefficient at 350 nm increases linearly with ZnO, obeying Beer's law at low particle contents. However, it levels off toward a value of about 5000 cm-1 and shows a negative deviation at high concentrations because of aggregation of the individual particles. Waveguide propagation loss coefficients of the composite films were examined by prism coupling. A steep increase of the loss coefficient was found with a slope of 52 dB cm-1 vol %-1 as the volume fraction of the particle increases. The refractive index of the composites depends linearly on volume fraction of ZnO and varies from 1.487 to 1.507 (φ = 7.76%) at 633 nm. The dispersion of refractive index was found to be consistent with Cauchy's formula.Article Citation - WoS: 69In-Situ Bulk Polymerization of Dilute Particle/Mma Dispersions(American Chemical Society, 2007) Demir, Mustafa Muammer; Castignolles, Patrice; Akbey, Ümit; Wegner, GerhardComposites of poly(methyl methacrylate) and various nanoscale inorganic particles (zinc oxide, titanium dioxide, zirconium dioxide, silicon dioxide, and aluminum nitride) were prepared by in-situ bulk polymerization using 2,2′-azobis(isobutyronitrile) as initiator. The particles of ZnO, TiO 2, and ZrO2 were surface-modified by alkylphosphonic acids to render them dispersible in the monomer. The effect of these nanoparticles on the free radical polymerization was investigated. Regardless of chemical nature and size, the particles suppress the autoacceleration which would otherwise occur in the bulk free-radical polymerization of methyl methacrylate (MMA). A degenerative chain transfer is proposed to take place between surface-adsorbed water on the particles and propagating chain radicals. This reaction competes with normal termination. Formation of vinylidene chains ends originating from disproportionation is suppressed. In consequence, thermal stability of PMMA produced in the presence of particles is improved. Aggregation of individual particles upon polymerization has been observed and presumably is due to interparticle depletion attraction, even though the particles are individually dispersed in the monomer. Formation of particle clusters is suppressed when a difunctional monomer (e.g., ethylene glycol dimethacrylate) is used as comonomer. The cross-linked medium slows down the diffusion of the particles and therefore interferes with particle aggregation via a depletion mechanism.Article Citation - WoS: 36Citation - Scopus: 36Surfactant-Assisted Formation of Organophilic Ceo2 Nanoparticles(Elsevier Ltd., 2012) Tunusoğlu, Özge; Muñoz-Espí, Rafael; Akbey, Ümit; Demir, Mustafa MuammerWe report a simple one-pot method to prepare organically functionalized CeO 2 nanoparticles by controlled chemical precipitation. The particles were nucleated by mixing aqueous solutions of Ce(NO 3) 3·6H 2O and ammonia at room temperature. Different small organic molecules were chosen as capping agents and injected into the reaction medium at the beginning of the synthesis: 3-(mercaptopropyl) trimethoxy silane (MPS), hexadecyltrimethyl ammonium bromide (CTAB), 3-mercapto propionic acid (3-MPA), and thioglycolic acid (TGA). The resulting nanocrystals were quasi-spherical and had a narrow mean size distribution with an average size smaller than 10nm. Dynamic nuclear polarization enhanced NMR (DNP-NMR) and FTIR measurements suggested a chemical grafting of the surfactant and a homogeneous surface modification. The colloidal stabilities were characterized by dynamic light scattering and zeta potential measurements. The stabilization by aliphatic groups was tested with a frequently used hydrophobic monomer, methyl methacrylate. According to the results, CTAB is the most effective of the used stabilizing surfactant. The mechanism of formation of the organophilic CeO 2 nanoparticles is discussed.