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Interfacial Electrostatic Properties of Hydrated Mesoporous and Nanostructured Alumina Powders by Spin Labeling EPR

  • Kovaleva, E. G.
  • Molochnikov, L. S.
  • Stepanova, D. P.
  • Pestov, A. V.
  • Trofimov, D. G.
  • Kirilyuk, I. A.
  • Smirnov, A. I.
Publication Date
Jan 01, 2017
Electronic archive of the Ural State Forest Engineering University
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Acid-base equilibria and interfacial electrostatic properties of hydrated mesoporous and nanostructured alumina powders are determining factors for the use of these materials in heterogeneous catalysis and as a sorption media for filtration and chromatographic applications including life sciences. Here spin probe electron paramagnetic resonance spectroscopy of pH-sensitive nitroxides was employed to evaluate the surface charge and interfacial acid-base equilibria at the pore surface of mesoporous powders of α-Al2O3, γ-Al2O3, Al2O3 × nH2O, and basic γ-Al2O3 and nanostructured Al2O3 in the form of pristine materials and modified with aluminum-tri-sec-butoxide, hydroxyaluminum glycerate, and several phospholipids. A new pH-sensitive nitroxide probe, 4-dimethylamino-5,5-dimethyl-2-(4-(chloromethyl)phenyl)-2-ethyl-2,5-dihydro-1H-imidazol-1-oxyl hydrochloride semihydrate (nitroxide R1), has been synthesized and characterized. It was found that conditions of preparation of alumina powders exert strikingly large effects on the apparent pKa of nitroxides measured from electron paramagnetic resonance titration curves. Specifically, while the electron paramagnetic resonance titrations curves for the nitroxide R1 in mesoporous powders prepared from basic γ-Al2O3 and Al2O3 × nH2O were shifted by ΔpKa≈ +0.6 and up to ≈ +1.2 pH units respectively, the shift for γ-Al2O3 was found to be much higher: ΔpKa = +3.5. Assuming approximately the same ∆pH = 0.5–1.0 arising from a difference in the hydrogen ion activity between the bulk solution phase and that in a confined pore volume, the samples were ranked in the following order of descending magnitude of the effective surface electrostatic potential Ψ: mesoporous γ-Al2O3 > Al2O3 × nH2O > basic γ-Al2O3 > α-Al2O3. Conditions of the Al2O3 synthesis as well as the surface modification procedures were found to have profound effects on the interfacial electrostatic properties of hydrated samples that are likely related to the nature and concentration of the active sites on the alumina surfaces. © 2016, Springer Science+Business Media New York. / This work was supported by the Russian Foundation for Basic Research (Grant 14-03-00898), the Program 211 of the Government of the Russian Federation No. 02.A03.21.0006 and the State Tasks of the Ministry of Education (Russian Federation) No. 4.1626.2014/K and No. 2014/239. Fabrication and SEM characterization of AAO, least-squares fitting of EPR spectra and the final preparation of the manuscript were supported by U.S. DOE Contract DE-FG02-02ER15354 to AIS. The authors are thankful to Mrs. Ksenia Kozhikhova (Department of Technology for Organic Synthesis, Ural Federal University, Yekaterinburg, Russia) for preparation of positively-and negatively-charged liposomes. The help and useful discussions with Prof. Maxim A. Voynov and Dr. Antonin Marek (both from NCSU) are gratefully acknowledged.

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