Interfacial Electrostatic Properties of Hydrated Mesoporous and Nanostructured Alumina Powders by Spin Labeling EPR.
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-Al2O3and nanostructured Al2O3in 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 pKaof 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-Al2O3and Al2O3× nH2O were shifted by ΔpKa≈ +0.6 and up to ≈ +1.2 pH units respectively, the shift for γ-Al2O3was 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 Al2O3synthesis 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.
Institute of Chemical Engineering, Ural Federal University, 19 Mira St., Yekaterinburg, 620002, Russian Federation. gek1969@bk.ru.
Elena G
EG
Kovaleva
Department of Chemistry, Ural State Forest Engineering University, 37 Siberian Highway, Yekaterinburg, 620100, Russian Federation. lsmolochnikov@gmail.com.
Leonid S
LS
Molochnikov
Institute of Chemical Engineering, Ural Federal University, 19 Mira St., Yekaterinburg, 620002, Russian Federation.
Darya P
DP
Stepanova
Laboratory of Organic Materials, I.Ya. Postovsky Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences, Akademicheskaya / S. Kovalevskoi, 22/20, Ekaterinburg, 620990, Russian Federation.
Alexander V
AV
Pestov
Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences, Akad. Lavrent'ev Av. 9, Novosibirsk, 630090, Russian Federation.
Dmitrii G
DG
Trofimov
Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences, Akad. Lavrent'ev Av. 9, Novosibirsk, 630090, Russian Federation.|Novosibirsk State University, Pirogova Str. 2, Novosibirsk, 630090, Russian Federation.
Igor A
IA
Kirilyuk
Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, NC, 27695-8204, USA. Alex_Smirnov@ncsu.edu.
Alex I
AI
Smirnov
D000537:Aluminum Oxide / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D006863:Hydrogen-Ion Concentration; D015335:Molecular Probes / Q000138:chemical synthesis*; D049329:Nanostructures / Q000737:chemistry / Q000648:ultrastructure; D009589:Nitrogen Oxides / Q000138:chemical synthesis*; D010743:Phospholipids / Q000737:chemistry; D016062:Porosity; D011208:Powders; D013113:Spin Labels / Q000138:chemical synthesis*; D055672:Static Electricity; D013499:Surface Properties
D015335:Molecular Probes; D009589:Nitrogen Oxides; D010743:Phospholipids; D011208:Powders; D013113:Spin Labels; C039900:nitroxyl; D000537:Aluminum Oxide
Aluminum alkoxides; Mesoporous alumina; Phospholipids; pH-sensitive nitroxides