Cell BioChem Biophys
DOI 10.1007/s12013-016-0767-0
ORIGINAL PAPER
Interfacial Electrostatic Properties of Hydrated Mesoporous and
Nanostructured Alumina Powders by Spin Labeling EPR
Elena G. Kovaleva
1
●
Leonid S. Molochnikov
2
●
Darya P. Stepanova
1
●
Alexander V. Pestov
3
●
Dmitrii G. Trofimov
4
●
Igor A. Kirilyuk
4,5
●
Alex I. Smirnov
6
Received: 1 May 2016 / Accepted: 14 October 2016
© Springer Science+Business Media New York 2016
Abstract Acid-base equilibria and interfacial electrostatic
properties of hydrated mesoporous and nanostructured alu-
mina 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 equili-
bria at the pore surface of mesoporous powders of α-Al
2
O
3
,
γ-Al
2
O
3
, Al
2
O
3
× nH
2
O, and basic γ-Al
2
O
3
and nanos-
tructured Al
2
O
3
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-(chlor-
omethyl)phenyl)-2-ethyl-2,5-dihydro-1H-imidazol-1-oxyl
hydrochloride semihydrate (nitroxide R1), has been syn-
thesized and characterized. It was found that conditions of
preparation of alumina powders exert strikingly large effects
on the apparent pK
a
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-
Al
2
O
3
and Al
2
O
3
× nH
2
O were shifted by ΔpK
a
≈ +0.6 and
up to ≈ +1.2 pH units respectively, the shift for γ-Al
2
O
3
was
found to be much higher: ΔpK
a
= +3.5. Assuming
approximately the same ΔpH = 0.5–1.0 arising from a dif-
ference 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-Al
2
O
3
> Al
2
O
3
× nH
2
O > basic γ-Al
2
O
3
> α-
Al
2
O
3
. Conditions of the Al
2
O
3
synthesis as well as the
surface modification procedures were found to have pro-
found 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.
Keywords Mesoporous alumina
●
Phospholipids
●
Aluminum alkoxides
●
pH-sensitive nitroxides
Introduction
Alumina, or Al(III) oxide, in either powdered or mesopor-
ous forms has found many industrial uses as an adsorbent or
* Elena G. Kovaleva
gek1969@bk.ru
* Leonid S. Molochnikov
lsmolochnikov@gmail.com
* Alex I. Smirnov
Alex_Smirnov@ncsu.edu
1
Institute of Chemical Engineering, Ural Federal University, 19
Mira St., Yekaterinburg 620002, Russian Federation
2
Department of Chemistry, Ural State Forest Engineering
University, 37 Siberian Highway, Yekaterinburg 620100, Russian
Federation
3
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
4
Institute of Organic Chemistry, Siberian Branch of the Russian
Academy of Sciences, Akad. Lavrent’ev Av. 9, Novosibirsk
630090, Russian Federation
5
Novosibirsk State University, Pirogova Str. 2, Novosibirsk
630090, Russian Federation
6
Department of Chemistry, North Carolina State University, 2620
Yarbrough Drive, Raleigh, NC 27695-8204, USA