Selective adsorption of nitro-substituted aromatics and accelerated hydrolysis of 4-nitrophenyl acetate on carbon surfaces

Article abstract of DOI:10.1039/b101963p

Nitrobenzene, 4-nitrotoluene and 4-nitrophenyl acetate are selectively adsorbed from aqueous 1% sodium dodecyl sulfate solutions onto aligned, randomly oriented and oxidised carbon nanotubes, as well as graphite and C₆₀ fullerene. The rate of h

Full text of DOI:10.1039/b101963p

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Selective adsorption of nitro-substituted aromatics and accelerated
hydrolysis of 4-nitrophenyl acetate on carbon surfaces
Adam G. Meyer,*a,b Liming Dai,a Qidao Chen,a Christopher J. Eastonb and Ling Xiab
a CSIRO Molecular Science, Bag 10, Clayton South, V IC 3169, Australia.
E-mail: Adam.Meyer=molsci.csiro.au
L e t t e r
b Research School of Chemistry, T he Australian National University,
Canberra, ACT 0200, Australia
Received (in Montpellier, France) 1st March 2001, Accepted 23rd April 2001
First published as an Advance Article on the web 1st June 2001
Nitrobenzene, 4-nitrotoluene and 4-nitrophenyl acetate are
selectively adsorbed from aqueous 1% sodium dodecyl sulfate
solutions onto aligned, randomly oriented and oxidised carbon
were exposed to the aforementioned other graphitised carbon
forms. A small decrease in the absorption (ca. 0.1 units) at j
max
was also seen for a solution of 2-nitrotoluene exposed to each
nanotubes, as well as graphite and C fullerene. The rate of
carbon form over a period of 14 days. The decrease in absorp-
tion over time is indicative of a reduction in the concentration
of the aromatic compound in solution and, since no chemical
reaction was detected, presumably adsorption onto the carbon
surface had occurred. This was supported by X-ray photoelec-
tron spectroscopy (XPS) measurements, which revealed that
aligned carbon nanotube samples exposed to solutions of
nitrobenzene or 4-nitrotoluene showed a characteristic peak
in the nitrogen region of the spectrum, attributable to the
presence of nitro groups on the surface. The quantity
adsorbed onto the surface area (1 cm2) of these nanotube
samples was calculated to be 0.02 mg of nitrobenzene or 4-
nitrotoluene over 7 days.14 Where possible, solutions of the
60
hydrolysis of 4-nitrophenyl acetate to 4-nitrophenolate is
modestly accelerated through this process. Selective adsorption
requires the presence of the surfactant and the hydrolysis of
pNPA is accelerated only by a carbon surface that is graphitic
in nature.
Graphite1 is a crystalline form of carbon that consists of
planar layers of atoms, known to adsorb surfactants2 and
other organic compounds3 from aqueous solutions. In the last
decade, two new forms of carbon, the fullerenes and carbon
nanotubes, have been discovered.4 Carbon nanotubes consist
of graphite planes forming the walls of nanometre diameter
cylinders and have attracted much attention because of their
potential applications.5 Although they are usually prepared in
a random and entangled manner, ourselves6 and others7 have
recently prepared them aligned orthogonally to a surface.
Only a limited number of studies of the adsorption character-
istics of fullerenes8 and carbon nanotubes9 have been reported
and, to our knowledge, no report on the adsorption of organic
compounds from aqueous solutions onto carbon nanotubes
has appeared. In the present study, therefore, the adsorption
of various organic compounds from their aqueous surfactant
solutions onto a wide range of graphitised carbon surfaces
was investigated.
Since it has been reported that nitrobenzene and its deriv-
atives are adsorbed from aqueous solution onto graphite,10
we chose to investigate the adsorption characteristics of pre-
dominantly nitro-substituted aromatic compounds (these are
listed in ref. 11). This class of compound also absorbs strongly
in the UV region, which facilitates monitoring of the solutions
by UV-visible spectroscopy. Each compound was solubilised
into a dilute anionic surfactant solution containing 1% (35
mM) sodium dodecyl sulfate (SDS) in water. The surfactant
was used to permit wetting of the carbon surfaces and to solu-
bilise the organic compounds. The solutions were then
exposed to aligned,6 randomly oriented12 and oxidised carbon
nanotubes13 as well as graphite powder and C fullerene. Of
60
the systems investigated,11 only the solutions of nitrobenzene,
4-nitrotoluene and 4-nitrophenyl acetate (pNPA) showed sub-
stantial changes in absorption intensity over time. In each
case, a decrease in the absorption at the absorbance maximum
(j ) was seen. The spectra of solutions of nitrobenzene and
Fig. 1 Spectra of (a) 1.62 ] 10~4
M
nitrobenzene and (b)
max
4-nitrotoluene exposed to aligned carbon nanotubes are
1.46 ] 10~4 M 4-nitrotoluene aq. 1% SDS solutions (5 ml) exposed to
aligned carbon nanotubes (sample area ca. 1 cm2) at 298 K. Spectra
recorded after (A) 0, (B) 3, (C) 5 and (D) 7 days.
shown in Fig. 1. Analogous spectral changes were also
observed when solutions of nitrobenzene and 4-nitrotoluene
DOI: 10.1039/b101963p
New J. Chem., 2001, 25, 887È889
887
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nitro-substituted aromatic compounds listed in ref. 11 (and
also nitrobenzene, 4-nitrotoluene and pNPA) were prepared in
the absence of SDS and exposed to the carbon surfaces. In all
adsorption process manifests itself as an inexact isosbestic
point at ca. 305 nm, which is in contrast to the precise isos-
bestic point observed for spectra from the control experiment
[Fig. 2(b)]. The control experiment shows that accelerated
hydrolysis of pNPA is not due to simple micellar catalysis,
there is a possibility that the hemimicelles on the carbon sur-
faces may have di†erent properties that induce the catalysis.
these cases, a decrease in the absorption at j was observed.
max
The selective adsorption from the surfactant solutions is
presumably attributable to SDS, which forms micelles in an
aqueous solution above the critical micelle concentration of
8.1 mM.15 This surfactant also aggregates at solidÈsolution
interfaces and is reported to assemble on the surface of graph-
ite in an organised periodic structure consisting of long paral-
lel Ñattened tubes (termed “hemimicellesÏ).16 Since it is known
that SDS complexes pNPA speciÐcally at the micelleÈsolution
interface,17 it can be postulated that pNPA complexed with a
hemimicelle is in a favourable environment to be adsorbed
because of its close proximity to the graphitised carbon
surface. Nitrobenzene and 4-nitrotoluene may behave and be
adsorbed in a similar fashion. The compounds listed in ref. 11
are perhaps complexed with SDS in a di†erent manner (for
example, sequestered within the hydrophobic core of a
micelle/hemimicelle) and are thus unable to be adsorbed.
We were intrigued to discover that pNPA not only
adsorbed onto the carbon surfaces, but this also accelerated
its rate of hydrolysis. The changes in the UV-vis spectrum in
the presence of aligned carbon nanotubes are shown in Fig.
Experimental
Aligned carbon nanotubes were prepared on quartz plates
according to the method described previously.6 Oxidised
nanotubes were obtained by treatment of aligned nanotubes,
scraped from the quartz surface, with boiling nitric acid.13
Optically transparent solutions of oxidised nanotubes were
prepared by their dispersion in an aqueous 1% SDS solution
(1 mg ml~1). Randomly oriented carbon nanotubes (“bucky-
tubesÏ), graphite powder (1È2 micron in diameter) and activat-
ed carbon were purchased from Aldrich Chemical Co. C
60
fullerene was supplied by Yin Han Hi-Tech C Co., Wuhan
60
University, China. SDS was purchased from Merck Chemical
Co. All of the chemicals from commercial sources were used
as received. De-ionised water was distilled and was of pH 6.0.
UV-vis spectra were recorded on a Hewlett Packard HP 8453
spectrophotometer. XPS analyses were performed on a Kratos
Analytical spectrometer using monochromatic Al-Ka radi-
ation at a power of 200 W. Compounds investigated were dis-
solved in an aqueous 1% SDS solution (2 mg per 100 ml) by
sonicating for 1 h. In a typical experiment, a solution of the
compound (5 ml) was added to a vial containing the carbon
2(a). With time, the absorbance of pNPA, (j \ 272 nm)
max
decreased and that of its hydrolysis product, 4-nitrophenolate
(j \ 401 nm), increased. Hydrolysis was also observed in
max
the absence of a carbon surface, but the rate was much slower
[Fig. 2(b)]. Moreover, only a graphitised carbon form such as
that used in this study accelerated the rate of hydrolysis. With
amorphous activated carbon, only adsorption of pNPA was
observed. An insight into the mechanism of hydrolysis in the
presence of graphitised carbon is provided by the observation
sample (5 mg for buckytube, graphite or C
fullerene
60
powders) and then sealed under an atmosphere of nitrogen.
An aliquot (3 ml) was removed and added to a quartz cell (1
cm pathlength), and the UV-vis spectrum was recorded every
24 h for a period of 7 days unless otherwise stated. The hydro-
lysis of a solution (5 ml) of pNPA (5 ] 10~5 M) containing a
carbon sample was monitored spectrophotometrically and the
increase in absorbance at 401 nm, due to 4-nitrophenolate lib-
eration, was recorded each day for a period of 21 days (i.e.,
monitored through to at least 90% completion).
that the absorption of 4-nitrophenolate [j \ 401 nm, Fig.
max
2(a)] increased to a greater extent between 7 and 14 days than
between 0 and 7 days. The hydrolysis is therefore not a
pseudo-Ðrst order process and can be assumed to be initiated
by slow adsorption onto the carbon nanotube surface. This
Acknowledgements
We are grateful to the Research School of Chemistry and
CSIRO Molecular Science for Ðnancial support, and to the
Australian Research Council for provision of a Postdoctoral
Research Fellowship (to A. G. M.).
Notes and references
1
2
3
D. B. Fischback, in Chemistry and Physics of Carbon, ed. P. L.
Walker, Marcel Dekker, Inc., New York, 1971, vol. 7.
J. H. Clint, Surfactant Aggregation, Chapman and Hall, New
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Z. Kiraly, I. Dekany, E. Klumpp, H. Lewandowski, H. D. Narres
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therein.
4
M. S. Dresselhaus, G. Dresselhaus and P. C. Eklund, Science of
Fullerenes and Carbon Nanotubes, Academic Press, London,
1996.
5
6
T. Ebbesen, Carbon Nanotubes, CRC Press, Boca Raton, FL,
USA, 1997.
(a) S. Huang, L. Dai and A. W. H. Mau, J. Phys. Chem. B, 1999,
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J. H. Wang, P. Bush, M. P. Siegel and P. N. Provencio, Science,
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(a) M. H. Abraham, C. M. Du, J. W. Grate, R. A. McGill and W.
J. Shuely, J. Chem. Soc., Chem. Commun., 1993, 1863; (b) V. Yu.
7
Fig. 2 UV-vis spectra showing hydrolysis of pNPA (5.0 ] 10~5 M)
in an aq. 1% SDS solution (5 ml) at 298 K (a) with and (b) without
exposure to aligned carbon nanotubes (sample area ca. 1 cm2) at 298
K. Spectra recorded after (A) 0, (B) 7, (C) 14 and (D) 21 days.
8
888
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Gusev, S. Ruetsch, L. A. Popeko and I. E. Popeko, J. Phys.
Chem. B, 1999, 103, 6498 and references cited therein.
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2-cyclohexen-1-one, 3-nitrobenzophenone, 1-nitronaphthalene, 2-
9
(a) S. A. Curran, P. M. Ajayan, W. J. Blau, D. L. Carroll, J. N.
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nitronaphthalene,
1-naphthol,
2-nitro-1-naphthol,
1-
naphthylacetic acid, phthalide, 2-nitroÑuorene.
12 Aldrich Chem. Co. “buckytubesÏ, powdered as-produced cylin-
ders.
13 S. C. Tsang, Y. K. Chen, P. J. F. Harris and M. L. H. Green,
Nature (L ondon), 1994, 372, 159.
14 BeerÈLambert law: A \ ecl. For nitrobenzene (j \ 267 nm) at
max
t \ 0: A \ 2.3729, c \ 1.62 ] 10~4 M, e \ 14 648 M~1 cm~1. At
t \ 7 days, A \ 1.9239, c \ 1.31 ] 10~4 M. Thus, 1.55 ] 10~7
10 For example, see: (a) J. S. Mattson, H. B. Mark, M. D. Malbin,
W. J. Weber and J. C. Crittenden, J. Colloid Interface Sci., 1969,
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11 No change in the UV-vis spectra over 7 days was seen for
aqueous 1% SDS solutions of: 1,2-dinitrobenzene, 1,3-dinitro-
benzene, 1,4-dinitrobenzene, 3-nitrotoluene, 3,4-dinitrotoluene,
mol or 0.02 mg was adsorbed from a 5 ml solution. For 4-
nitrotoluene
(j \ 283
nm)
at
t \ 0:
A \ 1.2865,
max
c \ 1.46 ] 10~4 M, e \ 8812 M~1 cm~1. At t \ 7 days,
A \ 1.0643, c \ 1.21 ] 10~4 M. Thus, 1.25 ] 10~7 mol or 0.02
mg was adsorbed from a 5 ml solution.
15 R. J. Williams, J. N. Phillips and K. J. Mysels, T rans. Faraday
Soc., 1955, 51, 728.
16 E. J. Wanless and W. A. Ducker, J. Phys. Chem., 1996, 100, 3207.
17 N. Pirinccioglu, F. Zaman and A. Williams, J. Org. Chem., 2000,
65, 2537.
2,4-dinitrotoluene,
1-bromo-4-nitrobenzene,
4-nitroaceto-
phenone, 4-nitrobenzaldehyde, methyl 4-nitrobenzoate, 4-nitro-
New J. Chem., 2001, 25, 887È889
889