Quantitative rate constants for radical reactions in the nanopores of cotton

Article abstract of DOI:10.1021/ja0201808

The understanding of radical reactions in nanostructured materials is important for developing new synthetic procedures and controlling degradation reactions. To develop this area, an easy method for measuring quantitative rate constants of some radical r

Full text of DOI:10.1021/ja0201808

Published on Web 06/27/2002
Quantitative Rate Constants for Radical Reactions in the Nanopores of
Cotton
†
†
†
,‡
Paula Hunt, David R. Worrall, Frank Wilkinson, and Stephen N. Batchelor*
Department of Chemistry, UniVersity of Technology Loughborough, Leicestershire LE11 3TU, United Kingdom, and
UnileVer Research Port Sunlight, Quarry Road East, Bebington, Wirral CH63 3JW, United Kingdom
Received February 4, 2002
Porous materials are crystalline or amorphous solids that allow
1
the reversible passage of molecules through their structure. Well-
known examples are zeolites with their regular networks of
The carbonyl radicals do not react with the dye on the current
nanochannels and cages, carbon nanotubes, and fibers such as
8
2
time scale, and in the absence of ketone the dyes are photostable.
Figure 1 shows the change in optical absorption of a solution of
the dye reactive red 3 and the change in reflectance of wet and dry
cotton, dyed with reactive red 3, following creation of 2-hydroxy-
-propyl radicals by laser flash photolysis, reaction 1. In all three
cases, a microsecond concentration-dependent bleaching of the dye
is observed.
cotton. In cotton, the porosity arises from the amorphous regions
which are well-defined slit pores of 1-3 nm situated between
3
crystallites of polysaccharide. For the technical use of all porous
materials, it is very useful to understand what controls chemical
reactions within the pores. Radical reactions are important in this
2
4
context, being widely used in synthesis and involved in the
5
degradation of many porous materials. In solution, radical chem-
In the solution experiment, an upper estimate of the initial radical
concentration may be calculated from the irradiated volume (0.7
mL), light absorbed by the ketone (<40%), quantum yield of
istry is well understood using a large set of quantitative rate
6
constants as a backbone. Unfortunately, in porous materials little
7
data exist, presumably because of measurement difficulties caused
1
0
-5
reaction 1 (0.3), and laser power (18 mJ/pulse) as <1.8 × 10
by the nature of the materials. Here a simple method is introduced
to obtain reliable rate constants of some radical reactions in porous
materials, using cotton as a model substrate.
-
1
mol L , which is smaller than the dye concentration of 5-30 ×
-5
-1
10
mol L . For R-hydroxy radicals, the rate constants for reaction
3
, kdye, and the competing second order reactions of radical self-
The best method to measure a rate constant is to measure and
6
6
and cross-termination are similar, and therefore reaction 3 will
dominate because of the high dye concentration.12 A simple kinetic
analysis then shows:
fit the kinetic behavior of one of the products/reactants. In liquids,
one of the easiest radical reactions to observe kinetically is the
8
bleaching of a dye, as the large color change provides a big signal
for transient optical absorbency measurements. This should also
be true in cotton; however, because of the opaque nature of the
substrate, signals must be observed via diffuse reflectance spec-
[
dye] ) [dye]t)0 - [(CH ) COH] exp(-kdye[dye]t) (4)
3 2 t)0
Hence, an exponential fit of the decay curve and then a plot of the
fitted rate constant kobs versus [dye] should give a straight line of
gradient kdye, as is observed, Figure 1.
If the radical reaction in cotton follows liquid-phase kinetics,
this approach should also yield kdye in cotton. Here the change in
reflectance rather than the absorbance is proportional to the change
9
troscopy. As a test case, the reactions of two R-hydroxy radicals
(2-hydroxy-2-propyl and 1-hydroxy-1-cyclohexyl) with azo dyes
in solution and in wet and dry nonmercerized cotton were
investigated.
The radicals were created by 355 or 260 nm laser flash photolysis
of the corresponding R-hydroxy ketones:
9
in dye concentration. Both reflectance and absorbance measure-
ments were made using a shuttered 300 W xenon arc lamp and a
9
fast photomultiplier attached to a digitizing oscilloscope. The
effective dye concentration in cotton is the dye concentration in
the amorphous regions, which is the pore space available to external
molecules.13 It is calculated from the dye loading in mol kg , the
-1
density (1.5 g/mL), and the accessible amorphous volume fraction
On excitation, both ketones intersystem cross from the initial
excited singlet state into a triplet state and then undergo R cleavage
3
(42%). Using this method, we obtained good fits to the data, Figure
1, supporting the liquid-phase assumption.
yielding R-hydroxy and carbonyl radicals, within a few nanosec-
onds.10 In cotton, the ketones were deposited into the amorphous
The measured rate constants are presented in Table 1. In
methanol, both radicals react with approximately the same kdye with
zone by soaking woven cotton fabric in a methanol solution of the
ketones for 30 s, removing, and then drying in air.11 This delivers
a known weight to the cotton.
reactive red 3, and the value for reactive orange 4/1-hydroxy-1-
14
cyclohexyl is one-half of this. Neta and Levanon measured a lower
7
-1
-1
value of 3 × 10 mol L s for 2-hydroxy-2-propyl radicals
bleaching azobenzene in propan-2-ol. However, the dyes used here
have electron-withdrawing subsistent sulfonate, hydroxy, and amine
groups which will aid reduction.15
In the presence of dyes, R-hydroxy radicals undergo a 1 electron
6
reduction with the dye, thereby giving the dye anion and bleaching,
that is
*
To whom correspondence should be addressed. E-mail: Stephen.Batchelor@
The values in dry cotton are smaller than those in wet cotton
unilever.com.
which are smaller than those in solution. This follows the respective
†
University of Technology Loughborough.
Unilever Research Port Sunlight.
‡
11,13
viscosities of the three environments of 30, ∼10, and 1 cP
and
8
532
9
J. AM. CHEM. SOC. 2002, 124, 8532-8533
10.1021/ja0201808 CCC: $22.00 © 2002 American Chemical Society

C O MMU N I C A T I O N S
liquid-phase kinetics in the nanopores of cotton. Previous spin probe
13
work showed that Heisenberg spin exchange of nitroxide radicals
and the development of electron spin polarization (ESP) in reactive
radical pairs11 also follow liquid-phase kinetics in cotton. Addition-
ally, work on the quenching of the excited triplet state of a dye by
7
-1
-1
19
oxygen gave a rate constant of 1 × 10 mol L s in wet cotton
which agrees with the current values.
The pore size in cotton is 1-3 nm, and it is surprising that even
for such narrow pores, surface effects play little or no role in the
chemical kinetics. However, a large cage effect can be inferred in
cotton, because twice as much dye is bleached in wet cotton than
in dry, Figure 1. The difference is too large to ascribe to the change
in viscosity, and control experiments showed it was not because
of changes in the amount of absorbed light. ESP experiments with
the ketone in reaction 1 showed that approximately 50% of the
radical pairs in dry cotton were caged11 and that the cages were
destroyed when the cotton was wet. Again, this shows a good match
between the two techniques.
Figure 1. Dye bleaching kinetics of reactive red 3 following laser flash
creation of 2-hydroxy-2-propyl radicals. The dye concentrations were
-
3
-1
methanol 0.12, 0.15, and 0.22 × 10 mol L ; wet cotton 0.5, 1.3, and
-
3
-1
-3
-1
1
.6 × 10 mol L ; dry cotton 0.2 and 1.6 × 10 mol L (cotton values
are for the amorphous region). The decay kinetics were fitted with single
exponentials, and the dependence of the observed rate constants on dye
concentration is also shown.
The simple method described here could be easily transferred
to other opaque porous media and could be extended to other
reactions using the dye bleaching as a competitive probe.
Table 1. Measured Rate Constants for Radical Dye Bleaching in
Methanol and Wet and Dry Cotton
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the calculated diffusion-limited rate constant (k
0
d
) 8RT/3η) of 0.22,
.66, and 6.6 × 10 mol L s . For wet and dry cotton, k is
lower than kdye measured in methanol, and therefore in cotton
9
-1
-1
(12) Radical addition to oxygen provides another major reaction path. In
solution, it was removed by bubbling with nitrogen; however, for technical
reasons this was not possible in cotton. This additional first order reaction
affects the intercept but not the gradient of the plot.
d
16
reaction should be diffusion-controlled. The dyes are reactively
bound to the pore walls in cotton17 and cannot diffuse, reducing
the diffusion-limited rate constant for reaction 3 by a factor of 2.
A further statistical drop of 2 should be included as only one side
(
13) (a) Batchelor, S. N. J. Phys. Chem. B 1999, 103, 6700. (b) Scheuermann,
R.; Roduner, E.; Batchelor, S. N. J. Phys. Chem. B 2001, 105, 11474.
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(
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1
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18
of the dye is available for reaction. Hence, rough estimates for
(16) (a) Von Smoluchowski, M. Z. Phys. Chem. 1917, 92, 129. (b) Noyes, R.
M. Prog. React. Kinet. 1961, 1, 129.
(
8
7
-1
k
s
dye in wet and dry cotton are 1.6 × 10 and 5.5 × 10 mol
L
17) The dye is attached to a polysaccharide chain via the triazinyl group, see:
Colourants and Auxiliaries; Shore, J., Ed.; Soc. of Dyers and Colourists:
Manchester, 1990; Vol. 1.
-
1
, respectively, and should be independent of the dye and the
radical. In agreement, the experimental kdye is smaller in dry than
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with the absolute values, Table 1. Thus, reaction 3 follows simple
(
18) Fischer, H.; Paul, H. Acc. Chem. Res. 1987, 20, 200.
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