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Langmuir
1992,8, 2455-2460 2455
Surfactant-Intercalated Clay Films Containing Metal
Phthalocyanines
James F. Rusling, Maryam
F.
Ahmadi, and Naifei Hut
Depar tment of Chemistry, Box U-60, niversity of Connecticut,
Storrs, Connecticut 06269-3060
Received Feb ruary
24, 1992.
n Final Form: Jun e
24, 1992
We previously showed tha t c lay-surfactant films containing metal phthalocyanines catalyze electro-
chemical reductive dechlorinations. Cobalt(I1) phthalocyanine (CoIIPc) was a much better catalyst than
the corresponding iron complex. Th is paper reports studies of these catalytic films by spectroscopic,
X-ray , and electron microscopic methods. Scan ning electron microscopy cross sectional images of films
of
didodecyldimethylmmonium
bromide, clay, and CoIIPc were considerably differen t from the stacked
layers observed for pure composites. Previously observed phase trans itions are ch aracteristic of surfac-
ta nt bilayers. Th e general morphology of these films appears to featu re a heterogeneous mixtu re of CoIIPc
crystals and surfactant bilayers intercalated between clay layers. Electronic spectra and X -ray diffraction
patterns suggest tha t iron phthalocy anine (FeIIPc)ispresen t in oxidized forms in these films. CoIIPc films
are better dechlorination catalysts partly because CoIIPc remains intact in the films, while FeIIPc is
decomposed.
Introduction
Orde red comp osites of clay and surfactants can be mad e
by reacting c ation exch anging clay colloids with insoluble
amp hiphilic cations. Films can be cast onto solid surfaces
from suspen sions of these co mposites in organic solvents.
Su rfac tan t films intercalate d between linear ionic poly-
me13293 hav e been prepare d by simila r metho ds. Gel-to -
liquid crystal phase transitions for surfac tan t composite
films occur at temperatures close to those of bilayer
suspensions of th e same surfactants. These results com-
bined with X-ray diffraction and electron microscopy
studies suggest tha t th e films are arranged in stacked sur-
factant bilayers intercalated between clay or polymer
layers.
1-3
Preparation and casting of composites
as
described
above are a convenient way to prepare ordered multiple
bilayer films of S urfactants with struc ture s and properties
related to biomembranes. Films with thicknesses in th e
micrometer range containing thousands of bilayers are
easily prepared. Perm eability is controlled by th e phase
of th e Neu tral, water soluble solutes pass through
the films in th e liquid crystal state bu t are blocked when
the films are brought to th e solidlike gel phase. In addition
to controlling permeability, other possible applications
include coatings for piezoelectric5 or a mpe rometric sen-
S O ~ S , ~ J
up por ts for ordering biological macromoleculesld
+
On leave from B eijing Normal University, Beijing, China.
1)
Okahata, Y.; Shimizu,
A.
Langmuir
1989,5,954-959.
2) (a) Shimomura,M.; Kunitake, T. Polym.
J. 984,16,187-190.
b)
Kunitake,
T.;
Tsuge,A.; Nakashima, N. Chem. Lett. 1984,1783-1786. (c)
Nakmhima, N.; Ku nitake, M ; Kunitake, T.; Tone, S.; Kajiyama, T.
Macromolecules
1986,18,1515-1516.
(d)Higashi, N.; Kajiyama,T .;Kuni-
take,
T.;
Praee,
W.;
Ringadorf, H.; Takahara, A. Macromolecules 1987,
20,29-33.
(e) Nakashima, N.; Eda, H.; Kunitake, M.; M anabe, 0 ;Na-
kano,I .J
Chem.
Soc., Chem.
Common. 1990,443-444.
3)
(a) Okahata,
Y.;
Enna,
G.;
Taguchi, K.; Seki,T.J Am. Chem. SOC.
1986,107,5300-5301.
(b) Okahata, Y.; Enna, G. J. hys. Chem.
1988,
92,4646-4551. c) Okahata, Y.; Enna, G.; Takenouchi, K.
J
Chem.
SOC.,
Perkin Tram. 2
1989,835-843.
4)
Fendler, J. H. Mem brane Mimetic Chemistry; Wiley: New York,
1982.
5)
Okahata,Y.; Ebato, H. Anal . Chem.
1991,63, 203-207.
6) Hu, N.; Rueling, J. F. Anal. Chem. 1991,63, 2163-2168.
7) Rusling, J. F.; Hu, N.; Zhang .; Howe, D.; Miaw, C.-L.; Couture,
E. In Electrochemistry
n
MicroheterogeneousFluids; Mackay, R.
A.,
Texter, J., Eds.; Plenum: New York, in press.
an d inorganic complexes,1c nd m embranes for controlling
vectorial electron transport.8
We recently evaluated clay-surfactant films containing
redox med iators for electrochemical catalysis. Com posite
films of didodecyl- an d dioctadecyldimethylammonium
bromide (DDAB and D ODAB ) an d clay cast on pyrolytic
graphite electrodes acted as barriers toward hydrophilic
multivalent ions6 bu t incorporated hydroph obic ions and
neutral m olecules from aqueous solutions. Clay-surfac-
tant films containing neutral metal phthalocyanines
catalyzed reductive dechlo rination of trichloroacetic acid?
Cobalt(I1) phthalocyanine was a much b ette r catalyst in
the composites than the corresponding iron complex.
Charge tran spo rt rates were excellent when the films were
in liquid crystal phases but poor in solidlike gel states.
Gel-to-liquid crystal phase transitions w ere detected by
voltammetry6 and differential scanning calorimetry.
Clay-su rfactant films containing me tal phthalocyanines
showed excellent stability in catalytic applications and
were usable for a mon th or more. In this paper, we report
results of several types of experiments to characterize
composite films containing metal phthalocyanines: (i)
square wave voltammetry to establish redox prop erties of
th e films; (ii) electronic ab sorption spectroscopy, which
provides insight into microenvironment, and oxidation
and
aggregation
states
of t he catalyst; (iii) scanning electron
microscopy
(SEM)
and energy dispersive X-ray (EDX )
analyses, which provide insight into film morphology,
structure, and catalyst distribution; (iv) X-ray powder
diffraction providing th e d-spacing of the composites.
Experimental Section
Chemicals and Solutions. Didodecyldimethylammonium
bromide (DDAB, 99+ ),
dioctadecyldimethylammonium
bro-
mide (DODAB, 99 +% ), and iron and cobalt phthalocyanine
(97+ were from Eastman Kodak. Cetyltrimethylammonium
bromide (CTAB, hexadecyltrimethylammonium bromide) was
Fisher certified grade, 99.8%. Solven ts were spectroscopic grade.
All othe r chemicals were reagent grade. Bentonite clay (Ben-
tolite H) as from Southern Clay Prod ucb and had a cation
exchange capacity of 80 meq uivI100 g.
Apparatus and Procedures.
A
Bioanalytical Systems BAS-
100electrochemistry system was used for Osteryoung-typesquare
(8)Gratzel,M.HeterogeneousPhotochemicalElectron ransfer;CRC
Press: Boca Raton,
FL,
1989.
0743-7463/92/2408-2455 03.00/0
1992
American Chemical Society
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Langmuir Vol
8,
No. 10
1992
wave voltammetry (SW V). Th e working electrode was a basal
plane pyrolyticgraphite (HPG-99,Un ionca rbide ) disk (geometric
A
=
0.2 cm2). Electrod es were pre par ed by sealing a disk in to
polypropylene pipet tips
as
describe d previouslf or by sealing
to glass tubes with heat shrinkable tubing. Pyrolytic graphite
(PG) electrodes were rough polished w ith 600-grit S i c paper on
a metallographic polishing wheel prior
to
coating.
Surfactant-clay composites were prepared by reacting clay
colloids and surfactant in aqueous suspension
as
described
previously.6 Purified, freeze-d ried com posite was suspen ded in
chloroform 2 mg mL-1) for preparing films. Metal phthaloc y-
anine (MPc) solutions in chloroform (10 mM) were mixed
1:l
with the composite suspension. For voltammetry and SEM I
ED X, 120 pL of the composite MP c suspension was deposited
with a micropipet onto a 0.2 cm2 pyrolytic graphite disk.
Chloroform was evaporated overnight in air. Dry
film
hicknesses
by SEM were 20-30 pm?
The three-electrode cell for SWV included the pyrolytic
graphite (PG) working electrode, a platinum wire counter
electrode, and a s atur ated calomel electrode (SCE) as eference.
Ohmic drop of the cell was abou t
90
compensated by the BAS-
100. Experimenta were therm ostated a t 25.0 f 0.1 OC. All
solutions were purged with purified nitrogen t o remove oxygen
before SWV.
Absorption spectroscopy was done using Perkin -Elm er Model
h3B or Milton Roy Spectro nic 3000 Array UV-Vis spectrop ho-
tometers. T he reference for composite films was usually a plain
glass slide. For visible spectrosc opy,MPc-composite suspensions
were prepared as bove, but with 1 M MP c to obtain measurable
absorbance spectra.
An 80-pL portion of the suspension was
cast onto a masked 0.5-cmZarea of a glass microscope slide and
chloroform was evaporated in air overnight. Dry thickness was
estimated at ca. 10
pm
by SEM.
Scanning electron micrscopy (SEM) and energy dispersive
X-ray analysis (ED X) were don e with an Amray 1810microscope
using a tungsten filament. DDAB films for SEM/ ED X analysis
were coated on PG electrodesusing the same preparation methods
as
for electroanalysis. Th e entire electrode assembly was fiied
on the mounting stage of the SEM with electrical connection
throug h the connecting wire. For cross-sectionalviews, compo site
coatings were prepared o n very th in disks of pyrolytic graphite
and freeze fractured after im mersion in liquid nitrogen. Prior
to analysis by SE M, 5 nm of gold was coated on to samples with
a Model SC 500 sputter coater (Bio-Rad). ED X was done using
a Phillips Nor th America PV-9800 EDA X system. Beam
diameter was 2 pm.
X-ra y diffraction studies were done w ith a Scintag XD S 2000
powder diffractometer using a Cu Ka source at 45 kV and
40
mA.
Scan rate was 0.5 deg/m in. Films for X-ray diffraction were
prepared on glass microscope slides from chloroform dispersions
with compositions described above. Before analyses, the d ry
films were soaked in 0.1 M KB r for 2-3 h, washed with w ater,
and stored in a closed desiccator with a small amo unt of water
in the bottom to m aintain hydration.
Results
Electrochemistry. Squarewave v oltamm etry was used
tomeasure reduction and oxidation potentials of th e m etal
phthalocyanines in the films. For the film containing
cobalt phthalocyanine (CoPc), he first reduction peak at
about -420 mV (allvsSCE) orresponds to
the
reversible
CdIPclCoIPc- redox coupleas eporte d previously6 (Fig ure
la). Ita shape reflects overlapped peaks due
to
adsorption
of CoPc on the electrode. By comparison with peak
poten tials reporte d for CoIIPc in organic solventa,9JO the
second reduction peak at about -1350 mV most likely
corresponds
to
the Co1Pc-/Co1Pc2- ouple. No oxidation
peak was observed for CoIIPc. Th e current increase at
700 mV (F igure lb ) is probably caused by oxidat ion of
bromide ions.
R u l i n g
et
al.
(9)
ever, A.
. .;Licoccia,
S.;
Magnell,
K.;
Minor,
P. C. Adu.
Chem.
10) R u l i n g , J.
F.;
wlia, A. J Electroana b Chem. Interfacial Elec-
Ser.
1982,
No .
201, 231-251.
trochem.
1987, 234,297.
0.001 .
- 1 0 0 300
700
1100 1500
-E, m V v 8 SCE
-0.25 1 1
-0.05
0.00
7
00 500 300 100
E, mV
v 8
SCE
Figure 1. Squa re wave voltammograms at 250Hz, 5 mV pulse
height for CoPcclay-DDA B films on PG disks in 0.1 M KBr:
(a) cathodic scan; (b) anodic scan.
0.30
A
0 400 800 1200 1600
-E, mV
VI
SCE
-0.16
I I
-0.10
I
700 5 0 0
3
00 100 -100
E, mV v 8 SCE
Figure
2.
Squere wave voltammograms a t 250
Hz,
5 mV pulse
height for FePcclay-DDAB films on PG disks in 0.1 M KB r:
(a) cathodic scan; (b) anodic scan.
Two reduction peaks were found for films
to
which iron
phthalocyanine (FePc) had been added (Figure 2). Th e
peak at -580 mV is in the potential range for an FeuP c
reduction. Th e peak at -1260 mV, by comparison with
literature?JO
is
in the same range
as
a Fe1Pc-/Fe*Pc2-
couple.
A n
oxidation peak for FePc films was observed
at
100 mV (Figure 2b).
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Clay-Surfactant Films Containing CdIPc
Langmuir Vol. 8, No. 10, 1992
2467
Table
I.
Electronic Absorbance
Peakr
for M eta l
Phthalocyanines in
Different M edia
.10
0.054
WAVELENGTH nm)
0.mo
0.060
0.040
5 0 0 5 5 0 ~ 7 5 0 8 0 0 8 5 0
WAVELENGTH nm)
WAVELENGTH nm)
Figure 3.
Visible absorbance spectra of (A)
1
pM ConPc in
DMSO (B) 1pM
ConPc
in
0.1
M
aqueous CTAB,
C)
ConPc in
clay-DDAB film.
UV Spectra. Electronic spectra of m etal phthalocy-
anines in the visible region reflect their state of aggre-
gation.11 In DM SO, the peak for CoPc a t 658
nm
and a
small peak at 596 nm are attributed to the monomer
(Figu re 3A).1J1
Peaks for association dimers of m etal phthalocyanine s
are generally found as shoulders on the sh ort wavelength
side of th e main monomer peak. When CoPc is dissolved
in aqueous micellar
0.1
M cetyltrimethylammonium
bromide (CTAB), a relatively large peak attributed to
dimer is found at
609
nm along with the monomer peak
at 668 nm (Figure 3B). This is presumably because the
~~
11) (a) Schelly, 2.
A.;
Farina,
R.
D.; Eyring, E.
M. Phya. Chem.
1970,74,617-620.
b)Schelly,2.
.;
Huward, D. J.; H e m e s ,P.; Eyring,
E.
M. . Phya. C h m . 1970, 74,3*3042.
(c) Gruen,
L.C.;
Blagrove,
R.
J. A u t . J . Chem.
1972,25,2553-2558; 973,26,319-323.
d) Boyd,
P.
D. W.; Smith,
T.
D.
J Chem. SOC.,
alton Tr0n.s.
1972,839-843.
e)
Farina,
R.
D.; Halko, D. J.; Swinehard, J. H.
J Phya. Chem. 1972, 76,
2343-2348. 0
bel, E.
W.;
Pratt,
J.M.;
helan,
R. J Chem. SOC.,
al-
ton
Trans. 1976,509-514. g)
Yang,
Y.;
Ward,
J. R.;
Seiders,R.
P.
Inorg.
Chem. l986,24, 765-1769.
A values, nm
cobalt phthalocyanine iron phthalocyanine
medium4
A*)*
DMSO
1.0) 658 596 653 630 590
CHzClz
(0.82) 665 706 657 552
hexane
(4.08) 671 746 709 680
tetradecane
(-0.08) 670 596 706 660
0.1
M
CTAB
668
609
707 616
clay/DDAB 670 639
606
146 681 594
clay/DODAB 668 638 603 748 682 592
4 Enough metal phthalocyanine was added to make each solution
1
pM. In the most hydrophobic solvents, some particles remained
which were removed by filteringbefore he spectra weretaken .
*
Taft
r
parameters from ref
12.
benzene (0.59) 667 599 708 674
7 Films CoPc FePc
COPC
6 9 0 1
u' I I
_
670
0 0
650
0 I
-0.30 0.10 0.50 0.90 1.30
-*
II
Figure
4.
Influen ce of Taft s
A*
solvent parameter on maxima
of longest wavelength peaks of met al phthalocyanines in various
solvents. A* values are given in Table I. Maxima for CoPc in
DDAB-
and
DODAB-clay
films are
arbitrarily located on the
linear regression line for th e CoPc solution
data.
water-insoluble CouPc is solubilized in restricted hydro-
phobic regions of t he micelles a t concentrations which
drive the form ation of dimers. Composite clay surfacta nt
films
containing CoPc had prominent m onomerpeaks
near
669
nm
with a relatively small dimer peak a t about 639
nm (Figure 3C).
Spec tra of CouPc were recorded in dimethyl sulfoxide
(DMSO ), methylene chloride, tetradecane, hexane , and
benzene, as well as in com posite clay films of DDAB and
DODAB (Table I). The main monomer peak showed a
definite shift toward longer wavelengths
as
solvent po-
larizability decreased.
A
linear plot was obtained when
A of the mon omer peak was plotted again st Taft's po-
larizability/dipolarityl2 arameter
x*
(Figure
4).
When
A
of CoPc in DDAB an d DODAB composites are placed
on thi s regressio n line, a value of
T
=
0.1 is found. This
is about midway between values for tetradecane and di-
ethyl ether.
In DM SO, spectra of FeP c were similar
to
those of CoPc,
with a monom er peak closeto653 nm an d a dimer shoulde r
a t about 630 nm (Figure 5A, Tab le I). However, in less
polar solvents, in 0.1 M CTAB, and in th e composite
films
(Figure 5B), broad peaks were observed at wav elength
about 700 nm (Table
I).
After a day or more, spectra
obtained from methylene chloride and DMSO solvents
showed the increasing developmen t of these peaks. Th e
positions of the longest wavelength peaks for freshly
(12) a)Taftsolv ent polarity indices were derived based on wavelength
shifta in absorption spectra for indicator solutes in a wide range of
solventa.'*b The
A*
parameter is an indicator
of
solvent polarity and
polarizability. (b) Kamlet, M.
.;
Abboud, J. L.-M.;braham, M. .;
Taft, R. W.
J .
Org.
Chem. 1983,48,2877.
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2468 Langmuir
Vol.
8, No. 10, 1992
540.0-
480
O
420.0-
Rwling
et
al.
BCINTA,ci/US
88.21
14.11 29.43
22.01
17.66 14.72 12.62 11.04 9.82
8 . 8 4
800.0 100
WAVELENQTM (nm)
BW em
700 750
ow 8
WAVELENGTH nm)
Figure
5.
Visible
absorbance spectra
of
(A)
1
WM
eIIPc
in
DMSO; (B) FePc-clay-DDAB film.
Table
11.
X-ray Diffraction
Results
for
DDABClay
Composite Films
film
additivea
28, ea basal macine.
A
~
clay alone 8.66 10.2
f
.4
composite alone 3.46 25.5
f
.5
composite CoPc 3.14 23.8f 0.8
composite FePc 3.03 29.1 .6
0 See Experimental Section
for preparation
and
compositions.
prepared solutions of Fe Pc gave no correlation with the
solvent parameter a* Figu re 4).
X-ray Diffraction. Th e lowest angle 28 reflection for
clay colloid
films
ntercalated with surfa ctants can be used
to obtain the interlayer basal spacing through the Bragg
relation.12J5 These small angle peak s were observed for
clay-DDAB composites with an d withou t MPcs (Table
11). These peaks w ere rather b road, suggesting a degree
of disorder in the films. Values for hydrated films were
roughly within experim entalerror of each other. Th e basal
spacings for unhydrated films were much smaller.
Althou gh only one peak a t 28 3.46O was fou nd for th e
pure clay-DDAB com posite, a series of addit iona l peaks
occurred in the 4-10 region when MPcs were prese nt in
the films. For CoPc films, the
first
peak marked 23.8 A
gives the clay basal plane spacing of th e film (Figure 6a).
Pea k between 28 4O and
l oo
in the CoPc film pattern are
at nearly identical positions
to
those found for CoPc
powder (Figure 6b). We conclude that there are crystals
of Con Pc in the film. Th e FePc composite
film
clear1
(Figure 7a). However, peaks betw een 28 4O and loo are
different from those in t h e X-ray spectrum of Fe*IPc
powder. We conclude th at these additional peaks in the
FePc films are not caused by crystals of Fen Pc.
ScanningElectronMicroscopy/Energy Dispersive
X-ray Analyris. SEM images of the films before use in
aqueous solution appeared distinctly different from those
of films tha t had been soaked in aqueo us solutions. Thu s,
shows the basal plane reflection peak marked 29.1
T
13) Shi, C.; R ul i ng , J. F.;Wang, Z.;Willis,
W.
S.; Winiecki,
A.
M.;
Suib,
5.
L. Langmuar 1989,5, 650.
5 6 0 . 0
480.0
400.0
3 2 0 . 0
240.0
160.0
8 0 . 0
0.0
i
t 7 0
IN: mjn205n5.ni I D : COP S B CI NT AO/ USA
DATE: 2 / 5 / 9 2 T I ME : 9:15 P T : 3 .600 S T L P : 0.030 W t : 1.51060
88.21
41.14 29.43
2 2 . 0 7
17.66 14.12 12.62 11.01 9.82
8.84
600.0j1 p o
b
60
s o
40
30
20
10
0
9 10
Figure
6.
X-ray
diffraction
powder
pattern for a) CoPcclay-
DDAB
film and b)CoPc powder.
all SEM and EDX experimenta were done on samples
that had been previously soaked in aqueous 0.1 M KB r,
then air-dried, to closer approximate the condition of the
films previously used for electrochemical catalysis.
SE M images of th e top of the film s were very differe nt
for composites with an d w ithout MPcs (Figure8). Films
containing MPcs appeared
to
include crystalline struc-
tures. These struc tures were differen t for CoPc films,
which appeared
as
a collection of need lelike crystals, th an
for FePc films. Th e FePc compo site filma appeared more
amorp hous, but a few needlelike crystals could be men.
The above morphologies were confirmed by cross-
sectional SEM images of t he films after freeze fracture.
Th e pure clay surfactan t composite films showed (Figure
9a) a layered structure similar to that reported for other
surfacta nt However, cross sections of the
CoPcf i i ppeared
as
crystals (Figure9b). Crosssections
of th e FeP c comp osite films revealed a rather thick layered
structure with a
few
needlelike structures (Figure 9c).
ED X of com posite films detected silicon, aluminum ,
and the m etal from MPcs. Traces of potassium and
chlorine were
also
found, but no bromine was detected.
Analyses at d ifferent apota on the sa mple surface revealed
a relatively constant Al/Si ratio consistent with the
aluminosilicate clay as he source for these two elem enta.
For CoPc films, this ratio had a 1 3% relative standard
deviation. In contrast, the C o/Si ratio varied considerably
from spot to spot and had a relative standard deviation
of 63
7 .
This suggesta a heterogeneous distribution of
CoPc in th e film.
Th e Al/Si ratio in FePc films measured by EDX had a
reproducibility of about 10%. The Fe/Si ratio had a
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Clay-Surfactant F i l m Containing
CdIPc
h n g m u i r
Vol. 8, No. 10, 1992
2459
88.27 44.14 2gk4 3 22.,07 17.,66 14:72 12.62 11.04 9.82 8.84
\ 90
80
-10
?
n loa
000.0
1800.0
1600.0
1400.0
O
540.0.
480.0-
420.0
360.0-,~~
300
.O
{
b
Figure7.
X-ray diffraction powder pattern for (a) FePc-clay-
DDAB film and (b) FePc powder.
standa rd dev iation of26 ron may
be
distributed more
homogeneously in its composites than is CoPc.
Discussion
Cobalt PhthalocyanineComposite Films.
Electronic
spectra and electrochem istry of th e films to which CoIIPc
was added are consistent with th e presence of CoHPc. In
organic solventa, th e m ain mo nomer peak of CoIIPc shows
a linear correlation with the Ta ft
polarizability/dipolarity
parameter A . In th e films, the monomer peak ap pears
a t values close
to
those in the nonpolar solvents (cf. Figure
4). Results suggest th at t he m olecules responsible for the
spectrum are pre sent in a relatively nonpolar environm ent,
which, however, is somew hat more polar th an a tetrad e-
cane environment. Th e fact th at no oxidation peak is
observed for CoIIPc in the film is consistent with the
presence of weak axial ligands which stabilize CorlPc
toward oxidation.9
The UV spectrum n
0.1M
TAB micelles reflects heavy
dimerization of CoIIPc. T he spectra of CorlPc n th e films
have a large monomer peak an d only a sm all dimer peak.
Th us, the degree of dim erization of CorlPc in th e films
seems relatively sm all com pared to CTAB micelles.
X-ray diffraction results are consistent with a stru cture
featuring surfactant intercalated between th e clay layers.
Th e interlayer spacing is increased greatly in th e surfac-
tant clay films compared
to
pure clay films (Table 11).
Thi s spacing, however, is somew hat smaller tha n the 30
A
found previously for DDAB-m ontmorillonite clay
composites.1
The observed spacing in our films, made
with a differen t clay with a
40
smaller cation exchange
Figure 8 SEM
top views
of (a,
top) clay-DDAB film and
(b,
botto m) CoPc-clay-DDAB film.
capacity, was on th e order of
15-20
A
after subtracting
9.8
A
for the thickness of t he clay layers from the values in
Tab le 11.
Th e X-ray diffraction pa ttern s suggest the presence of
crystallized Co rlPc n th e films. Th is is confirmed by the
SE M images which clearly show crystals. ED X spot
analyses suggesting heterogeneous distribution of Co in
th e films is also consis tent with th e presence of crystals.
SEM cross-sectionalviews of the pu re clay-DDAB films
clearly show th e layered str uc tur e observed previously for
polymerized sur fact an t bilayer composites.2dBk Th e lay-
ered structure is obscured when CoPc is present in the
films.
Iron Phthalocyanine Composite Films.
Th e volta-
mmetric oxidation peak a t
100
mV vs SC E for these films
showed that the iron present
is
quite easily oxidized.
Electronic spectra in DMSO reflect the presence of
monomer and dim er of Fer1Pc.l0J1However, in less polar
solvents, much broader pea ks a t longer wavelengths are
observed which showed no correlation with the Taf t A
solvent parameter.
Iron(I1)phthalocyaninetetrasuEonate issolved in water
is irreversibly oxidized by air.14 FeIIPc suspended in
organic solvent isoxidized by air to a dimeric p-oxo species
with
an
FerlLO-Ferrlinkage.15 Voltam metry of thi s dimer
in pyridine revealed a single-electron oxidation peak at
0.47 V
vs SCE and two one-electron reduction peaks a t
14)McLendon, G.; Martell, A.
E.
Inorg.
Chem. 1977,16, 1812.
15)
Ercolani, C.; Gardini, M.; Monacelli, F.; Pennesi, G.; h i , G.
Inorg. Chem.
1983,22,2584.
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2460
h n gmu ir
Vol.
8 No. 10 1992
Rusling
et al.
Table 111. Results of
EDX
Spot Analyses
of
Clay-DDAB
f i lms
atomic ratios
mot no.
Figure 9. SEM
cross-sectional views of
(a,
top) clay-DDAB
film,
b,
midd le) CoPc-clay-DDAB film,
and
(c, bottom) FePc-
clay-DDAB film.
-0.59
V
and -0.95 V.I6 In 96 sulfuric acid or aspowders,
the two crystalline forms of the p-oxo dimers gave
absorbance maxima
at 695
nm,
a
wavelength considerably
longer than th at of the Fe rlPc monomer. These p-oxo
dim ers ar e irreversibly oxidized by oxygen.15
The electrochemical behavior of the p-oxo dimer is
qualitatively similar to wha t we observe for th e composite
FeP c film. Quantitative comparison is difficult since the
(16) ottomley,L.A.; Ercolani, C.; Gorce,J.-N.;enneai,
G.;
i , .
Inorg. Chem.
1986,25,2338.
CoPc Film
AlISi CoISi
0.238 1.48
0.160
0.088
0.186 2.92
0.195
0.183 2.86
0.198 3.58
mean
0.193 .026 (*13%)
2J6f
1-29 163%)
3 1.45
FePc Film
AIISi FeISi
0.91
1.56
1.02
0.84
0.92
1.27
1.09 0.28 (i26%)
0.215
0.231
0.234
0.193
0.230
0.186
0.215 .021 (&lo%)
1
2
3
4
5
6
ean
experiments on t he dimer were do ne in pyridine,lGa strong
axial ligand which would have consid erable influence on
th e redox potentials. Thu s, the observed oxidation and
reduction peaks in th e films could come from FePcspecies,
p-oxo dimer, or other prod ucts of Fe Pc oxidation. How-
ever, the spectra in nonpolar organic solvents and in the
composite films clearly show large, broad electronic
absorbance bands at wavelengths longer tha n t ha t of the
FerlPcmonomer. Th is peak forms rapidly when FeIIPc is
dissolved in nonpolar solvents and its slow growth with
tim e was observed in DM SO and methylene chloride. This
behavior is consistent with the observed destabilization
of the FeIIPc oxidation
state
by weak axial ligand^.^
We conclude that the FerrPcoriginally placed in the
composite films is at least partly oxidized, probably
through form ation of th e p-oxo dimer and perhaps furth er
oxidation of th is species. Th is conclusion is supported by
the X-ray diffraction data, which show that t he peaks in
th e 4-10' 28 region are not due
to
FeIIPc crystals.
Conclusions
T he morphology of clay-DDAB composite
films
is
clearly changed by the presence of CoIIPc. T he gross
structure appears asa collection of C oIIPc crystals, rath er
tha n th e stacked layers observed in t he pu re composite.
However, gel-to-liquid crystal phase transitions were
clearly observed in electrochem ical catalysis at transition
temperatures close to those found for DDAB bilayer
suspension^.^*^ Th is suggests the presence of surfactan t
bilayers. However, the basal plane spacing of the clay a t
about 15-20
A
is considerably smaller than the 33.4
A
required
to
accommodate two extended DDAB chains
normal
to
the aluminosilicate layers. Thus , considerable
tilting' and self-intercalation of the hydrocarbon chains
must be considered. Th e general picture tha t suggests
itself is
a
rath er heterogeneous m ixture of CoIIPc crystals
and DDAB bilayers.
Our esults suggest ha t composite
films
ontaining Con-
P c are better catalysts tha n those containing FeIIPc partly
because Cor lPc emains intact in the films while FeIrPc s
presen t in irreversibly oxidized forms.
Acknowledgment. Th is work was supported by
U.S.
P H S Gra nt No. ES03154awarded by the National Institute
of Environmen tal Health Sciences. W e than k E. J. Neth,
Yanfei Shen, and
S.
Sui b for help with X-ray an d
SEM
analyses and David Howe for helpful discussions.
Registry No. DDAB,
3282-73-3;
oIIPc,
3317-67-7; eIIPc,
132- 6-
.