7/25/2019 la00046a018 xxx

1/6

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

7/25/2019 la00046a018 xxx

2/6

2456

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).

7/25/2019 la00046a018 xxx

3/6

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.

7/25/2019 la00046a018 xxx

4/6

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

7/25/2019 la00046a018 xxx

5/6

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.

7/25/2019 la00046a018 xxx

6/6

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-

.