Growth of spherulitic crystal patterns in a Belousov-Zhabotinski type reaction system

Article abstract of DOI:10.1039/c0nj00798f

The growth of spherulitic crystal patterns in a Belousov-Zhabotinski (BZ) type reaction system by using acetyl acetone (AA)-succinic acid (SA) as dual organic substrates has been reported. The reaction system in the liquid phase has been found to show concentric ring-like wave patterns. A colloidal phase composed of numerous fine particles has also been observed during reaction. The solid phase nucleation has been found to occur in the colloidal phase, which leads to the formation of some stable nucleus centers. The solid phase nucleus has been found to grow in symmetric crystal patterns, with the progress of reaction, exhibiting spherulitic morphology. The possible growth behavior of spherulites has also been discussed. The spherulitic structure composed of fine crystal fibrils diverging from a common center have been observed by a scanning electron microscopy (SEM) technique. The polymorphic crystalline phase, found in spherulites has been supported by thermal characterization (TGA/DTA) and X-ray diffraction (XRD) patterns of crystal materials. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011.

Full text of DOI:10.1039/c0nj00798f

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PAPER
Growth of spherulitic crystal patterns in a Belousov–Zhabotinski
type reaction system
Narendra Yadav*ab and P. K. Srivastava*a
Received (in Montpellier, France) 15th October 2010, Accepted 29th January 2011
DOI: 10.1039/c0nj00798f
The growth of spherulitic crystal patterns in a Belousov–Zhabotinski (BZ) type reaction system
by using acetyl acetone (AA)–succinic acid (SA) as dual organic substrates has been reported.
The reaction system in the liquid phase has been found to show concentric ring-like wave
patterns. A colloidal phase composed of numerous fine particles has also been observed during
reaction. The solid phase nucleation has been found to occur in the colloidal phase, which leads
to the formation of some stable nucleus centers. The solid phase nucleus has been found to grow
in symmetric crystal patterns, with the progress of reaction, exhibiting spherulitic morphology.
The possible growth behavior of spherulites has also been discussed. The spherulitic structure
composed of fine crystal fibrils diverging from a common center have been observed by a
scanning electron microscopy (SEM) technique. The polymorphic crystalline phase, found in
spherulites has been supported by thermal characterization (TGA/DTA) and X-ray diffraction
(
XRD) patterns of crystal materials.
Introduction
of these studies expanded considerably. These concepts are
more useful to describe the intricate pattern formations, such as
7,8
Some chemical systems, far from equilibrium occasionally
show regular changes of the concentrations of intermediate
species in space or time during the reactions. The well known
spatially periodic structures (Liesegang rings), liquid crystal
9
ordering, frontal polymerization,
2,10
11
self-oscillating gel,
12,13
oscillatory growth mode of dendrite and many others.
1
–4
example is the Belousov–Zhabotinski (BZ) reaction, which
involves oxidation of organic substrates by bromate ions in
acidic medium. The reaction is catalyzed by traces of transition
metal ions that possess two oxidation states differing by a
single electron, whether in native form or as complexes. If
the reaction mixture is efficiently stirred, its color changes
periodically as a function of time. The period of these oscilla-
tions is usually of the order of approximately one tenth of a
second to a few minutes, depending on the reagent concentra-
tions and the catalyst used. On the other hand, when the same
reaction is performed in a thin layer (thickness B1 to
These patterns are usually formed under certain conditions,
consistent with thermodynamic laws; their formation mecha-
nism is governed by the interplay between reaction, diffusion
and phase transitions. However, methodical investigations of
these phenomena were not carried out intelligibly, partly
because our present understanding of non-equilibrium thermo-
dynamics has not yet been fully developed. Such chemical
systems are represented by simple reactions, however spatio-
temporal patterns have given rise to the emergence of spontaneous
or coherent, often periodic, structure involving many molecular
units, which when exhibited during experiments with thin
layers of BZ solutions were found to be quite astonishing.
Reaction–diffusion mechanisms and pattern formation analysis
has continued to draw the attention of researchers over the
years for the elucidation of complex behaviour and their
2
millimetre (mm)) of the solution placed in the typical Petri
dish (with the upper surface free), particularly fascinating
spatio-temporal patterns such as concentric rings, spiral
bands, travelling waves, etc. are developed.
The BZ reaction is well-known for exhibiting temporal
and spatial oscillating phenomena, and this has laid a solid
foundation in understanding many physical phenomena
10,11
possible existence in various materials and systems.
On the other hand, spherulites are nearly spherical, densely
branched and polycrystalline clusters formed under highly
14,15
2
,5,6
through the reaction-diffusion (RD) mechanism.
With
non-equilibrium conditions.
They have been observed in
the systematic design of oscillating reactions in the 1980s
and the discovery of Turing patterns in the 1990s, the scope
a wide range of materials and systems, such as, polymers,
12–17
inorganic substances, proteins,
etc. Spherulitic crystal
patterns were found to arise when crystal growth rates are
large compared to diffusion rates, predominantly during phase
a
Department of Applied Chemistry, Birla Institute of Technology,
Mesra, Ranchi, India. E-mail: yadav_chemistry@rediffmail.com
Department of Space Engineering and Rocketry, Birla Institute of
Technology, Mesra, Ranchi, India. Fax: +91-651-2275983
1
3–15
b
separation process of multi-component materials.
At early
stages, they show fibrous crystal habits which develop into
1
080 New J. Chem., 2011, 35, 1080–1087
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non-crystallographic branching in later stages. They exhibit
various levels of organization whose length ranges from nm
to mm. The physical interaction among their constituent
molecules, temperature, pressure, compositions, etc. was
found to be the key factors which controlled the morphologies
of such complex organizations. Understanding the factors
which would influence the microstructure is imperative since,
most of the physical properties of the material systems are
strongly dependent on microstructure.
next B10 min. Subsequently, a single blue nucleation was
yet again observed on the red colored reaction solution at the
center. It was found to propagate almost radially in an
outward direction and after B 40 min was taken to cover
the whole surface area of the Petri dish (4.55 cm (id)). After
completion of this process, the reaction solution completely
transformed into a blue colored solution. It remained static for
the next 7 h.
For appropriate explanations of these findings, the role of
ferroin must be considered. It usually acts as a redox indicator
which can intensify the color variations under the effect of
The growth of spherulitic crystal patterns in this chemical
system comes under a new study towards crystal growth
associated with self-assembly and BZ reaction processes.
The present BZ reaction is carried out in a dual organic
substrate mode, in which two states of the reaction namely,
solution phase and colloidal phase, have been found to
show effective roles in the solid phase nucleation and sub-
sequent growth of spherulitic crystal patterns. A possible
reaction mechanism for the solution phase reaction has been
postulated, and its role for solid phase nucleation has been
discussed. Based on mesoscopic self-assembly, a graphical
scheme for the growth of spherulitic crystal patterns has been
proposed.
1
,2
the RD mechanism of the BZ reaction systems. Its color
can vary between red and blue, which indicates the state
of the reaction from reduced state to oxidized state respec-
tively. The coloration mechanism is also helpful in analysis
and the visual prediction of the spatio-temporal structure
of waves and patterns. During the reaction, ferroin can
change its form possibly between two physical states, as shown
below.
Results and discussion
The formation of concentric rings and propagation of a
single wave front has striking similarity with the target and
wave patterns and can be related with dual frequency oscilla-
These reaction processes exhibited complex pattern formation
behaviour and produced distinct pattern quality; however it
has some common characteristics for subsequent reaction
process. In order to simplify the complexity of the pattern
formation behaviour, the study has been carried out in the
three distinct sections, namely (a) development of concentric
rings, (b) colloidal state: solid phase nucleation center, and
1
9–21
tion of typical BZ reaction systems.
The high frequency is
attributed to the development of concentric ring like patterns,
while a single blue wave front propagation may be said
to form in the range of the low frequency regime of the
oscillatory reaction. The redox and RD mechanism signifi-
cantly controls these pattern behaviors. Moreover, the reac-
(
c) growth of spherulitic crystal patterns.
tion mechanism and description of pattern formations can be
22–29
Development of concentric rings
explained by fundamental BZ chemistry.
This suggests
4
+
2+
that, with trace amounts of Ce (or Fe ), acting as catalyst
ꢀ
Initially, BrO3 was introduced in the reaction solution which
ꢀ
and in acidic BrO3 , the AA (or SA) is oxidized. Some
raised a homogenous red colored solution mixture. After, a
short steady state (B5 min), a central single blue nucleation
occurred. It emanated from a point source producing a
circular front which propagated outwardly with uniform
speed. The repeating process led to the formation of concentric
circles that further organized into a concentric ring pattern as
shown in Fig. 1. This phenomenon continued for 25 min. After
that these ring patterns gradually started disappearing
from the outer ring and the reaction mixture returned to its
reduced state (red color). This state remained static for the
intermediates are also formed whose concentration is found
to vary periodically as a function of time. The RD mechanism
may relocate newly formed product phases at regular intervals.
The oscillatory behaviour of the solution phase reaction could
be elucidated by applying the described theory. However, the
reaction becomes complicated when it is are carried out under
solid phase nucleation and crystal growth is followed by a
colloidal phase reaction. Instead, the qualitative aspects of
pattern formation in the solution phase reaction and its
associated reaction mechanisms have been summarised in this
section as follows.
A number of results have been used to describe the chemistry
1
9,22,25,29
of the BZ experiments.
Based on experimental results
we can postulate the overall reaction mechanism as presented
in eqn (1).
ꢀ
þ
AA þ BrO þH ! ðBrÞAA þ ðHOÞAA þH2O þ P ð1Þ
3
ðAÞ
ðBÞ
where, product (A) is bromo acetyl acetone, (B) is hydroxy
acetyl acetone, and (P) corresponds to some unidentified
chemical entities. The reaction in eqn (1) results from a series
Fig. 1 Development of a concentric ring pattern.
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of reaction steps. The reactant AA does not react directly with
ꢀ
this, the formation of Br
2
is carried out during the reaction
ꢀ
ꢀ
BrO
3
+
; instead, BrO
3
is reduced to HOBr either by Br or by
eqn (2k). This reaction acts in two different ways: firstly, the
ꢀ
3
Ce as shown in eqn (1a) and (1b)
removal of Br ion during formation of Br which facilitates
2
reaction solution in oxidized state, and secondly, the release of
ꢀ
ꢀ
ꢀ
+
+ 2Br + 3H - 3HOBr
BrO
3
(1a)
Br ions during reaction with AA. Thus a cyclic reaction
ꢀ
3+
+
4+
network is accomplished between reduced state and oxidized
state during the reaction process. Instead, a large concentra-
BrO + 4Ce + 5H - HOBr + 4Ce 2H O (1b)
3
2
and then the HOBr formed in eqn (1a) or (1b) reacts with AA
to form product (A) and product (B), as shown in eqn (1c)
to (1e),
2
tion of the HBrO in the reaction system tends to diffuse into
neighbouring domains, and could produce a concentration
gradient between reduced and oxidized states. This results in
the development of wave fronts which consist of oxidizing
phase waves and reduced phase waves, as observed in the form
of concentric ring patterns during the study.
HOBr + AA - (A) + H
2
O
(1c)
(1d)
(1e)
ꢀ
+
HOBr + AA - (B) + Br + H
4
+
3+
+
Some recent studies on BZ reaction systems suggests that a
rise and fall of the concentration of substrates (or their
derivatives) also takes place during periodic color variation
phenomena, which plays a significant role in the oscillatory
2
2
Ce + AA + H O - 2Ce + (B) + 2H
The reaction process (1c), involves the formation of product
A) and consumption of bromide ions through the reduction
(
2
,19,22,23
of bromate via a series of oxygen transfers. The reaction
behaviour of the reaction system.
The coloration
processes (1d) and (1e) involve the formation product (B),
mechanism of the reaction suggests that the ferroin must
be involved in the oscillatory reaction processes. It is also
reported that ferroin alone could catalyze the BZ-like reac-
tion. Under such conditions, some common intermediates
2
followed by production of HBrO and autocatalytic oxidation
3
of Ce , at low bromide concentrations. These reaction
+
ꢀ
processes are very complex, wherever the BrO
3
is the key
ꢀ
3+
reagent which initiates the reaction at the beginning and
provides an oscillatory network during the progress of the
reaction process by forming number of intermediates such as
(often Br /HBrO
2
) coupled with [Fe(phen)
3
]
have been used
for the development of phase waves during the reaction
process. Based on this, possible reaction equations have been
shown as follows,
ꢁ
ꢀ
HOBr, BrO2 and Br etc. The concentration of these inter-
,
mediates effectively controls the oscillatory behaviour of
the reaction. The possible reactions for intermediates can be
summarized as below,
ꢁ
2+
+
2 3
+ H - HBrO + [Fe(phen) ]
3+
BrO
2
+ [Fe(phen)
3
]
(
2)
ꢀ
ꢀ
+
BrO + Br + 2H - HBrO + HOBr
The ferroin-catalyzed reaction is also monitored by Br and
(1f)
(1g)
(1h)
(1i)
3
2
HBrO , much like a cerium-catalyzed reaction. A brief summary
2
ꢀ
+
+ Br + H - 2HOBr
HBrO
2
of the way in which ferroin facilitates the oscillatory phase
reaction is explained as follows: the HBrO autocatalytically
2
ꢀ
+
BrO + HBrO + H " 2BrO2 H O
ꢁ
3
2
2
produced during the reaction process is balanced by excessive
ꢀ
ꢁ
3+
+
4+
+ Ce
BrO
2
+ Ce + H - HBrO
2
Br ions; thus, a dynamic equilibrium condition is supposed
ꢀ
between the concentration of Br and HBrO
2
by which ferroin
ꢀ
2HBrO
2
- BrO
3
+ HOBr + H
(1j)
is rapidly oxidized into ferriin and vice versa.
ꢀ
+
Br + HOBr + H - Br
2
+ H2O
(1k)
Colloidal state: solid phase nucleation
2
5–29
On the basis of modified version of the FKN mechanism,
After 7 h, a colloidal phase is observed during the reaction
process. It consists of numerous fine solid particles embedded
in the matrix of the reaction solution. These particles self-
assemble to form some discrete solid points, called solid phase
nucleation centres. Fig. 2a shows a solid phase nucleation
centre, surrounded by symmetrically distributed colloidal
it has been suggested that, the intermediate reaction steps are
involved initially to maintain the stoichiometry of reactions
ꢀ
1
a, and 1b. These are the Br ion dominating reaction steps
ꢀ
4+
which composed significant Br and almost no Ce , the
ꢀ
reaction 1a takes place followed by 1c, and Br is depleted.
ꢀ
Such a system is in a reduced state with relatively large [Br ]
4
+
and very low concentrations of HBrO , and of Ce . As Br is
ꢀ
2
consumed by a reaction system in a reduced state, the sequence
(
1h) + 2(1i) which produces HBrO , becomes ever more
2
important compared to step (1g) which consumes it. When
the rate of (1h) + 2(1i) is equal to that of (1g), [HBrO
2
]
increases autocatalytically and process (1b), becomes dominant
followed by (1c),(1d), (1e). Such a system is in an oxidized
2
state with relatively large concentrations of HBrO and of
4
+
Ce and very low Br . However, higher rates of (1h) + 2(1i)
ꢀ
ꢀ
could regenerate Br ions which may accelerate (1g) until it
attains a new maximum rate. Then HBrO is being consumed
2
more rapidly than it is being produced, and the system
switches back almost periodically to a reduced state. Besides
Fig. 2 (a) Colloidal phase just after solid phase nucleation at 5X,
(b) SEM microphotographs of colloidal particles.
1
082 New J. Chem., 2011, 35, 1080–1087
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particles. SEM microphotographs of the colloidal phase are
shown in Fig. 2b. It has also been observed that these particles
initially organized to form single fibre like crystals, from which
new crystals are branched.
The solid phase nucleation is found to be heterogeneous
1
5
in character which occurs spontaneously during complex
reaction sequences. A number of the products have been
estimated just before the nucleation process, as described in
eqn (1). Based on the proposed reaction mechanism, it has
been suggested that these products significantly control the
solid phase nucleation. A concentration gradient has also been
assumed between nucleation centres and uniformly distributed
fine solid particles. This concept is helpful to describe the
Fig. 4 Growth of spherulites, (a) after13 h and 15 min, (b) after 13 h
and 45 min.
1
2,13
2
nucleation process as a diffusive mode.
The excitability
(
tending to return back in its nearest steady state) is the most
essential factor of the BZ like reaction systems. If solid phase
nucleation is carried out under excitable conditions, some non-
equilibrium factors such as dynamic instability and surface
anisotropy etc. influence the nucleation process as explained: a
Mullins–Sekerka type of dynamic instability is expected in this
experiment which could generate a concentration gradient
between nucleation centres and uniformly distributed fine
solid particles. Depending upon the diffusion coefficients of
the colloidal particles, such instability controls the mobility to
fuse together or to form crystal interfaces. On the other hand,
surface anisotropy determines the disorder in the growing
crystal structure. It can be induced by non-uniform crystals
and also by a multi-crystal phase. The newly formed nucleation
centre exhibits some rudimentary crystal faces which might be
in the form of some mesoscopic crystals. These crystals possess
high surface energy, and also low order crystallographic
symmetry. The metal catalysts (cerium/iron) can destabilize
these stable faces in further crystallographic directions, because
they have both non-uniform crystal surfaces and well defined
crystallographic symmetry. Thus, an orderly nucleation
structure has been formed, taken as a whole, by a lowering of
the surface energy.
Fig. 5 Schematic growth mechanism of spherulites.
nucleation mechanism has been proposed schematically in
scheme-I, as shown in Fig. 3.
Growth of spherulite morphology
The solid phase nucleation results in an assembled form of some
tiny micro-crystals. At an early stage, these crystals exhibit an
ordered structure called the nucleation centre. When a stable
nucleus structure is produced they start to grow in size and
shape in stipulated directions, essentially driven by a decrease in
the surface energy.
A similar approach called particle mediated crystallization
0
3
theory, has been used to describe the solid phase nucleation.
This theory suggests that some mesoscopic transformation
The growth of the spherulitic crystal patterns is studied and
two sequential stages are depicted in Fig. 4. The typical
morphology of the spherulite has been found to resemble
1
8,25
process is involved followed by multiple nucleation events.
1
4,15,17
They initially form mesoscopic crystals and subsequently self-
assembled into an orderly mesostructure, which then fuse to
form a single crystal of specific shape and size. During self-
assembly, the mesoscopic crystals might be restructured in
such a way that the crystallographic orientation of the particle
is equal in all directions. Based on current analogy, the
polymeric spherulites of category-1.
It has also been
observed that spherulites grow radially from their nucleation
site, branching intermittently to maintain a space-filling charac-
ter. A schematic growth mechanism is illustrated in Fig. 5.
The spherulitic crystal pattern exhibits a spherical self-
assembled form of crystals or crystal units. The internal
structure of a spherulite may arise from a complex interplay
1
4,25
of heat and mass transfers.
However, coarse-grained phase
field theory (PFT) of Granasy et al. suggested that spherulite
1
5
patterns formed by growth front nucleation (GFN). They
claim that new crystal grains nucleate at the surface with a
different lattice orientation from that of the parent crystals.
The randomized orientation processes take place among
product phases which demonstrates the isotropy in the
spherulitic growth at large length scales and for long times.
The growth behaviours are described in Schemes I and II and
are attributable to comparable mechanisms. By applying the
knowledge of the PFT theory, possibly three factors of this
Fig. 3 Schematic mechanism of solid phase nucleation.
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system can significantly dictate the growth of spherulites. Due
to metallic traces (ions of cerium or iron), the long lived
dynamic heterogeneity is helpful in preventing the crystals or
particles from agglomerating, and also provides multiple
growth fronts for nucleation events. The disorder in the
growing structure emerges via growth front nucleation and
1
3–15
the existence of a multi-crystal phase,
which leads to
randomization of crystal units in directions of the crystallo-
graphic symmetry, while retaining isotropy at large scales. If
the disorder is sufficiently large, the growth form is directed
towards crystallographic symmetry, which resulted in robust
spherical crystal patterns. The radially elongated crystal struc-
ture may be attributed to self-organized selection of colloidal
particles or crystal units towards the direction of the interface.
The radially oriented crystal fibrils of spherulitic crystal
patterns in the non-crystallographic direction is explained
Fig. 6 Fully grown typical spherulitic crystal patterns, after 14 h and
1
6,17
as follows.
The imposed pressure between neighboring
1
0 min.
crystal fibrils could be a reason for such crystal organization.
However, splay among the crystal fibrils may be induced by
viscous forces or changes in the density during the crystal
growth process. This is also supported by the Keith–Padden
theory of spherulitic growth, which states that the highly
enriched liquid is trapped within the growing crystal inter-
faces, while narrow liquid channels separate adjacent needle
crystals towards the centre of the spherulite because crystal
surfaces are denser than the liquid and they generate a
1
4,15
pressure according to the Poiseuille equation.
The driving force is an essential factor which controls the
transportation of crystallizing particles or crystals during
the growth process. The emergence of the driving force can
be described in two different ways: firstly, the growth of
spherulitic crystal patterns under the phase separation process,
which is associated with a change in physical state of the
system. The difference in free energy between possible physical
states can generate a thermal gradient. Secondly, the Mullins–
Sekerka type dynamic instability under phase separation could
destabilize the liquid-solid interface. The high solvent evapora-
tion rate can also induce the emergence of higher driving
forces, which facilitate an increase in the branching frequency,
yielding more space filling characters.
Fig. 7 SEM micrographs, (a) organization of spherulitic crystal at
position 1, and (b) crystal organization at position 2.
a space filling character. Diverging crystal fibrils are also
emphasized and provide evidence of birefringency or Maltese-
1
6
cross extinction patterns similar to polymer spherulites.
Spherulitic crystal patterns located at position 2 (of Fig. 6)
have also been characterized by SEM, as shown in Fig. 7b.
This result shows number of fine crystal fibrils organized in the
form of bundles. Some crystal aggregates are also observed
just around the bundle of crystal fibrils. They might be the
residue of metal catalysts or non-crystallizing amorphous
material. These materials could facilitate the binding together
of the multi-crystal phases and also play a role in moulding the
spherulitic frames during the self-assembly process. On
the basis of this characterization, it has been suggested that
the morphology of spherulites (at position 2) closely resembles
The size of fully grown spherulites is in the range 2–3 cm
(
diameter), much larger than most of the studied polymers
or others spherulites. The observed spherulites were stable
indefinitely at room temperature if protected from moisture.
At high humidity, the spherulites deliquesced in 2–3 days. The
spherulitic crystal patterns are stable on heating up to 65 1C.
The crystal materials of spherulites are frequently soluble in
polar solvents such as ROH, RCOOEt, RCOOH, etc.
1
5,16
the spherulites of category II
which may emerge from a
The morphology of the spherulites has been examined by
SEM. Multiple nucleation centres have been found which
self-regulates the forms of the spherulitic crystal patterns of
different morphology. Two locations have been traced within
the domain of fully formed spherulites, as shown in Fig. 6,
which are attributable to dissimilar crystal organization.
Position 1 (of Fig. 6) is indicated by a dotted circle and its
corresponding SEM micrograph is shown in Fig. 7a; this figure
shows that the main architecture of the spherulite consists of
long crystal fibrils which diverge from a common centre. These
crystal fibrils have a well ordered branched structure which has
precursor via low-angle branching starting from the edges.
They successively branch to form nearly spherical crystal
patterns. A self-assembly mechanism could help to retain the
isotropy on the larger scale. The supersaturation condition has
also been used to explain such spherulitic crystal patterns, as
1
5
30
suggested by Granasy et al. and Beck and Andreassen. The
mesoscopic crystals are formed at low saturation conditions,
whereas the possible non-crystallographic branching and
growth front nucleation efficiency slows down at moderate
supersaturation conditions. With the progress of the reaction,
the branching frequency is higher where uncrystallized lobes
1
084 New J. Chem., 2011, 35, 1080–1087
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Fig. 8 Powder XRD patterns of spherulitic crystals at different
positions.
are diminished and crystal patterns are observed as spherulites
of category II.
Fig. 9 EDS of spherulitic crystal phase.
For this experimat, the SA is used as an additional organic
substrate; it can undergo oxidation/bromination reactions
A powder X-ray diffraction spectrum of spherulitic crystal
materials (Fig. 6) is shown in Fig. 8. The powder XRD pattern
of spherulite materials (position 1) shows a higher number of
peaks as compared with crystals of the spherulite at position 2.
The smaller peaks and some broad peaks in the XRD pattern
of spherulites (position 2) might be amorphous material
incorporated during the crystal packing. Some peaks appeared
at identical positions suggesting that a few common ingredients
are involved in the formation of both spherulitic crystal
patterns. It also confirms that these spherulites are formed
with similar constituents, although their morphology differs by
means of their growth modes.
similar to AA. However, SA has two active methylene (–CH –)
2
groups at reacting positions. This reveals a high electron density
region near the reacting positions. The high electron density may
cause a reduction of the efficiency of the oxidation/bromination
reactions. Thus AA can play a dominant role during such reaction
processes in the presence of SA. Instead, unreacted SA molecules
may dehydrate under these conditions, and could form a stable
5
-membered ring of succinic anhydride (SAA). The highly con-
centrated acidic medium (H SO ) and highly saturated solution
2
4
also supports the dehydration of SA. The possible reaction route
by which SA transforms into SAA, is shown below,
The diffraction peaks of the spherulite spectrum Fig. 8a, are
sharp and appear at higher intensity which reveals larger size
crystallites. This also confirmed that the crystal units which are
involved in the formation of spherulitic morphology are not
the consequence of the simultaneous aggregation of solid fine
particles but they resulted from a self-assembly mechanism.
A multi-crystal phase is supposed to play a significant
role in the formation of spherulitic crystal patterns. For
elucidating these multi-crystal phases, the described BZ
chemistry and the proposed reaction equations must be taken
as a reference. Eqn (1) remarkably suggested some major
products, designated product (A), product (B) and product (C),
as shown below,
Thus, it has been concluded that a number of products and
derivatives (product (A), product (B), product (C) or their com-
plexes, and also SAA) are formed during BZ reaction processes.
They possibly relate to the multi-crystal phases and are effectively
involved in the formation of initial nucleation centres. Whereas, a
self-assembly mechanism inevitably takes place for the organiza-
tion of crystal units in the form of spherulitic crystal patterns. The
multi-crystal phases are essential for randomization or synchroni-
zation during the self-assembly process.
It has also been reported that AA is readily brominated and
hydrolyzed during the BZ reaction process. The hydroxy-
derivative and bromo-derivatives were found to be unstable
1
9,22
under existing reaction conditions.
Thus, these products
potentially act to form some complexes with metal catalyst
2+ 31
4+
ions (Ce /Fe ). The fine solid particles which were observed
during the colloidal phase may be individual products of (A),
(
B), and (C) or their complexes. The presence of cerium and
iron in the spherulites has been confirmed from the EDS
spectra as shown in Fig. 9.
Fig. 10 FTIR of spherulitic crystal materials.
This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
New J. Chem., 2011, 35, 1080–1087 1085

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4+
cerium(IV) sulfate (Ce ), and potassium bromate (KBrO
3
)
)
(
E-Merck) of analytical grade were used for experimentation
2+
ꢀ3
without purification. Ferroin ([Fe (Phen)3] ) (0.025 mol dm
was prepared by homogenous mixing of 1,10-phenonthroline
and ferrous sulfate in a molar ratio of 3 : 1 in doubly distilled
water (DDW).
All experiments were performed in flat Petri dishes of
9
.1 cm (id). The experimental table and setups was levelled
and checked by sprit-level. Stock solutions of succinic acid
ꢀ
3
5% w/v) and acetyl acetone (1.59 mol dm ) were also
(
prepared in DDW. Stock solutions of potassium bromate and
ammonium cerium(IV) sulfate, (5% w/v) were prepared in
ꢀ
3
H
2
SO
TGA/ DTA analysis was carried out (Shimadzu DTC-60,
4
(1.5 mol dm ).
Fig. 11 TGA/DTA curve of spherulitic crystal materials.
Japan) between 0 1C and 500 1C. The average particle size
of spherulitic crystals was determined by DLS technique
(MALVERN; DTS Ver.4.10; Serial No. MAL500999) X-ray
analysis was carried out using a powder diffractometer
(Rigaku, model: Miniflex-2005B111, Japan) equipped with
More characteristics of the spherulitic crystal materials are
also observed in the FTIR spectrum Fig. 10. The spectrum
ꢀ1
shows the absence of a broad peak between 2500–3000 cm
ꢀ
1
,
due to carboxylic acid groups. The peaks at 1695 cm
ꢀ
1
ꢀ1
1
560 cm and 1516 cm are the characteristics peaks of carbonyl
CO) groups. These can be ascribed the carbonyl groups that
are bound to cyclic structures or to co-ordinate linkages. The
Cu-Ka (l = 1.54 A
and 50/60 Hz. Each sample was scanned in a (2y) range
˚
) operating at a tube load 100 V, 10 A
(
ꢀ
1
between 01 to 801 with a scan rate of 0.21 (3 s) . The study
of the morphology of the spherulitic crystal patterns has
been carried out by scanning electron microscopy (SEM)
(Jeol, JSM-6390 LV) coupled with energy dispersive spectro-
scopy (EDS) under vacuum, at an accelerating voltage of
20 kV. The infrared spectra (Shimadzu, IR Prestige- 21 FTIR
instrument, composed of Michelson interferometer and a
germanium-coated KBr plate beam splitter) of the spherulitic
ꢀ
1
ꢀ1
peaks at 1056 cm
and 1008 cm reveal the anhydride
ꢀ
1
linkage. Peaks near 3055 cm are suggestive of C–H stretching
associated with a highly ordered hydrocarbon chain. One peak
ꢀ
1
at 3741 cm may be ascribed to hydrogen bonding associated
with a co-ordinate structure. It has been observed that many
ꢀ1
peaks of the spectrum appear in the range 400–1500 cm . The
ꢀ
1
peaks between 1000–1200 cm are indicative of chemical
complexes associated with the structure. The possible M–O
bonds and their corresponding peaks are also observed in the
ꢀ
1
crystal material were recorded at a resolution of 4 cm and
64 scans. The evolution of chemical wave patterns and sequential
growth stages of the spherulitic patterns were monitored with a
digital camera (Nikon) equipped with 5X optical zoom.
ꢀ
1
spectrum in the region of 500–600 cm
.
The thermal analysis (TGA-DTA) of spherulitic crystal
materials has been carried out in the range 0–500 1C (Fig. 11).
The weight losses in the TGA curve were found at sequential
stages. This is related to the degradation behaviour of multi-
phase crystal materials having different groups or structures.
The initial 16.5% weight loss observed between 45–110 1C is
4
+
2.0 mL of SA, AA, Ce and H SO , along with 1.0 mL of
2
4
2
+
[Fe (Phen)3]
was mixed in a 250 mL beaker and made
homogenous by constant shaking and stirring for 5 min. Then
2 mL of the KBrO3 solution was added to the resultant
mixture and was well shaken to maintain the homogeneity
of the reaction solution. This stage was marked as initiation of
the reaction process. Thereafter, half of the reaction mixture
(6.5 mL) was immediately taken from the bulk solution and
spread in a Petri dish with uniform thickness (B1 mm). The
resultant concentrations of reactants were calculated as,
2
attributed to the liberation of CO or other gaseous products.
The major weight loss (62%) observed between 135–270 1C
suggests a transition temperature attributable to the multiple
crystal phases. The DTA results show two consecutive endo-
thermic peaks at 65 1C, 170 1C and an exothermic peak at
ꢀ
3
ꢀ3
4+
443 1C ascribed to the melting of different crystalline phases,
[AA] = 0.269 mol dm , [SA] = 0.091 mol dm , [Ce ] =
ꢀ
3
0.0122 mol dm , [[Fe (Phen) ] ] = 0.00208 mol dm
2+
ꢀ3
and the reorganization of crystals during the heating process
32,33
respectively.
3
ꢀ3
ꢀ3
The melting peaks also suggest that multi-
[KBrO ] = 0.055 mol dm , and [H SO ] = 0.54 mol dm
3
.
2
4
crystal packing is involved in the growth of spherulitic crystal
patterns, which melt at various energy levels.
Conclusions
The intervening crystallites formed during the sequential
growth front nucleation, which results in the primary crystals,
melt at low temperatures, while crystals which are involved in
the thickening or formation of secondary growth fronts melt
at comparatively higher temperature.
The growth of spherulitic crystal patterns has been studied in a
BZ reaction system. The reaction system has been found
to show the development of a concentric ring pattern and
propagation of a single blue wave front during the solution
phase reaction. Numerous fine solid particles which are uniformly
distributed in the matrix of the reaction solution have also
been observed during a colloidal phase of the reaction. These
particles self-assembled to form some active solid phase
nucleation centres during the initial phase. With the progress
of the reaction, these nucleation centres start to grow in size
Experimental
Succinic acid (SA), acetyl acetone (AA), and iron(II) sulfate
(
CDH India Ltd.), sulfuric acid (H SO ) (98%) (S. D. fine
2 4
chemicals, India Ltd), 1,10-phenonthroline, ammonium
1
086 New J. Chem., 2011, 35, 1080–1087
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and after B 2 h they grow into spherulitic crystal patterns.
Based on a modified version of the FKN mechanism, possible
reaction equations have been proposed. This suggests that
some reaction products have been formed during the solution
phase reaction. These products must be involved in the forma-
tion of ordered nucleation centres and the growth of spherulitic
crystal patterns. On the basis of particle mediated crystalli-
zation theory, a mesoscopic self-assembly scheme has been
proposed. Fibrous network and isotropy in the resulting
spherulites affirms the described scheme. The fibrillar organiza-
tion of spherulites was also supported by SEM results. Diffrac-
tion line broadening (XRD patterns) reveals that multi-crystal
phases must be involved in the fibrous network of the spherulitic
crystal patterns. The sizes of the spherulites have been found
to be 20–30 mm in diameter, much larger than most of the
studied polymers or other spherulites.
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Acknowledgements
We thank Dr A. K. Chatterjee, Professor & Head, Department
of Space Engineering & Rocketry and Dr Usha jha, Professor
2
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Head, Department of Applied Chemistry, BIT Mesra for
their fruitful suggestions during preparation of the manuscript,
and for providing infrastructure support during the entire
length of the experiments. We are grateful to Dr Prasana
Kumar David, Associate Professor & Head, Department of
English, Gossner College, Ranchi University, Ranchi, for his
valuable input for improvement of the manuscript content.
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