Calcium Carbonate Crystallization in the Presence of
BiopolymersMichael F. Butler,* Nicole Glaser, Antony C. Weaver,
Mark Kirkland, and Mary Heppenstall-ButlerUnileVer R&D
Colworth, Sharnbrook, Bedfordshire, MK44 1LQ, U.K. ReceiVed August
23, 2005; ReVised Manuscript ReceiVed NoVember 7, 2005
CRYSTAL GROWTH & DESIGN2006 VOL. 6, NO. 3 781-794
ABSTRACT: The influence on calcium carbonate crystallization of
a series of biopolymers that contain carboxylic acid or sulfate
functional groups was studied using pH and turbidity measurements,
optical microscopy, and scanning electron microscopy. Without
biopolymer, single calcite (104) rhombohedra were formed. In the
presence of nongelling biopolymers (xanthan and gellan) in the
conditions used, (104) rhombohedra formed aggregates that were
stack-like, but in the presence of gelling biopolymers (pectin,
-carrageenan, and sodium alginate) the aggregates were
rosette-like. The rosettes were proposed to form by the nucleation
of calcite on a gelled microparticle template to form a hollow
shell. Low methoxy (LM) pectin was particularly effective at
directing the growth of calcite rosettes and led to aggregates of
radially aligned crystals. The influence of biopolymer
concentration on calcite crystallization was studied for LM pectin
and -carrageenan. In the former case, an increasingly favorable
influence of the pectin molecules on the surface energy of calcite
nuclei was proposed to result in an enhanced propensity for
nucleation, until the pectin concentration was so high that all of
the calcium was sequestered. In the latter case, an increase in
calcium binding with increasing -carrageenan concentration
decreased the solution supersaturation and hence decreased the
propensity for calcite formation. Introduction The ability to
control crystallization is a critical requirement in the synthesis
of many industrially important materials. Selected examples of
these are pharmaceutical molecules, triglycerides, fatty acids, and
inorganic materials such as calcium carbonate. Scientifically,
calcium carbonate is of enormous interest in the context of crystal
control because it is produced by a wide variety of biological
organisms that exhibit exquisite control over the polymorph,
location of nucleation, crystal size and shape, crystallographic
orientation, composition, stability, and hierarchical assembly of
the crystals. Calcium carbonate can exist in several different
polymorphs that are, in order of increasing solubility: calcite,
aragonite, vaterite, calcium carbonate monohydrate, and calcium
carbonate hexahydrate. Although aragonite and calcite are the most
commonly found polymorphs in natural systems, vaterite is also
present in some systems, and all of the polymorphs may be obtained
in the laboratory. In the presence of water, all of the polymorphs
eventually transform into the most stable form, calcite. They may,
however, be stabilized in the presence of additives. Amorphous
calcium carbonate also exists but has only been observed in a
stabilized form as a biomineral.1 Many studies have investigated
the influence of additives on the crystallization of calcium
carbonate.2,3 In the simplest case, the addition of magnesium is
well known to lead to the formation of spicular aragonite crystals
rather than rhombohedral calcite. Simple organic acids such as
citric and maleic acid have been shown to strongly influence
crystal morphology, especially in the presence of magnesium ions,4
and studies have shown that acidic peptides, in particular,
aspartic acid, can alter the chiral morphology of calcite crystals
by binding to specific step edges at the crystal surface.5 In
general, aspartic acid has a large effect on calcium carbonate
crystal growth, causing either calcite or vaterite to form
depending on the acid concentration.6-8 Other amino acids, such as
alanine, lysine, and glycine, have similar effects.9 More
complicated sequences of amino acids, inspired* To whom
correspondence should be addressed. E-mail:
[email protected]. Phone: +44 (0)1234 222958. Fax: +44
(0)1234 248010.
by those present in biomineral forming systems, have also been
shown to strongly influence calcium carbonate growth. In some
cases, calcite is formed, but with morphologies other than
rhombohedra, whereas in others different forms of calcite, such as
hollow vaterite spheres, have been observed.10 In the more complex
case, polymers have been used to control the nucleation and growth
of calcium carbonate crystals. In many cases, the field of
biomineralization has motivated these studies. Polymeric systems
containing aspartic acid and glutamic acid have received much
attention because they are components present in several natural
shell-forming systems. Different polymorphs of calcium carbonate
were obtained depending on the amino acid sequence and polypeptide
conformation.11 Atomic force microscope (AFM) studies, combined
with molecular modeling, suggest that specific binding of natural
polypeptides, extracted from shells, to particular calcite crystal
faces, is responsible for the modification in morphology of calcite
crystals in these cases.12 In at least one case,13 a peptide was
designed to interact with a specific crystal face, and the
resulting crystals had the predicted morphology, demonstrating the
feasibility of this explanation for the natural polypeptides. Both
lysozyme14 and collagen15 have been shown to systematically
influence the calcium carbonate morphology with increasing
polypeptide concentration. In the case of lysozyme, the calcite
crystal habit was modified as the growth of different crystal faces
was sequentially inhibited. In the case of collagen, a transition
was observed with increasing collagen concentration, from
rhombohedral calcite crystals to spherulitic crystal aggregates,
via multiple layer crystals, that was explained by preferential
adsorption of collagen. In the presence of soluble proteins
extracted from coralline algae, amorphous calcium carbonate has
been stabilized.16 Synthetic polymers have also been used to alter
calcium carbonate growth. In some cases, simple anionic polymers,
such as poly(acrylic acid), poly(acrylonitrile), and poly(styrene
sulfonate), have been used to inhibit crystallization, for the
purpose of preventing limescale formation in boilers or water
pipes17-19 and, in the presence of chitin and cellulose films, to
confine crystallization to the film surface by preventing bulk
crystallization.20 Molecular modeling has shown the importance
10.1021/cg050436w CCC: $33.50 2006 American Chemical Society
Published on Web 02/14/2006
782 Crystal Growth & Design, Vol. 6, No. 3, 2006
Butler et al.
of the adsorbed polymer fine structure in controlling the
crystallization inhibition via the number and strength of the
polymer-crystal interactions.21 In other cases, specific
polymorphs, including calcite, vaterite, and calcium carbonate
hexahydrate, have been obtained in the presence of synthetic
polymers containing chemical groups, such as carboxylate, amide,
and sulfate, that are known to interact with inorganic
salts.17,22-26 More complex polymer architectures have also been
studied. For instance, anionic carboxylate terminated
poly(amidoamine) dendrimers27-29 have been found to stabilize
spherical vaterite particles. Double hydrophilic block copolymers
containing a weakly hydrophilic block and a polyelectrolyte block
that interacts with calcium carbonate,30 which are a simplified
model system for active polypeptides in biomineralization with
acidic domains, have also been found to stabilize vaterite as well
as amorphous calcium carbonate for short times.23,31-34 They lead
to the formation of well-defined microparticles with a shape that
predominantly depends on the chemical functionality of the
polyelectrolyte block. As well as showing crystallization
inhibition effects,35 a wide variety of crystal morphologies have
been obtained in the course of these studies, from the conventional
calcite rhombohedra, aragonite spicules, and vaterite spheres to
hollow shells23 (sometimes formed from aggregates of spheres36,37),
ellipsoids,38 dumbbells,23,26,39,40 plates,26 stacks of
rhombohedra,36 and disks.36 Helices41 and spheres42 have been
observed in the presence of synthetic block copolypeptides
containing serine and aspartate residues, respectively.
Interestingly, in the former case the chirality of the
copolypeptide was used to direct the sense of the helix. Many
polysaccharides are natural block copolymers and consist of two or
more different glycosidic monomer units. Alginates, which are
extracted from seaweed, are composed of mannuronic and guluronic
acid residues, for example. Despite their importance in the field
of biomineralization, where anionic polysaccharides provide the
template for the intricate hierarchical assemblies of calcium
carbonate crystals in coccoliths,43,44 relatively little work has
been done on the systematic study of the influence of simple
polysaccharides on calcium carbonate crystallization. Sodium
alginate and carboxymethyl inulin have been found to inhibit the
crystallization of calcium carbonate45,46 by interacting directly
with the surface of the growing crystals. Although sodium alginate
did not alter the standard rhombohedral habit of calcite,
carboxyethyl inulin caused spherical vaterite particles to grow in
addition to rhombohedral calcite crystals. For carboxymethyl
inulin, the inhibition efficacy increased with the carboxylate
content of the polymer. In another study, different calcium
carbonate polymorphs were obtained in the presence of different
polysaccharides.47 Calcite and vaterite were formed in the presence
of beta cyclodextrin and soluble starch, respectively. The presence
of the different polymorphs was explained by the differences in
geometrical matching and stereochemical complementarity between the
calcium ions and the hydroxyl groups on the biopolymers. The
importance of stereochemistry was also demonstrated in a study of
the influence of poly(alginic acid) and poly(galacturonic acid) on
calcium carbonate growth. Despite containing similar numbers of
carboxylate groups, poly(alginic acid) was found to be more
effective than poly(galacturonic acid) at influencing calcium
carbonate growth morphologies at equal concentrations.44 Some
studies on sulfated polysaccharides have been reported47-49 that
were inspired by the presence of sulfated
proteoglycans in some biomineralizing systems. Although
-carrageenan had a relatively small influence on calcium carbonate
growth,47 heparin was shown to form either aragonite crystals48 or
helicoidally arranged calcite aggregates49 depending on the
experimental conditions. Hyaluronic acid caused the formation of
columnar stacks of rhombohedral calcite crystals, whereas
individual cuboctahedral calcite crystals formed in the presence of
keratan sulfate.49 No systematic attempt was made to relate the
molecular structure of the sulfated polysaccharide to the crystal
habit in these studies. More detailed studies have been performed
on the formation of thin films of calcium carbonate on biopolymer
substrates, in an attempt to mimic the formation of the nacreous
layers of calcium carbonate in mollusc shells that grow on
biopolymer templates. Several studies have demonstrated the
formation of calcite films on insoluble polysaccharide (chitosan)
substrates in the presence of poly(acrylic) or poly(aspartic
acid).20,50-54 In these cases, the interaction of the polyanion
with the chitosan surface provided nucleation sites for the
initiation of calcium carbonate formation, while the presence of
free polyanion in solution prevented bulk nucleation and growth and
confined the crystal growth to the film on the insoluble substrate.
Calcite films have been grown in a similar way on simple glass
substrates, in the presence of poly(acrylic), poly(aspartic), and
poly(glutamic) acid.55 In the presence of magnesium, aragonite
films were grown on chitosan substrates.56 In the present study,
the crystallization of calcium carbonate in the presence of a range
of biopolymers that can be considered to be natural
double-hydrophilic block copolymers was investigated. They were
chosen so that the effects of calcium binding, number, and type of
side group (i.e., carboxylate and sulfate) on the resulting calcium
carbonate morphology and unit cell could be studied. As such, this
report presents the first investigation of the influence of a range
of readily available food biopolymers on calcium carbonate
crystallization. The biopolymers studied, shown in Figure 1,
consisted of two types. The first type, which were biopolymers that
did not gel in the presence of calcium, were gellan, xanthan, and
-carrageenan. Gellan and xanthan are acidic biopolymers that
contain residues with carboxylic acid groups. -Carrageenan is a
sulfated polysaccharide. The second type of biopolymer studied were
those that did chelate calcium to form gels. These were sodium
alginate and pectin, both of which contain residues with carboxylic
acid groups. Experimental SectionMaterials and Sample Preparation,
Control Samples. Calcium carbonate was formed by mixing and
continually stirring 10 mL of calcium chloride solution and 10 mL
of sodium bicarbonate solution (supplied by Sigma Chemical
Company), with the concentrations shown in Tables 1 and 2. In the
first and second set of experiments, where the ratio of calcium
chloride/sodium bicarbonate concentration was 1:2 and 1:1,
respectively, the mixture was raised to pH 10.5 by the dropwise
addition of 5 M sodium hydroxide, potassium hydroxide, or ammonium
hydroxide solutions (supplied by Sigma Chemical Company). To test
for the influence of pH, a similar procedure was adopted for
calcium chloride/sodium bicarbonate concentrations of 0.01 M:0.02 M
and 0.01 M:0.01 M, using sodium hydroxide to achieve pH values
between 8 and 12 inclusive, in increments of 0.5 pH units.
Measurements of pH were performed using a pH meter (HI8424, from
Hanna Instruments). Materials and Sample Preparation, Samples in
the Presence of Biopolymers. The biopolymers used in this study
were gellan (gel F, supplied by Kelco), xanthan (cold water soluble
Keltrol RD, supplied by Kelco), -carrageenan (Genugel X0909,
supplied by Kelco), sodium alginate (Manugel DMB, supplied by
Kelco), LM pectin (LM12, 35% de-esterified, supplied by Kelco), and
high-methoxy (HM) pectin (65% de-esterified, supplied by
Kelco).
Calcium Carbonate Crystallization
Crystal Growth & Design, Vol. 6, No. 3, 2006 783
Figure 1. Molecular structure of (a) xanthan, (b) gellan, (c)
-carrageenan, (d) sodium alginate, and (e) pectin.Table 1. Solution
Concentrations Used in Experiment 1a mix 1 cCaCl2 (mol/ L) cNaHCO3
(mol/ L)a
mix 2 0.025 0.050
mix 3 0.05 0.10
mix 4 0.075 0.150
mix 5 0.10 0.20
0.01 0.02
The pH was raised to 10.5. Table 2. Solution Concentrations Used
in Experiment 2a mix 6 mix 7 0.025 0.025 mix 8 0.05 0.05 mix 9
0.075 0.075 mix 10 0.10 0.10
cCaCl2 (mol/ L) cNaHCO3 (mol/ L)a
0.01 0.01
The pH was raised to 10.5.
Stock solutions of 4% (by weight) were made from sodium
alginate, -carrageenan, LM and HM pectin by stirring the dry
biopolymer powder into de-ionized water. The sodium alginate and
-carrageenan solutions were heated to 80 C for 30 min to ensure
complete dissolution, whereas the pectin solutions were stirred at
room temperature until completely dissolved. Stock solutions of
0.2% (by weight) of gellan and xanthan were made by adding the dry
biopolymer powder to de-ionized water and stirring at 80 C for 30
min to ensure complete dissolution. These stock solutions were
diluted to the required final concentrations for the calcium
carbonate growth experiments. The zero shear rate viscosity of the
different biopolymer solutions was measured for a variety of
concentrations, using a dynamic stress rheometer (Rheometrics
DSR200) equipped with a Couette geometry,
to determine the critical entanglement concentration that
separates the dilute concentration regime, where the polymers can
be regarded as separate entities, and the semidilute regime, where
the molecules are entangled. The critical entanglement
concentrations (%,w/v) were sodium alginate, 1.75; -carrageenan,
1.75; LM pectin, 1.83; HM pectin, 0.75; gellan, 1.12; xanthan,
0.005. Calcium carbonate was formed by mixing 10 mL of calcium
chloride (0.01 M) solution with 10 mL of sodium bicarbonate (0.02
M) solution containing the biopolymer at the required
concentration, made from the stock solution. The pH of the mixture
was increased to 10.5 using 5 M sodium hydroxide solution to
trigger the precipitation of calcium carbonate crystals. A
biopolymer concentration of 0.2% (by weight) was used for all of
the samples because this was below the critical entanglement
concentration for all of the biopolymers except for xanthan. pH
Measurements During Crystallization. A benchtop pH meter (Hanna
model 302) was used to continually measure the pH in mixtures
containing different biopolymers. 30 mL of 0.01 M calcium chloride
solution was added to 30 mL of biopolymer solution containing 0.02
M sodium bicarbonate in a glass beaker. The pH was increased to
10.5 by the dropwise addition of 5 M sodium hydroxide solution, and
its value was subsequently measured at intervals of 2 min. The
mixture was continuously stirred during the experiment. Optical
Microscopy. Transmission optical microscopy (Leitz Diaplan, set up
for Kohler illumination) was used to determine the crystal habit of
the calcium carbonate crystals, which had precipitated and aged for
3 days, in the control samples and in the presence of
biopolymers.
784 Crystal Growth & Design, Vol. 6, No. 3, 2006
Butler et al.
Figure 3. Typical calcium carbonate crystals observed in the
control samples: (a) single calcite rhombohedron and (b) aggregate
of calcite rhombohedra. Figure 2. Variation in turbidity (measured
by absorbance) with time for a calcium carbonate crystallization
control sample. The crystals were obtained by centrifuging the
suspension in which they formed and washing with 0.1 M sodium
hydroxide solution (to prevent dissolution during washing) three
times. A drop of the final, cleaned suspension was placed on a
microscope slide beneath a standard glass cover-slip and observed
under bright-field conditions with and without crossed polarizers.
Electron Microscopy. One drop of suspension of each sample, washed
(with 0.1 M sodium hydroxide) and centrifuged three times, was
placed onto a carbon-coated copper grid. The drop was allowed to
dry in air, and the residual crystals were sputter coated with a
10nm layer of gold/palladium metal. A JEOL1200EX transmission
electron microscope equipped with an ASID10 scanning attachment,
operating at an accelerating voltage of 20 kV, was used to obtain
highmagnification images of the crystal morphologies. The images
were recorded using software (INCA) supplied by Oxford Instruments.
Close-up images of the crystal surfaces were imaged using a JEOL
6301 scanning electron microscope. The samples were coated prior to
examination with a gold/palladium mixture in an Oxford CP2000
preparation chamber. Turbidity. In-situ crystallization experiments
were performed using an ultra-violet/visible wavelength (UV/Vis)
spectrophotometer (PerkinElmer Lambda 40) to measure the absorption
of light in a crystallizing suspension of calcium carbonate at 500
nm wavelength, every 20 s. The onset of crystallization was
detected via an increase in turbidity, as measured by increasing
absorbance of the light. Five milliliters of 0.01 M CaCl2 solution
was added to 5 mL of biopolymer solution containing 0.02 M NaHCO3
and 0.2% (w/v) biopolymer while being stirred using a magnetic
follower. The pH was increased to 10.5 using 1 M NaOH. Immediately
afterward, the mixture was transferred into the UV/Vis
spectrophotometer. The time taken between setting the pH to 10.5
and the turbidity measurements starting was measured and accounted
for during analysis of the results. X-ray Diffraction. X-ray
diffraction was performed at beamline 16.1 at the Synchrotron
Radiation Source, Daresbury. One-dimensional diffraction patterns
were obtained from samples consisting of a sedimented suspension of
calcium carbonate crystals with a thickness of approximately 1 mm.
The diffraction patterns were obtained on a curved Inel multiwire
detector. An exposure time of 30 s was used to collect the
diffraction patterns.
Results Control Samples. Turbidity, Optical Microscopy, and
Electron Microscopy. For all of the control samples,
crystallization occurred immediately upon reaching the final pH and
was measured by a sudden increase in sample turbidity using the
UV/Vis spectrophotometer (see Figure 2 for a sample measured at pH
10.5). The turbidity rapidly reached a maximum value and then
decreased again, owing to sedimentation of the crystals in the
cuvette. For all of the experimental conditions, the control
samples yielded rhombohedral crystals. Sometimes single crystals
were obtained, shown in the optical micrograph in Figure 3a for a
sample at pH 10.5, but in most cases aggregates of rhombohedra were
obtained, shown in the optical micrograph in Figure 3b,
also from a sample at pH 10.5. Scanning electron micrographs,
shown in Figure 4for samples at pH 10.5, revealed that the
aggregates were formed from several crystals that had intergrown.
pH Measurement. The pH variation after addition of alkali to the
target pH was different for the samples with 1:2 and 1:1 calcium
chloride/sodium bicarbonate ratios. The 1:2 samples remained
roughly at the target pH value that was attained after addition of
alkali, shown in Figure 5a. The 1:1 samples experienced a variation
in pH with time, shown in Figure 5b, which depended on the target
pH value. Above approximately pH 11, the pH remained constant.
Below pH 11, a rapid decrease was observed that reached a plateau
around pH 7.5. Because the pH was found to be constant after
addition of alkali for the 1:2 samples, subsequent experiments in
the presence of biopolymer were made with a 1:2 ratio of calcium
chloride to sodium bicarbonate. Crystallization in the Presence of
Biopolymers. Turbidity. For all of the biopolymers, there was an
induction time before the onset of crystallization was measured by
an increase in turbidity of the mixture. Figure 6 shows a typical
plot of the change in turbidity with time of a sample containing
0.2% (w/ v) biopolymer (-carrageenan) at pH 10.5, measured using
the UV/Vis spectrophotometer. The value of the induction time
varied within each sample, although the average induction time,
shown in Table 3, depended on the type of biopolymer for samples
with the same polymer concentration and final pH value. It also
depended on the biopolymer concentration, shown in Figure 7 for LM
pectin and -carrageenan. The induction time decreased with
increasing pectin concentration, although the entire sample formed
a gel at concentrations above 1%, and no crystallization was
observed at and above this concentration. In contrast, the reverse
trend was observed for -carrageenan that showed a positive
correlation between induction time and biopolymer concentration.
Optical and Electron Microscopy. Figure 8 shows optical micrographs
of samples, taken in bright-field mode and between crossed
polarizers, containing different biopolymers at 0.2% (w/ v) and at
pH 10.5. The bright-field images showed that in all cases the
majority of the crystals were in the form of aggregates. Single
rhombohedra were extremely rare. Crystals formed in the presence of
gellan and xanthan were of similar size and were anisotropic,
elongated aggregates. Close inspection of the polarizing optical
micrographs suggested that these aggregates were stacks of
rhombohedra. In contrast, dumb-bell-shaped aggregates were observed
in the presence of LM and HM pectin, and a mixture of spherical and
ellipsoidal aggregates was observed in the presence of -carrageenan
and sodium alginate. The crystalline aggregates formed in the
presence of -carrageenan appeared to be the smallest, whereas
similarly sized aggregates were formed in the presence of the
different pectins and sodium alginate. These observations were not
quantified,
Calcium Carbonate Crystallization
Crystal Growth & Design, Vol. 6, No. 3, 2006 785
Figure 4. Scanning electron micrograph of calcium carbonate
crystals grown in a control sample, showing a single crystal and
crystal aggregates.
Figure 6. Variation in turbidity (measured by absorbance) with
time for calcium carbonate crystallization in the presence of 0.2%
(w/v) -carrageenan, as a typical example of a system displaying an
induction time before the onset of crystallization.
Figure 5. Variation of pH with time for samples made with (a) a
1:2 ratio of calcium chloride/sodium bicarbonate and (b) a 1:1
ratio of calcium chloride/sodium bicarbonateTable 3. Induction
Times before the Onset of Calcium Carbonate Crystallization in the
Presence of Different Biopolymer Additives (0.2% w/v) biopolymer
gellan xanthan -carrageenan sodium alginate HM pectin LM pectin
induction time (minutes) 1.8 ( 0.5 2.0 ( 0.7 3.0 ( 1.1 82.0 ( 46.6
5.5 ( 1.0 46.0 ( 6.9
Figure 7. Variation of induction time before the onset of
crystallization with biopolymer concentration for LM pectin and
-carrageenan.
however. Inspection of the polarizing optical micrographs
revealed that for the -carrageenan, pectin, and sodium alginate
samples Maltese cross patterns were formed, although their
appearance did depend on the position of the crystalline aggregate
relative to the focal plane of the microscope. Scanning
transmission electron micrographs for these crystals are shown in
Figure 9. The higher magnification attainable using electron
microscopy confirmed the observations made using optical microscopy
for crystals grown in the presence of gellan and xanthan. In these
cases, the crystal aggregates were indeed
stacks of rhombohedra. The stacks were better defined for the
crystals in the xanthan sample. For the pectin, alginate, and
-carrageenan samples the ellipsoidal and dumb-bell-shaped
aggregates appeared to be formed from interconnected and radially
arranged crystals, which was most apparent for the LM pectin
sample. The size of the crystals in the sodium alginate sample was
greater than for the pectin and -carrageenan samples, which gave
the aggregates formed in the presence of sodium alginate a slightly
less well ordered appearance. In some cases, shown in Figure 10,
for the samples containing pectin and -carrageenan, hollow
crystalline shells were observed. The number of these was increased
when powdered samples of these crystals were crushed in a press.
Figure 11 shows scanning electron micrographs of the surface of the
crystals at higher magnification, and therefore in greater detail,
than the scanning transmission electron micrographs. The
786 Crystal Growth & Design, Vol. 6, No. 3, 2006
Butler et al.
concentrations of LM pectin and -carrageenan, at pH 10.5,
respectively. In all cases, similar types of crystalline aggregates
were formed that exhibited a Maltese cross pattern between crossed
polarizers in the case of LM pectin. For the LM pectin, fewer
crystals were observed at the higher biopolymer concentrations,
although the crystal size was similar. For the -carrageenan
samples, the crystals appeared to become more dumbbell shaped as
the biopolymer concentration increased from 0.3% (w/v) to 1.8%
(w/v), while crystal size and number were similar (being small and
high, respectively) at low and high concentrations, with a reduced
amount but larger size at intermediate concentrations. Scanning
transmission electron micrographs of calcium carbonate grown in the
presence of different concentrations of LM pectin and -carrageenan
are shown in Figure 14. Similar crystal morphologies were obtained
for all of the concentrations used. Figure 15 shows scanning
transmission electron micrographs of calcium carbonate grown in the
presence of 0.2% (w/ v) LM pectin at different pH values. For all
of the pH values studied, similar crystal morphologies were
obtained. pH Measurement. Figure 16 shows the variation in pH with
time after addition of 1 M sodium hydroxide to three different
samples containing 0.2% (w/v) LM pectin, to attain pH 10.5. The pH
initially increased to about 10.6 and after approximately 20 min
began to decrease again toward a plateau in the region between 10.0
and 10.2. On average, the time before the pH began to drop was less
than the time taken before crystallization was first detected from
the increase in turbidity measured by the UV/Vis spectrophotometer.
Crystalline Form. Figure 17 shows the wide-angle scattering
patterns from all of the samples, indexed according to calcite,
taken at least 2 days after crystallization occurred. For all of
the samples, calcite was the only crystalline form present. The
broad background scattering is caused by the biopolymer present in
the sample. Discussion Although calcite was formed in the control
samples and in the presence of biopolymers, the crystalline
aggregates that were formed in both cases were different shapes.
Whereas the control samples contained single and randomly clustered
rhombohedra, the rhombohedra that grew in the presence of
biopolymers were structured in particular arrangements.
Furthermore, the crystals that grew in the presence of the
biopolymers appeared after an induction time, signifying that there
was an interaction between the incipient calcium carbonate crystals
and the biopolymer that altered the nucleation kinetics. It should
be noted that, for the samples containing biopolymers, the 1:2
ratio of calcium chloride/sodium bicarbonate was used because, at
pH values greater than 10.5, only carbonate ions are formed, as
described in the following reaction. In this case, the hydrogen
carbonate ions were in excess and acted as a buffer in the presence
of sodium hydroxide to maintain a constant pH value.
Figure 8. Transmission optical micrographs (left-hand side) and
corresponding polarizing optical micrographs (right-hand side) of
calcium carbonate crystals grown in the presence of 0.2%
biopolymer: (a) gellan, (b) xanthan, (c) -carrageenan, (d) sodium
alginate, (e) LM pectin, and (f) HM pectin.
surfaces of the crystals grown in the presence of xanthan
appeared to be roughened, or composed of many nanocrystallites with
sizes of approximately 100 nm, on all of the crystal faces.
Crystals grown in the presence of -carrageenan and pectin appeared
to be roughened on some of the faces, but other faces were smooth.
All of the faces appeared to be smooth for the crystals grown in
the presence of sodium alginate. Figures 12 and 13 show
bright-field and polarizing optical micrographs of crystals formed
in the presence of different
H2CO3 + CaCl2 + 2NaHCO3 w CaCO3 + 2NaCl + HCO3 + H CO32- +
2H+Below pH 10.5, hydrogen carbonate is a weak acid, and it can
dissociate and liberate hydrogen ions, leading to a decrease in pH.
For the 1:1 ratio of calcium chloride/sodium bicarbonate, there was
no excess hydrogen carbonate, as it was all converted
|
Calcium Carbonate Crystallization
Crystal Growth & Design, Vol. 6, No. 3, 2006 787
Figure 9. STEM micrographs of calcium carbonate crystals grown
in the presence of 0.2% biopolymer: (a) gellan, (b) xanthan, (c)
-carrageenan, (d) sodium alginate, (e) LM pectin, and (f) HM
pectin.
to carbonate ions. The system therefore had no buffering
capacity, and the excess hydrogen ions caused the decrease in pH
that was measured at all of the pH values at which crystallization
was initiated. Since it was desirable to conduct experiments in
which the formation of calcium carbonate was as regulated as
possible, all of the experiments with biopolymers were performed at
pH 10.5, with the 1:2 ratio of calcium chloride/sodium bicarbonate.
The induction time that was measured prior to the onset of
crystallization in the presence of the biopolymers was indicative
of the crystallization inhibition that has been measured in the
presence of many polymers, including biopolymers such as
alginate,44,45 -carrageenan,48 and poly(galacturonic acid),44
which is similar to pectin. The current results, which showed that
sodium alginate was more effective at inhibiting calcium carbonate
crystallization than either pectin or -carrageenan, are consistent
with the previous studies on alginate, poly(galacturonic acid) and
-carrageenan.44,48 In those studies, it was argued that the main
factors that contributed to the crystallization inhibition were
charge strength, amount, and polymer conformation. The final factor
was proposed because some polyanions, such as chondroitin
sulfate,57 were investigated that, despite possessing many anionic
groups, had very little effect on calcium carbonate
crystalliza-
788 Crystal Growth & Design, Vol. 6, No. 3, 2006
Butler et al.
Figure 10. STEM micrographs of calcium carbonate shells grown in
the presence of (a) -carrageenan and (b) LM pectin.
Figure 11. SEM micrographs of the surfaces of calcium carbonate
crystals grown in the presence of (a) xanthan, (b) -carrageenan,
(c) sodium alginate, and (d) HM pectin.
tion. It was concluded that additional factors, such as polymer
conformation that may lead to a reduced availability or dissipation
of charge, were important. The first factor, strength of charge,
explains why -carrageenan had less effect on calcium carbonate
crystallization than did HM pectin, despite possessing similar
numbers of anionic groups. The lower partial charge on the oxygen
atoms of the anionic sulfate groups in -carrageenan compared to the
anionic carboxylate groups in pectin will result in a weaker
interaction with the calcium ions at the surface of calcium
carbonate crystals and therefore a weaker influence on calcium
carbonate crystallization. The second factor, number of charges,
explains the much smaller induction times for gellan and xanthan
compared to pectin and alginate, HM pectin compared to LM pectin,
and LM pectin compared to -carrageenan. All of these polymers
possess anionic carboxylate groups, but xanthan and gellan contain
far fewer carboxylate groups than pectin or alginate. Similarly, HM
pectin contains fewer carboxylate groups than LM pectin. The
smaller number of charges therefore leads to fewer possible
interactions between the polyanion and the surface of the calcium
carbonate crystal and weaker interaction overall. The third factor,
conformation and availability of charge,
is unlikely to be the reason for the difference in induction
time between LM pectin and sodium alginate. Pectin has been
reported to be generally more flexible than sodium alginate.58 It
might therefore be expected that the carboxylate groups in pectin
would be more available for binding to calcium, leading to more
efficient calcium sequestration by pectin and hence a longer
induction time. However, the reverse was actually observed. It
should be noted that the conformation of the polymers used in the
current study is not known. In conclusion, the difference between
the induction times of LM pectin and sodium alginate remains
unknown. The observation that, in the systems containing LM pectin,
the pH began to drop prior to the formation of calcium carbonate
detected by turbidity, suggests that events began to occur that
influenced crystallization prior to the nucleation event itself.
One explanation is that the drop in pH demonstrates the interaction
of LM pectin with incipient calcium carbonate nuclei, hindering
their formation and delaying crystallization until later times. A
previous study of calcium carbonate crystallization in the presence
of sodium alginate45 has shown the large influence that this
polyanion can have on calcium carbonate crystal growth kinetics. It
was suggested that the presence of the alginate altered
Calcium Carbonate Crystallization
Crystal Growth & Design, Vol. 6, No. 3, 2006 789
Figure 12. Transmission optical micrographs (left-hand side) and
corresponding polarizing optical micrographs (right-hand side) of
calcium carbonate crystals grown in the presence of different
concentrations of LM pectin: (a) 0.2%, (b) 0.3%, and (c) 0.4%.
the crystal growth mechanism from the normal spiral growth
mechanism,59 which has been observed using AFM in the presence of
some small molecular weight additives,5,60,61 to one controlled by
surface nucleation. The crystal morphologies were all related to
the (104) calcite rhombohedra obtained in the control samples, as
expected from the XRD data that showed that calcite was the only
crystal polymorph present in all cases. The effect of the
biopolymers was apparent, however, in the overall morphology of the
crystals. Most obvious was the distinction between the different
types of biopolymer: those that did not form gels in the presence
of calcium ions and those that did. The former biopolymers, gellan
and xanthan, formed aggregates of crystals that could be described
as roughly stack-like, whereas the latter biolymers, pectin (LM and
HM) and alginate, formed aggregates that could be described as
rosette-like. Furthermore, the rosette-like aggregates appeared to
be hollow shells. -Carrageenan, that forms a gel in the presence of
sodium but not calcium ions, formed a morphology that was
intermediate between stack- and rosette-like. Stack-like
morphologies have been obtained in the presence of hyaluronic
acid,49 a carboxylated biopolymer, a mixed solution of a
block-copolymer (poly(ethylene glycol)-block-poly(methacrylic
acid), PEG-b-PMAA) and a cationic surfactant
(cetyltrimethylammonium bromide, CTAB),36 and a watersoluble
terpolymer (poly(acrylamide-co-2-acrylamido-2-methyl1-propane
sodium sulfonate-co-n-vinyl-pyrrolidone)26 that contained several
functional groups known to interact with calcium ions. Although no
explanation was given for the formation of the stacks in the case
of hyaluronic acid, other than the acid
induced the formation of a monocrystalline aggregate, or for the
PEG-b-PMAA CTAB mixture, for the terpolymer it was suggested that
the molecule existed in an extended conformation owing to the large
number of ionizable sulfate units on the chain. Because the
terpolymer also possessed units (>CdO, >SdO, and >NdH)
that are known to interact with calcium ions, it was suggested that
they nucleated the formation of calcite crystals along the chain,
leading to the formation of stacks. A similar explanation may favor
the formation of stacks for hyaluronic acid and PEG-b-PMAA CTAB, as
well as for xanthan and gellan in the current study. All of these
polymers contain ionizable carboxylate groups that can become
charged at high pH values as well as interacting with calcium ions.
In addition, xanthan and gellan are rather stiff molecules by
virtue of being polysaccharides. It is therefore plausible to
suggest that, in the high pH conditions used in the present study
(>pH10.5), the gellan and xanthan molecules became ionized,
extended, and also nucleated calcite crystallization via the
interaction between the carboxylate groups on the polymer and the
calcium ions in solution, thereby forming stacks of calcite
rhombohedra. That there is an interaction between xanthan and
calcium carbonate is suggested by the high-resolution SEM image of
the calcite crystal surface grown in the presence of xanthan, which
shows a high degree of surface roughness consistent with the
presence of bound impurities, i.e., xanthan molecules, at the
interface. Rosette-like aggregates of calcium carbonate crystals
have been observed previously in the presence of biopolymers,
such
790 Crystal Growth & Design, Vol. 6, No. 3, 2006
Butler et al.
Figure 13. Transmission optical micrographs (left-hand side) and
corresponding polarizing optical micrographs (right-hand side) of
calcium carbonate crystals grown in the presence of different
concentrations of -carrageenan: (a) 0.2%, (b) 0.3%, (c) 1.0%, (d)
1.8%, (e) 2.0%, (f) 2.5%, and (g) 3.0%.
as heparin,47,49 tobacco mosaic virus,62 gelatin (with added
magnesium),63-65 and collagen.15 In the former two cases, it
was believed that lattice matching of the functional groups in a
helical conformation on the polymer, in the case of heparin, or
aggregates of proteins, in the case of tobacco mosaic virus, acted
as a template for the helical nucleation of calcite crystals into a
rosette-like aggregate. In the presence of gelatin and magnesium it
was shown that the rosette-like structures, that were very similar
in appearance to those formed in the current study in the presence
of LM pectin, were spherulitic structures formed from a nucleus of
aligned magnesium calcite prisms. Subsequent crystallization led to
an angular spread in the orientation of the prisms that increased
until the aggregate ended in a globular shape. A similar
spherulitic growth has been observed in similar inorganic systems:
octacalcium phosphate in the presence of poly(aspartic acid) or
poly(acrylate)66 and fluoroapatite in the presence of gelatin.67
The biopolymer systems in which rosette-like aggregates formed in
the present case, namely, pectin (HM and LM) and alginate, neither
form helical conformations nor aggregate into them. Therefore, the
rosettes are not likely to have formed via direct templating on the
biopolymer chains themselves. The spherulite growth mechanism
proposed in the gelatin study is also not a likely explanation for
the rosettes formed in the present study, since that relied upon
the formation of aligned magnesium calcite prisms, and no such
crystals were observed. A more likely explanation for the formation
of the rosettes in the presence of pectin and alginate is suggested
by previous studies of templated growth of calcium carbonate on
certain substrates, combined with the observations in the present
case that the rosettes formed only in the presence of biopolymers
that gelled in the presence of calcium ions and that, in some cases
at least, the calcite crystals were shown to form hollow shells.
Growth of calcium carbonate in the presence of p-mercaptophenol
colloidal gold seeds leads to the formation of rosettelike crystals
with a radial arrangement of calcite rhombohedra parallel to the
[001] crystallographic direction.68 In this case, the calcium
carbonate was believed to have been directly nucleated by the
modifed gold colloids, with the final morphology being influenced
by crystal-crystal interactions at later stages of growth that led
to mutual frustration of growth in the direction tangential to the
growing rosette. In another study, calcite platelets, formed in
lamellar surfactant structures used to stabilize aqueous foams,
have been shown to assemble into shells with a rosette-like
appearance.69 Rosettes have also been formed in the presence of
PEG-bPMAA,23,37,70 PEG-b-PMAA-aspartate (Asp),23
poly(styreneblock-acrylic acid) (PS-b-PAA),34 and PEO-b-PHEE30
double hydrophilic block copolymers as well as a solution
containing poly(ethylene oxide-block-methacrylic acid (PEO-b-PMAA)
and surfactant, sodium dodecyl sulfate (SDS).36 The block copolymer
was present in the calcite in all cases and was believed to
strongly interact by virtue of lattice matching between the
functional groups on the polymer and the calcium ions in the
crystal.30,34 For the PEG-b-PMAA-containing systems at least,
hollow aggregates of calcite crystals arose from the initial
formation of a calcium carbonate particle composed either of
amorphous calcium carbonate23 or an aggregate of vaterite
spheres.37 The block copolymer was then proposed to template
calcite nucleation and growth on the surface of the core calcium
carbonate particle. The outer layer of facetted calcite crystals
then grew at the expense of the core, which dissolved to provide
material for crystal growth. In a similar manner, surface
functionalized gold nanoparticles adsorbed onto vaterite spheres
have been shown to result in templated aragonite overgrowth
Calcium Carbonate Crystallization
Crystal Growth & Design, Vol. 6, No. 3, 2006 791
Figure 14. STEM images of calcium carbonate grown in the
presence of (a) LM pectin, 0.2%, (b) LM pectin, 0.4%, (c)
-carrageenan, 0.2%, and (d) -carrageenan, 2.5%.
leading to rosette-shaped crystal aggregates.71 Surface
functionalized dendrimer molecules, containing moieties that
interact with calcium at the outer surface of the dendrimer, also
lead to a spherical overgrowth of calcium carbonate crystals.27,28
Pectin and alginate are biopolymers that interact strongly with
calcium ions, to the extent that, at the right concentration, a gel
is formed whereby the calcium ions form physical cross-links
between different biopolymer chains. It is therefore likely that
the addition of calcium chloride to the solution containing pectin
or alginate and sodium bicarbonate led to the formation of small
regions of gelled biopolymer that contained bound calcium ions that
could act as nuclei for calcium carbonate crystal growth. In the
same manner as the double hydrophilic block copolymers or colloidal
gold particles templated the growth of calcite or aragonite on a
particle core, leading to the formation of a crystalline shell, it
is proposed that in the current study the pectin or alginate that
formed the core also templated the nucleation of the calcite
overgrowth. The common feature in all of the systems is that a
species is present that contains chemical groups, such as
carboxylate groups, that interact strongly with calcium. The
difference between the systems is that in the previous studies the
crystalline shell grew on a calcium carbonate core that
subsequently dissolved, whereas in the present study the interior
of the rosette-like aggregate is proposed to contained a biopolymer
core. Depending on the amount of free calcium ions present within
the biopolymer gel microparticle, it is therefore possible to form
calcite within the gel as well as at the surface, which explains
why, in some cases, separate calcite aggregates were observed that
fitted perfectly within the rosette-like shell. Presumably,
crystallization within pectin or alginate would be less likely to
occur than in the solution near the surface of the
gel because there will be less free calcium in the interior of
the gel that is available to form calcite crystals. An insufficient
number of open shells were observed to test this hypothesis,
however. Future studies will concentrate on developing an
understanding of the hollow calcite shells that are proposed to
form in the presence of pectin or alginate. Interestingly, the
rosette-like aggregates formed in the presence of LM pectin
appeared more ordered than those formed in the presence of HM
pectin and alginate, shown both by the appearance of the rosettes
in the STEM images and by the presence of Maltese crosses in the
rosettes observed between crossed polarizers in the optical
microscope. The Maltese cross patterns showed that, for LM pectin,
radial growth of the calcite rhombohedra occurred with the crystals
all possessing a uniform orientation, i.e., the growth occurred in
a directed manner. This marked degree of orientation implies that
there was a relation between the underlying LM pectin template and
the overgrowth of calcite crystals that formed the rosette-like
aggregate. The possibility of such a direct interaction is
suggested by studies of calcium carbonate growth on biopolymer
films, such as chitosan in the presence of poly(acrylic
acid)20,50-55 and cellulose.54,72 In the case of calcite grown on
poly(acrylic acid) bound to insoluble chitosan films, a direct
match between the separation of the calcium ions in the calcite
crystal on the (001), (110), and (104) faces and the spacing of the
carboxylate groups on the polymer was used to explain the crystal
morphologies obtained.50,51,55 Although detailed crystallographic
data are not available from the present study, future
investigations on templated growth on biopolymer films will explore
the nature of any potential crystallographic templating effect in
the presence of LM pectin.
792 Crystal Growth & Design, Vol. 6, No. 3, 2006
Butler et al.
Figure 15. STEM images of calcium carbonate grown in the
presence of 0.2% LM pectin at different pH values: (a) 10, (b)
10.5, (c) 11, and (d) 11.5.
Figure 16. Change in pH with time during crystallization in
solutions containing LM pectin.
Figure 17. X-ray diffraction patterns of the calcium carbonate
crystals grown in the presence of different biopolymers, indexed
according to calcite.
It was also observed that, in the present study, the rosettelike
aggregates often formed in the shape of dumb-bells. This effect was
most noticeable for LM pectin but could also be seen
to some extent for all of the other systems containing
rosettelike aggregates, including -carrageenan. In the case of
fluoroapatite grown in the presence of gelatin,67 dumb-bell-shaped
spherulites were obtained that were explained, and successfully
reproduced, by a fractal model of crystal growth. Modification of
crystal growth by local electric fields resulting from
crystalcrystal and crystal-polymer interactions were proposed to
lead to a divergence of crystal orientation from a central, flat,
hexagonal seed, eventually leading to a dumb-bell-shaped aggregate
with the crystals arranged in the same way as the electrical field
lines around a permanent dipole. However, although the fractal
model did not fully explain the dumb-bell morphologies that grew on
spherical calcium carbonate aggregates in the presence of double
hydrophilic block copolymers,39 enough similarities were observed
with the fluoroapatite aggregates to suggest that a similar
spherulitic growth mechanism could explain the calcium carbonate
dumb-bells in the later stages of aggregate growth. It is likely,
therefore, that the dumbbells formed in the present study were a
result of a spherulitic growth mechanism in which the gel particle
at the core of the aggregate acted as the precursor wheat sheaf
particle. If nucleation did not occur uniformly across the gel
microparticle, the initial crystals that formed at the particle
surface would produce the asymmetry present in the wheat-sheaf
shape of spherulite precursors that have been definitely shown to
produce dumb-bells. It would therefore be possible for a spherical
nucleus to form dumb-bells. In the case of -carrageenan, an
intermediate morphology was obtained that lay between the
stack-like morphology observed in the presence of nongelling
biopolymers and the rosette-like morphology obtained in the
presence of calcium-gelling biopolymers. However, the aggregates
formed in the presence of -carrageenan also appeared to be hollow,
in at least some cases. These observations can be explained using
the hypothesis
Calcium Carbonate Crystallization
Crystal Growth & Design, Vol. 6, No. 3, 2006 793
proposed for the calcium-gelling biopolymers, since -carrageenan
is known to form gels in the presence of sodium ions. Sodium ions
were present in the current study since the reaction to form
calcium carbonate was performed using sodium bicarbonate.
Therefore, it is likely that a weak gel of -carrageenan formed.
Furthermore, it is known that the cross-links in -carrageenan gels
are formed from two chains interacting in a double helix. The
presence of a gel therefore provides the template for the formation
of a hollow shell of calcite crystals in the same rosette form
observed in the presence of pectin and alginate, whereas the
presence of extended double-helical regions provides the reason for
the extended nucleation in the stack form observed in xanthan and
gellan. From the above discussion, it is possible to explain the
effect of pH and biopolymer concentration on the crystal
morphology. First, the overall similarity in aggregate morphology
for the crystals grown in the presence of LM pectin at different pH
values is to be expected, since in all cases crystal growth was
controlled only by the presence of the nucleating LM pectin gel
template and the presence of calcium and carbonate ions in
solution, which will be similar at all of the pH values studied.
The less-ordered aggregates observed at higher pH values reflect
the faster growth rates caused by the higher supersaturation of
carbonate ions in solution at higher pH. Second, the overall
similarity of the morphologies observed at different biopolymer
concentrations can also be explained for similar reasons. That the
biopolymers studied interact in solution to a lesser extent than
the double hydrophilic block copolymers reported in the literature
is shown by the presence of the rhombohedral (104) calcite form in
all of the biopolymer systems in the present study, over the range
of concentrations used, in the present study. For the block
copolymer systems, differences in crystal habit were also observed
over particular polymer concentration ranges, indicating a strong
interaction of those polymers with particular crystallographic
planes that markedly altered their growth rates.23,36 The
relationship between crystallization kinetics and the concentration
of LM pectin and -carrageenan is less straightforward to explain,
however. The nucleating ability of polymers is correlated with the
ability of the polymer to bind metal ions that intiates the
formation of subcritical nuclei that grow to the critical size
required for crystal growth.72 From standard nucleation theory, the
induction time, , is related to the solution supersaturation, , by
the following equation:
0.4%, however, a decrease in induction time was measured that
cannot be explained by the effect of the polymer on
supersaturation. Since LM pectin formed the most ordered radially
oriented aggregates of crystals and was therefore proposed to act
as a direct template for calcite growth, it is possible that the
decrease in induction time is indicative of a decrease in surface
energy that is increasingly favorable for nucleation despite the
reduction in solution supersaturation. Such an effect could occur
if, at the higher LM pectin concentrations, the separation between
the templating carboxylate groups became increasingly matched to
the lattice spacing between calcium ions on the templated crystal
plane. For -carrageenan, that does not gel in the presence of
calcium ions but is still expected to interact via the sulfate
groups, the overall increase in induction time is expected owing to
an increase in the amount of bound calcium with increasing polymer
concentration. The effect of supersaturation was therefore dominant
overall. Interestingly, however, a peak in the induction time was
measured, superimposed on the overall increase, at a -carrageenan
concentration around 2%, that coincided with the observation of
fewer nuclei and larger crystals using optical microscopy. The
subsequent reduction in induction time at the -carrageenan
concentration of 2.5% indicated that a compensating effect was
present at this concentration that served to increase the
likelihood of crystallization. As for the LM pectin case, it is
possible that at this concentration the average spacing of the
sulfate groups provided a better match between the polymer and the
incipient calcite crystals, thereby reducing the surface energy of
the calcite nuclei and promoting nucleation. Further experiments
will be performed on twodimensional biopolymer surfaces to
investigate these phenomena. Conclusions The influence on calcium
carbonate crystallization of a series of food biopolymers, which
contain carboxylic acid or sulfate functional groups, was studied
using a variety of techniques, including pH and turbidity
measurements, optical microscopy, and scanning electron microscopy.
The biopolymers chosen were gellan, xanthan, LM pectin, HM pectin,
and sodium alginate, which contain carboxylate groups, and
-carrageenan, which contains sulfate groups. In control samples
containing no biopolymer, single calcite (104) rhombohedra were
formed. In the presence of biopolymers, (104) rhombohedra were
formed as aggregates that were either stack-like or rosette-like.
Stacks were formed in the presence of nongelling biopolymers, which
were nucleated by the carboxylate groups on extended xanthan or
gellan chains. Rosettes were formed in the presence of
calcium-gelling biopolymers and were proposed to form by the
nucleation of calcite on a gelled microparticle template. LM pectin
was particularly effective at directing the growth of calcite
rosettes and led to aggregates of radially aligned crystals.
Evidence was found that the rosettes were hollow. The influence of
biopolymer concentration on calcite crystallization was studied for
LM pectin and -carrageenan. In the former case, an increasingly
favorable influence of the pectin molecules on the surface energy
of calcite nuclei was believed to result in an enhanced propensity
for nucleation, until the pectin concentration was so high that all
of the calcium was sequestered. In the latter case, an increase in
calcium binding with increasing -carrageenan concentration
generally decreased the solution supersaturation and hence
decreased the propensity for calcite formation. At certain higher
concentrations, however, it was possible that the -carrageenan
log
(
1 (2.303kBT)3 (log )2
2s2
)
where is a shape factor for the calcite nuclei () 16/3 for
spherical shapes), is the molar volume of calcite () 1.89 10-5
m-3), s is the surface energy of the calcite nuclei, kB is
Boltzmanns constant, and T is the temperature. Therefore, as the
polymer concentration changes the two factors that may be affected
are the solution supersaturation, as the polymer binds more calcium
from solution, and the nucleus surface energy, as the polymer may
become associated with the growing crystal. In the case of LM
pectin, it is known that the polymer binds calcium ions, as this is
how the pectin gel is formed. At concentrations above 1%, no
calcite crystals formed, and the entire solution became a single
lump of gel. In this case, the LM pectin had bound all of the
available calcium ions leaving none free for crystallization, which
is represented in the above equation for the induction time as the
limit when supersaturation tended to zero. At the concentrations
studied, between 0.2 and
794 Crystal Growth & Design, Vol. 6, No. 3, 2006
Butler et al.(34) Linhai, Y.; Dalai, J. Chin. Sci. Bull. 2004,
49, 235. (35) Sedlak, M.; Antonietti, M.; Colfen, H. Macromol.
Chem. Phys. 1998, 199, 247. (36) Qi, L.; Li, J.; Ma, J. AdV. Mater.
2002, 14, 300. (37) Yu, S.-H.; Colfen, H.; Antonietti, M. J. Phys.
Chem. B 2003, 107, 7396. (38) Marentette, J. M.; Norwig, J.;
Stockelmann, E.; Meyer, W. H.; Wegner, G. AdV. Mater. 1997, 9, 647.
(39) Colfen, H.; Qi, L. Chem. Eur. J. 2001, 7, 106. (40) Endo, H.;
Schwahn, D.; Colfen, H. J. Phys. Chem. 2003, 107, 7396. (41)
Sugawara, T.; Suwa, Y.; Ohkawa, K.; Yamamoto, H. Macromol. Rapid
Commun. 2003, 24, 847. (42) Euliss, L. E.; Trnka, T. M.; Deming, T.
J.; Stucky, G. D. Chem. Commun. 2004, 15, 1736. (43) Marsh, M. E.
in Biomineralization: From Biology to Biotechnology and Medical
Applications; Baeuerline, E., Ed.; Wiley-VCH: Weinheim, 2000; pp
251-268. (44) Didymus, J. M.; Oliver, P.; Mann, S.; DeVries, A. L.;
Hauschka, P. V.; Westbroek, P. J. Chem. Soc., Faraday Trans. 1993,
89, 2891. (45) Manoli, F.; Dalas, E. J. Mater. Sci.: Mater. Med.
2002, 13, 155. (46) Verraest, D. L.; Peters, J. A.; van Bekkum, H.;
van Rosmalen, G. M. J. Am. Oil Chem. Soc. 1996, 73, 55. (47)
Falini, G.; Gazzano, M.; Ripamonti, A. J. Cryst. Growth 1994, 137,
577. (48) Yang, L.; Zhang, X.; Liao, Z.; Guo, Y.; Hu, Z.; Cao, Y.
J. Inorg. Biochem. 2003, 97, 377. (49) Arias, J. L.;
Neira-Carrillo, A.; Arias, J. I.; Escobar, C.; Bodero, M.; David,
M.; Fernandez, M. S. J. Mater. Chem. 2004, 14, 2154. (50) Zhang,
S.; Gonsalves, K. E. J. Appl. Polym. Sci. 1995, 56, 687. (51)
Zhang, S.; Gonsalves, K. E. Mater. Sci. Eng. C 1995, 3, 117. (52)
Kato, T.; Suzuki, T.; Amamiya, T.; Irie, T.; Komiyama, M.; Yui, H.
Supramol. Sci. 1998, 5, 411. (53) Kato, T.; Suzuki, T.; Irie, T.
Chem. Lett. 2000, 2, 186. (54) Hosoda, N.; Kato, T. Chem. Mater.
2001, 13, 688. (55) Kotachi, A.; Miura, T.; Imai, H. Chem. Mater.
2004, 16, 3191. (56) Sugawara, A.; Kato, T. Chem. Commun. 2000, 6,
487. (57) Meyer, H. J. J. Cryst. Growth 1984, 66, 639. (58)
Harding, S. E. Prog. Biophys. Mol. Biol. 1997, 2/3, 207. (59) Liu,
X. Y.; Boek, E. S.; Briels, W. J.; Bennema, P. Nature 1995, 374,
342. (60) Reyhani, M. M.; Oliveira, A.; Parkinson, G. M.; Jones,
F.; Rohl, A. L.; Ogden, M. I. Int. J. Mod. Phys. B 2002, 16, 25.
(61) de Yoreo, J. J.; Dove, P. M. Science 2004, 306, 1301. (62)
Sinha, A.; Chakraborty, J.; Das, S. K.; Ramachandrarao, P. Curr.
Sci. 2003, 84, 1437. (63) Falini, G.; Fermani, S.; Gazzano, M.;
Ripamonti, A. J. Chem. Soc., Dalton Trans. 2000, 10, 3983. (64)
Falini, G.; Fermani, S.; Gazzano, M.; Ripamonti, A. Chem. Eur. J.
1997, 3, 1807. (65) Falini, G. Int. J. Inorg. Mater. 2000, 2, 455.
(66) Bigi, A.; Boanini, E.; Walsh, D.; Mann, S. Angew. Chem., Int.
Ed. 2002, 41, 2163. (67) Busch, S.; Schwarz, U.; Kniep, R. Chem.
Mater. 2001, 13, 3260. (68) Kunther. J.; Seshadri, R.; Nelles, G.;
Assenmacher, W.; Butt, H.-J.; Mader, W.; Tremel, W. Chem. Mater.
1999, 11, 1317. (69) Rautaray, D.; Sinha, K.; Shankar, S. S.;
Adyanthaya, S. D.; Sastry, M. Chem. Mater. 2004, 16, 1356. (70) Yu,
S. H.; Colfen, H.; Hartmann, J.; Antonietti, M. AdV. Funct. Mater.
2002, 12, 541. (71) Keum, D.-K.; Naka, K.; Chujo, Y. Chem. Lett.
2004, 33, 310. (72) Dalas, E.; Klepetsanis; Koutsoukos, P. G. J.
Coll. Int. Sci. 2000, 224, 56a.
conformation became important and favored calcite nucleation via
a surface energy decrease in the same way as for LM pectin, as an
enhancement in nucleation was observed. Acknowledgment. The authors
thank Unilever for permission to publish this paper. References(1)
(2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16)
(17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29)
(30) (31) (32) (33) Addadi, L.; Raz, S.; Weiner, S. AdV. Mater.
2003, 15, 959. Colfen, H. Curr. Opin. Coll. Int. Sci. 2003, 8, 23.
Mann, S.; Perry, C. C. AdV. Inorg. Chem. 1991, 36, 137. Meldrum, F.
C.; Hyde, S. T. J. Cryst. Growth 2001, 231, 544. Orme, C. A.; Noy,
A.; Wierzbicki, A.; McBride, M. T.; Grantham, M.; Teng, H. H.;
Dove, P. M.; DeYoreo, J. J. Nature 2001, 411, 775. Volkmer, D.;
Fricke, M.; Huber, T.; Sewald, N. Chem. Commun. 2004, 16, 1872.
Malkaj, P.; Dalas, E. Cryst. Growth Des. 2004, 4, 721. Tong, H.;
Ma, W.; Wang, L.; Peng, W.; Hu, J.; Cao, L. Biomaterials 2004, 25,
3923. Manoli, F.; Kanakis, J.; Malkaj, P.; Dalas, E. J. Cryst.
Growth 2002, 236, 363. Li, C. M.; Botsaris, G. D.; Kaplan, D. L.
Cryst. Growth Des. 2002, 2, 387. Levi, Y.; Albeck, S.; Brack, A.;
Weiner, S.; Addadi, L. Chem. Eur. J. 1998, 4, 389. Wierzbicki, A.;
Sikes, C. S.; Madura, J. D.; Drake, B. Calcif. Tissue Int. 1994,
54, 133. DeOliveira, D. B.; Laursen, R. A. J. Am. Chem. Soc. 1997,
119, 10627. Jimenez-Lopez, C.; Rodriguez-Navarro, A.;
Dominguez-Vera, J. M.; Garcia-Ruiz, J. M. Geochim. Cosmochim. Acta
2003, 67, 1667. Shen, F. H.; Feng, Q. L.; Wang, C. M. J. Cryst.
Growth 2002, 242, 239. Raz, S.; Weiner, S.; Addadi, L. AdV. Mater.
2000, 12, 38. Kawaguchi, H.; Hirai, H.; Sakai, K.; Sera, S.;
Nakajima, T.; Ebisawa, Y.; Koyama, K. Colloid Polym. Sci. 1992,
270, 1176. Rieger, J.; Thieme, J.; Schmidt, C. Langmuir 2000, 16,
8300. Shakkthivel, P.; Sathiyamoorthi, R.; Vasudevan, T.
Desalination 2004, 164, 111. Iwatsubo, T.; Sumaru, K.; Kanamori,
T.; Yamaguchi, T.; Sinbo, T. J. Appl. Polym. Sci. 2004, 91, 3627.
Hadicke, E.; Rieger, J.; Rau, I. U.; Boeckh, D. Phys. Chem. Chem.
Phys. 1999, 1, 3891. Verdoes, D.; van Landschoot, R. C.; van
Rosmalen, G. H. J. Cryst. Growth 1990, 99, 1124. Colfen, H.;
Antonietti, M. Langmuir 1998, 14, 582. Dalas, E.; Klepetsanis, P.;
Koutsoukos, P. G. Langmuir 1999, 15, 8322. Malkaj, P.;
Chrissanthopoulos, A.; Dalas, E. J. Cryst. Growth 2002, 242, 232.
Pai, R. K.; Hild, S.; Ziegler, A.; Marti, O. Langmuir 2004, 20,
3123. Naka, K.; Tanaka, Y.; Chujo, Y. Langmuir 2002, 18, 3655.
Naka, K.; Chujo, Y. C. R. Chimie 2003, 6, 1193. Naka, K. Top. Curr.
Chem. 2003, 228, 141. Dimova, R.; Lipowsky, R.; Mastai, Y.;
Antonietti, M. Langmuir 2003, 19, 6097. Sedlak, M.; Colfen, H.
Macromol. Chem. Phys. 2001, 202, 587. Kaluzynscki, K.; Pretula, J.;
Lapienis, G.; Basko, M.; Bartzcak, Z.; Dworak, A. J. Polym. Sci. A,
Polym. Chem. 2001, 117, 200. Bolze, J.; Pontoni, D.; Ballauff, M.;
Narayanan, T.; Colfen, H. J. Coll. Int. Sci. 2004, 277, 84.
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