Insight into shape control mechanism of calcium phosphate nanoparticles in reverse micelles solution

Article abstract of DOI:10.1080/15533170500302034

The present experiment demonstrates a systematic morphosynthesis of calcium phosphate crystals with controlled morphology in reverse micelles solution of CTAB/n-pentanol/ water/cyclohexane. Well-defined morphologies of calcium phosphate particles, such as nanowires, tablets, brushlike particles, and fiber bundles, can be prepared. The microstructural characteristics of the CTAB/n-pentanol/water/cyclohexane reverse micelles solution has been investigated by FTIR spectroscopic, ³¹P NMR, and UV-visible absorption spectra techniques, demonstrating that the molar ratio of water to surfactant (W_o) and n-pentanol to surfactant (P_o) showed significant effects on the morphology of the resulting particles. At lower W_o and P_o, CTAB played a role in guiding the growth direction. With increasing W_o and P_o, crystal growth lost the direction-guiding capability of CTAB. However, as crystal growth modifiers, water and cosurfactants follow different mechanisms. The solubility of water causes a decrease in bound water layer, resulting in a decrease in interactions between CTA⁺ and PO₄^3- which invalidates the shape control of surfactant molecules. The loading of cosurfactant (n-pentanol) results in decreasing the rigidity of the reverse micelles interface. This in turn favors the shape fluctuations of reverse micelles, inducing crystal development. Copyright

Full text of DOI:10.1080/15533170500302034

Synthesis and Reactivity in Inorganic, Metal-Organic and Nano-Metal Chemistry, 35:717–725, 2005
Copyright # 2005 Taylor & Francis, Inc.
ISSN: 1553-3174 print/1553-3182 online
DOI: 10.1080/15533170500302034
Insight into Shape Control Mechanism of Calcium Phosphate
Nanoparticles in Reverse Micelles Solution
Chen Lai and Shaoqiu Tang
College of Materials Science and Engineering, HuNan University, ChangSha, P.R. China
Yingjun Wang and Kun Wei
College of Materials Science and Engineering, South China University of Technology, GuangZhou,
P.R. China
Shiyin Zhang
College of Materials Science and Engineering, HuNan University, ChangSha, P.R. China
INTRODUCTION
The present experiment demonstrates a systematic morpho-
synthesis of calcium phosphate crystals with controlled mor-
phology in reverse micelles solution of CTAB/n-pentanol/
water/cyclohexane. Well-defined morphologies of calcium phos-
phate particles, such as nanowires, tablets, brushlike particles,
and fiber bundles, can be prepared. The microstructural charac-
teristics of the CTAB/n-pentanol/water/cyclohexane reverse
micelles solution has been investigated by FTIR spectroscopic,
31P NMR, and UV-visible absorption spectra techniques, demon-
strating that the molar ratio of water to surfactant (Wo) and
n-pentanol to surfactant (Po) showed significant effects on the
morphology of the resulting particles. At lower Wo and Po,
CTAB played a role in guiding the growth direction. With
increasing Wo and Po, crystal growth lost the direction-guiding
capability of CTAB. However, as crystal growth modifiers,
water and cosurfactants follow different mechanisms. The solu-
bility of water causes a decrease in bound water layer, resulting
in a decrease in interactions between CTA1 and PO432 which inva-
lidates the shape control of surfactant molecules. The loading
of cosurfactant (n-pentanol) results in decreasing the rigidity of
the reverse micelles interface. This in turn favors the shape
fluctuations of reverse micelles, inducing crystal development.
The controlled growth of crystals with respect to size, shape,
crystal structure, and so-called crystal design is regarded as a
key aspect of modern materials science because of the potential
to design new materials and devices in various fields such as
catalysis, medicine, electronics, ceramics, pigments, and cos-
metics. The morphologies of materials have been controlled
by mesoporous molecular sieves (Sasaki et al., 1999), porous
aluminum templates (Yin et al., 2001), or by surfactant mole-
cular self-assembly (Huang et al., 2002). In nonpolar solutions,
certain amphiphilic molecules (surfactant) form aggregates
with their polar head groups in the interior, separating the
organic solvent from the interior “water conduit.” A major
reason why reverse micelles are so useful for chemistry is
that they create an interface between the aqueous and
nonpolar phases, allowing reactants that prefer the two diffe-
rent phases to be brought together. The properties of this
interior phase and their variations with micelles size will
have great effects on the morphology of products produced
in them. By this method, many nanomaterials such as BaCO3
(Qi et al., 1997), CaSO4 (Rees et al., 1999), Ni(OH)2 (Liu
et al., 2003), ZnS (Wu et al., 2002), CaSO4 (Kuang et al.,
2002), and BaCrO4 (Li et al., 1999) are synthesized. The
patterns of bioceramic matter such as bone, teeth, or seashells,
with their properties superior to current man-made ceramics,
show that control of the growth of single nanocrystals as well
as their simultaneous alignment in some ordered superstructure
is a motive to generate water-borne, environmentally friendly
materials (Mann et al., 1989; Qi et al., 2000). Calcium
phosphate is an important biomaterial and the principal
inorganic constituent of bones and teeth. Its formula is
Ca102x(HPO4)x(PO4)62x(OH)22x(0 ꢀ x ꢀ 2). It is also useful
in other fields of industrial or technological interest as a
catalyzer, in choromatography or gas sensors, or it may be
Keywords calcium phosphates, crystal morphology, reverse
micelles
Received 11 June 2005; accepted 28 July 2005.
This study was supported by the National Nature Science Foun-
dation of China (59932050, 50272021) and Postdoctoral Fund
2004036103.
Address correspondence to Chen Lai, College of Materials Science
and Engineering, HuNan University, ChangSha 410082, P.R. China.
E-mail: laichen1110@hotmail.com
717

718
C. LAI ET AL.
used in water purification and fertilizer production (Marasaraju were analyzed by X-ray diffraction (XRD) using a Bru¨ker
and Phebe, 1996; Arendsa et al., 1987; Lazic, 1995). Its AXS, D8 Advance Diffractometer with Cu-Ka radiation at
morphology has a great effect on the functions. It is reported 40 kV and 40 mA. A high-resolution 31P NMR study was
that small changes in particle size and morphology in HA/ carried out with an Advance Digital 400-MHz NMR (Bru¨ker)
polyethlene composites have significant effects on the mecha- spectrometer at 258C. The instrument was 160.1 MHz.
nical properties of the composite (Wang et al., 1998; Pang and Chemical shifts of the sample were referenced to an external tri-
Bao, 2003). As a replacement for human bone or other tissues, methyl phosphate (TMP) standard contained in a capillary tube
its morphology would influence biocompatibility because bone in the center of the sample and positive shifts corresponded to
osteoblast cells (the cells that form new bone) are sensitive to upfield shifts. UV-visible absorption spectra were recoded
the physical properties of their immediate surroundings, using a Unico UV-2102PC spectrophotometer.
including surface composition, surface energy, roughness,
and topography (Schwartz et al., 1999).
RESULTS AND DISCUSSION
In previous articles, we described the synthesis of needle-
like, sheetlike, and wirelike nanoparticles of calcium phos-
phate (Lai et al., 2005) and discussed the effects of
temperature, ripening time, and the content of consurfactant
(Wang et al., 2005). In this article, we present more data to
demonstrate the general behavior of water and cosurfactant in
controlling the morphology of calcium phosphate by using the
techniques of 31P NMR, UV-visible, and FTIR spectroscopy.
Figure 1 shows the TEM images of nanoparticles prepared
at different Wo and Po value. The as-synthesized calcium phos-
phates at Wo ¼ 10, Po ¼ 3 were nanowire-like, as shown by the
TEM image in Figure 1a. The nanowires had straight edges,
uniforms widths of 30–50 nm, and a length of more than
900 nm. Increasing water content resulted in different mor-
phology. At Wo ¼ 20, Po ¼ 3, irregular tablets with sharp
edges could be observed in Figure 1b, and particles size
ranged from 200 to 300 nm. Keeping constant water content
of solution at Wo ¼ 10 and increasing the cosurfactant
EXPERIMENT
Allstartingmaterialsusedinthisinvestigationwereanalytical- content to Po ¼ 5 led to the formation of brushlike bundles
grade. CaCl2 was used as Ca source and (NH4)2HPO4 was used 5–7 mm long and 40–60 nm wide (as shown in Figure 1c).
as P source. The cation surfactant used in our experiments was The brushlike bundles consisted of many long nanowires
cetyltrimethylammonium bromide (CTAB), while the cosur- which all tended to aggregate toward middle points. Upon
factant was n-pentanol. Cyclohexane was used as continuous further increasing cosurfactant content to Po ¼ 10, the long
oil phase, and n-pentanol as cosurfactant. Throughout the nanowires bundles were obtained. Figure 1d shows that indi-
whole set of experiments the following parameters were kept vidual nanowires were about 30 nm in width, attached
constant: 0.1 M CTAB in cyclohexane, 1 M CaCl2, and 0.5 M parallel to one another, and continuous along the entire
(NH4)2HPO4 aqueous-phase solution, while the molar ratio length of the fibers. Figure 2 shows the XRD patterns of as-
between water and surfactant (Wo) was varied between 3 and prepared calcium phosphate nanopartles, showing that
35 and the molar ratio between n-pentanol and surfactant CaHPO4 was the dominant phase. From the left side of the
(Po) was 5–20. The pH was adjusted to a constant value of 5 spectrum, the significant peaks were CaHPO4 (001), (002),
by the addition of ammonia.
and (002). The peaks of (112) and (120), which could be
The Ca-containing reverse micelles solution was prepared observed in XRD patterns of tablets and brushlike bundles, dis-
by adding Ca2þ solution to the CTAB-containing cyclohexane appeared in XRD patterns of nanowires and bundles of nano-
and the n-pentanol, and stirred on a magnetic stirrer for half an wires. These confirmed that the long one-dimensional
hour until a transparent isotropic solution was obtained. The nanowires preferred to grow along the c axis.
same procedure was repeated to prepare P-containing reverse
Various morphologies of nanoparticles could be obtained
micelles. Then the two reverse micelles solutions were under different conditions, suggesting that amount of water
mixed, and stirred again for half an hour until the system and cosurfactant also had great influence on the final mor-
was transparent and uniform. The resulting solution was phology of the calcium phosphates. The effects of confinement
placed in a sealed Teflon container and statically heated in a and the strong interactions between the polar solvent molecules
furnace for 10 h at 1008C. The products were washed three and the head groups of surfactants, which are frequently ionic,
times with ethanol and ether. Finally, the white samples were produce a chemical environment that may differ dramatically
dried at 50 8C in oven.
from that of the bulk solvent. A large number of investigators
Morphology of powders as synthesized was characterized [20–26] have provided information about the structures and
by TEM (Philips CM300), using an accelerating voltage of properties of water in reverse micelles. There are three or
300 kV. FTIR spectra of the reverse micelles solutions were four different water structures: isolated water, bound water,
recorded in the range 1,000–4,000 cm21 with a Nicolet free water, and ion-bound water. As the reverse micelles
Nexus Spectrophotometer using CaF2 windows. The second solution in our experiment, the cation surfactant CTAB
derivative was found by a Thermo Nicolet program. The ionized completely in water phase, leading to the formation
phase composition and crystallinity of the calcined powders of positive amino head groups which might have a greater

SHAPE CONTROL MECHANISM OF CALCIUM PHOSPHATE NANOPARTICLES
719
FIG. 1. Typical TEM images of the manipulation of calcium phosphate morphologies prepared in reverse micelles solution. a. Nanowires prepared at Wo ¼ 10,
Po ¼ 3, b. Tablets prepared at Wo ¼ 20, Po ¼ 3, c. Brushlike particles prepared at Wo ¼ 10, Po ¼ 5, d. Fiber bundles prepared at Wo ¼ 10, Po ¼ 5.
influence on the characteristics of water in the reverse micelles. program. Moreover, by applying the second derative technique
Figure 3 shows typical IR spectra of the OH stretching region to the original OH stretching FTIR spectra, four subpeaks and
between 3,000 and 3,900 cm21 with the variation of Wo from 3 their positions were obtained. Figure 4 shows representative
to 35 at the constant cosurfactant content (Po ¼ 3). The total results of the second-derivative spectra of the OH stretching
peak area of the OH stretching band of water has been found peak. According to these results, the O-H stretching absorption
to increase with Wo, which is evident from Beer’slaw. Since bands of water in CTAB reverse micelles solution were fitted
the deconvolution technique has been proven to be successful as a sum of four subpeaks as shown in Figure 5. The
in studying the structure of solubilized water in reverse subpeaks centered at 3,195 + 10, 3,306 + 10, 3,489 + 10,
micelles (Tanoj et al., 1989; MacDonald et al., 1986; Bunton and 3,579 + 10 cm21 were assigned to the O-H stretching
et al., 2001; Peytcheva et al., 2002; Li et al., 2000, 1998; Caval- mode of free water, ion-bound water, bound water, and
laro et al., 1995) the FTIR spectra obtained for the given isolated water species, respectively. The bound water layer,
systems were subjected to curve-fitting to investigate the with the OH stretching vibration at 3,489 + 10 cm21, is
water structure with the use of a Gaussian curve-fitting featured as the water molecules are hydrogen-bonded to the

720
C. LAI ET AL.
FIG. 2. The XRD patterns of products as-prepared.
polar head groups of CTAB. The free water molecules, absorbed in the highest-frequency region. Owing to the many
however, had strong intermolecular hydrogen bonding, kinds of ions coming from the reactants, some water molecules
leading to an array to the lower side of the spectrum at inevitably bonded with them by electrostatic force (Faeder
3,306 + 10 cm21. In addition, there was a small amount of et al., 2000) leading to the formation of ion-bond water
water molecules dispersing among long hydrocarbon chains (Li et al., 2000). Figure 6 is a sketch of the structure of
of surfactant molecules. They existed as monomers or dimers CTAB reverse micelles including four different types of
and had no interaction with their surroundings (Tanoj et al., water molecules as described above. Because the total peak
1989). These will to be called “isolated water” and are area corresponding to the water band is the sum of the
FIG. 3. The IR spectra of the OH stretching region between 3000 to 3800 cm21 for the CTAB/n-pentanol/water/cyclohexane reverse micelles resolution with
the variation of Wo from 3 to 35 at Po ¼ 3 (up to down: pure water,35,30,25,20,15,10,5,3).

SHAPE CONTROL MECHANISM OF CALCIUM PHOSPHATE NANOPARTICLES
721
FIG. 4. FTIR spectra and second-derivative spectra of OH stretching vibrational absorption band of reverse micelles at Wo ¼ 20, Po ¼ 3.
subpeak areas of the different states of water, their respective structure. Meanwhile, the phosphate anion is also a tetrahedral
values could be calculated (MacDonald et al., 1996). Figure structure. It has been proposed that the charge and structure
7 depicted the dependence of the area fraction for various complementarily endow CTAB tight bonds with PO342 (Yan
water species as a function of water content in CTAB-stabilized et al., 2001). Filled with a large number of bound water in
reverse micelles. As can be seen, when Wo is between 3 to 10, reverse micelles, the reactions of ions coming from reactant
bound water occupies the major part of the reverse micelles. aqueous therefore occur in close proximity to the interfaces
Bound water, in comparison to free water, greatly restricts of the reverse micelles. Furthermore, the CTAB reverse
mobility—including translation, rotational, and diminished micelles prefer to form wormlike aggregations in solution, as
¨
hydrogen-bonding networking (Faeder et al., 2000). The high has been suggested by many researchers (Tornblom et al.,
charge density and small volume results in the rigid “shell” 1997; Hoffman et al.; Zielinski et al., 1987). The size of
´
of reverse micelles, providing stable surroundings for the reverse micelles is small enough to restrict crystal growth so
growth of crystals. As mentioned above, CTAB ionizes com- that the calcium phosphate growth process copies the charac-
pletely in aqueous phase and results in a cation with tetrahedral teristic and dimensions of the long wormlike microreactors.
FIG. 5. Curve-fitting results of the O-H stretching mode of water in CTAB/ FIG. 6. Sketch of a CTAB reverse micelle in which water molecules were
n-pentanol/water/cyclohexane reverse micelles resolution at Wo ¼ 20, Po ¼ 3. solubilized in three different regions.

722
C. LAI ET AL.
FIG. 8. 31P NMR spectra of PO342 in CTAB/n-pentanol/water/cyclohexane
reverse micelles solution with varing quantities of water.
FIG. 7. Variation in the area fraction of different water species with water
PO342. More PO34 and Br2 ions went into the core of reverse
micelles. Accumulating with Br2 ions in the core, the polarized
P atom could have lowered the electron-cloud density around
proton because of the much higher electronegativity of the
Br atom. This effect led the resonance peak to shift to lower
field, and thus d showed a higher numerical value. Proton
magnetic resonance further corroborated that there was less
interaction of PO342 with CTAþ owing to hydration of
CTAþ. At higher Wo value, CTAB would not act as a guide
to the growth direction.
content in CTAB/n-pentanol/water/cyclohexane.
Any water added in the reverse micelles solution above
Wo ¼ 10 swelled the micelles so that an enhancement of free
water and a decrease of bound water fraction were observed.
The concentration of PO432 ions in the bound water layer
decreased with increasing amounts of water molecules and
volume of reverse micelles. Herein PO342 ions went inside
the reverse micelles core. As a result, there was less interaction
of PO342 with CTAþ at the interface of reverse micelles owing
to hydration of CTAþ (Bunton et al., 2001). More and more
calcium phosphate nucleated and grew in the free water
region, gradually losing the growth direction-guiding
capacity of the CTAB. For Wo ¼ 20 the micelle size further
increased, and the process of nucleation and growth became
more similar to what was observed in aqueous solution.
Calcium phosphate developed with its generic features and
did not rely on surfactant and microreactor resulting in irregu-
lar tablets with sharp edges, which is its preferred crystal habit
under acidic conditions (Jinawath et al., 2002).
The morphologies shown in Figure 1c and 1d were prepared
at different Po values while keeping the water content constant
at Wo ¼ 10. The results suggest that n-pentanol also influences
the morphology of crystals. Alcohols are known to reduce
the rigidity of the water-in-oil interface and are generally
considered responsible for increasing the exchange rates of
The above results were further confirmed by 31P NMR. In
order to investigate the interaction between PO342 and CTAþ
in reverse micelles, we simplified the test by adding PO432
solution only. Figure 8 shows the 31P NMR spectra of P
proton for reverse micelles solutions in our experiments. The
patterns exhibited a single peak, indicating the rapid
exchange between P protons at various states (Hertz 1986).
Figure 9 shows the variation of P proton magnetic resonance
as a function of water content in the reverse micelles
solution. As seen in Figure 9, the chemical shift increased
with water content. In smaller micelles which were occupied
by bound water, water molecules, PO342 and counterions of sur-
factant molecules (Br2) were immobilized by the interaction of
ionized surfactant molecules. With the addition of water to the
reverse micelles, the ion density was decreased. More and more
water moleculars had opportunities to interact with surfactant
ions, resulting in reduced interaction between CTAþ and
FIG. 9. Variation of the chemical shift of the P proton magnetic resonance
with water content CTAB/n-pentanol/water/cyclohexane reverse micelles
solution.

SHAPE CONTROL MECHANISM OF CALCIUM PHOSPHATE NANOPARTICLES
723
micelles (Plalazzo et al., 2003). At lower cosurfactant (Po ¼ 3), where R is the radius of nanoparticles; m21 ¼ me21 þ mh21 (me
bound water occupied the majority of the inside region of and mh are the mass of electron and hole position, respec-
0
reverse micelles. CTAB reverse micelles in solution could be tively); Eg is band gap energy. Eg in the present work is
considered as “hard” droplets which will not be broken easily 3.4–3.2 eV, as reported in the literature for phosphate
when they collide with other reverse micelles in solution. (Chaudhry et al., 1995; Kutub et al., 1986); É ¼ h/2p (h is
Crystals nucleated and grown under this stable condition, 6.6262 ꢂ 10234 J S).
.
therefore, tend to copy the morphology of the reverse
Band gap energy Eg can be determined by Eqs. (2)–(4)
micelles. Moreover, the partitions of n-pentanol molecules at (Davis and Mott, 1970):
the interfaces of reverse micelles diminished the interaction
ꢁ
1=2
between the CTAþ and PO432 by decreasing the density of sur-
ðayhÞ
Eg ¼ hy ꢁ
ð2Þ
ð3Þ
ð4Þ
B
factant molecules per reverse micelle. The less strong inter-
actions between ionized surfactant molecules and reactant
ions and the unstable conditions resulted in the formation of
crystals with various morphologies instead of only spherical
or one-dimensional nanoparticles. Brushlike bundles could be
observed after increasing the pentanol content (Figure 1c).
The common middle starting points of nanofibers suggest
that they could be formed by heterogeneous nucleation from
single centers followed by crystal growth along a single
crystal direction (Qi et al., 2000). The results imply that the
shape fluctuations of reverse micelles are more favored by
the increase of cosurfactant content. When Po was enhanced
to 10, long nanofiber bundles were obtained (Figure 1d).
Each bundle consisted of 3–4 nanowires, and individual nano-
wires attached parallel to one another. One possibility is that
individual reverse micelles slowly self-assemble and fuse
along the side faces of the nanowires by interdigitation of the
organic chains of surfactant molecules (Kuang et al., 2002).
Figure 10 shows the UV-visible spectra of calcium phos-
phates prepared at different Po, suggesting that an increase of
Po results in a shift of absorption onset toward longer wave-
lengths and in a less structured peak. Theoretical work has
showed that the threshold wavelength of the UV-visible
absorption spectrum provides a reasonable estimate of the
particle size (Efros, 1982), as demonstrated by Eq. (1):
ꢀ ꢁ
A
a ¼
y ¼
ꢂ 2:303
d
c
l
where A is absorbance at the frequency, d is the thickness of the
glass sample, and c is the concentration of the solution.
Thus (ayh)1/2 was plotted against hy, and the graphs
obtained are shown in Figure 11. From the graphs, the Eg
values for various Po were obtained. The mean size of
calcium phosphate particles in the reverse micelles solution
seemed to be dependent on the alcohol content. It can be
seen in Figure 12 that as Po was increased, the particles’ dimen-
sion increased, went through a maximum, and then decreased.
n-Pentanol increased the fluidity of the reverse micelles inter-
face, making it easier to fuse with other reverse micelles in
the solution and deform their morphologies. On the other
hand, excessive addition of n-pentanol made reverse micelles
so frail that larger micelles tended to be broken into many
smaller micelles by collision. Overly frequent shape fluctu-
ations of reverse micelles disfavored the growth of crystal.
This would be responsible for the fact that in Figure 12 the
size of nanoparticles decreased after the maximum point of
Po ¼ 12. It is worth mentioning that the data of UV-visible
absorption were obtained at the beginning stage of the
ꢁ
h2p2
Eg ¼ Eg0 þ
ð1Þ
2mR2
FIG. 11. A plot of (ayh)1/2 versus hy for calcium phosphate as prepared at
FIG. 10. Absorption spectra of calcium phosphate formed at various Po values. different Po value.

724
C. LAI ET AL.
moderated the interaction between the CTAþ and PO432 by
decreasing the density of surfactant molecules per reverse
micelle. Moreover, n-pentanol reduced the rigidity of the
water-in-oil interface, favoring the shape fluctuations of
reverse micelles. As a result of these facts, it was possible to
obtain crystals with various morphologies instead of only
spherical or one-dimensional nanomaterials. Therefore, brush-
like bundles were observed at higher n-pentanol content
(Po ¼ 5). Long nanofiber bundles of attached parallel nano-
wires were obtained with the solution at Po ¼ 10 and
Wo ¼ 10. It could therefore be expected that individual
reverse micelles slowly self-assembled and fused along the
side faces of the nanowires by interdigitation of the organic
chains of surfactant molecules. From the results and analysis
given above, it is obvious that the morphology of crystal can
be fine-tuned by changing parameters such as the molar ratio
of water and surfactant (Wo) or the molar ratio of cosurfactant
and surfactant (Po).
FIG. 12. Diagram showing trends in the mean size of calcium phosphate as a
function of Po at the very beginning of reaction.
reaction, therefore, the diameters of calcium phosphate
particles were much smaller than those of the final, well-
crystallized products, which were in the autoclave at 1008C
for 12 h.
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