CHAPTER III
ROLE OF STRONTIUM ON THE CRYSTALLIZATION OF BRUSHITE (CHPD)
3.1 INTRODUCTION
Kidney stones, also known as a renal calculus, are one of the most painful
complications among the different urologic disorders and are not a product of modern
life. Scientists have found evidence of kidney stones in a 7,000-year-old Egyptian
mummy. Unfortunately, kidney stones are one of the most common disorders of the
urinary tract. Men tend to be affected more frequently than women. Approximately 85%
of kidney stones are calcium stones, in combination with either oxalate or phosphate, or
both in the form of apatite or brushite [63, 64].
Calcium phosphate stone formers
comprise about 15% of the stone forming population and the incidence of calcium
phosphate stones may be increasing [65]. Factors that promote the precipitation of oxalate
crystals in the urine, such as primary hyperoxaluria, are associated with the development
of calcium oxalate stones [66]. The formation of calcium phosphate stones is associated
with conditions such as hyperparathyroidism [67] and renal tubular acidosis [68].
Hydroxyapatite (HA, Ca5(PO4)3(OH), octacalcium phosphate (OCP,Ca8H2 (PO4)6·5H2O),
tricalcium phosphate (β-TCP, Ca3(PO 4)2), dicalcium phosphate dihydrate
or calcium
hydrogen phosphate dihydrate (CHPD, CaHPO4·2H2O), dicalcium phosphate anhydrous
(DCPA, CaHPO4), tetracalcium phosphate (TTCP, Ca4(PO4) 2O) and amorphous calcium
phosphate (ACP) [69] are different crystalline calcium phosphates that have applications
in biological mineralization.
3.1.1 Properties of CHPD crystals
Formula
: CaHPO4.2H2O
Mineral name
: Brushite
Colour
: white or gray crystalline mineral
Ca/P molar ratio
: 1.00
Structure
: Monoclinic
Space group
: Ia (9)
Lattice Parameters : a = 5.812 Å, b = 15.180 Å, c = 6.239 Å and β =116.47°
Moh’s hardness
: 2.5
Specific gravity
: 2.328 g/cm3
Calcium hydrogen phosphate dihydrate [CaHPO4· 2H2O], also known as brushite
mineral, is a stable form of calcium phosphate [70]. Recent studies used it as a precursor
55
to form apatite [Ca10(PO4)6(OH)2], an important bone forming mineral, as it contains 23%
calcium in its anhydrous form. The brushite mineral is found under various pathological
conditions including kidney stones, some forms of arthritis and caries [71, 72]. Brushite is
a transient precursor for phases such as octacalcium phosphate and hydroxyapatite. And
thus, calcium phosphate minerals are thought to be the initiator of stone formation in the
kidney and/or bladder, under favourable physiological environment. As brushite is the
phase that precipitates most readily in urine environments at pH less than 6.9, it has been
postulated that brushite enables the nucleation of calcium oxalate monohydrate, the major
component of kidney stones [73]. The brushite types of kidney stones are the main topic
of this research.
3.1.2 Structure of CHPD
Fig 3.1 Crystal structure of brushite [74]
A neutron diffraction measurement [75] on brushite has determined the hydrogen
positions, and confirmed the monoclinic system of space group Ia. Its unit cell contains
four molecules of CaHPO4 .2H2O. Brushite has a layer-like structure in which corrugated
56
sheets of [CaHPO4] and water layers alternate along the [0 1 0] direction. Each Ca2+ ion is
octahedrally surrounded by six phosphate oxygens from four phosphate groups (HPO 42-)
and two water oxygens from two water molecules which manifest themselves in the
formula, CaHPO4.2H2O.
3.2 CRYSTAL GROWTH OF CHPD
The crystallization of CHPD was obtained mainly by solution and gel method [76,
77]. Anee et al [78] have studied the influence of cobalt and malic acid on the
crystallization of DCPD by gel method. They reported that the presence of cobalt and
malic acid modify the crystal morphology. Kalkura et al [79] investigated the
crystallization of DCPD in the presence of iron in agarose gel. The presence of
magnesium reported to inhibit the formation of brushite crystals [80]. LeGeros [81] grew
single crystals of brushite in silica gel, and reported that the presence of Sr 2+ and P2O74−
causes marked effect on the crystal habit. Addition of Sr2+changed the morphology from
usual platelet to spiral aggregate and the presence of P2O 74− led to the growth of small
needle- shaped crystals. Abbona et al [82] have studied the crystal habit and growth
conditions of brushite crystals. Lundager Madsen [83] investigated the influence of 14
different di- and trivalent metal ions on brushite formation and reported that some ions
inhibit and some ions promote the formation of brushite. Sekar et al [84] reported that the
fluoride addition reduces the size and total number of brushite crystals. Thus, the
motivation for the present work is to study the influence of strontium on the
crystallization of CHPD under in vitro conditions by single diffusion gel method.
3.2.1 Role of trace elements
More than 40 chemical elements in the human body affect the biological processes
related to the health of body. These elements have different concentrations and functions.
For example, a small presence of some trace elements negatively affects the biological
processes in the body. Concentration of Co, Mg, and Ni affects Ca in the human body
through the increase of dissolution of crystallized calcium oxalate stones and many of
trace elements ions affecting the crystallization of urinary stones especially oxalate and
phosphate stones [85]. Furthermore, Levinson et al [86] explained the effect of some trace
elements on the dissolution and the crystallization of urinary stones. One of the main
problems is in determining whether nucleation is essentially homogeneous and takes
place spontaneously from highly supersaturated fluids or whether it is heterogeneous and
57
is initiated by some other agent such as trace elements (Ba, Sr, Pb, Mg, Zn, etc.). The
principal aim in this area is to identify the mechanism of crystal growth in conditions
prevalent in biological systems, and to formulate the means of inhibiting the crystal
growth by the addition of strontium. The functions of certain trace elements are given
below.
(i) Copper
The normal adult body contains 75-150 mg of copper. It is an antioxidant and
approximately one-third of the total body copper is found in the liver and brain at
high tissue concentrations [87]. Another one-third is located in the muscles at low tissue
concentrations. The
rest is found in the heart, spleen, kidneys, and blood [88]. It is
involved in the processes of skeletal development, electron transport, connective
tissue, and blood cells formation among others. Some authors have been reported that
copper as an inhibitor while some reports its promoting nature in urolithiasis [89]. Meyer
and Angino noticed the inhibitory activity of copper against growth of calcium phosphate
crystals but not on oxalate [90].
(ii) Iron
Iron is the most frequent trace element in human body. The deficiency of iron
causes anemia. This element is responsible for muscle and cognitive functioning, carrying
oxygen via hemoglobin and myoglobin. It is also highly involved in the enzymatic and
immune reactions. In serum, iron is in 60-70% bound to transferin. Its excess may cause
many disorders like liver damage, diabetes mellitus and skin pigmentation. The role of
iron in lithogenesis is not clear. The growth of calcium phosphate was inhibited by low
molecular weight Fe3+ - citric acid complex.
(iii) Nickel
Nickel plays important role in the biology of microorganisms and plants. It is
necessary for human health and, at the same time, Ni affects hormones, cell membranes
and some enzymes. The low nickel level causes liver and kidney diseases and higher level
have high incidence of heart disease, thyroid disorder or cancer.
In fact, urease (an
enzyme that assists in the hydrolysis of urea) contains nickel. It may disturb the activity
of urease, which is directly involved in the formation of struvite.
58
(iv) Lead
Lead, at certain exposure levels, is a poisonous substance to animals as well as for
human beings. It damages the nervous system and causes brain disorders. Excessive lead
also causes blood disorders in mammals. A prolonged intake of low level of lead (The
maximum daily intake of an individual should have 1.0 µg/g) could be hazardous to
human being. Changes in the trace elemental level in blood causes many diseases
including the kidney diseases.
(v) Chromium
Chromium is essential element in the human body. In the United States the dietary
guidelines for daily chromium uptake were lowered from 50-200 µ g for an adult to 35 µ g
(adult male) and to 25 µ g (adult female) [91]. The main physiological role of chromium is
a cofactor for insulin [92]. Chromium is involved in lipid and cholesterol metabolism, its
deficiency is a suspected risk factor for the development of atherosclerosis. Chromium is
excreted mainly in urine.
(vi) Barium
Barium is a toxic element, which may be absorbed via air by breathing. The
average consumption rate is one microgram per day. The second is via liquid by
liquid/water intake. The average consumption rate was 1-20 µ g per liter. Last one is via
solid-by food intake. The average consumption of barium present is 600-900 µ g per day.
For a normal man, the barium consumption bar limit is 1240 µ g per day, which is the
tolerance level, if exceeds the specific tolerance level of human physical system,
automatic renal stone deposition occurs.
(vii) Strontium
Strontium is a trace element widely found in nature. Strontium has fundamental
chemical similarities to calcium. The strontium and the calcium ions, members of the
alkaline earth series (Group IIB of the Periodic Table), have many properties in common,
both having a valency of 2, similar ionic radii, and the ability to form complexes and
chelates of various solubilities and various binding strengths.
The intake of Sr per day through food and fluids is 1.9 mg. In that the loss in urine
is 0.34 mg, loss in faeces is 1.5 mg , loss in sweat is 0.02 mg, and hair is 0.2 x 10 -3 mg
59
[93]. When dietary intake of strontium is raised, strontium begins to take the place of
calcium in developing bone. This replacement appears to be beneficial (at least with low
doses of strontium), leading to an increase in bone formation, a decrease in bone
breakdown, and an overall rise in bone density. Strontium renalate is a recommended
medication for osteoporosis which is found to increase the risk of stone formation in the
patients [94]. Thus the motivation for the present work is to study the influence strontium
on the crystallization of CHPD under in vitro conditions by single diffusion gel method.
3.2. 2 Preparation of gel
In the present investigation we have used analar grade calcium chloride
(CaCl2.2H2O) and orthophosphoric acid (H3PO4), sodium metasilicate (Na2SiO3 .9H2O,
SMS) and double distilled water. Sodium metasilicate gel was used as the medium to
grow crystals of brushite at room temperature. The stock solution was first prepared by
mixing 100 ml double distilled water and 35 gm SMS gel in a container. The contents
have been then and there stirred well for about 1 hour by magnetic stirrer and it was left
undisturbed for 1 hour, so that it was allowed to settle. The clear solution was then
filtered by using 110 mm pore size Whatmann filter paper and stored. The density of this
gel solution was measured and found to be 1.06 g/cm3 by hydrometer.
Subsequently, the aqueous solution of orthophosphoric acid was prepared by
mixing 1 M of H 3PO4 in double distilled water and stirred for 20 minutes. The clear
solution was then filtered and stored. One of the reactants, orthophosphoric acid (1 M)
was mixed with silica gel of density 1.06 gm/cm3 so that the pH of the mixture could be
set to 6.0. The mixture was then transferred into test tubes.
3.2.3 Growth of CHPD
The gel was set after 2 days and then the supernatant solution of pure CHPD was
prepared by mixing 1 M of CaCl2.2H2O in double distilled water in a beaker.
The
contents of the beaker were stirred for 15 minutes and the clear solution was then filtered.
The pH of the pure supernatant solution was found to be 7.82. About 10 ml of the calcium
chloride solution was poured carefully over the top of the gel medium without disturbing
the latter and the test tube was closed with airtight cover. Immediately after the addition
of the supernatant solution, a dense white precipitate has been formed at the gel- solution
interface within 12 minutes of adding supernatant solution. White colored rings known as
60
Liesegang rings have appeared just below the interface within 6 hours. The number of
these white colored rings increased with time and a total of 16 such rings were observed
after five days. In the mean time, the first few Liesegang rings started dissolving slowly
and tiny white spherulites have grown in that place. The colorless and transparent needle
shaped crystals have grown in between the Liesegang rings (Fig 3.2 a). Approximately
10-15 needle shaped crystals were harvested after three weeks.
3.2.4 Effect of strontium on the growth of CHPD
The gel was prepared as in the case of section 3.2.2. Preparation of supernatant
solution was done by mixing CaCl2.2H2O with water. Apprppriate amount of SrCl2 (0.1
& 0.2 M) was added to the supernatant solution. In strontium (0.1 M) added experiments,
the growth pattern was completely different. After adding the supernatant solutions white
precipitate was observed just below the interface. After three days, white spherulites have
appeared from the middle towards the bottom of the tube as shown in Fig.3.2 b.
Fig 3.2 Calcium phosphate crystals grown in the presence of Sr; (a) 0, (b) 0.1
and (c) 0.2 M
The number and size of spherulitic crystals increased with time. Simultaneously a few
tiny crystallites have appeared in the lower end of the gel medium which continued to
grow and assumed the coral (octopus) like morphology [95]. Contrary to the pure system,
Liesegang rings did not appear and there was no detectable amount of white precipitate at
the interface between supernatant solution and gel. As the concentration of Sr was
increased to 0.2 M, the size and
number of crystals got decreased further (Fig.3.2.c).
61
Thus, the presence of excessive amount of ‘Sr’ suppresses the formation of nuclei and
their further development into large crystals. Fig. 3.3 a-c show the needle and octopus
like crystal grown without and with different strontium concentrations.
Fig 3.3 CHPD crystals grown in the presence of Sr; (a) 0, (b) 0.1 and (c) 0.2 M
3.3 RESULTS AND DISCUSSION
3.3.1 XRD analysis
X- ray powder diffractometric technique is most widely used for the structural
characterisation of solid crystalline materials and determination of their probable crystal
structure.
This method is based on the fact that
each crystalline compound has a
characteristic unit cell. The relative intensities of the X-ray reflection from the atomic
plane of a crystal depends on the shape and type of the unitcell, arrangement of the atoms
in the unit cell and the absorption of X-rays by the crystal. The necessary condition for
the diffraction to occur depends on the Bragg law
nλ = 2dhkl sinθ
where dhkl is interplanar spacing having
Miller indices hkl, θ, the glancing angle of
incidence, n, order of reflection and λ wave length of monochromatic X-rays. The X-ray
detector moves around the sample and measures the intensity of these peaks and the
position of these peaks (diffraction angle 2θ). The highest peak is defined as the 100%
peak and the intensity of all the other peaks are measured as a percentage of the 100%
peak.
In the present investigation, a high resolution Bruker Advance D8 XRD
diffractometer in Bragg-Brentano geometry, with a CuKα monochromated beam (λ =
1.5406Ǻ) was used. The scanning range (2θ) was from 10º to 80º with a step size of
0.02°. The measurement of dhkl value corresponding to each peak position and the peak
62
intensity from the basis of the method of identification of materials organised in the
International Centre for Diffraction Data (ICDD) by directly comparing the X-ray
diffraction patterns to the Joint Committee for Powder Diffraction Standards (JCPDS)
files. Theoretically, the lattice parameters (a, b and c) for the monoclinic crystal structure
of CHPD, are determined by the equation
1
d2
1
sin 2
 h2

 a2

k 2 sin 2
b2
l2
c2
2 hl cos
ac




Volume V= abc sinβ
where a, b and c is the lattice parameters, λ is the wavelength (CuKα: λ = 1.5406 Ǻ) and
2θ is the diffraction angle.
The powder X-ray diffraction patterns were recorded for all the three types of
products grown without the addition of strontium i.e. the thick white precipitate formed at
the gel - solution interface, spherulitic and needle shaped crystals grown beneath the
interface.
Fig. 3.4 shows the powder XRD pattern of the CHPD crystals grown without
strontium (a) and with 0.1 M Sr (b) and 0.2 M Sr (c) respectively. The needle shaped
crystals were identified as pure CHPD by comparing the JCPDS data (72-0713). The
diffraction peaks of the crystals grown with Sr were strong and sharp when compared to
that of pure system. This result indicates that the presence of Sr enhances the crystallinity
of CHPD. A slight shift in the peak positions towards the lower angle side indicating an
increase in unit cell volume which confirms that the Sr replaces a certain amount of Ca in
CHPD.
63
Fig 3. 4 XRD patterns of CHPD crystal grown in the presence of Sr ; (a) 0, (b) 0.1
and (c) 0.2 M
The white precipitate was identified as hydroxyapatite (JCPDS data (09-0432)
(Fig.3.5). The spherulitic crystals were found to be a mixture of monetite (JCPDS data
(70-1425)) and brushite. Fig. 3.6 a, shows the powder XRD pattern of monetite crystals
grown without Sr. The minor peaks observed at 2θ values 11.76˚ and 21.05˚ could be
indexed for (0 2 0) and (1 2 1) planes of CHPD. The intensity of these CHPD peaks
increased with the addition of strontium as shown in Fig. 3.6 b. It can also be noted that
the peaks shift towards lower angle side which indicates that the ‘Sr’ enters into CHPD
crystal lattice.
64
Fig 3. 5 XRD pattern of hydroxyapatite
As already reported by Bigi et al [96] the transformation of CHPD to HA was not
observed in the present study. Instead Sr addition seems to suppress the crystallization of
HA and the monetite was found to be the secondary phase.
Single crystal XRD analysis
The lattice parameters determined from the single crystal X-ray diffraction data
acquired using four-circle Nonius CAD4 MACH3 diffractometer (MoKα, λ = 0.71073
Ǻ ) are shown in Table 3.1.
Table 3.1 Lattice parameters of pure and Sr grown CHPD crystals
Lattice parameters (Å)
a
b
c
β
Volume (Å3)
Pure CHPD
5.808
15.176
6.236
116.36
492.5
CHPD+0.1M Sr
5.814
15.184
6.239
116.00
495.0
CHPD+0.2M Sr
5.817
15.186
6.242
115.28
498.6
Samples
65
Fig 3.6 XRD patterns of monetite crystal grown in the presence of Sr; (a) 0 and
(b) 0.2 M
There is a small enhancement in the lattice parameters of Sr (0.2 M) grown CHPD
crystals. This increase in lattice parameters could be due to the larger ionic radius of Sr
(1.13 Ǻ) when compared to that of Ca (1.00 Ǻ). It may be that the Sr2+ provokes lattice
distortions which expand the brushite crystal lattice [97].
3.3.2 X-ray Fluorescence Spectroscopy studies
The XRF method depends on fundamental principles that are common to several
other instrumental methods involving interactions between electron beams and x-rays
with samples (e.g. SEM - EDX), X-ray diffraction (XRD), and wavelength dispersive
spectroscopy. The analysis of major and trace elements in materials by X-ray
fluorescence is made possible by the behaviour of atoms when they interact with
radiation. When materials are excited with high-energy, short wavelength radiation (e.g.
X-rays), they can become ionized. If the energy of the radiation is sufficient to dislodge a
tightly-held inner electron, the atom becomes unstable and an outer electron replaces the
missing inner electron. When this happens, energy is released due to the decreased
66
binding energy of the inner electron orbital compared with an outer one. The emitted
radiation is of lower energy than the primary incident X-rays and is termed fluorescent
radiation. Since the energy of the emitted photon is characteristic of a transition between
specific electron orbital in a particular element, the resulting fluorescent X-rays can be
used to detect the abundances of elements that are present in the sample.
Fig 3.7 X-ray fluorescence spectra of CHPD crystals grown in the presence of Sr;
(a) 0, (b) 0.1 and (c) 0.2 M
The elemental composition of the specimen was determined using an elemental
analyzer JEOL JSX 3222 equipped with energy dispersive X- ray fluorescence system
(XRF). X-ray fluorescence spectra of brushite crystals are shown in Fig. 3.7 which
reveals the presence of different elements such as Sr (Kα = 14.150 KeV), P (2.046 KeV)
and Ca (Kα= 3.691 KeV, Kβ = 4.012 KeV). In pure sample, the Ca/P value was found to
67
be very close to that of CHPD (1.00) according to the chemical formula. In doped
crystals, (Ca + Sr)/P ratio was found to be 1.18 and 1.54 for the crystals grown with 0.1
and 0.2 M Sr respectively. As a consequence the stoichiometry is no longer maintained in
case of Sr doping into CHPD.
3.3.4 SEM analysis
The scanning electron microscope (SEM) uses a focused beam of high-energy
electrons to generate a variety of signals at the surface of solid specimens. The signals
that derive from electron-sample interactions reveal information about the sample
including external morphology (texture), chemical composition, and crystalline structure
and orientation of materials making up the sample. In most applications, data are
collected over a selected area of the surface of the sample, and a two-dimensional image
is generated that displays spatial variations in these properties. Areas ranging from
approximately 1 cm to 5 microns in width can be imaged in a scanning mode using
conventional SEM techniques (magnification ranging from 20X to approximately
30,000X, spatial resolution of 50 to 100 nm). The SEM is also capable of performing
analyses of selected point locations on the sample; this approach is especially useful in
qualitatively or semi-quantitatively determining chemical compositions (using EDS),
crystalline structure, and crystal orientations (using EBSD). Accelerated electrons in a
SEM carry significant amounts of kinetic energy, and this energy is dissipated as a variety
of signals produced by electron-sample interactions when the incident electrons are
decelerated in the solid sample. These signals include secondary electrons (that produce
SEM images), backscattered electrons (BSE), diffracted backscattered electrons (EBSD
that are used to determine crystal structures and orientations of minerals), photons
(characteristic X-rays that are used for elemental analysis and continuum X-rays), visible
light (cathodoluminescence - CL), and heat. Secondary electrons and backscattered
electrons are commonly used for imaging samples. Secondary electrons are the most
valuable for showing morphology and topography on samples and backscattered electrons
are the most valuable for illustrating contrasts in composition in multiphase samples (i.e.
for rapid phase discrimination). X-ray generation is produced by inelastic collisions of the
incident electrons with electrons in discrete orbitals (shells) of atoms in the sample. As
the excited electrons return to lower energy states, they yield X-rays that are of a fixed
wavelength (that is related to the difference in energy levels of electrons in different
shells for a given element). Thus, characteristic X-rays are produced for each element in a
68
mineral that is "excited" by the electron beam. SEM analysis is considered to be "nondestructive"; that is, X-rays generated by electron interactions do not lead to volume loss
of the sample, so it is possible to analyze the same materials repeatedly.
In the present work, surface features and compositional analyses of the crystals
were done by using scanning electron microscopy (ZEISS LS10 EVO). Fig.3.8 a-b shows
the needle shaped CHPD crystals grown without Sr addition. At higher magnification, the
crystal surface shows a number of needle like crystals arranged in a single crystal. Fig.3.7
c-d shows the SEM images of octopus-like CHPD crystals grown in the presence of Sr
(0.2 M). The crystals grown without ‘Sr’ have needle-like morphology. The surfaces were
found to be fairly clean and defect free. On the other hand, the octopus-like crystals were
found to be branched dendrites (Fig.3.9 c). The branches exhibited curved needle shapes
which on further magnification showed the presence of large number of crystals arranged
in eagle’s wing-like shape (Fig.3.9 d). It may be that the presence of Sr enhances the
nucleation process of CHPD which results in innumerous crystals. Boanini et al [98]
synthesized CHPD crystal and reported that a relatively low Sr replacement of Ca induces
a decrease in the coherent length of the perfect crystalline domains and disturbs the shape
of the crystals.
Fig 3.8 SEM pictures of pure CHPD crystals
The spherulites formed in pure SEM contain submicron sized platy crystals radiating
from a central core, whereas incorporation of the Sr into the crystal increases the size of
the crystals to several microns (Fig.3.10 e-f).
69
Fig 3.9 SEM pictures of CHPD crystals grown in the presence of 0.2 M Sr
Fig 3.10 SEM pictures of monetite crystals grown in the presence of Sr; (a) 0 and
(b) 0.2 M
3.3.5 Thermal analysis
The temperature and rate at which the material undergoes physical and chemical
transitions during heating and cooling and the energy and mass changes associated with
temperature is the subject matter of thermal analysis. In thermogravimetric analysis the
amount and rate of change in the weight of a material is recorded as a function of
temperature or time in a controlled atmosphere. Changes in the mass of the sample occur
as a rupture and or formation of physical and chemical bond at elevated temperature that
lead to the evolution of volatile products or formation of reaction products. Hence, TGA
curve gives the information about the thermodynamics and kinetics of various chemical
reactions and reaction mechanisms, intermediate and final products.
70
In differential thermal analysis (DTA), the temperature difference between a
substance and a reference material is measured as a function of temperature whilst the
substance and reference material are subjected to the same controlled temperature
programme. DTA detects the release or absorption of heat, which is associated with
chemical and physical changes in materials as they are heated or cooled. Such
information is essential for understanding thermal properties of materials. The record is
the differential thermal or DTA curve; the temperature difference (∆T) should be plotted
on the ordinate with endothermic reactions downwards and temperature or time on the
abscissa increasing from left to right. The DTA studies in connection with TGA exhibit
weight loss or gain due to decomposition, oxidation, or dehydration.
Thermogravimetry of samples was performed here using TG instruments SDQ600
simultaneous TG-DTA instrument with thermal solution versions 1.2J controller
software. Data analyze was carried out using a TA Instrument Universal Analyser version
2.3C software in the temperature range of 30-1200°C at the heating rate of 20°C/minute
in nitrogen atmosphere.
Fig.3.11 and 3.12 illustrate thermal behaviour of pure and 0.2 M strontium grown
CHPD samples. In pure sample the weight loss occurs in two stages. The major weight
loss of about 21% occurs between 103ºC and 199ºC which indicates the loss of lattice
water. The endothermic peak in DTA around 128ºC with the associated shoulders
indicates the stepwise removal of water during this temperature range. In the region (199479ºC), two molecules of CaHPO4 combine and result in the elimination of a water
molecule leading to the formation of calcium pyrophosphate and nearly 74% of the
sample is stable.
The following chemical reactions are expected to occur during the
dehydration and decomposition stages [70].
2CaHPO4.2H2O → 2CaHPO4 + 4 H 2 O↑
2CaHPO4 → Ca2P2O7 + H2 O↑
71
Fig 3.11 TG-DTA curves of pure CHPD crystals
Fig 3.12 TG-DTA curves of CHPD crystals grown in presence of 0.2 M Sr
72
Nearly similar thermal behaviour occurred in Sr doped CHPD also. The major weight loss
of about 21% occurs between 110ºC and 200ºC in the crystals grown with Sr (0.2M)
addition. The mass loss corresponds well with the DTA results by the appearance of an
endothermic peak at 178ºC with the shoulders. Thus, there is an increase in the peak
temperature which indicates the improved thermal stability of CHPD due to Sr doping. In
the second stage, nearly 8% weight loss occurs and the rest of the sample (68%) was
stable. The excess weight loss (~ 6%) may be due to Sr doping into CHPD.
3.3.6 FTIR studies
FTIR studies provide information about the chemical bonds and molecular
structure of a material. The FTIR spectrum is equivalent to the "fingerprint" of the
material and can be compared with catalogued FTIR spectra to identify the material.
FTIR technique can be used to analyze organic materials and some of inorganic materials.
The FTIR technique is to measure the absorption of various infrared radiations by the
target material, to produce an IR spectrum that can be used to identify functional groups
and molecular structure in the sample. FTIR depends upon the absorption of infra-red
radiation arising from the vibrational and rotational characteristics of dipolar chemical
compounds. The arrangement and strength of chemical bonds within a molecule have a
direct effect on characteristic modes of vibration and vibrational bond frequencies of a
molecule, resulting in the formation of a series of characteristic mid-infrared absorption
bands (4000-400 cm-1) which can be used to characterize and quantify individual
compounds. Because each different material is a unique combination of atoms, no two
compounds produce the exact same infrared spectrum. Therefore, infrared spectroscopy
can result in a positive identification (qualitative analysis) of every different kind of
material. In addition, the size of the peaks in the spectrum is a direct indication of the
amount of materials present. With modern software algorithms, infrared is an excellent
tool for quantitative analysis.
In the present work, FTIR spectra of the grown crystals were recorded using
Perkin Elmer, Spectrum Rx1 detector and KBr beam splitter. The recorded FTIR spectra
for pure and Sr doped CHPD crystal were depicted in Fig.3.13. The observed wave
numbers, relative intensities and the assignments proposed for the crystals under
investigation were found to be in good agreement with the reported literature [99, 100].
73
Fig 3.13 FTIR spectra of CHPD crystals grown in presence of Sr ; (a) 0 , (b) 0.1 and
(c) 0.2 M
In the spectrum of pure CHPD, we can find two intense doublets; one with
components at 3544 and 3489 cm-1 and the other with components at 3284 and 3168 cm-1 .
In addition, a weak band around 2371 cm-1 and a sharp strong band around 1652 cm-1
have been observed. These two doublets have quite different shapes; the high-wave
number doublet consists of sharp bands whereas the low-wave number doublet is much
broader. The appearance of these two doublets is attributed to the existence of two
different types of water molecules in the unit cell of brushite [99]. Petro et al [101]
reported that the high-wave number lines are due to a loosely bound water molecule and
the low wave number doublet to vibrations of those water molecules which, according to
the crystallographic data of Beevers [102], forms direct bonds to calcium atoms. In the Sr
doped brushite, the low wave number doublets (3284 and 3168 cm-1) are slightly
broadened and the peaks are shifted to 3295 and 3171 cm-1 respectively. This may be due
74
to the presence of strontium. In case of high wave number doublet the intensity of peaks
got reversed in the doped samples when compared to that of pure sample.
Presence of sharp band around 872 cm-1 in all IR spectra confirms the brushite
mineral phase [99]. The sharp and strong band around 1652 cm-1 is assigned to the inplane bending of water molecules. CHPD is characterized by the splitting of
phosphate
bands in the region below 1600 cm-1 with more doublets. At 989 cm-1 a strong P-O
stretching mode (υ1) is observed. In the present work, the two bands at 576 and 527 cm-1
were assigned to the υ4 mode vibration. Peak intensity of hydrogen bonded HPO42- ions at
1387 cm-1 in the case of pure sample [103] increases significantly in the Sr doped
samples. The peak around 1215 cm-1 in the spectrum is due to O-H in plane bending of
HPO4 group. The peaks around 795 and 666 cm-1 are due to librations of water molecules
and the peak 795 cm-1 is assigned to a rocking and the 666 cm-1 peak to a wagging
librational motion respectively. No major shifts or additional peaks are found in the case
of Sr doped samples.
3.4 CONCLUSION
Simultaneous crystallization of calcium phosphates (brushite, hydroxyapatite, and
monetite) has occurred in sodium metasilicate gel under physiological conditions. The
presence of Sr has suppressed the formation of HA and promoted the monetite and
brushite formation. In addition, a significant change in morphology of CHPD from needle
shape to octopus-like shape has been observed. The XRD and XRF analyses have
confirmed the incorporation of Sr into brushite crystallites. TG-DTA studies indicated the
improvement in thermal stability of brushite due to Sr doping. The FTIR result shows that
there was no major shift in the peak position.
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