[Faculty of Science
Chemistry]
Synthesis of Colloidal (Mixed) Metal
Pyrophosphate Salts
Master Thesis
J.P.M. Pandelaar
Supervisors:
Y.M. van Leeuwen and Prof. W.K. Kegel
17-11-2011 – 31-01-2013
Abstract
Mineral deficiency is common worldwide and can lead to serious diseases in the long-term.
To prevent such diseases, nutrition can be fortified with dietary minerals. Fortification with
colloidal dispersions of water insoluble mineral compounds is an option for liquid products.
Colloidal dispersions of (mixed) metal pyrophosphate salts were prepared by different reprecipitation methods: an intermediate was dissolved in HCl and the pH was raised in a
controllable way. Stable dispersions of MgPPi, FeIIMgPPi, FeIIIMgPPi and ZnMgPPi were
obtained by injection and it was found how the stability depends on a combination of
particle size, surface charge and ionic strength of the dispersion medium. Characterization
was performed by dynamic light scattering, transmission electron microscopy, energy
dispersive X-ray spectroscopy and powder X- ray diffraction.
Table of Contents
1 Introduction .............................................................................................................................7
1.2 Colloidal dispersions .........................................................................................................8
1.2.1 Stability ......................................................................................................................8
1.2.2 Stabilization ...............................................................................................................8
1.2.3 Zeta potential and colloidal stability ...................................................................... 10
1.3 Nucleation and growth .................................................................................................. 11
1.4 Methods and surfactants .............................................................................................. 13
2 Experimental......................................................................................................................... 15
2.1 Intermediates ................................................................................................................ 16
2.1.1 Metal PPi................................................................................................................. 16
2.1.2 Mixed metal PPi ...................................................................................................... 16
2.2 Decomposition of urea .................................................................................................. 17
2.3 Titration ......................................................................................................................... 18
2.4 Injection ......................................................................................................................... 18
2.4.1 Metal PPi................................................................................................................. 18
2.4.2 Mixed metal PPi ...................................................................................................... 19
2.5 Characterization ............................................................................................................ 20
3 Results and discussion .......................................................................................................... 21
3.1 Decomposition of urea .................................................................................................. 21
3.2 Titration ......................................................................................................................... 22
3.3 Injection ......................................................................................................................... 23
3.3.1 Metal PPi................................................................................................................. 23
3.3.2 Mixed metal PPi ...................................................................................................... 27
3.3.3 X-Ray diffraction ..................................................................................................... 28
3.3.4 Energy dispersive X-ray analysis ............................................................................. 28
4 Conclusions ........................................................................................................................... 31
5 Outlook ................................................................................................................................. 32
6 Acknowledgements .............................................................................................................. 33
7 References ............................................................................................................................ 34
1 Introduction
For a good functionality, the human body needs sufficient minerals. Examples of important
dietary minerals are iron, calcium, magnesium and zinc. Mineral deficiency can develop
when insufficient mineral are absorbed from the diet and it is common in developing
countries as well as industrialized parts of the world [1]. When such a deficiency is not
treated in time it can lead to serious diseases such as osteoporosis (Mg) [2], anemia (Fe) and
growth retardation (Zn) [3]. One solution to prevent these diseases is eating food rich in
minerals. Unfortunately, this solution is not applicable in all cases. Another solution is
fortification of foods with these minerals [4]. This can easily be done for solid products as a
powder, but for liquid products it is more challenging.
For iron, food fortification can be achieved by addition of water soluble iron salts such as
ferrous sulfate. But while it is highly bioavailable, it leads to unacceptable colour and flavour
changes [5]. An alternative is using water insoluble compounds. Ferric pyrophosphate is
water insoluble and is already being applied in infant cereals and chocolate drink powders
[6]. The main advantage is that adverse colour or palatability changes are negligible [6, 7]. It
is slightly soluble in diluted acid, for example gastric juice, and as a result the iron is easily
and effectively absorbed in the human body [6, 7]. Additionally, pyrophosphates are
generated in ATP hydrolysis processes, which are essential for cellular functioning in living
organism [8]. This suggests that pyrophosphates are biocompatible.
Considering these advantages of pyrophosphate salts, colloidal dispersions of magnesium-,
zinc- and calcium pyrophosphate were prepared. Dispersions of magnesium pyrophosphate
were found to be stable for five months.
In a next step, mixed metal pyrophosphate salts were prepared by addition of a second
cation to the magnesium. There are two main reasons for this. Firstly, adding multiple
minerals in a single system makes it a more versatile and broadly applicable additive.
Secondly, some minerals by themselves are highly reactive with the foods to which they are
added (e.g. iron [5]). Diluting such a mineral with another, less reactive cation in a system of
colloidal pyrophosphate could reduce the overall reactivity of the system. Once stable
colloidal dispersions are obtained, they can finally be applied in liquid nutrition.
Synthesis was performed using re-precipitation based on supersaturation, which is induced
by raising the pH in a controllable way. Three different methods were used, which were
decomposition of urea, titration with sodium hydroxide and injection. Characterization was
performed by dynamic light scattering, transmission electron microscopy, energy dispersive
X-ray analysis and powder X-ray diffraction.
7
Figure 1. DLVO potential is the sum of
Van der Waals attraction and electrostatic
repulsion. The change in free energy is
plotted as a function of particle
separation [10].
1.2 Colloidal dispersions
Colloidal dispersions are systems where one state of matter (solid, liquid or gas) is finely
dispersed in another one, such as solid particles in liquid media, called a suspension or liquid
droplets in a liquid phase: an emulsion. The size range of colloidal particles is in between 1
and 1000 nm in at least one dimension. Colloidal dispersions have a higher state of free
energy compared to bulk material, which is thermodynamically unfavourable [9].
1.2.1 Stability
The DLVO theory describes the stability of charged particles in aqueous media. The stability
mainly depends on two opposite contributions acting on these particles in the dispersion
medium. These contributions come from the attractive Van der Waals forces and
electrostatic repulsion. When summed, the DLVO potential can be obtained. This potential
yields energy as a function of the separation distance between two particles, as shown in
figure 1 [10]
Due to Brownian motion colloidal particles in a dispersion approach each other, driven by
thermal energy. The separation between particles decreases and at a certain distance the
particles start to interact. The probability of flocculation depends on the height of the energy
barrier present in the DLVO potential. If the energy barrier is sufficiently high, larger than a
few kT, particles resist flocculation and the system remains well dispersed: the system is in a
metastable state referred to as kinetically stable. When the energy barrier is on the order of
kT, thermal fluctuations are sufficient to overcome the barrier and particles will aggregate.
An aggregate falling out of the colloidal size range usually phase separates or changes its
structure to a more dense form, which is called coagulation [11, 12].
1.2.2 Stabilization
To ensure stability, repulsive forces must be dominant. This can be achieved by either steric
stabilization or by electrostatic stabilization.
8
Figure 2. Steric stabilized colloidal particles at separation distance h, having an adsorbed layer of
thickness σ (A). The free energy increases if the polymers start to interact when h << 2σ, leading to
repulsion. The polymer tail length increases from I to III (B) [9].
Steric stabilization
Steric stabilization is achieved by the presence of surfactant molecules or polymeric layers
that are adsorbed or grafted onto the surface of the colloids. These molecules consist of
parts that are usually insoluble and have strong affinity for the surface, while other parts are
strongly solvated. These molecules will stabilize the particles and keep them in solution. The
extended stabilizing chains form an adsorbed layer having a thickness in the order of 5-10
nm, depending on the chain length [12].
If two particles of radius r, both containing an adsorbed layer of thickness σ, approach each
other, their layers start to interact at a separation distance (h) smaller than 2σ, see figure 2A.
The polymers start to overlap or the layers undergo compression, both resulting in an
increase in local density. First, unfavourable mixing between polymer chains occurs, rising
the local osmotic pressure. Second, a decrease in volume available for the polymer chains
reduces the configurationally entropy. These factors lead to a strong repulsion [12].
A combination of steric repulsive forces and the attractive Van der Waals forces result in a
total interaction potential as schematically represented in figure 2B. The separation at which
repulsion begins depends among other things on the polymer tail length. If the chains are
long enough to reduce the depth of the potential well in the order of kT, the system remains
well dispersed by Brownian motion [9].
Electrostatic stabilization
Most particles in aqueous media are charged as a result of ionizable groups located on the
surface, which dissociate in polar liquid. In an electrolyte solution, oppositely charged ions
are attracted to the particle, leading to an inhomogeneous ion distribution. This ion
distribution forms a so-called electrical double layer as illustrated in figure 3A. When two
particles approach each other, their electric double layers start to overlap, leading to
electrostatic repulsion [9].
9
Figure 3. Charge stabilized particles in electrolyte solution, surrounded by electric double layer (A).
DLVO potential depends on the ionic concentration, which increases from I to IV. At higher
concentration, (III, IV) the energy barrier disappears (B) [10].
The Debye-Huckel screening length κ-1 plays an important role in the electric double layer
thickness and depends on ionic strength and temperature, as shown in equation 1 [12, 13].
κ−1 = (
𝜀𝑟 𝜀𝑜 𝑘𝐵 𝑇 1/2
)
2𝑁𝐴 𝑒 2 𝐼
(1)
εr is the relative permittivity, εo the permittivity in free space, e is the elementary charge, NA
Avogadro’s number and I the ionic strength. The ionic strength depends on ci, which is the
ionic concentration and zi: the valency of the ion species including the sign of the charge, see
equation 2 [14].
I = (1/2) ∑𝑛𝑖=1 𝑐𝑖 𝑧𝑖2
(2)
The influence of ionic concentration on colloidal stability is shown in figure 3B. At a low ionic
strength, the double layer is extended. Repulsion dominates, resulting in a high energy
barrier resisting thermal fluctuations (I). If the electrolyte concentration increases, the
double layer is compressed and repulsive interactions will be reduced. As a result, a
secondary minimum appears in the total interaction potential (II). When the depth of this
minimum is on the order of a few kT, weak flocculation occurs. The flocs formed do not
completely dissociate by Brownian motion, but if externally forces are applied such as
stirring or by lowering the salt concentration these flocs can be redispersed [9, 10]. At high
ionic strength, the electric double layer is further compressed. Van der Waals forces
dominate and the height of the energy barrier is significantly reduced (III, IV). When dropped
to the order of kT or below, irreversible aggregation takes place and colloidal dispersions are
no longer stable.
1.2.3 Zeta potential and colloidal stability
The zeta potential is an easily accessible method that is used to indicate if a system might be
stable or not [13]. The electrical double layer, described in the previous section, can be
10
Figure 4. The electric double layer of charged
particles is composed of the Stern layer and
diffuse layer. The zeta potential can be
derived at the slipping plane in the diffuse
layer [11].
Figure 5. Plot of zeta potential versus pH. At
0 mV the IEP is reached. Expected stable and
unstable regions are indicated [11].
divided in two layers: the Stern layer and the diffuse layer. Ions close to the particle surface
are assumed to be strongly adsorbed, in the Stern layer. At a certain distance from the
surface, in the diffuse layer, ions are less strongly adsorbed. The diffuse layer can be split in
two regions: one in which ions will move with the particle when moving through the
medium and one where ions will remain in their places. These two regions are separated by
an notional boundary, called the slipping plane. At this slipping plane, the zeta potential can
be derived from the mobility of the particles. This is schematically represented in figure 4.
The zeta potential gives an indication of the potential stability of the system. Empirically, an
aqueous colloidal dispersion having a zeta potential higher than + 30 mV or lower than -30
mV can be considered stable [11]. Important factors that influence the zeta potential are pH
and ionic strength. If base is added to negatively charged particles, the zeta potential
becomes more negative. In the presence of acid, the zeta potential will be less negative until
the iso-electric point (IEP) is reached, where the charge is neutralized. Further addition of
acid leads to a positive zeta potential. The influence of pH on the zeta potential is shown in
figure 5.
As mentioned previously, the thickness of the double layer and thus the zeta potential
depends on the ionic concentration and valency of the dissolved ionic species. Additionally,
ions can interact with charged surfaces. Specific adsorption leads to a change in IEP and in
extreme cases to a reversal of the surface charge [11, 15].
1.3 Nucleation and growth
A general and often used method for synthesis of colloidal dispersions is nucleation and
growth. If a solution contains more dissolved material than its solubility limit, the solution is
supersaturated and precipitation is induced. This can be achieved by chemical reaction
producing a poorly soluble product or by a change in temperature, pH or solvent
11
Figure 6. Phase diagram of a
solution. Supersaturation is
achieved by cooling below Tc.
Nucleation and growth starts
when the system is in the
metastable region.
composition (by addition of an anti-solvent). If the solubility is reduced below a critical value,
a metastable region will be entered. In this region precipitation starts, leading to an increase
in Gibbs free energy. By further quenching an unstable region is entered, where no energy
barrier is present and phase separation occurs. This process is called spinodal
decomposition. A phase diagram of a supersaturated solution achieved by cooling is
schematically represented in figure 6 [16].
In homogeneous nucleation, particles will precipitate forming small clusters (nuclei) of
approximately 1-10 nm. In classical nucleation theory, the free energy of a cluster takes both
the bulk phase and the surface into account:
g(n) = nμb + σA,
(3)
where nμb is the free energy of the bulk. n the amount of molecules and μb the chemical
potential of the bulk material. σA is the free energy contribution from the surface. σ the
surface tension and A the surface area, which is proportional to bn2/3. b is a geometric factor
depending on the shape of the nucleus. The formation of a cluster of type A molecules can
be represented as (4).
nAAn
(4)
The free energy change per mole An is equal to the free energy of a cluster minus the free
energy of dissolved monomers:
ΔG = g(n) – nμ
(5)
For dilute solutions it can be assumed that:
μ = μ0 + kT ln x,
(6)
with μ0 is the standard chemical potential in solution and x the mole fraction. By inserting (3)
and (6) into equation (5) we obtain for the free energy:
ΔG = n[μb - μ0 - kT ln x] + σbn2/3
(7)
12
Figure 7. Free energy as a function of cluster size n. For increasing supersaturation (I, II) both nc and
ΔGc decreases (A). Concentration of dissolved species during formation of a uniform dispersion. If the
concentration exceeds critical supersaturation nucleation starts, lowering the concentration of
dissolved material. Depletion of the solution leads to a drop in concentration. Below critical supersaturation nucleation is stopped. The shorter τ, the more uniform the dispersion (B).
If a solution is saturated x =xsat and μb = μ0 + kT ln xsat, (7) becomes:
ΔG = - nkT ln [x/xsat] + σbn2/3
(8)
Here, the ratio x/xsat is the supersaturation ratio S of the solution. ΔG as a function of n is
shown in figure 7A. At the maximum, the derivative of this function is zero. Solving equation
(8) gives the critical cluster size nc and the activation energy barrier ΔGc, needed to
overcome for the formation of a cluster. If a cluster contains fewer molecules than nc the
cluster will dissolve, decreasing the Gibbs free energy. But when more molecules are present
compared to nc the cluster will grow. Both nc and ΔGc depend on the supersaturation ratio,
which decreases if S increases. Lowering the surface tension has a similar effect [9, 16]. At a
certain concentration ΔGc will be on the order of kT, where nuclei are produced by thermal
fluctuations. This is called critical supersaturation.
For synthesis of colloidal dispersions, growth has to be stopped when particles are in the
colloidal size range. If the concentration of dissolved material increases rapidly and critical
supersaturation is slightly exceeded for a short time nuclei will form. This nucleation leads to
a decrease in concentration of dissolved material. When dropped below critical
supersaturation, nucleation stops and particles will grow uniformly obtaining monodisperse
particles of controlled size [9]. The change in dissolved species during nucleation and growth
processes is shown in figure 7B.
1.4 Methods and surfactants
The synthesis of colloidal dispersions of metal pyrophosphate salts consists of two stages. At
first, an intermediate was prepared by precipitation. Since most metal pyrophosphate salts
are soluble in acid, in the second step the intermediate was dissolved in HCl and reprecipitated by raising the pH in a controllable way. Three different methods were used for
synthesis. These methods were decomposition of urea, titration and injection.
13
Figure 8. Decomposition of urea at a temperature of 90oC [18].
The first method is performed by the addition of urea to the dissolved intermediate solution
and heating [17]. The urea slowly decomposes at elevated temperature. Decomposition of
urea is a two-step reaction where NH3 is produced, acting as a base to raise the pH slowly, as
shown in figure 8 [18]. Critical supersaturation is slightly exceeded and nuclei will form.
When surfactants are used, they can adsorb onto the particle surface to encapsulate nuclei
and stop further growth. Titration is commonly used as a method for synthesis of particles in
dispersion. NaOH is added drop wise to the dissolved metal pyrophosphate solution. When
base is added, the solubility of metal pyrophosphate decreases and supersaturation is
reached. When critical supersaturation is exceeded, precipitation is induced. Injection
provides a short precipitation time. For short precipitation times, small particles having an
uniform size distribution are expected to be formed, as described in section 1.3.
Apart from encapsulation of nuclei, surfactants can stabilize particles. Surfactants are
classified being anionic, cationic, non-ionic or zwittterionic. Lots of variations are possible in
the structure of tail- and head group. Examples of such variations are single or double,
straight or branched hydrocarbon chain tails and the charge, type of molecules and the size
of the head groups [13]. These variations determine the affinity of the surfactant to the
particle surface and thus whether or not surfactants will adsorb.
Additionally, ATP can be used. ATP consists of a nucleotide part and three phosphate groups.
These phosphate groups can be incorporated into the metal pyrophosphate structure while
the nucleotide part blocks further crystal growth. An overview of molecules used for
synthesis is shown in table 1.
Table 1. Types of molecules used for synthesis of metal pyrophosphate.
Molecule
Type
CTAB
Cationic
SDS
Anionic
Igepal CO-720
Non-ionic
ATP
Incorporation
Structure
14
Figure 9. An overview of the synthesis methods.
2 Experimental
As mentioned before, a two-stage synthesis was performed for obtaining colloidal
dispersions of (mixed) metal pyrophosphate (PPi) salts. An intermediate was prepared first,
which was used as a starting material for further synthesis. Second, this intermediate was
dissolved in HCl and re-precipitated by decomposition of urea, titration or injection. For each
method, samples without any surfactants, referred as “bare’’, and samples with surfactants
were prepared. This is schematically represented in figure 9.
The sample names for the metal pyrophosphate salts were composed the metal used, PPi,
the amount of surfactant (mmol) and the surfactant. If more samples were made using the
same method, the sample name ends with the difference between them. The sample names
of the mixed systems were composed of the metals and their ratios. The naming
conventions for the samples used in this thesis are shown in table 2.
In all experiments Millipore water purified by a Synergy water purification system was used.
The intermediates dissolved in HCl were filtered using a syringe filter (Pall Acrodisc syringe
filters with 0.1 µm Supor membrane for MgPPi and Minisart disposable cellulose acetate
filter, 0.2 µm pore size, 16534-K for FeMgPPi), attached to a 20 mL syringe to remove dust.
The re-precipitated samples were washed to remove excess salt as described in their
respective sections. After washing, all samples were redispersed in 50 mL water.
Table 2. Sample names and their explanation for metal PPi and mixed metal PPi.
Metal PPi:
Explanation
Mixed metal PPi
Explanation
U
-
Method
I
Method
Mg
Cation
-
PPi _
1
CTAB
_
PPi
Amount
(mmol)
Surfactant
1
Fell
50
Mg
PPi
Ratio
Cation
Cation
PPi
Ratio
15
7
Difference between the
samples (e.g. pH) when
more were prepared using
the same method
2.1 Intermediates
2.1.1 Metal PPi
MgPPi
50 mmol Sodium pyrophosphate decahydrate (Na4P2O7·10H2O, Acros) was dissolved in 800
mL water. To this solution, a 50 mL 1 M hexahydrous magnesium chloride (MgCl2·6H2O,
Fluka) solution was added drop wise to the PPi in one hour while the solution was stirred
using a magnetic stirrer.
ZnPPi
50 mmol anhydrous ZnCl2 (Sigma-Aldrich) was dissolved in 150 mL water and added drop
wise in one hour to a 700 mL 72 mM PPi4- solution. During addition, the solution was
continuously stirred.
CaPPi
12.5 mmol CaCl2 (Sigma-Aldrich) was dissolved in 20 mL water. This was added drop wise in
30 minutes to a 200 mL solution, containing 12.5 mmol PPi4- while stirred.
2.1.2 Mixed metal PPi
FeMgPPi
Intermediates in different ratios Fe:Mg (shown in table 3) were prepared by dissolving 16 22 mmol MgCl2·6H2O and 0.2-2.0 mmol FellCl2·4H2O or FelllCl3·6H2O (both Sigma-Aldrich) in
20 mL water. This solution was added drop wise in 30 minutes to a 200 mL 60 mM PPi4solution while stirred. For simplicity, charge neutral salt complexes in stoichiometric ratios
were assumed to be formed.
ZnMgPPi, CaMgPPi
0.2 mmol ZnCl2 or CaCl2 and 10 mmol MgCl2·6H2O (ratio M:Mg 1:50) were dissolved in 20 mL
water. This was added drop wise to a 200 mL solution, containing 5.1 mmol PPi4- in 30
minutes under continuous stirring.
Table 3. Composition of Fe, Mg and PPi used for each intermediate of FeMgPPi.
intermediate
Fell/Felll
(mmol)
Mg
(mmol)
PPi
(mmol)
1Fell10MgPPi
1Fell20MgPPi
1Fell50MgPPi
1Fell100MgPPi
1.79
0.99
0.40
0.20
16.8
19.8
20.0
22.2
9.20
13.1
10.2
10.1
1FelIl10MgPPi
1FelIl20MgPPi
1FelIl50MgPPi
1FelIl100MgPPi
2.01
1.07
0.44
0.19
19.3
19.5
19.8
20.0
11.0
10.5
10.7
10.1
16
FeCaZnMgPPi
0.2 mmol of CaCl2, ZnCl2, FellCl2·4H2O and 33 mmol MgCl2·6H2O (ratio Fe:Ca:Zn:Mg
0.3:0.3:0.3:50) were dissolved in 30 mL water. This solution was added drop wise to a 200
mL 85 mM pyrophosphate solution in 30 minutes while stirred.
All dispersions were stirred for one hour after addition was complete. The precipitate was
washed three times with water and twice with acetone at 2000 rpm (931 g) for 20 minutes.
Finally, all metal pyrophosphate intermediates were dried in an oven at 37oC for two days
and all mixed metal PPi intermediates were dried at 35oC for one day [17].
2.2 Decomposition of urea
MgPPi
0.56 g MgPPi intermediate was completely dissolved in a 50 mL 0.3 M HCl solution. Urea
(Roht) and 1, 5 or 10 mmol CTAB (Sigma) or 5 mmol ATP-Na2 salt (Serva) were added. One
bare sample was prepared. The concentration of urea was 0.4-2.0 M. CTAB was dissolved by
ultrasonication for two hours. The solution was heated at 90oC using an oil bath, while being
stirred and refluxed [17]. The pH went up to a final pH of 7 in approximately two to three
hours. Precipitation started at pH 5 and a white suspension was obtained. During synthesis
of U-MgPPi5ATP the solution turned yellow while heated, which probably indicates (partial)
decomposition of ATP [19].
ZnPPi
0.76 g ZnPPi intermediate was dissolved in 50 mL 0.3 M HCl. Urea was added to yield a
concentration of 0.4 M. No surfactants were used. The solution was heated at 90oC for 3.5
hours under continuous stirring and refluxing. Precipitation started at pH 3 and heating was
continued until pH 6 was reached.
When the final pH was reached, heating was stopped and the dispersion slowly cooled down
to room temperature in 30 minutes while stirring was continued. All samples were washed
three times with water at 2000 rpm for 40 minutes. An overview of the samples prepared by
this method is shown in table 4.
Table 4. Samples prepared by decomposition of urea.
Sample
Mgppi (g)
[PPi]
(mM)
Surfactant
(mmol)
[Urea]
(M)
Urea releasing
time (h)
pH
U-MgPPi
U-MgPPi_5ATP
U-MgPPi_1CTAB
U-MgPPi_5CTAB
U-MgPPi_10CTAB
U-ZnPPi
0.55
0.56
0.55
0.56
0.55
0.76
50
50
50
50
50
50
4.6
1.0
5.0
10.5
-
2.0
0.4
1.0
0.4
0.4
0.4
0.5
5.5
2.0
3.0
3.0
3.5
7
5
7
6
7
6
17
Table 5. Samples prepared by titration.
Sample
Mgppi
(g)
Final Vol.
(mL)
[PPi]
(M)
Surf.
(mmol)
[HCL]
(M)
mL 1M
NaOH
Time
(min)
Final
pH
T (o )
T-MgPPi_30
T-MgPPi_4
T-MgPPi_1ATP
T-MgPPi_1CTAB
0.53
0.55
0.55
0.56
60
43
55
53
40
60
50
50
0.9
1.0
0.3
0.4
0.4
0.4
15
8
15
18
30
4
15
15
7
7
7
7
22
23
26
24
2.3 Titration
For each sample, 0.56 g MgPPi intermediate was dissolved in 35 mL 0.4 M HCl. To two
samples 1 mmol CTAB or ATP was added. About 15 mL 1 M NaOH was added drop wise to
the MgPPi solution in various time intervals, yielding a final volume of circa 50 mL. During
synthesis, pH and temperature were monitored and the solution was continuously stirred.
Precipitation started when the pH was in between 4.5 and 5. The addition of NaOH was
stopped when pH 7 was reached. Stirring was continued for 10 minutes. The precipitate was
washed three times with water at 2500 rpm (1455 g) for 40 minutes. An overview of these
samples is shown in table 5.
2.4 Injection
2.4.1 Metal PPi
MgPPi
0.56 g MgPPi intermediate was dissolved in 15 mL 1 M HCl and transferred to a 20 mL
syringe. 35 mL 0.36 M NaOH solution was vigorously stirred using a magnetic stirrer. 1 mmol
surfactant was added to the alkaline solution. CTAB was dissolved by ultrasonication and
Igepal co-720 (Aldrich) was heated at 500C until completely dissolved. The MgPPi solution
was quickly injected into the NaOH solution, which directly turned turbid. After injection,
stirring was continued for 10 minutes. The samples were washed three times with water:
two times at 2500 rpm for 40 minutes and once at 3000 rpm (2095 g) overnight. IMgPPi_1ATP was centrifuged two times at 2500 rpm for 40 minutes, followed by dialysis. 50
mL sample was dialyzed for two days, refreshing the water (500 mL) once a day.
ZnPPi
0.76 g ZnPPi intermediate was dissolved in 15 mL 1 M HCl and the solution was quickly
injected in a 35 mL 0.36 M NaOH solution under stirring. No surfactants were added.
Precipitation started immediately, yielding a final pH of 8. After injection, the dispersion was
stirred for another 10 minutes. ZnPPi was washed two times using a centrifuge at 2000 rpm
for 20 minutes and once at 3000 rpm for three hours.
18
CaPPi
0.63 g intermediate was dissolved in 15 mL 1 M HCL solution. The dissolved intermediate
was injected in a 0.39 M NaOH solution under continuous stirring. One sample was prepared
containing Igepal, as described previously. Stirring was continued for 10 minutes after
injection. Both samples were washed three times at 2500 rpm for 20 minutes.
2.4.2 Mixed metal PPi
FeMgPPi, ZnMgPPi, CaMgPPi and FeCaZnMgPPi
0.56 g intermediate was dissolved in 15 mL 1 M HCl. This was quickly injected in a vigorously
stirred 35 mL 0.39 M NaOH solution using a syringe. A final pH of 7 was reached and stirring
was continued for 10 minutes. The samples were washed once at 2500 rpm for 40 minutes
and once at 3000 rpm overnight. Exceptions are the 1: 10 ratios of FeMgPPi. These were
washed two times at 2500 rpm for 40 minutes. An overview of all samples prepared by the
injection method is shown in table 6.
Table 6. Samples prepared by injection.
Intermediate
(g)
[PPi]
(M)
Surfactant
(mmol)
Vol. NaOH
(mL)
[OH-]
(M)
Final pH
0.56
50
-
35
0.36
7
0.56
50
-
35
0.43
11
0.56
50
0.9
35
0.36
7
0.55
50
1.0
35
0.36
11
0.56
50
1.0
35
0.39
7
0.56
50
1.0
35
0.39
7
0.76
50
-
35
0.36
8
0.63
50
-
35
0.39
7
0.62
50
1.1
35
0.39
7
I-1Fell10MgPPi
I-1Fell20MgPPi
I-1Fell50MgPPi
I-1Fell100MgPPi
0.56
50
-
35
0.39
7
0.56
50
-
35
0.39
7
0.56
50
-
35
0.39
7
0.57
50
-
35
0.39
7
I-1Felll10MgPPi
I-1Felll20MgPPi
I-1Felll50MgPPi
I-1Felll100MgPPi
0.56
50
-
35
0.39
7
0.56
50
-
35
0.39
7
0.56
50
-
35
0.39
7
0.56
50
-
35
0.39
7
I-1Zn50MgPPi
I-1Ca50MgPPi
0.56
0.56
50
50
-
35
35
0.39
0.39
7
7
I-0.3Fe0.3Ca0.3
Zn50MgPPi
0.56
50
-
35
0.39
7
Sample
Metal PPi
I-MgPPi_7
I-MgPPi_11
I-MgPPi_1ATP
I-MgPPi_1CTAB
I-MgPPi_1Igepal
I-MgPPi_1SDS
I-ZnPPi
I-CaPPi
I-CaPPi_1Igepal
Mixed metal PPi
19
2.5 Characterization
The morphology of the dried samples was determined using a Transmission Electron
Microscope (TEM) Philips Tecnai 12 at an accelerating voltage of 120 kV. Elemental analysis
was performed by Energy dispersive X-ray analysis (EDX) using a Philips Tecnai 20
microscope at an acceleration voltage of 200 kV. X-ray diffractograms were obtained using a
Bruker D8 diffractometer with Cu-Kα radiation, operating at 30 kV and 45 mA. Dispersions
were analyzed by Dynamic Light Scattering (DLS) by using a Malavern Instruments Zetasizer
Nano series apparatus at a scattering angle of 173o and an equilibration time of five minutes.
20
3 Results and discussion
3.1 Decomposition of urea
TEM results of ZnPPi (figure 10A) showed aggregated amorphous spheres with a size ranging
from 30 - 100 nm and larger needle-shaped particles. Dispersions of these particles settled
out within a day.
DLS measurements of MgPPi (table 7) showed particle sizes above 1000 nm, which indicates
presence of large particles or aggregates. This was confirmed by TEM analysis, as shown in
figure 10B-D. Large crystalline platelets and aggregates were formed. Some of the platelets
were fragmented by centrifugation. The zeta potential could not be measured accurately
due to the fast sedimentation of these large particles. Decreasing the synthesis time by
increasing the urea concentration did not led to smaller particle sizes.
Summary
We were unable to obtain stable dispersions by using this method. A possible explanation
could be that the surfactants we used had too little affinity for the particle surface. As a
result, growth was continued and large particles were formed because it is
thermodynamically more favorable for molecules to grow on already existing particles rather
than creating new nuclei.
Figure 10. TEM results urea method: U-ZnPPi (A),U- MgPPi_bare (B), UMgPPi_ATP (C), U-MgPPi_CTAB (D). All samples that contained CTAB gave
comparable results.
21
Table 7. DLS results for the urea and titration method.
Sample
Size
(nm)
Zeta potential
(mV)
Conductivity
(mS/cm)
> 1000*
> 1000*
> 1000*
> 1000*
> 1000*
-1
- 24
0
- 10
+2
0.07
0.01
0.46
0.50
0.49
70
70
> 1000*
140
115
> 1000*
- 51
- 59
-1
- 45
- 65
- 12
0.34
2.00
1.39
0.16
4.73
0.29
Urea method
U-MgPPi
U-MgPPi_5ATP
U-MgPPi_1CTAB
U-MgPPi_5CTAB
U-MgPPi_10CTAB
Titration method
T-MgPPi_4
T-MgPPi_4 **
T-MgPPi_30
T-MgPPi_1ATP
T-MgPPi_1ATP**
T-MgPPi_1CTAB
* Sizes larger than 1000 nm cannot be measured and can only be used as indication.
** Measurements were performed eight months after synthesis.
3.2 Titration
Titration of MgPPi with sodium hydroxide in 30 minutes led to the formation of small
needles and the addition of CTAB had no effect on this. These needles arranged in star-like
structures and aggregated. TEM images are presented in figure 11A-B. The aggregates were
in the micron size range, as can be concluded from both TEM and DLS and the samples
sedimented within 24 hours. Addition of ATP led to stable dispersions of small needles and
platelets of circa 70 nm (figure 11C). For shorter titration times (T-MgPPi_4) stable
dispersions of platelets of approximately 20 - 50 nm were formed (figure 11D). DLS
measurements determined sizes of 140 nm (T-MgPPi1ATP) and 70 nm (T-MgPPi_4). The
negative zeta potential confirmed this stability. Aged dispersions of T-MgPPi_4 and TMgPPi1ATP stored in plastic centrifuge tubes were still stable after eight months, see table 7
for DLS results.
Summary
A decreased precipitation time resulted in stable dispersions. The disadvantage of this
method was that reaction conditions (i.e. final volume and titration time) were hard to
control, causing irreproducible results. Because of this irreproducibility, we have decided not
to attempt the preparation of ZnPPi and CaPPi dispersions with this method.
22
Figure 11. TEM results titration method: T-MgPPi_30 (A), T-MgPPi_1CTAB (B),
T-MgPPi_1ATP (C), T-MgPPi_4 (D).
3.3 Injection
3.3.1 Metal PPi
MgPPi
Addition of CTAB resulted in large thin needles of up to 10 μm, as shown by the TEM image
in figure 12A. DLS measurements (table 8) showed sizes larger than 1000 nm and a zeta
potential of + 4 mV, indicating rapidly sedimentation. For the bare samples (I-MgPPi_11 and
I-MgPPi_7) the structure depended on pH: a pH above 8 led to formation of amorphous
aggregated spheres (12B), while at neutral pH small platelets of 50 - 90 nm were present
(12C). Sizes obtained by DLS were > 1000 nm for the amorphous spheres and 95 nm for the
platelets. Additionally, I-MgPPi_7 was dried in an oven for one week at 55oC and 100oC,
obtaining crystalline spherical particles of approximately 150 nm (figure 12D). Unfortunately,
attempts to reproduce these spheres were unsuccessful.
Table 8. DLS results of MgPPi, prepared with the injection method.
Sample
Size
(nm)
Zeta potential
(mV)
Conductivity
(mS/cm)
I-MgPPi_7
I-MgPPi_11
I-MgPPi_1ATP
I-MgPPi_1CTAB
I-MgPPi_1Igepal
I-MgPPi_1SDS
95
> 1000*
> 1000*
> 1000*
115
150
- 47
- 59
-5
+4
- 40
- 52
0.92
1.05
0.01
0.69
0.23
0.23
* Sizes larger than 1000 nm cannot be measured and can only be used as indication.
23
Figure 12. TEM results injection method: I-MgPPi_1CTAB (A), I-MgPPi_11 (B), I-MgPPi_7 (C), IMgPPi_7_dried (D), I-MgPPi_11_ aged (E), I-MgPPi_7_aged (F).
Addition of ATP, Igepal or SDS had no effect on the particle shape and TEM results of these
samples were comparable to those of I-MgPPi_7. However, I-MgPPi1ATP particles were
aggregated and sedimented within a day, while the samples that contained Igepal and SDS
were stable for at least two months. This aggregation may be a result of the low
conductivity, as we will show in the next section.
In time, the amorphous spheres of I-MgPPi_11 had changed into small crystalline needles
while the platelets remained unchanged, see figure 11E-F. I-MgPPi_11 was stored in a plastic
centrifuge tube and was still stable after eight months, while I-MgPPi_7 was stored in glass
and started to aggregate after five months. Aging of I-MgPPi_7 is shown in figure 13.
Aggregates could not be redispersed by vigorous stirring or ultrasonication. The initial
decrease in particle size after 20 days can be explained by etching of curved surfaces [20]:
after washing, the ionic concentration was lowered and the MgPPi salt started to dissolve a
little, which was the fastest at the corners. This finally led to separate particles.
Figure 13. Aging of I-MgPPi_7,
measured using DLS. After synthesis
the size of the aggregates decreased,
as a result of etching. Particles started
to aggregate after five months.
24
Figure 14. DLS measurements of MgPPi_7 taken after various washing steps. Size versus conductivity
(A) and zeta potential versus conductivity (B) More detailed figure of conductivity versus size. At
conductivity in between 0.23 and 2.40 mS/cm stable dispersions were obtained (C).
Stability and conductivity
The zeta potential, size and conductivity were measured using DLS after various washing
steps, as shown in figure 14A-B. First, the sample was freshly prepared (a). Second, the
sample was centrifuged one to three times (b,c and d) and finally dialyzed (e) for one day (35
mL sample in 350 mL water). A minimum in size (circa 150 nm) appeared when the sample
was centrifuged the second and third time. Both dispersions were stable and had a zeta
potential of - 45 mV. In figure 14C DLS results of MgPPi dispersions at conductivity ranging
from 0 - 2.5 mS/cm are shown. A conductivity between 0.13 and 2.40 mS/cm resulted in
stable dispersions which were slightly turbid. These particles had dimensions of
approximately 100 - 150 nm, while at conductivity lower than 0.13 mS/cm particles were
aggregated and formed macroscopic clusters. These dispersions were more turbid and
sedimented within a day.
A minimum value of the zeta potential, - 63 mV, was reached after the first washing step. At
this minimum aggregates were too large for being stable. Further washing steps resulted in a
less negative zeta potential. This decrease in zeta potential and the increase in size might be
an indication for the loss of surface charge: the Debye screening length decreases and
particles aggregate. When the potential becomes too low, these aggregates could not be
redispersed either by ultrasonication or by bringing the conductivity back into the stable
range by addition of NaCl.
25
Figure 15. TEM results of ZnPPi (A) and CaPPi (B).
ZnPPi, CaPPi
In figure 15A, the TEM result of ZnPPi is presented. Generally, platelet-shaped particles of
about 200 nm were found, as confirmed by DLS results (table 9). Despite a zeta potential of
- 61 mV, the ZnPPi dispersions were not stable over time.
CaPPi consisted mainly of platelets having dimensions larger than 1 μm, with most of them
broken by centrifuging. TEM results (figure 15B) and DLS results (table 9) obtained
comparable sizes for both bare and Igepal containing samples. CaPPi dispersions settled out
within six hours.
Table 9. DLS results of ZnPPi, CaPPi, and mixed metal PPi obtained by the injection method.
Size
(nm)
Zeta potential
(mV)
Conductivity
(mS/cm)
215
> 1000*
> 1000*
- 61
- 58
- 50
0.25
0.03
0.13
I-1Fell10MgPPi
I-1Fell20MgPPi
I-1Fell50MgPPi
I-1Fell100MgPPi
> 1000*
140
150
155
- 14
- 47
- 46
- 44
0.14
0.50
0.27
0.14
I-1Felll10MgPPi
I-1Felll20MgPPi
I-1Felll50MgPPi
I-1Felll100MgPPi
> 1000*
170
120
155
- 36
- 42
- 37
- 43
0.33
0.24
0.20
0.22
I-1Zn50MgPPi
I-1Ca50MgPPi
I-0.3Fe0.3Ca0.3Zn50MgPPi
170
- 43
0.15
750
- 42
0.18
175**
- 42
0.14
Sample
ZnPPi, CaPPi
I-ZnPPi
I-CaPPi
I-CaPPi_1Igepal
Mixed metal PPi
* Sizes larger than 1000 nm cannot be measured and can only be used as indication.
** Two sizes were determined. Main size: 175 nm and > 1000 nm (small distribution).
26
3.3.2 Mixed metal PPi
FeMgPPi
Small thin needles, arranged in star-like structures were formed for I-1Fell10MgPPi and I1Felll10MgPPi, see figure 16A. DLS measurements (table 9) yielded sizes of several microns.
Both dispersions sedimented within eight hours. For the higher ratios (FeII or Felll:Mg 1:10,
1:50 and 1:100) stable, slightly turbid dispersions were obtained. Similar structures were
formed compared to I-MgPPi_7, as shown in figure 16B. Concerning conductivity, FeMgppi
(except 1:10 ratios) show similar behavior as MgPPi described previous.
ZnMgPPi
A stable slightly turbid dispersion of ZnMgPPi in a ratio Zn:Mg 1:50 was obtained. TEM
images (figure 16C) were comparable to those of I-FeMgPPi and I-MgPPi_7. DLS determined
particle sizes at 170 nm and a zeta potential of - 43 mV.
CaMgPPi
In TEM images (figure 16D) of CaMgPPi in a ratio Ca:Mg 1:50, amorphous structures were
observed. Part of this dispersion sedimented within one hour. After sedimentation, the
dispersion remained turbid. DLS measurements determined an average particle size of 750
nm and a zeta potential of - 42 mV.
Figure 16. TEM results of mixed metal PPi: 1Fell10MgPPi (A), 1Felll100MgPPi (B), 1Zn50MgPPi
(C), 1Ca50MgPPi, (D) and I-0.3Fe0.3Ca0.3Zn50MgPPi (E).
27
Figure 17. XRD results for
MgPPi and FeMgPPi in the
ratios 1:10 and 1:50. Both
FeMgPPi in the 1:10 ratios
showed crystalline
structures, while both 1:50
ratios and MgPPi were less
crystalline. This could be an
effect of particle size.
FeCaZnMgPPi
DLS analysis yielded two particle sizes: an average size of 175 nm and sizes larger than 1000
nm. The presence of large particles can be confirmed by the TEM image as shown in figure
16E. These large particles in sedimented within one hour and rest of the sample, small
platelet-shaped particles comparable to those of MgPPi, remained well dispersed and the
dispersion was slightly turbid. Previously, it was observed that dispersions of CaPPi and
CaMgPPi (partly) sedimented because the presence of large particles, while ZnMgPPi,
FeIIMgPPi, FeIIIMgPPi and MgPPi consisted of small particles and yielded stable dispersions.
This might be an indication that the large particles mainly contained CaPPi.
3.3.3 X-Ray diffraction
XRD measurements (figure 17) were obtained for I-MgPPi_7, FellMgPPi and FelllMgPPi (ratios
1:10 and 1:50). A crystalline structure was shown for FellMgPPi and FelllMgPPi in 1:10 ratio,
while for FellMgPPi and FelllMgPPi in 1:50 ratio and MgPPi a less crystalline structure was
observed. This can be an effect of peak broadening, which is common for small particles
[21].
3.3.4 Energy dispersive X-ray analysis
The elemental composition of the intermediates (table 9) and re-precipitated samples (table
10 of I-MgPPi_7, FellMgPPi and FelllMgPPi in the ratios 1:10 and 1:50 have been determined.
All intermediates contained a significant amount of Na and Cl, while the re-crystallized
samples contained neither Na nor Cl. An explanation might be that the precursor salts were
not completely dissolved during synthesis of the intermediates. In intermediates and final
products all FeII and FeIII was incorporated, while only a part of Mg was built in (50 - 80 % for
the intermediates and 15 - 60 % for the re-crystallized samples compared to the expected
ratios).
Considering the charge balance, both intermediates and re-precipitated samples had an
excess of negative charges. This could be an indication of incorporation of hydrogen into the
salt structure. An exception was the I-1FelI10MgPPi intermediate, which had a slightly
positive charge. This may be a result of the standard deviation in Fe, which was on the order
of the percentage Fe itself.
28
Table 9. Elemental composition determined by EDX of intermediates and expected ratios.
Sample
Na(%)
Cl( %)
Fe (%)
Mg (%)
P(%)
Ratio
Na
Cl
Fe
Mg
I-MgPPi_7
Expected
22.2 ± 8.3
10.1 ± 4.4
0.1 ± 0.2
23.1 ± 2.7
44.5 ± 9.7
1.00
0.45
0.01
1.04
1
-
-
-
2.00
1
I-1FelI10MgPPi
Expected
27.8 ± 6.8
1.54
0.23
0.29
1.48
1
-
-
0.18
1.82
1
I-1Fell50MgPPi
Expected
18.5 ± 8.1
0.84
0.33
0.05
1.61
1
-
-
0.04
1.96
1
I-1FelIl10MgPPi
Expected
20.4 ± 6.3
0.95
0.39
0.19
1.14
1
-
-
0.18
1.73
1
4.1 ± 1.1
7.4 ± 5.5
8.3 ± 3.2
5.2 ± 5.1
1.1 ± 0.5
4.0 ± 0.9
26.8 ± 3.3
35.6 ± 11.9
24.5 ± 1.0
36.1 ± 4.0
55.2 ± 7.8
42.9 ± 7.8
PPi
Table 10. Elemental composition determined by EDX of re-precipitated (mixed) metal
pyrophosphate salts and expected ratios.
Sample
Fe (%)
Mg( %)
P (%)
Ratio
Fe
Mg
PPi
I-MgPPi_7
Expected
0.2 ± 0.2
12.5 ± 3.7
87.4 ± 3.6
0.00
-
0.29
2.00
1
1
I-1FelI10MgPPi
Expected
7.3 ± 0.8
22.4 ± 6.9
70.3 ± 7.3
0.21
0.18
0.64
1.82
1
1
I-1Fell50MgPPi
Expected
1.3 ± 0.6
34.6 ± 2.2
64.2 ± 2.7
0.04
0.04
1.08
1.96
1
1
I-1FelIl10MgPPi
Expected
6.8 ± 0.8
13.6 ± 4.8
79.6 ± 5.5
0.17
0.18
0.34
1.73
1
1
I-1FelIl50MgPPi
Expected
1.3 ± 0.3
36.6 ± 1.7
62.1 ± 1.8
0.04
0.04
1.18
1.94
1
1
Summary
Stable dispersions of MgPPi and various mixed metal PPi salts were prepared. Particle sizes
of 50 - 90 nm according to TEM and 95 - 170 nm according to DLS were found. The
difference between these sizes can be explained by the fact that DLS determines the
hydrodynamic size of small aggregates in dispersion while with TEM only the size of
individual, dried particles was obtained.
The preparation time for this method was short, leading to controllable reaction conditions
and a high reproducibility. The stability of these dispersions depended on the cluster size of
aggregated particles, ionic strength (conductivity) and surface charge (zeta potential). IMgPPi_7 was stable for five months. Addition of Igepal and SDS had no effect on the stability
and shape of the particles, while CTAB led to different morphology and a decreased stability.
Properties of the mixed metal pyrophosphate salts were similar to I-MgPPi_7 if sufficient Mg
was used. Elemental analysis showed that the particles contained FeII or FeIII and Mg,
indicating that the metals indeed were incorporated into the salt structure. For all samples
29
containing Ca, large particle sizes were observed, which sedimented in at least one hour. A
possible explanation for this could be found in the elemental properties of Ca. Ca has a
larger atomic radius and is less electronegative compared to Fe and Mg. Possibly, the Ca
atoms could be too large for incorporation into the lattice structure of PPi. Additionally,
there is a difference between elemental Ca and Mg regarding the reactivity with water: Ca
reacts at room temperature while for Mg hot water is required [22].
30
4 Conclusions
Colloidal (mixed) metal pyrophosphate dispersions were prepared by a two-stage synthesis.
An intermediate was prepared first, which was subsequently dissolved in HCl and reprecipitated in a controllable way. To induce precipitation, the pH was raised by one of three
different methods. Decomposition of urea generally led to colloidally unstable dispersions of
platelet-shaped particles and aggregates with dimensions larger than 1 μm. Titration of
MgPPi led to stable dispersions only with short addition times. Reaction conditions were
hard to control, making the results of this method irreproducible. The injection method had
a short precipitation time and showed reproducible results.
Stable dispersions of MgPPi and mixed metal pyrophosphate salts were obtained by the
injection method and resulting systems consisted of platelet-shaped particles of
approximately 50 - 90 nm according to TEM and 95 - 170 nm according to DLS. The stability
depended on the ionic strength, the size of the aggregates and the surface charge. At
conductivity between 0.13 and 2.40 mS/cm stable dispersions were obtained. MgPPi
dispersion remained stable for five months. The use of Ca resulted in large particle sizes.
Addition of CTAB led to stronger aggregation of particles, while ATP, Igepal and SDS had no
influence on the particle shape and stability.
From elemental analysis of FellMgPPi and FelllMgPPi, it can be concluded that both metals
were incorporated in the particles. However, the amount of Mg was lower than expected. In
contrast to the intermediates, the re-precipitated samples contained no Na and Cl according
to EDX.
31
5 Outlook
Elemental analysis should be performed for ZnMgPPi, CaMgPPi and FeCaZnMgPPi to
determine if all the metals used were incorporated into the salt structure. For CaMgPPi and
FeCaZnMgPPi, the elemental composition of the large particles should be resolved to
determine if these particles indeed consist of CaPPi, indicating large-scale phase separation.
Synthesis of CaPPi, ZnPPi and CaMgPPi could be optimized to obtain stable dispersions. This
can be done by adjustment of the reaction conditions such as concentration, nucleation- or
growth time, surfactants or using different synthesis methods. For CaMgPPi, the ratio of Ca
could be lowered than it might be incorporated into the MgPPi structure and form a stable
dispersion.
For FeCaZnMgPPi, the ratios used for Fe:Ca:Zn:Mg were: 0.3:0.3:0.3:50. The amount of Ca
should be reduced when trying to avoid sedimentation. The other metals could be tuned to
comply with the requirement of minerals of the human body. More Fe could be used and
the amount of Mg can be lowered, but it should be taken to ensure that the amount of Mg is
not reduced too much, otherwise the dispersion will become unstable as we have shown in
section 3.3.2.
The long-term stability of I-MgPPi_7 stored in plastic bottles needs more attention. IMgPPi_7 was stored in a glass bottle and started to aggregate and sedimented after five
months while other samples stored in plastic centrifuge tubes remained stable for longer
time. After eight months, part of I-MgPPi_11 dispersion (stored in plastic) was transferred to
a glass bottle, which started to sediment after one week, while the part still in the plastic
remained stable. From this it can be concluded that glass had some influence on the stability
of the dispersion and when stored in plastic, dispersions can be stable for a longer time.
Finally, applications such as emulsions have to be researched in more detail. As a small
sidestep, emulsions of I-MgPPi_7 were made using octanol with an oil-water ratio of 1:10
[23]. CTAB, SDS, Igepal and PVA were used as surfactants. From this was found that particles
preferred the aqueous phase. More detailed study is necessary for preparation of stable
emulsions.
32
6 Acknowledgements
For this work, I want to acknowledge some people who were helping me during my time at
the Van ‘t Hoff laboratory for physical and colloid chemistry. In special, Mikal van Leeuwen
for his supervision, advices, recommendations and everything else I have learned. Janne
Mieke Meijer and Jan Hilhorst for supervising me on Thursday. Willem Kegel and the Friday
Brainstorm Sessions for tips, ideas and discussion. And last, but not least Rocio Costo for
making some of the TEM images.
33
7 References
[1] R.F. Hurrell, Nutr. Rev., 1997, 55(6), 210–222
[2] R.K. Rude, F.R. Singer and H.E. Gruber, J. Am. Col. Nutr., 2009, 28(2), 131–141
[3] H. Yanagisawa, JMAJ, 2004, 47(8), 359–364
[4] M.B. Zimmermann, R.F. Hurrell, Lancet, 2007, 370, 511–20
[5] R.F. Hurrell, J. Nutr. 2002, 132, 806S–812S
[6] M.C. Fidler, Th. Walczyk, L. Davidsson et al., Br. J. Nutr., 2004, 91, 107–112
[7] R. Blanco-Rojo, A. M. Pérez-Granados et al., Br. J. Nutr., 2011, 105, 1652–1659
[8] K. L. Manchester, Biochem. Educ., 1980, 8(3), 70–73
[9] D.H. Everret, Basic Principles of Colloid Science, The Royal Society of Chemistry, 1988
[10] H. N. W. Lekkerkerker, R. Tuinier, Colloids and the Depletion Interaction, Springer, 2011
[11] Malvern Instruments, Zetasizer Nano series technical note
[12] T. F. Tadros, Colloid Stability, Vol. 2, 2007
[13] T. Cosgrove, Colloid Science: Principles, Methods and Applications, second Edition, John Wiley &
Sons Ltd, 2010
[14] A. van Blaaderen, DLVO theory & measurement of Interaction Forces, SCM, Debye Institute,
Utrecht University, 2012, 191-193
[15] R. J. Hunter, Zeta Potential in Colloid Science: Principles and Applications, Academic Press, UK,
1998
[16] A.P. Philipse, Particulate Colloids: aspects of Preparation and Characterization, in J. Lyklema (ed.)
Fundamentals of Colloids and Interface Science, vol. 4, 2005
[17] P. J. Groves, 34 R. M. Wilson, 34 P. A. Dieppe, R. P. Shellis, J. Mater. Sci.: Mater. Med., 2007, 18,
1355–1360
[18] J.P. Chan, K. Isa, J. Mass Spectrom. Soc. Jpn., 1998, 46(8), 299-303
[19] R. B. Stockbridge, G. K. Schroeder, R. Wolfenden, Bioorg. Chem., 2010, 38, 224–228
[20] O.L.J. Gijzeman, Reaction kinetics and Diffusion, Master course 2011/2012
[21] L. R. Rodrigue et al.s, Materials Research. 2012, 15(6), 974-980
[22] Shriver & Atkins, Inorganic chemistry, fourth edition, Oxford University Press, 2006
[23]S. Droog, Emulsions stabilized by metal pyrophosphate salts, master thesis, 2012
34