Cent. Eur. J. Chem. • 11(1) • 2013 • 101-110
DOI: 10.2478/s11532-012-0136-9
Central European Journal of Chemistry
Effect of solution pH on the stability
of mixed silica –alumina suspension
in the presence of polyacrylic acid (PAA)
with different molecular weights
Research Article
Małgorzata Wiśniewska1*, Konrad Terpiłowski2, Stanisław Chibowski1,
Teresa Urban1, Vladimir I. Zarko3, Vladimir M. Gun’ko3
1
Department of Radiochemistry and Colloid Chemistry,
Faculty of Chemistry, Maria Curie Skłodowska University, 20-031 Lublin, Poland
2
Department of Physical Chemistry-Interfacial Phenomena,
Faculty of Chemistry, Maria Curie Skłodowska University, 20-031 Lublin, Poland
3Institute of Surface Chemistry, National Academy of Sciences of Ukraine,
03164 Kiev, Ukraine
Received 19 June 2012; Accepted 14 September 2012
Abstract: The influence of solution pH (in the range 3-9) on mixed silica-alumina suspension in the absence and presence of polyacrylic
acid (PAA) was studied. The composition of the adsorbent was SiO2 (97%) and Al2O3 (3%). The turbidimetry method was applied
to record changes in the stability of the investigated systems as a function of time. It was shown that the suspension without the
polymer is less stable at pH 3, whereas at pH 6 and 9, the systems were stable. PAA with molecular weights 100 000 and 240 000 at
pH 3 (improvement of system stability conditions) and PAA 2 000 at pH 6 (deterioration of suspension stability) have a great effect on
the silica-alumina suspension stability. The stabilization-flocculation properties of polyacrylic acid are a result of a specific conformation
of its chains on the solid surface where it depends on the solution pH and the polymer molecular weight.
Keywords: Polyacrylic acid • Mixed silica-alumina • Suspension stability • Turbidimetry • Macromolecule conformation
© Versita Sp. z o.o.
1. Introduction
The basic research of various substances’ adsorption
(simple ions, organic compounds, natural and synthetic
polymers) at the solid-liquid interface is very important
due to great possibilities of such system application in
many fields of human activity [1-4]. The wide practical
usage of polymers is a result of specific properties of
their chains on the solid surface. These chains consist
of a large number of monomers (segments). For this
reason their adsorption leads to the creation of the so
called train-loop tail structure on the adsorbent surface.
It means that the single polymer chain occupies many
adsorption places, in contrast to small molecules which
occupy only one place on the solid surface. The specific
arrangement of the adsorbed polymer chain, in which
one can distinguish segments located in train, loop and
tail structures, is referred to as a conformation. This
conformation influences directly the stability conditions
of colloidal solid dispersed in the polymer solution.
The polymer presence may result in both stabilization
and destabilization of solid particles. Adsorbed polymer
chains can cause steric stabilization or bridging
flocculation (depending on their molecular weight and
concentration in solution).
The entropy reduction of polymer layers adsorbed on
surfaces of two approaching solid particles causes the
effect of their repulsion, which leads to steric stabilization.
Moreover, the electrosteric stabilization often takes place
in many systems. It is a combination of both steric and
* E-mail: [email protected]
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Effect of solution pH on the stability of mixed
silica –alumina suspension in the presence
of polyacrylic acid (PAA) with different molecular weights
electrostatic repulsion. Polyelectrolytes, containing
functional groups capable of ionization, cause this type
of stabilization.
System destabilization due to formation of the
polymer bridges between colloidal particles is called
bridging flocculation. Two mechanisms of bridging can
be distinguished: (1) bridging by one polymer chain
attached to both particles, (2) bridging by the interaction
of polymer chains attached to different particles. Another
way of suspension destabilization is neutralization of
particle surface charge by the adsorbed ionic polymer.
In such a case, uncharged particles cease to repel and
undergo aggregation and sedimentation.
It is worth noting that unadsorbed polymer chains
remaining in the solution can also affect the suspension
stability due to depletion interactions. Such effects
are usually observed in the range of high polymer
concentration in the solution and can lead to both
stabilization (depletion stabilization) and destabilization
(depletion flocculation) of the colloidal system.
The stabilization properties of polymers are widely
used in the production of paint (i.e., stabilization of
titania particles [5,6]), cosmetics [7-9], washing agents
(detergents) [10-13], paper [14,15], in biomedical
engineering (controlled drug delivery) [16-18] and
food processing (improvement of taste, smell and
consistency) [19-21]. On the other hand, the most
important application of polymers in the role of flocculants
is waste water treatment [22-24] (removal process of
contaminants in the form of a colloidal suspension).
Such huge demand for efficient stabilizers and
flocculants of industrial colloidal suspensions makes
basic studies of the stability mechanism of solid
suspension in the presence of macromolecular
compound very important.
Thus, the main aim of this work is to determine the
changes in stability of mixed silica-alumina suspension
in the presence of anionic polyacrylic acid (PAA). Due
to the fact that conformation of the adsorbed polymer
chain (especially ionic) depends on many factors, the
influence of two factors in particular (solution pH and
polymer molecular weight) were investigated.
2. Experimental procedure
The samples of mixed silica-alumina of the chemical
composition SiO2 (97.0% ±0.1) and Al2O3 (3.0%±0.1)
were used in the study (pilot plant in the Chuiko Institute
of Surface Chemistry, Kalush, Ukraine). The silicaalumina suspension will be abbreviated to SA 3. The
adsorbent was characterized by the BET surface area
of 302 m2 g-1 and the mean pore diameter of 7.65 nm.
These two properties were determined by the lowtemperature nitrogen adsorption-desorption isotherm
method (Micrometritics ASAP 2405 analyzer). The point
of zero charge (pHpzc) of SA 3 was 3.0 (obtained from the
potentiometric titration) and its isoelectric point (pHiep)
was also 3.0 (zeta potential measurements - Zetasizer
3000, Malvern Instruments).
Polyacrylic acid (PAA, Fluka) with weight average
molecular weights 2000, 100000 and 240000 were used
in the experiments.
All measurements were carried out in the presence
of NaCl solution (1×10-2 mol dm-3) which was used
as the supporting electrolyte. Moreover, the stability
experiments were performed at solution pH 3, 6 and 9
at 25oC. The pH of the suspension was adjusted using
pH-meter PHM 240 (Radiometer) with accuracy ±0.1.
The polymer concentration was approximately equal to
500 ppm, which provided the surface coverage θ=1. The
solid content in the suspensions under investigation was
0.1%.
The stability measurements of silica-alumina
suspensions without and with PAA were carried out using
Turbiscan LabExpert with the cooling module TLAb Cooler.
This apparatus possesses the electroluminescence
diode
which
emits
collimated
light
beam
(λ = 880 nm) passing through the investigated suspension.
The apparatus has two synchronized detectors. The
transmission detector recorded light passing through
the probe at the angle 0o in relation to the incident light
direction. The other one is the backscattering detector
registering the light scattered at the angle 135o. The
obtained data are stored and converted by computer
program. The results are presented in the form of
curves which show the intensities of transmission and
backscattering as a function of time.
The analyzed suspension in a glass vial (7 cm long)
was placed in a thermostated measurement chamber.
The suspension with 0.02 g of oxide in 20 cm3 of NaCl
solution was sonicated for 1 min. Then the required pH
of the solution (i.e., 3, 6 or 9 ±0.1) was adjusted using
pH-meter PHM 240 (Radiometer). The suspension was
shaken in a water bath for 30 min and during this time
its pH was checked. The changes in the suspension
stability were monitored for 15 h (single scans were
collected every 15 min). The probes of the silica-alumina
suspension with polyacrylic acid were prepared in a
similar way. An appropriate volume of the PAA solution,
desired surface coverage θ=1 (CPAA≈500 ppm), was
added to the suspension after sonication.
Because the transmission for all samples was higher
than 2%, all stability parameters were calculated from
transmission graphs. These were the rate of particle
(aggregates, flocks) migration [µm min-1], the particle
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(aggregate, floc) diameter [µm] and the turbiscan
stability index (TSI). These data were calculated using
the programs TLab EXPERT 1.13 and Turbiscan Easy
Soft. The calculations of migration rate were based on
multiple-light scattering theory. Particle diameters were
obtained using the general law of sedimentation, which
is Stokes’ law extended to the concentrated dispersions
[25]:
(1)
where: V – particles migration velocity (µm min-1), ρc –
continuous phase density (kg m-3), ρp – particle density
(kg m-3), d – particle mean diameter (µm), v – continuous
phase kinematic viscosity (cP), φ – volume of dispersed
solid fraction (%).
The samples’ stabilities were estimated and
compared using the turbiscan stability index (TSI). This
parameter takes into account all single measurements
during experiments and the TSI value is obtained from
their averaging. All processes taking place in the sample
(including thickness of sediment and clear layer as well
as particles settling) were summed up. This parameter
was calculated with the special computer program
Turbiscan Easy Soft with the following formula:
(2)
where: xi – average backscattering for each minute of
measurement, xBS – average x1, n – number of scans.
The TSI values change in the range from 0 to 100. The
higher the TSI, the more unstable the system is.
3. Results and discussion
Figs. 1-3 present the transmission curves of the silicaalumina suspension at different pH values in the
absence and presence of polyacrylic acid. The level of
suspension in the measuring vial is along the x-axis and
the light intensity [%] transmitted through the suspension
is along the y-axis. The value of 40 mm in the x-axis
in these figures is the mark to which the investigated
suspension was poured in the measuring vial.
Additionally, Tables 1-3 and Figs. 4-6 present the
stability parameters characterizing the systems under
investigation (TSI, aggregates diameters and their
migration velocities).
As can be seen in Fig. 1a, the silica-alumina
suspension without polymer at pH 3 is the least
stable of all studied systems. It is evidenced by the
high transmission of around 85% and formation of
the thick sediment layer of about 2 mm on the vial
bottom (transmission increase in the range from
2.5 to 4.5 mm). The thickness of formed sediment in
the time of experiment (i.e., 15 h) was obtained from
initial rising part of the transmission graph. Such
thickness was calculated as the difference between the
height of suspension in the measurement vial, in which
the transmission started increasing and that in which
transmission reached a maximum. The transmission
changes in the range of 0-2.5 mm correspond with
changes of light intensity due to its passing through the
bottom of measuring vial (the thickness of this bottom
is about 2.5 mm). The schematic presentation of the
measuring vial-containing suspension with marked
sediment layer is presented in Fig. 7.
The thickness of sediment grows in a time of
3.5 h. Then the packing increases (without changing
sediment thickness). The greatest changes in system
stability occur during 30 min, then they are slowed as
evidenced by the accumulation of individual scans. The
greatest destabilization of SA 3 suspension without PAA
at pH 3 was proved by the highest value of TSI (28.8,
Table 1, Fig. 4) as well as high values of aggregate
diameter (0.26 µm - Table 2, Fig. 5) and its rate of
migration (2.93 µm mim-1 - Table 3, Fig. 6).
The addition of PAA 2 000 at pH 3 slightly improves
stability conditions of SA 3 suspension (Fig. 1b). In
this case, the thickness of formed sediment is four
times smaller as in the previous case and equals about
0.5 mm (transmission increase in the range from 2.5 to
3.0 mm). Moreover, the transmission reaches the level
of 79%. Nevertheless, SA 3-PAA 2 000 system is one of
the least stable among all studied systems (TSI=21.55).
The flocks’ size (0.12 µm) and their sedimentation
velocity (0.64 µm min-1) are not high, which can be a
result of the small molecular weight of polyacrylic acid.
Adsorption of PAA with higher molecular weights,
namely PAA 100 000 and 240 000 at pH 3, causes
greater improvement of mixed oxide suspension stability
(Figs. 1c and 1d). Manifestation of this fact is noticeably
lower transmission (about 70% for PAA 100 000 and 75%
for PAA 240 000) and thinner sediments (about 0.5 mm
and 1.5 mm for PAA 100 000 and 240 000, respectively).
The TSI values are lower (5.03 - PAA 100 000 and
8.38 - PAA 240 000) than for SA 3 system without
polymer. The flocks’ sizes and their sedimentation
velocities are dependent on polymer molecular weight
(Tables 2 and 3, Figs. 5 and 6).
The analysis of transmission curves at pH 6
(Fig. 2a) and respective stability parameters leads to
the conclusion that silica-alumina suspension without
polymer is successively stable at such pH conditions
(TSI=3.58). It is proved by poor separation of individual
scans (they overlap each other) and absence of
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Effect of solution pH on the stability of mixed
silica –alumina suspension in the presence
of polyacrylic acid (PAA) with different molecular weights
a)
b)
c)
d)
Figure 1.
Transmission curves for systems: a) SA 3/NaCl, b) SA 3/NaCl/PAA 2000, c) SA 3/NaCl/PAA 100000, d) SA 3/NaCl/PAA 240000 at pH=3.
The level of suspension in the measurement vial [mm] is along the x axis and the light intensity transmitted through the suspension [%]
is along the y axis.
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Table 1. The TSI indexes for SA 3 suspension in the absence and presence of polyacrylic acid.
System
TSI
pH=3
pH=6
pH=9
SA 3
28.80
3.58
5.76
SA 3 – PAA 2000
21.55
21.00
4.31
SA 3 – PAA 100000
5.03
6.49
5.85
SA 3 – PAA 240000
8.38
1.38
4.65
Table 2. The average size of aggregates (flocs) formed in SA 3 suspension in the absence and presence of polyacrylic acid.
System
d [µm]
pH=3
pH=6
pH=9
SA 3
0.26
0.08
0.19
SA 3 – PAA 2000
0.12
0.28
0.10
SA 3 – PAA 100000
0.12
0.14
0.11
SA 3 – PAA 240000
0.19
0.19
0.13
Table 3. The average velocity of aggregates (flocs) sedimentation in SA 3 suspension in the absence and presence of polyacrylic acid.
System
V [µm min-1]
pH=3
pH=6
pH=9
SA 3
2.93
0.30
1.55
SA 3 – PAA 2 000
0.64
3.39
0.41
SA 3 – PAA 100 000
0.67
0.86
0.57
SA 3 – PAA 240 000
1.58
1.62
0.71
sediment. In this case, the lowest values of d=0.08 µm
and V=0.3 µm min-1 were obtained.
The presence of PAA 2 000 in SA 3 suspension at
pH 6 significantly deteriorates its stability (TSI=21.00).
As can be seen in Fig. 2b, the transmission is high
(around 80%) and the thick sediment layer of about
3 mm on the vial bottom is formed. The individual
transmission curves collected during the first 4 hours
of the experiment are well separated from each other,
and then their concentration occurs. In this case, the
greatest values of d=0.28 µm and V=3.39 µm min-1 were
obtained among all the examined systems.
The addition of PAA 100 000 to the silica-alumina
suspension at pH 6 causes minimal deterioration of
its stability (TSI=6.49). In this case the transmission
is about 70% and the thickness of formed sediment is
1 mm (Fig. 2c).
The most stable suspension of all studied was
obtained at pH 6 in the presence of PAA 240 000
(TSI=1.38). The analysis of curves in Fig. 2d indicates
that the transmission is on the level of 60% (the lowest
transmission of all cases), the transmission curves
overlap and the sediment is practically not formed.
The behavior of the studied suspension at pH 9 is
similar to that observed at pH 6. The values of TSI for the
system without and with polymer are close to each other
and change from 4.31 to 5.85. The SA 3 suspension is
successively stable and the addition of polyacrylic acid
minimally influences its stability. As can be seen in Fig. 3
(all cases: 3a-3d) the transmission changes in the range
62-72%, transmission curves are largely overlapping
and the thickness of sediment is minimal or it is not
practically formed.
Of importance for the explanation of the obtained
changes in silica-alumina suspension stability (in the
absence and presence of PAA) is the analysis of both
polyacrylic acid functional group ionization and SA 3
surface charge density as a function of solution pH.
Each segment of PAA ([-CH2-CH(COOH)-])
contains the carboxyl group. These groups undergo
dissociation with increasing pH. The pKPAA occurs
at pH 4.5 [26] at which half of the carboxyl groups
are ionized. The characteristics of changes in PAA
macromolecule dissociation [27] and SA 3 surface
charges at different solution pH are presented in
Table 4.
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Effect of solution pH on the stability of mixed
silica –alumina suspension in the presence
of polyacrylic acid (PAA) with different molecular weights
a)
b)
c)
d)
Figure 2.
Transmission curves for systems: a) SA 3/NaCl, b) SA 3/NaCl/PAA 2000, c) SA 3/NaCl/PAA 100000, d) SA 3/NaCl/PAA 240000 at pH=6.
The level of suspension in the measurement vial [mm] is along the x axis and the light intensity transmitted through the suspension [%]
is along the y axis.
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M. Wiśniewska et al.
a)
b)
c)
d)
Figure 3.
Transmission curves for systems: a) SA 3/NaCl, b) SA 3/NaCl/PAA 2000, c) SA 3/NaCl/PAA 100000, d) SA 3/NaCl/PAA 240000 at pH=9.
The level of suspension in the measurement vial [mm] is along the x axis and the light intensity transmitted through the suspension [%]
is along the y axis.
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Effect of solution pH on the stability of mixed
silica –alumina suspension in the presence
of polyacrylic acid (PAA) with different molecular weights
Table 4.
Characteristics of PAA macromolecules ionization and SA 3 surface properties at different solution pH.
pH
PAA macromolecules
SA 3 surface
dissociation degree
ratio of concentration of
surface charge
zeta potential of the
(αdys)
nondissociated and dissociated
density of the solid
solid particles
carboxyl groups [-COOH]/[-COO ]
σ0 [µC cm ]
ζ [mV]
-
-2
0.03
31
0.03
0.2
6
0.97
0.031
-2.24
-26.5
9
0.99
0.000031
-11.67
-45.7
TSI (Turbiscan Stability Index)
3
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
pH=3
pH=6
pH=9
1 - S A3
2 - S A3 - PAA 2000
3 - S A3 - PAA 100000
4 - S A3 - PAA 240000
1
2
3 4
1
2
3 4
1
2
3 4
Figure 4. The TSI indexes for SA 3 suspension in the absence and
presence of polyacrylic acid at different solution pH.
0.40
pH=3
pH=6
pH=9
1 - S A3
2 - S A3 - PAA 2000
3 - S A3 - PAA 100000
4 - S A3 - PAA 240000
Particles average diameter µ[ m]
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
1
2
3 4
1
2
3 4
1
2
3 4
Figure 5. The
average diameter of aggregates (flocs) in SA 3
suspension in the absence and presence of polyacrylic
acid at different solution pH.
The analysis of the data in Table 4 indicates that only
at pH 3 the electrostatic repulsion between the polymer
chains and the solid surface does not take place. Under
such pH conditions, PAA macromolecules contain a few
dissociated carboxyl groups (dissociation degree (αdys)
0.03, [-COOH]/[-COO-] ratio equalled 31) and the solid
surface is practically neutral (pHpzc and pHiep≈3.0). The
previous investigations indicated [28] that polyacrylic
acid adsorption on the metal oxide’s surface decreases
with increasing pH and it occurs through hydrogen
bridges, even under conditions of strong adsorbateadsorbent repulsion.
The lack of electrostatic repulsion between neutral
solid particles (without polymer) is the main reason for
considerable destabilization of silica-alumina suspension
at pH 3. The coagulation of SA 3 particles takes place
in this system.
The polymer addition improves the stability of solid
suspension, but the greater effect of this improvement
is achieved in the case of PAA with higher molecular
weight. The polymer adsorbs on the solid surface
probably in the form of polymer coils. It is due to weak
repulsion between both its slightly dissociated carboxyl
groups and these same groups with the neutral surface
of the mixed oxide. The polymer adsorption layers
cause the appearance of steric repulsion between solid
particles. The higher the PAA molecular weight is, the
higher polymer adsorption is observed [29] and the
steric repulsion is more significant.
A different situation occurs at pH 6, at which the
solid surface is negatively charged (σ0= -2.24 µC cm-2,
ζ= -26.5 mV; Table 4). The repulsion between solid
particles is sufficient to ensure the stability of the system
without polymer.
The addition of PAA with the lowest examined
molecular weight (i.e., 2000) causes significant
deterioration of suspension stability. On the other hand,
in the presence of PAA with higher molecular weight,
the system is successively stable. The carboxyl groups
of polyacrylic acid are almost completely dissociated
(αdys=0.97, [-COOH]/[-COO-]=0.031, Table 4) at pH 6.
Thus their conformation on the solid surface is more
extended towards the liquid phase. It is due to repulsion
between both dissociated carboxyl groups in the
polymer macromolecules and between these groups
and negatively charged surface groups. For this reason,
the adsorption of PAA is also smaller than at pH 3. The
lower surface coverage with polymer (especially in the
case of PAA 2000) enables formation of polymer bridges
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The rate of migrationµm/min
[
]
M. Wiśniewska et al.
3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1 - S A3
2 - S A3 - PAA 2000
3 - S A3 - PAA 100000
4 - S A3 - PAA 240000
pH=3
pH=6
pH=9
covered by negatively charged polyacrylic acid chains
causes electrosteric stabilization of the suspension.
Additionally, the adsorption of polymer under such
conditions is small. Thus, a great number of ionized
polymer chains are present in the solution intensifying
the effect of solid particle repulsion.
4. Conclusions
1
2
Figure 6.
3 4
1
2
3 4
1
2
3 4
The rate of aggregate (floc) migration in SA 3 suspension
in the absence and presence of polyacrylic acid at
different solution pH.
Figure 7. The schematic presentation of measuring vial containing
suspension with marked sediment layer.
between solid particles leading to bridging flocculation
of suspension. The polymer molecular weight increase
results in higher coverage of adsorbent surface and
significant limitation of polymer bridging. On the other
hand, strong adsorbate-adsorbent repulsion in the SA
3 – PAA 240000 system at pH 6 causes the highest
stability of this suspension. In this situation, not only
steric interactions occur but electric ones are present
(electrosteric stabilization).
At pH 9 the silica-alumina suspension is
successively stable. This is a result of electrostatic
repulsion between negatively charged solid particles
(σ0= -11.67 µC cm-2, ζ= -45.7 mV; Table 4) leading to
electrostatic stabilization. The addition of PAA does not
significantly change its stability – the system remains
stable. The polymer functional groups were totally
dissociated (αdys=0.99, [-COOH]/[-COO-]=0.000031,
Table 4). Very strong repulsion between solid particles
The effect of anionic polyacrylic acid addition to the
mixed silica-alumina (SA 3) suspension as a function of
solution pH and polymer molecular weight was studied.
The turbidimetric results (transmission curves), as well
as calculated stability parameters (TSI, aggregate
diameter, and its rate of migration) indicate that the solid
suspension without polymer is successively unstable at
pH 3 (coagulation, pHpzc and pHiep≈3.0). On the other
hand, at pH 6 and 9 its stability conditions improve
significantly (electrostatic stabilization, high absolute
values of surface charge density and zeta potential).
Polyacrylic acid addition causes the smallest
changes in SA 3 suspension stability at pH 9, at which
PAA ensures electrosteric stabilization of the examined
system (strong repulsion between solid particles covered
with completely dissociated polymer chains).
The polymer addition at pH 3 results in improvement
of suspension stability (steric repulsion between solid
particles with minimally dissociated macromolecules).
This effect becomes larger for higher molecular weight
of polyacrylic acid.
The presence of PAA 2000 in the silica-alumina
suspension at pH 6 considerably deteriorates its
stability, whereas PAA 100000 and 240000 cause
its electrosteric stabilization (repulsion between the
adsorbed polymer chains for which carboxyl groups are
almost completely dissociated). In the case of the lowest
examined molecular weight of polyacrylic acid, a few
polymer bridges can be formed between the adsorbent
particles (the lowest surface coverage by adsorbate).
This leads to suspension destabilization.
Acknowledgement
The research leading to these results has received
funding from the European Community’s Seven
Framework Programme (FP7/2007 – 2013) under a
Maria Curie International Research Staff Exchange
Scheme, Grant Agreement No. PIRSES – GA – 2008
– 230790.
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Effect of solution pH on the stability of mixed
silica –alumina suspension in the presence
of polyacrylic acid (PAA) with different molecular weights
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