Cent. Eur. J. Chem. • 12(4) • 2014 • 480-491
DOI: 10.2478/s11532-013-0386-1
Central European Journal of Chemistry
Effects of dissolved metal chlorides
on the behavior of silica nanoparticles
in aqueous media
Research Article
Vladimir M. Gun’ko1* , Lyudmyla S. Andriyko1,
Vladimir I. Zarko1, Andrij I. Marynin2,
Valentyn V. Olishevskyi2, Wladyslaw Janusz3
Chuiko Institute of Surface Chemistry,
Department of Amorphous and Structurally Ordered Oxides
03164 Kyiv, Ukraine
1
National University of Food Technology,
Problem Research Laboratory
01033 Kyiv, Ukraine
2
Faculty of Chemistry, Maria Curie-Skłodowska University,
20031 Lublin, Poland
3
Received 25 September 2013; Accepted 16 November 2013
Abstract: Effects of chlorides of univalent (LiCl, NaCl, KCl), bivalent (MgCl2, BaCl2) and trivalent (AlCl3) metals at different concentration
(0.001-0.1 M) on the behavior of nanosilica A-200 (0.5-5 wt.%) in aqueous media are analyzed using photon correlation spectroscopy
(particle size distribution, PSD), electrophoresis (zeta potential z), potentiometric titration (surface charge density), and estimation
of screening length of primary particles and their aggregates. The zeta potential and the PSD are affected by silica content, pH, and
concentration and type of dissolved salts. Smaller but more strongly hydrated Li+ cations caused stronger nonlinear dependences of the
zeta potential on pH and salt content than Na+ or K+. This nonlinearity is much stronger at a lower content of silica (0.5-1 wt.%) than at
CA-200 = 2.5 or 5 wt.%. At a high concentration of nanosilica (5 wt.%) the effect of K+ ions causes stronger diminution of the negative
value of the zeta potential due to better adsorption of larger cations. Therefore, the influence of K+ on increasing screening length is
stronger than that of Na+ for both primary nanoparticles and their aggregates. A similar difference in the z values is observed for different
in size cations Ba2+ and Mg2+.
Keywords: Nanosilica • Metals chlorides • Aqueous medium • Zeta potential • Particle size distributions
© Versita Sp. z o.o.
1. Introduction
Changes in the characteristics of the electrical double
layer (EDL) and surface charging of particles due
to variations in pH and salinity caused by ionogenic
compounds (acids, bases, salts) strongly affect the
behavior of colloidal systems [1-6]. These effects,
due to a large outer surface area of nanoparticles
(SBET = 50-500 m2 g-1), are especially characteristic
for fumed oxides composed of weakly aggregated
nanoparticles of 5-100 nm in size. A significant amount
of these nanoparticles exist as individual particles [7].
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In concentrated suspensions the total volume of the
interfacial layers around the nanoparticles corresponds
to the lion’s share of the dispersion medium, and these
layers overlap. The nanoparticles can strongly affect the
structure of the dispersion medium [7-9]. The properties
of colloids (aggregative stability, viscosity, turbidity,
aging effects, etc.) containing nanoparticles depend
also on their concentration and aggregation degree,
long-range and short-range particle-particle interactions
(dependent on their chemical composition and surface
charging), the particle size distributions (PSD) and
particle shapes.
* E-mail: [email protected]
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According to Smoluchowski’s theory [3], there is a
linear relationship between the electrophoretic mobility,
U (measured experimentally) and the electrophoretic
potential, z (calculated): U =AzS, where A is a constant
for a thin EDL at ka >> 1 (where a denotes the particle
radius, and k is the Debye-Huckel parameter). For a
thick EDL (ka < 1), e.g. at pH close to the isoelectric
point (IEP), the equation with the Henry correction factor
is more appropriate
U = 2ezS,H/(3h),
(1)
where e is the dielectric permittivity; and h is the
viscosity of the liquid. The Debye length (k-1) of ions in
the solution is also an important characteristic affecting
the electrokinetic behavior of colloid particles [1-6]. At
pH close to the IEP, Eq. 1 gives the zS potential vs. pH
of silica particles, in the aqueous suspension with no
added inert electrolyte, close to the zO obtained using
the Ohshima equation [7,10,11] corresponding to
the improved Smoluchowski equation with the Henry
correction factor:
e re 0 z O  2
0 .5
(1 +
U=
1 + δ ( k a)
h  3
−1

) M 1 ( k a, ϕ ) + M 2 ( k a, ϕ ) 

(2)
where
,
2 ( ka ) 1 + f / 2
( f1 / 3 + f −2
9 P 1 −f
1
P = c o s h [ ( k ( b − a )] − s i n h [ k ( b − a )]
kb
1 − kb t a n h [ k ( b − a )]
,
Q=
t a n h [ k ( b − a )] − kb
M 2 ( ka, f ) =
δ=
2
/ 3
−
9
5 f1
/ 3
−
f
5
4 / 3
)
,
,
2 .5
,
1 + 2 e x p ( −ka )
f = (a/b)3,
where b denotes the radius of a virtual shell of a particle
[10,11]. Addition of 0.01 M NaCl leads to diminution of
the dependence of the ratio zO/zS or zD/zS on pH, where
zS, zO and zD are the zeta potentials calculated using
the Smoluchowski [3], Ohshima [10,11] or Dukhin
[12-14] (with consideration for the surface conductivity)
equations, respectively [7]. Thus, despite that the use
of the Smoluchowski equation can give certain errors
for nanosilicas, especially far from the IEP and without
addition of an inert electrolyte [7], one could use this
equation to compare the effects of different salts on
the behavior of nanosilica particles in the aqueous
suspensions under the same conditions (vide infra).
One can assume that the effective screening length
(l) of charged particles with counterions around them
can be estimated as a certain analog of the Debye length
for ions. Clearly in this case the standard definition of
the Debye length for ions is inappropriate. In this paper,
a method of calculation of the effective screening length
of nanoparticles is described.
Nanooxides (silica, titania, alumina, etc.) are
characterized by a certain degree of aggregation of
primary nanoparticles and agglomeration of aggregates
giving bi-, tri- or four-modal PSDs with one-two PSD
peaks of aggregates (d < 1 mm) and one-two PSD
peaks of agglomerates (d > 1 mm) [7-9,15-18]. The
aggregation/agglomeration degree depends on both the
dispersion medium and the particle characteristics. The
chemical bonds between nanoparticles in aggregates of
nanooxides are practically nonexistent. Their bonding is
mainly due to the electrostatic (including hydrogen bonds)
and dispersion interactions. Therefore, disaggregation
of the hydrophilic nanoparticles can easily occur in
aqueous media since interaction of a water shell with a
particle surface can be stronger than the particle-particle
interaction. The morphological characteristics of the
dispersion phase, the zeta potential (or electrophoretic
mobility U) and the surface charge density (s0) of
nanoparticles depend not only on pH but also on the
salinity (ionic strength) of the aqueous media, as well
as on the presence of other dissolved and dissociated
compounds, which can be adsorbed onto nanoparticles
[19-40]. These dependences can be nonlinear due to
several factors [7,21,22,29,41-44] such as the surface
charge density linked to ion adsorption and surface
hydroxyl deprotonation, the whole EDL characteristics,
perikinetic and orthokinetic aggregation of particles, their
agglomeration affected by adsorbates, sedimentation,
etc. characterized by different dependences on pH, the
salinity degree, and particle and salt concentrations.
Nanooxides are interesting materials to study their colloid
characteristics, since they weakly precipitate, possess
high Brownian and electrophoretic mobility, and show
essential sensitivity to changes in the dispersion media
and states of the interfaces due to adsorbates [7-9,2025,30,40]. Therefore, photon correlation spectroscopy
(PSD), electrophoresis (z potential), and potentiometric
titration (s0) are the most appropriate to study these
materials and the related systems in aqueous media
[7,45].
Cations K+ and Cs+ and anions Cl- are chaotropes
while Li+, Na+, Mg2+, Ba2+, and Al3+ are kosmotropes which
differently affect water structure and viscosity [46,47].
The cations influence on water structure depends on
their hydration degree and is greater when they decrease
in size. For instance, Jones-Dole viscosity B coefficients
are equal to 0.385 (Mg2+), 0.22 (Ba2+), 0.15 (Li+), 0.086
(Na+) for the studied kosmotropes, and -0.007 (K+, Cl-) for
the chaotropes [48,49]. It would be interesting to know
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Effects of dissolved metal chlorides on the
behavior of silica nanoparticles in aqueous media
the effects of cations of different metals (univalent alkali
Li+, Na+, K+, bivalent alkaline earth Mg2+ and Ba2+ and
trivalent Al3+) on the behavior of the dispersion phase
(nanosilica) and the characteristics of the dispersion
media. These phenomena are of importance from
both the theoretical and practical points of view since
nanosilica is produced in great amounts worldwide and
is widely used in industry (adsorbents, fillers, carriers,
etc.) and medicine (enterosorbents, drug fillers, carriers,
etc.). It is frequently used as aqueous suspensions that
contain a variety of dissolved compounds, among them
salts such as metal chlorides. The main characteristic of
nanosilica is the specific surface area, SBET, which can
range from 30 to 500 m2 g-1. Nanosilica A-200, the main
object of our study, has the mean SBET value (232 m2 g-1).
Therefore, the aim of this research has been the analysis
of the mentioned phenomena using PSD, surface charge
density, and zeta potential of silica particles versus
pH and the salinity degree (ionic strength) on samples
containing different concentrations of silica.
2.3. Potentiometric titration
To evaluate the surface charge density (s0),
potentiometric titrations were performed using a
thermostated Teflon vessel in nitrogen atmosphere,
free from CO2, at 25±0.2oC. The pH of the solution was
measured using a PHM240 Research pH-meter (G202C
and K401 electrodes) coupled with a REC-61 recorder.
The s0 values were calculated using the potentiometric
titration data for a blank electrolyte solution and silica
suspensions (CA-200 = 0.2 wt.%), at a constant salinity
of 10-3 M NaCl. From the difference of the acid or base
volume required to obtain the same pH value as that of
the background electrolyte of the same ionic strength
one can calculate the surface charge density according
to the following equation
,
(3)
2. Experimental procedure
where DV = Vs - Ve is the difference between the base
(acid) volumes added to the electrolyte solution Ve and
suspension Vs to achieve the same pH; F is the Faraday
constant, c is the concentration of base (acid), and m is
the weight of the oxide.
2.1. Materials
2.4. Effective screening length of silica particles
Fumed silica (nanosilica) A-200 (pilot plant at the
Institute of Surface Chemistry, Kalush, Ukraine,
99.8% purity) has the specific surface area
SBET = 232 m2 g-1 corresponding to the average
radius of primary nanoparticles aav = 3000/(r0SBET) =
5.88 nm, where r0 is the true density of fumed silica
(r0 = 2.2 g cm-3). The bulk density of the nanosilica
powder is very low (~0.05 g cm-3) due to formation of
loose aggregates (< 1 mm) of primary nanoparticles and
very loose agglomerates (>1 mm) of aggregates [7].
2.2. Electrophoretic mobility and particle size
distributions
Electrophoretic mobility and particle size distributions
were studied using either Zetasizer 3000 or Zetasizer
Nano ZS (Malvern Instruments) apparatus with a
universal dip cell (ZEN1002) and a disposable polystyrene
cell (DTS0012) for zeta potential measurements.
Distilled water with certain amounts of dissolved salt
(0.001-0.1 M LiCl, NaCl, KCl, MgCl2, BaCl2, AlCl3) and
nanosilica A-200 (5-50 g dm-3 of the aqueous solution of
a salt) were used to prepare the suspensions sonicated
for 2 min using an ultrasonic disperser (Sonicator
Misonix, power 500 W and frequency 22 kHz). The
suspensions were equilibrated for 24 h. The pH values
(mainly in the 2-7 range) were adjusted by addition of
either 0.1 M HCl or 0.1 M NaOH solutions.
The effective screening length (l) of a charged particle
surrounded by counterions can be roughly estimated
using the following equation [3,50]
q = 4 pee 0
 zez 
 zez  
k BT −1 2 
4
l a 2 s i n h 
 + −1 t a n h 
 ,
l
2
ze
k
T
a
 B 
 4 k BT  

(4)
where q is the particle charge, z the zeta-potential, z
the charge of the ions in the solution, e the electron
charge, ee0 the dielectric constant of the solution, and
a is the particle radius (either primary particles or their
aggregates). This equation uses the effective charge
density s = q/(4pa2) estimated from the potentiometric
titration. If the s, z and a values are measured
experimentally under the same conditions (fixed pH and
the concentrations of the particles and the salt) that l
can be determined as [51]
l = B/(s/A-C),
(5)
where
,
, and
.
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Figure 1.
Effective diameter of particles (determined from the light scattering intensity) as a function of (a) silica content without addition of any
electrolyte; (c, d) the salt content (0.001-0.1 M) for (c) AlCl3, BaCl2, and MgCl2; and (d) LiCl, NaCl, and KCl at different content of A-200
(0.5-5 wt.%) in the aqueous suspensions; and (b) PSD with respect of the particle volume (PSDV) or number (PSDN) for the aqueous
suspension of A-200 without electrolytes.
It should be noted that the aggregates of primary
nanoparticles are porous (these pores correspond
to voids between nanoparticles in aggregates and
agglomerates) and this porosity can be factored using
a volume fraction (f) of nanoparticles in the aggregates.
For the studied systems, the f value changes over the
0.01-0.3 range which is in agreement with the density of
the suspended fumed silicas [7,8,52,53].
3. Results and discussion
The effects of nanosilica composition, the type and
amount of metal chlorides, and pH values have been
analyzed with respect to the effective diameter of the
particles (Def), the particle size distributions (PSD),
the zeta potential (z), and the screening length of
nanoparticles (l). One can expect that some correlations
between these characteristics of nanosilica colloids
exist.
The effective diameter of particles is within a larger
range (170-920 nm) for suspensions containing bi- and
trivalent metal cations (Fig. 1c) than for these with
univalent cations (150-260 nm) (Fig. 1d) or for the
A-200 suspensions without electrolytes (170-250 nm)
(Fig. 1a). The Def values determined from the light
scattering intensity
Def = ∑ N i di6 /
i
∑N d
i
i
5
i
,
(6)
(where di and Ni are the diameter and the number of the
i-th particles) typically increase with increasing amounts
of A-200 and a salt in the suspensions (Fig. 1). The
effects become stronger (Li+ > Na+ > K+) with decreasing
size of the univalent metal cations (Li+ < Na+ < K+),
i.e., with increasing hydration degree and increasing
kosmotropic effect on the water structures resulting in
increase in the viscosity [7,8,46,47].
Besides the cations adsorbed onto silica
nanoparticles at pH higher than the isoelectric point
(pHIEP) or the point of zero charge (pHPZC), the changes
in the water structure caused by kosmotropes or
chaotropes can affect the electrophoretic mobility of
the silica particles and the PSD. For the alkaline earth
chlorides, the changes in the Def values are greater
for the larger Ba2+ cations than for the smaller Mg2+
(Fig. 1c). The effect is stronger for trivalent Al3+ at a
low concentration of salts (< 0.01 M). Overall, the
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Effects of dissolved metal chlorides on the
behavior of silica nanoparticles in aqueous media
Def values are larger for the systems with multivalent
cations (Fig. 1c) than those with univalent cations
(Fig. 1d) or without salts (Fig. 1a). The effect of univalent
cations is comparable only with the effect of Mg2+ at
CA-200 = 0.5 and 1 wt.%. This can be also explained by
poor adsorption of the alkali cations onto nanosilica in
comparison with the multivalent cations [52,53] over
the pH range studied. The silica surface possesses a
low ability in ion-exchange reactions due to weak acidic
properties of the silanol groups. According to Dove and
Craven [54], the surface charge density (s0) of silica
particles (Aerosil 380) decreases in the order K+ <
Na+ < Li+ and Mg2+ < Ba2+ in the presence of the alkali
metal chlorides and the alkaline earth chlorides in the
solution. The trend in the s0 values is opposite to the
crystallographic radius of the alkali and alkaline earth
cations [54]. The decrease in negative charges (at pH
> pHPZC) can enhance the coagulation of silica particles
since increasing negative charging of nanoparticles
provides stabilization of the silica colloids at increased
pH [7]. Therefore, an increase in the K+ content leads
to a relatively greater increase in the Def value than
that observed for Li+ or Na+ (Fig. 1d). However, the Def
values are minimal for the system with K+ (Fig. 1). This
order observed for the univalent cations is in agreement
with the changes in the Jones-Dole viscosity coefficient
B for them: 0.15 (Li+), 0.086 (Na+), and -0.007 (K+)
(larger B value leads to higher viscosity), since K+ is
a chaotropic cation. Thus, there are several effects
(some of them are opposite in nature) of dissolved
salts on the behavior of both the dispersion phase and
the dispersion medium. Overall, the coagulation effect
increases with increasing content of silica and the metal
chlorides (Fig. 1), and this effect depends on the pH
value (Figs. 2 and 3) due to the changes in the surface
charging and the EDL characteristics [7].
The Def value, as an average characteristic
determined from the intensity of light scattering with
Eq. 4, does not provide detailed information on the
particle size distributions (PSD). However, it shows
certain regularities in the PSD changes. Overall, the PSD
functions (Figs. 1b, 2 and 3) show a complex behavior
of the particles with respect to their aggregation vs.
content, ionic strength, and pH. The particles observed
in the PSD include primary silica nanoparticles, their
aggregates and the agglomerates, with the majority of
the aggregates corresponding to the Def value range
observed (see Fig. 1). The changes in the PSD shape
depend on the silica and the salt concentrations to a
greater extent than the Def value changes vs. Csalt and
CA-200. Despite the increase in the Def value with the
CA-200 and Csalt values (Fig. 1), the peak of the PSD,
with respect to the number of particles (PSDN), shifts
toward smaller sizes, up to the size range of the primary
nanoparticles (Figs. 2 and 3).
The pH values of the PZC and the IEP of fumed silicas
are around 2.5-3 [7]. The majority of the measurements
were performed at pH > pHIEP (Figs. 2 and 3). However,
at the pH in the range of 2.5-6, the s0 values are
relatively small (Fig. 4c) and cannot cause strong
repulsive interactions between the silica nanoparticles
which are aggregated in the secondary particles.
Therefore, the individual silica nanoparticles practically
do not exist in the suspensions (Figs. 2 and 3, compare
the PSDN2 peak of the primary particles with the PSD in
the suspensions). The salt effects in this pH range can
contribute to the particle coagulation, especially with
increasing content of both the silica and the salt (Fig. 1).
The stronger salt effects on the aggregation at higher
pH can be due to an increased influence of the charge
distribution in the system. The PSD shape with respect
to the particle volume (PSDV) becomes more complex
(bi- or trimodal) and broadened with an increase in the
CA-200 and Csalt values. One could expect that an increase
in pH can stabilize the silica colloids and the PSD peaks
can shift toward smaller particle size. This tendency is
observed; the shift is influenced by the content of the
salts and the silica.
Changes in the LiCl content significantly affect the
PSD in the range of aggregates (diameter d < 1 mm) at
CA-200 = 5 wt.% and in a narrow pH range of 3.39-3.59
(Fig. 2d).
At lower content of silica, LiCl weakly affects the
PSD of aggregates; however, the agglomerates appear
at d = 2-4 mm with increasing CLiCl and in narrow pH
ranges at pH > pHPZC (Figs. 2a-2d). In contrast to LiCl,
the changes in the NaCl content do not result in an
appearance of agglomerates (Figs. 2e-2h) with one
exception (Fig. 2f). This leads to the diminution in the
Def values in comparison with the systems containing
LiCl (Fig. 1). This difference can be explained by a
smaller diminution of the surface charge density of
silica nanoparticles in the presence of NaCl than LiCl.
Similar effects are observed for the suspensions with
KCl (Figs. 2i-2l). The relatively low PSDV peaks of
agglomerates of aggregates (d > 1 mm) of silica are
more intensive for the LiCl solutions (Figs. 2a-2d,
open symbols) than for the NaCl (Figs. 2e-2h) or KCl
(Figs. 2i-2l) solutions. The individual primary
nanoparticles (see PSDN2 at dav = 11.8 nm in
Figs. 2a, 2e, 2i, determined from the nitrogen adsorption
isotherm using a self-consistent regularization procedure
[8]) are mainly observed in the suspensions at CA-200 = 5
wt.% and their contribution to the PSDN increases with
the increase in the size of the alkali cations (Fig. 2).
This is due to the enhanced particle-particle interactions
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Figure 2.
Particle size distributions with respect to the particle number (solid symbols) and volume (open symbols) at different content of (a-c)
LiCl, (e-h) NaCl, (i-l) KCl and at CA-200 = (a, e, i) 0.5, (b, f, j) 1, (c, g, k) 2.5, and (d, h, l) 5 wt.% in the aqueous suspensions; (a, e, i) PSDN2
is the primary particle size distribution of A-200 determined from nitrogen adsorption isotherm.
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Effects of dissolved metal chlorides on the
behavior of silica nanoparticles in aqueous media
Figure 3.
Particle size distributions with respect to the particle number (solid symbols) and volume (open symbols) at different content of (a-c)
MgCl2, (e-h) AlCl3, (i-l) BaCl2 and at CA-200 = (a, e, i) 0.5, (b, f, j) 1, (c, g, k) 2.5, and (d, h, l) 5 wt.% in the aqueous suspensions.
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Figure 4.
Zeta potential as a function of (a) pH and calculated
using Smoluchowski (with the Henry correction factor
close to IEP) (curve 1), Ohshima (2), and Dukhin (3)
equations for diluted suspension at 0.02 wt.% of silica
and 0.01M NaCl; and (b) A-200 content in the aqueous
suspension without addition of any electrolyte; and (c)
surface charge density of nanosilica (0.2 wt.%, 0.001M
NaCl) vs. pH.
(more repulsive with increasing pH) with increasing
silica content that can stabilize the dispersion. However,
a contribution of individual nanoparticles to the PSDV is
small (Fig. 2) or none (Fig. 3).
Typically, the estimated diameters of particles
obey the relationship dPSDN < dPSDS < dPSDV < dPSDI with
respect to the particle number, surface area, volume
and light scattering intensity, respectively. This is due
to the features of light scattering efficiency depending
on the wavelength and the particle size ratio [7,45]. The
observed behavior of the PSDN and PSDV functions
is linked to the changes in the EDL state of both the
primary nanoparticles and their aggregates and
agglomerates with the changes in the A-200 and the salt
concentrations, as well as to the increasing pH which
strongly governs the EDL.
A more complex picture is observed for the systems
containing either the alkaline earth chlorides or AlCl3
in the aqueous solutions (Fig. 3) than the systems
containing the alkali metal chlorides (Fig. 2). First, the
PSDN and PSDV become broader, and the latter has the
main peak at d = 1 mm (Fig. 3). For the suspensions
with the alkali cations, the main peak of the PSDV is
at d = 0.1 mm (Fig. 2). The effect of increasing Csalt is
stronger than the effect of increasing CA-200 (Fig. 3). For
the suspensions with the alkali and the alkaline earth
chlorides and AlCl3, the measurements of the systems
with a certain content of silica and a varied content of a
salt were performed in relatively narrow, but different for
different systems, ranges of pH.
An increase in the amounts of A-200 alone (i.e.,
without addition of any electrolyte) in the suspension is
accompanied by a decrease in the negative value of the
zeta potential (Fig. 4b).
This can be due to two main effects: (i) aggregation
of the particles resulting in a decrease in their
electrophoretic mobility in comparison with the individual
nanoparticles (as described above), and (ii) a decrease
in the total charge (caused by deprotonation of silanols
to form ≡SiO-) of the silica surface and the EDL under the
slipping plane (due to the changes in the concentration
of dissolved ions in the EDL) with the increasing silica
content.
Corrections of the Smoluchowski equation (Eq. 1)
given by Ohshima [10,11] (see Eq. 2) or Dukhin [12-14]
lead to a certain increase in the modulus of the zeta
potential (Fig. 4a). The effects of the surface conductivity
[2,12-14] in the layer around the silica nanoparticles lead
to an underestimation of the zeta potential calculated
using the Smoluchowski equation. This deviation
becomes significant at pH > 6 (0.01 M NaCl) or at
pH > 4 (0.001 M NaCl) [55] as indicated by curves 1 and
3 (Fig. 4a). Additionally, a certain deviation of the zeta
potential curves from a smooth curve at the pH close to
the IEP can be due the errors in the measurements of
the electrophoretic mobility [7]. However, the observed
differences in the zS and zO or zD curves are not very
large for pure nanosilica at pH < 6, the pH values that
the majority of the measurements were performed
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Effects of dissolved metal chlorides on the
behavior of silica nanoparticles in aqueous media
Figure 5.
Zeta potential as a function of pH and salt content for (a)
LiCl, (b) NaCl, and (c) KCl at different content of A-200 in
the suspensions: 0.5 ( ), 1 ( ), 2.5 ( ), and 5 ( ) wt.%.
at. Therefore the subsequent calculations were
carried out using the zeta potentials calculated using
the Smoluchowski equation. For the more complex
nanoparticles, such as nanocarbon-nanosilica or mixed
oxides, the difference in the zeta potentials calculated
Figure 6.
Zeta potential as a function of pH and salt content for
(a) MgCl2, (b) AlCl3, and (c) BaCl2 at different content of
A-200 in the suspensions: 0.5 ( ), 1 ( ), 2.5 ( ), and
5 ( ) wt.%.
using the Smoluchowski, Ohshima (or Dukhin eq.
accounting the surface conductivity) equations can be
larger than these calculated for pure silica at pH < 6 [7].
Addition of different salts results in significant
changes in the zeta potential of the particles (Figs. 5
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and 6), especially at a lower content of silica (0.5 and
1 wt.%) in the presence of dissolved LiCl (Fig. 5a). For
the alkali cations, the nonlinearity in the z curves strongly
decreases with the value of CA-200 increasing to 2.5 and
5 wt.% (Figs. 5 and 7). The observed nonlinearity in the
z curves can be due to the several factors, including
changes in pH, which is the main factor affecting the
z values. For the alkaline earth chlorides (Fig. 6), the z
values increase with an increase of the salt content. A
similar tendency is observed for AlCl3 (Fig. 6b).
A decrease in the z values at CAlCl3 = 0.1M and
different CA-200 values can be partially caused by
diminution of pH. The effect of BaCl2 at Csalt = 0.05 and
0.1M (Fig. 6c) results in the positive z values at pH =
5-6.5 > pHPZC. A similar effect is observed for MgCl2 at
similar pH values (Fig. 6a). For AlCl3, at all Csalt values
used (Fig. 6b), z > 0 in a relatively narrow range of low
pH values (1.9-2.8) close to the IEP. The decrease in
the negative value of the zeta potential with an increase
in the neutral electrolyte content can be caused by
adsorption of metal cations onto the silica surface or by
their location in the EDL under the slipping plane. This
effect increases for the silica suspension when the pH
increases [56,57]. A larger z range vs. salt content is
observed for Ba2+ (Fig. 6c) than for Mg2+ (Fig. 6a) since
the latter is a smaller cation.
In the case of a maximal content of silica (5 wt.%)
the effect of K+, a large univalent cation, is greater than
that of Na+ and Li+ and the difference increases with an
increase in the salt content (Fig. 7).
Certain deviations in the curve course (Fig. 7) can
be due to a difference in the pH values. This effect
of K+ is due to a better adsorption (and ion exchange
efficiency) of larger cations since their hydration degree
is lower but the polarizability is greater. A similar order
Li+ < Na+ < K+ of the cation effect on the zeta potential
is also observed for the silica particles much larger
(~500 nm) [58] than that of A-200 (11.8 nm).
The effective screening length increases, i.e., the
EDL thickness grows, for aggregates in comparison
with the individual primary nanoparticles (Fig. 8). This
result can be caused by a decrease in the particles
charging during their aggregation due to the electrostatic
interactions of adjacent particles quenching the patches
of different (or opposite) charges (e.g. because of
interaction of adsorbed cations with deprotonated SiOgroups of adjacent nanoparticles) and affecting the
particle surface polarization. Overall the l values are
smaller than the sizes of the particles (either primary
or secondary). The effect of K+ on the l(pH) function is
stronger than that of Na+ due to a better adsorption of
the larger cations onto the silica surface which leads to
a decrease in the effective surface charging and to the
Figure 7.
Zeta potential as a function of salt content at
CA-200 = 5 wt.%.
Figure 8.
Effective screening length as a function of pH for
individual nanoparticles (curves 1 and 3) and their
aggregates (2, 4) at Csalt = 0.001M NaCl (1, 2) or KCl
(3, 4) and CA-200 = 5 wt.%.
diminution of the negative value of the zeta potential.
Therefore, the EDL thickness increases, as well as the l
value. The effective screening length tends to decrease
with an increase in pH since the EDL becomes thinner
with increasing negative charging of the silica surface
due to deprotonation of the surface silanol groups and
adsorption of cations or their location (in the EDL) close
to the silica surface. Overall, the l(pH) curve shape is
similar to that of the z(pH) for nanosilica [7] (Fig. 4a).
It should be noted that the Debye length (k-1) for
the salt ions in the systems shown in Fig. 8 is equal to
9.61 nm [1]. The effective screening length for the
individual A-200 nanoparticles (average diameter
11.8 nm) is less than the k-1 value for the univalent ions
489
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Effects of dissolved metal chlorides on the
behavior of silica nanoparticles in aqueous media
but for the aggregates l > k-1 at pH < 5.5 because the
EDL of silica aggregates becomes thicker with the pH
approaching the IEP. Thus, this rough approach in
calculation of the l values can be used to estimate the
EDL thickness for the primary and secondary particles
vs. pH and the salt content using the results of the
PCS (PSD, Def), electrophoresis (z) and potentiometric
titration (s0) methods applied to the same samples
under the same conditions. The l range corresponds
to 10% (high pH) – 70% (close to the IEP) of the size
of the primary particles. In the case of the aggregates,
it corresponds to 1-15% (depending on pH) of the
aggregate size. Overall these values are in agreement
with the difference between the geometrical and
hydrodynamic diameters of the particles (far from the
IEP) or the size of the ionic halo around the charged
particles [1-5,7,50,59].
4. Conclusion
The electrophoretic behavior (zeta potential) and particle
size distributions with respect to the particle number
and the particle volume of nanosilica A-200 depend
strongly on several factors such as silica content,
pH, concentration, and type of salts (LiCl, NaCl, KCl,
MgCl2, BaCl2, AlCl3). More strongly hydrated Li+ cations
cause a stronger nonlinear dependence of the zeta
potential on pH and the salt content than Na+ or K+. This
nonlinearity is much stronger at a lower content of silica
(0.5-1 wt.%) than at CA-200 = 2.5 or 5 wt.%. However,
at a high concentration of nanosilica (5 wt.%) the effect
of K+ ions leads to a greater diminution of the negative
value of the zeta potential due to a better adsorption
of the cations of a larger radius. Therefore, the effects
of K+ on the effective screening length l are stronger
(i.e., l is larger) than these of Na+ for both the individual
nanoparticles and their aggregates. The effective
screening length nonlinearly decreases with increasing
pH opposite to the changes in the negative zeta potential
vs. pH. Multivalent cations which better adsorb onto the
silica surface enhance particle aggregation to a greater
extent than the univalent cations that lead to greater
shifts the zeta potential toward positive values.
Based on these research results one can assume
that the efficiency of nanosilica, used as e.g. a medicinal
enterosorbent, can change depending on the state of
the biofluids (pH, salinity) due to the changes in the
colloidal properties of this nanomaterial following the
changes in pH and the ionic strength of the media.
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