Colloids and Surfaces A: Physicochem. Eng. Aspects 259 (2005) 173–177
Preparation and dispersive behaviors of reduced charge smectite
Chia-Chi Su, Yun-Hwei Shen ∗
Department of Resources Engineering, National Cheng Kung University, Tainan, Taiwan, ROC
Received 21 July 2004; accepted 14 February 2005
Available online 17 March 2005
Abstract
A series of reduced charge smectites (RCS) with gradually decreasing cation exchange capacity (CEC) were prepared by controlling the
amount of Li used to saturate SAz-1 to study the dispersive characteristics of RCS in water and provide insight into the influence of the layer
charge density on delamination of smectite in water. Results indicated that being well deliminated in water, high-CEC smectie suspension
resulted in small flocs with fluffy structure after coagulation presumably due to a more porous type of coagulation with intense face-to-edge
and edge-to-edge interactions of platelets. High-CEC smectie also resulted in suspension with higher viscosity due to the presence of larger
number of particles and secondary electroviscous effect. Finally, this study demonstrated the possibility of tailor-made smectite suspensions
with suitable dispersive properties for particular applications.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Reduced charge smectites; Layer charge; Delamination; Rheology
1. Introduction
Smectite, a 2:1 clay mineral, is characterized mainly by an
Al-octahedral sheet placed between two Si-tetrahedral sheets.
The isomorphous substitution of Al3+ for Si4+ in the tetrahedral layer and Mg2+ for Al3+ in the octahedral layer results in a
net negative layer charge on smectite. This charge imbalance
is offset by exchangeable cations (typically Na+ and Ca2+ ) at
the smectite surface. Na+ and Ca2+ are strongly hydrated in
the presence of water and facilitate smectite expansion after
wetting. The layer charge and its distribution are among the
most important characteristics of smectite leads to a complex
colloidal system which lends itself to a variety of applications
such as organoclay [1,2], polymer-clay nanocomposites [3],
coagulation aid [4] and drilling fluid [5]. The layer charge of
smectites can be modified by the fixation of small exchangeable cations, such as Li in the structure [6]. Li cations migrate
from the exchange sites in the gallery into the layers upon
heating and consequently the residual negative layer charge
is reduced [7–9]. Both temperature and duration of heating
affect Li-fixation in reduced charge smectite (RCS) [10].
∗
Corresponding author.
E-mail address: [email protected] (Y.-H. Shen).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.colsurfa.2005.02.010
The preparation of a series of reduced charge smectites
provides an opportunity to tailor-made smectite suspensions
with suitable dispersive properties for particular applications.
Some authors [11,12] claim that as the layer charge of smectite increases, the cohesion energy that holds the lamellae
closer also increases so that the dispersion of smectite in water become more difficult, resulting in larger particles. However, other researchers [13–15] found an increased number
of non-swelling interlayers in RCS that caused a decreased
specific surface area and increased particle size of smectite in
water. In this work we address the dispersive characteristics
of a series of prepared RCSs in water and provide insight into
the influence of the layer charge density on delamination of
smectite in water.
2. Materials and methods
2.1. Preparation of reduced charge smectites (RCS)
A reference Ca-saturated smectite (SAz-1) obtained from
the Clay Source Repository were used as received. The cation
exchange capacity (CEC) was 110 mequiv./100 g clay. Three
Li-saturated SAz-1 samples were prepared by ion exchange
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of 15.0 g-SAz-1 with 250 ml-LiCl solutions containing Li
ions equivalent to 135, 240 and 1170% of SAz-1’s CEC (0.9,
1.6 and 7.8 M of Li ions) respectively for 2 h at room temperature followed by centrifugation and washing with distilled
water several times, and freeze-dried. RCS samples were prepared by heating Li-saturated SAz-1 s for 24–72 h at temperatures of 100, 130, 160, 200 and 300 ◦ C, respectively. The
CEC of the prepared RCS was determined by the ammonium
acetate method at pH = 7 [16].
2.2. Hydrodynamic particle size distribution of RCS
The hydrodynamic particle size distribution of RCS was
determined by the Andreasen pipette method [17]. In this
method the concentration of solid changes occurring within
a settling suspension of RCS are followed by drawing off
definite volumes by means of a pipette and from these the
hydrodynamic size distribution may be found. Suspensions
(0.3% by weight) of RCS in distilled water were prepared
after ultrasonic disaggregation treatment for 60 min.
2.3. Visible spectra of MB-RCS suspension
Suspensions (0.05 wt.%) of RCS in distilled water were
prepared after ultrasonic disaggregation treatment for 60 min.
Water and methylene blue (MB) solution (10−3 mol/l) were
added with stirring to obtain MB-RCS suspensions with final
MB concentrations of 5 × 10−5 mol/l. Spectra of MB-RCS
dispersions were measured using a UV-VIS spectrophotometer (Jasco V-530) 2 min, 60 min and 24 h after mixing of the
MB solution with the smectite suspension.
2.4. Coagulation of RCS suspension
For the RCS coagulation test, a weighted sample of smectite was stirred in 500 ml of distilled water within a 600 ml
beaker, fitted with four 0.25 in. wide baffle plates and 1 in
diameter propeller, for 10 min. The suspension was then adjusted to pH 6.5 and further conditioned for 2 min. After
conditioning, different amounts of a cationic polyelectrolyte,
polydiallydimethylammonium chloride (PDADMAC), were
then added to the suspension, while the propeller was rotated at 300 rpm. After 3 min of rapid mixing, the sample
was stirred for a further 7 min at 30 rpm, then left to settle
for 45 min. The residual turbidity in the suspension was measured by a turbidimeter (HANNA HI93703). The volume of
settled flocs (sludge volume) was measured in 100 ml graduated cylinders.
distilled water were prepared after ultrasonic disaggregation
treatment for 60 min.
3. Results and discussion
Effects of the amount of saturating Li+ used, heating temperature and heating time on the preparation of RCS were
explored. Three Li-saturated SAz-1 samples were obtained
by ion exchange of SAz-1 with LiCl solutions containing Li
ions equivalent to 135, 240 and 1170% of SAz-1’s CEC, i.e.
RCS135, RCS240 and RCS1170, respectively. Fig. 1 shows
the effect of heating temperature on the CEC of Li-exchanged
samples saturated with different amount of Li+ . Increasing
the treatment temperature caused a more extensive reduction of CEC values in RCS240 and RCS1170 series with a
sharp decrease occurring after heating at 160 ◦ C. The temperature of 160 ◦ C seems to be a threshold value of treatment temperature to produce significant CEC reduction for
the preparation of RCS in this study. Previous studies [18,19]
indicated that the CEC-reduction of SAz-1 exchanged with
1 M LiCl and heated for 24 h at 150–200 ◦ C ranged from
87 to 79%. The CEC of RCS135 shown only a moderate
decrease with increasing treatment temperature presumably
due to the inadequate amount of Li+ on the exchange sites in
the gallery available to diffuse into the layers upon heating.
Upon heating at 200 ◦ C for 24 h the CEC of RCS135, RCS240
and RCS1170 were reduced to 83, 53, and 22 mequiv./100 g,
respectively. Alternatively, RCS with different layer charge
can be prepared at the same heating temperature from Lisaturated SAz-1 sample by varying the treatment time. Heating at 160 ◦ C from 24 to 72 h, the CEC of RCS135 decreased
gradually from 83 to 67 mequiv./100 g. Longer heating time
apparently allowed more Li+ ions in the gallery diffuse into
the layers. A suitable series of RCS samples were prepared
for the following experiments.
The hydrodynamic particle size distributions of RCS with
different CEC in water were determined by the Andreasen
2.5. Rheological measurements
The rheological behavior of the dispersions was measured
in a Brookfield HBDV-III rheometer with cone-plate adapter
at 25 ◦ C. The measurement required 0.8 ml of each dispersion. Suspensions of RCS with different solid content in
Fig. 1. Effect of heating temperature on the CEC of various Li-exchanged
samples.
C.-C. Su, Y.-H. Shen / Colloids and Surfaces A: Physicochem. Eng. Aspects 259 (2005) 173–177
175
Fig. 2. The hydrodynamic particle size distribution of various RCSs.
pipette method. The suspensions of RCS with different CEC
form particles with different size in suspension due to their
layer charge. Several authors [11,12] proposed that as the
layer charge increases, the cohesion energy that holds the
lamellae closer also increases so that the dispersion of smectite in water becomes more difficult, resulting in larger particles and smaller interlamellar spacing. However, results form
Fig. 2 indicated an opposite trend that particle size of smectite in water decrease with increasing layer charge or CEC.
Apparently, smectite with lower CEC forms tactoids, groups
of aligned layers, in water presumably due to weaker electrostatic repulsive force between layers. Smectite with higher
CEC was better deliminated in water and resulted in tactoids
with smaller number of layers and smaller particle size. Komadel et al. [14] reported that the decrease of layer charge
of smectite by Li-fixation causes the collapse of expandable
interlayers and the subsequent development of pyrophyllitelike (unexpandable) layers. The presence of non-swelling layers is also accountable for the increasing particle size of RCS
with lower CEC.
Fig. 3 shows the time evolution spectra of the suspensions containing MB and RCS with different CEC. The
distances between the adsorbed dye cations (MB) were controlled by the layer charge [20–24]. Short distances between
the MB, adsorbed on the untreated smectite with the highest layer charge, enable a high extent MB agglomeration
(570–590 nm) [20–24]. The spectra of RCS with a CEC of
121 mequiv./100 g show an absorbance peak only at 570 nm
(Fig. 4a) indicating the dominance of MB agglomerates.
On the other hand, spectra of RCS with a CEC of 83 and
53 mequiv./100 g (Fig. 4b and c) demonstrated the dominance
of monomeric MB (650–675 nm) and the appearance of protonated or J-aggregates MB (770 nm) [20–24]. Lower charge
smectite apparently decreased the amount of MB agglomerates in favor of monomeric (650–675 nm) and protonated or
J-aggregates MB (770 nm). In this case, larger distances between the neighboring negatively charged sites suppressed
the formation of MB agglomerates. The spectra of RCS
Fig. 3. The time evolution spectra of the suspensions containing MB and
RCS with different CEC.
with a CEC of 83 mequiv./100 g show a significant change
with time. The increasing intensity of the band assigned to
monomeric MB (650–675 nm) indicates the continuous formation of monomeric MB. It is also interesting to notice that
the 770 nm band, assigned to protonated or J-aggregates MB,
decreases for RCS with a CEC of 53 mequiv./100 g. The disappearance of this band with decreasing charge can be interpreted as a consequence of less water autoionization and/or
suppressed formation of J-aggregates at the surface of low
charge density as previously reported [22]. Apparently, the
change in spectra of MB-RCS suspensions reflect very sensitively the difference in the layer charge or CEC of RCSs
prepared in this study.
Smectite coagulation tests were run to explore dispersive
state for different RCS suspension. Fig. 4 presents a comparison of the coagulation of suspensions of RCS with different
CEC using a cationic polyelectrolyte, PDADMAC. Optimum
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Fig. 5. The shear stress–shear rate curves of various RCS dispersions.
Fig. 4. Coagulation of RCS suspensions with PDADMAC: (a) residual turbidity and (b) sludge volume as a function of PDADMAC dosage.
PDADMAC dosages for the coagulation of RCS with a CEC
of 83 and 121 mequiv.100 g suspensions were 2 and 20 mg l−1
PDADMAC, respectively (Fig. 4a). The optimum PDADMAC dosages for the coagulation of RCS suspensions did not
increase proportional to the increasing CEC of RCS. This result indicated that even a small difference in CEC may cause a
profound variation in the dispersive state of RCS suspensions.
In addition, Fig. 4b indicated a distinct difference among
volume of settled flocs (sludge volume) for different RCS
suspensions. The coagulation of smaller layer charge RCS
(CEC = 53 and 83 mequiv./100 g) suspensions produced sediment consisted of large aggregates with compact structure
and small sludge volume (less than 20 ml). In contrast, coagulation of high layer charge RCS (CEC = 121 mequiv./100 g)
suspension resulted in large volume of settled flocs (more than
30 ml) consisted of small aggregates with fluffy structure. It
is possible that smaller layer charge RCS has a tendency to
form aggregates of aligned (face-to-face interaction) smectite platelets in suspension and resulted in large flocs with
compact structure after coagulation. However, coagulation
of better delaminated high layer charge RCS suspension resulted in small flocs with fluffy structure presumably due to
a more porous type of coagulation with intense face-to-edge
and edge-to-edge interactions of platelets. Apparently, the
sludge volume after coagulation for different RCS suspensions reflected the dispersive state of RCS suspensions.
The shear stress-shear rate curves of the concentrated RCS
dispersions (23–33 wt.%) are shown in Fig. 5. Low layer
charge RCS (CEC = 53 mequiv./100 g) suspensions display
Newtonian flow behavior with a constant viscosity. Higher
layer charge RCS (CEC = 83 and 121 mequiv./100 g) suspensions has an initial yield stress at low shear rates, and afterwards presents pseudoplastic or shear-thinning type behavior
at higher shear rates. Fig. 6 shown change in viscosity determined at various shear rates as a function of CEC of RCS.
Clearly, the viscosity of RCS suspension increases with increasing CEC of RCS. Dispersing in water sheets of low layer
charge RCS are not well deliminated. Instead, several smectite sheets combine to form tactoids, groups of aligned sheets,
in water. This produces a small number of particles in water, so that the viscosity is reduced. Another possible cause
of the high viscosity for suspensions of high layer charge
RCS is the secondary electroviscous effect [25]. Rand et al.
Fig. 6. Change in viscosity determined at various shear rates as a function
of CEC of RCS.
C.-C. Su, Y.-H. Shen / Colloids and Surfaces A: Physicochem. Eng. Aspects 259 (2005) 173–177
[26] explained that the extended diffuse ionic layers around
the particles restrict their movement and enhance both the
shear stress and yield value. Reduced layer charge reduces
the thickness of the diffuse ionic layers, increases the translational and rotational freedom of the particles and decrease
viscosity.
177
number of particles and secondary electroviscous effect. Finally, this study demonstrated the possibility of tailor-made
smectite suspensions with suitable dispersive properties for
particular applications.
References
4. Conclusion
Heating Li-saturated smectites reduced the net negative
layer charge due to fixation of Li within the layers in smectite. A series of reuced charge smectites with gradually decreasing CEC were prepared by controlling the amount of
Li used to saturate SAz-1 in this study. Test results indicated
that high-CEC smecties posses higher layer charge density
as confirmed by spectra of MB-RCS suspension and smaller
hydrodynamic particle size in water due to better delimination (Fig. 7a). In contrast, smectite with lower CEC has a tendency to form aggregates of aligned (face-to-face interaction)
smectite platelets with larger hydrodynamic size in water presumably due to weaker electrostatic repulsive force between
layers (Fig. 7b). Being well deliminated in water, high-CEC
smectie suspension resulted in small flocs with fluffy structure after coagulation presumably due to a more porous type
of coagulation with intense face-to-edge and edge-to-edge
interactions of platelets. High-CEC smectie also resulted in
suspension with higher viscosity due to the presence of larger
Fig. 7. Dispersive state of (a) high layer charge RCS and (b) low layer charge
RCS.
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