Journal of Colloid and Interface Science 285 (2005) 443–447
www.elsevier.com/locate/jcis
Determination of the surface area of smectite in water
by ethylene oxide chain adsorption
Paul-Cheng Yuang, Yun-Hwei Shen ∗
Department of Resources Engineering, National Cheng Kung University, Tainan, Taiwan 70101, Republik of China
Received 4 December 2003; accepted 7 December 2004
Available online 29 January 2005
Abstract
This study investigates the feasibility of using ethylene oxide (EO) chain adsorption to determine the surface area of smectite in water.
Experimental results indicate that high-molecular-weight poly(ethylene oxide) (PEO) should be used to provide reasonable estimations for
monolayer capacity of PEO on smectite. The surface areas of smectites in water are calculated from the monolayer capacity of PEO adsorbed
on smectite by taking the area per EO unit as 8.05 Å2 . The method measures the actual surface area of smectite exposed when dispersed in
water, which is important to applications of smectite under aqueous conditions.
 2004 Elsevier Inc. All rights reserved.
Keywords: Surface area; Smectite; Adsorption; Ethylene oxide chain
1. Introduction
The determination of the exposed surface area of smectite, a swelling clay, deliminated in water is important in
many industrial applications such as organoclays, drilling
muds, binding agents, plasticizers, and soil technology. Surface area measurements are typically based on the amount
of probe molecule that a solid adsorbs at monolayer coverage and the molecular area of the probe molecule. Nitrogen,
a nonpolar molecule, is often used in the Brunauer, Emmett,
and Teller (BET) method to obtain the surface area of fine
particles [1]. However, it is widely recognized that nitrogen
does not interact with or have access to the interlayer surfaces of expandable clay minerals, such as smectite, which
collapse when the clay is dried during pretreatment. In the
presence of water, exchangeable cations that are associated
with expandable clays are hydrated; this forces a separation
of interlayer surface or delimination. The interlayer surfaces
of the clays then become accessible and are therefore considered an important component of the surface area of clays.
* Corresponding author. Fax: +886-6-2380421.
E-mail address: [email protected] (Y.-H. Shen).
0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcis.2004.12.056
The amount of some polar solvent (e.g., water or ethylene glycol) retained by a unit mass of the clay under certain
evacuating conditions has been used to determine the total
surface area of clay [2,3]. However, appropriate adsorbates
for surface area determination must be chemically inert so
that they neither alter the structure of the solid nor penetrate the molecular network of the solid [4]. Because some
polar solvents can potentially alter the solid structure, such
as by a solvation process with some clay minerals, the resulting surface area is often a measure of phenomena other
than physical adsorption. For example, it has been found
that the retention of polar molecules may be influenced by
the species of exchangeable cation of the clay [5] and polar
molecules tend to form multilayers at cation-exchange sites
prior to complete monolayer coverage [6]. The adsorption
of methylene blue dye by clay minerals in water has also
been used to determine either their cation exchange capacity
(CEC) or their surface area [7]. However, due to the cation
exchange nature of this adsorption process, what properties
are actually being measured is unclear.
Poly(ethylene glycol) (PEG) is a water-soluble nonionic
polymer, HO–(CH2 –CH2 –O–)n –H, which is available commercially in a wide range of molecular weights. Higher
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P.-C. Yuang, Y.-H. Shen / Journal of Colloid and Interface Science 285 (2005) 443–447
molecular weights are referred to as poly(ethylene oxide)
(PEO). The ether oxygen in the polymer chain interacts with
water, causing the polymer to be water-soluble, while the
CH2 –CH2 – groups are hydrophobic in nature. Adsorption
behavior of PEO and PEG for several different powders in
aqueous systems has been reported. Rubio and Kitchener
[8] proposed that the mechanism of PEO adsorption onto
silica was hydrogen bonding between the surface silanol
(SiOH) groups and the ether oxygen in the polymer. Koksal
et al. [9] discovered that no PEO adsorption occurred on alumina and hematite surfaces and postulated that the polymer
could not displace strongly adsorbed water molecules. Shen
[10] also confirmed that the ethylene oxide chain has strong
adsorption on SiO2 , but not on oxides such as Al2 O3 and
Fe2 O3 . Given that the ethylene oxide chain adsorbs strongly
onto SiO2 via hydrogen bonding in water, it is reasonable
to speculate that the ethylene oxide chain will fully coat the
interlamellar space of smectite, which is composed predominately of siloxane (–Si–O–Si–) surface. This caused us to
investigate the feasibility of using nonionic ethylene oxide
chain adsorption to determine the surface area of an expandable clay, smectite in water.
2. Experimental
2.1. Materials
Smectites (SWy-1, SAz-1, and STx-1, obtained from the
Clay Source Repository, University of Missouri) were used
as received. Montmorillonit K-10 was obtained from Aldrich
Chemical Co. and used as received. Table 1 lists the identification and CEC for the studied smectites. PEGs and
PEOs were obtained from Aldrich Chemical Co. and used
as received. Table 2 lists the molecular formulae, molecular
weights, and abbreviations of these compounds.
Table 1
Identification and CEC for studied smectites
Sample
Smectite
CEC (meq/100 g)
Smectite A
Smectite B
Smectite C
Smectite D
SWy-1
K-10
SAz-1
STx-1
75.9
40.0
101.8
83.2
Table 2
Molecular formulas, molecular weights, and abbreviations of polymers used
Formula
MW (g/mol)
Abbreviation
HO(CH2 CH2 O)n H
HO(CH2 CH2 O)n H
HO(CH2 CH2 O)n H
HO(CH2 CH2 O)n H
HO(CH2 CH2 O)n H
HO(CH2 CH2 O)n H
HO(CH2 CH2 O)n H
300
600
4000
20,000
100,000
400,000
1,000,000
PEO300
PEO600
PEO4000
PEO20000
PEO100000
PEO400000
PEO1000000
2.2. PEO adsorption isotherms
All adsorption tests were conducted at pH 7. Adsorption was conducted in 50-ml Pyrex beakers by contacting
the smectite suspension with polymer solution using magnetic stirrer at 600 rpm in a constant temperature chamber
of 22 ◦ C. A control with no smectite addition was included.
After a predetermined contact time (typically 8 h to reach
equilibrium), the sample was centrifuged at 10,000 rpm for
25 min and the supernatant was withdrawn. A total organic carbon analyzer (TOC 5000, Shimazdu) determined
the residual polymer concentrations. The standard errors of
the mean for the TOC concentrations were in the range of
0.02–0.08, which corresponds to a standard deviation of 0.2–
1.3 mg TOC/L. No detectable change in solution concentration of polymer was found in the control samples with no
smectite addition. The difference in the amount before and
after sorption gives the amount sorbed. All samples were run
in duplicate, and the average is reported.
2.3. EO unit area determinations
The surface area of a single EO unit was determined
based on the amount of PEO that a nonswelling clay, kaolinite, adsorbs at monolayer coverage and the BET surface area
of kaolinite measured with a Micromeritics ASAP 2000 instrument. Kaolinite is a nonswelling clay with 1:1 structure,
comprising an octahedral aluminum layer overlying a tetrahedral silicon sheet. This 1:1 structure implies an equal area
of exposed Si-oxide and Al-oxide surface. In other words,
one-half of the BET surface area of kaolinite arises from the
exposed Si-oxide surface. Assuming that the ethylene oxide chain has strong adsorption only on the Si-oxide surface,
PEO will form monolayer coverage only on that half of the
BET surface area of kaolinite. Thus, the surface area of a single EO unit may be obtained by dividing one-half of the BET
surface area of kaolinite by the total number of EO units adsorbed at monolayer coverage.
2.4. X-ray diffraction analysis
The d001 spacing of smectite was examined using an
X-ray diffractometer (Model Rad II A, Rigaku, Tokyo) with
CuKα radiation and Ni filter, operated at 30 kV, 200 mA, and
a scanning rate of 2◦ /min. The smectites were freeze-dried
and ground to pass a 100-mesh screen before XRD analysis.
3. Results and discussion
3.1. Monolayer coverage of PEO on smectite surface
Fig. 1 depicts the adsorption isotherms of the various
PEG and PEO polymers on smectite A (SWy-1). Notably, as
the molecular weight of the polymer increases, the isotherm
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Fig. 1. Sorption isotherms of the various PEO polymers on smectite A.
Fig. 2. The amount of PEO adsorbed on smectite A at the adsorption
isotherms platforms as a function of the square root of the PEO’s molecular mass.
also increases more steeply at low concentrations and attains a higher plateau region of adsorption. In general, it is
more difficult to desorb larger polymer molecules because
more segments must detach simultaneously for the larger
polymer molecule to desorb. Thus, for higher-molecularweight PEO, the isotherm is steeper at low solution concentrations. For a given fraction of surface area that adsorbed polymer segments cover, larger polymers will have
a greater mass in loops and tails, and therefore the amount
adsorbed will have a greater mass. Scheutjens and Fleer
[11,12] theoretically predicted that a linear correlation existed between the saturated amount of polymer adsorbed and
the square root of polymer molecular mass. However, when
we plot the amount of polymers adsorbed on smectite A at
the adsorption isotherms platforms against the square root
of the polymer’s molecular mass (Fig. 2), a linear relationship is not obtained. Fig. 2 indicates that the saturated
amount of polymer adsorbed initially increases with the increasing molecular weight of low-molecular-weight polymers but is almost independent of the molecular weight
445
Fig. 3. X-ray diffraction patterns of PEO-loaded smectite A.
for high-molecular-weight polymers. The convergent saturated amount of polymer that was adsorbed for smectite A
is 305 mg/g (Fig. 2). This observation obviously indicated
that for high-molecular-weight PEOs the polymer conformation adsorbed on smectite takes train segments predominantly and a monolayer coverage is formed on smectite
surface. Apparently, this is in direct conflict with accepted
polymer adsorption models, which indicate that the formation of loops and tails increases with molecular weight.
Research results [13–16] indicated that a large part of the
siloxane (–Si–O–Si–) surface in smectites has a hydrophobic nature. Thus, it is not surprising to observe a different
PEO adsorption model on smectites. We speculate that the
hydrophobic interaction between the siloxane (–Si–O–Si–)
surface and the ethylene group (CH3 –CH2 ) of the PEO is
the major adsorption mechanism of PEO on smectite surface. In this context, high-molecular-weight PEOs are less
soluble in water and tend to bind more strongly on hydrophobic siloxane surfaces, leading to a rapid approach to
monolayer coverage on smectite. The results from X-ray diffraction analyses of PEO-loaded smectites shown in Fig. 3
provide additional evidence for monolayer coverage of PEO
on smectite surface. At 125% saturated capacity loading of
PEO MW 100,000 and PEO MW 1,000,000 the d spacing
of both PEO-loaded smectites increase from 13.2 to 18.2 Å.
If it were not monolayer coverage of PEO, i.e. the adsorbed
polymer takes train segments predominantly on smectite, the
higher-molecular-weight PEO would have a greater mass in
loops and tails, and therefore a larger d spacing. At 50 and
125% saturated capacity loading of PEO MW 1,000,000 the
d spacing of PEO-loaded smectites increase from 13.2 to
17.2 and 18.2 Å, respectively. The slightly smaller expansion of d spacing for smectite with 50% saturated capacity
loading of PEO probably results from inadequate coverage
of PEO on the upper and lower surface of the interlamellar region in smectite. A reviewer of this manuscript raised
a concern with the speculation that high-molecular-weight
polymers adsorb essentially flat on a solid surface, especially at high surface coverage. However, without further
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Fig. 4. Sorption isotherms of various PEO polymers on kaolinite.
Fig. 6. The amount of PEO adsorbed on various smectites at the adsorption
isotherms platforms as a function of the square root of the PEO’s molecular
mass.
Table 3
Specific surface areas of smectites in water
Sample
Smectite
Surface area (m2 /g)
Smectite A
Smectite B
Smectite C
Smectite D
SWy-1
K-10
SAz-1
STx-1
335.5
115.5
176.0
198.0
posed Si-oxide surface, the area of a single EO unit was
determined to be 8.05 Å2 . This value is consistent with
the area of a single EO unit reported in other research
[17,18].
Fig. 5. The amount of PEO adsorbed on kaolinite at the adsorption isotherm
platforms as a function of the square root of the PEO’s molecular mass.
evidence from in situ surface analysis, we believe that this
admittedly crude model logically explain the experimental
observations.
3.2. Area of a single EO unit
The surface area of a single EO unit was obtained by dividing one-half of the BET surface area of kaolinite by the
total number of EO units adsorbed at monolayer coverage.
The total number of EO units adsorbed at monolayer coverage on kaolinite was obtained from Figs. 4 and 5. Fig. 4
depicts the adsorption isotherms of the various PEG and
PEO polymers on kaolinite and Fig. 5 plots the number of
polymers adsorbed on kaolinite at the adsorption isotherm
platforms against the square root of the polymer’s molecular mass. The convergent saturated amount of PEO that
was adsorbed for kaolinite is 6.32 mg/g and is a reasonable estimate of the monolayer capacity of EO on kaolinite. Quantity of 6.32 mg of PEO is equivalent to 0.000144
mole of the EO unit. Taking one-half of the BET surface
area of kaolinite (0.5 × 13.96 m2 /g) as the area of ex-
3.3. Surface area of smectite in water
At this stage, two requirements for surface area measurements, the amount of probe molecule that a solid adsorbs
at monolayer coverage and the molecular area of the probe
molecule, have been established for the adsorption of PEO
on smectite. In the following, the surface areas of smectites in water are calculated from the monolayer capacity
of PEO adsorbed on smectite by taking the area per EO
unit as 8.05 Å2 . The monolayer capacity of PEO was obtained by the convergent saturated amount of PEO adsorbed
on smectites form Fig. 6, which plots the saturated amount
of PEO adsorbed against the square root of PEO’s molecular mass. The resulting areas obtained for four smectites
are listed in Table 3. The calculated surface area for a typical smectite is 700–800 m2 /g [19] and there is a significant
difference between the areas determined by PEO adsorption
in this study and by calculation. The calculated surface area
of smectite (700–800 m2 /g) is the total area exposed when
individual smectite sheets are fully deliminated. Sheets of
smectite dispersed in water are unlikely to be fully deliminated due to the natural heterogeneity of smectite. Instead,
several smectite sheets combine to form tactoids, groups of
aligned sheets, in water. This produces a small number of
P.-C. Yuang, Y.-H. Shen / Journal of Colloid and Interface Science 285 (2005) 443–447
particles in water, so that the exposed surface area is reduced. We speculate that the surface area obtained by PEO
adsorption reveals the actual exposed area of smectite in
water. Comparing Tables 1 and 3, it is evident that the surface area of smectite in water decrease with increasing CEC
of smectite. The suspensions of SAz-1 form larger particles than SWy-1 in water due to their larger charge (higher
CEC) [20]. 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 tactoids and decreasing the available surface area. However, smectite K-10, with the lowest CEC,
shows relatively low surface area in water, presumably due
to the interlayer K+ ions effectively preventing delimination of smectite K-10 in water. This is confirmed by the
observation of the fast settling of smectite K-10 particles in
suspension.
4. Conclusion
This study demonstrates the feasibility of using the adsorption of on ethylene oxide chain to determine the surface
area of smectite in water. The method measures the actual
surface area of smectite exposed when dispersing in water
and is important to the application of smectite under aqueous conditions. High-molecular-weight PEO (MW 100,000
or higher) should be used to provide reasonable estimates for
monolayer capacity.
447
Acknowledgment
The authors thank the National Science Council of the
Republic of China for financially supporting this study.
References
[1] S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309.
[2] R.W. Mooney, A.C. Keenan, L.A. Wood, J. Am. Chem. Soc. 74 (1952)
1367.
[3] R.S. Dyal, S.B. Hendricks, Soil Sci. 69 (1950) 421.
[4] C.T. Chiou, Partition and Adsorption of Organic Contaminants in Environmental Systems, Wiley–Interscience, New York, 2002.
[5] A. Kellomaki, P. Nieminen, L. Ritamaki, Clay Miner. 22 (1987) 297.
[6] K.D. Pennell, R.D. Rhue, A.G. Hornsby, J. Environ. Qual. 21 (1992)
419.
[7] P.T. Hang, G.W. Brindley, Clays Clay Miner. 15 (1970) 259.
[8] J. Rubio, J.A. Kitchener, J. Colloid Interface Sci. 57 (1976) 132.
[9] E. Koksal, R. Ramachandran, P. Somasundaran, C. Maltesh, Powder
Technol. 62 (1990) 253.
[10] Y. Shen, Chemosphere 41 (2000) 711.
[11] J.M.H.M. Scheutjens, G.J. Fleer, J. Phys. Chem. 83 (1979) 1619.
[12] J.M.H.M. Scheutjens, G.J. Fleer, J. Phys. Chem. 84 (1980) 178.
[13] W.F. Jaynes, S.A. Boyd, Clays Clay Miner. 39 (1991) 428.
[14] R.K. Kukkadapu, S.A. Boyd, Clays Clay Miner. 43 (1995) 318.
[15] G. Sheng, S.A. Boyd, Clays Clay Miner. 46 (1998) 10.
[16] C. Breen, R. Watson, J. Colloid Interface Sci. 208 (1998) 422.
[17] S. Partyka, S. Zaini, M. Lindheimer, B. Burn, Colloids Surf. 12 (1984)
255.
[18] P. Somasundaran, E.D. Snell, E. Fu, Q. Xu, Colloids Surf. 63 (1992)
49.
[19] L.D. Baver, Soil Physics, Wiley, New York, 1948.
[20] M.G. Neumann, F. Gessner, C.C. Schmitt, R. Sartori, J. Colloid Interface Sci. 255 (2002) 254.