Clays and Clay Minerals, Vol. 49, No. 2. 168-173, 2001.
M E T H Y L E N E B L U E DIMERIZATION D O E S N O T INTERFERE IN
S U R F A C E - A R E A M E A S U R E M E N T S OF KAOLINITE A N D SOILS
MARCELO J. AVENA, LAURA E. VALENTI, VALERIA PFAFFEN, AND CARLOS P. DE PAULI
INFIQC, Departamento de Fisicoqufmica, Facultad de Ciencias Qu/micas, Universidad Nacional de C6rdoba,
Ciudad Universitaria, 5000 C6rdoba, Argentina
Abstract--Methylene blue (MB) was adsorbed from aqueous solutions onto a kaolinite and four soil
samples to determine the effects of MB dimerization on the measured surface area. Adsorption isotherms
were prepared using four adsorbing solutions containing, respectively, 9, 46, 71, and 83% of MB molecules in the dimeric state. Langmuir-type isotherms were obtained in each case. The results indicate that
equilibration occurs quickly. The aggregation state of MB molecules at the surface does not depend on
the aggregation state in the initial adsorbing solutions, but on the final equilibrium concentration of MB.
A comparison with the specific surface area measured by adsorption of ethylene glycol monoethyl ether
indicates that MB adsorbs as a monomer, regardless of the aggregation number in solution. This result
occurs owing to the strength of monomer-surface and monomer-monomer interactions. If monomer-surface
interactions are favored, the MB dimer adsorbs in the monomeric form. If monomer-monomer interactions
are favored, dimer adsorption may occur. The visible spectra of adsorbed molecules indicated that MB
was present at the surface as a mixture of monomeric and dimeric species. These results suggest that
dimers are formed in the contact region between two aggregating particles.
Key W o r d s - - D y e Adsorption, Ethylene Glycol Adsorption, Kaolinite Surface, Methylene Blue Dimerization, Specific Surface Area.
INTRODUCTION
T h e p r i n c i p l e for e s t i m a t i n g the specific surface area
(S) of a p o w d e r m a t e r i a l f r o m adsorption e x p e r i m e n t s
is simple. B y a d s o r b i n g a c h e m i c a l o f k n o w n m o l e c ular cross section a n d d e t e r m i n i n g the a m o u n t necessary for m o n o l a y e r c o v e r a g e , the surface area o f the
studied a d s o r b e n t is evaluated. H o w e v e r , the surface
o r i e n t a t i o n a n d the p a c k i n g density of an a d s o r b a t e c a n
c h a n g e f r o m solid to solid o w i n g to their intrinsic surface properties. Therefore, a precise e v a l u a t i o n o f the
effective m o l e c u l a r cross section o f the a d s o r b e d
c h e m i c a l species m a y b e difficult, a n d the calculated
surface area m a y b e inaccurate.
T h e m o s t suitable m e t h o d s for e v a l u a t i n g the specific surface area o f solids are those that do not c h a n g e
the surface a n d those that use the m o s t appropriate
adsorbate molecules. F o r e x a m p l e , if the area o f a catalyst used for g a s - p h a s e r e a c t i o n s o f small m o l e c u l e s
is required, the suitable t e c h n i q u e is gas a d s o r p t i o n o f
a s m a l l adsorbate. E i t h e r a p o l a r or a n o n - p o l a r adsorbate is u s e d d e p e n d i n g o n the properties o f the cata l y z e d molecules. I f the s a m p l e is l a k e - s e d i m e n t m a terial, a d s o r p t i o n e x p e r i m e n t s f r o m an aqueous solution are best. A d s o r p t i o n o f a polar liquid, such as
e t h y l e n e glycol m o n o e t h y l e t h e r ( E G M E E ) (Carter et
al., 1965), c o u l d also b e useful for these sediments.
A d s o r p t i o n o f m e t h y l e n e blue ( M B ) f r o m a q u e o u s
solution has frequently b e e n used for the d e t e r m i n a tion o f S (Potgieter, 1991; B r i n a a n d D e Battisti, 1987;
H e l m y et al., 1999). T h i s m e t h o d is s i m p l e a n d versatile b e c a u s e M B has a strong affinity for m o s t solid
surfaces ( B e r g m a n n a n d O ' K o n s k i , 1963; M a r g u l i e s
Copyright 9 2001, The Clay Minerals Society
et al., 1988; R y t w o et aL, 1998). M e t h y l e n e b l u e h a s
a special affinity for n e g a t i v e l y c h a r g e d surfaces, a n d
its c o n c e n t r a t i o n is easily quantified b y s p e c t r o p h o tometry. Thus, an a d s o r p t i o n i s o t h e r m w i t h a distinct
plateau c a n b e o b t a i n e d a n d u s e d for a n accurate det e r m i n a t i o n o f m o n o l a y e r mass. T h e M B m e t h o d is
c o n v e n i e n t for s u r f a c e - a r e a m e a s u r e m e n t s o n sedim e n t s f r o m w a t e r b o d i e s a n d clay or soil s a m p l e s
w h e r e the properties o f the c l a y - w a t e r or soil-water
interface are important. Moreover, b e c a u s e M B is a
heterocyclic cationic c o m p o u n d w i t h m o l e c u l a r dim e n s i o n s a n d c h e m i s t r y similar to s o m e herbicides,
such as diquat a n d paraquat, the M B m e t h o d is an
appropriate t e c h n i q u e to m e a s u r e the surface area o f
soil materials w h e r e i n t e r a c t i o n s w i t h h e r b i c i d e s are o f
interest.
T h e applicability o f the M B a d s o r p t i o n m e t h o d for
s u r f a c e - a r e a m e a s u r e m e n t s is often d i s p u t e d b e c a u s e
c h e m i s o r p t i o n c o m m o n l y occurs a n d the dye m o l e c u l e
m a y a d s o r b o n specific sites r a t h e r t h a n the total surface area (Van d e n Hul a n d L y k l e m a , 1968). Also, M B
m a y f o r m d i m e r s or larger a g g r e g a t e s either in solution or in the a d s o r b e d state ( S p e n c e r a n d Sutter, 1979;
Bujdiik a n d K o m a d e l , 1997). T h e r e a c t i o n (MB)2
2 M B w i t h a n e q u i l i b r i u m c o n s t a n t Kj o f 1.7 • 10 -4
( B e r g m a n n a n d O ' K o n s k i , 1963) indicates that d i m e r
is p r e s e n t in relatively dilute solutions a n d that dimerization is f a v o r e d b y i n c r e a s i n g M B c o n c e n t r a t i o n .
C o n c e r n i n g the a d s o r b e d state, B e r g m a n n a n d
O ' K o n s k i (1963) f o u n d that the visible s p e c t r u m o f
a d s o r b e d M B is s i m i l a r to that o f d i m e r i c or n - m e r i c
M B in solution. T h e y c o n c l u d e d that M B aggregates
168
Vol. 49, No. 2, 2001
Methylene blue dimerization and surface-area measurements
Table 1. Composition (%) and cation-exchange capacity
(CEC) of the studied solids.
Sample
HAPL 1
HAPL2
ARG1
ARG2
Kaolinite
Organic
material
Clay
Silt
4.7
5.3
3.7
5.0
.
24
25
33
36
.
46
55
51
57
.
.
Sa~d
31
20
16
7.0
10-~
CEC
(eq/g)
24
-26
-7.9
- - Not measured.
on the surface of montmorillonite e v e n at v e r y low
surface coverage. Bujd,~ik and Komadel (1997) determ i n e d that M B a g g r e g a t i o n on m o n t m o r i l l o n i t e
strongly depends on the charge density o f the clay and
the equilibration time. Koopal (1978) also reported dimer adsorption of M B on silver iodide and R y t w o et
al. (1998) postulated dimer formation on sepiolite. Bec a u s e the dimer is a face-to-face association of two
flat M B molecules (Bergmann and O ' K o n s k i , 1963),
the dimer and the m o n o m e r occupy similar areas if
they lay flat at the surface. Hence, the mass necessary
to f o r m a " m o n o l a y e r " of dimers is about twice the
mass necessary to f o r m a m o n o l a y e r o f monomers. If
dimerization at the surface is not considered, the estimated area will be twice the actual one.
It is sometimes assumed that dimer adsorption is
avoided by working with dilute M B solutions, where
dimers are absent (Brina and D e Battisti, 1987). The
assumption is that no change in the aggregation number occurs upon adsorption. Thus, if only m o n o m e r s
are present in solution, only m o n o m e r s are adsorbed
to the surface, and if only dirners are present, only
dimers are adsorbed. However, there is no experimental evidence to support this assumption. In the presence of an interface, dimerization m a y be induced by
special interactions with the surface or with previously
adsorbed molecules. These interactions could lead to
the formation of aggregates in the adsorbed state although they are absent in solution. The opposite phen o m e n o n m a y also occur, i.e., dimers from concentrated M B solutions m a y c o v e r a surface in the f o r m
of only monomers.
N o change in the aggregation number upon adsorption means that equilibration is kinetically inhibited by
the surface. A l t h o u g h dimer formation and splitting
are rapid processes in solution (Spencer and Sutter,
1979), the equilibration rate m a y be slowed by the
surface. If this is the case, equilibration is not attained
after a rapid adsorption o f m o n o m e r s and/or dimers,
and the aggregation state of attached M B is dependent
on the concentration o f the adsorbing M B solution.
The mass of M B required to saturate the surface will
be higher for samples treated with concentrated solutions containing a larger fraction of dimers. In contrast, if equilibrium is quickly achieved, the aggrega-
169
tion state of attached M B will not depend on the
m o n o m e r / d i m e r content o f the initial solution but on
the equilibrium concentration. Therefore, the mass of
M B required to saturate the surface will be always the
same regardless o f the initial solution concentration.
The aim of this article is to evaluate whether dimerization of M B affects specific surface-area measurements o f clay and soil samples. A possible condition for adsorption of M B in the m o n o m e r i c f o r m is
discussed. A comparison of M B surface areas with the
areas measured by E G M E E adsorption is g i v e n also.
MATERIALS AND METHODS
The studied solids were a kaolinite from Georgia,
U S A and four soil samples from the province of C6rdoba, Argentina. The kaolinite X-ray diffraction pattern indicated that the sample is low-defect and nearly
pure material. The soils were collected from the central region of the province o f C6rdoba, Argentina f r o m
Ap horizons at 0 - 1 8 cm. Two samples ( A R G 1 and
A R G 2 ) are typical A r g i u s t o l l s and two s a m p l e s
(HAPL1 and H A P L 2 ) were entic Haplustolls. The
A R G 1 and H A P L 1 soils were cultivated, whereas the
A R G 2 and H A P L 2 soils were uncultivated, having
only their native vegetation. The samples were airdried and sieved to separate the < 2 - r a m size fraction.
Prior to the studies, the soil samples and kaolinite were
v a c u u m dried. N o other special treatments were performed on the samples. Organic material (OM), silt,
cation-exchange capacity (CEC), clay, and sand contents of the samples are shown in Table 1. The inorganic phase o f these soils is dominated by illite with
small quantities of smectite, quartz, feldspar, and chlorite (Velasco and De Pauli, 1993).
M B adsorption
Three M B solutions were used in the experiments
with kaolinite, with concentrations o f M B o f 4.2 x
10 -6, 5.3 x 10 -5, and 2.3 x 10 -4 M. The KI value o f
1.7 X 10 4 indicates that the content of M B m o l e c u l e s
in the dimeric state was 9, 46, and 71%, respectively.
These solutions are hereafter referred to as 9, 46, and
71% adsorbing solutions. The solution used with the
soil samples was more concentrated (6.8 x 10 -4 M,
83% of molecules in dimeric state). The adsorption
experiments were designed so that only one o f the M B
adsorbing solutions was added to a solid sample.
Therefore, if no change in the aggregation n u m b e r occurs during adsorption, the fraction of attached dimers
is k n o w n and equal to that o f the bulk solution.
The adsorbing solutions were prepared by dissolving M B and NaCI in a phosphate buffer solution ( U S A
National Bureau of Standards, p H = 6.86). Because
phosphate specifically adsorbs to oxide, clay, and soil
surfaces, it helps to generate a surface with a relatively
h o m o g e n e o u s negative charge that increases the affinity of M B for the surface and that decreases specific
170
Avena, Valenti, Pfaffen, and De Pauli
2.5E-06
A 2.OE-O5i
1.oE~ 1
~.0E-O6
9
k
-
0.0E*00
0DE+00
9%
1.0E-06
2.0E-06
c(t~
3.0E-06
4.0~-O6
3 5E-05 3.0EX35 2.5E-.05 20E~35
-
1 0E4Y5 I
5.DE-06 0.0E,"00
0.0E*00
46%
1.0E-06
2.0E-05
3.0E-05 40E-05
5.0E-05 60E-05
35E.O5
3.0E-~5
2.5E-05
2.0E-O5
1 5E-O6I
I .OE.-06
5oe-~ I
0.0E+~
00E+00
71%
5 0E-O5
1.0E-O4
3 5E-05
1.5E-04
2.0E-04
BJI
310E~
2,5E-O5
I 5E-05
1.0E-05
5 0E-06
0.0E*00
0.0E+00
9%, 46% and 71%
50E435
1.0E-O4
1.5E~04
2 0E454
Figure 1. Adsorption isotherms of MB on kaolinite. The
number in the right corner of each plot is the percentage of
MB dimers present in the initial adsorbing solutions. Different symbols in the plots denote different experiments. Lines
were calculated using the Langmuir equation, and parameters
from Table 1. Figure 1D combines the data of Figure 1A to
1C.
sites present. T h e NaCI c o n c e n t r a t i o n u s e d w a s 0.1 M.
T h e h i g h ionic s t r e n g t h i m p r o v e d the c e n t r i f u g a t i o n of
the s u s p e n s i o n s a n d m a y h a v e d e c r e a s e d the lateral
r e p u l s i o n b e t w e e n a d s o r b e d M B molecules. Solid sampies ( 0 . 1 - 0 . 2 g) were d i s p e r s e d in a g i v e n a d s o r b i n g
s o l u t i o n in a c e n t r i f u g e tube. A f t e r a d s o r p t i o n o c c u r r e d
Clays and Clay Minerals
(--15 rain), the d i s p e r s i o n was centrifuged, the supern a t a n t t r a n s f e r r e d to a s p e c t r o p h o t o m e t e r cell for M B
quantification, a n d a n e w aliquot o f the a d s o r b i n g solution was a d d e d to the sample. T h i s p r o c e d u r e was
r e p e a t e d until the c o n c e n t r a t i o n o f M B in the supern a t a n t was n e a r that o f the a d s o r b i n g solution. T h e
m a s s of M B a d s o r b e d in e v e r y step was calculated
f r o m the difference in c o n c e n t r a t i o n s o f the a d s o r b i n g
solution a n d the supernatant. T h e total m a s s a d s o r b e d
was c a l c u l a t e d as the s u m o f the m a s s e s a d s o r b e d in
e a c h step, a n d a n a d s o r p t i o n i s o t h e r m was t h e n calculated for e a c h sample. T h e a d v a n t a g e o f the m e t h o d
is that it allows the d e t e r m i n a t i o n o f a n a d s o r p t i o n
i s o t h e r m b y treating the solid w i t h only o n e o f the M B
a d s o r b i n g solutions. A d i s a d v a n t a g e o f the m e t h o d is
that the m a x i m u m e q u i l i b r i u m c o n c e n t r a t i o n that is att a i n a b l e is that o f the a d s o r b i n g solution. A f t e r the
adsorption i s o t h e r m w a s obtained, the m a s s for m o n o layer coverage, Fro, a n d the a d s o r p t i o n constant, b,
were calculated b y u s i n g a n o n l i n e a r least-squares reg r e s s i o n analysis ( S c h u l t h e s s a n d Dey, 1996) o f the
L a n g m u i r equation, F = Fmbc/(1 + bc), w h e r e F is the
m a s s o f M B a d s o r b e d at the e q u i l i b r i u m c o n c e n t r a t i o n ,
c. T h e specific surface area was c a l c u l a t e d f r o m I"m
a s s u m i n g a cross section o f 120 ,~2 for the M B molecule.
T h e L a n g m u i r c o n s t a n t is not a true e q u i l i b r i u m
constant. A t the s o l i d - w a t e r interface, a d s o r p t i o n reactions are actually e x c h a n g e r e a c t i o n s w h e r e adsorbate m o l e c u l e s c o m p e t e w i t h o t h e r m o l e c u l e s for locations at the surface ( A d a m s o n , 1976). F o r the conditions here, M B m a y e x c h a n g e w i t h e i t h e r s o l v e n t
H 2 0 m o l e c u l e s or with s o d i u m ions f r o m the supporting electrolyte. In dilute M B solutions with a c o n s t a n t
c o n c e n t r a t i o n o f N a +, the activity o f w a t e r a n d N a +
are constant. T h e n , b is related to the c o n s t a n t o f the
a d s o r p t i o n ( e x c h a n g e ) reaction, KL, b y b = KL/ax ( A d a m s o n , 1976), w h e r e ax represents the activity o f either
w a t e r or N a §
T h e c o n c e n t r a t i o n s of M B in the s u p e r n a t a n t solutions were m e a s u r e d s p e c t r o p h o t o m e t r i c a l l y b y u s i n g
a U V - 1 6 0 1 S h i m a d z u a p p a r a t u s at 6 6 4 nm. Spectral
c u r v e s b e t w e e n 4 0 0 - 8 0 0 n m were o b t a i n e d for s a m p l e
dispersions containing adsorbed MB. The conditions
were selected so that I" > 0.9I"~. T h e a m o u n t o f M B
in the b u l k solution s u r r o u n d i n g the particles was negligible w i t h respect to the a m o u n t o f a t t a c h e d M B .
E G M E E adsorption
T h e m e t h o d u s e d to m e a s u r e S b y a d s o r p t i o n o f
E G M E E was m o d i f i e d f r o m Carter et al. (1965). T h e
s a m p l e s ( 7 - 1 0 g) w e r e w e i g h e d in a glass beaker, 3
m l o f E G M E E were t h e n added, a n d the b e a k e r was
p l a c e d in a v a c u u m c h a m b e r c o n t a i n i n g CaC12 saturated w i t h E G M E E . V a c u u m was applied to the c h a m b e r until free liquid w a s n o l o n g e r visible. Thereafter,
cycles of 1-h v a c u u m f o l l o w e d b y 1-h e q u i l i b r a t i o n
Vol. 49, No. 2, 2001
Methylene blue dimerization and surface-area measurements
171
Table 2. Langmuir parameters for the adsorption isotherms of the studied samples and surface-area values as obtained from
methylene blue and ethylene glycol rnonoethyl ether adsorption.
Sample
Kaolinite
HAPL1
HAPL2
ARG1
ARG2
Adsorbing
solution
concentration
(M)
4.3
5.2
2.3
6.8
6.8
6.8
6.8
•
•
•
•
•
•
x
10 6
10 3
10 -4
10 -4
10 4
10 -4
10 -4
MB
Log(b)
5.80
5.75
5.45
4.90
4.60
5.00
4.30
--_ 0.201
+- 0.20
+_ 0.20
~ 0.20
+_ 0.20
___ 0.20
+ 0.20
10SFo, (mol/g)
3.7
3.15
3.30
15.0
15.2
12.9
16.0
-+ 1 . 0 z
_+ 0.20
_+ 0.10
_+ 2
- 2
_+ 2
_+ 2
S (m2/g)
26
23
24
108
110
93
116
-
7 ~
+-- 2
-+ 1
_+ 12
_+ 12
-+ 12
-+ 12
EGMEE
S (mVg)
19
-
3 ~
--102 _+ 10
110 _+ 10
90 -+ 10
80 -+ 10
Error values are given as -+2~r.
a n d w e i g h t i n g steps w e r e applied. T h e cycles were rep e a t e d until c o n s t a n t w e i g h t was reached. U n d e r t h e s e
c o n d i t i o n s , a m o n o l a y e r of E G M E E r e m a i n s adsorbed.
T h e m a s s for m o n o l a y e r c o v e r a g e w a s d e t e r m i n e d b y
w e i g h t d i f f e r e n c e s a n d S was calculated a s s u m i n g a
cross-sectional area o f 52.3 ~2 for the E G M E E molecule (Carter e t a l . , 1965).
RESULTS AND DISCUSSION
F i g u r e 1A, 1B, a n d 1C s h o w s M B / k a o l i n i t e isot h e r m s o b t a i n e d u s i n g the 9, 46, a n d 7 1 % a d s o r b i n g
solutions, respectively. F i g u r e 1D c o m p a r e s the prev i o u s curves. T h e c o n c e n t r a t i o n o f M B in the 9 % adsorbing solution s e e m e d to b e too low to a c h i e v e surface saturation b e c a u s e the i s o t h e r m s in F i g u r e 1A do
not r e a c h a distinct plateau. I s o t h e r m s w i t h a distinct
plateau w e r e o b s e r v e d for the o t h e r a d s o r b i n g solutions. T h e relatively g o o d a g r e e m e n t b e t w e e n experim e n t a l a n d theoretical data suggests that the s y s t e m
o b e y s the L a n g m u i r e q u a t i o n u n d e r the studied c o n ditions. S e m i l o g a r i t h m i c plots (F v s . logc) a n d d o u b l e l o g a r i t h m i c plots (logF v s . logc) (not s h o w n ) s h o w e d
that the s h a p e o f the c u r v e s is i n d e e d L a n g m u i r i a n ,
a n d n o t either F r e u n d l i c h or F r u m k i n - F o w l e r - G u g g e n h e i m ( F G G ) (Lyklema, 1995). T h e s e results i n d i c a t e
that there is a relatively h o m o g e n e o u s distribution o f
surface sites a n d that lateral i n t e r a c t i o n s b e t w e e n ads o r b e d m o l e c u l e s are v e r y s m a l l or negligible. T h i s is
surprising b e c a u s e clay a n d soil surfaces are k n o w n to
b e h e t e r o g e n e o u s a n d M B m o l e c u l e s are c h a r g e d entities, w h i c h m a y result in lateral r e p u l s i o n b e t w e e n
t h e a d s o r b e d molecules. B e r g m a n n a n d O ' K o n s k i
(1963) o b s e r v e d a n o n - L a n g m u i r i a n b e h a v i o r for M B
a d s o r p t i o n o n m o n t m o r i l l o n i t e . T h e y p o s t u l a t e d the
p r e s e n c e o f two surface sites with different reactivity.
O n e site w a s o f h i g h affinity for M B m o l e c u l e s a n d
a c c o u n t e d for t h e i n t e r a c t i o n b e t w e e n p o s i t i v e l y
c h a r g e d M B a n d structural n e g a t i v e c h a r g e s o n t h e
clay. T h e o t h e r site was o f l o w e r affinity a n d a c c o u n t e d for i n t e r a c t i o n b e t w e e n M B a n d edge surfaces or
o t h e r n o n - n e g a t i v e l y c h a r g e d g r o u p s at the surface.
T h e L a n g m u i r i a n b e h a v i o r o f the M B / k a o l i n i t e s y s t e m
s e e m s to b e related to t h e c o n d i t i o n s selected for t h e
e x p e r i m e n t s , b u t there are n o clear e x p l a n a t i o n s for
this. O n e p o s s i b l e e x p l a n a t i o n is that p h o s p h a t e adsorption, w h i c h is k n o w n to largely o c c u r o n o x i d e s
a n d clays, p o p u l a t e s the surface i n a h o m o g e n e o u s
w a y to create sites w i t h similar affinity for M B . Tog e t h e r w i t h this, the h i g h ionic s t r e n g t h o f the adsorbing solutions m a y h a v e c o n t r i b u t e d to the L a n g m u i r i a n
b e h a v i o r b y s c r e e n i n g lateral electrostatic r e p u l s i o n
between adsorbed molecules.
F i g u r e 1D s h o w s that the e x p e r i m e n t a l points describe only o n e isotherm, regardless of the d e g r e e o f
d i m e r i z a t i o n o f M B . T h i s suggests t h a t e q u i l i b r a t i o n
occurs rapidly a n d that the a g g r e g a t i o n state o f the
a d s o r b e d M B m o l e c u l e s does n o t d e p e n d o n the agg r e g a t i o n n u m b e r initially in the a d s o r b i n g solutions,
b u t o n the final e q u i l i b r i u m c o n c e n t r a t i o n . T a b l e 2
s h o w s the e s t i m a t e d values o f Fm a n d b for three exp e r i m e n t a l conditions. T h e error in the v a l u e of F m for
the 9 % a d s o r b i n g s o l u t i o n was h i g h b e c a u s e the isot h e r m s d o n o t h a v e data p o i n t s n e a r saturation values.
T h e errors in F m for t h e other t w o M B c o n c e n t r a t i o n s
w e r e c o n s i d e r a b l y lower. F o r b, e a c h e s t i m a t e d v a l u e
h a s a relatively s m a l l s t a n d a r d deviation. T h e v a l u e s
o f the p a r a m e t e r s Fm a n d b w e r e nearly the s a m e for
e a c h c o n d i t i o n studied, w h i c h e m p h a s i z e s that the final
state o f a d s o r b e d m o l e c u l e s is i n d e p e n d e n t o f the initial a g g r e g a t i o n state o f M B . T h e relatively h i g h valu e s o f b i n d i c a t e that M B h a s a strong affinity for the
surface. T h e a d s o r p t i o n c a n n o t b e a s s u m e d to b e purely electrostatic b e c a u s e c o m p e t i t i o n w i t h N a ions
w o u l d i m p e d e any d e t e c t a b l e M B adsorption. T h e
c h e m i c a l , intrinsic, or n o n - e l e c t r o s t a t i c c o m p o n e n t o f
the s u r f a c e - M B i n t e r a c t i o n is m o r e i m p o r t a n t t h a n the
electrostatic c o m p o n e n t . T h i s e x p l a i n s w h y F m a n d
C E C are n o t c o r r e l a t e d quantities.
T h e p r e s e n t e d data d o n o t i n d i c a t e the a g g r e g a t i o n
n u m b e r o f M B at the surface. A c o m p a r i s o n b e t w e e n
the surface areas o b t a i n e d b y M B a d s o r p t i o n to another a d s o r p t i o n m e t h o d m a y p r o v i d e i n f o r m a t i o n a b o u t
a g g r e g a t i o n n u m b e r s . Table 2 c o m p a r e s the surface areas o f the s a m p l e s as m e a s u r e d b y a d s o r b i n g M B a n d
E G M E E . E x c e p t for s a m p l e A R G 2 , the a g r e e m e n t w a s
g e n e r a l l y g o o d b e t w e e n the values e s t i m a t e d w i t h b o t h
172
Arena, Valenti, Pfaffen, and De Pauli
Clays and Clay Minerals
Table 3. Equilibrium reactions and values of constants (order of magnitude) involving MB molecules.
Type
Reaction
E q u i l i b r i u m constant values ~
Dimer splitting
Monomer adsorption
Dimer adsorption
Splitting adsorption
(MB)2 ~ 2MB
MB + H2Oad ~-~ MB~d + H 2 0
(MB)2 + H2Oad ~ (MB)2.~d + H20
(MB)2 + 2 H2Oad 4-~ 2MB~a + 2 H 2 0
K~ ~ 1 • 10 4
K2 ~ KL ~ 55.5b ~ 1 • 107
K3 ~ K2 ~ 1 • 10v
K 4 = KIK22 ~ 1 X l0 w
~The equilibrium constant of the reaction aA + bB 6-~ cC + dD is defined as K = (ac)r (ao)d/(aA)a (aR)b. To estimate the
values of the constants it was assumed that the activity of water was 55.5. The suffix " a d " denotes adsorbed.
adsorbates, p e r h a p s b e c a u s e (1) the fractions o f m o n o mers, dimers, a n d n - m e r s in the a d s o r b e d state were
the s a m e for M B a n d E G M E E or (2) b o t h M B a n d
E G M E E a d s o r b in m o n o m e r i c form. R e a s o n (1) is improbable. M o n o l a y e r c o v e r a g e (2) is m o r e likely to
o c c u r at surface saturation. Therefore, a l t h o u g h a h i g h
p r o p o r t i o n o f M B m o l e c u l e s f o r m d i m e r s in the 46,
71, a n d 8 3 % a d s o r b i n g solutions, these d i m e r s s e e m
to f o r m m o n o m e r s d u r i n g adsorption.
D i f f e r e n t r e a c t i o n s w i t h their c o r r e s p o n d i n g equilibr i u m c o n s t a n t s are g i v e n in Table 3. In the a d s o r p t i o n
reactions, w a t e r is a s s u m e d to c o m p e t e w i t h M B for
adsorption. A s i n d i c a t e d a b o v e , c o m p e t i t i o n with sod i u m m a y also b e postulated. A l t h o u g h the values o f
the e q u i l i b r i u m c o n s t a n t s w o u l d c h a n g e in the latter,
their relative values w o u l d not v a r y a n d the f o l l o w i n g
analysis a n d c o n c l u s i o n s w o u l d n o t change. B e c a u s e
a d i m e r s e e m s to a d s o r b fiat to the surface, only o n e
o f its c o n s t i t u t i n g m o n o m e r s will b e in direct contact
with the surface. Therefore, the i n t e r a c t i o n forces bet w e e n a d i m e r a n d the surface a n d b e t w e e n a m o n o m e r
a n d the surface s h o u l d b e of the s a m e order o f m a g nitude. T h i s implies that the e q u i l i b r i u m c o n s t a n t for
a d s o r p t i o n of d i m e r s (K3) is similar to that for the ads o r p t i o n of m o n o m e r s (K2), e s t i m a t e d f r o m experim e n t a l b values. T h e e q u i l i b r i u m c o n s t a n t for adsorption o f d i m e r s f r o m s o l u t i o n to give m o n o m e r s at the
surface (K4) c a n b e e s t i m a t e d i f splitting o f the d i m e r
(KI) is considered. A c o m p a r i s o n of K3 a n d K4 values
indicates that the sorption o f M B is m o r e f a v o r a b l e i f
splitting to a m o n o m e r occurs. T h e similarity o f K4/
K3 ~ KIK2, that c a n b e o b t a i n e d b y c o m b i n i n g reactions in Table 3, indicates that splitting plus a d s o r p t i o n
600nm;
~
2GRA~
A
R
665nm
G
~
HAPL1
400
Figure 2.
ples.
450
500
550
6(~0
,~(nm)
650 '
700
750
800
Visible spectra of MB adsorbed on the solid sam-
is m o r e f a v o r a b l e for a large K 2 (strong m o n o m e r - s u r face interaction) a n d a large K~ ( w e a k m o n o m e r m o n o m e r interaction). Thus, d i m e r s c a n only a d s o r b
if K2 is relatively small. This m a y o c c u r for s a m p l e
A R G 2 . T h i s s a m p l e h a s the l o w e s t v a l u e o f K2, w h i c h
m a y lead to s o m e d i m e r a d s o r p t i o n a n d a n overestim a t i o n o f S.
F i g u r e 2 s h o w s the spectra o f M B a d s o r b e d o n e a c h
sample. N o t e the a b s o r p t i o n b a n d at - 6 6 5 n m and a
b a n d or s h o u l d e r at - 6 0 0 rim. T h e s p e c t r u m o f M B
a d s o r b e d o n s a m p l e A R G 1 is v e r y similar to that o f
m o n o m e r i c M B in a q u e o u s solutions ( B o u t t o n et al.,
1997) a n d to that o f m o n o m e r i c M B a d s o r b e d o n
m o n t m o r i l l o n i t e ( B e r g m a n n a n d O ' K o n s k i , 1963). T h e
spectra o f M B a d s o r b e d o n the o t h e r s a m p l e s h a v e a
relative i n c r e a s e a n d b r o a d e n i n g o f the 600-rim band.
(1) B e r g m a n n a n d O ' K o n s k i (1963) a n d J a c o b s a n d
S c h o o n h e y d t (1999) attributed this b e h a v i o r to the form a t i o n o f d i m e r s or h i g h e r a g g r e g a t e s at the surface,
b y a n a l o g y w i t h the spectral c h a n g e s that o c c u r in sol u t i o n w h e n d i m e r s are f o r m e d ( B e r g m a n n a n d
O ' K o n s k i , 1963); (2) G a r f i n k e l - S c w e k y a n d Yariv
(1997) a n d G r a u e r et aL (1987), a m o n g others, attrib u t e d the spectral c h a n g e s to ~ i n t e r a c t i o n s b e t w e e n
m o n o m e r i c M B a n d the solid surface a n d they conc l u d e d that n o a g g r e g a t i o n at the surface occurs. Also,
spectral c h a n g e s after a d s o r p t i o n c a n b e c a u s e d b y the
electrostatic field at the surface o f the particle a n d b y
shifts relating to c h a n g e s in p o l a r i t y o f the clay e n v i r o n m e n t as s u g g e s t e d b y B o u t t o n et al. (1997). T h e
state o f M B m o l e c u l e s at the surface is b e y o n d the
scope o f our study. T h e p r e s e n t data c a n n o t differentiate b e t w e e n (1) or (2). T h e possibility o f (2) is in
agreement with surface-area measurements suggesting
that M B saturates the surface f o r m i n g a m o n o l a y e r o f
m o n o m e r s w i t h a cross sectional area o f ~ 1 2 0 ,~2. T h e
possibility of (1) c o u l d also agree w i t h s u r f a c e - a r e a
m e a s u r e m e n t s , i f the flocculation o f particles saturated
with a m o n o l a y e r o f M B occurs. In this case, the form a t i o n o f d i m e r s b y the r e a c t i o n o f m o n o m e r s ads o r b e d to different particles is p o s s i b l e in contact reg i o n s b e t w e e n i n d i v i d u a l particles. T h e different adsorption p r o c e s s e s s h o u l d lead to different flocculation
states o f the particles, L i g h t scattering or flocculation
kinetics studies c a n h e l p in u n d e r s t a n d i n g w h a t occurs
d u r i n g adsorption.
Vol. 49, No. 2, 2001
Methylene blue dimerization and surface-area measurements
ACKNOWLEDGMENTS
The authors thank P. Depetris for the kaolinite sample, and
S. Hang for the soil samples, their chemical analyses, and
CEC measurements. This work was supported by CONICOR,
CONICET, and SECyT (UNC). M.J. Avena and C.P. De Pauli
are members of CONICET.
REFERENCES
Adamson, A.W. (1976) Physical Chemistry o f Surfaces. Wiley-lnterscience, New York, 697 pp.
Bergmann, K. and O'Konski, C.T. (1963) A spectroscopic
study of methylene blue monomer, dimer and complexes
with montmorillonite. Journal o f Physical Chemistry, 67,
2169-2177.
Boutton, C., Kauranen, M., Persoons, A., Keung, M.P., Katrien Y.J., and Schoonheydt, R.A. (1997) Enhanced secondorder optical nonlinearity of dye molecules adsorbed onto
Laponite particles. Clays and Clay Minerals, 45, 483-485.
Brina, R. and De Battisti, A. (1987) Determination of specific
surface area of solids by means of adsorption data. Journal
o f Chemical Education, 64, 175-176.
Bujdfik, J. and Komadel, P. (1997) Interaction of methylene
blue with reduced charge montmorillonite. Journal o f Physical Chemistry B, 101, 9065-9068.
Carter, D.L., Heilman, M.D., and Gonz~iles, C.L. (1965) Ethylene glycol monoethyl ether for determining surface area
of silicate minerals. Soil Science, 100, 356-360.
Garfinkel-Shweky, D. and Yariv, S. (1997) The determination
of surface basicity of the oxygen planes of expanding clay
minerals by acridine orange. Journal o f Colloids and Interface Science, 188, 168-175.
Grauer, Z., Grauer, G.L., Avnir, D., and Yariv, S. (1987)
Metachromasy in clay minerals. Sorption of pyronin Y by
montmoriilonite and Laponite. Journal o f the Chemical Society Faraday Transactions L 83, 1685-1701.
Helmy, A.K., Ferreiro, E.A., and de Bussetti, S.G. (1999)
Surface area evaluation of montmorillonite. Journal o f Colloids and Interface Science, 210, 167-171.
173
Jacobs, K.I. and Schoonheydt, R.A. (1999) Spectroscopy of
methylene blue-smectite suspensions. Journal o f Colloids
and Interface Science, 220, 103-111.
Koopal, L.K. (1978) Influence of polymer adsorption from
electrical double layer measurements. The silver iodidepolyvinyl alcohol system. Ph.D. thesis, Agricultural University of Wageningen, The Netherlands.
Lyklema, J. (1995) Fundamentals o f interface and colloid
science. Volume: Solid-liquid interfaces. Academic Press,
London.
Margulies, L., Rozen, H., and Nir, S. (1988) Model for competitive adsorption of organic cations on clays. Clays and
Clay Minerals, 36, 270-276.
Potgieter, J.H. (1991) Adsorption of methylene blue on activated carbon. An experiment illustrating both the Langmuir
and Freundlich isotherms. Journal o f Chemical Education,
68, 349-350.
Rytwo, G., Nir, S., Margulies, L., Casal, B., Merino, J., RuizHitzky, E., and Serratosa, J.M. (1998) Adsorption of monovalent organic cations on sepiolite: Experimental results
and model calculations. Clays and Clay Minerals, 46, 340348.
Schulthess, C.E and Dey, D.K. (1996) Estimation of the
Langmnir constants using linear and nonlinear least squares
regression analyses, Soil Science Society o f the America
Journal 60, 433-442.
Spencer, W. and Sutter, J.R. (1979) Kinetics study of the
monomer-dimer equilibrium of methylene blue in aqueous
solution. Journal o f Physical Chemistry, 83, 1573-1576.
Van den Hul, H.J. and Lyklema, J. (1968) Determination of
specific surface areas of dispersed materials. Comparison
of the negative adsorption method with some other methods. Journal o f the American Chemical Society, 90, 30103015.
Velasco, M.I. and De Panli, C.P. (1993) Surface charge properties of an Haplustoll from Argentina. Geoderma, 59, 345351.
E-mail of corresponding author: depauli@fisquim.fcq.unc.
edu.ar
(Received 6 March 2000; accepted 4 October 2000; Ms.
432; A.E. William F. Jaynes)