Clay Minerals (1991) 26, 19-32
ORGANO-BENTONITES WITH QUATERNARY
ALKYLAMMONIUM IONS
H. FAVRE
AND G . L A G A L Y
Institut fiir anorganische Chemic der Universitiit Kiel, OlshausenstraJ3e40, D-2300 Kiel, Germany
(Received 5 March 1990; revised 28 May 1990)
A B S T R A C T: Three bentonites, from Bavaria, Wyoming and Brazil, were separated into various
fractions. The layer charge was determined by alkylammonium ion exchange and increases with
particle size from 0.25 Eq/(Si,A1)4Ol0 (<0.06 gm) to 0-28 Eq/(Si,Al)4010 (l-10 #m). The charge
density corresponds to an interlayer cation density of 0-75-0.80 mEq/g silicate. Total amounts of
0.90-1-0 mEq of different surfactant cations (dimethyl dioctadecylammonium, trimethyl tetradecylammonium, alkylammoniumions) are bound per gram silicate. The difference between the total and
the interlayer amount of surfactant ions decreases with increasing particle size. The amounts
exceeding the interlayer CEC are bound at the edges. Tetramethylammonium (TM) ions restrict the
interlayer adsorption of long-chain quaternary alkylammonium ions such as trimethyl tetradecylammonium (TMTD) ions, and only monolayers of flat-lying surfactants are formed. A ratio of
TMTD and TM is attained which leads to densely packed monolayers of organic ions. The collapsing
effect is smaller for tetraethylammonium ions so that considerable amounts of TMTD ions are
adsorbed in bilayers. When bentonites are reacted with quaternary alkylammoniumions of technical
quality some selectivityis observed according to particle size and layer charge. Smaller particles with
lower charge density preferentially bind the longer chain compounds, whereas large particles with
higher charge density select smaller sized surfactants.
Organophilic bentonites are used in a wide range of practical applications (Jones, 1983;
Lagaly & Fahn, 1983). F o r practical and industrial uses quaternary a l k y l a m m o n i u m ions are
preferred to primary a l k y l a m m o n i u m ions because effects due to hydrolysis
(alkylammonium/alkylamine equilibrium) are absent, and desorption of free alkylamine is
strongly reduced. A further advantage is that the large amount of organic material
(30--40 w t % ) reduces the density of the dispersed particles.
Organo-bentonites are excellent gelling agents (Vold & Phansalkar, 1962; Granquist &
M c A t e e , 1963). High gel strengths in non-polar solvents require addition of small amounts
of a polar additive (Granquist & M c A t e e , 1963; Jones, 1983). Diluted dispersions of
organophilic bentonites in benzene/alcohol show maxima and minima of sedimentation
rates and sediment volumes with increasing alcohol content (Sz~int6 & Veres, 1963; Sz~int6
et al., 1972). A d s o r p t i o n studies from binary solutions (for instance hydrocarbons/alcohol)
by D6k~iny et al. (1986) have contributed to a b e t t e r understanding of the effect of polar
additives.
A further advantage of bentonites saturated with quaternary a l k y l a m m o n i u m ions is the
high adsorption capacity toward organic compounds. Even from aqueous solutions,
bentonites adsorb various organic c o m p o u n d s including phenols, amines and acids (Street
& White, 1963; Lee et al., 1989). The use of bentonites with quaternary a l k y l a m m o n i u m
ions as possible adsorbents for gas-solid c h r o m a t o g r a p h y has b e e n tested (Taramasso &
9 1991 The Mineralogical Society
20
H. Favre and G. Lagaly
Veniale, 1969; Taramasso, 1971; Slabaugh & Vasofsky, 1975; Vasofsky & Slabaugh, 1976;
McAtee & Robbins, 1980; Stul & Uytterhoeven, 1983).
In industrial production, bentonites are treated with quaternary alkylammonium salts at
low water contents (paste-like materials) or in more diluted dispersions. Fine products are
produced from diluted dispersions of Na-bentonite or soda-activated bentonite. The
amount of alkylammonium salt added corresponds to the experimental cation exchange
capacity (CEC) or thereabouts (80-100 mEq/100 g). Most of the alkylammonium ions are
bound by exchanging the cations in the interlayer space and at the edges. Some amounts of
alkylammonium salt are also retained, and these can be of great influence on the properties
of the gels and dispersions. (This will be discussed in a subsequent paper.)
It is important to note that quaternary alkylammonium salts of technical quality often
consist of a mixture of alkylammonium derivatives with different alkyl chain lengths and
different degrees of quaternization. The possibility must be considered that the
montmorillonite particles react preferentially with certain components of this mixture
whereas other components are excluded from the interlayer space.
Whereas primary alkylammonium ions are strong agents which successfully compete with
other cations for exchange positions, reaction of quaternary alkylammonium ions with clay
minerals is more delicate. The reactivity decreases with increasing layer charge. More
highly charged vermiculites do not exchange quaternary alkylammonium ions for interlayer
cations. Another observation (McAtee, 1962, 1963) is that cations displace quaternary
alkylammonium ions more easily from the interlayer space than primary alkylammonium
ions which are held very tightly between the layers. A further consequence concerns the
competition between short-chain and long-chain quaternary alkylammonium ions. It is to
be expected that the long-chain alkylammonium ions are preferentially adsorbed when a
mixture of short-chain and long-chain quaternary alkylammonium ions is reacted with
montmorillonite as occurs for primary alkylammonium ions. However, quite different
observations can be made.
MATERIALS AND METHODS
Bentonites
Wyoming bentonite (M 40, Greenbond) was supplied by Si~d-Chemie AG, Germany, as
was sample M 39, a bentonite from Bavaria (Niedersch6nbuch). Bentonite "25 de Maio" is
a Brazilian bentonite, supplied by Bentonit Uniao Nordeste SA, Brazil.
The bentonites were purified by removing organic materials by oxidation with H202, and
iron oxides by citrate-dithionite extraction. Fractions <2/~m were prepared by sedimentation in the centrifugal field (details: Samii & Lagaly, 1987).
The particle-size distribution was derived from the sedimentation velocity in the
gravitational and centrifugal field (Tributh & Lagaly, 1986).
Alkylarnmonium salts
The technical dimethyl dioctadecylammonium chloride (DMDO) as used in the
production of organo-bentonite (Siid-Chemie AG) had a chain length composition of 1.5%
Cl4, 0.8% C15, 27-5% C16, 2% C17 , 67.0% Cls and 1.2% C20.
Organo-bentonites with quaternary alkylammonium ions
21
Trimethyl tetradecylammonium (TMTD) bromide, tetramethylammonium (TM) chloride, and tetraethylammonium (TE) chloride (purum quality) were obtained from Hoechst
A G and Merck AG, Germany.
DMDO derivatives
Aqueous solutions of the technical D M D O were added to the dispersions of various
fractions of Na-montmoriUonite. The salt concentration was --0.1 mol/1, and the quantity of
D M D O was 1.5 mEq/g bentonite (calculated for bentonite dried at 110~
After 72 h at
70~ the solution was removed, and the DMDO-montmorillonite was washed with ethanol/
water, re-dispersed in a fresh D M D O salt solution, and allowed to react again for 72 h at
70~ The samples were washed several times with ethanol/water (60 vol% ethanol) and
dialysed at 70~ until the C/N content was virtually constant. The organo-montmorillonite
was dried at 80~ milled, sieved to <200 mesh, and dried once more at 70~ in vacuum
(<0.1 Pa) for 24 h.
Competitive cation exchange
The dispersions of Na-montmorillonites were reacted with TMTD chloride and TM or
TE ammonium bromide in the following quantities per g bentonite: 4 mEq TMTD; 1 mEq
TMTD + 3 mEq TM; 2 mEq TMTD + 2 mEq TM; 3 mEq TMTD + 1 mEq TM; 4 mEq
TM; 2.5 mEq TMTD + 2.5 mEq TE; 5 mEq TE. The salt concentration was again 0.1
mol/1, and the period of reaction was 72 h at 70~ The samples were washed several times
with ethanol/water (60 vol% ethanol) until the C/N ratio was nearly constant, and dried at
70~ in vacuum (<0-1 Pa).
Layer charge determination
The layer charge and charge distribution of the raw bentonites and the fractions were
measured by the alkylammonium method (Lagaly, 1981, 1982; Stul & Mortier, 1974; Malla
& Douglas, 1987a,b). Free alkylammonium salts and alkylamines were removed by careful
washing and dialysis in the same way as described for the D M D O derivatives.
Precision of the layer charge determination was _+0-005 Eq/(Si,A1)4010 and --2% for the
interlayer exchange capacity, Ci (eqn. 1).
Other analyses
The C, N-content of the samples was determined by combustion (Heraeus
Elementaranalysator CHN-O Rapid). The data reported are average values of at least three
parallel determinations. Deviations from the average value were in the range 2-5% which
gives a precision of 3-7% for Ct (eqn. 9).
The basal spacings dE of the washed and dried samples were measured by X-ray
diffraction using Debye-Scherrer cameras (diameter 114-6 mm, Cu-Ko~, Ni filter). For nonintegral 001 reflections, dE = d00l.
H. Favre and G. Lagaly
22
100
-
.-z
rr
1:7"
5O
I
0.01
I
I
I
J
I
I
0.06 0.2 0.6 2
6
particte size d/~um
I
20
F1G. 1. Particle-size distribution of b e n t o n i t e from B a v a r i a ( A ) , W y o m i n g ( A ) and Brazil ( 0 ) .
RESULTS AND DISCUSSION
Particle-size distribution
The bentonites from Wyoming and Bavaria consist of large amounts of fine particles
(Fig. 1), - 6 0 wt% being <0.06/~m. The size distributions are similar, but different from
the distribution for Brazilian bentonite which contains 35% of particles <0-06/an, and the
cumulative curve increases more steeply. The distribution has a maximum for particles
0.064).2 ~m. The different distribution below 0.6/~m illustrates the great importance of
particle size determinations at small particle sizes (Lagaly & Fahn, 1983; Lagaly et al.,
1985).
Layer charge and charge distribution
The three unfractionated montmorillonites have the same interlayer cation density
= 0.28 Eq/(Si,Al)4010 and similar charge distributions with a maximum at
0.23~1.26 Eq/(Si,Al)4Oa0. A charge density of ~ Eq/(Si,A1)4010 corresponds to an
interlayer exchange capacity, Ci (Lagaly, 1981):
Ci = ~/360 (Eq/g silicate)
(1)
As the average molecular mass of the (Si,A1)aOm unit of montmorillonites is 360 g, the
charge density ~ = 0.28 Eq/(Si,A1)4010 produces an interlayer exchange capacity of
0.78 mEq/g silicate.
Full details are shown for the Brazilian bentonite. The basal spacings of the
alkylammonium derivatives (no = 8-18) (Fig. 2) are highest for the 1-0-10/~m fraction and
decrease with decreasing particle size. The 0-06-0-2 #m fraction gives almost the same
spacings as the unfractionated bentonite. The large differences in the spacings between the
23
Organo-bentonites with quaternary alkylammonium ions
particles >0.06/~m and the smaller particles indicate dramatic changes in the charge
density. The steeper increase of the spacing with the alkyl chain length for particles
<0.06 ~tm reveals a narrower charge distribution.
The charge distribution of the different particle-size fractions was calculated under
consideration of a particle-size correction (Lagaly & Weiss, 1971; Stul & Mortier, 1974;
Lagaly, 1981). This correction is based on the assumption that the alkyl chains of the
alkylammonium ions lying very near the edges are squeezed out of the interlayer space. The
effective area per charge, Ae', is then somewhat larger than the equivalent area, Ae, which is
calculated for very large particles by
(2)
Ae = aobo/2~
A n alkylammonium ion (diameter 4.5 ,&) near the crystal edge may occupy only an area
of Ae/2. The number of chains in the edge region of a particle with a diameter d = 2r is
2n-r/4.5. The effective area per cation in the interlayer space is then
:rr2 - (2:rr/4-5) x Aft2
1 - 2Af14.5d
A~' =
:rre/Ae - 2Jrr/4.5
= A~I - 4Ae/4"5d
(3)
In the interlayer space the alkylammonium ions are close packed when
A c = Ae'
where A c is the area occupied by a flat-lying alkylammonium ion (Lagaly & Weiss, 1971):
A c = 1-27 x 4.5nc +
14 (/~k2)
(4)
20-
~15
ffl
El
~q
CJ
.121
10
J
L
I
10
I
I
i
I
1
i
I
15
i
I
J
20
nc
Flo. 2. Basal spacings of the alkylammonium derivatives (nc carbon atoms in the alkyl chain) of
bentonite from Brazil. Fractions (/~m): <0.02 (9 0.02~).06 (O); 0.064)-2 (A); 0.2-1.0 (1);
1.0-10 (~'); unfractionated ([2).
H. Favre and G. Lagaly
24
~
<0,02Fm
I
~=0249
10
50[
2 - 1pro
=0276
,
J
I- 1OHm
0_02-O.06pm
~:o2~s
i
i
I
I
i
1
I
unfractionated
0.06-0,2)um
~:0275
10
I
0,22
26
.30
.3&
.38
0'22
~/eq/(SiA[)4.Qo
.26
.30
3/.,
.38
Fro. 3. Charge distribution of various fractions of bentonite from Brazil.
With Ae' = A c we obtain from eqn. (3):
Ae = A c + 4-5d/4 - (Ac 2 + (4-5dI4)2) ~
and with eqn. (2)
aobo
-
2A~
aobo
1
2
A c + 4-5d/4 - ~ / A c 2 + (4-5d/4) 2
- -
x
(5)
The charge density and charge-density distribution is then obtained from A c , and A c is
calculated using eqn. (4) from the alkyl chain length nc at the mono/bilayer transition.
On the basis of the same correction, Stul & Mortier (1974) developed an iteration process
which leads to slightly higher charge densities for particles <0.04/~m. For larger particles,
both methods give identical results. A particle size correction is not necessary for particles
>0-2 #m.
The study of various fractions of several montmorillonites has shown that the particle-size
correction probably produces too high charge densities for particles <-0-02 #m. A simple
reason is that the particle-size fraction is given in Stokes equivalent diameters, but the real
diameter of the particle is certainly larger. The charge densities for particle-size fractions
<0-06 ~m were calculated on the basis of an apparent particle diameter of 0.04/*m. For
the fractions 0-06~)-2, 0.2-1 and 1-10/~m, values of d = 0.1, 0.6 and 2/~m were used.
Particles >0-06 ~m have a broad charge distribution (Fig. 3) with a maximum at
Organo-bentonites with quaternary alkylammoniurn ions
25
TABLE1. Layer charge and CEC of bentonite 25 de Maio, Brazil.
Particle
size ~m
<0.02
0.02-0-06
0.06-0.2
0.2-I
1-10
Unfractionated
~
Eq(Si,A1)4010
Ae
A2
mEq/g
Ct
mEq/g
Ci/C~
0.2492
0.255
0.276
0.276
0.282
0-275
93
91
84
84
82
85
0.69
0.71
0.77
0.77
0.78
0.76
0.94
0.94
0.96
0.97
0.57
0-95
0-73
0.76
0.80
0.79
-0,80
GI
C,, ct in mEqtg silicate, Ct [rom the decy~ammoniumderivative.
2 Three decimal digits are given only to reveal the variation in layer charge more clearly.
0.23-0-25 Eq/(Si,A1)4010 and an almost even participation of charge densities between 0-25
and 0.35 Eq/(Si,Al)4Ol0. The very narrow charge distribution of particles <0.06/~m
produces the impression that these particles are of different origin or have been p r o d u c e d
by different alteration processes.
The total amount of the a l k y l a m m o n i u m ions b o u n d is calculated from the nitrogen (cN)
or the carbon content (cc) when a d s o r b e d alkylammonium salt is r e m o v e d by washing. The
quantities cN and cr are usually o b t a i n e d in w t % . The total exchange capacity, Ct, is then
Ct =
CN/14
100 - M • CN/14
(Eq/g silicate)
(6)
when Ct is related to the silicate skeleton (amount of inorganic material). The molecular
mass of the a l k y l a m m o n i u m ion is M, the total a m o u n t of organic material is M x cN/14, and
100 - M x CN/14 is the amount of inorganic material (silicate).
The corresponding expression from the carbon content, co, is
Ct =
cc/12nc
(Eq/g silicate)
100 - M x cc/12nc
(7)
The total exchange capacity from the N and C content of the a l k y l a m m o n i u m derivatives
varies between 0-94 and 0.97 mEq/g silicate (Table 1). T h e interlayer cation density
increases with the particle size from Ci = 0.69 to 0.78 mEq/g, and the ratio Ci/Ct from 0.73 to
0.80. Thus, 20% of the cations are held at the crystal edges of the particles >0-06/~m.
The largest fraction contains high amounts of admixed minerals and materials; the total
exchange capacity, Ct, is 0.57 mEq/g. F o r the pure fraction, Ct is assumed to be
--0.97 mEq/g so that the montmorillonite content of the largest fraction is - 5 9 % .
DMDO derivatives
The total a m o u n t of the D M D O ions b o u n d per gram silicate, G , is calculated from the N
and C content (Table 2). A s the D M D O is of technical quality, the mean molecular mass,
3), is estimated from the C/N ratio:
H. Favre and G. Lagaly
26
TABLE2. DMDO derivativesof three bentonites.
Bentonite
Brazil
Wyoming
Bavaria
ci
Eq/(Si,Al)4010 mEq/g
0-28
0-28
0-28
Elementary
analysis
CN
CC
~(
0.78
0-78
0.78
0-906
0-896
0.880
28.72
28.34
28.47
Ct
mEq/g
Ci/Ct
d00a
]~
0.98
0.97
0.96
0.80
0-80
0-81
23-0
23-5
24.2
36.9
36.9
37.7
M ' ~ ( I x 14)+(Xx 14)+4
(8)
1 nitrogen + X methylene groups + 4 hydrogens of 4 methyl groups
A n a m o u n t of 100 g D M D O bentonite contains CN/14 mol D M D O (CN is the N content in
wt%) and an a m o u n t of ~r x CN/14 organic material (in g). The a m o u n t of organic material
per g silicate is then
C~ =
CN/14
(gq/g silicate)
(9)
100 - ~r • CN/14
The bentonites bind 0.96--0.98 m E q D M D O / g silicate (Table 2). Comparison with Ci
(from the alkylammonium exchange, eqn. 1, Table 1) indicates that 80% (81% for
Bavarian bentonite) of the total amounts of D M D O replace the exchangeable cations in the
interlayer space. This ratio is the same as for decylammonium ions (Table 1).
A n important observation is reported in Tables 2 and 3. The molar ratio C/N = Z of the
D M D O derivatives of the unfractionated Brazilian and Wyoming bentonite is almost the
same as in D M D O chloride (C/N = 36.7) but is enhanced for the Bavarian bentonite. The
bentonites reveal a tendency to enrich the longer chain compounds with decreasing particle
size (Table 3). For very small particles (<0.06 gm), the C/N ratio of 38 is distinctly larger
than in the D M D O chloride. Larger particles (>0.2 #m) have C/N ratios smaller than in the
TABLE3. Bindingof DMDO (technicalquality) by different fractionsof three bentonites,
Bentonite
Brazil
Wyoming3
Bavaria3
Particle
size/~m
~1
<0.06
0.0602
0.2-1-0
<0.06
0.06-0.2
0.2-1.0
<0.06
0-06-0-2
0.2-1-0
0.25
0.27
0-28
0-28
0-27
0-29
0.27
0.29
0-28
t In Eq/(Si,Al)4Oi0.
2 G, C, in mEq/g silicate.
3 Westfehling, 1987.
Elementary analysis
Cia
cN
Cc
0.70
0.77
0.78
0.77
0.74
0.81
0.74
0-79
0.79
0.905
0.915
0.895
0.873
0-882
0.849
0.917
0.887
0.893
29.48
28.48
26.98
28.29
27-18
25.89
30.06
27.70
27.20
d f~31
Z
32
Ct2
Ci/Ct
A
38.1
36-3
35.2
37.8
36.0
35-6
38-2
36.4
35-5
551
526
510
547
521
516
553
528
516
bOO
0.97
0-95
0-95
0.94
0.89
1.03
0.95
0.95
0.70
0.79
0-82
0-81
0-79
0-91
0-72
0-83
0-83
23.9
23-5
25.2
22.4
21.3
23.6
24.5
24.5
23-6
Organo-bentonites with quaternary alkylammonium ions
27
quaternary salt. The general conclusion is that very small particles enrich the longer chain
compounds, and larger particles the cations with smaller C/N ratios.
Preferential adsorption (longer chain compounds by small particles, compounds with
lower C/N ratio by large particles) is caused by the charge density variation and the particle
size. A larger equivalent area Ae (smaller ~) promotes binding of longer chain compounds,
in particular as the area of the dimethyl dioctadecyl ammonium ion ( - 2 3 0 A~2, McAtee,
1962) is much larger than the equivalent area (Table 1). The particle size effect results from
the enrichment of the longer chain compounds near the particle edges. As the long alkyl
chains are squeezed out of the interlayer space, larger amounts of longer chain compounds
are bound by smaller particles.
It is clear that the total amount of quaternary cations which decreases with increasing
particle size, is not decisive in determining the basal spacing which increases (Brazil,
Wyoming) or decreases (Bavaria) with particle size (Table 3). Rather, the spacing is
determined by the interlayer cation density Ci which increases with particle size.
A pseudotrimolecular arrangement (Lagaly, 1982) may be expected from the relation
between the area per molecule (230 &2 for the widely extended conformation) and the
equivalent areas of 82-93 A2/charge. A spacing of - 2 2 A is typical of this arrangement. The
larger spacings are indicative of more complicated conformations with gauche-bonds in the
alkyl chains. The number of possible conformations with one, two or even more gauche
bonds per alkyl chain is large so that details of the interlayer alkyl chain arrangements
cannot be deduced. It is evident from the data in Table 3 that small changes in the interlayer
cation density can induce conformations with distinctly higher spacings (d001 = 23.5 --->
25-2/~).
Competitive adsorption
An important observation is that the adsorbed amount of long-chain quaternary
alkylammonium ions can be strongly reduced in the presence of short-chain alkylammonium ions. This is exemplified for TMTD ions and TM or TE ions.
Various particle-size fractions of the three bentonites were reacted with mixtures of both
surfactants. The interlayer amounts of TMTD and TM or TE were calculated from the C
and N contents and the C/N ratio.
The N content, CN (in %), gives the total amount of interlayer surfactants
ct = 0.01 CN/14 (Eq/g organo-bentonite)
ct = cl + c2
(10)
The quantities cx and c2 are the amounts of TM or TE and TMTD ions in Eq/g organobentonite. The number of C atoms in the surfactants is no(l) and nc(2), X is the molar ratio
C/N, so that
C 1 • nc(1) + C2 X nc(2)
= Ca • nc(1) + ( G - el)no(2)
Ct X ~ =
cl -
G(X- nc(2))
nc(1) - nc(2)
The total amount of surfactants, Ct (in Eq/g silicate) is then
(11)
H. Favre and G. Lagaly
28
ct =
Ca + c2
( E q / g silicate)
(12)
1 - (c~M1 + c2M2)
w h e n M1 a n d M2 a r e t h e m o l e c u l a r m a s s e s o f s u r f a c t a n t 1 ( T M o r T E ) a n d s u r f a c t a n t 2
(TMTD).
T h e t o t a l a m o u n t o f T M T D a n d T M i o n s (G = 0 - 9 0 - 0 . 9 7 m E q / g ) is in t h e s a m e r a n g e as
f o r a l k y l a m m o n i u m i o n s ( T a b l e 1) o r D M D O i o n s ( T a b l e s 2,3). T h e a m o u n t s o f T E i o n s
(ct = 0 - 8 3 - 0 . 8 8 m E q / g ) a r e s o m e w h a t s m a l l e r b u t still g r e a t e r t h a n t h e ci v a l u e s .
The smallest particles which carry the smallest negative charge density, show a
p r e f e r e n t i a l a d s o r p t i o n o f T M T D i o n s o v e r T M i o n s (c2/G = 0 . 8 2 - 0 - 9 4 ) , w h e r e a s l a r g e r
p a r t i c l e s ( > 0 . 0 2 / ~ m ) b i n d a l m o s t e q u a l a m o u n t s o f b o t h s u r f a c t a n t s (c2/G ~ 0 . 5 - 0 - 6 ) . T h e
r a t i o c2/G t h e n c h a n g e s less w i t h t h e p a r t i c l e size, a n d is also n o t so d e p e n d e n t o n t h e r a t i o o f
t h e s u r f a c t a n t s in s o l u t i o n . A s t r o n g e r d e p e n d e n c e is o b s e r v e d f o r T E ions. S m a l l p a r t i c l e s
( < 0 - 0 6 ~ m ) a d s o r b e x c l u s i v e l y T M T D ions. F o r l a r g e r p a r t i c l e s t h e p r o p o r t i o n o f t h e longc h a i n c a t i o n s d e c r e a s e s f r o m 0.84 t o 0.43 ( T a b l e 4).
TABLE4. Competitive binding of tetramethylammonium ions (TM) or tetraethylammonium ions (TE) (= q ) and
trimethyl tetradecylammonium ions (TMTD) (= c2) by different fractions of bentonite from Brazil.
Fraction (/~m)
cN
<0.02
0.024).06
0.0643.2
0-24)-6
0.6-2
2-6
1.13
1.15
1.10
1.10
0-98
0-65
<0-02
0.024)-06
0.064).2
0-24).6
0.6--2
2-6
1-09
1.14
1.16
1.15
0.93
0.64
<0.02
0.024).06
0.064).2
0-2-0-6
0.6-2
2-6
1.08
1.11
1.16
1.12
0.98
0.64
<0-02
0.024).06
0-064).2
0-24)-6
0.6--2
2-6
1.05
1.07
1.10
1-11
0.99
0.79
1 In mEq/g organo-bentonite.
2 In mEq/g silicate.
Z
c,
cl 1
C21
3 mEq TM + 1 mEq TMTD/g bentonite
15-59
0.80
0-09
0.71
10-44
0.82
0.41
0.40
9.84
0.78
0.43
0-35
9.20
0.78
0-47
0.31
10-66
0.70
0.34
0-36
11.11
0-46
0.21
0.25
2 mEq TM + 2 mEq TMTD/g bentonite
14.72
0-77
0.14
0-64
12.30
0.81
0-29
0.52
12.01
0-82
0.32
0.51
1t-64
0.82
0-34
0-48
12.14
0.66
0.25
0.41
11.60
0.45
0-19
0.27
1 mEQ TM + 3 mEq TMTD/g bentonite
16-20
0.77
0-05
0.72
12-89
0.79
0-25
0.54
12.53
0.83
0-29
0.54
12.07
0.80
0.40
0-50
11.98
0.70
0.27
0.43
12.34
0.46
0-16
0.29
2-5 mEq TE + 2-5 mEq TMTD/g bentonite
17.00
0.75
0
0.75
17.14
0.76
0
0.76
15-37
0.79
0-13
0.66
14-98
0-79
0-18
0-62
13.24
0.71
0.30
0.41
11.86
0.56
0.32
0.24
Ct2
C2[Ct
dora (A)
0-99
0-95
0-90
0.89
0-80
0-50
0.89
0.49
0.45
0.40
0.52
0.55
14-8
13-8
13-7
13.6
13.7
14.1
0.94
0.96
0.98
0-97
0.76
0.50
0-82
0-64
0.62
0-59
0.63
0-58
14.0
13.8
13.9
14.1
13.7
15.2
0.96
0.95
0.99
0-95
0.81
0.50
0.94
0-68
0.66
0-62
0.61
0.64
15.2
13.9
13.9
14.2
13.7
15.5
0.93
0.96
0.97
0-97
0.83
0.62
1.00
1.00
0.84
0-78
0.58
0.43
15.9
16.0
15.9
16-0
15-6
13.9
Organo-bentonites with quaternary alkylammonium ions
29
Most basal spacings of the T M / T M T D derivatives are in the monolayer range
(13.6.14.0 A). The surfactants are arranged in monolayers, and the T M T D ions assume a
conformation as fiat as possible. The surfactants are closely packed when a distinct ratio of
TM and T M T D ions is attained. This ratio can be calculated in the following way. The
monolayer area per formula unit is aobo/2 = 23.25 A 2. There are, per formula unit, ~1 TM
ions (38 ~2/ion; Barrer et al., 1967) and ~2 T M T D ions (112 A2/ion) (~1 + ~2 = ~). The
monolayers are closely packed for
38~1 + 112(~ - ~1) = 23-25 (A2/unit)
(13)
Thus, ~1 can be calculated as a function of the layer charge ~. With increasing particle
size, ~1 increases strongly from 0.06 TM ions/unit for ~ = 0.25 to 0.11 ions/unit for
= 0.28 (Table 5). The ratio ~2/~ decreases from 0.74 to 0-61. The ratio TMFFMTD at the
external surfaces may be different from that in the interlayer space but data cannot be
obtained. If the differences are not too large, ~2/~ should be similar to c2/ct. This is in fact
observed when the concentration of T M T D in solution is sufficiently high (->2 mEq/g
bentonite).
Formation of dense monolayers of TM and T M T D ions in the interlayer space is
unexpected. The reason for the stability of monolayers is the strong tendency of the TM
ions to hold the silicate layers at close distances (13.6-13.9 A; Barrer & Kelsey, 1961;
Clementz & Mortland, 1974). The driving force is the keying of the methyl groups into the
six-membered rings of surface oxygen atoms (Barter & Kelsey, 1961). For very small
particles the number of these links is too small, and the T M T D ions can prise apart the
layers to some extent (dL > 13.8 ~ ) .
With mixtures of TE and T M T D ions, the montmorillonites react as expected. Small
particles (<0.06/~m) bind only T M T D ions. The ratio c2/ct decreases with increasing
particle size but is still large (20.8) for the 0.2-0-6/~m particles. Adsorption of the small
cations is preferred over the long-chain ions only for larger particles.
The fact that TE ions are not as strong competitors for interlayer sites as TM ions
probably follows from the reduced keying of the ethyl groups between the surface oxygen
atoms. Tetraethylammonium ions do not hold together the silicate layers as strongly as TM
ions and admit the binding of larger amounts of TMTD.
The spacings of --16 ~ are too small to be caused by conformations distinctly different
from a flat arrangement. The layer separation only allows insertion of a kink into the alkyl
TABLE5. Interlayer composition of close-packed monolayers of
surfactant cations. Number of cations per (Si,Al)4010 unit:
~1 tetramethylammonium ions; ~2 trimethyltetradecylammonium
ions; ~1 + ~2 = ~.
0.25
0-26
0.27
0.28
0.29
0-30
0-06
0.08
0.09
0.11
0-12
0.14
0-74
0-69
0.65
0.61
0-57
0.53
30
H. Favre and G. Lagaly
chain (Lagaly, 1976), which increases the spacing by 1-6-2 A . (The theoretical displacem e n t of the chain sections in the zig-zag plane by a kink is 1-8 A . ) It is m o r e likely that as a
consequence of charge heterogeneity, bilayers are interstratified with monolayers of
T E / T M T D ions. The short-chain and long-chain ions will not be distributed uniformly
between monolayers and bilayers. M o n o l a y e r s evolve in interlayer spaces with higher
charge densities and contain higher amounts of T E ions. M o r e lowly charged interlayers
enrich T M T D ions in bilayers. Consequently the most highly charged particles (2-6 ~m
fraction) contain surfactant monolayers, and the ratio Cz/Ct is low.
CONCLUSIONS
Bentonites develop a certain selectivity against distinct quaternary a l k y l a m m o n i u m ions
when reacted with mixtures of such surfactants. The extent of selectivity d e p e n d s on the
particle size. A s technical surfactants are generally mixtures of several c o m p o u n d s , two
important practical consequences arise. The real composition of the organo-bentonite
d e p e n d s not only on the layer charge, but also on the particle-size distribution. It also
depends on the technical quality of the surfactant used in industrial production. Thus,
organo-bentonites from different productions or manufacturers may differ considerably in
their properties.
Organo-bentonites are often p r o d u c e d by addition of the organic cations to paste-like
materials, and the product is dried without removal of excess salt by washing. W h e n the
surfactants are a d d e d to m o r e or less diluted bentonite dispersions, some of the excess salt is
r e m o v e d by the filtration steps but some of the salt m a y adhere to the dried material. Even if
these amounts are relatively small, they can strongly affect the properties of the
organo-bentonite.
Differences between similar organo-bentonites can be detected by the C and N contents
and C/N ratios. Even small differences can be indicative of widely differing properties. Very
sensitive are rheological properties (examples will be r e p o r t e d in a subsequent paper).
The use of different bentonites, surfactant mixtures, levels of adsorption and amounts of
excessive salts thus constitute the main confidential ways by which manufacturers produce
organo-clays with different properties.
ACKNOWLEDGMENT
The authors are indebted to the Alexander-von-Humboldt-Stiftungfor a fellowshipto Henry Favre during his stay
in Kiel. We also thank Mrs Gallay for an enormous number of C/N determinations.
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Organo-bentonites with quaternary a l k y l a m m o n i u m ions
31
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APPENDIX
List of symbols:
A~ = equivalent area, for montmorillonite Ae = 23-25/~ (/~2)
CN, CC = nitrogen and carbon content (in %)
cl, c2 = amounts of surfactant cations 1 and 2 (Eq/g organo-bentonite)
Ct
= C 1 + C2
Ci = amount of interlayer cations = interlayer exchange capacity (Eq/g silicate)
C, = amount of cations (internal and external) = total exchange capacity (Eq/g silicate)
H. Favre and G. Lagaly
32
dL=
M, M1, Me =
nc =
=
~1, ~2 =
basal spacing (.~)
molecular masses
number of carbon atoms in the organic cation
average charge density (layer charge) (Eq/(Si,A1)4Olo unit)
number of tetramethyl or tetraethylammonium ions ( e ) and trimethyl tetradecylammonium ions
(~2) per unit (Si,Al)4Olo, ~a + ~2 =
Z = molar ratio C/N