Clay Minerals (1993) 28, 475--481
D E T E R M I N A T I O N OF T H E C A T I O N E X C H A N G E
C A P A C I T Y OF C L A Y S BY S U R F A C E T E N S I O N
MEASUREMENTS
G. B U R R A F A T O
AND F. M I A N O *
AG1P S.p.A., Drilling Fluids Laboratory, via Maritano 26, 20097 S. Donato Milanese and *ENIRICERCHE
S.p.A., via Maritano 26, 20097 S.Donato Milanese, Italy
(Received 10 August 1992; revised 11 January 1993)
ABSTRACT: The cation exchange capacity (CEC) of clay minerals has been determined by
titration of aqueous suspensionof clayswith hexadecylpyridiniumchloride, monitoredusingsurface
tension measurements.In order to make the method suitable for an accurate analysisof the CEC of
the clays in drilling fluids or in soils, some parameters affectingthe CEC determination (e.g. the
presence of carboxymethylcellulose,mixingtime and measurementdevice)have been investigated.
The determination of the CEC is an important test in the evaluation of the quality of
commercial bentonites. Moreover, CEC is an index of the dispersibility of the formation
shales which may affect the properties of drilling fluids and borehole stability (Gray &
Darley, 1980).
The CEC may be defined as the quantity of exchangeable cations expressed in
milliequivalents per 100 g of fully dehydrated mineral (Van Olphen, 1977) or more
accurately expressed in mEq per 100 g of ignited weight of clay (Newman, 1987). The
principle on which current methods are based is the replacement of the original cations in an
aqueous clay suspension. The chemical substance replacing the previous cations in the clay
is detectable by some analytical method. Among the known methods, the most relevant
are: the ammonium exchange method (Mackenzie, 1951); the colorimetric titration by
hexaminocobaltichloride (Bardon et al., 1983); the methylene blue test (API, 1990); and
the barium exchange methods (Hendershot & Duquette, 1986). Work by Palumbo & Zucca
(pers. comm.) involved comparison of the (Co(NH3)6)C13 and methylene blue methods.
The CEC of a series of bentonite clays was consistently greater when tested with the
hexaminocobaltichloride technique.
The proposed method is based on the strong affinity of hexadecylpyridinium chloride
(CPC), a cationic surface-active agent or surfactant, with the clay surface (Greenland &
Quirk, 1964). However the tendency of CPC to form a multi-layer on the clay surface is a
limiting aspect of the method. Carminati et al., (1990) proposed titration of CPC by
monitoring the residual charge potential of the clay with an Electrokinetic Sonic Analyser
(ESA) and taking the charge reversal value as the equivalent point of titration. The
adsorbing organic cation will replace every available cation on the surface of the clay before
an excess of it would remain in solution. Once a mono-layer is formed, the adsorption of a
second layer leaves an amount of free surfactant in equilibrium which is able to drop to the
lower value of the surface tension of water, which can be detected with high sensitivity and
reliability.
9 1993 The MineralogicalSociety
476
G. Burrafato and F. Miano
SURFACE TENSION
Surface tension is the result of the existence of internal pressure, a force which draws
molecules into a liquid and which is perpendicular to the surface. This pressure is caused by
the action of the molecular forces and the greater the polarity of a substance, the higher the
internal pressure. The unit of measurement of surface tension is mN/m and this force is
always tangential to the liquid surface.
For a single liquid, the value of surface tension, in the formation of a surface, is
established instantaneously (after approx. 0.001 s).
The surface tension at the liquid-gas or liquid-vapour interface is determined by the
methods of the capillary rise, by the weighing or counting of drops, by the maximum bubble
pressure, by the break-off of a ring from the surface and by several other methods.
Those substances which accumulate in an interface layer are called surface-active agents
or surfactants. There is a positive adsorption at the interface and the surfactant must possess
a surface tension less than that of the solvent. In this way the accumulation of the substance
at the interface results in an advantageous change of surface tension.
Surfactants are generally made up of two parts: a polar group (ionic or non-ionic) and a
non-polar hydrocarbon chain. The interaction between a cationic surfactant and the clay
particles, and the ability of the surfactant to modify the surface tension of a water solution,
may lead to a new method for CEC determination.
MATERIALS AND METHODS
Bentonites (commercial samples) and shales (from well cores or quarry samples) have been
used, as were hexadecylpyridinium chloride (CPC, Merck) and commercial sodium
carboxymethylcellulose (CMC).
The amounts of montmorillonite, kaolinite and illite in the samples were determined by
X-ray diffractometry using a Philips PW 1730 instrument.
Suspensions of the clay minerals were titrated with an aqueous solution of CPC (0-025 N)
and the surface tension measured on the whole suspension. For comparison, the surface
tension was also measured on the supernatant after centrifugation at 8000 rpm for 30 min,
with a Kruss K10 tensiometer, equipped with a platinum plate or with a platinum ring, at a
temperature of 20~
The experimental sequence was as follows:
(1)
(2)
(3)
(4)
Grind the clay or shale sample.
Sieve the sample through a 200 mesh screen.
Dry the sample in an oven at 90~ for 2 h.
Suspend the clay in deionized water at a concentration of 0-5% w/v (250 mg in 50 cm 3
of water).
(5) Titrate 50 cm 3 of the suspension with CPC solution at a concentration of 25
mEq/dm 3.
(6) Stir, after every addition, for 20 min.
(7) Measure the surface tension.
As an alternative, different additions of CPC solution can be made to several different
aliquots of suspension and every sample can be centrifuged before carrying out the surface
tension measurement on the supernatant.
477
C E C by surface tension measurements
RESULTS AND DISCUSSION
The change in the surface tension of water whilst adding CPC and, in the absence and
presence of 0.5% w/v bentonite A, respectively, is shown in Fig. 1. As shown by Carminati
et al. (1990), the high repartition rate of CPC on the clay surface results in a high and
constant surface tension value as long as the surfactant does not reach the saturation of the
clay surface. Above the CEC, further adsorption of surfactant may occur. In this region the
adsorbed surfactant forms a second layer around the clay, in equilibrium with a
concentration of free surfactant which tends to its critical micellar concentration.
Consequently, a drop in surface tension can be detected easily at the CEC value, the
adsorption of CPC exceeding twice the CEC notwithstanding (Greenland & Quirk, 1964).
The addition of CMC and other polymers to commercial bentonites (e.g. for drilling
fluids) is a very common practice used to obtain better rheological properties in aqueous
dispersion and, usually, quantities of up to 1-2 % are added to obtain this result. The effect
of such addition had been evaluated and Fig. 2 shows the curves related to the untreated
bentonite and to the sample with addition of I and 10% of CMC, respectively (based on the
bentonite content). The evaluation of the CMC addition has been performed on bentonite
B because, in bentonite A, some CMC is often added by the producer. As can be seen on
Fig. 2, the usual quantity of CMC added does not influence the determination, while large
amounts of CMC may. The problem could be solved by treating the bentonite with H202
before performing the determination.
The influence of the delay time (the time after the addition of the surfactant to the
dispersion before reading the value) on the measured surface tension can be evaluated from
the data presented in Fig. 3. The three curves illustrate surface tension measurements as a
function of delay time for a 0.5% suspension of bentonite A. The first one is related to an
75.0
Surface tension (mN/m)
70.01
\
65.0
\\\
60.0
\\
'\\
55.0
50.0
\\\\\
\\
45.0
30.0 "
0.0
[]
CPC
a d d e d to d i s t i l l e d water
0
CPC a d d e d to 0.5 % b e n t o n i t e A
J]
I
I
I
I
J
1.0
2.0
3.0
4.0
5.0
CPC added
6.0
(cma)
FI6.1. Surfacetension vs. CPC (25 mEq/dm3) added to distilled water and to a 0-5% w/v suspension
of bentonite A.
G. Burrafato and F. Miano
478
Surface tension (mN/m)
75.0
_
[]
70.0 C
65.0 j
60.0
55.0
50.0
45.0
4O.0
35.0
i
[]
Bentonite B
O
B e n t o n i t e B * 1.0 % C M C
A
30.0
0.0
B e n t o n i t e B + 10.0 % C M C
I
r
I
I
I
r
i
I
r
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
CPC added (cma)
Fl6.2. Influence of CMC addition on surface tension measurements. Bentonite B, 0-5% wtv.
Surface tension (mN/m)
70.0 [
U
0
6 5 . 0 ~-
60.0
55.0
50.0
A
45.0
~-
[]
cPc
added - 4.0 cm3 (90 % of the calculated CEC)
0
CPC
added 9 4.5 cm a(100 % of the calculated CEC)
CPC
added 9 6.0 cm 3 (130 % of the calculated CEC)
40.0 ~
0
A
5
L
L
L
L
L
10
15
20
25
30
35
40
Delay time (min)
FIG. 3, Surface tension vs. delay time on a 0.5% w/v suspension of bentonite A with different amounts
of CPC added.
CEC by surface tension measurements
479
addition of CPC of - 9 0 % of the calculated CEC, the second one is related to an addition of
- 1 0 0 % and the last one to an addition of - 1 3 0 % . In every case, a period of 20 min is
sufficient to reach the equilibrium exchange of the organic cation, even at the very critical
point at which the total surface is almost completely covered by a monolayer of CPC.
To avoid the effect of suspended particles on the surface tension measurements, the
suspension was centrifuged at -8000 r.p.m, for 30 min. In this case, different volumes of
CPC were added to different aliquots of suspension, and, after centrifugation, the surface
tension measurements were carried out on the supernatant.
In Fig. 4, two curves illustrate data, obtained while using the platinum plate, on a
bentonite A suspension both with and without centrifugation as described above. The CEC
values were calculated from the intersection between the straight lines obtained from the
best fit analyses. The calculated values, 42-1 and 45.1 mEq/100 g, are very close and the
difference is < 7 % .
Following the reported procedure, the relationship between the CPC added (cm 3) and
the CEC value (mEq/100 g) is:
CEC (mEq/100 g)
= CPCadde d
(cm 3) • 10.
The use of different measuring devices can be evaluated from analysis of Fig. 5. The two
curves presented are related to different sets of measurements performed with the same
tensiometer equipped with a platinum plate or ring. Although the curves are shifted
slightly, the calculated CEC values are 45-1 and 45.6 mEq/100 g respectively, showing a
difference of --1%, negligible considering the presumed difference in water contents.
A comparison between the CEC values obtained from the "methylene blue test" (MBT)
and the "surface tension method" for five different bentonites, two shales and a kaolinite
sample is reported in Table 1. The latter determinations were performed on the whole
Surface tension (mN/m)
80.0
\
70.0
[]
60.0
50,0
40.0
30,0
20,0
0.0
[]
plate, no c e n t r i f u g a t i o n
9
plate, centrifugation
r
1.0
__,
2.0
~
3.0
_
_
4.0
5.0
6.0
CPC added (cm 3)
FIG.4. CEC determination from surface tension measurements.Bentonite A, 0-5% w/v.
7.0
G. Burrafato and F. Miano
480
suspension, using the platinum plate and without centrifugation. All of the values were
o b t a i n e d as a v e r a g e s o f a t l e a s t t h r e e d e t e r m i n a t i o n s a n d in b o t h cases t h e s t a n d a r d
d e v i a t i o n is - - 4 % . T h e r e is r e l a t i v e l y g o o d a g r e e m e n t b e t w e e n t h e d a t a o b t a i n e d f r o m t h e
t w o d i f f e r e n t m e t h o d s . T a b l e 1 also i l l u s t r a t e s t h a t t h e b e h a v i o u r o f t h e b e n t o n i t e s a n d
s h a l e s is n o t l i n e a r l y r e l a t e d t o t h e i r m o n t m o r i l l o n i t e c o n t e n t . T h i s is p r o b a b l y d u e t o
d i f f e r e n c e s b e t w e e n c o m m e r c i a l s a m p l e s s u c h as d i f f e r e n t t r e a t m e n t s (e.g. i n o r g a n i c a n d / o r
organic activation ) or different mineralogical components that can influence the CEC
Surface tension (mN/m)
80.0
0
70.0
60.0'
50.0
\
\
40.0
30.0
20.0 0.0
I:
\
plate, centrifugation
ring, centrifugation
-'
1.0
I
L
L
I
2.0
3.0
4.0
5.0
_
[
6.0
CPC added (crn 3)
F~6.5. CEC determination from surface tension measurements. Bentonite A, 0.5% w/v.
TABLEl, Montmorillonite content and CEC (mEq/100 g) values measured
by the surface tension method and the methylene blue test (MBT) for
various bentonites and shales.
Sample
Wyoming (bent.)
BDH 1 (bent.)
Bentonite A
Bentonite B
Bentonite C
Bentonite D
Shale A 2
Shale B
Shale C
Warren (kaol.)
MBT
Surface
tension
-46 _+ 2
65 _+ 3
63 _+ 3
47 _+ 2
35 +_ 1
8 -+ 0-3
--
98 _+ 4
67 _+ 3
45 _+ 2
56 _+ 2
58 _+ 2
49 _+ 2
34 +_ 1
7 _+ 0.3
8 -+ 0-3
5 _+ 0.2
% montmorillonite
99
92
86
75
91
-35
-9
97% kaolinite
BDH Bentonite powder (technical) Prod. 26022. BDH Ltd, England.
2 Illite + kaolinite + chlorite = 28%.
7.0
C E C by surface tension measurements
481
determination. Nevertheless, the analytical setup has been developed principally for
evaluation of commercial samples of clays.
CONCLUSIONS
A new method for the determination of C E C of clays and shales has been proposed. The
method is based on the "titration" of a clay suspension with an appropriate surfactant, the
first surfactant excess being determined by surface tension measurements. The method can
easily be performed in the laboratory and requires only 250 mg of sample for each
determination.
The results are comparable with those obtained from the methylene blue test (MBT) and
the method can also be used for the C E C determination of shales with a very low
montmorillonite content.
The method did not appear to be influenced by the presence, in the clay samples, of the
common additives used commercially to obtain better viscosity yields, and the results show
a relatively good level of reproducibility.
ACKNOWLEDGMENTS
The experimentalwork has been carriedout under the project "Additivesfor DrillingFluidsOptimisation"financed
by AGIP SpA and the ENI Group. Dr. Borgarello (Eniricerche)is thanked for helpful discussion.
REFERENCES
API (1990)RecommendedPractice StandardProcedure for LaboratoryTestingDrillingFluids.API Recommended
Practice 13 I.
BARDONC. et al. (1983) Recommandationspour la d6terminationexp6rimentalede la capacit6d'6changede cations
des milieuxargileux, Revue de l'lnstitute Franqais du Petrole, Vol. 38, no. 5, Sept-Oct, 621~i26.
CARM1NATIS., CARNIANIC. • MIANOF. (1990)Surface modificationof montmorillonitein aqueousdispersionswith
hexadecylpyridiniumchloride. Coll. Surf., 48, 209-217.
GRAYG.R. & DARLEYH.C.H. (1980) Composition and Properties of Oil Well Drilling Fluids. Gulf Publishing
Company, Fourth Edition, 21.
GREENLANDD.J. & QUIRKJ.P. (1964) Determination of the total specificsurface areas of soils by adsorption of
cetylpyridiniumbromide. J. Soil Sci. 15, 178-191.
HENDERSnOTW.H. & DUQUE'L'rEM. (1986) A simple barium chloride method for determiningcation exchange
capacity and exchangeablecations. Soil Sci. Soc. Am. J. 50, 605~608.
MACKENZIER.C. (1951) A micro-methodfor determination of cation exchange capacity of clay. J. Coll. Sci. 6,
219-222.
NEWMANA.D.C. (1987) Chemistry of Clays and Clay Minerals, pp. 237-271. MineralogicalSocietyMonograph no.
6, Longman, Essex.
VANOLPHENH. (1977) An Introduction to Clay Colloid Chemistry, John Wiley& Sons, (2nd Ed.) New York.