Water-Induced Dispersion/Flocculation of Colloidal Suspensions
in Nonpolar Media
CHRISTOPHE
A. MALBREL
AND P. SOMASUNDARAN
I
Langmuir Centerfor Colloids and Interfaces,Henry Krumb School of Mines, Columbia University,
New York, New York J(}()27
ReceivedJanuary 24,1989; acceptedMarch 31, 1989
Colloidal dispersionsin apolar media are used in a variety of technological applications, in most of
which water is presentand plays a major role in determining the behavior of the dispersion.In this work
the effect of water on the conoidal stability of a suspensionof alumina in cyclohexanein the presence
of a commonly usedstabilizer, the Aerosol OT, is investigated.A successionof flocculatedand dispersed
stateswas observedas the amount of water added to the suspension'Ya5increased.Basedon an analogy
between the adsorption of water in this system and the adsorption of a gas on a solid substrate, a
methodology is developed to help predict the suspensionbehavior in apolar media in the presenceof
water. e 1989
Academic
PI-. Ioc
INTRODUCfION
Stability of colloidal particles in liquids of
low dielectric constantis a subjectof increasing
interest, due to its widening range of technological applications. The processing of high
performance ceramics and magnetic tape ( I )
and the manufacturing of certain paints and
inks (2) require, at one point or another in
the process,the control of the suspensionstability. Dispersion of fine coal particles in a
nonpolar phase has also been considered for
coal cleaning (3). However, whether a stable
suspensionor a rapid flocculation for efficient
solid / liquid separation is desired, the mechanisms governing stabilization in nonpolar
media are poorly understood, partly due to
the lack of data obtained under carefully controlled experimental conditions.
In most of the applications mentioned
above, water is present in the dispersion. It
may be introduced into the systemasadsorbeQ
water on the solid particles or as water of hydration of the chemicalsusedor it can be present as a separateliquid phase. Some studies
have reported the effectsof water on the sus1
To whom all correspondenceshould be addressed.
pension stability ( 4-6 ) and the role it plays in
determining suspensionstabilities. Depending
on the nature of the colloid and the amount
of water present in the dispersion, water can
either flocculate a suspensionor help stabilize
it. The objective of this work is to investigate
systematicallythe effectof water on a colloidal
suspensionof alumina in cyclohexane stabilized by an anionic surfactant, Aerosol OT
(AOT). A model describing the behavior of
suspensionsin nonpolar media in the presence
of water is formulated based on an analogy
betweenthe adsorption of water from micellar
solution and vapor adsorption.
EXPERIMENTAL
SECTION
Materials
The alumina used in the presentstudy was
purchased from Union Carbide Corporation
as Linde Alumina Polishing Powder Type A.
X-ray diffraction and chemical analysisof the
powder show the mineral to be a we" crystallized corundum (alumina type a) of high purity (>99% AI2O3)' Morphologically, the
powder is constituted of p.m-sizeaggregates
composed of smaller particles (between 200
and 500 nm). Prior to the suspensionprepa-
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SUSPENSIONS IN NONPOLAR MEDIA
ration, the powder was subjected to a short
grinding step (5 min) in a mortar to break up
these aggregatesand increase the concentration of primary particles in the suspension.
Nitrogen adsorption gave a specific surface
area of 13.4 m2/g for this powder.
Cyclohexane, obtained from Fisher Scientific Co., was of certified ACS grade. The solvent was stored on Molecular Sieve 4A to
avoid contamination by water. The water used
wastriply distilled, of 10-6 mhos conductivity.
The surfactant Aerosol OT (sodium bis-(2ethylhexyl)-sulfosuccinate), obtained from
Fisher Scientific Co., was purified following a
procedure describedin the literature (7 ). The
dry surfactant was stored in a desiccatorusing
P20Sas desiccant. Prior to its use,the surfactant was vacuum desiccated overnight and
dissolvedin cyclohexaneand the solution was
stored for a week on dehydrated Molecular
Sieve4A to remove traces of water.
405
titrated against hexadecyltrimethyl ammonium bromide in chloroform with dimidium
bromide disulfine blue as the end-point indicator (8). Water concentration was measured
using a Karl FISher Coulometer. When required. the solubilization of water was determined by turbidity measurements,a technique
sensitiveenough to distinguish micellar solutions from W /0 emulsions.
RESULTS AND DISCUSSION
In order to investigatethe effect of water on
the colloidal stability, all experiments were
performed at constant surfactant concentrations. Figure I showsthe adsorption isotherm
of AOT on alumina in cyclohexane.Areas of
60 and 80 A 2 for AOT moleculesadsorbedat
the water/xylene and water/isooctane interfaces,respectively, have been reported in the
literature ( 4, 9). Using thesevalues,the surface
coverages corresponding to the adsorption
isotherm plateau (3.2 X 10-5 mole/g adsorpSample Preparation and
tion density) were estimated to be 1.14 and
Experimental Procedure
0.86. It is hencereasonableto assumethat the
plateau reached by the surfactant adsorption
The samples were prepared using the folon alumina correspondsto a monolayer covlowing procedure: ( 1) desiccation of the aluerage.All subsequentexperiments conducted
mina powder at 200°C for 6 h followed by a
with water were performed under the above
cooling period (2 h) at 25°C in a vacuum desconditions. No significant changein surfactant
iccator; (2) preparation of the solution byadadsorption density was observed when water
dition of a known amount of water to a cywas added to the system.
clohexanesolution of surfactant; (3) addition
of 15 ml of the solution to 1 g of alumina in
a graduatedtest tube; and ( 4 ) conditioning of
the sample (tumbling) at room temperature
for 24 h prior to settling experiments.
The stability of the dispersionwasmeasured
by optically monitoring the settling of the upper interface of the suspensionthat is allowed
to settle in a 15 cm3 graduated cylinder of 1
cm diameter. The suspensionsettling rate was
obtained by calculating the initial slope of the
plot of the upper interfaceposition versustime.
After a 24-h sedimentation period, the suspension was centrifuged and both the AOT
and water residual concentrations were measured. The AOT was analyzedby a two-phase
titration technique in which the surfactantwas
406
MALBREL AND SOMASUNDARAN
In Fig. 2a, the stability of the suspensionis
reported in terms of settling rate as a function
of residualwaterconcentrationat two different
surfactant concentrations (8.5 and 26 X 10-3
mole/liter). Figure 2b showsthe corresponding water adsorption isotherms. As the water
concentration in the system is increased,the
suspensionsexhibit a successionof flocculated
and stable states.At low water concentration,
the settling occurs rapidly. It can be seenthat
the onset of stabilization corresponds to a
sharp increase in the amount of water adsorbed on alumina, suggestingthat the ad-
10
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FIG. 2. (a) Effecl of water addition on the suspension
settling rate. Increase in surfactant concentration shifts
the suspensiondestabilization towards higher water concentrations.(b) Adsorption of water on alumina from the
AOT /cyclohexane solution. Increasein surfactant concentration expandsthe domain of water concentration in
which adSOrptiontakesplace.8: AOT = 8.5 X 10-3 mole/
liter; l:.; AOT = 26 X 10-3mole/liter.
Jouma/ td"ColJoidaJId,~
Science,Vol. 133, No. 2, Dc.1emIJe.1989
sorption of water plays a critical role in the
stabilization phenomenon. It is generally acceptedthat the stabilization by Aerosol OT of
oxide particles is due to the development of
electrostatic repulsive forcesbetweenparticles
when the dissociation of adsorbedsurfactants
and the subsequentdesorption of the anions
lead to the formation of chargesat the solidi
liquid interface ( 10). The results presented
here show that, even though a monolayer of
the surfactant is adsorbed at the interface in
all cases,stabilization takes place only when
trace amounts of water are added to the suspension.This observationis in agreementwith
the conclusion of McGown, ParfItt, and Willis
( 4) on the role played by water in chargedevelopment at the solidi AOT adsorbed layer
interface.Thus, it can be concludedthat water
plays a major role in the dissociation of the
surfactant ions.
At higher water concentrations, Fig. 2a
shows a sharp increasein the suspensionsettling rate at both surfactant concentrations
studied. But as the surfactant concentration
increases,so does the water concentration at
which the flocculation takes place. In these
rangesof water concentrations,no relationship
can be clearly establisheda priori betweenthe
suspensionstability and the water adsorption.
The shapeof the water adsorption isotherm
is similar to that of the one obtained for vapor
adsorption on solid substrates.An analogybetween the two adsorption phenomena can be
used to rationalize the water adsorption data.
Vapor adsorption on a solid substrate is a
function of the vapor pressure,P, in contact
with the solid. An increasein the vapor pressure increasesthe adsorption gradually until
the vapor pressurereachesvaluesat which the
vapor starts to condenseon the solid, leading
to a sharp increase in the amount of vapor
adsorbed. By analogy, water adsorption from
the AOT Icyclohexane solution is controlled
by the water concentration in the solution,
[H2O]. The adsorption of a gas is ultimately
limited by its saturating vapor pressure,Po.
Similarly, it is possibleto interpret the water
adsorption as being controlled by a critical
407
SUSPENSIONS IN NONPOLAR MEDIA
waterconcentrationat which a phasechange
in solution is observed. In the AOT /cyclo-
hexanesolution, a phasechangeis observed
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as the water concentration is increased at ..
which the solution goesfrom a clear,stable ~
micellar solution to a turbid. unstable W /0
emulsion. This critical concentration is referredto asthe critical emulsion concentration
(C.E.C.)
and
is domains
showninofFig.
3 asbehavior.
the limit
betweenthe
two
solution
Results presentedin this figure were obtained
at various surfactant concentrations by monitoring the sharp changein the turbidity of the
solution as water is gradually added to it. Gas
adsorption variesasa function of temperature:
as the temperature is increased,so is the saturating pressureof the gas,Po. Classically,gas
adsorption isothermsare plotted asa function
of relative vapor pressureof the gas,P/ Po, to
compensatethe effect of temperature on the
adsorption. Assuming that the surfactant concentration in solution is playing the role of
temperature in the gas adsorption, an analogous
normalization
of the
water in
adsorption
isotherm
is possibleand
is shown
Fig.4, in
which the data are plotted as a function
of
normalized
water
concentration,
[H2O]/
C.E.C. The good superimposition
of the two
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FIG. 4. (a) Normalized suspensionsettling rate data
showing the superimposition of the two data setsshown
in Figure 2a. (b) Normalized water adsorption data. The
two isotherms are also wen superimposedby this normaliZAtionprocedure.(8: AOT = 8.5 X lO andC.E.C.
= lSO x lO-3 mole/liter; ~ AOT = 26 X lO-3 and C.E.C.
= 540 X lO-3 mole/liter.
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CONCENTRATION
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FIG. 3. AOT IWater/Cyclohexane phase diagram
showing the change of critical emulsion concentration
(C.E.C.) as a function of surfactant concentration. The
points reported on the diagram representthe concentrations at which the turbidity measurementswereperformed
(.: optically clear; +: turbid).
water adsorption isotherms(Fig. 4b) indicates
that the C.E.C. is indeed a controlling parameter of the adsorption of water on the solid. It
justifies the useof the normalization procedure
for interpreting the settling rate data. Figure
4a shows the results of the normalization of
the two setsof settling rate data. Again the two
curves coincide, suggestingthat the flocculation phenomenon is also controlled by the
C.E.C. of the solution. However, from the figure, it can be seenthat the flocculation is not
directly due to the water condensing on the
mineral surface since water condensation
~
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Vol I)], No. 2, DecembeF
1989
408
MALBREL AND SOMASUNDARAN
starts above (H2OJ/C.E.C. = 0.70, whereas
the suspensionflocculation occursbetween0.2
and 0.4.
Two mechanismshave been put forward in
the literature to explain the suspensionflocculation at high water concentrations. McGown et al. postulated a decreasein the magnitude of the electrostatic repulsive forces between particles as the amount of adsorbed
water increases(4). On the other hand, Kandori et al. proposeda capillary bridging mechanism due to a layer of water "binding" the
particles together (6). The results presented
here do not help to choose between the two
mechanismsproposed.However, the normalization procedure proposed is thought to provide a meansto compare resultsobtained under different conditions, which is necessaryto
understand the flocculation phenomenon at
high water concentrations.
CONCLUSIONS
A systematic investigation of the effect of
water on the colloidal stability in apolar media
has demonstrated the critical role played by
water in stabilization phenomena.
Addition of trace amounts of water hasbeen
shown to control stabilization when an ionic
surfactant such as Aerosol OT is used as stabilizer. At high water concentrations, flocculation wasobservedand it wasestablishedthat
its onset varies with the surfactant concentration in solution. An increase in surfactant
concentration in solution expandsthe domain
of stability of the suspension.An analogywith
gasadsorption was found to be useful for describing the adsorption of water on alumina
in the presenceof AOT. This analogywasalso
used to interpret the stability data and to develop a model for the suspensionbehavior.
The effect of surfactant concentration on
the stability suggeststhat the suspensionbehavior is controlled by the solvation power of
the solution for water. When the surfactant
concentration is increased,the amount of wa-
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ter that can be dissolved in the micellar s0lution increases.The water concentration at
which water "condensation" takes place on
the surface of the colloid is shifted toward
higher water concentration and so is the water
concentration at which flocculation takes
place. The observed destabilization may be
due to a decreasein the magnitude of the electrostatic repulsive forcesbetweenthe particles
or to a capillary bridging phenomenon. The
normalization procedureproposedin this note
allows the comparison of data obtained under
different experimental conditions and should
therefore be useful for the determination of
the mechanism controlling the suspension
stability in the presenceof the water.
ACKNOWLEDGMENTS
The authors acknowledgethe financial support of the
NSF (MSM-86-17 183andCBT-86-15524 »and the New
Yorlc Mining and Mineral ResourcesResearchInstitute.
REFERENCES
I. "Interfacial Phenomena in the New and Emerging
TecIlnologjes,"(Proceedingsof the NSF Workshop
held at the Univ~ty of Colorado, Boulder, Colorado, May 1986)(W. B. Krantz and D. T. Wasan,
Eds.), HITEX PubI., 1987.
2. McKay, R. B., in "Interfacial Phenomenain Apolar
Media" (M. F. Eicke and G. D. Parfitt, Eds.), Surfactant Science Series, Vol. 21, p. 361, Dekker,
New York, 1987.
3. Capes,C. E., and Gernlain, R. J., in "PhysicalOeaning
ofCoaI" (Y. A. Liu, Ed.), Dekker, New York, p.
293 (1982).
4. McGown, D. N. L., Parfitt, G. D., and Willis, E., J.
Colloid Sci. 20,650 (1967).
5. Kitahara, A., Karasawa,S., and Yamada, H., J. Colloid Interface Sci. 2S,490 (1967).
6. Kandori, K., ~ma,
A., Kon-no, K., and Kitahara,
A., Bull. Chern. Soc. JapanS?, 1777(1984).
7. Kitahara, A., J. Phys. Chern. 69, 2788 (1965).
8. Reid, V. W., Longman, G. F., and Heinherth, E.,
TensideS, 90 (1968).
9. Maitra, A., and Patanjali, P. K., in "Surfactants in
Solution" (K. L. Mittai and P. BothoreL Eds.),
Vol. 5, p. 581, Plenum, New York, 1986.
10. Novotny, V., Colloids Surfaces2, 373 (1981).