CoUoidsand Surfaces,21 (1986) 355-369
ElsevierSciencePublishersB.V., Amsterdam
355
Printed in The Netherlands
Effect of Temperature on the Interfacial
Properties of Silicates*
R. RAMACHANDRAN and P. SOMASUNDARAN
Henry Kumb Schoolof Mines, Columbia University, New York, NY 10027(U.S.A.)
(Received2 April 1986;acceptedin final form 14July 1986)
ABSTRACT
Electrochemicalpropertiesof silicate minerals governtheir behavior in processessuch as flocculation and enhancedoil recoverythat can occur at elevatedtemperatures.Knowledgeof these
propertiesas a function of temperaturecan be helpful in developingan understandingof the role
of theseinterfacial propertiesat non-ambient temperatures.The zetapotential of sodium kaolinite and quartz hasbeendeterminedasa function of temperaturein this work. Both systemsexhibited markedly different behavior at higher temperaturesand also exhibited significant hysteresis.
The results wereexaminedin terms of possibledissolution of the minerals and surfacereactions
at different temperatures.
INTRODUCTION
Electrokinetic properties of minerals exert a governing influence on many
interfacial processesinvolving them. However,very little information is available in the literature on suchinterfacial properties as zetapotential at elevated
temperatures, although several processessuch as flotation and enhancedoil
recoveryoccur at high temperatures.The solid-solution equilibria of a system
will also be significantly affected by changesin temperature and precipitation
of various speciesdue to temperature fluctuations can markedly affect the
interfacial potential. Measurementof zeta potential using electrophoresis,as
a function of temperature, is inhibited due to the elaborate modifications
required to avoid interference from convectional currents and non-uniform
expansionof the cells. The streaming potential technique is most easily adaptable for zetapotential measurementsat non-ambient temperature conditions.
High temperature experiments were successfullyperformed by Kulkarni and
Somasundaran[1] using the streaming potential technique. In the present
study this procedurewas followed to investigate the zeta-potential behavior of
.Dedicated to the memoryof ProfessorG.D. Parfitt.
0166-6622/86/$03.50
@ 1986 Elsevier Science Publishers B. V.
356
Na-kaolinite and quartz as a function of temperature under different pH
conditions.
MATE~
AND METHODS
Brazilian quartz (- 28 to + 65 mesh) was prepared by roll crushing and
sizing. The samplewas leachedwith concentrated nitric acid till it was free of
iron and subsequentlywashed free of nitrate ions by repeatedwashing with
triply distilled water. The washing processwas continued till the pH of the
supernatant was constant and about the natural pH of quartz, 5.4-5.8. The
sampleswere stored in polypropylene bottles at pH 2.
A well crystallised sample of Georgia kaolinite was obtained from the clay
repository at the University of Missouri. The Na-kaolinite was subjectedto
repeatedwashing with NaCI using the procedureof Hollander et aI. [2] until
homoionic Na-kaolinite was obtained. The surface area of this sample was
determined by the BET method to be 9.4 m2g-l. The clay samplewas of submicrometer size and it was not possibleto make a reproducible stable porous
plug with it becausethe fines escapedeasily through the platinum electrode
(80 mesh). When the clay was contained using a porous membrane it was
observedthat even under high streaming pressure(15 cm of Hg) , there was no
significant motion of solution through the compact clay plug. These problems
weresuccessfullyovercomeby pelletising the clay using the following procedure.
About 15 g of the clay sample was transferred to a cavity in a 2cm mould.
The clay was compactedusing a plunger and a hydraulic pressat a pressureof
7X 106kg m-2. The pelletswerethen sintered at 500°C in an induction furnace
for 12 h. The hardenedpellets were crushed in an agate mortar and the - 28
to +48 mesh size fraction collected. This sample was washed 10 times with
distilled water and then with triply distilled water until a constant pH of the
supernatantwas obtained. The washedclay was then usedin streaming poten:tial experiments.
In order to determine the effect of heat treatment on the kaolinite sample
EDXRF and electrophoresisstudies were conducted on the heat-treated and
untreatedclay. EDXRF showedno significant variations betweenthe two samples (Fig. 1). Electrophoresis data of the two samplesalso showedno difference. Obviously, the present heat treatment does not alter the surface
significantly to affect the zeta potential.
EXPERIMENTALPROCEDURE
The procedurefollowed in the present study wassimilar to that describedin
an earlier work [1]. The streaming potential cell was filled with the solution
of desired pH and ionic strength and the solid (quartz or Na-kaolinite) was
then introduced.The mineral waspackedbetweenthe two platinum electrodes
357
Fig. 1. EDXRF of heat-treatedand untreated clay.
358
into a compact porousplug and conditioned in the test solution by repeatedto
and fro streaming for 1 h. The cell was immersed in a water bath maintained
at the desired test temperature. The two platinum electrodeswere connected
to a very high impedanceelectrometerto measurethe streaming potential. The
pressurewas measuredusing a mercury manometer.
The zeta potential was calculated using the Helmholtz-Smoluchowski
equation:
Zetapotential = 4nElI1
PE
whereA,.u and f are respectivelythe specific conductivity, viscosity and dielectric constant of the aqueousmedia and E the streaming potential under the
driving pressureP. E, P and A were determined experimentally and values for
viscosity and dielectric constants at the test temperature were obtained from
the literature [ 15] .
Extensive reviews on the precautions to be taken in streaming potential
experimentshave beenpublished [3,4] and were followed in the experiments.
The meanof at least 10readingsof E/ P were usedin the calculation of the zeta
potential. The experiments at room temperature were performed both using
electrophoresis (crushed samples of the - 400 mesh size fraction from the
samplesused for streaming potential) and streaming potential. The results
were comparablewithin ::!:5%.
R&gULTS
The performance of the cell was examined initially. A linear relationship
was obtained betweendriving pressureand streaming potential. In all experiments the mean value of 10 to 15 readingsof the ratio of streaming potential
to driving pressurewas usedto calculate the zeta potential.
The results obtained in this study are comparedwith those of other workers,
Fig. 2. The literature data is characterisedby a wide amount of scatter. Variations in data could be the result of non-equilibrium conditions usedas well as
due to differencesin mineralogical and chemical composition of solids and the
supporting electrolyte concentration.
Temperatureeffectson quartz
The results obtained for the zeta potential as a function of temperature at
10,35, and 75°C are shown in Fig. 3. The zeta potential is found to become
more negativewith increasingtemperature.Most interestingly, it was observed
during thesetests that the zeta potential did not return to its original value at
250C when the systemwastaken through a temperature cycle.A detailed study
359
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Fig. 2. Zetapotential of (a) quartz and (b) Na-kaolinite as a function of pH: Comparisonof data.
~
360
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r
BRAZILIAN
10.'N
QUARTZ -48 +6'
NaNO,
010.C
a :5'.C
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pH
Fig. 3. Effect of temperatureon zetapotential of quartz.
of the hysteresiseffect was conducted at two temperatures, 25 and 75°C, at
0.001M ionic strength.
Figure 4 showsthe zeta potential of quartz as a function of pH at 25 and
750C. The hysteresiseffect is schematically illustrated in Figs 5-7.
It can be seenfrom Fig. 6 that the zetapotential increasedfrom - 46 to - 82
mV upon increasing the temperature from 25 to 750C. Upon decreasingthe
temperatureback to 25°C the zeta potential remained at a value of -74 mV.
Even after washingthe'samplewith triply distilled water and introducing fresh
NaNO3 solution the zeta potential stayed at -74 mV. Similar results were
obtained at pH 4.4, 8.1, and 9.7. At alkaline pH, elevation of the temperature
causedsignificant changesin the final pH values partly due to the change in
the pK of water and also due to the mineral solution equilibria at this pH.
Tewari and Mclean [7) observedsimilar pH changesat elevatedtemperatures
for the alumina-water system.The zetapotential at the natural pH of 5.6 after
the sample had undergonea temperature cycle at alkaline pH was always significantly higher in magnitude than what it was initially.
361
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-4B
+65 MESH
10-3N
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QUARTZ
BRAZILIAN
NoNO3
t
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A 75.C
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pH
Fig. 4. Zetapotential of quartz at 25 and 750C.
Temperature effects on clay
Figure 8 showsthe zeta potential of clay at 25 and at 75° C. It can be seen
that at 75°C Na-kaolinite is more positive at acidic pH and more negative at
alkaline pH. Figures 9 and 10 illustrate the effect resulting from taking the
samplethrough a temperature cycle. Na-kaolinite exhibits significant hysteresis at all pH values. The zeta potential increasesfrom + 11 mV at pH 4 to
+ 22 mV at the samepH after a temperature cycle.An increasein the negative
direction, from -10 mV at pH 6.7 to - 30 mV at pH 7, is observedat alkaline
pH.
In order to understand the temperature dependenceof the interfacial properties such as zeta potential of quartz and sodium kaolinite it is necessaryto
look at the mineral solution chemical equilibria of these systemsat different
temperatures.
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TEST TIME: 0, 1,5,17. 21,35,40 ",.
SOLUTION pH( I
. ZETA POTENTIAL OF WASHEDPLUG
AFTER TEMPERATURE CYCLE
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TEMPERATURE,.C
75
Fig. 5. Schematicrepresentationof temperaturecycleat acidic pH.
Silica
r-
The hydrolysis of the surfacespeciesof silica can be representedby the following reactions [8-11,13].
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Si
rb
+ HQ1~ ~~-001
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TIME {pHI
BRAZILIAN QUARTZ
-48 +65MESH
10-3N NoNO3
TEST TIME: O,4,23,27,44,48h"
SOLuTION pH( )
. ZETA POTENTIAL OF WASHED PLUG
AFTER TEMPERATURE CYCLE
2~
75
TEMPERATURE,.C
Fig. 6. Schematicrepre.'lentationof temperaturecycleat natural pH.
The main causefor the surfacechargeis the dissociationof the silanol groups
at the interface.
Reactions (1) to (4) represent a continuous increaseof surface hydroxyl
groupsto form a silicic acid surface.The number of ionisable sites per silicon
atom is thus higher for a silicic acid surface than for a fresh quartz surface.
The silicic acid surfaceis therefore expectedto possessa higher surfacecharge
density than a quartz surface.Generation of suchsilicic acid surfacesitescould
be a major reasonfor the observedeffect of temperature.
De Bruyn et al. [12] have representedthe temperature dependenceof the
solubility of crystalline quartz by the following equations:
SiO2+2 H2O = H.SiO.
log( H"SiO,,)
O.151-1162jT
(5)
(6)
364
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QUARTZ
MESH
lcr3N
NoNO3
TEST TIME: 0.1.5.15.19,29hrs
SOLUTION pH
( I
-
. ZETA POTENTIAL OF WASHED
PLUG AFTER TEMPERATURE CYCLE
t
(pHI
z~
."
TEMPERATURE,"C
Fig. 7. Schematicrepresentationof temperaturecycleat alkaline pH.
The reaction is independent of pH and H4SiO. formation is favored at higher
temperatures.
It has also been noted by De Bruyn et al. that H3SiO4- is the only major
ionic speciesin solution.
In alkaline solutions [14] the equilibrium for the dissolution of quartz is
written as:
H.SiO4
-9.8
= H+ + H3SiOi pK = 9.8
= -log(H4SiO4) + log(H+)
-9.8 + log(H.SiO.) + pH
(7)
+ log(H3SiOi
= log(HaSiOi)
(9)
As the pH is increasedif K is constant log (HaSiO. -) must increase.It is
365
pH
Fig. 8. Zeta potential of Na-kaolinite at 25 and 75°C.
alsoclear from Eqns (7) -( 9) that as the temperature is increasedH.SiO. concentration must increase.Hence both increasein temperature and pH favour
formation ofH3SiO.-.
Data in the literature [8-12] indicate the presenceof a highly disturbed
amorphouslayer on the surfaceof quartz leadingto abnormally high solubility.
Dissolution of the amorphous layer in combination with adsorption/precipitation of H3SiO.- can be another major reason for the observedtemperature
effects. Further evidence for this hypothesis was seen when quartz treated
ultrasonically for 12 h' (to removethe amorphouslayer) was usedto measure
the zeta potential as a function of temperature. Most interestingly, ultrasonically treated quartz did not show any significant effect of hysteresis.Also the
zeta potential of ultrasonicated quartz at room temperature was about - 60
mV, which is comparableto the value of untreated quartz at 250C after subjecting it to a temperature cycle.
Na-Kaolinite
The speciesdistribution diagram for Na-kaolinite is shown in Fig. 11.In the
acidic region it can be seenthat the activity of the A13+ speciesis very high.
366
TEMP. CYCLE AT ACIDIC pH
SODIUM KAOLINITE 1-28+65)
IONIC STRENGT H: 10-4 NaCa
TEST TIME: O,1,12,14,17hrl
SOLUTION pH - 1 )
.
ZETA POTENTIAL
OF WASHED PLUG
AFTER TEMP. CYCLE
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17 (6.3).
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A
A
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r
TIME
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00(6.8)
I
75
25
TEMPERATURE
Fig. 9. Schematicrepresentationof temperaturecycle at acidic pH.
Increasein temperature would enhancethe dissolution resulting in increased
amount of A13+speciesin solution. Zeta potential studies at this pH show the
mineral to be in fact more positively charged at higher temperatures. Dissolution followed by readsorption of Al3+ and Al ( 0 H) 2+ can causethe increase
in potential due to their high activity at this pH.
Redissolution of these species,after adsorption at high temperature, could
be kinetically controlled, thus causingthe hysteresiseffect.
At natural pH (-7) the important species are AI(OH)3' H4SiO4 and
H3SiO 4-. The net negativepotential on the surfaceis attributed to the adsorption of H3SiO4- which is the only charged speciesthat is active at this pH.
Increasein temperature causeda decreasein pH resulting in a less negative
potential owing possibly to the adsorption of the AI( OH)2+.
In the alkaline region (pH -9) the major speciesare H3SiO4- and Al( OH) 4and adsorptionof theseions causesthe mineral to be highly negativelycharged
Again, increasein temperature resulted in a decreasein pH and a lessnegative
zetapotential.
367
TEW. CYCLE AT ALK. pH
SOOIUM KAOLINITE (-Z8+651
I<»8C STRENGTH 10-4 NaCi
TEST TIME 0,4,10, Z1,33 hr.
c
TIME I~)
SOLUTION
-
pH
. ~
-15
>
e
... -20
4
I
~
i
Z
-25
~
oC
~ -30
N
-3S
00(671
v
v
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v
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V
21 (71 OC < < < ' ,
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.4n
«««<
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~ ~')
')~')
4(8.7)
25
TEMPERATURE. -C
75
Fig. 10.Schematicrepresentationof temperaturecycle at alkaline pH.
CONCWSIONS
The zeta potential of quartz and Na-kaolinite were measuredas a function
of temperature.The zetapotential of quartz increasedin magnitude as a function of temperature at all pH conditions. Interestingly, significant hysteresis
was observedand the zeta potential did not return to the original values at
room temperatureevenafter severalwashings.However,ultrasonically cleaned
quartz did not exhibit measurablehysteresis.Quartz has been known to possessa disturbed amorphouslayer with very high solubility [8-12] . Dissolution
of surfacesilicic acid followed by adsorption of H3SiO. speciesis proposedto
-
be the major causefor the temperature dependenceof zeta potential. Desorption of H3SiO.- can be kinetically controlled and this could leadto the observed
hysteresiseffects.
The zeta potential of the Na-kaolinite was markedly sensitive to temperature changesin the system.The zeta potential becamemore positive at acidic
pH and more negativeat alkaline pH with increasing temperature.
The zeta potential changesas a function of temperature and pH have been
correlatedwith the speciesdistribution diagrams.Al3+ and the Al ( 0 H) 2 + species that predominate in the acidic pH range cause the mineral to be more
positively charged in this pH range. Presenceof neutral speciesH.SiO4 and
368
pH
Fig. 11.Speciesdistribution diagram of Na-kaolinite [16]
AI( OH) alower the effect of the negatively chargedHaSiO4- in the neutral pH
range.In the alkaline pH region HaSiO.- is the major specieswhich contributes to the negative potential on the surface. Increase in temperature of the
systemcan enhancethe dissolution of the speciesand affect the readsorption
as well as precipitation of the relevant species,resulting in marked changesof
the zeta potential.
REFERENCES
1 R.D.KulkarniandP. Somasundaran,
J. ColloidInterfaceSci, 45 (1973)591.
2 A.F. Hollander, P. Somasundaranand C.C. Grytte, in P.H. Tewari (Ed), Adsorption from
AqueousSolutions, Plenum, New York, 1981,pp. 143-161.
3 B. BailandD.W. Fuerstenau,Miner. Sci. Eng., 5 (1973) 267-275.
4 Grinell Jonesand Lloyd A. Wood,J. Chern.Phys., 13 (1945) 3.
5 Philip B. Lorenz, Clays Clay Miner., 17 (1969) 223-251.
6 D.J.A. Williams and K.P. Williams, J. Colloid Interface Sci., 65 (1978) 79.
7 P.H. Tewari and A.W. Mclean, J. Colloid Interface Sci., 40 (1972) 267.
8 A.J. Beal and A.L. Godbert, Researchreport No. 115,Safety in Mines ResearchEstablishment, Sheffield, U.K., 1955.
9 R. Tregan, C.R, Acad. Sci., 241 (1955) 219.
10 O.S. Heavens,Acta Crystailogr.,6 (1953) 571.
11 J.A. WaddaIns,Research(London), 11 (1958) 370.
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12
13
14
15
16
17
18
P.L. de Bruyn et aI., J. Phys. Chern.,64 (1960) 1675.
K.R. Langeand R.W. Spencer,Environrnental Sci. Technol., 2 (1968) 212.
P.S. Roller and G.E. Erwin,J. Am. Chern.Soc.,62 (1940) 461.
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G.L. Zucker, D.E.Sc.Thesis, Columbia University, 1959.