Effect of Temperature on the Interfacial Properties of Silic

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355CoUoids and Surfaces, 21 (1986) 355-369
Elsevier Science Publishers B. V., Amsterdam Printed in The Netherlands
Effect of Temperature on the Interfacial
Properties of Silicates*
R. RAMACHANDRAN and P. SOMASUNDARAN
Henry Kumb School of Mines, Columbia University, New York, NY 10027 (U.S.A.)
(Received 2 April 1986; accepted in final form 14 July 1986)
ABSTRACT
Electrochemical properties of silicate minerals govern their behavior in processes such as floc-
culation and enhanced oil recovery that can occur at elevated temperatures. Knowledge of these
properties as a function of temperature can be helpful in developing an understanding of the role
of these interfacial properties at non-ambient temperatures. The zeta potential of sodium kaolin-
ite and quartz has been determined as a function of temperature in this work. Both systems exhib-
ited markedly different behavior at higher temperatures and also exhibited significant hysteresis.
The results were examined in terms of possible dissolution of the minerals and surface reactions
at different temperatures.
INTRODUCTION
Electrokinetic properties of minerals exert a governing influence on many
interfacial processes involving them. However, very little information is avail-
able in the literature on such interfacial properties as zeta potential at elevated
temperatures, although several processes such as flotation and enhanced oil
recovery occur at high temperatures. The solid-solution equilibria of a system
will also be significantly affected by changes in temperature and precipitation
of various species due to temperature fluctuations can markedly affect the
interfacial potential. Measurement of 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
expansion of the cells. The streaming potential technique is most easily adapt-
able for zeta potential measurements at non-ambient temperature conditions.
High temperature experiments were successfully performed by Kulkarni and
Somasundaran [1] using the streaming potential technique. In the present
study this procedure was followed to investigate the zeta-potential behavior of
.Dedicated to the memory of Professor G.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 sample was leached with concentrated nitric acid till it was free of
iron and subsequently washed free of nitrate ions by repeated washing with
triply distilled water. The washing process was continued till the pH of the
supernatant was constant and about the natural pH of quartz, 5.4-5.8. The
samples were 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 subjected to
repeated washing with NaCI using the procedure of Hollander et aI. [2] until
homo ionic Na-kaolinite was obtained. The surface area of this sample was
determined by the BET method to be 9.4 m2 g-l. The clay sample was of sub-
micrometer size and it was not possible to make a reproducible stable porous
plug with it because the fines escaped easily through the platinum electrode
(80 mesh). When the clay was contained using a porous membrane it was
observed that even under high streaming pressure (15 cm of Hg) , there was no
significant motion of solution through the compact clay plug. These problems
were successfully overcome by 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 compacted using a plunger and a hydraulic press at a pressure of
7X 106 kg m-2. The pellets were then sintered at 500°C in an induction furnace
for 12 h. The hardened pellets 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
supernatant was obtained. The washed clay was then used in streaming poten:-
tial experiments.
In order to determine the effect of heat treatment on the kaolinite sample
EDXRF and electrophoresis studies were conducted on the heat-treated and
untreated clay. EDXRF showed no significant variations between the two sam-
ples (Fig. 1). Electrophoresis data of the two samples also showed no differ-
ence. Obviously, the present heat treatment does not alter the surface
significantly to affect the zeta potential.
EXPERIMENTAL PROCEDURE
The procedure followed in the present study was similar to that described in
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 was packed between the two platinum electrodes
357
Fig. 1. EDXRF of heat-treated and untreated clay.
358
into a compact porous plug and conditioned in the test solution by repeated to
and fro streaming for 1 h. The cell was immersed in a water bath maintained
at the desired test temperature. The two platinum electrodes were connected
to a very high impedance electrometer to measure the streaming potential. The
pressure was measured using a mercury manometer.
The zeta potential was calculated using the Helmholtz-Smoluchowski
equation:
4nElI1
PEZeta potential =
where A, .u and f are respectively the specific conductivity, viscosity and dielec-
tric constant of the aqueous media and E the streaming potential under the
driving pressure P. 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
experiments have been published [3,4] and were followed in the experiments.
The mean of at least 10 readings of E/ P were used in 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
samples used for streaming potential) and streaming potential. The results
were comparable within ::!:5%.
R&gULTS
The performance of the cell was examined initially. A linear relationship
was obtained between driving pressure and streaming potential. In all experi-
ments the mean value of 10 to 15 readings of the ratio of streaming potential
to driving pressure was used to calculate the zeta potential.
The results obtained in this study are compared with those of other workers,
Fig. 2. The literature data is characterised by a wide amount of scatter. Vari-
ations in data could be the result of non -equilibrium conditions used as well as
due to differences in mineralogical and chemical composition of solids and the
supporting electrolyte concentration.
Temperature effects on 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 negative with increasing temperature. Most interestingly, it was observed
during these tests that the zeta potential did not return to its original value at
250 C when the system was taken through a temperature cycle. A detailed study
359
>
E
...
C(
~
z
...
--
0
D-
C(
--
...
Fig. 2. Zeta potential of (a) quartz and (b) Na -kaolinite as a function of pH: Comparison of data.
360
.140 r BRAZILIAN QUARTZ -48 +6'
10.'N NaNO,
010.C
a :5'.C
'" 75"C
9'
1 4
,
Aj
>
e
-'
~.
I-
Z
III
I-
0
A.
C.
I-
III
N
'4j!
4'/
/
~
. '0 .,
I 4 . .
.J.
.
I I, 10?
pH
Fig. 3. Effect of temperature on zeta potential of quartz.
of the hysteresis effect was conducted at two temperatures, 25 and 75°C, at
0.001 M ionic strength.
Figure 4 shows the zeta potential of quartz as a function of pH at 25 and
750 C. The hysteresis effect is schematically illustrated in Figs 5-7.
It can be seen from Fig. 6 that the zeta potential increased from - 46 to - 82
m V upon increasing the temperature from 25 to 750 C. Upon decreasing the
temperature back to 25°C the zeta potential remained at a value of -74 mV.
Even after washing the'sample with 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
caused significant changes in 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) observed similar pH changes at elevated temperatures
for the alumina-water system. The zeta potential at the natural pH of 5.6 after
the sample had undergone a temperature cycle at alkaline pH was always sig-
nificantly higher in magnitude than what it was initially.
361
-"0 BRAZILIAN QUARTZ
-4B +65 MESH
10-3N NoNO3
025.C
A 75.C
tt-120
I
-100
/. p
AI
t I{
/
.t ;6'
>
e.-eo
-'
~
I-
Z
...
I-
0-60
a.
c(
I-
...
N
-40
4/0
-20
0 I . "',,"",,'jl
2 34. . " ' .-~ , IV
pH
Fig. 4. Zeta potential of quartz at 25 and 750 C.
Temperature effects on clay
Figure 8 shows the 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
sample through a temperature cycle. Na-kaolinite exhibits significant hyster-
esis at all pH values. The zeta potential increases from + 11 m V at pH 4 to
+ 22 m V at the same pH after a temperature cycle. An increase in the negative
direction, from -10 m V at pH 6.7 to - 30 m V at pH 7, is observed at alkaline
pH.
In order to understand the temperature dependence of the interfacial prop-
erties such as zeta potential of quartz and sodium kaolinite it is necessary to
look at the mineral solution chemical equilibria of these systems at different
temperatures.
362
-60,-
-40
>
E
-'
~
...
Z
III
f
~ -20
...
!'!
40(5.61~
A
A
A~(4.4Ie< < < < «< < < < < < < < ~':'7<.,o21(4.41TIME~ 15.6 'e '7'7'7'7'7 '7'7
(ptjl:!-J V -'7'7'7'7'7V '7'7"7 ' ~ -1714~IO<~'7< < < «< < < < < < < < ~.p-_(4.41
... '7'7'7
V '7'7'7
V '7'7'7
V '7'7'7
V '7'7'71 14 .41~'7 '7 '7
BRAZILIAN QUARTZ
- 48 +65 JESH
10-3N NoNO)
TEST TIME: 0, 1,5,17. 21,35,40 ",.
SOLUTION pH- ( I
. ZETA POTENTIAL OF WASHED PLUG
AFTER TEMPERATURE CYCLE
25
TEMPERATURE,.C
75
Fig. 5. Schematic representation of temperature cycle at acidic pH.
Silica
The hydrolysis of the surface species of silica can be represented by the fol-
lowing reactions [8-11,13].r-
~-oSi {I
(2)
+ HQ1 ~ ~~-001
~5I - 001
+
i '-Sl<OoI tOt~ 001
51<.001 001
r - $1-01
b
I
Si-OI
r~<oo ! Si~.g: OH.HOH OH
OH ~ OH
Si < Si~OH
OH ~af
(3)
363
;'"
,":
.,
O{5.610
t t
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. Schematic repre.'lentation of temperature cycle at natural pH.
The main cause for the surface charge is the dissociation of the silanol groups
at the interface.
Reactions (1) to (4) represent a continuous increase of surface hydroxyl
groups to 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 surface is therefore expected to possess a higher surface charge
density than a quartz surface. Generation of such silicic acid surface sites could
be a major reason for the observed effect of temperature.
De Bruyn et al. [12] have represented the temperature dependence of the
solubility of crystalline quartz by the following equations:
SiO2+2 H2O = H.SiO. (5)
(6)log( H"SiO,,) O.151-1162jT
364
-140.-
1~18.661at <><~\~~»> »»»»»>~ 1916.431
119.1~> »> > > ><>\<>\~<><>~<>S~~18.021
,. eo
,. &.
,. &.
,. &.
,. l-
I- ,. I-
,. &.
,. I-
,. &.
,. &.
..
,. ..
,. "
,. ..
...
0
."z~
TEMPERATURE,"C
Fig. 7. Schematic representation of temperature cycle at 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 species in solution.
In alkaline solutions [14] the equilibrium for the dissolution of quartz is
written as:
H.SiO4 = H+ + H3SiOi pK = 9.8 (7)
-9.8 = -log(H4SiO4) + log(H+) + log(H3SiOi
-9.8 + log(H.SiO.) + pH = log(HaSiOi) (9)
As the pH is increased if K is constant log (HaSiO. -) must increase. It is
I.
AI.
29(5.61J-
A
A BRAZILIAN QUARTZ
A -48 +65 MESH
A
A lcr3N NoNO3
A TEST TIME: 0.1.5.15.19,29hrs
A SOLUTION pH - ( I
A . ZETA POTENTIAL OF WASHED
A
A PLUG AFTER TEMPERATURE CYCLE
A
A
A
A
A
0(5.6)0
t t
TIME (pHI
>-e
-J
c(
I-
Z
...
I-
0
A.
'100
365
pH
Fig. 8. Zeta potential of Na-kaolinite at 25 and 75 °C.
also clear from Eqns (7) -( 9) that as the temperature is increased H.SiO. con-
centration must increase. Hence both increase in temperature and pH favour
formation ofH3SiO.-.
Data in the literature [8-12] indicate the presence of a highly disturbed
amorphous layer on the surface of quartz leading to abnormally high solubility.
Dissolution of the amorphous layer in combination with adsorption/precipi-tation of H3SiO. - can be another major reason for the observed temperature
effects. Further evidence for this hypothesis was seen when quartz treated
ultrasonically for 12 h' (to remove the amorphous layer) was used to measure
the zeta potential as a function of temperature. Most interestingly, ultrasoni-
cally treated quartz did not show any significant effect of hysteresis. Also the
zeta potential of ultrasonicated quartz at room temperature was about - 60
m V, which is comparable to the value of untreated quartz at 250 C after sub-
jecting it to a temperature cycle.
Na-Kaolinite
The species distribution diagram for Na-kaolinite is shown in Fig. 11. In the
acidic region it can be seen that the activity of the A13 + species is 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
14(4)0< < < < < < < < < < < <
< <~ < < < < < ~~
~ ?????12(3.7)
v ??
v???V ??
V ??
v,??V ?'
114)~??
A
A
A
A
A
A
A
A
A
A
A
17 (6.3).
A
A
A
A r TIME ~ rCPHI
00(6.8)I
25 75
TEMPERATURE
Fig. 9. Schematic representation of temperature cycle at acidic pH.
Increase in temperature would enhance the dissolution resulting in increased
amount of A13+ species in solution. Zeta potential studies at this pH show the
mineral to be in fact more positively charged at higher temperatures. Disso-
lution followed by readsorption of Al3 + and Al ( 0 H) 2 + can cause the 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 causing the hysteresis effect.
At natural pH (-7) the important species are AI(OH)3' H4SiO4 and
H3SiO 4 -. The net negative potential on the surface is attributed to the adsorp-
tion of H3SiO4- which is the only charged species that is active at this pH.
Increase in temperature caused a decrease in pH resulting in a less negativepotential owing possibly to the adsorption of the AI( OH)2+.
In the alkaline region (pH -9) the major species are H3SiO4- and Al( OH) 4-
and adsorption of these ions causes the mineral to be highly negatively charged
Again, increase in temperature resulted in a decrease in pH and a less negative
zeta potential.
367
TEW. CYCLE AT ALK. pH
SOOIUM KAOLINITE (-Z8+651
I<»8C STRENGTH 10-4 NaCi
TEST TIME 0,4,10, Z1,33 hr.
TIME I~) SOLUTION pH - ( I
. ~
00(671v
v
v
v
v
v
v
v
v
v
V
V
V
V
V
V
21 (71 OC < < < ' ,
V ~,«<
V «««<V < < < < f~IO(6.71
V ????
V ?????
V ????
v ')~')33(8.5I&? ?~')') ~ ~')
4(8.7)
c
-15
-20
I
i
-25
-30
>
e
...
4
~
Z
~
oC
~N
-3S
.4n
TEMPERATURE. -C
25 75
Fig. 10. Schematic representation of temperature cycle at alkaline pH.
CONCWSIONS
The zeta potential of quartz and Na-kaolinite were measured as a function
of temperature. The zeta potential of quartz increased in magnitude as a func-
tion of temperature at all pH conditions. Interestingly, significant hysteresis
was observed and the zeta potential did not return to the original values at
room temperature even after several washings. However, ultrasonically cleaned
quartz did not exhibit measurable hysteresis. Quartz has been known to pos-
sess a disturbed amorphous layer with very high solubility [8-12] . Dissolution
of surface silicic acid followed by adsorption of H3SiO. - species is proposed to
be the major cause for the temperature dependence of zeta potential. Desorp-
tion of H3SiO. - can be kinetically controlled and this could lead to the observed
hysteresis effects.
The zeta potential of the Na-kaolinite was markedly sensitive to tempera-
ture changes in the system. The zeta potential became more positive at acidic
pH and more negative at alkaline pH with increasing temperature.
The zeta potential changes as a function of temperature and pH have been
correlated with the species distribution diagrams. Al3 + and the Al ( 0 H) 2 + spe-
cies that predominate in the acidic pH range cause the mineral to be more
positively charged in this pH range. Presence of neutral species H.SiO4 and
368
pH
Fig. 11. Species distribution diagram of Na-kaolinite [16]
AI( OH) a lower the effect of the negatively charged HaSiO4 - in the neutral pH
range. In the alkaline pH region HaSiO. - is the major species which contrib-
utes to the negative potential on the surface. Increase in temperature of the
system can enhance the dissolution of the species and affect the readsorption
as well as precipitation of the relevant species, resulting in marked changes of
the zeta potential.
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1 R.D. Kulkarni and P. Somasundaran, J. Colloid Interface Sci, 45 (1973) 591.
2 A.F. Hollander, P. Somasundaran and C.C. Grytte, in P.H. Tewari (Ed), Adsorption from
Aqueous Solutions, Plenum, New York, 1981, pp. 143-161.
3 B. BailandD.W. Fuerstenau, Miner. Sci. Eng., 5 (1973) 267-275.
4 Grinell Jones and Lloyd A. Wood, J. Chern. Phys., 13 (1945) 3.
5 Philip B. Lorenz, Clays Clay Miner., 17 (1969) 223-251.
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7 P.H. Tewari and A. W. Mclean, J. Colloid Interface Sci., 40 (1972) 267.
8 A.J. Beal and A.L. Godbert, Research report No. 115, Safety in Mines Research Establish-
ment, 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.
369
12 P .L. de Bruyn et aI., J. Phys. Chern., 64 (1960) 1675.
13 K.R. Lange and R.W. Spencer, Environrnental Sci. Technol., 2 (1968) 212.
14 P.S. Roller and G.E. Erwin,J. Am. Chern. Soc., 62 (1940) 461.
15 CRC Handbook of Physics and Chemistry, 62nd edn, CRC Press, Boca Raton, FL, 1982.
16 Paul A. Siracusa, Ph.D. Thesis, Columbia University, 1986.
17 H.C. Li and P.L. de Bruyn, Surf. Sci., 5 (1966) 203.
18 G.L. Zucker, D.E.Sc. Thesis, Columbia University, 1959.