355CoUoids and Surfaces, 21 (1986) 355-369Elsevier Science Publishers B. V., Amsterdam Printed in The Netherlands

Effect of Temperature on the InterfacialProperties 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 theseproperties as a function of temperature can be helpful in developing an understanding of the roleof 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 reactionsat different temperatures.

INTRODUCTION

Electrokinetic properties of minerals exert a governing influence on manyinterfacial processes involving them. However, very little information is avail-able in the literature on such interfacial properties as zeta potential at elevatedtemperatures, although several processes such as flotation and enhanced oilrecovery occur at high temperatures. The solid-solution equilibria of a systemwill also be significantly affected by changes in temperature and precipitationof various species due to temperature fluctuations can markedly affect theinterfacial potential. Measurement of zeta potential using electrophoresis, asa function of temperature, is inhibited due to the elaborate modificationsrequired to avoid interference from convectional currents and non-uniformexpansion 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 andSomasundaran [1] using the streaming potential technique. In the presentstudy 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 pHconditions.

MATE~ AND METHODS

Brazilian quartz (- 28 to + 65 mesh) was prepared by roll crushing andsizing. The sample was leached with concentrated nitric acid till it was free ofiron and subsequently washed free of nitrate ions by repeated washing withtriply distilled water. The washing process was continued till the pH of thesupernatant was constant and about the natural pH of quartz, 5.4-5.8. Thesamples were stored in polypropylene bottles at pH 2.

A well crystallised sample of Georgia kaolinite was obtained from the clayrepository at the University of Missouri. The Na-kaolinite was subjected torepeated washing with NaCI using the procedure of Hollander et aI. [2] untilhomo ionic Na-kaolinite was obtained. The surface area of this sample wasdetermined 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 porousplug with it because the fines escaped easily through the platinum electrode(80 mesh). When the clay was contained using a porous membrane it wasobserved that even under high streaming pressure (15 cm of Hg) , there was nosignificant motion of solution through the compact clay plug. These problemswere 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 of7X 106 kg m-2. The pellets were then sintered at 500°C in an induction furnacefor 12 h. The hardened pellets were crushed in an agate mortar and the - 28to +48 mesh size fraction collected. This sample was washed 10 times withdistilled water and then with triply distilled water until a constant pH of thesupernatant 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 sampleEDXRF and electrophoresis studies were conducted on the heat-treated anduntreated 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 surfacesignificantly to affect the zeta potential.

EXPERIMENTAL PROCEDURE

The procedure followed in the present study was similar to that described inan earlier work [1]. The streaming potential cell was filled with the solutionof desired pH and ionic strength and the solid (quartz or Na-kaolinite) wasthen 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 toand fro streaming for 1 h. The cell was immersed in a water bath maintainedat the desired test temperature. The two platinum electrodes were connectedto a very high impedance electrometer to measure the streaming potential. Thepressure was measured using a mercury manometer.

The zeta potential was calculated using the Helmholtz-Smoluchowskiequation:

4nElI1PE

Zeta 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 thedriving pressure P. E, P and A were determined experimentally and values forviscosity and dielectric constants at the test temperature were obtained fromthe literature [ 15] .

Extensive reviews on the precautions to be taken in streaming potentialexperiments 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 zetapotential. The experiments at room temperature were performed both usingelectrophoresis (crushed samples of the - 400 mesh size fraction from thesamples used for streaming potential) and streaming potential. The resultswere comparable within ::!:5%.

R&gULTS

The performance of the cell was examined initially. A linear relationshipwas obtained between driving pressure and streaming potential. In all experi-ments the mean value of 10 to 15 readings of the ratio of streaming potentialto 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 asdue to differences in mineralogical and chemical composition of solids and thesupporting electrolyte concentration.

Temperature effects on quartz

The results obtained for the zeta potential as a function of temperature at10,35, and 75°C are shown in Fig. 3. The zeta potential is found to becomemore negative with increasing temperature. Most interestingly, it was observedduring these tests that the zeta potential did not return to its original value at250 C when the system was taken through a temperature cycle. A detailed study

359

>E

...C(~z...--0D-

C(--...

Fig. 2. Zeta potential of (a) quartz and (b) Na -kaolinite as a function of pH: Comparison of data.

360

.140 rBRAZILIAN QUARTZ -48 +6'

10.'N NaNO,

010.Ca :5'.C'" 75"C

9'

1 4,

Aj

>e

-'

~.I-ZIIII-0A.

C.I-IIIN '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, at0.001 M ionic strength.

Figure 4 shows the zeta potential of quartz as a function of pH at 25 and750 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 - 82m V upon increasing the temperature from 25 to 750 C. Upon decreasing thetemperature 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 freshNaNO3 solution the zeta potential stayed at -74 mV. Similar results wereobtained at pH 4.4, 8.1, and 9.7. At alkaline pH, elevation of the temperaturecaused significant changes in the final pH values partly due to the change inthe pK of water and also due to the mineral solution equilibria at this pH.Tewari and Mclean [7) observed similar pH changes at elevated temperaturesfor the alumina-water system. The zeta potential at the natural pH of 5.6 afterthe sample had undergone a temperature cycle at alkaline pH was always sig-nificantly higher in magnitude than what it was initially.

361

-"0BRAZILIAN QUARTZ

-4B +65 MESH

10-3N NoNO3

025.C

A 75.C

tt-120

I-100

/. pAIt

I{

/.t ;6'

>e.-eo-'~I-Z...I-0-60a.

c(I-...N

-404

/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 atalkaline pH. Figures 9 and 10 illustrate the effect resulting from taking thesample 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 negativedirection, from -10 m V at pH 6.7 to - 30 m V at pH 7, is observed at alkalinepH.

In order to understand the temperature dependence of the interfacial prop-erties such as zeta potential of quartz and sodium kaolinite it is necessary tolook at the mineral solution chemical equilibria of these systems at differenttemperatures.

362

-60,-

-40>E

-'~

...ZIII

f~ -20...!'!

40(5.61~AAA

~(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

... '7'7'7V '7'7'7V '7'7'7V '7'7'7V '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,.C75

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

bI

Si-OI

r~<oo ! Si~.g: OH.HOH OH

OH ~ OHSi < Si~OH

OH ~af

(3)

363

;'"

,":.,

O{5.610

t tTIME {pHI

BRAZILIAN QUARTZ-48 +65MESH10-3N NoNO3

TEST TIME: O,4,23,27,44,48h"SOLuTION pH- ( )

. ZETA POTENTIAL OF WASHED PLUGAFTER TEMPERATURE CYCLE

2~ 75TEMPERATURE,.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 groupsat the interface.

Reactions (1) to (4) represent a continuous increase of surface hydroxylgroups to form a silicic acid surface. The number of ionisable sites per siliconatom 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 chargedensity than a quartz surface. Generation of such silicic acid surface sites couldbe a major reason for the observed effect of temperature.

De Bruyn et al. [12] have represented the temperature dependence of thesolubility 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 <><~\~~»> »»»»»>~ 1916.431119.1~> »> > > ><>\<>\~<><>~<>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 highertemperatures.

It has also been noted by De Bruyn et al. that H3SiO4- is the only majorionic species in solution.

In alkaline solutions [14] the equilibrium for the dissolution of quartz iswritten 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-AA BRAZILIAN QUARTZA -48 +65 MESHAA lcr3N NoNO3A TEST TIME: 0.1.5.15.19,29hrsA SOLUTION pH - ( IA . ZETA POTENTIAL OF WASHEDAA PLUG AFTER TEMPERATURE CYCLEAAAAA

0(5.6)0

t tTIME (pHI

>-e-Jc(I-Z...I-0A.

'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 favourformation ofH3SiO.-.

Data in the literature [8-12] indicate the presence of a highly disturbedamorphous 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 treatedultrasonically for 12 h' (to remove the amorphous layer) was used to measurethe zeta potential as a function of temperature. Most interestingly, ultrasoni-cally treated quartz did not show any significant effect of hysteresis. Also thezeta potential of ultrasonicated quartz at room temperature was about - 60m 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 theacidic region it can be seen that the activity of the A13 + species is very high.

366

TEMP. CYCLE AT ACIDIC pHSODIUM KAOLINITE 1-28+65)IONIC STRENGT H: 10-4 NaCaTEST TIME: O,1,12,14,17hrlSOLUTION pH - 1 ). ZETA POTENTIAL OF WASHED PLUG

AFTER TEMP. CYCLE14(4)0< < < < < < < < < < < < < <~ < < < < < ~~

~ ?????12(3.7)v ??v???V ??V ??v,??V ?'

114)~??

A

A

AA

A

A

A

A

A

A

A

17 (6.3).AAAA r TIME ~ rCPHI

00(6.8)I

25 75TEMPERATURE

Fig. 9. Schematic representation of temperature cycle at acidic pH.

Increase in temperature would enhance the dissolution resulting in increasedamount of A13+ species in solution. Zeta potential studies at this pH show themineral 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 chargedAgain, increase in temperature resulted in a decrease in pH and a less negativezeta potential.

367

TEW. CYCLE AT ALK. pHSOOIUM KAOLINITE (-Z8+651I<»8C STRENGTH 10-4 NaCiTEST TIME 0,4,10, Z1,33 hr.

TIME I~) SOLUTION pH - ( I

. ~00(671vvvvvvvvvvVVVVVV

21 (71 OC < < < ' ,V ~,«<V «««<V < < < < f~IO(6.71V ????V ?????V ????v ')~')

33(8.5I&? ?~')') ~ ~')

4(8.7)

c

-15

-20Ii

-25

-30

>e

...4~Z

~oC

~N

-3S

.4n

TEMPERATURE. -C25 75

Fig. 10. Schematic representation of temperature cycle at alkaline pH.

CONCWSIONS

The zeta potential of quartz and Na-kaolinite were measured as a functionof temperature. The zeta potential of quartz increased in magnitude as a func-tion of temperature at all pH conditions. Interestingly, significant hysteresiswas observed and the zeta potential did not return to the original values atroom temperature even after several washings. However, ultrasonically cleanedquartz did not exhibit measurable hysteresis. Quartz has been known to pos-sess a disturbed amorphous layer with very high solubility [8-12] . Dissolutionof 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 acidicpH and more negative at alkaline pH with increasing temperature.

The zeta potential changes as a function of temperature and pH have beencorrelated 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 morepositively 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 pHrange. In the alkaline pH region HaSiO. - is the major species which contrib-

utes to the negative potential on the surface. Increase in temperature of thesystem can enhance the dissolution of the species and affect the readsorptionas well as precipitation of the relevant species, resulting in marked changes ofthe zeta potential.

REFERENCES

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) 223-251.6 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, 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.