The zero point of charge of alpha-alumina

JOURNAL OF COLLOID SCIENCE 19, 61-71 (1964)

T H E ZERO P O I N T OF CHARGE OF A L P H A - A L U M I N A J. A. Yopps I and D. W. Fuerstenau 2 Received April 29, 1963, revised July 16, 1963 ABSTRACT The zero point of charge of s-alumina was found to occur in aqueous solutions at pH 9.1 :t: 0.1. Three independent methods of measurement were used : potentiometric titration for determination of the adsorption density of the potential-determining ions, measurement of the electrophoretie mobility, and determination of maximum subsidence rate. Potassium, chloride, perchlorate, and nitrate ions were all found to be indifferent at the surface of alumina.

INTRODUCTION For colloids where reversible transfer of potential-determining ions across the solid-liquid interface is possible, a most important parameter describing the double layer is the zero point of charge of the surface (zpc). The objective of this paper is to present the results of experimental determinations of the zpe of a-alumina. In the case of oxides such as alumina the potential-determining role of H + and OH- has been well established (i-4). Charge transfer across the alumina-water interface is determined by the equilibrium of H + and OHwith Al +~ and O= in the lattice. Qualitatively, the mechanism by which the surface charge is established has generally been considered to involve a two-step process: surface hydration followed by dissociation of the surface hydroxide (2-4). Dissociation of the hydrated surface to give rise to a charged surface is shown schematically in the following manner: \ \ \ 0 0 0 \ + \ \ A1--OH(,.~f) __ AI--0H(,u~r) + + °/ H / / 0 0 / / / Positive surface

Zero point of charge

Negative charge

Parks and de B r u y n (1) have suggested t h a t the charging of the surface of an oxide in water occurs through the formation of hydroxo-complexes in 1 Research Assistant, Department of Mineral Technology, University of California, Berkeley. 2 Professor of Metallurgy, Department of Mineral Technology, University of

California, Berkeley. 61

62

YOPPS AND FUERSTENAU

solution and the transfer of these complexes across the interface. For Fe203 the adsorbed species that are responsible for the surface charge are postulated to be primarily Fe(OH) + and FeO;-, whose concentrations in saturated solutions are in turn determined by the pH. Thermodynamically it is not possible to distinguish between the two models: H + and OHadsorption followed by surface dissociation and hydroxo-complex adsorption. Parks and de Bruyn also suggest that the zpc of the solid coincides with the minimum solubility of the solid and show this to be true for Fe203. Determination of the zpc of a colloid with a reversible double layer must be carried out experimentally. For solids that do not function as satisfactory TABLE I Summary of the Zero Point of Charge of Aluminum Oxides and Hydrated Aluminum Oxides Material a-A120~

Experimental method

Investigators

Electrophoresis

Johansen and Buchanan Johansen and Buchanan Modi and Fuerstenau Robinson

5

Dobit~, Spurn?~, and Freudlov£ Fricke and Keefer Fricke and Keefer"

8

Streaming potential Streaming potential Streaming potential Electro-osmosis 7-A120a 7-A100H

AI-(OH)~

Electrophoresis Electrophoresis

Electrophoresis

Electrophoresis Minimum solubility Electrokinetic

Ref

6 3 7

9 9

tFricke and Leon- 10 / hardt Frieke and Keefel 9 I Fricke and Keefer 9 tFricke and Leon- 10 hardt Gayer, Thompson, 11 and Zajicek Tewari and Ghosh 12

Remarks Natural corundum Nat. corundum, 0.94% SiO2 Artificial sapphire Artificial sapphire Artificial sapphire Artificial Artificial Boehmite

Amorphous, artificial 7 form, artificial /~ form, artificial Artificial

Using a vertical electrophoresis cell, Schuylenborgh (13) and Schuylenborgh and Sanger (14) determined values of the zpc of different aluminas and hydrated aluminas, but their results indicate an acidic solid which is not realistic for alumina. Their results for the zpe of various forms of aluminum oxide are the following: natural corundum (14), pH 2.2; artificial alumina (14), pit 2.9; artificial -r-A1OOH (13), pH 6.5-8.8; artificial -~AI(OH)3 (13), pH 5.4-7.5; artificial a-AI(OH)a, pH 4.9 (13), pH 5.6 (14); natural a-AI(OH)a (14), pH 4.8-5.2.

ZERO POINT OF CHARGE OF ALPHA-ALUMINA

63

reversible electrodes, the methods for locating the zpc depend on knowing which ions are potential determining and on having the system free of specific adsorption effects. In the study of oxides, potentiometric titration of a suspension of the solid of high surface area can be carried out with a glass electrode to determine adsorption isotherms of the potential-determining ions, and thereby the zpc. If surface-active electrolytes are absent, the zero point of such electrokinetic effects as streaming potentials and electrophoretic mobility, maximum coagulation rate, and the absence of the suspension effect between a settled suspension and the supernatant liquid can all be used to determine the zpe. Only eleetrokinetic methods have been reported for the direct determination of the zpe of alumina. Table I presents the widely varying results that have been obtained for various forms of alumina. Differences in the results may arise from a number of factors, such as nonequilibrium of the hydroxylated surface with the solution, impurities in the solid, and possibly experimental procedures. In the present investigation, finely divided alumina of high purity was used so that the zpc could be determined by three independent methods: potentiometric titration for determination of the adsorption density of potential-determining ions, measurement of electrophoretic mobility, and determination of maximum subsidence rate of the colloidal suspension. EXPERIMENTAL MATERIALS Linde "A" alumina was used for the solid phase in this investigation. This material, which is prepared commercially by controlled firing of chemically pure ammonium alum, was found by spectrographic analysis to contain the following impurities: 0.02% Si, 0.003% Ga, 0.003 % Fe, 0.0007 % Ca, 0.0007 % Cu, 0.0005 % Mg, and a trace of Na. X-ray diffraction examination verified that the material is a-A1203, with no extraneous lines appearing in the diffraction pattern. BET surface area measurement showed the surface to be 15 m.2/g., indicating that the average particle diameter is 0.1 micron. The material as received was leached for several hours in concentrated hydrochloric acid, then washed with conductivity water until free of chloride ion, and stored for several days under water before use. If the material had not been allowed to age under water, equilibrium in the titration studies was slow to be reached. All solutions were prepared with conductivity water which had been triple distilled. The water was first distilled in a Barnstead laboratory still and then passed through a two-stage Heraeus quartz still. Conductivity water produced by this method was considered satisfactory if its conductivity was less than 8 X 10-7 ohm -1 cm. -1. The water was stored and dispensed in a nitrogen atmosphere free of carbon dioxide. 0ne-tenth normal acids and bases for pH control were made up with conductivity

64

YOPPS AND FUERSTENAU

water from pH Tamm volumetric solutions distributed by Bio-Rad Laboratories, Richmond, California. Basic solutions were stored in polyethylene bottles to avoid silicate contamination. EXPERIMENTAL PROCEDURES

Potentiolaetric titration has been used successfully for equilibrium studies of double layers on AgI (15, 16), Ages (17, 18), and Fe20~ (1). This technique necessitates working at high ionic strength of an indifferent electrolyte so that the adsorbed potential-determining ions can all be assigned to the solid side of the interface. Under these conditions, the surface charge, ¢~, on the alumina will be ~

= F(r~÷

-

ro~-),

[1]

where F is Faraday's constant and F~+ and PoE- are the adsorption densities of H + and OH-, respectively. The net adsorption density of potential-determining ions on alumina may be measured by the potentiometric titration of a suspension of solids in the reversible cell: aqueous suspension of glass electrode

AI~O~, KC1 supporting electrolyte; varying pH

saturated calomel reference electrode

At the zpc, there is no surface charge and the adsorption of H+ and OHwill be nil. Because the surface charge increases with increasing ionic strength, a series of titration curves will intersect at the only point common to all curves, the point corresponding to zero adsorption or the zpc. The titration of the A120~ suspension was carried out in a 600-ml. beaker which had the pouring lip removed. A rubber stopper was used to seal the titration cell but contained holes through which passed the electrodes, thermometer, burette, and inlet and outlet tubes for nitrogen. Details of this cell assembly can be seen in Fig. 1. Mixing was provided by means of a magnetic stirrer using a Teflon covered stirring bar. The titration cell was semi-immersed in a larger beaker through which thermostatically controlled water was pumped. The temperature of the system was kept constant at 25.0 ± 0.1°C. by means of a Labline constant temperature circulating system. Measurements of pH were made with a Beckman Model GS pH meter used in coniunction with a Beckman Type E-2 glass electrode and a Beckman saturated calomel electrode. Additions of acid and base were made with a Koch microburette to an accuracy of =h 0.001 ml. The electrokinetie or zeta potential is generally assumed to be the potential at the plane of closest approach of nonsurface-active counter ions

ZERO POINT OF CttARGE OF ALPHA-ALUMINA

65

~-~q,

TO pH METER

I

')V I

i

4

iH2o

2

PHOTOMETE

WATER,

25.(j'

I

-7

l0

FIG. 1. Titration and coagulation rate cell. 1. Glass electrode; 2. Calomel electrode; 3. Koch burette; 4. Thermometer; 5. Rubber stopper; 6. Water jacket; 7. Photoelectric cell; 8. Light source; 9. Stirrer bar; 10. Magnetic stirrer.

to the surface (19). In the absence of specific adsorption of surface-active counter ions, electrokinetic measurements can show a reversal in the sign of the zeta potential only at the zpc, at which condition both the surface potential, ¢0, and the zeta potential, p, are equal and zero. By determining the electrophoretic mobility of fine alumina particles as a function of pI-I, the zpc of alumina corresponds to the pit at which the mobility is zero. Electrophoretic mobility studies were carried out using a compact microelectrophoresis instrument manufactured by Zeta Meter, Inc. In these experiments, the electrophoretic mobility was measured as a function of pit in solution in which no supporting electrolyte was used, and in 10-3 37 KC1, 10-~ N I4C1Q, and 10-~ N KN03. The stability of colloidal suspensions results from the interaction of double layers (19). In the absence of specific adsorption, maximum coagulation occurs when double layers are absent. Thus, determination of maximum subsidence rate should provide a method of determining the zpc. By means of a photoelectric cell and a light source (see Fig. 1), the time required for light transmitted through the suspension to reach some selected intensity is obtained as a function of pH. The minimum time occurs at the zpc because of the larger floes that are formed from the uncharged particles. On either side of the zpc, the degree of coagulation of the suspen-

66

YOPPS AND FUERSTENAU

sion decreases and permits one to determine the zpc fairly easily and rapidly. The best experimental results are obtained at ionic strengths of 10 -3 N, the influence of the zpc on coagulation being less as the double layer is compressed at greater ionic strengths. The key to obtaining reproducible results was found to be working at constant temperature so that temperature gradients are absent. EXPERIMENTAL I~ESULTS

Figure 2 presents adsorption isotherms of H + and OH- obtained by the titration of an alumina suspension in the presence of 10-~, 10-2, and 10-1 N KC1 as the indifferent electrolyte. Adsorption density is given in arbitrary units. It can be seen that the intersection of the curves of different ionic strengths occurs at pH 9.1. I

i

I

i

I

i

I

-2 O3 z

n.,~

_

n~

l--m n."

if0 ~ i

4-

I--z o z o FZPC = pH 9.1

a

I !

8

~

pH

li

9

I

I

I0

FIG. 2. Adsorption density of p o t e n t i a l - d e t e r m i n i n g ions on alumina at 25.0°C. as a function of pI-I and ionic strength, using t{C1 as supporting electrolyte. KC1, normality: A--10-8; O--10-2; O - - 1 0 -1.

67

ZERO POINT OF CHARGE OF ALPHA-ALUMINA

Figure 3 presents the results of the study of the electrophoresis of alumina. The data are reported as electrophoretic mobility because only the pll of zero mobility is of significance in determining the zpc. It can be seen that the electrophoretic mobility of the suspended material is zero at pll 9.1 in all the solutions. When the solid is negatively charged in the presence of the potassium salts as supporting electrolyte, the counter ions are K + and the curves for the various salt solutions coincide, but when the solid is positively charged, differences in the size of the anionic counter ions change the mobility of the particles. Figures 4 and 5 present the results of a coagulation study of alumina in the presence of three different supporting indifferent electrolytes. Figure 4, which presents data for i0 -~, 10 -2, 10 -I, and i0 ° N KCI, shows that the maximum subsidence rate at all ionic strengths of KCI is at approximately pH 9.0 to 9.1. As the ionic strength of the supporting electrolyte is inJ

~ d o

-4

o

-3

1

I

,f

W

Z

I

-2

o

n,.-

_o

S b-

Y

-I

0

d

m

+r

0 0 b..kLI

0 klA

J

I

°

I

0 7-

+2

0 U3

n--

I O-

oQ=

l i,I

+4

o ZPC

= p H 9.1

I r I

+5

6

i

I

7

8

PI

9

I

10

I

pH

FIG. 3. E l e c t r o p h o r e t i c m o b i l i t y of a l u m i n a in a q u e o u s s y s t e m s as a f u n c t i o n of p H . O - - t t 2 0 ; O - - 1 0 -a N • C I 0 4 ; A - - 1 0 -a N K N O 3 ; G - - 1 0 -3 N KCI.

68

YOPPS

AND

FUERSTENAU

i

I

I

140 U') 0

12.0 0 LJJ U)

>:1o0 I--

,z, 80 Z

-,- 60

0

12)

4C

ul

~ 2o

ZPC= p H 9.0

I-kd

> b--

o

i

\

,

_J

u.l

& I

8

JJ I

pH

9

1

I0

FIG. 4. Relative subsidence time of alumina as a function of ptI at different ionic strengths of KC1 (the minimum in each curve is displaced downward 10 seconds for each of the KC1 concentrations. KC1, normality: Q--10-~; O--10-2; A--10-1; A--10 °. creased, the shape of the curve changes from a v e r y definite U to an extremely shallow U, probably because of coagulation due to compression of the double layer. No particular significance should be placed on the magnitude of the settling time at the different ionic strengths because the level of liquid and placement of the beaker in the assembly varied between runs. T h e actual settling time for all the minima was between 2 and 3 minutes with no particular p a t t e r n being observed between different ionic strengths. The m i n i m u m of each curve was arbitrarily set equal to zero, and the zero value of the curves at 10-3, 10-1, and 10 ° are displaced downward to prevent crowding of the data. Figure 5 presents the subsidence rate curve at 10 -3 N KC104, showing a m i n i m u m in the curve at p i t 9.0. I n this figure, flocculation curves for 10-3 N KNO3 are also presented. To show t h a t the p H at which the maxim u m settling rate occurs is independent of the light intensity chosen for

ZERO POINT

I

OF CHARGE

OF ALPHA-ALUMINA

I

69

I

300 280 0 Z 0

2.60 O3

>: 240 I--.

ZPC =lpH 9.0

I

~ 220 Z

200 Ld

o

180

Ld

~60

140

120

I

8

I

9 pH.

10

FIG. 5. Subsidence time of alumina as a function of pH in KC104 and KNO3 systems. 10-3 N KC104; 0 - - 4 ft. cdl.; 10-3 N KNO3; 0 - - 5 ft. cdl.; A--4 ft. cdl. the experiments, d a t a are given for the times to reach an intensity of 4 ft.-candles and 5 ft.-candles in 10 -3 N K N 0 3 . Only vertical displacement of the curves occurs. DISCUSSION OF RESULTS Electrophoresis of alumina in the absence of a n y supporting electrolyte shows t h a t the zpc occurs at p H 9.1. Figure 3 shows t h a t there is no shift of the zpc in the presence of KC1, KCI04, or KN03. M a x i m u m subsidence rate occurs at p H 9.0 in solutions with 10 -3 N KCI04 as supporting electrolyte. I n solutions containing KC] as the supporting electrolyte, maxim u m coagulation at different ionic strengths was found to occur at p H values ranging from 9.0 to 9.1. I n solutions containing KNO3 as supporting electrolyte, m a x i m u m coagulation occurs at p H 9.1. Adsorption densities of H + and O H - in the presence of KC1 as supporting electrolyte show t h a t zero adsorption occurs at p H 9.1, independent of ionic strength.

70

YOPPS AND FUERSTENAU

Thus, from these three independent means of measurement, it is concluded that the zero point of charge of a-alumina occurs at pH 9.1. These experiments also indicate that chloride ions, perchlorate ions, and nitrate ions are all indifferent counter ions at the alumina-water interface. Nitrate and perchlorate ions are known to be indifferent counter ions at the surface of AgI (15, 16). Nitrate ions are known to be indifferent counter ions for Fe~03 (1). Since chloride ions do not form complex ions with aluminum ions in solution (20), their nonsurface-active nature on alumina could be anticipated. The zpc of this finely divided a-alumina is in close agreement with the zpe of synthetic sapphire (see Table I). In addition there is close agreement with Fricke's values for the zpe's of hydrated aluminas. Natural corundum has a zpc about one pH unit lower than the pure synthetic aluminas, the lower value probably resulting from effects of impurities in the natural mineral. The low value of the zpc (pH 6.7) of synthetic sapphire obtained by Dobi£~ et al. (8) probably results from their method of preparation of the solid. They leached their crushed samples for five minutes in 6 % HCt, washed the material until it was free of chloride ion, dried the samples at 120°C., and stored them dry. With this procedure, they obtained the same zpc that is produced by heat treating alumina. Robinson (7) determined the zpe to be pH 9.1 for synthetic sapphire that has been aged under water, but by a heat treatment procedure he reduced the zpc to pH 6.7. The minimum solubility of hydrated alumina (pH 7.7) found by Gayer, Thompson, and Zajicek (11) does not appear to coincide with the zpc of alumina. SUMMA_RY AND CONCLUSIONS

1. The zero point of charge of a-alumina occurs at pH 9.1 ± 0.1, as shown by electrophoresis, adsorption isotherms of potential-determining ions, and maximum coagulation rate. 2. Potassium chloride, potassium nitrate, and potassium perchlorate appear to be indifferent electrolytes at the surface of alumina. 3. A method is given for the rapid determination of the zero point of charge of a colloid by means of measurement of the maximum subsidence rate of the suspension. ACKNOWLEDGMENT

The authors wish to thank the National Science Foundation for its support of this work. REFERENCES 1. PARKS,G. A., A.NDDE BRUYN, P. L., J. Phys. Chem. 66, 967 (1962). 2. V~RwEv, E. J., in W. D. Harkins, ed., "The Physical Chemistry of Surface Films," Ch. 6. Reinhold, New York, 1950.

ZERO POINT OF CHARGE OF ALPHA-ALUMINA

71

3. MODI, H. S., AND :FUERSTENAU,I). W., J. Phys. Chem. 61,640 (1957). 4. O'CONNOR, D. J., JOHANSEN, P. G., AND BUCHANAN,A. S., Trans. Faraday Soc. 52, 229 (1955). 5. JOHANSEN, P. G., AND BUCHANAN,A. S., Australian J. Chem. 19,398 (1957). 6. JOHANSEN, P. G., AND BUCHANAN,A. S., Australian J. Chem. 10, 392 (1957). 7. ROBINSON,M., Master's Thesis, University of California, Berkeley, 1962. 8. DOBIig, B., SPVRN~, J., ANn FREUDLOV£, E., Collection Czechoslov. Chem. CorntoURS. 24, 3663 (1959). 9. FRIC~E, R., ANn KEEFER, H., Z. Naturforsch. 4A, 76 (1949). 10. FRIC~E, R., ANn LEON~IARDT,I., Naturwiss. 37,428 (1950). 11. GAYE~, K. H., THOMPSON, S. C., AND ZAJICEI~I:,O. T., Can. J. Chem. 36, 1268 (1958). J2. TE'WARI, S. N., ANn GHOSI:[, S., Proc. Natl. Acad. Sci. India, Sect. A 21, 41 (1952). 13. SCHUYLENBORGH,J., Rec. tray. chim. 70, 985 (1951). 14. SCHUYLENBORGH,J., AND SANGER, A. M. i . , Rec. tray. ehim. 68,999 (1949). 15. MACKOR,E. L:, Rec. tray. chim. 70, 763 (1951). 16. LIJKLEMA, J., Kolloid-Z. 175, 129 (1961). 17. FnEYBV,RGEn, W. L., ANn DE BRUYN, P. L., J. Phys. Chem. 61,586 (1957). 18. IV~'ASAKI,I., AND DE BRUYN, P. L., J. Phys. Chem. 62,594 (1958). 19. XnUYT, H. R., "Colloid Science," Vol. 1. Elsevier, New York, 1952. 20. YATSIMIRSKII,K. B., ANn VASIL'EV, V. P., "Instability Constants of Complex Compounds." Pergamon Press, New York, 1960.