Electrokinetic Phenomena at the Thorium Oxide-Aqueous Solution

agreement with streaming potential data except in the presence of acidic solutions. The cell constant for bulk electrical conductance was observed, in...
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Electrokinetic Phenomena at the Thorium Oxide-Aqueous Solution Interface'

by H. F. Holmes, Charles S . Shoup, Jr., and C. H. Secoy Oak Rid& National Laboratory, Oak Ridge, l'enmsaee

(Received April 19, 1966)

Electrokinetic effects of aqueous solutions in porous plugs of thorium oxide have been investigated from the standpoint of irreversible thermodynamics. A kinetic electroosmotic technique was used to determine the electrokinetic potential. This method gave results in agreement with streaming potential data except in the presence of acidic solutions. The cell constant for bulk electrical conductance was observed, in some cases, to be dependent on the nature of the electrolyte within the plugs. Despite variations in cell constants, the measured surface conductances of several plugs and capillaries of thorium oxide could be interpreted in terms of the specific surface conductivity. The specific surface conductivities were much larger than those predicted from the classical theory of surface conductance. The mechanism of surface conductivity in this system was ascribed to the conductivity of ionizable surface hydroxyl groups.

Introduction According to the theory of irreversible thermodyn-cs,2 fluxes, J, (such as electric current, US flow, and chemical reaction rate), can be expressed by phenomenological relations of the type n

Jc = C L a X t k-1

(i

1,2,

.-..

TI)

(1)

Thus, a given irreversible flux is a linear function of the forces (here represented by x k ) present in the system, provided that the system is in a state of microscopic reversibility. If the fluxes and forces are chosen properly, the Onsager reciprocal relationa states that the matrix of the phenomenological coefficients must be symmetric, ;.e.

Lik = Lki

(2)

In the case of electrokinetic phenomena, the fluxes are electric current, I, and volume flow rate, dV/dt = while the forces at constant temperature are the potential gradient, E , and the pressure gradient, P. Thus

v,

I

=:

LiiE

+ LJ'

V=LJZ+LnP

Onsager's reciprocal relation has been shown to be valid for a wide variety of electrokinetic stem^.' Electrokinetic effects can now be expressed in t e m of the Phenomenological coefficients; for example, the streaming potential

The study of electrokinetic effects in porous plugs is fraught with pitfalls, both experimental and theoretical. Unless otherwise noted, subsequent discussion will be based on the following assumptions which are common to this type of investigation. (1) The electrokinetic system is in a state of microscopic reversibility. (2) The ratio of the dielectric constant to the viscosity of the solution (D/T) is constant throughout the double layer. (3) Either the geometry of the plug is independent of time and the nature of the electrolyte within its pores, or changes in geometry must be recognized and their effects determined. (4) The radius of curvature of the porea is much larger than the thickness of the double layer. For a plug of arbitrary geometry, the electroosmotic

(3) (4)

where Ln is the electrical conductance, & is the permeability, and Le is the electrokinetic coefficient with the Onsager reciprocal relation taken into account.

(1) Researoh sponsored by the U. S. Atomio Energy Commission under contract with the Union Carbide Corp. (2) 8. R. deGroot, “Thermodynamics of Irreversible Proceases," Interscience Publishers, h a . , New York, N. Y., 1961. (3) L. Onsager, Phys. Reo., 37, 406 (1931); 38, 2266 (1931). (4) D.G. Miller, C h . Rev., 60, 15 (1960).

ELECTROKINETIC PHENOMENA AT THE THORIUM OXIDE-AQUEOUS SOLUTION INTERFACE

flow rate due to an applied potential in the absence of a pressure gradient is5

rm,

3149

m CI

II I

integrated over the total cross-sectional area, A . Thus

vp4= DP - AE 47V

kb

(7)

where AE is the applied potential between the ends of the plug, is the electrokinetic potential at the slipping plane, and k b is the cell constant for electrolytic conductance through the bulk solution within the pores of the plug. From eq. 4 and 7

Le = DPl/4xqhb

(8)

where 1 is the length of the plug. The present paper describes a series of experiments undertaken for the purpose of studying the electrokinetic properties of the thorium oxide-aqueous solution interface in. porous plugs.

Experimental The porous plugs were constructed of powdered thorium oxide which had been prepared by calcining two different batches of thorium oxalate at 650" for 4 hr. in air. Part of batch A was divided into smaller portions, which were subsequently fired for 4 hr. more at 800, 1000, 1200, and 1400", respectively. Batch B was subsequently fired at 1600". Most of the investigations reported here were carried out on the material from batch A. The specific surface areas of the various samples, as determined by the B.E.T. nitrogen adsorption technique, ranged from 0.99 to 14.7 m.2/g., while the Xray crystallite sizes varied from 200 to more than 2500 8, As expected, the specific surface areas decreased, and the crystallite sizes increased with increasing firing temperature. The average particle sizes, which were independent of firing temperature, were 2.7 and 1.2 1.1 for batches A and B, respectively. Spectrographic analyses revealed that no impurities were present to an extent greater than a few parts per million. Deionized water and reagent grade chemicals were used at all times. The experiments were carried out at room temperature (24.7 * 0.4"), and the system wa.s protected from the atmosphere by Ascarite. The glass electrokineticcell is illustrated schematically in Figure 1. The powder, in the form of a thick aqueous slurry, was introduced into the electrokinetic cell and retained by a disk of filter paper at each end of

ill] Figure 1. Porous powder electrokinetic conductance cell: C, capillaries of identical known radii; F, filter paper disks; R, polyethylene or Teflon retaining rings; P, probe electrodes; T, porous plug of Tho*; W, working electrodes.

the plug. Probe electrodes of platinized platinum gauze were pressed against the filter paper by a tightly fitting polyethylene or Teflon ring. The plug w&s packed by a vacuum applied to the end opposite that which was being filled, resulting in a void space of 60 to 70%. The cell was constructed in such a way as to make each plug 1.00 cm. in length with a diameter of about 0.6 cm. A bright platinum electrode was installed at each end of the cell, a few centimeters removed from the plug itself, the pair serving as working (current-carrying) electrodes in the electroosmotic experiments. The cell was provided with vertical identical capillaries of known radius near each end of the plug. Blank runs were carried out with the cell completely assembled except for the thorium oxide, and no electrokinetic effects were observed. A.c. resistance measurements at 2000 C.P.S. were made with a Jones bridge coupled to an oscillator and tuned amplifier, with an oscilloscope used as the detector. Whenever the oscilloscope pattern indicated the presence of polarization, the resistance measurements at 2000, 1000, and 500 C.P.S. were extrapolated to infinite frequency and the extrapolated resistance used in all calculations. The plug resistance was measured across the probe electrodes. Previously calibrated conductivity cells were placed on both the entrance and exit sides of the electrokinetic cell in order to monitor the specific conductivity of the bulk solution, Ab, within the system. The cell constant for the plug was determined from the product of these two values (plug resistance and Ab) after Ab on the exit ( 5 ) J. Th. G. Overbeek in "Colloid Science," Vol. I, H. R. fiyt,, Ed., Elsevier Publishing Co., New York, N. Y., 1952, Chapter V.

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H. F. HOLMES, C. S. SHOUP, JR.,AND C. H. SECOY

3150

side of the plug was found to agree within 1%of the value of A b on the entrance side. Streaming potentials were determined as a function of the applied pressure by the use of a high input impedance pH meter with a millivolt scale. The POtential drop across the plug was measured with and without the application of a known pressure, and the difference was taken as the streaming potential. The applied pressure seldom exceeded 50 cm. of water although a residual pressure drop of up to 100 cm. of water due to gravity was sometimes present. The residual pressure drop was the same before and after the application of the known pressure and had no effect on the streaming potential measurement itself. The electrokinetic coefficient, Le, can be determined from e!ectroosmotic experiments in a number of ways. The two most common methods are: (1) determination of the electroosmotic flow rate under an applied potential by measuring the volume flow rate in a horizontal capillary of known radius and (2) determination of the electroosmotic pressure developed by an applied potential under the condition that no net flow occurs through the plug. In the present investigation, a kinetic electroosmotic technique was used to determine Le and Lz2 simultaneously. An electrical potential was applied across the working electrodes and adjusted to give a known potential drop across the probe electrodes. The latter was measured potentiometrically, with a Leeds and Northrup K-3 potentiometer serving as the primary potential-measuring instrument. The height of the liquid level in one of the two capillaries at the ends of the porous plug was then monitored as a function of time. If ho is the equilibrium height of the liquid levels with the electrodes shorted and hi is the height in either capillary at time t, then from eq. 4 arz dhldt = L e a

+ 2Lzzghp

(9)

where r is the radius of the two identical capillaries, p is the density of the solution, g is the acceleration due to gravity, h is hl - ho, and AE is the applied potential as measured across the probe electrodes. By following the change in height of one of the liquid levels with a cathetometer or traveling microscope as a function of time it is possible to use the linear relation

Ah/&

=

(Lelrr2)A.E

+ (2LZzpg/ar2)h

(10)

Here h is the average value of h during the time interval At, and Ah is the linear distance traveled by the same liquid level during that time interval. Potential gradients of 1 to 7 v./cm. and time in-

tervals of 100 to 300 sec. were used to obtain plots similar to that shown in Figure 2. From the intercept and slope of these plots, Le and Lz2, respectively, were determined by the method of least squares. It is to be noted in Figure 2 that line A does not pass through the origin as would be expected from eq. 10 for a truly constant permeability. Small deviations (always less than 10%) in both Le and Lzzwere characteristic of all the data obtained. Repeated experiments with the polarity of the electrodes reversed and, with the flow in first one direction and then in the other, indicated the presence of small degrees of both electrical asymmetry and hydrodynamic asymmetry. No method of either eliminating or reproducibly measuring these asymmetries was found. It was assumed that the best data were obtained by using the mean values found for Le and Lzzfrom the four combinations of electrical polarity and flow direction. In actual practice, it was only necessary to measure a few points in each of two groups well separated on the h axis and draw the best straight line through them.

-6

-5

-4

-3 -2

-f

i

0

2

3

4

5

6

7i (em) Figure 2. Typical electroosmotic data: NHdOH (pH 10.10) on porous plug of Tho2 ( k b = 9.70 cm.-'); A, E = 0.00 v./cm. (probe electrodes shorted); B, E = 3.30 v./cm. (open and closed points represent different experimental runs); C, E = 4.40 v./cm.; average = -20.7 i 5% mv.; average Ln = 4.13 X 10-9 i 5% cm.6 dyne-' see.-'.

r

Results and Discussion Cell Constants and Surface Conductance. Experimental measurements of the cell constant in porous plugs of Thoz using KC1 solutions revealed that it is highly dependent on the specific conductivity of the bulk solution, Ab, particularly at low electrolyte concentrations. The apparent cell constant, k', was observed to increase with increasing KCI concentration,

ELECTROKINETIC PHENOMENA AT THE THORIUM OXIDE-AQUEOUS SOLUTION INTERFACE

approaching a constant value with relatively concentrated solutions. This behavior implies that one or more additional conductance mechanisms are present. I n subsequent discussion the additional conductance is treated as surface conductance. With solutions of specific conductivity sufficiently high for the surface conductance to be negligible compared to the total conductance, the apparent cell constant was independent of A b and was therefore assumed to be equal to kb. Although k b for the various plugs ranged from 4.69 to 10.57 cm.-', the ratio k'/kb was very nearly the same at any given specific conductivity (Figure 5). In an attempt to determine if this phenomenon is unique with porous plugs, two ceramic Tho2 capillaries were obtained with geometrically measured cell constants of 94.5 and 144.4 cm.-l. The apparent cell constant of each capillary was measured with KC1 solution flowing through it. Initial resistance measurements showed a strong time dependence, indicating that the solution was penetrating slowly into the porous structure of the capillary walls. (The ceramic Tho2 was many orders of magnitude less porous than the porous plugs.) In an attempt to correlate the resistance measurements with the macroscopic capillary geometry, the solution was permitted to flow through a previously rinsed (deionized H20) and dried (120") capillary and its resistance measured as a function of time. Extrapolation of the resistance readings to zero time gave reproducible results. By following this procedure, the apparent cell constants were determined as a function of KCI concentration and are illustrated in Figure 3. The extrapolated values of k b for the two capillaries were found to be 97.3 and 145.6 cm.-l. These values compare quite favorably with the geometrically determined cell constants. This fact lends additional support to the use of k b values for the porous plugs as determined in concentrated solutions. The ratio k'/kb was observed to follow a curve similar to those for the porous plugs (Figure 5). If the electrokinetic system is represented as a pair of parallel conductance paths, the total conductance can be expressed as

where A, and k, are the specific surface conductivity and its associated cell constant. This equation can be rearranged to yield an expression for the specific surface conductivity

160

I

3151

I

I

0

140 kb (GEOM.) = f44.4 cm-

(20

-

'

io0

1

E

-2 80 4

60

40

20 2 5 0 (0-7

1

1

4

(0-6

10-5

40-4

40-3

40-2

10-4

100

(ohm-' cm-I)

Figure 3. Cell constants for Thoz capillaries; KCl solutions.

For a single capillary, k,/kb = a/2, where a is the radius of the capillary. In the case of the two Thoz capillaries used in the present investigation, this ratio is 0.045, and thus the specific surface conductivity of the Tho2 capillaries can be readily determined. Since A. is independent of geometry, it should be the same for the porous plugs as for the capillaries, thus making it possible to determine k, for the various porous plugs whose apparent cell constants have been determined by the use of KCI solutions at different concentrations, Once k, and k b have been established for a particular plug, it should be possible to determine A. for a variety of electrolytes. Measurements of the apparent cell constant as a function of the concentration of other eIectroIytes, however, revealed that the value obtained for k b was dependent on the electrolyte used for its evaluation. In general, the variation of k' with concentration of a given electrolyte followed a smooth curve. The use of different electrolytes often resulted in displaced curves, with k' approaching a constant value characteristic of the electrolyte used at high concentration within a particular plug. The nature of the variation of k b with the electrolyte used for its determination was complex and not entirely reproducible. In general, however, lod4 N or greater concentrations of Na&03 or acid solutions caused a downward shift in the values obtained for k' and thus a Volume 69,Numbm 9 September 1966

H. F. HOLMES, C.S.SHOUP,JR.,AND C. H. SECOY

3152

I

I

I

I

I

I

7

6

5 I

E

.

0 4

Y

-k

3

2

4

2 5

1.0

0.8 Q6

%) 0.4

Despite the complexities involved in the electrolyte shifts observed for the cell constants, the ratio k'/kb was found to be essentially independent of the history of the sample, provided the appropriate value of k b is used. This independence is illustrated in Figure 5 in which the variation of k'/kb with specific conductivity of the solution is shown to be the same for the capillaries and porous plugs, even in the presence of electrolyte shifts. The value of LB obtained for the porous plugs varied considerably from plug to plug, but all were of the order of magnitude of cm.6 dyne-1 sec.-l. For a given plug, LZ2was reproducible to within &lo%. In contrast to the variations of k' and k b with the electrolyte studied, the measured permeability of the plugs showed no dependence on the electrolyte present within the plug. Since permeability is more sensitive to geometry than the cell constant is, this implies that the cell constant variation is not necessarily due to a gross change in the geometry of the plug but may be due to some sort of surface effect. Since the ratio k'/kb was essentially the same for the plugs and capillaries investigated, the variation in the specific surface conductivity, A,, of "indifferent" electrolytes such as KCl must be less than the experimental errors involved in determining it. By use of eq. 11 and A, for KCl obtained from the capillaries, k, w a evaluated ~ for a limited number of plugs, making it possible to determine A, for electrolytes other than KC1. The specific surface conductivity so determined is illustrated in Figure 6 for a variety of plugs and capillaries. No significant differences were observed among the Tho2 plugs from different batches of Tho2 nor among the plugs which were prepared from Tho2 calcined at different temperatures.

0.2

t0-t i 0

IO-^

40-5

io-4

io-3

4b2

to-'

1 Ill

Ab (ohm-' Cm-')

Figure 5. Normalized apparent cell constant, k'/kb, aa a function of electrolyte specific conductivity: 0, A ThOz, plug No. 52 (after electrolyte shift); 0, A Thoz, plug No. 52 (before electrolyte shift); A, A ThOz, plug No. 31 and No. 50, HzO, KCI; A, capillaries, HzO, KCI.

lower value for k b than determined previously by the use of other solutions. Subsequent use of KC1 usually resulted in a variation of k' identical with that of the electrolyte used immediately prior to the introduction of the KC1 solution. Some of these effects are illustrat.ed in Figure 4 for a porous plug of Thoz from batch A. The Journal of .Physical Chemistry

I 11

I Ill

IIII

LiIM R I l

0

a

1

E

-2 c

L ! !!AL!Jl ! ! ! ! W 40-7

40-8 ._ 10-~

io+

10-~ Ab (ohm-'

163

cm-')

Figure 6. Specific surface conductivity aa a function of electrolyte specific conductivity: 0, He0 and KC1 in porous plugs; 0, HzO and KCI in Capillaries; 0, NH40H in porous plugs; A, NazCOa in porous plugs.

(0-2

ELECTROKINETIC PHENOMENA AT THE THORIUM OXIDE-AQUEOUS SOLUTION INTERFACE

An important feature of the results shown in Figure 6 is the fact that they are some two to three orders of magnitude larger than would be predicted from the classical theories of surface conductance.6 Most of these theories attribute surface conductivity solely to an excess of ions in the diffuse part of the double layer while assuming that those ions in the Stern layer are immobile. Most of the reported surface conductivity values have been obtained from experiments with the glass-aqueous solution interface. Although highly divergent, these reported results are consistently larger than would be predicted from the classical theories. However, Urban, White, and StrassneP succeeded in agreeing fairly well with the experimental results by adding to the classical theory a term involving the mobility of ions in the Stern layer. McBain and Fosteflb also concluded that their experimental surface conductivities for glass were much too large to be attributed solely to ions in the diffuse layer. Failure of the classical theories of surface conductance is even more strikingly illustrated by considering reported results for materials other than glass. For comparison with Figure 6, theoretical values for specifk surface conductivity range from about to 10” ohm-’, with the larger values obtained by also considering the mobility of ions in the Stern layer. This discrepancy between the experimental and theoretical values is also apparent in the reported surface conductivities for Ti02,’ Sn02,8 and Fe20J although the authors attributed this excess surface conductivity to the semiconducting properties of the bulk oxide.’ Although Thoz has some semiconductor properties at very high temperatures, this characteristic is negligible at ordinary temperatures. It would appear that there is an extra surface conduction mechanism which can be many times larger than those considered in the classical theory. Many workers, in interpreting electrokinetic data for oxide surfaces in aqueous media, have used an acid-base model involving ionization of surface hydroxyl groups. It seems equally probable that this same model can qualitatively explain the large surface conductivity of oxide surfaces. Several features of Figure 6 point to this mechanism. One of these is that the surface conductivity is relatively independent of the bulk conductivity (bulk ionic concentration). This fact indicates that the concentration of the surface conducting species is also relatively independent of bulk ionic concentration. Actually, one would expect that, in the absence of strong specific adsorption, the surface would be saturated with hydroxyl groups regardless of bulk ionic concentration. If the major mechanism for surface conduction is ionization of surface hydroxyl

3153

groups, it should be dependent on pH. Evidence for this is seen in Figure 6 with the consistently high values obtained with NH40H. Further evidence for this may be seen in the consistently high values for the surface conductivity of glass in solutions of HC1 and HN03.6 In all fairness it should be pointed out that the results for the Tho2 surface conductivity in Na&03 solutions are slightly, but consistently, lower than those in KC1 solutions. According to the present hypothesis, the results for Na2C03 should more closely resemble those for NH40H since Na2C03 solutions are basic. It is quite possible that, in this case, we are dealing with strong specilk adsorption. As a matter of fact, the concept of an immobile surface carbonate ion has been used to explain the adsorption of C02 on ThOz.g It should also be pointed out that a model involving ionizable surface hydroxyl groups has been used in interpreting electrokinetic data obtained for the Thoz surface in aqueous solutions. The relationship between surface conductance and the electrokinetic potential will be discussed in a subsequent section. Electrokinetic Polentiah. The observed streaming potential was a linear function of the applied pressure in all cases, as it should be in the absence of turbulent flow. With neutral or basic solutions the values of Le determined from streaming potential and electroosmotic measurements under identical conditions were in good agreement. Once the plug had been exposed to acidic solutions, however, the electroosmotic experiments almost invariably gave values for Le that were considerably smaller in absolute value than those from streaming potential measurements. According to the Onsager reciprocal relationJa identical values of Le should have been obtained from the two types of experiments. Differences in the electroosmotic and streaming potential data may be related to the unexplained variation of the cell constant with the nature of the electrolyte which was discussed in the previous section. Experimentally identical values of Le could be obtained by using cell constants calculated from electrical conductivity measurements taken with KCI as the electrolyte before and after exposure to acidic solutions for the streaming potential and electroosmotic data, respectively. However, the data were not (6) (a) F. Urban, H. L. White, and E. A. Strassner, J. Phys. Chem., 39, 311 (1936); (b) J. W.McBain and J. F. Foster, ibid., 39, 331 (1936). (7) D. J. O’Connor,N. Street, and A. S. Buchanan, Australian J . Chem., 7 , 245 (1946). (8) D.J. O’Connor and A. S. Buchanan, ibid., 6,278 (1953). (9) C. H.Pitt and M. E. Wadsworth, “Carbon Dioxide Adsorption on Thoria,” Technical Report, University of Utah, Salt Lake City, Utah, Feb. 15, 1968. (10) P. J. Anderson, Trans. Faraduu SOC.,54, 130 (1968).

Volume 69,Number 9 September 1966

3154

H. F. HOLMES,C . S. SHOUP, JR.,AND C . H. SECOY

treated in this manner, for there seem to be no valid reason for using two different cell constants for the identical porous plug. This anomaly should not be taken as implying failure of the Onsager reciprocal relation in acidic solution as it is probably due to an inadequacy in the classical experimental definition of the phenomenological coeBcients. Failure of one or more of the four basic assumptions listed in the introduction may be responsible. Resolution of this important and presently unexplained anomaly must, of necessity, await further experimental work. Some of the pertinent electrokinetic potential data are shown in Figure 7. Tho2 samples from batches A and B are seen to have significantly different electrokinetic potentials in KC1 solutions. As a matter of fact, no positive electrokinetic potential was ever observed with Tho2from batch B. If one compares the electrokinetic potential of batch A in Na&Oa and NazSO, solutions, one infers that there is specific adsorption of the carbonate ion. This is in line with the postulated effect of N&Oa on surface conductance. The effect of pH on the electrokinetic potential is illustrated in Figure 8. Although the Thoz from batch A had an isoelectric point at a pH of approximately 9.5, no positive electrokinetic potential was observed for batch B over the pH range studied. The isoelectric point for batch A agrees quite favorably with that obtained from electrophoresis measurements on other Thoz samples.11J2 However, the values of the reported electrokinetic potentials were different in all cases. Brief investigations of Thoz plugs from a variety of sources, including single crystals, indicate that the electrokinetic potential is strongly influenced by the method of preparation. For example, an increase in the calcining temperature of batch A produced an increase in the electrokinetic potential of Tho2 in pure water or very dilute KCI solutions. The effect of calcining temperature has also been noted in heat of immersion measurement~l~on Tho2 samples from batch A. The effect of calcining temperature on the electrokinetic potential of Thoz has been noted briefly by Anderson.1" D o u g h and Burden1' obtained different electrokinetic potentials for Tho2 before and after grinding the sample to a smaller particle size. The three samples of Tho2 studied by Sowden and Francis12had approximately the same isoelectric point but gave very different results for the electrokinetic potential and surface charge density. The complex behavior of the electrical double layer is not limited to the thorium oxide-aqueous solution interface. Of the three samples of Sn02 investigated by O'Connor and Buchanan,*two gave positive electro-

30

The JOUTVAZ~ of Physical Chamistry

20

-

(0

>

E o

&Jl

-io -20

-30

(o-'

to+

io-6

40-~

Ab (ohm-' C m - ' )

Figure 7. Electrokinetic potential a.a a function of electrolyte specific conductivity; open points, by streaming potential, closed points, by electroosmosis: e, A Tho2, plug No. 30, KC1; AA, A ThOz, plug NO. 51, NazSOa; 0, A Thoz, plug NO. 52, N@COa; OB, B Tho?, plug NO. 60, KCl. 60 40

5 &n

20

0

-20 -40

-60 4

5

6

8

7

( 0 1 1 4 2

9

DH

Figure 8. Electrokinetic potential 88 a function of pH; open points, by streaming potential; closed points, by electroosmosis: O.,A Thoe, plug No. 50, NHlOH and HC1; 0, A Thoz, plug No. 52, N%COa; Om, B Thoz, plug No. 60, NH40H and HC1.

kinetic potentials while the third was negative. The initial positive electrokinetic potential of A1203could be changed to negative by ignition above 1000" and then back to positive by grinding lightly.14 Morimoto and Sakamoto15 attributed their results for the dependence of the electrokinetic potential of Ti02 on calcining temperature to a phase transition between ~

~~

~

(11) H. W. Douglas and J. Burden, Trans. F u T u Soc., ~ ~ ~55, 350

(1959). (12) R. G . Sowden and K. E. Francis, Nucl. Sci. Eng., 16, 1 (1963). (13) H.F. Holmes and C. H. Secoy, J. Phys. Chem., 69, 151 (1965). (14) D. J. O'Connor, P. G . Johansen, and A. S. Buchanan, Trans. F a r d a y Soc., 52, 229 (1956). (15) T. Morimoto and M. Sakamoto, Bull. Chem. SOC.Japan, 37, 719 (1964).

ELECTROKINETIC PHENOMENA AT THE THORIUM OXIDE-AQUEOUS SOLUTION INTERFACE

anatase and rutile. Quantitative differences in their three samples of TiOz were ascribed to differences in impurities. However, Tho2 has no solid phase transition, and samples from the same batch (identical impurity concentration) show a dependence on calcining temperature. The electrokinetic properties of the Ti02surface has also been shown to be dependent on the size of the particles.lB It is interesting to compare the pH dependence of the electrokinetic potential for batch B with the results of Streetll for synthetic sapphire. He found that while powdered specimens exhibited an isoelectric point, bulk specimens did not. Apparently, the properties of the electrical double layer at an oxide-aqueous solution interface are complex functions of the chemical and thermal history of the sample. The behavior of amorphous surfaces such as glass is not quite so complex.6 Until this dependence can be put on a concrete basis, electrokinetic results for such systems must be assumed to be valid only for a specific sample. This fact has also been emphasized by Andemonlo in his work on Thoz. Surface Conductance and the Electrokinetic Potential. Classical theories of surface conductance treat this

3155

phenomenon as an exponential function of the electrokinetic potentiaL6 An inspection of Figures 6, 7, and 8 fails to reveal any obvious relationship between surface conductivity and the electrokinetic potential. The exponential dependence in the classical theories arises from a consideration of excess ions in the double layer due to the potential difference. It seems logical that these excess ions would make a contribution to surface conductance. However, in the present system, and others a9 well, it is quite probable that the contribution of the excess ions in the double layer is masked by a second mechanism which is much larger in magnitude. It is equally probable that this second mechanism is the conductivity of ionizable surface hydroxyl groups. Perhaps a better test of the classical theories of surface conductance would be a system in which the solid surface is not so intricately associated with the liquid a9 is the case in oxide-aqueous solution systems.

E. J. W. Verwey, Rec. trav. chim., 60, 026 (1941). (17) N. Street, Australian J. Chem., 17,828 (1904). (10)