THE ELECTROKINETIC (ZETA) POTEYTL4L OF T H I N METAL FILiVIS1 GRANT W. SMITH
AND
L. H. REPERSON
School of Chemistry, University of Minnesota, Minneapolis, Minnesota
Received September 6, 1958
One of the problems in the study of the electrokinetic potential has been the proof that the potentials as measured are different from the thermodynamic potentials of Nernst. The development of the glass electrode by Haber and Klemensiewicz enabled investigators to compare the potentials of this electrode in various solutions of electrolytes with the potentials obtained by streaming the same solutions through glass capillaries made from the same glass as the glass electrode. Measurements of this kind have shown a real difference in the values of the potential as measured by the two methods. The glass electrode behaved essentially as a hydrogen electrode, while the measurements of potentials by streaming solutions through glass capillaries showed that these potentials were sensitive to small changes in ion concentrations, especially if the ions were polyvalent. In addition it has been shown that the glass electrode is far from a simple system. As a result it is doubtful whether a real comparison can be made between the two potentials by the use of glass. The desirability and importance of finding a simple metal-solution system for streaming potential study becomes clearly evident. Previous work in this laboratory has shown that silica gel could be metallized so that the extensive surface of the gel was well covered by a metal deposit. The evidence further showed that metal layers could be made thick enough to show the crystal structure of the metal in x-ray studies. Accordingly it was felt that such metallized gels might be used in streaming potential measurements and that a direct comparison with the thermodynamic potential thus could be obtained. The gel itself would act as an insulator having the metal film on it. In the following investigation, streaming potential measurements are reported for nickel and silver surfaces in solutions containing the ions of the metals used. These metals were chosen so as to have one metal above and the other below hydrogen in the electromotive force series. The results on two such metals would 1 The material here presented formed a part of a thesis submitted t o the graduate faculty of the University of Minnesota by Grant W. Smith in partial fulfillment of the requirements for the degree of Doctor of Philosophy, June, 1932. 133 THE JOURNAL OF PHYSICAL CHEMISTRY, VOL. XXXYIII, NO.
2
134
GRANT W. SMITH AND L. H. REYERSON
more nearly answer the question of the similarity or dissimilarity of the two , potentials. EXPERIMENTAL PROCEDURE
The metallized silica gels were prepared by allowing a purified silica gel to adsorb the complex ammonia ions of silver, Ag(NH&+, or of nickel, Ni(NH8)4++,from an aqueous solution. Silica gel adsorbs these cations very strongly, 1 g. of silica gel adsorbing as much as 0.32 g. (1.692millimoles) of the silver complex and 0.31 g. (1.10 millimoles) of the nickel complex.2 After adsorption had reached equilibrium the liquid was drained from the gel, the gel was dried, and finally heated in a current of hydrogen until the complex salt in the gel was reduced and the metal left in a free state on the surface of the gel. The product was a deep black, and microscopic examination of the granule showed no uncovered silica surfaces. In order to make sure that the metallized gels were sufficiently coated, they were analyzed by means of x-rays. Using copper radiation, definite patterns of the metals deposited on the gel were obtained. Silver wire and nickel wire produced patterns which were identical with those of the corresponding powdered gels. The lines of the nickel gel were faint but definite, Both gels gave broader lines than the metal wire, indicating that the metallic film was very thin and of approximately colloidal dimensions. The apparatus used in this investigation consisted of three essential parts: a diaphragm of finely divided material packed between two perforated electrodes, with a system for forcing the liquid through the diaphragm under known hydrostatic pressures; a potentiometric device for measuring the E.M.F. developed between the two electrodes; and a conductivity bridge. The streaming potential cell was of the same general type as that used by Briggs, except that some of the dimensions were altered to suit the particular case a t hand. The ring in which the diaphragm material was packed was made of clear Bakelite, furnished by the Bakelite Corporation. Its outside diameter was 4.5 cm.; its inside diameter, 3.5 cm. It was 2.5 cm. in width. The two electrodes were made to fit on either side of the ring. They were of 14 K gold, 4.5 cm. in diameter and 1 mm. thick. The electrodes were perforated with 0.5 mm. holes t o within 0.5 cm. of the circumference, the holes lying as closely together as they could be drilled without weakening the electrodes too much. A platinum wire was soldered with gold solder to the edge of each electrode to form a connection with the electrical measuring instruments. The two flared tubes which formed the rest of the cell were made of Pyrex glass. The cell was assembled with thin rubber gaskets to insure against legks, and the whole was clamped tightly together with a specially constructed clamp. 2 Unpublished results of a series of similar experiments in this laboratory confirm the strong adsorption of complex ammonia ions in ammoniacal solution.
ELECTROKINETIC POTENTIAL OF THIN METAL FILMS
135
Figure 1 shows the complete cell and pressure system. Z is the zeta potential cell which has just been described. M is a mercury manometer by which pressures up to about 80 em. can be read. N is a cylinder of nitrogen which was used to supply the pressure on the streaming liquid; V is a reducing valve for controlling the pressure. SI,Sz,Sa,and SA are stopcocks, and A1 is a “valve” controlled by a screw clamp. F1 is a 5-liter Pyrex flask in which the liquid which is to be streamed through the cell is placed. It is fitted with a three-hole stopper through which go the inlet tube from the pressure line, the outlet tube to the cell, and a closed-end glass tube holding a piece of ruled paper for measuring the water level. E is a clamp. F2 is another 5-liter flask which is used to catch the waste liquid
FIQ. 1. THE COMPLETE CELLAND PRESSURE SYSTEM
from the cell. C is a piece of flexible metal tubing which is grounded, and is used to carry off the waste liquid after it has streamed through the cell. This was found to be necessary, for the flow of liquid down a glass tube caused very disturbing electrical effects. D is a sulfur block holding two mercury cups in which dip the leads from the cell and from the measuring instruments. A and B are “rocker” switches, consisting of mercury cups set in sulfur blocks. B is a reversing switch which reverses the direction of the E.M.F. of the cell with reference to the potentiometer in the circuit. When A is to the left, the cell is in the circuit of the conductivity bridge; when it is to the right, the cell is in the potentiometer circuit. HI shows the leads to the bridge, while Hz shows those to the potentiometer. GS is
136
GRANT W. SMlTH AND L. W. REYERSON
grounded shielding about the leads from the cell, the cell itself, and the switches, A and B. The shielding about the lead wires consists of 1$inch brass tubing. The cell is shielded with copper gauze. The switches are in a copper switch box, SB, made of copper sheeting. It was originally intended to use a quadrant electrometer as the null instrument in the measurement of the E.M.F. of the cell, but vibrations in the room used for this investigation made it impossible, Several cushion and suspension mountings were tried without avail. A vacuum tube potentiometer was then built for the purpose. The circuit used was much
j i
i /
I
1 I I
i I
iI i
I
FIQ.2. THE ELECTRICAL CIRCUIT
AND THE W I R I N Q DIAQRAM
the same as described by Fosbinder (4) (see also Nottingham (6)). The set-up as constructed employs a potentiometer as a null instrument, and a screen grid vacuum tube as amplifier. It draws only a negligible amount of current from the cell during adjustment and no current a t all a t the balance point. Furthermore it is not affected by ordinary mechanical vibrations. Figure 2 gives both the electrical circuit and the wiring diagram. The principle of operation is as follows: With the grid circuit open, the plate current through a sensitive galvanometer (G) is adjusted by means of a
ELECTROKINETIC POTENTIAL OF THIN YETAL FILMS
137
shunt resistance and battery (R1, Rz, R3, and B3) until there is no galvanometer deflection. Now the grid circuit is closed by means of a switch (S1), and adjusted by a shunted resistance and battery (R4, Rg,Rs, and Bg) until there is again no deflection on the galvanometer in the plate circuit. This adjustment has brought the grid to the same potential it was a t when the grid was open, Le., a t “floating potential,” as it is sometimes called. The switch (S1) is now opened, and another switch (S3) is closed, which brings the cell and the potentiometer into the grid circuit. The potentiometer is now balanced against the cell in the usual manner, until there is again no galvanometer deflection, and the reading on the potentiometer is the potential difference across the cell. KO current is drawn from the cell a t ’the balance point, for the grid is always at “floating potential” just as though it were an open circuit, Le., as though the resistance of the grid circuit were “infinite” as it was when the first adjustment was made. The vacuum tube is of the screen grid type, UX 222. MA is a milliameter with a full scale deflection of 1.5 milliamperes; during use this reads about 0.5 milliampere. R1, Rz, and R3 are radio rheostats with resistance8 of 10,000 ohms, 5,000 ohms, and 100 ohms, respectively. R4 and Rg are 6-ohm and 100-ohm rheostats, and R7 is a 30-ohm rheostat. Re is a radio potentiometer of 2000 ohms resistance. B1 is an ordinary dry cell. Bz is the potentiometer battery, two dry cells in series. B3 is a 4.5-volt battery composed of nine dry cells in series-parallel. B4 is the plate battery, two heavy duty 45-volt “B” batteries. Bg is a lead storage battery of 6 volts. A is a Leeds and Northrup 40,000 ohm Ayrton shunt. G is a Leeds and Northrup # 2420c galvanometer. L indicates the leads to the zeta potential cell (see Ha, figure 1). P is a Leeds and Northrup Type K potentiometer. SI, Sz,and S3 are specially constructed switches of small electrical capacities. SC is a Leeds and Xorthrup Weston standard cell, #7307. SAis a double throw double pole switch by means of which the galvanometer may be brought into the Type K potentiometer circuit so that the latter may be checked with the standard cell, or into the vacuum tube plate circuit. When Sq is in the latter position, Sg must be closed to short the galvanometer connections on the Type K potentiometer. GS is grounded shielding, indicated throughout the wiring diagram by broken lines. The wire leads are shielded with metal tubing. The switches, 51, Sz,and S3are enclosed with the vacuum tube in a large copper box, and are controlled from outside conveniently by means of threads which run over pulleys to some little distance from the box. The vacuum tube is mounted on a cast sulfur base, inside a small copper box which is suspended by rubber bands in the larger copper box which also encloses the switches mentioned above. It was found necessary to shield all vital parts carefully, as shown in the diagram. The whole apparatus was shielded as completely as was practical by building a large “cage” of copper gauze about
138
GRANT W. SMITH AND L. H. REYERSON
it. The potentiometer was found to be very satisfactory in this work, being unaffected by vibration in the building. It proved to be very sensitive also, for a change of 0.0001 volt on the potentiometer from the balance point was easily detected on the galvanometer. Figure 3 shows the conductivity bridge. The arrangement is adapted from the Wheatstone bridge circuit used by Jones and Josephs ( 5 ) . The apparatus was modified in several particulars, but the working principle is the same. The oscillator is shown at 0; it is a vacuum tube oscillator, and although it was constructed so that it created little external field, it was
0
R
R
W
-
A
WB FIG.3. THECONDUCTIVITY BRIDGE
placed a t a distance of about ten feet from the rest of the apparatus. TI is a “power tube,” UX 171 A. BI is the filament battery, a 6-volt storage or Edison battery. BZis a “B” battery of 67.5 volts. L1, Lz, and LBare the grid, plate, and output coils, respectively. They are wound about an iron core, with B 30 B & S gauge enameled wire, and mounted in a copper box to shield them from the rest of the apparatus. The plate coil, Lz. is tapped in four places, and by means of the multiple switch shown in the diagram the pitch of the sound may be varied readily. C8 is a fixed condenser of 0.11 microfarad capacity, found suitable by trial.
ELECTROKINETIC POTENTIAL O F THIN METAL FILMS
139
A vacuum tube amplifier is shown a t A. Tz is a UX 201 A tube. B3 is a 6-volt storage battery; BAis a “B” battery of 90 volts. P i s a set of phones. T R is an audio transformer of four to one ratio. The Wheatstone bridge itself is a t WB. W is a Wheatstone bridge of the dial type, equipped with a multiplier. R and R are variable resistances of 10,000 ohms each. C1 is a variable condenser of 0.0005 microfarad capacity. Cz is a variable capacity consisting of seven condensers in parallel: a variable capacity of 0.0005 microfarad, and six fixed condensers of respective capacities, 0.0005, 0.001, 0.002, 0.002, 01.004, and 0.1 microfarad, so that any desirable values from 0.0 to 0.02 microfarad may be obtained by manipulating the switches properly. Sr is a switch used to connect the phones either to earth or to the bridge, as shown. L indicates the leads to the cell (see HI, figure 1). The operation of the bridge is as follows : With S3in position so that the phones are in the bridge circuit, the bridge, W, and the capacity, Cz, are adjusted so that the sound is as near a minimum as possible; S3is then thrown to the other position, grounding the are adjusted until phones, and the resistances, R, R, and the condenser, the sound is again a minimum. This adjustment brings the phones to earth potential. Now S3is put back in its original position, and Wand Czfurther adjusted until a minimum is reached. If the position of W is much different from its former setting, the process should be repeated. In this manner, a very distinct minimum point is obtained. The two points on the bridge (see upper diagram) which are joined to the phones are now both a t earth potential, and noises which are due to the difference in potential between the phones and the earth are eliminated. In fwt, in the operation of this conductivity bridge, a point of dead silence wafi practically always obtained at a very sharply defined dial position on W. The resistance can be determined accurately to four figures, and the multipliers on W can be used for very large or very small resistances. The water which was used for streaming through the (:ell and for making all solutions was conductivity water which was reboiled just before use. Its specific conductivity a t room temperature ranged between 0.9 X and 1.6 X mhos, and was generally about 1.1 X lo-+ mhos. Mallinckrodt’s “Reagent Quality” nickel nitrate and silver nitrate were used without further purification for making up the streaming solutions. Since from three to five liters of solution were prepared for each streaming potential run, they could be made up with considerable accuracy even though the concentrations were only fractions of millimoles per liter. The volumetric apparatus and the weights used to weigh the salts were calibrated. The greatest care was exercised a t all times to prevent contamination of the water and solutions while handling, and the air over the liquids was displaced by nitrogen from the pressure tank (N, figure 1) as soon as they were placed in the apparatus in preparation for a run.
140
GRANT W. SMITH A S D L. H. REYERSON
The diaphragm was prepared in the following manner. The diaphragm material, Le., the silica gel, was ground in an agate mortar and separated by screening into four groups according to size, (1) larger than 28 mesh, (2) between 28 and 65 mesh, (3) between 65and 200 mesh, and (4) smaller than 200 mesh. The diaphragm was made up almost entirely of the finest material (4). The coarser gels were used only t o fill in the holes in the electrodes so that the finely powdered material would not be washed away when the liquid was forced through the cell. That is, right next to the electrodes on either side, was, first, a thin layer of the coarsest gel, followed by a thin layer of the next size, and then a layer of the next finest gel. With the holes well stopped in this manner, the rest of the diaphragm was made up with the gel of finest particle size. This was kept moist with pure water and packed very tightly. METHOD OF MAKIXG A RUN
The cell was now assembled in place as shown in figure 1. The water or aqueous solution was put into the reservoir flask (figure 1, F1) and the air above it displaced by nitrogen. By applying pressure, the liquid was then streamed through the cell at a very slow rate for at least twenty-four hours before any readings were taken. It was found that about this length of time was required for the diaphragm and liquid to reach equilibrium. If too short a time were allowed for this operation, reproducible stream potentials could not be obtained. It is to be noted that, owing to the porous nature of the diaphragm materials, a considerably longer time was required for equilibrium conditions to be set up than would be the case with such materials as glass, quartz, cellulose, and other substances which have been used hitherto in streaming potential work. About twenty minutes were required for the potentiometer to reach a steady state so that readings could be made. The liquid was streamed through at a given pressure until a t least three consecutive readings in good agreement were obtained over a period of several minutes. Immediately after measurement of the E M . F . the mercury manometer was read, the difference in level between the liquid in the reservoir and the liquid leaving the cell was taken (pressure correction), the temperature read, and the conductivity was measured by switching the cell over to the bridge circuit. The pressure was then changed and the process repeated. The potentiometer was frequently checked with the Weston standard cell. Since the laboratory in which this work was done was an inner room, the temperature was quite constant, and was always near 25°C. After all of the desired runs were made upon any one diaphragm, the cell constant was determined by streaming a standard N / 5 0 potassium chloride solution through it until a constant resistance was reached. The specific conductivity of the liquid in the diaphragm could then be calculated for each run.
ELECTROKINETIC POTENTIAL OF THIN METAL FILMS
141
RESULTS
The values of the electrokinetic (zeta) potential are calculated according to the expression used by Briggs (l),
where H is the electromotive force set up across the diaphragm, K~ is the specific conductivity of the liquid in the diaphragm, rl the viscosity of the solutions, P the hydrostatic pressure, and t the dielectric constant. Using the proper constants at 25°C.) r(volts)
=
97.165 X
HK,
P
(2)
where H is measured in millivolts, K~ in reciprocal ohms, and P in centimeters of mercury. Bull and Gortner (2) suggest, in view of the existing doubt as to the correct value of the dielectric constant which should be used in the formula, that the value of the electric moment of the double layer be used to express the capillary electric conditions a t the interface instead of the electrokinetic potential. The electric moment of the double layer is equal to the charge per unit area multiplied by the distance between the layers. This expression was met in the derivation of the formula for the streaming potential. It is (3)
If the value of { from equation 1be substituted in equation 3 we obtain the following expression for the electric moment
Thus the value of the dielectric constant has been eliminated from the calculation. In the tables given in the following sections the values of { are negative unless otherwise indicated. The value of the electric moment p was also calculated and included in the results. The negative value of [ means that the fixed wall bears a negative charge with reference to the movable liquid layer. I n the preliminary measurements purified silica gel was used for'the diaphragms. The H I P ratio was found to be constant a t pressures above about 25 em., but the values fell a t lower pressures. Bull and Gortner (3) ascribe a similar result on their quartz diaphragms t o a difference in the size of the particles composing the diaphragm. In the present case the fall in H I P values in the low pressure range may be attributed logically to the
142
GRANT W. SMITH AND L. H. REYERSON
porous character of the material, rather than to variations in particle size. It was found that gels prepared under different conditions gave different potential values, so that samples of the same gel were used throughout the entire investigation. Table 1 gives a typical set of determinations on pure silica gel. TABLE 1 r-Potential of water-silica gel t = 24.9% Average K~ = 2.04 X
,
TABLE 2 r-Potential of water-nickel t = 24.8% Average K~ = 2.6 x 10-5
P
cm.Hg
72.6 53.4 52.0 58.9 48.5 37.3 22.0 11.6 12.3
mu.
mv. ~
153.8 115.2 112.0 125.5 103.8 78.5 48.5 17.7 18.9
2.12 2.16 2.15 2.13 2.14 2.11 1.97 1.53 1.54
4.2 4.3 4.3 4.2 4.2 4.2 3.9 (3.0) (3.0)
f
___-
cm. Hg
mv.
47.2 69.6 69.3 55.5 54.9 30.4 29.7 18.1 38.6
110.0 172.8 172.7 134.0 132.0 67.7 65.9 37.6 88.9
mv
2.33 2.48 2.49 2.41 2.40 2.22 2.22 2.08 2.30
.
6.0 6.3 6.3 6.2 6.1 5.7 5.7 (5.3) 5.9
Average, . . . . . . . . . . . . . . . . . 6 . 0
TABLE 3 r-Potential of N/10,000 nickel nitrate-nickel t = 25.4"C. K~ = 4.47 X P
1
H
em. Hg
mc .
48.7 47.9 65.8 65.6 75.5
74.8 72.8 95.6 95.2 113.0
I H I P I
f
TABLE 4 r-Potential of N/6000 nickel nitratenickel t = 25.4%. Average K~ = 5.09 x
r
mu.
1.54 1.52 1.45 1.45 1.50
Average. . . . . . . . . . . . . . . . .
6.6 6.6 6.3 6.3 6.5 6.5
cm. Hg
mv .
49.3 73.5 72.9 52.0 63.9 34.5 25.8
58.4 86.4 85.0 61.4 74.2 41.6 31.4
mu.
1.19 1.18 1.17 1.18 1.16 1.20 1.21
5.9 5.8 5.8 5.8 5.7 5.9 6.0 (over)
~~
Average, . . . . . . . . . . .
5.9
Following the preliminary determinations of the electrokinetic potentials of water-silica gel, it was decided to limit the present investigation to the study of gels metallized with nickel and with silver. This gives one metal above and one below hydrogen in the E.M.F. series.
143
ELECTROKINETIC POTENTIAL OF THIN METAL FILMS
Tables 2, 3, 4,5, and 6 give the results of electrokinetic potential measurements on nickel-coated silica gel in pure water, N/10,000 nickel nitrate, N/5000 nickel nitrate, N/2000 nickel nitrate, and N/1000 nickel nitrate solutions, respectively . Figure 4 shows these results graphically and compares the electrokinetic potential with the Nernst potentials for the same concentrations of Ni++. The full curve gives the character of the change of the {-potential with TABLE 6 r-Potential of N/1000 nickel nitrate-nickel t = 25.0"C. Average K~ = 1.71 x 10-4 P
H
cm. Hg
mv.
59.7 73.8 73.5 41.5 40.7
0.0 0.0 0.0 0.0 0.0
HIP
-__________
0.0 0.0 0.0 0.0 0.0
Average.. . . . . . . . . . . . . . . . .
I mv .
1
H
em. H g
mv.
58.2 73.6 73.4 63.6 46.4
8.6 14.9 14.5 10.5 3.7
P
_
H
_
HIP
r
_
em. Hg
mv.
0.0 0.0 0.0
36.6 75.7 74.7 46.7 32.0
0.0 0.0 0.0 0.0 0.0
0.0
Average . . . . . . . . . . . . . . . . . . 0.0
0.0 0.0
TABLE 7 I-Potential of N/lO,OOO silver nitrate-silver t = 26.2"C. Average K~ = 2.32 x 10-4 P
~
~
mu.
0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0
TABLE 8 f-Potential of water-silver t = 25.2"C. Average K~ =
I
] H I P I
I
cm. Hg
mn.
mv.
0.148 0.202 0.197 0.165 0.079
3.3 4.5 4.3 3.6 1.7
75.4 66.0 57.4 41.0 53.9
112.9 101.3 95.6 70.1 86.4
mu.
1.50 1.53 1.66 1.71 1.60
7.3 7.2 7.8 8.0 7.5 7.6
concentration, while the broken line shows the €-potential values through a like change in concentration. In order to get both curves on the same figure, it was necessary to use a different scale for plotting the two potentials. This is indicated on the margin in figure 4. Following the work on the nickel-coated gels, a series of measurements were carried out on the silver-coated gel. Table 7 gives a typical set of results. It was apparent that the silver gels were not giving uniform
-
144
GRANT W. SMITH AND L. H. REYERSON
results. Figure 5 shows the dependence of H on the pressure for three solutions of silver nitrate as well as for pure water. The values given in table 7 are shown as No. I1 in figure 5 . Theoretically the curves in the pressurepotential diagram should be straight lines passing through the origin. The curve No. I for water is the only one that passes through the origin. I t appears as though some positive potential were being superimposed on the negative streaming potentials. This is approximately given by the intercept of the extended curves on the ordinate. If this is corrected for by -7
-6
-5
-4 ? . . .
-”
- -3 4
e .2
,u
-2
-1
0
tl 1
2
3
4
5
Concentration (Nor&.ity
7
6
x
0
9
10
io4)
FIG.4. NICKEL-NICKELNITRATE
moving the curves parallel to themselves until they pass through the origin, then the values seem reasonable as shown in figure 6. Here the curves of figure 5 have been moved parallel to themselves until they passed through the origin. An additional set of values for N/1000 silver nitrate has been added. The anomalous behavior of the silver gels was early shown t o be due to other factors than the silver ions. Measurements of the streaming potentials for dilute silver nitrate solutions on pure silica gel gave uniform results. It was then suspected that the silver gels might be acting as an
-40
-30
-
-20
-
-10
-
I 1
Pure H20
N/IOOOO Ay NO, .2l N/SOOO AJN03 .E N A O O O AyN03
0
10
20
40
30
P
50
60
70
80
(cm. Hq)
FIG.5 . ABNORMAL SILVER-SILVER NITRATECURVES
+Lot 0
P N/IO 0 0 AjN0,
10
20
30
40
P
50
60
70
(cm.Hg)
FIG.6. CURVESFROM FIG.5 MOVEDTO 145
A
COMMON ORIUIN
80
146
GRANT W. SMITH AND L. H. REYERSON
oxygen electrode. A sample of the silvered gel was ground and resilvered. Every precaution was taken t o keep oxygen from the gel or the solutions used. The solutions were prepared and stored under nitrogen. The silvered gel was kept in a hydrogen atmosphere until used. The streaming potential results on these gels gave the same anomalous results. I n fact TABLE 9 r-Potential of N/IO,OOO silver nitrate-silver t = 25.5%. Average K~ =
TABLE 10 f-Potential of N/b000 silver nitrate-silver i = 25.8%. Average Ks =
P cm. Hg
mu.
70.6 77.1 63.7 46.3 59.0
42.4 44.4 35.6 24.0 32.4
mv .
0.600 0.575 0.560 0.519 0.549
-I
Average . . . . . . . . . . . . . . . . . .
5.2 5.0 4.8 4.5 4.8
P H HIP r ~~-~ cm.Hg
mu.
74.5 70.1 43.1 56.4
39.3 36.6 21.3 27.9
Average 4.9
TABLE 11 r-Potential of N/BOOO silver nitrate-silver t = 25.9"C. Average Ks = 1.67 X lo-*
.........I
mu.
0.528 0.522 0.495 0.494
5.9 5.9 5.5 5.5 5.7
TABLE 12 r-Potential of N/1000 silver nitrate-silver t = 25.3%. Average K* =
r
_ _ _ _ _ _ _HIP _ _ _r ~ P
H
cm. Hg
mv.
4.6 4.3 4.4 4.2 4.4 4.2
64.1 76.1 49.9 36.5 60.2
8.0 9.6 7.2 4.7 7.8
(0.000)
(0.0)
Average..
. . . . . . . . . . . . . . . . 2.9
0.287
4.6
crn.Hg
mu.
77.8 66.4 43.3 63.2 51.9 70.3
22.6 18.1 12.0 16.5 14.5 18.2
0.290 0.272 0.277 0.261 0.279 0.259
(00.0)
(00.0)
52.9
15.2
mv
.
mu.
0.124 0.126 0.143 0.129 0.130
2.7 2.8 3.1 2.8 2.9
-
4.4
a t the end of a run using N/2000 silver nitrate, a positive potential was registered after the pressure on the flowing liquid was brought to zero. This potential persisted for more than an hour after the liquid had ceased to flow. Sjnce the metallized gels were reduced and stored in an atmosphere of hydrogen it was felt that there might be some hydrogen electrode effect,
1
0
3
2
4
5
7
6
9
10
Concentration ( ~ o r d i t yx lo4)
FIG.7. SILVER-SILVER NITRATE
0
1
2
3
4
5
6
7
Concentration (Normality x lo4) FIG.8. SILICAGEL-SILVERNITRATE 147
0
9
10
148
GRANT W. SMITH AND L. H. REYERSON
since oxygen had been eliminated from the system. Accordingly a fresh sample of silvered gel was prepared as previously described. This was placed in bulbs and sealed to a vacuum line. The bulbs were heated for two hours a t a temperature above 350°C. while the vacuum pump was running. The bulbs were sealed off under vacuum and later opened under freshly boiled water. As a result a diaphragm of silverized gel was prepared under conditions which practically eliminated both oxygen and hydrogen from the system. A series of measurements were made on this gel and the results are given in tables 8, 9, 10, 11, and 12. These results
CONCENTRATION
r
normality
mu.
ox
x x x io x 1 2 5
10-4 10-4 10-4 10-4 10-4
P e.8.u.
CONCENTRATION
r
normality
mu
X 10%
o x 10-4
11.2 12.1 11.0
6.0 6.5 5.9 0.0
x x x io x 1 2 5
0.0 0.0
0.0
7.6 4.9 5.7 4.4 2.9
10-4 10-4 10-4 10-4
P e.8.u. X
10%
14.1 9.1 10.6 8.2 5.4
TABLE 15 S u m m a r y of results for the system silica gel-silver nitrate CONCENTRATION
normality
ox 1 2 5 10
x x x x
10-4 10-4 10-4 10-4 lo-'
1
r mu.
4.7 6.0 6.1 3.0 2.9
P
e.s.u. X
lOJ
8.7 11.1 11.3 5.6 5.4
show that the disturbing influence was eliminated by the above treatment. Figure 7 graphically shows the effect of the concentration of silver nitrate on the zeta potential. As in figure 4 the broken curve shows the change of the Nernst potential with concentration. The values of the Nernst potential in both cases are those referred to the standard hydrogen electrode. The results of a similar series of determinations on pure silica gel, using silver nitrate solutions, are shown graphically in figure 8. A summary of the results obtained on nickel-surfaced gel, silver-surfaced gel, and pure silica gel is given in tables 13, 14, and 15. The last column gives the values of the electric moment of the double layer.
ELECTROKINETIC POTENTIAL OF THIN METAL FILMS
149
DISCUSSION
The streaming potential measurements in this investigation plainly indicate the difference in character between the t and p potentials. The electrokinetic potential is negative for both the nickel and silver systems, while the silver-silver ion system would normally be expected to be positive. The actual change in the electrokinetic potential is usually small as compared to the €-potential changes in the same concentration range, but the relative change in the zeta potential is much larger. The zeta potential in the nickel-nickel nitrate system reached a zero value at a concentration of N/2000. This is characteristic of the effect of divalent ions in streaming potential studies. No such results are found in €-potentials. The curve obtained for silver nitrate-silver is different from any hitherto reported for monovalent ions. In the case of monovalent cations of K+ and Naf, an initial rise in the zeta potential is observed and then a gradual decrease. In the silver nitrate-silver case there is the initial sharp decrease, then a rise which is followed by the usual gradual decrease. This was at first attributed to experimental error, but check runs gave the same results. In order to explain this unusual result it IS necessary to consider the factors affecting the potential a t the interface. If the diffuse double layer is considered as a plane condenser with the planes representing the average positions of the two layers, Le., the electrogravitational planes, then the potential across these planes depends upon the distance between them and upon the charge density. If the charge density is constant, then the difference in potential is directly proportional to the distance across the double layer, whereas, if the distance is kept constant, the potential is proportional to the charge density of the layers. This assumes that the dielectric constant does not change. However it may be considered as a constant factor, and as such will not affect the conclusions. The distance across the double layer varies with the concentration of ions in the solution, the thickness being greatest in pure water and dilute solution. The first addition of electrolyte causes the greatest decrease in the distance, with successive additions causing less and less change. The potential should thus decrease sharply at first with a further fall occurring on addition of electrolyte. On the other hand, the addition of electrolyte will affect the charge on the double layer. If adsorption into the interface tends to increase the net charge of the double layer, i.e., by preferential adsorption of negative ions on a negatively charged fixed layer, then the potential will increase. If positive ions are adsorbed into the negative layer then the potential must decrease, for the charge is lowered. The resultant change in potential must therefore depend on the relative magnitude of these three factors: (1) change in thickness of the double
150
GRANT W. SMITH AND L. H. REYERSON
layer, (2) adsorption of anions, and (3) adsorption of cations. Changes in these three factors have been used to account successfully for the changes which occur when solutions of such salts as potassium chloride, sodium chloride, etc., are streamed through glass capillaries. In the case of the silver-silver ion system, changes in the relative magnitude of the three factors give rise to a different type of potential curve. It is probable that the heavy metal ion is adsorbed to a much greater degree than was the case with the alkali cations. This effect, together with any drop in potential due to the shrinkage in the double layer, probably more than balances the effect of anion adsorption. On increased concentration the effect of shrinkage in the double layer becomes less significant, and we have the usual rise in potential due to preferential anion adsorption. As the double layer becomes more nearly saturated with ions, we obtain the final gradual fall in potential due to the decrease in double layer thickness. The electrokinetic potentials of the silver-silver ion system differ from those of the nickel-nickel ion system. However they both exhibit changes in the l-potential which are unpredictable on the basis of consideration of the €-potential theory. The Nernst potentials over this same concentration range vary in a perfectly regular manner. The variations in the electrokinetic potential depend upon the nature of the substances concerned and the concentration of the electrolyte. The Nernst potential, e, depends upon one variable factor, the osmotic pressure of those ions which the metal supplies and a factor which is constant for each metal, the electrolytic solution pressure. SUMMARY
A streaming potential method for the determination of the electrokinetic potentials of metals in contact with their ions has been developed, and measurements were made on the systems silver-silver ion and nickel-nickel ion, as well as on pure silica gel. Metallized silica gels were used in the investigation. They were prepared by the new process of adsorbing complex ions of ammonia on the silica gel and later reducing to the metallic state. A vacuum tube potentiometer and a vacuum tube conductivity bridge have been adapted to this type of work with excellent results. Fundamental differences between the electrokinetic potentials and the Nernst potentials were found for the systems studied. REFERENCES
(1) BRIGGS,D. R.: J. Phys. Chem. 32, 641-75 (1928). The Determination of the Zeta Potential on Cellulose.-A Method. (2) BULL,H. B., AND GORTNER,R. A . : Physics 2, 21-32 (1932). Electrokinetic Potentials. XI. The Effect of Sodium Soaps on the Electrical Moment of the Double Layer a t an Aqueous-Cellulose Interface.
ELECTROKINETIC POTENTIAL OF THIN METAL FILMS
BULL,H. B.,
151
AND GORTNER, R. A.: J. Phys. Chem. 36, 111-9 (1932). Electrokinetic Potentials. X. The Effect of Particle Size on the Potential. (4) FOSBINDER, R. J. : J. Phys. Chem. 34,1294-1302 (1930). A Vacuum Tube Potentiometer for the Determination of the True E. M. F. of a High-resistance Cell. (5) JONES, G., AND JOSEPHS, R. C.: J. Am. Chem. SOC.60, 1049-92 (1928). The Measurement of the Conductance of Electrolytes. (6) NOTTINCIHAM, W. B.: J. Franklin Inst. 209, 287-348 (1930). Measurement of Small D. c. Potentials and Currents in High-resistance Circuits by Using Vacuum Tubes.
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