METALLIC COUPLES. I1 CONCERNING THE PREPARATION OF ALUMINA HYDROSOLS'' M I K E A. MILLER
Aluminum Research Laboratories, Aluminum Company of America, New Kensington, Pennsylvania Received December 91, 1987 INTRODUCTION
The author has recently pointed out (35) that practically pure alcoholic alumina jellies can be prepared by a modification of the KuteelniggWagner reaction (32). Kuteelnigg and Wagner allowed a 2 per cent copper chloride solution in 95 per cent ethyl alcohol to act on metallic aluminum in order to produce an alcoholic alumina gel. Analysis showed the final transparent product to be free from copper ion but to contain a high percentage of chloride ion. The modified method, described by the present author (35), produced gels apparently free from both of these ions. Further studies have indicated that an extension of the modified method can be used to prepare reasonably pure colloidal alumina. The purpose of the present note is to describe a simple method of preparing large quantities of apparently unprotected, but remarkably stable, alumina hydrosols. Preliminary work on other than the aluminum-copper couple is described, and a mechanism for the reaction is proposed. Data for rate of settling of alumina hydrosols are given. ALCOHOL-ACTIVATED ALUMINUM-COPPER
COUPLE
The Kutzelnigg-Wagner reaction (32) was allowed to proceed for several days. The copper-coated aluminum sheet was then removed, washed thoroughly in alcohol, and placed in distilled water. After several days an alumina hydrosol-producing couple was obtained. The first portion of the hydrosol was discaitled, as it contained some copper, and fresh water was added. As the hydrosol concentration became of sufficient strength for whatever purpose it was intended, it was filtered through fine-pored filter paper into a steamed flask or stock b ~ t t l e . ~More distilled Metallic Couples. I. uber kolloides Kupfer und alkoholisches Aluminiumoxyd Gel. Kolloid-2. 131 77, 310-12 (1937). * The experimental work described in this paper was carried out in the Chemical Laboratories of the University of Michigan, Ann Arbor, Michigan, 1932-36. a Since the hydrosol appears to be very sensitive to traces of electrolytes, care should be taken t o steam all containers, if flocculation is to be avoided. 419
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MIKE A. MILLER
water was added to the couple each time. I n this manner, with occasional stirring, such couples continued t o produce the hydrosol until virtually all of the metallic aluminum was used up. The resulting hydrosols were neutral t o litmus. Hydrosols containing up t o 1 g. of A1203 per liter hare been prepared. REACTIONS OF OTHER METALLIC COUPLES
Table 1 summarizes data relative t o the physical character of the alumina produced by several metallic couples in various media. The couples were made as follows: Sheets of 99.5 aluminum (ea. 2 in. x 1 in.) TABLE 1
Physical character of a l u m i n a from several metallic couples METALLIC COUPLE
AI-Hg AI-Cd I AI-Cd I1
FORM OF PRODUCT I N QIVEN MEDIA
METHOD OF PREPARINO
COUPLE: ALUMINUM SHEET IN
Aqueous HgClz Aqueous acidic CdCI, Alcoholic CdCh
Distilled H20
Fibrous
I ~
0.02 NAlCls
Hydrosol
1 Flocculent
I
95 per
j
Flocculent
alcohol
I
AI-Cu I AI-Cu I1 A1-Cu I11
Aqueous CuCI2 Alcoholic CuClz CuA12intermetallic compound
A1-Fe I
Aqueous acidic FeC1" Alcoholic FeC13
Trace of hydro- Hydrosol
Aqueous arnrnoniacal Sic12 Alcoholic Sic12
Hydrosol Hydrosol Trace of gel formed slowly' formed slowlyi Gel Hydrosol Hydrosol
AI-Fe I1 AI-Xi I AI-Ni I1
Hydrosol Gelatinous
Coarse, floccu-
Hydrosol Flocculent
i Fibrous
I
I
Gel Gel Trace of gel; coarse black precipitate Gel Gel
~
~
'
were cleaned in 5 per cent caustic, thoroughly rinsed with distilled water, and air-dried. Into each of the designated salt aolutions (ca. 2 per cent) a sheet of this aluminum was then immersed for approximately one minute. After washing and air-drying, they were placed in about 50 cr. of the reaction medium and left undisturbed until the character of the product was apparent. In general, this period did not exceed two weeks. Besides the couples listed in table 1, a preliminary investigation was made on couples of aluminum with tin, zinc, cobalt, lead, and silver. Of these only the aluminum--cobalt couple appeared t o produce hydrosols in pure water. From table 1 it is apparent that the manner of preparing the couple is
PREPARATION OF ALUMINA HYDROSOLS
421
of more than minor importance, The particle size of the deposited metal, the porosity of the coating, and the activity of the deposited metal are evidently functions of the deposition medium. Factors such as the concentration of the depositing solutions and the effect of the temperature of deposition and of reaction, of the pH of the depositing solutions, of the thickness of the metallic coating, and of colloids were not investigated. I n the opinion of the author, these factors have an important bearing on the decomposition of water by metallic couples. We note that in contrast to the well-known aluminum-mercury couple (6, 49), which ordinarily produces with water a flocculent, fibrous alumina (39, 50), and evidently produces true sols (41) only in the presence of stabilizing ions, the alcohol-activated aluminum-copper couple generates a remarkably stable, apparently unprotected hydrosol. Experiments performed with an aluminum-copper couple which had been generated in an aqueous cupric chloride solution invariably resulted in the formation of flocculent alumina, incapable of more than transitory suspension in the absence of stabilizing ions. Mabb (33) has recently discussed certain reactions of the aluminum-copper couple. The results of the interaction of various metals, including aluminum and copper, with anhydrous ethyl alcohol, gasoline, and mixtures of these two liquids, have recently been reported by Zdarsky (51). This investigator states that ethyl alcohol which contains moisture will give a soft jelly with aluminum. The author has been unable to produce either hydrosols or gels by the action of either water or alcohol on aluminum alone. Apparently a coupled reaction is necessary (35). On the other hand, Bouchet (2) states that he has produced blue sols by allowing natural waters to react on pure zinc. It is possible that certain ions in the waters stabilized the corrosion product produced by the salts present in the waters. SETTLING EXPERIMENTS WITH ALUMINA HYDROSOL
An attempt was made to determine the rate of settling, if any, of the hydrosol particles resulting from the interaction of the alcohol-activated aluminum-copper couple. The hydrosol used in these experiments contained 0.22 g. of A1203 per liter.* Tubes of various diameters, ranging from 0.5 to 2.0 cm., were used. Evidently tube length is of more importance than tube diameter. Some gradient settling took place and was noticeable after several weeks. No visible boundaries resulted, however, until after about three months, though a trace of sediment was apparent at the bottom of the settling tubes before this time. At the end of about six months sharp boundary effects became visible. By the proper adjustment of a light Value obtained from caIcining i-midw from given sol volume,
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MIKE A. MILLER
PREPARATION OF ALUMINA HYDROSOLS
423
source, these boundaries could be read to within 2 mm. Representative result9 are recorded in table 2. It will be seen that a t first a few fairly sharp boundaries appear, that after about a thousand hours more the number has increased, and that as equilibrium is approached, aweral of the boundaries tend to disappear. It would be interesting to carry out similar experiments in tubes of the same diameter but of various lengths, in analogy to The Svedberg’s experiments with the centrifuge. The figures do not readily lend themselves to quantitative calculations of particle size, since D O attempt was made to measure the concentration of alumina in each of the zones. Visual observation indicated that there were more particles of an intermediate size than either very small particles or very large particles. In short, the distribution was what might be expected. Pavlov (42) has recently discussed the mechanism of coagulation. The Smoluchowski theory (43) of liquid envelopes and mutual attractions and repulsions has been further developed. It is difficult to apply the theory of velocity of coagulation to a distribution type of normal settling. One would have to assume a slow but continuous coagulation of hydrosol particles, and hence the absence of an apparent equilibrium stratification, such as was actually obtained. DISCUSSION OF RESULTS
From a consideration of the action of water on an active metallic couple we would infer that a distribution of particle sizes would be expected. We may tentatively consider the hydrosol particles to result from the action of one or both of two series of processes. Firstly, electrochemical reaction may produce aluminum ions, which migrate into the solution before forming aluminum oxide molecules. Thcse oxide molecules then agglomerate sufficiently to produce a range of colloidal-sized particles in agreement with experiment. The second mechanism would also begin as an electrochemical reaction, with the production of films of oxide, impermeable to aluminum ions; these films would then disrupt, under the influence of growth and pressure of generated hydrogen gas, to produce a gradation of colloidal sizes in harmony with the actual figures. The latter mechanism seems more in keeping with experiments on aluminum-mercury couples, where the formation of films is obvious, as well as with the fact that particle sizes range from the colloidal to the macroscopic. On the other hand, the first mechanism would more adequately account for the stability of the sols, since ionic aluminum is known to peptize the fibrous alumina ordinarily produced by aluminum-mercury couples (38, 41). It is possible that alcohol-activated aluminum-copper couples produce hydrosols in pure water because of stabilization by an ethylate.
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hHKE A. MILLER
It remains to consider the individual couples. Table 3 lists the normal electrode (single) potentials and the relative hydrogen overvoltage. of some of the metals investigated. The E.M.F. of a metal indicates its replacrment or deposition ability; any metal will tend to replace any metal below it in the E.M.F. series. Likewise, the total E.M.F. of a cell is equal to the difference bet\$een the single electrode potentials of the electrodes. For example, the aluminum-copper couple will h a w an E.M.F. of 1.63 volts; the aluminum-nickel couple nil1 have an m 1 . F . of I .06 volts. The hydrogen overvoltages are a function of the position in the periodic arrangement of the elements, dependent upon the group. Oliverio and Belfiori (40), after investigating the decomposition of water by metals and mci-,iilic couples, conclude that this deconiposition depends more on the TABLE 3 S t n g l e potentials a n d relative hydrogen over~oltaqesof several metals MB'IAL
I
NORMAL ELECTRODE POTENTIALS
METAL ~
_______
,
oolts
Si . . . . . . . . . . . . . . . . . . . . . . Sn. . . . . . . . . . . . . . . . . . . . . . . Ph. ......................... H ........................... cu.........................
+1.28 0.76 0.41 0.40 0.29 0.22 0.14 0.12 0.00 -0.35 -0.80 -0.80
RELATIVE HYDROGEN OVERYOLTAOES
UOllS
Zn.. . . . . . . . . . . . . . . . . . . . . . .
.~
H g . ........................ . I C'd... . . . . . . . . . . . . . . . . . . . . . A l . . . . . . . . . . . . . . . . . . . . . . . . . .~ Sn . . . . . . . . . . . . . . . . . . . . . . . . Pb. . . . . . . . . . . . . . . . . . . . . . . . . ~
~
cu
........................
,,
.4g., . . . . . . . . . . . . . . . . . . . . . . .
.:
0.72 0.70 0.66 0.50 0.45
8: 0.30
Xi. . . . . . . . . . . . . . .
overvoltage of the more noble metal of the couple with respect to hydrogen than on the position in the E.M.F. series. They found, for instance, that from the metals which follow zinc in the E.M.F. series only couples containing iron decompose water. These workers also investigated the effect of temperature on the decomposition of water by zinc-nickel couples, the decomposition increasing with rise in temperature. Superficial considerations make it a t once apparent that the heredity of the created couple, particularly the interagent of its formation, and the environment of the acting couple, especially the nature of the medium in which it generates the product, have also a major responsibility in determining the physical, and often the chemical, properties of the product. This conclusion is sensibly identical with that previously advanced by the author in connection with studies on the physical character of iodine
PREPARATION O F ALUMINA HYDROSOLS
425
and insoluble iodides precipitated in various media (34, 36, 37). It is notable that somewhat similar summarizations have been made as follows: ( a ) concerning the genesis of somatoidal aluminum oxide (25, 26, 27), basic aluminum sulfate (8, 9, lo), calcium carbonate (21, 22, 23, 24), iodides (30), and electrolytic deposits and corrosion products (28, 29) by Kohlschutter and his students; (b) regarding (12, 14, 15, 18), interaction (11, 13, 17, 19, 20, 31), and sorption (7, 16) by solid substances, especially active oxides by Huttig and his coworkers; (c) respecting crystal polymorphism (1, 3, 4, 5) by Buerger; and (d) pertaining to spatially influenced origin and growth of various substances (44, 45, 46, 47, 48) by Urazovskii and f e l l o ~workers. We are once more forced to the conclusion that the vectorial properties attending a given substance, by virtue of the mode of its formation, are largely responsible for the subsequent characterizations which that substance displays during the course of its chemical history. A detailed discussion of the importance of vectorialism as related to the physical and chemical character of a product and to the growth of crystals in general will be discussed in a different connection. SUMMARY
A simple method of preparing large quantities of alumina hydrosols, apparently in the absence of stabilizing ions, has been described. It has been shown that alcohol-activated aluminum-copper couples produce sols which settle to an apparent equilibrium stratification after a period of about 5000 hours. Preliminary experiments on other than the aluminum-copper couple have been described. A mechanism for the reaction of metallic couples with water has been proposed. It is concluded that the vectorial properties attending a given substance, by virtue of the mode of its formation, are largely responsible for the subsequent characterizations which that substance displays during the course of its chemical history. The author wishes to thank Professor F. E. Bartell, in whose laboratory the experimental work described in this paper was carried out, for permission to publish these results. REFERENCES (1) (2) (3) (4)
BLOOMAND BUERGER: Z. Krist. 96, 365-75 (1937). W . : Compt. rend. 204, 1068 (1937). BOUCHET, BUERGER, hl. J . : Proc. Natl. Acad. Sci. U. S. 20,444-53 (1934). BUERGER, M. J . : Proc. Katl. Acad. Sci. U.S. 22,682-5 (1936). “Erinnerungsvermogen” or “Gedachtnis” (13).
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MIKE A . MILLER
(5) RZTERGER, XI. J.: Proc. Natl. Acad. Sei. 22,685-9 (1936). (6) ~ Z o c I i R a L s K I AND WAJGMAN: Wiadomoeci Inst. Metal 3, 90-4 (1936); Chem. Ahstracts 31,7023 (1937). (7) HAMPEL!J.: 2. Elektrochem. 42, 185-7 (1936). (8) WUEER, K. : Helv. Chim. Acta 18, 858-91 (1935). (9) H U B E R , K . : IIelv. Chim. Acta 18, 1316-26 (1935). (10) HUBEB,K.: IIelv. Chim. Acta 18, 1327-35 (1935). G STEFFEL:Kolloid-2. 68, 178-84 (1934). (11) H ~ T T IASD (12) IIUTTIG,G. F.: Z. anorg. digem. Chem. 2 1 7 , 2 2 4 (1934). (13) TXBTTIG, G. F.: Z. anorg. allgem. Chem. 219, 256-62 (1934). I GF.: , -4ngew. Chem. 49,882-92 (1936); eighty-one references. (14) H ~ ~ T TG.
T H ~ T T E RAND
EGG:Hclv. Chim. Acta 8, 697-703 (1925).
(29) ~
