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ARTHUR A. VBIRNON AND RARRISON A. NELBON
(8) HAUSEBAND LEBEAU:J. Phya. Chem. 42,1031 (1938). AND REED: J. Phys. Chem. 41,911 (1937). (9) HAUEER AND WILM: Z. Krist. 66,340 (1933). (10) HOFUNN, ENDELL, (11) HOUWINK:B h t i c i t y , Plasticity, and Structure of Matter, p. 336. University Press, Cambridge (1937). (12) HOUWINK:Eketicity, P h t i c i t y , and Structure of Matter, p. 338. University Press, Cambridge (1937). (13) LANGMUIB: J. Chem. Phys. 6, 873 (1938). AND THOMPEON: Trans. Am. Inst. Mining Met. Engrs. 114, (14) LEWIS,SQUIBES, 39 (1936). (15) MABBHALL: Trans. Faraday SOC.26, 173 (1930). (16) MABEHALL: J. Phys. Chem. 41, 935 (1937). (17) MATTSON:Soil Sci. SS, 301 (1931). (18) MELLOR:Treatise on Inorganic Chemistry, Vol. VIII, p. 990. Longmans, Green and Company, London (1928). (19) REED:Petroleum Eng. 9, No. 4, 48 (1938). (20) SALMANG: Kolloid-Z. 46, 377 (1929). Soil Sci. 41, 26 (1936). (21) WINTEBKOBN:
CHEMICAL PREPARATION OF COLLOIDAL SUSPENSIONS I N NON-AQUEOUS SOLVENTS. I METHYLALCOHOL AND BENZENE' ARTHUR A. VERNON'
AND
HARRISON A. NELSON8
Department of Chemistry, Rhode Island State College, Kingston, Rho& Island deceived February bS, 19S9
Tomaschewsky (9) prepared fairly stable alkali-metal organosols by the simultaneous condensation of the vapors of the metal and the dispersion medium. Fodiman and Kargin (1) used a method by which they condensed metallic vapors in an organic liquid. The change of solvent method was used by Horiba, Otagari, and Kiyota (5) and by Von Hoessle (lo), while Svedberg (7) used an electrical dispersion system. A silver suspension in alcohol was prepared by Formstecher (2) by reduction of silver nitrate with formaldehyde. Hydrogen sulfide was used by Lottermoser (6) to suspend cupric sulfide and mercuric sulfide in alcohol. In view of the scant information on the subject it seemed of 1 This article is condensed from a thesis submitted by Harrison A. Nelson t o the Faculty of Rhode Island State College in partial fulfillment of the requirements for the degree of Master of Science in Chemistry, June, 1938. * Present address: Department of Chemistry, Northeastern University, Boston, Massachusetts. a Present address: Department of Chemistry, The Rice Institute, Houston, Texas.
COLLOIDAL SUSPENSIONS IN NON-AQUEOUS SOLVENTS
13
value to try the methods of Gorbowski (3) and Gutbier (4) with organic solvents. EXPERIMENTAL
Materials All materials used were of C.P. grade. The solvents were methyl alcohol and benzene and were redistilled before use. Procedure The procedure was essentially the same in all cases; a dilute solution was prepared in the dispersion medium to be used and the protective colloid waa added. The reducing agent was added directly or hydrogen sulfide was bubbled through the solution. All colloids were filtered and examined with a dark-field microscope attachment. It was found preferable to filter unprotected sols immediately, rubberprotected sols after 24 hr., and cellulose nitrate-protected sols in from 2 to 4 days. A suction filter was used, with a medium-pore filter paper. Owing to the general insolubility of metallic salts in benzene, a concentrated solution of the salt was prepared in ether or acetone, and enough of this solution added to the benzene to produce the desired concentration. The solutions prepared for reductions were generally very dilute. In the case of colored solutions, a dilution was made up in which the ion coloration was scarcely visible, so that the optical properties of the sol might not be interfered with to too great an extent. Non-colored ions were used in a range of concentrations from 0.001 to 2.0 per cent. Two compounds were found especially suitable as protective agents. Collodion, added on the basis of cellulose nitrate in amounts of 0.2 per cent or more, successfully protected suspensions in alcohol. In benzene, rubber waa generally the protecting agent. Two methods of producing a suitable rubber material for this purpose were employed with almost equal success. I n one method, a 40 per cent commercial latex suspension waa dried out in a film which waa then cut into small pieces and shaken with benzene. Not all of the material dissolved, but the solution adequately protected suspensions when added in small amounts before reaction. In the other procedure, pale crepe rubber wm cut up and dissolved in benzene. This method was found to produce a more viscous solution with less undinsolved material. In both cases standing for 24 hr. was necessary to produce a suitable solution. Filtering the rubber solution before adding it to the solution to be reduced did not materially impair its stabilizing effect, but it was generally found more satisfactory to add the mixture, reduce or react the solution in the presence of the undissolved lumps, and filter the resulting suspension. “Gold nuclear solution”, as mentioned in connection with the formation of certain sols, waa merely a stable red gold
14
ARTHUR A. VERNON AND HARRISON A. NELSON
sol, made up in the dispersion medium in use and reduced by hydrazine hydrate. Reducing agents (1) Hydrazine hydrate: A very small quantity of a 40 per cent aqueous solution of this material waa introduced into the salt solution on the end of a glass rod. (2) Phosphorus: A solution of phosphorus in carbon disulfide was diluted twenty times with the dispersion medium and a few drops of the dilute solution were added to the solution to be reduced. (3) Phosphorus and hydrazine (phosphorus-hydrazine) : This combination of the two preceding reducing agents was found capable of reducing many salts upon which other reagents had no effect. The phosphorus was first added in slight excess, eight to ten drops of the dilute solution, followed by one drop of 40 per cent hydrazine hydrate solution. (4) Tannin: A freshly prepared alcoholic solution of tannic acid was added dropwise until the first faint signs of color developed. (5) Stannous chloride: A concentrated alcoholic solution of stannous chloride was added dropwise until the development of color in the reduction of auric chloride solution. (6) Hydrogen: Hydrogen gas was passed through a solution of silver oxide a t a moderate rate for about 20 min., or until the development of a color. Reductions at a raised temperature were attempted with the first five reducing agents mentioned. The reagent was added to the protected salt solution, and the mixture sealed in a tube and heated to 100OC. for 20 min. Before filtering the benzene sols, the ether or acetone added in the salt' solution was removed by boiling. RESULTS
The investigation has resulted in the preparation of many metallic and metallic sulfide organosols and the compilation of data on various possibilities of organosol formation. I n tables 1, 2, and 3 are listed all the more stable sols produced from alcohol and benzene, which were observed over a period of several months. The more significant results of the experiment are summarized below. 1. Alcohol as dispersion m d i u m
(a) Gold: It was not possible to produce red gold sols without first adding cellulose nitrate. Reducing an unprotected auric chloride solution gave a blue sol which had a life of 90 min. Phosphorus, tannic acid, and other organic acids did not reduce auric chloride a t room temperature. (b) Silver: Silver oxide was not reduced by hydrogen at room temperature. At a temperature of 5OoC. hydrogen when passed through the solution produced a stable sol with cellulose nitrate; without a protective colloid the sol waa stable for only 5 hr.
COLLOIDAL SUSPENSIONS IN NON-AQUEOUS SOLVENTS
15
(c) Lead: Lead nitrate solution was reduced to a good sol in the presence of a small amount of gold nuclear solution, but without the nuclear solution a white cloud formed. Phosphorus a t 100OC. reduced lead nitrate to a brown sol, but the phosphorus-hydrazine combination was the only other successful reducing agent. Phosphorus reduced lead acetate in 7 hr. a t room temperature, producing a black sol st, 'de for 3 days. ( d ) Bismuth: Bismuth nitrate and bismuth chloride crystals added to alcohol produced the white flocculent oxide complex. Filtering removed the undissolved material and left a solution reducible to a good brown TABLE 1 Metallic sols in alcohol (with protective colloid) DIBPEBBED PHASE
COXPOUND REDUCED
Au , . . . . . . . Au . . . . . . . . Au . . . . . . .
AuCla AuCls AuCla
Au , . . . . . . .
AuCls
Ag . , . . . . . . Ag . , . . , . . Ag . . . . . . , Ag . . . . . . Pb . , . . . . .
AgNOs AgNOs AgNOa AgzO Pb(K0s)z
Bi . , . . . . . . , Sb , . , , . . . . Ni . , . . . . . . Cr . . , . . . . .
BiOCl SbCls NiNOs CrCL coc19 CuBrl HgNOs SnCll
,
co . . . . . . . . cu,,... . . . Hg . . . . . . . . Sn . , . . . . . .
COLOB REDUCINQ AQENT
B y transmitted light
Hydrazine .Hydrazine Stannous chloride Stannous ohloride Hydrazine Tannin Phosphorus Hydrogen Phosphorushydrazine Hydrazine Hydrazine Hydrazine Phosphorus Hydrazine Hydrazine Hydrazine Hydrazine
B y reflected light
Ruby Purple Ruby
Brown Gray-brown Red-brown
Purple
Dull gold
Amber Deep red Brown Yellow Yellow to red (slow change) Brown Yellow Amber Yellow Amber Yellow Yellow Red-amber
Olive Gray-brown Black Brown Green to black (slow change) Black Yellow Lavender Green Gray-green Green Gray-green Tan
sol. The phosphorus-hydrazine combination was the only successful reducing agent for bismuth. ( e ) Antimony: Gold nuclear solution was necessary in antimony trichloride solution for the formation of stable sols. The phosphorushydrazine combination was the only suitable reducing agent. (j) Nickel: In the presence of gold nuclear solution, hydrazine hydrate reduced nickel nitrate to nickel. Without the nuclear solution, the suspension was an unstable cloudy white. Other reducing agents produced unstable sols or no sols at all. ( 8 ) Copper: Cupric chloride and cupric acetate were precipitated by
16
ARTHUR A. VERNON AND HARRISON A. NELSON
reducing agents. Hydrazine hydrate reduced them to suspensions with a life of about 24 hr. Cupric bromide was reduced only with the phosphorus-hydrazine combination. ( h ) Cobalt: Cobaltous chloride was precipitated by hydrazine hydrate, reduced by phosphorus-hydrazine, and unaffected by other reducing TABLE 2 Metallic sols in benzene (w'th protective colloid) C0M P 0U N D REDUCED
DIBPERBED PHABE
XEDUCINQ ACENT
Hydrazine Excess hydrazine Stannous chloride Phosphorushydrazine Phosphorus Phosphorushydrazine
Au . . . . . . . . Au . . . . . . . Au . . . . . . . . . Ag . . . . . . . . . Ag . . . . . . . . . B i . .. . . . . . . . ,
Red Purple-blue Pink Yellow
Brown-purple Gray-tan Pink-gold Green-gray
Amber Orange
Brown Brown
TABLE 3 Stable metallic sulfide sols i n methyl alcohol and benzene (with . .protective colloid) BCLFIDEOF
I
Ag . . . . . . . . . . Pb. . . . . . . . Bi . . . . . . . . . . Sb. . . . . . . . . Hg . . . . . . . . . Ni. . . . . . . . . . Co, . . . . . . . . .
Sn . . . . . . . . . . Fe . . . . . . . . . . Cu. . . . . . . . . . As..........
~
COLOR I N AICOHOL BALTUBED
By
transmitted light
By
reflected light
COLOR I N BENZENE
By
transmitted light
By
reflected light
Red-brown Brown Red Orange Brown Black Black Amber
Black Black Black Tan Black Black Black Tan
Yellow Brown Yellow
Gray Black Tan
Black Gray Amber
Black Black Tan
Light green Amber
Gray Yellow
Light green Red
Gray Yellow
Cd.. . . . . . . . .I
agents. A small amount of cobaltous sulfate, added to the cobaltous chloride solution to be reduced, was found to increase the stability very greatly. (i) Chromium: Reducing agents, with the exception of phosphorushydrazine, had no effect on solutions of chromic chloride. Dilute solutions which had been reduced with phosphorus-hydrazine precipitated
C’OLLOIDAL SUSPENSIONS I N NON-AQUEOUS SOLVENTS
17
in a few weeks, but more concentrated chromic chloride solutions, in which the green color was very evident, were reduced to form very stable sols. (j) Mercury: Mercurous nitrate was similar to chromic chloride in that concentrated solutions were necessary for stability. Phosphorus reduced mercurous nitrate to a black sol stable for 2 months, while other reducing agents gave precipitates. ( k ) Strontium: Strontium nitrate was reduced by phosphorus-hydrazine, in the presence of gold nuclear solution, to a yellow sol stable for 2 months. (1) Tin: Stannous chloride was reduced by phosphorus-hydrazine alone. Only very dilute solutions gave stable sols. Sols produced from solutions of a concentratign of 0.2 per cent or more flocculated almost immediately. 2. Benzene as dispersion medium The possibility of sol formation in benzene depended on the use of salts soluble in acetone or ether, since most salts are insoluble in benzene. Those salts giving satisfactory acetone or ether solutions were added to benzene as described in the method of procedure, and the various reducing agents were used with each. Although salts of antimony, gold, silver, lead, bismuth, tin, nickel, chromium, cobalt, copper, mercury, and silver were tried, only with salts of gold, silver, bismuth, and tin were stable sols possible. A pink suspension of gold in benzene, prepared by the use of stannous chloride, was stable for 24 hr. without the addition of a second organic protective colloid. Silver sols were stable fo: 4 hr. without a protective colloid, but bismuth and tin suspensions were possible only in the presence of rubber. DISCUSSION
The following observations are given to show the general behavior of various sols and the problems involved in their preparation : (1) Gold: This was the most easily prepared suspension. It was found possible to form almost any shade of blue, purple, or red gold suspension desired. For example, if a light red sol were desired, a dilute solution of auric chloride was reduced with hydrazine in the presence of cellulose nitrate. If a blue suspension were desired, it could be prepared by introducing the reducing agent before adding the cellulose nitrate. Deeper colors were obtained by using higher concentrations of auric chloride. During the course of these experiments, it was noticed that the color of gold sols reduced in the presence of cellulose nitrate developed from a blue to a deep red, and in no case was a red sol formed immediately. This phenomenon was very strongly evident in solutions containing a high concentration of both salt and cellulose nitrate. In this case the introduc-
18
ARTHUR A. VERNON AN0 HAP#!BC)N A. NI?.!LSON
tion of hydrazine hydrate produced a very light blue color, which slowly darkened to black, then to purple, then to red. It has been suggested by Svedberg that the difference between blue and red gold sols may be due to a difference in the size of the particles, the blue sols containing larger particles than the red. This suggestion, however, is not supported by the above observations. It seems improbable that reduction should first form large aggregates which on standing would break down into smaller particles. The tendency is rather for condensation than for dispersion. But the proposition of Thiessen (8),that colored aurous oxide may be partly responsible for the color of blue sols, is apparently supported. It is quite possible that reduction is at first incomplete, thus producing aurous oxide instead of gold in suspension, giving a blue color. Reduction continuing, the metal stage is finally reached, and the sol becomes red. (2) Silver: The silver sols prepared were quite uniform in color, transmitting yellow to brown light, and reflecting brown-gray to black. These sols were unique in appearance under the dark-field apparatus, showing white particles in a deep blue field. The microscopic field was practically colorless with other sols. (3)Lead: Some reduction of lead salts was possible with phosphorus a t lOO"C., but phosphorus-hydrazine was much more successful. With the latter reducing agent, the first color observed was light yellow. This slowly changed to a deeper yellow in transmitted light and a green-yellow in reflected light. There was then no change in coloration for several days, when a gradual darkening set in, finally producing a typical dark lead sol transmitting brown light and appearing black in reflected light. This gradual change of appearance might be analogous to the blue or red color change observed in gold sols. It might then be assumed that the reduction is slow, and that the first sol contains oxides, or other incompletely reduced compounds. * In the case of lead, as with several other metals, the addition of a small amopnt of a gold nuclear solution was found to assist greatly in stabilizing sols. The action of the nuclear solution was merely to furnish a large number of nuclei, upon which the lead molecules might deposit. In the absence of the nuclear material, the first few particles of metal acted as nuclei for the agglomeration of the lead subsequently reduced. These particles enlarged rapidly and soon precipitated, subsequent lead produced by reduction in the solution depositing on the precipitated mass. (4) Cobalt: Gold nuclear solutions greatly facilitated the preparation of stable cobalt sols, but in this case the addition of a small amount of cobaltous sulfate apparently served the same purpose. Evidently the cobaltous sulfate was unreduced and partially dissociated, providing some stabilizing ions on the particle surface. This procedure might conceivably
COLLOIDAL SUSPENSIONS IN NON-AQUEOUS SOLVENTS
19
be as successful with other metals, but in this investigation it was not attempted with metals other than cobalt and nickel. (6) Iron: Iron sols prepared with phosphorus-hydrazine were first deep red, very similar to the red of a gold sol, but later changed to a green or black. The red sol was proved by dark-field examination to contain colloidal particles. The other colored suspensions might possibly have contained compounds instead of the metal. It is even possible that the red sol was of ferric oxide, as a slight amount of water was introduced with the hydrazine in reduction. (6) Chromium: The unusual behavior of chromium sols in the matter of stability with respect to concentration might well be investigated. The fact that higher dilutions are less stable is in direct opposition to what appeared to be the general rule in all other cases. It is possible that a strong excess of unreduced ions is necessary to form the stabilizing layer.
Protective colloids A wide variety of material was used in an attempt to stabilize the sols, but few substances were satisfactory. The protective properties of cellulose nitrate have not been investigated thoroughly. Rubber, on the other hand, has often been used. Among the many compounds employed were stearates, linoleates, glycols, and Glyco products, shellac, waxes, and paraffin. Ammonium stearate prolonged the life of a blue gold sol in alcohol by 8 hr., but was otherwise ineffectual, as were all other compounds tried.
Eflect of aging During the first few weeks after the formation of a stable sol, some changes were noticed. By the end of the first week, a slight precipitate was often present in the bottom of the vessel. This preliminary precipitation was almost universallynoted in greater or lesser degree. The characteristic appearance and stability of the residual sol were, however, in no wise affected,and if the sol were filtered after several weeks’ standing, no further change or precipitation was noted. It is probable that the precipitated material consisted of irregular shaped particles and particles of greater than colloidal dimensions. It is also possible that some organic compounds were included in the precipitate as the result of side reactions in the organic liquids. Sulfide sols The table of sulfide sols is rather complete, and little need be said of the results. It is fairly simple to suspend sulfides in organic liquids, owing to their general insolubility. But some attempts were unsuccessful, owing to precipitation of the sulfide. Removal of the excess hydrogen sulfide
20
ARTHUR A. VERNON AND HARRISON A. NELSON
after reaction could be accomplished by passing a current of air through the solution or by boiling, but a somewhat greater general stability was evident if some hydrogen sulfide were left, it being apparently sufficiently dissociated to supply stabilizing ions. SUMMARY
1. By the use of hydrazine hydrate, phosphorus, stannous chloride, tannin, and other reducing agents, stable colloidal suspensions of several metals were prepared in methyl alcohol and benzene. The concentration of salts used varied from 0.001 to 2.0 per cent. A combination of phosphorus and hydrazine was necessary to reduce salts of copper, lead, tin, and antimony. 2. Colloidal suspensions of metallic sulfides were prepared in the same dispersion media by passing dry hydrogen sulfide through dilute solutions of metallic salts. 3. Protective colloids were generally found necessary for stability. Cellulose nitrate, in concentrations of 0.2 per cent or more, successfully protected sols in alcohol, while rubber was found to be necessary for benzene. Some sulfide sols were very stable without protective colloids, but unprotected metal sols were short-lived. REFERENCES
(1) FODIMAN AND KARGIN: J. Phys. Chem. (TJ.S. S. R.) 6, 423-30 (1934). Phot. Ind. 30, 6 '(1932). (2) FORMSTECAER: (3) GORBOWSKI: Ber. 36, 125 (1903). Z. anorg. Chem. 31, 448;32, 348 (1902). (4) GUTBIER: (5) HORIBA,OTAGARI, AND KIYOTA: Japanese patent 101,948 (July 13, 1933). (6) LOTTERMOSER: J. prakt. Chem. [2]76, 293-306 (1907). Ber. S6, 3616 (1905);39, 1705 (1906);J. Am. Chem. SOC. 46, 1980 (7) SVEDBERQ: (1924);Kolloid-Z. 26, 154 (1919). (8) THIESSEN: Z. anorg. Chem. 134, 393 (1924). Kolloid-Z. 64, 79 (1931). (9) TOMASCHEWSKY: (10) VONHOESSLE:German patent 623,075 (December 12, 1935).
