THE CONSTITUTION OF HYDROUS OXIDE SOLS FROM X-RAY DIFFRACTION STUDIES‘ HARRY B. WEISER AND W. 0. MILLJIGAN Departmnt o j Chemistry, Tha Rice Institute, Houston, Texan
Received June So, 1B36
Sols of the hydrophobic type which includes most of the hydrous oxides, can be prepared fairly free from electrolytes, but it was demonstrated first by Thomas Graham and confirmed repeatedly thereafter, that in the absence of protecting colloids some electrolyte must be present in the sols to ensure their stability. Thus ferric oxide sol formed by hydrolysis of ferric chloride or by peptization of the hydl‘ous oxide by ferric chloride always contains traces of chloride however long the dialysis is continued.2 The presence of chloride in the dialyzed sols led Wyrouboff and Verneuil (13) to suggest that the various preparations contain basic salts or chlorides of “condensed” hydroxides. This idea was further extended and developed by several investigators especially by Duclaux (3), Malfitano (7), Hantzsch and Desch (4),and LiLler and Picton (6). Thus, the constitution of the sols was represented by formulas such m: [2OFe(OH)s.FeCls],
and
[45Fe(OH)s.FeCls],
Pauli (8) considers the colloidal particles to be complex ions resulting from ionization of complex electrolytes allied to the Werner compounds. Since the constitution of a given sol varies with the conditions of preparation he represents it by a general formula. I n the case of ferric oxide sol this is [zFe(OH)s.yFeOCl. FeO]+, (C1-) in which z = 32 to 350 and g = 4 to 5.7 in sols formed by hydrolysis. It is difficult to justify this formulation, since no one has established the existence of Fe(OH)a, and FeOCl is obtained only under special conditions in a bomb tube at elevated temperatures. Thomas and coworkers (9, 10) suggest that the dispersed phase in 1 Presented at the Twelfth Colloid Symposium, held at Ithaca, New York, June 2042,1935. 9Sorum sols (J. Am. Chem. SOC.60, 1264 (1928)) containing no detectable chloride either contain a trace of some other electrolyte or are protected by some material derived from the dialyzing membrane during the prolonged dialysis.
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HARRY B . WEISER AND W. 0. XILLIGAN
hydrous oxide sols, such as alumina sol formed by peptization of the gel with hydrochloric acid, consists of olated and possibly oxolated aluminum oxychloride complexes of the Werner type resembling the poly-ol basic chromic salts formulated by Bjerrum (2). Thomas formulates one such hypothetical complex as given in figure 1. The assumption that such hypothetical compounds exist in the sol was made to account for the observation that the pH value of the sol is raised by the stepwise addition of neutral electrolytes, the anion order being: oxalate > acetate > sulfate > halides > nitrate. The increased pH value was attributed to replacement of the OH groups by the anion of the added salt, followed by the union of the displaced OH radicals with hydrogen to form water. Since hydrous oxide sols formed in the presence of chloride, say, always contain more or less chloride, Thomas believes that such sols should be designated as metallic oxychloride sols rather than as hydrous oxide sols. He recognizes that the term ferric oxychloride hydrosol is objectionable,
FIQ.1. Formula of a hypothetical alumina complex (after Thomas)
since it connotes a definite chemical compound when no such meaning is intended. Nevertheless he prefers to regard the hydrous oxide sols as oxysalt sols, since the colloidal particles are not pure hydrous oxides. To be consistent, one should rename the metallic sols and the salt sols, which, like the oxide sols, are not pure insoluble metal or salt. It is difficult to see what would be gained by introducing such a change in our terminology. The ratio of iron to chlorine in a ferric oxide sol has been variously reported as 6, 42,84, 396, 2700, and higher. To designate a sol with a low chloride content as an oxychloride sol is like calling precipitated barium sulfate a chlorosulfate because it contains some adsorbed barium chloride. It is now quite generally recognized that the stability of a positive silver halide sol is due to the preferential adsorption of silver ion by unsaturated secondary valence forces on the surface of the crystals. The crystals will also contain some adsorbed silver nitrate. Similarly a hydrous oxide sol formed in the presence of metallic chloride, hydrochloric acid, and their
3
CONSTITUTION OF HYDROUS OXIDE SOLS
corresponding ions will contain varying amounts of the several components, depending on the precise method of formation and the purification of the sol. The positive charge on a given sol is due to the preferential adsorption of hydrogen and metallic ion on the surface of the hydrous oxide just as the positive charge on a silver halide sol is due to preferential adsorption of silver ion. It is unnecessary to attribute the charge to the presence of an ‘‘ionogenic complex attached to the surface’’ (Pauli) unless the presence of such a complex has been rendered highly probable. Thus to assume the presence of even a simple salt like FeOCl in a ferric oxide sol goes well beyond the established facts. I n recent years investigations (11.) have been made of the various phenomena which take place on adding electrolytes stepwise to sols of the hydrous oxides of iron and aluminum. To account for the several phenomena, including the form of the chloride displacement curves and of curves showing the change in pH value, the constitution of the particles may be represented by the general formulas: [(zFezOd.yHCl .z H z O ) F e ~ + + H ~(m l* Solid phase
Inner layer
+ n - q)Cl-, qC1-
Diffuse outer layer
and,
[(zA1203. HzO. yHCl z H ~ O ) A l ~ ’ + H ~ (m ] +n Solid phase
Inner layer
- q)Cl-,
qCl-
Diffuse outer layer
Iver (5) accepts this formulation of the constitution of certain oxide sols, but suggests that the outer portion of the double layer contains hydroxyl ions as well as chloride ions. The displacement of the former on titrating with neutral salts would account for the increase in the pH value of the mixture. It seems rather questionable, however, whether hydroxyl ions will be present in the outer portion of the double layer in sols having a p H value of 4 to 5. The increase in the pH value on the addition to the sols of neutral salts, especially those with multivalent anions, is doubtless due to the increase in adsorption of hydrogen ion in the presence of a strongly adsorbed anion. This increased adsorption of cations in the presence of strongly adsorbed anions is a general phenomenon that has been observed with various types of adsorbents such aa carbon and fibers a8 well as with the hydrous oxides. With salts such as citrate, oxalate, and acetate, buffer action increases the pH value of the sol-electrolyte mixture above that of the sol alone. From the evidence obtained by potentiometric titration of the sols, there would appear to be no necessary reason for assuming that the sols are
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HdRRY B . WEISER AND W. 0. MILLIGAN
colloidal electrolytes consisting of basic salts or Werner complexes. On the contrary the indirect evidence suggests that the solid phase consists essentially of the hydrous oxide (or simple hydrate). Since x-ray diffraction studies have proven helpful in determining the composition of gels, it was believed that similar studies on sols might give direct evidence of the constitution of the colloidal particles in the sols. The chief complication in the examination of the systems is the relatively low concentrations of the solid phase and the scattering of the x-rays by the water in the samples. Bohm and Niclassen (1) showed that the gels from certain oxide sols were not amorphous; but in most cases it was not stated whether the air-dried or moist gel was examined. Since Thomas believes that the elements of water in certain oxide sols are combined in the form of poly-ol basic Werner salts, it is not permissible to air-dry the samples before examination. On the contrary, the sols must be studied directly, or the undried gel, obtained preferably by ultrafiltration of the sols, must be examined. Preliminary experiments were carried out with sols of the hydrous oxides of aluminum, tin, and indium. 1.
I I
yAltO?.H,O &I (bu ultrafiltratton of sol from A I
amalgam)
yAI,03.Ha0 Ge\ (by ultrafiltration o'f sol trom pptd,
2.
alumina
)
yAI,O,.HLO
3.
I I
Powdor
Precipitated at 100'
FIQ.2. X-ray diffraction patterns ALUMINUM OXIDE SOLS
Two sols of aluminum oxide were employed. The first was prepared by peptization, with hot 0.01 N hydrochloric acid, of the gel formed by the action of amalgamated aluminum on water. The method of Thomas (10) was used except that the gel was not dried. The second sol was obtained by peptization, with hot 0.05 N hydrochloric acid, of the gel thrown down from hot aluminum chloride solution with ammonia and washed by decantation until almost free from chloride. I n both cases a large excess of the gel remained unpeptized, from which the sol was decanted. After standing quietly for several days to allow further traces of unpeptized gel to settle, the slightly cloudy sols were examined Portions of the sols were ultrafiltered through a cellophane membrane, and the resulting moist gels were analyzed by the x-ray diffraction method, using a camera of the Seemann-Bohlin type. Copper radiation filtered through nickel foil, a t GO milliamperes and 50,000 volts, was employed. Under these conditions but five to ten minutes exposure was necessary, so that little or no drying of the gel took place. The results of the observa-
CONSTITUTION O F HYDROUS OXIDE SOLS
5
tions are shown in diagram form in figure 2 (1 and 2). For the purpose of comparison the diagram of yAlzOs.HzO powder is included in the figure. Reproductions of the negatives are shown in figure 3. From these observa-
FIQ.3. X-ray diffraction patterns 1. -,-A1203-H20gel (ultrafiltration of sol from aluminum amalgam) 2. -,-A1203,H,O gel (ultrafiltration of 801 from precipitated alumina) 3. -,-Al1O3.H,O powder (precipitated a t l0O'C.)
I 3.
I
I
I I I I
I
I l l
1 I
I
Sno,sl
(Thixotropicl
SnO, G ~ I
(By u l t r a f i l t r a t i o n )
sn'+
Stlo ( Powder HNO,
SnO,
4. I I I
Ill I
I I
II
(No. 3 ignited 1 t
FIG.4. X-ray diffraction patterns tions, it would appear that the particles in the alumina sol consist essentially of hydrous y-AlzOs.HtO. There is no reason to believe that they are made up of simple basic salts or basic salts of the Werner type. STANNIC OXIDE
Stannic oxide sol was prepared by the method of Zsigrnondy (14). Twenty-five grams of hydrated stannic chloride was dissolved in 20 liters
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HARRY B . WEISER AND W. 0. MILLIGAN
of water and allowed to hydrolyze. The resulting gel was washed until the wash-water was free from chloride. After suspending in 250 cc. of water, it was peptized with three drops of concentrated ammonia and the resulting sol was boiled to remove excess ammonia. The moist gel obtained by ultrafiltration of the clear sol gave the x-ray diffraction pattern of anhydrous stannic oxide or cassiterite, as shown diagrammatically in figure 4 (2). For the purpose of comparison the patterns of the so-called “P”-stannic oxide and of anhydrous stannic oxide are included in the diagram. The sol concentrated to 100 cc. became thixotropic. This sol gave the pattern shown in figure 4 (1). INDIUM OXIDE SOL
A solution of indium nitrate was precipitated in the cold with ammonia, the precipitate thoroughly washed, and peptized in the cold with dilute hydrochloric acid. The gel obtained by ultrafiltration of the sol gave the
FIG.5. X-ray diffraction patterns for In201.3H20gel (from ultrafiltration of sol)
x-ray diffraction pattern shown in figure 5. This is the pattern for Inz03.3H20 or Iii(OH)3 (12). From the above x-ray studies, it would appear that the particles in typical oxide sols consist essentially of aggregates of minute crystals of hydrous oxides or of simple oxide hydrates. I n the sols containing chloride, the latter is not bound in the form of a basic salt in most cases, but is adsorbed in an amount depending on the size and physical character of the particles. Such sol systems are properly referred to as hydrous oxide sols. In the light of the above, if one prefers to regard the hydrous oxide sols as electrolytes with colloidal ions it must be emphasized that there is a fundamental difference between sols and non-colloidal complex electrolytes such as potassium ferrocyanide, the cobalt amines, the complex platinic salts, etc., formulated by Terner. There is also a distinct difference between a hydrous oxide sol and such colloidal electrolytes as the soaps and Congo red, in that the latter contain ionic micelles made up of groups of ions which have a definite composition and Tvhich carry one charge for each equivalent of the ion, whereas the micelles of the former have no definite
CONSTITUTION OF HYDROUS OXIDE SOLS
7
composition and may carry hundreds or thousands of equivalents for each free charge. SUMWY
X-ray diffraction examination of the moist gels from typical hydrous oxide sols indicates that the particles of the solid phase in such sols consist essentially of aggregates of minute crystals of hydrous oxides or of simple oxide hydrates. In the sols containing chloride, the latter is not bound in the form of a basic salt in most cases, but is adsorbed in an amount depending on the size and physical character of the particles. REFERENCES (1) B ~ H AND M NICLASBEN: Z. anorg. Chem. 192, 7 (1924).
(2) BJERRUM: Z. physik. Chem. 79, 724 (1910). (3) DUCLAUX:J. chim. phys. I,79 (1907); 7, 405 (1909). (4) HANTESCH AND DESCH:Anr. 323, 38 (1902). (5) IVER:Proc. Indian Acad. Sci. 1, 372 (1934). (6) LINDBRAND PICTON:J. Chem. 800. 87, 1919 (1905). (7) MALFJTANO: Compt. rend. 148,1047 (1909); 8.physik. Chem. 68,232,248 (1910). (8) PAULIAND VALIKO: Elektrochemie der Kolloide. Julius Springer, Vienna (1929). (9) THOMAS AND WHITEHEAD: J. Phys. Chem. 96,27 (1931). THOMAS AND VON WICKLEN: J. Am. Chem. SOC.66, 794 (1934). (10) THOMAS AND TAI: J. Am. Chem. Soo. 64, 841 (1932). AND VARTANIAN: J. Am. Chem. Soo. 67, 4 (1935). THOMAS (11) WEIBER:J. Phys. Chem. 96, 1, 1368 (1931). WEISER AND GRAY:J. Phys. Chem. 96, 2178 (1932). X-ray Studies on the Hydrous Oxides. VIII. Gallium, (12) WEISERAND MILLIQAN: Indium, and Thallic Oxides. Reported at the Meeting of the American Chemical Society, April, 1935. (13) WYROUBOFF AND VERNEUIL: Bull. 800. chim. [3] 21, 137 (1899). (14) ZSIQMONDY:Ann. Sol, 361 (1898).
