HYDROCARBOKS AS D I S P E l W O S MEDIA: A REVIEW* BY BASIL C. S O T E S K O F F
Methods of Preparation In a previous communication,’ evidence was brought to show that peptization and colloid stability can occur in the absence of ions. I t still remained to find whether such behavior was exceptional or usual in the case of non-polar liquids as dispersion media. Chemical literature contains numerous references to colloid systems in benzene and other hydrocarbons. For the purposes of a preliminary discussion, we shall classify these organosols according to the methods employed in preparing them, since the latter a t least indicate the chemical composition of the dispersed phase,-a point on which other data often are lacking. Among the dispersion methods, electrical dispersion was resorted to by Svedberg2 to obtain sodium and potassium sols in ligroin, pentane and liquid methane. In spite of the painstaking precautions taken to insure the purity of the solvent’ and prevent its decomposition by the oscillatory discharge, the resulting sols were unstable. Hallw prepared dispersions of aluminium (and apparently also of silver, copper, lead and iron) by means of a 1000v. a.c. arc in “high grade transformer oil.” In some cases, the dispersed phase settled out rapidly (f. inst. copper in 2 4 hours). Haurowitz3 prepared the benzinosols of nickel, iron, aluminium, lead, tin, zinc, copper, manganese and brass. High frequency discharge was produced among small particles of metallic foil suspended in benzine, optimum results being obtained with 0.5-1.5 amp. a t 180 v. Since no special effort was made to prevent the decomposition of the dispersion medium, Haurowitz’s sols probably contained some colloid carbon. The sols remained stable for months provided rubber had been previously added. Colophony, cholesterol and lecithin had no protective action. Berl, Barth and W i n n a ~ k e r ,using ~ high-frequency alternating potentials of about 2 joo-0 v., obtained hexane dispersions of magnesium, zinc, lead, tin, copper and iron, which remained stable only in the presence of dissolved rubber. The most concentrated lead sol contained 1 . 3 2 7 ~P b and 0.7jqh rubber, the amount of P b in stable dispersion being in general proportional to that of the added rubber. I t s protective action was therefore considered as possibly due to the formation of addition compounds. Among the colloid systems prepared by mechanical dispersion we have emulsions in which an aqueous solution of soap, gelatin etc., constitutes the dispersed phase surrounded by a hydrocarbon (benzene, paraffin oil) as the continuous phase.
* Contribution from the Department of
Chemistry, Pi. Y. C . College of Dentistry
2994
BASIL C. SOTENKOFF
Thermal dispersion has been used in several instances. Thus von Keim a n 5 stratified glycerine under xylene and heated the lower layer; glycerine vapor condensed to an emulsion in xylene. Pochettinoj’ heated selenium above its melting point in contact with an “inert” solvent (paraffin, naphthalene etc.) which was solid at room temperature. The mixture assumed a red coloration due to the diffusion of selenium vapor through the hydrocarbon. Solid colloidal systrms were obtained on cooling, which yielded coarse and rather unstable sols on treatment with benzene, carbon bisulfide etc. Xdding selenium to boiling aniline produced colloid dispersions which settled out in 2 4 hours (in a few minutes when placed in direct sunlight). Very dilute dispersions in glycerine resulted in a similar manner. The method described by Semenoff, Schnalnikoff and collaborators deserves a special mention here. The two components (the metal and the frozen dispersion medium) are simultaneously vaporized in the vacuum of a diffusion pump, and the mixed vapor allowed to condense on the outer surface of a glass tube filled with liquid air. Solid mixtures result in which the dispersed phase is present in a state of very fine subdivision (not more than 2 j atoms in a particle according to X-ray evidence). Using a similar method, v. Bogdandy, Boehm and Polanyi6 obtained an Ag-naphthalene mixture containing 1 . 2 ~ ; of the metal and yielding, when treated with xylene, a red suspension which existed only for a short time. Roginsky and Schalnikoff’ prepared the orpanosols of sodium, pot assium, rubidium, caesium, cadmium and mercury. I?enzene, hexane, toluene, and xylene (in a few cases also ethyl ether and ethyl alcohol) served as the dispersion media; they were carefully purified by vacuum distillation. The stability of the sols varied from a few minutes to two hours at room temperature. A further study of these sols was made by Tomaschewsky.8 The solid mixtures containing sodium were ruby red immediately on melting; the transmitted light was not polarized. Before the room temperature was reached, however, a condensation of the particles occurred; the sols became polydisperse and violet colored in reflected light, the violet component being polarized. Colloid S a in ether remained stable for a t least a month,inxylene for 1 2 - 2 0 hours, in benzene and hexane was difficult to prepare. The organosols of the other alkali metals were dark blue in color; K in ether was stable after two month’s time, K in xylene lasted for about ten days, and R b and Cs in ether for less than half an hour. Many molecularly complex substances yield colloid dispersions in hydrocarbons merely on the addition of the solvent and, sometimes, heating. Rubber, gums, synthetic products like polystyrenes, phospholipin~~etc., belong in this class. Lyophilic organosols have been comprehensively discussed by Whitbylo and by Staudinger.]’ Some of the substances soluble in benzene can act as peptizing agents. Thus Evers’? obtained a black benzenosol of platinum by hydrogenating rubber under pressure in the presence of Pt powder.
HYDROCARBONS AS DISPERSION MEDIA
2995
Loewe9 notes that methylene blue, insoluble in chloroform, can be dispersed in a chloroform sol of kephalin, and the amounts of the dissolved dye are in agreement with those calculated from the adsorption isotherm. Amberger’3 triturated a concentrated solution of silver nitrate in water with lanolin and allowed the mixture to stand for a few hours; aqueous NaOH was then added in small portions. The resulting brown paste was soluble in chloroform, ethyl ether, petrolic ether, etc. The addition of ethyl alcohol produced a precipitate, containing as much as 74Yc Ag, which redispersed in the organic liquids mentioned above. Gold sols were similarly obtained by reducing gold chloride with hydrazine hydrate; they were purple in chloroform, blue in ethyl ether, petrolic ether, paraffin etc. The peptizing action of lanolin was traced to its non-saponifiable fraction. Other instances of lipins acting as protective colloids are given by Dean:“ “a chloroform solution of lecithin is able to take up or dissolve such substances as cobra venom, trypsin, rennet, and even oxide of iron.” Fatty acids (propionic to stearic) facilitate the dispersion of soaps in hydrocarbons;ls in the case of nickel stearate peptized by stearic acid, it’ has been shown that the resulting dispersions are colloid.1 More complex colloid systems can result in this way, as for instance the gold and silver organosols, prepared by von Weimarn16 as follows. A mixture of aqueous gold chloride and sodium oleate is treated with ammonium nitrate, whereupon x dark colored precipitate rises to the surface. This coagulum yields ruby or purplish-red sols on shaking with organic solvents. According to Haller, “adsorption compounds” of fatty acids and certain dyes form colloid dispersions in xylene.16 The condensation methods of colloid synthesis seem to have found more extensive application than the dispersion methods outlined above. The CD = AC B D (BD being insoluble, and the other three reaction AB compounds soluble), frequently resorted to in the preparat,ion of hydrosols, has also proved useful with hydrocarbons as the dispersion media. A sodium chloride sol, for instance, was prepared by Paal” by refluxing sodium malonic ester with chloracetic ester in benzene or toluene. The addition of petrolic ether precipitates the dispersed phase which contains 6 0 - 7 0 7 ~XaC1. The precipitate, if immediately transferred into pure benzene, will redisperse. A trace of moisture will cause the sol to coagulate. Paal concluded that the colloid was an “adsorption compound” of sodium chloride and an undetermined organic complex. Na-ethyl-malonic ester, w-chloracetophenone, chloroacetone, Na acetone dicarboxylic ester and acetyl chloride were used in subsequent preparations. Sodium bromide sol obtained with the corresponding bromine compounds was less stable, did not redisperse on precipitation apd passed readily into jelly. Ton TYeimarn18 prepared colloid halides, sulfates and nitrates of the alkali and alkaline earth metals in mixtures of butyl and propyl alcohols with benzene, xylene etc., from the corresponding thiocyanates. Colloid
+
+
2996
BASIL C . SOTESKOFF
chloride of copper was obtainedIg by mixing dilute benzene solutions of cupric oleate and hydrogen chloride, an excess of the oleate being necessary for the stability of the sol. The benzenosols of nickel, cobalt, iron and chromium chlorides resulted similarly from the oleates of these metals.?O I n the case of nickel and iron, ~ the sols were formed over the concentration range of o . o o o o ~ - o . o onormal. By varying the moisture content of benzene and the relative proportions of the reagents, suspensoid and emulsoid systems were produced of varying color and composition. Thus pink, blue, yellow and green varieties of the cobalt chloride sol were prepared. The color changes exhibited by the above-mentioned benzenosols ryere further discussed by von Weimarn and coworkers,?’ as well as the protective action of rubber on suspensoids in benzene.?’ X copper chloride sol, Lvhich coagulated after j minutes in pure benzene, was kept for 40 days in 0.1per cent rubber solution without any signs of settling out. The greater transparence and deeper color of the stabilized sols was also noted. Eiharichko~?~ used both the oleates and naphthenates of sodium, potassium, iron etc., in the preparation of dispersions of the corresponding chlorides in hydrocarbons. A copper sulfate sol was obtained on treatment of a benzene solution of copper naphthenate with sulphur trioxide. A dispersion of arsenious sulfide, stable for at least six months, results when hydrogen sulfide is passed through a solution of arsenic triiodide in transformer oil.” Dispersions of other insoluble compounds (mercuric sulfide, barium sulfate, Prussian blue) in the same medium were obtained in “an analogous manner.” Wo. Ostwald?? obtained colloid nickel by boiling nickel carbonyl in benzene; the dispersed phase settled out on standing. Hatschek and Thorne?j used pale cr&pe and colophony to stabilize Ostwild’s nickel sols in benzene and toluene. Greenish sols only were obtained in the latter which sharply turned black on addition of benzene. The green suspension was found to contain basic nickel carbonate.:6 Since nickel carbonyl does not ionize, and its dissociation in the vapor phase begins a t zooo, the possibility of a reaction between the carbonyl and benzene catalyzed by nickel was suggested. On passing sulfur dioxide and hydrogen sulfide through benzene until the formation of precipitate, a dark yellow sol results?’ which can be concentrated by evaporation and dialyzed against benzene. The sols remain stable for at least four months; pyridine, resorcinol, hydrogen chloride, benzoic and formic acids etc., have no effect, while potassium hydroxide produces a white emulsion. The colloid particles pass through most of the dialyzing membranes, beef bladder and cellulose acetate being the only suitable ones. Most of the colloid remains in dispersed form after freezing. The decomposition of an organic solvent by electric discharge yields colloid carbon besides other products. TarczynskP arced benzene, carbon
HYDROCARBON3 AS DISPERSION YEDIA
2997
tetrachloride, chloroform etc. (at 8-10 amp. d.c.) between carbon or platinum electrodes. The resulting sols were brown in transmitted, olive green in reflected light; they were stable for at least two years. Lastly, we have various procedures which involve a replacement of solvent; the starting point being either a true solution or colloid dispersion in a liquid miscible with some hydrocarbon. For. instance, on pouring an alcoholic (or acetone) solution of glucose into benzene a colloid suspension of the sugar resulted which varied in color froni greenish blue to violet with opalescence in complementary colors.cy (:olloid glucose could be preserved for a month, sucrose for a week, while lactose gelated in about ten minutes. Sols of sodium chloride arid copper chloride in aromatic hydrocarbons were similarly prepared.3o On shaking the crystalline hydrate of gold chloride with xylene, von Keimarn and Yanek”’ observed that some of it passed into yolution; the liquid became pink colored when boiled, due t o the formation of metallic gold. h number of other hydrated salts (magnesium, calcium, strontium, barium, zinc, cobalt and copper chlorides) disperscd in boiling xylene.3’ \Then a xylene sol of rubber was used as the dispersion medium, which was diluted quickly with more xylene after hoiling to prevent the separation of an aqueous 1 a y r on cooling, there resulted stable suspensions of the corresponding salts which could be dried over phosphorus pentoxide.”’ S e ~ b e r gobtained ~~ colloidally dispersed carbonates by pasing carbon dixode through the solutions of magnesium, calcium and barium oxides in methyl alcohol. The alcosols were miscible with benzene, chloroform ctc. displaced the water in a hydrogel of silica with alcohol. He further states that “the alcogel may be made a starting point in the formation of a gre:rt variety of other substitution jellies. . . . Compounds of ether, benzole and bisulphide of carbon have thus been produced.” Benzenogel of calcium germanate can be obtained in a similar manner.,i6 Such structures differ from rubber jellies, being rigid and brittle, and perhaps are more properly classed with gelatinous precipitates rather than true gels. Evidence of Colloidality Pome of the above authors made an effort to determine the dcgree of subdivision of the dispersed phase. Others apparently assumed a dispersion to be colloid because the dispersed substance was ordinarily insoluble in the medium; or, if soluble, separated in a gelatinous rather than crystalline form on cooling. X sol should appear optically heterogeneous unless the colloid particles imbibe the solvent to a great extent. The ultramicroscopic observations recorded in literature are compiled below. The writer also ventures to add some of his own (made with the aid of a cardioid condenser and I j-2 j amp. carbon arc as the light source).
2998
BASIL C . SOYENKOFF
Dispersed phase
Ultramicr. appearance
Lead protected with rubber Rubber (smoked sheets) Rubber (cr&pe) Kickel protected with rubber Selenium “Adsorption compound” of fuchsine and stearin “Ads. compd.” of Nile blue and stearin Lecithin, kephalin and mists. of cerebrosides Chlorides of heavy metals Kickel protected with rubber Higher polymers of styrene and isoprene Gold (prepared according to Amberger) Silver (from silver oxide and oleic acid) Carbon (according to Tarczynski) Iron sulfide
Numerous yellow particles Few ultramicrons
Arsenic Copper chloride
,,
,,
Reference 4
ibid. 3
Sumerous ultramicrons Circular particles Ultramicrons
ibid.
Tyndall cone only
ibid.
51 16
37
Ult ramicrons Numerous particles Particles Tyndall cone, sometimes a few ultra-microns Sumerous bright particles Tyndall cone, no ultramicrons Tyndall cone, no ultramicrons Xumerous particles which vary widely in size Very pale (barely visible) particles Tyndall cone, no particles
21 ?S
1
cf. p. 3002 P. 3003 p.
300‘
P. 300 P. 3003 P. 3003
The colloid solutions examined by the writer were both filtered through 602 (extra hard) Schleicher and Schull paper and centrifuged for one hour.
The latter treatment proved the more effective one in removing coarse particles. Tarczynski’s carbon sol, which is strongly opalescent (red brown in transmitted and olive green in reflected light) no longer contained ultramicroscopically visible particles after centrifuging. I t is possible that, because of the dispersed phase being uncharged (cf. p. 3006) and due to the lower viscosity of hydrocarbons compared to water, the larger particles settle out more rapidly in those sols than in hydrosols. Some of the ultramicroscopic observations tabulated above were made on lyophilic sols (rubber, lipins, Staudinger’s molecular colloids) ; when aplied to such sols, however, optical methods yield doubtful information which must be supplemented by other evidence. Since subsequent discussion will be limited to non-lyophilic (or nonswelling) colloids, the methods particularly applicable to lyophilic sols will be only briefly treated of here.
HYDROCARBONS AS DISPERSIOS XEDIA
2999
The coefficient of free diffusion offers perhaps the most reliable means of distinguishing between true and colloidal solutions. Northrop's method has been employed in approximate measurements.' StaudingeP be:ieves cryoscopic measurements to be reliable under certain conditions when the diffusiometric method is not likely to yield the correct values of molecular weight. On the other hand, a freezing point determination is valid only when the solute remains d:spersed in the liquid portion of the solvent which is in equilibrium with the solvent crystals. hloreover, the degree of dispersion should not change on cooling to the freezing point. Since the cryoscopic method involves the use of concentrated solutions when applied to substances of high molecular weight, the solute may precipitate before the freezing point is reached. Sometimes (f. inst. nickel stearate) a suspension of finely divided jelly is formed which is quite fluid and exhibits only a trace of opalescence, although the suspended phase can be separated by filtration through ordinary paper. Ebullioecopic determinations have been resorted to by n'alden40 to show that tetraisoamylammonium iodide is highly polymerized in chloroform and benzene solution. Barger's vapor pressure method was used by Loewe.9 Osmotic pressure, dialysis and ultrafiltration experiments require membranes of known and reproducible permeability. A beginning in this direction has been made by Bechhold?' and by McBain and Kistler.?* Using Rechhold's precipitation membranes, Kroepelin and Brumshagen determined the osmotic pressure of rubber b e n z e n ~ s o l s . ~Mark ~ and hleyer" used porcelain filters in similar measurements. Special methods based on viscosity measurements have been described by 8ta~dinger.~g Evidence of Electric Charge Several authors have observed the electrophoresis of benzenosols. Hatschek and ThorneZ5(cf. p. 2996) made a detailed study of the behavior of their nickel sols in an electric field. A potential drop of about Iov./cm. caused no noticeable shift in the sol boundary after 8 hours. K h e n 200-400 v./cm. were applied the sol deposited on both the electrodes in nearly equal amounts. The weight of the deposit was roughly proportional to the voltage drop and not t o square of the same (as might be expected for particles originally uncharged). Ultramicroscopic observations showed that the particles often continued in the same direction upon the reversal of the poles, or, on the contrary, described curved paths and proceeded towards the opposite pole while the field remained constant. The particles which happened to be midway b e h e e n the electrodes when the voltage was applied were not affected by the field. Rubber, which was used as the protective colloid, did not migrate to either pole under similar experimental conditions. The specific resistance of a nickel sol in toluene equaled 0.96 X IO'? ohms, while that of the dispersion
3000
BASIL C. SOTESKOFF
medium (toluene sol of rubber plus a little benzene) was 1.37 X IO’? ohms. No deposit was produced by zoo v. a.c. in 1.5 hours. The green-colored nickel carbonate s o h 6 migrated only to the anode. When precautions were takrn to exclude oxidation, nickel sols were prepared which similarly contained only negative particles. All of the ahove sols contained between 1-8 g. nickel per liter. More dilute sols (0.1-0.2 g. per liter) did not migrate to either pole when subjected to l o o v..’cm. Hatschek and Thorn? :ire inclined to think that their sols contain charged particles, :md that the mspnitude and sign of the charge changes during the preparation. Humphry and JaneJs observed the effect of an electric field ( 2 0 0 v./cm.) on benzenosols which w r e caused to flow downward, in a thin stream, betn-een charged plates immersed in benzene. They resorted to the optical method of striae in order t o render colorless sols visible in benzene. Amberger’s silvcr sols (cf. p. 2 9 0 ; ) Twre deflected towards the anode,16 Hatschek and Thorne’s nickel sols both ways. Rubber sols’j also migrated to both the poles; after careful drying, however, no deflection )vas observed. Buchner and Royen17 investigated Ilumphry and Jane’s method further. I n every case which they observed (hydrosols and salt solutions in water) a spreading of the stream took place rather than unilateral deflection. Any solution or colloid of greater conductivity than the liquid, through which it was caused to flow, tended to spread hetween the electrodes. Yo spreading occurred when the conductivities were nearly equal. This might be expected because, at the boundary between two solutions of different conductivity, electric forces change in a manner equivalmt to the effect of a surface electric charge. 1Iuch higher field intensities (several kilovolts per cm.) were used by other investigators. Pocheltino’l connected platinum electrodes immersed in a selenium so1 to the poles of a “small electrostatic machine of Wimshurst.” A compact, strongly adherent deposit of selenium formed rapidly on the anode. The sols studied were olitained by dissolving the dispersions of selenium in solids (fluorene, retenr, thymol, diphenylamine and triphenylamine) in carbon bisulfide and in benzene. Dispersions in phenanthrene, when dissolved in benzene, deposited on the anode; when dissolved in carbon bisulfide, on the cathode. Von Jl’eimarn’s organosols uf gold and silverIFmigrated to the anode in the field of an electrostatic machinr. It is not stated, however, whether hydrocarbons or other organic solvents served as the dispersion media in those experiments. Hall“ subjected suspensions of metals and their salts (cf. pp. z 9 : 3 , 2996) in transformer oil to 10-200 kv. for 1-3 hours, in order to determine whether the Cottrell method of precipitation was applicable to liquid media. A platinum plate and needle served as the electrodes. (The distance between the electrodes was given as I cm. in the calculations at the end of the article). The polarity was reversed in a number of the experiments, and high voltage
HYDROCARBONS AS D I S P E R S I O S MEDIA
3001
a.c. was also used. There was no movement to either electrode, and the rate of sedimentation was not influenced by the electrical treatment. Evers'? (cf. p. 2994) subjected the platinum sol prepared by him to 40 kv./cm.; the whole of the dispersed phase deposited in half an hour. More material separated on the anode than on the cathode. Few data are available regarding the electrical conductivity of colloid dispersions in hydrocarbons. The measurements made by Hatschel: and Thorne have been referred to above. The conductivity of nickel stearate benzenosols does not exceed that of hpnzene of ordinary purity (ca. 1 0 d 3 mhos.)' Tomaschewsky* found the resistance of a potassium sol in xylene to approximate IOI! ohms. Additional Electrophoresis Experiments JYhile studying the behavior of nickel stearate in benzene, the writer subjected a IC: dispersion of the soap to 20 kv./cm. for 6 hours. S o change in color and no deposit resulted (an unpublished observation). The rectified output of a high-voltage transformer served as the source of potential. Xniberger's silver sol (cf. p. 2 9 9 5 ) was next investigated. It did not move to either electrode in the field of an electrostatic machine (ca. I j kv'cm.), the time of observation again being 6 hours. Partial coagulation occurred; some of the coarse particles formed settled out between the electrodes, others remained in temporary suspension. The filtrate was colored like the original sol diluted with two volumes of the solvent. The above experiments led the writer recently to undertake further work in order to find out whether charged benzenosols were the rule, or, rather, an exception. Colloid systems were chosen in which the dispersed phase was, as far as it was known, insoluble in the dispersion medium; i.e. systems analogous to hydrophobic sols in water in which the electric charge would more likely be the main stability factor. Experimental conditions further restricted the choice of sols to those which were colored and did not decompose when in contact with air. \Yith the aid of a battery ( 7 2 0 v.) and the line voltage (240 v. d.c.) in series, potential drops of up to 5000 v.:'cni. were easily realized by varying the distance between the electrodes. The only source of higher voltage available at that time was an induction coil. A rectifying device was therefore built (Fig. I ) , which consisted of two disks mounted on the same motor shaft. One of the disks, while rotatinq, closed the primary contacts (P) and allowed them to open again. The second disk, covered with copper, carried a spring which, in a certain poFition, made contact with the ball K ; another spring, connected to the high voltage end of the secondary, pressed against the disk. The condenser C'Z discharged, about once a second, through the spark gap Sp which consisted of two brass balls, z cm. each in diameter and 1 . 2 cm. apart.
BASIL C. SOYENKOFF
3002
The sparking potential under the above conditions equals 38 kv. when one of the balls is grounded.49 The distance between the electrodes of the cell was 6-7 mm., and hence the potential drop of the order of 60 kv./cm. The design of the electrophoresis cells is illustrated on Fig. 2 . The electrodes are finely polished and plated with platinum The cells used in high voltage experiments (60 kv./cm.) were made of thick capillary tubing. The p.d. was applied for an hour, a t the end of which time any change in color of the sol was noted, and the electrodes were examined for deposit with the aid of a microscope.
FIG.I B
=
6 v.; C, = 4pf. (this value is rather critical); G is an ammeter (30 amp. scale); C2 = ca. 150 ppf.
d
A!-!+
FIG.2
Since the field strength mas of such a magnitude as to drive to the electrodes all charged colloid particles within a fraction of one hour, the above observations were deemed sufficient. In another set of experiments, where j kv./cm. were used, the inner diameter of the cell was 3 mm. The time of observation was six hours or longer The preparation and properties of the sols used in the electrophoresis experiments are given below. (The non-volatile matter content was deter mined by evaporating a sample of the sol a t room temperature in vacuo. Ultramicroscopic observations are given on p. 2998). I. Amberger's gold sol in xylene. This sol was prepared according to the original direction^'^ and reprecipitated with 4 vols. of ethyl alcohol, the precipitate dried in vacuo and dissolved in xylene. There was considerable sediment on centrifuging. Analysis of the supernatant sol showed the presence of 14.1g. non-volatile matter and 2.44 g. gold per 1. The sol is dark brown, almost opaque, and strongly opalescent, the reflected light being purple. There was no further sediment after two months.
HYDROCARBOXS AS DISPERSION MEDIA
3003
Colloidal silver in toluene (from silver oxide and oleic acid). Silver 2. oxide (0.1g.) was heated on a water bath with IOO cc. of jqc oleic acid in toluene for 4 hours. On cooling to - IO', a gelatinous precipitate of the silver soap separated. The sol, filtered and centrifuged, appeared red brown in transmitted and purple in reflected light. The silver content was 0 . 2 2 g./l. 3. Colloid arsenic i n xylene. The arsenic sol was prepared by refluxing a dilute solution (I~;'C or less) of arsenic trichloride over sodium; it, was concentrated by distilling off As Cla and xylene. The remainder appeared greenish gray and exhibited a faint Tyndall cone. S o sediment separated in 3 years' time. Colloid tin was also prepared by passing carbon monoxide through a solution of stannic chloride; it required an escess of the chloride for stability, reacted rapidly with moisture in the air and was therefore unsuitable for the electrophoresis experiments. 4. Colloid nickel in. The following mixture was reflused for I hour: 20 cc. toluene, 2 0 cc. 17nickel carbonyl and I O C C . of Centrifuged 15: crepe sol in toluene.* Metallic S i precipitated; the filtrate was an opxlescent black sol which showed no sediment on centrifuging. The so1 contained 0,394 g. Xi per 1. j . Tarczynski's carbon sol in Xylene (30cc.) was arced between carbon electrodes a t 9 amp. d.c. or 1 5 min. The resulting suspension was turbid and polydisperse. I t was quite clear when centrifuged, brown red in transmitted and olive green in reflected light. Yon-volatile matter content, was I g./l. When five cc. of the sol in a collodion bag were dialyzed against continuously renewed solvent for 2 days, the dispersed phase (about I mg.) precipitated as 3 purple powder. 6. Pochettino's selenium sol in p a r a f i n oil.b1 Red (amorhpous) selenium was precipitated by passing SO2 through an aqueous solution of selenium oxychloride, filtered and dried. It was ground to a fine powder, and a small amount (about I O mg.) suspended in IO cc. paraffin oil (nujol). The suspension was stirred for 3 hours and finally filtered. The filtrate was colorless, and the possible amount of dissolved selenium did not exceed I mg. S e x t , 50 mg. selenium were added to 30 cc. paraffin oil at 18oO, and the temperature raised to zooo in 5 minutes. A red coloration developed. The dispersion was rapidly cooled (by immersing the container in running water) and centrifuged. The portion which had not dispersed, together with the small amount of sediment, weighed 2 9 mg. The sol thus contained about 0,; g.11. of selenium, although a small amount might have been lost to the atmosphere. It was markedly opalescent and orange red in color. There was no sediment after a month. Also, the sol is not sensitive towards direct sunlight, ;. Colloid copper chloride 1'11 toluene (according to von l17eimarn). To 62 mg. copper oleate dissolved in I j cc. toluene were added gcc. of ;'1 crepe * Samples of crepe and smoked sheets were ohtained through the courtesy of the Rubber Company.
P.S.
3004
BASIL C. S O T E N K O F F
sol and 5 cc. 0.1 .V HCl. -1turbid orange suspension resulted which was evaporated to j cc. volume and centrifuged, yielding an orange green sol of 0.2 g./l. copper content. Pols of similar appearance and properties are made by adding a n ether solution of cupric bromide to toluene. The dispersion referred to above is the most concentrated one which could be obtained. 8 . Hatschek a n d Thorne’s nickel carbonate sol iii To 30 cc. toluene were added IOCC. of I(> rubber (smoked sheets) sol and I O cc. of 15 nickel carbonyl in toluene. The temperature was raised to 100’ in 3 hours and allowed to rise further to boiling point overnight. The filtered sol was pale green; its nickel content was 0.4 g.,’l. solution of ferric stearate 9 . I r o n sulfide so1 in toliteiie. To I O cc. of 0.17~ in toluene were added 7 cc. of 15: rubber (smoked sheets) sol in toluene followed, drop by drop, by toluene saturated with HZS until the color deepened to black (3 cc. altogether). The sol was strongly opalescent; it passed unchanged through hardened filter paper, and there was no sediment on centrifuging. The iron content was 0.43 g./L The color changes to light brown in 4-5 days, unless the sol is preserved in a sealed vessel. Freshly prepared sols only were used in the electrophoresis experiments. I
*
A particular effort was made t o obtain dispersions which, according t o previous investigators, migrate in an electric field. Thus Hatschek and Thorne’s nickel sol was prepared and studied, as well as Pochettino’s seleniumsols. The concentrated nickel carbonate sols, migrating t o the anode, could not be readily prepared: for the addition of nickel carbonyl could be continued only u p to a certain point, beyond which metallic nickel \vas formed. Ton Keimarn‘s colloid goldI6 in benzene usually precipitated within a few hours after its preparation, and in no case lasted longer than 4 days. l h e silver sols were more stable, but could more readily be prepared by the method outlined above (sol S o . 2 ) . Eyers’s platinum sols:’ could not be obtained at atmospheric pressure, and t h e writer lacked facilities for high-pressure work. Xmberger’s gold sol (sol S o . I ) was prepared; the effect of high electric potentials on the silver sol has already heen referred to (p. 0 0 0 0 ) . The only sol affected by potential drops of j kv. lcm. and lower was Hatschek and Thorne’s colloid Xi, Indeed, Iooov./’cm. were already sufficient t o drive the dispersed phase to the electrodes within I O minutes’ time. A number of sols, which showed no change when subjected to j kv.Jcni. for 6 hours and longer, underwent complete or partial coagulation at 60 kv./cm. Colloid iron sulfide precipitated instantly and completely. Sediment appeared in Amberger’s gold and Tarczynski’s carbon sols after a few minutes. I n the case of t,he carbon sols, centrifuging removed both the coarser particles and the tendency to precipitate in the electric field. Colloid arsenic (sol S o . 3), selenium (sol S o . 6), copper chloride (sol S o . 7 ) were not visibly changed after I hour at 60 kv./cm. These sols exhibited a
H Y D R O C A R B O S S AS D I S P E R S I O S MEDIA
3005
distinct Tyndall cone even when viewed with the aid of a relatively weak source of illumination (f. inst. a flashlight). Selenium sol was the only one which contained bright particles visible particles; a very powerful illumination source ( 2 5 amp. arc) revealed the presence of numerous ultramicrons, just on the boundary of visibility, in the arsenic sol. Cnstable dispersions of selenium, similar to those described by P o ~ h e t t i n o , ~ ~ were prepared by fusing selenium under phenanthrene and dissolving the solidified mixture in benzene or carbon bisulfide. No deposit could be noticed on the electrodes when such dispersions were subjected to 3 kv./cm. for one hour, but at 30 kv./cm. it required less than a minute for the anode to acquire a red coating of selenium. Reversing the connections from the induction coil, so that the grounded electrode became the anode, caused selenium to leave the high potential electrode (now the cathode) and to deposit on the new anode. The deposit of Se was thus transferred back and forth a number of times by reversing the polarity. Selenium deposited on the anode from both the benzene and carbon bisulfide dispersions. I n this respect the writer’s observations differ from those of Pochettino, who states that the electrophoresis proceeds towards the cathode in phenanthrene-carbon bisulfide as the dispersion medium. S o deposit resulted when a solution of amorphous selenium in carbon bisulfide was subjected to the same p.d. for one hour. Discussion of the Electrical Properties The experimental evidence regarding electrophoresis in hydrocarbons as dispersion media is summarized below. Dispersed phase
Gold
Evidence of electrophoresis
Reference
S o n e after 6 hrs. a t j kv./cm.; Pol s o . I 60 kv./cm. cause partial pptn. Deposits (chiefly on the anode) at 50 kv./cm. 12 Platinum Silver (Amberger’s). Migrates to the anode. 6 Undergoes partial pptn. at Ijkv.,’cm. p. 3001 but does not move to either electrode. Silver (from Ag S o n e after zo hrs. a t j kv./cm. Sol s o . 2 oleate). 0 Silver (by electric S o n e after 2 hrs. a t ca. 2 0 0 kv./(cm?) dispersion) SO Lead. S o n e after I hrs. at ca. zoo kv.,/(cml) SO Copper. S o n e after IOO min. a t ca. zoo kv.,’(cm?) ?5 Sickel. S o n e after Iov./cm. for 8 hrs.; 90 v./cm. and higher cause the sol to deposit either on both the electrodes or only on the anode. Mobility ca. z X IO-^ cm./sec. (p.d. = Sol S o . 4 960 v., electrodes Icm. apart). 50 Iron. S o n e after z hrs. a t ca. zoo kv./(Lm?) 53 Aluminium. S o n e after 3 hrs. at ca. zoo kv./(cm?)
3006
Dispersed phase
Arsenic. Potassium Carbon (U). Selenium (in benzene).
Selenium (in paraffin oil). Sickel carbonate.
Copper chloride. Arsenious sulfide. Mercuric sulfide. Ferrous sulfide. Barium sulfate. Prussian blue.
BASIL C. SOTESKOFF
Evidence of electrophoresis
Reference
Xone after 60 kv./cm. for I hr. Electr. resist. approximates IO^* ohms. S o n e at 5 kv./cm. for 2 2 hrs.; none a t 60 kv./cm. for I hr. PIIoves to the anode in the field of an electrostatic machine. Deposits rapidly on the anode at 30 kv.,'cm., no change for I hr. a t 3 kv./cm. S o n e at 5 kv./cm. for 26 hrs.; none at 60 kv./cm. for I hr. Deposits on the anode from concentrated sols; does not migrate in dilute sols (p.d. about 100-400v./cm.). .; kv./:cm. for 2 0 hrs. has no effect on a dilute sol. 5 kv./cm. for zo hrs. has no effect. Yone after ca. zoo kv./(cm?) for 3 hrs. ?;one after z hrs. at ca. zoo kv.j(cm?) Xone after 16 hrs. a t j kv./cm., instant pptn. at 60 kv./cm. ?;one after z hrs. at ea. zoo kv./(cm.?). S o n e after I hrs. at ea. zoo kv./(cm?)
Sol ?io. 3 8
Sol s o . 5 51
p.
0000
Sol S o . 6 TG
Sol S o . 8 Sol s o . 7 50
50
Sol s o . 9 jo
50
I t is thus seen that potential drops of less than j kv./cm. have no noticeable effect on colloid dispersions in hydrocarbons, with the exception of Hatschek and Thorne's nickel sols. IVhen we recall that the particles in hydrosols exhibit nearly the same cataphoretic mobility (ca. I O + cm./sec. a t I v./cm.); and furthermore that similarily charged particles should move faster in hydrocarbons due to the lower dielect'ric constant,-it follows that the majority of colloid dispersions in hydrocarbons either are uncharged or carry only a small fraction (less than IO-^) of the charge possessed by the particles in water. An electric field of great intensity (corresponding to several kv./cm.) exerts ponderomotor effects on particles suspended in a hydroca.rbon which are not necessarily electrophoretic. The nature and magnitude of these phenomena depends on the size and shape of the particles, viscosity of the medium, distance between the electrodes and their shape, etc. Thus if a suspension of dust particles in toluene is observed through the microscope, some of them will be seen to proceed towards one of the electrodes as soon as 5 kv./cm. is applied and to remain attached there. Oblong particles will form chains which orient themselves perpendicularly to the electrodes and eventually form a continuous path between them; while some of the smaller and nearly round particles begin to oscillate rapidly between the electrodes without actually touching the latter. Also, the free ends of the filaments adhering to the electrodes exhibit an undulatory niovement.
HYDROCARBONS AS D I S P E R S I O S MEDIA
3007
Particles of higher dielectric constant than the medium would be expected t o coalesce when subjected to a sufficiently strong electric field. Conductors (metallic particles) suspended in a liquid insulator can be considered to have an infinitely large dielectric constant. Alternating potentials are equally effective in this case, and have been successfully used in the precipitation of water-in-oil emulsions: the water droplets “arrange themselves . . like the iron filings between the poles of the magnet, dance about and jiggle themselves into larger drops.”$? At potential drops of the order of 60 kv.icm. particles which are too small to be resolved under the microscope come within the range of the forces which cause coalescence, as evidenced in the precipitat ion of Amberger’s gold and silver sols, iron sulfide sol, etc. At the same time, sols which contain smaller particles (colloid arsenic and carbon) are not visibly changed. It is rather difficult to explain, in view of the above, why high electric potentials, both d.c. and a x . , should have no effect on coarse dispersions of metals and their salts in transformer oil,j0 unless we assume, perhaps, that the viscosity of the medium was sufficiently high to prevent rapid coagulation. Another manner in which the influence of an electric field manifests itself is to cause any suspended body of higher conductivity or dielectric constant thzn the medium to move towards the region of highest field intensity; for instance, to the anode if the negative pole is grounded. Furthermore, ions are formed in increasing numbers as the voltage across the electrodes approaches the sparking potential: there is a rapid rise in conductance. Particles which were originally uncharged may then assume a charge, the phenomenon being similar to the electrification of smoke particles in the Cottrell precipitation process. The precipitation of a sol on the electrodes (or between them) when subjected to several kv./cm. does not prove therefore that the dispersed phase is charged, although the behavior of some sols (Pochettino’s colloid selenium) will not allow of a different interpretation. The conclusion follows that stable sols can result (and do result, in the majority of cases) in the absence of an electric charge, if we choose a nonpolar solvent as the dispersion medium. The determination of electrophoretic velocity in benzenosols which are charged presents difficulties: colloid particles are often driven to the walls of the vesselz3or deposited on the glass in the neighborhood of the electrodes (in case of the nickel sol). That fraction of the electrical conductivity of a sol which represents the transport of electricity by the colloid particles should be more capable of nieasurement when a hydrocarbon serves as the dispersion medium instead of water. However, the calculation of migration velocities from conductivity data requires that we know the thickness of the Helmholtz double l~yer,~SR quantity which cannot be independently determined. Stnbility Fnctom. All of the dispersions studied by the writer showed no sediment after 3 months, and some of them have been preserved unchanged
3008
BASIL C . SOTENKOFF
for over three years (colloid arsenic, carbon, hmberger’s gold and silver). Since the viscosity of the dispersion media (benzene, toluene and xylene) is less than half that of Mater, the above colloid systems compare favorably with hydrosols as regards their stability. The electric charge cannot therefore be an important stability factor in the case of benzenosols. The obvious reason for this is that the dispersion medium itself does not ionize and, although a number of substances yield ions when dissolved in benzene, the degree of ionization is extremely small except in concentrated solutions. The orientation of solvent molecules, contributing to the stability of charged particles, is absent in “dipole free” liquids like benzene, hexane and carbon tetrachloride.>‘ Only those colloid systems, in which at least part of the dispersed phase is insoluble in the medium, and which possess the optical properties of lyophobic sols, form the subject of this discussion. I n the absence of an electric charge, one is reduced to the assumption t h a t the colloid particle derives its stability from a surrounding layer of some substance or substances soluble in the dispersion medium. I n numerous instances the behavior of benzenosols bears out this supposition. Thus, aside from a variety of sols in which rubber serves as the protective colloid and Amberger’s sols in which lanolin is similarly used, we note that copper chloride sols require an excess of copper oleate for their stability,’g colloid tin requires stannic chloride (p. 3003) while colloid carbon precipitates when the diffusible components are removed by dialysis. Perhaps the most fundamental criterion whereby we distinguish between lyophobic and non-lyophobic (lyophilic and protected) sols is that of reversibility. Hydrophobic sols are metastable systems which precipitate irreversibly on evaporation and freezing. Benzenosols (colloid carbon, silver, glucose etc.) on evaporation leave a residue which yields a sol of similar properties as soon as the solvent is added; rubber protected sols redissolve more slowly, possibly because the swelling and dispersion of rubber in benzene requires time. All of the sols show no change whatever on freezing. The results of the writer‘s own experiments, together with observations of similar nature made by previous investigators, are given below. Dispersed phase
Potassium. Silver (lanolin). Gold (lanolin).
Silver. h’ickel (rubber). Arsenic.
Evidence of reversibility
Redisperses on evaporation. Redissolves on pptn. and drying. Redissolves on pptn. and drying. Unchanged by freezing in liquid air; redissolves instantly on evaporation. Redissolves instantly on evaporation. Redisperses after precipitation with alcohol. Freezing in liquid air produces no change. Freezing in liquid air produces no change.
Reference 8
13 13
Sol KO.
I
Sol KO.
2
25
Sol s o . 4 Sol x o . 3
3009
HYDROCARBOSS AS DISPERSIOS MEDIA
Dispersed phase
Carbon(’?)
Evidence of reversibhty
Vnchanged on evaporation and freezing in liquid air. Plight ppt. after repeated freezing. Pnchanged on freezing in liquid air. Unchanged on freezing in liquid air.
Sulfur. Selenium Nickel carbonate (rubber) rnchanged on freezing in liquid air. Copper chloride (rubber) Iron sulfide (rubber) Pnchanged on freezing in liquid air. Redisperses after being precipitated with Sodium chloride. petrolic ether.
Reference
Sol
10. j 1
Sol S o . 6 Sol S o . 8 Sol KO. 7 Sol s o . 9 1;
This difference in behavior from the hydrophobic sols, where the electric charge is an important and probably dominant stability factor, is significant. The above benzenoeols can most logically be classed with protected sols, except that the protecting substance is not always colloid.
Summary The literature relative to the preparation of benzenosols and their behavior in the electric field is reviewed, as well as some additional experiments. Electrical factors of stability are shown to be unimportant in hydrocarbons as dispersion media. References Soyenkoff: J. Phys. Chem., 3 4 , 2 j 1 9 ( 1 9 3 0 ) . Svedberg: “Heratellung kolloider Loesungen,” pp. 477-56 (1922). 3 Haurowitz: Kolloid-Z., 40, I39 (1926). 4 Berl, Barth and Winnacker: Z. physik. Chem., 145, 298 (1929). 5 Van Weirnarn: Kolloid-Z., 36, 176 (192s). 6 v. Bogdandy, Boehm and Polanyi: Z. Physik, 40, 2 1 2 (1926). 7 Roginsky and Schnalnikoff: Koiioid-Z., 43, 67 1192;). 8 Tomaschewsky: Kolloid-Z., 54, 79 (1931). Freundlich: “Colloid and Capillary Chemistry,” 1). 6 j 5 (1926); Loewe: Biochem. 2. 42, 1 5 0 , 207 (1912). 1OWhitby: Colloid Symposium Monograph, 4, 203 (1926); 6, 225 (1928). l1 Staudinger: Z. angew. Chem., 42, 37, 67 (1929); 2. physik. Chem. 153.4, 391 (1931). 12 Evers: Kolloid-Z., 36, 206 (1925). ‘3 Amberger: Kolloid-Z., 11, 97, 100 (1912). l 4 Dean: Lancet, 192, 4 j (1917). 15 Yon Weimam: J. Russ. Phys. Chem. Sac., 46, 624 (1914); Dn Fano: Chem. Ahs., 23, 1
2
Q
5386. 16 Von Weimam quoted by Hatschek and Thorne: Kolloid-Z., 36, 14 (192j); Hailer: 22, 127-8 (1918). P a d : Ber., 39, 1436 (1906); P a d and Kuhn: 39, 28j9, 2863 (1906); 41, j I , j 8 (1908). Also Svedherg: “Herstellung kolloider Loesungen,” pp. 346-61 (1922). l X Von Weimam: J. Russ. Phys. Chem. Soc., 40, 1124 (1908). l9 Von Weimam: J. Russ. Phys. Cheni. Soc., 38, 268 (1906); Kagan: 42, 372 (1910). *” Van Weirnarn and Anosov: J. Russ. Phys. Chem. Sac., 46, 622; v. If-. and hlorozov: 6 2 3 (1914). 21 Von Weimarn: J. Russ. Phys. Chem. Sac., 48, 532, i o 5 f1916j; Anosov: p. j o q ; Anosov and Morozov: p. ;oj.
3010
BASIL C. SOYENKOFF
22Von Weimarn, Anosov and Morozov: J. R u m Phys. Chem. SOC.,48, 706, (1916). 23Kharichkov: J. Russ. Phys. Chem. SOL, 52, 91 (1920). z4 Ostwald: Kolloid-Z., 15, 204 (1914). 25 Hatschek and Thorne: Kolloid-Z., 33, I (1923). leHatschek and Thorne: Kolloid-Z., 36, 12 (1925). 2 i Garard and Colt: J. Am. Chem. SOC.,49, 630 (1927). 28 Goldschmidt and Tarczynski: Bull. SOC. sc. med. et nat. Bruxelles, 65, 40 (190j); Tarczynski: Z. Elektrochemie, 22, 2j2 (1916). 28VonWeimarn: Kolloid-Z., 36, 118 (1925). 3 0 \‘on Weimam: Kolloid-Z., 36, 176 (1925). 31Von Weimam and Yanek: J. Russ. Phys. Chem. SOC.,48, 1060(1916). 32 Von Weimarn: J. Russ. Phys. Chem. SOC.,48, 1061(1916). 33Von Weimarn: J. Ruus. Phys. Chem. Soc., 48, 1749 (1916). 34 Freundlich: “Colloid and Capillary Chemistry,” 6jj (1926). 35 Graham: J. Chem. SOC.,17, 322 (1864). 36 Muller and Gulesian: J. Am. Chem. SOC.,51, 2032 (1929). 3i Loewe: Biochem. Z., 42, 21 j (1912). Staundinger: Ber., 59, 3036 (1926). 39 Staudinger: Z. physik. Chem., 153A, 403 (1931). 4 o Walden: Kolloid-Z., 27,99 (1920). 41 Bechhold: Kolloid-Z., 36, (Erganzungsh.), 2 j y (192j). 4 2 McBain and Kistler: J. Phys. Chem., 35, 130 (1931). 4 3 Kroepelin and Brumshagen: Ber., 61B, 2441 (1928). 44 Meyer and Mark: Ber., 61B, 1946(1928). 45Humphry and Jane: Trans. Faraday SOC.,2 2 , 420 (1926);Kolloid-Z., 41, 293 (1927). 4G Humphry: Kolloid-Z., 38, 306 (1926). 4i Buchner and van Royen: Kolloid-Z., 49, 249 (1929). 4 3 Freundlich: “Colloid and Capillary Chemistry,” p. 39j (1926). 49V0igt:Ann. Physik, (4) 12, 403 (1903). ‘OHalI: J. .4m.Chem. SOC.,44, 1246(1922). 51 Pochettino: Atti Accad. Lincei, ( 5 ) 20 I, 428 (1911). 62 Speed: J. Ind. Eng. Chem., 11, I j3 (1919). 63 Sherrick: J. Ind. Eng. Chem., 12. 13j (1920). Ostwald: Kolloid-Z., 45, 331 (1928).
