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EMULSIFICATION SYMPOSIUM Papers presented before the Section of Petroleum Chemistry a t the Band Meeting of the American Chemical Society, New York, N. Y., September 6 to 10, 1921.
Emulsions with Finely Divided Solids By T. R. Briggs CORNELL UNIVERSITY,
ITHACA, N E W YORK
Pickeringl was apparently the first to recognize clearly that h e l y divided solids are capable of acting as emulsifying agents. He wrote as follows: Apparently, a precipitate consisting of any insoluble substance which is wetted more easily by water than by oil, if in a sufficiently fine state of subdivision, will equally act as an emulsifier ***** and it is possible under the microscope to see the coating of solid particles which envelop the oil particles.
Pickering prepared emulsions of a petroleum oil in water with the aid of so-called basic copper sulfate and other precipitated finely divided solids. He showed that the drops of oil were kept from coalescing by being coated with a pellicle of the finely divided solid material. I n all of his emulsions, however, oil formed the drops, and he did not complete his experiments by preparing emulsions in which the drops consisted of water, suspended in oil as the outside phase. A few years later Kcwman,2 at Bancroft's suggestion, showed that water may be emulsified in benzene by the use of magnesium or calcium oleate, and in this way called the attention of chemists to emulsions of the water-in-oil type, which, though long recognized by pharmacists, had more or less escaped consideration by others. Thereupon the work of Pickering with solid emulsifiers was continued by SchIaepfer,a who succeeded in obtaining emulsions of water in kerosene by the use of carbon black or soot as the emulsifying agent. As the result of these and other researches, it is now generally recognized that finely divided solids may play an important role in the formation and existence of many well-known emulsions, such as ready-mixed paints and crude p e t r ~ l e u m . ~ It is part of the purpose of this paper to emphasize the fact that emulsions with finely divided solids are essentially similar to those in which an apparently soluble emulsifier is used, such as sodium oleate or gum arabic. In every case, investigation has shown that the apparently soluble emulsifier is in colloidal suspension in the outside phase of the emulsion and therefore constitutes a third phase in the system, exactly as a solid emulsifier can be seen to constitute a third phase in S U C ~systems. Furthermore, it is accepted by nearly everybody that the globules of liquid in all ordinarys emulsions are invariably coated by some kind of film or pellicle6 which tends to prevent coalescence, and that this film or pellicle is present whether the emulsifier is apparently soluble or consists of a finely divided solid. In view of the vast majority of the experimental evidence, it seems safe to assume that the presence of a film of emulsifying agent is absolutely essential to the existence of all ordinary emulsions. The discussion being limited for the moment to finely divided solids or to their suspensions as emulsifying agents, one may ask how the necessary interfacial films are produced. Separation and concentration of finely divided solid, such J . Chem. Soc., 9 1 (1907),2001. J . Phys. Chem., 18 (1914),34. 8 J . Chem. SOL, 118 (1918), 522; cf. Moore, J. Am. Chenz. Soc., 41 (1919). 940. 4 Padgett, Chem. Met. Eng., 26 (1921),189. 1
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5 Excepting possibly suspensions of liquid-in liquid in which electrical peptization is brought about by t h e adsorption of ions. Ellis, Z . phys. Chem , 80 (1912),597;Powis, I b i d . , 89 (1914). 91,179. e Briggs, J . Phys. Chem , 19 (1915),810.
as hydrous oxide of iron, a t the interface between benzene and water, for instance, can sczrcely be due to a lowering of the interfacial tension, for hydrous iron oxide does not affect the surface tension of mater. For all those cases, therefore, in which the interfacial solid is known not to affect the interfacial tension, it seems necessary to postulate that the solid is wetted to some extent by both liquids comprising the emulsion.' This must be done in the case of the solid emulsifiers studied by Pickering, and possibly also in the case of saponin and gum arabic which have relatively little effect on thc surface tension of water. The next question which arises is in regard to the type of emulsion-oil-in-water or water-in-oil-that may be formed through the agency of an interfacial, finely divided solid. It is generally agreed that the liquid which wets the solid emulsifier the more strongly under the conditions of the experiment, tends to become the outside phase, the less strongly wetting liquid being broken up into drops. Just why this is so is difficult to explain satisfactorily for the moment. It was pointed out in a previous paragraph that a finely divided solid can serve as an emulsifying agent only if it possesses the power to separate out or concentrate at the interface between the two liquids. At the same time it must be capable of forming a film which is sufficiently continuous and elastic; to do this the solid particles apparently should be extremely minute, as Pickering has stated in the paragraph quoted previously. Hence it follows that influences opposing separation of the solid a t the interface or increasing the size of the individual particles which do pass into the interface, will tend to prevent the formation of a stable emulsion. The process of interfacial concentration may be looked upon as a process of distribution of finely divided solid between the interface and one a t least of the component liquids. Anything therefore which carries the solid particles into one liquid must tend to remove them from the interface. Hence a peptizing agent tending to carry the solid into suspension in one of the liquids will make the solid less interfacial, other things being equal, and will oppose the formation of an emulsion. For instance, Reinders2 found that gold separated out as a semi-metallic, interfacial film, blue by transmitted and gold by reflected light, when a gold hydrosol was shaken with isobutyl alcohol. On adding to the gold hydrosol gum arabic as peptizing colloid, however, metallic gold no longer passed into the interface with isobutyl alcohol. Further evidence bearing directly on emulsions will be offered in experiments shortly to be described. On the other hand, if a solid is already in suspension in one of the liquid emulsion components and is prevented from being sufficiently interfacial because of strong peptization, it ought to be possible, by adding a flocculating agent, to counteract the peptizing influence and to force the solid into the interface. There should result accordingly an increas in the emulsifying action of the solid material. This is often the case in practice as the experiments to be described indicate. If, however, a very powerful flocculating agent is used, the finely divided solid may be caused to agglomerate into large masses or flocks, so rendering the formation of a suitable emulsion film impossible. It thus follows that if an aqueous suspension of a solid is used to emulsify oil in water, a substance having 1 Hofmann, Z . p h y s . Chem., 88 (1913),385;Reinders, 2'. KoR&dzenz., 13 (1913). 235;Bancroft, J . Phys. Chem., 19 (1915),286, 513.
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a weak flocculating effect on the suspension may be expected t o help in the emulsifying process, while a powerful flocculating agent will be harmful. Experiment has confirmed this expectation.
EMULSIONS WITH HYDROUS FERRIC OXIDE A suspension of hydrous ferric oxide was prepared by adding crystalline ferric chloride to boiling water and submitting the suspension to partial purification by dialysis in collodion sacs. The suspension contained approximately 10 g. of oxide per liter. Forty cc. of this nearly clear suspension were shaken in a bottle with 10 cc. of benzene, but no emulsion was produced, the benzene and water separating into two continuous layers shortly after shaking ceased. When, however, 1 g. of sodium chloride (free from sulfates) was added to the oxide suspension and the latter was shaken anew with benzene, a coarse, unstable emulsion of benzene in water was produced. As the concentration of salt was increased in the oxide suspension, the stability and the degree of dispersion of the emulsified benzene were greatly enhanced, fairly permanent emulsions being ultimately obtained. I n time the emulsified benzene floated to the top of the water layer in the form of a brownish yellon. cream and carried with it a large proportion of the oxide originally suspended in the aqueous phase. To make certain that the sodium chloride itself contained no emulsifying impurity, a blank experiment was carried out by shaking benzene with solutions of the salt; but no trace of emulsion-building was observed in any case. When kerosene was substituted for benzene, similar results were obtained. These experiments indicate that sodium chloride, a weak flocculating agent, drives ferric oxide out of suspension in water ox" a t least destabilizes the suspension, and forces the oxide into the interface. By doing so, it causes the oxide to act &s an emulsifying agent. That weak flocculation does actually occur is evident by inspection, for the suspension becomes noticeably more turbid on the addition of salt and settles out after a considerable interval of time. Barium chloride, also a weak flocculating agent toward the suspension of ferric oxide. acted quite as sodium chloride did in bringing about emulsification. But when sodium sulfate, a powerful flocculating agent, was added, no emulsions were produced, the ferric oxide being precipitated in flocks which were too coarse in size to form a suitable emulsifying film.
EMULSIONE WITH ARSENIOUS SULFIDE Six grams of arsenious oxide were dissolved in water and the solution was treated with washed hydrogen sulfide. The filtered suspension of the trisulfide was purified partly by dialysis and was diluted to 600 cc. Forty cc. of the yellow suspension were shaken with 10 cc. of benzene, and, as was the, case with the iron oxide suspension, no benzene was emulsified. On adding sodium chloride, however, emulsions of benzene in the aqueous phase were obtained on shaking, exactly as in the previous work. Experiment showed that 0.16 g. of sodium chloride added to 40 cc. of the sulfide suspension was sufficient to cause the complete emulsification of 10 cc. of benzene. This quantity of salt was observed slightly to coagulate 40 cc. of the suspension after an hour. It WFW apparent, too, that the sulfide became strongly interfacial as salt was introduced and emulsions were obtained, for the drops of benzene could be seen to carry a yellow film, and much of the sulfide was floated up. When large amounts of salt were used the emulsions became less complete and coarser, although the sulfide became increasingly interfacial. Evidently flocculation had gone too far, and the solid was no longer able to form about the benzene a suitably fine-grained film. For the same reason barium chloride, a strong flocculating agent analogous to sodium sulfate with ferric oxide, produced far less stable
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and effective emulsions than sodium chloride did, when added to benzene and sulfide suspension. It is evident that the experiments with arsenious sulfide are quite in line with the experiments with ferric oxide, and that the two sets of experiments afford a striking confirmation of the theory. ANTAGONISTIC SOLIDEMULSIFIERS It has been pointed out in a former paragraph that the two types of oil-and-water emulsions have now been prepared, not only with so-called soluble emulPjfiers, but also with finely divided solids. Among the emulsifiers of the first class, sodium oleate produces emulsions of oil in water, while calcium oleate produces emulsions of water in oil (benzene). Clowes has shown1 that if sodium oleate and calcium oleate are present in approximately equivalent quantities in a mixture of water and benzene, no emulsion a t all is produced on shaking. That is, sodium oleate and calcium oleate are mutually antagonistic emulsifying agents, and when they are both present in the right proportions each neutralizes the emulsifying action of the other. Since among solid emulsifiers carbon black is known to emulsify water in oil while finely divided silica emulsifies oil in water, one should expect to find a similar antagonistic action between these two solids when they are used together. This was sho'wn to be true in the following manner: To 15 cc. of kerosene was added 0 . 8 g. of carbon black, which formed a black and fairly stable suspension in the oil. To this suspension in a bottle 25 cc. of water were introduced slowly and with shaking. I n this way an emulsion of water in oil was formed, which mixed freely with fresh oil but did Finely pulverized silica which not do so with water. had been passed through a 350-mesh sieve was added to a second bottle containing the above-mentioned quantities of kerosene and carbon black, and water was introduced as before. It was found that the silica exerted a marked antagonistic effect on the carbon black, and rendered it much more difficult to emulsify the water. When the amount of silica exceeded 0 . 1 g. it was found impossible to emulsify any water a t all, the mixture in the bottle separating after shaking into two sharply defined layers, an upper one of kerosene and carbon black and a lower one of water containing some silica in suspension. Likewise no emulsion was formed when equal parts of silica and carbon black were mixed. On adding 15 cc. of kerosene to 25 cc. of water containing only pulverized silica, the oil was completely emulsified on shaking. Water WRS now the outside phase, for the emulsion mixed freely with water but not with oil. A small amount of carbon black added to the silica, however, sufficed to keep the oil from being emulsified, though unfortunately the quantity needed completely to prevent emulsification was not determined. A similar experiment was, however, carried out with kerosene, water, silica, and powdered mercuric iodide, the latter like carbon black being readily wetted by oil and tending to emulsify water in oil. It was found that approximately one part of mercuric iodide in 20 parts of silica was enough to prevent the latter from emulsifying 25 cc. of kerosene in 25 cc. of water. SUMMARY
l-l3mulsions with finely divided solids are similar to emulsions in which so-called soluble emulsifiers are used. 2-The general theory of solid emulsifiers has been reviewed and amplified. 3-1tj is absolutely necessary that the finely divided solids form a suitable film a t the interface between the twoliquids which are to be emulsified. 4-If the finely divided solid forms a stable suspension' in one of the liquids, it may be necessary to add a weak floccuJ . Phus. Chem., 20 (1916). 407.
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lating agent before a satisfactory emulsion can be produced; but a powerful flocculating agent will usually prevent emulsification. 5-Certain finely divided solids, such as carbon black and silica, are mutually antagonistic so far as the formation of emulsions is concerned, and in this respect are analogous to sodium and calcium oleates.
Emulsifying Agents in Oil-Field Emulsions By J. L. Sherrick MELLON INSTITUTE OF INDUSTRIAL. RESEARCH, PITTSBURGH, PA.
Since publishing the reeults of a research on oil-field emulsions1 carried on in the winter of 1918 to 1919, the writer has received many inquiries from widrspread sources. Although the subject matter of the present contribution represents no additional laboratory research, certain points therein have been developed in discussion with associates and are offered in the hope that they will encourage further discussion. Before considering the question as to the substance in crude oil which serves as the emulsifying agent, it is first necessary to review briefly the general conditions for emulsion formation. EMULSIFYING AGENTS The necessity for the presence of some third component to serve as an emulsifying agent has been recognized by almost all workers in emulsions. The substances, which function as emulsifiers, by collecting a t the interface of the two liquids to form protective films for the dispersed phase are, however, quite varied in their nature. The most common emulsifying agents are the emulsoid colloids like soap, gelatin, gums, starches, beeswax, etc. It seems certain also that suspensoid colloids may so serve, although they are not generally used. Pickering2 however, has gone a step beyond this and used finely divided solids for such a purpose. It has been claimed that ions adsorbed on the surface of liquid particles may serve as emulsifiers, and Ellis has prepared an emulsion which owes its stability to ion adsorption. It is probable, however, that for the formation of emulsions of relatively high concentration of dispersed phase, say 50 per cent or more, the presence of an emulsifier which will form a relatively tough and elastic film by collecting at the interface is necessary.
TYPES OF EMULSIONS Emulsions may be classified according to the position of the several phases. Thus we have oil-in-water emulsions, in which the oil is dispersed as small particles in a continuous water phase, and water-in-oil emulsions, in which the water is dispersed as small particles in a continuous oil phase. The type of emulsion formed by any given oil with water depends primarily upon the nature of the emulsifying substance. Bancroft has explained this from the standpoint of surface tension. I n a general way, however, the following applies: An oil-in-water emulsion is formed by the use of a water-soluble colloid as emulsifier; a water-in-oil emulsion is formed by the use of an oil-soluble colloid. With finely divided solid material, the liquid which more easily wets the solid material will be the continuous phase. OIL-FIELD EMULSIONS Oil-field emulsions belong to the water-in-oil type and as such must be formed by an oil-soluble colloid or a solid ma1 THISJOURNAL, 2.T.
12 (1920),133. Chcm. Soc., 91 (1907),2001.
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terial more easily wetted by oil acting as emulsifier. The character of this emulsifier may most easily be shown by the actions of the emulsions under certain conditions of test. It has been shown by the author that the water particles are negatively charged electrically, and indeed both alternating and direct current electrical treatment have been used to discharge these emulsions. As a matter of fact we have found that a static charge is sufficient toprecipitate the water. The water particles may be made to coalesce and precipitate by the addition of certain electrolytes, such as acids and iron salts. Such an effect may be due to the preferential adsorption of certain ions, but it is rather hard to believe that these emulsions, ranging from 10 to 60 per cent in content of dispersed water phase, owe their stability to ion adsorption alone. Even though Ellis has prepared emulsions stabilized by ion adsorption, these were extremely dilute emulsions and were of the oil-in-water type. Certain water-soluble colloids, such as sodium oleate and the sodium salts of certain sulfonic acids render these emulsions unstable and precipitate the water if added in proper proportion. This is indeed what one might expect if t h e original emulsifying agent were an oil-soluble colloid as the action of two such colloids must be antagonistic, the one tending to form a water-in-oil and the other tending to form an oil-in-water emulsion. The precipitating colloid must, however, be added in exactly sufficient quantity to neutralize the effect of the original emulsifying colloid. If tQ0 large an excess is added it may bring about simply a phase reversal, changing the emulsion from the water-in-oil type to the oilin-water type. The action of certain organic solvents upon these emulsions probably throws more light on the nature of the emulsifying agent present than any of the other reactions. A standard method among oil men for determining the percentage of water in an emulsion consists in adding light gasolene to the emulsion and centrifuging the mixture when the amount of water in the sample can be read off from the graduated centrifuge glass. It has been found that ether serves even better than gasoline for this test. The effect of these organic solvents in breaking the emulsions was thought to be due to the fact that the solvent mixing with the oil phase increased the difference in density of the two phases to such an extent that the emulsifier was no longer powerful enough to hold the emulsion. It was found, however, that a mixture of ether and carbon bisulfide of the same density RS the emulsion served to break the emulsion more easily even than the light liquid ether. THEEMULSIFYING AGENT The substances, other than water and oil, present iu these emulsions are the following: Electrolytes in solution in the water phase. Hydrated earthy material. Heavy hydrocarbons, such aa aspllalt, asphaltenes, etc., probably present in colloidal solution in the oil.
The ions of the electrolytes present in the water phase probably affect both the magnitude and nature of the elece on the water particles but, as previously stated, ow they could stabilize such concenall, it seems hardly likely that ions could stabilize a water-inemulsion, even granting that they might be effective for an emulsion of the opposite type. We would expect hydrated earthy matter to be wetted more easily by water khan by oil and, if a colloid, to be water-soluble. We seem to recall, however, having read that precipitated hydrous ferric oxide would occlude or adsorb small amounts of oil or grease contained in the solution from which it was precipitated.
