THE T IEORY OF EMULSIFICATIO J. BY WILDER D. BANCROFT
If two non-miscible or partially miscible liquids are shaken together vigorously, each tends t o break into drops and to form an emulsion in the other. The emulsions thus formed are very instable and it seems probable that the presence of a third constituent is necessary to the formation of a stable or even of a fairly stable emulsion. Wa. Ostwaldl has pointed out that it is theoretically possible to have two series of emulsions containing two non-miscible or partially miscible liquids, A and B. Since one of the liquids is practically always water, and since the other liquid is usually an oil, we can speak of water and oil instead of liquids A and B. In one of the two series referred to by Wa. Ostwald, water will be the dispersed phase while i t will be the dispersing phase in the other. If we use the vernacular we shall say that water is present as drops in one series of emulsions and oil as drops in the other series. The physical properties of the two series of emulsions may differ considerably. If we are impregnating wood with an emulsion of water and creosote, the oil will come in contact with the wood if the water is present as drops suspended in the oil. If the oil is present as drops suspended in the water, the wood will be wetted first by the water and will consequently not take up the oil readily. If one dips a piece of filter paper into an oil-water emulsion in which the oil is present as drops, the filter paper becomes wetted with water and the oil drops run off. If the filter paper is dipped into an emulsion in which water is present as drops, we get an oiled paper from which the water runs off. Wa. Ostwald points out that milk is an emulsion in which fat occurs in drops, while butter is an emulsion in which water is present as drops. He then proceeds t o discuss the possible concentrations for which the double series of emulsions occur. Zeit. Kolloidchemie, 6, 103 (1910).
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While his reasoning is inaccurate, it is interesting as showing how far astray one can go if one starts from slightly inaccurate premises. “ I t seems at first as though the nature of the emulsion would depend on the relative amount of the two phases forming the emulsion. It is obvious [in Clifford’s sense of the word] that with 98 percent water and 2 percent oil, one can only make an emulsion in which oil is present in drops. I t is equally clear that we can only make the emulsion in which water is present as drops in case we start with 97 percent oil and 3 percent water. There is nevertheless an extended series of concentrations within which i t is possible to have either constituent occur in drops depending on the way in which the emulsion is made. I t is easy to prove stereometrically that this is possible. Although it is often not quite correct to do so, we will make the assumption that the drops in an emulsion are spherical and of equal size and that the dispersing phase changes to drops when the original drops come in contact. “To take a concrete case we will consider oil emulsified in water. The water forms the dispersing phase and the oil the drops. If we keep on adding oil, the distance between any two drops of oil will become less and less until, finally, the oil drops will come in contact and will coalesce, forming the dispersing phase and breaking the water film into drops. This is one of the ‘critical points’ which we can calculate according to our assumptions. We find the second ‘critical point’ by emulsifying water in oil and by adding water until the oil changes from being the dispersing phase t o being drops. If neither liquid affects the other, the two critical points must be situated symmetrically, which simplifies very much the calculation (based on our assumption). If the usual assumption is correct, that only the percentage composition counts, the two critical points must coincide at, and must occur at, 50 percent [presumably volume concentration]. ”
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The author then calculates' that the double series of emulsions may occur from 25.96-74.04 volume percents. Although the author is careful to say that these limits hold only in case his assumptions hold, it is quite clear that he considers them approximately accurate and that he looks upon the range of about 48 percent as distinctly a first approximation. This is the more surprising because Pickering2 had previously made an emulsion containing 99 volumes kerosene to I volume water in which the kerosene occurred in drops with the water as dispersing phase. One trouble with the theory is the postulate that the drops must all be the same size. It is probably very rarely true that all the drops are even approximately of the same size and nobody is interested in the limiting case of an emulsion prepared under such artificial restrictions. This however is not of much importance. The great difficulty with the theory of Wa. Ostwald is the double assumption that the drops in an emulsion are necessarily spherical and that they will coalesce before being forced out of shape. This might perhaps be true for mixtures of two pure liquids but even then it is of no interest if one cannot make even a semi-stable emulsion of them. The assumption is certainly not true for any actual emulsion because, in these cases, we always have a surface film of some sort which tends to prevent coalescence. Under these circumstances the drops may squash down until the space between them is only vanishingly small. Nobody knows what the real limiting value is. I n a gel we have drops of water in a net-work of the other phase. If we consider the other phase a liquid, a gel is merely a very viscous emulsion. I prefer t o look upon a two-component gel as a limiting case of an emulsion in which we have one liquid and the emulsifying agent, while the concentration of the second liquid has become zero. That, however, is a minor matter. The important thing is that a dilute gel gives us some data as to the amount of the second Wa. Ostwald: Zeit. Kolloidchemie, 7, 64 (1910). Jour. Chem. SOC.,91,zoo2 (1907).
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phase necessary t o keep water or some other liquid in drops. As long ago as 1853, Cruml prepared an aqueous gel containing 1/600alumina and 1/75oo sulphuric acid. Doubtless one could do better now; but this is good enough for the time being. While the limits for the double series of emulsions, as calculated by Wa. Ostwald, are faulty owing t o mistaken premises, this does not detract from the real value of the paper. He is the first, so far as I know, to lay stress on the existence of two over-lapping series of emulsions and the first to point out the possible technical importance of the two series. So far as I can see, it is theoretically possible to take any pair of non-miscible or partially miscible liquids and t o make two series of emulsions over the whole range of concentrations by adding suitable third substances. One cannot yet tell whether it will be possible t o find suitable third substances for every pair of liquids, but in any case, it ought to be possible to predict what the properties of the third substance should be. Before describing our own experiments] it has seemed to me desirable to give a detailed account of what is now known in regard to emulsions. This is the more necessary because I cannot find that anybody has ever even made an attempt to look up the literature and to explain the many curious observations which are on record. I n spite of the wide-spread household use of some emulsions, there is no article on emulsion either in the Encyclopaedia Britannica or in Watts Dictionary of Chemistry. In the previously cited paper, Wa. Ostwald2 describes his experiments on the two series of emulsions. Most of his experiments seem t o have been made with a final mixture of sixty parts water and forty parts kerosene. The oil, or water as the case might be, was placed in a tall glass cylinder and the other liquid emulsified in it gradually by means of an electrically-driven stirrer. In this way a snow-white emulsion of kerosene and water was obtained which began Jour. Chem. SOC.,6, 216 (1854). Zeit. Kolloidchemie, 6, 106 (1911).
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to clear, on standing, a t the top and a t the bottom. When everything was clean, it was possible to note two fairly sharp meniscuses; the rate of movement of each meniscus was noted and plotted on coijrdinate paper. Experiments with a shaking-machine gave no comparable results even though glass beads were added. Coloring with dyes or with iodine changed the whole nature of the phenomenon. Instead of two fairly sharp meniscuses, there was formed a thick creamy layer which lasted for days. This was really a first step towards preparing a stable emulsion, but Wa. Ostwald did not look a t it in that light. He considered that he was dealing with complicated and superfluous phenomena. While the first experiments went as Wa. Ostwald expected them to, there was trouble when the measurements were repeated. Careful tests finally showed that the trouble was due to the difficulty in cleaning the surface of the cylinders and the stirrer. Washing with hot water was quite insufficient. I n order to duplicate results it was necessary t o wash out the cylinders with benzene or some other volatile solvent for fats, t o treat carefully with chromic acid mixture, and then to pass live steam through the cylinders for several hours. The purified and dried surfaces were carefully covered with oil in case an oily emulsion (0) was wanted and with water in case a watery emulsion (W) was to be made. The other liquid was added and the stirring begun. “The shortlived emulsions had very different properties depending on whether the surface was water or oil. The oily emulsions separated very quickly into a lower layer of practically clear water and an upper, oil layer which consisted of a very fine and distinctly stable emulsion of a small amount of water as drops in a large amount of oil. With’an oily emulsion the meniscus was convex. The walls of the lower part of the cylinder also had adhering drops of oil. “ The watery emulsions-the usual type-were very much more stable. They separated slowly into a practically
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clear oil layer and a water layer containing finely-divided oil drops. This emulsion persisted a long time just as did the drops of water in the oil layer of the oily emulsion. The walls of the cylinder were clean and so was the meniscus.” Wa. Ostwald made a couple of rather interesting observations which he puts in a footnote and from which he apparently drew no conclusions. He poured oil into a carefully purified and dried cylinder, and then ran in the water drop by drop without stirring. He says that even with this very coarse emulsion one can recognize the appearance of the critical point. He did not, however, try the other experiment of running oil into the water, which probably would not have turned out as well. He did observe, however, that sometimes the change to oil as drops did not take place as scheduled and that the oil formed very thin films round the deformed drops of water. Even this did not shake his views as to an emulsion of water in oil being possible only when the volume concentration of the water was less than 74 percent. He considered the matter purely as a case of supersaturation and let it go at that. According to Wa. Ostwald an emulsion of oil and water settles rapidly, forming two liquid layers, one of which is clear and the other turbid. He accounts for this by saying that when one has an oily emulsion the scattered drops of oil are made to coalesce by the stirrer while this is not the case with scattered drops of water. In other words an oily emulsion, with oil as surface liquid, tends to destroy drops of oil and to conserve drops of water while a watery emulsion, with water as surface liquid, tends to destroy scattered drops of water and t o conserve drops of oil. Wa. Ostwaldl sums up his paper as follows : ( I ) When the concentration of one constituent is increased continuously an emulsion must go through a critical point, where v-hat has been the dispersed phase becomes the dispersing phase. Starting with the same constituents b u t 1
Zeit. Kolloidchemie, 6, 108 (1910).
T h e Theory
of
Emulsification
183
at the other end of the series, increasing the concentration of the other constituent continuously will cause the emulsion to pass through a second, analogous, critical point. ( 2 ) These two critical points do not coincide. Calculation shows that, as a first approximation, they overlap about 48 percent' [from 26-74 percent]. Anywhere within this range it is possible by special means to prepare two emulsions having the same percentage composition but entirely different properties. (3) As a result of supersaturation phenomena this critical range can be extended and systems can be obtained consisting practically of two liquids, in which the outside phase (the dispersing one) occurs as an extraordinarily thin film around fairly coarse drops of the dispersed phase. The result is something like a foam with coarse air-bubbles.' (4) The experiments showed the correctness of the first two conclusions and showed that the limiting critical concentrations for the two sets of emulsions can be determined by having the containing vessel completely wetted first by one constituent and then by the other. (5) The fundamental influence of the nature of the surface upon the phenomenon is shown by the precautions necessary when one wishes to duplicate results, by the characteristic form of the meniscuses, and especially by the fact that when the two liquid layers separate, the constituent, which has been the external phase, retains drops of the other phase so as t o form a fairly permanent, dilute emulsion, while the other constituent separates as a clear phase. (6) These phenomena become intelligible in view of the macroscopic observation that wetted surfaces destroy the dispersity of that phase and conserve the dispersity of the other phase. The figure is actually given as 56 but was changed to 48 in the second paper, Zeit. Kolloidchemie, 7, 64 (1910). It is possible that there are more liquid-liquid foams than people realize a t present and that such dispersed systems may be of considerable theoretical importance.
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Quincke’ discusses the stability of emulsions as follows : J. Plateau2 first showed that many liquids seem to be more viscous a t the surface than within the mass. It was found later by myself3 and by Marangoni4 that the surface of a liquid in contact with air became less mobile the moment a thin film of another liquid spreads over it. The same thing holds when a third liquid (soap solution) spreads out over ‘the surface of contact between two such liquids as oil and water. The immobility or the permanency of the surface modified by the thin film of the third liquid is shown by the fact that any hole in the film is a t once closed by the molecular forces because we have in the hole a clean surface of a liquid and consequently a greater surface tension than where the surface is contaminated by the film of the third liquid. “ F o r equal mobility of the surfaces a hole in the film will be closed more rapidly and the stability of the surface will be greater, the greater the d i f f e r e n ~ e . ~
- (a13 + a d L‘An emulsion consists of a large number of spherical drops of fat dispersed through a watery liquid. Ordinary milk is an emulsion for instance. The smaller the drops of f a t are, the greater the resistance as they rise through the denser, surrounding liquid. The smaller the drops of fat, the longer they remain floating in the surrounding liquid and the more perfect the emulsion. The emulsion is more permanent, the less readily the small drops of fat coalesce to form larger drops and the lower the rate with which the globules of fat rise through the surrounding aqueous liquid. The lower this rate, the less the density of the emulsion varies ai2
Wied. Ann., 35, 589 (1888). M6m. d. Brux., 37, 3 (1868). Quincke: Pogg. Ann., 139,3 (1868). Nuovo Cimento, [2] 5, 239 (1872). 8 [a,,is the surface tension between oil (Liquid I ) and water (Liquid 2 ) ; q8 between oil and the soap film (Liquid 3 ) ; ag2between the soap film and 1vater.I a
from that of an actual solution of the fat in the surrounding liquid. “ B y shaking oil with soda or gum arabic solution, one can prepare emulsions which look like milk. When fatty oils are emulsified in soda solution, the coalescing of the fat globules is prevented by a thin film of soap solution,2 any opening in which is a t once closed by the molecular forces. I n the emulsions as prepared by the druggists, each small globule of fat is separated from the aqueous solution by a film of gum arabic solution. The figures in Table I show that the surface tension between the fatty oils and the gum arabic solution is less than the surface tension of the fatty oils against water. TABLEI Surface tension at the surface between ~~
oil arid water
Colza oil I .564 Olive oil 2.296 Almond oil 2.370 Cod-liver oil 0.878 Castor oil I ,624
I
oil and gum solution
1.474-o.415 1.491-0.020 1.237-0.487 0.600-0.333
0.95.5-0.785
“After the oil and the solution of gum arabic come in contact, the surface tension of the surface separating the two decreases a good deal a t once. I n the first minute it falls off I O percent and in the course of several hours it drops 35-50 percent of the original maximum value. This unusually rapid decrease appears to indicate a chemical compound which is formed from the oil or the free fatty acid in it by the action of the gum arabic solution. It spreads over the surface between the two liquids, covers the drops of oil, and keeps them from coalescing. The viscosity of the gum arabic a
Bondy: Pogg. Ann., 146,323 (1865). E. Mach: Ibid., 126,329 (1865). Quincke: Wied. Ann., 35, 562 (1888).
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186
solution also checks the rise of the oil drops and therefore keeps them to some extent from coalescing. “If one shakes together mercury, water and olive oil, there is formed a grayish white viscous mass, an emulsion of mercury, formed of many small globules of mercury, each one covered with a film of oil. According to my measurements, we have the following surface tensions : Mercury Mercury Oil
1 water I oil 1 water
aI2= 42.58 mg =
aS8=
34.19 mg 2 . 3 0 mg
The force, with which a break in the oil film is drawn together, is a fairly large one,
+ a3J
a12- (el3
=
6.09 mg
As a matter of fact, such mercury emulsions last for months. Addition of acid destroys the film of oil and consequently the emulsion. Gray mercury salve is really an emulsion of mercury in viscous lard.” Quincke’ has also something to say in regard to spontaneous form tion of emulsions. “During the process of digestion the first step in the assimilation of fat is the formation of an emulsion with the liquid in the intestine and this is facilitated by the presence of bile. W. Kuhne2 and Brucke3 have called attention t o the effect which the soaps, formed in the small intestine, have on the formation of emulsions. According to Brucke, rancid oil, which contains free fatty acids, forms perfect emulsions more readily than a neutral fat, when shaken with dilute aqueous solutions of white of egg, borax or sodium carbonate. “ Joh. Gad4 first made the interesting observation t h a t when drops of oil contain free fatty acids, they form perfect emulsions by mere contact of the drops with alkaline liquids a
*
Wied. Ann., 35, 594 (1888). Physiol. Chemie, 129 (1866). Sitzungsber. Akad. Wiss. Wien, 61,11, 362 (1870). E. du Bois Reymond: Arch. Anat. Phys., 1878, 181.
I
The Theory of Enaulsification and without any mechanical shaking. This phenomenon can be shown particularly well with a drop of cod-liver oil in 0.25 percent of soda solution. The ease of emulsification depends on the amount of acid, the viscosity of the oil, the concentration of the soda solution and the solubility, in the dispersing medium, of the soaps formed from the fatty acids. The solubility of the soaps can be so changed by the addition of sodium chloride and bile that the conditions are more favorable to the formation of emulsions. With viscous castor-oil no formation of emulsions could be observed. “When emulsions were formed, tentacles developed on the drops of oil, the changes of shape and the movements being very like those of an Amoeba. Later, smaller drops of oil broke away and these were to some extent emulsified further. ” “When the oil comes in contact with the soda solution, a solid soap is formed by the action of the alkali on the free fatty acid contained in the oil. I n time a part of this soap dissolves in the surrounding aqueous solution. When the liquid soap solution comes in contact with the oil, it spreads out suddenly on the interface of oil and water, carrying with i t the undissolved particles of soap and the adhering masses of oil. In this way threads of oil are drawn out into the aqueous solution. These threads of oil have a tendency to change so as to present the smallest surface and they therefore break up into larger or smaller spherical drops just as a stream of water in air breaks into larger or smaller drops. The formation of drops is retarded to some extent by the solid or liquid soap either previously present or freshly formed. This tends t o increase the length of the oil threads and to decrease the size of the resulting drops of oil. Through the spreading out of the soap solution, fresh surfaces of oil are brought in contact with the soda solution, and new amounts of soap are formed which then dissolve and spread out as before. “This periodical spreading of the soap solution over the interface of the oil and water does not take place simultane-
ously at all points of the surface. It is connected with the previously described eddies1 inside the two liquids. These draw the aqueous solution and especially the viscous oil toward the vortices, thus producing the amoeba-like movements a t the edge of the mass of oil. When the oil drops split off, they form the emulsion. With an average value for the viscosity of the oil and for’the rate of spreading a t the interface of oil and aqueous solution, the eddies and the number of drops of oil formed will be especially large. The phenomena can be produced with films of dilute soap solution only a few millionths of a millimeter in thickness, and an incredibly small amount of solid soap is sufficient. “The free fatty acid to form the soap is nearly always present in the oil and is brought to the surface by diffusion, I n the liquids in the intestines, the free fatty acid can be formed by the action of the pancreatic juice, while, in the open air we get it by the action of carbonic acid on a neutral alkali oleate. “If soap is formed too rapidly, the surface between the oil and the aqueous solution becomes covered with a membrane of solid soap. The surface ceases to be readily mobile and the spreading out, with its consequences, is either retarded or prevented entirely, just as the formation of vapor is either retarded or prevented in the Leidenfrost experiment where one pours water on a white hot metal. “The motion of the oil drops, and the formation of emulsions, ceases also if the spreading out does not occur, owing to too little soap being formed or to the soap being dissolved too rapidly by the dispersing liquid. I n the latter case each small amount of soap will spread out as a soap solution, b u t the energy of the spreading out will not be sufficient to start violent eddies and to cause particles of oil to split off. When the water in a mill stream is low, the mill wheel will not turn if the water is allowed to run down in drops, whereas work can be obtained if the water is dammed back and then allowed t o flow out rapidly. Quincke: Wied. Ann., 35, 562 (1888).
.
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“When the oil drops, swimming on a soda solution, are covered with a membrane of solid soap, this can be dissolved by an aqueous solution of ox-bile. The bile may aid in the spontaneous formation of emulsions within the animal if the solid soap at the surface between oil and soda solution dissolves too slowly. It may check the formation of an emulsion in case it causes the solid soap t o dissolve too rapidly. Both these phenomena have actually been observed by Gad.” In a later paper, Quincke’ discusses the spontaneous formation of bubbles and foam with salts of oleic acid. This is really the case of an emulsion of air in water, which is not our problem strictly speaking; but it throws so much light on the behavior of the oleates in the case of emulsions of two liquids that I quote from it without further apology. “ Up to now very little work has been done on the alkali salts of oleic acid. According to Heintz’ the potassium, sodium and ammonium salts of oleic acid form, with small amounts of water, slimy or gelatinous masses which are soluble in a little hot water and in alcohol. Addition of much water decomposes these salts into free alkali and acid salts, t h e x last precipitating. Neutral potassium oleate (15 parts caustic potash and 85 parts oleic acid) takes up moisture from the air; neutral sodium oleate does likewise, but not to the same e ~ t e n t . Potassium ~ bioleate is a slimy or gelatinous mass, soluble in alcohol and insoluble in water, by which it is not decomposed. “ I have studied the behavior of the instable and easily decomposed oleates either in an ordinary test tube or under the microscope in a watch-glass which was filled with pure water and was covered in the usual way with a thin coverglass. By means of a pointed, fire-polished glass rod the oleate was placed in the middle of the under side of the coverglass, which latter was then set down upon the water of the watch-glass. When necessary, a slow stream of liquid was Wied. Ann., 53, 598 (1894). Zoochemie, 439 (1853). Lowig: Chemie organ. Verbindungen,
2,
203 (1846).
Y
I90
L
Wilder D . Bancroft
made to flow between the glasses, the water being introduced a t the edge of the cover-glass and drawn off by means of a strip of filter paper at the opposite side. “Sometimes dyes were added to the water so that we could test the acid or alkaline nature of the oleates under consideration. The dye must not make the liquid surface immobile and it must give a very intense coloration so t h a t the smallest particles can be detected. Henna and litmus do not give a sufficiently intense coloration. I have usually used methylene blue or hectograph ink, or an alcoholic solution of phenolphthaleine which is colored an intense red by alkalies. The neutral potassium, sodium and ammonium oleates were prepared by heating oleic acid with usually a I O percent solution of the alkali in question. The alkali was added gradually until the alcoholic solution of phenolphthaleine began to show a reddish color. These neutraI salts are soluble in alcohol. When the solution is evaporated on the water bath or is cooled, the salts precipitate as solid substances, occasionally in the form of small, rather instable crystals. These salts form slimy or gelatinous masses when treated with a little water. With more water an acid salt precipitates, while free alkali remains in solution.‘ “ O n adding a few drops of an alcoholic solution of phenolphthaleine to a dilute aqueous solution of a neutral alkali oleate, a red color is always obtained. If we heat the slimy masses of neutral alkali oleates, dissolved in a very little water, red streaks often appear and gradually disappear again, This proves that decomposition takes place here and there; but that the neutral salt is formed again later. “ O n long standing in contact with water the acid salts break down into oleic acid, which separates in small liquid globules and rises to the surface of the liquid, and into a slimy or gelatinous mass of neutral oleate dissolved in water. This According to the recent investigations of Krafft and Stern [Ber. cheni. Ges. Berlin, 27, 1755 (1894)]no basic oleates are formed.
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slimy mass dissolves in more water to a mobile liquid, oleic acid precipitating. ‘‘ While oleic acid decomposes aqueous solutions of alkali carbonates, forming oleates, these oleates are also decomposed by carbon dioxide when this gas is passed through a slightly turbid and originally neutral solution of one of these salts. The solution clouds at once through precipitation of solid particles of sparingly soluble acid oleates. After a few hours globules of oleic acid collect on the surface of the liquid. If crystals or solid particles of sodium or potassium oleate are precipitated by water and then suspended in water, they become less dense if carbon dioxide is passed into the solution and they collect a t the surface of the liquid, presumably carried up by the less dense oleic acid. At the same time there is also formed a readily soluble oleate which spreads out over the surface between air and water, dragging with it the soap crystals suspended in the water, and rising several centimeters up on the wetted wall of the test tube. When much alkali is added to a little oleic acid, we usually get oleates readily soluble in water; with a small amount of alkali and much oleic acid we usually get acid oleates which are sparingly soluble or insoluble in water. Crystals or solid masses of neutral potassium, sodium or ammonium oleate are soluble to a considerable extent in pure oleic acid, whereby the density of the latter may be increased until it is denser than water. “ D r y oleic acid remains colorless when placed in contact with dry methylene blue. On the other hand oleic acid takes methylene blue out of an aqueous solution of the dye. I n capillary tubes of 0 . 5 - 2 . 0 mm diameter were sucked up water containing methylene blue (1/500, 1/5000, I /50000) and then oleic acid in such a way that equal lengths of each liquid were in contact. The tubes were protected from the action of light by a metal case. Oleic acid containing dissolved potassium, sodium or ammonium oleate takes up the blue dye faster than does pure oleic acid. Under the influence of light the blue color disappears in pure oleic acid ‘I
W i l d e r D . Bancrojt
192
more rapidly than in oleic acid containing alkali oleates. Even in the latter there is a marked disappearance of the color under the action of intense light. “Sodium oleate also takes methylene blue out of an aqueous solution of the dye, assuming a deep blue, greenish blue or whitish blue color depending on whether the salt contains an excess of acid, is neutral, or contains an excess of alkali. The absorption spectrum of an aqueous solution of methylene blue shows two dark bands in the red and yellow at 600 pp and 606 pp. The absorption spectrum of oleic acid which has taken up methylene blue from water shows the two bands displaced towards the blue, the more refrangible band being faint or even non-existent. Thus one finds either two bands a t 657 pp and 571 pp or one band at 652-642 pp. Oleic acid containing dissolved potassium, sodium or ammonium oleate shows one dark band at 654641 pp or also at 596 and 522 with sodium oleate;’ and two bands running together between 640 and 632 with ammonia. I n this last case the band towards the red is darker than the one towards the blue. ‘~B
+
0A-B.
The first drop does not suffice to saturate A and B and to lower aA to u t A such that a first drop of B shall not spread out. We still have a'A>aB
+
0A-B
and this may continue to hold while many more drops are added. It ceases t o be true when A becomes covered with a film of B. We get cases of this type with pairs of liquids giving small values for aAWB, for instance water and ether, water and turpentine, water and linseed oil, etc. Freundlich: Kapillarchemie, 58 (1909).
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231
“Anything that decreases uA-B tends t o make the drop spread out. When dissolved substances decrease uA-B very much they cause the drop to spread out. Pure kerosene, u ~ = - 48.3, ~ stays as a drop on water; but kerosene containing mastic spreads out readily over water,l the value of uA-B being 16.3. This is especially true for substances which are soluble in B and which reduce the surface tensions uB and aA-B. With substances which dissolve in A and which lower the surface tension uA, the tendency for B to form lenticular drops on A is increased. If we have an oil which contains fatty acids and place a drop of it on the water, the fatty acid will first spread out because its uB and, still more, its uA-B are smaller than the corresponding uB and the uA-B of the oil. The film of fatty acid on the surface of the water lowers the surface-tension of the water so much that the oil does not spread out a t all because uIA is no longer larger than uB + uA-B. It is also possible that hydrolysis of the oil may take place more in the surface because oleic acid lowers the surface-tension very much. If so, we should get the same phenomenon even with an oil which originally contained no free oleic acid. “The same is true if the surface of A originally contains an impurity which lowers the surface-tension. In that case uA may be so small from the start that it does not overbalance uB uA + B . Under some circumstances it is therefore a sign of the purity of a surface, for instance of mercury, that a drop of another liquid spreads out on it. “ W e have now to consider the question whether there really is an equilibrium
+
G’A
=
u’B
+
0A-B
when we have a lenticular drop resting on a liquid. Is the component for dB uA-B equal but opposed t o u’* or do the two liquids meet at a finite contact angle in which case the cosine of the angle should appear in the formula? As a
+
Pockels: Wied. Ann., 67, 668 (1899).
Wilder D . Bancroft
232
_ _
-~
_ _ ~
Water, benzene Water, isobutyl alcohol Water, isoamyl alcohol Water, ethyl ether
- .I-_ _! 1 600
2 8 2
12;; 1 ii:
Jour. Chim. Phys., 5 , 372 (1907).
‘ 4
4 6 9 4
’1
326 I 76 4 4 9 I2
The Theory of Emulsification
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two sets of emulsions are theoretically possible over practically the whole range of concentrations. I n one series of emulsions “oil” will be present as drops while “water” will be present as drops in the other series. ( 2 ) No one has prepared the two sets of emulsions for any pair of liquids. ( 3 ) By shaking a fairly pure oil with water Lewis obtained an emulsion containing 2 percent of oil. With water and a hydrocarbon oil containing not more than 0.03 percent oleic acid Ellis only obtained an emulsion containing one part of oil to ten thousand of water. (1) It seems probable that no stable emulsion can be prepared with a two-component system consisting of two mobile liquids. (5) To obtain a more stable and a more concentrated emulsion a third substance must be added as an emulsifying agent. ( 6 ) When water is the dispersing phase, the emulsifying agent should lower the surface tension of water (Quincke, Donnan) and should be viscous (Quincke, Hillyer) . (7) When water is the dispersing phase, the emulsifying agent should be a hydrophile colloid (Hober). Hober does not draw the conclusion that the emulsifying agent should be an oleophile colloid in case the emulsion is to contain water in drops. (8) There is as yet no published theory connecting emulsification with the relative surface-tensions of the two pure liquids which are to be emulsified. (9) Foam is an emulsion in which the dispersed phase is a gas and not a liquid. A gel is a limiting case of an emulsion; the dispersing liquid being lacking. (IO) The electrical charge on the drops of an emulsion is analogous in nature and value t o that on suspended particles. Cornell University
