THE THEORY OF EMULSIFICATION. IV BY WILDER D. BANCROFT
Robertson1 has published some results on emulsions of olive oil and water which are of especial interest because he succeeded in obtaining an emulsion of water in oil. “If one shakes up, thoroughly, equal parts of slightly alkaline water and olive oil, a very stable emulsion is formed in which olive oil forms the. internal phase and water the external; that is, the olive oil is suspended in the form of spherical droplets within the water. If we decrease the proportion of water, the resultant emulsion still continues to consist of droplets of oil suspended in water until the proportion of water t o oil reaches a definite, critical value. When this critical ratio is reached, the character of the emulsion undergoes an abrupt change. From being a viscous, creamywhite emulsion of oil in water, it becomes a fluid, yellow emulsion of water in oil. The oil is now the external phase and the water the internal one. One can very readily detect which phase of the emulsion is external, without microscopical examination, by means of the following simple device: The bright red dye Soudan I11 is insoluble in water but readily soluble in oils; on sprinkling a few grains of Soudan I11 upon the surface of an emulsion of water in oil the color spreads rapidly over the surface. If, however, one sprinkles the Soudan I11 upon an emulsion of oil in water, the color remains confined t o the droplets of oil with which the grains are in actual contact, since it cannot spread from them t o adjacent drops through the intervening water. “ I n many cases I observed an intermediate form. Suspended in the oil were drops which apparently consisted of an emulsion of oil in water; in other words the oil was external phase.with an emulsion of oil in water as the internal phase. It is possible that a similar intermediate form exists in the Zeit. Kolloidchemie, 7, 7 (1919).
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case of emuliions of oil in water, especially since it is difficult t o tell by direct microscopical examination whether the drops suspended in oil are really drops of water or are a dilute emulsion of oil in water. In the tabulated data any’ernulsion with oil as the external phase is called an emulsion of water in oil unless it was recognized definitely as an emulsion in oil of drops of water containing oil. “ I think that the following nomenclature will prove serviceable because it shows what the constituents of the emulsion are, which phase is external, and which is internal, I use it throughout this paper and I recommend that an emulsion of oil in water be designated ‘oil’-water and that an emulsion of water in oil be designated ‘water’-oil. The intermediate case, t o which I have referred, would be designated as ‘ oil-water ’-oil. “ My experiments have been devoted chiefly t o ascertaining the influence of the proportion of alkali to the total volume of emulsion upon the critical ratio of water to oil at which the emulsion changes character and ceases to be an ‘oil’-water emulsion. It is a well-known fact that neutral, distilled water forms no stable emulsions with olive oil. The stability of emulsions of olive oil and water in presence of alkali is due t o the action of the alkali on the fatty acids of the oil. Consequently one must conclude that the effect which alkali has on the stability of an emulsion of oil in water will depend on the amount of fatty acids in the oil. The olive oil used in these experiments was extremely pure; it was California oil and contained scarcely any free fatty acids, the absolute amount not being determined. TGe same oil was used in all the experiments. ‘‘TOprepare the emulsion, measured amounts of oil, distilled water and caustic soda were placed in an ordinary, narrow-necked, glass bottle holding about 150 cc. The flask was corked tightly, was placed in an upright position in a shaking machine, and was shaken vigorously for about 2 0 minutes. The machine made a thousand reversals per minute and displaced the flask horizontally and vertically
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in a vertical plane so that the centre of gravity of the flask described a series of small ellipses in a vertical plane. During the shaking a good deal of air became mixed with the emulsion; but, in the less viscous emulsions, this rose very rapidly to the surface. When samples were taken 2 or 3 hours after the shaking had ceased, they contained very few air-bubbles. “TWOmethods were used in studying the emulsion: the Soudan 111 method and the microscopical examination. I n the following sets of observations, the experiments were so arranged that the caustic soda concentration remained constant while the concentration of the water increased by I percent in each succeeding experiment. The critical ratio of water to oil is the value at which the character of the emulsion changes. To obtain this I took the mean of the smallest ratio for which an ‘oil’-water emulsion could be obtained and of the largest ratio for which a ‘ water ’-oil emulsion could be maintained. If a mixture containing 8 cc water plus NaOH gave an ‘oil’-water emulsion and if a mixture containing 7 cc water plus NaOH gave a ‘water’-oil emulsion, the alkalinity being the same in the two cases, the critical ratio was taken as 7.5,’p.s because the total volume was always IOO cc. Of course the error in such a determination is w 9 2 . 5
9
“The data are given in Tables I-VI.
TABLEI In each emulsion Oil cc
99 98 96 92
91 90 89
i w:ier I - 1
3 7 8 9 IO
I
cc 5 N / I NaOH
Character of emulsion
‘Water ’-oil, fluid, yellow ‘Water ’-oil, fluid, yellow ‘Water’-oil, fluid, yellow Water ‘-oil, fluid, yellow cream ‘Water’-oil ‘Oil’-water, creamy white, very viscous ‘Oil’-water, creamy white, very viscous
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742
oil
Water
cc
cc
93
6 7 8
92
91 89
87
';: I 93
Io I2
6 7
8
Oil
Water
cc
cc
1
I
I .
Character of emulsion
' Oil-water '-oil, fluid, yellow 'Oil'-water, creamy white, very viscous ;Oil'-water, creamy white, very viscous ' Oil'-water, creamy, fairly viscous 'Oil'-water, creamy, fairly viscous
' Oil-water '-oil, fluid, yellow ' Oil'-water, creamy white, very viscous 'Oil'-water, creamy white, very viscous
Character of emulsion
91
8
89
IO
' Oil-water '-oil, fluid, instable ' Oil-water '-oil, fluid with granulated struc-
87
I2
' Oil-water '-oil, fluid with granular structure,
86 85
I3 I4 15
as though coagulated 'Oil'-water, creamy white, very viscous ' Oil'-water, creamy white, viscous 'Oil'-water, creamy white, very fluid
84
ture, as though coagulated
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743
TABLEV
cc Oil
87
86 85
In each emulsion
'
1 Water cc 12
I4 I3
I 1
!
I
cc N / 6 NaOH
Character of emulsiou
' Water1-oil, fluid, yellow ' Oil'-water, creamy white, very viscous ' Oil'-water, creamy white, still distinctly
I
Critical ratio
viscous 13.5/86.5
=
=
0.156 f 0.006
TABLE: VI In each emulsion Oil cc
87
85 83 49
wt:er i 1
I
cc N / 8 NaOH
Character of eniulsion
' Water'-oil, fluid, yellow Not stable; two layers are formed, one ' water '-oill the other ' oil'-water 16 Not stable; two layers are formed, one ' water'-oil, the other 'oil'-water 50 Not stable; two layers are formed, one ' water'-oil, the other 'oil'-water No stable ' oil'-water emulsion could be obtained 12
~
' ' These experimental results indicate that above a certain alkali concentration (hr/zoofor this oil), the critical ratio of water to oil remains constant (at 0.08 for this oil); but that after the concentration of alkali falls below this limit the amount of oil which a given amount of water will hold in suspension diminishes progressively until, when the alkaliconcentration falls below N/8oo (for this oil), no stable emulsion of oil in water can be obtained. I am inclined t o attribute these phenomena t o the fact that the action of alkali in securing a stable emulsion of oil is due to the soap which it forms with the free fatty acid in the oil. So long as the alkali is in excess of the amount required to neutralize this acid, therefore, the concentration of soap in the system will be constant; when the concentration of alkali falls below this limit, the amount of soap in the system will be approximately
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Wilder D. Bancroft .
proportional to the amount of alkali and, as this diminishes progressively, the power of the water t o surround the oil, as we have seen, grows less. On decreasing progressively the proportion of water t o oil, one observes that the resulting emulsions are more and more viscous as one approaches the critical proportion until, just before that proportion is reached, the emulsion is so viscous as scarcely to flow a t all. Immediately the critical ratio is passed, howevei, the emulsions of water in oil which are then formed are quite fluid. “The permanence of an emulsion or of an extended surface of contact between two immiscible phases indicates, as I have pointed out, that the surface tension a t the surface of separation is so small that the force tending to minimize the surface, that is, to coalesce the droplets of the internal phase is evanescent. Now Quincke’ has pointed out that soap diminishes the surface tension a t the surface of contact of oil and water, and GibbsZ and Thomson3 have pointed out .that substances which diminish the surface tension tend to become concentrated at the surface of which they diminish the tension. The reason for the permanence of oil and water emulsions in the presence of alkali is therefore clear; the soap formed by the interaction of the alkali and fatty acid becomes concentrated at the surface of the droplets and so reduces the surface tension that the force tending t o restore a minimum surface of contact is exceedingly small. At the critical ratio, however, the soap is spread over so large an area that it is only just able to cover the surface of the oil droplets without leaving spaces of more than molecular dimensions. Upon Sitzungsber, Akad. Wiss. Berlin, 1888, 791. Scientific Papers, I, 235, 265 (1906). “Applications of Dynamics to Physics and Chemistry,” 190 (1888). The fact that not all of the soap will be present in the superficial layer of the oil (or water) a t the critical ratio does not, of course, invalidate the above reasoning. The proportion of soap contained in the surface of contact of the oil and water must be determined by the coefficient of distribution of the soap c do between the oil and the water and by the Gibbs equation r = - (J. WilRT dc
-
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passing this ratio the system breaks down to that possessing the next largest surface, namely, that in which the water is suspended in oil. The mechanical force exerted in emulsification, of course, secures, ,transitorily, the maximum possible surface of contact; should this surface, however, possess an appreciable suiface tension as would be the case were the soap unable fully to cover the droplets, the system would’break down to the arrangement securing the next largest area of contact, namely, in the case under consideration, that: of droplets of water suspended in oil. “The reason for the high viscosity of the oil-in-water emulsions, when the ratio of water t o oil is very nearly that a t which the character of the emulsions changes, is also clear. I n these emulsions the soap is just able to cover the oil droplets without leaving spaces of more than molecular dimensions. Any strain leading to deformation of these droplets, such as would occur in flowing, would necessarily increase the surface of the dtoplets, since the sphere is the body which possesses the least surface for a given volume. Hence, in flowing, gaps would be produced in the soap-covering of the droplets; these gaps, however, would have a high surface tension; tending to bring together again the particles of soap, to restore the spherical form of the droplets, and thus to offer a resistance to the force deforming them, that is, to the flow. The resistance to flow a t the critical ratio is therefore qecessarily high, since any flow which occurs must result in lobal description of the system. It is possible that the absence of Brownian movements in droplets of protein separating out in the initial stages of gel-formation1 is due t o similar factors . “ I am inclined to believe that the mechanism leading to the formation of stable gels is of the same character as that lead to the formation of stable emulsions.” lard Gibbs: LOC. cit.); when this proportion is insufficient to cover the surface without leaving spaces of molecular dimensions, the system must break down. W. B. Hardy: Jour. Phys. Chem., 4, 255 (1900).
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This paper of Robertson’s is valuable because he has actually prepared the two sets of emulsions. It is unsatisfactory because he worked with indeterminate solutions. He, himself, recognizes that the formation of the emulsion depends on the presence of salts of the fatty acids and yet we do not know how much sodium soap we have in any given case.‘ There is no way of telling how much reaction has taken place in any given case between the olive oil and caustic soda. We do not even know definitely what sodium soaps are formed. Olive oil is sometimes given as containing 7 0 percent oleine, 2 5 percent palmitine and 5 percent linoline; but we do not know whether the oleine alone has been saponified or whether the other two constituents have also been decomposed to some extent. Consequently, we do not know whether the change in the character of the emulsion is or is not due to the sodium oleate. We also have the glycerol and the uncombined caustic soda as unknown and disturbing factors. It would have been more satisfactory if Robertson had added definite quantities of sodium oleate instead of free alkali. Assuming, however, that sodium oleate is the important factor in forming the surface films, the question arises why we should get the two sets of emulsions with olive oil while we do not with kerosene or benzene. I believe that the difference is due to the insolubility of sodium oleate in kerosene and benzene and to its solubility in olive oil. For high concentrations of oil, we may easily have the solubility of the oleate in the oil a more important factor than its solubility in water. I cannot accept Robertson’s conclusion that there necessarily is a critical ratio with one type of emulsion existing below it and the other above it. It seems to me quite conceivable that an emulsion might become instable a t a given point without there being necessarily any formation of the other type of emulsion. That is certainly what seems to happen experimentally in most cases. It is interesting to note that Robertson’s shaker was apparently so efficient that he was able to mix the ingredients all a t once and then to emulsify them. It would have been a
The Theory of Emulsification.
747
good thing if the observed limits had been checked by adding oil or water t o an emulsion and then shaking again. It is always possible that this might have led to different results. The preparation of emulsions is a regular practice in pharmaceutical chemistry; but the point of view differs radically from that held by the chemists who have been quoted hitherto. The pharmacist is quite clear in his mind that an emulsion consists essentially of capsules containing one liquid, suspended in another liquid. The problem that worries him is how best to get one liquid into capsules of a suitable size. The subject of emulsions is treated a t length in Remington’s Practice of Pharmacy,’ from which I quote. “ Emulsions are aqueous liquid preparations in which oily or resinous liquids are suspended by the agency of gummy or viscid substances. They may be conveniently divided into two classes : I . Natural emulsions. 2 . Manufactured emulsions. They are opaque liquids, generally of a thick consistence. I . Natural emulsions are those which are found in nature, ready formed, as the milky juices of plants, the milk of animals, yolk of egg, etc. 2. Manufactured‘ emulsions are those which are made artificially by various processes. The art of producing them is termed emulsification “ Manufactured emulsions are usually made from two classes of substances: I . Those which contain an oily or a resinous compound associated naturally with either gum or some other emulsifying agent. 2 . Oils, fatty and resinous bodies containing no emulsifying substance. “Gum resin emulsions and seed emulsions are included in the first class. These arc usually made by simple triturating in contact with water. ‘‘ Gum resin emulsions are made by reducing to a coarse powder, in a mortar, selected pieces of the gum resin, triturating with a small quantity of water so as to form a smooth, uniform paste, and then adding the remainder of the water,, ( 6
‘6
Fifth edition, 1153 (1907).
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finally straining the mixture through a cloth strainer or a plug of absorbent cotton contained in a funnel. Powdered gum resin should never be used for making emulsions, because of the loss or deterioration of the volatile constituents which always take place when the substance is dried so that it may be powdered. “ Seed emulsions are so termed because they are made by rubbing seeds or the kernels of fruits which contain fixed oils with water, the emulsifying agent being a gummy or albuminous substance found naturally in the seed or kernel associated with the oil. Emulsions of almond, castor oil bean, croton oil bean, etc., are examples of this kind. “The theory of emulsification is based upon a study of the best type of a natural emulsion-namely, milk. This liquid is found, on examination, to consist of innumerable globules of a fatty substance (butter) enveloped in a thin membrane of viscid matter (casein) suspended in water. The object sought by the pharmacist in making emulsions is first to thoroughly divide the oily or resinous liquid into minute globules, and then to surround each globule with an adhesive envelope (mucilage of acacia, yolk of egg, etc.). The globules, when completely enveloped, are suspended in water, and if the emulsion is properly made, there will be no tendency on the part of the oily or resinous liquid to recombine. Several methods are employed in making emulsions, the most important of which, however, may be grouped under two typical methods, named from the geographical locations where they are used most frequently: I . The English method. 2 . The Continental method. Both are , equally useful, and should be employed according to circumstances. I . The English Method.-In this mode of making emulsions the emulsifying agent, consisting of mucilage, yolk of egg, etc., is first placed in a dry mortar, and small quantities of oil and water are gradually and alternately added a t intervals. The pestle is rapidly and lightly rotated (counterclock wise), with the effect of dashing the oil into globules, (6
The Theory of Emulsification
749
which are a t once enveloped by the viscid emulsifying agent. If the oil or water is added too rapidly a t the beginning, or the mucilage has not been thick enough, the accident of (cracking’ the emulsion occurs. This may be known by the &pearly’appearance assumed by the mixture, and on close examination the globules of unenveloped oil may be seen floating about. If each stage of the process is successful, the emulsion presents, upon thorough mixing after each addition, a smooth, opaque, glistening appearance like cream. Success depends largely upon the care exercised in forming the nucleus a t the beginning, and this, therefore, should not be too hastily made. When an emulsion is icracked,’ it need not be thrown away. It may be restored by placing an additional quantity of mucilage in the mortar and gradually adding the (cracked’ emulsion to it, triturating after each addition, when finally the satisfaction of seeing the uncombined globules disappear will gene1ally be experienced. “The English method of making emulsions is the best to use in general prescription practice, where the proportions of gum, oily or resinous liquids, and water must necessarily vary. A typical formula is appended : B Olei Morrhuae f ounce ii Pulv. Acaciae ounce ss Aquae q. s. ft. f ounce iv “ Place the acacia, which should not be finely powdered, but granulated, in a mortar with one fluidounce of water. This should be triturated until the mucilage is perfectly smooth and free from lumps. The oil should be added a t first in quantities not greater than half a fluidrachm a t a time, stirring rapidly with the pestle after each application, care being taken not to add a fresh portion of oil until the last one has been thoroughly emulsified. When the liquid becomes too thick t o be easily stirred, a fluidrachm of water should be mixed with it, and the gradual addition of oil continued until the whole quantity has been used. The larger quantity of water may be added rapidly after the nucleus is once properly formed, without risk.
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WiZder D. Bancroft
2. The Continental Method.-This has the great merit of never failing to produce a good emulsion if the proper proportions are used to form the nucleus, and if the directions are strictly followed. The most satisfactory proportions may be easily remembered. Half as much water is taken as of oil, and half as much gum as of water; or it may be expressed as oil, 4; water, 2 ; gum, I . The four parts of oil must be placed in a dry mortar and one part of finely powdered gum added to it, stirring with the pestle; when a uniform mixture is made, two parts of water are added, not gradually, but all at once, when, upon stirring, thc emulsion is quickly made. An additional quantity of water may be added to this nucleus without risk. The explanation of making an emulsion by this method is, that the particles of gum, being insoluble in the oil and surrounded by it, are prevented from separating and dissolving in the water so as to form lumps; by stirring the mixture actively the water gradually dissolves the gum, the oil becomes incorporated a t the same time, and a homogeneous mixture is produced, the quantities of oil, gum, and water being in exactly the right proportions to form an emulsion.’ ’ “ Casein Emulsions.-The use of casein as an emulsifier has been developed by Leger, a Parisian pharmacist. He recommends the preparation of saccharated casein, a fine white powder, which is used for emulsifying just as is powdered acacia. The advantage claimed for casein are that its emulsions are more readily retained by the stomach, and that greater stability and perfection are secured through its use. - “Saccharated casein is prepared by heating one gallon of cow’s milk to 104’ I?, adding two fluidounces of ammonia water, allowing the whole to stand a day, and separating the lower milky liquid from the oily liquid on top. The milky liquid (lactoserum) is treated with acetic acid until the casein is precipitated. After washing the precipitate thoroughly with water at 1 0 4 O F it is collected on a muslin strainer, pressed, and dried; a weighed portion of the casein is dried and the percentage of moisture ascertained; the damp cake of
The Th.eory of Emulsification
751
casein is then triturated with three and a half ounces of powdered sugar and eight parts of sodium bicarbonate for every one hundred parts of dry casein. " Prolonged trituration and the' addition of more powdered sugar, until it amounts to nine parts in one hundred, result in the formation of a paste, which must now be dried by a gentie heat not above 86' to 90' F. After complete drying, it is powdered and sifted. To make a casein emulsion of a fixed oil, fifteen parts of the oil are gradually incorporated with a mucilage previously made with fifteen parts of saccharated casein and five parts of water, when a perfect emulsion is formed and other ingredients are added. Chondrus Emulsions.--Since acacia is sometimes subject to fluctuations in price and is often expensive,' various substitutes havc appeared which have been tried as emulsifying agents, one of the most successful being the gelatinous substance obtained from chondrus or Irish moss. " In the formulary, Part VI, under the heads of gelatinum chondri, mucilago chondri, and emulsio olei morrhuae, full information as to the methods of using it will be found. In this place it will only be necessary to say that a gummy substance in scales is produced by evaporating and desiccating a decoction of chondrus, and that a mucilage may be made from this Irish moss gelatin by heating eight grains of it in contact with one ounce of boiling water until it is completely dissolved. The mucilage, after being cooled, is then used for preparing emulsions exactly as mucilage of acacia. " Ouillaja Emulsions.-Quillaja, or quillaya bark contains the principle saponin, a glucoside which is capable of emulsifying oils. Senega contains an analogous principle. The property which both possess, of causing frothing in aqueous solutions, suggested the use of quillaja as an emulsifier. It has not come into extensive use, and care is necessary in employing it, as it is not without medicinal and irritating properties. One of the essentials of a good emulsifier is that it should be inert. Quillaja has been adopted in the National Formulary. (See emulsio olei morrhuae, Part VI, which ' I
-
752
I
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Wilder D. Bancroft
illustrates the method of using it.) Where an active medicine is to be made into an emulsion, and its properties are not antagonized by the quillaja, it may be judicious to employ it. Another disadvantage that it possesses is that a large quantity of tincture is required to be effective. ((CompoundEmulsions.-As a general rule, the addition of alcoholic liquids to emulsions destroys their homogeneity. When it is necessary to add them in compounding prescriptions, they should be diluted, if possible, with a portion of the water, and added after the emulsion is nearly finished. Alkaline solutions generally aid emulsification, by forming soaps with the resinous or oily liquids; volatile oils make better emulsions if they are first mixed with an equal volume of fixed oil.” There is no obvious reason why a pharmacist should be interested in preparing an emulsion containing water as drops and therefore it is not surprising to find that the emulsions, referred to in Remington’s book, are all of the other type. I confess t o being puzzled by the statement that the proportion of 4 : 2 : I for oil, water and gum is exactly right. Since the relative amounts of oil and water can vary within wide limits, there is no such thing as a best ratio unless one specifies an emulsion of definite properties, which is probably what was meant though it was not so stated. Even then, the statement is misleading because different gums have different emulsifying powers, and a proportion which is the best for one gum will not be the best for another. The matter seems to be expressed more clearly by Scovi1le.l “Acacia is the emulsifying agent par excellence for general use. Emulsions made with it are attractive in appearance, palatable, and permanent. Its range of power is exceeded only by the albuminous agents mentioned (milk and yolk of egg). Emulsion of chloroform, oleo1esins, resinous tinctures, etc., can be made readily with acacia, but separates into 4 layers quicker than when made with yolk of egg. t
“The Art of Compounding,” 88 (1895).
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753
‘‘ Either dry acacia or mucilage of acacia can be used for emulsions. Both have their advocates in point of preference, but dry ac?cia has proved itself a quicker and more certain agent to use, a t least in the hands of novices. This is probably due to the fact that dry acacia must always be used in definite proportions, as must also the water first added. Two rules are in common use for making emulsions with dry acacia. “Rule I.--For one patt of gum use four parts of fixed oil (or two parts of voEatiZe oil), and once and a half as much water as gum. Rule 2 varies only in using twice as much water1 as gum. Exceptions to these may be met with in that the proportions of oil to gum vary with diffennt oils; most fixed oils being emulsified well in proportion of four of oil to one of gum, while most volatik oils require one of gum to two of oil. Occasionally, however, a fixed oil is found which requires one-third its weight of acacia, or a volatile body which requires an equal weight. The amount of dilution t o which the primary emulsion is subject also affects the proportions. In all cases, once-and-a-half or twice as much water as gum must be used for the primary emulsion.’’ “Tragacanth is, next to acacia, the most popular ernulsifying agent. It is a type of the
