BREAKING EMULSIONS
T. G.ROCHOWL AND C. W. MASON Cornel1 University, Ithace, N. Y.
BY FREEZING ItEAKIX‘ci by freezing is important in siicli n variety of emulsions that no investigation can deal adequately with all cases. The aim of the present work is to examine some of the simpler typical systerns, under conditions siwh that the microscopical ohservat.ion of the phenomena is possihle. The prevailing theories of emulsificatiom, ns sonimarieerl in some detail by Tlionias (fO),are as follows:
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Interfacial tension bet,wern two imnriscible liquid8 lowered by the rmuhifying agent. Oriented srrangenrent of wedgeshaped molecules of the emulsifier, to form a curved layer st the interface. 3. A plastic film of the emabifying agent, costing the droplets of the dispersed phhese. 1. 2.
These theories are not necessarily mut,ually exclusive; indeed, all the factors mentioned may he 6imult.aneously effective. In studying the freezing of emulsions, it is of less hnmediate interest to know which is the “correct” theory of emulsification than to obtain experimental evidence that will actually show how breaking proceeds and will pennit intelligent application of whatever theoretied explanation best fits the observed pheiiomena.
Advantages of Microscopical Studies In this investigation microscopical methods were chosen to overcome three disadvantages inherent in ordinary mncroscopical methods:
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globules and breaking.
If contact IS niado hetween tlie envelopes of emulsifying a g e n t s u r -
1.0) was added with shaking to a 0.5 per cent s o d i u m o l e a t e (Kahlbaum) solution in water,
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the emulsion obtained was fairly stable, and the photomicrographs shown in Figure 1 were taken when it was 37 days old. When slowly frozen a t -6" C.,z the globules were brought into contact by being concentrated between the growing dendrites of ice (Figure I d ) . They were distinctly flattened against one another, although the surfaces in contact were marked by clearly visible membranes, just as in a mass of soap bubbles. The remarkable feature of this emulsion was that the operations of freezing and thawing did not break it; the reason is apparent from the microscopical evidence. Figure 1A was taken immediately after freezing, and the ice was then melted by an air jet directed on the cover glass. Figure 1B shows thawing beginning around the clusters of globules, with a large amount of ice as the continuous phase. The globules, although undoubtedly in mechanical contact while the emulsion was frozen, have not coalesced, and the emulsion has not broken. When this emulsion was frozen and held for 9 minutes a t -7.8' C. (Figure IC), the membranes separating the globules became much thinner, but the emulsion still recovered on quick thawing. A third freezing, with chilling t o -11.8' C. for 10 minutes, caused disappearance of the membranes; the droplets coalesced and the emulsion broke. This behavior can be duplicated with considerable variation of time and temperature. The essential factor that determines breaking or recovering of the emulsion is whether the membrane of emulsifier is still existing when thawing releases water and permits mobility of the droplets. It is possible to predict, with a high degree of certainty, whether a frozen emulsion will recover when thawed or whether it will break, simply from the appearance of the membrane between adjacent droplets. Similar behavior is shown by other emulsions. For example, an emulsion of 30 per cent of carbon tetrachloride by volume in water, with 2 per cent of a triethylammonium oleate soap prepared from stoichiometric quantities of triethanolamine of equivalent weight 135 (Carbide and Carbon Chemicals Corporation) and oleic acid (Kahlbaum), endured freezing a t -8" C. if thawed immediately. The emulsion broke, however, when it was kept in the frozen state long enough for the membranes to disappear. The process is shown in detail in Figure 2. The critical stages are the segregation of the droplets by the dendrites of ice ( B and C), the disappearance of the membrane boundaries between the droplets (D and E ) , and coalescence of droplets released by thawing.
Influence of Amount of Emulsifier The fact that mere contact, or even marked flattening of the globules against each other, does not cause breaking if an emulsifying agent is used raises the question as to how the amount of emulsifier affects the extensibility of the membrane surrounding them. Increasing the concentration of soap as an emulsifying agent increases the amount adsorbed on the dispersed phase, as Briggs has shown ( 2 ) . However, it likewise increases the fineness of the emulsion by supplying an excess of membraneforming material to stabilize the droplets as they are newly sheared off in the process of making the emulsion by stirring or shaking. These finer emulsions show a greater tendency towards clustering of the globules, and this behavior tends t o nullify any improvement in stability towards freezing, because the clumps are more readily entrapped and compressed by the growing ice crystals than are isolated globules. A series of 33.3 per cent by volume of benzene-carbon tetrachloride (specific gravity 1.0) emulsions in water, with 2 Temperatures are given as read on the thermometer of the cold stage. The previous article (6) discusses their accuracy.
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sodium oleate ranging from 0.01 to 5 per cent, was studied. The "cream" of these e m u l s i o n s was taken for microscopical study to give a higher concentration of droplets. All showed marked coalescence after freezing and thawing. Soap was precipitated from concentrations greater than 3 per cent when the emulsion was chilled. In e m u l s i o n s c o n t a i n i n g more than 0.5 per cent soap, there was a period after freezing when the membranes persisted between tightly pressed globules, but the durability of the membranes was not markedly greater with increasing soap content.
Viscous Internal Phase Viscosity of the internal phase does not prevent coalescence, as exemplified by an emulsion of 33 per cent olive oil by volume in 1 per cent aqueous sodium oleate solution. dlthough it is highly viscous a t freezing temperatures, droplets of the oil will run together slowly if the ice holds them in contact long enough for the membrane to disappear. An emulsion with a plastic internal phase (butter fat in cream) was frozen a t -10" C. Large ice crystals collected the fat globules and squeezed them together into polygonal shapes, most of which remained adherent after thawing, even though membranes were still visible surrounding the droplets in the mass. Repeated freezing a t -10' C. increased the size of the clusters but did not produce appreciable coalescence. Another sample, frozen and held at -20" C. for 15 minutes, showed the dendrites of ice closed tightly around the fat globules, and the membranes were no longer visible. However, the globules did not coalesce to larger round globules, but remained in chains of two or three, because they were plastic instead of viscous a t room temperature.
Solute in Aqueous Phase In the process of freezing an emulsion, any water-soluble constituents are concentrated in the interstices between the growing ice crystals. The ultimate liquid may have a relatively high concentration of emulsifying agent or other dissolved material and will, of course, not freeze completely unless the cryohydrate temperature of the solution is reached. The influence of such removal of water from the emulsion, or from the membranes surrounding the "oil" droplets, deserves consideration. An emulsion of 33 per cent carbon tetrachloride by volume in water with 1 per cent sodium oleate and 2 per cent glycerol was frozen. Approximately 10 per cent of the aqueous phase remained melted, although most of the droplets of carbon tetrachloride had been brought into contact by the growing ice crystals. The separating membranes could be observed t o disappear on standing a t this stage, even though fluid surrounded the clusters. A similar emulsion with 5 per cent glycerol by volume showed greater durability on freezing. Most of the membranes surrounding the droplets persisted for an hour at -10" C. and even withstood cooling to -27" without marked thinning. Evidently the primary governing factor is not temperature but some change that takes place in the membrane with time. Besides being subjected to stretching when the globules are flattened against each other, the membrane of emulsifying agent is in contact with a relatively concentrated glycerol solution and may be dehydrated thereby, with a decrease in stability.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Effect of Dehydration Dehydration is obviously a possible cause of membrane rupture, because few emulsions are unbroken after drying. Merely allowing water to evaporate from an aqueous emulsion causes the “oil” droplets to become so closely concentrated that they are pressed into polyhedral shapes, but this contact alone is not sufficient to cause breakage. Further evaporation results in breaking at the outside of the emulsion; if the dispersed phase is volatile, the emulsion continues to break as evaporation proceeds. A milder dehydration is effected by introducing a strip of dry gelatin into a concentrated emulsion, when coalescence occurs between the globules nearest the gelatin as i t abstracts water from the emulsion. The effects of moderate chilling and of contact between droplets are not alone sufficient to cause breaking if an emulsifier is present. An emulsion containing 80 per cent carbon tetrachloride by volume in 0.5 per cent aqueous sodium oleate was centrifuged until any excess carbon tetrachloride or soap solution had separated, and the resulting “cream,” consisting of carbon tetrachloride droplets in contact except for a minimum of soap solution, was held a t 0 ” C. for 24 hours with no apparent breaking.
Nature of the Film of Emulsifying Agent Since Bancroft’s early formulation ( 1 ) of the role of a film of emulsifier in stabilizing the droplets of an emulsion, various workers have confirmed and extended this idea (4,7--9), The effect of distortion of the droplets in an emulsion throws some light upon the nature of the film that surrounds them. Where no emulsifier or definite film is present, as in the first emulsion described, mere contact between globules results in their coalescence, and isolated globules show the elasticity due to surface tension when they are compressed or released. Solid emulsifiers, consisting of a layer of particles coating the droplets, yield more stable emulsions. The use of watersoluble nigrosine (aniline black) as a coloring material to improve the visibility of the aqueous phase of some of the preceding emulsions led to the discovery that it alone was an effective emulsifying agent for benzene-carbon tetrachloride emulsions in water. A concentration of 0.5 per cent in the aqueous phase is sufficient. The solid dyestuff is precipitated a t the interface by the benzenecarbon tetrachloride mixture in which it is practically insoluble, and is visible as a distinct film on the droplets. Such an emulsion is stable for months a t room temperature. When such an emulsion is frozen, the droplets are forced together and behave just as did those with soap as emulsifier; a separating membrane is shown whose persistence is an indication of whether or not the droplets are potentially c:oaleeced. When this membrane is broken, fragments often rem:iin within the resulting drop. When the drops are deformed by flattening, the membrane may actually show cracks; when the pressure is released, it may wrinkle as if it were a plastic skin. Antimony oxychloride, used as an emulsifying agent in some polishes, shows this behavior more clearly because the film of emulsifier consists of visible granules coating the “oil” droplets. Such an emulsion may be made by dissolving antimony trichloride (butter of antimony) in oil or benzene, and shaking the mixture with water. During freezing the skin of granular material on the droplets cracks in places, but coalescence takes place only if they are near enough to each other to touch during the formation of ice. This situation is fairly rare because the droplets often carry clumps of excess precipitate. Other emulsifiers show definite skin formation; saponin, for example, gives a tough film that can be stretched by pres-
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sure on the oil droplets and is markedly wrinkled when the pressure is released. (Saponin behaves similarly when used in blowing bubbles. If the bubbles are made to contract, they appear as wrinkled bags.) Gum arabic emulsifies cod liver oil in water, forming skins which can be wrinkled as described. Serrallach and Jones (7),in discussing the importance of film formation as essential to emulsification, point out that the ability of films to wrinkle is an indication of their toughness. The existence of markedly nonspherical droplets free in the continuous phase of an emulsion is further evidence of the reality of a plastic skin surrounding them, which prevents surface tension from drawing them into spheres. Such ‘%ear drops” or elongated globules are often present in the above emulsions as a result of shearing large drops into smaller ones and are a well-known feature of rubber latex. A soap film as emulsifier is ordinarily believed to be oriented, with the oleate chains towards the oil and the alkali end of the molecules towards the water. There is evidence (9) that such films are more than one molecule thick; certainly their influence extends farther into either or both phases, for small droplets can often be observed under such conditions that they seem clearly to be separated by a layer of water of finite thickness and appear as if they could approach each other only up to a certain point. (A similar appearance occurs in alumina sols where flocculated particles are separated as much as 1 or 2 p , and have a loose Brownian movement even within their clumps.) Some direct evidence as to the oriented structure of films is afforded by emulsions prepared from 33 per cent water by volume in a 0.5 per cent solution of cholesterol (Merck) in benzene, The droplets of water are surrounded by a distinct film, visible best when two are pressed together. This film shows marked double refraction with its vibration axes tangential and perpendicular to its surface. The effects of possible polarization by surface reflection are eliminated by comparison with droplets having no emulsifier present and such behavior is undoubtedly due to the parallelism of submicroscopic cholesterol crystals, which are known to have excep tionally strong double refraction. The effect is strongest where two droplets touch, because here the film is of double thickness. Single large drops also show some birefringence a t their surfaces. The emulsions of water-in-benzene and cholesterol show the same behavior on freezing as the emulsions previously described, with the same persistence of a film between mutually flattened droplets so that the point of breaking can be recognized. Before actual breaking occurs, the partitioning film between adjacent droplets becomes thinner and thinner, and its birefringence leqs distinct; but, if the emulsion is thawed before the stage of potential coalescence is reached, the original thickness and birefringence of the film are regained.
Behavior of Emulsions In order t o emulsify one liquid in another, it is desirable that their interfacial tension should be low, t o permit the enormous increase in surface that occurs. The ideal liquids for emulsification would be a consolute pair; the organic chemist often has to contend with such systems-for example, when he extracts an aqueous solution with ether or precipitates phenol from an aqueous solution of the alkaline phenolate. In the absence of an emulsifier, such emulsions coalesce by “digestion” or by contact in gravitational settling. However, the primary role of an emulsifying agent is not t o facilitate disintegration of the dispersed phase into droplets but to preserve these droplets once they have been formed ( 9 ) . By agitation methods of making emulsions, droplets are sheared off from the main body of the liquid and must be instantly coated with a protective membrane (8).
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The role of the emulsifying agent in lowering the interfacial tension between the two liquids has been overemphasized, however. It is admitted that sodium oleate greatly lowers the surface tension between most oils and water, and that this is an advantage in emulsification. On the other hand, saponin does not appreciably lower the interfacial tension between oil and water (3) but is an excellent emulsifier. The film-forming properties of the emulsifier are of primary importance, since they not only govern the initial stabilization of the droplets but also their ability to be stretched or sheared into smaller droplets and to suffer compression against each other without true contact. Two factors bring about the localization of the emulsifier a t the interface. It may be precipitated by chemical reaction (as in the case of the antimony oxychloride) or by its property of being insoluble in one phase though soluble in the other. Sodium oleate is insoluble in benzene, and its localization a t the water-benzene interface may be due partly to precipitation but also to a process analogous t o adsorption, by which the dissolved emulsifying material is concentrated by surface forces and is more or less oriented in the process. Both factors tend to make the film self-healing, though diffusion of material within the film itself is an additional means of restoring local weakness. The aging of an emulsion, which usually results in marked strengthening against breaking, is due either to this last-mentioned process of equalization, or to precipitation of additional emulsifier a t the interface. More perfect orientation of the molecules of the emulsifying agent can also occur on standing. The membranes around the droplets of an emulsion are optically perceptible and mechanically demonstrable. They therefore simulate a third phase in the system. But they may themselves be composite, in the sense that they are of the nature of gels, permeated by the phase in which they are more soluble. The water in the soap film surrounding the droplets of an oil-in-water emulsion may be physically different from the bulk of the external phase, perhaps being bound in this gel and frozen only a t very low temperatures, if a t all.
Process of Breaking by Freezing The process of freezing an oil-in-water emulsion may now be recapitulated, in terms of the above characteristics. As ice forms, the droplets are collected in advance of the growing crystals and entrapped in clusters in the interstices. Simultaneously, any water-soluble constituents are concentrated in the last of the melt and, if the solution becomes saturated, may be precipitated (as soap is). As the growing crystals push the droplets closer together, flattening occurs, the membranes surrounding the globules are stretched and thinned, and an unstable situation is created between two adjacent droplets. The films are no longer in contact with free water nor subject to its orientating influence. They form a double layer surrounded by the internal phase (oil), and this layer would not be expected to have elastic or self-healing properties. As the residual water freezes, they are subject to further
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mechanical stress; also their bound water tends to be abstracted by the ice or by the concentrated aqueous solution last to freeze, which may possess quite different orienting power from that of water. It is a t this stage that the ability to recover is lost, and the membrane partition between two juxtaposed droplets becomes thinner and disappears. Time is an essential factor in the destruction of this membrane; this is consistent with the slowness of freezing of bound water, emphasized by Jones and Gortner (6). If the freezing point of the water is not depressed by the presence of a solute, ice crystals will grow up to the surface of the films surrounding the oil droplets, and the orientating effect of xater in contact with the molecules of the film will be lost. No data appear to be available on interfacial tensions between ice and other materials of emulsions. Furthermore, if bound water is assumed to be absent, the films may be completely dehydrated or may be penetrated by small ice crystals; both of these factors will tend to destroy stability. In the freezing process, dehydration seems t o be essential. Prolonged contact of droplets a t ordinary temperature does not break a reasonably stable emulsion, nor does contact in the cold, unless most of the water is frozen. The great sensitiveness of emulsions to any form of drying confirms this assumption. Breaking by freezing may therefore be ascribed to the following causes, in sequence: 1. Withdrand of free and/or bound water from the films between touching droplets, by crystallization as ice, or by concentrating of any solutes present. 2. Establishment of true contact between adjacent films of emulsifier, with loss of the orienting influence of water. 3. Diffusion of the emulsifier in the film away from these thick regions. 4. Decrease in fdm area and coalescence of droplets as soon as thawing of the ice permits them to change shape.
In any further study of the resistance of einulsions towards freezing, it would be in order to examine in detail the behavior of the membrane, including its hydration, the freezing point of any bound water, and the time, temperature, concentration, and extensibility as influencing the stability of films of various emulsifiers between pairs of immiscible liquids. By analogy with the results obtained on films at air-liquid interfaces, valuable information should be forthcoming from such an investigation.
Literature Cited
(;Oj
Bancroft, J . P h y s . Chem., 17, 501 (1913). Briggs, Ibid., 19, 215 (1915). Hillyer, 2. p h y s i k . Chem., 47, 336 (1904). Holmes, J . A m . Chem. Soc., 44, 66 (1922). Jones and Gortner, J . P h y s . Chem., 36, 387 (1932). Mason and Roohow, IND.ENG.CHEM.,Anal. Ed., 6, 367 (1934). Serrallach and Jones, ISD. EXQ.CHEX.,23, 1016 (1931) Serrallach. Jones, and Owen, Zbid., 25, 816 (1933). Stamm and Kraemer. J . Phus. Chem., 30, 992 (1926). Thomas, J. -4m. Leather C h e h . Assoc.. 22, 173 (1927).
RECEIVED June 4, 1936.
