- Email: [email protected]
EMULSIONS
CHAPTER 16
EMULSIONS T H E term emulsion refers to any dispersion of one liquid in another. The liquids, of course, must be immiscible. Water is one of the most common components, and the other is usually an oil or some other lipophilic liquid. Cream, rubber latex, and a dispersion of lubricating oil in water are examples of emulsions. Concentration and particle size. The amount of solid substance dispersed in sols, relative to the medium, is small. The concentration of sols is usually only about 1-2%. The amount of one liquid dispersed in another is usually greater, e.g. in cream about 15-20% of fat is dispersed in water. A geometrical calculation shows that the maximum amount of one liquid which can be dispersed in another is 7 4 % of the total available volume, if the droplets are spheres of equal size. It can also be shown that this maximum amount does not depend on the diameter of the droplets. Emulsions even more concentrated than 7 4 % are known. This is explained by the fact that the droplets can be deformed upon close packing, as well as by the possibility of introducing small droplets between large ones. Such a highly concentrated emulsion may have a rather solid consistency. For instance, the creams used in cosmetics are coherent, jelly-like systems. It is sometimes difficult to decide whether such cream or grease is a gel or an emulsion, especially when the system contains more than two components. Thus emulsions of oils in glycerine have a high consistency. Examination of emulsions shows that they contain droplets of diameter 0-001 to 0-050 mm. Hence the emulsions are coarsely disperse systems. The fat droplets in milk, for instance, can be seen in ordinary microscopes. By the use of special emulsification machines and h o m o genisers it is possible to prepare very fine emulsions possessing droplets of diameter only about 0-0001 mm. Mesophases. Liquid crystals. There is a continuity between suspensions and emulsions, and it is often difficult to classify a dispersion into one or the other class. F o r instance, some kinds of natural rubber latex contain rod-shaped particles, which are soft but not liquid. Emulsions of many fats also contain particles of consistency intermediate between solid and liquid. If the particles are composed of long or flat molecules, they may be pseudo-crystalline or mesomorphic, e.g. they may be doubly refracting (optically anisotropic). VORLÄNDER (1908) described about 250 mesomorphic compounds. Examples include emulsions of phenol ether in glycerin, ammonium oleate in alcohol, /7-azoxyanisole in water, and cholesteryl benzoate in water. 414
EMULSIFYING
AGENTS
415
The high molecular fatty acids and soaps often appear in this liquidcrystalline state. The long or flat molecules in the liquid crystals may be oriented in different degrees. For example, a triglyceride in a droplet may not be oriented at all, in which case the droplet is truly liquid ; if the molecules are oriented in groups with a certain regular structure (Fig. 178) the droplets still flow steadily, but they may be anisotropic. This
a b F I G . 178. Fibrous molecules in the nematic state (in a). Asymmetric molecules in the smectic state (in b). partly oriented state is called the nematic state. The bundles of long molecules which are oriented parallel in the nematic state are like micelles which are distributed randomly in the droplet. If they lie in parallel layers, we have a higher degree of orientation, the so-called smectic state. Smectic liquid crystals do not flow but glide. They are composed of series of planes. Examples are ammonium oleate and ( 1) cholesteryl b e n z o a t e . Preparation of emulsions. Emulsifying agents. The simplest method of making an emulsion would be to shake two immiscible liquids together. On shaking an oil or benzene with water the oily liquid can certainly be dispersed in droplets, but the emulsion is not stable ; the droplets quickly flow together again, and the liquids separate into two layers. It has been found that the ease with which two immiscible liquids can be emulsified becomes greater as the differences between their surface tensions and densities become less. However, even by choosing the most favourable cases such emulsions have a poor stability. In order to prepare stable emulsions a third component—an emulsifying agent—must be introduced into the system. The emulsifying agent, in the first instance, may lower the interfacial tension (see Chap. 5), though this is not the only purpose in using it. The lowering of the interfacial tension facilitates the formation of droplets, but it does not necessarily increase their stability. An emulsifying agent must also stabilise the droplets formed, as explained in the following section. Numerous emulsifying agents are known, and they can be classified into several groups. The largest and most significant group is that of to G.
F R I E D E L in J. A L E X A N D E R ' S Colloid Chemistry, Vol. I, p. 102 (1926); R . D . and M. V O L D ; ibid. Vol. V, p. 266 (1944); A . S. C. L A W R E N C E ; Trans. Faraday Soc. 34, 660 (1938).
VOLD
416
EMULSIONS
the soaps and detergents, and other substances of similar chemical structure. The essential feature of the structure of these emulsifiers is the combination of a large lipophilic radical with a polar group in the same molecule. For instance, the sodium stéarate molecule is composed of a long hydrocarbon tail and a polar head of the ionisable carboxyl group. Sodium dodecylbenzene sulphonate molecules contain a long hydrocarbon tail and a polar head of the sulphonate. Lecithin molecules are composed of several lipophilic chains and the ionogenic phosphate and tertiary amine groups. The proteins, agaragar, the pectins and the saponins are further examples of emulsifiers. It is interesting that a number of finely ground insoluble solids such as some clays and certain basic sulphates may also act as emulsifying agents ; they are solids which are easily wettable by one of the liquids. Emulsification in industry is accomplished by means of emulsifying
F I G . 178a. Emulsifying machine (the Charlotte colloid mill). (By courtesy of Chemicolloid Laboratories Inc., Garden City Park, Ν . Υ . )
EMULSIFYING
417
AGENTS
machines which work on the same principle as colloid mills (p. 294). The mixture of the two liquids containing an emulsifying agent may, for example, be forced through fine slots, a n d then be subjected t o further shear when it strikes the wall of the machine. The mixture may be sheared between two grooved disks or funnels, one being static and the other rotating rapidly (see Fig. 178a). The structure of the particles in emulsions. T h e structure of the particles in stable emulsions has been elucidated by the work of LANGMUIR, H A R K I N S , RIDEAL, SCHULMAN a n d o t h e r s .
( 2)
A stable emulsion
is a three-component system composed of two immiscible liquids a n d an emulsifier. T h e latter is usually introduced only in an a m o u n t of i t o 2 % . Let us consider first what happens when an oil is emulsified in water by means of some soap or similar partly lipophilic a n d
F I G . 179. Oil droplet in an emulsion protected by a layer of soap molecules. The lipophilic tails of the soap molecules are immersed in the oil; the polar heads extend to the water phase (dispersion medium).
partly polar compound. The lipophilic part of the soap molecule has a n affinity for the oil, the polar head for the water phase. Hence the soap molecules will be packed on the surface of the dispersed oil droplets as illustrated in Fig. 179. The hydrocarbon tails of the soap molecules will be immersed in the oily droplet, but the polar heads will be turned into the water. Consequently, the droplet will be covered by a more or less dense protecting layer which will hinder the coalescence of the droplets. In the absence of such an emulsifier, which acts as a stabiliser, the droplets will coalesce instantly, because of the tendency to decrease the free surface energy, i.e. because of the cohesive forces acting between the oily surfaces as they come in contact. D r o p lets whose surfaces are coated with soap molecules are, however, electrically charged by the ionised carboxyl groups of the soap. Hence ( 2)
I. L A N G M U I R and W. D. H A R K I N S in J. A L E X A N D E R ' S Colloid
Chemistry,
Vol.
I
(1926). W. C L A Y T O N ; The Theory of Emulsions and Their Technical Treatment, 4th ed. (Churchill, London 1943). J. H . S C H U L M A N and D . P. R I L E Y ; / . Colloid Sei. 3, 383 (1948). E . G . C O C K B A I N ; Trans. Faraday Soc. 48, 185 (1952); P . B E C H E R ; Emulsions: Theory and Practice (Reinhold, New York 1957).
EMULSIONS
418
the droplets repel each other before they collide, and the emulsion is stable. Very stable emulsions can be prepared by using a combination of two or more emulsifying agents. F o r example, while cholesterol o r sodium cetyl sulphate alone are poor emulsifying agents, a mixture of these compounds can be used to form very stable emulsions of oil in water. Cholesterol is t o o soluble in oil, a n d too insoluble in water to be a good emulsifying agent ; moreover, it does n o t contain ionogenic groups. Sodium cetyl sulphate has such groups, but it is relatively insoluble in oil. When both are present, a complex film of cholesterol ( 2 a) and the cetyl sulphate is formed on the surface of the d r o p l e t s . It is also interesting that the short molecules of the unsaturated eis isomers have a weaker emulsifying effect than the long trans isomers For example, the stability of oil emulsions with cholesterol a n d sodium oleate (eis isomer) is poor, but with cholesterol a n d sodium elaidate (trans isomer) the emulsion is quite stable. The thickness of the interfacial film which stabilises the droplets of emulsions has been studied by K R E M N E V
( 2 B)
a n d by C O C K B A I N .
( 2 c)
The
Russian investigators studied chiefly the emulsions of benzene in aqueous solutions of 5 % sodium oleate. The emulsions were characterised by the maximum volume of benzene which could be dispersed in 1 ml of the oleate solution, and the droplets were measured under a microscope. The thickness of the stabilising film was found to be of the order of ΟΌΙ/ι. Similar studies were extended also to other types of emulsions with other emulsifying agents, and in some instances thinner films of 20 Â were found. Cockbain studied the film thickness of H-decane emulsions which were stabilised by serum albumin a n d sodium dodecyl ( 2 c} sulphate. He found that in the absence of dodecyl suphfate the albumin film was monomolecular in thickness (13-5 Â), whereas the thickness of a complex albumin-dodecyl sulphate film was 20-5 Â. The stability of emulsions depends chiefly on the following two factors : (1) thickness and compactness of the protecting film (interfacial layer), and (2) electrical charge on the droplets or film. Moreover, the stability depends on the viscosity of the dispersion medium, a n d on the density difference between the two liquids. The emulsifying power of finely ground powders depends o n h o w they are wetted by the liquids. In the most suitable wetting conditions the droplets are surrounded by a layer of the solid particles which thus prevent coalescence (Fig. 180). 2a
( 2 b) J. Η. S C H U L M A N and E. G. C O C K B A I N ; Trans. Faraday Soc. 36, 651, 661 (1940). ( ) L. Y. K R E M N E V and S. A. S O S K I N ; Zh. Obsch. Chim., Moskva, 16, 2000 (1946); Koll. Zh. 9, 269 (1947); 11, 24 (1949); K R E M N E V and N. K U I B I N A ; Koll. Zh. 13, 38 (1951). 2c ( ) E. G. C O C K B A I N ; / . Colloid Sei. 11, 575 (1956).
EMULSIFYING AGENTS
419
F I G . 180. Stabilisation of a droplet by a layer of solid particles.
Two types of emulsions. Two types of emulsions can be distinguished : the oil in water type (O/W), and the water in oil (W/O) type. The emulsions of the O/W type conduct electricity, the W / O emulsions do not. The former can be diluted with water and coloured with water-soluble dyes, the latter can be diluted with oils and lipophilic liquids only, and they can be coloured with oil-soluble dyes. Two liquids, e.g. oil and water, can give emulsions of either type depending on the emulsifying agent. Ordinary soaps, such as sodium stéarate, will produce emulsions of the O/W type. If, however, a mixture of oil and water is shaken with powdered calcium stéarate, the continuous phase will be oil, with the water dispersed in it. The droplets of water in this example are stabilised by a layer of calcium stéarate with the calcium heads turned into water and the hydrocarbon tails immersed in the oil phase. The water droplets in this type of emulsion are protected from coalescence not by an electrical charge, but by a protective layer of calcium stéarate. Emulsions of this type are also obtained by using certain resins or lamp-black as emulsifying and stabilising agents. The inversion of phases. CLOWES has thoroughly studied emulsions of both types, and he found that one type can easily be converted into the other. F o r example, an emulsion of oil in water, which is stabilised by a sodium soap, can be converted into a W / O emulsion on addition of calcium chloride. The calcium ions combine with the anions of the fatty acid, and the calcium salt of the fatty acid covers and protects the water droplets formed upon shaking the mixture. Inversion of phases is believed to occur also in the complex membranes of living cells. Certain parts of the membrane contain lipids which are dispersed as a stiff emulsion or a labile jelly. Such a membrane will be permeable to water soluble substances if water is the continuous phase. In the presence of calcium ions the phases may be inverted and the film may become impermeable to water soluble substances, but will now be permeable to lipids.
420
EMULSIONS
Creaming and breaking of emulsions. Creaming and breaking of emulsions are processes similar to the coagulation of sols. In creaming the separation of the two liquid phases is incomplete, but in breaking the separation is complete. In creaming the droplets of the dispersed oil separate slowly into a more concentrated layer. This occurs by virtue of the difference in density between the oil and water. The droplets, however, do not coalesce upon such creaming. Creaming can be facilitated by centrifugation, or by increasing the density of the continuous phase by adding a dense substance not interacting with the components, but soluble in the continuous phase. The choice in such substances, however, is very limited. As an example of this, a concentrated salt solution usually breaks the emulsion, but is undesirable for other reasons. Creaming can be prevented or slowed down on adding substances (e.g. glycerin or a solution of gelatin), which increase the viscosity of the continuous phase. The breaking of emulsions is important in certain technical processes in which spontaneous emulsification takes place. For example, when emulsions are not desired they must be broken either to free oils from dispersed water or to recover oils from O/W emulsions. Emulsions may be broken up by physical or by chemical means. The physical methods include heating, centrifugation, electrophoresis, and irradiation with ultrasonic waves. Heating and high frequency sound waves obviously increase the intensity of motion of the droplets, and may thus promote coalescence, although the same methods may also produce emulsification. Centrifugation usually produces only creaming, and the concentrated emulsion must then be treated chemically in order to induce coalescence. Agents producing O/W emulsions will break W / O emulsions and vice versa. The droplets in O/W dispersions are usually negatively charged, and the emulsions are broken upon addition of salts. The W / O emulsions are broken by adding sodium soaps or sulphonated oils. Sometimes the separation of the phases can be achieved by adsorbing the dispersed material on activated carbon or other adsorbents. Emulsions of fats or fatty oils in water can be broken by adding acid. The added hydrogen ions diminish the dissociation of the carboxyl groups on the surfaces of the droplets ; this reduces the charge and the droplets can flow together. Alkalis, on the other hand, are good emulsifying agents for fats and fatty oils. Some practically important emulsions. Milk is one of the most important emulsions. Only hot milk, however, is a true emulsion, containing truly liquid droplets of butter fat. At a temperature below the melting point of butter fat the droplets are solid, or they may be in states intermediate between liquid and solid. The droplets of fat in milk are protected from coalescence by a layer of protein. As the density of the fat is lower than that of the milk serum (containing
SOME IMPORTANT EMULSIONS
421
proteins, sugar and salts dissolved in water), creaming occurs if the milk is allowed t o stand (the separation can be facilitated by means of a centrifuge). The cream is a relatively concentrated emulsion of the O/W type. Butter is a consistent, more stable, and still more concentrated milk fat. U p o n very intense agitation of cream the fat particles coalesce, the emulsion breaks, and the fat separates in semi-solid lumps. It is separated from the liquid (called butter-milk) by centrifugation or filtration. Butter is a coarse, consistent, pseudo-emulsion of the W / O type, the consistency depending on the temperature. Molten butter is a true emulsion of the W / O type. The resistance of milk against creaming can be increased by means of homogenisation. This is a process of increased dispersion by means of mechanical treatment. T h e finer the droplets the more resistant they are to creaming. One method of achieving homogenisation would be to force milk through fine capillaries. I n the dairy industry homogenisation is effected b y special machines in which warm milk is pressed through valves with small clearances or by the use of devices like the disk-type colloid mills. The originally polydisperse emulsion is thus made more homogeneous a n d the degree of dispersion is greatly increased. The properties of the protective film which stabilises the fat globules of milk have been studied recently. Washed cream was churned, and the stabilising substances were isolated from the buttermilk (serum) after separation of the fat (butter). It was found that thus prepared buttermilk contained a relatively large amount of a y-globulin, similar to that found in blood serum. This globulin is the protein which forms the protecting membrane around the fat globules in ordinary milk. In the buttermilk from homogenised milk, however, many other protein ( 2 d) This is explained components attached to this globulin were f o u n d . as being due t o the high adsorption capacity of the extremely large surface developed upon homogenisation. In the ordinary milk only the protein is adsorbed o n the globules which is the most lipophilic (i.e. least hydrophilic) ; this then, probably together with some phospholipids, forms the protecting layer. U p o n homogenisation, an extremely large fresh surface area is formed, and everything present in the system and capable of adsorption is bound at these surfaces. Hence, most of the proteins of the milk, such as the caseins, the ß-lactoglobulin, as well as the milk albumin, besides the y-globulin, participate in the stabilisation of the small globules of the homogenised milk. The oleo-margarine industry is now becoming increasingly important. The best sorts of margarine hardly differ from butter either in food value o r in taste, and margarine can be produced a t about one2D
( ) J . R . B R U N N E R , C. W . D U N C A N and G. M . T R O U T ; B R U N N E R et al. ; ibid. 4 6 3 , 4 6 9 .
Food Res.
18, 4 5 4 ( 1 9 5 3 ) ;
422
EMULSIONS
half the price of butter. The basic ingredients are the oils of ground nuts (peanuts), soya beans, cotton seeds, corn seeds, or coconuts. The oils may be hydrogenated and deodorised, and the warm oils are then emulsified in water. Lecithin and monoglycerides are used as emulsify( 3) ing agents and stabilisers. The emulsion, like butter, is of the W / O type. Vitamins and flavouring substances are added to improve the quality. Various emulsions of both the O/W and W / O types are used in pharmacy and in cosmetics. The pharmaceutical emulsions of the O/W type for internal use are usually stabilised by proteins or gums. The lotions, creams and ointments for external use are stabilised by lanoline, ( 4) The skin-penetrating the monoglycerides, soaps, cholesterol, e t c . ' vanishing creams ' are O/W emulsions ; the fatty creams are emulsions of the W/O type. At ordinary temperatures the creams and ointments have the consistency of gels, and in many instances it is rather difficult to classify these systems. Mayonnaise and salad cream are semi-solid emulsions of edible vegetable oils in diluted vinegar or diluted lemon juice. Egg yolk, lecithin, and certain proteins are used as stabilisers in these foods. The emulsions of asphalt are of great practical importance, especially in road construction, as well as for covering roofs, floors, as binders for briquets, or as insulation materials. The principal advantage of applying asphalt emulsions instead of molten asphalt in road surfacing lies in the ease of application. Also important is the fact that the aqueous dispersions of asphaltic bitumen can be spread on moist surfaces. The dispersions are prepared by intense agitation of asphaltic bitumen with dilute alkali, sometimes also by addition of some cheap soap. The alkali reacts with the naphthenic acids of the asphalt, and the naphthenate ions form a protecting layer round the particles. The dispersed asphalt phase usually comprises approximately one half of the total mass of the ( 4 a) dispersion. Very important W / O emulsions are those of natural petroleum. It has been estimated that about one fourth of all the crude oil produced in the U.S.A. is pumped from the ground in the form of a W / O emulsion. The water content varies strongly, but oils containing emulsified 50-70% water are not uncommon, and for further processing of the oil the removal of the water is essential. This is achieved for example by heating, settling and filtration through porous masses, electrical methods, or by addition of chemicals. The stability of these emulsions varies considerably, depending on the composition of the oil, (3
> H. W . V A H L T E I C H in J. A L E X A N D E R ' S Colloid Chemistry, Vol. VII, p. 673 (1950). 4 ( 4)a J. T. D A V I E S ; The Perfumery and Essential Oil Record, Sept.-Oct. 1952. ( ) S. B E R K M A N and G . E G L O F F ; Emulsions and Foams (Reinhold, New York 1941) p. 396 ff.; M. V A N D E R W A A R D E N ; Kolloid-Z. 156, 116 (1958).
EMULSION
423
POLYMERISATION
as well as on the minerals with which the oil and water have been in contact in the particular case. Addition of certain complex organic surface-active agents, combined with heating and settling, is the most ( 4 a 4 b) common method of breaking these emulsions. « Emulsion polymerisation. This is one of the most important branches of the modern industrial chemistry of macromolecular substances. The emulsions are not the final products, but are intermediates in preparing stable dispersions of high polymers obtained from the emulsions. Huge quantities of such dispersions obtained in emulsion polymerisation are manufactured, a n d a large proportion of them are coagulated by acids and salts in order to obtain the polymer as a compact solid. The manufacture of synthetic rubber, chiefly a copolymer of butadiene and styrene (GR-S rubber), is accomplished by means of emulsion polymerisation. Large quantities of the dispersions of polymers obtained in emulsion polymerisation are applied also in liquid form, for example, in paints, and in treatment of paper, as well as for leather finishing and impregnation. The polymerisation reactions of emulsified monomers are much more easy to control than in other types of polymerisations, and this is the chief reason that emulsion polymerisation is so important in industrial chemistry. The basic starting material in emulsion polymerisation is a monomer, i.e. some micromolecular substance capable of polymerising. As examples may be mentioned butadiene, styrene, vinyl acetate, a n d the esters of acrylic a n d methacrylic acids. T h e liquid monomer is emulsified into water by means of a soap, sulphonated castor oil (turkey red oil) or similar agent. The polymerisation is initiated by some peroxide or other agent added in small quantities, and by warming the emulsion a n d stirring it. Since the polymerisation is an exothermic reaction, the activated molecules of the monomer combine spontaneously, and the temperature of the mixture rises. Various substances are added in small quantities in order to regulate the polymerisation rate, a n d to produce dispersions of the polymers with certain exactly ( 5) reproducible properties. The mechanism of the reactions is n o t yet fully explained, a n d indeed it may vary from case to case. As the monomer is usually somewhat soluble in water, some authors have pointed out that the polymerisation occurs n o t in the droplets, b u t in the water phase which ( 6) contains also the peroxide. The monomer used up in the polymerisa4 B
( ) J . C . M O R E L L and G . E G L O F F ;
in Colloid
Chemistry
(J. A L E X A N D E R ,
ed.),
Vol. III (Reinhold, New York 1931) p. 505 ff. to I. M . K O L T H O F F and E . J. M E E H A N ; / . Polymer Sei. 9, 327, 343 (1952). W . P. H O H E N S T E I N and H . M A R K in High Molecular Weight Organic Compounds (Interscience Publishers, New York/London 1949), pp. 1-170. ( Β) E . T R O M M S D O R F F , H . K O H L E and P. L A G A L L Y ; Makromol. Chem. 1, 185 (1948). J. B A X E N D A L E , M . G . E V A N S and J. K . K I L H A M ; / . Polymer Sei. 1, 466 (1946).
424
EMULSIONS
tion in the water phase is then replaced by new portions of the monomer from the droplets. The latter thus decrease in size and number, and a new phase of the solid particles of the polymer is formed. The particles become stabilised by the same stabilising agent which protected the droplets. In the instances of monomers practically insoluble in water the polymerisation occurs on the surfaces of the droplets of the emulsion. The loci for the initiation of polymerisation have been stated, in some instances, to be in the micelles of detergent which also ( 7) functions as the emulsifier. This is in accord with the fact that the molecular weight of the polymer formed in such emulsion polymerisation depends on the soap concentration. 7
( ) W. D . I.
M.
HARKINS;
KOLTHOFF,
A.
J. Amer. Chem. Soc. 69, 1428 (1947); see also: F. A . I.
MEDALIA
and
(Interscience, New York, London 1955).
E.
J.
MEEHAN;
Emulsion
BOVEY,
Polymerisation
EMULSIONS T H E term emulsion refers to any dispersion of one liquid in another. The liquids, of course, must be immiscible. Water is one of the most common components, and the other is usually an oil or some other lipophilic liquid. Cream, rubber latex, and a dispersion of lubricating oil in water are examples of emulsions. Concentration and particle size. The amount of solid substance dispersed in sols, relative to the medium, is small. The concentration of sols is usually only about 1-2%. The amount of one liquid dispersed in another is usually greater, e.g. in cream about 15-20% of fat is dispersed in water. A geometrical calculation shows that the maximum amount of one liquid which can be dispersed in another is 7 4 % of the total available volume, if the droplets are spheres of equal size. It can also be shown that this maximum amount does not depend on the diameter of the droplets. Emulsions even more concentrated than 7 4 % are known. This is explained by the fact that the droplets can be deformed upon close packing, as well as by the possibility of introducing small droplets between large ones. Such a highly concentrated emulsion may have a rather solid consistency. For instance, the creams used in cosmetics are coherent, jelly-like systems. It is sometimes difficult to decide whether such cream or grease is a gel or an emulsion, especially when the system contains more than two components. Thus emulsions of oils in glycerine have a high consistency. Examination of emulsions shows that they contain droplets of diameter 0-001 to 0-050 mm. Hence the emulsions are coarsely disperse systems. The fat droplets in milk, for instance, can be seen in ordinary microscopes. By the use of special emulsification machines and h o m o genisers it is possible to prepare very fine emulsions possessing droplets of diameter only about 0-0001 mm. Mesophases. Liquid crystals. There is a continuity between suspensions and emulsions, and it is often difficult to classify a dispersion into one or the other class. F o r instance, some kinds of natural rubber latex contain rod-shaped particles, which are soft but not liquid. Emulsions of many fats also contain particles of consistency intermediate between solid and liquid. If the particles are composed of long or flat molecules, they may be pseudo-crystalline or mesomorphic, e.g. they may be doubly refracting (optically anisotropic). VORLÄNDER (1908) described about 250 mesomorphic compounds. Examples include emulsions of phenol ether in glycerin, ammonium oleate in alcohol, /7-azoxyanisole in water, and cholesteryl benzoate in water. 414
EMULSIFYING
AGENTS
415
The high molecular fatty acids and soaps often appear in this liquidcrystalline state. The long or flat molecules in the liquid crystals may be oriented in different degrees. For example, a triglyceride in a droplet may not be oriented at all, in which case the droplet is truly liquid ; if the molecules are oriented in groups with a certain regular structure (Fig. 178) the droplets still flow steadily, but they may be anisotropic. This
a b F I G . 178. Fibrous molecules in the nematic state (in a). Asymmetric molecules in the smectic state (in b). partly oriented state is called the nematic state. The bundles of long molecules which are oriented parallel in the nematic state are like micelles which are distributed randomly in the droplet. If they lie in parallel layers, we have a higher degree of orientation, the so-called smectic state. Smectic liquid crystals do not flow but glide. They are composed of series of planes. Examples are ammonium oleate and ( 1) cholesteryl b e n z o a t e . Preparation of emulsions. Emulsifying agents. The simplest method of making an emulsion would be to shake two immiscible liquids together. On shaking an oil or benzene with water the oily liquid can certainly be dispersed in droplets, but the emulsion is not stable ; the droplets quickly flow together again, and the liquids separate into two layers. It has been found that the ease with which two immiscible liquids can be emulsified becomes greater as the differences between their surface tensions and densities become less. However, even by choosing the most favourable cases such emulsions have a poor stability. In order to prepare stable emulsions a third component—an emulsifying agent—must be introduced into the system. The emulsifying agent, in the first instance, may lower the interfacial tension (see Chap. 5), though this is not the only purpose in using it. The lowering of the interfacial tension facilitates the formation of droplets, but it does not necessarily increase their stability. An emulsifying agent must also stabilise the droplets formed, as explained in the following section. Numerous emulsifying agents are known, and they can be classified into several groups. The largest and most significant group is that of to G.
F R I E D E L in J. A L E X A N D E R ' S Colloid Chemistry, Vol. I, p. 102 (1926); R . D . and M. V O L D ; ibid. Vol. V, p. 266 (1944); A . S. C. L A W R E N C E ; Trans. Faraday Soc. 34, 660 (1938).
VOLD
416
EMULSIONS
the soaps and detergents, and other substances of similar chemical structure. The essential feature of the structure of these emulsifiers is the combination of a large lipophilic radical with a polar group in the same molecule. For instance, the sodium stéarate molecule is composed of a long hydrocarbon tail and a polar head of the ionisable carboxyl group. Sodium dodecylbenzene sulphonate molecules contain a long hydrocarbon tail and a polar head of the sulphonate. Lecithin molecules are composed of several lipophilic chains and the ionogenic phosphate and tertiary amine groups. The proteins, agaragar, the pectins and the saponins are further examples of emulsifiers. It is interesting that a number of finely ground insoluble solids such as some clays and certain basic sulphates may also act as emulsifying agents ; they are solids which are easily wettable by one of the liquids. Emulsification in industry is accomplished by means of emulsifying
F I G . 178a. Emulsifying machine (the Charlotte colloid mill). (By courtesy of Chemicolloid Laboratories Inc., Garden City Park, Ν . Υ . )
EMULSIFYING
417
AGENTS
machines which work on the same principle as colloid mills (p. 294). The mixture of the two liquids containing an emulsifying agent may, for example, be forced through fine slots, a n d then be subjected t o further shear when it strikes the wall of the machine. The mixture may be sheared between two grooved disks or funnels, one being static and the other rotating rapidly (see Fig. 178a). The structure of the particles in emulsions. T h e structure of the particles in stable emulsions has been elucidated by the work of LANGMUIR, H A R K I N S , RIDEAL, SCHULMAN a n d o t h e r s .
( 2)
A stable emulsion
is a three-component system composed of two immiscible liquids a n d an emulsifier. T h e latter is usually introduced only in an a m o u n t of i t o 2 % . Let us consider first what happens when an oil is emulsified in water by means of some soap or similar partly lipophilic a n d
F I G . 179. Oil droplet in an emulsion protected by a layer of soap molecules. The lipophilic tails of the soap molecules are immersed in the oil; the polar heads extend to the water phase (dispersion medium).
partly polar compound. The lipophilic part of the soap molecule has a n affinity for the oil, the polar head for the water phase. Hence the soap molecules will be packed on the surface of the dispersed oil droplets as illustrated in Fig. 179. The hydrocarbon tails of the soap molecules will be immersed in the oily droplet, but the polar heads will be turned into the water. Consequently, the droplet will be covered by a more or less dense protecting layer which will hinder the coalescence of the droplets. In the absence of such an emulsifier, which acts as a stabiliser, the droplets will coalesce instantly, because of the tendency to decrease the free surface energy, i.e. because of the cohesive forces acting between the oily surfaces as they come in contact. D r o p lets whose surfaces are coated with soap molecules are, however, electrically charged by the ionised carboxyl groups of the soap. Hence ( 2)
I. L A N G M U I R and W. D. H A R K I N S in J. A L E X A N D E R ' S Colloid
Chemistry,
Vol.
I
(1926). W. C L A Y T O N ; The Theory of Emulsions and Their Technical Treatment, 4th ed. (Churchill, London 1943). J. H . S C H U L M A N and D . P. R I L E Y ; / . Colloid Sei. 3, 383 (1948). E . G . C O C K B A I N ; Trans. Faraday Soc. 48, 185 (1952); P . B E C H E R ; Emulsions: Theory and Practice (Reinhold, New York 1957).
EMULSIONS
418
the droplets repel each other before they collide, and the emulsion is stable. Very stable emulsions can be prepared by using a combination of two or more emulsifying agents. F o r example, while cholesterol o r sodium cetyl sulphate alone are poor emulsifying agents, a mixture of these compounds can be used to form very stable emulsions of oil in water. Cholesterol is t o o soluble in oil, a n d too insoluble in water to be a good emulsifying agent ; moreover, it does n o t contain ionogenic groups. Sodium cetyl sulphate has such groups, but it is relatively insoluble in oil. When both are present, a complex film of cholesterol ( 2 a) and the cetyl sulphate is formed on the surface of the d r o p l e t s . It is also interesting that the short molecules of the unsaturated eis isomers have a weaker emulsifying effect than the long trans isomers For example, the stability of oil emulsions with cholesterol a n d sodium oleate (eis isomer) is poor, but with cholesterol a n d sodium elaidate (trans isomer) the emulsion is quite stable. The thickness of the interfacial film which stabilises the droplets of emulsions has been studied by K R E M N E V
( 2 B)
a n d by C O C K B A I N .
( 2 c)
The
Russian investigators studied chiefly the emulsions of benzene in aqueous solutions of 5 % sodium oleate. The emulsions were characterised by the maximum volume of benzene which could be dispersed in 1 ml of the oleate solution, and the droplets were measured under a microscope. The thickness of the stabilising film was found to be of the order of ΟΌΙ/ι. Similar studies were extended also to other types of emulsions with other emulsifying agents, and in some instances thinner films of 20 Â were found. Cockbain studied the film thickness of H-decane emulsions which were stabilised by serum albumin a n d sodium dodecyl ( 2 c} sulphate. He found that in the absence of dodecyl suphfate the albumin film was monomolecular in thickness (13-5 Â), whereas the thickness of a complex albumin-dodecyl sulphate film was 20-5 Â. The stability of emulsions depends chiefly on the following two factors : (1) thickness and compactness of the protecting film (interfacial layer), and (2) electrical charge on the droplets or film. Moreover, the stability depends on the viscosity of the dispersion medium, a n d on the density difference between the two liquids. The emulsifying power of finely ground powders depends o n h o w they are wetted by the liquids. In the most suitable wetting conditions the droplets are surrounded by a layer of the solid particles which thus prevent coalescence (Fig. 180). 2a
( 2 b) J. Η. S C H U L M A N and E. G. C O C K B A I N ; Trans. Faraday Soc. 36, 651, 661 (1940). ( ) L. Y. K R E M N E V and S. A. S O S K I N ; Zh. Obsch. Chim., Moskva, 16, 2000 (1946); Koll. Zh. 9, 269 (1947); 11, 24 (1949); K R E M N E V and N. K U I B I N A ; Koll. Zh. 13, 38 (1951). 2c ( ) E. G. C O C K B A I N ; / . Colloid Sei. 11, 575 (1956).
EMULSIFYING AGENTS
419
F I G . 180. Stabilisation of a droplet by a layer of solid particles.
Two types of emulsions. Two types of emulsions can be distinguished : the oil in water type (O/W), and the water in oil (W/O) type. The emulsions of the O/W type conduct electricity, the W / O emulsions do not. The former can be diluted with water and coloured with water-soluble dyes, the latter can be diluted with oils and lipophilic liquids only, and they can be coloured with oil-soluble dyes. Two liquids, e.g. oil and water, can give emulsions of either type depending on the emulsifying agent. Ordinary soaps, such as sodium stéarate, will produce emulsions of the O/W type. If, however, a mixture of oil and water is shaken with powdered calcium stéarate, the continuous phase will be oil, with the water dispersed in it. The droplets of water in this example are stabilised by a layer of calcium stéarate with the calcium heads turned into water and the hydrocarbon tails immersed in the oil phase. The water droplets in this type of emulsion are protected from coalescence not by an electrical charge, but by a protective layer of calcium stéarate. Emulsions of this type are also obtained by using certain resins or lamp-black as emulsifying and stabilising agents. The inversion of phases. CLOWES has thoroughly studied emulsions of both types, and he found that one type can easily be converted into the other. F o r example, an emulsion of oil in water, which is stabilised by a sodium soap, can be converted into a W / O emulsion on addition of calcium chloride. The calcium ions combine with the anions of the fatty acid, and the calcium salt of the fatty acid covers and protects the water droplets formed upon shaking the mixture. Inversion of phases is believed to occur also in the complex membranes of living cells. Certain parts of the membrane contain lipids which are dispersed as a stiff emulsion or a labile jelly. Such a membrane will be permeable to water soluble substances if water is the continuous phase. In the presence of calcium ions the phases may be inverted and the film may become impermeable to water soluble substances, but will now be permeable to lipids.
420
EMULSIONS
Creaming and breaking of emulsions. Creaming and breaking of emulsions are processes similar to the coagulation of sols. In creaming the separation of the two liquid phases is incomplete, but in breaking the separation is complete. In creaming the droplets of the dispersed oil separate slowly into a more concentrated layer. This occurs by virtue of the difference in density between the oil and water. The droplets, however, do not coalesce upon such creaming. Creaming can be facilitated by centrifugation, or by increasing the density of the continuous phase by adding a dense substance not interacting with the components, but soluble in the continuous phase. The choice in such substances, however, is very limited. As an example of this, a concentrated salt solution usually breaks the emulsion, but is undesirable for other reasons. Creaming can be prevented or slowed down on adding substances (e.g. glycerin or a solution of gelatin), which increase the viscosity of the continuous phase. The breaking of emulsions is important in certain technical processes in which spontaneous emulsification takes place. For example, when emulsions are not desired they must be broken either to free oils from dispersed water or to recover oils from O/W emulsions. Emulsions may be broken up by physical or by chemical means. The physical methods include heating, centrifugation, electrophoresis, and irradiation with ultrasonic waves. Heating and high frequency sound waves obviously increase the intensity of motion of the droplets, and may thus promote coalescence, although the same methods may also produce emulsification. Centrifugation usually produces only creaming, and the concentrated emulsion must then be treated chemically in order to induce coalescence. Agents producing O/W emulsions will break W / O emulsions and vice versa. The droplets in O/W dispersions are usually negatively charged, and the emulsions are broken upon addition of salts. The W / O emulsions are broken by adding sodium soaps or sulphonated oils. Sometimes the separation of the phases can be achieved by adsorbing the dispersed material on activated carbon or other adsorbents. Emulsions of fats or fatty oils in water can be broken by adding acid. The added hydrogen ions diminish the dissociation of the carboxyl groups on the surfaces of the droplets ; this reduces the charge and the droplets can flow together. Alkalis, on the other hand, are good emulsifying agents for fats and fatty oils. Some practically important emulsions. Milk is one of the most important emulsions. Only hot milk, however, is a true emulsion, containing truly liquid droplets of butter fat. At a temperature below the melting point of butter fat the droplets are solid, or they may be in states intermediate between liquid and solid. The droplets of fat in milk are protected from coalescence by a layer of protein. As the density of the fat is lower than that of the milk serum (containing
SOME IMPORTANT EMULSIONS
421
proteins, sugar and salts dissolved in water), creaming occurs if the milk is allowed t o stand (the separation can be facilitated by means of a centrifuge). The cream is a relatively concentrated emulsion of the O/W type. Butter is a consistent, more stable, and still more concentrated milk fat. U p o n very intense agitation of cream the fat particles coalesce, the emulsion breaks, and the fat separates in semi-solid lumps. It is separated from the liquid (called butter-milk) by centrifugation or filtration. Butter is a coarse, consistent, pseudo-emulsion of the W / O type, the consistency depending on the temperature. Molten butter is a true emulsion of the W / O type. The resistance of milk against creaming can be increased by means of homogenisation. This is a process of increased dispersion by means of mechanical treatment. T h e finer the droplets the more resistant they are to creaming. One method of achieving homogenisation would be to force milk through fine capillaries. I n the dairy industry homogenisation is effected b y special machines in which warm milk is pressed through valves with small clearances or by the use of devices like the disk-type colloid mills. The originally polydisperse emulsion is thus made more homogeneous a n d the degree of dispersion is greatly increased. The properties of the protective film which stabilises the fat globules of milk have been studied recently. Washed cream was churned, and the stabilising substances were isolated from the buttermilk (serum) after separation of the fat (butter). It was found that thus prepared buttermilk contained a relatively large amount of a y-globulin, similar to that found in blood serum. This globulin is the protein which forms the protecting membrane around the fat globules in ordinary milk. In the buttermilk from homogenised milk, however, many other protein ( 2 d) This is explained components attached to this globulin were f o u n d . as being due t o the high adsorption capacity of the extremely large surface developed upon homogenisation. In the ordinary milk only the protein is adsorbed o n the globules which is the most lipophilic (i.e. least hydrophilic) ; this then, probably together with some phospholipids, forms the protecting layer. U p o n homogenisation, an extremely large fresh surface area is formed, and everything present in the system and capable of adsorption is bound at these surfaces. Hence, most of the proteins of the milk, such as the caseins, the ß-lactoglobulin, as well as the milk albumin, besides the y-globulin, participate in the stabilisation of the small globules of the homogenised milk. The oleo-margarine industry is now becoming increasingly important. The best sorts of margarine hardly differ from butter either in food value o r in taste, and margarine can be produced a t about one2D
( ) J . R . B R U N N E R , C. W . D U N C A N and G. M . T R O U T ; B R U N N E R et al. ; ibid. 4 6 3 , 4 6 9 .
Food Res.
18, 4 5 4 ( 1 9 5 3 ) ;
422
EMULSIONS
half the price of butter. The basic ingredients are the oils of ground nuts (peanuts), soya beans, cotton seeds, corn seeds, or coconuts. The oils may be hydrogenated and deodorised, and the warm oils are then emulsified in water. Lecithin and monoglycerides are used as emulsify( 3) ing agents and stabilisers. The emulsion, like butter, is of the W / O type. Vitamins and flavouring substances are added to improve the quality. Various emulsions of both the O/W and W / O types are used in pharmacy and in cosmetics. The pharmaceutical emulsions of the O/W type for internal use are usually stabilised by proteins or gums. The lotions, creams and ointments for external use are stabilised by lanoline, ( 4) The skin-penetrating the monoglycerides, soaps, cholesterol, e t c . ' vanishing creams ' are O/W emulsions ; the fatty creams are emulsions of the W/O type. At ordinary temperatures the creams and ointments have the consistency of gels, and in many instances it is rather difficult to classify these systems. Mayonnaise and salad cream are semi-solid emulsions of edible vegetable oils in diluted vinegar or diluted lemon juice. Egg yolk, lecithin, and certain proteins are used as stabilisers in these foods. The emulsions of asphalt are of great practical importance, especially in road construction, as well as for covering roofs, floors, as binders for briquets, or as insulation materials. The principal advantage of applying asphalt emulsions instead of molten asphalt in road surfacing lies in the ease of application. Also important is the fact that the aqueous dispersions of asphaltic bitumen can be spread on moist surfaces. The dispersions are prepared by intense agitation of asphaltic bitumen with dilute alkali, sometimes also by addition of some cheap soap. The alkali reacts with the naphthenic acids of the asphalt, and the naphthenate ions form a protecting layer round the particles. The dispersed asphalt phase usually comprises approximately one half of the total mass of the ( 4 a) dispersion. Very important W / O emulsions are those of natural petroleum. It has been estimated that about one fourth of all the crude oil produced in the U.S.A. is pumped from the ground in the form of a W / O emulsion. The water content varies strongly, but oils containing emulsified 50-70% water are not uncommon, and for further processing of the oil the removal of the water is essential. This is achieved for example by heating, settling and filtration through porous masses, electrical methods, or by addition of chemicals. The stability of these emulsions varies considerably, depending on the composition of the oil, (3
> H. W . V A H L T E I C H in J. A L E X A N D E R ' S Colloid Chemistry, Vol. VII, p. 673 (1950). 4 ( 4)a J. T. D A V I E S ; The Perfumery and Essential Oil Record, Sept.-Oct. 1952. ( ) S. B E R K M A N and G . E G L O F F ; Emulsions and Foams (Reinhold, New York 1941) p. 396 ff.; M. V A N D E R W A A R D E N ; Kolloid-Z. 156, 116 (1958).
EMULSION
423
POLYMERISATION
as well as on the minerals with which the oil and water have been in contact in the particular case. Addition of certain complex organic surface-active agents, combined with heating and settling, is the most ( 4 a 4 b) common method of breaking these emulsions. « Emulsion polymerisation. This is one of the most important branches of the modern industrial chemistry of macromolecular substances. The emulsions are not the final products, but are intermediates in preparing stable dispersions of high polymers obtained from the emulsions. Huge quantities of such dispersions obtained in emulsion polymerisation are manufactured, a n d a large proportion of them are coagulated by acids and salts in order to obtain the polymer as a compact solid. The manufacture of synthetic rubber, chiefly a copolymer of butadiene and styrene (GR-S rubber), is accomplished by means of emulsion polymerisation. Large quantities of the dispersions of polymers obtained in emulsion polymerisation are applied also in liquid form, for example, in paints, and in treatment of paper, as well as for leather finishing and impregnation. The polymerisation reactions of emulsified monomers are much more easy to control than in other types of polymerisations, and this is the chief reason that emulsion polymerisation is so important in industrial chemistry. The basic starting material in emulsion polymerisation is a monomer, i.e. some micromolecular substance capable of polymerising. As examples may be mentioned butadiene, styrene, vinyl acetate, a n d the esters of acrylic a n d methacrylic acids. T h e liquid monomer is emulsified into water by means of a soap, sulphonated castor oil (turkey red oil) or similar agent. The polymerisation is initiated by some peroxide or other agent added in small quantities, and by warming the emulsion a n d stirring it. Since the polymerisation is an exothermic reaction, the activated molecules of the monomer combine spontaneously, and the temperature of the mixture rises. Various substances are added in small quantities in order to regulate the polymerisation rate, a n d to produce dispersions of the polymers with certain exactly ( 5) reproducible properties. The mechanism of the reactions is n o t yet fully explained, a n d indeed it may vary from case to case. As the monomer is usually somewhat soluble in water, some authors have pointed out that the polymerisation occurs n o t in the droplets, b u t in the water phase which ( 6) contains also the peroxide. The monomer used up in the polymerisa4 B
( ) J . C . M O R E L L and G . E G L O F F ;
in Colloid
Chemistry
(J. A L E X A N D E R ,
ed.),
Vol. III (Reinhold, New York 1931) p. 505 ff. to I. M . K O L T H O F F and E . J. M E E H A N ; / . Polymer Sei. 9, 327, 343 (1952). W . P. H O H E N S T E I N and H . M A R K in High Molecular Weight Organic Compounds (Interscience Publishers, New York/London 1949), pp. 1-170. ( Β) E . T R O M M S D O R F F , H . K O H L E and P. L A G A L L Y ; Makromol. Chem. 1, 185 (1948). J. B A X E N D A L E , M . G . E V A N S and J. K . K I L H A M ; / . Polymer Sei. 1, 466 (1946).
424
EMULSIONS
tion in the water phase is then replaced by new portions of the monomer from the droplets. The latter thus decrease in size and number, and a new phase of the solid particles of the polymer is formed. The particles become stabilised by the same stabilising agent which protected the droplets. In the instances of monomers practically insoluble in water the polymerisation occurs on the surfaces of the droplets of the emulsion. The loci for the initiation of polymerisation have been stated, in some instances, to be in the micelles of detergent which also ( 7) functions as the emulsifier. This is in accord with the fact that the molecular weight of the polymer formed in such emulsion polymerisation depends on the soap concentration. 7
( ) W. D . I.
M.
HARKINS;
KOLTHOFF,
A.
J. Amer. Chem. Soc. 69, 1428 (1947); see also: F. A . I.
MEDALIA
and
(Interscience, New York, London 1955).
E.
J.
MEEHAN;
Emulsion
BOVEY,
Polymerisation