Strength of Emulsifier Films a t LiquidLiquid Interfaces J. A. SERRALLACH, Massachusetts Institute of Technology, Cambridge, Mass., GRINNELLJONES, Harvard University, Cambridge, Mass., AND RANDOLPHJ. OWEX, The E. L. Patch Company, Stoneham, hIass.
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N A PREI’IOUS paper (7i the interface. In the instruA method is described for making quantitatice t h e e a r l i e r investigations ment as supplied by the makers, measurements of the strength of coherent films at on the formation of films t h e p l a t f o r m supporting the the interface between emulsijier solutions and of emulsifiers a t liquid-liquid inbeaker is raised by a thumbvarious oils. Data are given for the first time terfaces and a simple method of screw until the interfacial surface on the strength of such films and their changes on forming such films were deexerts an upward force on the s c r i b e d . Qualitative informaring. After each movement of standing. These data are interpreted f r o m the t i o n w a s given on the characthe thumbscrew which raises the point of view of practical emulsification. This teristics and speed of formation platform, the torsion head which type of measurements, combined with deterof films which appeared a t inmeasures the force is adjusted. minations of particle size, cataphoretic velocity, terfaces between aqueous soluMore definite and reproduchydrogen-ion concentration, and viscosity, hace tions of eight different emulsiible results can be obtained if fiers and four different oils. the platform is raised a t a conhelped in the decelopment of methods of preparing stant slow rate by a suitable In this paper a method of stable and highly dispersed emulsions. m a k i n g quantitative measuremotor-driven mechanism instead of intermittently a n d i r r e g u ments of the strength of coherent interfacial film; is described and new data are shown in larly by the thumbscrew. For this purpose a telechron the form of curves. The method depends upon the use of a motor (Warren Telechron Company, type CM, Model M-43) modified du Kouy interfacial tensiometer which is an instru- was used whose shaft has a constant velocity of four revolument designed for the measurement of the surface tension tions per minute, and which raises or lowers the platform a t a rate of 3.8 mm. per minute by means of the mechanism a t the interface of two liquid phases. This instrument as first described by its inventor in 1919 shown in Figure 1. The hands and attention of the operator (4) was suitable for use only with a liquid-gas interface. An are thus left free to control the torsion head which measures improved model which can also be used for measurements on the force exerted by the rising film on the platinum ring. The platinum ring is delicate and easily bent out of shape, a liquid-liquid interface was reported in 1925 ( 5 ) . Its construction and manipulation are more fully described by the which causes errors. To reshape the ring whenever necessary, the devices shown in Figure 2 were used. The ring manufacturer (3). The instrument consists essentially of a platinum ring of and its supports ( A ) were pressed into two planes a t right known diameter mounted horizontally and held in a sensitive angles to each other by means of two brass angle pieces and mechanism which measures any vertical force exerted on the a brass plate held together by suitable clamps (Ba, Bb). To ring in dynes per centimeter of ring circumference. The insure that the ring was circular, it was slipped over a short films whose strength waB to be measured were prepared in cylindrical steel piece ending in a cone (Ca, Cb) around which multiple by adding a fresh, aqueous solution of the emulsify- it fitted tightly. The conical part had a one-mm. groove starting a t the top into which the supporting wire fitted. The ring was then gently hammered over the conical-cylindrical steel piece until it attained its proper shape. The procedure for making a series of measurements was as follows: The ring, after reshapff,?mJ ffa/y~: ing if necessary, was washed and heated to redR ~ . C ~ L Q W ~ness in a flame to insure cleanliness, and then mounted in the instrument. At the beginning of each day’s measurements the calibration of the scale was checked by adding a known weight to the ring and adjusting if necessary. The zero reading was checked and adjusted if necessary before each measurement. The beaker of oil and FIGURE1. DEVICE TO LIFTINTERFACE AT UNIFOFKM SPEED emulsifier solution was placed on the platform with extreme care to avoid disturbing the intering agent of known concentration to a number of glass jars facial film. The ring was then lowered into the upper layer and then gently adding to each a layer of oil on top of the of oil by the rack and pinion. The ring was unclamped, and solution of the emulsifying agent by means of a pipet, taking the motor started to raise the platform; when the film touched the ring and tended to raise it, the ring was maintained a t its care to avoid mixing. The beakers were then allowed to stand quietly for many proper position (as shown by the indicator) by turning the days to permit the film to form and age, and measurements torsion head. When the film broke, the force per centimeter were made a t intervals. For this purpose a selected beaker was read directly from the vernier on the torsion head. Successive measurements made on duplicate specimens was placed on the platform of the instrument and the platform raised until the platinum ring was about one mm. above prepared simultaneously from the same solutions of emulsifier 816
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and the same sample of oil rarely differed by more than 0.4 dyne per cm. and usually agreed within 0.2 dyne per cm. The temperature during the measurements was 25’ * 2’ C. The variation in temperature may have caused an error of 1 0 . 5 dyne per cm.
PROPERTIES OF FILX AT ACACIASOLUTIONS-OLIVE OIL
INTERFACES CHAXGESOF FILMSTREKGTH WITH TIME. The acacia used in these experiments met the U. S. P. standards (moisture content 12.00 per cent, ash content 3.08 per cent). The olive oil was a good edible grade and had a 0.62 per cent free fatty acid content. BQ
DOTT€D L IN[ SHOWS POSITION **RING.
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FIGURE2.
. 1)EVICE TO ALIGN PLATINUM
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interfacial tension is measured rather than film strength. At the interface the molecules do not remain in their original state, I n most cases they eventually form a coherent film which can be seen with the naked eye during the measurements. These experiments explain why the so-called Continental method of making pharmaceutical emulsions is better than the so-called English method. The Continental method of making pharmaceutical acaciaolive oil emulsions is as follows: Four parts of oil and two parts of acacia are ground to a paste in a mortar. Three parts of water are then added in bulk and a thick creamy nucleus is obtained by further trituration, The nucleus can then be diluted with water a t will. According to Briggs, DuCass6, and Clark (2) the grinding of the solid acacia with the oil insures a uniform suspension of the acacia in oil. As acacia is easily wetted by water, the aqueous phase tends to displace the oil from the surface of the solid particles and so helps in the dispersion of the oil. Roon and Oesper (6) state that “The agitation disperses the internal phase and the resulting droplets are immediately coated with hydrated colloid formed a t the same instant, this coating being the sine qua non of emulsification.” The rapidity of film formation (Figure 3) increases with the concentration of the acacia solution. Therefore, in the preliminary concentrated acacia solution prepared by the Continental method, accumulation of the emulsifier (film formation) will be rapid around the globules as soon as they have been formed by trituration. Thereby their coalescence is prevented. If the emulsifier solution is diluted before the oil is dispersed, film formation will be slow (Figure 3, curve I ) and coalescence of the oil globules will occur before film formation has had a chance to take place.
RIKG
The solution of acacia was prepared by heating water to boiling, stirring in the emulsifier, cooling to 50-55’ C., adding 0.1 per cent benzoic acid to act as preservative, and stirring again. When the solution had cooled, 450 cc. were poured into a glass jar (diameter 7.7 cm., height 9.5 cm.) and immediately 25 cc. of oil were added gently from a pipet. The samples were carefully covered with a screw cap and stored a t room temperature near the tensiometer. The changes in film strength a t the interface of various concentrations of acacia solution and olive oil were determined a t intervals during a period of 9 days. The results are given in Figure 3. The arrows indicate the first of the measurements in which the film showed visible heavy wrinkles when pressure was applied on it with the ring. Each point of the curve was determined by two independent measurements. For each determination a new interface was used. In order to facilitate plotting the results for each case on a single sheet, the horizontal scale was made nonuniform. At the left side each scale division represents 4 hours up to 24 hours, and beyond this each division represents one day. For all concentrations of acacia with olive oil the film strength decreases a t first, passes through a minimum, and then rises. Increasing the concentration of the acacia shortens the time required to give the minimum film strength and decreases the difference between the initial interfacial tension and the minimum value of the film strength. After sufficient time has elapsed for the formation of a coherent film, the film strength increases with the concentration of the acacia. The drop in the interfacial tension immediately after formation of the interface is probably due to the accumulation of emulsifier material a t the interface according to Gibb’s law of adsorption. The higher the concentration of the acacia solution, the quicker this accumulation takes place. In the first moments of the formation of the interface, the
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FIGURE 3. CHANGES IN F I L n i STRENGTH OF ACACIA SOLUTION-OLIVE
OIL INTERF.4CES
The latter procedure is followed in the English method of preparing pharmaceutical emulsions in which a mucilage of the acacia is prepared first (which is more dilute than the nucleus of the Continental method) and the oil is added with remaining water in small portions alternately, triturating after each addition. The resulting emulsions are unsatisfactory, as pointed out by Arny ( I ) . Roon and Oesper also found that unstable emulsions result if the emulsifier is diluted before the addition of the dispersed phase. The authors have confirmed this explanation when emulsifying cog liver oil with malt. The more concentrated the malt sirup, the greater was the dispersion of the oil (even after dilution to the same malt concentration). Some commercial cosmetic and food emulsions are manufactured by emulsifying with a concentrated solution of the emulsifying agent and then diluting with n-ater. CONCEXTRATIOX OF EMULSIFIER.AT INTERFACE. The changes in dry matter content in the various emulsifier solutions in contact with the various oils were determined whenever a film measurement vas carried out. The concentra-
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FIGURE 4. STRENGTH OF FILMS FORMED AT SAPONIN SOLUTION-OIL INTERFACES
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T/ML-DAYS.
FIGURE5 . STRENGTH OF FILMS AT SODIUM OLEATE SOLUTION-OIL INTERFACES
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FIGURE 6. SOLUTION-OIL INTERFACES
tions remained practically the same as a t the start. All results were within the limit of error, either above or below the original figure-that is, *0.02 per cent. This shows that the weight of emulsifier required to form the films is small.
PROPERTIES OF FILMS AT INTERFACES OF VARIOUS EMULSIFIER SOLUTIONS AND VARIOUS OILS CHANGESIN FILMSTRENGTH WITH TIME. I n these experiments the following emulsifiers were used a t the concentrations given in Figures 4 to 11: sodium oleate (Sterling Products Company), saponin, sodium glycocholate (Eastman Kodak Company), triethanolamine (Union Carbide & Carbon Corporation), gelatin (Atlantic Gelatine Company, 225 blooms, pH 5.3), Irish moss (U. S. P.), acacia (U. S. P ), tragacanth (U. S. P.). The interfaces were formed with fresh, cold-pressed, cod liver oil (E. L. Patch Company, free fatty acids 0.5 per cent), heavy mineral oil (Stanolind), olive oil (best market quality, free fatty acids 0.4 per cent), and castor oil. I n these experiments 30 cc. of emulsifier solution and 10 cc. of oil were placed in 50-cc. beakers. Storage was a t room temperature. No preservatives were used in order to obtain results uninfluenced by the presence of any foreign compound. The solutions of the emulsifiers were made by dissolving in hot water. After they had attained rbom temperature in the course of 1to 2 hours, the interface was formed by adding the oil. As in the preceding experiments each interface was measured only once; after that it was discarded. An arrow on the curves (Figures 4 to 11) indicates the fist time that the film became distinctly visible when pressure was exerted upon it with the ring during the measurements.
T / M DAYS. ~.
T//YL- DAYS.
FIGURE7 . STRENGTH OF FILMS AT TRAGACANTH SOLUTION-OIL INTERFACES The form of the curves in Figure 3 (acacia solutions-ohve oil) which were obtained with emulsifier solutions containing benzoic acid as preservative, and that of Figure 6 (olive oil in contact with acacia solutions without any preservative) are similar. We may deduce from this that the curves of the other putrifiable emulsifiers used in this set of experiments would have been approximately the same with preservative as they were without it for the short time they were kept under examination. The outstanding facts about all these curves are that (1)the interfacial relations between emulsifiers and oils are specific; (2) the interfacial tension or film strength is constant in some cases, whereas in others the value changes by many fold in the course of 8 days; (3) the different oils, even when used with the same emulsifier, give wide differences in the initial tension and its changes, and in the film strength. For these reasons an emulsifier that will give a stable and highly dispersed emulsion with one oil may be comparatively ineffective with another. Although each interface has its own specific and characteristic properties, they may be divided into four groups: 1. Those at which the film strength decreases throughout the period of these experiments: triethanolamine (mineral oil, olive oil), Figure 10; and sodium glycocholate (cod liver oil), Figure 11. 2. Those a t which it remains constant after a preliminary drop within the first few days: sodium oleate (mineral oil, olive oil, castor oil at the lower concentration of sodium oleate), Figure 5; triethanolamine (castor oil, cod liver oil), Figure 10; saponin (mineral oil, olive oil, cod liver oil with higher concentrations of saponin), Figure 4; tragacanth (mineral oil), Figure 7; acacia (mineral oil), Figure 6. 3. Those at which the film strength increases slightly: saponin (castor oil), Figure 4; tragacanth (olive oil, castor oil, cod liver oil), Figure 7 ; acacia (olive oil, castor oil), Figure 6; Irish moss (castor oil), Figure 8; sodium glycocholate (castor oil), Figure 11. 4. Those at which the film strength increases considerably: sodium oleate (cod liver oil), Figure 5 ; saponin (cod liver oil with
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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OF FILMS AT IRISH Moss FIGURE8. STRENGTH SOI~UTION-OIL INTERFACES
7/ffE DAYS
T/MF DA YS.
OF FILMS A T TRIETHANOLFIGURE10. STRENGTH AMINE SOLUTION-OIL IXTERFACES
low concentration of saponin), Figure 4; acacia (cod liver oil), Figure 6; gelatin (mineral oil, olive oil, castor oil, cod liver oil), Figure 9; Irish moss (mineral oil, olive oil, cod liver oil), Figure 8; sodium glycocholat,e (mineral oil, olive oil), Figure 11. Gelatin and Irish moss interfaces form films with all four of the oils studied which become stronger with time, some of them considerably. The other emulsifiers give films which, depending on the oil, are constant, increase, or decrease wit’h time. It would be wrong to conclude from these experiments that the efficiency of a n emulsifier depends primarily on the strength of the films which are formed. The preliminary lowering of the interfacial tension and the rapidity of film formation are probab!y the main factors. Any increase of the film strength later will be favorable. A considerable lowering of the interfacial tension will insure a small particle size, and quick film formation will give a n early protection of the globules against coalescence. A medium viscosity and a p H to give the particles a n adequate charge will further contribute to stability. Since it is seldom that one emulsifier alone fulfills all these conditions, it has been found helpful, especially with gums! t o give the oil particles multiple protection by using blends of emulsifiers. A well-known, stable, cod liver oil emulsion on the market contains three emulsifiers (tragacanth, acacia, agar). I n the light of the above results, tragacanth gives a quick film formation (Figure 7 ) ,acacia acts as film strengthener (Figure 6), and agar increases the viscosity. Saponin is a quick film former for cod liver oil and also lowers the interfacial tension (Figure 4). I n countries where saponin is permitted, it is used for pharmaceutical cod liver oil emulsions. A remarkable fact is the low interfacial tension between cod liver oil and sodium glycocholate (Figure 11). Therefore, bile salts are effective in producing a high degree of dis-
T / f f TDA YS.
OF FILMSAT GELATIN FIGURE9. STRENGTH SOLUTION-OIL INTERFACES
J/fl€ DAYS,
FIGURE11. STRENGTH OF FILMSAT SODIUM GLYCOCHOLATE SOLUTIOX-OIL INTERF.4CE
persion of cod liver oil which may account for the easy digestibility of this oil. Some of the films showed remarkable elongation during the measurements. Before the ring broke through, they stretched as much as 8 mm. under the conditions of these experiments. INFLUENCE OF COXCENTRATION. For the emulsifiers which were used in two different concentrations, the film was dist’nctly stronger for the higher concentration in the case of acacia (Figures 3 and 6), but lower for saponin (Figure 4), and sodium oleate (Figure 5). I n the case of tragacanth (Figure 7 , for the concentrations chosen, it made little difference. INFLUENCE OF TEMPERATURE OF STORAGE. The film strength was determined for interfaces kept during storage a t 10” C. The increase of film strength with time was retarded for those emulsifiers with which it would have taken place. LITERATURE CITED (1) Amy, “Principles of Pharmacy,” p. 267, W. B. Saunders, Philadelphia, 1911. (2) Briggs, DuCass6, and Clark, J. Phys. Chem., 24, 147 (1920). (3) Central Sci. Co. (Chicago), Bull. 104. (4) Nouy, du, J . Gen. Physiol.,1, 521 (1919). (5) Ibid., 7, 626 (1925). (6) Roon and Oesper, J. IKD. ESG. CHEM.,9, 156 (1917). (7) Serrallach and Jones, I b i d . , 23, 1016 (1931). RECEIVEDDecember 8, 1932.
CORRECTION. Owing t o an oversight on my part, three figures are wrong in my article on acetic acid and water [IND. ENQ. 25, 569 (1933)]. In Table I1 solution 1 has a mole per CHEM., cent of water in vapor of 59.3 for 100 mm.; solution 3 has a mole per cent of water in vapor of 85.66 at 350 mm., and 84.76 at 200 mm., instead of the figures given. D. B. KEYES
