T H E JOURhTAL OF I N D U S T R I A L A N D E-VGINEERING C H E M I S T R Y
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PERCENTAGE ANALYSESOF M ~ X T T I R E S No. 1 h-0. 2 No. 3 No. 4 W-S Ins. W-S Ins. TV-S Ins. W-S Ins. i l ~ r i l 21st. Immedi-~ ‘ately after mixing.. 6 . 1 5 1.07 8.30 1 . 1 5 10.50 1 . 2 4 2.40 0.83 h l a y 10th . . . . . . . . . . . 3.05 . 1.30 84 1 26 8.66 1.37 0 . 9 9 0 . 9 8 J u n e 1 2 t h . . . . . . . . . . . 3.31 1.39 2:39 1.66 8.15 1.59 1.10 1.11 Sept. 13th . . . . . . . . . . . 3.38 1.46 3.31(a)1.33 8.05 1 46 0.87 1.08 ( a ) This is probably a n error as an anafysis in November, 1913, gave about five per cent water-soluble Mixture
T‘ol. 9! S o . z
A CONTRIBUTION TO THE THEORY OF EMULSIFICATION BASED ON PHARMACEUTICAL PRACTICE’ Ry LEO ROONA N D RALPHE. OESPER
~
Received Xovember 2 2 , 1916
ISTRODUCTORY
The practical knowledge of emulsions, which has I was surprised t h a t more insoluble phosphate been handed down t o the pharmacists from generation was not formed in these mixtures, especially t h e half t o generation, has been surveyed by physical chemists, and half mixture. but this pharmaceutical experience’ has been neglected I n order t o test t h e effect of larger mixtures t h a n almost entirely as an aid in developing a rational theory used by Dr. Brackett and also to t r y the effect of of emulsification. Several theories of emulsification fresher acid phosphate. I made u p some mixtures last have been proposed which differ according t o t h e factor summer a n d used acid phosphate a month old and held responsible for t h e stability of t h e emulsion, but some which h a d been made only a few days and was objection has been raised t o all of them. It is probable warm when mixed. I used ground oyster shells in- t h a t none of these theories is of universal application, stead of ground limestone. and though each may cover special cases or classes TABLE I-MATERIALS USED of emulsions, their inadequacy is generally admitted. Percentages Phosphoric Acid The surface tension theory upheld by Plateau,Z Material Total Insoluble Water-Soluble New acid phosphate.. . , . . , . . . . , , . . . . . 19.33 2.41 15.78 Quincke3 and notably Donnan4 was shown t o be Month-old acid phosphate., . . . . . . . . . , . 18.13 1.04 14.80 Ground oyster shells.. . . , . . . . . . . . . . . , . 0 . 5 1 .... ,.... inadequate when Pickering5 emulsified “solar distilTABLE 11-MIXTURFS A N D RESULTSOF ANALYSES late” with t h e aid of such materials as t h e basic sulAcid Phosphate Used: -OLD--NEW-fptes of iron a n d copper, hydrous ferric. oxide, etc., Date Phosphoric Acid (%) Phosphoric Acid (70) MIXTURE Analyzed Insol. Water-Sol. Insol. Water-Sol. materials which do not influence surface tension. The 190 lbs. Acid Phos.. , . 8/16 ’2.50 15.05 0.82 13.72 viscosity theory, whose origin is obscure, was attacked 14.00 0.24 11.28 10 lbs. Oyster Shells. , 9/20 2.70 Theory 2.32 14.99 1.02 14.06 by Hillyer,G who pointed out t h a t although extreme 0.79 13.38 180 Ibs. Acid Phos.. , . 8/16 2.45 14.50 l.5Q 3.05 20 lbs. Oyster Shells. , 9/20 3.44 7.08 viscosity was an important factor in t h e preparation Theorv 2.22 14.20 0.99 13.32 of some pharmaceutical emulsions, nevertheless, viscous 160Ibs. Acid Phos .... 8/16 2.40 12.15 0.84 11.10 40 Ibs. Oyster Shells.. 9/20 2.99 4.30 1.57 1.05 materials, such as j o per cent glycerine or 6 per cent Theory 2.03 12.62 0.93 11.84 gum solution. are not able t o emulsify kerosene or 1.65 10.08 1.14 10.58 140 lbs. Acid Phos.. , . 8/16 3.26 1 .SO 1.95 0.65 60 Ibs. Oyster Shells., 9/20 cottonseed oil, though dilute, comparatively mobile 0.89 10.36 Theory 1 .84 11 .05 5.55 1.12 7.60 100 lbs. Acid Phos ... . 8/16 1.95 soap solutions do so easily. 2.32 0.30 1.11 0.35 100 lbs. Oyster Shells 9/20 7.89 0.77 7.40 Theory 1.46 While admitting t h e favorable influences of high I n studying this table there are some inconsistencies, viscosity and low surface tension, Pickering7 p u t forth t h e view t h a t the deciding factor is t h e formation of b u t taking t h e results as a whole it is seen: I-That t h e new acid phosphate is acted on more films of insoluble particles around t h e droplets. Donnan* attacked this hypothesis bitterly, b u t BancroftQ readily b y t h e oyster shells t h a n the older. pointed out t h a t t h e criticism was beside t h e point a-That action begins a t once. a n d t h a t t h e theory was not intended t o be a com3-That t h e action continues on standing. plete one. Clowes*o has described experiments in which 4-That t h e greater t h e amount of oyster shells these films were distinctly visible and he attributes t h e greater t h e action. 5-That with 30 a n d jo per cent shells virtually their action in part t o a purely mechanical action in keeping t h e particles from coalescing, in part t o lowerall of t h e water-soluble phosphoric acid disappears. These results, on t h e whole, agree with Dr. Brackett’s, ing of surface tension a n d in part t o repulsive action of t h e electrically charged particles. I intend t o make further analyses of these samples. Fischer” has recently proposed t h a t emulsification I t has generally been considered t h a t water-soluble or mono-calcium phosphate is more valuable t o crops is due t o , or accompanied b y , the formation of a hyt h a n citrate-soluble, or di-calcium phosphate. If dration colloid compound. He says: “ I n reviewing t h a t is t r u e then mixing larger quantities of calcium the empirical instructions for t h e preparation of emulcarbonate with acid phosphate would reduce t h e sions, and in our own attempts t o formulate such as value of t h e phosphate very greatly, b y reducing would always yield permanent results, we were struck the water-soluble phosphoric acid t o the vanishing point with the fact t h a t their production is always associated and also by increasing t h e insoluble phosphoric acid. 1 The work reported in this article constitutes the basis of a thesis -4s t h e fertilizer laws are a t present, fertilizer manu- submitted by Leo Roon to the Faculty of the Graduate School of New University, in part fulfillment of the requirements for the degree of facturers certainly could not afford t o mix much calcium York Master of Science. carbonate with their acid phosphates on account of t h e 9 Pogg. A n n , 14 (1870), 44. 3 117cd .4nn., 35 (ISSS), 589. high insoluble phosphoric acid which would he produced. 4 Z . phys. Chem., 31 (1899). 42. As this question is a very important one t o both 6 J . C h e m . Soc., 9 1 (1907). 200. 6 J . A m . Chem. Soc., 25 (1903), 513. farmers and fertilizer manufacturers, I have thought 7 2. Koll. C h e m . . 7 (1910), 11. i t well t o make public my results in order t o have more a Ibid., 7 (1910), 214. light thrown on t h e subject. 9 J . Phys. Chem., 16 (1912), 504. F. S. ROYSTERGUANOC O M P A N Y NORFOLK, VIRGINIA
10 11
I b i d . , 20 (1916), 415. Science, 43 (1916), 468.
Feb., 1917
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quantities of oil, gum and water. This creamy, viscous nucleus may then be diluted TTith any quantity of water t o form a good emulsion. I n this case all of the e m u l s i f y i n g agent i s hydrated at o n e t i m e a n d irz the preseitce of the internal phase. Remington’ says concerning this method: “This has t h e great merit of never failing t o produce a good emulsion if t h e proper proportions are used t o form t h e nucleus, and if t h e directions are strictly followed.” Text-books and practitioners are divided as t o what may be considered the proper proportions of oil, gum and water for making t h e nucleus. The majority of pharmacists use the materials in the proportion of 4 parts oil, 2 parts acacia, 3 parts mater. Remington and Xrny both recommend 4 parts oil, 2 parts water and I part acacia; t h e National Formulary2 recommends 8 parts oil; 2 parts acacia a n d 3 parts water. I n a n y case, it is evident t h a t t h e proportion of water t o t h e emulsifying agent, acacia, is practically constant. The proportion of 4 parts oil, z parts acacia and 3 parts water is considered most reliable for making t h e nucleus P H A R 11A C E U TIC 4 L E &I U L SI 0 N S and has been used as t h e standard for t h e purposes The preparation of emulsions is a regulxr practice of this research. T o make a good emulsion nucleus, in pharmacy, but the pharmacist, in contrast t o t h e strict adherence t o the mode o j procedure is just as imphysical chemist, recognizes only the oil-in-ivater t y p e portant as taking t h e proper proportions of materials. of emulsions, even though he has, as official pharma- The standard method consists in placing t h e oil (4 copoeial preparations, lanolin, carron oil, etc., which parts) in a dry mortar, adding the acacia ( 2 parts) are of t h e water-in-oil type. These latter preparations, and triturating until a smooth paste is formed. The however, he classes with t h e ointments a n d liniments, water (3 parts) is added, not in portions, b u t all a t respectively. Every pharmaceutical emulsion consists once, when, upon trituration, a thick creamy nucleus of three principal parts: ( I ) t h e liquid t o be emulsified; is formed. This is now perfectly miscible with water ( 1 ) t h e emulsifying agent; (3) water. The liquids in all proportions. used are limited t o fixed a n d volatile oils and other A survey of t h e literature shows t h a t some physical liquids immiscible with water, such as chloroform, chemists also have found t h a t t h e use of certain definite ether, etc. The emulsifying agents most generally proportions of materials leads t o good emulsions. For employed are gum arabic (acacia), gum t-agacanth, example, Gad3 found t h a t when oil containing t h e Irish moss, dextrin. gelatin, extract of malt, yolk of proper percentage of f a t t y acid was placed on t h e suregg. Of these, powdered or granulated acicia is by face of a sodium carbonate solution, a beautiful sponfar t h e best. I n addition t o t h e standard emulsifiers taneous emulsion resulted, and from this he held t h a t mentioned above. rondensed milk, casein, starch and neither shaking nor a n y other mechanical process m s various mixtures of any of those named have been used necessary for t h e formation of a n emulsion. His with more or less success. method consisted in taking a series of watch glasses There are two methods of preparing pharmaceutical containing 0.25 per cent sodium carbonate solution emulsions, t h e English a n d t h e Continental. a n d t h e n placing drops of oil, containing various amounts I-The Eraglish method consists in triturating t h e of free f a t t y acid on t h e surface of t h e solutions in the gum with water t o make a mucilage. and then adding glasses. He observed t h a t t h e oil which contained oil a n d water alternately in small portions, 1 riturating j . j per cent f a t t y acid emulsified spontaneously a t after each addition until the emulsion is completed. room temperature and t h a t t h e oils containing very This method is not considered very satisfactory by little more or less t h a n 5 . 5 per cent yielded incomplete most pharmacists. For example, Arny’ sxates t h a t emulsions. He, therefore, concluded t h a t t h e limits “this method of manufacturing emulsions is slow and of good spontaneous emulsion were not only constant uncertain a n d cannot be compared with t h e Conti- but quite narrow. nental method in simplicity.” I t has been our exRobertson4 has published some interesting results perience t h a t not more t h a n two per cent of t h e average which also serve t o illustrate t h e importance of t h e students of pharmacy are able t o make good emulsions by this method. It is of interest t o note t h a t t h e use of definite proportions of t h e materials for emulsihydratiort of the eniulsifyiiig agent occurs progressively fication. He states: “If one shakes up thoroughly, a n d not all at otie time a n d f o r the greater part is n o t equal parts of slightly alkaline water a n d olive oil, a very stable emulsion is formed in which olive oil accomplished i n the presetice of the interrial phase. I “Practice of Pharmacy,” p. 1155 (5th edition). 2-The Continental method consists in first making 2 “National Formulary,” 1906, p. 46 (3rd edition). what is called t h e emulsion nucleus with certain definite a Arch Anal. Physiol., 1878, 181. with t h e discovery of a method whereby t h e water (or other medium) which is t o act as t h e dispersing agent is all used in t h e formation of a colloid hydration (solvation) compound. I n other words, when i t is said t h a t t h e addition of soap favors t h e formation a n d stabilization of a division of oil in water, i t really means t h a t soap is a hydrophilic colloid, which, with water, forms a colloid hydrate with certain physical characteristics and that t h e oil is divided in this. The resulting mixture cannot, therefore, be looked upon as a subdivision of oil in water, b u t rather as one of oil in a hydrated colloid.” Fischer gave no experimental details of his work a n d though he promised their future publication, neither his method nor results were available t o us. The work described in this paper had been under way for several months before t h e appearance of Fischer’s article, b u t our results a n d conclusions coincide 50 closely with his \.iews, t h a t in t h e absence of his own data, our findings seem t o constitute a t least a partial confirmation of his theory.
1
“Principles of Pharmacy,” 1911, p. 267.
4
Z.Koll.
Chem., 7 (1910). 7.
158
T H E JOC7RN,4L O F I i V D U S T R I A L Ah-D E X G I S E E R I i l - G C H E M I S T R Y
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forms t h e internal phase a n d water t h e external phase; or proportions therein, but t o excess of dilution, and t h a t is, t h e oil is suspended in spherical droplets in a little shaking will quickly rediffuse it. The same t h e water. If we increase t h e proportion of water, thing occurs in milk-the best type of natural emult h e resultant emulsion still continues t o exist as drops sion-in which t h e t r u e emulsion portion separates as of oil suspended in water until t h e proportion of water cream.” t o oil reaches a definite critical value. When this EXPERIMEKTAL critical ratio is reached, t h e character of t h e emulsion The following materials were used in this research: undergoes a n abrupt change. From being a viscid, I. Hexane, specific gravity 0.691. creamy white emulsion of oil in water, i t becomes a 2. Chloroform, specific gravity 1.526. fluid yellow emulsion of water in oil.” Bancroft’ 3. Carbon tetrachloride, specific gravity 1.597. does not agree with Robertson’s conclusion t h a t there 4. Benzene, specific gravity 0.877. is necessarily a critical ratio, with one t y p e of emulsion j. Oil of turpentine rectified. existing below this value a n d t h e other t y p e above i t , 6. Mineral oil, specific gravity 0.8 j3. although he admits t h e possibility of there being a 7. Cottonseed oil, winter yellow, specific gravity given point a t which a n emulsion might become un- 0.921. stable. 8. Squibb’s soft soap. An alcoholic solution conA search of t h e literature revealed very little con- taining jo grams of soap in I O O cc. of solution was cerning quantitative measurements of t h e critical used. proportions of emulsions, nor could we find records 9. Powdered acacia (gum arabic) first selection. of t h e results of varying t h e order in which t h e inI O . Mucilage of acacia, 33 grams of acacia contained gredients of emulsions were added t o each other a n d in I O O cc. of mucilage. our work was undertaken with a view of studying I-ACACIA EMULSIONS several typical emulsions from these standpoints. The results were rather striking a n d we believe throw All t h e acacia emulsions were made b y shaking in some light on t h e process of emulsification. I t is 3 0 cc. test-tubes, except those containing cottonseed probably true t h a t t h e critical values determined are oil and mineral oil, which, because of their high visvalid only for t h e quantities used in our work and apply cosities, h a d t o be made by trituration in a mortar. only t o emulsions prepared by shaking in test tubes, The method a n d order of adding t h e necessary conb u t our basic conclusions are probably of universal stituents, was, however, t h e same in all cases. T h e application. acacia was added t o a n d mixed with I O cc. of t h e inOnly two emulsifying agents, acacia a n d soap, ternal phase; 7. j cc. of water was then added a t once, and were employed. ilcacia (gum arabic) is considered t h e mixture rapidly triturated (cottonseed or mineral b y pharmacists to be t h e best emulsifying agent, oils), or briskly shaken in a test-tube. The quantia n d soap has been mentioned by t h e physical chemists ties of internal phase, gum and water taken were in as t h e most efficient emulsifier. Only those emulsions accord with t h e standard pharmaceutical proportions were dealt with in which water constituted t h e ex- of 4 : 2 : 3, respectively, or in other words, I O cc. ternal phase. As internal phases, substances were of internal phase, 5 g. acacia and 7. 5 cc. of water. T h e employed which represented different types of liquids volumes of internal phase (IO cc.) and of water (7.5 immiscible with water: hexane, chloroform, carbon cc.) were kept constant throughout, b u t t h e amount tetrachloride, benzene, white mineral oil a n d cotton- of acacia was varied a n d t h e effect on t h e condition seed oil. The work dealt with practical emulsions of emulsification noted in order t o determine t h e presrather t h a n with theoretical emulsions such as were ence of a critical point, i. e., t h e minimum quantity which would emulsify t h e given volumes of water a n d prepared b y Lewis? a n d Ellis.3 Those dispersions were considered as good emulsions oil. Such points were observed a n d are indicated in which were uniform throughout in color and consis- t h e results given in Table I. tency, which were perfectly miscible with water, which I t was found on taking I O cc. mineral oil, adding uniformly wetted t h e malls of t h e containing vessel 5 g. acacia,triturating t o a smooth paste and thenadding a n d which did not separate on standing into three water in 0 . 5 cc. portions, t h a t t h e acacia immediately parts-a layer of internal phase; a layer of external absorbed t h e water a n d formed large translucent lumps. phase; with a n interfacial ring of partially emulsified On adding more water, vigorous trituration being internal phase separating t h e two layers. On standing. maintained, t h e mixture grew whiter a n d whiter until most good emulsions cream, or separate into two layers, suddenly a perfectly white, creamy emulsion rea layer of completely emulsified internal phase a n d a sulted. At this point of emulsification, however, layer of external phase. I n this connection, Scoville4 several of t h e rubbery lumps of acacia still remained, says: “Good emulsions often separate into layers showing t h a t j g. acacia were in excess of t h e amount on standing, b u t without showing a n y separated oil. necessary t o emulsify t h e I O cc. of oil. For this An emulsion should not be condemned for this, be- treatment, 11.5 cc. of water, in small portions, h a d been cause t h e separation is due, not t o faulty manipula- used, whereas when 7.5 cc. of water (pharmaceutical tion in making t h e emulsion. or t o improper ingredients ratio 4 : 2 : 3) mas added all at o n c e , a beautiful 1 J . Phys. Chem., 16 (1912). 746. white creamy viscid emulsion was produced. Evi* Z Koll. Chem., 4 (19091, 211. dently then, j g. of acacia are more t h a n sufficient t o 8 Z . g h y s . C h e m , 78 (1911). 352. emulsify I O cc. of oil, a n d 7 . j cc. of water added a t 4 “Art of Compounding,” 1895, p. 82.
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burette. A definite quantity of water was t h e n added from another burette, t h e contents of t h e test-tube suddenly a n d briskly shaken, a n d t h e degree of emulsification noted. T h e quantity of water was varied over a considerable range t o determine t h e possible presence of critical points, i . e . , t h e minimum quantity of water required t o bring about t h e emulsification of t h e given volumes of t h e other ingredients. Such points were found in all cases as is indicated in t h e results, shown i n Table 11. TAFILE II-SOAP SOLUTIONS INTERNAL
PHASE Cottonseed Oil..
.......
Oil of Turpentine..
Mineral Oil.,
........
.............
Mixture H 1 2 3 I 1 2 3 4
’;
3 Benzene . . . . . . . . . . . . . . . . . . K 1 2 3 4 Carbon Tetrachloride. ...... L 1 2 3 Chloroform M 1 2 3 4 H e x a n e . . . . . . . . . . . . . . . . . .N 1 2 3 *Hexane-Carbon Tetrachlo. 0 1 ride 2 3 *Hexane-Mineral Oil.. P 1 2
...............
....
3,.
H (a)
(b) (c)
J (a)
(b)
(6)
K (a) (b)
L M
N
O&P
Water Size of Globules Cc. Microns 2.0 1 andless 1.5 1 and less 1.4 12.0 11.0 10.0 3-10 9.9 Av. 2 2.8 2.7 Av. 2 2.6 4.5 1-2 1-2 4.4 1-2 4.3 ...... 4.2 7.2 1-2 7.1 1-2 7.0 ...... 6.0 2 (approx.) 5.7 2 (approx.) 2 (approx.) 5.6 ...... 5.5 15 (approx.) 7.0 15 (approx.) 6.9 6.8 10.1 10.0 ...... 9.9 ...... 3.0 2.7
...... ...... ...... ......
EMULCritical SION Point Good 1 4 t o Good 1:s cc. None J water
1
......
...... ......
2L.3 .6
...... ...... ...... ......
Good Good Good None
2.5 to 2.6 cc. water
!
*Specific gravity 1.0 (see Special Cases below). SPECIAL CASES Oil 10 CC. plus soap solution 1 cc. plus water in portions of 0 2 cc. emulsifies when a total of 5.6 cc. of water has been added. Compare above. 1.0 cc. soap solution diluted with 1.5 cc. water (critical amount) plus 10 cc. oil gave partial emulsion. Oil 10 cc. soap solution 1.0 cc. plus 5.0 cc. water gave emulsion but not so stable as one made by making Nucleus 2 and diluting i t with 3.5 cc. additional water. Oil 10 cc. plus soap solution 1 cc. plus water in 0.2 cc. portions emulsifies when a total of 6.5 cc. have been added. Oil 10 cc. plus soap solution 1 cc. plus water 5 cc. gave emulsion not so stable as one made by dilutingNucleus 2 with 2.3 cc. additional water. 1 cc. soap solution plus 2.7 cc. water (critical amount) plus 10 cc. oil gave emulsion. This is an exception to the results obtained in all other cases. Benzene 10 cc. plus soap solution 1 cc. plus water in 0.5 cc. portions emulsifies when the total of 10.5 cc. water have been added. 1 cc. soap solution diluted with 4.3 cc. water (critical amount) and 10 cc. benzene added gave no emulsion. 1 cc. soap solutions plus 7.1 cc. water to which 10 cc. carbon tetrachloride were added, gave no emulsion. Observations of this emulsion with high magnification showed that the very small globules contained nuclei closely resembling those in protoplasmic cells. These nuclei were seen to vibrate with a motion, which especially for the smaller globules, resembled very closely the so-called Brownian Movement. The larger globules contained many of these highly refractive particles of varying sizes. T h e larger particles vibrated rather slowly. but the smaller ones darted about very vigorously, often colliding with one another. T h e phenomenon was only observed in the chlorofbrm emulsion. 1.0 cc. soap solution diluted with 6.9 cc. water (critical amount) plus 10 cc. hexane gave no emulsion. A mixture possessing a specific gravity of 1.00 a n d a relatively low viscosity was prepared from 59.7 cc. hexane and 30.9 cc. carbon tetrachloride. Another mixture also of a specific gravity of 1.00, b u t of a relatively high viscosity, was prepared from 59.7 cc mineral oil and 14.7 cc. carbon tetrachloride. These mixtures, of the same density as water, were then treated in the same way as the pure substances had been, t o determine whether emulsibility was in any way dependent upon specific gravity. No relation was found.
Vol. 9, No.
2
I n connection with some other work, i t has been our experience t h a t , in t h e preparation of large quantities of soap emulsions of certain volatile oils, in which definite volumes of soap solution and volatile oil were taken. t h a t t h e addition of a n excess of water t o t h e soap and oil mixture produced much foaming and a comparatively thin emulsion resulted; t h a t the addition of a n insufficient amount of water produced a n incomplete emulsion which streaked down the sides of t h e container; t h a t between these limits of overemulsification a n d incomplete emulsification there existed a point at which perfect emulsification occurred without foaming and streaking. This experience was found useful a n d was employed as a means of judging t h e excellence of t h e emulsions prepared from I O cc. internal phase, I cc. soap solution and varying volumes of water. T h e foaming is plainly indicative of excess of water and t h e non-wetting of t h e sides of t h e container is a sign of incomplete emulsification due t o insufficient water. T h e critical point between t h e incomplete a n d t h e perfect emulsion is strikingly sharp a n d may be determined within less t h a n 0.1 cc. of water. At t h e critical point t h e emulsion seems t o form almost spontaneously, and only one t o two seconds shaking are necessary for complete emulsification. However, those systems which are within a few tenths of a cc. below t h e critical point in water content, will emulsify if allowed t o “age” for about I O minutes before being vigorously shaken. These critical points are applicable only t o emulsions prepared with t h e quantities used in this research. For instance, if 20 cc. of benzene are t o be emulsified with 2 cc. of soap solution, then 2 times 4.3 cc. water (critical volume for I O cc. benzene a n d I cc. soap solution) will not be t h e correct value for emulsification. I n general, as t h e total volume of t h e emulsion t o be formed increases, mTe note a relative decrease in t h e critical volume of water. If t h e water be added in small portions t o t h e mixture of internal phase and soap solution, a larger v o h m e is required t o bring about emulsification. For instance, in Mixture Ha, 5.6 cc. of water must be added in 0 . 2 cc. portions before emulsification occurs, while if I . j cc. of water (critical volume) are added all at once t o t h e mixture of internal phase and soap solution, and t h e whole then shaken, a rich, creamy emulsion is produced. The same fact is shown in Mixture Ja, where 6. j cc. are used instead of 2 . 7 cc. (critical volume); in Mixture Ka, where I O . j cc. are used instead of 4.3 cc. (critical volume); in Mixtures Hc a n d J b , we see t h a t adding more t h a n t h e critical volume of water at orie time a n d then shaking yields a thin, foamy emulsion incomparable in general excellence t o those emulsions prepared by first diluting a comhleted nucleus (made with t h e critical volume of water) t o t h e required volume. Mixtures Hb, Kb and La show t h a t no emulsification results if t h e requisite I cc. of soap solution be first diluted with t h e critical volume of water a n d this diluted (hydrated) solution of soap then shaken with t h e I O cc. of internal phase. This is analogous t o t h e use of mucilage of acacia in acacia emulsions. Hence
