The Theory of Emulsification, III - The Journal of Physical Chemistry

T H E THEORY O F EMULSIFICATION.

I11

BY WILDER D. BANCROFT

Pickering’ has published an experimental study on emulsions which seems to me so important that I quote it in full : In the Sixth Report of the Woburn Experimental Fruit Farm (Eyre and Spottiswoode, 1906) were published the results of an examination of emulsions of paraffin oil in solutions of soft soap, such as are used for insecticidal purposes; this examination has now been extended with the double object of obtaining an emulsifying agent which would, for practical purposes, not be open to the objections presented by those containing soap, and also of elucidating the nature of emulsification. The subject had already been investigated by RarnsdeqL but his work, unfortunately, did not come under the notice of the writer until that here described had been completed. It is satisfactory to find, however, that Ramsden, pursuing a different line of enquiry, should have arrived a t an explanation of emulsification which is essentially the same as that given here. I‘

Emulsions with Soluble Emulsifiers “When any liquid insoluble in water is mixed with the latter and broken up into minute globules, as when it is forced through a syringe, the globules remain more or less permanently emulsified whenever the water contains soap or some other emulsifying agent in solution. The whole of the liquid, however, is not an emulsion, properly so called, but a mixture of the emulsion with excess of soap solution; on standing, the true emulsion separates, and, if the substance emulsified is lighter than water, rises to the surface. This is precisely analogous to the separation of cream from milk. “The time required for an emulsion t o rise, and for the Jour. Chem. SOC.,91, 2001 (1907). Proc. Roy. SOC.,72, 156 (1903).

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Wilder D . Bancroft

volume occupied by it to become constant, depends, naturally, on various circumstances, and varies between several days and many weeks. The line of demarcation between the emulsion and the excess of liquid is generally well marked, for the liquid becomes almost clear, and, if it contains much soap, quite clear, while the emulsion is opaque. In many cases, the emulsion itself separates into two layers, evidently of different composition, the line of demarcation between these being fairly distinct. " A rise of temperature facilitates emulsification with soap solutions, but there appears to be no difference between emulsions of the same composition when made at different temperatures. For each temperature, and for each proportion of paraffin oil, the quantity of emulsifier which must be present ranges between certain definite limits; thus, to emulsify 75 volumes of oil with 25 volumes of water, the latter must contain from about 0.7-1.8 percent of potash soap; but for 40 volumes of oil to 60 of water, emulsification is possible with from 0.3 to over 25 percent of soap. The explanation of the existence of such limits (especially of the superior limit) will be discussed below, but i t may be remarked that the above figures apply to only one particular soap, and that soaps, even when of the same brand, vary greatly in their emulsifying power, i t being difficult to obtain emulsification at all in some cases, even with the aid of heat. '' Emulsification appears to occur suddenly : when soap is used, and the solution is not very strong, it is possible t o judge immediately by the working of the syringe whether it has occurred or not; but when it has not occurred, no continuance of the pumping, without altering other conditions, will produce emulsification. The increase of viscosity on emulsification is easily explained by the great increase in the extent of the oil-water surface on the oil being reduced to minute globules. An ordinary emulsion, although containing 70-80 percent of mobile paraffin oil, is as viscid as thick cream, and its viscidity increases with the proportion of oil present, even when that oil is a very light one, such as a

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benzine. With very high percentages of oil, the emulsion becomes practically solid, resembling a blancmange. Emulsions containing as much as 99 percent of ordinary lighting oil have been made, the remaining I percent being a I percent solution of soap. Such strong emulsions, however, cannot be obtained directly; they must be made by taking a weaker emulsion, and gradually increasing the paraffin in it, churning it after each addition. Even these strongest emulsions are true emulsions of the oil; that is, it is the oil which is in globules, and the water, although so small in amount, which is the menstruum. There are no signs of the existence of an emulsion of water in oil. As a consequence of this, an emulsipn (even a 99 percent one) will mix perfectly with water to form a weaker emulsion, but it will not mix with more paraffin,' unless, of course, that paraffin is emulsified with it by further churning. Indeed, the addition of any paraffin in bulk to an emulsion eventually causes the de-emulsification of the whole, the globules of the emulsion being attracted by, and coalescing with, the mass of unemulsified paraffin. It is for this reason that an imperfectly made emulsion will eventually de-emulsify on standing, and so will a very strong emulsion, for it is generally an imperfect emulsion also, as it is so viscid that the whole of the paraffin in it cannot be churned up with it properly. Spontaneous de-emulsification is usually a question of weeks or months, sometimes of years; possibly a very perfect emulsion would never de-emulsify spontaneously at all, but once de-emulsification has started, it progresses a t an increasing rate. " Strong emulsions, especially the semi-solid ones, are very bad conductors of heat. When one of them is heated in a wide test tube, it partially melts, de-emulsifies, and boils; but, although the boiling point may be 1 6 0 ~ - 1 7 0 ~ebullition , can be continued for five or ten minutes, while a nucleus of the emulsion remains at a temperature far below I O O O in the center of the boiling paraffin. The water becomes [Kerosene.]

Wilder D . Bancroft

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gradually concentrated in this nucleus, and finally volatilizes explosively. Although one of these semi-solid emulsions, when kept in a closed vessel, will only gradually de-emulsify in the course of months, yet, when exposed to dry air, it will do so in a very short time; the edges of the lumps assume the appearance of a transparent, slightly blue jelly, and the whole liquefies in the course of some hours. No such change occurs in moist air. It is evident, therefore, that the change to the transparent condition must be due t o the loss of moisture, but it is difficult t o see in what the change consists. The opaque emulsion appears to contain a few globules of about 0.0003 mm in diameter, but the bulk of it seems to be devoid of structure, and, if composed of globules, as presumably it is, these must be less than 0.0001 mm in diameter. The transparent jelly presents nothing a t all visible under the microscope. Thus the difference between them seems to consist in the presence or absence of a few larger globules, but why these should disappear throughout the mass on exposure to air is not a t all clear. The transparent jelly is as stable as the opaque emulsion, when kept in a closed vessel. On placing a portion of it in water it instantly becomes opaque, and gradually disseminates in the water, forming a weak, milky emulsion. '' In a collection of closed-packed spheres of uniform size, whatever that size may be, the volume of the spheres is 74.048 percent of the total volume of the mass; withan emulsion of uniform spheres, the volume of the oil would be less than this, as the spheres are not in actual contact, the separating film of medium being of appreciable thickness. But in emulsions, the spheres are not uniform, and there does not therefore appear to be any reason why the ratio of the volume of the oil to the total volume of emulsion should be of any particular magnitude. The ratios actually found, however, do not differ yery widely from that for close-packed uniform spheres, being generally from 65-82 for every IOO volumes of emulsion. This is independent of the proportions 'I

,

The Theory of Emulsi,fication

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in which the oil and solution are taken to start with, so long as the former does not exceed 80 percent. “ I n a number of experiments which were quoted in the Woburn report, the proportions for the oil to water composing emulsions were found to be in the simple ratios of 2 : I , 3 : I , or 4 : I . It was not possible to suggest any explanation of this, and the results (although similar ones have been obtained subsequently) were probably fortuitous. This seems to be established by the fact that these emulsions vary more or less in composition throughout the mass. The nature of this variation will be seen best by an inspection of Fig. I . Three emulsions were made by churning a I percent soap solution with ‘Solar Distillate, ’ a paraffin oil distilling almost entirely between 240’ and 350°, sufficient to make mixtures containing, respectively, 67, 50 and 33 percent by volume of the oil. These were left undisturbed for twelve weeks, after which the emulsions, which had risen to the top, contained, on the average, 81.0, 81.9 and 77.7 percent of oil, in the three cases, respectively. The emulsions were analyzed from top to bottom, removing for the purpose successive layers, and analyzing each by adding to it a trace of acid, which de-emulsified it. The percentage (by volume) of oil diminishes throughout from the top of the column down-

go

-.0

80 c

t

a,

70

$1 100 (top)

80

40 Position

60

20

0 (bottomj

Composition of emulsions. Solar distillate in soap solutions.

Fig. I

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Wilder D . Bancroft

wards, but very slowly at first, and more rapidly towards the bottom. The examination cannot well be extended to the very bottom of the column, as it is difficult to remove the emulsion without drawing with it some of the watery liquid from below. The three emulsions, as will be seen, do not differ much in composition, in spite of the different proportions of the oil and solution taken in their manufacture, and such differences as exist do not vary regularly with these proportions. I n none of these cases were there visible any different zones, such as zre often noticed, but the analysis gives some evidence of the existence of one in the case of the curve marked 67, for the first three portions removed, embracing a quarter of the whole emulsion, gave the same values, namely, 84.7, 84.5 and 84.6 percent. A more conspicuous instance of the arrangement in zones of equal strength will be mentioned below. “The emulsification of 50 percent of paraffin was repeated with a 5 percent, and with a 0 . 2 percent solution of soap as well. The results with the former, when plotted, were almost coincident with those with the I percent solution shown in the figure (marked 5 0 ) ; those with the 0 . 2 percent solution gave lower values throughout, and are represented by the lowest curve in the diagram. ‘‘ Differences, such as those exhibited between the emulsions portrayed in Fig. I , are, no doubt, merely due t o differences in the size of the globules, and must be largely accidental. An examination under the microscope was sufficient t o indicate that the average size of the globules in the emulsion giving the lowest curve was somewhat greater than t h a t in the others, although no differentiation of these others among themselves was possible. “All emulsions contain globules of different sizes, and it is possible to form only a very rough estimate as to what is the average, or, rather, most frequent, size of the globules present. With most paraffin oils emulsified in soap solutions containing from 5-0.5 percent of soap, the most fi-equent size seems t o be about 0.0075 mm in diameter, b u t there‘are present some


481

globules of three of four times this size, as well as smaller ones down to one-tenth of it, or less. I n the semi-solid emulsions, as already stated, the globules are much smaller. Imperfect churning of the mixture results in larger globules, but a dozen vigorous strokes of a syringe fitted with a rose jet seems to be sufficient to break up the oil as far as i t is possible. A deficiency in the proportion of soap to the paraffin results in larger globules, and, if reduced too far, no permanent emulsification can be obtained. “The character of the oil, also, has an effect on the size of the globules; emulsions with the lighter paraffins, such as the various benzines and motor spirits (sp. gr. 0,8) or ‘Solar Distillates’ (sp. gr. 0.86) ; but with mineral naphtha and benzene, the reverse is the case, emulsions with these being exceptionally fine-grained. With crude petroleum, sp. gr. 0.955 (which contains much Vaseline), a very thick, unmanageable emulsion is obtained. Turpentine and olive oil do not emulsify with soap solution in the same way as do the paraffin oils, a large portion of the material remains unemulsified, and the line of demarcation between the layers of different composition is not clearly marked. With turpentine, that portion of the mixture which most resembles an emulsion contains only 5-6 percent of the oil, but with olive oil there is a small layer, intermediate between the free oil on the top and the solution a t the bottom, yhich appears to be a true emulsion, as it contains about 7 0 percent of oil. “Compared with the oil globules in cream, paraffin emulsions are more coarse-grained, and much less uniform. I n cream, the globules are 0.003-0.005 mm in diameter. They are, however, collected together in groups, and are not distributed uniformly throughout the medium as in the case of paraffin emulsions. “So long as thorough emulsification has been produced, there does not appear t o bemarked variations in the size of the globules of an emulsion throughout the mass; the only difference observable between the upper and lower layers is

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Wilder D . Bancroft

that in the former the globules are more closely packed than in the latter. " Other soluble substances, besides soap, will act as emulsifiers for paraffin, and some of them are used practically in America. Sour milk and scorched flour are among the latter, and the author has also examined the results obtained with solutions of glue, starch, egg-albumen, saponin and extract of quillaia bark. All these substances give emulsions more or less closely resembling those with soap, but they all appear to be inferior to the latter as regards the fineness of texture of the product, and, consequently, the emulsions separate from the excess of liquid more rapidly. With saponin and quillaia, this is notably so, and with proportions of these materials which have been recommended for use (0.025 percent of quillaia), emulsification is so unsatisfactory that de-emulsification sets in after twenty-four hours, while, if the proportion of paraffin taken for the mixture is large (67 percent), only partial emulsification is obtainable. With some of the other substances mentioned above, for example, milk, it is not possible to obtain a strong emulsion unless the paraffin is added gradually, churning after each addition.

De-emulsi Acation ( ' The de-emulsification of an emulsification may be brought about, ( I ) by the presence of unemulsified paraffin, as already mentioned; ( 2 ) by adding a liquid, such as alcohol, in which the baraffin and water are both soluble; or (3) by destroying the emulsifier. The most convenient de-emulsifier to use when determining the percentage of oil in an emulsion made with soap is a small quantity of mineral acid. With a very strong semi-solid emulsion, a mere trace of hydrochloric acid is sufficient to convert it almost instantly into a mass of liquid oil, with a few drops of water below it. Any sodium salt produces de-emulsification, owing to the conversion of the potassium soap into the comparatively insoluble sodium soap. De-emulsifying action does not appear t o be in any way dependent on the de-emulsifying agent

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being an electrolyte, for potassium salts are without effect on emulsions in potassium soap, and even hydrochloric acid will not produce de-emulsification (as will be shown immediately) in cases when the emulsifying agent is not destroyed by it. ( ' I n the Woburn report, it was shown that a paraffin emulsion containing 2 percent of caustic soda possessed valuable insecticidal and detergent properties, but the practical de-emulsification produced by soda detracted from the merits of the mixture, making constant agitation of it necessary during use, and rendering a uniform distribution difficult. The amount of paraffin recommended for this wash was six volumes percent, and the soap was reduced to 0.5 percent, so as to minimize the bulk of solid thrown out when the soda was added. From such a mixture, the soda soap generally separates as a flocculent mass, which rises to the surface, mixed with the paraffin, and forms with it a bulky scum which occupies about one-third of the total volume. This scum, as will be shown below, is what may be termed a quasi-emulsion. It varies considerable in its nature according to the character of the soap used, and with one example of soap in our possession (although of the same brand as other samples) i t is scarcely distinguishable from a true emulsion. (Caustic potash, it may be mentioned, cannot well be used as a substitute for caustic soda, on account of its price, and, if much soap is present, the soap is gelatinized, forming a magma which cannot be used for spraying.) " To avoid the disadvantage attaching to the separation of soap in these caustic emulsions, some more suitable emulsifier was sought. When quillaia or glue was used, the addition of caustic soda produced de-emulsification, and the same occurred t o a slight extent with egg-albumen or starch; milk, also, is not very satisfactory, as it clots, even when caustic soda is not present. Attention therefore was turned to the possibility of obtaining an insoluble substance to act as an emulsifier. This did not seem impracticable, from the fact that lime had recently been stated to produce emulsification.

Wilder D .Bancroft

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Emulsions with Insoluble Emulsiflers Lime united with petroleum was first used for spraying purposes under the name of ‘limoid.’ A certain magnesian lime of a very”fine-grained character was found t o absorb double its weight of petroleum, and the mixture thus obtained, disseminated in three to nine times its volume water, was used as a spray fluid. Subsequently it was found that any ordinary lime would make an emulsion with petroleum, and the proportions recommended in America are 2 . 2 grams of quicklime to I I cc of kerosene and 88 cc of water. There does not appear, however, to be any reason for adhering to these particular proportions, for they may be varied largely without materially affecting the results. “Much work was done on these lime emulsions before a clue was obtained as to their real nature, but only a brief reference to this work will be necessary here. Limes from various sources were examined, and all gave substantially the same results as a calcium oxide prepared from marble, to which the following remarks apply. “ O n churning up lime with paraffin and wa.ter, a sort of emulsion is obtained, which either rises or sinks in the excess of liquid, according to the proportions used. The separation of the emulsion from the excess of water is much more rapid than in the case of emulsions in soap solutions. By suitably adjusting the proportions, an emulsion of a specific gravity of unity can be obtained; there is then no sinking or rising, so long as the proportion of water present is not more than 5-70 percent, but, on dilution, part of the emulsion rises and part sinks; both these parts contain some oil, but the upper part most. “The volume occupied by emulsions with lime, and the percentage of oil contained in them, present no features analogous t o emulsions in soap solutions, for here the lime itself occupies much of the total volume, and by its weight it drags down the oil and prevents any close-packing of the oil globules. “The general behavior of these lime emulsions suggests ‘ I


1

485

that they are merely gross mechanical mixtures of lime and paraffin, and not true emulsions. An examination under the microscope confirms this view; the oil globules present in them are very large, and show great variations in size, being, under the most favorable conditions, ten times the diameter of the globules in a soap emulsion: they appear to be merely caught by, and entangled in, the particles of lime. Moreover, their size, instead of being practically constant, independent of the proportions of emulsifier and of oil, as in the case of soap emulsions, seems to be entirely dependent on them, and, if the lime is not present in considerable quantity, the globules are so large that they become visible to the naked eye, the liquid then being an evidently non-homogeneous mixture, which can claim no title to be called an emulsion. With a further reduction in the amount of lime, or an increase in that of oil, a separate layer of the latter appears. “ I n one series with ‘Water White’ oil in proportions ranging from 1.5-67 percent by volume, and with lime equal to 2 . 2 grams CaO per IOO cc of the mixture, the product became visibly non-homogeneous when about 35 percent of paraffin was reached. A rather lower limit (about 30 percent) was obtained in a similar series with ‘Solar Distillate.’ I n another series with the latter oil, where the paraffin was kept constant a t 50 percent, and the amount of lime varies, an apparently homogeneous emulsion was not obtained until the lime reached I O grams per IOO cc of the mixture. With a very light oil, such as motor spirit, no homogeneous emulsion could be obtained a t all, even when the percentage of lime exceeded that of the oil. Other finely-divided substances have been examined, and found t o give results similar to those with lime, although minor peculiarities are noticeable in the case of each substance ; plaster of Paris, precipitated chalk, precipitated silica and alumina were among those which more closely resembled lime in their behavior. Mechanical mixtures of such substances with paraffin may conveniently be termed quasi-emulsions, for, with certain I‘

I‘

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Wilder D . Bancroft

proportions between the solid and the paraffin, they approximate to emulsions in character, and may serve the same practical purposes as true emulsions do. Yet, as will be shown immediately, they are essentially different in constitution from true emulsions. “ A quasi-emulsion with lime may be mixed with caustic soda without any change being produced, beyond rendering it somewhat more viscid, and it can be mixed with other insecticides and fungicides, notably with copper sulphate, thus giving it an advantage over emulsions with soap. On the other hand, it presents decided disadvantages, in the coarseness of the emulsion produced, the large proportion of lime required, and the difficulty of spraying it without clogging or destroying the nozzles of the machines. “When copper sulphate was added to the lime before emulsification, it was noticed that the emulsion produced was altogether different from that with lime alone, and, following up this indication, it was ascertained that the basic sulphate of copper, precipitated by the action of lime on the copper sulphate, is, by itself, capable of emulsifying paraffin and water, almost as finely as, and much more easily than, ’ soap. If excess of lime is to be avoided, and all the copper is to be precipitated, the basic sulphate should be obtained by adding about 134 cc of lime-water to every gram of CuS0,.5H20; if otherwise, solid lime may be added to the copper sulphate solution, and, although the quantity of pure lime required is only one-seventh of the weight of CuS0,.5H20, it is advisable in practice to increase this to one-third if commercial lime is used, and to make this into a milk, running i t through sacking into the copper sulphate, so as to eliminate any gross particles. On adding paraffin oil to this, and churning, one stroke of the syringe is sufficient to produce a perfect emulsion, and even shaking the mixture will form an emulsion good enough for practicable purposes. ‘‘ Such an emulsion possesses nearly all the characteristics of one made with soap, except that, there being a heavy and bulky solid present, the volume occupied by the emulsion

’

The Theory of Emulsificatiolz

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487

proper does not depend solely on the oil globules, and the emulsion may rise or sink in the excess of solution according to the proportions of oil and basic sulphate used. I t generally forms a very smooth cream, with a perfectly clear-cut line of division between it and the excess of liquid. In texture, it appears under the microscope to be identical with emulsions in soap, although the average size of the component globules is about twice as great. Nothing but a mass of oil globules can be seen, as the particles of basic sulphate are generally too small to be visible. If enough of the basic sulphate is present, the proportion of paraffin can be increased up to a very high point, the emulsions becoming thicker, until they are almost solid. With an insufficient proportion of basic sulphate, emulsification is imperfect ; the minimum with strong emulsions of ‘ Solar Distillate ’ is, approximately, I gram of CuS04.5H,0, converted into basic sulphate, for every 1 2 0 cc of oil; if the emulsion is weak, the proportion of basic sulphate required is less. In the case of the sulphate of iron, which, as will be mentioned immediately, may be used in the same way, the proportions are of the same order of magnitude, I gram of FeS04.7H,0 being required to emulsify about 2 0 0 cc of oil to make a 6 percent emulsion, IOO cc t o make a 2 0 percent efnulsion, and 30 cc to make a 60 percent emulsion. “TO obtain satisfactory emulsions with the basic sulphates, it seems necessary to use a paraffin oil of high boiling point, such as a ‘Solar Distillate;’ with ordinary lighting oil, emulsification is not perfect, unless the proportion of basic sulphate is very large, and with motor spirit the paraffin is never more than partially emulsified. For practical purposes, an emulsion with basic copper sulphate presents many advantages over one made with soap. I n - t h e first place, it is a fungicide as well as an insecticide, for it contains all the elements of Bordeaux mixture; secondly, it is very bulky, and the tendency to separate from the liquid is much less. If the copper sulphate taken’ is 1.6 grams t o IOO cc (the quantity used in ‘ normal ’ Bordeaux mixture made ‘ I

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Wilder D. Bancroft

with milk of lime), 3.5 cc of ‘Solar Distillate’ (sp. gr. 0.858) makes with it an emulsion which neither sinks or rises, and if the ‘Solar Distillate’ is increased to 6 cc, as it might be for winter use, or reduced to 1-2 cc for summer use, the tendency to rise or sink, respectively; would be but small, the slightest agitation being sufficient t o keep such emulsions disseminated throughout the liquid. With Bordeaux mixture made with lime-water, as recommended in the preceding communication, only 0 . 7 gram of copper sulphate to every 100 cc is used, and I .4 cc of ‘ Solar Distillate ’ will make this into an eniulsion which will neither sink or rise, and which is of a most convenient strength for general summer use. “Besides the facility and certainty with which these emulsions can be made, they possess the advantage of being much more permanent than emulsions made with s’oap. No single instance has yet occurred in which any one of them has de-emulsified spontaneously. Also, they can be mixed with caustic soda (2 percent being generally used) to make a detergent winter wash, without becoming de-emulsified, as in the case of soap emulsions. “ I n connection with the use of soda with these emulsions, one or two points must be mentioned. Copper hydroxide will not emulsify paraffin as the basic sulphate does, and, although caustic soda precipitates the basic sulphate ~ C U O S O , ,when added in the proper proportions, excess of alkali converts it into the hydroxide, so that soda cannot be used for the preparation of caustic copper emulsion. But if the 4Cu0.SO3 is precipitated by lime, and then emulsified, the excess of soda may be added without producing any appreciable change; it is only after the lapse of a month or more that an emulsion made in this way will show any signs of the presence of copper hydroxide, and begin to blacken. The, difference in behavior, according t o which alkali is used for the precipitation, naturally caused much perplexity, until the investigation of the basic sulphates, detailed in a previous communication, had shown that the precipitate with lime is Pickering: Jour. Chem. SOC.,91, 1997 (1907).


489

not merely ~ C U O S O , but , a compound of that sulphate with calcium sulphate. “ I f fungicidal properties are not required in the caustic wash, ferrous sulphate may be substituted for the more expensive copper sulphate. The emulsions with the iron salt have certain advantages over those with the copper salt. The oil globules in them are smaller (about one-third to onefifth of the diameter) and the emulsions are more bulky, and separate from the excess of liquid more slowly. Moreover, ferrous hydroxide, unlike copper hydroxide, is itself capable of emulsifying, and hence the precipitation with lime may be dispensed with and the caustic soda alone be used. A much finer emulsion, however, is obtained if lime is used as the precipitant, as in the case of copper. Where soda is used, t h a t required for the precipitation, as well as the extra z percent, may all be added to the iron sulphate at once before emulsifying with paraffin. The basic ferrous sulphate, or ferrous hydroxide, as the case may be, is, of course, partially oxidized, and the emulsion becomes nearly black. On keeping it in a closed vessel, i t sometimes (especially if excess of lime is present) becomes almost colorless, organic substances in the paraffin apparently reducing the ferric compounds; on exposure t o air, it becomes orange-red, ferroso-ferric oxide being formed. These changes do not appear to affect the emulsification. One drawback to the copper emulsion is that it cannot be prepared in a concentrated form like a soap emulsion. The percentage of oil present may be increased, but not that of the copper, for lime must be used as the precipitant, and, if this is added t o a strong solution of copper sulphate, the precipitated basic sulphate forms a compact mass which will not emulsify the oil properly. With ferrous sulphate, however, where soda can be used as the precipitant, this difficulty does not exist, and it is possible to make an emulsion of which IOO cc contain 60 cc of oil, this being ten times the strength generally required for spraying. The amount of iron which can be got into such a strong emulsion, however, is not large,

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and, consequently, the emulsion is light, and rises rapidly t o the surface when diluted, necessitating agitation during use. " Some of the copper emulsions were examined by analysis in the same way as soap emulsions. It will be sufficient to quote the results with one of them only, and these are given in the accompanying table and in Fig. 2 . The mixture taken

50

-.0

40

rc

0 e:

k 30

0

h 20

100

60P)

80

60

40

Position

Composition of emulsions.

20

0 (bo t to m)

Solar distillate in soap solutions. Fig. 2

for emulsification contained 2 0 cc of ' Solar Distillate ' and 0.18gram of CuO, in the form of sulphate, per IOO cc. Portions of the emulsion proper were analyzed throughout their mass after twenty-four hours, ten days, and twelve weeks, the average proportions of oil in them then being 33.3, 38.0 and 42.6 percent, respectively. The emulsion becomes more concentrated as time elapses, and, a t the same time, the curve representing its composition becomes flatter, although in all cases there is a rapid fall in concentration towards the bottom of the column. The most marked feature, however, is the ultimate arrangement of the emulsion into layers of uniform composition, which, although not visible as separate layers t o the eye, are made evident by the analysis. Such arrangements have already been noticed in the case of emulsions with soap, As will be seen from the table, the percentage of basic


491

sulphate in the emulsion decreases from the top to the bottom with the' percentage of oil, but not so rapidly as the latter, COMPOSITION O F

EMULSIONS OF

__

. .

~

After 24 hours

"

SOLAR

DISTILLATE "

100-83 83-66 66-50 50-34 34-19 19-7

After

IO

-

'ercenl Position oil

36.8 36.0 35.4 33.5 30. I 22.8

-

-

-

-

-

BASICC O P P E R

___

days ___

~

Position

WITH

SULPHATE

I 00-85

85-69 69-53 53-37 3 7-22 2 2-8

-

Percen oil

Position

40. I 39.9 39.4 38.7 36.7 26.4

100-93 93-86 86-79 79.72 72-66 66-60 60-53 53-47 47-39 39-31 31-25

-

25-22 22-17

17-11

Percei oil

45.3 43.5 45.3 45.3 45-3 43.4 43.3 43 * 1 43.3 41.3 41.3 40.0 37.2 28.5

'ercen' CUO

CuO : oil

0.907 :

IOO

0.916 : IOO

0.928 : I

,071 :

IOO

IOO

so that the proportion of it to the oil increases downwards. It is noticeable, however, that this increase is very small; in spite of the differences in specific gravities, about 2 . 5 and 0.86, there is very little separation of these two substances, and the union between them must, therefore, be of a decidedly intimate character.

The Nature of Emulsions It is evident that emulsions with basic copper sulphate are strictly similar in nature to those with soap, and are quite different from the quasi-emulsions with lime. Many other substances act in a manner similar to basic copper sulphate, but few of them so satislactorily. The action, however, is not distinctive of basic salts, some of them, such as the basic sulphates of copper, iron and nickel, form true emulsions, whereas those of zinc, aluminum and cadmium form quasi"

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emulsions only; other substances when freshly precipitated behave in a similarly arbitrary manner, calcium hrsenate, for instance, forms a good emulsion, whereas with barium sulphate, no signs of emulsification are obtained. On the other hand, no connection between emulsification and the nature of the metal present can be traced; copper hydroxide will not emulsify, although the basic sulphate will, whereas both ferrous hydroxide and basic ferrous sulphate are good emulsifiers. Nor is the mere bulkiness of the precipitate any guide to its emulsifying powers; precipitated aluminum silicate, although very bulky, does not emulsify, nor does gelatinous silica. The colloidal condition of the substance, also, has no influence on the results, for paraffin will not emulsify in a dialyzed solution of silica. “The view which the writer was eventually led to adopt as to emulsification, was that it depended solely on the size of the particles constituting the precipitate. When the oil is broken up into small globules by being forced through the syringe, and these globules find themselves in the presence of a number of very much more solid particles, the latter will be attracted by the globules, and will form a coating or pellicle over the globules, preventing them from coming in contact and coalescing with their neighbors. Whether gravitation alone is sufficient to account for such a result, or whether other forces come into play, must be left for others to determine, but that the solid particles do congregate closely round the globules, there can be no doubt, for they can often be seen under a powerful microscope. With basic copper sulphate this is so, although the particles are so nearly ultramicroscopic that they cannot be resolved sufficiently to admit of any estimate being made as to their actual size; with basic iron sulphate, they are still smaller, and quite invisible, but their aggregation round the globules is evident from the brown ring encircling the latter. “Further evidence that the globules must be enveloped in some covering is found in the fact that these emulsions do not make the containing vessel oily; indeed, this forms a very


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good distinctive test between a true emulsion and a quasi-, or imperfect, emulsion. The safne test may be varied by dropping a little of the liquid into a basin of water; if unemulsified paraffin is present, the surface of the liquid becomes oily, but not so if the paraffin is present as a true emulsion. “ A n emulsion on this view, however much it is diluted with water, will suffer no de-emulsification, whereas with a quasi-emulsion, where we simply have oil globules entangled with the gross particles of a solid, it should always be possible to separate the greater part of the oil by adding enough water. This, as a matter of fact, is found to be the case. It is also possible t o remove the oil from a substance with which it forms only a quasi-emulsion, by adding anothel- substance with which it forms a true emulsion. Thus quasi-emulsion of lime and paraffin of a suitable strength will rise in a mass t o the surface of water, but, if some sodium arsenate is added, calcium arsenate is formed, and, as this is a true emulsifier, it forms an emulsion with the oil, and rises to the surface, while the lime, deprived of the oil, sinks to the bottom. “Neither in the case of a true emulsion nor in that of a quasi-emulsion does the nature of the’ menstruum appear t o have any influence on the emulsification, so long as it does not react chemically with the emulsifier or the oil; a variety of salts may be dissolved in water containing an emulsifier without affecting the emulsion, beyond making it in some cases (notably when caustic soda has been added) more viscid due, no doubt, to an increase in the coefficient of friction. Even strong mineral acids have no effect in a case such as that of clay (some clays will act as emulsifiers), where the solid emulsifier is insoluble in acids. “ Although emulsions and quasi-emulsions are radically different in their nature, various substances will yield results showing every possible gradation between the two, for the simple reason that substances may consist of particles of every gradation of magnitude, some small enough to emulsify the oil globules, and others too large to do more than form quasiemulsion. In the same way, there is every gradation between

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a quasi-emulsion and the complete separation of the oil and water into independent layers. . “The power of emulsifying, not being an inherent property of the substance, but simply depending on the size of its particles, it follows that this power should be destroyed by aggregation; that this is so, can easily be proved. The basic sulphates, calcium arsenate, Oxford clay, etc., all of which form true emulsions, are found to be quite incapable of doing so after they have been dried, however finely they may then be powdered. “The average size of the globules in an emulsion seems to be dependent on the size of the particles of the emulsifier. Thus the particles of basic ferrous sulphate are more minute than those of basic copper sulphate, and the emulsion is decidedly finer-grained also ; with nickel basic sulphate, both the particles and the oil globules are of about the same size as with the copper salt, whereas with cadmium and zinc basic sulphates the particles are gross and the emulsion is very coarse, being t o a large extent only a quasi-emulsion. Whether, however, the chemical nature of the emulsifier is altogether without effect on the results or not, and whether the size of the particles is the sole determining factor, it is impossible to decide with certainty, without a more extended series of observations, aided by better eyesight and better microscopic appliances than were available in the present case. It seems, however, that the attraction between the oil globules and the solid particles may be sufficient to modify somewhat the chemical behavior of the latter. The basic sulphate of copper precipitated by lime, when treated with excess of caustic soda, will blacken aftkr a few weeks, owing t o the formation of copper oxide, but, when emulsified with ‘ Solar Distillate, ’ this blackening does not commence nearly so soon; it is more rapid when a lighting oil is used, this forming a coarser emulsion, and much more so if petroll is used, when the emulsification is very imperfect indeed. Similar evidence is obtained by making emulsions with the same oil, [Gasolene.]

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but using different proportions of it; that which contains more oil will blacken more slowly. Analogous results are obtained with ferrous sulphate, the color of the emulsions, due to different degrees of oxidation, differing according to the nature of the oil used. I t is possible, however, that in these cases the retardation of the action of the soda on the basic sulphate, or the oxidation of the compounds present, may be due to a chemical action of impurities in the oil. “A few instances exist which seem a t first sight to negative the view that the power of emulsifying is conditioned by the size of the solid particles. Thus, precipitated sulphur, purple of Cassius, and ferric ferrocyanide, all of which are in an extreme state of sub-division, will not emulsify paraffin at all. The behavior of sulphur gives a clue to this result. When flowers of sulphur, thoroughly incorporated with water, are churned with paraffin, the latter abstracts the whole of the sulphur from the water, forming with it a greasy mixture, which sticks obstinately to the syringe, containing vessel, etc., the paraffin evidently wetting the sulphur in a way which the water does not. A similar effect is observed in the case of purple of Cassius and ferric ferrocyanide; although the paraffin is not emulsified at all, it abstracts the whole of these substances from the water, becoming intensely colored by them, and leaving the water colorless, just as if they dissolved in the paraffin. The view therefore that emulsification depends solely on the minuteness of the solid particles must be so far modified as to apply only to such solids as are wetted more easily by water than by oil, otherwise there is no emulsification a t all. The explanation of emulsification here advanced would hardly be acceptable unless it could be applied equally to cases where the emulsifier is a liquid, and there seems to be b u t little difficulty in doing so; for these liquid emulsifiers are all substances which, under the conditions obtaining, partly separate from the solution, and yield sufficient solid particles to form a pellicle over the oil globules. Liquid emulsifiers are all distinguished, so far as we know, by a feeble ‘ I

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affinity for water; in many cases there are, or contain, protein substances, which, as Ramsden has shown, form a skin of solid matter at any boundary between the solution and another fluid, whether liquid or gaseous. I n other cases, the deposition of some particles of the solid would result from this solid being insoluble in the paraffin. The water in the neighborhood of a paraffin globule would become impregnated with paraffin, and the dissolved substance, having but a small affinity for water, would be thrown out of solution. Solutions of egg-albumen, glue and starch, when covered with a layer of paraffin, all form a slight opaque film a t the junction of the two liquids. I n this way, each globule of oil would obtain for itself the solid particles necessary for its emulsification; but these particles would, no doubt, often be redissolved, while others, in their turn, would be deposited, and, in the course of such changes, opportunities would occur for the coalescence of neighboring globules. This explains why emulsions in soap will generally de-emulsify spontaneously, if left long enough, whereas emulsions with a really insoluble emulsifier seem to be quite permanent. Formed in this way by solid deposited in situ, an emulsion in soap, or similar substances, would naturally be more fine-grained, arid the globules would be more closely packed, than in cases where a separately formed precipitate is the emulsifier. A rise of temperature, or an increase in the volatility of the oil used, would result in the soap solution becoming more permeated with paraffin particles, and depositing more of the dissolved soap, thus facilitating emulsification. With soap, a rise of temperature does facilitate emulsification, and an increase in the volatility of the oil sometimes does the same. With insoluble emulsifiers, however, neither a rise of temperature nor an increase in volatility favors emulsification, indeed, the latter materially hinders it, owing, no doubt, to the more volatile oils being more mobile. " Whether any soluble inorganic substance exists which would emulsify oil in the same way as does soap, appears to be very doubtful; it is certainly probable that any substance I'


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which is crystalline would be incapable of emulsifying. Saturated solutions of calcium hydroxide and sulphate, as well as of sucrose, dextrose and dextrin, have been tried, with negative results. “ With soap, the particles requisite for emulsification are also provided in another way; for when soft soap is dissolved in much water, or when a strong solution of it is diluted, a considerable amount of a very fine deposit is obtained, and this is often in such a minute state of division that it remains suspended in the liquid for weeks. The separation of this precipitate on dilution, and its redissolution or concentration, explains a fact previously noticed, which otherwise would present considerable difficulty, namely, that for each proportion of paraffin and water the amount of soap necessary for emulsification must not only be in excess of a certain minimum, but also must not be in excess of a certain maximum. The existence of the superior limit is explained if the emulsification depends to any large extent on the presence of solid particles precipitated by dilution, for the tendency to deposit such particles would be less in stronger than in weaker soap solutions. (It should be mentioned, however, that a superior limit does not seem to exist in the case of all soaps. That is not surprising, for soaps vary largely in composition, even when they are of the same brand, and from the same factory, and they differ considerably in their behavior on dilution.) “ T h a t the formation of a pellicle of solid particles over the oil globules affords an explanation of emulsification, is, as has been mentioned, the conclusion to which Dr. Ramsden also was led, and the view that such a pellicle exists in the case of the oil globules in milk was advocated long ago. So far as can be seen, this view seems to harmonize with all the observed facts. Behavior of Various Substances “ I t would be tedious and unprofitable t o describe at length the behavior of the many substances which have been

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examined as regards their emulsifying powers; but they may be briefly enumerated, classing them roughly into those which give true emulsions, those which give imperfect emulsions or quasi-emulsions, and those which do not seem to emulsify a t all. In some cases, however, it is rather difficult to decide in which class to place a particular substance, and, in others, the method of preparation affects the behavior of the substance in question. Good Emulsifiers.-Among soluble, or partially soluble, substances, soft soap seems to be the best; dissolved starch, milk and flour are good, although the latter forms a flocculent, and not a creamy, emulsion, and milk gives rise t o solid clots; glue emulsifies well, and so does egg-albumen, but the emulsion with the latter is rather frothy, owing to enclosed air bubbles; saponin and quillaia bark give good emulsions when the proportion of oil present is not large. " Among insoluble emulsifiers, the basic sulphate of iron is the best, followed by those of copper and nickel; the basic sulphates of zinc and aluminium generally give good emulsions a t first, but aggregation of the particle seems to occur, and causes partial de-emulsification. Ferrous hydroxide and the higher oxides of iron (hydrated) are good. The precipitate obtained by adding sodium carbonate to copper sulphate is a very good emulsifier, so are calcium carbonate and calcium arsenate when first precipitated, but the latter soon becomes crystalline, and de-emulsification follows ; lead arsenate, reshly precipitated, is good, and zinc oxychloride, or basic chloride, is fair, although the emulsion with it is flocculent; some fine clays (unheated), such as Oxford clay, give good emulsions, but they, naturally, contain many gross particles which sink to the bottom unemulsified ; ferrous hydrosulphide gives a good emulsion if the proportion of oil present is small. ' (Substances Producing Quasi-Emulsions, or Producing Partial EmuZsions.-I.,ime, silica, alumina, plaster of Paris and many fine powders which have been dried; also the following precipitated substances when in the liquids from


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which they have been precipitated : basic cadmium sulphate, magnesium hydroxide, copper hydroxide, the basic sulphates of zinc and aluminium, stannous oxychloride, purple of Cassius, lead arsenate paste (a commercial preparation for insecticidal purposes), some clays and brick earths, copper hydrosulphide, ferrous hydrosulphide (unless the proportion of paraffin is very low), and precipitated soda soap. ‘‘ Substances Showing Little or no Power of Emulsijying.Precipitated lead chloride, lead sulphate, barium sulphate, neutral silica solution, sulphur precipitated by adding acid to sodium thiosulphate, flowers of sulphur and dried Paris green (in both of which cases the powders seem to absorb the oil, forming a sticky mass which adheres to the syringe and containing vessel in an obstinate manner); an alcoholic solution of resin precipitated by water, ferrous ferricyanide, ferric ferrocyanide, and purple of Cassius (in the last two cases the precipitate is carried up by the oil in sufficient quantities t o color it strongly); and, lastly, any course, or even fine powders, especially when present in small proportion s.

Summary When paraffin oil is churned up with a solution of soft soap, an emulsion is formed which rises t o the surface and generally contains 65-82 percent by volume of oil. Emulsions containing as much as 99 percent can, however, be obtained, and these are so stiff as to be almost solid. The character of the oil does not much affect the results, neither does the extent of the churning nor the proportion of soap, so long as this proportion is within certain limits. “The amount of oil in an emulsion which has arrived at a state of equilibrium decreases somewhat from the top downwards, but there is often a tendency to arrange itself in layers of approximately uniform composition. “Solutions of other organic substances, such as glue, flour, milk, starch, albumen, saponin, etc., act as emulsifiers for paraffin oil. All these emulsions will often spontaneously become de-emulsified, and this occurs a t once if the emulsifier

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is destroyed; for example, if an acid or any sodium salt is added 'to an emulsion with soap. Electrolytes, as such, seem to have no de-emulsifying action. The addition of unemulsified paraffin to an emulsion will gradually de-emulsify the whole. "The oil globules in an emulsion are probably prevented from coalescing by being enveloped in a pellicle consisting of particles of solid much more minute than the globules themselves. The solid particles would be derived from the solution, which in all cases contains a substance with but little affinity for water, and insoluble paraffin, it being, therefore, precipitated in the neighborhood of the paraffin globules. '' Apparently, a precipitate consisting of any insoluble substance which is wetted more easily by water than by oil, if in a sufficiently fine state of division, will equally act as an emulsifier, and in some cases it is possible under a microscope to see the coating of solid particles which envelop the oil globules. Emulsions made with an insoluble emulsifier are in every respect similar to those made with soap, etc., except that they never seem t o de-emulsify spontaneously, spontaneous de-emulsification being, no doubt, do to the fact that the solid particles, as in the case of soap, are soluble in water, and are continually being redissolved and reprecipitated, thus affording opportunities for the coalescence of the oil globules. " The basic sulphates of iron and copper are among those substances which give excellent emulsions. They may be formed by adding lime, or lime-water, to the normal sulphates, and then the paraffin, when the slightest churning, or even shaking of the mixture, produces emulsification. Besides the ease of manufacture, and the absence of spontaneous deemulsification, these emulsions possess the advantage of not being decomposed by the addition of caustic soda, as are emulsions with soap, and caustic soda is required when the emulsion is used as a winter wash for trees. When the copper salt is used, the emulsion possesses all the fungicidal properties of Bordeaux mixture.

The Theoyy of Emulsification " Many other precipitated substances, act as emulsifiers; but this property is destroyed as soon as they have been dried, or have by any other means been deprived of their fine. grained structure. " Solids which are not sufficiently fine-grained to emulsify . will, in many cases, when present in considerable proportions, form quasi-emulsions. I n these, the particles of oil are merely entangled with the particles of solid, and may be separated from them by such simple means as dilution with water. Lime is an instance of a substance which forms a quasiemulsion which can be used for spraying purpose. Other substances, including many recently formed precipitates, and, probably, all crystalline solids, seem to be incapable of forming even quasi-emulsions. " Pickering afterwards published a lengthy abstract of this paper1 in which he added some paragraphs on the effect of surface tension. '' It is generally assumed that the formation of emulsions is dependent on a low surface tension and a high viscosity of the medium.2 It is clear that a viscous liquid must have a tendency to hinder the coalescence of oil drops; but it is not permissible to attribute t o viscosity any really important part in emulsification. Dilute soap solutions are not very viscous and yet their power of forming an emulsion may even be destroyed by increasing the concentration and thereby the viscosity, above a certain point. Hillyer also showed that a very viscous 50 percent solution of glycerine or a 6 percent gum acacia solution did not yield an emulsion. The author has found recently that a perfect emulsion (of potassium cupric tartrate) can be made in a mixture of alcohol and water though this mixture is not viscous at all. "The surface tension of the medium doubtless plays an important part in emulsification in many cases; but i t seems impossible to assume that it is the only, or even the most

Pickering: Zeit. Kolloidchemie, 7, 1 1 (1910). Plateau: Pogg Ann., 141, 44 (1870);Quincke: Wied. Ann., 35, 592 (1888);Hillyer: Jour. Am. Chem. SOC.,25, 511, 524 (1903).

* J.

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important, factor. In many cases it seems to be of no importance in the formation of an emulsion. This is ‘shown by the recently discovered fact that emulsification may be caused by the mere presence of solid particles, without there being any change in the surface tension of the medium. When we treat such solutions as copper sulphate with lime water, the resulting dilute solutions have practically the same surface tension as water. On the other hand a number of solitble substances have no effect on emulsification. One can even add strong acids without destroying the emulsions provided they do not attack the solid emulsifying particles. This can be realized with certain clays. There are also reasons for considering surface tension as an insufficient explanation even in the case of emulsions formed by means of soap. If we add soap to water and thereby decrease the surface tension of the latter, films of water are formed around the drops of oil and keep these from coalescing. Looking a t the matter in this way we should expect that the emulsification would become more perfect, the more concentrated the soap solution was made, since the smallest drops would be emulsified most easily, and the largest ones with the most difficulty. This is not the case, however. There is no emulsification a t all until a certain concentration of soap has been reached; but then we get a perfect emulsion at once, whereas we cannot get an emulsion a t all when the concentration of the soap goes above a certain limit. “ W e must therefore ask ourselves whether the emulsification of oil in soap solution or in other similar liquids may not be explained more satisfactorily by applying the same principles which have been shown to be found in the case of emulsification by solid particles. It certainly seems as though this presents no difficulties in the case of soap solutions for these are never clear and always contain insoluble particles, notably stearine, which precipitate gradually on standing and more rapidly when the solution is diluted. We have only to assume that the drops of oil are surrounded by the necessary film of solid particles. Since the stearine particles precipitate less


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readily as the concentration increases, this accounts for the difficulty that no emulsion is formed after the soap concentration exceeds a certain limit. We know that the drops in such an emulsion are actually covered with some sort of a film because they do not make the containing vessel oily, as is the case when the drops are not emulsified. Even when the soap solution does not contain enough solid particles to coat the drops, it is quite probable that more will be precipitated from the soap solution by the kerosene itself, for Ramsden’ has shown that, when kerosene is brought in contact with different liquids containing protein substances, an opaque film or membrane is formed a t the surface separating the two liquids. The author has found that this also takes place with solutions of starch, glue, albumen, etc. Such a film is probably not stable, but is always being dissolved and then formed again. This gives the drops of oil a chance to coalesce and this may account for the fact that emulsions prepared with soap usually go to pieces when allowed to stand for a long time. On the other hand cracking has never been observed when an emulsion has been properly prepared by means of solid particles (a basic sulphate for instance), always provided that nothing is present which would decompose the solid film. “The conclusion to be drawn is that emulsification may take place in a liquid when the viscosity is not high and the surface tension is not low, though i t is admitted that a high viscosity and a low surface tension often facilitate the formation of an emulsion. The only-or rather the chief-cause of emulsification seems t o be the presence in the liquid medium of small, insoluble particles, which form a film round the drops of oil and keep these from coalescing. In order that such particles should have the power of forming an emulsion, they must have only a very slight tendency to agglomerate, they must be, more readily wetted by the water than by the oil, and they must not be crystalline.” Donnan’ comments on this paper as follows : Proc. Roy. SOC.,72, 156 (1903). Zeit. Kolloidchemie, 7, 2 14 (1910).

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In a recent article S. U. Pickering discusses the theory of emulsions, and apparently comes t o the conclusion that surface tension has very little to do with the phenomenon of emulsification and that emulsions owe their formation chiefly t o solid particles. S. U. Pickering has made no accurate measurements on this point and seems also not to know about the close relation between surface adsorption and lowering of the surface tension. He also does not consider the electrical forces which we know to be acting. We do not believe that the true nature of emulsions is t o be explained by S.U. Pickering’s theory of solid particles. ” It does not seem t o me that Donnan is quite fair t o Pickering in this criticism. It is true that Pickering does not seem to be familiar with Donnan’s earlier papers; but it is also true that Donnan has never referred to Pickering’s original paper. Donnan himself assumes the existence of a film of soap about the drops of oil in soap emulsion and yet he does not consider that as ruling out electrical phenomena. It is possible that Donnan considers the film as liquid and not solid; but, as Pickering’s film is explicitly not crystalline, the question of whether it is a solid film or a liquid film depends upon one’s definition of a solid. Pickering has postulated the existence of a surface film in the case of an emulsion; b u t he has made no attempt t o formulate quantitatively all the factors determining the formation and stability of a surface film. Donnan reproaches Pickering with not having made any accurate measurements of surface tension and with ignoring the conditions governing surface adsorption; but this, while true, is not t o the point. Donnan has been interested in cases in which there was a real or an apparent connection between the lowering of the surface tension and the formation of an emulsion. Pickering does not deny all this. What he does is to point out t h a t there are cases in which there is no apparent connection between surface tension and formation of emulsion. In this he is entirely correct. There are accurate measurements which show that colloidal ferric oxide has little or no effect on the surface tension of ‘ I


’

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water.l On the other hand hydrous ferric oxide is an excellent emulsifier in some cases. I am quite willing to admit that the discrepancy may be apparent and not real; but it has not yet been explained away and one does not quite see why Donnan should ignore these facts. It is quite true t h a t Pickering has not given us a complete theory of emulsification; but on the other hand he has not claimed t o do so. There are a number of interesting things in Pickering’s paper which it will be profitable t o consider somewhat in detail. I have already cited the formation of an emulsion with 99 percent of kerosene as the dispersed phase to illustrate the futility of considering an emulsion as necessarily made up of absolutely spherical drops. Curiously enough, Pickering seems to feel that there is or should be something especially significant about a volume percentage of 74.048 percent, which is the percentage volume of the spheres in a collection of closely-packed spheres of uniform size. In order to get high concentrations of the dispersed phase, Pickering found it necessary to add the oil by degrees. This is a familiar practice in pharmaceutical work where it is even necessary not to reverse the direction of grinding when emulsions are prepared in a mortar. We have confirmed Pickering’s results, substituting benzene for kerosene; but I am inclined to think that it is all a matter of inefficient stirring. It is harder to break up a large mass of oil than a small one, and churning with a syringe is not really an effective method. I n Robertson’s work’ the shaking was much more effective and he did not have any difficulty in emulsifying high percentages of oil. Pickering found that satisfactory emulsions could be obtained only between two limiting concentrations of soap, the limiting values varying with the nature of the soap and with the relative volumes of oil and water. According to Pickering the minimum soap concentration increases and the 2

Freundlich: Kapillarchemie, 313. Jour. Phys. Chem., 16,179 (1912). Zeit. Kolloidchemie, 7, 7 (1910).

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maximum soap concentration decreases with increasing percentage of oil. The minimum concentrations seem to present no especial difficulties. If we postulate that a soap film is essential t o an emulsion, it is clear that there will necessarily be some concentration which will behave very much like pure water. Since there are more drops of oil in an oil-rich emulsion than in one containing less oil, it is not surprising that somewhat more soap should be necessary in order to provide for the emulsion. The question of the maximum concentration is a more serious one. It bothered Pickering a good deal as is shown by his attempts a t an explanation. I believe that the difficulty lies in the nature of the film around the drops of oil. If the conditions are such that a rigid film is formed instead of a flexible one, it will be ruptured by shaking. Instead of an emulsion, we shall get a coagulation of the soap. When the ratio of water to soap gets too low, we shall have no satisfactory emulsion formed. A consequence of this is that the maximum value for the soap will be lower, the greater the concentration of oil, which is exactly what Pickering found. The agreement is a qualitative one and not a quantitative one because Pickering’s maximum value was 25 percent soap when he had sixty volumes of water to forty of oil whereas it was 1.8 per&nt soap, when he had twenty-five volumes of water t o seventyfive of oil. In view of the inefficient stirring I am not inclined to lay much stress on the absolute figures obtained by Pickering. The change in stability when changing from a potassium soap to the less soluble sodium soap is probably due also t o a decreased flexibility of the soap films. I have seen emulsions of benzene in water made up with sodium oleate and with a regular sodium soap, containing stearate and palmitate. When first made up, there was little to choose between the two emulsions but the sodium oleate one stood up very satisfactorily for a t least six weeks, while the soap emulsion went to pieces within a few days. The color and viscosity of an emulsion are interesting things. We get the same result with kerosene, benzene, or

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olive oil. With high concentrations of the dispersed phase, the emulsion becomes creamy and viscous, finally approaching the consistency of tallow or even of cheese as the concentration gets very high. While it is possible that the smallness of the globules may be a factor in determining the viscosity, I am more inclined to believe that the viscosity of the surface film of soap is the most important item. If we have an infinite number of very small, semi-solid films or membranes, we have practically a cellular structure and consequently the maximum rigidity. The opacity is undoubtedly due to the soap films. That a semi-solid emulsion should gradually liquefy when exposed to dry air is simple enough. The water evaporates from the film and the film coagulates, letting the kerosene run out. I t is not quite clear, however, why the edges of the lump should assume the appearance of a slightly transparent, slightly blue jelly on drying; but I imagine that that could easily be accounted for if I knew a little more about the properties of soap. It seems probable that we are dealing with the production of films of transparent soap. Pickering believes that the emulsions made with basic copper sulphate as emulsifier tend to separate into layers of uniform composition which, although not visible as separate layers t o the eye, are made evident by analysis (Fig. 2 ) . I feel certain that this is merely a case of experimental error. The evidence on which the conclusion rests is meagre and unsatisfactory. After standing twenty-four hours, there was no sign of a separation into a series of uniform layersr nor was there after ten days. At the end of twelve weeks, one set of analyses is said to have shown the existence of these layers, each of uniform composition. The experiment was not allowed to run longer, so we do not know whether these alleged three layers persisted or whether their occurrence was purely accidental. The experiment seems never t o have been duplicated, which is distinctly unfortunate under the circumstances. It is very doubtful also whether the data, such as they are, bear out the conclusions. Unless there is a

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misprint, the uppermost layer varied in composition from 45.3 percent oil to 43.5 percent, while the composition of the next layer varies from 43.4 percent oil to 43.1 percent. I should be inclined t o doubt the probability of a misprint because the 43.5 percent point has been omitted from the diagram. It is the only one of the tabulated data which has not been plotted and it looks as though this point had been omitted intentionally because it did not come where it should. The last six analyses gave the figures 43.3, 41.3, 41.3, 40.0, 37.2, 28.5 percent oil. Pickering makes the 43.3 part of the second layer, and he groups the 41.3, 41.3 and 40.0 as a third layer. When these data are plotted by Pickering it is quite clear that five of them lie on a smooth curve, the first 41.3 being below the curve. Pickering evidently draws the tacit conclusion that the 40.0 point is over a percent too low. If we draw the conclusion that this point is approximately right and that the first 41.3 point is over a percent too low, all six points would lie on a smooth curve and the third layer would be non-existent. It must also be remembered that there may be other errors beside the analytical ones. Differences in the sizes of the globules of oil certainly existed and would have some effect. Differences in the subdivision of the basic copper sulphate presumably existed and had some effect. There is therefore no justification a t present for assuming that we get discontinuous layers of uniform composition as constituting the final equilibrium when “ Solar Distillate ” and water are emulsified by means of basic copper sulphate. Our experiments have not confirmed Pickering’s conclusion t h a t once de-emulsification has started, it progresses a t an increasing rate. We have had many creamy emulsions from which a little benzene has separated fairly quickly, after which there was apparently no further change for months. On the other hand some emulsions have gone all to pieces soon after they began t o crack. The complete breaking down of an emulsion is due to the coagulation of the films surrounding the drops of oil or benzene. Whether such coagulation takes place and how fast it takes place depends on the nature of the

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two liquids and on the nature of the substance forming the film. The question of the permanency of an emulsion is a very important one practically; but it can best be considered after we have straightened out the problem of the formation of an emulsion. Pickering’s quasi-emulsions seem to be cases in which the oil has carried the solids into suspension. While there is doubtless some formation of a true emulsion, i t seems probable that the adsorption of oil by the solid is also an important factor. I am inclined to think that Pickering is right in considering the emulsions with basic copper sulphate and basic ferric sulphate as true emulsions. We have obtained similar results using hydrous ferric oxide as the eniulsifying agent. I cannot agree with him however in his conclusion that emulsification “ depends solely on the size of the particles constituting the precipitate.” I cannot understand how he reconciles the following two sentences : ‘‘ The power of emulsifying, not being an inherent property of the substance, but simply depending on the size of its particles, it follows that this power should be destroyed by aggregation; that this is so, can easily be proved. The basic sulphates, calcium arsenate, Oxford clay, etc., all of which form true emulsions, are found to be quite incapable of doing so after they have been dried, however finely they may then be powdered.” According t o Pickering’s hypo thesis any very finelydivided substances should act as an emulsifier provided it is wetted more easily by water than by oil. I n fact he says so definitely. ‘‘ Apparently, a precipitate consisting of any insoluble substance which is wetted more easily by water than by oil, if in a sufficiently fine state of division will equally act as an emulsifier.” If this is true the basic sulphates, calcium arsenate, etc., should act as emulsifiers after being dried, provided they are ground finely enough. Such substances as barium sulphate and lead sulphate ought to act as good emulsifiers if obtained in a fine enough form and yet Pickering himself says that they do not. The important

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W i l d e r D.Bancroft

thing is that we shall have a coating round each globule of oil preveii ting i t from coalescing with other globules. Pickering considers that this coating can be made up of discrete particles, but the experimental evidence does not bear him out in this. It seems to me that the essential thing is t o have a coherent film around each globule. In that case it is not enough to have a finely-divided precipitate. It must be of such a nature and in such a state that it can and does form a coherent film around the globules of oil. The conditions for this are still to be worked out. The importance of Pickering’s work is that he has shown experimentally that certain precipitates can cause the formation of emulsions even though they have practically no effect on the surface tension of water. I t is interesting to note that Pickering found no trace of an emulsion of water in oil, and that it never occurred to him to turn his definition round. If it is true that one gets an emulsion of oil in water when one introduces insoluble particles which have only a very slight tendency to agglomerate, which are more readily wetted by water than by oil and which are not crystalline, it ought t o follow that we should get an emulsion of water in oil if the insoluble particles were more readily wetted by oil than by water. The general conclusions to be drawn from this paper are as follows: I Stable emulsions can be made using as emulsifying agents certain precipitates or suspensions which are known not t o have any marked effect on the surface tension of water. 2 The theory of surface tension, as it has hitherto been applied, is not so important for the understanding of emulsification as has generally been supposed. 3 In many cases the existence of an emulsion depends upon the formation of a film or membrane around each globule of the dispersed phase. This does not exclude the possibility of an electrical charge being the whole, or a partial, cause for the drops not coalescing. 4 Though it is often necessary to add the dispersed phase by degrees when working up to a high concentration,

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it is probable that this is true only when relatively inefficient methods of dispersion are used. 5 The breaking down of an emulsion is due to the coagulation of the film around the globules of the dispersed phase. 6 It is not true that, when once de-emulsification has started, it progresses a t an increasing rate. 7 I t is not true that the emulsifying action of insoluble particl s depends only on the fineness of division and that it is in ependent of the nature of the particles. 8 the concentrations are such that a non-elastic film tends to form, it will be difficult to form an emulsion by shaking. This accounts for Pickering not being able to prepare an emulsion starting with too concentrated a soap solution. 9 The color and viscosity of an emulsion depends very largely upon the properties of the protect ng films. I O The formation of a transparent jelly under the conditions noted by Pickering is probably due to the formation of a film of transparent soap around each globule of oil. 1 1 It is probably not true that the emulsification of “Solar Distillate” and water by means of basic copper sulphate results in “ t h e ultimate arrangement of the emulsion into layers of uniform composition, which, although not visible as separate layers to the eye, are made evident by analysis.” I 2 Pickering found no evidence of any emulsification of water in oil. I 3 Pickering considers that emulsification will be produced by any insoluble particles in a sufficiently fine state of division, “which have only a slight tendency to agglomerate, which are more readily wetted by water than by oil, and which are not crystalline.” This statement is inadequate because it does not state the conditions under which the particles will coalesce to form a coherent film around each globule of the dispersed phase. 1 4 Pickering does not point out that a corollary of his

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hypothesis is that insoluble particles, which are more readily wetted by oil than by water, should tend to promote the emulsification of water in oil. I 5 The quasi-emulsions of Pickering are primarily due to solids suspended in oil to such an extent as t o make the density approximately equal to t h a t of water. There is probably also some formation of a true emulsion-of drops of oil with a' film around each. Caritell University

The Theory of Emulsification, III - The Journal of Physical Chemistry

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