856
INDUSTRIAL AND ENGINEERING CHEMISTRY
Limit specified by the Army for dehydrated veget.ables. Samples for these determinations, in contrast to t,hose sent out for vitamin analysis, r e r e not prepared immediately: t,herefore, some gain or loss of moisture may have taken place. I n general, it is perhaps fair to state that the major point iii favor of dehydrated products is their concentrated form w d adaptability to storage. The common observation is that thow products showing good vitamin retention are usually those POPsessing superior palatability and acceptance. From this standpoint alone the criterion of high vitamin content in thr prodiict is to be encouraged. ACKNOWLEDGBIEST
I n this survey the assistance of t'he laboratories of Y.H. Cheldulin, Joseph H. Roe, P. B. Pearson, G. 0. Kohler, E. 11.Nelson, and C. A. Elvehjem is acknowledged in connection with t,he vitamin assays. A. Kramer is responsible for the mineral and prosimate data, and his contributions to the eurvcy are also a c k n o ~ ~ l edged. LITERATURE CITED
I
1) Booher, L. F., Harteler, E. R.. a n d H m i t o n . E. 1T , U. S.Deprt .4gr , Cwc. 638 ( I 942).
Vol. 38, No. 8
'urn. L. E., and Hehorleill. 1). G., Ixu. LNG.CHEM.,36, 912(1944). (3'1 F c n t o n . F., Barilea, B., >foyer. J. C.. Wheeler. K. A., and Tressler. I).I. E:NG. CHEJI.,12, 337-8 (1940). r6) Loeffler, H. J., a n d P o n t i x g , J . I]., IYD.EYG.CHGM.. ..\N.+L. ED., 14, 846-9 (1942). elson. W. I,,, a n d Gortner, '7) ;\Iallotte, 51. F., Dawson, C . I1 W. A , ISD.E N G .CHEX.,38, 437-41 (1946). (Si Aforgan, :I. F., Carl. B. C., Hunner, 1f. C.. K i d d e r . L. E., Hurnrnel, hl., and Peat, ,J. 11..F r i d Products .I., 23, 207-11, 219-21 (1944). (9) I{m, J. E.. a n d Kuethrv, C. .I..J . Hid. Chem.. 147, 399-407 (1943). (10) S a r o t t , H . P., and CliPldeliii. V~ H.. Ibid., 155, 153-60 c1944). (11) Srhults, -1.S.,Atkin, L.. a n d P r e y , C . N., IND. ENU. CHEJI.. ANAL.ED.,14, 35-9 (1942). (12) Snell. E. E.. a n d S t r o n g , F. .\I.,IDi?., 11, 346-50 (1939). (13) Snel!. E. E.,and TTright, L. D.. J . Biol. Chem.. 139, G75-8G (1941). 14: T r e s d e r , D. K., Mloyer, J. C.. and W h d e r . K. A.. A m . J . Pub. Health, 33, 975-9 (1943).
Accelerated Breaking of Unstable Emulsions H. P. AIEISSNER AND B. CHERTOWMassachusetts I n s t i t u t e of Technology, Cambridge, Mass.
T
A method is discussed for accelerating the break of I n the investigations deEMPORARY emulsions emulsions containing no surface active agents such scribed in this paper, a may be encountered inas are sometimes encountered in steam distillation, quantitative study was initidustrially in liquid-liquid exsolvent extraction, and other processes. The method inally undertaken to determine traction processes and in volves agitating the emulsion w i t h one to four times its the effect of phase ratio on steam distillations; they may volume of dispersed phase material and then allowing the the rate of breaking unstable result when one liquid phase system to stand idle, w hereupon the originally cloudy emulsions. It was found that is dispersed in another in the phase clariiies rapidly. This method was effective in some quantitative reproducibility absence of a stabilizer or after systems, called totally recolerable, regardless of which of of breaking rate could be the emulsifying agent has the two phases present was dispersed. In all other cases, obtained only with a high been destroyed er othernise called semirecolerable, the method vas effectil e only degree of control. As was removed from a stabilized when one of the two phases was dispersed. Successful to he expected from results emulsion. Unlike the large cladlcation was usually attained with a polar but not a of other investigators, results amount of work reported on nonpolar dispersed phase. A totally recoverable system, varied, depending upon traces stabilized systems ( f - 4 ) , relatherefore, usually showed that both phases present conof impurities, slight differtivelylittle has been published tained polar components, whereas a semirecoverable sysences in amounts of impurion unstabilized emulsions. I n tem contained a nonpolar and a polar phase. ties, amount of air bubbles 1910 Ostwald (6) showed that drawn into the system during if an emulsion can be conaeitation. variation in tvue sidered to consist of equaland violence of agitation, upon which phase of the system under sized spherical droplets of one phase dispersed in a second study wet the container walls first, and so on. Many of these continuous phase, the droplets would all touch when the ratio of variables could not be practically controlled in industrial scale dispersed to continuous phase volume was increased to 74.02: operations. Certain qualitative observations, and also a re25.98 or, roughly, about 3: 1. Inversion in a n unstabilized system covery procedure for accelerating the clarification of these would then occur, with the dispersed phase becoming the cont,inuemulsions. however, were unaffected by these factors. The ous phase. Stamm and Iiraemer ( 9 ) studied the rates of brea,k object of this paper is to describe the lat,ter findings. of unstable systems as influenced by various factors, and Roberts (7) investigated their inversion behavior. Hauser and EXPERIMENTAL PROCEDURE Lynn ( 5 ) among others state that phase volume ratio (hereafter Expcrinicntal work involved preparation of c-riiulsions by called phase ratio) has an important influence on the stability of various methods and observation of their qualitative behavior emulsions generally. ' 4 recent development in breaking unstable during brcak. For the systems stutlictl (Tables I and II), a emulsions has been reported by the Selas Corporation (8) nhereby range of phase ratios from 20:l to 1:20 F a s usually explored. 3eparation is effected through the use of a porous medium which .\gitation methods of rmulsion preparat ion were chosen t o cover permits the passage of only that phase Fhich wets it. I
_
August, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 1. Unstabilized System Butanol-Water Undergoing Primary Break, after 3 3Iinutes of Vigorous Hand Shaking Followed by 10 Seconds of Settling rhe volume ratios of the saturated phases (water : butanol) in the tubes counting from left to right are: 95:5,80:20,65:35. .50:50,36:65, The middle tube, with a ratio of 1 : l . is evidently breaking most rapidly; the other tubes show R decreasing rate of break as the phase ratio deviate further from unity. In the two end tubes primary hreak has not yet progressed very far, in that no hottom or top layer# have >-etformed.
20:80, 5:95.
as wide a range of conditions as possible. 1-arious types of high speed mechanical mixers and hand shaking of emulsions in containers of various shapes were investigated. The violence and duration of agitation were varied over wide limits. h rack holding seven test tubes which could be shaken as a unit (Figures 1 and 2) was found convenient for illustrating qualitative results. Emulsions m r e also prepared by precipitation-for example, by cooling butanol saturated with water. The reduced solubility of vater caused the water t o precipitate out in the form of a fog of tiny droplets dispersed in the butanol phase. Since the objective of this work was t,he study of unstabilized emulsions, care was exercised a t the outset to eliminate any surface active agents. Tests on the systems studied were therefore made with chemically pure reagents, and with conductivity water in the aqueous systems. Trials were then repeated with technical grade reagents and t,ap water. S y s t e m to which ,Jarious impurities were deliberately added were also investigated. A determination was made of the charge carried by t,he dispersed phase particles in several of the systems studied. This involved placing a sample of a very dilute emulsion in a cataDlioresis cell and observing, n i t h a slit ultramicroscope, the movement of the dispersed phase particles in a n electric field of 25 to 45 volts per inch. KO correlation was found between recoverability and the presence or absence of charge.
a57
clarification which follows primary break is cal.:ed the sccontlary hreak. With the exception of certain semirecoverable casc's, the rate of 1)rimai-y break in any given system is of an order of magnitude greater than that of secondary i)i.cak. This is true even though it is difficult to duplicate the rates of primary and secondary hrcak from one case to another. The primary break itself is usually rapid enough to present no seriom separation problem, unless the viscosity of the continuous phase is very high. The secondary break, honever, is often extremely slow; sonietimes hours or even days are required for t,he break to occur in some systems. I t is desired to accelerate the rlisappearance of these secondary fogs. Immediately after primary break, a secondary cloud is usually present in the majority phase-that is, the pha5e romprising the larger volume fraction of the system under observation. The greater the phase ratio, the worse the secondary fog in t,he majority phase and the clearer the minority layer immediately after primary break. When the phase ratio q u a l s or exceeds Ostnald's 3:l ratio, the minority phase is usudly crystal-clear, although in some cases the minority layer is clt,ar even a t phase ratios approaching unity, as in the butanol-waw system shown in Figure 1. The technique for accelerat,ing the break of secondary clouds ie illustrated in Figure 3. The technique is based on the observation that, in many systems, the minority phase is always clear immediately after primary break, regardless of the clarity of the phases from which the emulsion was prepared. Therefore, an initially cloudy layer, such as the nitrobenzene layer in Figure 3, can be clarified by agitation n i t h an excess of cloudy or clear water layer. A clouded water layer can be clarified by the same technique. The water-nitrobenzene system in Figure 3 is interesting in that both phases can be clarified simultaneously with a 1:l phase ratio. The phase ratio required for recovery varies from case t o case and must be determined by trial, but is oft,en near Ostwald's 3 : l ratio. '4s expected, the majority p h a s ~is - a t this rclntivitly Iiigh ratio usually clouded aftcsr r w o
RESULTS
Immediately after a n unstabilized emulsion is freshly prepared :for example, by agitating two immiscible phases in the absence of surface active agents), it starts to separate into its two original Layers when permitted t o stand idle. Two distinct stages are usually observed during separation and these are called primary Etnd secondary break. During primary break, which is the first stage observed, three regions are usually visible-namely, a Father clear bottom layer of the heavier phase, a rather clear top Layer of the lighter phase, and a middle region separating these t ~ layers o composed of the dispersion itself undergoing further 5reakdom-n (Figure 1). Primary break is considered complete vt hen the middle region has disappeared and a clearly defined .nterface has been established between the bottom and top nyers. After primary break (Figure 2 ) , usually one and sometimes twth layers are cloudy-that is, still contain a fog of tiny droplets d the other phase. Eventually the secondary fog disappears in the sense that the tiny droplets of which it is composed travel -lon.ly to the interface and merge n i t h their parent phase. The
1
2
3
4
5
6
7
Figure 2. n-Butanol-Water System 20 Seconds after Figure 1, with Primary Break Complete in .Ul Tubes Tubes 3, 4, and 5 show both layern fairly clear following primary b r e h ; therefore they define the critical ratio range of 65:35 to 35:65 for thio case. The majority phases of tubes 1,2,6, and 7 show secondary fog which will not clear up for a longer period of time. The minority phasea of theae tubes are crystal-clear.
Systems like nitrobenzene-water, in which either phase can be clarified by this technique, are called totally recoverable. Table I lists the systems found which could be so classified; mutual solubility is high in some cases and low in others znd, thus, cannot be a prerequisite to recoverability. Similarly, no relation is evident between phase density or interfacial tension and recoverability. The only characteristic common to these systems appears to be the presence, in each phase, of components exhibiting substantial polarity. 11utiixl solubility is evidently importari t
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
858
(:ulrlpuIielil
Compuirents A arid 14 Water-71-butanol Water-nitrobenzene Water-ethsl acetate Water-ethyl ether Water-nierh?-lethylketoxir Water-chloroform Water-trichloroethylene W-ater-aniline Water-methylene chloride Glycerol-ethyl acetate Furfural- wheptane
Phase Densities. G. per 311. 0.988/0.843 0.998/1.201 1 . ooa/o. 905 0.717/0.987 0.968/0.836 1.00+/1.483 1.00+/1.464 0.999/1.216 1.002/1.329 1.247/0.895 1,131/0.688b
.1 in Each Phase. Wt. % 92.2/20 99.8/0.22 92,013 93.5/1.2 77.4110 99.210 1 99.8/0.1b 96.5/5 98/? ?/O.ib 98.613. O b
1uterfyci:ii Tenvon, ErgsiCm. 24.6
25.
10:7ci
...
32,s
...
5.77 28.31 9.9b
a Other totally recoverable systems: saturated aqueous potasaiun. chloride--n-butanol, 1057, aqueous hydrochloric acid-nitrobenzene, 107, caustic soda-nitrobenzene, water-benzene-hydrochloric acid water-nheptane-n-butanol, glycerol-ethyl acetate-water, glycerol-ni'trob~nzenrethyl acetate. 811 other data from standarc b D a t a determined b y standard methods. references.
T IBLI: 11. PROPER TIC^
SEMIRECOVERABLE SYSTCM AT~ ~ 20" c.
OF
Coniponents A and B
Phase Densities, G. per MI.
Watern-heptane Water-benzene Water-carbon tetrachloride Water-carbon disulfide Glycerol-nitrobenzene Glycerol-carbon bisulfide Glycerol-benzene
0.997/0.68 0.998/0.879 0.997/1.585 0.997/1.26 1.247/1.20b 1.217/1.256b 1.247/0.872b
Component . I in Each Phase, Wt. 70
mjo: 13 99.9/0.01 99.8/0.8 O.lb
O.1b l.Ob
Interfacial Tension. Ergs/Cm. 61 35 45 48
12.1s
53 b
19b
a Other semirecoverable systems: water-tetrachloroethylene, glycerolcarbon tetrachloride. glycerol-trichloroethylene, glycerol-tetrachloroethyl. ene saturated aqueous potassium chloride-n-heptane, 10% aqueous caustic solution-benzene, water-lubricating oil (S..4.E. 20). glycerol-lubrieating oil (S.A.E. 20). b See footnote b , Table I.
Vol. 38, No. 8
the violence or type of agitatioii u r uther mealis ubed to furni the vmulsions, of the number of components present, etc. The recovery t,echnique is usually not operable with a dispersed nonpolar phase. An apparent exception to this generalization wae encountered in the system glycerol-nitrobenzene, xhich showed semirecoverable behavior in spite of the polarity of both phases. Careful rcdistillat,ion of the nitrobenzene t o remove any possible nonvolatile impurities failed to improve recoverability of the dispersed nitrobenzene phase. Since exceptions to the polarity rule appear to exist, and since it is often difficult to predict polarity, especially in multicomtems, a laboratory test should be made of the recoverthe system under study. The following procedure might br adopted:
Prepare two samples of the system in appropriate containers, wirh phase ratios of about 1:l5 to 15:1, respectively. Agitate violently, then allow to stand idle, and observe primary break, n-hereupon the majority layers in both tubes will show a secondary cloud. Take a small amount of the clouded majority layer from tube 8, add about four times its volume of the majority layer from tube B, shake vigorously for a few minutes, and alloa to settle. If the layer from tube A is clear after primary break, then clarification of this phase is possible. Repeat this test on the clouded majority layer from the second tube by adding an excess of its companion layer. shaking, and allowing to settle. If this layer can also be clarified, the system is completely rei:ovprable; other\vise it is semirecoverable. Tile observations presented here apply only when no significant arnounts of surface active components'are present. A stabilized emulsion is possible even in a two-component system when one of t,he components shows surface activity-for example, in a waterliquid soap system. On t,he other hand, small t,races of powerful emulsifiers are sometimes insufficient t o stabilize a syatrm. Each cape. therefore, must be studied individually. I S D b STRIAL APPLlCATION
to the extent that it serves to bring a polar coniponent into art otherwise nonpolar phase, as in the system furfural-heptane. Further tests showed that all other systems encountered coulii is, only one of the twube classified as semirecoverable-that component, phases responded t o the clarification technique described. In the system benzene-water, for example, a cloudy benzene phase could be clarified, just as in the case of nitrobenzene, by adding a n excess of the water phase, agitating, and allowing the system t o undergo primary break to yield a clear benzene layer. A clouded water layer with benzene dispersed in it, however, did not show any marked improvement, when subjected t o this clarification technique.
The recovery technique described will be found ineffective on a ilispersion of nonpolar droplets. This difficulty can be resolved by making the system totally recoverable by the addition of a polar material which is soluble in the nonpolar phases; butanol, for example, can be added to the semirecoverable system nraterbenzene. Such a procedure is usually not practical for obvious reasons. However. if the cloudy phase is formed by agitation,
EFFECT OF POLARITY
Table I1 lists the semirecoverable systenla encountered. They consist of components of lov mutual solubility in which one of the phases is largely made up of nonpolar components. It, was found, as before, that primary break is relatively rapid arid sharply distinguishable from secondary break ivhen the polar phase is dispersed. When the nonpolar phase is dispersed, however, primary break is slow and sometimes difficult to distinguish from secondary break. It was found, further, that the recovery technique always fails on the polar phase containing a cloud of dispersed nonpolar droplets. On the other hand, the technique is effective in the opposite case of dispersed polar droplets in a nonpolar phase. It is interesting t o note that the nonrecoverable benzene-water system could be converted to a recoverable system by addition of hydrochloric acid, which distributed itself behveen the two phases and thereby brought a polar component, into each (Table I). Addition of sodium hydroxide or potassium chloride, both of which are insoluble in benzene, did not improve the recoverability of the system (Table 11). This recovery technique, therefore, is usually operable when t,he dispersed phase is composed of polar droplets, regardless of
.A
B
Figure 3. RccoTery Demonstrated with the Totall\ Kecoverable System Water-Sitrobenzene, Which Teiids to Form Stable Secondary Fogs In 4, the middle tube is empty, the tube on the left contains a secondary fog of water in nitrobenzene, and the tube on the right contains a secondary fog of nitrobenzene in water. B was taken 3 minutes after miring the content8 of the end tubes in the middle tube, shaking, and allowing primary break to occur. In this case both layers are very clear (the dropletn visible in the bottom layer of the center tube adhere to the walle, and are not dispersed in the liquid). It should be noted that the small amount of minority layer accidentally present in B is crystal clear, aa expeeted at this phase ratio. The secondary toga in the majority phases of the end tubes did not clear up after many houn standing.
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
August, 1946
PHASE DROPLETS TABLE 111. C’HARGES os DISPERSED System .inilirie in water Water in aniline Nitrobenzene in waLer Water in nitrobenzene Carbon tetrachloride i n water Water in carbon tetrachloride n-Heptane in tiater Water in n-heptane G1ycerol in ethyl acetate Ethyl acetate in glycerol Carbon tetraetiloride in glycerol
’
Droplet Charge Negative Nolne
Negative None Negative None Negative None None None Sone
Type of System Recoverable
Recoverable Semirecoverable Semirecoverahle Recoverable Selnmcoverable
t,heii consideration should be given to the possibility of wduciug the proportion of polar phase to 20% and/or of redtieing the violence of primary agitation. Under these conditions the undesired secondary fog of nonpolar droplets dispersed in the pollir. phase were not easily formed in these experiments. If t’he clouded phase proves to be recoverable, some sort of ri?cycling operation can be undertaken in the industrial application of this technique. For example, if an A layer contacts a B layer. in an extraction procedure, and the phase ratio is such that the A Layer is fogged and the B layer clear after primary break, the former is first separated from the latt’er product by decantation. I t is then mixed and agitated, batchwise or continuously, with an excess of fogged B layer, from which operation the d layer becomes clear after primary break. The B layer used for this clarification is recycled indefinitely, so that no loss of material results. Other schemes of this sort can be readily worked out. It has already been indicated that the foregoing procedure applies only to simple, unst’abilized emulsions. This technique i R , apparently, sometimes applicable to stabilized emulsions aft,er conversion to the unstabilized type by removal of the surface agent. For example, if the emulsion is stabilized by a cationic surface active agent, this agent can often be effectively precipitated by the addition of an appropriate nmonnt of anionic.wetting agent, and vice versa. THEORETICAL DISCUSSIOS
Observation indicates that emulsion breaking iwolves growth o f the dispersed phase droplets, which results in their increasingly rapid travel to the interface where they unite with their parent phase. There are probably two important mechanisms by which this droplet growth occurs-by merging during collision and by the “solubility effect”. Collision between droplets in the colloidal size range is probably due chiefly to Brownian movement. For larger siee droplets Brownian movement becomes less pronounced, and collisions are much more apt to be caused by differences in displacement velocities. Displacement velocity is caused by the difference between droplet density and continuous phase density. Displacement velocity is small for droplets of colloidal siee but becomes substantial for larger droplets, since it increases with increasing droplet diameter. The faster moving, larger particles, therefore, overtake the smaller ones, with which they collide and merge. The second mechanism for droplet growth, solubility effect, is probably significant only when very small droplets are present along with larger on=. Under these conditions the material in the very small droplets is more soluble than that in the larger droplets; consequently the latter grow a t the expense of the former. Before the solubility effect can be substantial, mutual Jolubility must probably be relatively high. Its effectiveness, however, is indicated by the comparatively short life of secondary fogs in systems such as butanol-water, methyl ethyl ketone-water, and others of high mutual solubility. The rapidity of primary break can be explained on the grounds that all the droplets are well supplied -4th close neighbors, so that groTth is rapid. At increasingly high phase ratios, the con-
859
centration of dispersed phase droplm decreases, and so an increasing number have no opportunity to grow. These droplets remain behind to form a secondary fog in the continuous phase layer. The stability of this fog is evidently due to the slow displacement velocity of the tiny droplets of which i t is composed, which are now so far apart that, collision and growth occur only rarely. These fogs may be expected to become even more stable when the droplets are charged, for charge causefi them to repel one another. High stability is also evident when mutual phase soluhilitiea are low and when traces of impurities act as stabilizers. The clarity of the dispersed phase layer after primary break and a t higher phase ratios suggests that no dual emulsions (consisting of tiny droplets of continuous-phase material enclosed in disperse phase droplets) tend to exist’. The secondary fog sometimes observed in dispersed phase layers and a t phase ratios close to unity may be due to the presrnre of dual emulsions formed under these conditions. The recovery procedure proposed for clarifying secondary fogs iuvolves adding an excess of dispersed phase material to the clouded layer, agit’ating, and allowing the system to stand idle n-hile primary break occurs. This procedure provides the original dispersed-phase droplets with near-by neighbors with which they can readily merge. Often the quantity of dispersed phase material which must be added for clarification causes inversion of the emulsion to occur; under these conditions the originally dispersed droplets are closely surrounded with large masses of parent phase material Kith which they can readily unite. Explanations for the stability of dispersions of nonpolar d r o p lets in semirecoverable systems may now be considered. This stability does not appear to be due to an electrical charge, since, just as in the completely recoverable cases, these droplets were found to be charged in some systems but uncharged in others-for example, carbon tetrachloride-glycerol, Table 111. The presence of trace impurities which have surface active properties can also be ruled out in view of the purity of the reagents used in preparing many of these systems. Moreover, a system can be completely recoverable even though small amounts of surface active substances are present, as is shown by the behavior of a butanolwater system t’owhich traces of a commercial stabilizer like Daxad have been added. Similarly, low mutual solubility cannot serve to explain the stability of these droplets, since solubility in the totally recoverable system nitrobenzene-water ifi about the same as that in the semirecoverable system benzene-water. No correlation between interfacial tension or phase densities appears to exist. The evidence therefore suggests that there may be attractive or repulsive forces acting among dispersed phase d r o p lets, depending upon polarity, which also affect, the stability of the emulsion. ACKNOWLEDGMENT
The data on the electrical charge of dispersed droplets were obtained through the kind cooperation of E. A. Hauser of the Department of Chemical Enginerring, Massachusetts Institute of Technology. LITERATURE CITED
Alexander, J., ”Colloid Chemistry”, Vol. VI, New York, Reinhold Pub. Corp., 1943. Bennett, H., “Practical Emulsions”, Brooklyn, Chemical Pub, Co., 1943. Berkmrsn, S., and Egloff, G., “Emulsions and Foams”, Sew York, Reinhold Pub. Corp., 1941. Clayton, W.. “The Theory of Emulsions”, 4th ed., London, J. & A. Churchill, Ltd., 1943. Hauser, E. A , , and Lynn, J. E., “Experiments in Colloid Chemistry”, McGraw-Hill Book Co.. Inc.. 1940. Ostwald, W., Kolloid-Z., 6 , 103 (1910). Roberts, C. H. M., C o Z Z o ~ d S y m p o s i u . m ~ ~ ~ o n o g r12, a p h111 , (1035~. Selas Corp. Bulletin, Philadelphia, 1943. Stamm, -4. J., and Kraemer. E. 0..J . P h ys. Chem., 30,992 (1926).
