Oct., 19G21
ULTRACENTRIFUGAL METHOD FOR EMULSIOS STABILITY
1969
AK IJLTKACEKTRIFUGAL METHOD FOR THE QUAKTITATIVE DETERMINATION OF EMULSION STABILITY' BY ROBERT D. VOLDAND ROBERT C. GROOT Department o j Chemistry, UrLiversity of Southern California, Los Anyeles 7 , California Received March 12, 1062
B method was developed for use of the ultracentrifuge to determine the rate of separation of Sujol from Nujol-water emulsions stabilized with sodium dodecyl sulfate (SDS). This rate remains constant throughout separation of a large proportion of the oil in the emulsion, and can be used as a measure of the stability. The effect of differences in the purity and concentration of the SDS and of changes in the speed of centrifugation on the ultracentrifugal stability was determined. It is desirable to maintain constant drop size distribution despite differing SDS concentration in order to obtain unambiguous results. The ultracentrifugal stability increases uTith increasing bulk concentration of SDS until the equilibrium concentration in the aqueous phase reaches the critical micelle concentration, and thereafter is independent of Concentration. The results are :ilso correlated with the extent of adsorption of SDS at the oil-water interface.
Introduction The process of demulsification can be separated into two steps: flocculation, which decreases the number of kinetically independent units but causes no change in the total number of primary particles, and coalesceiice, which results in the ultimate separation of the oil as a bulk phase. In this paper emulsion stability is used exclusively to refer to the rate of coalescence. The chief objective of the present work was development of a satisfactory measure of this rate so as to permit quantitative comparison between experiment and the predictions of different theories as t'o the rate-determining step in the process. More particularly, it mould be valuable to ascertain whether the rate of drainage of water from the thin lamellae separating approaching oil drops, or the rate of rupture of the adsorbed film of stabilizer coat'ing the drops, is the more important fact,or. Many attempts have been made to measure the st'ability of emulsioiis in terms of changes in size distribution,2-5 interfacial or othermise.11312However, t'hose based on counting techniques are exceedingly tedious while those based on change of interfacial area or drop volume with time may be very insensitive to small but important changes occurring in the emulsion. Moreover, the latter methods may be useful only with quite unstable emulsions since Sujol emulsions stabilized with sodium dodecyl sulfate (SIX) (present work) showed very litt'le change in total interfacial area over a period of three weeks. Likewise, the average drop size in a 20% heat-bodied linseed oil (31 37 oi1)-80% water emulsion stabilized with (1) A partial report of work done under contract with the E. S. Department of Agriculture a n d authorized b y the Research and Marketing Act. T h e contract was supervised b y the Northern Ctilization Research and Development Division of the Agricultural Research Service. (2) S. Berkman, J . Phys. Chem., 39, 527 (1935). (3) E. K. Fisoher and W. 13. Harkins, ibid., 36, 98 (1932). (4) H. €1. 0. Jellinek, J . SOC.Chem. Ind., 69, 225 (1950). (5) E. S. Rajagopal, Kolloid Z., 167, 17 (1960). (6) A. King a n d L. N. Mukerjee, J . SOC.Chem. Ind., 87, 431 (1938). ( 7 ) A. Kine: and L. N. Mukerjee, ibid., 68, 243 (1939). (8) L. N. :Mukerjee a n d S. K. Srivastsva, Kolloid Z., 147, 146 (1956). (9) H. Lotzkar a n d D. Maclay, Ind. Eng. Chem., 36, 1294 (1943). (IO) W. E. Lloyd, J . Colloid Sei.,14,441 (1959). (11) 31.van den Tempel, Rec. tras. chim., 72, 433 (1953). (12) A. 5. (1. Lawrence a n d 0. Mills, Discussions Faradav SOC.,18, 98 (1954).
1% technical SDS did not change significantly over a period of a month.13 Earlier attempts14-16 to use rate of separation of free oil under gravity as a measure of emulsion stability were not regarded as promising because of the long time required for each measurement and the uncertainty as t o whether all the demulsified oil had separated sufficiently so as to be visually detectable. Interesting results had been obtained1' using a bucket type centrifuge, but preliminary experiments showed that S u j01-water emulsions did not separate oil sufficiently rapidly under these conditions. In extensive experiments with a Serval1 angle centrifuge at 10,000 r.p.m., oil separation was sufficiently rapid but reproducibility was unsatisfactory because of boundary disturbances occurring during deceleration or during the handling of the system necessary to separate and measure the demulsified oil. Hence a method was developed using a Spiiico ultracentrifuge permitting observation of the emulsion while undergoing ceiitrifugation.18 This method is described in detail and its use illustrated by application to determination of the effect of the concentration of SDS on the stability of Nujolwater emulsions. However, it, must be emphasized that results of the ultracentrifugal method will not necessarily correlate with the shelf stability of tEe same emulsion since they pertain to the highly concentrated cream remaining after rapid separation of most of the water in the ultrqcentrifugal field. The adsorption of SDS at the oil-water interface was also determined in these systems so as to permit correlation of the ultracentrifugal stability with the extent of coverage of the oil droplets with adsorbed emulsifier. Materials and Techniques Ultracentrifugal Method.-A Beckman Spinco Model ultracentrifuge was used in this work with a 12 mm. 4 (13) Unpublished data obtained in this Laboratory by B Levy a n d
R. L. Vold. (14) E. L. Lederer, Kollozd 2 ,7 1 , 61 (1935). (15) D. F. Cheesman a n d A. King, %bad.,83, 33 (1938) (16) D. F. Cheesman and 4. King, Trans. Faraday S o c ,
36, 211
(1940). (17) R C. WIerriIl, Ind. Eng. Chem (Anal. Ed 1, 16, 743 (1943) (18) While this work was in process it came t o the authors' attention t h a t Dr. E. R. Garrett was using a n ultracentrifuge to study pharmaceutical emulsions. This u o r k has nou been published [ J . Pharm. SCZ.,61, 35 (1962)1, b u t deals primarily with rate of flocculation and creaming although some measurements of oil separation are also reported.
ROBERT D. VOLDASD ROBERF C. GROOT
1970
standard cell having a capacity of about 0.8 ml. The emulsion is introduced through a hypodermic needle. Since small slicks of free oil often were observed in these emulsions after standing overnight they were swirled gently and inverted 20 times or so prior to sampling in order to obtain a uniform sample without changing the drop size distribution. On starting the ultracentrifuge, the current is held at 6 amp. until a speed of 10,000 r.p.m. is reached, and then a t 14 amp. until the desired operating speed is attained. The time required for acceleration to a constant speed of 39,460 r.p.m. was 8 min. Zero time in the oil separation experiments was taken as the time at which constant speed was reached. All runs were made a t 25". Upon centrifugation the emulsion separates into transparent layers of oil and water separated by a layer of opaque, concentrated emulsion. These give sharp boundaries on a photographic plate which change position with time as more oil is separated from the emulsion. Determination of the positions of the boundaries on a series of such plates taken a t successive time intervals permits calculation of the rate of separation of oil from the emulsion. Measurements were made with a travelling microscope reading direct1 to 0.01 mm., determining the distance from the top of tze cell to the oil-emulsion and emulsion-water interfaces, and to the bottom of the cell. Each distance was measured eight times and the best average value used. The volume corresponding to a length, A, in the sectorhaped cell is given by
where IZ. is the distance from the center of rotation to the top of the cell, and h is the thickness of the cell. A graph was plotted of the volume as a function of the measured value of A from the top of the cell using an optical enlargement of 2.17. This plot then was used to determine the volume from the differencesbetween the measured distances to the different interfaces. The per csnt of the initially emulsified oil which has separated at any given time is determined by dividing the volume of the oil layer by the sum of the volumes of the oil layer and the creamed emulsion. This implies that so little water remains in the creamed emulsion by the time steady speed centrifugal velocity is reached that its amount is immeasurable by the technique used. That this assumption is correct is proved by the fact that the sum of the volumes of clear oil and creamed emulsion remained constant throughout all runs independent of the amount of oil separated, as did that of the water layer. For example, in a typical run lasting 135 min. the length of oil plus emulsion was constant a t 1.625 f 0.0034 cm., while the water layer was 1.413 & 0.0040 cm. That the layer of clear oil contains no emulsified water was shown by adding 0.05Oj, of Orange I1 to one emulsion and finding after centrifugation that the aqueous layer was colored orange while the oil layer was completely colorless. The presence of residual traces of water in the emulsion layer is Ehown by its opacity and also by the fact that it showed a trace of color when Orange I1 was present in the system. Moreover, although slightly translucent during and immediately after centrifugation, it gradually became slightly more opaque on standing in contact with the aqueous layer, indicative of slow imbibition of additional water. The per cent of the initially emulsified oil which has separated then is plotted against the time of centrifugation. The rate of separation a t the given speed is determined from the slope of the linear portion of the curve. Materials.-In the earlier experiments testing reproducibility, for which numerical data are not recorded, Fischer Scientific Co. technical SDS was used without purification. In all other experiments a s ecially purified sample of SDS was used prepared by H. Princen following Dreger's method.lg The pure SDS, prepared from a mixture of t 5 o fractions of lauryl alcohol of setting points 23.40 and 23.71 , was extracted for a few days with ether in a Soxhlet extractor to remove traces of lauryl alcohol, and then stored in a desiccator over calcium chloride until used. There was no minimum in the surface tension-concentration curve, this showing that the sample was nearly free of lauryl alcohol.20 In
h.
(19) E. E. Dreger, Ind. Rng. Chew,., 86, RID (1944). (20) G. D. Nilen aud L Shedlovsky. J. Phys. Chem., 48, 37 (1044).
Vole 66
one set of experiments this SDS was again extracted with ether for 2 hr. just before use to further reduce the concentration of auryl alcohol, since even very small quantities can change the adsorbed layer of SDS a t a suiface from a mobile to a rigid film.21 Sujol from Plough, Inc., S e w York, was used without purification. The manufacturer's values for its constants are: Saybolt viscosity 360 to 390 a t 100'F. and specific gravity 0.880 to 0.900 at 60°F. Bottled distilled water WRY used without further purification. Preparation of Emulsions.-In general the requisite volumes Qf Nujol and an aqueous solution containing the desired concentration of SDS were poured into a cut-off 500-ml. graduated cylinder and stirred 5 min. at 5000 r.p.m. with a Rrookfield counter-rotating mixer, a stirrer with two counter-rotating blades which reduces absorption of air and foaming. The resultant emulsion was further dispersed by passing i t four times through a Cenco hand homogenizer. All results reported pertain to emulsions about 20 hr. old except for one sat dealing specifically with the effect of aging. In earlier experiments establishing the reproducibility of the method and studying the effect of changes in the speed a t which the ultracentrifuge was run, for which numerical data are not reported, the emulsions used were from 50 ml. of Nujol and 50 ml. of 0.2% technical &!!p~f tion. Emulsiona used to determine the effect of changing the rate of acceleration of the centrifuge were prepared the same way except that purified SDS was used. It is nwessnry that emulsions used to study the effect of SDS concentration on stability all have the same distribution of drop size so that they will have the same interfacial area and calculable degrees of surface coverage. Since preparation by the same procedure but with different initial concentration& of SDS would result in different drop size distributions the following method was employed. A series of emulsion8 was prepared by adding 150 ml. of Nujol to 120 ml. of 0.2y0 SDS solution and emulsifying as described above. After standing 20 hr. the emulsion wm stirred gently by hand with several slow inversions to reincorporate any separated oil or cream, and divided into 50-ml. portions. From each of these 5 ml. was removed, and then 5-ml. portions of SIX solutions of varying concentrations added with gentle hand stirring to give 50-ml. samples of 50-50 oilwater emulsions of presumably identical drop size distribution but varying concentrations of SDS. Ultracentrifuge runs were enerally made on the same day on which the additional EDS solution was added to the 20-hr. old stock emulsion. In Table I all emulsions made from the same stock b this addition process are given the same Roman numeraysince they are more likely to be directly comparable than others of the same composition but made from a different stoglc emulsion. That this procedure was generally successfu! is shown by the agreement between results obtained on difierent emulsions prepared by this technique. The error in the case of occasianal discordant runs probably arises either because of irrepwducibility in the preparation of the emulsion or from the dificulty of obtaining a completely representative sample in withdmwing the very small volume used in the ultracentrifuge cell. Determination of Adsorption Isotherms.--These were determined with the same emulsions used for study of the effect of concentration on stability following Cockbain'q except that the aqueous phase was separated for anaIysis by centrifugation. Twelve hours after preparation of the final emulsions from the 20-hr old stock, 25-ml. samples were centrifuged for 30 min. a t 5000 r.p.m. in IL Serval1 angle centrifuge subsequent to gentle hand mixing prior to sampling. The separated aqueous layer was removed with a hypodermic needle, and recentrifuged an additional 30 min. to ensure complete removal of any oil or emulrsion. Errors may orcur due t o incomplete separation of the emulsion or to coalescence occurring during centrifugation but the reproducibility and internal consistency of the results suggest that these are generally not seriouc, although occasionally results on a series of emulsions from a given preparation may vary erratically. The c0ncentr.r-
4
(21) M. B. Epstein, A. Wilson, C. W. Jakob, L. E. Conroy. and J . Ross, ihid., 58, 800 (1954). (22) E. G . Coekbain, Trone. Pmudag Soe., 60, 874 (19.54).
l ; ~ ~ r z h ~ ~ ~ ~ i 'An~ Er Th~' IuO ~DFOR . ~ ENMULSION ~ STABILITY
Oct., 19c2
CLTRACEKTRIBUGAL STABILITY AT 39,460 R.P.M.
Stock,a emulsion
Emulsion number
Initial concn. of SDS, % on water phase
AXD
1971
TABLE I INTERFACIAL ADSORPTION OF NUJOL-LVATIR EMUISI~NS STABIIJZED
WITH SODIUM DOUECYL &JLFA'l'E ---Equilibrium conon. SDS-Extrapolated Moles X loa/ Moles X 106 R a t e of % 011 liter in adsorbed, separated oil sepn., aqueous phase per ml. oil at zero time %/min.
Fraction of saturation adsorption
*. 22 2.96 0.398 0.51 0.2 0.64 3.00 .409 .55 25 .2 -66 2.84 .410 21 '60 .2 .65 2.78 .416 23 .49 .2 .70 5.90 .451 14 .32 .3 .76 5.70 ,472 10 .3 .31 5.38 .504 .78 16 .3 .30 * 86 6.59 .557 10 .35 .28 .. 8.11 .57i .15 1.5 .4 .86 8.40 .549 (4.4)b .4 ( .24)b .96 7.93 .596 I11 * 17 2.0 .4 .93 9.66 .597 IV .45 3.8 .I6 .. 2.6 ... ... .5 I .I4 .88 11.73 ,563 I1 ( .20)b (3.3Ib .5 .89 XI . i 8 ,558 ITT .14 0.7 .5 .. I ... ... .6 .I4 .3 .93 7.89 ,600 IV .4 *. ... .88 11.68 .568 '!I .5 .. ( .98)' 14.51 ,633 IS' .6 .. ... (1.03)b 17.68 .662 TV .7 .. ... 0.96 25.03 .622 IV .9 .. * * . .. V' 200 25 ... .2 .62 .64 VIS 3.45 ,350 210 .2 24 .63 1'1 -71 6 51 ,391 -3 .. ... .. V 203 ... ... .35 .26 7.1 .78 VI 213 .35 7.88 .427 6.6 .25 V 204 ... ... .. 5.7 .4 -22 214 VI .81 9.48 .441 .23 5.4 .4 V 205 .. ... ... ,45 .23 4.7 215 VI ( .76)' I1 44 ,418 .45 .21 4.1 140d A .. .2 26 .62 ,.. B 142d .. .2 ... ... 26 .48 A 1416 .. 26 .2 .62 R 143" .2 ... .56 25 144 c .2 ... ,.. .. 24 .62 1) 147 .. .2 25 .53 E' 145 .2 .. 19 .62 Stock emulsions I through VI were prepared with 150 ml. of Nujol and 120 ml. of water, and then adjusted to H 50-50 volume ratio before measurement. Emulsions A through E were prepared using 50 ml. of Nujol and 50 ml. of water. Xumbers enclosed in parentheses can be assumed to be in error, The SDS used for preparation of emulsions V and VI was extracted with ether for 24 hr. just prior to use. d In these cases the ultracentrifuge 'was accelerated more rapidly than usual. reaching final speed in 5 min. e In these cases the ultracentrifuge was accelerated much more slowly than usual. The speed was increased gradually over a 35 min. period to 13,000 r.p.m., and then to the final speed of 39,460 r.p.m. over the next 5 min. Orange I1 (0.05y0 on the water phase) was added to this system before emulsification. 1
120h 125A 131A 160 12613 132B 161 162 123C 127C 133c 163 124D 128D 134D 122E
I1 I11 IV I1 I11 IV IV I I1
...
...
.
.
I
. I .
(I
'
tion of SDS in the aqueous phase was determined by titralion with cetylpyridinium b r ~ m i d e ,e1imi;ating ~ ~ . ~ ~ the need for a blank by using a constant volume of l o ml. of chloroform and E;.5 ml. of water. The cetylpyridinium bromide was standardized on a relative basisa&against pure SDS, The data obtained fitted the linear form of the Langmuir equation
C'/.r/m = I/ab
+ C/a,
Over the .c&ole range from 3.0 to 25 x 10-3 mole SDS/1. of slope at the critical micelle mrith no detertable concentration26 (0.oogl mole/l.), as was also found to be - - .(23) S. R. Epton, Trans. Fnrod,z?j Suc., 44, 226 (1948). (24) A. 8. hveatherburn, J . Am. 0 2 1 Chemists Xoc., 28, 233 (1951). (25) R. D. Vold and N. €1. Sivaramakrishnan, J . Phz/a. Chem , 62, 984 (1958).
the case with Cockbain'g data.22 The equilibrium concentrations of SDS in solution and a t t,he interface, together with the fraction of saturation adsorption reached, i.e., ( z / m ) / u , which i s related to the fraction of the surface covered with adsorbed SDS,are given in Table I. To determine the rate of change of int,erfacial area wit)h time, three emulsions of 150 ml. Nujol-120 ml. 0.201, SDS were prepared as described previously, combined, and stored in a I-liter graduated cylinder a t room temperature. At different time intervals 200-ml. portions were withdrawn and divided into four 45-m1. samples, to each of which was added 5 ml. of more concentrated SDS solution so as to deternline four points on the adsorption isotherm of the resultant 5050 oil-water emulsion as described above. The interfacial area was determined from the Langmuir plots for these (28) R. .J. Williams, J. N. Phillips, and K. J. Mysels, Trans. Farodav Sue., 61, 728 13925).
centration of SDS, as is strikingly shown in Fig. 2 , all emulsions prepared with SDS concentrations between 0.4 and 0.6y0 SDS separating oil at the same rate within experimental error. I /-Quantitative data on the effect of SDS concentration on the rate of separation of oil, the el”fect of difference in the time of acceleration of the ultracentrifuge to its operating speed on the rate of oil separation, and the extent of adsorption of SDS at the oil-water interface in these emulsions are shown in Table I. Frequently coiisiderable differences were found in the absolute quantity of oil separated by supposedly identical emulsions although the slopes of the linear portions of the curves of oil separated us. time were nearly the same. The linear portions of the curves were therefore extrapolated to zero time, as shown in Fig. 1, and these 1 I I I I 20 40 6C 80 100 120 140 extrapolated values of per cent oil separated at zero time also listed in Table I. Time of Centrifugolion in Min Fig. 1.-Rate of separation of Sujol at 39,460 r.p.m. Generally there was no difficulty in drawing a from a 50 volume yGSujol-in-water emulsion stabilized with straight line through the points obtained in the 0.2% SDS. interval between 20 and 60 min. of centrifugation, emulsions after 1, 96, and 504 hr. aging, the data being and extrapolating to zero time so as to obtain the given in Table 11. slope and the intercept. In order to avoid arbiTABLE I1 trariness, however, the values recorded in Table I, and used in the construction of Fig. 2 , were obEFFECT O F B G I S G O N INTERFACIAL ADSORPTION OF SDS I N tained from the best line calculated by the least NUJOL-WATEREMULSIOSS squares method. Comparison of results obtained Equilibrium conon. SDS Initial conon. Moles X 106 Fraction of by the two methods showed they agreed within 3 of SDS, % on Moles X IOa/liter adsorbed Der saturation water phaae in aqueous phase ml. oiladsorption to 4% of the value of the slope although occaEmulsion 1 hr. old sionally differences as high as 10% were found, the least squares method tending to give lower slopes 0.2 2.76 0.418 0.61 and higher intercepts. .3 5.26 ,514 * 74 .4 8.18 .571 Figure 2 shows the effect of the concentration of -83 .5 11.39 .597 .87 SDS on the rate of separation of oil from the emulsion in the ultracentrifuge. In order to facilitate Emulsion 96 hr. old comparison of runs on different emulsions with the 0.2 2.79 0.416 0.59 same concentration of SDS the extrapolated per .3 5.55 ,487 .69 cent oil separated at zero time is subtracted from .4 8.22 .566 80 the actual amounts separated at any given time. .5 11.34 .605 .85 Figure 2 also shows the degree to which the loss of Emulsion 504 hr. old oil by the different emulsions conforms to the 0.2 2.75 0.419 0.55 straight line relationship, and the extent of agree.3 5.56 .485 -63 ment between runs at the same composition but on .4 8.10 .580 .75 different preparations of the emulsion. .5 11 16 .620 .81 With respect to reproducibility, it can be seen Results from Table I that rates of oil separation generally Figure 1 is characteristic of the curves of per agreed within 4 to 8% in runs on comparable samcent oil separated us. time of ultracentrifugation ples, although occasionally an individual sample when using low concentrations of SDS. Initially differed widely from the average for its composition. oil separates very rapidly but the rate-determined Runs at 39,460 r.p.m. on six samples from four from the slope of the curve-decreases rapidly with separately prepared 50% Nujol-joye water-0.270 lime during the first twenty minutes or so, during tech. SDS emulsions gave an average value for the which period with 0.2% SDS as emulsifier slightly rate of separation of oil of 0.136 O.O04%/min. more than 20% of the emulsified Kujol separates as The larger irreproducibility in the values for the a layer of clear oil. The rate of separation then amount of oil which separates rapidly at the beginremains constant for a considerable period until ning of the run may be due to uncontrolled fluctuaabout 60% of all the oil in the system has separated, tions in the degree of dispersion obtained during and thereafter decreases slowly with further separa- preparation of the emulsions, or to lack of represention of oil. tative sampling on withdraming the small samples Increasing the concentration of SDS a t first de- used in the ultracentrifuge cell. A few large creases the rate of separation of oil from the emul- droplets of poorly emulsified oil would have little sion and also the amount of oil which is separated effect on the measured values of interfacial area rapidly on initial centrifugation before the steady but would make a disproportionately large contristate rate is reached. At higher concentrations bution to the volume of oil appearing, and would be the rate beccmes independent of the initial con- likely to separate very rapidly on initial application I
I
1
1
1
I
I
Oct., 19162
ULTRACEKTRIFTTGAL RImI-Ion FOR E M ~ L S I STABILITY OS
*
,>-n
l!Ji,i
3 Time of Centrifugation in Minutes Fig. 2.--Effect of concentration of SDS on the rate of separation of Nujol from Xujol-in-water emulsions centrifuged a t 39,460 r.p.m.: 0, 0 , 8, 8 , all 0.2y0,from emulsions 120A, 125A, 131A, and 160; D, 0 , m, all 0.37,, from emulsions 0.457,, from 126B, 132B, and 161; V , 0.3570, from emulsion 162; A , AL, both 0.4%, from emulsions 123C, 133C; 0-, emulsion 163; 0,0, both 0.5Yc, from emulsions 124D and 134D; e, 0.6%, from emulsion 122E.
of the iiiltracentrifugal field. Consequently, two of the SDS just before use increased the rate of nearly identical emulsions may separate quite separation of oil from 0.16% to 0.22%,/min. Effect of Centrifugal Speed.-Rates of oil sepadifferent quantities of oil in the initial stage of centrifugation but thereafter behave alike with ration were determined on duplicate samples of 50 ml. Nujol-50 ml. water emulsions stabilized respect t,o the rate of further separation of oil. That Lhe emulsification technique used produced with 0.2% technical SDS at speeds of 10,589, 25,nearly the same initial distribution of droplet sizes 890, 33,450, 39,460, and 56,100 r.p.m. The rate of in most cases is shown by the relatively constant oil separation decreased rapidly with decreasing value found for the oil-water interfacial area of speed of centrifugation, and with the emulsions the different stock emulsions. Assuming an area used was zero for at least 2.5 hr. of centrifugation of 50 per adsorbed SDS m o l e ~ u l e , ~the ~ - ~ ~at 10,589 r.p.m. This suggests the interesting interfacial areas of stock emulsions 11, 111, and IV possibility that coalescence may not occur in exof Table I were found to be, respectively, 1.92, periments of this type until the force of the centrif1.85, and 1.93 X lo4 cm.2/ml. oil. Adsorption ugal field just exceeds the rupture strength of the data obtained with emulsion I, however, were so adsorbed layer of emulsifier, and so may provide a irregular as to prevent determination of its area. means of measuring the latter. Emulsion YI, prepared in the same way as the Effect of Aging on Interfacial Area.-From the others but using freshly extracted SDS, had an Langmuir plots of the adsorption data in Table I1 interfacial area of 1.64 X lo4cm.2/ml. oil. it can be calculated that the interfacial area after 1, Effect of Iml~uritieson Ultracentrifugal Stabil- 96, and 504 hy. is, respectively, 2.07, 2.12, and 2.25 ity.-The ultracentrifugal stability decreases the X lo4 cm.2/ml. oil if the aTea per adsorbed SDS greater 1,he purity of the SDS used as an emulsifying molecule is taken to be 50 A.2 It is evident that agent. With 50% Xujo1-50% water emulsions the rate of change of area with time in this emulsion centrifuged a t 39,460 r.p.m. (113,154 g) the rate is too small and too slow to afford a satisfactory of loss of oil was 0.136%/min. when 0.2% technical criterion of emulsion stability. Moreover, it SDS was used as emulsifier but rose to 0.51% using might have been expected that the area would depure SDS. Ether extraction of the SDS immedi- crease with time as the drops coarsened, whereas ately prior to use further increased the rate to 0.63%/ instead it increases somewhat. Consequently the min. Likewise at 0.45y0 SDS, ether extraction larger drops must disappear from the emulsion (27) F. van Voorst Vader, Trans. FaTaday Soc., 66, 1067 (1960). more rapidly than the small drops, either by (28) W. ]
