1966
SELWYB J. REHFELD
provement in the equipment which may provide differential diffusion data of accuracy approaching 0.1 to 0.27,, a t very low concentrations. For certain poorly conducting systems involving extensive solute-solute interaction, such data would be quite significant if they could be obtained. Acknowledgments.-This work was iiiikiated with the aid of a grant from Research Corpora-
Vol. 66
tion. Further support was obtained under U. S. Public Health Service Grant A-3508. IBhI Computer calculations were made possible by the availability of the M.I.T. Computation Center at Cambridge, Mass., through the Kew England Cooperating Colleges arrangement. We rnish to thank Dr. J. L. Bethune for help in aligning the optics.
STABILITY OF EMULSIONS TO ULTltACENTItIFUG,4TION : DISCONTINUITY THE CRITICAL MICELLE COXCENTRATION BY SELWYN J. REHFELD Shell Development Company, Emeryville, California Reeeiced March Y , 1981
A new and rapid method for quantitatively studying the mechanical stability of emulsions has been developed employing the ultracentrifuge. This experimental method offers the possibility of separating the coagulation of emulsions into distinct steps: aggregation and coalescence. Emulsions of benzene in an e ual volume of water, stabilized with various concentrations of sodium dodecyl sulfate, were evaluated for mechanical stabjity by measuring the volume fraction of emulsion that remains after various times in the ultracentrifuge a t 25,980 r.p.m. The mechanical stability decreases xith decreasing emulsifier concentration. Plots of volume fraction of emulsion remaining stable us. the logarithm of the concentration of emulsifier show a definite discontinuity. This sharp change in mechanical stability occurred at an emulsifier concentration corresponding closely to the critical micelle concentration (c.m.c.) determined in earlier studies. The results indicate that a t emulsifier concentrations below the c.m.c., a nbn-linear relationship between mechanical stability and time occurs.
Introduction Coagulation of emulsions is generally considered to take place in two steps; the first of these is aggregation, in which droplets of the dispersed phase form aggregates, while the second step is coalescence of the droplets. The present work was undertaken to evaluate the mechanical stability of oil-water emulsions as a function of concentration of surface active agent. Ultracentrifugation was found to offer a means for accomplishing the aggregation step very rapidly and thus producing a highly concentrated emulsion before appreciable coalescence can occur. The coalescence process then could be studied independently .
removal of water (aggregation step) a sizeable reduction in the volume of the emulsion occurs while the rotor is coming up to speed. With unstable emulsions various degrees of coagulation also occur leading to the development of a benzene layer distinct from the concentrated emulsion layer. The volume fraction of the cell1 occupied by the remaining Plots then were made emulsified layer was degignated as of 60 and +PZO vs. the logarithm of the emulsifier concentration. (The subscript on 4 denotes the time in minutes after the centrifuge reaches constant speed.)
+.
Experimental Results Boundary Changes during CentrifugationThe boundary between the uncoalesced emulsioii and the aqueous phase forms some time within the first three miiiutes of centrifugation and remains in a constant position during the entire run even when there is complete coalescencc of the emulsion. This fact indicates that almost all of the aqueous Experimental phase of the emulsion is separated from the benzene Materials.-The emulsifying agent, sodium dodecyl sulfate (NaDS), was synthesized a t this Laborat,ory. An droplets very early in the centrifugation process, a,queous solution of this material showed no minimum in leaving a very concentrated emulsion consisting of surface tension-concentration curves. Reagent grade ben- benzene droplets, adsorbed emulsifier, and a very zene, manufactured by Allied Chemical, wras the oil phase small amount of water. At this stage thc system in these experiments. Procedure.-A series of emulsions of benzene in water can be described as an aggregated and compressed was prepared with various concentrations of emulsifying emulsion. The droplets must assume some shape a ent. A 1: 1 phase ratio by volume was used. A Giffordother than spherical (polyhedral?) in order to have g o o d Eppenbach homo-mixer was used to emulsify a dispersed phase constituting so nearly 10070 by these systems a t a speed of about 7000 r.p.m. (Variac setting of 110 volts) and for exactly 5 min. Care was taken to volume of the concentrated emulsion. Any further reproduce conditions exactly in pre aring the emulsions. decrease in the volume of this aggregated emulsion Immediately after preparation 0.7 m f of the emulsion was is the result of coalescence of the benzene droplets injected into an ultracentrifuge cell (12 mm. light path and a and consequent separation into a distinct benzene 4 degree sector angle). The cell then was placed in an analytical rotor and subsequently centrifuged in a Beckman phase. Figure 1 shows the results of centrifugation of an Spinco Model E ultracentrifuge a t 25,980 r.p.m. (other speeds also can be used) and at 25.0 f 0.1". Photographic emulsion of 50% v. benzene in 50% v. water records were made of the cell a t regular time intervals (4 stabilized with 2.1% w. sodium dodecyl sulfate. min.) during the experiment. The first picture was taken immediately upon attainment of the pre-set centrifugal The actual volumes found, 48y0 v. benzene droplets speed. The time required to attain this speed was 2 min. in 52% v. aqueous phase, indicate a loss in volume and 45 seconds 15 seconds. During centrifugation three of benzene by evaporation in the emulsification layers develop in the cell : benzene (coagulated emulsion), the remaining stable emulsion, and the aqueous layer. As a result of the increasing emulsion concentration due to
( 1 ) T. Svedberg and K. 0. Pedersen, "Ultracentrifuge," Oxford Presa. New York. N. Y.. 1940 p. 7.
Oct., 1962
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7 967
STABILITY O F EMULSIONS TO ITLTRdCENTRIFUGATIO~
0 Emulsion Emulsifier: 2 . 1 % ~Sodium Dodecyl Sulfate
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3
7
11
15
t, minutes.
19
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480 720 960 1200 t, seconds. Fig. 3.-Frartion of emulsion rrmainin$ after t , seconds of centrifugation.
23
Fig. l.-Volume changes occurring in a stable emulsion as a function of time.
240
0 Aqueous Phase 0 Emulsion (Uncoagulated) Benzene (Coagulated Emulsion)
0
Emulsifier: 0 . 1 2 5 % Sodium ~ Dodecyl Sulfate
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3 ‘i 11 15 19 23 27 Fig 2 --Vohnne changes occurring in a n unstable emulsion as a function of time (in min.).
step. In {,hisrun the emulsion was stable throughout, the only effect being the removal of almost all of tJhe aqueous phase to leave a flocculated but stable benzene emulsion. Figure 2 shows the result for an unstable emulsion of 50% v. benzene in 50% v. water stabilized with only 0.125% w. of sodium dodecyl sulfate. During the first three minutes the aqueous phase is separated from the benzene droplets and the aggregated emulsion has coalesced to some extent, producing a definite benzene layer in addition to the remaining emulsion layer and aqueous phase. Plots of the volume fraction of emulsion, 4, us. time for a series of emulsions stabilized with NaDS are shown in Fig. 3. (Note that emulsions prepared with concentrations of NaDS below 0.18% m.gave non-linear plots.) The 4,, and +20 for each of the emulsions were plotted against the logarithm of YaDS concentration. A discontinuity was found at a, conceiitration of 0.26% w. KaDS (see Fig. 4). It is interesting that this value closely approximates the accepted value of the critical micelle concentration (c.m.c.) of KaDS. In the literature the values reported for the c.m.c. of NaDS by various methods are in the re. ~ spectroscopic gion of 0.18 to 0.25y9 t ~ The method utjlizing dyesa gives the low value and the pH method4the high value. T’alues of 0.18% w. (spectroscopic method), (2) W. C. Preston, J . Phys. and Colloid. Chem., 62, 84 (1948). ( 3 ) W. D. ICarklns, “Physical Chemistry of Surface rilms,” Reinhold Piibliuliin,: Coiy., New k o l h , N. Y.,1952.
0
I
Linear Coagulation Region
I + -
IA E ~ -
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t
0.01 0.1 1 10 Conceniration, yo w., sodium dodecyl sulfate. Fig, 4.-Fraction of emulsion a t time to and tzo as a function of concentration.
0.22% w. (conductivity method), and 0.22% w. (surface tension method) were found in this Laboratory for the c.m.c. of the NaDS used in these experimen ts. Discussions and Conclusions.-In the experiments described here, the oil droplets in oil-inwater emulsions have been forced by centrifugation into very closely packed arrays which can be described as aggregated emulsions. The resistance of the aggregated droplets to coalescence arises from the existence of very thin water films between droplets and from the presence of ionized emulsifier adsorbed on the particle surfaces. The coalescence of emulsion drops has been shown by Gillespie and Ridealj to be related to the rate of drainage of the thin films separating them, the probability of film rupture being an inverse function of film thickness. Van den Tempel’s6studies also indicated that the rate of coalescence of aggregated emulsions is determined by the average lifetime of the films separating the droplets. He derived equations which indicate that coalescence is a first-order process but his experimental data did not conclusively show a first-order process. Our results, obtained by a quite different experimental approach, indicate that the rate of coales(4) A. S. C. Lawrence and M. P. ILlcDonald. “Second International Congress of Surface Activity.” Vol. I. Academic Press, New York, N. Y., 1957, p. 385. (5) T. Gillespie a n d E. K. Rideal, T7ans. Faiudav Soe., 63, 173 (1 956). (6) M. Van den Tempel, “Second International Congress of Surface Activity,” Academic Preas, New Yolk, N. Y., 1957, kol. 1, p. 439.
1968
SELWYN. J. REI~FELD
cence varies as a function of emulsifier concentration, Further studies of the kinetics of coalescence will be reported at a later date. The sharp change observed in stability of emulsiolis to centrifugation when the emulsifier concentration passes through the critical micelle concentration is believed t o be associated with the density of packing of adsorbed emulsifier on the particle surface. As the emulsifier concentration is raised from very low values, the amount of emulsifier adsorbed on the emulsion particle increases and the area of the particle surface per adsorbed emulsifier molecule decreases. At the critical micelle concentration the density Of packing is at its highest value with the area per adsorbed emulsifier molecule approaching the actual molecular cross section. At this point, the mechaiiical stability of the adsorbed emulsifier layer can reasonably be expetted to be greatest in view Of the known force-area curves for adsorbed monolayers. The addition of emulsifier in excess of the critical micelle concentration should only increase the llumber of micelles in the aqueous phase without appreciable effect on the density of packing of the adsorbed emulsifier. Acknowledgments.-The writer wishes to thank Drs. F. M. Fowkes and W. M. Sawyer for determining the c.m.c. of the emulsifier used in these experiments.
DISCUSS103 S. J. REHFELD.-~wish to add the following to the experimental details given in this paper: The original particle size of the emulsions mas approximately 1 p ; this was determined microscopically. After 3 min. of centrifugation, the time required to bring the rotor to 26,980 r.p.m. and then stopping the run, the particle size had increased approximately tenfold. This increase in particle size also occurred above the critical micelle concentration with no visible co-
Yol. OB
agulation occurring. Therefore, the interfacial area was reduced drastically and the emulsiher concentration in the aqueous phase will be almost equal to but less than the emulsifier concentration originally used to prepare the emulsion. This increase in droplet size also has been observed using Oil, n-hexadecane. a more R.D. VOLD(Cniversity of Southern California).-Would you not agree that the separated oil should not be called a coagulated emulsion? The rate of appearance of clear oil be a measure of a rate of flocculation, a rate of coalescence, a rate of mass transfer of larger oil “drops” through the flocculated compressed emulsion, or a rate of coalescencr of discrete drops with the bulk oil. Calling the separated clear oil “coagulated emulsion” is merely likely to confuse the S, J. REHFELD -The process of coagulation is considered to take place in two steps, first aggregation, which results in t w o or more particles coming into close contact with no change in particle diameter (this step is reversible), and secondlv, coalescence, the floming of two or more particles together to form a new droplet of larger diameter. At this point a problem in terminology does exist. When does a droplet cease to be called a droplet but a continuous oil phase? The rate of coagulation is defined as the rate of transition of oil droplets t o a ckar continuous Oil phase; it would be proper, therefore, to refer to the continuous oil phase as coagulated emulsion, vrThich also drscribes the final Droduct in the coagulation Drocess. ITT. C. SIMPSON(Shell Development Company).-Can you comment on the applicability of this technique to emulsions stabilized with surface active agents other than sodium dodecyl sulfate? s. J. REHmLD.-The ability of Dresinate 214, a commercial emulsifier composed predominantly of the potassium salt of dehydroabietic acid, to stabilize oil-water emulsions to ultracentrifugation was examined using the same technique reported in this paper. A sharp break in the mechanical stability was found to occur between 1.1 and 1.2 weight .% Dresinate 214. The critical micelle concentration of this emulsifier was determined by Dr. TTT. W.Saqyer using interfacial and surface tension techniques. A sharp break in both surface and interfacial tension us. concentration occurred also between 1.1 and 1.2 wt. % Dresinate 214. I
