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oil emulsion formation
Water-Cyclohexane-"Span 80"-"Tween 80" Systems: Solution Properties and Water/Oil Emulsion Formation ROBIN DAVIES,* DAVID E. GRAHAM,~" AND BRIAN VINCENT *'1 *Department of Physical Chemistry, University of Bristol, Bristol BS8 1TS, UK, and ~fB. P. Research Ltd., Sunbury-on-Thames, Middx, TW16 7LN, UK Received March 19, 1986; accepted June 30, 1986 The solution properties ofsorbitan monooleate (Span 80) and ethoxylated sorbitan monooleate (Tween 80) in cyclohexane are discussed. It is shown that aggregates of nominal "radius" ~65 nm are formed in the former case at concentrations up to ~0.5 mole dm -3. The aggregation behavior is very sensitive to the presence of water. On the other hand, the Tween 80 + cyclohexane system shows phase separation behavior. At ambient temperature a two-phase (initially turbid) region appears at very low Tween 80 concentrations returning to single phase at higher concentrations. Lamellar-type structures form in the two-phase region while spherical aggregates (radius < 100 nm) seem to form in the single-phase region. With (approximately) equimolar mixtures of Span 80 and Tween 80 in cyclohexane, small spherical micelles (~7-nm radius) are formed. The addition of water to Span 80 (HLB 4.3) did not lead to useful W/O emulsions, but addition of water (or NaC1 solution) to equimolar solutions of Span 80 and Tween 80 did, at least over the water volume fraction range ~ 1-5%. These droplets were reasonably monodisperse (in the range 30-100 nm), but an unexpected feature was that their mean radius decreased with increasing aqueous phase volume fraction. © i987AcademicPress,Inc.
INTRODUCTION
tivity to changes in conditions (e.g., dilution, temperature). In this paper we discuss the formation and characterization of aqueous/cyclohexane emulsions, containing monodisperse droplets of radius 40 to 80 nm, in which the internal aqueous phase contains up to 10 wt% ( ~ 2 mole dm -3) sodium chloride. The surfactants used were sorbitan monooleate (Span 80), ethoxylated sorbitan monooleate (Tween 80), and their mixtures. The droplets were characterized using photon correlation spectroscopy and freeze-fracture electron microscopy. The only previously reported extensive study of the use of Span/Tween mixtures for the formation of water/oil (and oil/water) emulsions, to the author's knowledge, is that of Boyd et aL (1), who correlated the stability of the emulsions to coalescence with the interfacial rheological characteristics of the oil + Span/water + Tween systems.
Water-in-off emulsions have received much less attention in the academic literature than oil-in-water emulsions. Yet their importance in a wide variety of technologies is well recognized, e.g., in the oil industry (recovery and dewatering processes), the food industry (margarine, etc.), cosmetics, cleaning fluids. In particular, there have been few studies in which the internal aqueous phase contains electrolytes. In order to study the properties of emulsions of this type, model reproducible systems containing monodisperse droplets are required. The so-called "microemulsions" achieve these objectives in part, despite their requirement for high concentrations of surfactants (frequently > 10% of the system) and their sensi1Author to whom correspondence should be addressed.
88 0021-9797/87 $3.00 Copyright © 1987 by Academic Press, Inc. All fights of reproduction in any form reserved.
Journal ofColloidandlnterface Science, Vol. 116, No. 1, March 1987
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EXPERIMENTAL
Materials All water was doubly distilled from an allPyrex apparatus prior to use. Cyclohexane and
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"SPAN" AND "TWEEN"
other hydrocarbons (B.D.H. "AnalaR" grade) were dried over calcium hydride, where necessary, and distilled. Sodium chloride (B.D.H. "AnalaR" grade) was used as supplied. The formulas of sorbitan monooleate (I) and ethoxylated sorbitan monooleate (II) are OH
I
HLB = 4.3 Span 80 0 (CH2CH20)x H
0
II
O,.,.v.....~ O(CH2 CH20 )z H
HLB= 15Tween80(x+y+z=20)
Both were obtained from Koch-Light laboratories as Span 80 and Tween 80, respectively.
Emulsification Two methods were used: (a) Homogenization. A Silverson homogenizer unit was used, operating at 5000 rpm for 1 min. No further changes in droplet size were generally observed after this time, in line with Gopel's previous observation (2). (b) Sonification. A Dawe "Soniclean" (Type 7532A) ultrasonic bath, operating at 20 KHz for about 5 min, was used. Again a limiting droplet size was generally obtained within this period.
Droplet Size Determination Photon correlation spectroscopy (PCS) was the main method used. The apparatus comprised a Coherent CR-2000K krypton ion laser, with "PS 100" Malvern optics and a Malvern K7025 multibit correlator. Data storage and analysis were carried out using an on-line "Apple II" microcomputer. "Particle" radii were calculated in the usual manner from
the translational diffusion coefficient, obtained from the time autocorrelation function, g(t). The correlation delay time (r) was chosen such that g(t) decreases to about g(0)/2 within about 10-20 r; this normally resulted in good exponential fits of the g(t) data. Measurements were made over a range of scattering angles, 80-120 °. All solutions were filtered before study, using an appropriate pore-size Millipore filter. Particle radii were also determined in some cases using a time-averaged light-scattering technique, based on a modification of the standard Zimm plot suggested by Yang (3). A Sophica 4200 light-scattering apparatus was used. Further determinations of particle radii were also made from sedimentation coefficients using a Beckman Spinco Model E ultracentrifuge.
Freeze-Fracture Electron Microscopy (FFEM) Electron microscopy of liquid droplets was not possible prior to the advent of freeze-fracture techniques, developed in the main for the Journal of Colloid andlnterface Science, Vol. 116, No. 1, March 1987
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study of biological systems. Even so most studies have been confined to emulsions in which water was the external phase; there have been very few studies, as far as the authors are aware, of hydrocarbon-continuous emulsions (4). For this reason the techniques used here are discussed in some detail. In the first method replicated samples were prepared prior to investigation by transmission electron microscopy (TEM). A small drop of emulsion was placed on a gold supporting stub which was then dipped into a suitable freezing medium (Freon or nitrogen "slush"). The stub with the frozen sample was then transferred to the cold stage (maintained at - 1 5 0 ° C ) of a Balzers 360M freeze-etch unit, the chamber of which was evacuated to a pressure of < 10 -5 Torr. The sample was then sliced with a blade precooled with liquid nitrogen. The fractured surface was then immediately replicated and shadowed with platinum and carbon. The replica was "floated off" from the sample in a suitable solvent: normally water would be used for this purpose, but in this case a cyclohexane-miscible solvent was necessary and hence toluene was used, even though the replica tended to sink rather than float. Some other FFEM experiments were carried out in which the fractured surface was prepared in situ on the cryostage of a scanning electron microscope (SEM). In this method two small metal rivets are filled with the emulsion (or surfactant solution) to be studied; one is inverted and placed on top of the other. The pair is then transferred to the cryostage contained in a vacuum chamber (pressure <0.2 Torr). The top rivet is "knocked away," revealing the fractured surface of the frozen liquid which is then replicated with gold.
Dilatometry In order to study the transfer of water into cyclohexane solutions containing surfactants a dilatometry technique was developed. The apparatus is shown schematically in Fig. 1. Water contained in a dialysis bag is fixed to a glass support with a protruding capillary atJournal of Colloid and InterfaceScience, Vol. 116,No. 1, March 1987
C
D
0
FIG. 1. Dilatometer. A, aqueous phase; O, cyclohexane; C, capillary; D, dialysis tubing; M, magnetic stirrer.
tachment. The water level is adjusted to be near to the top of the capillary. The dialysis tubing, carefully dried on the exterior, is then placed in the cyclohexane solution as shown. The whole apparatus is mounted in a thermostat bath, and the change in the water level in the capillary monitored using a cathometer.
Interfacial Tension The interfacial tension between the cyclohexane phase, containing the dissolved surfactant, and bulk water or aqueous electrolyte solution was determined using a Kruss Spin-
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W/O EMULSIONS WITH "SPAN" AND "TWEEN"
ning Drop Interracial Tensiometer. A 4-/~liter droplet of the cyclohexane phase was suspended in the spinning tube. The concentration of the surfactant (mixture) was chosen to give approximately the same number o f molecules per unit interfacial area as in the emulsion studies.
~
°
"
RESULTS AND DISCUSSION
Prior to discussing the formation and characterization of water/cyclohexane emulsions, the phase structure of the individual surfactants (Span 80 and Tween 80) in cyclohexane and cyclohexane/water mixtures will be discussed.
Span 80 + Cyclohexane The concept of micellization in nonpolar solvents has been much debated in the literature; in particular, the existence of a critical micelle concentration (CMC) per se has been questioned and casts doubt on reported CMC values (5, 6). However, some form of aggregation usually occurs, and the resultant aggregates, in general, solubilize water. For example, "Aerosol O.T." (sodium diethylhexylsulphosuccinate) does form inverse micelles in various hydrocarbons (6, 8); in dry isooctane they have a radius of 1.5 nm and swell to a limiting size of ~ 18 n m with solubilized water at ambient temperature (9). Dry Span 80 dissolves readily in dry cyclohexane to give solutions whose turbidity rises linearly with increasing concentration. There is no indication of a discontinuity over the concentration range studied. Moreover the particles present appear to have a constant average size (equivalent spherical radius) of "~ 65 + 5 nm, as determined by PCS measurements, at least up to concentrations of 0.05 mole d m -3. This implies that there are no problems associated with multiple scattering and/or particle interactions, at least over this concentration range. Clearly, however, the particles in this case cannot be simple spherical micelles. Assuming, as a limiting case, the particles to be composed purely of Span 80 mol-
csx 10-2/(mo1 din-3)
FIG. 2. Turbidity (r) versus concentration (cs) of Span 80 in cyclohexane.
ecules (i.e., no solubilization of cyclohexane), then the aggregation number per particle would be ~ 106. Some form of multibilayer structure is probably involved, maybe in extended (rod or sheetlike) form or possibly in the form of vesicles having a more or less spherical morphology. In the latter case, since the extended length of the sorbitan monooleate chains is 2.9 nm, this would correspond to an average of some 10 or 30 concentric bilayer tings per vesicle. H bonding between hydroxyl groups on the sorbitan rings is probably the main factor controlling the aggregation behavior. The presence of even trace amounts of water in the cyclohexane would facilitate this H bonding. Indeed, it was observed that if the system is not rigorously dry (and maintained that way), then both the turbidity of the sample and the apparent size of the particles, as measured by PCS, increased slowly with time. For example, in a solution of 0.05 mole dm -3 Span 80 in cyclohexane, which had not been dried and which was left open to the atmosphere, the apparent particle size reached 300 + 30 nm after about 1 h; after this time some setting of the aggregates was observed. This increase in size cannot be due solely to solubilization of water alone; some aggregation of the basic (65-nm) particles must also occur, possibly as a consequence of water "bridging" between exposed sorbitan ring hydroxyls during particle collisions. Journal of Colloid and Interface Science, Vol. I 16, No. 1, March 1987
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In order to investigate the effect of water and the PCS results indicated that the aggrefurther, dilatometry experiments were carried gation process was still occurring slowly with out using the apparatus described earlier (Fig. time, even after ~ 3 h. 1), maintained at 20°C. As well as monitoring the decrease in the water level in the capillary, Span 80 + Cyclohexane + Water," the apparent particle size was determined peEmulsification riodically by PCS by sampling the cyclohexane phase. Clearly the presence of Span 80 particles or The PCS results are shown in Fig. 3. The aggregates complicates the emulsification procorresponding decreases in the volume of the cess of bulk water into cyclohexane containing water phase after 1 h were 6 m m 3 for pure this surfactant. Nevertheless, reasonably satcyclohexane, 18 m m 3 for 10 -3 mole d m -3 isfactory water/cyclohexane emulsions, over a Span 80, and 24 m m 3 for 10-2 mole d m -3 wide range of water volume fractions, could Span 80. The value for cyclohexane corre- be prepared with Span 80 dissolved in the cysponds to a dissolved water concentration of clohexane phase. For example, for a 5 vol% 0.026 wt%. For the 10 -3 mole dm -3 solution water/cyclohexane emulsion, a m i n i m u m o f " S p a n 80" in cyclohexane, if all the excess concentration of 5 × 10-3 mole d m -3 Span water (i.e., over and above that required to 80 in cyclohexane was found to be necessary saturate the cyclohexane) had been solubilized to produce droplets with long-term stability; into the particles, then their volume (assumed that is, there was no indication of phase sepspherical) would have approximately doubled aration after some 3 h centrifugation at 10,000 (i.e., from -~9 × 105 n m 3 to ~ 17 × 105 nm3). rpm in an MSE Hi-Spin 21 centrifuge. This would be equivalent to a particle radius In Fig. 4 typical TEM pictures from FFEM increase from 65 to ~ 80 n m after 1 h. For the studies are shown for a 20 vol% water/cyclo10-2 mole dm -3 solution, the corresponding hexane emulsion prepared by homogenization increase in radius would be even smaller, i.e., with 5 × 10-2 mole dm -3 Span 80 in cyclofrom 65 to ~ 6 8 nm. These results, when hexane. It would seem that most of the dropcompared to the apparent radius determined lets have radii typically in the range 100 to from PCS (Fig. 3), suggest that particle aggre- 2000 nm. gation in the presence of water, and not just Because of the presence of Span 80 particles swelling, is occurring. Both the dilatometer and maybe aggregates, as well as the emulsified water droplets, dispersed in the cyclohexane, it was felt that light-scattering studies (PCS or total intensity) would be difficult to interpret E and therefore inappropriate. Moreover, at 5 E / vol% water problems o f multiple scattering/ o J particle interactions are likely to arise, so that dilution of the emulsion is necessary. The acA cepted practice is to dilute with the continuous phase, i.e., in this case cyclohexane containing 5 X 10 -2 mole dm -3 Span 80, in order to avoid the possibility of stabilizer desorption from the droplet interfaces. Indeed, it was observed that dilution with pure cyclohexane did lead to ini 5~0 100 150 stability and phase separation. However, dit / rain lution with Span 80 solutions in cyclohexane FIG. 3. Apparent aggregate"radius" (a) as a function would result in the introduction o f further of time (t). A, 10-3 mole dm-3 Span 80 in cyclohexane; particles. B, 10-2 mole dm-3 Span 80 in cyelohexane. Journal of Colloid and Interface Science, Vol. 116, No. 1, March 1987
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W / O EMULSIONS W I T H "SPAN" A N D " T W E E N "
FIG. 4. Freeze-fracture transmission electron micrograph for a 20 vol% water/cyclohexane emulsion prepared by homogenization of water in cyclohexane containing 5 × 10-2 mole d m -3 Span 80. (Magnification: 2250×.)
Thus, although Span 80 alone forms reasonable water/cyclohexane emulsions, the presence of excess surfactant particles renders them somewhat unsatisfactory as model systems. For this reason studies were undertaken on mixtures of Span 80 and Tween 80.
Tween 80 + Cyclohexane Solutions of Tween 80 in dry cyclohexane behave rather differently from solutions of Span 80 in this solvent. The Tween 80/cyclohexane system shows a two-phase region, with a lower consolute point. Hence at 20°C, if the initial turbidity of the system is determined as a function of Tween 80 concentration (CT),the turbidity rises very rapidly at very low concentrations. In this concentration region, two translucent phases separate on standing. Beyond about 7 X 10-2 mole d m -3 the (initial) turbidity drops suddenly to give a stable translucent system. This corresponds to the tran-
sition from the two-phase to the single-phase region. I f a solution in the single-phase region (i.e., Cz > 7 X 10 --2 mole dm -3) is heated, then at a certain temperature it becomes turbid, indicating that the two-phase region has been entered. The Tween 80 + cyclohexane system, in the vicinity of the two-phase --~ one-phase transition concentration region (at ambient temperature), was investigated using PCS and FFEM (in this case SEM). The PCS data are 7 × 10 - 2 mole shown in Fig. 5. Beyond CT dm -3, particles having an apparent "'radius" of 60-70 nm are formed, whereas below CT 7 × 10-2 mole dm -3, the apparent size rises very rapidly (with large error bars), indicative of some form of aggregation process. The SEM picture shown in Fig. 6 refers to the structure of the Tween 80 + cyclohexane system in the single-phase region (CT = 8 X 10 -2 mole dm-3). There is evidence for the existence o f spherical units (typically <100 ~
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two-phase region (CT = 1 × 10 -2 mole rim-3). The lower phase is rather similar to Fig. 6 (i.e., t~ o spherical units). In Fig. 7 there is evidence o f sheetlike (lamellar-like) structures, i.e., this phase is probably liquid crystalline in nature. (two phase) ( o n e phase) Because of the presence of ethylene oxide moieties in both the particulate and lamellarI o like entities for the Tween 80 + cyclohexane m I systems, the structures formed are likely to be I fig5 1fig-7 much less sensitive to the absorption of ato -o.'o~ -&o oi~ mospheric water than those in the Span 80 cT/(mot dm-3) + cyclohexane systems, since water molecules FIG. 5. Apparent aggregate "radius" (a) versus concen- may be solubilized in the ethylene oxide retration (CT) of Tween 80 in cyclohexane (20°C). gions. nm), which would be consistent with the PCS data in the single-phase region (Fig. 5). The more extensive structures present in Fig. 6 seem to be composed of fused spheres, and may well be artifacts which occur as a result of the solvent evaporation step in the FFEM preparation technique. Figure 7 refers to the upper phase of the
Span 80 + Tween 80 + Cyclohexane It was observed that the addition of Span 80 to turbid solutions o f Tween 80 (at concentrations < 7 × 10 -2 mole d m -3) led to optical clarification of the system and presumably, therefore, to a breakdown of the lamellarlike aggregates.
FIG. 6. Freeze-fracture scanning electron micrograph for a Tween 80 + cyclohexane mixture in the onephase region (CT = 8 × 10-2 mole dm -3) at 20°C. (Magnification: 1650;<.) Journal of Colloid and Interface Science, V o l ,
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FIG. 7. Freeze-fracture scanning electron micrograph for a Tween 80 + cyclohexane mixture in the twophase region (cT = 1 × 10-2 mole dm -3) at 20°C. This micrograph represents the upper phase which separates. (Magnification: 750×.)
The results of titrating mixtures of Span 80 and Tween 80, at various mole ratios (r), with cyclohexane is shown in Fig. 8; the total concentration of surfactant (Cs+T) at which the mixture changes from clear to turbid is plotted. From the earlier studies of Boyd et al. (1), it is known that mixtures of Spans and Tweens form mixed molecular layers at the hydrocarbon/water interface. A solution of the mixed surfactants, at r = 1, CS+T = 1.8 × 10 - 2 mole dm -3 (clear region), was studied using PCS and ultracentrifugation. In both cases the mean particle radius was found to be ~ 7 rim, implying the existence of simple spherical micelles (Tween 80 chain length ~ 5 nm), even though, as has been demonstrated, neither Span 80 nor Tween 80 alone form simple micelles. For this reason it was felt that mixtures of Span 80 with Tween 80 might form better model water/ cyclohexane emulsions than Span 80 alone, even though the HLB of the mixed surfactants would be higher than that of Span 80 alone.
Span 80 + Tween 80 + Cyclohexane + Water,"Emulsification Water/cyclohexane emulsions, containing up to 5 vol% water (or aqueous NaC1 solution), were prepared by the addition of the required quantity of the aqueous phase (using a microsyringe) to a (initially clear) solution of 1.8 × 10-2 mole dm -3 total surfactant concentration (r = 1) and then by the application of ultrasonics to the mixture for ~ 5 rain. The resulting emulsions were mostly translucent in appearance, except that those prepared at low water content (< 1.2 vol% water) were turbid and eventually broke on standing. Emulsions prepared at water contents > 5 vol% were also turbid. The droplet sizes of the emulsions in the range 1.2 to 3 vol% water were determined using PCS time-average light scattering and ultracentrifugation (Fig. 9a). A surprising feature is that the average droplet size apparently decreases as the water content increases over Journal of Colloid and InterfaceScience, Vol. 116, No. 1, March 1987
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sification the inverted mixed micelles in the cyclohexane phase solubilize (up to the saturation limit) only a small fraction of the water present, under equilibrium conditions. Ultrasonification then emulsifies the remaining Y (excess) water, the droplets produced being clear stabilized by adsorption of the surfactant, much of which is tied up in the water-solubilized micelles which have to be broken ~4 -X down. It is possible that the larger the volume turbid of water present, the larger will be the energy ols 11o 1--~s dissipated in the system during the emulsifir cation process, and so the more readily the FIG. 8. Total molar concentration ( C S + T ) of surfactant micelles can be broken down; hence smaller (Tween 80 + Span 80) at which the systemchanges from turbid (two-phase)to clear (single-phase)as a function of water droplets will form more easily. In Fig. I 0 a scanning electron micrograph, molar ratio (r) Span 80: Tween 80. X: Composition of Span 80 + Tween 80 + cyclohexaneused to preparemodel obtained by FFEM, is shown for the system 5 W/O emulsions. vol% water in cyclohexane, at CS+T= 1.8 mole d m -3 (r = 1). There is evidence for spherical entities, presumably W / O droplets. These this concentration range. However, this is droplets are < I00 n m in radius, which is in consistent with the decrease in turbidity over reasonable agreement with the PCS data (Fig. this range and also with the observation that the lower the water content of the emulsion, the longer the emulsification time required to reach a "steady" droplet size, in the ultrasonic bath. Moreover, the trend in particle size obc served in Fig. 9a appears not to be on artifact (e.g., due to multiple scattering/interdroplet × interactions in the PCS measurements) since x x o x o ~ very similar droplet sizes were obtained using i i the three methods. Indeed, the data agree remarkably well considering the different assumptions implicit in the three techniques. ? The explanation for the decrease in droplet size with increasing water content over the range of water concentrations studied cannot be based simply on thermodynamic (equilibo rium) arguments. Since the total interfacial area which can be supported by a fixed amount of surfactant must itself be fixed, then the 6 o larger the water volume fraction, the larger the o i. i o 1.5 2.0 2.'5 3.0 expected average droplet size. Therefore, the Cw explanation must presumably be a kinetic one, FIG.9. Droplet radius (a) as a function ofvol% aqueous probably connected with the emulsification phase (¢w), in 1.8 X 10 -2 mole d m -3 Span 80 + Tween process itself. However, it is still not exactly 80 (r = 1) + cyclohexane (point X, Fig. 8). ©, PCS; 0, clear what is happening and it is difficult to time-average light scattering; X, ultracentrifuge. (a) give a definitive explanation. Prior to emul- Aqueous phase: water. (b) Aqueous phase: 10%(w/v)NaC1. Journal of Colloid and Interface Science, V o l ,
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FIG. 10. Freeze-fracture scanning electric micrograph for 5% water in cyclohexane at Cs+T= 1.8 × 10-2 mole dm -3 (r = 1). Magnification (1600X.) 9a). Again, it could be that artifacts are introduced during the preparation stage o f the F F E M method; some coalescence could occur. With 10 wt% NaC1 present in the aqueous phase the emulsification took longer and larger droplets, at a given aqueous phase content, were p r o d u c e d (Fig. 9b). This could have to do with the decreased affinity o f the P E O chains o f the Tween surfactant for the cyclohexane/10% NaC1 solution interface, c o m pared to the cyclohexane/pure water interface. The effect on droplet size o f varying r, over the range 0.8-1.2, but keeping the total surfactant concentration and water content fixed at 1.8 X 10 -2 mole d m -3 and 1.8 vol%, respectively, is shown in Fig. 11. There is some evidence for a m i n i m u m in droplet size a r o u n d r = 1. In order to try to understand the effect o f r on droplet size, water/cyclohexane interfacial tension measurements were carried out, using the spinning drop method. T h e results are shown in Fig. 12 for water/cyclohexane and 10% NaC1/cyclohexane interfaces in the pres-
ence o f 5 X 10 -4 mole d m -3 surfactant mixtures, at various mole fractions x [ x = r/ (1 + r)] o f Span 80 dissolved initially in the cyclohexane phase. The results for the water/ cyclohexane interface show that ~ increases steadily from a value o f 1.5 m N m -1 in pure Tween 80 to 11.5 m N m -~ in pure Span 80. However, the droplets produced a r o u n d r = 1 seem to be the smallest and m o s t monodis-
o c
A
r
P]G. 11. Droplet radius (a) as a function of r for 1.8% aqueous phase vd. fraction, in 1.8 X 10-2 mole dm -3 Span 80 + Tween 80 + cyclohexane (pt. X, Fig. 8). A, pure water; B, 10 w/v% aqueous NaC1 solution. Journal of Colloid and Interface Science, Vol. 116, No. 1, M a r c h 1987
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electrolyte concentrations, the time taken for Tween 80 molecules to cross the interface into the aqueous NaCI solution will be significant compared with the time scale of the spinning drop experiment. With pure water the transfer would be much faster. A CONCLUSIONS o
i
o
// O25
0.5
0Y5
PiG. 12. Interfacialtension (3,)for aqueous/cyclohexane interface, with total initial concentrationof surfactant in cyclohexane= 5 × 10-4 mole dm-3, as a function of Span 80 mole fractionx [ x = r/(l + r)]. A, aqueousphase:water; B, aqueous phase: 10 w/v% NaC1.---: initial 3' value; -equilibrium 3' value.
perse. At this r value presumably the packing of the Span 80 and Tween 80 molecules in the interface is more efficient, even though the interfacial tension is not at a minimum. The 3' values for the aqueous 10 wt% NaC1/ cyclohexane interface are generally lower than those for the water/cyclohexane interface. This again could be due to the decreased H bonding in strong electrolyte solutions, compared to water, and, in addition, it is likely that the PEO chains are able to pack closer (i.e., are less expanded) at the interface since the (aqueous) solvent environment is less favorable at high electrolyte concentrations. It was also observed that the interfacial tension values recorded with pure water did not change with time, whereas those with aqueous NaC1 solutions did. Both the initial and equilibrium values are shown in Fig. 12. The fact that at high Tween 80 mole ratios the initial value is in fact lower than the equilibrium value may have to do with the time required for the Tween 80 molecules to redistribute themselves between the cyclohexane and aqueous phases. Gerbacia and Rosano (10) have shown, for example, that the interfacial tension may drop transiently (below the equilibrium value) while (co)surfactant molecules are transversing an interface. Clearly, at high Journal of Colloid and Interface Science, Vol. 116,No. 1, March 1987
These studies have shown that neither pure Span 80 nor pure Tween 80 form simple micelles in cyclohexane at room temperature, rather that aggregate structures form, which may be spherical (vesiclelike) or more extended (sheetlike). However, in mixtures of the two surfactants, over certain concentration ranges, simple spherical inverted miceUes do form in cyclohexane. Emulsification of aqueous solutions into these cyclohexane solutions leads to small, spherical, reasonably monodisperse water droplets, whose size decreases with increasing water content over the range of water contents 1.2-5.0% at a total surfactant concentration of 1.8 × 10 -2 mole d m -3, and at a Span 80/Tween 80 mole ratio of r = 1. This ratio would seem to be the optimum one for forming water/hydrocarbon emulsions, even though the HLB value is not as low as that for pure Span 80 and the interfacial tension is not a minimum. The packing efficiency of the mixed surfactants at the water/ hydrocarbon interface seems to be the controlling factor. This formulation was therefore used to prepare model water/cyclohexane and aqueous NaC1/cyclohexane droplets for the emulsion breakdown studies, which will be reported in a subsequent paper. Finally, on a point of semantics: are these systems "microemulsions"? They certainly fall within the generally accepted definition (11) in terms of their size, but they are not equilibrium systems. They contain dispersed water at concentrations well in excess of the solubilization limit of the mixed micelles. ACKNOWLEDGMENTS The authors gratefully acknowledge the help given by Dr. Kevin Meade (B. P. Research, Sunbury-on-Thames)
W/O EMULSIONS WITH "SPAN" AND "TWEEN" and Dr. R. Hockham (Hexland Electron Microscopy Technology, Wantage, Oxon) in carrying out the freezefracture/scanning electron microscopy experiments. They also thank M. R. Fisher (Bristol University) for carrying out the static light-scattering and ultracentrifugation experiments reported here. Finally, one of us (R.D.) would like to thank the SERC for financial support through a CASE Award during the course of this work. REFERENCES 1. Boyd, J., Parkinson, C., and Sherman, P., J. Colloid Interface Sci. 41, 359 (1972). 2. Gopas, E. S. R., KolloidZ. 167, 17 (1959). 3. Yang, J. T., J. PolymerSci. 26, 305 (1957).
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4. Eley, D., J. Colloidlnterface Sci. 54, 462 (1976). 5. Kertes, A. S., and Gutmann, H., in "Surface and Colloid Science" (E. Matijevir, Ed,), Vol. 8, Ch. 3. Wiley-lnterscience, New York, 1976. 6. Ruckenstein, E., and Nagarajan, R., J. Phys. Chem. 84, 1349 (1980). 7. Corkhill, J. M., et al., Trans. Faraday Soc. 61, 589 (1965). 8. Frank, S. G., and Zografi, G., J. Colloid Interface Sci. 29, 27 (1969). 9. Zulauf, M., and Eicke, H. F., J. Phys. Chem. 83, 480 (1979). 10. Gerbacia, W., and Rosano, H. L., J. Colloidlnterface Sci. 44, 242 (1973). 11. Prince, L. M., "Microemulsions: Theory and Practice," Academic Press, New York, 1977.
Journalof Colloidand InterfaceScience, Vol.116,No. I, March 1987
INTRODUCTION
tivity to changes in conditions (e.g., dilution, temperature). In this paper we discuss the formation and characterization of aqueous/cyclohexane emulsions, containing monodisperse droplets of radius 40 to 80 nm, in which the internal aqueous phase contains up to 10 wt% ( ~ 2 mole dm -3) sodium chloride. The surfactants used were sorbitan monooleate (Span 80), ethoxylated sorbitan monooleate (Tween 80), and their mixtures. The droplets were characterized using photon correlation spectroscopy and freeze-fracture electron microscopy. The only previously reported extensive study of the use of Span/Tween mixtures for the formation of water/oil (and oil/water) emulsions, to the author's knowledge, is that of Boyd et aL (1), who correlated the stability of the emulsions to coalescence with the interfacial rheological characteristics of the oil + Span/water + Tween systems.
Water-in-off emulsions have received much less attention in the academic literature than oil-in-water emulsions. Yet their importance in a wide variety of technologies is well recognized, e.g., in the oil industry (recovery and dewatering processes), the food industry (margarine, etc.), cosmetics, cleaning fluids. In particular, there have been few studies in which the internal aqueous phase contains electrolytes. In order to study the properties of emulsions of this type, model reproducible systems containing monodisperse droplets are required. The so-called "microemulsions" achieve these objectives in part, despite their requirement for high concentrations of surfactants (frequently > 10% of the system) and their sensi1Author to whom correspondence should be addressed.
88 0021-9797/87 $3.00 Copyright © 1987 by Academic Press, Inc. All fights of reproduction in any form reserved.
Journal ofColloidandlnterface Science, Vol. 116, No. 1, March 1987
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EXPERIMENTAL
Materials All water was doubly distilled from an allPyrex apparatus prior to use. Cyclohexane and
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"SPAN" AND "TWEEN"
other hydrocarbons (B.D.H. "AnalaR" grade) were dried over calcium hydride, where necessary, and distilled. Sodium chloride (B.D.H. "AnalaR" grade) was used as supplied. The formulas of sorbitan monooleate (I) and ethoxylated sorbitan monooleate (II) are OH
I
HLB = 4.3 Span 80 0 (CH2CH20)x H
0
II
O,.,.v.....~ O(CH2 CH20 )z H
HLB= 15Tween80(x+y+z=20)
Both were obtained from Koch-Light laboratories as Span 80 and Tween 80, respectively.
Emulsification Two methods were used: (a) Homogenization. A Silverson homogenizer unit was used, operating at 5000 rpm for 1 min. No further changes in droplet size were generally observed after this time, in line with Gopel's previous observation (2). (b) Sonification. A Dawe "Soniclean" (Type 7532A) ultrasonic bath, operating at 20 KHz for about 5 min, was used. Again a limiting droplet size was generally obtained within this period.
Droplet Size Determination Photon correlation spectroscopy (PCS) was the main method used. The apparatus comprised a Coherent CR-2000K krypton ion laser, with "PS 100" Malvern optics and a Malvern K7025 multibit correlator. Data storage and analysis were carried out using an on-line "Apple II" microcomputer. "Particle" radii were calculated in the usual manner from
the translational diffusion coefficient, obtained from the time autocorrelation function, g(t). The correlation delay time (r) was chosen such that g(t) decreases to about g(0)/2 within about 10-20 r; this normally resulted in good exponential fits of the g(t) data. Measurements were made over a range of scattering angles, 80-120 °. All solutions were filtered before study, using an appropriate pore-size Millipore filter. Particle radii were also determined in some cases using a time-averaged light-scattering technique, based on a modification of the standard Zimm plot suggested by Yang (3). A Sophica 4200 light-scattering apparatus was used. Further determinations of particle radii were also made from sedimentation coefficients using a Beckman Spinco Model E ultracentrifuge.
Freeze-Fracture Electron Microscopy (FFEM) Electron microscopy of liquid droplets was not possible prior to the advent of freeze-fracture techniques, developed in the main for the Journal of Colloid andlnterface Science, Vol. 116, No. 1, March 1987
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study of biological systems. Even so most studies have been confined to emulsions in which water was the external phase; there have been very few studies, as far as the authors are aware, of hydrocarbon-continuous emulsions (4). For this reason the techniques used here are discussed in some detail. In the first method replicated samples were prepared prior to investigation by transmission electron microscopy (TEM). A small drop of emulsion was placed on a gold supporting stub which was then dipped into a suitable freezing medium (Freon or nitrogen "slush"). The stub with the frozen sample was then transferred to the cold stage (maintained at - 1 5 0 ° C ) of a Balzers 360M freeze-etch unit, the chamber of which was evacuated to a pressure of < 10 -5 Torr. The sample was then sliced with a blade precooled with liquid nitrogen. The fractured surface was then immediately replicated and shadowed with platinum and carbon. The replica was "floated off" from the sample in a suitable solvent: normally water would be used for this purpose, but in this case a cyclohexane-miscible solvent was necessary and hence toluene was used, even though the replica tended to sink rather than float. Some other FFEM experiments were carried out in which the fractured surface was prepared in situ on the cryostage of a scanning electron microscope (SEM). In this method two small metal rivets are filled with the emulsion (or surfactant solution) to be studied; one is inverted and placed on top of the other. The pair is then transferred to the cryostage contained in a vacuum chamber (pressure <0.2 Torr). The top rivet is "knocked away," revealing the fractured surface of the frozen liquid which is then replicated with gold.
Dilatometry In order to study the transfer of water into cyclohexane solutions containing surfactants a dilatometry technique was developed. The apparatus is shown schematically in Fig. 1. Water contained in a dialysis bag is fixed to a glass support with a protruding capillary atJournal of Colloid and InterfaceScience, Vol. 116,No. 1, March 1987
C
D
0
FIG. 1. Dilatometer. A, aqueous phase; O, cyclohexane; C, capillary; D, dialysis tubing; M, magnetic stirrer.
tachment. The water level is adjusted to be near to the top of the capillary. The dialysis tubing, carefully dried on the exterior, is then placed in the cyclohexane solution as shown. The whole apparatus is mounted in a thermostat bath, and the change in the water level in the capillary monitored using a cathometer.
Interfacial Tension The interfacial tension between the cyclohexane phase, containing the dissolved surfactant, and bulk water or aqueous electrolyte solution was determined using a Kruss Spin-
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ning Drop Interracial Tensiometer. A 4-/~liter droplet of the cyclohexane phase was suspended in the spinning tube. The concentration of the surfactant (mixture) was chosen to give approximately the same number o f molecules per unit interfacial area as in the emulsion studies.
~
°
"
RESULTS AND DISCUSSION
Prior to discussing the formation and characterization of water/cyclohexane emulsions, the phase structure of the individual surfactants (Span 80 and Tween 80) in cyclohexane and cyclohexane/water mixtures will be discussed.
Span 80 + Cyclohexane The concept of micellization in nonpolar solvents has been much debated in the literature; in particular, the existence of a critical micelle concentration (CMC) per se has been questioned and casts doubt on reported CMC values (5, 6). However, some form of aggregation usually occurs, and the resultant aggregates, in general, solubilize water. For example, "Aerosol O.T." (sodium diethylhexylsulphosuccinate) does form inverse micelles in various hydrocarbons (6, 8); in dry isooctane they have a radius of 1.5 nm and swell to a limiting size of ~ 18 n m with solubilized water at ambient temperature (9). Dry Span 80 dissolves readily in dry cyclohexane to give solutions whose turbidity rises linearly with increasing concentration. There is no indication of a discontinuity over the concentration range studied. Moreover the particles present appear to have a constant average size (equivalent spherical radius) of "~ 65 + 5 nm, as determined by PCS measurements, at least up to concentrations of 0.05 mole d m -3. This implies that there are no problems associated with multiple scattering and/or particle interactions, at least over this concentration range. Clearly, however, the particles in this case cannot be simple spherical micelles. Assuming, as a limiting case, the particles to be composed purely of Span 80 mol-
csx 10-2/(mo1 din-3)
FIG. 2. Turbidity (r) versus concentration (cs) of Span 80 in cyclohexane.
ecules (i.e., no solubilization of cyclohexane), then the aggregation number per particle would be ~ 106. Some form of multibilayer structure is probably involved, maybe in extended (rod or sheetlike) form or possibly in the form of vesicles having a more or less spherical morphology. In the latter case, since the extended length of the sorbitan monooleate chains is 2.9 nm, this would correspond to an average of some 10 or 30 concentric bilayer tings per vesicle. H bonding between hydroxyl groups on the sorbitan rings is probably the main factor controlling the aggregation behavior. The presence of even trace amounts of water in the cyclohexane would facilitate this H bonding. Indeed, it was observed that if the system is not rigorously dry (and maintained that way), then both the turbidity of the sample and the apparent size of the particles, as measured by PCS, increased slowly with time. For example, in a solution of 0.05 mole dm -3 Span 80 in cyclohexane, which had not been dried and which was left open to the atmosphere, the apparent particle size reached 300 + 30 nm after about 1 h; after this time some setting of the aggregates was observed. This increase in size cannot be due solely to solubilization of water alone; some aggregation of the basic (65-nm) particles must also occur, possibly as a consequence of water "bridging" between exposed sorbitan ring hydroxyls during particle collisions. Journal of Colloid and Interface Science, Vol. I 16, No. 1, March 1987
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In order to investigate the effect of water and the PCS results indicated that the aggrefurther, dilatometry experiments were carried gation process was still occurring slowly with out using the apparatus described earlier (Fig. time, even after ~ 3 h. 1), maintained at 20°C. As well as monitoring the decrease in the water level in the capillary, Span 80 + Cyclohexane + Water," the apparent particle size was determined peEmulsification riodically by PCS by sampling the cyclohexane phase. Clearly the presence of Span 80 particles or The PCS results are shown in Fig. 3. The aggregates complicates the emulsification procorresponding decreases in the volume of the cess of bulk water into cyclohexane containing water phase after 1 h were 6 m m 3 for pure this surfactant. Nevertheless, reasonably satcyclohexane, 18 m m 3 for 10 -3 mole d m -3 isfactory water/cyclohexane emulsions, over a Span 80, and 24 m m 3 for 10-2 mole d m -3 wide range of water volume fractions, could Span 80. The value for cyclohexane corre- be prepared with Span 80 dissolved in the cysponds to a dissolved water concentration of clohexane phase. For example, for a 5 vol% 0.026 wt%. For the 10 -3 mole dm -3 solution water/cyclohexane emulsion, a m i n i m u m o f " S p a n 80" in cyclohexane, if all the excess concentration of 5 × 10-3 mole d m -3 Span water (i.e., over and above that required to 80 in cyclohexane was found to be necessary saturate the cyclohexane) had been solubilized to produce droplets with long-term stability; into the particles, then their volume (assumed that is, there was no indication of phase sepspherical) would have approximately doubled aration after some 3 h centrifugation at 10,000 (i.e., from -~9 × 105 n m 3 to ~ 17 × 105 nm3). rpm in an MSE Hi-Spin 21 centrifuge. This would be equivalent to a particle radius In Fig. 4 typical TEM pictures from FFEM increase from 65 to ~ 80 n m after 1 h. For the studies are shown for a 20 vol% water/cyclo10-2 mole dm -3 solution, the corresponding hexane emulsion prepared by homogenization increase in radius would be even smaller, i.e., with 5 × 10-2 mole dm -3 Span 80 in cyclofrom 65 to ~ 6 8 nm. These results, when hexane. It would seem that most of the dropcompared to the apparent radius determined lets have radii typically in the range 100 to from PCS (Fig. 3), suggest that particle aggre- 2000 nm. gation in the presence of water, and not just Because of the presence of Span 80 particles swelling, is occurring. Both the dilatometer and maybe aggregates, as well as the emulsified water droplets, dispersed in the cyclohexane, it was felt that light-scattering studies (PCS or total intensity) would be difficult to interpret E and therefore inappropriate. Moreover, at 5 E / vol% water problems o f multiple scattering/ o J particle interactions are likely to arise, so that dilution of the emulsion is necessary. The acA cepted practice is to dilute with the continuous phase, i.e., in this case cyclohexane containing 5 X 10 -2 mole dm -3 Span 80, in order to avoid the possibility of stabilizer desorption from the droplet interfaces. Indeed, it was observed that dilution with pure cyclohexane did lead to ini 5~0 100 150 stability and phase separation. However, dit / rain lution with Span 80 solutions in cyclohexane FIG. 3. Apparent aggregate"radius" (a) as a function would result in the introduction o f further of time (t). A, 10-3 mole dm-3 Span 80 in cyclohexane; particles. B, 10-2 mole dm-3 Span 80 in cyelohexane. Journal of Colloid and Interface Science, Vol. 116, No. 1, March 1987
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FIG. 4. Freeze-fracture transmission electron micrograph for a 20 vol% water/cyclohexane emulsion prepared by homogenization of water in cyclohexane containing 5 × 10-2 mole d m -3 Span 80. (Magnification: 2250×.)
Thus, although Span 80 alone forms reasonable water/cyclohexane emulsions, the presence of excess surfactant particles renders them somewhat unsatisfactory as model systems. For this reason studies were undertaken on mixtures of Span 80 and Tween 80.
Tween 80 + Cyclohexane Solutions of Tween 80 in dry cyclohexane behave rather differently from solutions of Span 80 in this solvent. The Tween 80/cyclohexane system shows a two-phase region, with a lower consolute point. Hence at 20°C, if the initial turbidity of the system is determined as a function of Tween 80 concentration (CT),the turbidity rises very rapidly at very low concentrations. In this concentration region, two translucent phases separate on standing. Beyond about 7 X 10-2 mole d m -3 the (initial) turbidity drops suddenly to give a stable translucent system. This corresponds to the tran-
sition from the two-phase to the single-phase region. I f a solution in the single-phase region (i.e., Cz > 7 X 10 --2 mole dm -3) is heated, then at a certain temperature it becomes turbid, indicating that the two-phase region has been entered. The Tween 80 + cyclohexane system, in the vicinity of the two-phase --~ one-phase transition concentration region (at ambient temperature), was investigated using PCS and FFEM (in this case SEM). The PCS data are 7 × 10 - 2 mole shown in Fig. 5. Beyond CT dm -3, particles having an apparent "'radius" of 60-70 nm are formed, whereas below CT 7 × 10-2 mole dm -3, the apparent size rises very rapidly (with large error bars), indicative of some form of aggregation process. The SEM picture shown in Fig. 6 refers to the structure of the Tween 80 + cyclohexane system in the single-phase region (CT = 8 X 10 -2 mole dm-3). There is evidence for the existence o f spherical units (typically <100 ~
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two-phase region (CT = 1 × 10 -2 mole rim-3). The lower phase is rather similar to Fig. 6 (i.e., t~ o spherical units). In Fig. 7 there is evidence o f sheetlike (lamellar-like) structures, i.e., this phase is probably liquid crystalline in nature. (two phase) ( o n e phase) Because of the presence of ethylene oxide moieties in both the particulate and lamellarI o like entities for the Tween 80 + cyclohexane m I systems, the structures formed are likely to be I fig5 1fig-7 much less sensitive to the absorption of ato -o.'o~ -&o oi~ mospheric water than those in the Span 80 cT/(mot dm-3) + cyclohexane systems, since water molecules FIG. 5. Apparent aggregate "radius" (a) versus concen- may be solubilized in the ethylene oxide retration (CT) of Tween 80 in cyclohexane (20°C). gions. nm), which would be consistent with the PCS data in the single-phase region (Fig. 5). The more extensive structures present in Fig. 6 seem to be composed of fused spheres, and may well be artifacts which occur as a result of the solvent evaporation step in the FFEM preparation technique. Figure 7 refers to the upper phase of the
Span 80 + Tween 80 + Cyclohexane It was observed that the addition of Span 80 to turbid solutions o f Tween 80 (at concentrations < 7 × 10 -2 mole d m -3) led to optical clarification of the system and presumably, therefore, to a breakdown of the lamellarlike aggregates.
FIG. 6. Freeze-fracture scanning electron micrograph for a Tween 80 + cyclohexane mixture in the onephase region (CT = 8 × 10-2 mole dm -3) at 20°C. (Magnification: 1650;<.) Journal of Colloid and Interface Science, V o l ,
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FIG. 7. Freeze-fracture scanning electron micrograph for a Tween 80 + cyclohexane mixture in the twophase region (cT = 1 × 10-2 mole dm -3) at 20°C. This micrograph represents the upper phase which separates. (Magnification: 750×.)
The results of titrating mixtures of Span 80 and Tween 80, at various mole ratios (r), with cyclohexane is shown in Fig. 8; the total concentration of surfactant (Cs+T) at which the mixture changes from clear to turbid is plotted. From the earlier studies of Boyd et al. (1), it is known that mixtures of Spans and Tweens form mixed molecular layers at the hydrocarbon/water interface. A solution of the mixed surfactants, at r = 1, CS+T = 1.8 × 10 - 2 mole dm -3 (clear region), was studied using PCS and ultracentrifugation. In both cases the mean particle radius was found to be ~ 7 rim, implying the existence of simple spherical micelles (Tween 80 chain length ~ 5 nm), even though, as has been demonstrated, neither Span 80 nor Tween 80 alone form simple micelles. For this reason it was felt that mixtures of Span 80 with Tween 80 might form better model water/ cyclohexane emulsions than Span 80 alone, even though the HLB of the mixed surfactants would be higher than that of Span 80 alone.
Span 80 + Tween 80 + Cyclohexane + Water,"Emulsification Water/cyclohexane emulsions, containing up to 5 vol% water (or aqueous NaC1 solution), were prepared by the addition of the required quantity of the aqueous phase (using a microsyringe) to a (initially clear) solution of 1.8 × 10-2 mole dm -3 total surfactant concentration (r = 1) and then by the application of ultrasonics to the mixture for ~ 5 rain. The resulting emulsions were mostly translucent in appearance, except that those prepared at low water content (< 1.2 vol% water) were turbid and eventually broke on standing. Emulsions prepared at water contents > 5 vol% were also turbid. The droplet sizes of the emulsions in the range 1.2 to 3 vol% water were determined using PCS time-average light scattering and ultracentrifugation (Fig. 9a). A surprising feature is that the average droplet size apparently decreases as the water content increases over Journal of Colloid and InterfaceScience, Vol. 116, No. 1, March 1987
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sification the inverted mixed micelles in the cyclohexane phase solubilize (up to the saturation limit) only a small fraction of the water present, under equilibrium conditions. Ultrasonification then emulsifies the remaining Y (excess) water, the droplets produced being clear stabilized by adsorption of the surfactant, much of which is tied up in the water-solubilized micelles which have to be broken ~4 -X down. It is possible that the larger the volume turbid of water present, the larger will be the energy ols 11o 1--~s dissipated in the system during the emulsifir cation process, and so the more readily the FIG. 8. Total molar concentration ( C S + T ) of surfactant micelles can be broken down; hence smaller (Tween 80 + Span 80) at which the systemchanges from turbid (two-phase)to clear (single-phase)as a function of water droplets will form more easily. In Fig. I 0 a scanning electron micrograph, molar ratio (r) Span 80: Tween 80. X: Composition of Span 80 + Tween 80 + cyclohexaneused to preparemodel obtained by FFEM, is shown for the system 5 W/O emulsions. vol% water in cyclohexane, at CS+T= 1.8 mole d m -3 (r = 1). There is evidence for spherical entities, presumably W / O droplets. These this concentration range. However, this is droplets are < I00 n m in radius, which is in consistent with the decrease in turbidity over reasonable agreement with the PCS data (Fig. this range and also with the observation that the lower the water content of the emulsion, the longer the emulsification time required to reach a "steady" droplet size, in the ultrasonic bath. Moreover, the trend in particle size obc served in Fig. 9a appears not to be on artifact (e.g., due to multiple scattering/interdroplet × interactions in the PCS measurements) since x x o x o ~ very similar droplet sizes were obtained using i i the three methods. Indeed, the data agree remarkably well considering the different assumptions implicit in the three techniques. ? The explanation for the decrease in droplet size with increasing water content over the range of water concentrations studied cannot be based simply on thermodynamic (equilibo rium) arguments. Since the total interfacial area which can be supported by a fixed amount of surfactant must itself be fixed, then the 6 o larger the water volume fraction, the larger the o i. i o 1.5 2.0 2.'5 3.0 expected average droplet size. Therefore, the Cw explanation must presumably be a kinetic one, FIG.9. Droplet radius (a) as a function ofvol% aqueous probably connected with the emulsification phase (¢w), in 1.8 X 10 -2 mole d m -3 Span 80 + Tween process itself. However, it is still not exactly 80 (r = 1) + cyclohexane (point X, Fig. 8). ©, PCS; 0, clear what is happening and it is difficult to time-average light scattering; X, ultracentrifuge. (a) give a definitive explanation. Prior to emul- Aqueous phase: water. (b) Aqueous phase: 10%(w/v)NaC1. Journal of Colloid and Interface Science, V o l ,
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FIG. 10. Freeze-fracture scanning electric micrograph for 5% water in cyclohexane at Cs+T= 1.8 × 10-2 mole dm -3 (r = 1). Magnification (1600X.) 9a). Again, it could be that artifacts are introduced during the preparation stage o f the F F E M method; some coalescence could occur. With 10 wt% NaC1 present in the aqueous phase the emulsification took longer and larger droplets, at a given aqueous phase content, were p r o d u c e d (Fig. 9b). This could have to do with the decreased affinity o f the P E O chains o f the Tween surfactant for the cyclohexane/10% NaC1 solution interface, c o m pared to the cyclohexane/pure water interface. The effect on droplet size o f varying r, over the range 0.8-1.2, but keeping the total surfactant concentration and water content fixed at 1.8 X 10 -2 mole d m -3 and 1.8 vol%, respectively, is shown in Fig. 11. There is some evidence for a m i n i m u m in droplet size a r o u n d r = 1. In order to try to understand the effect o f r on droplet size, water/cyclohexane interfacial tension measurements were carried out, using the spinning drop method. T h e results are shown in Fig. 12 for water/cyclohexane and 10% NaC1/cyclohexane interfaces in the pres-
ence o f 5 X 10 -4 mole d m -3 surfactant mixtures, at various mole fractions x [ x = r/ (1 + r)] o f Span 80 dissolved initially in the cyclohexane phase. The results for the water/ cyclohexane interface show that ~ increases steadily from a value o f 1.5 m N m -1 in pure Tween 80 to 11.5 m N m -~ in pure Span 80. However, the droplets produced a r o u n d r = 1 seem to be the smallest and m o s t monodis-
o c
A
r
P]G. 11. Droplet radius (a) as a function of r for 1.8% aqueous phase vd. fraction, in 1.8 X 10-2 mole dm -3 Span 80 + Tween 80 + cyclohexane (pt. X, Fig. 8). A, pure water; B, 10 w/v% aqueous NaC1 solution. Journal of Colloid and Interface Science, Vol. 116, No. 1, M a r c h 1987
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electrolyte concentrations, the time taken for Tween 80 molecules to cross the interface into the aqueous NaCI solution will be significant compared with the time scale of the spinning drop experiment. With pure water the transfer would be much faster. A CONCLUSIONS o
i
o
// O25
0.5
0Y5
PiG. 12. Interfacialtension (3,)for aqueous/cyclohexane interface, with total initial concentrationof surfactant in cyclohexane= 5 × 10-4 mole dm-3, as a function of Span 80 mole fractionx [ x = r/(l + r)]. A, aqueousphase:water; B, aqueous phase: 10 w/v% NaC1.---: initial 3' value; -equilibrium 3' value.
perse. At this r value presumably the packing of the Span 80 and Tween 80 molecules in the interface is more efficient, even though the interfacial tension is not at a minimum. The 3' values for the aqueous 10 wt% NaC1/ cyclohexane interface are generally lower than those for the water/cyclohexane interface. This again could be due to the decreased H bonding in strong electrolyte solutions, compared to water, and, in addition, it is likely that the PEO chains are able to pack closer (i.e., are less expanded) at the interface since the (aqueous) solvent environment is less favorable at high electrolyte concentrations. It was also observed that the interfacial tension values recorded with pure water did not change with time, whereas those with aqueous NaC1 solutions did. Both the initial and equilibrium values are shown in Fig. 12. The fact that at high Tween 80 mole ratios the initial value is in fact lower than the equilibrium value may have to do with the time required for the Tween 80 molecules to redistribute themselves between the cyclohexane and aqueous phases. Gerbacia and Rosano (10) have shown, for example, that the interfacial tension may drop transiently (below the equilibrium value) while (co)surfactant molecules are transversing an interface. Clearly, at high Journal of Colloid and Interface Science, Vol. 116,No. 1, March 1987
These studies have shown that neither pure Span 80 nor pure Tween 80 form simple micelles in cyclohexane at room temperature, rather that aggregate structures form, which may be spherical (vesiclelike) or more extended (sheetlike). However, in mixtures of the two surfactants, over certain concentration ranges, simple spherical inverted miceUes do form in cyclohexane. Emulsification of aqueous solutions into these cyclohexane solutions leads to small, spherical, reasonably monodisperse water droplets, whose size decreases with increasing water content over the range of water contents 1.2-5.0% at a total surfactant concentration of 1.8 × 10 -2 mole d m -3, and at a Span 80/Tween 80 mole ratio of r = 1. This ratio would seem to be the optimum one for forming water/hydrocarbon emulsions, even though the HLB value is not as low as that for pure Span 80 and the interfacial tension is not a minimum. The packing efficiency of the mixed surfactants at the water/ hydrocarbon interface seems to be the controlling factor. This formulation was therefore used to prepare model water/cyclohexane and aqueous NaC1/cyclohexane droplets for the emulsion breakdown studies, which will be reported in a subsequent paper. Finally, on a point of semantics: are these systems "microemulsions"? They certainly fall within the generally accepted definition (11) in terms of their size, but they are not equilibrium systems. They contain dispersed water at concentrations well in excess of the solubilization limit of the mixed micelles. ACKNOWLEDGMENTS The authors gratefully acknowledge the help given by Dr. Kevin Meade (B. P. Research, Sunbury-on-Thames)
W/O EMULSIONS WITH "SPAN" AND "TWEEN" and Dr. R. Hockham (Hexland Electron Microscopy Technology, Wantage, Oxon) in carrying out the freezefracture/scanning electron microscopy experiments. They also thank M. R. Fisher (Bristol University) for carrying out the static light-scattering and ultracentrifugation experiments reported here. Finally, one of us (R.D.) would like to thank the SERC for financial support through a CASE Award during the course of this work. REFERENCES 1. Boyd, J., Parkinson, C., and Sherman, P., J. Colloid Interface Sci. 41, 359 (1972). 2. Gopas, E. S. R., KolloidZ. 167, 17 (1959). 3. Yang, J. T., J. PolymerSci. 26, 305 (1957).
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4. Eley, D., J. Colloidlnterface Sci. 54, 462 (1976). 5. Kertes, A. S., and Gutmann, H., in "Surface and Colloid Science" (E. Matijevir, Ed,), Vol. 8, Ch. 3. Wiley-lnterscience, New York, 1976. 6. Ruckenstein, E., and Nagarajan, R., J. Phys. Chem. 84, 1349 (1980). 7. Corkhill, J. M., et al., Trans. Faraday Soc. 61, 589 (1965). 8. Frank, S. G., and Zografi, G., J. Colloid Interface Sci. 29, 27 (1969). 9. Zulauf, M., and Eicke, H. F., J. Phys. Chem. 83, 480 (1979). 10. Gerbacia, W., and Rosano, H. L., J. Colloidlnterface Sci. 44, 242 (1973). 11. Prince, L. M., "Microemulsions: Theory and Practice," Academic Press, New York, 1977.
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