Langmuir 1993,9, 25-28
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Relationship between Microemulsion Phase Behavior and Macroemulsion Type in Systems Containing Nonionic Surfactant B.P.Binks School of Chemistry, University of Hull, Hull HU6 7RX, U.K. Received July 8,1992. In Final Form: October 9,1992 In systems containing the nonionic surfactant C I ~ Eheptane, ~, and 0.01 M aqueous NaC1, the effect of surfactant concentration on emulsion type with respect to temperature has been studied by conductivity, in relation to the equilibriumdistribution of both monomeric and aggregated surfactantin microemulaion systems. At high enough concentrations (e.g. 6 wt 5% or 84 mM initially in oil) emulaions invert from o/w to w/o at temperatures close to where middle-phase microemulaions are formed (Winsor I11 systems) and the oil-water interfacialtension is minimum. As the surfactant concentration is reduced, emulsions pass progressivelyfrom those which invert to those which remain highly conductingat all temperatures. The resdta are discussedin relation to Bancroft's rule and to the partitioningof both monomeric and aggregated Surfactant.
Introduction The equilibrium microemulsionphase behavior of oil + water systems containingnonionic surfactant of the poly(oxyethylene) glycol ether type (C,E,) is well under~ t o o d . ' ~Interest ~ now focuses on understanding the relationship between these properties and the type and stability of the macroemulsions formed from microemulsions plus their conjugate excess This paper is aimedat investigatingunder what conditions of surfactant concentration and temperature the microemulsion type (oil-in-water,o/w, or water-in-oil,w/o) is the same as that of the emulsion. Let us consider equilibrium systems of heptane and water (equal volumes) containing C12E5. At low concentrations of surfactant, monomer distributes between oil and water but heavily in favor of the The partition coefficient defined as (molar concentration in heptane/ molar concentration in water) increases from 130 at 10 "C to 1500 at 60 "C. The oil-water interfacial tension ( 7 ) decreases with increasing surfactant concentration. Above a critical concentration, reached in both phases and designated cphabr (typically 5 X 10-5 M)and cpc0a (typically between (6 and 60) X M), all additional surfactant in excess of this concentration is present in the form of aggregates. At low temperatures T (99.5% by GLC) sample obtained from Nikko of Japan. The cloud point of a 1wt 7% aqueous solution was 31.9 "C, in good agreement with that quoted by Schubert et al." of 32.0 OC after purification. Sodium chloride, used in low concentration (0.01 M) to detect emuleion inversion, was BDH AnalaR grade. Methods. Low oil-water interfacial tensions were measured for preequilibrated phases using a Kruas Site 04 spinning drop tensiometer, thermostated to fO.l "C. Equilibrium two- and three-phase systems were prepared using equal volumes (20 cm3) of C&S in heptane and 0.01 M aqueous NaC1. Emulsions made from such systems were obtained either by vigorous shaking for 2 min at the required temperature or by homogenization for 1 min at 8O00 rpm using a Janke and Kunkel Ultra Turrax T25 homogenizer. Very similar conductivities were obtained by both methoda. Conductivity was measured using a PtlPt black dip cell and a FTI-18 digital conductivity meter either toward the end of homogenization or immediately after hand shaking. All measurements were made in a g h cell thermostated usbg a Haake thermostat. In some experiments, the conductivity of an emuleion prepared at low temperature was monitored at 2 OC intervah while increasing the temperature. No hysteresis was found in asimilar experiment but on decreasingthe temperature.
Results and Discussion (a) Emulsion Type for Cl& ConcentrationsGreater Than cpcoilat All Temperatures. Figure 2 shows the variation in the post-cpc interfacial tension with temperature for the + heptane + 0.01 M aqueous NaCl system. The low tension minimum is associated with a changeover from Winsor I systems at T < 28 "C (containing o/w microemulsion) to Winsor I1 systems at T > 30 "C (containingw/o microemulsion) via Winsor I11 (11) Schubert,K.-V.; Strey, R.; Kahlweit, M.J. Colloid Interface Sci. 1901,141, 21.
Table I. Variation of the Partition Coefficient for Monomeric Surfactant, P,cpc,,ll, Winsor Microemulsion Type, and Macroemulsion Type with Temperature in the System C I ~ + E 0.01 ~ M Aaueous NaCl + HeDtane Winsor macroemulsion type microemulsion CfiCoid TIT type P mMa cpcd 10 I 132 6.7 otw otw 13.7 20 I 256 otw otw 18.8 olw olw 25 I 352 24.9 otw intermediate 30 I11 478 32.2 olw wto 35 I1 642 40.5 40 I1 856 olw wlo olw wto 60.4 50 I1 1478 a Interpolated from data in ref 4. cphaur= (5.6 i 1.6) X M, independent of T.
systems between 28 and 30 "C. The conductivitiesof the corresponding emulsions are high (indicating o/w) at low T and increase with Tup to -29.5 "C. Between 29.5 and 31 O C the emulsion conductivity falls drastically by over 2 orders of magnitude, indicative of emulsion inversion to w/o emulsions which form at the higher T, The correspondencebetween the lowest tension (wherethree phases form at equilibrium) and emulsion inversion is clearly evident. In line with previousr e p ~ r t e ~emulsions J ~ - ~ ~ were very unstable (breaking within minutee) in the intermediate T range over which inversion occurred, stability increasing either side of the phase inversion temperature, PIT (-30 "C). In 1913 Bancroft wrote that "a hydrophile colloid will tend to make water the dispersing phase while a hydrophobic colloid will tend to make water the disperse phase".15 With reference to emulsions, the so-called "Bancroft rule" states that the phase of greater surfactant concentrationtends to be the continuousphase. Referring to Table I, for T < 30 O C , surfactant in excess of the CCLC~U will be present in the form of o/w microemulsiondroplets, present in the continuous phase of the o/w emulsions. At the concentration of surfactant used for the results in Figure 2 (84 mM initially in oil) it may be estimated that 16 mM surfactant is required to saturate micrometersized emulsion drops of area per surfactant molecule 0.6 nm2. It is seen that although monomericsurfactant is lost to the dispersed oil drops at low T, the emulsion type (o/w) is the one in which the continuous phase contains most of the surfactant (i.e. cpc0ais never >34 mM between 10 and 30 "C). At higher T,the continuous phase of the wlo emulsions contains both a high concentration of monomer (cpco~) and w/o microemulsion droplets, which together exceed the amount of surfactant present within the dispersed drops (cpcwater).Thus, Bancroft's rule holds at this higher overall surfactant concentration, but it is impossibleto distinguishwhether preferred emuleiontype is determined by the distribution of monomeric or of aggregated surfactant. (b) Emulsion Type for ClsEs Concentrations Less Than cpcoil. Figure 3 shows how the conductivities of emulsionschangewith T for different initial concentrations of C12E5 in heptane. The behavior with respect to T may be described in three concentration regimes: (i) between 28 and 84 mM, emulsions invert from o/w to w/o at the PIT of -30 "C; (ii) between 0 and 16 mM, emulsions (which were very unstable at all T) remained highly conducting (and so probably o/w) between 10 and 60 "C; (iii) for concentrations of 20 and 23 mM, the behavior is
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(12) Shinada, K.;Saito, H. J . Colloid Interface Sci. 1969,.?9,268. (13) Saito, H.; Shinoda, K.J. Colloid Interface Sci. 1970,92,647. (14) Bell, S. A.; Binlre, B. P. Unpublished resulta. (15) Bancroft, W. D. J. Phys. Chem. 1913,17,W1.
Letters
Langmuir, Vol. 9, No.1, 1993 27
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m-
eao700
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Figum 3. Conductivity of stirred macroemulsions versus temperature for different initial concentrationsof surfactant in the system r mM ClzEs in heptane + 0.01 M aqueous NaC1.
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intermediate between these two extremes. Emulsion conductivity is high and increases up to 30 O C and then falls rapidly. At 20 mM, the conductivity decreases to -250 pS/cm at 31 O C and then rises gradually with T reaching -740 pSlcm at 50 O C . At 23 mM, inversion to wlo emulsions (conductivity < 0.1 pS/cm) occurs up to -44.5 O C , above which the conductivity rises to overlap with the values obtained at 20 mM. Further valuable information can be obtained by recasting the data in Figure 3 in order to show the effects of surfactant concentration on emulsion type at various T. Figure 4 shows that at T < PIT, Le. at 10,20,and 25 "C, emulsions are olw both below and above cpcoa. Thus emulsion type below c p is the ~ same as the microemulsion typeabove it. At T > PIT,i.e. at 35,40, and 50 O C , inversion of emulsions from olw to wlo occurs with increasing surfactant concentration. However, it is seen from the figure that the concentration required for inversion is significantly less than the equilibrium cpcoa (indicated by the arrows), it being approximately half this value. It has been shown elsewhere (ref 4) from interfacial tension studies that the surface concentration r of CIPE~ at the oil-water interface becomes apparently constant at concentrations less than cpcoa (it is realized of course that r cannot be strictly constant below the cpc since the tension continues to fall-this has been discussed in ref 16). Therefore it can be concluded that emulsion type is determined once a close-packed monolayer of surfactant is obtained at the oil-water interface. For these higher T whichexhibit two phases at equilibrium (>30 O C ) , we have also shown that inversion of emulsion type occurs without passing through a three-phase region, consistent with the findings of Smith et ala1'using C& as Surfactant. At the PIT of 30 O C , emuleion conductivity increases initially with C12E6 concentration (from -660 to -750 rSlcm) and then decreases (to -430 pS1cm) at a concentration close to cpcoil. Assuming three phases are (16)Si", E.A.; Thomas, R. K.; Penfold, J.; Aveyard, R.; Binke, B. P.; Cooper, P.; Fletcher, P. D. I.; Lu, J. R.; Sokolowski, A. J. Phys. Chem. 1992, a,1383. (17) Smith, D. H.; Covatch, 0.C.; Lim, K.-H. J. Phy8. Chem. 1991,95, 1463.
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10'
'02
25'
1, 1 1
0 1 0 2 0 3 0 4 0 5 0 6 0
,
[c 2E5]0ilhM
Figure 4. Conductivity of stirred macroemuleiom versus initial concentration of surfactant in oil at different temperatures in the system ClzEa + heptane + 0.01 M aqueous NaCI. The cl.cq,a values at various temperatures are indicated by the arrows.
formed in situ during emulsification above cpcoa, the relatively high conductivity observed implies that the continuous phase of these emulsions is reasonably conducting. It is difficult to speculate on the type of emulsion preferred, be it olw, water-in-third-phase, third-phasein-water, oil-in-third-phase,or even a multiple emulsion. One other important aspect related to the results in Figure 4 requires comment; it is a consideration of the partitioning of monomeric surfactant (at or below cpqd) and preferred emulsion type. Daviesl* put forward a kinetic theory of emulsions. He assumed that the type of emulsion is determined by the relative rates of coalescence of oil and water drops after emulsion formation. The slower coalescence rate for one type of drop, say water, would result in a wlo emulsionpreferentially. The process of coalescence can be thought of as (i) the approach of the drops and formation of a plane-parallel f i i and (ii) thinning of that film to a critical thickness at which the f i i ruptures and the two drops coalesceinto a single larger drop. The kinetics of thinning of such emulsion f i i has been predicted theoretically using a hydrodynamic model.lpP1 This velocity of thinning is dependent upon the balance of forces acting at the interface of the approaching drops. As the two drops come close to each other, liquid flows out of the film toward ita thicker parta (bulk) resulting in the convective flux of surfactant at the surfacethus perturbing its equilibriumdistribution. This generates reverse fluxes tending to restore equilibrium, including surface diffusive flux and bulk fluxes from the film and the drop. Interfacial flow gives rise to interfacial stresses. The difference in surface concentration along the surface results in a variation in the local values of the (18)Daviee, J. T. In 'Proceeding8 of the 2nd International Congreer on Surface Actioity; J. H., Schulman, Ed.; Butterworth London, 1967; Vol. 1,p 426. (19)Traykov, T. T.; Ivanov, I. B. Int. J. Multiplurse Flow, 1977,3,471. (20) Ivanov, I. B. Pure Appl. Chem. 1980,62,1241. (21)+pryanov, 2.; Malhotra, A. K.; Aderangi, N.; Wanan, D. T. Znt. J . Multrphaue Flow lSM, 9,106.
28 Langmuir, Vol. 9, No. 1, 1993
interfacial tension which produces a surface force (interfacial tension gradient) opposite to the liquid flow. The rise in tension depends on the Gibbs elasticity G of the film, equal to 2c if sufficiently thick, where t = dr/d In A (A = area of surface)and is the surface dilational modulus. G is a measure of the film’sresistanceto localizedthinning. With this model, it is shown that the veIocity of thinning depends on the location of nonadsorbed surfactant. If surfactant is solublemainly in the continuous(film)phase, it has to diffuse a long way from the film perimeter in order to reduce G. Since the driving force for this process is the gradient of surfactant concentration along the surface,this diffusioncannot eliminatethe tension gradient which opposes thinning. However, if surfactant is soluble mainly in the dispersed (drop) phase, it must diffuse a much shorter distance, and since this flux is driven by the normal gradientof the concentration, it can counterbalance the convectiveflux and so relieve the elasticity and increase the thinning velocity. Summarizing, theory that emulsions containing most of the surfactant in the dispersed phase will coalesce faster than those where surfactant is mainly in the continuousphase, and so will not be the preferred emulsion type, in apparent accordance with Bancroft’s rule. In this context, it is interesting to use the present results to test these predictions. From Table I and Figure 4 (for concentrationsSC~C,,~) we see two occasionswhere,despite the surfactant (monomer) distribution being heavily in favor of oil,emulsion type remainso/w violatingBancroft’s rule. This occurs at T < PIT and concentrations S C ~ C , ~ , where P in the worst case is at least -400, and at T > PIT for concentrations
