Langmuir 1994,10, 2570-2577
2570
Nonequilibrium Structure of Water in Oil Gel Emulsions Hironobu Kunieda,*>tVijay Rajagopalan,? Etsuko Kimura,? and Conxita Solanst Department of Physical CheGistry, Division of Materials Science and Chemical Engineering, Yokohama National University, Tokiwadai 156, Hodogaya-ku, Yokohama 240, Japan, and Departamento de Tensioactivos, CID, C.S.I.C., Jordi Girona 18-26, 08034 Barcelona, Spain Received December 14, 1993. I n Final Form: May 19, 1994@ Nonequilibrium structure of water in oil (W/O) type gel emulsions (or highly concentrated emulsions) was investigated by means of the ESR spin probe method. The change in apparent order parameter “S” and the isotropic hyperfine splitting constant “UN” of spin probes (5- and 16-doxylstearic acids) in gel emulsions, ordinary W/O (middle- or low-internal-phase-ratio) emulsions, and single oil phases were determined along a tie line in which the oihurfactant ratio is kept constant and only water content is changed in a watedtriethylene glycol dodecyl ether (or tetraethylene glycol hexadecyl ether)/cyclohexane and the isotropic hyperfine splitting constant “a”’ of system. The apparent order parameter “S, 5-doxylstearicacid increase slowly but continuouslyin an ordinaryemulsion region beyond the solubilization limit and then abruptly increases in the gel emulsion. Whereas in case of 16-doxylstearicacid, both S and U N remain unchanged over a wide range of water contents except in the gel emulsions. Using a simple model, we have calculated the distribution of surfactant molecules between the reverse micelle and the water-oil interface in emulsions. The result predicts that there is no reverse micelle in the continuous oil phase and the spin probe exists at the water-oil interface in gel emulsions, and the water droplet size increasesin the gel emulsionregion with increasingwater content. These predictions are in good agreement with the ESR data and the microscopic observations.
Introduction Phase equilibria of ternary watednonionic surfactant/ oil systems have been widely investigated and their complexbehavior as a function of temperature is a t present well under~tood.l-~ One of the most characteristic features of the ternary phase diagram is the existence of a miscibility gap expanding from a water-oil axis. At equilibrium, the miscibility gap consists of two- or threeliquid phases depending on composition variables and temperature. The present knowledge in nonequilibrium conditions is by far less understood as different emulsion types can be formed a t a given composition and temperature. It is known that high-internal-phase-ratio emulsions (HIPREs) or highly concentrated emulsions can be formed in the water- and oil-rich regions under certain cond i t i o n ~ . ~In- ~this context, in the recent years, we have undertaken a systematic study of W/O highly concentrated emulsions. Due to their characteristic features such as large water content, high viscosity, and translucence they are also referred to as gel emulsions.10-16
* To whom correspondence should be addressed. t Yokohama National University. Departamento de Tensioactivos, CID. Abstract published in Advance A C S Abstracts, J u l y 1, 1994. (1) Shinoda, K.; Kunieda, H. J . Colloid Interface Sci. 1972,42,381. (2) Kunieda, H.; Shinoda, K. Bull. Chem. SOC.Jpn. 1982,55, 1777. (3) Kunieda, H.; Sato, Y. Organized Solutions; Friberg, Stig E., Lindman, B., Eds.; Surfactant Science Series; Marcel Dekker: New York, 1992; Vol. 44. (4) Lissant, K. J. J . Colloid Interface Sci. 1966, 22, 462. (5) Lissant, K. J.; Mayhan, K. G. J . Colloid Interface Sci. 1973,42, @
ani (6) Groves, M. J.; Mustafa, R. M. A,; Carless, J. E. J . Pharm. Pharmacol. 1974,26, 616. (7) Mi, A. A.; Mulley, B. A. J . Pharm. Pharmacol. 1978, 30, 205. ( 8 ) Princen, H. M. J . Colloid Interface Sci. 1979, 71, 55. (9) Sagitani, H.; Hattori, T.; Nabeta, K.; Nagai, M. Nippon Kagaku Kaishi l9ES, 1399. (10) Solans, C.; Azemar, N.; Comelles, F.; Sanchez Leal, J.; Parra, J . L. Proc. XVII Jorn CED/AID. 1986. 109. (11) Kunieda, H.; Solans, C.;’Shida, N.; Parra, J. L. Colloids Surf. 1987, 24, 225. (12) Solans, C.; Azemar, N.; Parra, J . L. Prog. Colloid Polym. Sei. 1988, 76, 224.
These gel emulsions can be formulated with large amount ofwater (i.e. higher than 99%(w/w))and very low surfactant content (i.e.lower than 0.5%(w/w)).11-14There is a huge interfacial area (water-oil interfaces) compared with the volume of continuous media inside the gel emulsions. Therefore, it is possible that the structure of the continuous oil phase in the gel emulsions is different from that in its equilibrium state. Electron spin resonance (ESR) is a powerful technique extensively used to study biological systems17J8and also in surfactant chemistry, to study the s t r u c t ~ r a land ~~,~~ dynamic properties21-28of micelles and microemulsions. The dynamic nature of the association of monomers in micelles, the dynamics of solubilization of compounds by micelles, and the rate of motion of solubilizates within micelles have been investigated by means of the ESR spin probe method. In W/O emulsion systems, the surfactant molecules would be distributed between the reverse micelles and surfaces of dispersed water droplets. Spin (13) Solans, C.; Dominguez, J. G.; Parra, J. L.; Heuser, J.; Friberg, S . E. Colloid Polym. Sei. 1988,266, 570. (14) Kunieda, H.; Yano, N.; Solans, C. Colloid Surf. 1989, 36, 313. (15) Kunieda, H.; Evans, D. F.; Solans, C.; Yoshida, M. Colloid Surf. 1990, 47, 35. (16) Pons. R.: Solans. C.: Stebe. M. J.: Erra., P.:. Ravev. J . C. Prop. Colloid Polym. Sci. 1992, 89, 110.’ (17) McConnel, H. M.; McFarland, B. G. Q.Reu. Biophys. 1970,3, 91. (18) Smith, I. C. P. BiologicalApplicatwnsofElectronSpinResonance; Swartz, H. M., Bolton, J. R., Borg, D. C., Eds.; Wiley-Interscience: New York, 1972, pp 483-539. (19)Waggoner, A. S.; Griffith, 0. H.; Christensen, C. R. Proc. Natl. Acad. Sci. U S A . 1969, 57, 1198. (20)Wagonner, A. S.; Keith, A. D.; Griffith, 0. H. J . Phys. Chem. 1968, 72, 4129. (21) Artherton, N. M.; Strach, S. J . Chem. SOC.,Faraday Trans. 2 1972, 68, 374. (22) Brotherus, J.;Tormala, P. Kolloid 2.2.Polym. 1973,251, 774. (23) Fox, K. K. Trans. Faraday SOC.1971, 67, 2802. (24) Nakagawa, T.; Jizonoto, H. Kolloid 2.2.Polym. 1972,250,594. (25) Oakes. J . Nature 1971.231. 38. (26) Ohnishi, S.; Cyr, T. J. R.; Fdkushima, H. Bull. Chem. Soc Jpn. 1970. 43. 673. (271DiMegli0,J.-M.; Dvolaitzky, M.; Taupin, C. J . Phys. Chem. 1985, 89, 871. (28) Bglioni, P.; Gambi, C. M. C.; Goldfarb, D. J . Phys. Chem. 1991, 95, 2577. “
0 1994 American Chemical Society
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\
(b)
Figure 1. Typical ESR spectra of 5-doxylstearic acid in emulsion (a) and gel emulsion (b) states.
probes are also likely to be distributed in these two states if amphiphilic spin probe molecules are used. Hence, it is also possible to figure out the microstructure in a n emulsion state. In this context, the nonequilibrium structure of gel emulsions has been investigated by means of the ESR spin probe method.
Experimental Section Materials. Homogeneoustri- and tetraethylene glycoldodecyl ether (abbreviated as R12EO3 and R12E04, respectively) and tetraethylene glycol hexadecyl ether (abbreviated as R16E04)) were kindly suppliedby Nihon Surfactant Co. Extra-puregrade cyclohexane and heptane were obtained from TokyoKasei Kogyo Co. The spin probes, 5-doxylstearicacid and 16-doxylstearicacid were obtained from Molecular Probes, Inc., and Aldrich, Inc., respectively. All the materials were used without further purification and always double distilled water was used. Procedures. Preparation of Gel Emulsions. The surfactant (R12E03and R16E04)and the spin probes were weighed in the ratio of 100:1,respectively, and then shaken in a vortex mixer for about 5 min until we obtain a clear homogeneous mixture of the surfactant and the spin probe. As reported ealier, we have confirmed that there is no direct interaction between the spin probe molecules at this mixing ratio of surfactant to spin probe.29 Gel emulsionswere prepared by shakinga test tube containing oil, water or aqueous solution,and surfactantwith the spin probe. If necessary, the formation of gels was aided by the presence of glass beads to enhance the local agitation.ll Procedures To Determine Phase Diagram. Procedures to determine phase boundaries and gel regions are described in the previous papers.11J5 ESRMeasurements. The ESR measurementswere recorded using a JEOL-ME-3X spectrometerwith 100-kHz field modulation. Since the present systemscontain a large amount ofwater, a capillarytube (9 = 1.54mm)was used to avoid heating samples by microwave radiation. The ESR spectra of 5- and 16-doxylstearicacid in oil phases and emulsions were measured without neutralization and the observed spectra were anisotropic as shown in Figure 1. (29) Rajagopalan, V.; Solans, C.; Kunieda, H. Colloid Polym.Sci., in press.
Figure 2. Phase diagram of a water/RlzEO&yclohexane system at 25 "C. I and I1 are one- and two-phase regions. VI is a viscous isotropic phase. RH is a reverse hexagonal liquid crystal. LC is the region in which a lamellar liquid crystal is present. The broken lines are tie lines. The gel emulsion region is indicated by a black area.
These anisotropic spectra were characterized by using the apparent order parameter S30 given by the following equation S = ( U ~ U , ) (-AA)/(A,,
- 1/2(A,
= Ayy))
(1)
where the principal values of the hyperfine splitting tensol31 were taken as A, = 6.3 G, A, = 5.8 G, and A,, = 33.6 G. The polarity correction factor involves ao = 1/3(& + A, + AZz)= 15.23G and U N = 1/3(A+ 2A). The isotropic hyperfine splitting constant U N is a measure of the polarity of the spin probe. VEM (Video Enhanced Microscopy). A differential interference phase-contrast (Nomarski-type)microscope (Nikon, X2F-NTF-21)with a video-enhancedsystem (HamamatsuPhotonics Co., Argus 10) was used to observe gel and ordinary emulsions.
Results and Discussion Phase Diagrams of Water/R18Os/Cyclohexane and Water/RlsEOdCyclohexane systems. The HLB (hydrophile-lipophile balance) numbers of R12EO3 and R16EO4 are nearly the same. Their Griffin's HLB numbers are 8.3 and 8.4,respectively. Using the empirical relation ofHLB number, the HLB t e m p e r a t ~ ror e ~the ~ three phase temperature in water-cyclohexane are calculated to be -29 and -27 "C,respectively. Their phase behaviors are quite similar and both surfactants act as lipophilic emulsifiers at room temperature (25 "C), as a result of which WIO type gel emulsions can be formed in the waterrich region of these systems. Phase diagrams of water/R12EOdcyclahexaneand water/ R16EOdcyclohexanesystems at 25 "C are shown in Figures 2 and 3. A two-phase region consisting of oil phase and excess water is extended from a water-oil axis. At room temperature, which is much higher than the respective HLB temperature of the systems, the surfactant is lipophilic and mainly dissolves in oil and forms reverse micelles. The tie lines are focused to the water apex in the two-phase region. The tie lines in the main miscibility gaps were estimated by analyzing the water content in (30) Grafiey, B. J. Spin probeing; Berliner, L. J., Ed.; Academic Press: New York, 1976; p 567. (31)Graffney, B. J.; McConell, H. M. J. Magn. Reson. 1974,16, 1. (32) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1986,107, 107.
2572 Langmuir, Vol. 10, No. 8, 1994
Kunieda et al.
Y P
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P
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tiQ
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Figure 3. Phase diagram of a water/R&04/cyclohexane system at 25 "C. The phase equilibria in the region near the water-surfactant axis is omitted in the figure.
the oil phase. There is a region including liquid crystal (LC) in the vicinity of a water-surfactant axis but the details are omitted, especially in Figure 3. W/Otype emulsions are formed in the main two-phase regions. The internal water phase in the W/O emulsions increases with increase in water content. The W/O emulsion close to the water apex includes a large amount of water, and the system is very viscous and translucent. Since the volume fraction of excess water phase exceeds that of closest packing of spheres, the water droplet is not spherical but p ~ l y h e d r a l . ~The J ~ change in appearance from ordinary emulsions to gel emulsions is continuous and it is very difficult to determine the boundary accurately. The central area of translucent and stiff gel regions are indicated in Figures 2 and 3. The regions were determined by visual observation. The structure of gel emulsions resembles that of aged foams and the water droplets are covered by a very thin layer of continuous phase, oil phase. Therefore, the interfacial area is very large although the volume fraction of the oil phase is very small. Note that although the gel emulsions form almost a t the same oil/surfactant ratio in both the systems, the visual inspection results show that the gel emulsions formed with R16E04 are more stable than the R12E03 gel emulsions. Thus we see that longer chain surfactants can stabilize the gel emulsions. Structure in the Single Oil Phase. If we fix the surfactant to oil ratio and follow a n experimental path toward the water apex, we gradually proceed from the one phase to the low- and then high-internal-phase-ratio (gel-) emulsions in the two-phase region with increasing water concentration. The line corresponds to a tie line in the two-phase regions (e.g. lines A and B in Figures 2 and 3). In order to get a clear picture of the structural changes associated to the continuous phase along this tie line, we have made use of 5-doxylstearic acid and 16-doxylstearic acid as the two spin probes. The reason to use two spin probes arises from the fact that for 5-doxylstearic acid, the nitroxide moiety of the probe in the system would be located nearer to the hydrophilic head group and thus it would throw light on the packing of surfactant head groups, whereas for 16-doxy1stearic acid, the position of the nitroxide molecule is a t the end of the hydrophobic chain and consequently information on the hydrophobic surface of the surfactant layers could be obtained.
0.12
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Water content1 Wt% Figure 4. Apparent order parameter S of 5-doxylstearicacid aa a function of water content in water/RlzEO&yclohexane and water/Rl~E04/cyclohexanesystems. The compositions are along tie lines A and B of Figures 2 and 3. uanand "b" denote
the solubilization limits in R12E03 and R16E04 systems, respectively. R12E03: a = 0.132, K = 1.5. R16E04:a = 0.2, K = 1.63. The anisotropic ESR spectra suggest a change in the apparent order parameter S and the isotropic hyperfine splitting constant aN, thus suggesting a change in the packing of the hydrocarbon chains. The degree of restriction of spin probe motion is reflected by the high field line, which has broadened appreciably. The broadening leads to a noticeable reduction in the peak-to-peak amplitude. The high field line has the shortest transverse relaxation time and hence is most sensitive to changes in the mobility of the spin probe. The distance between the low field and high field lines does not significantly change by a very large value for all our spectra, thus suggesting that although the packing of surfactant molecules increases, the motion of surfactant molecules in the films a t the water-oil interface does not change significantly. We measured the apparent order parameter S and the isotropic hyperfine splitting constant aN of the spin probes as a function of water content along the lines A and B of Figures 2 and 3 and the results are shown in Figures 4-7. It is known that there is no self-organizing structure at a n oil-surfactant axis in the absence of wateF' and the ESR spectra is fairly isotropic; however, the degree of anisotropy increases as the water content increases due to the formation of reverse micelles. The reason for the increase in spectral anisotropy is mainly due to the two types of processes prevalent; namely, the Brownian tumbling of the reverse micelles and the lateral diffusion of surfactant molecules along the interface of micelle.34 These two processes modulate the hyperfine coupling (33) Olsson, U.; Jonstromer, M.; Nagai, K.; Soderman, 0.; Wennerstrom, H.; Hose, G . Prog. Colloid Polym. 1988,76, 75. (34) Kommareddi,N. S.;John,V.T.;Waguespack,Y.Y.;McPherson, G.L.J . Phys. Chem. l99S,97,5752.
Structure of W I O Gel Emulsions
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a
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Figure 5. Isotropic hyperfine splitting constant aN of 5-doxylstearic acid as a function of water content in water/RlzEO$ cyclohexane and water/Rl~EOdcyclohexanesystems. The compositions are along tie lines A and B of Figures 2 and 3. “a” and “b” denote the solubilizationlimits in R12E03 and R16E04 systems, respectively. R12E03: a = 0.132, K = 1.5. R16E04: a = 0.2, K = 1.63.
20
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Water content 1 Wt% Figure 6. Apparent order parameter S of 16-doxylstearicacid as a function of water content in water/R~~E03/cyclohexane and water/R16E0dcyclohexane systems. The compositions are along tie lines A and B of Figures 2 and 3. 15
tensor, A, and the g tensor. When the frequency of the two averaging processes falls below lo8 s-l, the resulting spectra would be isotropic. As the size of the reversed micelles increases with increasing water content, Brownian tumbling deceases and the frequency of the averaging processes falls below lo8 s-l, which is not rapid enough to average out the anisotropies. This results in highly anisotropic spectra for the high water content systems.35 Such spectra are normally discussed in terms of the apparent order parameter. Order parameters provide information regarding the degree of organization of the surfactant molecules and the isotropic hyperfine splitting constant, aN, is used as a measure of the polarity of the spin probe environment. Figure 5 illustrates the variation of aN with water content. With increasing water content, aN increases, indicating that the environment ofthe spin probe changes from an apolar one to a more polar one. The change as expected is more pronounced in the present system because the doxy1 group in the spin probe used (5-doxylstearic acid) is closer to the COO- group. This could be due to the molecular and segmental motion and the penetration of water into the surfactant layer, both of which will contribute to the increase in aN with increasing water content. This behavior is not unexpected if one considers the location of the spin probe to be like in the model proposed by Haering et al.35 In fact, a recent electron spin echo modulation study by Piero Baglioni et al.36using n-doxylstearic acids confirms the location of the para-
Figure 7. Isotropic hyperfine splitting constant aN of 16doxylstearic acid as a function of water content in water/Rlz-
(35)Haering, G.; Luisi, P.; Hauser, H. J.Phys. Chem. 1988,92,3574. (36)Baglioni, P.;Nakamura, H.; Kevan, L. J.Phys. Chem. 1991,95, 3856.
magnetic moieties and the solubilization ofthe spin probes at the micellar interface.
14.8
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Water content1 Wt%
E03/cyclohexane and water/R&Odcyclohexane systems. The compositions are along tie lines A and B of Figures 2 and 3.
Kunieda et al.
2574 Langmuir, Vol. 10, No. 8,1994 The order parameters increases up to the solubilization limits a and b due to the existence of reverse micelles. The change in S and the corresponding change in aN for 5-doxylstearic acid are consistent with the existence of reverse micelles in a single oil phase. On the other hand, both S and aN remain almost unchanged when we use 16-doxylstearic acid as the spin probe for a very large range of water concentration. Structure in the Emulsion Phase. At the solubilization limits indicated by a and b in Figures 4 and 6, W/O type emulsions form due to the phase separation of a n excess water. Beyond these points both S and aN were measured in the nonequilibrium emulsion states in which a n excess water phase is dispersed as water droplets in the continuous medium, the oil phase containing reverse micelles. Judging from Figures 2 and 3, the compositions of oil and water phases are practically unchanged on the lines A and B in the two-phase region a t equilibrium because the dilution path is along the tie line. Therefore, no change in S and aN is expected in the two-phase region. In fact, as shown in Figures 4-7 both S and aN are almost unchanged in the ordinary emulsion region implying that both the order and the polarity are almost unchanged in this region. However, the apparent order parameter S increases in the water-rich region where gel emulsions form, as is shown in Figures 4 and 6. As the spin probes are waterinsoluble and only monodispersed surfactant is dissolved in the excess water phase, the spin probes do not exist in the excess water phase. In a n emulsion state, the wateroil interfacial area (total interfacial area ofwater droplets) increases with increasing water content, and the surfactant molecules are preferentially adsorbed at water-oil interface. With a n increase in water-oil interfacial area, the number of reverse micelles should decrease. Therefore, the spin probes are moved from reverse micelles to the water-oil interface with increasing water content. The spin probe is incorporated in reverse micelles at the maximum solubilization points, a and b, in Figures 2 and 3. On the other hand, in the gel emulsion system, most of surfactant molecules are considered to be a t the wateroil interface (water droplet film) because surfactant concentration is very small (0.5wt %) and large interfacial area exists. Although gel emulsions can be formed a t almost the same surfactant to oil ratio for both the R16E01 and the R12EO3 systems, visual observation results show that the gels formed with longer chain surfactants are more stable when compared to the short chain surfactants. In fact, it is well-known that the stability for coalescence of emulsion droplets dramatically increases with the sizes of surfactant molecules.37 We see from Figure 4 that the apparent order parameter for the longer chain surfactant (R16EO4)is seen to be consistently higher than that for the shorter chain surfactant (R12E03). Hence, the longer-chain surfactant is more tightly packed a t the interface which is consistent with the visual observation of higher stability against the coalescence. Figures 6 and 7 show that there is almost no change in S and aN of 16-doxylstearic acid along the tie line. This can be attributed to the fact that in case of 16-doxylstearic acid the nitroxide moiety being further away from the hydrophilic group and the surfactant chains being more relatively “free’),a constant value of S and aN could be expected over a wide range of water content. However, the spin probe is mainly located at the water-oil interface (the interface of water droplet) in the gel emulsions. It is considered that since the curvature of the water droplet
is extremely small compared to that of reverse micelles, the surfactant molecules at the water droplet are more tightly packed. Distribution of Surfactant Molecules between Reverse Micelles and Oil-Water Interface. The ESR study suggests that the positions of the spin probes are different in ordinary and highly concentrated emulsions. The surfactant molecules are distributed between the interface and reverse micelles. Surfactant molecules are preferentially adsorbed at the interface since it is wellknown that surfactant starts to form micelles beyond the critical micellar concentration. As a result, in nonequilibrium state (in emulsion state), the number of reverse micelles decreases with increasing water content, and finally no reverse micelles are present in the continuous media a t very high water content. In order to confirm this idea, we calculated the surfactant ratio a t the interface and in the reverse micelles in emulsions. We assume that monodisperse solubilities of surfactant in water and oil are zero. The monodisperse solubility of nonionic surfactant in oil is negligible (although not very low), because the surfactant concentration in oil is always very high in our present study. It is also assumed that the structure of reverse micelles in the continuous media remains unchanged even ifthe water content is changed. Namely, the waterlsurfactant ratio in the reverse micelles is constant and is the same as that a t the solubilization limit
(37) Shinoda, K.; Saito, H.; Arai, H. J. Colloid Interface Sci. 1971, 35,624.
where WSIm is the weight of surfactant at the droplet interface and WsRMis the weight of surfactant in the reverse
a = WwsL/W,SL where a is waterlsurfactant weight ratio in reverse micelles and WwsLand WssLare weight fractions of water and surfactant a t the solubilization limit corresponding to the points a and b in Figures 2 and 3. In the emulsion state the water droplets are covered by surfactant monolayers. If we assume that the droplets are homogeneous in size (radius r), then the water1 surfactant ratio can be expressed as:
where p is the waterlsurfactant weight ratio in the water droplet, A is the section area of surfactant per 1mol of surfactant, M , is the molecular weight of surfactant, and ew is the density of the water phase. At a certain composition on the tie line (W,,, W,, Ww, are the weight fractions of oil, surfactant, water, respectively),
w,= + y l + a l+/3 Ww=-
lSa
(4)
+A 1+8
where x is the weight fraction of reverse micelles (water + surfactant) in the system and y is weight fraction of water droplets (water surfactant) in the system. Hence, the weight ratio of surfactant at the water droplet interface and in reverse micelles can be expressed as follows
+
(6)
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1.6
2 2@ 1.2 \
c
I
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0
0
0.25
0.5
0.75
1
WATER CONTENT/ Wt% Figure 8. Theoretical value of WsINTiWsRM as a function of water content along the line A in Figure 1. The values are calculated using eq 6 for cases where the radius of the water droplets are 1or 5pm. a = 0.2,K = 1.63. The values ofg were 253 for 1pm and 1263 for 5 pm. micelles. Since WdW, on the tie line is constant, K,and W, WO W, = 1we have
+
+
w, w, = 1-
l+K
(7)
Using eqs 3-6, we can eliminate x , y , and W, and the resultant equation is W,INTlWSRM =
+ +
a - (1 K a)W, (1 + K + p>w, - p
(8)
The variation of W,"TIW,RM as a function of water content (W,) is shown in Figure 8, in which the section area of surfactant molecule a t the water-oil interface is assumed to be 40 A2.38The value is zero at the solubilization limit and a t first it increases very slowly and most of surfactant molecules are incorporated in reverse micelles in a wide range ofwater content. In the water-rich region, the WSm/ WsRMsuddenly increases and most of the surfactant is located a t the water-oil interface (interface of water droplets). Finally it becomes infinite a t a certain value of W, (-0.99). Beyond this infinite value, reverse micelles do not exist in the continuous oil phase and surfactant moleculesare located only a t the interface ofwater droplets if one neglects monodisperse surfactant in water and oil. This prediction is also supported by the SAXS experimental data of gel emulsions in a similar system.39 Figure (38)Rosen, M. J. Surfactants andlnterfacial Phenomena; Johnviley & Sons: New York, 1989.
(39)Pons, R.; Ravey, J. C.; Swage, S.; Stebe, M. J.; Erra, P.; Solans, C. Colloids Surf., in press.
0
0.25
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1
Aqueous Solution (10% Na, SO, ) Content / Wt% Figure 9, Theoretical value of W,'TWsRM as a function of water content along tie lines in a 10 wt % NazS04(aq)/RlzE03/ cyclohexane and 10wt % Na2S04(aq)/Rl~EO&eptanesystems. "c" and "d" indicate the solubilization limits for R12E03 and R12E04 systems, respectively. The radius of the water droplet is assumed to be 1pm. R12E03: a = 0.16, K = 1.5,3!, = 281. R12E04: a = 0.04, K = 1.38, 4, = 246.
8 also shows that the droplet size does not significantly change the variation until the time one does not assume a very small droplet size. If the spin probe molecules are distributed between the interface and reverse micelles, then we should observe the ESR spectrum mainly from the reverse micelles a t low water content and in the water-rich region or gel emulsion region; the ESR spectrum would come from the interface. Hence, this calculation explicitly confirms that the resonance observed in the gel emulsion systems is purely from the interface. Water Droplet Sizes in W/O Gel Emulsions. At certain water content, W, in the gel emulsion region, the value of WsmT/WsRM becomes infinite. In the water-rich region with highest water concentration, the total interfacial area of water droplets should be constant, because reverse micelles are not present in the continuous oil phase and all the surfactant molecules are adsorbed a t the interface of water droplets. The change in S in this region is very small as is seen from Figure 4; therefore, the water droplet size should increase with increasing water content in the gel emulsions. In fact, the former experimental results support this a s s u m p t i ~ n . ~ ~ * ~ ~ In order to verify this idea, the water droplet size was investigated in two systems, 10 wt % Na2SO4(aq)/Rl2E03/ cyclohexane and 10 wt % NaZS04(aq)/R12EOdjheptane systems. The addition of Na2S04 was due to the fact that gel emulsions are highly stabilized in the presence of inorganic salts.14 The theoretical values of WsINT/WsRM (40) Pons, R. Ph.D. Thesis, University of Barcelona, 1992.
2576 Langmuir, Vol. 10, No. 8, 1994
Kunieda et al.
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Figure 10. VEM pictures of ordinary and gel emulsions along the tie lines in a 10 wt % Na~S04(aq)/R12EO&yclohexane and 10 w t % NazSOdaq)/R12E04/heptane system a t 25 "C. 10 wt % Na2SO4(aq)/R12EO3/cyclohexane system, (a)W, = 0.500, (b) W, = 0.980, (c) W, = 0.995, 10 wt % Na2SOAaq)/R12E04/heptane,(d) W, = 0.980, (e) W, = 0.988, (0 W, = 0.995. for the two systems were calculated using eq 7 and are shown in Figure 9. It is assumed that the radius of water droplet in the ordinary emulsion is 1pm judging from the microscopic observation. Figure 10shows the VEM (videoenhanced microscopy)pictures of ordinary and gel emulsions. As predicted before, water droplet size in the gel emulsions is increased with increasing water content in both the systems. In our theory, the monodisperse solubilities of surfactants in oil and water phases are neglected. If both values are taken into account, we would get more accurate curves. However, for W/O gel emulsions, the deviation is very small, because the cmc in water is very small, about 5 x M.38 Although the monodisperse solubility of swfactant in oil is not low, the oiVsurfactant weight ratio is low in the present system and hence it can also be neglected.
Conclusion W/O gel emulsions (high-internal-phase-ratio emulsions) form in waterhomogeneous polyethylene glycol alkyl ether (kE0,)hydrocarbon systems above the threephase temperature called the HLB temperature. The gel emulsion consists of an excess water phase and an oil phase containing reverse micelles. We measured the apparent order parameter S and the isotropic hyperfine splitting constant a~ of two spin probes (5- and 16doxylstearic acids) along a tie line. Since the spin probe is amphiphilic and not water-soluble, it is incorporated with surfactant aggregates or in surfactant molecular layers. When one makes use of 5-doxylstearicacid as the spin probe, we notice that all along the tie line the apparent order parameter in the R16E04 system is higher than the R12E03 system which implies a tight packing in the longer chain surfactant than in the shorter chain surfactant. This
Structure of W I O Gel Emulsions is consistent with our visual inspection of stability results. On the tie line in the two-phase region, the compositions of each phase should be invariant a t equilibrium and independent of water content. However, although the apparent order parameter S of the spin probe in the single oil phase at the solubilization limit is similar to that in an ordinary (low internal-phasevolume-ratio) emulsion, there is a n increase in S in the gel emulsion region. The distribution of surfactant between reverse micelles and the oil-water interface (surface of water droplet) is calculated. It is expected from the calculation that most of surfactant molecules
Langmuir, Vol. 10,No.8,1994 2577 form reverse micelles over a wide range of water content in the emulsion (two-phase) region. On the other hand, when the weight fraction of water in the system is -0.99, most of the surfactant is located at the water-oil interface and no reverse micelles are present in the continuous oil phase. The ESR data and VEM observations support this idea.
Acknowledgment. The authors gratefully acknowledge Mr. M. Akimaru (Nihon Surfactant Co.) for having supplied pure nonionic surfactants. Support by DGICYT (Grant No. PB92-0102) is gratefully acknowledged.
