Langmuir 1992,8, 833-831
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Phase Behavior and Physicochemical Properties of Sodium Octyl Sulfateln-Decane/l -Hexanol/Aqueous AlC13 Middle-Phase Microemulsion Masahiko Abe,*ltJ Tadao Yamazaki,? and Keizo OginofJ Faculty of Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278, Japan, and Institute of Colloid and Interface Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162, Japan
Myung Ja Kim Department of Chemistry, Sookmyung Women's University, Chungpa-dong 2-kaJ Yongsan-ku, Seoul 140- 742, Korea Received March 12, 1991. In Final Form: December 31, 1991
The phase behavior and physicochemical propertiesof sodium octyl sulfateln-decanell-hexanol/aqueous AlC13middle-phase microemulsion have been studied as a function of salinity to develop an experimental investigation for better understanding of the microstructureof a middle-phase microemulsion. The system exhibits a Winsor-type phase transition (Winsor I Winsor I11 Winsor 11) with increasing salinity. Over an appreciable salinity (from 0.50% to 9.2%),the formation of Winsor 111, composed of a middlephase microemulsion in equilibrium with the excess water and oil phases, was observed. It has been observed that as the salinity is increased, the phase volume of the middle-phase microemulsion undergoes a drastic decrease at a specific brine concentration (3.8 76 ). Furthermore, the physicochemical properties such as water content, electrical conductivity,diffusion coefficient,.and solubilization of 1-hexanol in the AlC13 middle-phase microemulsion all show abrupt changes at this salinity. The drastic change in the phase volume and physicochemical properties at the specific salinity of 3.8% may be attributed to a phase inversion of the AlC13 middle-phase microemulsion from oil-rich to water-rich continuous phase with increasing AlC13concentration,which is quite a different behavior from that observed for monovalent and divalent salt systems. Specifically, it may be assumed that a fluctuating structure of bicontinuous type and a liquid crystal structure overcome the droplet structure in the phase equilibrium at a certain salinity during the increase in the trivalent salt concentration.
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It is generally believed that a middle-phase microemulsion system can have a bicontinuous structure consisting Microemulsions are homogeneous mixtures of hydroof an equal mixture of oil-in-water and water-in-oil type carbons and water with large amounts of surfactants.'V2 micro emulsion^.^^^ Although the physical meaning of biThey usually contain a cosurfactant such as a medium continuity has not been defined, it is frequently considered chain alcohol in combination with a primary s ~ r f a c t a n t . ~ ~ ~ to consist of ill-defined oil and water regions coexisting in Microemulsions form spontaneously on contact among the absence of long-range order. However, the basic components; these systems can have ultralow interfacial concept of oil/water phase classification and the discrete tensions when a surfactant-rich middle phase exists in nature of the dispersed phases of microemulsions still need equilibrium with phases of both excess oil and water, to be clarified. separated from each other by sharp phase b~undaries.~-~ In studies of the phase behavior of middle-phase miIt has been recognized that the middle-phase microemulcroemulsions,it has been established that the phase volume sions exhibit remarkable properties which have considof the middle phase becomes a minimum at the optimum erable practical and scientific importance in the field of condition of salinity. Previously we have reported on the enhanced oil recovery&13 and ultra micro particle^.^^-'^ Winsor-type phase behavior of a middle-phase microemulsion system containing varying concentrations of + Faculty of Science a n d Technology. monovalent inorganic However, there have no * Institute of Colloid a n d Interface Science. studies of the effect of a trivalent salt on the formation (1)Schulman, J. H.;Stoeckenius, W.; Prince, L. M. J. Phys. Chem. of middle-phase microemulsions. 1959,63,1677. (2)Stoeckenius, W.; Schulman, J. H.; Prince, L. M. Kolloid-Z. 1960, In this paper, we report the phase behavior and phys169, 170. icochemical properties of sodium octyl sulfateln-decane1 (3)Barakat, Y.; Fortney, L. N.; Schechter, R. S.;Wade, W. H.; Yiv, S.; 1-hexanovaqueous (or FeCl3) microemulsion systems Graciaa, A. J. Colloid Interface Sci. 1983,92,561. (4)Lalanne-Cassou, C.; Carmone, I.; Fortney, L. N.; Samii, A.; as a function of the concentration of the trivalent inorganic Schechter, R. S.; Wade, W. H.; Weerasooriya, U.; Weerasooriya, V.; Yiv, salt. Results obtained from measurements of water S. J. Dispersion Sci. Technol. 1983,8, 137.
Introduction
(5)Saito, H.;Shinoda, K. Bull. Chem. Sci. Jpn. 1970,32, 647. Shinoda, K. J. Colloid Interface Sci. 1982,55,1777. (6)Kunieda, H.; (7)Reed, R.I.; Healy, R. N. Improved OilRecouery by Surfactant and Polymer Flooding; Shah, D. O., Schechter, R. S., Eds.; Academic Press, Inc.: New York, 1977;p 383. (8)Abe, M.; Schechter, D.; Schechter, R. S.; Wade, W. H.; Weerasooriya, U.; Yiv, S. J. Colloid Interface Sci. 1986,114, 342. (9)Abe,,M.; Schechter, R. S.; Selliah, R. D.; Sheikh, B.; Wade, W. H. J. Dispersron Sci. Technol. 1987,8, 157. (IO) Baviere, M.;Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1981,81, 266.
0743-74631921240a-0a33$03.0010
(11)Abe, M.; Nakamae, M.; Ogino, K. Sekiyu Gakkaishi 1988,32,458. (12)Nakamae, M.; Abe, M.; Ogino, K. Sekiyu Gakkaishi 1988,31,466. (13)Yamazaki, T.;Nakamae, M.; Abe, M.; Ogino, K. Sekiyu Gakkaishi 1990,33, 241. (14)Gobe, M.; Kon-no, K.; Kandori, K.; Kitahara, A. J. Colloid Interface Sci. 1983, 93, 293. (15)Kandori, K.; Kon-no, K.; Kitahara, A. J. Colloid Interface Sei. 1987,155,579. (16)Boutonet, M.; Kizling, J.; Stenius, P. Colloid Surf. 1982,5,209. (17)Ogino, K.;Nakamae, M.; Abe, M. J.Phys. Chem. 1989,93,3704.
0 1992 American Chemical Society
Abe et al.
834 Langmuir, Vol. 8, No. 3, 1992
content, electric conductivity, diffusion coefficient, and solubilization of 1-hexanol in the system, are examined in an attempt to get a deeper insight into the microstructure of middle-phase microemulsion.
Experimental Section Materials. Sodium octyl sulfate (SOS) was supplied by Nihon Surfactant Industries Co., Tokyo, Japan. The sulfate was recrystallized from 1-propanol and then was extracted with diethyl ether in a Soxhlet extractor and dried in a vacuum oven. The purity was ascertained by surface tension measurements and DSC measurements. n-Decane, purchased from Tokyo Chemical Industry Co., Tokyo, Japan, was reagent grade and used without further purification. 1-Hexanol purchased from Tokyo Chemical Industry Co. and aluminum chloride (hexahydrate) and iron(II1) chloride (hexahydrate) from Wako Pure Chemical Industries Co. were reagent grade and used without further purification. Water used in this experiment was twice-distilled and deionized by an ion-exchangeinstrument (NAN0 pure D-1791of Barnstead Co.). Methods. Preparation of Microemulsion. Sodium octyl sulfate (2.0 wt %), n-decane (47.0 wt %), 1-hexanol(4.0 wt %), and brine (47.0 wt %) of varying salinities were placed in a measuring cylinder and sealed. The tubes were shaken and allowed to attain equilibrium over a period of 10 days a t 30 f 0.1 "C. After equilibrium, the microemulsion phase volumes were recorded; the recordings were repeated until no further changes in volume were observed, indicating the attainment of equilibrium. Measurement of Water Content. Water content in the middle-phase microemulsion was measured with a Karl-Fisher moisture titrator (Kyoto Electronics Co., Model MKA-3P) a t 30 f 0.1 "C. Measurement of Alcohol Content. The measurement of 1-hexanol content in the middle-phase microemulsion was performed by gas chromatography using a GC-8A instrument (Shimadzu Co. Ltd.). A differential scanning calorimeter was used to determine the concentrations of 1-hexanola t which large changes in alcohol solubility in the middle phase occur. Differential Scanning Calorimetry (DSC). The analysis by differential scanning calorimetry using DSC-8420B (Rigaku Co., Tokyo, Japan) has permitted the determination of temperatures a t which changes of state occur, as well as the related enthalpy changes. The selected heating rate was 1"Clmin and the weightof sample (themiddle-phase microemulsion mentioned above) was approximately 3 mg. We have checked all the samples to confirm that this system shows no phase separation up to a temperature slightly higher than the freezing point of 1-hexanol. Measurement of Electrical Conductivity. Electrical conductivity was measured by a conductivity meter (TOA Electronics Co. Ltd., CM-40s) with an immersion cell (cell constant of 9.49 cm-l) a t 30 f 0.1 "C. Measurementof Diffusion Coefficient. Dynamic laser light scattering (DLS) experiments were performed to determine mutual diffusion coefficients of the middle-phase microemulsion. These studies used a 4700-type submicrometer particle analyzer (Malvern Instrument, U.K.), with a multibit (8 X N) Malvern correlator with delayed channels. The light source was an argon ion laser (Coherent Co., Innova go), having a wavelength of 488 nm and a power of 5 W or less, and the time-dependent correlation function of the scattered light intensitywas measured a t a scattering angle of 90". Data were analyzed by the combined use of the cumulant methodsls and the model-free algorithm.18 Samples were contained in 8-mm-diameter high-precision Burchard cells, placed in a temperature-controlled vessel a t 30 "C.
Results Phase Behavior of the SOS/n-Decane/1-Hexanol/ Aqueous AlC13 System. Figure 1 shows the change in the phase volume fractions of the SOSln-decanell-hex(18) Berne, B. J.; Pecora, R. Dynamic Light Scattering; Wiley-Interscience: New York, 1976; p 195.
0""
os1
I
2
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I
'
I
6 SALINITY I Yo 4
I
"
8
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10
Figure 1. Volume fraction of each phase in sodium octyl sulfate/ n-decanell-hexanollAlC13aqueous multiphase microemulsion systems as a function of salinity a t 30 "C(- - -,boundary): ML, lower-phase microemulsion; MM,middle-phase microemulsion; MU,upper-phase microemulsion. 1.o
0.5
SALINITY I Yo
Figure 2. Volume fraction of each phase in sodium octyl sulfate/ n-decanell-hexanollFeCl3 aqueous multiphase microemulsion systems as a function of salinity a t 30 "C (- - -,boundary); ML, lower-phase microemulsion; Mu, middle-phase microemulsion; MU,upper-phase microemulsion.
anol/aqueous AlC13 multiphase microemulsion with increasing salinity. Winsor type I, which is the two-phase region composed of a lower-phase microemulsion and excess oil, appears at a salinity lower than 0.5% (SI).On the other hand, Winsor 11,which is the two-phase region made of an upper-phase microemulsion and excess water phase, forms at the higher salinityregion above 9.2 % (SII). In the intermediate region between SI and SII, the formation of Winsor 111, which is a three-phase state containing a middle-phase microemulsion, is observed. As shown in Figure 1, it is noteworthy that the phase volume of the AlC13 middle-phase microemulsion undergoes a drastic decrease at a salinity of 3.8 %. The oil content in the AlC13 middle-phase microemulsion is found to be much larger than that of water a t the salinity region lower than 3.8%,at which concentration the oil content shows a sudden decrease and then gradually increases with further increase in salinity. A similar and even more abrupt decrease in phase volume at a specific salinity can also be recognized in the results of the FeC13 system, as shown by Figure 2. Water Content in Middle-Phase Microemulsion. The water content in a middle-phase microemulsion is generally known to decrease with increasing salinity. However, as depicted in Figure 3, the change in water content of the middle-phase microemulsion is quite unusual, the water concentration remains at approximately 20 % , independent of salinity, until the salinity is 3.7 % , at which point it shows an abrupt increase to about 70% in the vicinity of 3.8% salinity. There is a more gradual decrease to approximately 45 % as the salinity increases up to SII(where the second phase transition takes place). The water content at optimum salinity, which is the intermediate between SI and SII,is about 50 % .
Properties of a Middle-Phase Microemulsion
Langmuir, Vol. 8, No. 3, 1992 835
? 1 3
0 0 SI
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SII 10
SALINITY I Figure 3. Water content in sodium octyl sulfateln-decanellhexanol/AlCl3aqueous middle-phase microemulsion systems as a function of salinity at 30 OC (- - -, boundary).
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0
1
2
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Figure 6. Reciprocal of diffusion coefficients in sodium octyl sulfateln-decane/l-hexanol/AlCl3aqueous middle-phase microemulsion systems as a function of scattering vector at 30 OC: 8 , upper-phase microemulsion; 0 , lower-phase microemulsion. 0.4
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SALINITY I Yo
Figure 4. Electrical conductivity in sodium octyl sulfateln-decane/l-hexanol/AlC13aqueous middle-phase microemulsion systems as a function of salinity at 30 "C (- - -,boundary).
01 0 SI
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4 6 SALINITY I Yo
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Figure 7. Diffusion coefficient (D)in sodium octyl sulfatelndecane/ l-hexanol/AlC13 aqueous middle-phase microemulsion systems as a function of salinity at 30 O C (- - -, boundary). Salinily
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181
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1
2
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4
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Figure 5. Reciprocal of diffusion coefficients in sodium octyl sulfateln-decane/l-hexanol/A1Cl~ aqueous middle-phase microemulsion systems as a function of scattering vector at 30 "C.
Electrical Conductivity in Middle-Phase Microemulsion. The electrical conductivity of a middle-phase microemulsion is generally known to decrease with increasing salinity. As shown in Figure 4, the conductivity of the AlC4 middle-phase microemulsion remains very low (below 2 mS/cm) and essentially unchanged until the salinity is 3.796, after which the conductivity increases sharply to about 9 mS/cm at a salinity of 3.8%. Finally, the conductivity decreases gradually back to the initial value as the salinity is increased up to SII. Diffusion Coefficient of Middle-Phase Microemulsion. Figures 5 and 6 depict the reciprocal of the diffusion coefficient (D-I) as a function of the magnitude of the scattering vector (q), which may be expressed as
where n is the refractive index of the solution, h the
wavelength of light, and 6 the scattering angle. Rushforth et al.19 and Fijnaut20have suggested that values of D-' for spherical structure without interparticle interactions do not depend on q, while D-' values for spherical structures with interparticle interactions and for nonspherical structures do depend on q. Figure 5 shows that values of D-' for the middle-phase microemulsion at the salinities of 3.7% and 3.8% do not vary with q. As mentioned in a previous paper," D-' values for the middle-phase microemulsions where the salinity is far from the two characteristic salinities (SI and SI*) show no dependence on q, while those at salinities of 1.0% and 6.0% decrease gradually with increasing q, perhaps owing to increasing interparticle interactions. For the lower phase and upper phase microemulsions, respectively, Figure 6 shows that the D-I of the lower phase increases gradually with increasing q, whereas that of the upper phase decreases gradually with increasing q. From the results of Figures 5 and 6, it should be noted that the plot of D-I versus q at a salinity of 1.0%,near the lower phase boundary, shows a decreasing trend, which is opposite the increasing trend observed with a lower phase microemulsion of an o/w type. Figure 7 displaysthe dependence of diffusioncoefficients ( D ) for the AlCb middle-phase microemulsion on salt concentration at 30 "C. As has been found by the change (19)Rushforth, D. S.; Sanchez-Rubio, M.; Santos-Vidals, L. M.; Wormuth, K. R.; Kaler, E. W.; Cuevas, R.; Puig, J. E. J. Phys. Chem. 1986, 90,6668. (20) Fijnaut, H.M. J . Chem. Phys. 1981, 74, 6857.
Abe et al.
836 Langmuir, Vol. 8,No. 3, 1992
SALINITY I
Yo
Figure 8. Partition ratio of cosurfactant in sodium octyl sulfate/ n-decane/ 1-hexanoVNaC1aqueous middle-phase microemulsion systems as a function of salinity at 30 OC (- - -,boundary).
in other physicochemical properties including the phase volume, the diffusion coefficient plot also exhibits a sharp break at the specific salt concentration, the D values decreasing from about 14 X 10-8 to 4 X 10-8 cm2/sin the vicinity of 3.7 5% salinity, after which they increase rapidly to around 24 X 10-8 cm2/s and remain essentially unchanged. In our previous studies,21,22the content of anionic surfactant in each phase of a multiphase microemulsion system was determined accurately with an extractionspectrophotometry method. In the present study, we have qualitatively determined the surfactant fraction in the AlCls middle-phase microemulsion system. It was confirmed that most of the SOS molecules exist in the middlephase microemulsion, and the SOS fraction therein is independent of the salt concentration. Alcohol Content in Middle-Phase Microemulsion. The solubilization of 1-hexanol in the middle-phase microemulsion was studied by the techniques of gas chromatography and DSC. Figure 8illustrates the dependence of the partition ratio of cosurfactant (1-hexanol) in multiphase microemulsion formed with the monovalent salt NaC1. Approximately 23 % hexanol was found in the lower phase microemulsion in the salinity region below l o % , whereas 98% hexanol was found in the upper phase microemulsion in the salinity region above 15%. In the middle-phase microemulsion zone between them, the fraction of 1-hexanol remains at about 50% and is independent of NaCl concentration. On the other hand, the filch middle-phase microemulsion shows quite different behavior; as is shown in Figure 9, the solubilization of 1-hexanol in the middle-phase microemulsion depends on the salinity, showing a maximum at the specific salinity of 3.8%, where all the other physicochemical properties exhibit break-points. The partition ratio of 1-hexanol undergoes an appreciable change from 44% to 60% at a salinity of 3.8%; otherwise, the ratio remains at approximately 47 7% throughout a wide range of salinities. A similar tendency is also evident in the results of DSC measurement. Following Boned et aL23 and Senatra et al.,24who applied DSC to characterize microemulsion, we have used DSC to obtain heating thermograms of the middle-phase microemulsion incorporating 4.0 % l-hexan01 at various AP+ concentrations. As shown in Figure (21) Nakamae, M.; Ogino, K.; Abe, M. Colloid Polym. Sci. 1988,266, 475. (22) Nakamae, M.; Abe, M.; Ogino, K. J. Colloid Interface Sci. 1990, 135, 449. (23) Boned, C.;Peyrelasse, J.;Moha-Ouchane, M.J. Phys. Chem. 1986, 90. 634. (24) Senatra, D.; Guarini, G. G. T.; Gabrielli, G.; Zoppi, M. J. Phys. (Paris) 1984, 45, 1159. ~~
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SALINITY I % Figure 9. Partition ratio of cosurfactant in sodium octyl sulfate/ n-decane/ l-hexanol/AlClSaqueous middle-phase microemulsion systems as a function of salinity at 30 "C (- - -, boundary).
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Figure 10. Heating thermograms for sodium octyl sulfatelndecane/l-hexanoVAlCb aqueous middle-phase microemulsion at various salinity.
10, three peaks appear at salinities above 3.8%, whereas only two peaks appear at salinities below 3.7%. The two peaks at highest temperatures correspond to the fusion temperature of water (0 "C) and n-decane (-30 "C), and the third peak is that of 1-hexanol (-48.5 "C). The peak intensity curve deepens as the salt concentration is progressively increased from 3.8% to 4.0%, indicating the further migration of 1-hexanol molecules into the middlephase microemulsion. The lack of a hexanol peak at ' may imply that the presence of salinities less than 3.7% surfactant, oil, and water in the continuous phase will prevent condensation of hexanol. However, hexanol in the dispersed phase may be concentrated enough to condense as pure hexanol.
Discussion It is remarkable that the composition of the A1Cb middle-phase microemulsion becomes drastically richer in water and correspondingly depleted of oil in the vicinity of a specific salinity of 3.8% and that all other physicochemical properties including the water content, electrical conductivity, and diffusion coefficientof the A1C13middlephase microemulsion show a corresponding change at this salinity. The partition of cosurfactant molecules also increases at this salinity. Although it is expected that higher valent ions should more strongly affect the behavior of microemulsion system, these abrupt changes in physicochemical properties were not observed at all in the results for the divalent salt systems MgClz and C a c l ~ . ~ ~ In rationalizing the observed effects, it is reasonable to attribute the sharp increase in the electrical conductivity vs salinity curve at 3.8% salinity to a structural change, accompanied by an abrupt increase in the concentration (25) Abe, M.; Yamazaki, T.; Nakamae, M.; Ogino, K. To be submitted for publication.
Properties of a Middle-Phase Microemulsion
and mobilities of conducting species, and coinciding with the observed increase in water content and the observed decrease in oil content of the middle-phase microemulsion. Taken together, these results imply that the system undergoes a phase inversion from an oil-rich to a waterrich continuous phase with increasing salinity. The diffusion coefficient data also support the idea of phase inversion and consequent structural change by showing a drastic decrease and then an increase in the vicinity of 3.8%. Furthermore, the data independently suggest that there occurs a drastic increase in the hydrodynamic radius of the droplets just before the salinity of 3.8%,after which the droplet size decreases to a nearly constant value. This inference is based on use of the Stokes-Einstein equation of R = (kT/GrqD)to calculate the hydrodynamic radius of the droplet from the diffusion coefficient data, where k is the Boltzmann constant, q the viscosity of the continuous medium, and R the hydrodynamic radius of droplet. The drastic change in droplet size may owe to the growth of a water core in a water-in-oil type microemulsion by incorporating more salt. This effect may be followed by rupture of the droplet to give a transient layered structure; however, after inversion the system may be considered to be richer in an oil-in-water type microstructure with steady droplet size. In our previous application of the microstructural model of middlephase mi~roemulsi~n~,'~ we described the middle phase microemulsion (with monovalent salt) as a mixed dispersion containing oil, water, surfactant, and cosurfactant. The structure may form throughapartial fracture of oil/water interfaces in the middle-phase microemulsion, caused by stretching motions of the interfaces. Similar microstructures may exist in the system under investigation here. The remarkable discontinuities in the physicochemical properties of the trivalent middle-phase microemulsion may be explained in terms of the action of the trivalent Al3+ ions on the inversion and stabilization of a dispersion medium. The SchulzeHardy rule says that a coagulating ability of an ion will increase with increasing ionic charge. Multivalent cations have a much stronger tendency than monovalent ions to associate with anionic surfactants,
Langmuir, Vol. 8, No. 3, 1992 831
which would cause them to concentrate in the middlephase microemulsion to a much greater extent than in the NaCl system. It is known that A13+ion is present in the hexahedral hydration complex (A13++ 6H20 ;=rA1(H20)s3+)in aqueous solution, and that the hydrated species undergoes hydrolysis (Al(H20)s3++ H2O ==A1(OH)(H20)s2++ H2O+). In addition, polymeric multiply charged cations, as well as monomeric cations, are present in dilute solutions of A13+ ions. It is likely that hydrated A13+ complexes in the interior of the water core will break up owing to geometrical constraint as the salinity is increased and that the hydrated ions should also play an important role through a bridging mechanism in stabilizing the interface between the water and oil layer in the bicontinuous fluctuating structure. The role of 1-hexanol molecules should also be taken into consideration in relation to the unusual structural changes; it is significant that the solubilization of the cosurfactant, hexanol, becomes enhanced near the specific salinity of 3.8% in view of the fact that the migration of hexanol molecules to the interfacial film may cause an increase in the curvature of the droplet.
Conclusion The experimental results presented here suggest that the AlC13 middle-phase microemulsion undergoes a type of phase inversion from oil-continuous droplets to a waterrich fluctuating transient medium with increasing trivalent salt concentration, although the middle-phase microemulsion remains nearly the same in appearance. It must be concluded that the nature of the middle-phase microemulsion is very complicated and that its microstructure involves an intermicellar equilibrium incorporating various types of droplets, bicontinuous fluctuating, and/or a rigid liquid crystal phase states, depending on the type of salt in the system. Acknowledgment. The research was supported by Japan National Oil Corp. and the Saneyoshi Scholarship Foundation. Registry No. AlC13, 7446-70-0; n-decane, 124-18-5; 1-hexmol, 111-27-3;sodium octyl sulfate, 142-31-4.
