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Langmuir 1995,11, 3302-3306
Effect of Oil on the Solubilization in Microemulsion Systems Including Nonionic Surfactant Mixtures Hironobu Kunieda," Akihiro Nakano, and Ma Angeles Pes Department of Physical Chemistry, Division of Materials Science and Chemical Engineering, Faculty of Engineering, Yokohama National University, Tokiwadai 156, Hodogaya-ku, Yokohama 240, Japan Received February 27, 1995. In Final Form: May 15, 1995@ The effect of molecular weight of oil on the three-phase behavior and maximum solubilization was investigatedin microemulsionsystemsofmixtures of di-and octaethyleneglycol dodecyl ethers and mixtures of hexanol and octaethylene glycol dodecyl ether at constant temperature (25 or 35 "C). It was found that the minimum weight fraction of surfactant to make equal weights of water and oil to a single phase ( x b ) increases with increasing molecular weight of oil (hydrocarbon). Nonionic surfactant mainly distributes between micro-oil domains and the interface between microwater and oil domains inside the microemulsion phase. Surfactant molecules at the interface are responsible for the solubilization. Assuming that the monomeric solubilityof each surfactant in oil is the same as that in the micro-oil domains ofthe microemulsion and the microwater domains consist of pure water, the weight fractions of each surfactant at the interface in the microemulsion (C1 or C2) were obtained by a mass-balanced equation. The mixing fraction of surfactant at the interface was also determined by another method based on the geometrical relation of the three-phase tie triangle in the composition tetrahedron. In the hexanol systems, the monomeric solubility of hexanol in water should be taken into account. C1+ C2 indicates the net solubilizing power and it dramatically decreases with increasing the molecular weight of oil.
Introduction It is well-known that the solubilization capability of a nonionic surfactant reaches its maximum a t a particular temperature called the HLB (hydrophile-lipophile balance) temperature a t which a microemulsion phase coexists with excess water and oil This kind of three-phase behavior was observed not only in nonionic surfactant but also in ionic surfactant systems and has been extensively studied for enhanced oil r e ~ o v e r y . ~ - ~ Phase inversion in emulsions occurs and ultralow interfacial tensions are attained a t the HLB t e m p e r a t ~ r e . ~ , ' The HLB temperature is always k e d in a ternary water/ single nonionic surfactantdoil system a t constant pressure according to the phase r ~ l e . ~Therefore, ,~ the HLB temperature to reveal the maximum solubilization is different in each system depending on the types of surfactants andor oils. In order to compare the solubilization capability of surfactant in microemulsion systems, mixtures of nonionic surfactants are used to adjust the HLB temperatures to a fixed temperature. Since microemulsions have a bicontinuous structure a t the HLB t e m p e r a t ~ r e , ~ surfactant ,'~ molecules are distributed among water domains, oil domains, and the water-oil interfaces inside micro emulsion^.^^-^^ It is very important
* To whom correspondence should be addressed.
Abstract published in Advance ACSAbstracts, August 1,1995. (1)Shinoda, K.; Kunieda, H. J . Colloid Interface Sci. 1973,42,381. (2) Kunieda, H.; Shinoda, K. Bull. Chem. SOC.Jpn. 1982,55, 1777. (3) Kunieda, H.; Shinoda, K. J . Colloid Interface Sci. 1985,107,107. (4) Winsor, P. A. Solvent Properties ofAmphiphilic Compounds;Butterworths: London, 1950; p 68. ( 5 ) Kunieda, H.; Hanno, K.; Yamaguchi, S.; Shinoda, K. J. Colloid Interface Sci. 1986, 107, 129. (6) Bourrel, M.; Schechter,R. S. MicroemulswnsandRelated Systems: Formulation, Soluency, and Physical Properties; Marcel Dekker: New York, 1988; pp 335-395. (7) Shinoda, K.; Saito, H. J . Colloid Interface Sci. 1988, 26, 70. (8)Kunieda, H.; Sato, Y. Organized Solutions; Friberg, S. E., Lindman, B., Eds.; Marcel Dekker: New York, 1992; pp 67-88. (9) Olsson, U.; Shinoda, K.; Lindman, B. J . Phys. Chem. 1986, 90, @
to know this distribution in order to estimate the net solubilization capability of surfactant, i.14 As described above, the hydrophile-lipophile property of nonionic surfactants, especially polyethylene glycol-type nonionic surfactants, is dramatically changed with increasing temperature due to the conformation change of the hydrophilic chain.15 Therefore, at least, two surfactants have to be mixed to adjust the HLB temperature to a fxed temperature. In a previous paper,14we investigate the effect ofmixing nonionic surfactants on the maximum solubilization in a microemulsion system. It was found that the net solubilizing power increases dramatically when nonionic surfactants, whose HLBs are far separated, are mixed. At constant temperature, the HLBs of mixed surfactants in surfactant monolayers inside the microemulsion phases are the same in the case where oil is fixed. The attention has been focused on the hydrocarbon or oil effect on the solubilization in a microemulsion system including mixtures of nonionic polyethylene glycol alkyl ester as well as how these mixtures favor the oil solubilization comparing with nonionic-alcohol (cosurfactant) mixtures. In this context, we analyzed the three-phase behavior of mixtures of di- and octaethylene glycol dodecyl ethers and mixtures of hexanol and octaethylene glycol dodecyl ether in different oil systems a t constant temperature (25 or 35 "C).
Experimental Section Materials. Homogeneous polyethylene glycol dodecyl ethers (C12E0,) were kindly supplied from Nihon Surfactant Co. Extrapure grade decane, hexadecane, and 1-hexanol were obtained from Tokyo Kasei Kogyo Co. Twice distilled water was used in all the experiments. Procedures. Procedure To Determine Phase Diagram.
Procedures to determine phase boundaries are described in ref 3.
4083.
(10)Jahn, W.; Strey, R. J . Phys. Chem. 1988, 92, 2294. (11)Kunieda, H.;Ushio,N.;Nakano,A.;Miura,M. J . ColloidInterface Sci. 1993, 159, 37. (12) Kunieda, H.; Yamagata, M. Colloid Polym. Sci. 1993,271,997.
(13) Kunieda, Hl.; Yamagata, M. Langmuir 1993, 9, 3345. (14) Kunieda, H.; Nakano, A.; Akimaru, M. J . Colloid Interface Sci. 1996, 170, 78. (15)Karlstrom, G. J . Phys. Chem. 1986, 89, 4962.
0743-746319512411-3302$09.0010 0 1995 American Chemical Society
Effect of Oil on Solubilization
Langmuir, Vol. 11, No. 9, 1995 3303 35.0"C
35.0"C
0.6 I
3 0.4
I1
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0 0
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X Figure 1. Phase diagram of a water/ClzEO$ClzEOddecane system at 35 "C. Xmeans the weight fraction oftotal surfactant in system. Wl indicates the weight fraction of lipophilic surfactant in total surfactant. I, 11, and I11 are single-phase, two-phase, and three-phase regions. Point b shows the composition of the maximum solubilization. x b and Wlb are the weight fractionoftotal surfactantin systemat the maximum solubilization and the weight fraction of lipophilic surfactant (C12E02)in total surfactant. The monomeric solubility of each surfactant in excess oil phase was measured at pointsp and q . Gas Chromatography. Monomericsolubilities of surfactants in excess oil were determined by means of gas chromatography (Yokogawa-HewlettPackard Co., HP-5890,FID as detector)and column chromatography (J & W Co., DB-1). Helium was used as carrier gas (2.5 psi flow speed). Temperature was incrased from 60 to 320 "C (10 "C/min) and held for 25 min at the final temperature.
Results Solubilization in Mixed Nonionic Surfactant Systems. Phase behavior of water/C12EO$CIzEOz/decane, water/C12EO$C12EOz/hexadecane, water/ClzEO$l-hexanolheptane, waterlC12EO$l-hexanol/decane,and water/ C12EOdl-hexanolhexadecane systems were studied a t constant temperture (35 and 25 "C)and at constant water/ oil weight ratio (50/50 (w/w)). The phase diagrams are shown in Figures 1-5. The weight fraction of total surfactant in the system (X) is plotted horizontally and the weight fraction oflipophilic surfactant or cosurfactant (hexanol) in the surfactant mixture (W1) is plotted vertically. Mixed surfactant forms aqueous micelles and aqueous micellar solution phase solubilizing oil coexist with excess oil phase a t low WI. With increasing W1, surfactant aggregates separate from water, and a three-phase body consisting of excess water, surfactant (microemulsion), and excess oil phases appears as shown in Figures 1-5. With further increase in W1, surfactant mainly dissolves in oil and reverse micellar solution phase solubilizing water coexists with excess water phase. The three-phase region is shifted to higher W1 with decreasing surfactant concentration (at lower X).3 As described later, the monomeric solubility of lipophilic surfactant or cosurfactant is much higher than that of a hydrophilic one. The mixing ratio of nonionic surfactants a t the interface of micro-water and oil domains inside the microemulsion phase is directly related to the HLB
11
0.2
0
0.1
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X Figure 3. Phase diagram of a water/C12EO$l-hexanol/heptane system at 25 "C. The notation is the same as that in Figure 1.
temperature or the three-phase temperature. Therefore, in order to keep the mixing ratio, it is needed to add more lipophilic surfactant in a dilute region. This is the reason why three-phase body is shifted to the higher Apparent Solubilizing Power. Point b in Figures 1-5 indicates the intersection between three-phase and single-phase regions, and it reveals the minimum surfactant concentration required to make equal weights of water and oil to a single phase a t the f i e d temperature. This concentration is indicated by x b in Figure 1. The smaller the value, the larger the solubilizing power is. T h e x b is shown in Table 1 together with the mixing fraction, Wlb, a t this point. As the molecular weight of oil increases, the Xb increases. Compared with the C12EO$C12E02 system, a higher& is needed to make a single microemulsion phase in the W1.338J2-14
Kunieda et al.
3304 Langmuir, Vol. 11, No. 9, 1995 25.0"C
1.o
ty
0.8
I1
0.6 1
3 0.4 I1
0.2
0
0.1
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X Figure 4. Phase diagram of a water/C12EO$l-hexanoUdecane system at 25 "C. The notation is the same as that in Figure 1. 25.0 "C
'
,
O
7
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0.6 1
3
I
0.41
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distributed in microwater and oil domains and monolayers in the microemulsion phase. In the three-phase region, the microemulsion phase coexists with excess water and oil phases. It is known that the micro-oil domains inside the microemulsion phase is almost the same as bulk oil phase when the solubilization is large.16 Therefore, we can assume that the composition of the excess oil phase is the same as that of the micro-oil domains.14 On the other hand, the monomeric solubility of surfactant in water is usually very small and negligible. In order to estimate the monomeric solubility of surfactant in the micro-oil domains, we measured the concentration of each surfactant in the excess oil phase a t points p and q in the midst of three-phase region in Figures 1-5 by means of gas chromatography. Since the weight fractions of excess oil and water phases are almost identical in the midst of the three-phase region, the microemulsion phase dissolves approximately equal weights of water and oil. In other words, the midpoints of the three-phase body are included in a three-phase tie triangle containing the microemulsion phase a t point b. The average values of SI at points p and q are also listed in Table 1. In both CIZEO$C~ZEO~ systems, the monomeric solubility of C12E02 (SI)is not different very much and that of C12EOs ( S 2 ) is small. In the hexanol systems, the monomeric solubility ofhexanol (SI)increases with increasing molecular weight of oil and that of C1zEO8 slightly decreases. 5'1 is always much larger than SZ in all the present systems.
Discussion Net SolubilizingPower of Mixed Surfactant. As described before, surfactant molecules are distributed in micro-oil and water domains and their interface inside a single microemulsion phase because a so-called bicontinuous structure is present. Although there is an argument as for the detail structure of microemulsion,17 the following discussion is not affected by the difference in the detail structure. Since surfactant molecules at water-oil interfaces inside a microemulsion phase are directly related to the solubilization and the HLB temperature,l* it is important to estimate the mixing fraction of each surfactant. If it is assumed that excess water phase is pure water and the composition of excess oil phase is the same as that in the micro-oil domain of microemulsion, we obtain by using the simple mass balance equation14
0.3
X Figure 5. Phase diagram of a water/C12EO$l-hexanoYhexadecane system at 25 "C. The notation is the same as that in Figure 1. C12EO$l-hexanol system. Since the hydrocarbon chain of 1-hexanol is much shorter than that in a nonionic surfactant, the average hydrocarbon chain of the mixed surfactant in the later system is short and the solubilizing capability is rather low. On the other hand, W1b in a hexanol system is smaller than that of a C12EOdC12E02 system. It means that hexanol is more lipophilic than ClzEOz and a low W1b is needed to balance the hydrophilelipophile property of the mixed surfactant in a given water-oil system. Monomeric Solubility of Surfactant in Excess Oil Phase. It is considered that the surfactant layers at the interfaces of micro-oil and water domains inside the microemulsion phase are responsible for the solubilization of water and oil. In order to compare the net solubilizing power of surfactant, one has to know the concentration of surfactant in the monolayers. Surfactant molecules are
and
where C1 and Cz are the weight fractions of lipophilic and hydrophilic surfactants a t the water-oil interface inside a single microemulsion phase at point b in Figures 1-5, and SI and S 2 are monomeric solubilities of lipophilic and hydrophilic surfactants in oil, respectively. Rowmeans the weight fraction of oil in water oil. The calculated C1 and C2 are given in weight fractions in system and are shown in Table 1.
+
(16)Kawai, K.;Hamada, K.; Shindo, N.; Konno, K. Bull.Chem.SOC. Jpn. 1992,65,2715. (17)Zemb, T.N.;Barnes, J. S.; Derian, P. J.;Ninham, B. W. Prog. Colloid Polym.Sci. 1990,81,20.
Effect of Oil on Solubilization
Langmuir, Vol. 11, No. 9, 1995 3305
Table 1. Values of xb,Wlb, SI,SZ, CI, CZ,c1 f CZ,cl/cl f CZ,and SI' Wlb s 1 Sa C1" CZ" C 1 f CZ" 0.662 0.0566
