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Comparison between Phase Behavior of Anionic Dimeric (Gemini-Type) and Monomeric Surfactants in Water and Water-Oil Hironobu Kunieda,*,† Nagahiro Masuda,† and Kazuyuki Tsubone‡ Division of Artificial Environment and Systems, Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan, and Cosmetics Laboratory, Kanebo Ltd., Kotobuki-cho 5-3-28, Odawara 250-0002, Japan Received January 27, 2000. In Final Form: June 5, 2000 The phase behavior of dimeric (gemini-type) anionic surfactant (sodium 1,2-bis(N-dodecanoyl β-alanate)N-ethane, GS) in water is similar to that of the corresponding monomeric surfactant (sodium N-dodecanoylN-methyl β-alanate, MS). Aqueous micellar phase (Wm), hexagonal (H1), bicontinuous cubic (V1), and lamellar (LR) phases are successively formed with increasing the surfactant concentration. SAXS data for cross-sectional area per surfactant molecule in H1 and LR phases reveal that GS molecules are more tightly packed in the aggregates than MS molecules, and this is due to the bridging of two hydrocarbon chains by the short spacer chain (-C2H4-). In the ternary water/surfactant/dodecane or m-xylene systems, the phase behavior of both surfactants is also practically similar. Upon addition of oil, the H1 phase is changed to the discontinuous micellar cubic (I1) phase in both surfactant systems. The phase transition mechanism is discussed according to the geometrical packing model. In the 3 wt % NaCl aqueous/MS or GS/cosurfactant(hexanol or butanol)/dodecane systems, a single-phase microemulsion is formed at an equal brine/dodecane ratio over a wide range of surfactant concentration in the MS systems whereas the LR phase is mainly formed and the microemulsion is produced only in a very narrow region in the GS/hexanol system. By replacement of hexanol with butanol, wide microemulsion regions appear, even in the GS system. Hence, GS molecules in the surfactant layer are considered to be packed more tightly than MS, even in the presence of oil.
Introduction Recently, dimeric (gemini-type) or oligomeric surfactants, which have plural hydrophilic and lipophilic moieties connected by a spacer chain, have attracted attention in both basic and industrial aspects. Solution properties of dilute aqueous gemini surfactant solutions have been extensively studied.1-6 In comparison with ordinary monomeric surfactants, the cmc of the gemini surfactant is considerably low if the spacer chains are polymethylene or polyoxyethylene.7 The minimum surface tension of an aqueous gemini surfactant solution above its cmc is also low, especially when the spacer chain is short.8,9 However, the phase behavior of a gemini surfactant over a wide range of composition has not been completely figured out yet, although it is known that the phase sequence of aqueous micellar solution, hexagonal, and lamellar liquid crystals is similar to that in ordinary monomeric ionic surfactant-water systems.10 Moreover, * To whom correspondence should be addressed. † Yokohama National University. ‡ Kanebo Ltd. (1) Okahara, M.; Masuyama, A.; Sumida, Y.; Zhu, Y.-P. J. Jpn. Oil Chem. Soc. 1988, 37, 746. (2) Damino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448. (3) Sommerdijk, N. A. J. M.; Hoeks, T. H. L.; Synak, M.; Feiters, M. C.; Nolte, R. J. M.; Zwanenburg, B. J. Am. Chem. Soc. 1997, 119, 4338. (4) Rosen, M. J.; Mathias, J. H.; Davenport, L. Langmuir 1999, 15, 7340. (5) Pinazo, A.; Wen, X.; Perez, L.; Infante, M.-R.; Franses, E. I. Langmuir 1999, 15, 3134. (6) Oda, R.; Huc, I.; Homo, J.-C.; Heinrich, B.; Schmutz, M.; Candau, S. Langmuir 1999, 15, 2384. (7) Zana, R. In Novel Surfactant; Holmberg, K. Ed.; Marcel Dekker: New York, 1998; Surfactant Science Series, Vol. 74, Chapter 8. (8) Yun-Peng, Z.; Masuyama, A.; Nagata, T.; Okahara, M. J. Jpn. Oil Chem. Soc. 1991, 40, 473. (9) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9, 1465.
comparative studies of the phase behavior of gemini-type surfactants with that of the corresponding monomeric surfactants are scarce. Surfactants form a variety of self-organized structures such as micelles, vesicles, various liquid crystals, and microemulsions in water as well as in water-oil. The shapes of the surfactant aggregates are highly dependent on the surfactant layer curvature determined by the balance between the repulsion of hydrophilic moieties and the attraction due to the interfacial tension between water and the hydrocarbon part of the surfactant at the interface.11-13 In general, when the surfactant layer curvature is convex toward water, the curvature is defined to be positive. Since the surfactant layer curvature is influenced by oil solubilization in surfactant aggregates, a phase transition often takes place. For example, when short alkanes or aromatic hydrocarbons are solubilized in hexagonal liquid crystal (H1), the surfactant layer curvature becomes less positive because these oils tend to penetrate the surfactant palisade layer. As a result, the H1-lamellar liquid crystal transition takes place. On the other hand, long alkanes have an opposite effect because they tend to be solubilized in the deep core of the aggregates and form an oil pool.12,14 In the case of dimeric surfactants, there are very few studies on the effect of added oil on the self-organizing structures.15,16 (10) Alami, E.; Levy, H.; Zana, R.; Skoulios, A. Langmuir 1993, 9, 940. (11) Israelachvili, J. W.; Mitchell, D. J.; Ninham, B. W.; J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (12) Kunieda, H.; Ozawa, K.; Huang, K.-L. J. Phys. Chem. 1998, 102, 831. (13) Kunieda, H.; Umizu, G.; Yamaguchi, Y.; Suzuki, M. J. Jpn. Oil Chem. Soc. 1998, 47, 879. (14) Kunieda, H.; Umizu, G.; Aramaki, K. J. Phys. Chem. B, 2000, 104, 2005. (15) Dreja, M.; Tieke, B. Langmuir 1998, 14, 800.
10.1021/la0001068 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/14/2000
Phase Behavior of Dimeric and Monomeric Surfactants
Figure 1. Chemical structures of sodium N-dodecanoyl-Nmethyl β-alanate (MS) and sodium 1,2-bis(N-dodecanoyl β-alanate)-N-ethane (GS).
In this context, we investigated the phase behavior of a water-gemini surfactant (sodium 1,2-bis(N-dodecanoyl β-alanate)-N-ethane) system over a wide range of composition and the effect of adding oil to the hexagonal liquid crystal to figure out the change in surfactant layer curvature. Finally, the formation of a microemulsion is reported. The results will be compared with that of the corresponding monomeric surfactant (sodium N-dodecanoyl-N-methyl β-alanate). Experimental Section Materials. Sodium 1,2-bis(N-dodecanoyl β-alanate)-N-ethane (gemini-type surfactant) was synthesized as follows: an aqueous solution (20 mL) of NaOH (3.2 g, 0.08 mol) was added dropwise to a stirred suspended solution of (CH2)2(NHC2H4CO2H) 2‚2HCl (Tokyo Chemical Industry Co. Ltd., Japan, 6.1 g, 0.02 mol) in 200 mL of acetone/H2O (4:6, v/v) at room temperature. While the pH of the solution was held between 8 and 10, a solution of dodecanoyl chloride (10.9 g, 0.05 mol) in acetone (20 mL) was added dropwise and stirring was continued at room temperature for 3 h. Concentrated HCl was added to the reaction mixture to bring the pH to 1. The precipitated crude product was filtered off, washed with water, and dried. The crude product was suspended in MeOH (200 mL). A few drops of concentrated H2SO4 were added to the MeOH solution and the mixture was stirred under reflux conditions for 5 h. The product was passed through benzene to remove methyl dodecanoate derived from dodecanoyl chloride and then the solvent was evaporated off and the product was purified by chromatography on a silica gel column using chloroform/acetone (97:3, v/v) as a mobile phase. The gemini surfactant was prepared by hydrolysis of the methyl ester with an equivalent weight of NaOH in MeOH at 80 °C for 3 h, followed by evaporation under reduced pressure. The total yield was 85%. The gemini thus obtained did not contain sodium dodecanoate, which was confirmed by the 13C-NMR spectrum. The purity of the compound was checked by elemental analysis. Theoretical values: C, 62.72%; H, 9.54%; N, 4.57%. Experimental values: C, 62.66%; H, 9.45%; N, 4.61. The corresponding monomeric surfactant (extra-pure-grade sodium N-dodecanoyl-N-methyl β-alanate) was obtained from Nihon Surfactant Industry Co., Ltd. Gemini and monomeric surfactants are abbreviated as GS and MS, respectively, in the present paper. The chemical structures of these surfactants are shown in Figure 1. GS is exactly the dimer of MS. MS was used after repeating recrystallization five times with a mixed solvent consisting of tetrahydrofuran and water. Doubledistilled water was used as the solvent. All aqueous solutions were controlled at pH 11 by NaOH to avoid the formation of acid soaps, although the aqueous solution is designated by “water” in the text and figures. Guaranteed-grade sodium hydroxide and sodium chloride were obtained from Junsei Chemical Co. Guaranteed-grade n-dodecane and n-hexanol were obtained from Tokyo Kasei Kogyo Co. (16) Dreja, M.; Pyckhout-Hintzen, W.; Mays, H.; Tieke, B. Langmuir 1999, 15, 391.
Langmuir, Vol. 16, No. 16, 2000 6439 N-Butanol and m-xylene were obtained from Wako Pure Chemical Industries Ltd. These chemicals were used without further purification. Procedures To Determine the Phase Diagram. Samples of required compositions were sealed in ampules. They were well shaken and were kept in a thermostat ((0.1 °C). Phase separation was detected by visual observation. Liquid crystals were detected by crossed polarizers, and their type was identified by smallangle X-ray scattering. The phase boundaries between the regions containing surfactant solid were determined by a differential scanning calorimeter (Rigaku, DSC 8240). Density Measurement. Surfactant densities were measured at 25 °C by a digital density meter, Anton Paar model DMA40. Since both surfactants are in a solid state, the densities of their NaOH aqueous (pH 11) solutions were measured at various surfactant concentrations. The reciprocals of these values were plotted against surfactant concentration and the densities of the pure surfactants in a liquid state were estimated from the intercepts of the density lines at 100% surfactant. As a result, the density of monomeric surfactant was 1.123 (g/cm3) and the that of gemini-type surfactant was 1.137 (g/cm3). Hence, the calculated molar volume of monomeric surfactant is 273 (cm3/ mol), and that of gemini-type surfactant is 539 (cm3/mol). The molecular volume of the gemini-type surfactant is almost 2 times larger than that of the monomeric surfactant. Small-Angle X-ray Scattering (SAXS). The interlayer spacing of liquid crystal was measured using SAXS, performed on a small-angle scattering goniometer with an 18 kW Rigaku Denki rotating anode goniometer (Rint-2500) at about 25 °C. The samples of liquid crystals were lapped by plastic films for the measurement (Mylar seal method). The types of liquid crystals were identified by the SAXS peak ratios.17
Results and Discussion Phase Behavior in a Binary Surfactant-Water System. Figures 2a and 2b show the phase diagrams of binary water-GS (gemini-type surfactant) and waterMS (monomeric surfactant) systems as a function of temperature. The Krafft temperature for GS is lower than that of MS, and above the Krafft temperature, aqueous micellar solutions (Wm) are formed in both systems. A single Wm phase becomes turbid and a two-phase region appears at a very low surfactant concentration in both systems in a few days or in a week after preparation of the sample, although the pH of aqueous solutions was adjusted to 11. Perhaps, atmospheric carbon dioxide is dissolved in the solutions and some part of the surfactant is changed to acidic soap. In fact, this two-phase region does not appear for a long time if the pH of the aqueous solution is above 13. Hence, the samples for SAXS measurement or that corresponding to high surfactant concentrations were completely flame-sealed in glass vials during the mixing process. The Wm phase forms up to around 40 wt % for both systems. With increasing surfactant concentration, hexagonal liquid crystal (H1), optically isotropic cubic phase (V1), and lamellar liquid crystal (LR) are successively formed in both systems. Although there is a solid-present region at a high surfactant concentration, we did not investigate it in detail. This phase sequence is very typical for a binary ionic surfactant-water system such as SDS or soap systems.18 Although both phase diagrams are quite similar, the lowest temperature of the V1 phase is considerably different. Since the V1 phase is present between H1 and LR phases, it is considered to be a bicontinuous cubic phase in which the average surfactant layer curvature is slightly positive.19 As described in the (17) Kunieda, H.; Shigeta, K.; Ozawa, K.; Suzuki, M. J. Phys. Chem. 1997, 101, 7952. (18) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: San Diego, 1994; Chapter 5.
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Figure 3. The interlayer spacing, d (square), the effective crosssectional area per surfactant, aS (circle), and the radius of a cylindrical micelle, rH (triangle), in the H1 phase at 25 °C as a function of surfactant weight fraction in the system. Open marks are for the MS system whereas filled marks are for the GS systems. The broken curve indicates the twice the value of aS for the MS system.
Figure 2. Binary phase diagrams of water/GS (a) and water/ MS (b) systems at 25 °C. Wm means an aqueous micellar solution phase and V1 is a bicontinuous micellar-type cubic phase. H1 is a hexagonal liquid crystal and S is solid. II is a two-phase region containing two types of liquid crystals.
following section, the hydrocarbon chain of the GS molecule is more rigid than that of the MS molecule because two hydrocarbon chains are bridged by the short spacer chain (-CH2CH2-). It is considered that the V1 phase is formed only at high temperatures in the GS system due to the difference in the rigidity of the hydrocarbon chains. However, we need further investigation. SAXS Measurement in a Binary Water-Surfactant System. Interlayer spacings of H1 and LR phases, d, were measured by means of small-angle X-ray scattering (SAXS) at 25 °C, and the results are shown in Figure 3. It is assumed that infinitely long cylindrical micelles are packed in a hexagonal array in the H1 phase. The schematic structures of H1 phase, lamellar (LR), and discontinuous cubic (I1) phases are shown in Figure 4. The radius of a cylindrical micelle, rH, and the effective cross-sectional area per surfactant molcule at the interface of aggregates aS, for the H1 phase are calculated by a simple geometric relation,12
rH )
(
)
2 φS x3π
aS )
1/2
2MS rHNAFS
d
(1)
(2)
where φS is the volume fraction of surfactant in the system, (19) Joensson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons: Chichester, 1998; Chapter 3.
Figure 4. Schematic structures of hexagonal (H1) (a), lamellar (LR) (b), and discontinuous micellar cubic (I1) (c) phases.
MS is the molecular weight of the surfactant, FS is the density of the surfactant, and NA is Avogadro’s constant. The aS and rH are also shown in Figure 3. Although the aS and rH are the values for the whole surfactant molecule, they are similar to the effective cross-sectional area and the radius of the lipophilic part of the surfactant because the ionic headgroup is very small.20 In fact, the aS value per ionic headgroup is around 0.5 nm2 and this is similar to that in the H1 phase of nonionic surfactants.17 Since the net sectional area of a linear hydrocarbon chain is 0.2-0.22 nm2, the maximum length of both surfactants is approximately 2.1-2.3 nm in their fully extended forms, which is longer than rH in the H1 phase. The d and aS gradually decreases, whereas the radius of the cylinder gradually increases with surfactant concentration. Since the counterion concentration is high at a high surfactant concentration, the dissociation of the carboxylic group of surfactant is reduced.21 This may cause the decrease in repulsion between ionic headgroups and (20) Stokes, R. J.; Evans, D. F. Fundamentals of Interfacial Engineering; Wiley-VCH: New York, 1997; Chapter 5.
Phase Behavior of Dimeric and Monomeric Surfactants
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Table 1. Interlayer Spacing, d, Effective Cross-sectional Area per Surfactant, aS, and Half-Thickness of the Bilayer, dLr, in the Lr Phase at 75 wt % of the Surfactant surfactant MS temp./°C d/nm dLR/nm aS/nm2
50 3.60 1.31 0.346
GS 60 3.55 1.29 0.351
50 4.17 1.51 0.594
60 4.08 1.48 0.607
thus aS decreases. The aS for GS is considerably smaller than the twice the value of aS for MS (refer to the broken curve in Figure 3). Hence, GS molecules are tightly packed at the interface of the cylindrical micelle. The small aS means that the hydrophobic chain of GS is close to that in its fully extended form compared with that of the MS molecule as is revealed by rH in Figure 3, in which the GS molecule is ∼5% longer than the MS molecule. This is perhaps due to the fact that the spacer chain is very short; it may restrict the conformation of the hydrocarbon chain of the GS surfactant, which is longer than MS. However, the difference in aS or rH is not large enough to form the different type of liquid crystal in the same composition range. For the LR phase, it is assumed that infinitely wide bilayers are stacked in a parallel way as is shown in Figure 4. Then, the half-thickness of the bilayer, dLR, and aS are calculated by12,17
1 dLR ) dφS 2 aS )
2MS FSNAdφS
(3) (4)
The results are shown in Table 1. The difference in aS for the LR phase between both surfactant systems is similar to that in the H1 phase. The aS tends to be large with increasing temperature as is shown in Table 1. Although the two hydrocarbon chains of GS are bridged by the spacer chain and their conformation is restricted, the aS for the GS molecule is still smaller than double the value of aS for the MS molecule. dLR is also considerably different between the two systems. Ternary Water/Surfactant/Dodecane Systems. Figures 5a and 5b show the ternary phase diagrams of water/ GS/dodecane and water/MS/dodecane systems at 25 °C. The solubilization of dodecane in the aqueous micellar solutions (Wm) is very low for both surfactant systems, as is shown in Figure 5. It is known that the solubilization is dramatically increased when the spacer chain is long.15 However, the structure of the present gemini-type surfactant is exactly the dimer of the corresponding monomeric surfactant, and its spacer chain is very short. This may be the reason the phase behavior is similar in both systems. Beyond the solubilization limit, the excess oil is separated and the two-phase region appears. When oil is solubilized in the H1 phase, the H1-I1 phase transition takes place in both surfactant systems. The I1 phase is identified as a face-centered cubic structure by the ratio of three SAXS peak ratios, 1:x3/4:x3/8, as is shown in Figure 6. Since the I1 phase is formed between Wm and H1 phases, as is shown in Figure 5, it is considered to be a discontinuous micellar cubic phase in which spherical or (21) Bleasdale, T. A.; Tiddy, G. J. T.; Wyn-Jones, E. J. Phys. Chem. 1991, 95, 5385.
Figure 5. Ternary phase diagrams of water/GS/dodecane (a) and water/MS/dodecane (b) systems at 25 °C. pH of water was adjusted to 11 by adding NaOH. I1 is a discontinuous micellar cubic phase. O is an excess oil phase. Only single-cubic-phase regions for m-xylene systems (filled circles) are also indicated in Figure 5.
Figure 6. SAXS pattern for the cubic phase in the water/GS/ dodecane system.
almost spherical micelles are packed in a cubic array. The surfactant layer curvature is changed to be more positive upon addition of oil. The I1 phase is narrow and an excess oil phase is separated at the solubilization limit. With increasing surfactant concentration, the solubilization of oil in the I1 phase increases and the single-phase region is skewed toward the oil-rich region in both systems. It is known that long alkanes such as decane, hexadecane, etc., are mainly solubilized deep inside the hydrocarbon chains of surfactant and form an oil pool. In
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Table 2. Interlayer Spacing, d, Effective Cross-sectional Area per Surfactant, aS, and Radius of Spherical Micelle, rΙ, in the I1 Phase water/GS/dodecane system GS in aq. soln. (wt %)
dodecane in system
d/nm
aSa/nm2
rΙ/nm
Agg. #
30.0 34.9 39.9 45.1
2.0 2.4 2.4 2.9
5.96 5.81 5.45 5.35
1.11 1.08 1.09 1.07 (1.03)
2.68 2.75 2.69 2.76
82 88 83 89
MS in aq. soln. (wt %)
dodecane in system
d/nm
aSa/nm2
rΙ/nm
Agg. #
39.7 45.1 49.9 54.9 56.9
2.4 3.5 4.9 9.2 11.5
5.29 5.25 5.38 6.22 6.64
0.57 0.56 (0.55) 0.54 (0.52) 0.50 (0.52) 0.48 (0.51)
2.62 2.73 2.90 3.51 3.81
151 167 195 312 381
water/MS/dodecane system
water/GS/m-xylene system GS in aq. soln. (wt %)
m-xylene in system
d/nm
aSa/nm2
rΙ/nm
Agg. #
35.0 35.0 35.0 40.0 40.1 40.1 45.1 45.1
4.9 7.4 10.0 7.7 10.0 12.4 9.9 12.2
5.85 6.63 6.95 6.44 6.84 7.36 6.17 6.95
1.21 1.17 1.24 1.13 1.15 1.16 1.19 (1.03) 1.13
2.65 2.98 3.09 3.03 3.18 3.39 3.00 3.34
73 95 97 102 111 124 95 124
MS in aq. soln. (wt %)
m-xylene in system
d/nm
aSa/nm2
rΙ/nm
Agg. #
34.9 35.2 35.2 39.9 44.8
5.0 7.6 10.0 10.0 13.1
5.81 6.40 6.90 6.13 6.40
0.62 0.62 0.63 0.65 0.64 (0.55)
2.64 2.89 3.08 2.86 3.06
142 170 191 158 184
water/MS/m-xylene system
a a in parentheses: the effective cross-sectional area in the H S 1 phase at the same surfactant concentration in water.
this case, the aS is almost unchanged with increasing oil solubilization, and the repulsion between the hydrophilic chains is increased due to a decrease in the surfactant layer curvature. As a result, the surfactant layer curvature is changed to be more positive and the H1-I1 phase transition takes place by solubilizing these oils. The radius of micelles, rI, and the effective crosssectional area per surfactant molecule in the micelle in the single I1 phase, aS, are calculated by the following equations,12, 17
rI )
(
)
9x3 (φ + φO) 16π S
aS )
(
1/3
)
3vS φS + φO rI φS
d
(5) (6)
where φO is the volume fraction of oil in the system. It is assumed in eqs 5 and 6 that spherical micelles are packed in a face-centered cubic array in the I1 phase, as is shown in Figure 4. The results are shown in Table 2. We could not perform the SAXS measurement at very high surfactant concentrations in the GS system because of the narrowness of the single I1 phase. In the column for aS in Table 2, aS for the H1 phase at the corresponding water/ surfactant ratio in the absence of oil is cited in parentheses.
In both systems, aS gradually decreases with an increase in surfactant concentration. aS in the I1 phase is similar to that in the H1 phase in the dodecane systems when the surfactant/water ratio is the same. Hence, dodecane is not solubilized at the interface of aggregates but makes an oil core deep inside the micelle. If there is no oil, the I1 phase cannot be formed because the packing parameter, vS/aSrI, is too large to form a spherical micelle (vS is the volume of a surfactant molecule).22 However, in the presence of oil, a long rI is possible because oil makes an oil core inside the micelle.23 If aS is small, a large amount of oil is necessary to form a spherical micelle because the maximum length of surfactant is limited. This is the reason the I1 phase is formed in an oil-rich region at a high surfactant concentration, as is shown in Figure 5. Since aS for GS is slightly larger than that for MS at the same surfactant/water ratio, the solubilization of dodecane in the I1 phase of the GS system is slightly larger than that in the MS system. The aggregation number of micelles (Agg. # in Table 2) is calculated by the following equation:
Agg. # ) 4πrI2/aS
(7)
The half of the MS aggregation number corresponds to the GS aggregation number because GS is the dimer of MS. When the solubilization of oil increases, the aggregation number also increases, as is shown in Table 2. Since the number of micelles decreases, the interlayer spacing is observed large at a high solubilization. Formation of I1 Phase in Water/Surfactant/mXylene Systems. Only the single I1 phase region for the m-xylene system is indicated in Figure 5 (filled circles). The solubilization of m-xylene in the I1 phase is much larger than that of dodecane for all surfactant/water ratios. Since m-xylene tends to penetrate the surfactant palisade layer, it changes the surfactant layer curvature to less positive in polyoxyethylene-type nonionic surfactant systems.12 However, in the present ionic surfactant systems, the H1-I1 phase transition takes place in the case of the dodecane systems. Perhaps, the repulsion between the headgroups is too large to induce a negative or zero curvature of the surfactant layer. Since aS is considerably increased upon addition of m-xylene, oil is considered to penetrate the surfactant palisade layer, as is shown in Table 2. It means that the total surface area of micelles is large and the micelles have more capacity to solubilize m-xylene compared with dodecane because the aggregation number in the m-xylene systems is smaller than that in the dodecane systems at the same solubilization amount. Formation of Microemulsions. As described in the former sections, the solubilizing capabilities of MS and GS aqueous micellar solutions (Wm) are both very low. To increase the solubilization and form so-called microemulsions, one has to add cosurfactant such as middle-chain alcohol to adjust the hydrophile-lipophile balance of surfactant in a given water-oil system. Figures 7a and 7b show the phase diagrams of 3 wt % NaCl aqueous/MS or GS/hexanol/dodecane systems at equal weights of water and dodecane at 25 °C. Inorganic salt is usually added to form microemulsions in ionic surfactant systems because the distribution of cosurfactant in the surfactant layer (22) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, 1992; Chapter 17. (23) Hoffmann, H.; Ulbricht, W. J. Colloid Interface Sci. 1989, 129, 388.
Phase Behavior of Dimeric and Monomeric Surfactants
Figure 7. Phase diagram of 3 wt % NaCl aqueous/GS (a) or MS (b) /hexanol/dodecane systems at equal weights of brine and dodecane at 25 °C. pH of water was adjusted to 11 by adding NaOH. Om is a reverse micellar solution phase or W/O microemulsion. D is a microemulsion phase or surfactant phase. I, II, and III mean single-, two-, and three-phase regions, respectively.
increases upon the addition of salt and the salt suppresses the formation of liquid crystal.24,25 Both phase diagrams are quite different concerning the microemulsion areas, as is shown in Figure 7. In the MS system, there is a wide isotropic microemulsion area (I(D)) and a three-phase region containing water (W), microemulsion (D), and oil (O) phases appears at a low surfactant concentration, whereas the liquid crystal (LC) present region is dominant and a very narrow single microemulsion phase is formed in the GS system. Since lamellar liquid crystal is formed in most of the LC present region of the GS system, the surfactant layer is more rigid than that in the MS system.26,27 Judging from the SAXS analysis in binary water-surfactant systems, the aS for the GS molecule is considerably smaller than that of MS, and GS is more tightly packed at the interface of aggregates. (24) Kunieda, H.; Nakamura, K. J. Phys. Chem. 1991, 95, 8861. (25) Kunieda, H.; Aoki, R. Langmuir 1996, 12, 5796. (26) Leaver, M. S.; Olsson, U.; Wennerstroem, H.; Strey, R. J. Phys. II France 1994, 4, 515. (27) Aramaki, K.; Hayashi, T.; Katsuragi, T.; Ishitobi, M.; Kunieda, H. J. Colloid Interface Sci., submitted.
Langmuir, Vol. 16, No. 16, 2000 6443
Figure 8. Phase diagram of 3 wt % NaCl aqueous/GS (a) or MS (b) /butanol/dodecane systems at equal weights of water and dodecane at 25 °C. pH of water was adjusted to 11 by adding NaOH.
Hence, the hydrocarbon chains of GS are more elongated and are less flexible due to the presence of the spacer chain. To form isotropic fluid microemulsions, the surfactant layer should be more flexible. Since hexanol is not enough for this purpose, we constructed the same phase diagrams as Figure 7 by replacing the cosurfactant, hexanol, with butanol. The results are shown in Figures 8a and 8b. In both systems, there are wide single-phase microemulsion regions. Although the three-phase regions appear in both systems, their position is located in the butanol-rich region in the GS system. It means that more butanol is needed to have a balanced state.25 Judging from the phase behavior of binary water-surfactant systems shown in Figure 2, the HLB of both surfactants are more or less similar. Hence, it is considered that the cosurfactant, butanol, hardly penetrates the GS surfactant layer in the microemulsion compared with the MS layer. Conclusions Phase diagrams for water-gemini surfactant (sodium 1,2-bis(N-dodecanoyl β-alanate)-N-ethane, GS) and the corresponding monomeric surfactant (sodium N-dodecanoyl-N-methyl β-alanate, MS) systems were constructed. The phase behavior is almost the same and the aqueous
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micellar solution phase (Wm), hexagonal phase (H1), bicontinuous cubic phase (V1), and lamellar phase (LR) are successively formed with increasing surfactant concentration. The main difference is that the V1 phase in the GS system forms at a higher temperature than that in the MS system. The effective cross-sectional area per ionic headgroup for both H1 and LR phases in the GS system is considerably smaller than that in the MS system. Hence, the GS molecules are more tightly packed in the surfactant layer because the two hydrocarbon chains are bridged by a short spacer chain (-CH2CH2-). The phase behavior of ternary water/surfactant/dodecane systems are also similar in both GS and MS systems. Upon addition of dodecane to the H1 phase, a discontinuous micellar cubic phase (I1) appears. aS is not largely changed in both liquid crystals, and dodecane is mainly solubilized deep inside the aggregates and makes an oil core. Since aS is small, the solubilized oil is necessary to form a spherical micelle. Since aS is decreased with increasing the surfactant concentration, solubilization in the I1 phase
Kunieda et al.
largely increases at a high surfactant concentration. On the other hand, m-xylene penetrates the surfactant palisade layer and the aS increases. However, the same phase transition from H1 to I1 phases takes place, even in the m-xylene systems. Since aS is large and the aggregation number of the surfactant is not very different from the dodecane systems, the solubilization of m-xylene in the I1 phase is much larger than the that of the dodecane systems. In the 3 wt % NaCl aqueous/MS/hexanol/dodecane system, a wide microemulsion region is formed at equal weights of brine and oil, whereas the liquid crystal region is dominant in 3 wt % NaCl aqueous/GS/hexanol/dodecane system. When hexanol is replaced with a short-chain alcohol such as butanol, a wide microemulsion region appears in both systems. The difference in rigidity of the surfactant layers may cause the difference in the phase behavior. LA0001068
