Langmuir 1998, 14, 5113-5120
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Phase Behavior of Polyoxyethylene Trisiloxane Surfactant in Water and Water-Oil Hironobu Kunieda,*,† Hidemi Taoka,† Tetsuro Iwanaga,‡ and Asao Harashima§ Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan, Basic Research Laboratory, Noevir Co., Ltd., Okada-cho 112-1, Youkaichi 527-0057, Japan, and Dow Corning Toray Silicone Co., Ltd., Chigusa-Kaigan 2-2, Ichihara 299-01, Japan Received March 11, 1998. In Final Form: July 1, 1998 A phase diagram of the polyoxyethylene trisiloxane surfactant-water system was constructed as a function of polyoxyethylene (EO) chain length at 25 °C. The HLB (hydrophile-lipophile balance) of surfactant corresponds to the volume ratio of the EO chain to that of surfactant molecule (φEO/φS), where φS and φEO indicate the volume fractions of surfactant and hydrophilic moiety in the system, respectively. Aqueous micellar (Wm), hexagonal liquid crystalline (H1), lamellar liquid crystalline (LR), and reverse micellar (Om) phases are formed with decreasing φEO/φS. A sponge phase (D2) is also formed near the LR phase region. The effective cross sectional area per one surfactant molecule, aS, in liquid crystals in the present systems depends only on their EO chain lengths and are the same as that in ordinary linear hydrocarbon surfactant systems. Since the maximum length of the trisiloxane moiety in its extended form is short, the LR phase is formed at φEO/φS between 0.45 and 0.6 in the present system, whereas an ordinary lineartype nonionic surfactant forms the H1 phase in the same range of φEO/φS. The H1 phase, which was observed in a narrow range of φEO/φS, becomes stable upon addition of oil. Although the H1 phase in the absence of oil is considered to have the hexagonal structure confirmed by polarized optical microscopy, the calculated radius of the cylinder is much longer than the hydrophobic chain length in its extended form. Perhaps, the present H1 phase is different from an ordinary hexagonal liquid crystalline structure. The three-phase behaviors of microemulsions in the present systems were also examined.
Introduction Silicone surfactants with siloxane derivative structures have been widely used in many industrial fields because of their thermal stability, ultraviolet ray resistant property, low surface tensions, biocompatibility, etc. The siloxane chain is highly flexible and even high-molecularweight chains are in a liquid state at room temperature. Polyoxyethylene-modified silicones are extensively used as emulsifying agents and detergents in cosmetics, toiletries, etc.1 The properties of silicone surfactants with polydimethylsiloxane chains are very surface active resulting in rapid wetting for low surface energy2-7 because the silicone chains have many methyl groups whose cohesive energy is very low compared with a methylene group.8 The molecular structures of polyoxyethylenemodified silicones are typically classified into a branchtype, a triblock-type, and a comb-type. The phase behavior and the properties of these copolymer-type surfactants in aqueous solution have been studied as a function of * To whom correspondence should be addressed. † Yokohama National University. ‡ Noevir Co. Ltd. § Dow Corning Toray Silicone Co., Ltd. (1) Riess, G.; Hurtrez, G.; Bahadur, P. Encyclopedia of Polymer Science and Engineering, 2nd ed.; John Wiley and Sons: New York, 1985; Vol. 2, p 324. (2) Bernard K.; Reid, W. G.; Petersen, I. H. Ind. Eng. Chem. Prod. Res. Dev. 1967, 6, 88. (3) Lin, Z.; Hill, R. M.; Davis, H. T.; Ward, M. D. Langmuir 1994, 10, 4060. (4) Goddard, E. D.; Ananthapadmanabhan, K. P.; Chandar, P. Langmuir 1995, 11, 1415. (5) Stoebe, T.; Lin, Z.; Hill, R. M.; Ward, M. D.; Davis, H. T. Langmuir 1996, 12, 337. (6) Tiberg, F.; Cazabat, A. M. Langmuir 1994, 10, 2301. (7) Lin, Z.; Stoebe, T.; Hill, R. M.; Davis, H. T.; Ward, M. D. Langmuir 1996, 12, 345. (8) Shafrin, E. G.; Zisman, W. A. J. Phys. Chem. 1960, 64, 519.
temperature and surfactant concentration.9-13 Ionic (anionic or cationic or zwitterionic) silicone surfactants have also been studied.14-18 However, there is no systematic study on the correlation between the HLB (hydrophile-lipophile balance) of silicone surfactants and the phase behavior. On the other hand, ordinary polyoxyethylene nonionic surfactants (CmEOn) have been also extensively investigated.19 Recently, the phase behavior of polyoxyethylene oleyl or dodecyl ethers in water were studied as a function of the oxyethylene (EO) chain length at constant temperature.20,21 Almost all the self-organizing structures, such as lamellar liquid crystal, (reverse) hexagonal liquid crystals, and discontinuous-type cubic, bicontinuous cubic, and isotropic bicontinuous surfactant phases, appear by changing the EO chain length. Types of the self-organizing structures are highly related to the curvature of surfactant (9) Yang, J.; Wegner, G.; Koningsveld, R. Colloid Polym. Sci. 1992, 270, 1080. (10) Hill, R. M.; He, M.; Lin, Z.; Davis, H. T.; Scriven, L. E. Langmuir 1993, 9, 2789. (11) Lin, Z.; Hill, R. M.; Davis, H. T.; Scriven, L. E.; Talmon, Y. Langmuir 1994, 10, 1008. (12) Kanellopoulous, A. G.; Owen, M. J. J. Colloid Interface Sci. 1971, 35, 120. (13) Kendrick, T. C.; Kingston, B. M.; Lloyd, N. C.; Owen, M. J. J. Colloid Interface Sci. 1967, 24, 135. (14) He, M.; Scriven, L. E.; Davis, H. T.; Snow, S. A. J. Phys. Chem. 1994, 98, 6148. (15) Snow, S. T.; Fenton, W. N.; Owen, M. J. Langmuir 1990, 6, 385. (16) Snow, S. T.; Fenton, W. N.; Owen, M. J. Langmuir 1991, 7, 868. (17) Snow, S. T. Langmuir 1993, 9, 424. (18) Lin, Z.; He, M.; Scriven, L. E.; Davis, H. T. J. Phys. Chem. 1993, 97, 3571. (19) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975. (20) Kunieda, H.; Shigeta, K.; Ozawa, K.; Suzuki, M. J. Phys. Chem. B 1997, 101, 7952. (21) Huang, K. L.; Shigeta, K.; Kunieda, H. Prog. Colloid Polym. Sci., in press.
S0743-7463(98)00298-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/15/1998
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molecular layers at the interface and the packing of surfactant molecules in the aggregates.19,22 In the past, it was difficult to investigate the phase behavior and self-organizing structures of silicone surfactants in detail in view of fundamental aspects, because most of available silicone surfactants were essentially the mixtures of unspecified compounds. However, recently, polyoxyethylene trisiloxane surfactants have been used for the fundamental study on the phase behavior in water and surface properties.5-7,23-25 Trisiloxane surfactants contain very pure hydrophobic parts, which is a bulky shape called an umbrella structure,25 and the effective cross sectional area and the hydrophobic part per one surfactant molecule in the self-organizing structures could be accurately analyzed. Although these trisiloxane surfactants have distributions in their EO chains, it is known that the influence of the distributions on the phase behavior in water is not large.26 It was reported that the phase behavior of polyoxyethylene-type trisiloxane surfactants in water is similar to that of ordinary polyoxyethylene-type hydrocarbon surfactants, CmEOn, having longer hydrophilic EO chains.23,24 However, the effect of the bulky trisiloxane hydrophobic moiety on the phase behavior has not been investigated. Moreover, the correlation between the curvature of surfactant layer and types of self-organizing structures was not studied in detail. In this context, we investigated the phase behavior of pure polyoxyethylene trisiloxane surfactant in binary water-surfactant or ternary water-surfactant-oil systems at constant temperature. The structures of liquid crystals were also investigated by means of small-angle X-ray scattering. Three-phase microemulsion behavior was studied in order to understand the HLB of the siloxane surfactant in water-oil. Experimental Section Materials. Polyoxyethylene trisiloxane surfactants, AH301, AH302, AH303, and AH304, were experimentally prepared from Dow Corning Toray Silicone Co., Ltd. The average oxyethylene (EO) chain lengths of the above surfactants are 4 for AH301, 8 for AH302, 15.7 for AH303, and 32.8 for AH304, respectively. The synthesis was essentially the same as the hydrosilylation method described earlier.23 To minimize the production of a possible side product, CH2dCHCH2(OCH2CH2)nOH, excess 1,1,1,3,5,5,5-heptamethyltrisiloxane was added in the synthetic process using platinum catalyst. The unreacted siloxane compound can be easily removed from the produced surfactants due to its high volatility. The chemical structures of the trisiloxane surfactants are shown in Figure 1. The trisiloxane surfactant is abbreviated as M(D′EOn)M, where n is the average number of the EO units. The thickness of the hydrophobic part parallel to the hydrophilic chain is approximately 0.92 nm in its extended form as is shown in Figure 1. Although there are distributions on the EO chain lengths, their purities are at least much greater than 95%. Silicone surfactants with other EO lengths were obtained by mixing the above AH301-4. Extra-pure-grade decane was obtained from Tokyo Kasei Kogyo Co. Octamethylcyclotetrasiloxane (SH244) was obtained from Dow Corning Toray Silicone Co., Ltd. Doubly distilled water was used. Procedure To Determine the Phase Diagram. After the above surfactants were mixed to adjust the EO chain, water was (22) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (23) Hill, R. M.; He, M.; Davis, H. T.; Scriven, L. E. Langmuir 1994, 10, 1724. (24) He, M.; Hill, R. H.; Lin, Z.; Scriven, L. E.; Davis, H. T. J. Phys. Chem. 1993, 97, 8820. (25) Gentle, T. E.; Snow, S. A. Langmuir 1995, 11, 2905. (26) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press, Inc.: San Diego, CA, 1994; p 304.
Kunieda et al.
Figure 1. Chemical structure of polyoxyethylene trisiloxane surfactant. added to the mixture in ampules. These ampules, kept in a thermostat, were well shaken and left at constant temperature (25 °C) from several hours to 1 week. Phase equilibria were determined by visual observation. The types of liquid crystals were identified by means of a polarized optical microscope and small-angle X-ray scattering (SAXS). Small-Angle X-ray Scattering (SAXS). Interlayer spacing of liquid crystal was measured using SAXS, performed on a smallangle 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 film for the measurement (Mylar seal method). Formation of Microemulsions in Ternary Water-Surfactant-Oil Systems. To form three-phase regions consisting of water, oil (decane or SH244), and surfactant phases, the weight ratio of water/oil was fixed at 1/1 and the total surfactant concentration was 10 wt %. These ampules were kept in a thermostat, were well shaken, and were left at 25 °C for 2 weeks. Volume fractions of respective phases were measured by a scale. Calculation of the Volume Fraction of the Lipophilic Part. The densities of surfactants were measured by a digital density meter (Anton Paar 40). We assume that the densities of surfactants in the liquid state are unchanged even in aqueous solutions or liquid crystals at constant temperature. The molar volume of surfactant is calculated by the following equation
VS ) MS/Fs
(1)
where MS and VS are the molecular weight and the molar volume of surfactant. It is also assumed that arithmetic additivity holds concerning the molar volumes of each functional groups in the surfactant.27 Then, the molar volume of surfactant is the sum of molar volumes of each group in the surfactant and the following relation holds
VS ) VL + nVEO + VOH
(2)
where VL, VEO, and VOH are the molar volumes of the lipophilic part, the oxyethylene (EO) group, and the hydroxyl group, respectively, and n is the number of EO units. Experimentally, the densities of AH301, AH302, AH303, and AH304 are 0.968, (27) Tanford, C. J. Phys. Chem. 1972, 76, 3020.
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Figure 2. Molar volume of polyoxyethylene trisiloxane surfactant as a function of EO chain length at 25 °C. 1.013, 1.055, and 1.103 g/cm3, respectively. From these data, we can calculate the molar volume of these surfactants by eq 1. The linear relationship between the molar volumes and the number of EO units holds as is shown in Figure 2. From the slope of the line in Figure 2, VEO is calculated to 37.9 cm3/mol. VOH is 8.84 cm3/mol according to the previous data on C12EOn.20 From these data and eq 2, VL is calculated to 313 cm3/mol. The hydrophobic volume of the trisiloxane moiety is almost the same as that for the oleyl group. The above values were used to calculate the volume fraction of the lipophilic part of the surfactant, φL, in the system
φL )
VL MS 1 - W S VS + FW W S
(
)
(3)
where FW is the density of water and WS is the weight fraction of surfactant in water-surfactant systems. In the presence of oil, eq 3 is rewritten to the following form
φL )
VL MS 1 - W O MS WO VS + -1 + FW WS FO WS
(
)
(4)
where WO is the weight fraction of oil in the system. Notation. We use the following notations to describe each phase. The subscript 1 denotes “hydrophilic” or “normal”-type self-organizing structure or phases, whereas the subscript 2 indicates “lipophilic” or “reverse”-type one. The following abbreviations are used: H1, hexagonal liquid crystal; LR, lamellar liquid crystal; D2, isotropic bicontinuous surfactant phase, this phase is denoted by D′ or L3 in the previous papers,28,29 Wm, micellar solution phase; Om, reverse micellar phase or surfactant liquid; W, excess water; O, excess oil; D, middle-phase microemulsion or surfactant phase.
Results Phase Diagram of Polyoxyethylene Trisiloxane Surfactant-Water System at 25 °C. The phase diagram of the water-M(D′EOn)M surfactant system as a function of surfactant concentration is shown in Figure 3. The weight fraction of surfactant in the aqueous (28) Kunieda, H.; Shinoda, K. J. Dispers. Sci. Technol. 1982, 3, 233. (29) Strey, R.; Schomacker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. Soc., Faraday Trans. 1990, 86, 2253.
Figure 3. Phase diagram of polyoxyehylene trisiloxane surfactant-water system as a function of the volume fraction of EO chain in the surfactant molecule at 25 °C.
solution, WS, is plotted horizontally. The volume fraction of the hydrophilic part (oxyethylene chain) in the total surfactant molecule, φEO/φS, is plotted vertically. The φEO/ φS ) VEO/VS is related to Griffin’s HLB number30 as
HLB number ) 20
( )( ) FEO(n) φEO FS φS
(5)
where FEO(n) and FS are the densities of EO chain and surfactant, respectively. The number of EO units, n, is also plotted on the right-hand axis, vertically. Although the surfactants are the mixture of surfactants with the different EO chain lengths, the phase behavior would not be largely different from that of pure surfactant except in a very dilute region. When the EO chain length of the surfactant is approximately longer than 15, an isotropic phase is present in a wide range of compositions. A lamellar liquid crystalline (LR) phase is formed and coexists with excess water (W) at φEO/φS between 0.45 and 0.6. The LR phase is changed to the reverse micellar solution or surfactant liquid (Om) with the further decrease in the number of the EO units. In the upper region of LR phase in the phase diagram, the isotropic isolated fluid phase, D2, appears. This phase is often called L3, or sponge phase.28,29 The surfactant curvature in the D2 phase is slightly negative, and the notation, D2, is used in the present paper. This phase appears in the phase behavior of the M(D′EO5)M-water system as a function of temperature.24 However, the D2 phase gradually became turbid from several hours to 1 day after preparing samples, although the LR phase is rather stable. Some part of the surfactant may be gradually decomposed in water. Perhaps, the hydrolysis of the Si-O-C bond takes place. The LR phase is confirmed by the optical microscopic texture and the SAXS data in which the peak ratio is 1:1/2. Hexagonal liquid crystal (H1) is formed in the very narrow area of the phase diagram as is shown in Figure 3. Since the second SAXS peak was not observed in the H1 phase, the H1 phase was confirmed only by the optical microscopic texture. The maximum temperature for the phase is (30) Griffin, W. C. J. Soc. Cosmet. Chem. 1954, 5, 249.
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Figure 5. Change in interlayer spacing (d), effective cross sectional area (aS), and a half thickness of lipophilic part of the bilayer (rLR) in the LR phase as a function of φEO/φS at constant φL ) 0.3. The broken curve corresponds to the change in aS in H1 and LR phases of polyoxyethylene oleyl ether systems.
Figure 4. Change in interlayer spacing (d), effective cross sectional area (aS), and a half thickness of lipophilic part of the bilayer (rLR) in the LR phase as a function of φL at constant φEO/φS (n ) 8).
slightly higher than 25 °C. In the previous study for the M(D′EO18)M-water system, the H1 phase is present even above 50 °C.24 In our systems, the H1 phase is formed in the shorter EO-chain surfactant system as is shown in Figure 3, but the reason for the difference in the position and maximum temperature has not been known. The H1 phase is usually considered to include infinitely long cylindrical aggregates that are packed in a hexagonal array. As described later, however, the SAXS analysis does not support this model. The range of φEO/φS for the LR phase is 0.45-0.6 in the present silicone surfactant system. On the other hand, the ranges of φEO/φS for the LR phase in the waterpolyoxyethylene oleyl ether and water-polyoxyethylene dodecyl ether systems are 0.3-0.4820 and 0.28-0.43,21 respectively. Lamellar Liquid Crystal in the M(D′EO8)M System. Interlayer spacing of lamellar liquid crystals of M(D′EO8)M was determined by the SAXS measurement. The results are shown in Figure 4. The half distance of the lipophilic part, rLR, and the effective cross sectional area of one surfactant molecule at the interface of lipophilic moieties, aS, in the LR phase are calculated by the following equations
rLR ) dφL/2
(6)
aS ) νL/rLR
(7)
where d is a measured interlayer spacing and vL is the volume of lipophilic group of one surfactant molecule. In Figure 4, the interlayer spacing gradually increases with decreasing the surfactant concentration. The effective cross sectional area and the half distance of lipophilic
part are almost constant. That means the bilayer structure in the LR phase is maintained or unchanged and only the distance between the bilayers is expanded upon addition of water. The length of the hydrophobic part is about 1 nm, whereas the aS is approximately 0.52-0.53 nm2. The length corresponds to that of the trisiloxane moiety in the all-trans form, 0.92 nm, as is shown in Figure 1. In general, a hydrophobic thickness is considerably shorter than the hydrocarbon chain length in its extended form of ordinary linear nonionic surfactant because the effective cross sectional area is much larger than the section area (0.22 nm2) of the hydrocarbon chain in a liquid state.31 However, the present trisiloxane surfactant is considered to keep an “umbrella” structure in the LR phase, and the thickness is almost the same as that in the extended form. Change in Structure of Lr Phase as a Function of OEO/OS. The interlayer spacing was also measured as a function of the EO chain length at constant φL ) 0.3 as is shown in Figure 5. The d and rLR values gradually decrease, whereas the aS increases with increasing φEO/ φS. The change in aS in this trisiloxane surfactant by the EO chain length is almost coincident with that in an ordinary polyoxyethylene oleyl ether system (the broken curve in Figure 5).20 In other words, the aS in this system is almost the same as that in the ordinary linear-shape surfactant system when the EO chain length is the same. It is known that the aS depends on the hydrophilic part in the ionic hydrocarbon chain surfactant system.32 In the polyoxyethylene-type surfactant system, the aS is mainly dependent on the EO chain length, because the hydrocarbon part is in an amorphous liquid state and the cohesive force is similar even if the hydrocarbon chain length is different. In the case of trisiloxane surfactant, it is expected that the cohesive force would be different from that of hydrocarbon because the lipophilic chain consists of seven methyl groups and three methylene groups. In fact, the surface tension minima and the solubility parameter of siloxane moiety are considerably (31) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; WileyInterscience: New York, 1967; p 133. (32) Gallot, B.; Skoulios A. Kolloid Z. Z. Polym. 1966, 208, 37.
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Figure 7. Effect of added decane on the phase behavior in the water-M(D′EO15.7)M system at 25 °C. Figure 6. Change in interlayer spacing (d), effective cross sectional area (aS), and a radius of lipophilic part of the cylinder (rH) in the H1 phase as a function of φO.
different from hydrocarbon moiety.33 Nevertheless, the present result shows that the effect of silicone chains on aS is similar to that of hydrocarbon chains if the EO length is the same. The aS is not dependent on the shapes and sizes of hydrophobic moieties but is dependent on the EO chain length. However, it is not known if this concept is applicable for extremely long or short hydrophobic chains. Structure of the H1 Phase and the Oil Effect. The interlayer spacing (d) of the H1 phase in the M(D′EO15.7)M system with or without oil was also determined by SAXS. The surfactant concentration is 55 wt %. Since the H1 phase is easily melted slightly above 25 °C in the absence of oil, the SAXS measurement was performed around 10 °C. We assumed that the hexagonal phase consists of infinitely long cylinders. Then, the radius of the cylinder, rH, and the effective cross sectional area, aS, are calculated by the following equations.
rH )
{
2 (φL + φO) x3π
aS )
(
}
)
2νL φO 1+ rH φL
1/2
d
(8)
(9)
where φO is the volume fraction of added oil in the system. The interlayer spacing of the H1 phase was measured as a function of decane content by SAXS, and the result is shown in Figure 6. The aS is almost unchanged, whereas the radius of the cylinder increases upon addition of decane as is shown in Figure 6. It is known that decane molecules do not penetrate into the palisade of the surfactant molecular layer. In the present system, also, decane molecules are mainly solubilized in the core of aggregates because the aS is unchanged. In the absence of oil, the aS is 0.64 nm2, and this value is similar to that of polyoxyethylene oleyl ether having the same EO chain length (0.60 nm2). Namely, even in the H1 phase, the aS is highly dependent on the EO chain length. On the other hand, the radius of the cylinder is 1.63 nm and is considerably longer than 0.92 nm in Figure 1. Therefore, it is considered that the (33) Kunieda, H.; Nakano, A.; Pes, M. A. Langmuir 1995, 11, 3302.
structure of the present H1 phase is distorted from the ideal hexagonal structure. When decane is added to the water-M(D′EO15.7)M system, the H1-phase region is widened as is shown in Figure 7. Another liquid crystal is formed at high surfactant concentration, and it was confirmed by polarized microscopy that a LR phase is produced. We also constructed the phase diagrams of the waterM(D′EOn)M system as a function of temperature as is shown in Figure 8. Figure 8a shows the phase diagram without decane and Figure 8b shows the phase diagram in the presence of 3 wt % decane. According to the phase rule, a three-phase region should be present between two two-phase regions, but we could not find it because of the narrowness. The maximum temperature for the H1 phase considerably increases upon addition of decane. Hence, added oil stabilizes the H1 phase. Formation of Microemulsions in the Ternary System. We observed the change in volume fractions of respective phases as a function of the EO chain length at equal weights of water and oil in water/trisiloxane surfactant/ decane or SH244 system. The results are shown in Figures 9 and 10, respectively. When the hydrophile-lipophile property is optimum, three-phase regions consisting of water, oil, and the surfactant phases are formed. With decreasing the hydrophilicity (EO chain length) of surfactant, a microemulsion (Om) phase coexists with excess water. On the other hand, with increasing the hydrophilicity of surfactant, a microemulsion (Wm) phase coexists with excess oil. The volume fraction of the middlephase microemulsion is extremely low in both oil systems, although the system contains 10 wt % of M(D′EOn)M. It is considered that a considerable amount of surfactant is monomericly dissolved in oil and the net surfactant concentration at the micro-interface inside the microemulsion would be low.34 Since the present surfactant is a mixture of surfactants, which have distribution in hydrophilic moieties, and the lipophilic ones with short hydrophilic chains are more soluble in water than hydrophilic ones, the average EO chain of the surfactant at the microinterface inside the middle-phase microemulsion may be longer than the original surfactant. (34) Iwanaga, T.; Shiogai, Y.; Kunieda, H. Prog. Colloid Polym. Sci., in press.
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a
Figure 9. Volume fractions of respective phases in the water/ silicone surfactant/decane system at 25 °C. Total surfactant concentration in the system is 10 wt %. The weight ratio of water/decane is unity.
b
Figure 8. Effect of temperature on the phase behavior in the water-M(D′EOn)M system as a function of the EO chain length: (a) without decane; (b) with 3 wt % decane. The surfactant concentration is 50 wt %.
Discussion Phase Behavior and Self-Organizing Structures. There is a significant difference between trisiloxane surfactant and polyoxyethylene oleyl ether (POlE), CH3(CH2)7CHdCH(CH2)8-(OCH2CH2)nOH. In the POlE surfactant system, hexagonal liquid crystal is formed whereas the LR phase appears in the present system in the same EO chain range. This is due to the difference in the hydrophobic chain length. From the density measurement, the molecular volume of trisiloxane chain is 0.52 nm3/molecule. The trisiloxane chain length in the all-trans form is estimated about 0.92 nm, and the cross sectional area is 0.57 nm2. The hydrophobic chain length in the LR phase is slightly longer than this value as is shown in Figures 4 and 5. This means that the surfactant molecule is considered to be slightly tilted or distorted in the LR phase. However, judging from the molecular structure in Figure 1, it is not energetically favorable for the hydrophobic part to have the longer length perpendicular to the interface of the hydrophobic part of
Figure 10. Volume fractions of respective phases in the water/ silicone surfactant/SH244 system at 25 °C. Total surfactant concentration in the system is 10 wt %. The weight ratio of water/SH244 is unity.
aggregates. When the LR-H1 transition takes place by extending the EO chain length in the linear hydrocarbon surfactant systems, the radius of the cylinder should be much longer than the half thickness of the LR phase because the aS is not largely changed at the transition.20 The molecular volume of the oleyl and dodecyl chain is 0.51 and 0.36 nm3/molecule, respectively. The cross sectional area of closed-packed hydrocarbon chains is 0.22 nm2 per chain in a liquid state.31 Hence, the oleyl and dodecyl chain length is considered to be 2.33 and 1.62 nm in the most extended form. In the H1 phases at the H1LR transition, the radius of hydrophobic part approaches these values.20,21 It is considered that the aS is mainly dependent on the EO chain length and the aS is not very large even if a trisiloxane surfactant has a long hydrophilic chain. For
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Figure 11. H1 phase in the presence of oil.
example, the aS of the POlE (20) is only 0.62 nm2.20 If the aS is the same value in the trisiloxane surfactant having a 20 EO chain, the packing parameter, v/aSlC, for the trisiloxane surfactant is still too large to form a H1 phase when lC is not changed. To form the H1 phase, aS should be more than 1 nm2 if the hydrophobic thickness is fixed to 1 nm. This is not the case. This is the reason the present trisiloxane surfactant mainly forms the LR phase whereas the H1-phase region is extremely narrow. In the H1 phase in the absence of oil, the radius of the cylinder is calculated to 1.63 nm, assuming that the cylindrical aggregates are packed in a hexagonal array. This is an unrealistic value. One possibility is that the structure of the H1 phase is distorted from the ideal model. Another possibility is that part of the EO chain penetrates into the hydrophobic core. Consequently, the aS in both polyoxyethylene linear alkyl ether and trisiloxane ether systems are almost dependent only on their EO chain lengths, and the shape of the hydrophobic parts do not influence the aS. However, both phase behaviors are very different because of the defference in the maximum lengths of the hydrophobic chains. Formation of H1 Phase by Added Oil. To stabilize the H1 phase in the present siloxane surfactant system, addition of oil is very effective. Since the aS is dependent on the EO chain length and it is difficult to reach 1 nm2, it is important to increase the effective thickness of the hydrophobic part. Hence, if oil is solubilized in the aggregates, the hydrophobic length would be extended as is shown in Figure 11. If it is assumed that oil does not penetrate into the palisade layer of surfactant, the hydrophobic chain length of surfactant part, lC, is calculated by simple geometrical relation
( x
lC ) rH 1 -
)
φΟ φL + φ O
(10)
where φO and φL are the volume fractions of oil and the hydrophobic part of surfactant in the system, respectively. We can calculate the hydrophobic thickness of the surfactant layer by eq 10 with the data in Figure 6. The results are shown in Figure 12. The hydrophobic thickness approaches the original thickness of the surfactant, 0.92 nm. Hence, upon addition of decane, the distorted hydrophobic moiety of the H1
Figure 12. Change in thickness of hydrophobic moiety of the trisiloxane surfactant in the H1 phase as a function of the decane content.
structure would be changed and the normal hexagonal structure is formed. In fact, the typical pattern of hexagonal liquid crystal was observed by polarizing microscopy, although the pattern is not very clear in the absence of oil. Balanced Microemulsions. In the case of ordinary surfactant, polyoxyethylene dodecyl ether (C12EO4) forms a balanced microemulsion (three-phase body) in a waterdecane system around 25 °C.35 The three-phase body appears in the present water/silicone surfactant/decane system when the EO chain length of the surfactant is around 9, as is shown in Figure 9. In the balanced microemulsions, the curvature of surfactant molecular layer is considered to be zero. If the packing theory can be applied, v/aSlC should be 1. Since the lC value in the present silicone surfactant is very short, the aS should be large to form the balanced microemulsion. This is the reason the EO chain length is very long to form the threephase body compared with ordinary linear-chain nonionic surfactant systems. Conclusions Phase behavior and structures of liquid crystals were investigated in water-polyoxyethylene trisiloxane surfactant systems as a function of VEO/VS ) φEO/φS. Only lamellar (LR) and hexagonal (H1) liquid crystals were observed as an anisotropic phase in the present system at 25 °C. In both liquid crystals, the effective cross sectional area per one surfactant molecule at the interface of the hydrophobic part in the aggregates, aS, is the same as that of an ordinary linear nonionic surfactant such as polyoxyethylene oleyl ethers having the same EO chain length. Hence, aS is almost dependent only on the EO chain length of surfactant. However, since the present silicone surfactant has an “umbrella” structure, the hydrophobic thickness is considerably shorter than that of linear-type nonionic surfactant. As a result, the lamellar phase is formed in the range of φEO/φS ) 0.450.6 in which the H1 phase is produced in the linear surfactant systems. If the ideal model for the structure of the H1 phase is applied, the calculated hydrophobic (35) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1985, 107, 107.
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chain length is much longer than the trisiloxane chain in its extended form in the H1 phase. Probably, the H1 phase is distorted from the ideal hexagonal structure or a part of the EO chain penetrates into the hydrophobic core. When oil, decane, is added, it is solubilized in the core of cylinder, and a normal hexagonal structure is formed because the radius of the cylinder can be expanded. Hence, the stability of the H1 phase increases upon addition of decane. This bulky and short hydrophobic part of the
Kunieda et al.
present silicone surfactant also affects the three-phase behavior of microemulsions in a water/silicone surfactant/ oil system. Consequently, the unique phase behavior of the trisiloxane surfactant in water is attributed to the short length of the hydrophobic part and not to the large effective cross sectional area, aS. LA980298V
