Langmuir 2008, 24, 3111-3117
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Phase Behavior and Microstructure of C12E5 Nonionic Microemulsions with Chlorinated Oils G. Roshan Deen* and Jan Skov Pedersen Department of Chemistry and Interdisciplinary Nanoscience Center (iNANO), UniVersity of Aarhus, Langelandsgade 140, Aarhus C, Denmark ReceiVed October 24, 2007. In Final Form: December 14, 2007 New non-ionic microemulsions consisting of pentaethyleneglycol dodecyl ether, water, and 1-chloroalkanes were prepared, and their phase behavior was studied. A homologous series of five different 1-chloroalkanes from 1-chlorooctane to 1-chlorohexadecane was studied. The phase behavior of the microemulsions was determined by vertical sections through the Gibbs’ phase prism (“fish” plots), from which valuable information such as the microemulsion balance temperature (T0), efficiency of the surfactant (φ*s), temperature extension of the three-body phase (∆T), mean temperature (Tm), and the monomeric solubility in oil (φmon,oil) was obtained. The chlorinated alkanes in the microemulsions shift the balance temperature to about 14 °C lower compared with their n-alkane counterparts. This indicates the polar nature of the chlorinated oils and their ability to penetrate the surfactant film. The chlorinated alkanes thus behave as short n-alkane molecules and lower the spontaneous curvature of the microemulsion droplets. The efficiency of the surfactant and the monomeric solubility in oil systematically depend on the alkyl chain length of the oil, with the efficiency and solubility decreasing with increasing alkyl chain length of 1-chloroalkane. The size and shape of the microemulsion droplets in the microemulsion phase were studied by small-angle X-ray scattering (SAXS). For a surfactant-to-oil volume fraction ratio of 0.80, the droplets can be described by ellipsoidal shapes, and the size of the droplets increased with increasing alkyl chain length.
Introduction Microemulsions are multicomponent systems consisting mainly of oil, water, and surfactant. These systems have low viscosity, are optically clear and thermodynamically stable, and have characteristic properties such as ultralow interfacial tension, large interfacial area, and the capacity to solubilize both aqueous and oil-soluble compounds.1 Over the past decade, microemulsions have attracted considerable attention because of their applications in cosmetics, food, pharmaceuticals, nanoparticles synthesis, membranes, and so forth.2-6 The different types of microemulsions are described by Winsor phases:7 Winsor I (oil-in-water), Winsor II (water-in-oil), Winsor III (bicontinuous), and Winsor IV (single phase). By suitable adjustment of the composition (and temperature) of the microemulsion constituents, interconversion between the various Winsor phases can be achieved. The characteristic length scales of these microemulsion structures normally range from 1 to 100 nm.8 The surfactant present in the microemulsion plays the key role in solubilizing the immiscible components, namely, oil and water, by forming an interfacial film at the oil-water interface. The surfactant can be cationic, anionic, zwitterionic, or non-ionic.9 Interest in non-ionic surfactants of the alkyl polyoxyethylene type CH3-(CH2)j - 1(OCH2CH2)i-OH (CjEi) has been mainly due to microemulsion phase structures that can be tuned by * Corresponding author. E-mail:
[email protected]. (1) Schulmann, J. H.; Stoeckenius, W.; Prince, L. M. J. Phys. Chem. 1959, 53, 1677. (2) Yaghmur, A.; Aserin, A.; Garti, N. Colloids Surf., A 2002, 209, 71. (3) Solans, C.; Pons, R.; Kuneida, H. In Industrial Applications of Microemulsions; Kuneida, H., Ed.; Marcel Dekker: New York, 1997. (4) Sjo¨blom, J.; Lindberg, R.; Fribers, S. E. AdV. Colloid Interface Sci. 1996, 65 125. (5) Moulik, S. P.; De, G. C.; Panda, A. K.; Bhowmik, B. B.; Das, A. R. Langmuir 1999, 15, 8361. (6) Paul, B. K.; Moulik, S. P. Curr. Sci. 2001, 80, 990. (7) Winsor, A. P. In SolVent Properties of Amphiphilic Compounds; Butterworths: London, 1954. (8) Strey, R.Colloid Polym. Sci. 1994, 272 ,1005. (9) Hamley I. In Introduction to Soft Matter; Wiley: New York, 2005.
varying the temperature, where j represents the number of carbon atoms in the alkyl chain and i represents number of ethylene oxide units of the surfactant. At the oil-water interface, a lateral head-head and tail-tail interactions of the surfactant exist. Because of the strong temperature-dependent interaction of the ethylene oxide group with water, the curvature of the surfactant film at the oil-water interface can be varied by changing the temperature.10-12 At lower temperature, the ethylene oxide groups form hydrogen bonds with water, and the surface area of the head group of the surfactant is increased, resulting in a preferred curvature toward oil. At higher temperature, the ethylene oxide groups are dehydrated, and the surfactant film curves toward water. The spontaneous mean curvature H0 of the surfactant film is a strong function of temperature, and it can be described as13
H0(T) ) C(T0 - T)
(1)
T0 is the temperature where the surfactant film prefers zero mean curvature and is called the balance temperature or the phaseinversion temperature (PIT). Coefficient C depends slightly on the nature of the surfactant, and for pentaethylene glycol dodecylether (C12E5) it is ∼10-3 (K nm).8,14 Among the non-ionic microemulsions, C12E5-water-n-alkane ternary systems have been most widely studied with respect to their phase behavior, interfacial properties, and microstructures.15-18 The oil component of the non-ionic microemul(10) Olsson, U.; Wennerstro¨m, H. AdV. Colloid Interface Sci. 1994, 49, 113. (11) Olsson, U.; Nagai, K.; Wennerstro¨m, H. J. Phys. Chem. 1988, 92, 6675. (12) Anderson, D.; Wennerstro¨m, H.; Olsson, U. J. Phys. Chem. 1989, 93, 4243. (13) Morris, J.; Olsson, U.; Wennerstro¨m, H. Langmuir 1997, 13, 606. (14) Rajagopalan, V.; Bagger-Jo¨rgensen, H.; Fukuda, K.; Olsson, U.; Jonsson, B. Langmuir 1996, 12, 2939. (15) Kuneida, H.; Shinoda, K. J. Colloid Interface Sci. 1973, 42, 381. (16) Kahlweit, M.; Strey, R.; Firman, P.; Haase, D.; Jen, J.; Schoma¨cker, R. Langmuir 1988, 4, 499. (17) Gradzielski, M.; Langevin, D.; Farago, B. Phys. ReV. E 1996, 53, 3900.
10.1021/la703323n CCC: $40.75 © 2008 American Chemical Society Published on Web 02/29/2008
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sions, however, is not limited to n-alkanes but can be varied, which is the subject of the present article. Earlier, the phase behavior of non-ionic microemulsions containing n-alkyl β-D-monoglucoside as a surfactant and oxygenated oils such as alkyl ethylene glycol ethers was reported by Ryan and Kaler.19 Alexandridis et al.20 used polar oils such as xylene with pluronic systems to study the phase behavior. The use of toluene, cyclohexane, and ethylbenzene in non-ionic microemulsions was described by Burauer et al.21 The phase behavior of polymerizable non-ionic microemulsions with n-alkyl methacrylates as oil was reported by Lade et al.22 The use of pharmaceutically accepted oils such as ethyl butyrate, ethyl caprylate, ethyl oleate, triglycerides, soya bean oil, miglyol, and tributyrin in non-ionic microemulsions was reported by Warisnoicharoen et al.23 Despite these various investigations using a wide variety of oils in non-ionic microemulsions, the use of chlorinated oils as reported in the present work is rather scarce. Related to the present work is the work by Schubert and Kaler.24 They used perfluorinated oils with mixtures of fluorinated and hydrogenated surfactants in ionic and non-ionic microemulsions. Egger et al.25 used short chlorinated oils such as 1,2-dichloroethane, carbon tetrachloride, and dichloromethane in non-ionic microemulsions. These ternary systems were used as reaction media in the catalytic epoxidation of olefins. Baran et al.26 used tetrachloroethylene, carbon tetrachloride, and trichloroethylene in ionic microemulsions. The same group27 also reported using chloroform, 2-butyloctanoyl chloride, dichlorobenzene, and dichloroethylene as oils in microemulsions containing N-methylN-D-glucalkanamide as the surfactant, with the aim of developing systems for enhanced oil recovery. To the best of our knowledge following a literature search, the phase behavior of non-ionic microemulsions containing C12E5 and long-chain chlorinated alkanes has not been investigated in detail. In this article, we report the phase behavior of non-ionic microemulsions of the type C12E5-water-1-chloroalkane. The polar nature of the chlorinated oils can impart (good) solubility and hence (better) oil penetration. Five different 1-chloroalkanes were used, ranging from 1-chloroalkane (C8) to 1-chlorohexadecane (C16), to study the effect of chain length on the phase behavior of these microemulsions. These new systems with polar oils are envisaged to be good model systems for studying microemulsion droplets with surface barriers (partitioning of polar groups at the surface of the droplets), emulsification, and material transport properties.28
(C12E5) of >98% purity was obtained from Nikko Chemicals. Milli-Q water was used for all sample preparations. Density Measurements. The densities of chlorinated oils and water were measured at the desired temperature with a DMA 5000 densitometer (Anton-Paar, Graz, Austria), which employs the oscillating-tube technique. The instrument was calibrated at 20 °C using air and water as references prior to sample measurements. Approximately 1.5 mL of the sample was required for each measurement, and the measurement was repeated at least three times on the same day until reproducible results were obtained. For the SAXS study, the electron densities of the surfactant, water, and chlorinated oils were calculated using their appropriate molar volumes.29,30 Small-Angle X-ray Scattering (SAXS) Measurements. The size of the microemulsion droplets in the L1 phase was determined using modified NanoSTAR SAXS31 in a high-resolution setup. The instrument is a modified version of commercially available SAXS equipment supplied by Bruker AXS. It is optimized with respect to flux and background and is therefore suited for solution scattering. The configuration of the instrument in the high-resolution configuration at a sample-to-detector distance of 108.45 cm provides a range of scattering vector moduli, q, from 0.004 to 0.22 Å-1 with a flux of about 106 photons/s. The sample is held in a home-built quartz capillary holder (sealed with caps and o-rings) and placed in a thermostated sample block for good thermal contact. The capillary and the thermostated block are placed inside the integrated vacuum chamber of the instrument. Sample Preparation for Phase Diagram Studies. The samples were prepared by weight at room temperature and later converted to volume using their respective densities. Accordingly, the following density values (g/cm3 as determined by densitometry) were used: C12E5, 0.967;32 water, 0.9980; C8-Cl, 0.875; C10-Cl, 0.868; C12Cl, 0.867; C14-Cl, 0.859; and C16-Cl, 0.860. Appropriate amounts of C12E5, water, and the respective 1-chloroalkane were accurately weighed into screw-capped glass vials and mixed thoroughly using a vortex mixer for complete homogenization. The samples were heated to 40 °C in an oven and air cooled to room temperature to ensure complete homogenization. All samples for the determination of phase diagrams were prepared at a constant water-to-oil volume ratio of φ ) 0.5. The compositions of the microemulsion droplets were defined in terms of volume fractions rather than mass fractions as follows:
Experimental Section
where Vo, Vs, and Vw are the volumes of oil, surfactant, and water, respectively. Determination of Phase Diagram (“Fish”) and Method. The microemulsion samples were placed in a thermostated water bath connected to a chiller (Ika labortechnik and Julabo F33 chiller) to observe the phase behavior. The temperature was increased from 20 to 80 °C and then gradually decreased (1 °C min-1) to 5 °C. The phase changes that occur as a function of surfactant concentration and temperature were recorded. The presence of the lamellar phase was observed between crossed polarizers. The droplet (L1) phase region of the microemulsion was clear and transparent, but the lamellar phase (LR) had a faint blue tinge and showed optical birefringence. In addition, a coexisting region between the L1 and LR phases was also observed that turned to a gel (elastic phase) at high surfactant concentration and at low temperature. The upper and lower phase separations (2h and 2) and the three-phase region (3)
Materials. All chemicals and reagents were used as received. 1-Chlorooctane (C8-Cl), 1-chlorodecane (C10-Cl), 1-chlorododecane (C12-Cl), 1-chlorotetradecane (C14-Cl), 1-chlorohexadecane (C16-Cl) and 1,10-dichlorodecane (1,10-C10-Cl2) were obtained from Sigma-Aldrich. High-grade pentaethylene glycol dodecylether (18) Leitao, H.; Somoza, A. M.; Telo da Gama, M. M.; Sottmann, T.; Strey, R. Chem. Phys. 1996, 105, 2875. (19) Ryan, L. D.; Kaler, E. W. Langmuir 1997, 13, 5222. (20) Alexandridis, P.; Olsson, U.; Lindman, B. Macromolecules 1995, 28, 7700. (21) Burauer, S.; Sottmann, T.; Strey, R. Tenside Surf. Det. 2000, 37, 8. (22) Lade, O.; Beizei, K.; Sottmann, T.; Strey, R. Langmuir 2000, 16, 4122. (23) Warinoicharoen, W.; Lansley, A. B.; Lawrence, M. J. Int. J. Pharm. 2000, 198, 7. (24) Schubert, K. V.; Kaler, E. W. Colloids Surf., A. 1994, 84, 97. (25) Egger, H.; Sottmann, T.; Strey, R.; Valero, C.; Berkessel, A. Tenside Surf. Det. 2002, 39, 17. (26) Baran, J. R, Jr.; Pope, G. A.; Wade, W. H.; Weerasooriya, V.; Yapa, A. J. Colloid Interface Sci. 1994, 168, 67. (27) Arenas, E.; Baran, J. R, Jr.; Pope, G. A.; Wade, W. H.; Weerasooriya, V. Langmuir 1996, 12, 588. (28) Roshan Deen, G.; Pedersen, J. S. To be submitted for publication.
Vo Vo + Vw
(2)
Vs Vs + V o + V w
(3)
φ)
φs )
(29) Maccarini, M.; Briganti, G. J. Phys. Chem. A 2000, 104, 11451. (30) Sommer, C.; Pedersen, J. S.; Stein, P. C. J. Phys. Chem. B 2004, 108, 6242. (31) Pedersen, J. S. J. Appl. Cryst. 2004, 37, 369. (32) Vollmer, D. Fett/Lipid 1999, 101, 379.
C12E5 Nonionic Microemulsions with Chlorinated Oils
Langmuir, Vol. 24, No. 7, 2008 3113 surfactant required to completely homogenize equal amounts of water and oil).
Results and Discussion
Figure 1. (A) Sketch of the phase prism for ternary system C12E5water-1-chloroalkane. (B) Sketch of a fish plot indicating the locations of various phases and physical quantities. were all turbid. Determinations of these phases are laborious, and phase changes have to be awaited because the appearance of some of them is limited by slow kinetics. All measurements of the phase diagrams were repeated three times to confirm the phase boundaries. The method introduced by Kahlweit33 and later by Strey34 to characterize the phase behavior of a microemulsion system as a function of temperature and composition was followed. This is conveniently done by considering a ternary mixture of water (A), a hydrocarbon (oil) (B), and a non-ionic surfactant (C). At constant pressure, this is represented in an upright phase prism with an isothermal Gibbs’ triangle with A-B-C as the base and temperature (T) as the ordinate. This is shown in Figure 1A.24 In this Figure, the vertical section of the prism shows a constant water-to-oil volume ratio of 0.5. The shape of the phase diagram obtained in such a section is that of a fish, which starts at corner C of the prism and strikes the A-B axis at φ ) 0.5 (Figure 1B). The fish plot, as it is commonly called, provides valuable information about the partial ternary microemulsion system of oil, surfactant, and water.33,34 From Figure 1B, it can be observed that as the concentration of the surfactant and temperature are increased various phases are formed. At low temperature (T < TL) and intermediate surfactant concentration, the mixture forms an oil-in-water microemulsion that is in equilibrium with the excess oil phase (L1 + O phase) whereas at high temperature (T > TU) the surfactant-rich oil phase coexists with an excess water phase. These two cases are denoted by 2h and 2, respectively, and the corresponding boundaries are also called the water emulsification boundary (web) and oil emulsification boundary (oeb), respectively.32 Between these two temperature regions (TL < T < TU), a water excess, an oil excess, and a surfactantrich phase coexist. This is denoted by 3. With increasing surfactant concentration and moderate temperature, a single homogeneous microemulsion phase (L1) and liquid-crystalline phases (LR) are formed. The phase boundaries between the upper and lower twophase regions and the one-phase region form the fish tail, which is a measure of the efficiency of the surfactant (minimum amount of (33) Kahlweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1990, 94, 3881. (34) Jakobs, B.; Sottmann, T.; Strey, R. Tenside Surf. Det. 2000, 37, 6.
Phase Behavior of C12E5-Water-1-Chloroalkane. Figure 2A-E shows the fish cut plots for the system C12E5-water1-chloroalkane with various 1-chloroalkanes at constant φ ) 0.5. The phase diagrams resemble that of a fish with a welldefined head-body (three-phase region) and tail (one-phase region). The head-body region shows three regions corresponding to oil emulsification (Winsor I), water emulsification (Winsor II), and the middle three-phase body (Winsor III). From the tail of the fish, a distinct droplet phase (L1), a lamellar bilayer phase (LR), and a coexisting droplet and lamellar phase can be identified. These coexisting phases form an elastic gel at high surfactant concentration and with decreasing temperature, which is a typical feature of long-chain surfactants.22 The appearance of the so-called fishes is almost horizontal, indicating the high purity of the surfactant C12E5 used in the preparation of microemulsions. For technical grade or polydisperse surfactants, usually an upturn toward higher temperature is observed.35 In all of the phase diagrams, a small three-body region (fish head) is observed, which is related to the efficiency of C12E5 in solubilizing equal amounts of 1-chloroalkanes and water. From the respective phase diagrams, a series of characteristic parameters of the microemulsions were extracted, and the results are discussed in the following section. (a) From the intersecting point of the one- and three-phase regions (the fish tail point, χ*), the volume fraction of surfactant that is required for complete solubilization of equal volumes of water and oil was determined (φ/s). The lower this value, the more “efficient” the surfactant. It can be seen from Figure 3 that the efficiency of C12E5 in solubilizing equal amounts of water and 1-chloroalkane decreases from 1-chlorooctane to 1-chlorohexadecane, which is not an unexpected observation. In spite of the polar nature of the 1-chloroalkanes, an increase in the alkyl chain length affects the phase inversion, which leads to a decease in the efficiency of the surfactant. Non-ionic microemulsions of C12E5-water-n-alkanes also show similar behavior.35-37 However, compared with n-alkane systems, C12E5 microemulsions with 1-chloroalkanes were found to be 3033% more efficient in solubilizing capacity. This implies that C12E5 can effectively solubilize 1-chloroalkanes with moderate alkyl chain length and above. (b) The temperature that corresponds to φs/ is called the balance temperature (T0) or the phase-inversion temperature (PIT) of the microemulsion. At this temperature, the curvature of the surfactant is zero (on average) and it has no preferred curvature either toward oil or toward water, and the microemulsion is in the fully swollen state. This temperature also depends on the nature of the oil through its ability to penetrate into the apolar part of the surfactant film.22,25 The effect of the alkyl chain length of 1-chloroalkanes on T0 is shown in Figure 3. It can be observed that the PIT increases with increasing alkyl chain length following the general trend for n-alkanes.36,37 Any increase in hydrocarbon chain length would shift the PIT to higher temperature, also causing the efficiency of the surfactant to decrease. Interestingly, the T0 values of the 1-chloralkanes are about 14 °C lower than those of C12E5 microemulsions containing n-alkane as an oil. This marked decrease is attributed to the decreased hydrophobicity (35) Sottmann, T.; Strey, R.; Chen, S. H. J. Chem. Phys. 1997, 106, 6483. (36) Sottmann, T.; Strey, R. J. Chem. Phys. 1997, 106, 8606. (37) Gosh, S. K.; Komura, S.; Matsuba, J.; Seto, H.; Takeda, T.; Hikosaka, M. Jpn. J. Appl. Phys. 1998, 37, 919.
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Figure 2. Fish plots for ternary system C12E5-water-1-chloroalkane: (A) 1-chlorooctane, (B) 1-chlorodecane, (C) 1-chlorododecane, (D) 1-chlorotetradecane, and (E) 1-chlorohexadecane.
Figure 3. Effect of the alkyl chain length of 1-chloroalkanes on T0 and φ/s.
(increased polar nature) of the 1-chloroalkanes. As a result of this, these chlorinated alkanes swell the alkyl chain of the surfactant, leading to marked changes in interfacial curvature
properties. This in turn lowers the spontaneous curvature of the surfactant film, which leads to a decrease in PIT. Thus, 1-chloroalkanes with C12E5 behave like small n-alkane molecules, providing better oil penetration and any surface barrier on the microemulsion droplets. Surface barrier here refers to the partitioning of the hydrophilic chlorine atom between the interface of the microemulsion droplet and the bulk medium. This is similar to the partitioning of the methacrylate groups between the surface of the microemulsion droplet and the bulk as described by Lade et al.22 It has to be noted that the investigation of microemulsions containing very small n-alkanes is cumbersome or extremely difficult because of their high vapor pressure at the temperature of interest. (c) At the fish-head point, the three-phase body appears at a surfactant volume fraction (φ0s ). Below this value, the surfactant molecules are dissolved in the excess phases. The values for the non-ionic microemulsions containing 1-chloroalkanes are summarized in Table 1. It can be seen that φ0s decreases with the increasing alkyl chain length of 1-chloroalkanes as a result of
C12E5 Nonionic Microemulsions with Chlorinated Oils
Langmuir, Vol. 24, No. 7, 2008 3115
Table 1. Characteristic Physical Parameters for Termary Systems C12E5-Water-1-Chloroalkane and C12E5-Water-1,10-Dichlorodecane at O ) 0.5, Extracted from Fish Plots chlorinated oil C8-Cl C10-Cl C12-Cl C14-Cl C16-Cl 1,10-C10-Cl2
TL ∆T Tm TU (°C) (°C) (°C) (°C) 13.6 23.6 27.4 33.0 39.0 32.1
11.2 19.2 21.8 26.0 31.4 23.3
2.4 4.4 5.6 7.00 7.6 8.8
12.4 21.4 24.6 29.5 35.2 27.7
φ0s
φis
φmon
φmon,oil
0.024 0.023 0.021 0.020 0.018 0.038
0.0011 0.0011 0.0012 0.0014 0.0016 0.184
0.0228 0.0217 0.0202 0.0183 0.0165 0.03
0.0467 0.0446 0.0420 0.0386 0.0356 0.073
the increasing hydrophobic nature of the oil,22 with the effect of the temperature-dependent solubility of C12E5 playing a major role. (d) From the upper and lower temperatures of the three-phase body (fish head), the temperature extension ∆T and the mean temperature Tm were calculated using the following respective expressions:
∆T ) (TU - TL) Tm )
(
)
TU + T L 2
(4) (5)
where TU and TL are the temperatures corresponding to the upper and lower phase boundaries of the three-phase body. The results are summarized in Table 1. Both ∆T and Tm increase with increasing alkyl chain length of the 1-chloroalkanes, showing the same general trend as for non-ionic microemulsions with n-alkanes as the oil.21,36,37 Generally, Tm and T0 do not differ much because of the symmetry of the three-phase region, but deviations can be expected in some cases. With increasing alkyl chain length of the 1-chloroalkanes, it is observed that the fish head becomes asymmetrical close to φ0s , which could be attributed to the partitioning of the polar chlorine group between the bulk and the surfactant interface. It is further qualitatively envisaged that the chlorine group partially acts as a co-surfactant at the fish-head point, which is an effect observed for alcohols.38 Similar behavior has been observed for C12E5 microemulsions containing n-alkyl methacrylates as a result of partitioning of the relatively polar methacrylate groups between the bulk and the surfactant film.22 (e) Replacing 1-chloroalkane with a dichloroalkane such as 1,10-dichlorodecane in the microemulsion leads to different phase behavior of the system. Figure 4 shows the fish-cut plot of the system C12E5-water-1,10-dichlorodecane in which a large threebody phase, an L1 droplet phase, and a small LR region can be readily observed. The physical parameters obtained from the fish-cut plots are summarized in Table 1. The large three-body phase extends all the way from a surfactant volume fraction of 0.057 to 0.304. This indicates that 1,10-dicholordecane is not a suitable oil for non-ionic microemulsions containing C12E5 in spite of its more polar nature as a result of the presence of two chlorine atoms. This system rather requires a large volume fraction of surfactant (0.304) before a microemulsion can be formed, thus making the oil less compatible. The two chlorine atoms at the 1st and 10th positions do not permit oil penetration and swelling of the alkyl chains of the surfactant. Because of the presence of two polar groups at either end of the molecule, some kind of strong partitioning of the hydrophilic chlorine atoms between the bulk phase and the droplets can be qualitatively envisaged. When the microemulsion samples that contained low (38) Penders, M. H. G. M.; Strey, R. J. Phys. Chem. 1995, 99, 10313.
Figure 4. Fish plot of ternary system C12E5-water-1,10-dichlorodecane.
amounts of surfactants were cooled at 10 °C for 2 h, crystallization of the surfactant was observed, thus rendering the system very unstable. The non-ionic microemulsion system of C10E8-1,2dichloroethane-water25 shows only an uninterrupted coexistence line between the one- and two-phase regions with a surfactant efficiency of 0.4 in terms of weight fraction. According to Egger et al.,25 this high value was due to the polar nature of oil that renders the n-alkyl polyglycol ether surfactant inefficient. When the oil was replaced with carbon tetrachloride (CCl4), a nonpolar chlorinated compound, the efficiency increased and was comparable to that observed with n-alkanes. These results clearly demonstrate that the presence of two or more highly polar groups (such as chlorine) in the oil molecule greatly affects the efficiency of alkyl polyglycol ether surfactants and consequently the stability of the microemulsion. Volume Fraction of Surfactant in the Internal Interface. The surfactant interfacial film that forms the internal interface of a microemulsion plays a vital role. Therefore, using the fishhead and fish-tail points, it is possible to deduce the volume fraction of surfactant at the oil-water interface, φis. Moreover, from the fish-head point the solubility of the surfactant monomer in the excess phases can be calculated. The volume of monomerically dissolved surfactant (φmon) and the volume fraction of surfactant in the internal interface (φis) can be determined using the following expressions:21,22
φmon )
φ0s (1 - φ0s )
(1 - φs/)
φis ) φs/- φmon
(6) (7)
The volume of monomerically dissolved surfactant in the excess phase decreases with the increasing chain length of 1-chloroalkanes, indicating the increased hydrophobic character of the oil. To evaluate the strength of a surfactant, it is important to know the amount of surfactant that is in the internal interface of the fish-tail point, and this is of practical importance. From Table 1, it can be seen that (φis) increases less significantly. This behavior could be due to unfavorable interaction between the strong polar chlorine atom of the oil and the alkyl chain of the surfactant. Monomeric Solubility of C12E5 in 1-Chloroalkanes. The determination of the actual solubility of the surfactant in the oil phase is an important parameter for industrial applications. This is done conveniently by the method described by Burauer et al.21 and Lade et al.22 using the following expression:
3116 Langmuir, Vol. 24, No. 7, 2008
φmon,oil )
φ0s φ0s + φ(1 - φ0s )
Deen and Pedersen
(8)
The values are summarized in Table 1. In this expression, it is generally assumed that φmon,water, which corresponds to the critical micelle concentration (cmc) of long-chain surfactants (i > 8), is at least a factor of 10 less than φmon,oil, and thus it is neglected in the above equation. The results indicate that φmon,oil decreases with the increasing alkyl chain length of the chlorinated oil, further demonstrating the unfavorable interaction between the polar chlorine atom and the surfactant’s alkyl group. Nonionic microemulsions of the C10E6-water-alkyl methacrylate type behave in a similar way as a result of the unfavorable interaction between the relatively polar methacrylate group and the alkyl chain of the surfactant.22 For non-ionic microemulsions with n-alkanes, the solubility of the surfactant in the oil phase is found to increase less significantly or remain more or less constant.35,36,39 For practical purposes, φ/s is important because it indicates the amount of surfactant that is actually required to form a single-phase bicontinuous microemulsion. SAXS Study of the Microemulsion Droplets. The structure of microemulsion droplets (with a fixed surfactant-to-oil volume fraction ratio of 0.80) in the L1 phase was studied by SAXS at the appropriate temperature. This surfactant-to-oil volume fraction ratio defines the constraint of the area to the enclosed volume ratio that gives the maximum curvature toward oil. This value was chosen because under this condition the system gives spherical microemulsion droplets with low polydispersity. To analyze the data quantitatively on an absolute scale, a polydisperse ellipsoidal model with hard-sphere interactions was used to model the microemulsion droplets.40,41 Formally, the intensity I(q), which is a function of the modulus of the q scattering vector, can be written as
I(q) )
Figure 5. SAXS data for microemulsion droplets for ternary system C12E5-water-1-chlorooctane at 7 °C with a droplet volume fraction of 0.022 and a fixed surfactant-to-oil volume fraction ratio of 0.80. The solid line is the fit of the polydisperse ellipsoid model described in the text. Table 2. SAXS Results on the Size and Shape of Microemulsion Droplets at a Fixed Droplet Volume Fraction of 0.021 1-chloroalkane
temp (°C)a
size (Å)
eccentricity
as (Å2)
σrelative
C8-Cl C10-Cl C12-Cl C14-Cl C16-Cl
7.0 7.0 20.0 24.0 33.0
72.3 ( 0.22 75.4 ( 0.22 76.1 ( 0.21 78.0 ( 0.23 81.3 ( 0.29
1.49 1.71 1.77 1.98 1.98
50.9 49.7 48.8 47.1 46.5
0.14 0.14 0.14 0.16 0.17
a Refers to the temperature of the L1 phase where measurements were performed. The temperature range of the L1 phase increases with increasing alkyl chain length of 1-chloroalkane.
∫0∞ n(R)〈F(q, R)2〉 dR + N1 ∫0∞ ∫0∞ n(R) n(R′) ×
〈F(q, R)〉〈F(q, R′)〉[S(q, R, R′) - 1] dR dR′ (9)
where n(R) is the number size distribution, N is the total number of particles, and S(q, R, R′) represents the partial structure factors between spheres with radii R and R′. 〈F(q, R)〉 is the orientationally averaged amplitude of a single ellipsoidal object with volumeequivalent radius R and eccentricity e. The objects are taken to be being ellipsoids of revolution, and the volume-equivalent radius is the radius of a spherical particle with the same volume as that of the ellipsoid. 〈F(q, R)2〉 is the corresponding orientationally averaged amplitude square of an ellipsoidal object. The particles in the model (microemulsion droplets) consist of a core and two shells: a core for the chloroalkane, one shell for the hydrocarbon tails, and the other shell for the head groups of the surfactant. The scattering-length density of each component was calculated from the determined partial specific densities, and water penetration in the surfactant head group region was allowed. By fitting the experimental data with this model, the size of the microemulsion droplets, the relative polydispersity, and the shape of the droplets were obtained. Figure 5 shows representative SAXS data along with the model fit for a microemulsion droplet volume fraction of 0.022 for the system C12E5-water-1-chlorooctane at 7 °C. The experimental data are in good agreement with the described hard-sphere model. (39) Burauer, S.; Sachert, T.; Sottmann, T.; Strey, R. Phys. Chem. Chem. Phys. 1999, 1, 4299. (40) Arleth, L.; Pedersen, J. S. Phys. ReV. E 2001, 63, 61406. (41) Roshan Deen, G.; Pedersen, J. S. Z. Metallkd. 2006, 97, 285.
Figure 6. Size distribution D(r) plots of microemulsion droplets for the various 1-chloroalkanes.
Table 2 summarizes the fit results for the microemulsion droplets of volume fraction 0.021 containing various 1-chloroalkanes, and the droplet size distribution is shown in Figure 6. The volumeequivalent radius, taken as the one defined at the neutral plane between the head and tail of the surfactant that forms the microemulsion droplets, increases from 72.3 Å for the system containing 1-chlorooctane as an oil to 81.3 Å for 1-chlorohexadecane. This change in droplet size is attributed to the change in curvature of the droplet as a result of the increasing hydrophobic character of the oil resulting in swelling of the droplets. The shape of the droplets (eccentricity e) also changes significantly with the increasing alkyl chain length of the 1-chloroalkanes. From an eccentricity of 1.5 for 1-chlorooctane, it evolves to an eccentricity of 1.98 for 1-chlorohexadecane. This change in microemulsion droplet shape is attributed to changes in the spontaneous curvature of the droplet and also to changes in the
C12E5 Nonionic Microemulsions with Chlorinated Oils
elastic constants of the surfactant film.42,43 Also note that the temperature at which the stable L1 microemulsion phase forms increases with increasing hydrophobicity of the 1-chloroalkanes. The relative polydispersity (σrelative) varied by about 3% with increasing alkyl chain length of the chlorinated oil. The area that the surfactant occupies at the polar-apolar interface (as) was also calculated according to the method described earlier.40 The results are summarized in Table 2. The head group area of the surfactant decreases with increasing hydrophobicity of the chlorinated oils, reflecting the swelling of the droplets. It can also be envisaged that an increase in the alkyl chain length of the oil can influence the elastic constants of the interfacial surfactant monolayer of the microemulsion droplet, thereby leading to changes in curvature.
Conclusions The effect of the oil chain length on the phase behavior and structure of a new type of non-ionic microemulsion of C12E5water-1-chloroalkane was investigated. The fish-plot phase diagram was used to study the phase behavior, and characteristic physical quantities were obtained. Striking differences in the (42) Balogh, J.; Olsson, U. J. Dispersion Sci. Technol. 2007, 28, 223. (43) Leaver, M.; Furo´, I.; Olsson, U. Langmuir 1995, 11, 1524.
Langmuir, Vol. 24, No. 7, 2008 3117
balance temperature and solubilizing capacity were observed on comparing with similar non-ionic microemulsions containing n-alkanes. The 1-chloroalkanes shifted the microemulsion balance temperature (T0) to about 14 °C lower than those of the corresponding n-alkane systems. This indicates that the 1-chloroalkanes behave as small molecules and can efficiently penetrate the surfactant film and lower the spontaneous curvature. Partitioning of the oil between the bulk and the surfactant film was also observed. SAXS studies show that the microemulsion droplets increase in size and the shape changed to slightly elongated droplets as a function of alkyl chain length. This effect is attributed to a combination of various effects such as the hydrophobic nature of oil, the change in spontaneous curvature, and elastic constants of the surfactant film. It is thus concluded that the behavior of non-ionic microemulsions of C12E5-water1-chloroalkane may be regarded as a continuation of the n-alkane series toward shorter alkyl chain length. Acknowledgment. We thank Ulf Olsson, Joakim Balogh, and Stefan Egelhaaf for stimulating discussions, Bente Olsen for assistance in the laboratory, and The Danish Natural Science Research Council (FNU) for financial support. LA703323N
