Phenomenological Properties and Phase Behavior of

Phenomenological Properties and Phase Behavior of. Benzylalkyldimethylammonium Salts in the Presence of. Benzene and Electrolyte Solutions. Noritaka ...
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Langmuir 2001, 17, 3829-3835

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Phenomenological Properties and Phase Behavior of Benzylalkyldimethylammonium Salts in the Presence of Benzene and Electrolyte Solutions Noritaka Ohtani,* Yoshiyuki Morimoto, Masao Naitou, Yuki Kasuga, and Daisuke Tsuchimoto Department of MaterialssProcess Engineering and Applied Chemistry for Environments, Faculty of Engineering and Resource Science, Akita University, Akita, 010-8502 Japan Received October 26, 2000. In Final Form: April 5, 2001 The solubilities of benzylalkyldimethylammonium salts (BADAX) in benzene were characterized by the specific Krafft boundaries, depending on their molecular structures. BADAX affords not only the Krafft boundary but also the immiscibility gap in water or aqueous electrolyte solutions. BADAX formed aggregates in aqueous solution or in benzene when its concentration was above the critical solubility concentration (csc) of the Krafft boundary. It is assumed that the aggregate structure varies successively with the change in electrolyte concentration or oil-water ratio. The phase behavior of the mixture of BADAX, benzene, water, and electrolyte was examined as a function of temperature or a function of component composition. The conditions under which a microemulsion phase appears or coexists with other phases are clarified, and the change in aggregate structure is discussed, on the basis of the solubility and immiscibility gap.

Introduction Quaternary ammonium salts have been used as the catalysts of phase-transfer catalysis (PTC)1 or as the catalytic moieties of the corresponding polymer-supported phase-transfer catalysis.2 Previously the active species of PTC was assumed to be a monomeric ion pair that is extracted into the organic layer under liquid-liquid twophase conditions. However, our early studies on polymersupported phase-transfer catalysts suggested that a specific microstructure like reverse micelles was formed within the insoluble catalyst polymer matrix.3 Benzylalkyldimethylammonium salts are one of the quaternary salts that have been most often used for being attached to an insoluble polymer matrix. The solution behavior and reactivity of the corresponding linear polymers containing the quaternary salts (cationic ionomers) has shown that the quaternary salts aggregate in nonpolar oils.4 Furthermore, the recent development of studies on both theoretical and experimental aspects of microemulsions has provided a better understanding of the formation, properties, and phase behavior of microemulsions, which include the w/o microemulsions made of cationic surfactants.5,6 It has already been reported that benzylhexadecyldimethylammonium chloride (BHDACl) (1) (a) Starks, C. M.; Liotta, C. Phase Transfer Catalysis Principles and Techniques; Academic Press: New York, 1978. (b) Weber, W. P.; Gokel, G. W. Phase Transfer Catalysis in Organic Synthesis; SpringerVerlag: Berlin, 1977. (c) Dehmlow, E. V.; Dehmlow S. S. Phase Transfer Catalysis, 2nd ed.; Verlag Chemie: Weinheim, 1983. (d) Makosza, M. Pure Appl. Chem. 1975, 43, 439. (e) Brandstrom, A. Adv. Phys. Org. Chem. 1977, 15, 267. (2) (a) Regen, S. L. Angew. Chem. 1979, 91, 464. (b) Montanari, F.; Landini, D.; Rolla, F. Top. Curr. Chem. 1982, 101, 147. (c) Hodge, P.; Sherrington, D. C. Polymer-supported Reactions in Organic Synthesis; John Wiley & Sons: New York, 1980. (3) (a) Ohtani, N.; Wilkie, C. A.; Nigam, A.; Regen, S. L. Macromolecules 1981, 14, 516. (b) Ohtani, N.; Regen, S. L. Macromolecules 1981, 14, 1594. (4) (a) Ohtani, N.; Inoue, Y.; Mizuoka, H.; Itoh, K. J. Polym. Sci., Polym. Chem. Ed. 1994, 32, 2589. (b) Ohtani, N.; Nakaya, M.; Shirahata, K.; Yamashita, T. J. Polym. Sci., Polym. Chem. Ed. 1994, 32, 2677. (c) Ohtani, N.; Inoue, Y.; Kaneko, Y.; Okumura, S. J. Polym. Sci., Polym. Chem. Ed. 1995, 33, 2449. (d) Ohtani, N.; Inoue, Y.; Kaneko, Y.; Sakakida, A.; Takeishi, I.; Furutani, H. Polym. J. 1996, 28, 11.

forms w/o microemulsions at ambient temperature in benzene and other aromatic hydrocarbons.7 However, an understanding of the thermodynamics of microemulsion systems has not yet progressed to the point of being able to predict all the details of the phase behavior of PTC reaction systems. The usual PTC system contains as many as seven components, among which quaternary salt, oil, water, and inorganic salt are the main components that determine the phase equilibrium. The reaction temperature is usually much higher than ambient temperature. The PTC systems contain a relatively small amount of quaternary salts as well as an almost concentrated solution of aqueous inorganic salts, the volume of which is approximately equal to that of oil. Thus, the phenomenological knowledge about the phase behavior is essential for elucidating the kinetics and mechanism of PTC as well as the microstructure formed in each layer of PTC systems. Recent development of PTC studies has already implied that one particular equilibrium state should not be premised when a given quaternary salt is mixed with an aqueous solution and a water-immiscible organic solvent.8 Recently, we reported on the phase behavior of tetrabutylammonium salts (TBAX) in oils or in aqueous electrolyte solutions.9 They afford Krafft boundaries in oils, which are closely dependent on the oil molecular volume and on their counterions. The behavior was rather similar to the solubility of common ionic surfactants in (5) (a) Evans, D. F.; Mitchell, D. J.; Ninham, B. W. J. Phys. Chem. 1986, 90, 2817. (b) Chen, S. J.; Evans D. F.; Ninham, B. W.; Mitchell, D. J.; Brum, F. D.; Pickup, S. J. Phys. Chem. 1986, 90, 842. (c) Kawai, T.; Hamada, K.; Kon-No, K. Bull. Chem. Soc. Jpn. 1993, 66, 2804. (d) Eastoe, J.; Hetherington, K. J.; Dalton, J. S.; Sharpe, D.; Lu, J. R.; Heenan, R. K. J. Colloid Interface Sci. 1997, 190, 449. (e) Leung, R.; Shah, D. O. J. Colloid Interface Sci. 1987, 120, 320. (f) Leung, R.; Shah, D. O. J. Colloid Interface Sci. 1987, 120, 330. (6) (a) Jada, A.; Lang, J.; Zana, R. J. Phys. Chem. 1990, 94, 381. (b) Jada, A.; Lang, J.; Zana, R.; Makhloufi, R.; Hirsch, E.; Candau, S. J. J. Phys. Chem. 1990, 94, 387. (c) Verrall, R. E.; Milioto, S.; Zana, R. J. Phys. Chem. 1988, 92, 3939. (7) (a) McNeil, R.; Thomas, J. K. J. Colloid Interface Sci. 1981, 83, 57. (b) Borsarelli, C. D.; Previtali, C. M.; Cosa, J. J. J. Colloid Interface Sci. 1996, 179, 34. (c) Chatenay, D.; Urbach, W.; Cazabat, A. M.; Langevin, D. Phys. Rev. Lett. 1985, 54, 2253.

10.1021/la001508i CCC: $20.00 © 2001 American Chemical Society Published on Web 05/30/2001

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water. The quaternary salts afford a lower consolute behavior in aqueous solutions. The behavior was very similar to those of micelle-forming quaternary ammonium surfactants.10 We also disclosed that, under adequate conditions, TBAX/benzene/water/NaBr four-component systems form microemulsions, with which an oil phase and aqueous phase coexist depending on temperature and constituent composition.11 These similarities in phase behavior between symmetric quaternary ammonium molecules and unsymmetrical surfactant molecules somewhat contradict the significance to classify them. It may be necessary to further clarify the relationship between amphiphile structure and property. In this article, we examine the phase behavior of benzylalkyldimethylammonium chlorides (BADACl; alkyl (A) ) butyl (B), octyl (O), dodecyl (D), tetradecyl (T), and hexadecyl (H)) and benzylhexadecyldimethylammonium salts (BHDAX; X ) Cl, Br, I, SCN, OAc, and OMs) as a function of temperature or composition. We show how the parameters, such as temperature, alkyl chain length of quaternary cation, counteranion, or electrolyte concentration, influence the formation of aggregates or microemulsions. The conditions where the phase containing these aggregates coexists with other phases under the BADAX/ benzene/water/KX four-component systems are given to correlate to the corresponding PTC systems that involve these quaternary salts.

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Figure 1. Solubility of BADAC in benzene. The label, f(BADACl), represents the weight fraction of BADACl in the mixture. The region above the solubility curves is an isotropic reverse-micellar solution (L2). The region at low temperatures below the solubility curves is a liquid-solid two-phase region (O + Q). Q denotes a solid phase of quaternary ammonium salts. O denotes an oil phase that coexists with other phase(s) and that contains a small amount of ammonium salts. A broken line represents the solubility curve of BBDACl.

Results and Discussion Solubility of BADACl in Benzene and Aggregate Formation. Simple solubility measurements supply valuable information regarding the aggregation of quaternary salts in oils. The solubility of BADACl in benzene is shown in Figure 1 as a function of temperature. The solubility curve shows a striking resemblance to those of ionic surfactants in water, which are characterized by Krafft point and cmc. This type of solubility curve suggests that the quaternary salts form an aggregate in the medium, as is the case for a number of ionic surfactants in water.12 In benzene, accordingly, BHDACl is assumed to form a reverse-type aggregate (L2 phase) instead of a normal micelle above the critical concentration. It is also noted that the solubility behavior of BHDACl is similar to that of symmetric tetrabutylammonium salts in benzene while the bending of the solubility curve is much sharper for BHDACl.9 Compared with BHDACl, BTDACl gave a slightly lower Krafft point and a somewhat obscure bending in the solubility curve (Figure 1). This tendency was more strengthened for BDDACl. The BTDACl and BDDACl may form aggregates with a smaller association number or with a wider size polydispersity. On the other hand, BODACl showed a rather high Krafft point. BBDAC was (8) (a) Wang, D.-H.; Weng, H.-S. Chem. Eng. Sci. 1995, 50, 3477. (b) Mason, D.; Magdassi, S.; Sasson, Y. J. Org. Chem. 1991, 56, 7229. (c) Ido, T.; Yamamoto, T.; Jin, G.; Goto, S. Chem. Eng. Sci. 1997, 52, 3511. (d) Holmberg, K. Adv. Colloid Interface Sci. 1994, 51, 137. (e) Cerichelli, G.; Mancini, G.; Luchetti, L.; Savelli, G.; Bunton, C. A. Langmuir 1994, 10, 3982. (f) Menger, F. M.; Elrington, A. R. J. Am. Chem. Soc. 1991, 113, 9621. (g) Ohtani, N.; Inoue, Y.; Shinoki, N.; Nakayama, K. Bull. Chem. Soc. Jpn. 1995, 68, 2417. (h) Ohtani, N.; Inoue, Y.; Mukudai, J.; Yamashita, T. In Phase-Transfer Catalysis; Halpern, M. E., Ed.; ACS Symposium Series, Vol. 659; American Chemical Society: Washington, DC, 1996; Chapter 19. (9) Ohtani, N.; Hosoda, Y. Bull. Chem. Soc. Jpn. 2000, 74, 2263. (10) (a) Buckingham, S. A.; Garvey, C. J.; Warr, G. G. J. Phys. Chem. 1993, 97, 10236. (b) Kubota, K.; Kuwahara, N.; Sato, H. J. Chem. Phys. 1994, 100, 4543. (11) Ohtani, N.; Yamashita, T.; Hosoda, Y. Bull. Chem. Soc. Jpn. 2000, 74, 2269. (12) Ohtani, N. Stud. Surf. Sci. Catal. 2001, 132, 189.

Figure 2. Influence of BADAX concentration on the 1H NMR chemical-shift difference between the benzyl methylene proton and the N-methyl proton of BADAX in benzene-d6 at 60 °C.

hardly soluble in benzene, as shown roughly by the broken line in Figure 1. Other BHDAXs similarly gave their individual Krafft temperatures in benzene, depending on their counteranions.12 The 1H NMR analysis using the method of Lindman13 is useful to examine the change in state of quaternary salts in benzene. The chemical-shift difference of two singlet peaks, benzyl methylene proton and N-methyl proton of BADAX, is plotted against the inverse BADAX concentration, as shown in Figure 2. If only monomer and aggregates with a definite association number are present, the plots should be the two straight lines that intersect at the critical solubility concentration. The results rather show the gradual change in aggregate state with the concentration. The chemical-shift difference began to decrease at some concentrations that were slightly lower than the bending point of the solubility curve in Figure 1. The appearance of a wider transition region for BODACl probably indicates a more gradual growth of reversedtype aggregates and/or a wider size polydispersity of the aggregates.14 The shifting of the peak position of the residual water was also observed; the peak identified as free water in benzene (0.41 ppm) at low BADAX concen(13) Persson, B.-O.; Drankenberg, T.; Lindman, B. J. Phys. Chem. 1979, 83, 3011. (14) Moroi, Y.; Matuura, R. Bull. Chem. Soc. Jpn. 1988, 61, 333.

Properties and Phase Behavior of BADAX

Figure 3. Solubility of the BHDACl/BBDACl mixture in benzene. The sum of the weight fractions of BHDACl and BBDACl is represented by f(Q). Symbols for phases are the same as in Figure 1. The region surrounded by a broken line represents a liquid-solid two-phase region (O + Q) of the 52 mol % BBDACl system.

Figure 4. Solubility of BADAX in water. The region above the solubility curves is an isotropic micellar solution (L1). The region at low temperatures below the solubility curves is a solidliquid two-phase region (Q + W). Q denotes a solid phase of quaternary ammonium salts. W denotes an aqueous phase that coexists with other phase(s) and that contains a small amount of ammonium salts.

trations moved downfield with an increase in BADAX concentration. This indicates that, at low BADAX concentrations in the absence of reversed-type aggregates, most of the water in the solution is freely dissolved in bulk benzene and that, at high BADAX concentrations, water resides in the BADAX aggregates.15 Merely through solubility measurement, it is possible to ascertain the presence of aggregates. Figure 3 shows the solubility behavior of the mixture of BHDACl and BBDACl in benzene. Interestingly, the solubility of BHDACl was rather increased by the addition of benzeneinsoluble BBDACl. This is due to the lowering of the Krafft point. BBDACl is dissolved in benzene accompanied by the dissolution of BHDACl, clearly indicating that BHDACl forms aggregates. In fact, at such low concentrations as those at which BHDACl was present in a monomer form, the solid, excess BBDACl remained undissolved in benzene, which is shown as a broken line in the Figure 3. BBDACl probably functions as a cosurfactant in a similar manner to that of alcohols. Phase Behavior of BADAX in Water or Aqueous KX Solution. As shown in Figure 4, the solubility behavior of BADACl in water was similar to those of common ionic surfactants that were characterized by the Krafft bound(15) (a) Heatley, F. J. Chem. Soc., Faraday Trans. 1 1988, 84, 343. (b) Seno, M.; Sawada, K.; Araki, K.; Iwamoto, K.; Kise, H. J. Colloid Interface Sci. 1980, 78, 57.

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Figure 5. Effect of the concentration of the KCl aqueous solution on the solubility of BHDACl. L1, Q, and W denote an isotropic micellar solution, a solid phase of quaternary ammonium salts, and an aqueous phase that contains a small amount of ammonium salts, respectively. Broken lines represent the phase boundaries of the system with the 10 wt % KCl aqueous solution. L is a so-called microemulsion phase that is an isotropic quaternary ammonium salt-rich liquid.

Figure 6. Solubility of BODACl in the 12.5 wt % KCl aqueous solution. L denotes a liquid isotropic quaternary ammonium salt phase. The W phase that coexists with the L phase is assumed to be an aqueous micellar solution (L1).

ary. The liquid-solid biphase was converted into a uniphase (L1) at a relatively constant temperature. A decrease in the alkyl chain length of BADACl lowered the Krafft temperature: BHDACl > BTDACl > BDDACl. The Krafft boundaries of BODACl and BBDACl were not observed. As shown in Figure 5, the Krafft point of BHDACl is elevated by an increase in the KCl concentration. The bending of the solubility curve became sharper and the concentration of BHDACl at the bending point became lower, indicating that the cmc was lowered. When the KCl concentration was higher than 7.5 wt %, BHDACl no longer formed a homogeneous solution unless the BHDACl concentration was extremely low. A liquid-liquid twophase separation (L + W) occurred beyond 39 °C with a 10 wt % KCl aqueous solution, as shown by the broken line in Figure 5. L denotes a liquid phase formed by quaternary ammonium salts. W represents an excess aqueous electrolyte solution. BHDACl melted at 39 ˚C, but the melt was not fully miscible with the KCl aqueous solution. The diagram with a 10 wt % KCl aqueous solution was very similar to that reported for the tetrabutylammonium iodide-water system9 and the tetradecyltripentylammonium bromide-water system.10 In the presence of KCl, one can observe the Krafft boundary of BODACl. Figure 6 shows the phase behavior of BODACl with a 12.5 wt % KCl aqueous solution. At BODACl weight fractions higher than 0.07, a liquid-liquid

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Figure 7. Effect of an addition of KOAc on the solubility of BHDAOAc in water. The original concentration of BHDAOAc was 0.125 M before the addition of KOAc. The notation of phases is the same as in Figure 5.

biphase (L + W) occurred on warming. It is noted that, at BODACl weight fractions from 0.05 to 0.07, the system affords a lower consolute boundary corresponding to the change from L1 to L + W on warming. At a certain temperature, the system suddenly became cloudy; that is, the cloud point was observed. However, further elevation of temperature changes the system again back to a homogeneous L1 solution. This indicates the presence of an upper consolute boundary as well. This also suggests that the liquid-liquid immiscibility gap is a closed-loop, as reported for the tetraisopentylammonium bromide/ water system.16 The solubility in aqueous solutions is also dependent on the counterions. The solubility temperature of BHDABr in the absence of KBr was 41.0 °C, which was not so high compared to the corresponding temperature of 31.0 °C for BHDACl. However, the addition of as low as 2 wt % KBr was enough to induce an L + W phase separation. The solubility of BHDAI in water was very poor. At ambient temperature, BHDAI is hardly soluble in water and solid BHDAI coexists with almost pure water. The solid melts at 56.5 °C to give a liquid-liquid two-phase separation at higher temperatures. An addition of KI heightened the melting point, leaving the solid BHDAI undissolved even at considerably high temperatures. BHDAOAc is very soluble in water. It affords a homogeneous solution even at 5 °C irrespective of its concentration. However, we could observe the Krafft point in the presence of KOAc (Figure 7). At the weight fractions of KOAc between 0.36 and 0.37, furthermore, a cloud point was observed, at which temperature the system becomes cloudy due to an L + W two-liquid separation. As shown in Figure 8, the isothermal phase diagrams of BHDACl/water/KCl, BHDAOAc/water/KOAc, and BODACl/water/KCl three-component systems resemble each other; there is commonly a L + W two-phase region. The change in state from L to L + W (line ab) occurred at a relatively constant KX weight fraction in water irrespective of BADAX content. This is the case particularly for BHDACl, as already mentioned. As the W phase along the ac boundary and the L phase adjacent to the zero-electrolyte line are both L1, it seems that the microstructure of the L phase varies with the electrolyte concentration. The L region of BHDAOAc is much wider than that of BHDACl, indicating that the high dissociating ability of acetate ions in water may retard the progressive growth of the aggregate size. (16) Weingartner, H.; Steinle, E. J. Phys. Chem. 1992, 96, 2407.

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If the temperature was above the Krafft point, the effect of temperature on the ternary phase diagrams was not significant except that the point, c, corresponding to the saturated KX aqueous solution, moved upward with the temperature elevation. For the systems that show the lower consolute boundary (BHDAOAc and BODACl systems), however, the point a, which probably corresponds to the plait point, slightly moves toward the water corner with the temperature elevation. Phase Behavior of BADAX/Benzene/Water/KX Four-Component Systems. An addition of water to the BADAX/benzene system usually lowered the Krafft temperature if the counteranion X is hard. Therefore, it becomes possible that, even at ambient temperatures, we make a homogeneous solution if the amount of water is within the solubilization limit. As shown in Figure 9, the BODACl/benzene/water threecomponent system affords a broad homogeneous liquid one-phase region (L), which is attached both to the zero benzene line (L1) and to the zero water line (L2). This means that the microstructure of the L phase successively varies with oil-water ratio. Excess oil (O) coexisted with the quaternary-salt phase (L). This phase separation (O + L) is often referred to as Winsor I type. BODACl always gave two clearly separated transparent phases when phase separation took place. Some tie lines obtained from 1H NMR analysis are drawn in Figure 9. The distinct region of a liquid-liquid-liquid (O + L + W) three-phase equilibrium was not found for any threecomponent systems of BADAX/benzene/water. However, an addition of electrolyte, KX, as a fourth component sometimes induced this type of phase separation (Table 1). Except for the case of BODACl, the electrolyte concentration that gave the O + L + W three-phase separation was restricted to a limited range and the system was converted to an L + W state on adding another small quantity of electrolyte. The presence of KX made it easy to analyze the phase behavior. For the BHDAOAc/benzene/water system, a minimal amount of KOAc was enough to make a clear two-phase separation (O + L) from the stable emulsion formed by a BHDAOAc/benzene/water system. The bottom layer was a slightly bluish microemulsion phase (L). The influence of KOAc on the phase transition is shown in Figure 10. The system transformed its state from O + L to O + L + W with an increasing concentration of KOAc. Further addition of KOAc changed the state into an L + W phase equilibrium. On the other hand, an elevation of temperature tends to vary its state from O + L, through O + L + W, to L + W. This temperature dependence is similar to those of nonionic surfactants17 and also to those of tetrabutylammonium salts (TBAX).11 However, it was the reverse that was reported for the dioctyldimethylammonium bromide/toluene/water/NaBr system.18 The change in the volume ratio of each layer at 60 °C is also given in Figure 10. The W phase appears suddenly at a certain KOAc concentration, and the O phase disappears at another certain KOAc concentration. This suggests that a sudden change in the microstructure of the L phase occurs beyond and below some KOAc concentrations. Figure 11 shows the partial phase diagram of the BHDAOAc/benzene/water/KOAc four-component system at 60 °C. The volume ratio of benzene to water was maintained constant at 50:50. The change in state along (17) (a) Shinoda, K. Prog. Colloid Polym. Sci. 1983, 68, 1. (b) Shinoda, K.; Friberg, S. Emulsion and Solubilization; Wiley-Interscience: New York, 1986. (18) Kahlweit, M.; Strey, R.; Schomacker, R.; Haase, D. Langmuir 1989, 5, 305.

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Figure 8. Partial ternary phase diagram of BADAX/water/KX at 60 °C. The scales are shown in weight fraction of each component. The notation of phases is the same as in Figure 6. S represents a KX solid crystalline phase.

Figure 9. Partial ternary phase diagram of BODACl/benzene/ water at 60 °C. L represents a microemulsion phase. O denotes an excess oil phase that coexists with the L phase. Some tie lines that are obtained by 1H NMR analyses are shown. The mixtures with the compositions represented by filled squares were equilibrated, and the compositions of the resulting two phases were shown by open squares. Table 1. Effect of Electrolyte Concentration on the Phase Transition of BADAX/Benzene/Water/KX Systems at 60 °Ca wt fraction of KX in water, f(KX)/(f(water) + f(KX))

BADAX BHDASCN BHDAI BHDABr BHDAOAc BHDAOMs BHDACl BTDACl BDDACl BODACl

Figure 10. Phase transition of the BHDAOAc/benzene/water/ KOAc four-component system depending on KOAc concentration and the change in the volume fraction of each layer at 60 °C. The system was made of 0.50 mmol of BHDAOAc, 2 mL of benzene, and 2 mL of a KOAc aqueous solution. O and W represent an excess benzene phase and an excess aqueous phase, respectively. L is a microemulsion phase. O + L + W means that a microemulsion phase (L) is in equilibrium with an excess oil (O) and an excess electrolyte aqueous phase (W).

I: I: I: I: I: I:

0.05 0.019 (0.016) 0.007 0.015 0.04

II: II: II: II: II:

0.21 0.037 0.019 0.014 0.078

III III III III III III III III IV

a

BADAX, 0.1 mmol; benzene, 1.5 mL; water, 2.0 mL. Phase transition type I, O + L/O + L + W; II, O + L + W/L + W; III, L + W/L + W + S; IV, O + L + W/O + L + W + S. Phase transitions of type III and IV took place closely at the saturated concentration of KX in water at 60 °C.

the dotted line ab roughly corresponds to the results as already shown in Figure 10. When the KOAc weight fraction in water was between 0.05 and 0.21, the addition of BHDAOAc forced the system to change from a liquidliquid biphase, via O + L + W, to L + W along the line cd, for an example. After a middle layer (L phase) appeared, its volume increased with a decrease in the volume of the upper O layer until the L phase entirely absorbed the O phase to form an L + W equilibrium. When the weight fraction of KOAc in water was higher than 0.21, the system kept a two-phase equilibrium even if BHDAOAc was added. No three-phase separation was observed. It can be estimated that most of the BHDAOAc added was dissolved in the upper benzene layer with imbibing water from the bottom W phase, leading to the formation of the L phase. On the other hand, BHDAI or BHDASCN affords an L + W biphase with benzene and water even in the absence

Figure 11. Phase diagram of the BHDAOAc/benzene/water/ KOAc four-component system at 60 °C. The volume ratio of benzene to water was kept at 50:50. S denotes the KOAc solid phase. L + W + S means that a microemulsion phase (L) is in equilibrium with an excess KOAc aqueous phase (W) and a solid KOAc phase.

of the corresponding electrolytes. The L phase was an oil-continuous w/o microemulsion (L2). Therefore, an addition of KX to the BHDAX/benzene/water threecomponent system did not change the equilibrium state. The added salt was merely dissolved in the water layer. Further addition of KX leads to the precipitation of the excess amount of KX beyond its solubility in water. As shown in Figure 12, the partial phase diagram of a BODACl/benzene/water/KCl four-component system at 60 °C is greatly different from that of the BHDAOAc system; an O + L + W + S four-phase body is present. The diagram was very similar to that of TBABr/benzene/water/NaBr.11

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behavior of the BHDAX/water/KX system. There was a lower consolute boundary. On warming, an L1 phase liberates a W phase to form an L + W biliquid. The hydrophobic hydration to the o/w droplets may be destroyed by the temperature elevation, leading to an increase in the attractive interaction among o/w droplets. The structure of the interface film in an L phase may vary from an o/w to a bicontinuous type because of an increase in the surfactant-tail volume when temperature is elevated.5a An absorption of oil by the surfactant tails further increases their volume. Thus, an L phase tends to liberate an aqueous W phase (Figure 10) and to absorb an oil O phase on warming. Conclusions Figure 12. Phase diagram of the BODACl/benzene/water/KCl four-component system at 60 °C. The volume ratio of benzene to water was kept at 50:50. The notation of phases is the same as in Figure 11 except that S means the KCl solid phase. A four-phase equilibrium, O + L + W + S, means that a microemulsion (L) coexists with an excess oil (O), an excess aqueous solution (W), and an excess KCl solid (S).

In this study, we could observe an O + L + W + S fourphase equilibrium state only for the BODACl system (Table 1). The four-phase region is located at high KCl fractions and low BODACl fractions of the diagram. The presence of the four-phase body means that this L phase cannot absorb the whole amount of benzene even under the conditions where the coexisting W phase is a saturated KCl aqueous solution. The sequence of phase equilibrium along line gh was greatly different from that along line ef. The disappearance of the O phase along line gh was gradual. An incremental addition of BODACl to a benzene/ water/KCl system (along line ij) always leads to the change in state from O + W, through O + L + W, to L + W unless the KCl concentration is too low. The sudden change in state according to the KOAc concentration (Figure 10) strongly suggests the structural change of the L phase. The sharp increase in the miscibility with oil at least shows that the structure above 21 wt % KOAc is an oil-continuous w/o type (L2). The sharp increase in the miscibility with water below 5 wt % KOAc shows that a water-continuous o/w structure (L1) is formed. The addition of electrolytes to an O + L1 biphase reduces the electrostatic repulsion between o/w droplets in the L1 phase and also changes the surfactant layer curvature to less positive. The aggregates grow in average size. At a certain electrolyte concentration, a two-phase separation occurs to form a O + L + W three-phase. The resulting L phase may be a bicontinuous microemulsion. Further addition of KX increases the KX content in the L phase and reduces the water content, leading to a w/o interface (L2). If, beyond a certain KOAc concentration, the entropy effect to dilute w/o droplets exceeds the relatively weak interaction between w/o droplets, benzene is unlimitedly absorbed by the L phase to form an L2-W state. On the other hand, the L phase made by BODACl may persist in keeping a bicontinuous form even with the concentrated KCl aqueous solution because of the relatively strong interaction between w/o droplets. Thus, the system affords an O + L + W + S quaternary phase. If the content of BODACl in the bicontinuous L phase is high enough to absorb the whole amount of benzene (O phase), the system gives an L + W biphase while keeping the bicontinuous structure. The effect of temperature on the phase behavior for the BHDAOAc/benzene/water/KOAc system was the reverse of that anticipated for common ionic surfactants.18 The behavior was similar to that of nonionic surfactants. This is related to the temperature dependence of the phase

Simple drawing of the solubility curves offers valuable information about the aggregation of BADAX in benzene. The solubility behavior in aqueous solution was characterized by the Krafft boundary and immiscibility gap. Above the critical solubility concentration, BADAX aggregates in the medium. The aggregates can absorb oil or aqueous solution, leading to the formation of a microemulsion (L phase). When the L phase is assumed to be an o/w (L1) phase, the L phase unlimitedly imbibes a dilute electrolyte solution against the short-range attractive interaction between the o/w droplets. When the electrolyte concentration is beyond a certain value, however, the attractive interaction sets a limit to the absorption of the aqueous solution and the L1 phase separates into a liquidliquid L + W biphase. When the L phase is assumed to be a w/o microemulsion, on the other hand, the L phase unlimitedly imbibes oil while the absorption of aqueous solution is limited. When the L phase is assumed to be bicontinuous, the L phase can imbibe considerable amounts of both oil (O phase) and aqueous solution (W phase). If either phase is completely absorbed by the L phase, the O + L or L + W phase equilibrium is realized. If the volumes of the O and W phases exceed the absorption limits, an O + L + W three-liquid phase equilibrium is realized. The type of L phase was controlled by the alkyl chain length and counterion of BADAX, the concentration of inorganic salt (KX), and the temperature. Experimental Section Materials and Equipment. All of the solvents and inorganic reagents are commercially available and guaranteed reagent grade. Deionized water was used throughout the experiments. BHDACl and BTDACl were purchased from Tokyo-Kasei and used without further purification. GLC analysis was done using a Hitachi Model 163 FID instrument with an 1 m column of SE-30 or PEG-20M at 170 °C. 1H (270.05 MHz) NMR spectra were recorded on a JEOL EX-270 spectrometer or a Varian Mercury 300. Chemical shifts were referenced to proton impurities in deuterized benzene (δ 7.20) and were reported downfield of TMS. Preparation of Quaternary Salts. BDDACl, BODACl, and BBDACl were synthesized by the quaternization of N,Ndimethyldodecylamine, N,N-dimethyloctylamine, and N,N-dimethylbutylamine with benzyl chloride, respectively. The quaternization reactions were performed in toluene at about 70 °C for 36-48 h. They were recrystallized from benzene or ethyl acetate three to four times and dried under vacuum at 60 °C. The purity was checked by 1H NMR using deuterized chloroform as a solvent. BHDAOAc was prepared through the ion exchange of BHDACl. BHDACl was mixed with a large excess of potassium acetate in water at 60 °C. The mixture was cooled to precipitate the quaternary salts. The recovered quaternary salts were treated with aqueous potassium acetate solution several times in the same way. To remove potassium acetate, the obtained white solid was dissolved in dichloromethane, filtered, evaporated, and then recrystallized from benzene. BHDAOMs was prepared by the

Properties and Phase Behavior of BADAX reaction of BHDACl with methyl methanesulfonate in benzene. BHDAX compounds with other counterions (X ) Br, I, and SCN) were prepared through the extraction method (BHDACl in dichloromethane was treated several times with a concentrated aqueous solution of the corresponding potassium salt). The organic layer was evaporated, and the recovered quaternary salt was purified in the same way. The purities of the prepared quaternary salts were determined by GLC analyses of the decyl derivatives that were formed by the reaction of decyl methanesulfonate with the quaternary salts.4 All the quaternary salts had purities beyond 99%. Solubility of BADAX. The solubility of BADAX in oil was measured in the following way. Given amounts of BADAX and benzene were added to a 10 mL Teflon-coated screw-capped test tube with an inside diameter of 10.5 mm. The tube was transferred to a variable-temperature water bath, and the mixture was stirred with magnetic stirring. The temperature was raised at a rate of 1 °C min-1. The temperature was read when all the BADAX crystalline solid or BADAX-rich liquid disappeared. The solubility of BADACl in a KCl aqueous solution was measured in the same way. The temperature was read when all of the BADAX crystalline solid disappeared. Generally, it takes a long time to precipitate crystalline BADACl from the supercooling mixture. Thus, the mixture was sometimes cooled below 0 °C to solidify the whole system and allowed to stand at 2 °C for solid crystals to be precipitated. The cloud points of BADACl in aqueous KCl solution were measured in such a way that the temperature was raised at a rate of 1 °C min-1 and the temperature of the phase transition was read when the solution became turbid. The temperature uncertainty about the above experiments was (0.3 °C. Phase Equilibrium. Given amounts of a quaternary salt (BADAX), an aqueous KX solution, and benzene were added to a 10 mL Teflon-coated screw-capped test tube with an inside diameter of 10.5 mm. The mixture was vigorously stirred with

Langmuir, Vol. 17, No. 13, 2001 3835 a tube shaker and maintained at a given temperature for 10 min. When the mixture was homogeneous, a small amount of one component or a homogeneous solution with a given concentration was added successively via microsyringe accurate to within 1 µL. The mixture was stirred each time with a tube shaker until phase separation took place. The phase transition was determined as that point where the first visual evidence of phase separation occurred, that is, the solution became cloudy. When the system consisted of multiphases at the temperature, the tube was shaken vigorously with a tube shaker again and allowed to stand still to wait for phases to clearly separate at the temperature. The composition of the mixture was then slightly varied by adding a small amount of one component or a homogeneous solution with a given concentration. The composition uncertainty was (0.005 in weight fraction. To measure the volume of each separated phase, a calibrated 10 mL screw-capped test tube with an inside diameter of 10.5 mm was used. After a heterogeneous mixture was vigorously stirred with a tube shaker, it was allowed to stand in a temperature-controlled bath maintained at a given temperature. The volume of each phase was read after the phases clearly separated. After the volume of each layer was measured at one composition, a given amount of one component or their homogeneous mixture was added via microsyringe to vary the composition. The uncertainty of the calculated volume fraction was estimated to be (0.05. Without measuring the composition of each phase, the type of phase separation could be easily judged from the change in volume ratio of the separated phases. If it was necessary to analyze the phase composition of a liquid layer, an aliquot of the layer was added to a given amount of methanol-d4 and the composition was determined by 1H NMR on the basis of the residual proton in the methanol.

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