3986
Langmuir 1996, 12, 3986-3990
Pressure-Area Isotherms for Double-Chain Amphiphiles Bearing Two Hydroxyl Groups Derived from Diepoxides Yasushi Sumida,† Araki Masuyama,‡ Toshihiro Oki,‡ Toshiyuki Kida,‡ Yohji Nakatsuji,‡ Isao Ikeda,*,‡ and Masatomo Nojima‡ Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565, Japan, and Cosmetic Laboratory, Kanebo Corporation, Kotobuki-cho 5-3-28, Odawara, Kanagawa 250, Japan Received March 20, 1996. In Final Form: May 20, 1996X The pressure-area (π-A) isotherms for a series of double-chain amphiphiles bearing two hydroxyl groups were measured by the conventional film-balance technique. These double-chain compounds were prepared by reaction of epichlorohydrin with various dihydroxy compounds which become the connecting moiety, followed by reaction of the resultant diepoxides with fatty alcohols. They are synthetic precursors of double-chain surfactants bearing two anionic head groups which have been developed as high-performance “gemini surfactants” by the authors’ group. The effect of the following factors on the π-A isotherms for these compounds was examined: the length of the saturated hydrophobic alkyl chains, the difference between saturated and unsaturated aliphatic chains, temperature, the structure of the connecting group between the two hydrophobic chains, and the difference in configuration of the chiral molecules. Some relationships between the structures of these double-chain amphiphiles and their adsorptive behavior at the air-water interface were clarified: (1) A more tightly packed monolayer was formed as the length of hydrophobic alkyl chain increased and as the length of the alkylene chain of the connecting part was shortened, (2) an unsaturated bond in the middle of the hydrophobic chain contributed to loose packing of the monolayer, (3) the profile of the π-A isotherms varied with temperature, and (4) no influence of chirality in the molecule on the adsorptive behavior was observed.
Introduction Because almost all the surfactants in general use consist simply of one hydrophobic alkyl chain and one hydrophilic head group, there are limitations to modifying or improving the surface-active properties within the range of the above-mentioned structure. For example, one will be faced with a contradiction that if the micelle-forming property is improved by increasing the alkyl chain length, the water solubility then becomes worse. A new strategy for molecular design free from the conception of a singlechain surfactant is therefore required to break through these problems. We have developed new types of double-chain amphiphiles bearing two ionic head groups. It was found that the double-chain surfactants (2) derived from diepoxy compounds, of which the structure is shaped as a bundle of two single-chain surfactant molecules in appearance, have higher hydrophilicity, lower critical micelle concentration, and equal or greater ability to lower surface tension than conventional single-chain surfactants bearing the corresponding one ionic head group1 (Scheme 1). Double-chain surfactants bearing two hydrophilic ionic head groups, which are generally called “gemini surfactants”,2 have attracted the attention of many researchers. For example, Menger et al. synthesized some “gemini †
Kanebo Corporation. Osaka University. X Abstract published in Advance ACS Abstracts, July 1, 1996. ‡
(1) (a) Okahara, M.; Masuyama, A.; Sumida, Y.; Zhu, Y.-P. J. Jpn. Oil Chem. Soc. (Yukagaku) 1988, 37, 746. (b) Zhu, Y.-P.; Masuyama, A.; Okahara, M. J. Am. Oil Chem. Soc. 1990, 67, 459. (c) Zhu, Y.-P.; Masuyama, A.; Okahara, M. J. Am. Oil Chem. Soc. 1991, 68, 268. (d) Zhu, Y.-P.; Masuyama, A.; Nagata, T.; Okahara, M. J. Jpn Oil Chem. Soc. (Yukagaku) 1991, 40, 473. (e) Masuyama, A.; Hirono, T.; Zhu, Y.P.; Okahara, M.; Rosen, M. J. J. Jpn Oil Chem. Soc. (Yukagaku) 1992, 41, 301. (f) Zhu, Y.-P.; Masuyama, A.; Nakatsuji, Y., Okahara, M. J. Jpn Oil Chem. Soc. (Yukagaku) 1993, 42, 86. (g) Zhu, Y.-P.; Ishihara, K.; Masuyama, A.; Nakatsuji, Y.; Okahara, M. J. Jpn Oil Chem. Soc. (Yukagaku) 1993, 42, 161. (h) Zhu, Y.-P.; Masuyama, A.; Kobata, Y.; Nakatsuji, Y.; Okahara, M.; Rosen, M. J. J. Colloid Interface Sci. 1993, 158, 40. (2) Rosen, M. J. CHEMTECH 1993, 30.
S0743-7463(96)00268-5 CCC: $12.00
Scheme 1
surfactants”, which are amphiphiles possessing, in a sequence, a long hydrophobic chain, an ionic group, a rigid connecting group, a second ionic group, and another hydrophobic tail and reported that they showed unique interfacial properties.3 Zana et al. studied the solution properties of double-chain bis(ammonium) types of surfactants and proposed possible modes of self-assembly of these “gemini surfactants” by computer simulation.4 By the way, studies on a monolayer of amphiphiles at the air-water interface have now assumed an important position in the field of interfacial chemistry.5 Much information on the orientation and/or packing behavior of amphiphilic molecules at the interface can be obtained (3) (a) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451. (b) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (4) (a) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (b) Zana, R.; Talmon, Y. Nature 1993, 362, 228. (c) Karaborni, S.; Esselink, K.; Hilbers, P. A. J.; Smit, B.; Kartha¨user, J.; Van Os, N. M.; Zana, R. Science 1994, 266, 254. (5) Arnett, E. M.; Harvey, N. G.; Rose, P. L. Acc. Chem. Res. 1989, 22, 131. (b) Knobler, C. M. Adv. Chem. Phys. 1990, 77, 397. (c) Aston, M. S. Chem. Soc. Rev. 1993, 22, 67.
© 1996 American Chemical Society
π-A Isotherms for Gemini Surfactants
by analysis of surface pressure-surface area (π-A) isotherms of their monolayer films.6 Examination of the adsorptive behavior of surfactants at the interface is regarded as one of the effective methods for understanding the relation between their surface-active properties and their molecular structure.7 In our previous paper,8 we reported the monolayer behavior of water-insoluble double-chain amphiphiles bearing two decyl groups and two hydroxyl groups (1), which are the synthetic precursors of surfactants 2. In that paper, we proposed a new parameter, Elasticity Index, to estimate the effect of the connecting group (-O-YO-) between the two hydrophobic chains of double-chain amphiphiles on the behavior of their monolayer films at the air-water interface. In this work, we have measured the π-A isotherms for a variety of double-chain diols (1) by the conventional film balance technique and evaluated the effect of the alkyl chain length, the presence of unsaturated aliphatic chains, the type of connecting group, and the measuring temperature on the profiles of their π-A isotherms. Results and Discussion In Table 1 are listed the structures of the double-chain amphiphiles bearing two hydroxyl groups (1a-m) derived from racemic epichlorohydrin along with their abbreviations. The π-A isotherms for these amphiphiles were recorded by a computer-controlled film balance system in an equilibrium-relaxation compression mode at constant temperature (see Experimental Section). Three parameters, the liftoff area (AL, the molecular occupation area value on the isotherm where the curve emerges from the base line, i.e., π ) 0), the limiting area (A∞), and the collapse pressure (πC), for these compounds at 25 °C are summarized in Table 2. In the case of monolayers which collapsed in a state of a liquid-expanded phase upon compression, A∞ values were calculated by regression analysis of the following equation concerning a twodimensional variation of a nonideal gas law, which has been proposed by Menger.9
π[A - A∞(1 - kπ)] ) nRT
Langmuir, Vol. 12, No. 16, 1996 3987 Table 1. Abbreviations of the Double-Chain Amphiphiles Bearing Two Hydroxyl Groups (1a-m) Derived from Racemic Epichlorohydrin 1
R-
-O-Y-O-
abbreviation
a b c d e f g h i j k l m
n-C10H21n-C10H21n-C10H21n-C12H25n-C14H29n-C16H33n-C16H33n-C16H33n-C18H37n-C18H37n-C18H37n-C18H35- [(Z)-9-octadecenyl] n-C18H35- [(E)-9-octadecenyl]
-O(CH2)2O-O(CH2)4O-O(CH2)6O-O(CH2)2O-O(CH2)2O-O(CH2)2O-O(CH2)4O-O(CH2)6O-O(CH2)2O-O(CH2)4O-O(CH2)6O-O(CH2)2O-O(CH2)2O-
2C10-C2 2C10-C4 2C10-C6 2C12-C2 2C14-C2 2C16-C2 2C16-C4 2C16-C6 2C18-C2 2C18-C4 2C18-C6 2CZ-18:1-C2 2CE-18:1-C2
Table 2. Liftoff Area (AL), Limiting Area (A∞), and Collapse Pressure (πC)a of the Double-Chain Amphiphiles Bearing Two Hydroxyl Groups (1a-m) at 25 °C A∞/Å2 πC/mN AL/Å2 entry compound abbreviation molecule-1 molecule-1 m-1 1 2 3 4 5 6 7 8 9 10 11 12 13
1a 1b 1c 1d 1e 1f 1g 1h 1i 1j 1k 1l 1m
2C10-C2 2C10-C4 2C10-C6 2C12-C2 2C14-C2 2C16-C2 2C16-C4 2C16-C6 2C18-C2 2C18-C4 2C18-C6 2CZ-18:1-C2 2CE-18:1-C2
211 218 239 133 129 123 149 171 120 148 138 143 121
112 137 174 118 43 46 135 146 57 57 130 129 117
50 43 41 50 54 65 39 32 61 50 28 40 45
a The liftoff area, A , is defined as the first point of the isotherm L where a monolayer shows detectable resistance to compression. The limiting area, A∞, is a parameter approximating the area occupied by the molecule on the surface at zero pressure. The method to determine the A∞ value is mentioned in the text in detail. The collapse pressure, πc, is the maximum pressure of the isotherm and in the case of the plateau is observed in the final part of the isotherm. When the plateau part is not observed, πc is defined as the point of the isotherm where the steep part of a liquid-condensed or solid phase in the curve begins to bend.
where π and A are the observed pressure and molecular occupation area, respectively, and k is a constant. This equation is generally applicable to the liquid-like region of the monolayer film (the 3-10 mN m-1 region for each corresponding amphiphile). If a liquid-condenced or solid phase is observed in the isotherm for a compound, the A∞ value is conventionally given by extrapolation of the steepest portion of the π-A curve (i.e., the behavior of the liquid-condenced or the solid phase) to π ) 0. Effect of the Length of Saturated Aliphatic Chains on the π-A Isotherms. Figure 1 shows the π-A isotherms at 25 °C for the homologues of compounds having a connecting group derived from ethylene glycol (2Cm-C2 series; m ) 10, 12, 14, 16, and 18). Whereas the monolayers of 2C10-C2 and 2C12-C2 collapsed in a state (6) (a) Arnett, E. M.; Chao, J.; Kinzig, B. J.; Stewart, M. V.; Thompson, O.; Verbiar, R. J. J. Am. Chem. Soc. 1982, 104, 389. (b) Menger, F. M.; Richardson, S. D.; Wood, M. G., Jr.; Sherrod, M. J. Langmuir 1989, 5, 833. (c) Naumann, C.; Dietrich, C.; Lu, J. R.; Thomas, R. K.; Rennie, A. R.; Penfold, J.; Bayerl, T. M. Langmuir 1994, 10, 1919. (d) van Esch, J. H.; Nolte, R. J. M.; Ringsdorf, H.; Wildburg, G. Langmuir 1994, 10, 1955. (7) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1989; Chapter 2. (8) Masuyama, A.; Kawano, T.; Zhu, Y.-P.; Kida, T.; Nakatsuji, Y. Chem. Lett. 1993, 2053. (9) Menger, F. M.; Wood, M. G., Jr.; Richardson, S.; Zhou, Q.; Elington, A. R.; Sherrod, M. J. J. Am. Chem. Soc. 1988, 110, 6797.
Figure 1. Effect of the length of saturated alkyl chains on the π-A isotherms at 25 °C.
of a liquid-expanded phase, a phase transition of the 2C14C2 monolayer from the liquid-expanded to the liquidcondensed state was observed upon compression. Liftoff areas for these homologues decreased in the order of increasing length of the alkyl chains. The AL value for 2C10-C2 was much higher than that of other 2Cm-C2 types of homologues. In the case of 2C16-C2 and 2C18-C2, the liquid-condensed state also appeared, and the collapse
3988 Langmuir, Vol. 12, No. 16, 1996
Sumida et al.
Figure 2. Effect of an unsaturated bond in the middle of the hydrophobic chain at 25 °C: (a) 2C18-C2 (1i); (b) 2CE-18:1-C2 (1m); (c) 2CZ-18:1-C2 (1l); (d) octadecanol; (e) (E)-9-octadecenol; (f) (Z)-9-octadecenol.
Figure 3. Stylized illustration of the typical adsorptive behavior of compounds 2C18-C2 (1i), 2CZ-18:1-C2 (1l), and 2CE-18:1-C2 (1m) at the air-water interface.
pressures of these two amphiphiles were higher than those of other lower homologues (entries 1, 4, 5, 6, and 9 in Table 2). These results clearly indicate that a more tightly packed monolayer is formed as the alkyl chain length of this type of double-chain amphiphile increases, which will be attributed mainly to stronger hydrophobic interaction between the two alkyl groups in the molecule. Effect of Unsaturated Linkage in the Middle of the Hydrophobic Chain. The influence of an unsaturated linkage in the hydrophobic chain was demonstrated by comparing the π-A isotherms for 2C18-C2, 2CZ-18:1C2, and 2CE-18:1-C2, all of which have the same connecting group. These results are shown in Figure 2 along with the results of the corresponding single-chain alcohols, octadecanol, (Z)-9-octadecenol, and (E)-9-octadecenol. A phase transition from the liquid-expanded to the liquid-condensed state was observed in the isotherm for (E)-9-octadecenol. The saturated 2C18-C2 showed the liquid-condensed phase as mentioned above. However, both 2CZ-18:1-C2 and 2CE-18:1-C2 collapse in a state of a liquid-expanded phase upon compression, and their collapsed pressures were much lower than that of 2C18-C2 (entries 9, 12, and 13 in Table 2). The A∞ values for 2C18C2, 2CZ-18:1-C2, and 2CE-18:1-C2 were about three times that for octadecanol (20 Å2 molecule-1), (Z)-9-octadecenol (38 Å2 molecule-1), and (E)-9-octadecenol (33 Å2 molecule-1), respectively. Both AL and A∞ values for the 2CE-18: 1-C2 isomer were smaller than those for the corresponding (Z)-isomer. A stylized illustration of the typical adsorptive behavior of these three compounds is shown in Figure 3. It seems that this illustration reflects the practical AL and A∞ results for these three compounds well. Especially, typical adsorptive behavior III, which is the most spreading structure on the surface among the illustrated structures in Figure 3, will contribute to the larger AL and A∞ values for 2CZ-18:1-C2 isomer compared with the type V behavior of the corresponding (E)-isomer.
Figure 4. Effect of temperature on the π-A isotherms for (A) 2C14-C2 (1e), (B) 2C16-C2 (1f), and (C) 2C18-C2 (1i).
Effect of the Measuring Temperature. Figure 4 shows the π-A isotherms for a series of 2Cm-C2 (m ) 14, 16, and 18) at various temperatures. In each compound, the transition state region narrowed and the liquidexpanded phase region widened with a rise in temperature. In the case of 2C14-C2, both the transition state and the liquid-condensed phase were clearly observed below 25 °C. Although the liquid-condensed phase of the 2C16-C2 monolayer was slightly observed even at 40 °C, it collapsed with the liquid-expanded phase at 50 °C. In the case of 2C18-C2, although no transition state was observed below 25 °C, the liquid-condensed phase appeared even at 50 °C. We have additionally measured the DSC behavior of these three amphiphiles dispersed in water. Sharp endothermic peak tops were monitored at 32 °C for 2C14C2, 47 °C for 2C16-C2, and 57 °C for 2C18-C2, respectively, which are surmised to be the phase transition temperature (Tc) of a molecular aggregate from a gel to a liquid-crystal phase. No liquid-condensed phase and/or any transition state was not observed in their π-A isotherms at any temperature above each Tc.
π-A Isotherms for Gemini Surfactants
Langmuir, Vol. 12, No. 16, 1996 3989
Figure 6. Effect of chirality in the molecule on the π-A isotherms at 25 °C: (a) (S,S)-2C16-C2 (3a); (b) (R,R)-2C16-C2 (3b); (c) equimolar mixture of 3a and 3b. Scheme 2
Figure 5. Effect of the connecting group between the two hydrophobic chains of (A) 2C10-Cn series, (B) 2C16-Cn series, and (C) 2C18-Cn series: (a) n ) 2, (b) n ) 4, and (c) n ) 6 in each chart.
Effect of the Connecting Group between the Two Hydrophobic Chains of Double-Chain Amphiphiles. We have already investigated the effect of the number of oxyethylene units in the connecting group on the behavior of monolayer films of double-chain diols bearing two decyl groups at the air-water interface.5 Zana et al. have also pointed out the importance of the connecting group structure of their gemini type surfactants for interfacial properties.4b In this paper, we mention the effect of a simple alkylene-connecting group on the π-A isotherms of a series of 2Cm-Cn (m ) 10, 16, and 18; n ) 2, 4, and 6) at 25 °C. Figure 5 clearly shows a considerable change in the π-A isotherms depending on the combination of the connecting group and the length of the alkyl chains. All monolayers consisted of the 2C10-Cn amphiphiles (n ) 2, 4, and 6; Figure 5A) collapsed in the liquid-expanded state upon compression. The A∞ values for these compounds increased with an increase in the length of the alkylene chain in the connecting part (entries 1, 2, and 3 in Table 2). In the case of 2C16-Cn (Figure 5B), only the 2C16-C2
monolayer showed the transition state and the liquidcondensed phase. Thus the increase in the length of the alkylene chain makes the packing of the monolayer at the interface loose in the 2C16-Cn types of compounds. Probably because of strong intramolecular hydrophobic interactions, the limiting areas for 2C18-C2 and 2C18-C4 (Figure 5C) are almost the same (entries 9 and 10 in Table 2). In the case of 2C18-C6, however, the monolayer collapsed in the state of a liquid-expanded film. These results indicate that a hexamethylene connecting group contributes to loose packing of the monolayer of 2Cm-Cn types of amphiphiles at the interface. Effect of Chirality in the Molecule on the π-A Isotherms. Finally, because each enantiomer of chiral epichlorohydrins with high optical purity is commercially available, we prepared two enantiomers of the chiral double-chain amphiphiles (3a,b) (Scheme 2). The π-A isotherms for each enantiomer and an equimolar mixture of these enantiomers are shown in Figure 6. Three isotherms were almost identical within experimental error, indicating that the influence of mixing of two enantiomers on the adsorptive behavior of compounds 3 at the air-water interface cannot be monitored by this film balance technique.
3990 Langmuir, Vol. 12, No. 16, 1996
In previous papers, we discussed only the surface-active properties of micellar solutions of double-chain surfactants bearing two anionic head groups (2).1a-f,h Although it is expected that these types of surfactants (2) with longer hydrophobic alkyl chains will form bilayer vesicles in water, the behavior of 2 as a vesicle has not yet been investigated. Information on the packing behavior of a homologous series of 1 at various temperatures proposed in this work will be very useful for exploring the behavior and properties of vesicles composed of the double-chain amphiphiles 2. Experimental Section Materials. Double-chain diols 1 were synthesized by the previously reported method.10 Chiral double-chain diols (3a,b) were prepared using chiral epichlorohydrin obtained from Daisoh (Amagasaki, Hyogo, Japan) according to similar procedures for preparation of 1 (Scheme 2): (R)-(-)-epichlorohydrin, [R]D20 ) -24.0° (chloroform, c 10), and S-(+)-epichlorohydrin, [R]D20 ) +24.8° (chloroform, c 10). Compound 3a was derived from the former, [R]D20 ) +3.40° (chloroform, c 10), and 3b was derived from the latter, [R]D20 ) -3.10° (chloroform, c 10). Samples for measuring the monolayer behavior in this study were isolated and purified by silica gel column chromatography with an appropriate eluent system and/or recrystallization from hexane. The purity of each sample was confirmed by TLC, spectral (IR, MS, and 1H NMR), and elemental analyses. The purification methods and the characteristic data are available in the supporting information. Dodecanol and (Z)-9-octadecenol were purchased from Nacalai Tesque, Inc. (Kyoto, Japan), and used after purification as follows: The former was purified by recrystallization from ethanol three times (mp 60.0-61.0 °C). The latter was distilled under reduced pressure using a Widmer distilling column. The distillate at 128-132 °C (0.08 mmHg) was used. (E)-9-Octadecenol was synthesized according to the authentic reduction of (E)-9-octadecenoic acid with LiAlH4 in THF. Purification was carried out by recrystallization from ethanol (mp 30.5-32.0 °C). The purity of these three alcohols (10) Nakatsuji, Y.; Tsuji, Y.; Ikeda, I.; Okahara, M. J. Org. Chem. 1986, 51, 78.
Sumida et al. was ascertained by TLC, 1H NMR, and elemental analyses (within an error of (0.30%). Methods. 1H NMR spectra were recorded in CDCl3 with a JEOL JNM-GSX-400 (400 MHz) or a Bruker AM-600 (600 MHz) spectrometer using tetramethylsilane (TMS) as an internal standard. The IR spectra and MS spectra were measured on a Hitachi 260 spectrometer and a JEOL JMS-DX-303 mass spectrometer, respectively. Differential scanning calorimetric (DSC) analysis for 1e,f,i was carried out with a DSC-20 apparatus (SEIKO Instruments & Electronics, Ltd.) using a dispersion of each sample in water (10 µL) sealed in an aluminum pan at the heating rate of 2 °C/min. Optical rotation of chiral epichlorohydrins and compounds 3a,b was measured with a JASCO DIP181 digital polarimeter (Japan Spectroscopic Corp.) in chloroform at 25 °C. The π-A isotherms were recorded with a computer-controlled film balance system (Nippon Laser & Electronics Lab. type NLLB80S-MTC) installed in an acrylic box on a stone table in an air-conditioned room. The trough of the system was made of Teflon. Pure water used as a subphase was finally obtained with a Milli-Q-Labo apparatus (18 MΩ cm). The sample was dissolved in freshly distilled benzene (0.1 g L-1) and was spread on the subphase water evenly from the solution with a 100-µL microsyringe. Surface pressure (π) as a function of molecular area (A) was measured in an equilibrium-relaxation compression mode (surface-compression conditions: permitted range of equilibration surface pressure per compression step ) 0.5 mN m-1, compression step time ) 10 s, and waiting time of equilibration surface pressure per one compression step ) 10 s), and the temperature of the subphase was maintained at the appointed temperature (0.5 °C by circulation of water slowly from a constant temperature bath to the inner jacket of the trough. A film balance experiment requires a delicate operation. The measurement was carried out carefully according to the guidelines of the operation described previously.6a,9 The π-A isotherms for each compound were measured at least five times to confirm their reproducibility.
Supporting Information Available: Spectral and elemental data for 1a-m and 3a,b (4 pages). Ordering information is given on any current masthead page. LA960268X
