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Langmuir 1998, 14, 7450-7455
π-A Isotherms for Triple-Chain Amphiphiles Bearing Two or Three Hydroxyl Groups. Effect of the Backbone Structure on the Adsorption Behavior of the Molecules on the Surface Yasushi Sumida,† Toshihiro Oki,‡ Araki Masuyama,‡ Hiroshi Maekawa,‡ Masahito Nishiura,‡ Toshiyuki Kida,‡ Yohji Nakatsuji,‡ Isao Ikeda,*,‡ and Masatomo Nojima‡ Cosmetic Laboratory, Kanebo Corporation, Kotobuki-cho 5-3-28, Odawara, Kanagawa 250-0002, Japan, and Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan Received July 6, 1998. In Final Form: October 6, 1998 Three types of triple-chain surfactants bearing three sulfonate groups showed unusual behavior; that is, their critical micelle concentration measured by the Wilhelmy method for their homologous series increased with an increase in the hydrophobic alkyl chain length. Thus, the difference in the backbone structure of these surfactants, whether glycerol type (glycerol or 2-methylglycerol) or 1,1,1-tris(hydroxymethyl)ethane (i.e., trimethylolethane) type, significantly affects their surface properties. To clarify this unusual behavior, the adsorption manner of triple-chain amphiphiles bearing two or three hydroxyl groups, which are synthetic precursors of triple-chain surfactants bearing two or three anionic headgroups, was studied by measuring pressure-area (π-A) isotherms with a computer-controlled filmbalance technique. Some clear-cut profiles with respect to the relationship between the structure of these amphiphiles and their adsorption behavior on the surface were revealed as follows: (1) The packing of hydrophobic alkyl chains of triple-chain diols was tighter than that of the corresponding double-chain diols with the same alkyl chain length; (2) as to both triple-chain diols and triple-chain triols, the π-A isotherms were greatly changed depending on their backbone structure, whether glycerol, 2-methylglycerol, or trimethylolethane; (3) three additional isolated oxyethylene units connecting to the backbone of triplechain triols contribute significantly to the increase in hydrophilicity of the molecule. These results indicate that the choice of the backbone structure of a triple-chain surfactant is important to predict and to understand the packing of hydrophobic chains, which directly relates to its surface properties in water.
Introduction Research on double-chain surfactants bearing two hydrophilic ionic headgroups, generally called “gemini surfactants”,1 is still a stimulating subject in the chemistry of surfactants. More than 30 papers have been published in the last two years by several research groups, including ours. Many attractive items can be easily extracted from these papers as follows: preparation and characterization of a new series of gemini surfactants,2 unique properties of gemini surfactants with rigid and flexible spacers,3 the effect of structural factors in bis(ammonium halide) surfactants on their surface properties,4 behavior of mixed micelles or systems containing gemini surfactants in water,5 other marvelous physicochemical properties in† ‡
Kanebo Corporation. Osaka University.
(1) (a) Okahara, M.; Masuyama, A.; Sumida, Y.; Zhu, Y.-P. J. Jpn. Oil Chem. Soc. (YUKAGAKU) 1988, 37, 746. (b) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451. (c) Rosen, M. J. Chemtech 1993, March, 30. (d) Rosen, M. J. In New Horizons, An AOCS/CSMA Detergent Industry Conference; Coffey, R. T., Ed.; AOCS Press: Champaign, IL, 1996; Chapter 8. (2) (a) Perez, L.; Torres, J. L.; Manresa, A.; Solans, C.; Infante, M. R. Langmuir 1996, 12, 5296. (b) Dam, T.; Engberts, J. B. F. N.; Karthauser, J.; Karaborni, S.; van Os, N. M. Colloids Surf., A 1996, 118, 41. (c) Castro, M. J. L.; Kovensky, J.; Fernandez Cirelli, A. Tetrahedron Lett. 1997, 38, 3995. (d) Duivenvoorde, F. L.; Feiters, M. C.; van der Gaast, S. L.; Engberts, J. B. F. N. Langmuir 1997, 13, 3737. (3) (a) Song, L. D.; Rosen, M. J. Langmuir 1996, 12, 1149. (b) Rosen, M. J.; Song, L. D. J. Colloid Interface Sci. 1996, 179, 261. (4) (a) Rosen, M. J.; Liu, L. J. Am. Oil Chem. Soc. 1996, 73, 885. (b) Oda, R.; Huc, I.; Candau, S. J. Chem. Commun. 1997, 2105. (c) Zana, R.; In, M.; Levy, H.; Guy, D. Langmuir 1997, 13, 5552.
cluding vesicle formation,6 and so on. Recently, we also reported the pressure-area (π-A) isotherms for a series of double-chain amphiphiles bearing two hydroxyl groups (1)7 besides a study on the synthesis and surface-active properties of multiple quaternary ammonium salts.8 The following clear-cut features were revealed in the adsorption behaviors of these double-chain diols at the air/water interface from their π-A isotherms measured under various conditions: A more tightly packed monolayer was formed as the length of hydrophobic chain increased and the length of the connecting part was shortened. And, an unsaturated bond in the middle of the hydrophobic chain contributed to loose packing of the monolayer.7 By the way, we have already clarified that glycerolbackbone triple-chain surfactants with two anionic headgroups show excellent surface-active properties, such as (5) (a) Liu, L.; Rosen, M. L. J. Colloid Interface Sci. 1996, 179, 454. (b) Zana, R.; Levy, H.; Danino, D.; Talmon, Y.; Kwetkat, K. Langmuir 1997, 13, 402. (6) (a) Esumi, K.; Goino, M.; Koide, Y. Colloids Surf., A 1996, 118, 161. (b) Sommerdijk, N. A. J. M.; Hoeks, T. H. L.; Synak, M.; Feiters, M. C.; Nolte, R. J. M.; Zwanenburg, B. J. Am. Chem. Soc. 1997, 119, 4338. (c) Sommerdijk, N. A. J. M.; Lambermon, M. H. L.; Feiters, M. C.; Nolte, R. J. M.; Zwanenburg, B. Chem. Commun. 1997, 1423. (d) Bhattacharya, S.; De, S.; George, S. K. Chem. Commun. 1997, 2287. (e) Manne, S.; Schaeffer, T. E.; Huo, Q.; Hansma, P. K.; Morse, D. E.; Stucky, G. D.; Aksay, I. A. Langmuir 1997, 13, 6382. (7) Sumida, Y.; Masuyama, A.; Oki, T.; Kida, T.; Nakatsuji, Y.; Ikeda, I.; Nojima, M. Langmuir 1996, 12, 3986. (8) (a) Kim, T.-S.; Hirao, T.; Ikeda, I. J. Am. Oil Chem. Soc. 1996, 73, 67. (b) Kim, T.-S.; Kida, T.; Nakatsuji, Y.; Hirao, T.; Ikeda, I. J. Am. Oil Chem. Soc. 1996, 73, 907. (c) Kim, T.-S.; Kida, T.; Nakatsuji, Y.; Ikeda, I. Langmuir 1996, 12, 6304. (d) Kim, T.-S.; Tatsumi, T.; Kida, T.; Nakatsuji, Y.; Ikeda, I. J. Jpn. Oil Chem. Soc. 1997, 46, 747.
10.1021/la980814h CCC: $15.00 © 1998 American Chemical Society Published on Web 12/04/1998
π-A Isotherms for Triple-Chain Amphiphiles Chart 1
micelle-forming ability at lower concentration and the ability to lower surface tension, compared not only with the corresponding single-chain surfactants but also with the corresponding double-chain surfactants having two anionic headgroups.9 We have additionally reported unique surface properties of a homologous series of novel triple-chain surfactants bearing three sulfonate groups derived from 1,1,1-tris(hydroxymethyl)ethane (2).10 In the present work, two additional types of triple-chain surfactants with three sulfonate groups (3 and 4) were prepared and some of their surface properties were estimated by the Wilhelmy method. The effect of the difference in the backbone structures of these triple-chain surfactants on their physicochemical properties was discussed on the basis of these results for 2, 3, and 4. And the π-A isotherms for triple-chain diols (5 and 6) and triple-chain triols (7-10) were measured by the conventional film balance technique. In general, information on the orientation and packing manner of amphiphilic molecules at the air/water interface, which can be obtained by analysis of π-A isotherms of the monolayer film, is important and useful for understanding the relation between surface properties and molecular structure.11 In this paper, we will analyze the π-A isotherms of compounds 1 and 5-10 from various aspects and speculate on their adsorption manner on the surface. Compounds which have been mentioned in this paper are listed in Charts 1 and 2. Results and Discussion π-A Isotherms for a Series of Double- or TripleChain Amphiphiles Bearing Two or Three Hydroxyl (9) Zhu, Y.-P.; Masuyama, A.; Kirito, Y.; Okahara, M.; Rosen, M. J. J. Am. Oil Chem. Soc. 1992, 69, 626. (10) Masuyama, A.; Yokota, M.; Zhu, Y.-P.; Kida, T.; Nakatsuji, Y. J. Chem. Soc., Chem. Commun. 1994, 1435. (11) (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.
Langmuir, Vol. 14, No. 26, 1998 7451 Chart 2
Groups. In Chart 2 are listed the structures of waterinsoluble amphiphiles subjected to the π-A experiments. In this work, two kinds of III-II type (the former Roman numeral means the number of alkyl chains, and the latter means the number of hydroxyl groups) and four kinds of III-III type amphiphiles have been studied along with II-II type ones as references. Among III-II type compounds, 5a-e are the synthetic precursors of glycerolbackbone triple-chain surfactants bearing two anionic (sulfonate, sulfate, and carboxylate) groups which have shown excellent surface-active properties.9 Compound 6c was prepared to clarify the effect of the difference in the backbone structure between glycerol and trimethylolethane on their adsorption manner on the surface. Four kinds of III-III type compounds 7-10 were designed to explore the contribution of the backbone structure and the additional oxyethylene units to the adsorption manner for these types of triple-chain amphiphiles bearing three hydrophilic groups. The π-A isotherms in this work were recorded with a computer-controlled film balance system in an equilibrium-relaxation compression mode at 25 °C (see Experimental Section). Three parameters (AL, A∞, and πc) for these amphiphiles are summarized in Table 1. AL is the molecular occupation area on the isotherm where the curve emerges from the baseline (i.e., π ) 0). A∞ is the limiting area.12 In the case of monolayers which collapsed in a state of a liquid-expanded phase upon compression, A∞ was calculated by regression analysis of the following equation for a two-dimensional variation of a nonideal gas law, which has been proposed by Menger.13
π[A - A∞(1 - kπ)] ) nRT where π and A are the observed pressure and molecular occupation area, respectively, and k is a constant. This equation is generally applicable to a liquid-expanded region of the monolayer film and is valid in the 3-10 mN m-1 region for each amphiphile in this work. πc is the collapse pressure. Comparison of π-A Isotherms between III-II and II-II Types. The effect of chain length for a homologous series of single-chain amphiphiles on the behavior of π-A isotherms is well-established. For example, in the homo(12) If a liquid-condensed or solid phase is observed in the isotherm for a compound, the A∞ value is conventionally given by extrapolation of the liquid-condensed or the solid phase to π ) 0. (13) Menger, F. M.; Wood, M. G., Jr.; Richardson, S.; Zhou, Q.; Elington, A. R.; Sherrod, M. J. J. Am. Chem. Soc. 1988, 110, 6797.
7452 Langmuir, Vol. 14, No. 26, 1998
Sumida et al.
Table 1. Liftoff Area (AL), Limiting Area (A∞), and Collapse Pressure (πc)a at 25 °C entry compound 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
1a 5a 9a 10a 1b 5b 9b 10b 1c 5c 6c 7c 8c 9c 10c 1d 5d 1e 5e
R
type
n-C10H21 n-C10H21 n-C10H21 n-C10H21 n-C12H25 n-C12H25 n-C12H25 n-C12H25 n-C14H29 n-C14H29 n-C14H29 n-C14H29 n-C14H29 n-C14H29 n-C14H29 n-C16H33 n-C16H33 n-C18H37 n-C18H37
II-II III-II III-III III-III II-II III-II III-III III-III II-II III-II III-II III-III III-III III-III III-III II-II III-II II-II III-II
A∞/Å2 πc/ AL/Å2 molecule-1 molecule-1 mN m-1 211 185 181 281 133 141 184 246 129 140 150 167 226 174 243 123 95 120 90
112 153 155 215 118 132 163 219 43 60 75 158 220 169 231 46 56 57 55
50 40 37 35 50 40 37 38 54 59 38 40 37 37 39 65 59 61 62
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. The collapse pressure πc is the maximum pressure of the isotherm in the case of the plateau and 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 in the curve of a liquidcondensed or solid phase begins to bend.
Figure 1. Comparison of π-A isotherms between III-II type (above) and II-II type (below) compounds.
logues of alkyl carboxylic acids, a phase transition from a liquid-expanded to a liquid-condensed state was observed for tetradecanoic acid whereas hexadecanoic and octadecanoic acids showed a liquid-condensed phase without a liquid-expanded state.14 Figure 1 shows the π-A isotherms for the homologues of III-II (5a-e; above) and II-II type (1a-e; below)7 compounds. (14) Adam, N. K.; Jessop, G. Proc. R. Soc. London, Ser. A 1926, 112, 364.
Figure 2. Effect of the backbone structure in III-II type compounds on the π-A isotherms.
While the monolayers of 5a (R ) n-C10H21) and 5b (R ) n-C12H25) collapsed in a liquid-expanded state such as did 1a (R ) n-C10H21) and 1b (R ) n-C12H25), a clear phase transition from a liquid-expanded to a liquid-condensed state was noticed upon compression in the case of 5c (R ) n-C14H29). For the II-II type compound, a transition state was observed for 1c (R ) n-C14H29) and 1d (R ) n-C16H33), but only 5c of a tetradecyl derivative showed the transition for III-II type compounds. In the case of III-II type, 5c, 5d (R ) n-C16H33), and 5e (R ) n-C18H37) showed a liquid-condensed state, while only 1d and 1e did so for the II-II type. The collapse pressures of these compounds were higher than those of 5a and 5b (entries 2, 6, 10, 17, and 19 in Table 1). The AL values for 5d and 5e were considerably smaller than those of other III-II type homologues. The A∞ values for III-II type compounds are smaller than the expected values based on the II-II type results. If each A∞ value is converted to the limiting occupation area per hydrophobic alkyl chain in a molecule [A∞′/Å2 (alkyl chain)-1], almost the same figures result for 1a and 5a [ca. 56 and 51 Å2 (alkyl chain)-1, respectively]. The A∞′ values for 5b-e, however, are much smaller than those for 1b-e bearing the corresponding alkyl chains. The A∞′ values for 5c-e are about only 20 Å2 (alkyl chain)-1. The resulting A∞ (A∞′) values suggest that the packing of hydrophobic alkyl chains of III-II type amphiphiles is tighter than that of the corresponding II-II type compounds. So we propose, on the basis of this effective adsorption of the III-II type on the surface, that tight packing of the corresponding triple-chain surfactants bearing two anionic headgroups and a glycerol backbone plausibly accounts for the excellent surface-active properties, such as lower cmc (the critical micelle concentration) and γcmc (the surface tension at the cmc) values, and so on, mentioned in our previous paper.9 Effect of the Backbone Structure in III-II Type Amphiphiles on the π-A Isotherms. The π-A isotherms for 5c and 6c, both of which have three tetradecyl chains in a molecule, are presented in Figure 2. Similar curves were observed for both compounds in the region from their liftoff to about 10 mN m-1 of surface pressure. In the case of 5c bearing a glycerol backbone was noticed a transition state at 13 mN m-1. Compound 5c also showed both liquid-condensed and solid-film regions, and collapsed at about 59 mN m-1. Although 6c bearing a trimethylolethane backbone assumed a narrow transition state and a liquid-condensed region, it collapsed at lower pressure than that for 5c, 38 mN m-1. The difference in behavior at the higher pressure region of the isotherms for these compounds indicates that the triplechain amphiphiles with a glycerol backbone adsorb more effectively on the surface than that with a trimethylole-
π-A Isotherms for Triple-Chain Amphiphiles
Langmuir, Vol. 14, No. 26, 1998 7453
Figure 3. π-A isotherms for III-III type compounds bearing different backbone structures.
thane backbone. The result that the A∞ for 5c is smaller by 15 Å2 molecule-1 than that for 6c (entries 10 and 11 in Table 1) also supports this speculation. Variation of π-A Isotherms in III-III Type Amphiphiles Bearing Different Backbone Structure. The difference in the backbone structure also influenced the adsorption behavior of the III-III type amphiphiles, as shown in Figure 3. Compounds 7c, 8c, 9c, and 10c all have three tetradecyl chains in the hydrophobic part of the molecule. The collapse pressures πc for these four compounds were in the range 37-40 mN m-1, and there is no remarkable difference. The difference in A∞ between 7c and 9c is 11 Å2 molecule-1 (entries 12 and 14 in Table 1), and the difference between 8c and 10c, both of which have three additional isolated oxyethylene units in the molecule, is also the same (entries 13 and 15). These results quantitatively indicate that a molecule of the III-III type amphiphiles bearing the trimethylolethane backbone (8c and 10c in this case) takes a broader adsorption style on the surface than a molecule of the III-III type ones bearing the 2-methylglycerol backbone (7c and 9c), as originally expected. A similar trend was observed in the AL values for these four compounds. In any event, it becomes apparent that the presence or the absence of one methylene unit in the backbone structure of triple-chain amphiphiles significantly affects the adsorption manner of these types of triple-chain amphiphiles with three hydrophilic groups on the surface. Effect of the Alkyl Chain Length on the π-A Isotherms in the Case of III-III Type Amphiphiles Bearing a Trimethylolethane Backbone. Figure 4 shows the π-A isotherms for a homologous series of 9a-c bearing the trimethylolethane backbone (above) and 10a-c having three additional isolated oxyethylene units in the backbone (below). Both the A∞ and the occupation area of a molecule at any constant surface pressure in the 5 to approximately 30 mN m-1 region for both 9 and 10 increase sparingly with an increase in the alkyl chain length. These results are quite different from the order of increasing A∞ for the homologues of III-II (5a-e) type compounds. It is surmised that the cross-sectional area of the hydrophilic moiety containing three hydroxyl groups of these III-III type compounds on the surface is so large that the hydrophobic interaction between three alkyl chains cannot work effectively. So the enhancement of hydrophobic interaction accompanied with an increase in the hydrophobic chain length, like the case of the III-II type homologues, is not expected for these two III-III type amphiphiles (9a-c and 10a-c) bearing the trimethylolethane backbone. Additional oxyethylene units in 10
Figure 4. Effect of the alkyl chains on the π-A isotherms in III-III type compounds bearing a trimethylolethane backbone (top, a homologous series of 9; bottom, 10). Table 2. Cmc, γcmc, and pC20 Values of Surfactants 2,a 3, and 4 Measured by the Wilhelmy Method at 20 °C in Water surfactant
R
cmc/mm
γcmc/mN m-1
pC20
2a 2b 2c 3a 3b 4a 4b
n-C10H21 n-C12H25 n-C14H29 n-C10H21 n-C12H25 n-C10H21 n-C12H25
0.0068 0.050 0.25 0.010 0.018 0.0080 0.027
31.5 33.0 34.0 31.0 32.5 33.5 34.0
7.4 5.8 4.8 6.5 7.5 7.1 6.4
a
Reference 10.
had a negative contribution to the adsorption on the surface, which was demonstrated by the significant increase in the AL and A∞ values for 10a-c as compared with the corresponding 9a-c ones (entries 3, 4, 7, 8, 14, and 15 in Table 1). Comparison of the π-A Isotherms for II-II, IIIII, and III-III Types Bearing Two or Three Tetradecyl Chains. A comparison of the π-A isotherms for 1c, 5c (both in Figure 1), and 7c (in Figure 3) is appropriate to this subject. The collapse pressure of compound 7c (40 mN m-1) was lower than those of 1c and 5c. The A∞′ values, the limiting area per hydrophobic alkyl chain in a molecule, for these compounds are 22 (1c), 30 (5c), and 53 (7c) Å2 (alkyl chain)-1, respectively. These results reflect the looser packing mode of 7c on the surface, probably due to its bulkier hydrophilic part than that of 1c and 5c. Interpretation of the Surface Properties of Surfactants 2, 3, and 4 on the basis of the Adsorption Manner of the Corresponding Triols 8, 9, and 10. Peculiar surface properties of a homologous series of triplechain surfactants bearing three sulfonate groups derived from 1,1,1-tris(hydroxymethyl)ethane (i.e. trimethylolethane, 2) were reported in a previous paper.10 As shown in Table 2, unusual behavior, that is, an increase in the cmc of a homologous series of the surfactants 2a-c with an increase of the hydrophobic alkyl chain length, was observed when these data were estimated by the con-
7454 Langmuir, Vol. 14, No. 26, 1998
Sumida et al. Scheme 1
ventional surface tension method using a Wilhelmy tensiometer. To clarify the effect of the backbone structure on the surface properties, two other types of related triplechain surfactants bearing three sulfonate groups (3a,b and 4a,b) were newly synthesized in this work. Table 2 summarizes their cmc, γcmc, and pC20 (the efficiency of adsorption at the air/water interface) values,15 which were obtained from each surface tension versus concentration (on a log scale) plot, along with the data for 2a-c. We have surmised that surfactants 2a-c form “premicelles” at lower concentrations than their cmc’s estimated by the Wilhelmy method in the previous communication.10 The possibility of premicelle formation in certain gemini surfactants was suggested and studied previously by other research groups.1b,3a,16 The trimethylolethane backbone seems to be a key structural factor in this unusual surface behavior of a homologous series of 2. In other words, we speculate that the backbone of this type of triple-chain surfactant is an unfavorable structure for effective adsorption at the air/water interface. Similar to the case for 2, the cmc’s of newly prepared 3a and 4a were lower than those of the corresponding higher homologues 3b and 4b, respectively. But the difference in cmc between 3a and 3b both bearing a 2-methylglycerol backbone, is smaller than that between 2a and 2b or 4a and 4b, all of which have a trimethylolethane backbone. The effectiveness of the adsorption of 3 on the surface is higher than that of 2 and 4 with the same alkyl chain, which can be shown as γcmc results. These results imply that the 2-methylglycerol backbone will be a more effective structure for tightly packed adsorption of triple-chain surfactants on the surface than the trimethylolethane backbone. As already known, pC20 is proportional to the free energy of adsorption on the surface (-∆G°ad).15 Concerning the conventional single-chain surfactants, these values generally increase with an increase in the hydrophobic chain length,15 while the pC20 decreases with an increase in the length of alkyl chains in the cases of 2a-c and 4a,b. The pC20 of 3b with three dodecyl chains is higher than that of the corresponding decyl homologue 3a. Compounds 3 with the glycerol backbone showed this normal tendency in pC20, supporting the above speculation that the 2-methylglycerol backbone is a more favorable structure for effective adsorption than the trimethylolethane backbone. In any event, it is evident from the results in Table 2 that not only the difference in the backbone structure but also the presence or the absence of oxyethylene units connecting to the backbone will significantly influence the adsorption manner of these amphiphiles on the surface. So water-insoluble double- or triple-chain amphiphiles having these backbone structures were newly designed to estimate the effect of these structural factors on the adsorption behavior through measurement of their π-A isotherms. The triple-chain triols 9, 8, and 10 are the synthetic precursors of the triple-chain surfactants 2, 3, and 4, respectively. Discussion concerning the relation between the specific surface properties and the π-A results of the triple-chain triols should be restricted within narrow limits because the π-A isotherms for the triple-chain triols corresponding to the synthetic precursors of the triplechain surfactants 2, 3, and 4 have not been measured comprehensively in this work; nevertheless, we can explain fairly well the unusual tendency of three surface properties
(cmc, γcmc, and pC20 in Table 2) in a homologous series of three types of triple-chain surfactants on the basis of the results of the π-A isotherms for the corresponding triols. The following two significant points are revealed from the π-A isotherm measurement: (1) whether the presence or the absence of only one methylene unit in the backbone structure affects the adsorption behavior of these types of triple-chain triols on the surface (compare the results of 7c and 9c or of 8c and 10c) and (2) three isolatedoxyethylene units in the backbone act as hydrophilic groups to spread the occupation area of the molecule on the surface (compare the results of 7c and 8c or of 9c and 10c). Additionally, in connection with the unusual order of increasing cmc for a homologous series of triple-chain surfactants 2a-c and 4a,b, the behavior of π-A isotherms for 9a-c and 10a-c, which are the corresponding triols of 2 and 4, respectively, clearly indicates that the effective hydrophobic interaction between three alkyl chains will be difficult for these compounds, as mentioned in the previous section. In conclusion, the glycerol backbone including the 2-methylglycerol structure is of greater advantage than the trimethylolethane backbone for effective adsorption on the surface (compare the results of 7 and 9 or of 8 and 10). Previously, we have prepared a series of C-pivot tripodal ligands bearing the 2-methylglycerol or trimethylolethane backbone and have measured their complexation properties toward alkali metal cations.17 In that work, the ligand bearing the former backbone was found to possess much higher complexing ability than the corresponding other one. The result was explained by considering that the 2-methylglycerol structure is more effective for arranging the three arms in one direction
(15) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1989; pp 84-90. (16) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083.
(17) (a) Nakatsuji, Y.; Kita, K.; Kida, T.; Masuyama, A. Chem. Lett. 1995, 51. (b) Kita, K.; Kida, T.; Nakatsuji, Y.; Ikeda, I. J. Org. Chem. 1997, 62, 8076.
π-A Isotherms for Triple-Chain Amphiphiles Scheme 2
than the trimethylolethane structure.17b A similar consideration would be applied for explaining the effective adsorption of the glycerol backbone including the 2-methylglycerol structure observed in this study. In accordance with the work for the molecular design of C-pivot tripodal ligands, it has become clear that the choice of the backbone structure of a triple-chain surfactant is also important for predicting and understanding its physicochemical properties, such as cmc, γcmc, and pC20 estimated by the Wilhelmy method, which are resulted depending on the adsorption manners on the surface. Experimental Section Materials. Schemes 1 and 2 give an outline of synthetic routes to the compounds studied in this work. Scheme 1 includes compounds bearing a glycerol or 2-methylglycerol backbone, and Scheme 2 deals with the preparative route from 1,1,1-tris(hydroxymethyl)ethane.
Langmuir, Vol. 14, No. 26, 1998 7455 Procedures for preparation of triple-chain diols 5a-c have already been reported.9 Both triple-chain triols 9a-c and triplechain surfactants bearing three sulfonate groups 4a,b were synthesized by the previously reported method.10 The purity of each sample was confirmed by TLC and spectral (IR, MS, and 1H NMR) and elemental analyses. Concrete procedures for preparation of the newly synthesized compounds in this work are given in the Supporting Information along with the purification methods, spectral data, and elemental analytical data for all compounds subjected to π-A experiments. 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. To determine the cmc values for surfactants 3a,b and 4a,b, the surface tension of a surfactant solution was measured with a Wilhelmy tensiometer (Shimadzu ST-1) at 20 °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 for equilibration surface pressure per one compression step ) 10 s), and the temperature of the subphase was maintained at 25 ( 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 delicate operation. The measurement was carried out carefully according to the guidelines of the operation described previously.11a,13 The π-A isotherms for each compound were measured at least five times to confirm their reproducibility.
Supporting Information Available: Procedures of preparation of 3, 4, 6-8, and 10, and spectral and elemental analytical data for 3-10 (8 pages). Ordering and Internet access information is given on any current masthead page. LA980814H
