Langmuir 1996, 12, 4039-4041
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Notes Aqueous Properties of Multichain Quaternary Cationic Surfactants Kunio Esumi,*,† Kazuhiro Taguma,† and Yoshifumi Koide‡ Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan, and Department of Applied Chemistry, Faculty of Engineering, Kumamoto University, Kurokami, Kumamoto 860, Japan Received March 11, 1996. In Final Form: April 23, 1996
Introduction Conventional single-chain surfactant molecules consist of hydrophobic and hydrophilic parts. With increasing concentrations, they form micelles and then lyotropic mesophases. These organized microstructures are considerably dependent on their own structure. Further, double-chain and triple-chain surfactants usually form bilayer sheets or vesicles. Recently, Zana et al.1,2 synthesized dimeric or trimeric surfactants in which two or three quarternary ammonium species are linked at the level of the head groups by a hydrocarbon spacer and reported their physicochemical properties in aqueous solution.3,4 Menger and Littau5 also synthesized a new type of surfactants in which the molecules possess a long hydrocarbon chain, an ionic group, a rigid spacer, an ionic group, and another hydrocarbon tail. These surfactants are often called “gemini” surfactants. It was found6 that the microstructures of gemini surfactants are significantly affected by the chain length of the spacer. When gemini surfactant is expressed as m-n-m, where m and n are the carbon numbers of the side alkyl chains and of the alkanediyl spacer, the solution of 12-2-12 showed shear-thickening and became viscoelastic with the concentration, while only spherical or spheroidal micelles were present in solutions of 12-4-12, 12-8-12, and 12-12-12. Thus, the gemini surfactants tend to form aggregates of lower curvature in aqueous solutions than the corresponding “monomeric” surfactants. The SANS analysis also showed7 that the extent of growth and the variation of shape of the dimeric surfactant micelles strongly depend on the spacer chain length. We have recently synthesized di- and tri(quaternary ammonium) surfactants such as 1,2-bis(dodecyldimethylammonio)ethane dibromide and methyldodecylbis[2(dimethyldodecylammonio)ethyl]ammonium tribromide. It is still interesting to compare the physicochemical properties of the trimeric surfactant with those of the corresponding monomeric and dimeric surfactants. Al* To whom correspondence should be addressed.
[email protected]. † Science University of Tokyo. ‡ Kumamoto University.
E-mail:
(1) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (2) Zana, R.; Levy, H.; Papoutsi, D.; Beinert, G. Langmuir 1995, 10, 3694. (3) Alami, E.; Levy, H.; Zana, R. Langmuir 1993, 9, 940. (4) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9, 1465. (5) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451. (6) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448. (7) Hirata, H.; Hottori, N.; Ishida, M.; Okabayashi, H.; Furusaka, M.; Zana, R. J. Phys. Chem. 1995, 99, 17778.
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though a series of gemini surfactant studies have been reported by Zana et al.,1-4 the physicochemical properties of multichain quaternary cationic surfactants with a spacer (n ) 2) have not been elucidated in detail. In this paper, the aqueous properties of the quarternary cationic surfactants mentioned above were characterized by surface tension, electrical conductivity, fluorescence probe, and light-scattering measurements. Experimental Section Materials. Dodecyltrimethylammonium bromide (1RQ) was obtained from Tokyo Kasei Co. and used after crystallization from acetone. Dimeric (1,2-bis(dodecyldimethylammonio)ethane dibromide, 2RenQ) and trimeric (methyldodecylbis[2-(dimethyldodecylammonio)ethyl]ammonium tribromide, 3RdienQ) surfactants were synthesized as follows. The reaction of N,N,N′,N′tetramethylethylenediamine with lauryl bromide in ethanol under reflux at 80 °C for 8 h yielded 2RenQ, which was recrystallized from mixtures of hexane and ethanol. The elemental analytical values of the recrystallized 2RenQ were in good agreement with the calculated values (C (%) found 56.61, calculated 56.98; H (%) found 10.64, calculated 10.44; N (%) found 4.51, calculated 4.43). Also, for 3RdienQ, lauryl bromide was added dropwise to N,N,N′,N′′,N′′-pentamethyldiethylenetriamine in 1-propanol under reflux at 100 °C for 1 h and the reaction was carried out for a further 40 h. The obtained samples were washed with acetone. The elemental analysis of 3RdienQ thus obtained was not satisfactory because the samples were mixtures of 3RdienQ and 2RdienQ. Accordingly, 3RdienQ was separated from the mixtures with a column (alumina; elusion solvent, ethyl acetate/ethanol ) 1/1). The confirmation of 3RdienQ was carried out by TLC. Then, the solid 3RdienQ sample was obtained by removing the solvent under vacuum. The elemental analytical data of 3RdienQ purified were as follows: C (%) found 53.98, calculated 53.97; H (%) found 10.09, calculated 10.78; N (%) found 4.35, calculated 4.20 (where a formula of 3RdienQ‚4.5H2O was used for calculation). Pyrene was obtained from Aldrich Chemical Co. and was recrystallized several times from ethanol. The water used in this study was purified through a Milli-Q System until the electrical conductivity fell down below 0.1 µS cm-1. The chemical structures of the surfactants used in this study are given in Figure 1. Measurements. The surface tensions of the surfactant aqueous solutions were measured with a KRUSS tensiometer K12. Static light scattering measurements were performed on a light-scattering spectrophotometer, Model DLS-700Ar (Otsuka Electronics Co., Ltd). The light source used was an argon ion laser (wavelength 488 nm). The electrical conductivities of the surfactant solutions were also measured with a TOA electrical conductivity meter. Fluorescence spectra of pyrene were obtained using a fluorescence spectrophotometer (Hitachi 650-10S). The excited wavelength was 342 nm. All measurements were carried out at 25 °C.
Results and Discussion The surface tension as a function of the logarithm of the concentration for the aqueous solutions of 1RQ, 2RenQ, and 3RdienQ is shown in Figure 2. The surface tension decreased gradually with increasing surfactant concentration and then showed a break point, which was taken as the critical micelle concentration (cmc). The cmc values of 1RQ, 2RenQ, and 3RdienQ were 1.4 × 10-2, 9.0 × 10-4, and 8.0 × 10-5 mol dm-3, respectively, indicating that the cmc values are markedly decreased with increasing dodecyl chain number of the surfactants. In the case of 3RdienQ, the surface tension above the cmc decreased linearly with the surfactant concentration. Alami et al.4 © 1996 American Chemical Society
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Figure 1. Chemical structures of 1RQ, 2RenQ, and 3RdienQ.
Notes
Figure 3. Change of the electrical conductivity with the concentration of 1RQ, 2RenQ, and 3RdienQ at 25 °C. Table 1. Physicochemical Properties of 1RQ, 2RenQ, and 3RdienQ cmc γcmc -[dγ/ 106Γ A (nm2/ surfactant (mmol dm-3) (mN m-1) d log C] (mol m-2) molecule) 1RQ 2RenQ 3RdienQ
14.0 0.9 0.08
38.6 31.4 25.2
39.1 39.5 29.5
3.42 2.31 1.29
0.49 0.72 1.28
0.19 for 2RenQ; 0.93 for 3RdienQ. The micelle ionization degree of 3RdienQ is quite large compared with those of 1RQ and 2RenQ, suggesting 3RdienQ micelles of fairly small aggregation number. Very interestingly, the micelle ionization degree of 3RdienQ is extremely large compared with 0.242 of 12-3-12-3-12 3Br- in spite of only one methylene chain difference as a spacer. The surface excess Γ at the air-water interface can be calculated by applying the Gibbs adsorption isotherm equation Figure 2. Change of the surface tension with the concentration of 1RQ, 2RenQ, and 3RdienQ at 25 °C.
Γ ) -(1/iRT)(dγ/d ln C)
reported a similar reduction in the surface tension above the cmc for gemini surfactants. The decrease in the cmc with the chain number was also observed for the 12-3-12 and 12-3-12-3-12 series.2 In addition, the surface tension values at the cmc (γcmc) were in the order 3RdienQ (25.2 mN m-1) < 2 RenQ (31.4 mN m-1) < 1RQ (38.6 mN m-1). The γcmc of 2RenQ was much lower compared with that of 12-3-12, indicating that the shorter the spacer the greater the reduction of surface tension of the gemini surfactants. These cmc values were in good agreement with those determined by the electrical conductivity measurements: the variation of the electrical conductivity with the surfactant concentration shows a break which is taken as the cmc (Figure 3). Furthermore, the micelle ionization degree at the cmc was taken as the ratio of the values of dκ/dC above and below the cmc:8 0.24 for 1RQ;
where γ is the surface tension and C is the surfactant concentration. i equals 2, 3, and 4 for 1RQ, 2RenQ, and 3RdienQ, respectively. The occupied area (A) per surfactant molecule is calculated from A ) 1/NΓ, where N is Avogadro’s number. The occupied areas were 0.49, 0.72, and 1.28 nm2 for 1RQ, 2RenQ, and 3RdienQ, respectively. The data obtained from the surface tension measurements are given in Table 1. It has been reported4 that with a short spacer (s < 8), the spacer is in contact with water, lying more or less stretched at the air-water interface. From the occupied area obtained by the surface tension data, it is inferred that the molecular compactness of 2RenQ and 3RdienQ at the air-water interface is much higher than that of 1RQ. (8) Zana, R. J. Colloid Interface Sci. 1980, 78, 330.
Notes
Figure 4. Change of the I1/I3 ratio of pyrene with the concentration of 1RQ, 2RenQ, or 3RdienQ at 25 °C.
The fluorescence spectrum of micelle-bound pyrene is sensitive to the polarity of the microenvironment at the site of solubilization of the fluorophore.9 In this study, the polarity of the micelle interior was evaluated by the intensity ratio I1/I3 of the first and third vibronic bands of monomeric pyrene. Figure 4 shows the variations of I1/I3 as a function of the surfactant concentration. As the (9) Kalynasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.
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surfactant concentration increased, the I1/I3 ratio decreased abruply at the cmc, indicating that pyrene is solubilized in the hydrophobic interior of the micelles. Above the cmc, the I1/I3 ratio decreased with increasing chain number from 1RQ to 2RenQ, whereas the I1/I3 ratio of 3RdienQ was located between those of 1RQ and 2RenQ. This suggests that the polarity of the 3RdienQ micellar interior sensed by pyrene is higher than that of 2RenQ. Static light-scattering measurements were performed to estimate the aggregation number of the micelles. From the Debye plots the aggregation numbers of 1RQ and 2RenQ at their cmc’s were determined to be about 51 and 60, respectively. However, we were unable to determine the aggregation number of 3RdienQ, since the intensity of light scattering was very low even above the cmc. This result for 3RdienQ is also understood by a high micelle ionization degree determined by the electrical conductivity method. It seems likely that the aggregation number for 3RdienQ is very small due to steric hindrance of the hydrophobic chains in the micelle and due to the electrostatic repulsion force. According to Zana et al.,2 the aggregation number expressed as the number of dodecyl chains per micelle is increased in the following order 1RQ (DTAB) < 12-3-12 < 12-2-12 < 12-3-12-3-12. However, in this study since the aggregation number of 3RdienQ is very small compared to that of 12-3-12-3-12, the spacer carbon number in the trimer series is a critical factor to consider concerning micelle formation. Acknowledgment. We are grateful to M. Mizuta for synthesizing the surfactants. LA960230K
