Zwitterionic Heterogemini Surfactants Containing Ammonium and

Feb 25, 2005 - IR Studies of Interfacial Interaction of the Succinic Surfactants with Different Head Groups in Highly Concentrated W/O Emulsions. I. M...
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Zwitterionic Heterogemini Surfactants Containing Ammonium and Carboxylate Headgroups. 1. Adsorption and Micellization Tomokazu Yoshimura,*,† Kanae Nyuta,† and Kunio Esumi†,‡ Department of Applied Chemistry, Faculty of Science and Institute of Colloid and Interface Science, Tokyo University of Science, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received September 7, 2004. In Final Form: January 6, 2005 Zwitterionic heterogemini surfactants with two hydrocarbon chains and two different hydrophilic groups, N,N-dimethyl-N-[2-(N′-alkyl-N′-β-carboxypropanoylamino)ethyl]-1-alkylammonium bromides (2CnAmCa, where n represents the hydrocarbon chain lengths of 8, 10, 12, and 14), were synthesized by N,N-dimethylethylenediamine with alkyl bromide, followed by reaction with succinic anhydride. One of the hydrophilic groups is a carboxylate anion, and the other is an ammonium cation. Their physicochemical properties were characterized by measuring equilibrium and dynamic surface tension, fluorescence intensity of pyrene, and light-scattering intensity. A relationship between a logarithm of critical micelle concentration (cmc) and hydrocarbon chain length showed a linear decrease upon increasing chain length and then a departure from linearity at n ) 14. This is due to the existence of premicellar aggregations at concentrations below the cmc for n ) 14. The surface tension of 2CnAmCa reached 27-30 mN m-1 at each cmc, indicating efficiencies typical of hydrocarbon chain surfactants. The adsorbing rate at the air/water interface became slow with an increase of the chain length. From the fluorescence intensity ratios of 373 and 384 nm using pyrene as a probe, for n ) 8, 10, and 14, the pyrene was solubilized in surfactant micelles at around the cmc, whereas for n ) 12 the pyrene was solubilized from a concentration of 10-fold the cmc. The scattering intensities by dynamic light scattering also increased from around these concentrations for each chain length, showing the formation of aggregates in solution.

Introduction Gemini or dimeric surfactants contain two hydrocarbon chains and two hydrophilic groups in a molecule and make up two amphiphilic moieties having the structure of conventional monomeric surfactants connected by a spacer group.1 Until now, a large number of gemini surfactants have been designed and synthesized by many researchers and investigated for their properties at the air/water, water/oil, or water/solid interface and in solution.2-4 For instance, gemini surfactants provide higher efficiency in reducing the surface tension at low concentration than the corresponding monomeric surfactants. The vast majority of work on gemini surfactants is made with symmetrical geminis, which possess identical hydrocarbon chain lengths and identical polar headgroups. In recent years, new classes of gemini surfactants with different types of headgroups, which are referred to as “heterogemini”, were reported by Alami and Holmberg.5 The heterogemini surfactants were first synthesized by Jaeger et al. in 1996.6 They are cleavable zwitterionic surfactants with two hydrocarbon chains (lengths ) 12 and 14) and two nonidentical headgroups containing both ammonium and carboxylate. It was concluded by optical * Corresponding author. E-mail address: yoshimura@ ch.kagu.tus.ac.jp (T. Yoshimura). † Department of Applied Chemistry and Faculty of Science. ‡ Institute of Colloid and Interface Science. (1) Zana, R.; Xia, J. In Gemini Surfactants; Synthesis, Interfacial and Solution-Phase Behavior, and Applications; Zana, R., Xia, J., Eds.; Dekker: New York, 2003; Chapter 1, p 1. (2) Rosen, M. J. CHEMTECH 1993, 23, 30. (3) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (4) Zana, R. J. Colloid Interface Sci. 2002, 248, 203. (5) Alami, E.; Holmberg, K. Adv. Colloid Interface Sci. 2003, 100102, 13. (6) Jaeger, D. A.; Li, B.; Clark, T. Langmuir 1996, 12, 4314.

microscopy that the cleavable heterogemini surfactants form giant vesicles. Later on several zwitterionic heterogemini surfactants with nonidentical headgroups, such as phosphodiester-quaternary ammonium salt (anioniccationic),7-10 sulfate-polyoxyethylene (anionic-nonionic),11,12 and polyoxyethylene-hydroxyl (nonionic-nonionic),13,14 have been studied. Further the zwitterionic surfactants are adsorbed on to both negatively charged and positively charged surfaces without changing the charge of the surface significantly, since they carry both negative and positive charges. The zwitterionic surfactants exhibit pH-dependent behavior, and they are less irritating to skin and eyes than anionic and cationic surfactants.15 Because of these useful characteristics, the zwitterionic surfactants are often combined with anionic or cationic surfactants in many consumer products, such as shampoos and detergents.16 In this work, we synthesized zwitterionic heterogemini surfactants of N,N-dimethyl-N-[2-(N′-alkyl-N′-β-carboxy(7) Seredyuk, V.; Alami, E.; Nyden, M.; Holmberg, K.; Peresypkin, A.; Menger, F. M. Langmuir 2001, 17, 5160. (8) Seredyuk, V.; Holmberg, K. J. Colloid Interface Sci. 2001, 241, 524. (9) Seredyuk, V.; Alami, E.; Nyden, M.; Holmberg, K.; Peresypkin, A. V.; Menger, F. M. Colloids Surf., A 2002, 203, 245. (10) Kumar, A.; Alami, E.; Holmberg, K.; Seredyuk, V.; Menger, F. M. Colloids Surf., A 2003, 228, 197. (11) Renouf, P.; Mioskowski, C.; Lebeau, L.; Hebrault, D.; Desmurs, J.-R. Tetrahedron Lett. 1998, 39, 1357. (12) Alami, E.; Holmberg, K.; Eastoe, J. J. Colloid Interface Sci. 2002, 247, 447. (13) Alami, E.; Holmberg, K. J. Colloid Interface Sci. 2001, 239, 230. (14) Abrahmsen-Alami, S.; Alami, E.; Eastoe, J.; Cosgrove, T. J. Colloid Interface Sci. 2002, 246, 191. (15) Tsubone, K.; Uchida, N.; Mimura, K. J. Am. Oil Chem. Soc. 1990, 67, 455. (16) Tsujii, K.; Okahashi, K.; Takeuchi, T. J. Phys. Chem. 1982, 86, 1437.

10.1021/la047773b CCC: $30.25 © 2005 American Chemical Society Published on Web 02/25/2005

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Langmuir, Vol. 21, No. 7, 2005 2683 Scheme 1

propanoylamino)ethyl]-1-alkylammonium bromides (2CnAmCa, where n represents the hydrocarbon chain lengths of 8, 10, 12, and 14) with nonidentical headgroups containing ammonium and carboxylate by two-step reactions and investigated their physicochemical properties such as equilibrium and dynamic surface tensions, steadystate fluorescence spectrum of pyrene, and light scattering intensity. Scheme 1 shows the synthesis route of novel zwitterionic heterogemini surfactants 2CnAmCa. Experimental and Methods Materials. N,N-Dimethylethylenediamine, n-octyl bromide, n-decyl bromide, n-dodecyl bromide, n-tetradecyl bromide, succinic anhydride, and triethylamine were obtained from Tokyo Kasei Co., Ltd. (Tokyo, Japan), and used without further purification. Acetone, chloroform, ethyl acetate, hexane, methanol, ethanol, tetrahydrofuran, and sodium hydroxide were purchased from Kanto Chemicals Co., Inc. (Tokyo, Japan). Synthesis of N,N-Dimethyl-N-[2-(alkylamino)ethyl]-1alkylammonium Bromides. n-Octyl, n-decyl, n-dodecyl, or tetradecyl bromide (66-94 g, 0.34 mol) was added to a stirred solution of N,N-dimethylethylenediamine (15 g, 0.17 mol) in about 250 mL of methanol. The mixture was refluxed for 10 h under an alkaline condition by adding NaOH. After the solvent was removed, the residue was dissolved in acetone, and the mixture was filtered to remove the inorganic salt. After acetone was removed, the product was washed with ethyl acetate and then hexane and recrystallized from mixtures of ethyl acetate and ethanol to give N,N-dimethyl-N-[2-(alkylamino)ethyl]-1-alkylammonium bromides (refer to 2CnAm, n ) 8, 10, 12, and 14) as white solids. The yields were 15, 18, 19, and 21% for 2C8Am, 2C10Am, 2C12Am, and 2C14Am, respectively. 1H NMR (JEOL JNM-EX 500 MHz, CDCl3): δ 0.882 (t, 6H, 2CH3-CH2-), 1.271.36 (m, (4n - 12)H, 2CH3-(CH2)n-3-CH2-), 1.45 (m, 2H, -CH2CH2-NH(CH2-)-), 1.74 (m, 2H, -CH2-CH2-N+(CH3)2CH2-), 2.58 (t, 2H, -CH2-CH2-NH(CH2-)-), 3.10 (t, 2H, -CH2-NHCH2-CH2- N+(CH3)2CH2-), 3.44 (s, 6H, -CH2-N+(CH3)2CH2), 3.59 (t, 2H, -NH-CH2-CH2- N+(CH3)2CH2-), and 3.82 ppm (t, 2H, -NH-CH2-CH2- N+(CH3)2CH2-). Elemental analysis (Perkin-Elmer 2400II CHNS/O): 2C8Am. Calcd for C20H45N2Br: C, 61.05; H, 11.53; N, 7.12. Found: C, 60.75; H, 11.89; N, 7.07. 2C10Am. Calcd for C24H53N2Br: C, 64.11; H, 11.88; N, 6.23. Found: C, 63.99; H, 12.36; N, 6.26. 2C12Am. Calcd for C28H61N2Br: C, 66.50; H, 12.16; N, 5.54. Found: C, 66.72; H, 12.94; N, 5.59. 2C14Am. Calcd for C32H69N2Br: C, 68.41; H, 12.38; N, 4.99. Found: C, 68.60; H, 12.98; N, 5.05. Synthesis of N,N-Dimethyl-N-[2-(N′-alkyl-N′-β-carboxypropanoylamino)ethyl]-1-alkylammonium Bromides. A 5-fold molar excess of succinic anhydride was added to a stirred solution of 2CnAm (3 g, 0.0053-0.0076 mol) dissolved in about 150 mL of tetrahydrofuran containing triethylamine (0.54-0.77 g, 0.0053-0.0076 mol). The mixture was refluxed for about 50 h. After being cooled to room temperature, the solution was filtered to remove insoluble materials. The filtrate was evaporated under reduced pressure, and the residue was dissolved in the mixture of hexane and ethanol. After the solution was cooled to 0 °C, it was filtered to remove the precipitate obtained. The solvent of filtrate was removed, and the residue was dissolved in methanol containing NaOH. The mixture was filtered, and the solvent of filtrate was removed by evaporation. The residue was put into acetone, and the mixture was filtered to remove inorganic salt. This process was carried out repeatedly using chloroform instead of acetone. After the solvent was removed, the product was washed

with hexane and dried under reduced pressure to give N,Ndimethyl-N-[2-(N′-alkyl-N′-β-carboxypropanoylamino)ethyl]-1alkylammonium bromides (2CnAmCa, n ) 8, 10, 12, and 14) as pale brown solids. The yields were 31, 39, 43, and 55% of 2CnAm for 2C8AmCa, 2C10AmCa, 2C12AmCa, and 2C14AmCa, respectively. 1H NMR (CD3OD): δ 0.902 (t, 6H, 2CH3-CH2-), 1.321.37 (m, (4n - 12)H, 2CH3-(CH2)n-3-CH2-), 1.64 (m, 2H, -CH2CH2-N+(CH3)2CH2-), 1.80 (m, 2H, -CH2-CH2-NH(CH2-)-), 2.49 (t, 2H, -CH2 (-CH2) NC(dO)CH2-), 2.58 (t, 2H, -N(dO)CH2CH2-COO), 3.11 (s, 6H, -CH2-N+(CH3)2CH2-), 3.31-3.34 (m, 4H, -CH2N(CH2-)C(dO)-CH2CH2- N+(CH3)2CH2-), and 3.72 ppm (t, 2H, -CH2CH2N(CH2-)C(dO)CH2CH2-COO). Elemental analysis: 2C8AmCa. Calcd for C24H48N2O3NaBr: C, 55.91; H, 9.38; N, 5.43. Found: C, 55.77; H, 9.75; N, 5.26. 2C10AmCa. Calcd for C28H56N2O3NaBr: C, 58.83; H, 9.87; N, 4.90. Found: C, 58.52; H, 10.01; N, 4.71. 2C12AmCa. Calcd for C32H64N2O3NaBr: C, 61.23; H, 10.28; N, 4.46. Found: C, 61.21; H, 10.52; N, 4.45. 2C14AmCa. Calcd for C36H72N2O3NaBr: C, 63.23; H, 10.61; N, 4.10. Found: C, 62.95; H, 10.86; N, 4.01. Measurements. Equilibrium Surface Tension. The surface tensions of aqueous solutions of surfactant were measured with a Kru¨ss K100 tensiometer by the Wilhelmy plate technique. Sets of measurements to obtain equilibrium surface tension were taken until the change in surface tension was less than 0.01 mN m-1 every 3 min. The critical micelle concentration (cmc) and surface tension at the cmc were determined from the break point of the surface tension and logarithm of concentration curve. The solutions of the heterogemini surfactants with concentrations above the cmc reached equilibrium within 2 h, whereas those below the cmc required a long time to stabilize. The adsorption amount of surfactants Γ for heterogemini surfactants is calculated using the following Gibbs adsorption isotherm equation17

Γ ) -(1/iRT) (dγ/d ln C)

(1)

where γ is the surface tension in mN m-1, Γ the adsorbed amount in mol m-2, R the gas constant (8.31 J mol-1 K-1), T the absolute temperature, C the surfactant concentration, and (dγ/d ln C) the slope below the cmc in the surface tension plots. The occupied area per molecule at the cmc, Acmc, is obtained from the saturated adsorption; that is, the Γcmc is the surface excess concentration at the cmc

Acmc ) 1/NΓcmc

(2)

where N is Avogadro’s number. The value of i (the number of species at the interface whose concentration changes with the surfactant concentration) is taken as 2 for the heterogemini surfactants because their solutions are regarded as 1-1 electrolyte type anionic ones in the pH range studied. Dynamic Surface Tension. The dynamic surface tension was measured using a Kru¨ss bubble pressure tensiometer BP2, a method which involves measuring the maximum pressure necessary to blow a bubble in a liquid from the tip of a capillary. The measurements were conducted with effective surface ages from 5 ms to 30 s. Conductivity. The electrical conductivity measurements of the heterogemini surfactant solution were performed on a model CM-20E TOA electrical conductivity meter. Steady-State Fluorescence. The fluorescence measurements were performed using a Hitachi 650-10S fluorescence spectro(17) Rosen, M. J.; Cohen, A. W.; Dahanayake, M.; Hua, X. Y. J. Phys. Chem. 1982, 86, 541.

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photometer. The spectra were recorded between 360 and 400 nm with the excitation wavelength at 335 nm, where the concentration of pyrene was 1 × 10-6 mol dm-3 for each solution. The fluorescence intensity ratio of the first (373 nm) to the third (384 nm) vibronic peaks, I1/I3, depends on the environment of the pyrene molecules.18,19 The environment of the pyrene is more hydrophobic when the I1/I3 ratio decreases. The I1/I3 ratio is influenced not only by the solvent polarity but also by the aggregation number and core cavity.20 Dynamic Light Scattering. Dynamic light scattering (DLS) measurements were performed with DLS-7000, Otsuka Electronics Co., Ltd., spectrophotometer using a 488 nm argon laser and at scattering angle of 30°. All solutions were filtered with a 0.2 µm membrane filter of mixed cellulose acetate before measurements. All the surfactant solutions were prepared by using Milli-Q Plus water (resistivity ) 18.2 ΜΩ cm). The pH of all the solutions was adjusted to approximately 11 by NaOH, because these surfactants could not dissolve in neutral water. All measurements were carried out at 25 °C.

Results and Discussion cmc and Surface Tension. The surface tension curves as a function of concentration for zwitterionic heterogemini surfactants 2CnAmCa with n ) 8, 10, 12, and 14 are shown in Figure 1. The surface tension of 2CnAmCa decreases with increasing surfactant concentration, reaching clear break points, which are taken as the cmc. The values of the cmc, surface tension at the cmc (γcmc), surface excess concentration (Γcmc), and occupied area per molecule at the cmc (Acmc) of 2CnAmCa are listed in Table 1, along with the data of zwitterionic monomeric surfactants of N-alkylbetaines.21a-c The values of γcmc of 2CnAmCa are much smaller than those of betaine-type monomeric surfactants with the chain length of 10-16 (36-40 mN m-1 at 23 °C),21b indicating that the heterogemini surfactants provide great efficiencies in lowering the surface tension of water. For homologous straight-chain ionic surfactants, a relation between the number of carbon atoms in the hydrophobic chain and the cmc can be written in the form22

log cmc ) A - BN

(3)

where A is a constant for a particular ionic head at given temperature and B is close to 0.3 at 35 °C for the conventional anionic and cationic surfactants. Nonionic and zwitterionic surfactants also obey this relation, but the value of B is close to 0.5. The relationship between hydrocarbon chain length of 2CnAmCa and cmc is shown in Figure 2, along with the plot of the betaine-type monomeric surfactant.21a The betaine-type monomeric surfactants show linear decreases in cmc with increasing chain length, while heterogemini surfactants studied deviate from linearity at chain length of 14. This is often the case with gemini surfactants because of premicellar aggregation at concentrations below the cmc. This plot is linear up to chain lengths of 16 or 18 for the quaternary ammonium salt type gemini surfactants23-26 and up to 12 (18) Kalyansundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (19) Ananthapadmanabhan, K. P.; Goddard, E. D.; Turro, N. J.; Kuo, P. L. Langmuir 1985, 1, 352. (20) Turro, N. J.; Kuo, P. L. J. Phys. Chem. 1986, 90, 4205. (21) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley and Sons: New York, 1989: (a) p 127, (b) p 223, (c) p 70, (d) p 84, (e) p 143. (22) Klevens, H. B. J. Phys. Colloid Chem. 1948, 52, 130. (23) Zana, R.; Levy, H. Colloids Surf., A 1997, 117, 229. (24) Devinsky, F.; Lacko, I.; Bittererova, F.; Tomeckova, L. J. Colloid Interface Sci. 1986, 114, 314. (25) Frindi, M.; Michels, B.; Levy, H.; Zana, R. Langmuir 1994, 10, 1140.

Figure 1. Variation of the surface tension with the surfactant concentration for 2CnAmCa at 25 °C: 1, n ) 8; 0, 10; [, 12; O, 14.

for nonionic gemini surfactants with glucopyranoside headgroups.27 The value of B of 2CnAmCa using the linear part of the plot where premicellization does not occur is 0.84. This value is larger than that for betaine-type monomeric surfactant (0.51) and for many gemini surfactants (0.40-0.46). This means that the decrease in the cmc with an increase of the chain length for heterogemini surfactants studied is very large. It is also noteworthy that the cmc of 2CnAmCa is smaller by about 2-3 orders of magnitude than that of the betaine-type monomeric surfactants with the same hydrocarbon chain length. Further, the heterogemini surfactants can be compared to cationic (12-2-12)28 and anionic (2C12enAm)29 gemini surfactants that possess the same two headgroups such as ammonium and carboxylate, respectively. For example, in the case of chain length of 12, the cmc of heterogemini surfactant is 1/710 of 12-2-12, and 1/7 of 2C12enAm. It is apparent that the heterogemini surfactants have the excellent micelle-forming ability at fairly low concentration, because the driving forces occur by the interaction between hydrocarbon chains connected by short spacer chain as well as by a decline of electrostatic repulsion between ammonium and carboxylate headgroups. Micellization in Solution. The micellization of surfactant has been often investigated using pyrene as the fluorescent probe. The I1/I3 ratio is usually taken as a measure of the polarity of the microenvironment near pyrene. The surfactant molecules are not micellized when the I1/I3 ratio is close to approximately 1.8, while micelles are formed when it is close to approximately 1.2. Figure 3 shows the variations of the pyrene polarity ratio I1/I3 with the concentration for 2CnAmCa with n ) 8, 10, 12, and 14, along with the scattering intensities obtained by DLS measurement. At low concentrations, maximum I1/ I3 ratio values of four heterogemini surfactants are approximately 1.8. For n ) 8 and 10, at the concentration around the respective cmc’s obtained by surface tension measurements, the I1/I3 ratio starts to decrease and the scattering intensity also starts to increase. In general, the scattering intensity increases with the number and (26) Menger, F. M.; Keiper, J. S.; Azov, V. Langmuir 2000, 16, 2062. (27) Castro, M. J.; Kovensky, J.; Cirelli, A. F. Langmuir 2002, 17, 2477. (28) Yoshimura, T.; Yoshida, H.; Ohno, A.; Esumi, K. J. Colloid Interface Sci. 2003, 267, 167. (29) Yoshimura, T.; Esumi, K. J. Colloid Interface Sci., 2004, 276, 231.

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Table 1. Physicochemical Properties of Heterogemini Surfactants 2CnAmCaa surfactant

cmc (mmol dm-3)

γcmc (mN m-1)

106Γcmc (mol m-2)

Acmc (nm2 molecule-1)

2C8AmCa 2C10AmCa 2C12AmCa 2C14AmCa C10H21N+(CH3)2CH2COO- b C12H25N+(CH3)2CH2COO- b C14H29N+(CH3)2CH2COO- b C16H33N+(CH3)2CH2COO- b

3.45 0.0501 0.00155 0.00340 18 1.8 0.18 0.020

29.8 28.2 27.7 27.2 39.7 36.5 37.5 39.7

1.21 1.22 1.22 1.24 4.15 3.57 3.53 4.13

1.37 1.37 1.36 1.34 0.40 0.47 0.47 0.40

a cmc, critical micelle concentration; γ cmc, surface tension at cmc; Γcmc, maximum surface excess concentration at cmc; Acmc, occupied area per molecule at cmc. Measured at pH 11 and 25 °C. b cmc (from ref 21a), γcmc (from ref 21b), and Γcmc and Acmc (from ref 21c), measured at 23 °C.

Figure 2. Relationship between the cmc and hydrocarbon chain length for 2CnAmCa and monomeric surfactant N-alkylbetaines: b, 2CnAmCa; 0, N-alkylbetaines.

size of particles in solution. These results indicate that the heterogemini surfactants with shorter chain lengths form the micelles from these concentrations of cmc. However, this trend is not observed for n ) 12. The I1/I3 ratio of n ) 12 is a large value even at a concentration of 10-fold of the cmc. This suggests that the surfactant may form the loose micelles at concentrations above the cmc, because the pyrene molecule makes it difficult to be solubilized in the micelles. For n ) 14, the I1/I3 ratio of the surfactant starts to decrease at a concentration around the cmc. In this case, although the premicellar aggregations such as dimers and trimers are formed in solution at the concentration below the cmc as described above, the pyrene is not solubilized into them due to less hydrophobic environment. The scattering intensity is also low within the premicellar concentrations. It seems that the premicellar aggregations grow to micelles at around cmc. It is known that the micellization of monomeric surfactants is accompanied by an abrupt decrease in I1/I3. This abrupt decrease is observed for n ) 8, whereas the I1/I3 ratio for n ) 10, 12, and 14 decreases slowly. This trend is similar to the fluorescence result of heterogemini surfactants reported by Alami and Holmberg.13 According to their suggestion, this is due to a broad micelle size distribution. At high concentrations, the minimum I1/I3 ratio values for all the heterogemini surfactants are 1.21.4, indicating that the pyrene is fully solubilized in micelles formed. Adsorption at Air/Water Interface. The occupied area per molecule, Acmc, gives the information of the packing degree of surfactant molecule adsorbed at the air/water interface. The Acmc of heterogemini surfactants does not depend on the hydrocarbon chain lengths, and

the values are 1.34-1.37 nm2 molecule-1. The Acmc values of 2CnAmCa are also approximately 3-fold of those of betaine-type monomeric surfactants and close to the addition of that of cationic monomeric dodecyltrimethylammonium bromide (0.49 nm2 molecule-1) and that of anionic monomeric sodium dodecanoate (0.69 nm2 molecule-1). This indicates that the heterogemini surfactants adsorb at the air/water interface and orient themselves without strong interactions between two hydrocarbon chains. In the case of n ) 12 and 14, it also takes 6-10 h to reach the equilibrium of surface tension in the solution of heterogemini surfactants at concentrations below the cmc. The information on micellization and adsorption of surfactants can be obtained by pC20 and cmc/C20 parameters, and standard free energy. The efficiency and effectiveness can be characterized by the value of logarithm of the surfactant concentration C20 at which the surface tension of water is reduced by 20 mN m-1 (pC20) and by the value of cmc/C20, respectively. The pC20 value measures the efficiency of adsorption of surfactant at the air/water interface; the larger the value of pC20, the greater the tendency of the surfactant to adsorb at the air/water interface, relative to its tendency to form micelles, and the more efficiently it reduces the surface tension.21d The value of cmc/C20 ratio is a convenient way of measuring effectiveness which can be correlated with structural factors on the micellization and adsorption processes; the larger the values of cmc/C20 ratio, the greater the tendency of the surfactant to adsorb at the interface, relative to its tendency to form micelles.21e The values of pC20 and cmc/ C20 of 2CnAmCa are listed in Table 2. The pC20 values of heterogemini surfactants are larger than those for the betaine-type monomeric surfactants with chain lengths of 10-16 (pC20 ) 2.59-5.5421c). It is known that the gemini surfactants have large pC20 values in comparison with the conventional monomeric surfactants30 that are consistent with the results in this study. The pC20 values also increase with the hydrocarbon chain length up to 12, and the high homologue with chain length of 14 deviates from this trend, resulting in a decrease in the pC20. This is also attributed to premicellar aggregation when the chain length becomes longer as described above. On the other hand, the cmc/C20 ratio values of heterogemini surfactants increase with an increase in hydrocarbon chain length, and they are much larger than those for the betaine-type monomeric surfactants (cmc/C20 ) 6.5-7.521e). This indicates that the heterogemini surfactants make it easy to adsorb at the air/water interface. The larger cmc/C20 ratio probably reflects the difficulty of packing by two hydrocarbon chains in micelle. The standard free energy of micellization (∆G0mic) for ionic surfactants can be calcu(30) Rosen, M. J.; Tracy, D. J. J. Surfactants Deterg. 1998, 1, 547.

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Figure 3. Variation of the pyrene fluorescence intensity ratio I1/I3 and scattering intensity by DLS with the surfactant concentration: (a) n ) 8, (b) 10, (c) 12, (d) 14. cmc, determined by surface tension measurement. Table 2. Parameters on Micellization and Adsorption of Heterogemini Surfactants 2CnAmCaa surfactant

pC20

cmc/C20

∆G0mic (kJ mol-1)

∆G0ads (kJ mol-1)

2C8AmCa 2C10AmCa 2C12AmCa 2C14AmCa

4.10 6.05 7.58 7.25

43.0 55.6 59.4 60.4

-24.2 -37.2 -54.7 -52.2

-59.2 -73.3 -91.2 -88.5

a C , the bulk surfactant concentration required to reduce the 20 surface tension of the solvent by 20 mN m-1; cmc/C20, the relative effects of some structural or microenvironmental factor on adsorption; pC20, the efficiency of surface adsorption. Measured at pH 11 and 25 °C.

lated by the equation31

∆G0mic ) (1 + β)RT ln(cmc/55.5)

(4)

where β is the apparent degree of counterion binding to the micelle/solution interface calculated from β ) 1 - R. Here R is calculated as the ratio of the slopes above and below the cmc in the electrical conductivity measurements. The values of R are taken as 0.99, 0.92, 0.73, and 0.73 for 2C8AmCa, 2C10AmCa, 2C12AmCa, and 2C14AmCa, respectively. The standard free energy of adsorption (∆G0ads) at the air/water interface is calculated by the equation32

∆G0ads ) ∆G0mic - πcmc/Γ

(5)

where πcmc is the surface pressure at the cmc ()γ0 - γcmc; γ0 is the surface tension of water and γcmc is the surface tension of surfactant solution at the cmc). The values of (31) Zana, R. Langmuir 1996, 12, 1208. (32) Rosen, M. J.; Aronson, S. Colloids Surf. 1981, 3, 201.

∆G0mic and ∆G0ads of 2CnAmCa with n ) 8-14 are also listed in Table 2. The values of ∆G0mic and ∆G0ads of the heterogemini surfactants studied are negative, indicating that they have a great ability to form micelles in solution and to adsorb at the air/water interface. Their magnitudes reveal that ∆G0ads is more spontaneous than ∆G0mic, indicating that the adsorption is promoted more than the micellization. This result is supported by the values of large pC20 and large cmc/C20 as described above. The -∆G0mic becomes large with an increase from 8 to 10 and 12 in hydrocarbon chain length, while a further increase of chain length results in a decrease in the -∆G0mic. This may be due to the stereo inhibition of two longer hydrocarbon chains of heterogemini surfactant, as it forms the micelle. Dynamic Surface Tension. The dynamic surface tension measurements of heterogemini surfactants were performed by the maximum bubble pressure technique. The adsorption rate of the heterogemini surfactants becomes slow with an increase in the hydrocarbon chain length, and those with n ) 12 and 14 do not show decrease of the surface tension even at the high concentrations above the cmc. This indicates that the dynamics of heterogemini surfactants are very slow, and the adsorption to the air/water interface is inhibited as the chain length becomes long, probably due to the steric hindrance caused by long hydrocarbon chains connected by short spacer chain. The dynamic surface tension for n ) 8 is reduced to about 42 mN m-1 even at the concentration below the cmc. On the other hand, for n ) 10, the decrease of surface tension is not observed at the concentrations below the cmc, whereas above the cmc it decreases markedly. Figure 4 shows the relationship between dynamic surface tension and logarithm of time for 2C10AmCa at the concentrations of cmc × 2.5, 5, 10, 25, and 50. The higher the concentra-

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Langmuir, Vol. 21, No. 7, 2005 2687

Figure 4. Dynamic surface tension with the surface age for 2C10AmCa at 25 °C: b, cmc × 2.5; 0, cmc × 5; 1, cmc × 10; O, cmc × 25; [, cmc × 50.

Figure 5. Dynamic surface tension plotted against t1/2 measured 2C10AmCa solutions: b, cmc × 2.5; 0, cmc × 5; 1, cmc × 10; O, cmc × 25; [, cmc × 50.

tions of heterogemini surfactants, the faster the adsorptions at the air/water interface. At the concentrations of 25- and 50-fold of cmc, the values of reduced dynamic surface tension are nearly close to the equilibrium ones, suggesting a fast adsorption process of heterogemini surfactants with chain length of 10. The decrease in surface tension can be described according to the Ward and Tordai model, as a diffusioncontrolled adsorption process to a clean surface without convection. The process can be analyzed quantitatively using the integral equation33

Γ(t) ) (4D/π)1/2(C0t1/2 +

∫0t Cs(τ) d(t - τ)1/2)

(6)

where t is time, Γ(t) the surface concentration, D the monomer diffusion coefficient, C0 the bulk concentration, Cs(t) the concentration at the subsurface, and τ a dummy time-delay variable. In an attempt to bypass the need for complicated numerical solutions when analyzing the experimental data, asymptotic equations for both shortand long-time adsorption behavior have been derived.34,35 Short-time behavior is obtained by considering only the first term of eq 6

γ - γ0 ) -2C0RT(Dt/π)1/2

(7)

where γ0 is the surface tension of solvent. The equation for long-time behavior has been derived by Hansen and Joos36,37

γt - γe ) RTΓ2/C0(π/4Dt)1/2

(8)

where γe is the equilibrium surface tension (at infinite time) and Γ is the surface excess concentration, which can be obtained from equilibrium surface tension. Note that the long-time approximation solution of eq 8 is only for

Figure 6. Dynamic surface tension plotted against t-1/2 measured 2C10AmCa solutions: b, cmc × 2.5; 0, cmc × 5; 1, cmc × 10; O, cmc × 25; [, cmc × 50.

estimating the adsorption mechanism. For the 2C10AmCa at different concentrations, the plots of dynamic surface tension versus t1/2 and t-1/2 are constructed and shown in Figures 5 and 6, respectively. These plots show a linear behavior over the shorter time scales (low t1/2) and the longer time scales (low t-1/2). From the gradients of the plots, values of D can be derived and are shown in Table 3. Using a linear fit when t equals zero in Figure 5, the intercept γ0 is obtained, and they are close to surface tension of water. From Figure 6, the lines are least-squares fits for t > 6, with the intercepts nearly equal to the equilibrium surface tension by the Wilhelmy method. The coefficient diffusion of heterogemini surfactants obtained by short-time behavior is not consistent with that in longtime behavior. According to the suggestion by Eastoe et

Table 3. Diffusion Coefficient of 2C10AmCa Calculated from Dynamic Surface Tension short time

long time

concentration (mmol dm-3)

gradient (mN m-1 s-1/2)

D (m2 s-1)

gradient (mN m-1 s1/2)

D (m2 s-1)

0.125 (cmc × 2.5) 0.251 (cmc × 5) 0.501 (cmc × 10) 1.25 (cmc × 25) 2.51 (cmc × 50)

11.6 15.9 20.8 43.5 53.9

1.09 × 10-9 5.13 × 10-10 2.20 × 10-10 1.54 × 10-10 5.92 × 10-11

63.7 54.3 25.7 8.37 5.63

1.68 × 10-13 5.76 × 10-14 6.43 × 10-14 9.70 × 10-14 5.36 × 10-14

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al.,12,38 this is due to the uncertainty of surface excess concentration obtained from surface tension measurement and a presence of an adsorption barrier. In addition, the coefficient diffusion at the long time is too small a value, indicating the diffusion of the solute molecules to the subsurface and adsorption of the solute from the subsurface to the surface.10 To investigate the shape and size of aggregates of heterogemini surfactants in solution, further details using DLS and small-angle neutron scattering (SANS) are in progress. Conclusion Novel heterogemini surfactants with ammonium and carboxylate headgroups were synthesized, and their (33) Ward, A. F. H.; Tordai, L. J. Chem. Phys. 1946, 14, 453. (34) Fainerman, V. B.; Makievski, A. V.; Miller, R. Colloids Surf., A 1994, 87, 61. (35) Eastoe, J.; Dalton, J. S. Adv. Colloid Interface Sci. 2000, 85, 103. (36) Hansen, R. S. J. Phys. Chem. 1960, 64, 637. (37) Rillaerts, E.; Joos, P. J. Phys. Chem. 1982, 86, 3471. (38) Eastoe, J.; Dalton, J. S.; Rogueda, P. G. A.; Crooks, E. R.; Pitt, A. R.; Simister, E. A. J. Colloid Interface Sci. 1997, 188, 423.

Yoshimura et al.

adsorption and micellization were investigated by equilibrium and dynamic surface tension, pyrene fluorescence, and DLS measurements. The heterogemini surfactants show high surface activities such as low cmc and high efficiency in lowering surface tension. The shorter the hydrocarbon chain lengths of surfactants are, the faster the adsorption rate to the interface becomes and the greater the tendency to adsorb at the interface, relative to the tendency to form micelles. The fluorescence intensity ratio of the heterogemini surfactants using pyrene as a probe is influenced significantly by their hydrocarbon chain lengths. It was found that for n ) 8 the micelles are formed at around the cmc, whereas for n ) 10, 12, and 14 the broad micelles in size distribution are formed at the concentrations above the cmc. Particularly, in the case of n ) 12 loose micelles are formed, and they grow to the micelles with the concentration. On the other hand, for n ) 14, the premicellar aggregations are formed at the concentrations below the cmc, and then above the cmc the micelles are formed. LA047773B