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Sugar-Based Gemini Surfactants with Peptide BondssSynthesis, Adsorption, Micellization, and Biodegradability Tomokazu Yoshimura,* Kana Ishihara, and Kunio Esumi Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received June 16, 2005. In Final Form: August 9, 2005 The sugar-based gemini surfactant with peptide bonds, N,N′-bisalkyl-N,N′-bis[2-(lactobionylamide)ethyl]hexanediamide (2CnpeLac, in which n represents hydrocarbon chain lengths of 12 and 16), was synthesized by reacting adipoyl chloride with the corresponding monomeric surfactant N-alkyl-N′lactobionylethylenediamine (CnpeLac), which was obtained by reacting ethylenediamine with alkyl bromide and lactobionic acid. The adsorption and micellization properties of CnpeLac and 2CnpeLac were characterized by the measurement of their equilibrium and dynamic surface tension, steady-state fluorescence using pyrene as a probe, dynamic light scattering (DLS), and time-resolved fluorescence quenching (TRFQ), and their biodegradability was also investigated. The critical micelle concentration (cmc) decreases with an increase in the hydrocarbon chains from monomeric to gemini surfactants, whereas it increases with an increase in the chain length from 12 to 16 for both systems. The increases in both the hydrocarbon chain and the chain length of sugar-based surfactants reduce surface activities such as the ability to lower the surface tension, the occupied area per molecule, and the adsorption rate at the air/water interface. The sugar-based surfactants CnpeLac and 2CnpeLac exhibit unique aggregation behavior in aqueous solution. The DLS results indicate that the apparent hydrodynamic diameter of CnpeLac micelles decreases sharply with increasing concentration, whereas that of 2CnpeLac micelles decreases gradually. From the TRFQ measurement, it was observed that, as concentration increases, the aggregation numbers are almost constant for CnpeLac, whereas they increase for 2CnpeLac. These results imply that loosely packed micelles formed by sugar-based surfactants become tightly packed micelles as the concentration increases. Furthermore, it was found that 2CnpeLac shows lower biodegradability than does CnpeLac because it contains tertiary amines in the molecule.
Introduction Gemini surfactants contain two hydrophobic groups and two hydrophilic groups, which are connected by a linkage close to the hydrophilic groups. Thus far, hundreds of papers and patents regarding gemini surfactants have appeared in the literature, and they were reviewed by Rosen,1a Menger,2 and Zana.3 The gemini surfactants are known to exhibit properties such as lower critical micelle concentration (cmc), greater efficiency in lowering the surface tension of water and the interfacial tension between water and oil, and better solubility in water than conventional monomeric surfactants. Many papers pertaining to the synthesis and properties of anionic and cationic gemini surfactants have been published; however, there are very few studies on nonionic and zwitterionic gemini surfactants. The majority of the conventional nonionic surfactants are polyoxyethylene- or propylene chain-containing compounds.4 From the viewpoint of human health and the conservation of the environment, sugar-based nonionic surfactants with a carbohydrate moiety as a hydrophilic group have attracted considerable attention.5 They are used in applications such as detergents, dishwashing agents, and personal care products.6,7 * Corresponding ch.kagu.tus.ac.jp.
author.
E-mail
address:
yoshimura@
(1) Rosen, M. J. Surfactants and Interfacial Phenomena, 3rd ed.; John Wiley and Sons: New York, 2004; (a) p 415. (b) p 60. (c) p 83. (d) p 149. (e) p 157. (2) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906. (3) Zana, R. In Gemini Surfactants; Synthesis, Interfacial and Solution - Phase Behavior, and Applications; Zana, R., Xia, J., Eds.; Dekker: New York, 2003; p 141. (4) Schick, M. J., Ed. In Nonionic Surfactants; Dekker: New York, 1966.
Interestingly, the cloud point observed in polyoxyethylenebased nonionic surfactants are not observed in the sugarbased surfactants.8-10 The study of nonionic gemini surfactants is relatively new, and it is likely that these surfactants will be further investigated in the future. Nonionic gemini surfactants with polyoxyethylene headgroups were previously synthesized,11 and they showed a much lower cmc than the corresponding monomeric and ionic gemini surfactants. Recently, a few sugar-based nonionic gemini surfactants were designed and synthesized.10,12-17 Wilk et al.12,13 reported the synthesis of aldonamide-type gemini surfactants with a glucose- and (5) Burczyk, B. In Novel Surfactants; Preparation, Applications, and Biodegradability, 2nd ed.; Holmberg, K., Ed.; Dekker: New York, 2003; Chapter 4, p 129. (6) Andree, H.; Middelhauve, B. Tenside, Surfactants, Deterg. 1991, 28, 413. (7) Busch, P.; Hensen, H.; Tesmann, H. Tenside, Surfactants, Deterg. 1993, 30, 116. (8) Eastoe, J.; Rogueda, P.; Harrison, B. J.; Howe, A. M.; Pitt, A. R. Langmuir 1994, 10, 4429. (9) Eastoe, J.; Rogueda, P.; Howe, A. M.; Pitt, A. R.; Heenan, R. K. Langmuir 1996, 12, 2701. (10) Pestman, J. M.; Terpstra, K. R.; Stuart, M. C. A.; van Doren, H. A.; Brisson, A.; Kellogg, R. M.; Engberts, J. B. F. N. Langmuir 1997, 13, 6857. (11) Paddon-Jones, G.; Regismond, S.; Kwetkat, K.; Zana, R. J. Colloid Interface Sci. 2001, 243, 496. (12) Wilk, K. A.; Syper, L.; Domagalska, B. W.; Komorek, U.; Maliszewska, I.; Gancarz, R. J. Surfactants Deterg. 2002, 5, 235. (13) Komorek, U.; Wilk, K. A. J. Colloid Interface Sci. 2004, 271, 206. (14) Bergsma, M.; Fielden, M. L.; Engberts, J. B. F. N. J. Colloid Interface Sci. 2001, 243, 491. (15) Johnsson, M.; Wagenaar, A.; Stuart, M. C. A.; Engberts, J. B. F. N. Langmuir 2003, 19, 4609. (16) Johnsson, M.; Wagenaar, A.; Engberts, J. B. F. N. J. Am. Chem. Soc. 2003, 125, 757. (17) Castro, M. J. L.; Kovensky, J.; Cirelli, A. F. Langmuir 2002, 18, 2477.
10.1021/la051614q CCC: $30.25 © 2005 American Chemical Society Published on Web 10/07/2005
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Yoshimura et al. Scheme 1
lactobionic acid-derived hydrophilic part, along with its surface and biological properties. Cirelli et al.17 synthesized sugar-based nonionic gemini surfactants from protected alkyl glucosides and dibasic acid dichloride and investigated the effects of the position of linkage, anomeric configuration, and the nature of the spacers on their properties. The main purpose of this study is to develop novel sugarbased gemini surfactants with better properties than those already reported. In this paper, we describe the synthesis of a novel sugar-based gemini surfactant, N,N′-bisalkyl-N,N′-bis[2-(lactobionylamide)ethyl]hexanediamide (2CnpeLac, in which n represents hydrocarbon chain lengths of 12 and 16), along with the corresponding monomeric surfactant N-alkyl-N′-lactobionylethylenediamine (CnpeLac, n ) 12, 16) for comparison, as well as their adsorption and micellization properties. Because the sugar-based gemini compounds in this study contain two peptide bonds and two amide bonds, they can be expected to be environmentally acceptable surfactants. Scheme 1 shows the synthesis route of the novel sugar-based gemini surfactant 2CnpeLac. Experimental Methods Materials. Lactobionic acid from Aldrich and ethylenediamine from Kanto Chemicals Co., Inc. (Tokyo, Japan) were used without further purification. n-Dodecyl bromide, n-hexadecyl bromide, and adipoyl chloride were obtained from Tokyo Kasei Co., Ltd. (Tokyo, Japan) and used without further purification. Acetone, acetonitrile, diethyl ether, ethanol, ethyl acetate, hexane, methanol, NaOH, and tetrahydrofuran were purchased from Kanto Chemicals Co., Inc. Synthesis. a. N-Dodecylethylenediamine Hydrochloride (C12). n-Dodecyl bromide (49.8 g, 0.2 mol) was added to a stirred solution of ethylenediamine (60.1 g, 1.0 mol) in methanol containing NaOH. The mixture was refluxed for over 30 h under alkaline conditions by adding NaOH. The mixture was filtered under hot conditions to remove the inorganic salt formed. After the filtrate was evaporated, the residue was poured in the NaOH solution, adjusted to a pH of ∼12, and stirred. The solution was extracted with diethyl ether, and hydrochloric acid gas was injected into the diethyl ether for 3 h. The precipitate obtained was collected by filtration; the product was washed twice with hexane, and it was recrystallized from ethanol to produce C12 as a white solid in 39% yield. 1H NMR (JEOL JNM-EX 500 MHz, D2O): δ 0.864 (t, 3H, CH3-CH2-), 1.26-1.38 (m, 18H, CH3-(CH2)9-CH2-), 1.70 (m, 2H, CH3-(CH2)9-CH2-), 3.11 (t, 2H, -CH2-N-CH2CH2-N), and 3.38 ppm (m, 4H, -CH2-N-CH2-CH2-N).
Elemental analysis (Perkin-Elmer 2400II CHNS/O) calcd for C14H32N2 2HCl: C, 55.80; H, 11.37; N, 9.30. Found: C, 55.70; H, 11.60; N, 9.19. b. N-Haxadecylethylenediamine Hydrochloride (C16). C16 was synthesized using the same procedure as described above (section a) with n-hexadecyl bromide instead of n-dodecyl bromide. Yield: 40% (white solid). 1H NMR (D2O): δ 0.823 (t, 3H, CH3CH2-), 1.20-1.36 (m, 18H, CH3-(CH2)9-CH2-), 1.66 (m, 2H, CH3-(CH2)9-CH2-), 3.04 (t, 2H, -CH2-N-CH2-CH2-N), and 3.35 ppm (m, 4H, -CH2-N-CH2-CH2-N). Elemental analysis calcd for C18H40N2 2HCl: C, 60.48; H, 11.84; N, 7.84. Found: C, 61.40; H, 11.77; N, 7.56. c. N-Dodecyl-N′-lactobionylethylenediamine (C12peLac). A solution of C12 in 1N NaOH was stirred while being heated for 3 h. After it was cooled to room temperature, the oily material was extracted with diethyl ether, and the solvent was evaporated under reduced pressure. The solution was cooled to 0 °C after the residue was dissolved in ethanol, and the precipitate obtained was removed by filtration. The removal of the ethanol yielded N-dodecylethylenediamine. N-Dodecylethylenediamine (11.4 g, 0.05 mol) was added dropwise to a stirred solution of lactobionic acid (17.9 g, 0.05 mol) dissolved in methanol under reflux, and the reaction was carried out for 40 h. The mixture was filtered under hot conditions to remove the lactobionic acid; there was no reaction, and the solvent was evaporated under reduced pressure. The residue was washed twice with acetone and then dissolved in mixtures of methanol and ethyl acetate. The solution was cooled to 0 °C, and the precipitate obtained was removed by filtration. This process was carried out several times. The removal of the solvents produced C12peLac as a pale brown solid in 53% yield. 1H NMR (D2O): δ 0.882 (t, 3H, CH3-CH2-), 1.22-1.42 (m, 18H, CH3(CH2)9-CH2-), 1.52 (t, 2H, CH3-(CH2)9-CH2-), 2.62 (m, 2H, -CH2-N-CH2-CH2-N-CO-), 2.77 (m, 2H, -CH2-N-CH2CH2-N-CO-), 3.41 (m, 2H, -CH2-N-CH2-CH2-N-CO-), and 3.57-4.57 ppm (m, 13 H, protons of sugar group). Elemental analysis calcd for C26H52N2O11: C, 54.91; H, 9.22; N, 4.93. Found: C, 54.72; H, 9.27; N, 4.82. d. N-Hexadecyl-N′-lactobionylethylenediamine (C16peLac). C16peLac was synthesized using the same procedure described above (section c). Yield: 55% (pale brown solid). 1H NMR (D2O): δ 0.860 (t, 3H, CH3-CH2-), 1.15-1.40 (m, 26H, CH3-(CH2)13CH2-), 1.50 (t, 2H, CH3-(CH2)13-CH2-), 2.59 (m, 2H, -CH2N-CH2-CH2-N-CO-), 2.75 (m, 2H, -CH2-N-CH2-CH2-NCO-), 3.39 (m, 2H, -CH2-N-CH2-CH2-N-CO-), and 3.554.54 ppm (m, 13 H, protons of sugar group). Elemental analysis calcd for C30H60N2O11: C, 57.67; H, 9.68; N, 4.48. Found: C, 57.19; H, 9.44; N, 4.47. e. N,N′-Bisdodecyl-N,N′-bis[2-(lactobionylamide)ethyl]hexanediamide (2C12peLac). Adipoyl chloride was added to a stirred
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solution of C12peLac in tetrahydrofuran at 5 °C, and the mixture was stirred for 24 h. After the solvent was removed by evaporation, the residue obtained was washed with ethyl acetate and then recrystallized several times from the mixtures of ethanol and acetonitrile, producing 2C12peLac as a pale brown solid in 20% yield. 1H NMR (D2O): δ 0.849 (t, 6H, 2CH3-CH2-), 1.20-1.35 (m, 36H, 2CH3-(CH2)9-CH2-), 1.62 (m, 4H, -N-CO-CH2(CH2)2-CH2-CO-N-), 1.68 (t, 4H, 2CH3-(CH2)9-CH2-), 2.342.45 (m, 4H, -N-CO-CH2-(CH2)2-CH2-CO-N-), 3.05 (m, 4H, 2-CH2-N(-CH2-CH2-NH-CO-)-CO-), 3.23 (m, 4H, 2-CO-N(-CH2-)-CH2-CH2-NH-CO-), and 3.33-4.60 ppm (m, 30H, 2-CO-N(-CH2-)-CH2-CH2-NH-CO-, protons of sugar groups). Elemental analysis calcd for C58H110N4O24 2H2O: C, 54.27; H, 8.95; N, 4.37. Found: C, 54.11; H, 9.08; N, 4.30. f. N,N′-Bishexadecyl-N,N′-bis[2-(lactobionylamide)ethyl]hexanediamide (2C16peLac). 2C16peLac was synthesized using the same procedure described above (section e). The purification of 2C16peLac was carried out by recrystallization from methanol. Yield: 25% (pale brown solid). 1H NMR (D2O): δ 0.829 (t, 6H, 2CH3-CH2-), 1.15-1.39 (m, 52H, 2CH3-(CH2)13-CH2-), 1.61 (m, 4H, -N-CO-CH2-(CH2)2-CH2-CO-N-), 1.70 (t, 4H, 2CH3-(CH2)13-CH2-), 2.31-2.45 (m, 4H, -N-CO-CH2(CH2)2-CH2-CO-N-), 3.04 (m, 4H, 2-CH2-N(-CH2-CH2NH-CO-)-CO-), 3.22 (m, 4H, 2-CO-N(-CH2-)-CH2-CH2NH-CO-), and 3.30-4.55 ppm (m, 30H, 2-CO-N(-CH2-)CH2-CH2-NH-CO-, protons of sugar groups). Elemental analysis calcd for C64H126N4O24 H2O: C, 57.54; H, 9.36; N, 4.07. Found: C, 57.14; H, 9.29; N, 4.06. Measurements. All of the surfactant solutions were prepared using Milli-Q Plus water (resistivity ) 18.2 ΜΩ cm). All measurements were carried out at 25 °C. Equilibrium Surface Tension. The surface tensions of aqueous solutions of the surfactant were measured with a Kru¨ss K100 tensiometer using the Wilhelmy plate technique. To obtain equilibrium surface tension, sets of measurements were performed until the change in the surface tension was less than 0.01 mN m-1 every 80 s. The cmc and the surface tension at the cmc were determined from the breakpoint of the surface tension and the logarithm of the concentration curve. The solutions of the surfactants with concentrations above the cmc reached equilibrium within 3 h, whereas those with concentrations below the cmc required 12-20 h to stabilize. The surfactant surface excess concentration at the air/solution interface (Γ) in mol m-2 was calculated using the following Gibbs adsorption isotherm equation:1b
Γ ) -(1/iRT) (dγ/d ln C)
(1)
in which γ represents the surface tension in mN m-1, R is the gas constant (8.31 J mol-1 K-1), T is the absolute temperature, C is the surfactant concentration, and (dγ/d ln C) is the slope below the cmc in the surface tension plots. The area occupied by the surfactant molecule at the air/solution interface, Acmc, was obtained from the saturated adsorption as follows:
Acmc ) 1/NΓcmc
(2)
in which N is Avogadro’s number, and Γcmc represents the surface excess concentration at the cmc. The value of i (the number of species at the interface for which the concentration changes with the surfactant concentration) is taken as 1 for the dilute solution of a nonionic surfactant. To estimate the adsorption and aggregation properties of sugarbased surfactants, we use parameters such as pC20, cmc/C20, ∆ 0 G0mic, and ∆Gads . Here, C20 represents the surfactant concentration required to reduce the surface tension of water by 20 mN m-1.1c The values of pC20 and the cmc/C20 ratio represent the efficiency of adsorption of the surfactant at the air/water interface and the effectiveness that can be correlated with structural factors with regard to the adsorption and micellization processes, respectively. The larger the value of pC20, the greater the tendency of the surfactant to adsorb at the air/water interface.1c The larger the value of the cmc/C20 ratio, the greater the tendency of the surfactant to adsorb at the interface, relative to its tendency to form micelles.1d The standard free energy of micellization ∆G0mic for nonionic surfactants can be calculated by the equation1e
0 ∆Gmic ) RT ln xcmc
(3)
When the cmc is less than 10-2 mol dm-3, this can be approximated without significant error by the equation 0 ∆Gmic ) RT ln (cmc/ω)
(4)
in which xcmc represents the molar fraction of the surfactant in the liquid phase at the cmc, which is expected to be in molar units, and ω represents the number of moles per liter of water at the absolute temperature (55.3 at 25 °C). The standard free 0 at the air/water interface is calcuenergy of adsorption ∆Gads lated by the equation18 0 0 ∆Gads ) ∆Gmic - πcmc/Γcmc
(5)
in which πcmc represents the surface pressure at the cmc () γ0 - γcmc; γ0 and γcmc represent the surface tension of water and that of the surfactant solution at the cmc, respectively). Dynamic Surface Tension. The dynamic surface tension was measured using a Kru¨ss bubble pressure tensiometer BP2sa method that 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 0.1 to 1.7 s. Dynamic Light Scattering. Dynamic light scattering (DLS) measurements were performed with a DLS-7000, Otsuka Electronics Co., Ltd. spectrophotometer. Vertically polarized light of 488-nm wavelength from an Ar ion laser (65 mW) was used as the incident beam. The measurement was conducted at a scattering angle of 30°. All the solutions were filtered with a 0.2-µm membrane filter of mixed cellulose acetate before the measurements. For spherical particles, the diffusion coefficient extrapolated to zero concentration (D0) is converted into the apparent hydrodynamic radius (RH) by the Stokes-Einstein equation:19
D0 ) kT/(6πηRH)
(6)
in which k represents the Boltzmann constant, T is the absolute temperature, and η is the viscosity of the solution. The size distribution was estimated from the correlation function profile using the histogram method. Time-Resolved Fluorescence Quenching. Time-resolved fluorescence quenching (TRFQ) measurements were performed with a HORIBA-JOBIN-YVON FluoroCube 5000U lifetime system by the time-correlated single photon counting (TCSCO) method. The excitation wavelength was 316 nm, and the emission was monitored at 402 nm. Pyrene and cetylpyridinium chloride were used as the fluorescence probe and the quencher of the probe, respectively. The TRFQ data for the surfactant solutions were analyzed in the generalized version of the equation proposed by Infelta20 and Tachiya:21
I(t) ) A1 exp{-A2t - A3[1 - exp(-A4t)]}
(7)
The parameters A1, A2, A3, and A4 are given by
A1 ) I(0); A2 ) 1/τ; A3 ) Cq/Cm; A4 ) kq
(8)
in which I(0) is the fluorescence intensity at time t ) 0, τ is the fluorescence lifetime, and kq is the rate constant for intramicellar quenching. Cq and Cm represent the concentration of the quencher and micelle, respectively. The micelle aggregation number N can be calculated from
N ) A3(C - cmc)/Cq
(9)
in which C represents the surfactant concentration. The molar ratio Cp/Cm, in which Cp represents the concentration of the probe, must be maintained at a low value (below 0.02 to prevent the formation of pyrene excimers). A3 is also adjusted to be ∼1.25. (18) Rosen, M. J.; Aronson, S. Colloids Surf. 1981, 3, 201.
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Yoshimura et al. Table 1. Parameters of Micellization and Adsorption at the Air/Water Interface of the Sugar-Based Surfactants 2CnpeLac and CnpeLac γcmc 106Γcmc Acmc cmc surfactant (mmol dm-3) (mN m-1) (mol m-2) (nm2/molecule) 2C12peLac 2C16peLac C12peLac C16peLac
Figure 1. Variation in the surface tension as a function of surfactant concentration for sugar-based surfactants at 25 °C: 9 ) C12peLac; 0 ) C16peLac; b ) 2C12peLac; and O ) 2C16peLac. Values of A3 that are larger than 2 must be avoided for theoretical reasons22 and because such a high micelle quencher loading may affect the micelle structure.23 Steady-State Fluorescence. The fluorescence measurements were performed using a Hitachi 650-10S fluorescence spectrophotometer. The spectra were between 360 and 400 nm at an excitation wavelength of 335 nm and a pyrene concentration of 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.24,25 The environment of pyrene is more hydrophobic when the I1/I3 ratio decreases. Biodegradability. The biodegradability of the surfactants was evaluated with an oxygen consumption method based on the modified MITI test.26 The activated sludge was obtained from a municipal sewage treatment plant in Osaka City (Japan). Biochemical oxygen demand (BOD) after 7, 14, 21, and 28 days was determined by the quantity of oxygen consumed. The biodegradability was estimated using the following equation:
Biodegradability (%) ) [(BOD - blank)/TOD] × 100 (10) in which blank represents the oxygen consumption (mg) in the blank dispersion, and TOD represents the theoretical oxygen demand (mg).
Results and Discussion Adsorption Properties. The sugar-based monomeric CnpeLac surfactants and the 2CnpeLac gemini surfactants show good water solubility at 25 °C. The surface tension curves as a function of the concentration for CnpeLac and 2CnpeLac when n ) 12 and 16 are shown in Figure 1. The surface tensions decrease as the concentrations increase, reaching clear break points, which are assumed to be the cmc. The values of the cmc, surface tension at the cmc (19) Einstein, A. Ann. Phys. 1905, 17, 549. (20) Infelta, P. P.; Gra¨tzel, M.; Thomas, J. K. J. Phys. Chem. 1974, 78, 190. (21) Tachiya, M., Chem. Phys. Lett. 1975, 33, 289. (22) Almgren, M.; Lo¨froth, J.-E.; van Stam, J. J. Phys. Chem. 1986, 90, 4431. (23) Alargova, R. G.; Kochijashky, I. I.; Sierra, M. L.; Zana, R. Langmuir 1998, 14, 5412. (24) Kalyansundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (25) Ananthapadmanabhan, K. P.; Goddard, E. D.; Turro, N. J.; Kuo, P. L. Langmuir 1985, 1, 352. (26) Organization for Economic Cooperation and Development. Guidelines for Testing of Chemicals, 301C; Modified MITI Test, Paris, 1981.
0.000507 0.0127 0.106 0.139
39.0 50.2 26.1 31.4
1.40 1.57 4.17 4.33
1.19 1.06 0.398 0.384
pC20
cmc/C20
0 ∆Gmic (kJ mol-1)
∆G0ads (kJ mol-1)
8.01 5.08 5.13 4.77
52.3 1.52 14.4 8.27
-45.8 -37.9 -32.6 -31.9
-69.9 -51.9 -43.7 -41.4
(γcmc), surface excess concentration (Γcmc), and area occupied by the surfactant molecule at the air/solution interface (Acmc) at the cmc are summarized in Table 1. The cmc’s of 2C12peLac and 2C16peLac are 1/209 and 1/11 of C12peLac and C16peLac, respectively. As expected, the gemini surfactants have a lower cmc than the monomeric surfactants. The increase in the hydrocarbon chain length from 12 to 16 results in an increase in the cmc for both the monomeric and gemini surfactants. In particular, a marked difference in the cmc is observed for the gemini types. In general, the cmc of conventional surfactants decreases as the hydrocarbon chain length increases. However, the sugar-based surfactants studied reveal an opposite trend in the cmc. According to the suggestion by Rosen27,28 and Menger29 et al., this indicates that premicellar aggregates, such as dimer and trimer, are formed at concentrations below the cmc as the chain length of the surfactants increases. Cirelli reported that the cmc’s of gemini surfactants based on alkyl glucopyranoside with chain lengths from 4 to 14 are fairly identical because of self-coiling or premicellar aggregates.17 It is also noteworthy that the cmc of the sugar-based gemini surfactants in this study is 1 order of magnitude smaller than that of the aldonamide-13 and glucopyranoside17-type gemini surfactants with the same chain length. Furthermore, the increases in chain length from 12 to 16 and from monomer to dimer in chain numbers also increase γcmc. It was found that the sugar-based monomeric surfactants adsorb more efficiently than gemini surfactants and orient themselves at the air/water interface. The low surface activity of the gemini surfactants in this study might be due to the bulky structure caused by a long hexanediamide spacer and by lactobionyl sugar moieties. The data on the orientation at the air/water interface can be judged from the values of Γcmc and Acmc. The area occupied by the gemini surfactant molecules is 2-3 times larger than that for the monomeric surfactants, and the excess concentration on the surface is much smaller. In addition, the values of the area for CnpeLac are close to those for the aldonamidetype monomeric surfactants (0.39-0.45 nm2/molecule),30,31 whereas the values for 2CnpeLac are much larger than those for the aldonamide-type gemini surfactants with a short spacer chain (0.64-0.75 nm2/molecule)13 and those for the glucamide-type gemini surfactants with no spacer (0.68-0.81 nm2/molecule).9 On the other hand, the area per headgroup for the sugar-amine gemini surfactant (27) Song, L. D.; Rosen, M. J. Langmuir 1996, 12, 1149. (28) Rosen, M. J.; Liu, L. J. Am. Oil Chem. Soc. 1996, 73, 885. (29) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (30) Syper, L.; Wilk, K. A.; Sokołowski, A.; Burczyk, B. Prog. Colloid Polym. Sci. 1998, 110, 199. (31) Burczyk, B.; Wilk, K. A.; Sokołowski, A.; Syper, L. J. Colloid Interface Sci. 2001, 240, 552.
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Figure 3. Variation in the apparent hydrodynamic diameter as a function of the surfactant concentration for sugar-based surfactants: 9 ) C12peLac; 0 ) C16peLac; b ) 2C12peLac; and O ) 2C16peLac.
Figure 2. Variation in the dynamic surface tension as a function of time for CnpeLac: (a) n ) 12; cmc × 3.8, 9.4, and 110 from the top, (b) n ) 16; cmc × 2.2, 7.2, 22, and 43 from the top.
hexane-1,6-bis(hexadecyl-1′-deoxyglucitylamine) with pH 6 is 1.09 nm2/molecule.14 This indicates that the sugarbased gemini surfactants in this study widely adsorb at the air/water interface because of the bulky structure containing a long spacer chain. As listed in Table 1, the values of pC20 and the cmc/C20 ratio for 2C12peLac are larger than those for 2C16peLac, which has a longer chain length, and CnpeLac, indicating that the highest adsorption at the air/water interface is by 2C12peLac. This result is also supported by the values of ∆G0ads; that is, the value of -∆G0ads is the largest for 2C12peLac. In addition, the value of -∆G0ads is larger than the value of -∆G0mic for all of the surfactants, showing that the adsorption at the interface is promoted more than the micellization in solution. For gemini surfactants, the increase in chain length makes it difficult for them to be adsorbed at the air/water interface because of the large molecular size. To investigate the adsorption rate at the air/water interface of sugar-based monomeric and gemini surfactants, the dynamic surface tension was measured by maximum bubble pressure techniques. Figure 2 shows the variation of the dynamic surface tension as a function of the surface age for the monomeric surfactants C12peLac and C16peLac. The sugar-based monomeric surfactants exhibit a general lowering of surface tension as the surface age increases. Among the monomeric surfactants, the
adsorption rate of C12peLac is higher than that of C16peLac, indicating that the adsorption to the interface is inhibited as the hydrocarbon chain length increases. The values of the reduced dynamic surface tension for the monomeric surfactants are also much larger than the equilibrium values. This suggests that the sugar-based surfactants show slow adsorption because of the bulky structure of the sugar moiety. On the other hand, the dynamic surface tension of the gemini surfactants is almost close to that of water, even for the longest measurement time (not shown). This indicates that the dynamics in these gemini surfactants are very slow because of the bulky structure containing two sugar hydrophilic groups and a hexanediamide spacer. Micellization Behavior. To investigate the size of the micelle formed in sugar-based monomeric and gemini surfactants, DLS measurements were performed at various concentrations above each cmc. Figure 3 shows the variation in the apparent hydrodynamic diameter of micelles as a function of the concentration of CnpeLac and 2CnpeLac. The apparent diameter of micelles decreases as the concentration for all surfactants increases. However, the degree of decrease that occurs as a result of gemini surfactant concentration differs from that which occurs as a result of the monomeric surfactants; that is, the micelle size of gemini types decreases gradually, whereas that of monomeric types decreases sharply. The aggregation numbers of the sugar-based surfactant micelles were also investigated by the TRFQ method. Figure 4 shows an example of the decay curve that agrees with the fitting data. The aggregation number was determined by using the fitting data and eqs 7-9. Figure 5 shows the variation of the aggregation number as a function of the concentration of CnpeLac and 2CnpeLac micelles. From the plots, the aggregation numbers of the gemini surfactants increase as the concentration increases, whereas those of the monomeric surfactants are nearly constant. The aggregation numbers are in the following order: C16peLac > C12peLac > 2C12peLac > 2C16peLac; that is, those of the gemini surfactants are much smaller than those of the monomeric surfactants. Among the monomeric surfactants, the aggregation numbers of C16peLac are much larger than those of C12peLac. This indicates that, in a monomeric surfactant, a longer hydrocarbon chain length makes it easy to form large
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Langmuir, Vol. 21, No. 23, 2005
Figure 4. Fluorescence decay curve for 2C16peLac; c ) 1 mmol dm-3. Fitting curve (smooth line) is superimposed on the experimental data plots.
Figure 5. Variation in the aggregation number as a function of the surfactant concentration for sugar-based surfactants: 9 ) C12peLac; 0 ) C16peLac; b ) 2C12peLac; and O ) 2C16peLac.
micelles in solution because of the strong hydrophobic interactions between the hydrocarbon chains. This suggests that the monomeric surfactants form loosely bound micelles in solutions at low concentrations above the cmc; subsequently, the surfactant molecules that form the loosely bound micelles become closely packed as the concentration increases, with almost constant aggregation numbers. On the other hand, the aggregation behavior of the gemini surfactants differs from that of the monomeric surfactants; that is, they form relatively compact micelles within a wide concentration range. The variation in aggregation number as a function of the concentration of gemini surfactants shows an opposite trend compared to that of the micelle size. This result indicates that the sugarbased gemini surfactants tends to form tightly packed micelles in solution with increasing concentrations. This may be due to the strong interactions between the two hydrocarbon chains and the hydrogen bonds between the hydroxyl groups in the sugar moieties. Some gemini surfactants show increasing aggregation numbers as the concentration increases, indicating micelle growth.32-35 (32) Zana, R.; Le´vy, H.; Papoutsi, D.; Beinert, G. Langmuir 1995, 11, 3694. (33) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448. (34) Wang, X.; Wang, J.; Wang, Y.; Ye, J.; Yan, H.; Thomas, R. K. J. Phys. Chem. B 2003, 107, 11428.
Yoshimura et al.
Figure 6. Variation in the pyrene fluorescence intensity ratio I1/I3 as a function of the surfactant concentration for sugarbased surfactants: 9 ) C12peLac; 0 ) C16peLac; b ) 2C12peLac; and O ) 2C16peLac.
Furthermore, some gemini surfactants with long hydrocarbon chains show a transition from micelles to vesicles and from disk- to rodlike micelles as a function of concentration;3a,36 in addition, the sugar-amine gemini surfactant hexane-1,6-bis(hexadecyl-1′-deoxyglucitylamine) shows a transition from a micelle to a vesicle as pH increases.14 However, interestingly, the sugar-based nonionic gemini surfactants in this study can maintain the micelle structure, even at a fairly high concentration of cmc × 40 000. The pyrene polarity ratio I1/I3 for four sugar-based surfactant micelles was also investigated. The results as a function of concentration are shown in Figure 6. The I1/I3 ratio of the surfactants begins to decrease at the approximate cmc determined by the surface tension measurement as a function of increasing concentration, and the curves show a gradual decrease within a wide concentration range. This is because these sugar-based surfactants form loosely bound micelles in solution at a low concentration above the cmc as mentioned above. The final values of the I1/I3 ratio of sugar-based surfactants are 1.08, 1.05, 1.15, and 1.16 for C12peLac, C16peLac, 2C12peLac, and 2C16peLac, respectively, indicating that the polarity in the gemini surfactant micelles is slightly higher than that in the monomeric surfactant micelles. Apparently, the pyrene molecules can be solubilized with difficulty in the small micelles of gemini surfactants that are packed tightly in comparison with the monomeric surfactants. Biodegradability. The biodegradabilities of C12peLac and 2C12peLac were investigated by the BOD method with an activated sludge. The biodegradability data are shown in Table 2, along with the data of the sugar-based gemini surfactant N, N′-didodecyl-N,N′-dilactobionylamideethylenediamine (2C12Lac) and the heterogemini surfactant with a sugar moiety and an ammonium headgroup N,Ndimethyl-N-[2-(N′-dodecyl-N′-gluconamide)ethyl]-1-dodecylammonium bromide (2C12AmGlu). It is evident that the biodegradation rate of sugar-based surfactants depends on the number of hydrocarbon chains; that is, the monomeric type C12peLac is more rapidly biodegraded than the gemini types 2C12peLac, 2C12Lac, and 2C12AmGlu (35) Wang, X.; Wang, J.; Wang, Y.; Yan, H.; Li, P.; Thomas, R. K. Langmuir 2004, 20, 53. (36) Aswal, V. K.; De, S.; Goyal, P. S.; Bhattacharya, S.; Heenan, R. K. J. Chem. Soc., Faraday Trans. 1998, 94, 2965.
Sugar-Based Gemini Surfactants with Peptide Bonds Table 2. Biodegradability of the Sugar-Based Surfactants C12peLac and 2C12peLac BOD/TOD (%) surfactant
7 days
14 days
21 days
28 days
C12peLac 2C12peLac 2C12Lac 2C12AmGlu
26.0 3.5 11.9 4.6
54.6 24.2 22.8 5.1
57.1 30.7 28.0 5.9
63.1 33.2 31.6 6.3
with the same dodecyl chain lengths. The test chemicals that showed a result of greater than 60% BOD should be regarded as readily biodegradable. The biodegradation rates of sugar-based gemini surfactants are markedly slow, which suggests that the presence of tertiary amines of sugar-based surfactants causes low biodegradation rates.37,38 Wilk et al. reported that three types of aldonamide-type gemini surfactants show lower biodegradability than their corresponding monomeric surfactants.12,39 It was also reported that gemini surfactants with an amide bond show a lower biodegradability than those with an ester bond.40 In addition, the biodegradability of sugar(37) Yoshimura, K.; Machida, S.; Mosuda, F. J. Am. Oil Chem. Soc. 1980, 57, 238. (38) Van Ginkel, C. G.; Pomper, M. A.; Stroo, C. A.; Kroon, A. G. Tenside, Surfactants, Deterg. 1995, 32, 355. (39) Maliszewska, I.; Wilk, K. A.; Burczyk, B.; Syper, L. Prog. Colloid Polym. Sci. 2001, 118, 172. (40) Tatsumi, T.; Zhang, W.; Kida, T.; Nakatsuji, Y.; Ono, D.; Takeda, T.; Ikeda, I. J. Surfactants Deterg. 2001, 4, 271. (41) Swisher, R. D. Surfactant Biodegradation, 2nd ed.; Dekker: New York, 1987; p 751.
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based surfactants is lower than that of the oxyethylenetype nonionic surfactant dodecylpolyoxyethyleneether (72-97% after 30 days at 4-10 mol of ethylene oxide).41 Conclusion We investigated the adsorption and aggregation properties of novel sugar-based nonionic monomeric and gemini surfactants with a peptide bond. It was found that the gemini surfactants have a low cmc, a high surface tension of water, a large area occupied by the surfactant molecule at the air/water interface, and a slow adsorption rate at the interface, in comparison with the monomeric surfactants. Interestingly, the sugar-based surfactants exhibit unique aggregation behavior in solution. At low concentrations above the cmc, the sugar-based surfactants are formed with relatively loosely bound micelles, and as the concentration increases, they are packed tightly, resulting in the formation of closely packed small micelles. It was also confirmed that the hydrophobic environment of the micelles formed by the gemini surfactants is lower than that of those formed by the monomeric surfactants. Furthermore, the sugar-based gemini surfactants show slower biodegradation rates than do the monomeric surfactants, and the presence of tertiary amines in the molecule gives low biodegradability. Acknowledgment. We are grateful to Dr. Y. Nakata of HORIBA, Ltd. (Tokyo, Japan) for the TRFQ measurements. LA051614Q
