pubs.acs.org/Langmuir © 2010 American Chemical Society
Molecular Conformation-Controlled Vesicle/Micelle Transition of Cationic Trimeric Surfactants in Aqueous Solution Chunxian Wu,† Yanbo Hou,† Manli Deng,† Xu Huang,† Defeng Yu,† Junfeng Xiang,‡ Yu Liu,§ Zhibo Li,§ and Yilin Wang*,† ‡
† Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid and Interface Science, Center for Physiochemical Analysis & Measurement, and §State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China
Received December 21, 2009. Revised Manuscript Received March 8, 2010 Two star-like trimeric cationic surfactants with amide groups in spacers, tri(dodecyldimethylammonioacetoxy)diethyltriamine trichloride (DTAD) and tri(dodecyldimethylammonioacetoxy)tris(2-aminoethyl)amine trichloride (DDAD), have been synthesized, and the aggregation behavior of the surfactants in aqueous solution has been investigated by surface tension, electrical conductivity, isothermal titration microcalorimetry, dynamic light scattering, cryogenic transmission electron microscopy, and NMR techniques. Typically, both the surfactants form vesicles just above critical aggregation concentration (CAC), and then the vesicles transfer to micelles gradually with an increase of the surfactant concentration. It is approved that the conformation of the surfactant molecules changes in this transition process. Just above the CAC, the hydrophobic chains of the surfactant molecules pack more loosely because of the rigid spacer and intramolecular electrostatic repulsion in the three-charged headgroup. With the increase of the surfactant concentration, hydrophobic interaction becomes strong enough to pack the hydrophobic tails tightly and turn the molecular conformation into a pyramid-like shape, thus leading to the vesicle to micelle transition.
Introduction It is well-known that surfactants are widely applied in many fields. For a long time, most of the research on surfactants was focused on surfactants bearing a polar headgroup and a hydrophobic chain. In 1971, a newcomer, gemini surfactant, joined in the surfactant family.1 Gemini surfactants, which have two hydrophilic headgroups and two hydrophobic tails connected by a linking spacer at the headgroup level, have been intensively studied in the past 20 years.2-4 Comparing with conventional counterparts, gemini surfactants have affluent aggregate morphologies which depend on molecular structure and experimental conditions, such as ionic strength, temperature, UV light, pH values, etc.5-8 Admittedly, the dimeric structure of gemini surfactants has brought about a profound improvement in the performance of surfactants. Hence, tremendous curiosity drives people to comprehend the aggregation behavior of trimeric, tetrameric, and other higher oligomeric analogues. Since the early understanding of oligomeric surfactants documented in the review of Laschewsky in the mid-1990s,9 the investigation *To whom correspondence should be addressed. E-mail: yilinwang@ iccas.ac.cn. (1) Bunton, C. A.; Robinson, L.; Schaak, J.; Stam, M. F. J. Org. Chem. 1971, 36, 2346–2350. (2) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906–1920. (3) Zana, R. Adv. Colloid Interface Sci. 2002, 97, 205–253. (4) Gemini Surfactants: Synthesis, Interfacial and Solution-Phase Behavior, and Applications; Zana, R., Xia, J., Eds.; Marcel Dekker: New York, 2004. (5) Minami, H.; Inoue, T. Langmuir 1999, 15, 6643–6651. (6) Yin, H.; Huang, J.; Gao, Y.; Fu, H. Langmuir 2005, 21, 2656–2659. (7) You, H.; Tirrell, D. A. J. Am. Chem. Soc. 1991, 113, 4022–4023. (8) Huang, X.; Cao, M.; Wang, J.; Wang, Y. J. Phys. Chem. B 2006, 110, 19479– 19486. (9) Laschewsky, A. Adv. Polym. Sci. 1995, 124, 1–86. (10) Daninod, D.; Talmon, Y.; Levy, H.; Beinert, G.; Zana, R. Science 1995, 269, 1420–1421.
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of oligomeric surfactants has been expanded in many aspects.10-23 The aggregate morphology of cationic ammonium trimeric surfactant (12-3-12-3-12 3 3Br-) reported by the pioneering work of Zana et al.10 indicated it forms unique branched threadlike micelles in aqueous solution. Their further investigation found that closedlooped micelles are formed by cationic ammonium tetrameric surfactant (12-3-12-4-12-3-12 3 4Br-).21 Menger et al.12 synthesized two series of cationic oligomeric surfactants named “multiarmed” surfactants. They conceived that the loose ends associating with each other might produce “dendritic-like” aggregates even gel network, but this was not proved by experimental results. Yoshimura and co-workers15 synthesized a series of ring-type trimeric surfactants and found large particles exist in aqueous solution. However, they did not define the aggregate structure. Although we expect the exciting outcome of the young field, the aggregation behavior of oligomeric surfactants, especially the structure of the aggregates, has not yet been completely revealed.
(11) Zana, R.; Levy, H.; Papoutsi, D.; Beinert, G. Langmuir 1995, 11, 3694– 3698. (12) Menger, F. M.; Migulin, V. A. J. Org. Chem. 1999, 64, 8916–8921. (13) Esumi, K.; Taguma, K.; Koide, Y. Langmuir 1996, 12, 4039–4041. (14) Sumida, Y.; Masuyama, A.; Maekawa, H.; Takasu, M.; Kida, T.; Nakatsuji, Y.; Ikeda, I.; Nojima, M. Chem. Commun. 1998, 2385–2386. (15) Yoshimura, T.; Esumi, K. Langmuir 2003, 19, 3535–3538. (16) Yoshimura, T.; Yoshida, H.; Ohno, A.; Esumi, K. J. Colloid Interface Sci. 2003, 267, 167–172. (17) Yoshimura, T.; Kimura, N.; Onitsuka, E.; Shosenji, H.; Esumi, K. J. Surfactants Deterg. 2004, 7, 67–74. (18) Gao, C. L.; Millqvist-Fureby, A.; Whitcombe, M. J.; Vulfson, E. N. J. Surfactants Deterg. 1999, 2, 293–302. (19) Murguı´ a, M. C.; Cabrera, M. I.; Guastavino, J. E.; Grau, R. J. Colloids Surf., A 2005, 262, 1–7. (20) Murguı´ a, M. C.; Cristaldi, M. D.; Porto, A.; Di Conza, J.; Grau, R. J. J. Surfactants Deterg. 2008, 11, 41–48. (21) In, M.; Bec, V.; Aguerre-Chariol, O.; Zana, R. Langmuir 2000, 16, 141–148. (22) Hou, Y.; Cao, M.; Deng, M.; Wang, Y. Langmuir 2008, 24, 10572–10574. (23) Rosen, M.; Tracy, D. J. Surfactants Deterg. 1998, 1, 547–554.
Published on Web 04/28/2010
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In this work, we synthesized two star-like cationic trimeric surfactants (Figure 1), tri(dodecyldimethylammonioacetoxy)diethyltriamine trichloride (DTAD) and tri(dodecyldimethylammonioacetoxy)tris(2-aminoethyl)amine trichloride (DDAD), and investigated their aggregation behavior in aqueous solution. DDAD is a completely symmetric molecule, whereas DTAD is not symmetric and only has a slight difference in spacer from DDAD. It is found that both DTAD and DDAD mainly form vesicles just above their critical aggregation concentrations (CAC); however, vesicles transform into micelles with an increase in concentration. The conformational change of the surfactant molecules is the key factor in controlling the unique vesicle to micelle transition.
Experimental Section Materials. N,N-Dimethyldodecylamine and tris(2-aminoethyl)amine were obtained from Acros and Alfa Aesar, respectively. Both of them were used without further purification. Chloroacetyl chloride, triethylamine, and all organic solvents were purchased from Beijing Chemical Co. All of the organic solvents were dried and distilled. Triply distilled water was used in all experiments. Synthesis. The synthesis of DTAD has been reported in our previous article.22 Compound DDAD was synthesized according to Scheme 1 and was characterized by 1H NMR, 13C NMR, mass spectrum, and elemental analysis. The Krafft temperatures of DTAD and DDAD are below 0 °C. Tris(2-choracetyl)amine. Chloroacetyl chloride (3.12 g, 7 mmol) in 100 mL of dichloromethane was added to a mixture of tris(2-aminoethyl)amine (1.02 g, 7 mmol) and triethylamine (2.83 g, 28 mmol) in 50 mL of dichloromethane dropwise at 0 °C
Figure 1. Chemical structures and 1H NMR signal assignments of DTAD and DDAD.
with vigorous stirring for 3 h. Upon completion, the solvent was removed in vacuo, and the residue was column separated with methanol/ethyl acetate=1:20 to afford a light yellow solid and then recrystallized from methanol to afford tris(2-choracetyl)amine as white crystals in 31% yield. 1H NMR (D2O, ppm): δ 2.62 (t, 6H, N-CH2), 3.25 (t, 6H, CH2NHCO), 4.04 (s, 6H, CH2Cl). 13C NMR (D2O, ppm): 35.7, 39.9, 51.1, 165.9. MS-ESI (m/z): calcd, 374; found, 375 (M þ 1).
Tri(dodecyldimethylammonioacetoxy)tris(2-aminoethyl)amine Trichloride (DDAD). N,N-Dimethyldodecylamine (3.54 g, 12 mmol) was added to a solution of tris(2-choracethyl)amine (1.13 g, 3 mmol) in 30 mL of methanol and heated at 45 °C for 72 h. The solvent was removed in vacuo, and the residue was recrystallized from methanol/ethyl acetate three times to afford DDAD as white solid in 38% yield. 13C NMR (D2O, ppm): 13.5, 22.1, 22.3, 25.7, 28.5, 29.1, 29.2, 29.4, 29.5, 29.6, 31.6, 36.8, 52.3, 52.7, 61.4, 63.2, 163.5. MS-ESI (m/z): calcd, 1016; found 303 ([M - 3Cl-]/3). Anal. Calcd for C54H114Cl3N7O3 3 1.5H2O: C, 62.19; H, 11.31; N, 9.40. Found: C, 62.17; H, 11.02; N, 9.20. 1H NMR analyses of DDAD is given in the Results and Discussion. Surface Tension Measurement. Surface tension measurement was carried out using the drop volume method. To attain the surface adsorption equilibrium, the drop formation consisted of two steps: first, a pendant drop whose size was about 90 vol % of a falling drop was squeezed out rapidly. Then, it was permitted to stand for enough time until the whole drop was exposed and dropped automatically. Each surface tension value (γ) was determined from at least five measured values. The standard error of the surface tension data was 0.2 mN/m. The measurement temperature was controlled at 25.00 ( 0.05 °C using a thermostat. Electrical Conductivity Measurement. The conductivity of the surfactant solutions was measured as a function of concentration using a JENWAY model 4320 conductivity meter. The measurements were performed in a temperature-controlled, doublewalled glass container with a circulation of water. Sufficient time was allowed to the system equilibrium between successive additions. The temperature of the solution was controlled at 25.0 ( 0.1 °C. Isothermal Titration Microcalorimetry (ITC). A TAM 2277-201 isothermal titration microcalorimeter (Thermometric AB, J€arf€alla, Sweden) was used to measure the CAC values and the enthalpy change for the aggregation of these surfactants. Both the sample cell and the reference cell of the microcalorimeter are 1 mL, which were initially loaded with 0.6 mL of pure water. Concentrated solution was injected consecutively into the stirred sample cell in each portion of 10 μL using a 500 μL Hamilton syringe controlled by a Thermometric 612 Lund pump until the desired concentration range had been covered. During the whole titration process, the system was stirred at 60 rpm with a gold propeller, and the interval between two injections was sufficiently
Scheme 1. Synthetic Procedure of DDAD
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long for the signal to return to the baseline. The observed enthalpies (ΔHobs) were obtained by integrating the areas of the peaks in the plot of thermal power against time. The accuracy of the calorimeter was periodically calibrated electrically and verified by measuring the dilution enthalpy of concentrated sucrose solution. The reproducibility of experiments was within (4%. All of the measurements were performed at 25.00 ( 0.01 °C. NMR. 1H NMR measurements were carried out at 20.7 ( 0.3 °C on a Bruker AV400 FT-NMR spectrometer operating at 400.1 MHz. Deuterium oxide (99.9%) was purchased from CIL Cambridge Isotope Laboratories and used to prepare the stock solution of the studied surfactants in D2O. About 0.7 mL of each of the solutions was transferred to a 5 mm NMR tube for the measurement. The center of the HDO signal (4.79 ppm) was used as the reference in the D2O solutions. In all NMR experiments, the number of scans was adjusted to achieve high signal-to-noise ratios depending on the surfactant concentration and was recorded with a digital resolution of 0.04 Hz/data point. Spin-spin relaxation time (T2) measurement and 2D DOSY pulsed-gradient spin-echo (PGSE) NMR spectra were carried out at 25 °C on a Bruker Avance 600 spectrometer. The T2 values were measured by the Carr-Puecell-Meiboom-Gill (CPMG) sequence (PD-90°x-[τ-180°y-τ]2n-AC). The 2D DOSY NMR spectra were obtained with stebpgp1s pulse program and with maximum gradient strength of 50 G cm-1. Bipolar spoil gradients were used with total duration of 100 ms. The gradient field was linearly increased in 32 steps, resulting in an attenuation of 1H NMR from 2% to 95%. Dynamic Light Scattering (DLS). Measurements were carried out using an LLS spectrometer (ALV/SP-125) with a multi-τ digital time correlater (ALV-5000). Light of λ=632.8 nm from a solid-state He-Ne laser (22 mW) was used as the incident beam. The measurement was conducted at a scattering angle of 90°. All of the solutions were filtered through a 0.45 μm membrane filter of hydrophilic PVDF before the measurements. The correlation function was analyzed from the scattering data via the CONTIN method to obtain the distribution of diffusion coefficients (D) of the solutes. The apparent hydrodynamic radius Rh was deduced from D by the Stokes-Einstein equation Rh = kBT/(6πηD) for spherical particles, where kB represents the Boltzmann constant, T is the absolute temperature, and η is the solvent viscosity. All of the measurements were performed at 25.0 ( 0.1 °C.
Cryogenic Transmission Electron Microscopy (CryoTEM). CryoTEM samples were prepared in a controlled envir-
onment vitrification system (CEVS) at 28 °C.24 A micropipet was used to load 5 μL surfactant solution onto a lacey support TEM grid, which was held by tweezers. The excess solution was blotted with a piece of filter paper, resulting in the formation of thin films suspended on the mesh holes. After waiting for about 10 s to relax any stresses induced during the blotting, the samples were quickly plunged into a reservoir of liquid ethane (cooled by the nitrogen) at its melting temperature. The vitrified samples were then stored in the liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and examined with a JEM 2200FS TEM (200 KeV) at about -174 °C. The phase contrast was enhanced by underfocus. The images were recorded on a Gatan multiscan CCD and processed with Digital Micrograph.
Results and Discussion Figure 2 shows the surface tension curves of DTAD and DDAD as a function of concentration logarithm. The surface tension of these two trimeric surfactants decreases with an increase of concentration, and the CAC values of DTAD and DDAD determined from the clear breakpoints are 0.20 and 0.33 mM, (24) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y. J. Electron. Microsc. Tech. 1988, 10, 87–111.
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Figure 2. Variation of surface tension with the DTAD and DDAD concentrations (C) at 25.00 °C.
Figure 3. Variation of the electrical conductivity (κ) with the surfactant concentrations (C) for DTAD and DDAD at 25.0 °C.
respectively. Compared with the widely reported cationic gemini surfactants, the CAC values are only slightly lower and the surface tensions at the CAC are larger. It is noted that the surface tension still decreases rather sharply even far beyond the CAC, which implies that the adsorption of these two surfactants at air/water interface still keeps changing above the CAC. This suggests that the aggregation behavior of the surfactants may also keep changing beyond the CAC. Electrical conductivity measurement and ITC have been applied to study the aggregation behavior of the surfactants. Figure 3 presents the variation of electrical conductivity against the surfactant concentration. The CAC values of DTAD and DDAD can be determined from the breakpoints. In addition, aggregation ionization degree (R) can be estimated as the ratio of the slopes of the two straight lines above and below the CAC. The observed enthalpy changes (ΔHobs) of the dilution of DTAD and DDAD concentrated solutions into water were plotted against the final concentration (C) in Figure 4. The titration curves are approximately sigmoid in shape, and both the CAC and the enthalpy changes for aggregation (ΔHagg) can be calculated following the procedure described previously.25-28 The CAC values obtained by the three techniques above, R values, and ΔHagg are all summarized in Table 1. The CAC values from the (25) Johson, I.; Olofsson, G.; J€onsson, B. J. Chem. Soc., Faraday Trans. 1 1987, 83, 3331–3344. (26) Kiraly, Z.; Dekany, I. J. Colloid Interface Sci. 2001, 242, 214–219. (27) Bhattacharya, S.; Haldar, J. Langmuir 2004, 20, 7940–7947. (28) Li, Y.; Reeve, J.; Wang, Y.; Thomas, R. K.; Wang, J.; Yan, H. J. Phys. Chem. B 2005, 109, 16070–16074.
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Figure 6. CryoTEM micrographs of (a) vesicles and micelles formed by 1 mM DDAD and (b) micelles formed by 10 mM DDAD. Figure 4. Observed enthalpy changes (ΔHobs) of DTAD and DDAD with the final concentration (C) at 25.00 °C. Table 1. Critical Aggregation Concentration (CAC), Aggregation Ionization Degree (r), and Enthalpy Changes for Aggregation of DTAD and DDAD at 25.0 °C CAC (mM) surface tension conductivity calorimetry DTAD DDAD
0.20 0.33
0.32 0.39
0.22 0.32
R
ΔHagg (kJ mol-1)
0.45 0.50
-17.9 -18.9
Figure 5. DLS measurements of the size distributions of DTAD and DDAD at various concentrations at 25.0 °C.
three different techniques are consistent with each others considering experimental errors. Both the CAC and R of DTAD are only slightly lower than those of DDAD. Moreover, the ΔHagg values are significantly exothermic for both DTAD and DDAD. These results indicate that DTAD and DDAD have similar aggregation behavior and ability, and their aggregation is dominated by hydrophobic interaction. The size distributions and morphologies of the DTAD and DDAD aggregates were investigated by DLS and CryoTEM. As shown in Figure 5, both DTAD and DDAD exhibit very similar size variation with the concentration increase beyond the CAC. Just above the CAC, DTAD and DDAD present a large size distribution around 80 nm and a very small one around 1 nm. With the increase of the surfactant concentration, the relative intensity of the large size distribution decreases while the relative intensity of the small size distribution increases gradually and Langmuir 2010, 26(11), 7922–7927
obviously. This is an abnormal phenomenon. Normally, surfactants form small aggregates first, and then the aggregates may become large with an increase of the surfactant concentration. In order to confirm the present result, the morphologies of the surfactants at different concentrations were observed with CryoTEM. Taking the case of DDAD, the representative images of the surfactant aggregates just above the CAC and far beyond the CAC are illustrated in Figure 6. More CryoTEM images of DDAD are supplied in the Supporting Information (Figure S1). The CryoTEM micrograph at the low concentration just above the CAC displays the coexistence of globule vesicle of ∼30 nm and micelles of ∼3 nm. The aggregates at high concentration far beyond the CAC only present ∼3 nm micelles, where the amount of vesicles may be too small to be observed. The results imply that DTAD and DDAD form vesicles just beyond the CAC, and then the vesicles transfer to small micelles with the increase of the concentration. It is necessary to illustrate that the small size distribution around 1 nm from DLS is too small and not true. Such a small size distribution could be wrongly caused by strong electrical repulsion among the highly charged aggregates.29-31 That is to say, the present small aggregates should be micelles of around 3 nm as observed from CryoTEM. Possibly since the aggregate size from DLS is the hydrodynamic one, the size of the vesicles from DLS is larger than that from CryoTEM. To understand why and how DTAD and DDAD form larger vesicles first and then transfer to smaller micelles afterward, several NMR techniques were applied as described below. The 1H NMR spectra of DTAD and DDAD at different concentrations are given in Figure 7. With the surfactant concentration increasing from just above the CAC to far beyond the CAC, several sets of resonance changes are characteristic in the 1H NMR spectra of both DTAD and DDAD. Normally, chemical shifts of surfactant protons will not change beyond the CAC, since environment of protons will not obviously change anymore. However, almost all the protons for DTAD and DDAD move to downfield upon the surfactant aggregation beyond the CAC. The signals of the protons in the ends of hydrophobic chains (Ha and Hb) have a minute downfield shift while the signals of protons in the region of headgroups (He and Hg for DDAD; Hg, Hg0 , and Hg0 0 for DTAD) show significant downfield shifts. The large narrow singlet ascribed to N-methyl (Hf) becomes broaden and slightly moves to downfield. The downfield shifting of the protons of DTAD and DDAD indicates the protons of these two (29) Dorshow, R.; Briggs, J.; Bunton, C. A.; Nicoli, D. F. J. Phys. Chem. 1982, 86, 2388–2395. (30) Dorshow, R. B.; Bunton, C. A.; Nicoli, D. F. J. Phys. Chem. 1983, 87, 1409–1416. (31) Biresaw, G.; McKenzie, D. C.; Bunton, C. A.; Nicoli, D. F. J. Phys. Chem. 1985, 89, 5144–5146.
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Wu et al. Table 2. Spin-Spin Relaxation Time T2 (ms) of the Corresponding Protons of DDAD at 1 and 10 mM
1 mM 10 mM
Figure 7. 1H NMR spectra and the proton assignments of DTAD and DDAD in D2O at different concentrations.
surfactant molecules sense less polar microenvironment as the concentration increases. This suggests that the hydrocarbon chains of the surfactant molecules may pack more densely with the increase of the concentration. Especially, DTAD displays some obvious differences from DDAD in 1H NMR spectra. For DDAD, the protons of the methylenes connected with the quaternary ammonium groups with the amide groups (Hg) only exhibit a singlet at 4.17 ppm, which implies that these protons are located in the same chemical environment due to the completely symmetric molecular structure. However, the corresponding protons of DTAD (Hg, Hg0 , and Hg0 0 ) show three singlet peaks at 3.97, 4.07, and 4.39 ppm, respectively. Although the protons (Hg0 and Hg0 0 ) of DTAD are chemical equivalent, they have different chemical shifts. This fact may be rationalized by assuming the presence of hydrogen bonds in DTAD spacer between the amide proton and the tertiary carbonyls with proper distance, as shown in Supporting Information (Figure S3). Correspondingly, the 13C NMR spectra of DTAD presented in Figure S4a show three peaks around 164 ppm, which are ascribed to the carbons of a tertiary amide group and two second amide groups. This directly supports our deduction. The phenomenon of the protons (Hg0 and Hg0 0 ) also happens on the protons near the center of the spacer (Hh and Hi) of DTAD. The protons (Hh and Hi) display a series of broad peaks from 3.16 to 3.47 ppm, and these protons have quite similar 7926 DOI: 10.1021/la9048067
Ha
Hb
Hc
Hd
He
Hf
452 414
250 173
142 99
101 55
65 41
130
Hh
Hi
97
77 45
94
chemical shifts, resulting in the difficulty for accurate assignment of the peaks. Another distinct difference in the 1H NMR spectra between these two surfactants is that, between 0.25 and 1 mM, the signals of Hg0 and Hg0 0 of DTAD split into doublet peaks from singlet peaks just above the CAC and then merge into singlet peaks again. Normally, there are fast exchanges between monomers and micelles in surfactant solution. Nevertheless, if vesicles coexist with micelles in aqueous solution, they cannot exchange fast with each other due to the relative long-term stability of the vesicles, leading to signal splitting. So the present splitting signals of Hg0 and Hg0 0 can be thought as the result of the micelle/vesicle coexistence. However, the changes taking place in the signals of Hg0 and Hg0 0 of DTAD do not occur in the protons of DDAD. Compared to DTAD, the motion of DDAD molecules is relatively free possibly because of the lack of the intramolecular hydrogen bonding and the resultant fast exchanges between the vesicles and micelles. The 2D DOSY PGSE NMR spectra and spin-spin relaxation time measurement were utilized to achieve further understanding of the unique vesicle to micelle transition.32,33 Because DTAD and DDAD carry the similar molecular structure and show similar aggregation behavior and ability as discussed above, DDAD has been investigated as an example by these two NMR techniques. The 2D DOSY NMR spectra of DDAD at different concentrations are presented in the Supporting Information (Figure S5). Visually, the self-diffusion coefficient at 1 mM is smaller and spreads over a wider range than that at 10 mM, which implies the following possible situations. On one hand, the coexistence of a considerate number of vesicles with micelles generates a polydispersed solution at 1 mM, while the solution at 10 mM is micelle-dominated. On the other hand, the aggregation number of vesicles may vary in a large range. The spin-spin relaxation time (T2) values of all the protons of DDAD are shown in Table 2 except for Hg, which is too weak to be observed. The T2 values of all the protons are smaller at 10 mM than at 1 mM, suggesting that the motion of the protons is hindered more seriously at higher concentration. At 1 mM, the T2 values of Hd is close to those of Hh and Hf but much larger than those of He and Hi. However, at 10 mM, the T2 value of Hd becomes almost the same as those of He and Hi. The following simulations on the T2 of Hd reflect that proton Hd is in dramatically different motional states at these two different concentrations. On the basis of two-step model,34,35 the spin-spin relaxations of some protons, usually near the headgroup, obey the biexponential behavior. These T2 values are composed of two parts: the fast relaxing component and the slow relaxing component. The T2 values of the two components can be obtained by fitting the data to the following equation MðtÞ ¼ Mð0Þ½P2f expð- t=T2f Þ þ P2s expð- t=T2s Þ
ð1Þ
(32) Villeneuve, M.; Ootsu, R.; Ishiwata, M.; Nakahara, H. J. Phys. Chem. B 2006, 110, 17830–17839. (33) Jiang, Y.; Chen, H.; Cui, X.; Mao, S.; Liu, M.; Luo, P.; Du, Y. Langmuir 2008, 24, 3118–3121. (34) Wennerstr€om, H.; Lindman, B.; S€oederman, O.; Drakenberg, T.; Rosenholm, J. B. J. Am. Chem. Soc. 1979, 101, 6860–6864. (35) Halle, B.; Wennerstr€om, H. J. Chem. Phys. 1981, 75, 1928–1943.
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Table 3. Spin-Spin Relaxation Parameters for Hd and Hi of DDAD at 1 and 10 mM
1 mM 10 mM
Hd Hi Hd Hi
T2f (ms)
T2s (ms)
P2f
31 10 39
101 93 75 310
0.21 0.20 0.95
Figure 8. Possible model of the conformational transition of the trimeric surfactant molecules.
where P2f þ P2s=1 and P2f and P2s stand for the fractions (weighting factors) of the protons with fast and slow relaxation, respectively, and T2f and T2s are the corresponding spin-spin relaxation times. Simulation results of the T2 values of Hd and Hi are listed in Table 3. At 1 mM, the spin-spin relaxation of Hd does not obey the biexponential decay, while it obeys the biexponential decay at 10 mM. The T2 of Hd at 10 mM consists of two components, implying that the motion of Hd is in two distinguished states. The motion of the slow relaxation portion of Hd is rather free, which is similar to Ha and Hb, while the motion of the fast relaxation part of Hd is restricted just like Hi. These results suggest that the alkyl chains near the ammonium groups are packed more closely in the micellar palisade layer at 10 mM. Moreover, the increase of P2f of Hi with the increase of surfactant concentration also indicates that the motion of protons in the headgroups is more restricted at high concentration, which is the micelle-dominated solution. Normally, surfactant molecules are organized more loosely in micelles than vesicles, which is contrary to the present situation. Combining the above discussion, it can be inferred that the conformation of these trimeric surfactant molecules changes during the vesicle/micelle transition with the increase of surfactant concentrations (Figure 8). At low surfactant concentration, strong electrostatic repulsion and the rigid spacer may make the hydrophobic chains difficult to close to each others tightly. The molecules prefer to form vesicles based on this loose molecular conformation. With the increase of surfactant concentration, hydrophobic interaction among more hydrophobic chains becomes strong enough to pack the hydrophobic tails more tightly and turn the molecular conformation into a pyramid-like shape;
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thus, the volume ratio of hydrocarbon chain part to the headgroup part becomes smaller and the vesicles are converted into micelles spontaneously. In our previous study,36 we have found that a star-like cationic ammonium tetrameric surfactant PATC can form premicellar network-like aggregates far below its critical micelle concentration (CMC) and the network-like aggregates transfer to micelles beyond its CMC accompanying with the variation of the PATC conformation. Both the star-like tetrameric and trimeric surfactants share the similar driving force for the aggregate transition. At low surfactant concentration, these star-like surfactants present a stretched conformation because of their rigid spacer and strong intramolecular electrostatic repulsion in the headgroups, which generates large aggregates with small curvature, such as vesicles or network-like aggregates. When hydrophobic interaction among the hydrocarbon chains becomes strong enough at high concentration, the hydrophobic chains can be packed tightly and the molecular conformations are turned into the pyramid-like shape, which makes the large aggregates transfer into small aggregates with large curvature, such as micelles.
Conclusion The aggregation behavior of two star-like trimeric cationic surfactants DTAD and DDAD in aqueous solution has been investigated. It is revealed that the molecular conformational transition driven by hydrophobic interaction is the controlling factor for the aggregate formation and transition. This factor endows these surfactants with a unique aggregation behavior. Both DTAD and DDAD first form vesicles just beyond the CAC because of the loosely packing of the hydrophobic chains caused by the electrostatic repulsion and rigid spacer in the headgroups. When the hydrophobic interaction of the hydrocarbon chains becomes strong enough to turn the molecular conformation into a pyramid-like shape, the vesicles transfer to micelles. Along with the previous work on a star-like tetrameric surfactant,35 this work enriches the approaches of constructing and adjusting surfactant aggregates through changing molecular conformation. Acknowledgment. We are grateful for financial support from National Natural Science Foundation of China and National Basic Research Program of China (Grants 20633010, 20973181, and 2005cb221300). Supporting Information Available: TEM images for DTAD and DDAD vesicles, illustration of hydrogen bond formation, expanded 13C NMR spectra, and 2D DOSY PGSE NMR spectra. This material is free of charge via the Internet http://pubs.acs.org. (36) Hou, Y.; Han, Y.; Deng, M.; Xiang, J.; Wang, Y. Langmuir 2009, 26, 28–33.
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