pubs.acs.org/Langmuir © 2009 American Chemical Society
Aggregation Behavior of a Tetrameric Cationic Surfactant in Aqueous Solution Yanbo Hou,† Yuchun Han,† Manli Deng,† Junfen Xiang,‡ and Yilin Wang*,† † ‡
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid and Interface Science and Center for Physiochemical Analysis & Measurement, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China Received April 23, 2009. Revised Manuscript Received November 9, 2009
A star-shaped tetrameric quaternary ammonium surfactant PATC, which has four hydrophobic chains and charged hydrophilic headgroups connected by amide-type spacer group, has been synthesized in this work. Surface tension, electrical conductivity, ITC, DLS, and NMR have been used to investigate the relationship between its chemical structure and its aggregation properties. Interestingly, a large size distribution around 75 nm is observed below the critical micelle concentration (cmc) of PATC, and the large size distribution starts to decrease beyond the cmc and finally transfers to a small size distribution. It is proved that the large size premicellar aggregates may display networklike structure, and the size decrease beyond the cmc is the transition of the network-like aggregates to micelles. The possible reason is that intramolecular electrostatic repulsion among the charged headgroups below the cmc leads to a star-shaped molecular configuration, which may form the network-like aggregates through intermolecular hydrophobic interaction between hydrocarbon chains, while the hydrophobic effect becomes strong enough to turn the molecular configuration into pyramid-like shape beyond the cmc, which make the transition of network-like aggregates to micelles available.
Introduction Compared with traditional monomeric surfactants, gemini (dimeric) surfactants exhibit much better performance, such as higher surface activity, lower critical micelle concentration, and more diverse aggregate structures.1-3 Obviously this has stimulated the interesting to explore the behavior of higher oligomeric analogues, such as trimeric or tetrameric surfactants,4-15 which have three or four hydrophilic headgroups and hydrophobic chains linked by spacer groups. The early investigations on oligomeric surfactants have been well reviewed by Laschewsky.16 Oligomeric surfactants have been found to show very special aggregation behavior. Here a few examples are presented as follows. One is a linear type4-10,15 such as m-s1-m-s2-m-s3-m 3 3Xor m-s1-m-s2-m-s3-m-s4-m 3 4X- (m and s are the carbon numbers of the alkyl chain and of the alkanediyl spacer, respectively, and *To whom the correspondence should be addressed. E-mail: yilinwang@ iccas.ac.cn. (1) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083–10090. (2) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906–1920. (3) Zana, R. Adv. Colloid Interface Sci. 2002, 97, 203–251. (4) Daninod, D.; Talmon, Y.; Levy, H.; Beinert, G.; Zana, R. Science 1995, 269, 1420–1421. (5) Zana, R.; Levy, H.; Papoutsi, D.; Beinert, G. Langmuir 1995, 11, 3694–3698. (6) Esumi, K.; Taguma, K.; Koide, Y. Langmuir 1996, 12, 4039–4041. (7) Yoshimura, T.; Yoshida, H.; Ohno, A.; Esumi, K. J. Colloid Interface Sci. 2003, 267, 167–172. (8) In, M.; Bec, V.; Aguerre-Chariol, O.; Zana, R. Langmuir 2000, 16, 141–148. (9) Laschewsky, A.; Wattebled, L.; Arotcarena, M.; Habib-Jiwan, J.; Rakotoaly, R. V. Langmuir 2005, 21, 7170–7179. (10) Wattebled, L.; Laschewsky, A.; Moussa, A.; Habib-Jiwan, J. Langmuir 2006, 22, 2551–2557. (11) Menger, F. M.; Migulin, V. A. J. Org. Chem. 1999, 64, 8916–8921. (12) Sumida, Y.; Oki, T.; Masuyama, A.; Maekawa, H.; Nishiura, M.; Kida, T.; Nakatsuji, Y.; Ikeda, I.; Nojima, M. Langmuir 1998, 14, 7450–7455. (13) Murguia, M. C.; Grau, R. J. Syn. Lett. 2001, 8, 1229–1232. (14) Murguia, M. C.; Cabrera, M. I.; Guastavino, J. F.; Grau, R. J. Colloids Surf., A 2005, 262, 1–7. (15) In, M.; Aguerre-Chariol, O.; Zana, R. J. Phys. Chem. B 1999, 103, 7747– 7750. (16) Laschewsky, A. Adv. Polym. Sci. 1995, 124, 1–86.
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X is counterion Cl- or Br-), which looks like extended gemini surfactant. Zana and co-workers4,15 studied 12-3-12-3-12 3 3Brand 12-3-12-4-12-3-12 3 4Br- and found that branched threadlike and closed-loop micelles are formed in aqueous solution. Another one possesses star-shaped configuration,11-14 whose spacer groups radiate from a central moiety. Menger et al.11 synthesized two series of tetrameric cationic surfactants and called them “multiarmed” surfactants. They pointed out “dendritic growth” of aggregates may possibly exist, but only small size aggregate was observed from dynamic light scattering results. In previous work,17 we synthesized a trimeric surfactant molecule DTAD and observed its selectivity of self-assembly on solid surfaces. Since the special aggregation behaviors of oligomeric surfactants have not been completely revealed, much more investigations are expected. Herein, a tetrameric cationic surfactant PATC is synthesized, and its aggregation behavior in aqueous solution is studied (Figure 1). Ethylenediamine is used as the core moiety of the spacer group which is linked to four quaternary ammonium headgroups and four 12-carbon chains through four amide groups. The ethylenediamine moiety increases the flexibility of the spacer, and the amide groups raise the solubility as well as the self-assembly ability of PATC in water. A unique aggregate transition process revealed is that PATC forms network-like aggregates before its critical micellar concentration (cmc), and the network-like aggregates transfer to micelles beyond its cmc.
Experimental Section Materials. Dodecyl bromide was purchased from Lancaster, England, N,N-dimethylethylenediamine was purchased from Alfa Aesar, and ethylenediamine was purchased from Beijing Chemical Co. All of the organic solvents were dried and distilled. Triply distilled water was used in all experiments. (17) Hou, Y.; Cao, M.; Deng, M.; Wang, Y. L. Langmuir 2008, 24, 10572–10574.
Published on Web 11/30/2009
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Figure 2. Variation of surface tension with the PATC concentration (C) at 25.00 ( 0.05 °C. Figure 1. Chemical structure of PATC.
Synthesis. The synthesis of PATC and the characterization data by 1H NMR, 13C NMR, mass spectrum, and elemental analysis are presented in the Supporting Information. Surface Tension Measurements. Surface tension measurement was carried out using the drop volume method.18 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, and then it was permitted to stand for enough time until the whole drop was exposed and dropped automatically. One surface tension value (γ) was determined from at least five measured values. 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, double-walled 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 25.00.1 °C. This method was used to determine both the cmc and micelle ionization degree (R) of the surfactant. Isothermal Titration Microcalorimetry (ITC). A TAM 2277-201 isothermal titration microcalorimeter (Thermometric AB, J€arf€alla, Sweden) was used to measure the cmc value and the enthalpy change for micelle formation of PATC. 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 PATC 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 50 rpm with a gold propeller, and the interval between two injections was sufficiently 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 (18) Wang, C. Z.; Huang, J. B.; Tang, S. H.; Zhu, B. Y. Langmuir 2001, 17, 6389–6392.
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Cambridge Isotope Laboratories and used to prepare the stock solution of PATC 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 the NMR experiments, the number of scans was adjusted to achieve good signal-to-noise ratios depending on the surfactant concentration and was recorded with a digital resolution of 0.04 Hz/data point. Heteronuclear multiple bond correlation (1H-13C HMBC) spectrum, 1D selective nuclear Overhauser effect (NOE) spectrum, and 2D DOSY pulsed-gradient spin-echo (PGSE) NMR spectra were all carried out at 25 °C on a Bruker Avance 600 spectrometer. The 2D DOSY PGSE NMR spectra were obtained with stebpgp1s pulse program. The experiments were carried out at 25 °C with maximum gradient strength 50 G cm-1. Bipolar spoil gradients were used with total duration of 100 ms. Gradient recovery delays were 1 ms, and diffusion times were within 2.2, 3.0, and 3.0 ms for 0.06, 2, and 30 mM PATC samples, respectively. 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 the measurements were performed at 25.0 ( 0.1 °C.
Results and Discussion The surface tension curve as a function of the PATC concentration is shown in Figure 2. The surface tension decreases as the PATC concentration increases, reaching an obvious breakpoint, i.e., the cmc. The cmc is 0.08 mM, and the surface tension at the cmc (γcmc) is 47.6 mN m-1. The cmc value of PATC is at least 1 order of magnitude smaller than those of cationic gemini surfactants; however, the γcmc value is much higher than most of the reported cationic gemini surfactants.3 Surprisingly, the surface tension does not become constant beyond the cmc but continues to decrease significantly and does not stop this decreasing DOI: 10.1021/la903672r
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Figure 3. Variation of surface tension with the PATC concentration (C) at 25.0 ( 0.1 °C.
tendency far beyond the cmc. This indicates that the PATC may already form premicellar aggregates before the critical point cmc and start to form micelles at the cmc. Moreover, the aggregates may still keep changing beyond the cmc. To confirm the cmc, electrical conductivity measurements and ITC have also been carried out. Figure 3 presents the variation of electrical conductivity versus the PATC concentration. The breakpoint corresponds to the cmc, which is 0.12 mM and is consistent with the value from the surface tension curve considering experimental error. In addition, the micelle ionization degree (R) can be estimated as the ratio of the slopes of the two straight lines above and below the cmc. The obtained R of PATC is 0.73, which is much higher than those of normal ionic single-chain surfactants and gemini surfactants The observed enthalpy change (ΔHobs) of the concentrated PATC solution diluted into water is plotted against the final PATC concentration (C) in Figure 4. The titration curve is approximately sigmoidal in shape, and the value of the enthalpy for the PATC aggregation (ΔH) is determined from the enthalpy difference between the two linear segments of the differential enthalpy curve extrapolated to the cmc. The derived cmc and ΔH per mole PATC at 25.00 °C are 0.13 mM and -30.2 kJ mol-1, respectively. The cmc values from surface tension measurement and ITC are consistent within experimental error. The large exothermic enthalpy may be resulted from the strong hydrophobic interaction of the PATC hydrocarbon chains and the conformation changes of PATC during the transition of the premicellar aggregates to the micelles beyond the cmc. To further understand the unique aggregation behavior of PATC, DLS experiments are performed in a large concentration range, which are 0.03 and 0.06 mM below the cmc and 2, 10, 20, 30, and 40 mM above the cmc, as shown in Figure 5. Interestingly, at very low concentration of 0.03 mM, a very large size distribution of around 75 nm and a weak size distribution of around 1.7 nm are observed. When the concentration increases to 0.06 mM but is still lower than cmc, the large distribution around 75 nm does not change, but the intensity of the 1.7 nm size distribution becomes weak. When the PATC concentration increases to above cmc, i.e., 2 and 10 mM, the size distribution of around 75 nm has no obvious change in intensity, but the 1.7 nm size distribution is replaced by the 0.9 nm size distribution and its relative intensity becomes obviously larger. When the PATC concentration 30 DOI: 10.1021/la903672r
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Figure 4. Observed enthalpy change (ΔHobs) with the final PATC concentration (C) at 25.00 °C.
Figure 5. DLS measurements of the size distributions of PATC at various concentrations at 25.0 °C.
increases to far beyond cmc, i.e., 20, 30, and 40 mM, the intensity of the 0.9 nm size distribution increases significantly, while the 75 nm size distribution gradually decreases and finally disappears. That is to say, the PATC molecules already form large aggregates below cmc, and the large aggregates transfer to small aggregates beyond the cmc. Usually, a small size distribution with a hydrodynamic diameter of several nanometers for a surfactant belongs to spherical micelles. Here, the size value 1.7 or 0.9 nm fitted out by DLS is smaller than the real value because the measured diffusion coefficients by DLS are strongly influenced by interaggregate repulsion among the highly charged PATC aggregates. This kind of phenomenon has been reported and explained in the literature.19-21 So the observed 0.9 nm size distribution is thought to correspond to the PATC micelles, while the 75 nm large size distribution below the cmc may come from some a kind of special premicellar aggregates. (19) Dorshow, R.; Briggs, J.; Bunton, C. A.; Nicoli, D. F. J. Phys. Chem. 1982, 86, 2388–2395. (20) Dorshow, R. B.; Bunton, C. A.; Nicoli, D. F. J. Phys. Chem. 1983, 87, 1409– 1416. (21) Biresaw, G.; McKenzie, D. C.; Bunton, C. A.; Nicoli, D. F. J. Phys. Chem. 1985, 89, 5144–5146.
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Figure 6. (A) 1H NMR spectra and proton assignments of PATC in D2O at different concentrations. (B, C) Enlarged selected regions of 1H NMR spectra of PATC at various concentrations.
To understand the aggregation behavior of PATC, the 1H NMR technique is used to reveal the aggregate structure and solvation during the aggregation process.22-24 The spectra of PATC at different concentrations are presented in Figure 6. The variations in chemical shifts (Δδobs) of the representative protons in the hydrophobic chain, headgroup, and spacer group with the PATC concentration are given in Figure 7 for more clear observation. Normally, changes in the chemical shifts with aggregation are discussed in terms of medium effects and conformation effects.25-27 The former is caused by monomers transferring from water into aggregates, where surfactant alkyl chains are immersed into a hydrophobic core, while hydrophilic headgroups are solvated by interfacial water.28 Herein, when the (22) Das, S.; Bhirud, R. G.; Nayyar, N.; Narayan, K. S.; Kumar, V. V. J. Phys. Chem. 1992, 96, 7454–7457. (23) Goon, P.; Das, S.; Clemett, C. J.; Tiddy, G. J. T.; Kumar, V. V. Langmuir 1997, 13, 5577–5582. (24) Xing, H.; Lin, S.; Yan, P.; Xiao, J.; Chen, Y. J. Phys. Chem. B 2007, 111, 8089–8095. (25) Persson, B.-O.; Drakenberg, T.; Lindman, B. J. Phys. Chem. 1976, 80, 2124–2125. (26) Shimizu, S.; Pires, P. A. R.; El Seoud, O. A. Langmuir 2003, 19, 9645–9652. (27) Huang, X.; Han, Y.; Wang, Y. L.; Wang, Y. X. J. Phys. Chem. B 2007, 111, 12439–12446. (28) Tada, E. B.; Novaki, L. P.; El Seoud, O. A. Langmuir 2001, 17, 652–658.
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Figure 7. Δδobs with the PATC concentration (C) in D2O.
PATC concentration beyond 5 mM concentration, all the protons of PATC shift to downfield, which indicates that all the PATC protons sense less polar environment with the concentration increasing at this period. In other words, the PATC molecules DOI: 10.1021/la903672r
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Figure 8. 1D NOE difference spectrum upon irradiating the Ha at 0.82 ppm.
pack more tightly with the increase of concentration. However, from 0.05 to 5 mM, the variation of the chemical shifts is different. The protons in hydrophobic chains and headgroups, such as Ha, Hb, Hc, He, and Hg, still move to downfield, whereas the protons in spacer group, such as Hj and Hk, oppositely move to upfield. These results indicate that the spacer group feels a more polar environment, reflecting the curving of the spacer group toward bulk phase. Moreover, the magnitude of these chemical shift variations is greater in the concentration range of 0.05-5 mM than beyond 5 mM, indicating the aggregate transition in the range of 0.05-5 mM. For surfactants having fast exchange between monomers and micelles in bulk solution, the observed chemical shift of the resonance peak can be expressed as the weighted mean of chemical shifts of the micelles and the monomers, appearing as a single peak. However, as shown in Figure 6B, from 0.05 to 1 mM, the peak of N-methyl groups in the headgroup (Hf) experienced splitting to two peaks and finally merging to one peak. This indicates that two kinds of aggregates coexist in the solution and cannot form fast exchange between each other, which confirms that the large aggregates already exist below the cmc. Another interesting phenomenon is the inconsistent extent of the splitting of the different protons at 0.05 mM (Figure 6C). It is noted that Hg and Hh well split into triple peaks, and He protons in alky chains show characteristic multiple peaks. Differently, the signals of the protons in the spacer group (Hj and Hk), which should split to triple peaks theoretically, become smooth and broad. This indicates that the spacer group is spin-restricted below the cmc and that the aggregates are already formed well below the cmc. To further understand the structure of PATC aggregate, a 1D selective nuclear Overhauser effect (NOE) experiment was carried out, and the NOE difference spectrum of PATC is shown in Figure 8. Obviously, irradiation of the methyl proton Ha at the end of hydrocarbon chains produced the NOE effect with all the other protons of PATC, which indicates that the chain-tailed methyl group is spatially correlated with the headgroup, the spacer moieties, and other protons in the hydrocarbon chains in the PATC aggregates. Because of the limitation of the molecular structure, the hydrophobic chains of PATC cannot be intramolecularly curved to close to the headgroup and the spacer. Therefore, the possible explanation is that the hydrophobic chains stretch out and form intermolecular contacts with the other parts of the hydrocarbon chains and the spacer in different extents. The 32 DOI: 10.1021/la903672r
Figure 9. (A) Possible model of transition of the networklike aggregates to micelles. (B) Possible model of transition of star-shaped molecular configuration to pyramid-like configuration.
following proposed network-like aggregates may make this kind of contacts possible. The 2D DOSY PGSE NMR spectra (Figure S2 of the Supporting Information) were also obtained with stebpgp1s pulse program at 25 °C with maximum gradient strength 50 G cm-1. The obtained surfactant self-diffusion coefficients at 0.06, 2, and 30 mM are around 2.04 10-10, 0.89 10-10, and 0.66 10-10 m2/s, respectively, i.e., decreasing with an increase of the surfactant concentration. Since the cmc is very low, the contribution from free monomers to the surfactant self-diffusion at low concentration is not negligible and will increase the value of the surfactant self-diffusion coefficients. Even so, from the value of the self-diffusion coefficient at 0.06 mM (low than the cmc), we can conclude that the premicellar aggregates are already formed before cmc. Normally, a decrease of surfactant self-diffusion coefficient reflects a size increase of the surfactant aggregates or an increase of the surfactant density in the aggregates. Herein, because the aggregates size before the cmc is larger than that beyond the cmc (from DLS), the decrease of the surfactant selfdiffusion coefficients with the increase of the surfactant concentration from 0.06 to 2 and 30 mM suggests that the surfactant molecules display more densely packing in the micelles than in the premicellar aggregates. In addition, the self-diffusion coefficients at low concentration distribute over a wider range than those at high concentration. Obviously, continuously changing aggregates exist at each concentration. The proposed network-like aggregate is possible to have such a characteristic since its aggregation number could vary in a large range. Considering all the results described above, the possible model of the PATC aggregation is proposed. As shown in Figure 9A, when the concentration is below the cmc, network-like large aggregates already exist in aqueous solution, while above the cmc, the large aggregates begin to transform into the micelles; finally, all the network-like large aggregates completely transform Langmuir 2010, 26(1), 28–33
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to the micelles. In the view of the PATC molecular structure, at the low concentration, the star-shaped molecular configuration is dominant (Figure 9B). Due to high charge density of PATC molecule, strong electrostatic repulsion results in the headgroups leaving each other as far as possible accompanied by the hydrocarbon chains stretching out. Thus, the free spin around the central spacer moiety N-CH2-CH2-N (yellow part in Figure 9B) is restricted. When the concentration is high enough, i.e., higher than the cmc, the hydrophobic interaction becomes strong enough and hydrophobic chains close together to escape water, making the molecular configuration turn into a pyramidlike shape. This configuration transition is proved by the spacer group being curved toward the bulk phase stated above. The starshaped configuration should favor to form the network-like aggregates, whereas the pyramid-like configuration should favor to form micelles.
Conclusion A star-shaped tetrameric quaternary ammonium surfactant PATC has been synthesized in this work. Surface tension, electrical conductivity, ITC, DLS, and NMR are used to investigate the relationship between its chemical structure and its aggregation properties. Below the cmc, intramolecular electrostatic
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repulsion in the headgroups generates the star-shaped molecular configuration. Based on this configuration, PATC molecules may form premicellar network-like aggregates through hydrophobic interaction among hydrophobic chains of different molecules. Beyond the cmc, the hydrophobic interaction becomes strong enough to turn the molecular configuration into pyramid-like shape, which leads to a network aggregate-to-micelle transition, and finally the network-like aggregates totally convert into micelles. The unique aggregation behavior sheds new light on the approaches of constructing novel self-assemblies through adjusting amphiphile structures. Acknowledgment. We are grateful for financial support from the National Natural Science Foundation of China and National Basic Research Program of China (Grants 20633010, 20973181, and 2005cb221300). Supporting Information Available: PATC synthesis, characterization data by 1H NMR, 13C NMR, mass spectrum, and elemental analysis, HMBC spectrum and 2D DOSY PGSE NMR spectra obtained with stebpgp1s pulse program. This material is free of charge via the Internet http:// pubs.acs.org.
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