3118
Langmuir 2008, 24, 3118-3121 1
H NMR Study on Pre-micellization of Quaternary Ammonium Gemini Surfactants Yan Jiang,†,§ Hong Chen,‡ Xiao-Hong Cui,†,§ Shi-Zhen Mao,*,† Mai-Li Liu,*,† Ping-Ya Luo,‡ and You-Ru Du†
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, People’s Republic of China, State Key Laboratory of Oil and Gas ReserVoir Geology and Exploitation Engineering, Southwest Petroleum UniVersity, Chengdu 610500, People’s Republic of China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed October 28, 2007. In Final Form: NoVember 20, 2007 Two quaternary ammonium Gemini surfactant series, 12-s-12, ([C12H25N+ (CH3)2]2(CH2)s‚2Br-) and 14-s-14 ([C14H29N+(CH3)2]2(CH2)s‚2Br-), where s ) 2, 3, and 4, have been studied by the use of 1H NMR in aqueous solution at concentrations below their critical micelle concentrations (CMC) at 25 °C. The appearance of a second set of peaks for the 14-s-14 series and the changes in chemical shifts, line widths, and line shapes of the 12-s-12 series with increasing concentration below the CMC are interpreted as evidence for the formation of premicelle aggregates (oligomers) that appear at approximately one-half their CMC values. Self-diffusion coefficients (D) and transverse relaxation times (T2) have also been detected and support the results obtained by 1H NMR.
Introduction Surfactants aggregate into micelles in aqueous solutions at concentrations above CMC. Pre-micellization is a molecular preaggregation stage before self-aggregating to integrated micelles, which includes too much micro information about self-aggregation. There are many surfactant-accelerated reactions with surfactant concentrations below CMC. Thus, it seems very necessary and important to understand pre-micellization more clearly, especially at the molecular level. Gemini surfactants, owing to their tunable molecular geometries and unusual physical properties of their aggregates in aqueous solutions and at the interfaces, have recently attracted considerable attention. Gemini surfactants also form micelles in aqueous solutions at concentrations above CMC. However, there are different ideas about the behavior of the Gemini surfactant molecules at concentrations below their CMC: whether they form oligomeric or keep monomers twisting within themselves. Menger and Littau1 first gave some evidence of Gemini surfactants forming premicelles in aqueous solutions by surface tension method. Rosen et al.2 also made the same conclusion by surface tension and steady-state fluorescence methods. However, Rosen et al. admitted that measurements of small aggregation numbers as detailed in their work is challenging.3 Song and Rosen4 also used the surface tension method to study the micellization and pre-micellization of two kinds of Gemini surfactants. The * To whom correspondence should be addressed. Fax: 86-(0)27-87199291. E-mail:
[email protected];
[email protected]. † State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences. ‡ State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation Engineering, Southwest Petroleum University. § Graduate University of the Chinese Academy of Sciences. (1) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115 (22), 1008310090. (2) Rosen, M. J.; Mathias, J. H.; Davenport, L. Langmuir 1999, 15 (21), 73407346. (3) Mathias, J. H.; Rosen, M. J.; Davenport, L. Langmuir 2002, 18 (20), 77617761. (4) Song, L. D.; Rosen, M. J. Langmuir 1996, 12 (5), 1149-1153.
Figure 1. Chemical structures and proton numbering of quaternary ammonium Gemini surfactants m-s-m, concretely, 12-2-12 (m12-s2-m12), 12-3-12 (m12-s3-m12), 12-4-12 (m12-s4-m12), 142-14 (m14-s2-m14), 14-3-14 (m14-s3-m14), and 14-4-14 (m14-s4m14).
deviation in surface activity is concluded to be due to the forming of premicelles. Mathias and Rosen et al.5 used steady-state and time-resolved fluorescence spectroscopy to study the premicellar aggregation of cationic Gemini surfactants, although Zana6 questioned the veracity of the quantitative aggregation numbers for premicelle oligomers obtained by the latter authors. Zana7 used electrical conductivity measurements to investigate ion pairing and premicellar association in solutions of eight bisquaternary ammonium bromide dimeric surfactants. The results conclusively show that the 12-s-12 series with s e 10 show neither ion pairing nor premicellar association but that premiclle association occurs for the 12-s-12 series with s g 12 and the m-s-m series with m g 14, having either an octanediyl or a p-xylylene spacer. Li and Wettig et al.8,9 also produced evidence of premicelle aggregates by using conductance and surface tension measurements. 1H NMR is a new method useful in studying surfactants aggregating in aqueous solutions. Gillitt et al.10 found changes (5) Mathias, J. H.; Rosen, M. J.; Davenport, L. Langmuir 2001, 17 (20), 61486154. (6) Zana, R. Langmuir 2002, 18 (20), 7759-7760. (7) Zana, R. J. Colloid Interface Sci. 2002, 246 (1), 182-190. (8) Li, X. F.; Wettig, S. D.; Wang, C. Z.; Foldvari, M.; Verrall, R. E. Phys. Chem. Chem. Phys. 2005, 7, 3172-3178. (9) Wettig, S. D.; Wang, C. Z.; Verrall, R. E.; Foldvari, M. Phys. Chem. Chem. Phys. 2007, 9, 871-877.
10.1021/la703346p CCC: $40.75 © 2008 American Chemical Society Published on Web 02/12/2008
1H
NMR Study of Gemini Surfactants
Langmuir, Vol. 24, No. 7, 2008 3119
Figure 2. Parts of the 500 MHz 1H NMR spectra and proton assignments of 14-s-14 at various concentrations in D2O at 25 °C, a prime was used to represent premicelles to distinguish from monomers.
in chemical shifts, line widths, and line shapes of 1H NMR spectra of dimethyl di-n-dodecyl ammonium chloride, which indicate the formation of premicelles. However, there have been few reports on studying quaternary ammonium Gemini surfactants at concentrations below CMC by NMR method. The aim of this paper is to study two series of quaternary ammonium bromide Gemini surfactants with m ) 12 and 14, s ) 2, 3, and 4 in aqueous solutions at concentrations below their CMC using 1H NMR method. Experimental Section Materials. The quaternary ammonium Gemini surfactants 12s-12 and 14-s-14 (where s ) 2, 3, and 4) series were synthesized by the Southwest Petroleum University of China. Heavy water (D2O) was obtained from Beijing Chemical Factory of China and was 99.8% deuterated. The samples were recrystallized in the mixture of ethyl acetate and chloroform for several times until a minimum in the surface tension profile was no longer observed. The reagents were used as received, without any further purification. Experiments. 1H NMR experiments were performed on a Varian INOVA-500 spectrometer with a proton frequency of 500.13 MHz at 25 °C. TSP (Me3Si-CD2CD2-CO2Na) was used as the external reference. For the sake of saving time, a small pulse flip-angle 30° was used rather than 90° in the conventional single pulse sequence. A longitudinal eddy-current delay with a bipolar pulse pair (LEDBPP) pulse sequence11 was used to determine the self-diffusion coefficients D. Transverse relaxation times (T2) have been determined by CPMG pulse sequences.12 Both D and T2 were gotten by origin 7.0 fitted from the experimental data. The spectra are plotted by the software of Corel Draw 12 after processed by TOPSPIN 2.0.
Results and Discussions Chemical structures and proton numbering of the studied quaternary ammonium dibromide Gemini surfactants are shown in Figure 1. (10) Gillitt, N. D.; Savelli, G.; Bunton, C. A. Langmuir 2006, 22 (13), 55705571. (11) Wu, D. H.; Chen, A. D.; Johnson, C. S. J. Magn. Reson., Ser. A 1995, 115 (2), 260-264. (12) Slichter, S. P. Principles of Magnetic Resonance; Springer-Verlag; New York, 1990, Chapter 8.
Figure 3. Variations of self-diffusion coefficient (D) of different components with the surfactant concentrations for 14-3-14 in D2O at 25°C.
Micelles are in direct equilibrium with the monomers in the bulk solution. When the exchange between the monomer and the micelle is fast, such as conventional single chain surfactant, a set of weighted average peaks results in the 1H NMR spectrum. Huc and Oda13 were the first to report the slow surfactant exchange between micelles and the bulk solutions on the NMR time scale for some cationic Gemini surfactants. They showed that when the concentration of the surfactant is above its CMC, a second set of signals appears in the 1H NMR spectrum, which corresponds to the micelle. However, they have not detected any information about the appearance of this set of signals in solutions at concentrations below CMC. We found that this second set of signals appears in 14-s-14 solutions far below their CMC, shown in Figure 2. We also found that when the concentration became higher, but still below CMC, integral of not only H6′ (prime is used to represent premicelles) but also H6, both increased. This is an indication of the premicelle formation. It is evident that the (13) Huc, I.; Oda, R. Chem. Comm. 1999, 2025-2026.
3120 Langmuir, Vol. 24, No. 7, 2008
Jiang et al.
Figure 4. Parts of the 500 MHz 1H NMR spectra and proton assignments of 12-s-12 at various concentrations in D2O at 25 °C.
premicellar peak of proton H6, i.e., H6′, which is most easily detectable in these very dilute solutions, appears already when the concentrations are 0.039-0.076 mM, 0.054-0.074 mM, 0.059-0.063 mM for 14-2-14, 14-3-14, and 14-4-14, respectively (all of their CMCs are about 0.16 mM). Although Zana7 has only observed premicellar association in solutions of dimeric surfactants with m g 14, having either an octanediyl or a p-xylene spacer, the above results definitely suggest that 14-s-14, with s ) 2, 3, and 4, form premicelles in solutions at concentrations which are well below their CMC. Variation in self-diffusion coefficients (D) of 14-3-14 surfactants as monomers and in premicelles with concentrations below its CMC is shown in Figure 3. It is easy to see that when the concentration reaches 0.07 mM, a second set of D values, corresponding to H6′, represented as premicelles appeared, which were remarkably smaller than those of the monomer surfactants. To be more accurate, D of monomers was gained from the average of values of all the protons instead of merely H6. This also proves the forming of aggregates. Unfortunately, exchange of monomers with micelles for 12s-12 surfactants is out of the NMR time scale.13 No regular micellar peak can be observed. Huc and Oda13 state that the borderline between fast and slow exchange of surfactant from the bulk to regular micelle on the NMR time scale at 25 °C lies between a total of 24 and 26 carbon atoms in the two alkyl tails for s ) 2. However, the changes in chemical shifts with increasing concentration and the changes in line widths and line shapes, as depicted in Figure 4, give evidence about the formation of premicelles in 12-s-12 systems at concentrations below their CMC. Parts of their 1H NMR spectra depicted in Figure 4, are similar to those reported by Gillitt et al.10 for dimethyl di-n-ammonium chloride. It is not difficult to see that the δ of all protons of each Gemini surfactant moves to a lower field as the concentrations become higher. Concretely, for 12-2-12, 12-3-12, and 12-4-12, the δ of all of the protons become larger at about 0.6-0.7× the CMC (with CMC ) 0.84, 0.90, and 0.95 mM respectively, according to the refs 14 and 15). Generally, proton chemical
Figure 5. Variations of transverse relaxation times (T2) of different protons with the surfactant concentrations for 12-3-12 in D2O at 25 °C.
shifts of a surfactant monomer in solution do not change with concentration before aggregation occurs, i.e., keep constant in the concentration range below its CMC. This change in chemical shift at concentrations below their CMC implies that the environment of the monomer molecules is varied. Besides, the fact that proton chemical shifts move to lower fields is the result of the effect of the positively charged head group, association of monomers. Transverse relaxation time (T2) measurements can be used to monitor the molecular motion of protons. It is sensitive to the slower translational diffusion provides information about motions in a wide range of frequencies. Knowledge of the change in T2 value of the proton will help understand the change in molecular environment in the solution.16 Variation in transverse relaxation times (T2) of different protons of 12-3-12 with concentrations below its CMC is shown in Figure (14) Wettig, S. D.; Verrall, R. E. J. Colloid Interface Sci. 2001, 235 (2), 310-316. (15) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7 (6), 1072-1075.
1H
NMR Study of Gemini Surfactants
5. It is obvious that the T2 of all of the protons of 12-3-12 decrease as the concentration reaches 0.53-0.68 mM, which is 0.6-0.7× its CMC, referring to the references.14,15 This implies that the motion of surfactant molecules is seriously hindered at this concentration. Dimeric surfactant 12-3-12 molecules are aggregated at concentration which is 0.6-0.7 times its CMC. It should be noted that this concentration agrees well with the result obtained from chemical shift changes.
Langmuir, Vol. 24, No. 7, 2008 3121
CMC in aqueous solutions have been studied by 1H NMR. The appearance of a second set of resonance peaks in the 1H NMR spectra with self-diffusion coefficients remarkably below the monomers for the 14-s-14 series, and the changes in chemical shift, line width, and shape and the diminishing of transverse relaxation times with concentration for the 12-s-12 series, strongly show premicellization of these two series of surfactants at concentrations of about one-half that of their CMC values.
Conclusions In conclusion, two series of quaternary ammonium Gemini surfactants 12-s-12 and 14-s-14 at concentrations below their
Acknowledgment. This work was financially supported by the National Science Foundation of China (20610104, 20635040).
(16) Mao, S. Z.; Du, Y. R. Acta Physico-Chimica Sinica 2003, 19 (7), 675680.
LA703346P
