FT-IR Study on Hydrogen Bonds between the Headgroups of

Hideya Kawasaki* and Hiroshi Maeda ... protonated cationic species [CnH2n+1(CH3)2N+-OH X-]. ... this hydrogen bond, it is important to focus on the OH...
12 downloads 0 Views 58KB Size
Download PDF
2278

Langmuir 2001, 17, 2278-2281

FT-IR Study on Hydrogen Bonds between the Headgroups of Dodecyldimethylamine Oxide Hemihydrochloride Hideya Kawasaki* and Hiroshi Maeda Department of Chemistry, Faculty of Science, Kyushu University 33, Hakozaki, Higashi-ku, Fukuoka, 812-8581, Japan Received September 25, 2000. In Final Form: December 27, 2000

Introduction Alkyldimethylamine oxides (CnDAO) solutions are mixtures of the nonionic [CnH2n+1(CH3)2NfO] and the protonated cationic species [CnH2n+1(CH3)2N+-OH X-]. The composition (the degree of ionization R) is determined by pH under a given ionic strength. On dodecyldimethylamine oxide (C12DAO) and tetradecyldimethylamine oxide (C14DAO), several marked effects of the protonation have been found,1-13 and they have been reviewed recently.14 (1) At R ) 0.5, the micelle size is the largest and the critical micelle concentration, cmc, is a minimum.3-6 (2) The maximum value of the surface excess Γ is at R ) 0.5.7 (3) For the intrinsic proton dissociation constants of the micelle KM and the monomer K1, pKM > pK1.8,9 (4) The stable complex is found in the solid state only at R ) 0.5.10 These protonation effects cannot be understood in terms of the introduced electric interaction alone, and we have proposed a hydrogen bond between the headgroups (-N+-OH‚‚O-N-), the cationic-nonionic pair. However, the spectroscopic evidence of the hydrogen bond between the two headgroups is not yet quite clear. In the present study, Fourier transform infrared spectroscopy (FT-IR) coupled with attenuated total reflection (ATR) was used to investigate the proposed hydrogen bond between the headgroups of C12DAO (R ) 0.5) in both an aqueous medium and the solid state. To clarify this hydrogen bond, it is important to focus on the OH band of the headgroup because the hydrogen bond between the headgroups is expected to be reflected significantly in this vibrational mode. Therefore, we focused on the OH band of the headgroup of C12DAO (R ) 0.5 and 1) in this study. Experimental Section Samples. Dodecyldimethylamine oxide, C12DAO (Fluka), was freeze-dried and recrystallized twice from hot acetone. Samples of different compositions R were prepared as follows.10 The (1) Herrmann, K. W. J. Phys. Chem. 1962, 66, 295. (2) Herrmann, K. W. J. Phys. Chem. 1964, 68, 1540. (3) Ikeda, S.; Tsunoda, M.; Maeda, H. J. Colloid Interface Sci. 1979, 70, 448. (4) Rathman, J. F.; Christian, S. D. Langmuir 1990, 6, 391. (5) Maeda, H. Colloids Surf., A 1996, 109, 263. (6) Zhang, H.; Dubin, P. L.; Kaplan, J. I. Langmuir 1991, 7, 2103. (7) Maeda, H.; Muroi, S.; Ishii, M.; Kaimoto, H.; Kakehashi, R.; Nakahara, T.; Motomoura, K. J. Colloid Interface Sci. 1995, 175, 497. (8) Tokiwa, F.; Ohki, K. J. Phys. Chem. 1966, 70, 3437. (9) Maeda, H.; Tsunoda, M.; Ikeda, S. J. Phys. Chem. 1974, 78, 1086. (10) Kawasaki, H.; Fukuda, T.; Yamamoto, A.; Fukada, K.; Maeda, H. Colloids Surf., A 2000, 169, 117. (11) Brycki, B.; Szafran, M. Magn. Reson. Chem. 1992, 30, 535. (12) Brycki, B.; Szafran, M. J. Mol. Liq. 1994, 59, 83. (13) Maeda, H.; Kanakubo, Y.; Miyahara, M.; Kakehashi, R.; Garamus, V.; Pedersen, J. S. J. Phys. Chem. B 2000, 104, 6174. (14) Maeda, H.; Kakehashi, R. Adv. Colloid Interface. Sci. 2000, 88, 275.

Figure 1. (A) FT-IR spectra of solid C12DAO with different degrees of protonation, R (R ) 0, 0.5, and 1), at 25 °C. (B) Comparison of the IR spectra of C12DAOHCl with that of C12DAOHNO3. prescribed amounts of aqueous HCl solutions were added to aqueous nonionic C12DAO, and finally crystalline samples of C12DAO with different compositions were obtained by freezedrying. Trimethylamine N-oxide hydrochloride (C1DAOHCl) was obtained by adding the prescribed amounts of aqueous HCl solutions to nonionic trimethylamine N-oxide (C1DAO) (Aldrich). The trimethylamine N-oxide (>98%) was used without further purification. Methods. All IR spectra were obtained at a 4 cm-1 resolution with a FT-IR 8600 spectrometer (Shimadzu Co.). The scans (256) were coadded. Happ-Genzel apodization was used in the subsequent Fourier transform. Samples in the solid state were spread on calcium fluoride windows and were measured in transmission mode. For the IR spectra in the aqueous solutions, an ATR attachment (ATR8100H) was used. The ATR plate is made from germanium crystal with a 45° end. The penetration depth of IR radiation in the crystal is about 0.66 µm at 1000 cm-1. The spectral subtraction of water from the IR spectra was performed only on the C1DAOHCl spectra in the aqueous solution of 5 wt %, and this was done by making use of the combination band of water at ∼2125 cm-1.15

Results and Discussion Effects of Protonation on the IR Spectra of C12DAO in the Solid State. Figure 1A shows the IR spectra of C12DAO (R ) 0, 0.5, and 1) in the solid state. For C12DAO (R ) 1), the protonation of the oxygen atom of C12DAO (R ) 0) with hydrochloride acid results in a (15) Dousseau, F.; Therrien, M.; Pezolet, M. Appl. Spectrosc. 1989, 43 (3), 538.

10.1021/la0013594 CCC: $20.00 © 2001 American Chemical Society Published on Web 03/09/2001

Notes

Figure 2. Dependence of a continuous absorption band (1300800 cm-1) of the solid C12DAO on the degree of ionization, R. The peak intensity IN is normalized by that of the symmetric CH2 stretching band.

broad band over 2300-2750 cm-1. The replacement of the proton of the OH group by deuterium caused a shift of the broad band to the lower wavenumbers near 2000 cm-1 (not shown). This indicates that the broad band over 23002750 cm-1 originates from the OH stretching mode (νOH) of the C12DAO (R ) 1). In the case of trimethylamine N-oxide perchlorate (C1DAOH‚ClO4) in acetonitrile solution, the νOH band has been reported to appear near 3260 cm-1.16 The lower wavenumber band position of νOH for the C12DAO (R ) 1) strongly suggests that the OH group of the headgroup is hydrogen bonded. We found that the intensity of the νOH band of the C12DAO (R ) 1) remarkably decreased by changing the counterion from Cl- to NO3- (Figure 1B). This suggests a hydrogen bond between the OH group and the chloride ion in the case of the C12DAO (R ) 1). It should be noted that the weak νOH band of the C12DAO (R ) 1) remains around 2500 cm-1 even though the counterion changed from Cl- to NO3-. This suggests that another type of hydrogen bond stronger than that between the OH group and the chloride ion also exists in the solid C12DAOHNO3 (R ) 1). A short O-Cl distance (2.94 Å), which is a smaller value than the van der Waals distance (3.2 Å), was reported for the solid C1DAOH‚Cl by X-ray diffraction,17 suggesting a hydrogen bond between OH and Cl-. In this solid C1DAOHCl, the νOH band has been reported to appear around 2900 cm-1.18,19 It is most likely that the band over 2300-2750 cm-1 for C12DAO (R ) 1) mainly originates from the hydrogen-bonded OH group to the counterion (-N+-OH‚‚Cl-). For the C12DAO (R ) 0.5), on the other hand, the IR spectra showed no detectable band in the range of 18002800 cm-1 but showed the two broad bands around 1650 cm-1 (weak) and 800-300 cm-1 in the low-frequency region. As to a continuous band around 800-300 cm-1, we found that the peak height of this continuous band, IN, was the maximum around R ) 0.5, as shown in Figure 2. This indicates that the continuous band originates from a complex with R ) 0.5, the nonionic-cationic pairs. It has been reported that the IR spectra of the solid N-pyridine oxide and its derivatives showed an intense broad band over 1700-600 cm-1 for R ) 0.5 (ClO4counterion) and for R ) 1 (SO42- counterion) because of (16) Krzywda, S.; Jaskolski, M.; Gdaniec, M.; Dega-Szafran, Z.; Grundwald-Wyspianska, M.; Szafran, M.; Dauter, Z.; Davies, G. J. Mol. Struct. 1994, 321, 57. (17) Rerat, P. C. Acta Crystallogr. 1960, 13, 63. (18) Jerslev, B. Acta Crystallogr. 1948, 1, 21. (19) Nakamoto, K.; Margoshes, M.; Rundle, R. E. J. Am. Chem. Soc. 1955, 77, 6480.

Langmuir, Vol. 17, No. 7, 2001 2279

a very strong hydrogen bond (OH- -O).20,21 From X-ray analysis, the very short OH- -O distances have been found to be 2.43 and 2.44-2.54 Å for the former and the latter, respectively.20,21 A characteristic continuous absorption in the 1000-800 cm-1 region has also been reported for C12DAO for R ) 0.5 (ClO4- counterion) in dichloromethane.12 It is known from theoretical as well as experimental results that in the case of strong hydrogen bonds with a large proton polarizability caused by proton motion an intense continuum appears in the region of 700-1500 cm-1 and is most intense around 1000 cm-1.22 The continuous band over 800-1300 cm-1 observed for the solid C12DAO (R ) 0.5) is consistent with the IR spectra of the systems with short and strong hydrogen bonds.12,20-22 A small fraction of the vibrational energy is still localized, and this is expected to give a weak, broad band around 1650 cm-1. It is well-known that the OH stretching band (νOH) remarkably shifts to the lower band position over the range from 3650 to 1650 cm-1 depending on the strength of the hydrogen bond.19 The weak, broad band around 1650 cm-1 observed for the C12DAO (R ) 0.5) falls within this frequency range of the νOH band. We may assign this band to the νOH of the headgroups. Approximately linear relations have been found between the OH‚‚O distance and the νOH band position for the crystalline state.19 According to this relationship, the 1650 cm-1 of νOH corresponds to the OH‚‚O distance of 2.4-2.5 Å, implying the very short OH- -O distances due to the hydrogen bond. Thus, the two bands around 1650 and 800-1300 cm-1 strongly suggest very strong hydrogen bonds between the headgroups (-N+-OH‚‚O-N-) in the case of the solid C12DAO (R ) 0.5). We have reported that the melting point of the solid C12DAO at R ) 0.5 is much higher than those at R ) 0 and R ) 1.10 The expected strong hydrogen bonds likely contribute to the higher melting point of the C12DAO (R ) 0.5) in the solid state. We can rule out the possibility of assigning those two broad bands to the OH band of water molecules. If hydration to the headgroups is significant, we should observe the OH stretching band in the range from 3600 to 3000 cm-1, but no significant absorption was detected, as seen in Figure 1A. In this figure, a sharp band was found around 3030 cm-1 for both R ) 0.5 and R ) 1. This band can be ascribed to the asymmetric stretching mode of the methyl groups attached to the N+ atom in the headgroup.23 Effects of Protonation on the IR Spectra of C12DAO in an Aqueous Medium. Figure 3A shows the spectra of C12DAO (R ) 0, 0.5, and 1) in aqueous solutions of 55 wt % surfactant. According to the phase diagrams of C12DAO-water systems,24 the solutions of C12DAO (R ) 0, 0.5, and 1) shown in Figure 3A correspond to the liquid crystalline hexagonal phase. In an aqueous medium, we expect to observe νOH of the hydrated OH group. A broad νOH band in the range of 2400-2800 cm-1 was found for R ) 1, but no peak was found in this range for R ) 0.5. The frequency band position of the νOH of the C12DAO (R ) 1) is close to that in the solid state. This indicates that the OH group of the headgroup forms a hydrogen bond in the aqueous medium. For the hydrogen bond in the case of R ) 1 in the liquid crystalline state, the following (20) Wasicki, J.; Jaskolski, M.; Pajak, Z.; Szafran, M.; Dega-Szafran, Z.; Adams, M. A.; Parker, S. F. J. Mol. Struct. 1999, 476, 81. (21) Szafran, M.; Tykarska, E.; Dega-Szafran, Z. J. Mol. Struct. 1997, 416, 81. (22) Bohner, U.; Zundel, G. J. Phys. Chem. 1986, 90, 964. (23) Kawai, T.; Umemura, J.; Takenaka, T.; Kodama, M.; Seki, S. J. Colloid Interface Sci. 1985, 103, 56. (24) Fukada, K.; Kawasaki, M.; Kato, T.; Maeda, H. Langmuir 2000, 16, 2495.

2280

Langmuir, Vol. 17, No. 7, 2001

Notes

Figure 4. FT-IR-ATR spectra of C12DAO with different degrees of protonation, R (R ) 0, 0.5, and 1), in aqueous solutions at 25 °C. The weight percent of the surfactants is 55.

Figure 3. (A) FT-IR-ATR spectra of C12DAO with different degrees of protonation, R (R ) 0, 0.5, and 1), in the aqueous liquid crystalline phase (hexagonal) at 25 °C. The weight percent of the surfactants is 55. (B) FT-IR-ATR spectra of C1DAOHCl in aqueous solutions of various concentrations at 25 °C. The weight percent of C1DAOHCl (from top to bottom) was 40, 30, 20, and 5. For comparison, the FT-IR-ATR spectrum of the C12DAOHCl (55 wt %) is also shown (a). The spectral subtraction of water from the IR spectra was performed on only C1DAOHCl spectra of 5 wt %.

three possible types are examined: (1) hydration of the headgroup (-N+-O-H‚‚‚OH2), (2) the hydrogen bond between the two headgroups on the micelle surface [-N+O-H‚‚‚O(-H)-N+-], and (3) the hydrogen bond between the headgroup OH group and the chloride ion (-N+-OH‚‚‚Cl-). Type 3 may be ruled out, because the position and intensity of the νOH band did not change by replacement of the counterion from Cl- to NO3- (not shown). To examine the possibility of the other two types of hydrogen bond (type 1 or type 2), we compared the IR spectra of the C12DAO (R ) 1) with that of C1DAOHCl in aqueous solutions. It was expected that the νOH band over 24002800 cm-1 would be observed for both the C1DAOHCl and the C12DAO (R ) 1) if the νOH band originates from the hydrated OH group (i.e., type 1). If the νOH band of the C12DAO (R ) 1) can be attributed to the hydrogen bond between the headgroups on the micelle surface (i.e., type 2), on the other hand, the νOH band positions of the C12DAO (R ) 1) and the C1DAOHCl would differ from each other, because C1DAOHCl cannot form micelles. Figure 3B shows the IR spectra of the C1DAOHCl in aqueous solutions of various concentrations. For comparison, the IR spectra of the C12DAO (R ) 1) in the hexagonal phase (55%) is also shown in the figure. The νOH band appears over 2400-2800 cm-1 for C1DAOHCl and the band position of the νOH is almost independent of the concentration, indicating type 1. The results shown in Figure 3B strongly suggest that the νOH band over 2400-2800 cm-1

of the C12DAO (R ) 1) can be attributed to the OH stretching modes of the hydrated OH groups (-N+-OH‚‚‚OH2). In contrast to C12DAO (R )1), there is no detectable band over 2400-2800 cm-1 for the C12DAO (R ) 0.5), as shown in Figure 3A. This indicates that the hydration of the OH group of the headgroup is inhibited for some reason for C12DAO (R ) 0.5). The hydrogen bond to the neighboring headgroup on the micelle surface (-N+OH‚‚O-N-) is suggested instead of the hydrated OH group (-N+-OH‚‚OH2). This headgroup-headgroup hydrogen bond could explain why the broad band due to the hydrated OH group does not appear for the C12DAO (R ) 0.5) even in an aqueous medium. For C12DAO (R ) 0.5) in the hexagonal phase, we could not clearly discriminate the continuum band (800-1300 cm-1) and the νOH band found in the solid state, because of the superposition of the intense absorption of H2O (3600-2900, 1750-1500, and 1000-500 cm-1) and D2O (2800-2100, 1300-1100, and 700-400 cm-1). Enlarged spectra of Figure 3A in the range of 1400-950 cm-1 are shown in Figure 4. The following two results shown in Figure 4 should be noted. (1) The absorption around 1330 cm-1 (CNO bending + CH2 wagging) disappeared for the C12DAO (R ) 0.5), whereas this band was observed for the C12DAO (R ) 0) and the C12DAO (R ) 1). This suggests that the C-N-O bending is greatly restricted for R ) 0.5. This is expected if the packing of the C12DAO micelle is the closest at R ) 0.5 with respect to the headgroup as a result of the hydrogen bond. This is consistent with the previous observation.25 (2) A broad band extending over 10001200 cm-1 was observed for C12DAO (R ) 0.5) but not for R ) 1 and 0. This broad band likely corresponds to the continuum band found in the solid C12DAO (R ) 0.5) (Figure 1A). In summary, this paper has shown the hydrogen bond between the headgroups (-N+-OH‚‚O-N-) for the C12DAO (R ) 0.5) in aqueous solutions as well as in the solid state by focusing on the OH band of the headgroup in the IR spectra. As for C12DAO (R ) 1), it has been suggested that the OH group of the headgroup forms (25) Rathman, J. F.; Scheuing, D. R. ACS Symp. Ser. 1990, 447, Chapter 7, 123.

Notes

hydrogen bonds to water molecules in the aqueous solution and to the counterion (Cl-) in the solid state. The present spectroscopic results on the C12DAO (R ) 0.5) provide a molecular basis for the hydrogen bond mechanism proposed in the previous study on the micelle size, the cmc, the titration property (i.e., pKM > pK1), and the solidphase behavior.

Langmuir, Vol. 17, No. 7, 2001 2281

Acknowledgment. We thank Dr. Y. Ozaki for giving us enlightening comments on the IR spectra of the C12DAO. This work is supported, in part, by the Grantin-Aid for Scientific Research (B) (No. 20022626) from The Monbukagaku-shou, Japan. LA0013594