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Langmuir 2005, 21, 9440-9445
Surfactant Quaternary Ammonium Hydrotris(1-pyrazolyl)borates† David A. Jaeger,*,‡ Alvaro Mendoza,‡ Jason Bragdon,‡,§ Navamoney Arulsamy,‡ Zhihong Li,‡ and Robert P. Apkarian| Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071, and Integrated Microscopy and Microanalytical Facility, Emory University, Atlanta, Georgia 30322 Received May 27, 2005. In Final Form: July 27, 2005
Quaternary ammonium surfactants with the hydrotris(1-pyrazolyl)borate anion as an unusual counterion were prepared and characterized. Dodecyl- (1a), tetradecyl- (1b), and hexadecyltrimethylammonium hydrotris(1-pyrazolyl)borate (1c), as well as didodecyldimethylammonium hydrotris(1-pyrazolyl)borate (2), were prepared by metathesis reactions with the corresponding quaternary ammonium bromides and potassium hydrotris(1-pyrazolyl)borate (4). Although the surfactants have limited stabilities in water at 23 °C, they were characterized by measurement of their Krafft temperatures and critical aggregation concentrations. 1H NMR spectroscopy suggested that the single-chain surfactants 1 form small aggregates in water and revealed an association of the quaternary ammonium and hydrotris(1-pyrazolyl)borate ions above their critical aggregation concentrations. Cryo-etch high-resolution scanning electron microscopy indicated that 1a and 1b most likely form segregation patterns upon plunge-freezing and cryo-etching of their aqueous solutions, and a single-crystal X-ray diffraction study of 1a was performed.
Introduction Most quaternary ammonium surfactants contain simple counterions such as halide anions, although, in principle, any anion can serve as a counterion. Examples of more complex anions that have been used as counterions include alkanoates1 and perfluoroalkanoates,2 benzoates,3 sulfates,4 sulfonates,3a,5 O,O′-dialkylphosphorodithioates,6 oximates,7 glucuronates,8 and dyes.9 We have an interest in quaternary ammonium surfactants containing uncommon counterions, which may bestow interesting properties on such surfactants. Herein we report the preparation and characterization of quaternary ammonium surfactants 1a-c and 2, which contain an unusual counterion, namely, the hydrotris(1-pyrazolyl)borate anion [HB(Pz)3-] (Pz ) 1-pyrazolyl). To our knowledge, this anion, which † Dedicated to the memory of Ms. Zhihong Li (deceased May 25, 2003). * To whom correspondence should be addressed. E-mail: DAJ@ uwyo.edu. ‡ University of Wyoming. § NSF Research Experiences for Undergraduates participant, Summer 2003. | Emory University.
(1) (a) Chung, Y.-C.; Regen, S. L. Langmuir 1993, 9, 1937. (b) Toullec, J.; Couderc, S. Langmuir 1997, 13, 1918. (c) Nascimento, D. B.; Rapuano, R.; Lessa, M. M.; Carmona-Ribeiro, A. M. Langmuir 1998, 14, 7387. (d) Chung, Y.-C.; Lee, H.-J. Bull. Korean Chem. Soc. 1999, 20, 16. (2) Regev, O.; Leaver, M. S.; Zhou, R.; Puntambekar, S. Langmuir 2001, 17, 5141. (3) (a) Bijma, K.; Engberts, J. B. F. N. Langmuir 1997, 13, 4843. (b) Kreke, P. J.; Magid, L. J.; Gee, J. C. Langmuir 1996, 12, 699. (4) (a) Tomasic, V.; Popovic, S.; Tusek-Bozic, L.; Pucic, I.; FilipovicVincekovic, N. Ber. Bunsen-Ges. 1997, 101, 1942. (b) Brasher, L. L.; Kaler, E. W. Langmuir 1996, 12, 6270. (5) (a) Persson, G.; Edlund, H.; Hedenstro¨m, E.; Lindblom, G. Langmuir 2004, 20, 1168. (b) Jaeger, D. A.; Sayed, Y. M. J. Org. Chem. 1993, 58, 2619. (6) Jaeger, D. A.; Roberts, T. L.; Zelenin, A. K. Colloids Surf. A 2002, 196, 209. (7) Balakrishnan, V. M.; Han, X.; VanLoon, G. W.; Dust, J. M.; Toullec, J.; Buncel, E. Langmuir 2004, 20, 6586. (8) Menger, F. M.; Binder, W. H.; Keiper, J. S. Langmuir 1997, 13, 3247. (9) Buwalda, R. T.; Engberts, J. B. F. N. Langmuir 2001, 17, 1054.
can function as a tridentate ligand,10 has not been employed previously as a surfactant counterion.
Results and Discussion Single-chain surfactants 1a-c and double-chain surfactant 2 were prepared by simple metathesis reactions as illustrated for the former (eq 1). Aqueous solutions of an alkyltrimethylammonium bromide (3) and potassium hydrotris(1-pyrazolyl)borate (4) were mixed and extracted with chloroform. The resultant chloroform solution was rotary-evaporated to give 1, which was dried and recrystallized from hexanes to give analytically pure surfactant.
Although they are stable indefinitely as pure materials at 23 °C, surfactants 1a-c and 2 have limited stabilities in water, as determined by 1H NMR spectroscopy. In particular, the hydrotris(1-pyrazolyl)borate anion decomposes, as indicated by the appearance of signals corresponding to pyrazole (5) in the 1H NMR spectra of 1a-c and 2 in D2O. The identity of pyrazole was confirmed by (10) Trofimenko, S. Chem. Rev. 1993, 93, 943.
10.1021/la0514011 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/14/2005
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Figure 1. Plots of surface tension for surfactants 1a (circles), 1b (squares), and 1c (triangles) in water.
a spiking experiment with 1b (see the Experimental Section). In D2O at 23 °C, 1a-c are stable for up to 180 min, and 2 is stable for up to 20 min. At present, it is unknown why the HB(Pz)3- counterion of double-chain surfactant 2 is less stable than that of single-chain surfactants 1 (but see below). All of the characterization of 1a-c and 2 in water was performed within 180 and 20 min, respectively, after dissolving a given surfactant in water. Compounds 1a-c and 2 are more stable in aqueous K2CO3 than in pure water. At 23 °C, 1a-c are stable for up to 15 h in 0.10 M K2CO3 in D2O, and 2 is stable for up to 5 h in 0.0010 M K2CO3 in D2O. The decomposition of hydrotris(1-pyrazolyl)borate ligands to give the corresponding pyrazoles has been reported.11,12
The surfactants were characterized by measurement of their Krafft temperatures (Tk) and critical aggregation concentrations (cac). Although the solubility of an ionic surfactant in water generally increases with increasing temperature, it typically increases dramatically at a point known as its Krafft temperature.13 Aggregation of an ionic surfactant into assemblies such as micelles or vesicles can occur only above its Tk and cac values. Aggregated surfactants 1 in water were evaluated by 1H NMR spectroscopy and dynamic laser light scattering (DLLS). Aqueous 1a and 1b were also examined by cryo-etch high-resolution scanning electron microscopy (cryo-etch HRSEM), and a single-crystal X-ray diffraction study of 1a was performed. The Tk values of 1a-c in water and aqueous 0.10 M K2CO3 are 4σ(F) giving the data-to-parameter ratio of 3.4:1. Additional X-ray diffraction results and data are found in the Supporting Information.
Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant No. 0092560. Supporting Information Available: 1H NMR spectra of surfactants 1a (6.5 × 10-3 M) and 3c (CTABr) (23 × 10-3 M) in D2O at 23 °C. X-ray diffraction results for surfactant 1b consisting of tables of crystal data and structure refinement; atomic coordinates and equivalent isotropic displacement parameters; bond lengths and angles; anisotropic displacement parameters; and hydrogen coordinates and isotropic displacement parameters. This information is available free of charge via the Internet at http://pubs.acs.org. LA0514011
