Langmuir 2008, 24, 673-677
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Unusual Aqueous-Phase Behavior of Cationic Amphiphiles with Hydrogen-Bonding Headgroups Syed A. A. Rizvi, Lei Shi, Dan Lundberg,† and Fredric M. Menger* Department of Chemistry, Emory UniVersity, 1515 Dickey DriVe, Atlanta, Georgia 30322 ReceiVed NoVember 30, 2007. In Final Form: December 17, 2007 Two cationic surfactants with hydroxyl and carbamate hydrogen-bonding sites at their headgroups were synthesized. Both surfactants are ionic liquids (one of them at room temperature). Samples are isotropic solutions over the entire 0-100% concentration range, which is highly unusual for ionic surfactants. Surface tension, NMR, and conductivity measurements indicate classical micelle formation in aqueous solutions with CMCs below 10 mM. Pulse-gradient spin-echo (PGSE) NMR confirms micelle formation and provides micellar hydrodynamic radii of about 3.8 nm. Because this value is larger than the length of the extended surfactant molecules, about 2.7 nm, it appears that hydrogen-bonded water of hydration contributes substantially to the effective micelle size. At higher concentrations (above 25 wt %), surfactant solutions become viscous, but line broadening in the NMR is small relative to that found with a conventional cationic surfactant (CTAB). Thus, long rod formation, the source of line broadening in the latter, is absent with the new surfactants. Finally, PGSE NMR data show a 5-fold decrease in the diffusion coefficient between 5 and 20 wt %, above which the diffusion coefficients remain constant. The results are best explained by micelle clustering that is likely aided by intermicellar hydrogen bonding. The possibility of an isotropic liquid crystal (LC) phase with cubic symmetry is discussed and dismissed, demonstrating that LC formation of ionic surfactants at high concentrations, the usual behavior in past work, need not occur. Nor is there a definite connection between ionic liquid behavior and isotropic morphology.
As is the case with many scientific discoveries, it took nearly a century for ionic liquids to evolve into an area of intense research.1 The term and concept of ionic liquids, first enunciated in 1914,2 were not popularized until the 1990s. Several key advantages sparked widespread interest in developing new ionic liquids for specific tasks. Advantageous properties include miscibility with both aqueous and organic phases and low vapor pressure. Ionic liquids have the potential to be used as designer solvents because, by appropriate choice of anions and cations, their physicochemical characteristics can be easily tailored.3 Ionic liquids are now commonly used as media for liquid-liquid extractions,4,5 additives in high-performance liquid chromatography (HPLC) mobile phases,6 matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS),7 pseudostationary phases in micellar electrokinetic chromatography (MEKC),8 and solvents and catalysts in organic synthesis.3 The unique properties of ionic liquids have also earned them the attention of theoreticians from the broad scientific community, and several theories and models have thereby been proposed to explain their physicochemical behavior.9-11 Recently, Krossing et al.12 proposed a model to explain the low melting points of * Corresponding author. E-mail:
[email protected]. † Present address: Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal. (1) Welton, T. Chem. ReV. 1999, 99, 2071. (2) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (3) Parvulescu, V. I.; Hardacre, C. Chem. ReV. 2007, 107, 2615. (4) Carda-Broch, S.; Berthod, A.; Armstrong, D. W. Anal. Bioanal. Chem. 2003, 375, 191. (5) Carmichael, A. J.; Earle, M. J.; Holbrey, J. D.; McCormac, P. B.; Seddon, K. R. Org. Lett. 1999, 1, 997. (6) Poole, S. K.; Shetty, P. H.; Poole, C. F. Anal. Chim. Acta 1989, 218, 241. (7) Carda-Broch, S.; Berthod, A.; Armstrong, D. W. Rapid Commun. Mass Spectrom. 2003, 17, 553. (8) Rizvi, S. A. A.; Shamsi, S. A. Anal. Chem. 2006, 78, 7061. (9) Eike, D. M.; Brennecke, J. F.; Maginn, E. J. Green Chem. 2003, 5, 323. (10) Seddon, K. R.; Deetlefs, M.; Shara, M. Phys. Chem. Chem. Phys. 2006, 8, 642. (11) Abbott, A. P. Chem. Phys. Chem. 2005, 6, 2502. (12) Krossing, I.; Slattery, J. M.; Daguenet, C.; Dyson, P. J.; Oleinikova, A.; Weinga¨rtner, H. J. Am. Chem. Soc. 2006, 128, 13427.
ionic liquids. These authors suggest that unsymmetrical large ions with conformation flexibility impede effective packing in the solid state and thus promote ionic liquid formation. The choice of ions also plays an important role in the phase behavior of ionic liquids. For example, Bradley et al.13 observed that certain long-chained imidazolonium with large, flexible counterions ((CF3SO2)2N-) show ionic liquid behavior, whereas small counterions (Cl-, Br-, I-, and [BF4]-) impart liquid-crystalline behavior. A number of substances with ionic liquid behavior are amphiphilic (i.e., they have distinct hydrophobic and hydrophilic moieties). Because such compounds are known to form selfassemblies (micelles, liquid crystals, etc.) in aqueous solutions, it is worthwhile to study the aqueous phase behavior of amphiphilic ionic liquids in order to investigate if the ionic liquid properties of these compounds have any specific influence on their surfactant behavior. Indeed, the ionic liquid 1-decyl-3methylimidazolium bromide has been examined with respect to its aqueous phase behavior.14 It was found that the addition of 5-40% w/w water resulted in a homogeneous gel. An inspection of the gel in an optical microscope with sample placed between crossed polarizers revealed optical birefringence, an observation suggesting formation of an anisotropic lyotropic liquid crystalline (LC) phase. Although no specific data were reported regarding aggregate formation (e.g., critical micelle concentration (cmc) or critical aggregation concentration (cac)), such data on the surfactant were subsequently reported in a more recent paper.15 We now report on the aqueous phase behavior studies of two amphiphiles, A [(1-hydroxymethyl-3-methyl butyl)-dimethyl(2-undecyloxycarbonylamino ethyl) ammonium bromide] and B [(1-hydroxymethyl-3-methyl butyl)-dimethyl-(2-undec-10enyloxycarbonylamino ethyl) ammonium bromide] shown in (13) Bradley, A. E.; Hardacre, C.; Holbrey, J. D.; Johnston, S.; McMath, S. E. J.; Nieuwenhuyzen, M. Chem. Mater. 2002, 14, 629. (14) Firestone, M. A.; Dzielawa, J. A.; Zapol, P.; Curtiss, L. A.; Seifert, S.; Dietz, M. L. Langmuir 2002, 18, 7258. (15) Blesic, M.; Marques, M. H.; Plechkova, N. V.; Seddon, K. R.; Rebelo, L. P. N.; Lopes, A. Green Chem. 2007, 9, 481.
10.1021/la7037608 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/12/2008
674 Langmuir, Vol. 24, No. 3, 2008
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Figure 1. Molecular structures of the L-leucinol-derived amphiphilic ionic liquids.
Figure 1, that happen to be ionic liquids. A had been synthesized by one of the authors for chiral separation purposes.8 Both A and B carry a large, flexible leucinol-based headgroup with bromide as the counterion. A is a room-temperature ionic liquid (RTIL) and is miscible with water in all proportions, forming an isotropic liquid over the whole composition range. The pure compound remains liquid even after drying for 48 h under reduced pressure in the presence of P2O5. However, B is a solid at room temperature and has a small region of coexistence between liquid and solid (55-58 °C) while being an ionic liquid above 58 °C. With aqueous systems of most single-chain surfactants, LC phases are formed at concentrations above 25-50 wt % (depending on the chain length and the type of headgroup).16,17 In contrast, no texture characteristics of the LC phase were observed for either A or B when samples were placed between crossed polarizers in an optical microscope up to 65 wt % in water. The isotropic behavior observed for A and B is highly unusual, particularly for ionic surfactants. Other than a few nonionic surfactants that form isotropic solutions over the entire concentration range (usually at elevated temperatures),16 examples of highly concentrated isotropic surfactant solutions at room temperature are extremely rare. Thus, the question arose as to a possible connection between ionic liquid and isotropic behavior. We began our study by investigating whether A and B form conventional micelles in dilute solution. Surface tension versus log C plots for A and B (Figure 2) show sharp breaks that correspond to CMCs of 4.8 and 1.7 mM, respectively. Two other methods used for the determination of CMCs, 1H NMR chemical shift and conductivity, also showed sharp breaks at similar concentrations (Supporting Information). To obtain more detailed information on the assemblies formed by A and B, the pulsed gradient spin echo NMR (PGSE-NMR) technique was utilized to determine the self-diffusion coefficients of the amphiphiles at varying concentrations. Because selfdiffusion coefficients of solution components are dependent on aggregation, among other interactions, such data are useful in assessing the self-assembly process as well as the character of the assemblies. PGSE-NMR is a well-established noninvasive approach for characterizing surfactant aggregation.18-21 The observed self-diffusion coefficients D of A or B are plotted versus the reciprocal concentration in Figure 3a,b, respectively.21 It is evident from the graphs that both A and B indeed yield two sharply intersecting straight lines, as indicative of highly (16) Holmberg, K.; Jo¨nsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution, 2nd ed.; John Wiley & Sons: Chichester, U.K., 2002. (17) Laughlin, R. G. The Aqueous Phase BehaVior of Surfactants; Academic Press: London, 1994. (18) Furo, I. J. Mol. Liq. 2005, 117, 117. (19) Price, W. S. Concepts Magn. Reson. 1997, 9, 299. (20) Price, W. S. Concepts Magn. Reson. 1998, 10, 197. (21) So¨derman, O.; Stilbs, P.; Price, W. S. Concepts Magn. Reson. A 2004, 23A, 121. (22) Stejskal, E. O.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288.
Figure 2. Surface tension versus concentration of A (a) and B (b).
cooperative self-assembly.21 Estimated CMCs of A and B are 5.7 and 2.2 mM, respectively, in acceptable agreement with the results from the three other techniques. The difference between the CMCs of A and B follows an expected trend, namely, that surfactants with unsaturation in their hydrophobic chains generally have a higher CMC than those with a saturated chain of the same length.23 By extrapolating (to the y axis) the sloping lines in Figure 3, diffusion coefficients of the micelles, Dmic, were estimated to be ∼0.5 × 10-10 m2 s-1 for both A and B in dilute solutions. Selfdiffusion coefficients (D) of the different components were obtained from the attenuation of relevant echo peaks by a linear least-squares fit to the Stejskal-Tanner equation (eq 1)22
ln
()
I ) -(γGδ)2D(∆ - δ/3) I0
(1)
where I is the measured signal intensity, I0 is the signal intensity in the absence of gradient pulses, and γ is the magnetogyric ratio of protons, with the remainder of the parameters being defined above. In all experiments, the observed echo decays gave good fits to eq 1, which shows that they represent single self-diffusion coefficients. Dmic can be related to the hydrodynamic radius of the aggregates, RH, via the Stokes-Einstein equation (eq 2)
D0,agg )
kBT 6πηRH
(2)
where kB is the Boltzmann constant and η is the viscosity of the solvent at the experimental temperature T. With 1.132 mPa s as the viscosity of D2O, eq 1 gives the hydrodynamic radii of A and B of ∼3.8 nm. This value is somewhat larger than what is expected for a spherical micelle composed of these molecules (the extended (23) Patist, A. Determining Critical Micelle Concentration. In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; John Wiley & Sons: New York, 2001; Vol. 2, p 239.
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Figure 3. Observed self-diffusion coefficients, D, of A (a) and B (b) plotted versus the reciprocal of their concentrations (D2O, 25 °C). The dashed line is the predicted D for a conventional micelleforming amphiphile with Dmicelle ) 0.5 × 10-10 m2 s-1.
length of either of the molecules is about 2.7 nm).24 However, the presence of three polar functionalities in the headgroups may bind considerable amounts of water of hydration, and the resulting larger effective size of the micelles may, at least in part, be accounted for by this effect. Taken collectively, the results from surface tension (Figure 2), 1H NMR, conductivity, and PGSENMR (Figure 3) suggest that A and B form conventional micelles at low concentrations. From a visual inspection of the aqueous solutions of both A and B, it was noted that there is a significant increase in their viscosity at concentrations above 25 wt %. At about 25 wt %, the viscosity of the solutions approximates 10-20 Poise. An increase in the viscosity of a surfactant solution with increasing concentration is often attributed to the occurrence of significant micellar growth into rods or worms. Solutions of large wormlike aggregates have much in common with certain polymer solutions where both systems derive their viscosity from the entanglement of the aggregates.16 Evidence for micellar growth can be obtained by investigating the width and shape of the 1H NMR spectral peaks as a function of concentration. Resonances from a surfactant residing within large micelles are expected to show a characteristic (24) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992.
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band shape with a broad base and a narrow apex, features that reflect the presence of slow-motion components.25,26 We performed detailed NMR studies on surfactant A because it is a room-temperature ionic liquid. Figure 4 shows the 1H NMR spectra of two different solutions of A with concentration ranging from 5 wt % (∼120 mM) to 30 wt % (∼720 mM). One can see that the peaks of A remain reasonably narrow in this concentration range. Although a slight broadening is present, the broadening is negligible compared to that observed for CTAB in Figure 4b, which is attributable to growth into rod- or wormlike micelles. This finding indicates that there is no dramatic rearrangement of growth into linear aggregates where motional freedom would have been restricted. Moreover, the observed increase in viscosity cannot be accounted for by entangled rods. Observed self-diffusion coefficients (D) for A and water at high concentrations of the compound are shown in Figure 5. Plots of D of A versus concentration show two interesting features: (a) Between 5 and 20 wt % of solute, the value of D decreases to about one-fifth of the values for micelles at low concentrations. (b) D is essentially independent of surfactant concentration between 20 and 65 wt %. The first observation indicates an increase in the apparent aggregate size in the 5-20 wt % range. A decrease of D in this range could be explained either by minor micellar growth or by micelle clustering into what has been called “secondary aggregates”.27 No further growth or clustering is evident after 20 wt % of solute. Note that as the volume fraction of the aggregates increases and as the aggregates therefore come into closer contact, aggregate-to-aggregate transport of monomer might increase concomitantly. Thus, the actual D of the aggregates may in fact be somewhat lower than the observed value. As can be seen from Figure 5b, the decrease in D of water with increasing surfactant concentration is more gradual than the decrease in the D of A. We recapitulate the observations based on Figures 2-5: (a) ionic liquid behavior in dehydrated samples; (b) isotropic solutions at all concentrations; (c) conventional micelle formation at low concentrations (
