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Kinetics of Fusion and Fragmentation Nonionic Micelles: Triton X-100 Yahya Rharbi and Mitchell A. Winnik* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
Kenneth G. Hahn, Jr. ICI Paints Research Center, 16651 Sprague Road, Strongsville, Ohio 44136-1739 Received January 21, 1999. In Final Form: May 13, 1999 We describe the kinetics of exchange of a pyrene-labeled triglyceride 1 between micelles of Triton X-100, an octylphenol ethoxylate with a mean degree of ethoxylation of 9.5. These micelles have an aggregation number of ca. 110 surfactants/micelle and are capable of solubilizing more than four molecules of 1 per micelle while maintaining Poisson solubilization statistics. When excess surfactant is added to a solution of TX100 micelles containing sufficient 1 that the fluorescence spectrum is dominated by excimer emission, the monomer emission intensity grows in at the expense of excimer emission. Since 1 is insoluble in water, exchange requires fusion of two micelles followed by fragmentation. This process follows second-order kinetics, from which we calculate a second-order rate constant kfus ) 1.15 × 106 M-1 s-1. Micelle fusion is approximately 5000-fold slower than diffusion controlled. We infer that the barrier to micelle fusion is due primarily to entropic and hydration effects of the ethylene oxide stabilizer chains. This barrier is relatively easily overcome to allow rapid micelle fusion-and-fragmentation to take place in dilute solution at room temperature.
Introduction For many years there has been a deep interest in the dynamics of micellar solutions. Many micellar systems have been examined by stopped flow, pressure jump, temperature jump, and ultrasonic relaxation measurements.1 These measurements frequently identify two wellseparated relaxation times, a rapid relaxation which occurs on a time scale of microseconds and a slower process which requires milliseconds to seconds. 2-4 Aniansson and Wall 3,4 assigned the fast process to an associationdissociation process involving the exchange of individual surfactant molecules between the micelles and the water phase and attributed the slower process to a more deepseated rearrangement of the system involving the creation or destruction of micelles.1-4 Aniansson et al. have shown that one can calculate the exit (k-) and re-entry (k+) rate constants for the surfactant molecules from the concentration dependence of the fast relaxation time. For the nonionic surfactant Triton X-100,5 which we examine in this paper, they found k- ) 1.1 × 106 s-1 and k+ ) 3.7 × 109 M-1 s-1 at 25 °C. When the micelles contain hydrophobic solutes, another type of dynamics becomes important: the exchange of (1) For reviews of dynamic processes in micelles, see: (a) Muller, N. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum: New York, 1979; Vol. I, pp 267-295. (b) Gormally, J.; Gettins, W. J.; WynJones, E. In Molecular Interactions; Wiley: New York, 1980; Vol. 2, pp 143-177. (c) Lang, J.; Zana, R. In Surfactant Solutions: New Methods of Investigation; Zana, R., Ed.; Marcel Dekker: New York, 1987; pp 405-452. (d) Huibers, P. D. T.; Oh, S. G.; Shah, D. O. In Surfactants in Solution; Chattopadhay, A. K., Mittal, K. L., Eds.; Marcel Dekker: New York, 1995; Vol. 64, pp 105-121. (2) Lang, J.; Tondre, C.; Zana, R.; Bauer, H.; Hoffmann, H.; Ulbricht, W. J. Phys. Chem. 1975, 79, 275. (3) Aniansson, E. A. G.; Wall, S. N. J. Phys. Chem. 1974, 78, 10241030; 1975, 75, 857-858. (4) Aniansson, E. A. G.; Wall, S. N.; Almgren, M.; Hoffmann, H.; Kielmann, H.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J. Phys. Chem. 1976, 80, 905. (5) Hoffmann, H.; Kielmann, H.; Pavlovic, D.; Platz, G.; Ulbricht, W. J. Colloid. Interface Sci. 1981, 80, 237.
solute molecules between micelles.6 As in the case of surfactant exchange, the dominant mechanism is an “evaporation-condensation” process involving passage of individual solute molecules through the water phase. When the solute is a luminescent dye or a quencher of the fluorescence or phosphorescence of a dye localized in the micelles, time-resolved luminescence quenching experiments can be used to determine the exit and entry rates of the solute from and into the micelles.6-11 One finds that the exit rates are rate-limiting, whereas the entry rates are close to diffusion controlled. Within a series of solutes containing a different number of aromatic rings or different alkyl substituents, the exit rate decreases rapidly with decreasing water solubility. 8,9 Fluorescence quenching experiments6-9 are sensitive to fast exchange rates, whereas triplet quenching measurements,10-13 because of the longer-lived excited state, are sensitive to slower exit rates. One can imagine a very different dynamic process that exchanges both solutes and surfactant molecules between micelles. This process, involving micelle fusion followed by fragmentation, has been suggested as a mechanism to explain rapid exchange for nonionic surfactant micelles at high concentrations or at elevated temperatures. It is analogous to the fusion process observed for single lamellar vesicles in solution.14 For example, Zana et al.15 studied (6) Zana, R. In Surfactant Solutions: New Method of Investigation; Zana, R., Ed.; Marcel Dekker: New York, 1987. (7) Infelta, P. P.; Gra¨tzel, M.; Thomas, J. K. J. Phys. Chem. 1974, 78, 190. (8) Bolt, J. D.; Turro, N. J. J. Phys. Chem. 1981, 85, 4029. (9) Almgren, M.; Grieser, F.; Thomas, J. K. J. Am. Chem. Soc. 1979, 101, 2021. (10) Scaiano, J. C.; Selwyn, J. C. Can. J. Chem. 1981, 59, 2368. (11) Selwyn, J. C.; Scaiano, J. C. Can. J. Chem. 1981, 59, 663. (12) Pileni, M. P.; Gra¨tzel, M. J. Phys. Chem. 1980, 84, 1822. (13) Hruska, Z.; Piton, Mark; Yekta, A.; Duhamel, J.; Winnik, M. A.; Riess, G.; Croucher, M. D. Macromolecules 1993, 26, 1825. (14) Duzgunes, N.; Allen, T. M.; Fedor, J.; Papahadjopoulos, D. Biochemistry 1987, 26, 8435. (15) Zana, R.; Weill, C. J. Phys. Lett. 1985, 46, 953.
10.1021/la990064r CCC: $18.00 © 1999 American Chemical Society Published on Web 06/16/1999
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the kinetics of pyrene excimer formation in several nonionic surfactant micelles at temperatures close to the cloud point. They analyzed their data using a model that incorporated pyrene exchange among micelles as an important step in the excimer-forming reaction and inferred that a fusion-fragmentation exchange takes place in some of the systems. In these experiments, the solubility of pyrene in water, although small (7 × 10-7 M at 23 °C), is still large enough that exit-re-entry kinetics competes with the micelle fusion-fragmentation exchange. Designing an experiment to measure micelle fusion rates directly has remained a challenge for many years. There is a discussion in the literature, summarized in the review by Gehlen and De Schryver,16 about whether the exchange process referred to above actually involves micelle fusion to form a transient supermicelle as depicted in Scheme 1 or whether exchange occurs through “sticky collisions”. The two processes are kinetically equivalent. The results which we describe below are consistent with either mechanism. In this paper we show that if one selects a pyrene derivative that has a negligible solubility in water, then the process shown in Scheme 1 will lead to exchange of solutes between micelles. We employ the pyrene-substituted triglyceride 1 as a solute to study exchange in Triton X-100 (TX100) micelles in dilute solution in water at room temperature (23 °C). Solutions can be prepared in which some micelles contain more than one molecule of 1. The fluorescence spectra of these solutions are characterized by a strong excimer emission. When one of these solutions is mixed under stopped flow conditions with an excess of empty micelles, exchange takes place and exhibits secondorder kinetics. One can follow this process in a time-scan experiment by monitoring either the growth in intensity of the blue “monomer” emission (IM) or the decrease in excimer intensity (IE).
A somewhat similar experiment was reported nearly a decade ago by Bohne and Scaiano.17 They found that
aggregates of dodecylpyrene (DPy) in water in the presence of sodium dodecyl sulfate (SDS) micelles initially gave excimer emission but underwent very slow exchange to form a solution of micelle-bound molecules of DPy which gave monomer fluorescence. The authors found no dependence of the exchange rate on the SDS concentration, which led them to conclude that the exchange occurs via the exit of solute from the micelles. Slow exchange was also reported by Schreier et al.18 for a nitroxide spin probe in a cationic micellar system. One expects that ionic surfactant micelles, because of Coulombic repulsion, have very slow rates of fusion. We will show that solutions of 1 in SDS micelles do not exchange on the time scale of months unless salt is added to the solutions. At high ionic strength, the exchange process can be monitored even for ionic surfactants. Experimental Section Materials. The molecule 1 (Mw ) 882) is a triglyceride in which 4-(1-pyrene) butyric acid is one of the constituent fatty acid esters. Its synthesis and characterization are described elsewhere.19 Triton-X 100 (TX100, Aldrich) is an octylphenol ethoxylate with an average of 9.5 ethylene oxide (EO) groups per molecule. It was used as received. Distilled water was further purified through a Millipore Milli-Q purification system. Surfactant Solutions Containing 1. A solution of Triton X-100 (21.66 g/ L) was mixed with 0.1 mg of 1. The mixture was heated at 75 °C (above the melting point of 1) and strongly agitated for 15 min with a Vortex Genie 2, model G 560, mechanical shaker at its maximum frequency (>10 Hz). The solution was then allowed to cool to room temperature over 2 h and then filtered through a 0.2 µm filter in order to remove a tiny amount of solid. This transparent solution was diluted with aqueous surfactant solution and then with water to yield a solution containing 0.68 mM of TX100 (critical micelle concentration (cmc) ) 0.15 mM) and 2.99 µM of 1. For TX100, with an aggregation number (Nagg) of 100,20 this solution has a mean occupancy number 〈n〉 of 0.44 molecule of 1 per surfactant micelle, where 〈n〉 ) [1]/[Mic] and brackets imply molar concentration. Micelle concentrations ([Mic]) were calculated from the expression [Mic] ) {[CTX] cmc)/Nagg}. The amount of 1 solubilized in each TX100 solution was determined by UV spectroscopy using the value 346 ) 4.7 × 104 M-1 cm-1 determined previously.19 Absorption spectra of 1 were measured with a Hewlett-Packard 8452A diode-array spectrometer, using a 1.00 cm cell. The background was subtracted using a TX100 solution of the same concentration as a reference. The absorbance of 1 was calculated relative to that at 398 nm, which was considered as the baseline. (16) Gehlen, M. H.; De Schryver, F. C. Chem. Rev. 1993, 93, 199221. (17) Bohne, C.; Konuk, R.; Scaiano, J. C. Chem. Phys. Lett. 1988, 152, 156. The authors imagine that exit of DPy from the SDS micelle and transport through the water are assisted by interaction of the solute with a few SDS molecules. (18) Schreier, S.; Ernandes, J. R.; Cuccovia, I.; Chaimovich, H. J. Magn. Reson. 1978, 30, 283. (19) Rharbi, Y.; Kitaev, V.; Winnik, M. A.; Hahn, K. G. Langmuir 1999, 15, 2259. The value of 〈n〉 ) 0.44 employed here is based on data reported in this paper. (20) Streletzky, K.; Phillies, G. D. J. Langmuir 1995, 11, 42-47. (b) Brown, W.; Rymden, R.; van Stam, J.; Almgren, M.; Svensk, G. J. Phys. Chem. 1989, 93, 2512-2519. (c) Alargova, R. G.; Kochijashky, I. I.; Sierra, M. L.; Zana, R. Langmuir 1998, 14, 5412-5418.
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Fluorescence Measurements. Fluorescence measurements were carried out with a SPEX (2.1.2) Fluorolog spectrometer in the S/R mode. The intensity was kept below 2 × 106 counts/s to maintain the linearity of the detector response, which was calibrated prior to running these experiments. For emission spectra and for time-scan kinetics experiments, λex ) 346 nm, whereas excitation spectra were obtained for both λem ) 375 nm (monomer) and λem ) 480 nm (excimer). In kinetics measurements, the samples were mixed in the sample chamber using a home-built stopped-flow injector. In each injection, 0.35 mL of a solution containing [1] (2.99 µM) + TX100 (0.68 mM) was mixed in with 0.35 mL of pure TX100 solution ([TX100] ) 0, 0.3, 7.7, 1.54, 3, 5.3, 7.7, or 11 mM). The signal was monitored at either λem ) 375 nm or λem ) 480 nm, with integration and interval times of 1 ms and a total experiment time ranging from 0.5 to 2 s. All experiments were carried out at 25.1 °C.
Results and Discussion The interpretation of the kinetics experiments described here depends on an understanding of how a molecule as large as the triglyceride 1 is solubilized in TX100 micelles. We have recently reported time-resolved measurements of excimer formation of 1 in aqueous solutions of TX100.19 To our surprise, the fluorescence decay behavior was entirely consistent with the Poisson-distribution model up to a mean occupancy of 〈n〉 > 4, where 〈n〉 is defined as the mean number of chromophores per micelle. Analysis of the fluorescence decay profiles of these solutions, at surfactant concentrations well above the surfactant cmc, gives 〈n〉 values consistent with 100 ( 5 surfactant molecules per micelle. This value was found constant for a large range of surfactant and solute concentrations. Measurements of the TX100 aggregation number Nagg by light scattering give a value of 105 molecules per micelle at 25 °C,20a with similar values obtained by fluorescence quenching.20b While exchange experiments can in principle be carried out for any solution of 1 in TX100, solutions characterized by large values of 〈n〉 would require multiple exchanges with empty micelles to produce micelles containing a single dye. To simplify the data analysis, we chose a solution with 〈n〉 ) 0.4419 as one of the partners in the exchange reaction. Most of the micelles in this solution contain zero, one, or two molecules of 1. The small fraction of micelles containing three or more molecules of 1 can safely be neglected. The Exchange Process. In Figure 1, we present the fluorescence spectrum of a solution of 1 in water at a bulk concentration of 1.5 µM in the presence of 0.34 mM TX100. This spectrum, labeled “Prior” has a broad excimer emission with a peak at 480 nm, in addition to the monomer fluorescence with a (0,0) band at 375 nm. When an aliquot of a concentrated TX100 solution is mixed with this solution, to bring the total TX100 concentration to 5.89 mM, the spectrum evolves to that labeled “After exchange”. This spectrum has a strong monomer emission, but no discernible excimer band. Consistent with the absence of excimer, we find that upon pulsed excitation, the pyrene of 1 in the solution after exchange exhibits an exponential decay with a lifetime of 170 ns. In Figure 2, we show the results of two time-scan experiments in which we monitor the decrease in excimer emission intensity (IE, 480 nm) and the growth in monomer emission intensity (IM, 375 nm) following the rapid addition of excess TX100 surfactant to the solution with 〈n〉 ) 0.44. The decay in IE is characterized by a higher signal-tonoise ratio than the growth in IE, and the flat baseline at 480 nm after equilibration is due only to weak monomer emission at this wavelength. When these experiments are repeated at different TX100 concentrations, we can
Figure 1. Emission spectra (λex ) 346 nm) of 1 solubilized in aqueous solutions of TX100 micelles. In the spectrum labeled “Prior to exchange”, [1] ) 1.5 µM and [TX100] ) 0.34. The spectrum “After exchange” refers to the solution obtained by mixing the original solution with an equal volume of TX100 solution to raise the surfactant concentration to [TX100] ) 5.89 mM.
Figure 2. Time-scan experiments monitoring the increase in the monomer emission (λem ) 375 nm) and the decrease in the excimer emission (λem ) 480 nm) after stopped-flow mixing of a solution of 1 in TX100 micelles ([1] ) 2.99 µM, [TX100} ) 0.68 mM) with an equal volume of TX100 solution ([TX100] ) 7.7 mM). The solid lines represent the exponential curves which best fit the data.
identify four features of particular interest: (1) the relatively short time scale of the exchange process; (2) the identical rates for the growth of monomer and the decay of excimer emission; (3) the dependence of the exchange rate on the concentration of unoccupied micelles; (4) the exponential shape of the individual decay profiles. Exchange occurs on a time scale of ca. 50 ms when the TX100 concentration is 3.7 mM (Figure 2) and about 200 ms when [TX100] ) 0.77 mM, and individual decays can be fitted to an exponential function with a relaxation rate kobs (s-1).
kobs ) ko + kfus[micelle] ) ko + kfus[TX100 cmc]/Nagg (1) kdiff ) 4πNA(r1 + r2)(D1 + D2)/1000
(2)
A striking observation is that the exchange rate observed here (kobs ) 6-60 s-1) is more than 104 times slower than the exit rate of TX100 molecules from the micelles (k- ) 1.1 × 106 s-1).5 Even though the individual surfactant molecules are in a state of rapid exchange, the triglyceride
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Figure 3. Relaxation rates kobs calculated from the fits of the data from individual stopped-flow experiments plotted vs the concentration of empty TX100 micelles. These decays were monitored at λem ) 480 nm with λex ) 346 nm. The error bars refer to one standard deviation in repeated measurements at each empty micelle concentration.
molecules 1 remain inside individual micelles waiting to exchange via an event of micelle fusion. Micelle fusion should be characterized by bimolecular kinetics with a second-order rate constant kfus. With the assumption that fragmentation is much faster than fusion, this implies that under our conditions of excess unoccupied micelles, kobs is a pseudo-first-order rate constant proportional to the concentration of unoccupied micelles. It will be interesting to compare the magnitude of kfus with that of kdiff, the diffusion-controlled rate constant. In eq 2, r1 and r2 are the radii of the diffusing species and D1 and D2 are their diffusion coefficients. NA is Avogadro’s number. In Figure 3 we plot values of kobs vs [micelle], where the kobs values are calculated from the decay of IE. We obtain a straight line and calculate the second-order rate constant from the slope, kfus ) 1.15 × 106 M-1 s-1. The linearity of the plot in Figure 3 provides strong evidence that exchange occurs via a second-order kinetic process, i.e., the fusionfragmentation mechanism or the kinetically equivalent “sticky collision” mechanism. If the exchange occurred via an exit-re-entry mechanism, the exchange rate would be independent of micelle concentration as found in the experiments of Bohne and Scaiano.17,21 One might assume that the finite intercept in Figure 3 (ko ) 6 s-1) describes the exit rate of 1 from the micelles, but this is unlikely to be the case. We imagine that the exit of 1 into the water phase should occur at similar rates for different micelles, and in the absence of added salt, the exchange of 1 among
Letters
SDS micelles is too slow to be detected. The intercept may point to a first-order exchange process of the type proposed by Zana6 and by Bohne and Scaiano:17 a solute molecule encapsulated in surfactant exits the micelle in a ratelimiting step, followed by a more rapid growth of this submicelle through adsorption of additional surfactant molecules. Barrier to Micelle Fusion. Although the measured rate of exchange is relatively rapid (kfus ) 1.15 × 106 M-1 s-1), it is still 5000 times slower than the diffusioncontrolled rate. The diffusion-controlled rate constant kdiff in this system can be estimated with eq 2, from the known values of the micelle diffusion constant (D1 ) 5 × 10-7 cm2 s-1) and micelle radius (r1 ) 2 nm) by assuming that D2 and r2 values for the pyrene-laden micelles are similar to those of the empty micelles. In this way we calculate kdiff ) 6.0 × 109 M-1 s-1.
Although only one encounter in 5000 leads to micelle fusion, the fact that fusion is even this efficient implies a rather weak barrier to the fusion process. There are two sources to this barrier. One is the interaction of the ethylene oxide steric stabilizer chains, which provides both an entropic and a hydration barrier to close approach of the micelle cores. The second is the need for the micelle core to rearrange to accommodate fusion of the contents. In the case of TX100, it is likely that the short EO chains provide the major barrier to fusion. A measure of the micelle core fluidity is provided by the rate of excimer formation between two molecules of 1 solubilized in an TX100 micelle. The first-order rate constant for this process is provided as a fitting parameter in the fluorescence decay analysis of solutions of 1 in TX100. As reported previously,19 we find a value of 7 × 106 s-1 over a wide variety of surfactant and solute concentrations. Thus the micelle core acts as a fluid droplet on the time scale of seconds and is unlikely to act as a barrier here to the fusion process. Coulombic interactions appear to be much more effective at preventing close micelle approach. Preliminary experiments in which 1 is solubilized in SDS micelles indicates that no exchange takes place at low ionic strength over a time scale of weeks. The exchange process can be detected only when salt is added to the solution. These results will be described in detail in a subsequent publication.22 Acknowledgment. The authors thank ICI, ICI Canada, and NSERC Canada for their support of this research. LA990064R
(21) Almgren, M. Chem. Phys. Lett. 1980, 71, 539
(22) Rharbi, Y.; Winnik, M. A.; Hahn, K. Manuscript in preparation.
