Dynamics of Micro-and Macrophase Separation of Amphiphilic Block

Jul 29, 1999 - at Buffalo, Buffalo, New York 14260-4200; and Department of ... studied using the iodine laser temperature-jump and stopped flow techni...
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Macromolecules 1999, 32, 5539-5551

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Dynamics of Micro- and Macrophase Separation of Amphiphilic Block-Copolymers in Aqueous Solution Matthias J. Kositza,† Cornelia Bohne,†,⊥ Paschalis Alexandridis,‡ T. Alan Hatton,§ and Josef F. Holzwarth*,† Abteilung Physikalische Chemie, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin-Dahlem, Germany; Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260-4200; and Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received March 23, 1999

ABSTRACT: The dynamics of purified poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) block-copolymer micellization and phase separation in aqueous solutions were studied using the iodine laser temperature-jump and stopped flow techniques. The changes in the micellar solutions were followed by either light scattering or fluorescence of 1,6-diphenyl-1,3,5-hexatriene (DPH), which is a probe located in the micelle interior. Three different relaxation processes were observed for the temperature range covering the micro- and macrophase separation of the EO13PO30EO13 (Pluronic L64) block-copolymer. The fastest process corresponds to the incorporation of unimers into micelles which leads to larger micelles that are not thermodynamically stable. This process is followed by a relaxation with negative amplitude during which the micellar core is dehydrated and a redistribution of micellar sizes is achieved. The third relaxation process corresponds to the clustering of micelles into larger aggregates which is associated with the initial step of macrophase separation. Other PEO-PPO-PEO block-copolymers, like EO19PO43EO19 (Pluronic P84) and EO27PO61EO27 (Pluronic P104), were investigated to provide additional information concerning the second relaxation process. Mixed micelles containing sodium dodecyl sulfate were studied to support the assignment of relaxation processes involving micellar collisions. This study of the dynamics of purified PEO-PPO-PEO block-copolymers clarifies several controversial points because the dynamics were investigated over a wide temperature and concentration range and avoid impurity effects.

Introduction Block-copolymers, which are macromolecules consisting of parts (blocks) that differ in chemical nature, have a fascinating tendency to attain, through block segregation and microphase separation, diverse ordered morphologies such as lamellae, cylinders, and spheres, as well as cocontinuous minimal surface structures.1-3 The ordered morphologies exhibit rheological and other properties that are very different from those of the disordered state. Important materials, such as thermoplastic elastomers, are based on ordered morphologies generated by block-copolymers. The tendency for the block-copolymers to aggregate at a molecular level is also manifested when they are dissolved in solvents which are selective for one of the block types.4 In this case, block-copolymers form micelles of finite size and usually of spherical shape, with a core made of the less soluble blocks and a corona consisting of the better solvated blocks. The formation of micelles by block-copolymers in selective solvents parallels the micellization of common surfactants in water.4 Water-soluble poly(ethylene oxide)block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO, or commercially called Pluronics (BASF) and Poloxamers (ICI)) are block-copolymers that can form micelles in water, because water is a better †

Fritz-Haber-Institut der Max-Planck-Gesellschaft. State University of New York at Buffalo. § Massachusetts Institute of Technology. ⊥ On sabbatical leave from the Department of Chemistry, University of Victoria, BC, Canada V8W 3V6. * Corresponding author: phone +49-30-84135516, Fax +49-3084135385, e-mail [email protected]. ‡

solvent for PEO than PPO.5,6 One way to control the micellization properties of the PEO-PPO-PEO blockcopolymers is to vary the PPO/PEO composition ratio and molecular weight of the copolymer. This variation can be achieved during the synthesis of these compounds, and it allows for the production of molecules with hydrophilic/hydrophobic properties that meet the specific requirements of diverse applications such as detergency, foaming, dispersion stabilization, emulsification, and lubrication.7,8 Control of the PEO-PPOPEO micellization properties can also be achieved by physical means, i.e., by either varying the solution temperature or modifying the aqueous solvent quality with, for example, the addition of salts. The effects of temperature on the properties and structure of PEOPPO-PEO block-copolymer solutions have been studied extensively. An increase in temperature has been shown to induce micelle formation in aqueous solutions of a number of these block-copolymers, with the micelle core dominated by the hydrophobic PPO and the corona composed of hydrated PEO segments.6,9 The temperature at which micellization starts is called the critical micellar temperature (cmt); this is equivalent to the critical micellar concentration (cmc) at constant temperature. The worsening of the aqueous solvent quality by the addition of electrolytes, such as simple salts, is another way to promote the micellization of PEOPPO-PEO block-copolymers.10 When the solvent conditions become bad (at sufficiently high temperatures or high salt concentrations), the so-called cloud point (CP) is reached, where macroscopic phase separation into a block-copolymer-rich phase and a water-rich phase takes place.

10.1021/ma9904316 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/29/1999

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The transition from unimers (nonassociated blockcopolymers) to micelles is a process inherent in many applications of block-copolymer solutions, such as dispersion/emulsion polymerization and formulation of lubricants. Indeed, the micellar lifetime has been correlated to the rates of solubilization and detergency in ionic surfactant solutions.11,12 The consideration of block-copolymer micellar kinetics also involves fundamental questions concerning the desorption dynamics of polymeric surfactants.13 The high molecular weight and chainlike structure of block-copolymer molecules is expected to complicate the processes of micelle formation and exchange of block-copolymer molecules between micelles and the bulk solution. Disentanglement of copolymer molecules from the micelles should result in time scales slower than the ones observed in conventional, low-molecular weight, surfactants.14,56 Despite the practical and fundamental implications of dynamic processes in the micellization of blockcopolymers, the understanding of the kinetic processes involved is still limited.15-29 The aggregation dynamics are very dependent on the structure of the blocks, and the mechanistic assignments of the various relaxation processes observed are still controversial. In the case of PEO-PPO-PEO block-copolymers, earlier studies based on gel permeation chromatography suggested that the residence time of the unimer of the Pluronic F127 was very long (∼hours).18,19 In contrast, pulse-field gradient NMR measurements suggested that the residence time of the same block-copolymer was less than 3 ms.30 More recently direct spectroscopic methods, such as ultrasonic relaxation and Joule heating or laser temperature-jump experiments, have been employed to study the dynamics of PEO-PPO-PEO micellization.22,23,25,26,28,29 These studies showed a dynamic behavior that is much more complex than for simple shortchained surfactants. From ultrasonic relaxation studies, the unimer exchange for the Pluronic L64 was attributed to occur around 2 µs.25 In contrast, in temperature-jump experiments the unimer exchange was assigned to a slower relaxation process.22,23,28 Furthermore, additional relaxation processes were observed at time domains longer than for the unimer exchange.22,25,28 A second process with negative amplitude was attributed to the redistribution of the micellar sizes after the initial incorporation of unimers had been achieved.25,28 We recently showed that at temperatures close to the CP a third relaxation process occurs which is probably related to the clustering of micelles that precedes macrophase separation.28 The objective of the present investigation is to expand on the studies previously published28,29 in order to provide a comprehensive mechanistic picture of the dynamics of aggregation of PEO-PPO-PEO blockcopolymers over the temperature range between the CMT and the CP and over a wide concentration range. The studies presented here are with Pluronic PEOPPO-PEO block-copolymers, but the general features of the aggregation dynamics in solution should be common to other block-copolymer types. We describe here a comprehensive study presented on iodine laser temperature-jump (ILTJ) and stopped-flow (SF) measurements. The former technique explores the relaxation of the system when a small temperature perturbation is induced to an equilibrium distribution of states of aggregation. The latter technique is based on a concentration perturbation and can be employed to

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study reversible as well as irreversible processes. ILTJ can be used from nanoseconds to seconds, whereas SF covers the time range from 1 ms to hours. The micellization dynamics were followed by monitoring “external features” such as micellar size and number density with light scattering measurements or “internal features” such as the micellar core hydrophobicity sensed with an environmentally sensitive fluorescence probe. Experimental Section The Pluronics L64 (EO13PO30EO13, nominal MW ) 2900 g/mol), P84 (EO19PO43EO19, nominal MW ) 4200 g/mol), and P104 (EO27PO61EO27, nominal MW ) 5900 g/mol) were a generous gift from BASF (Parsipanny, NY). All these blockcopolymers have a PEO weight content of 40% (PEO/PPO ) 2/3). These block-copolymers frequently contain impurities. In the case of L64, it was shown that the presence of impurities significantly alters the dynamics of micellization of this blockcopolymer.28 Therefore, we purified L64 by extraction with hexane,28 whereas the impurities in P84 and P104 were removed by filtering the solutions in water twice through 0.22 µM Millipore filters. The purity of the block-copolymers was checked by light scattering until the extra peaks below the cmt,28 known to originate from hydrophobic impurities, were no longer observed ([impurity] < 0.001% w/v). 1,6-Diphenyl1,3,5-hexatriene (DPH, Fluka 99%), NaCl (Merck p.a.), sodium dodecyl sulfate (SDS, Fluka, g98%), and methanol (Merck p.a.) were used without further purification. Purified water (Milli-Q Ultrapure Water System) was employed to prepare the aqueous Pluronic solutions. The block-copolymer solutions were kept for 4-6 h at 15 °C before being filtered through 0.22 µM Millipore filters at temperatures below the cmt. The concentration of the block-copolymers was checked by differential scanning calorimetry (DSC, Microcal MC-2) where a linear relationship was observed between ∆H, the enthalpy associated with the micellization, and the block-copolymer concentration. The total mass before and after filtration was established to be the same within 5%. Samples containing DPH were prepared by injecting a methanolic stock solution (ca. 0.5 mM) into the L64 solution leading to final concentrations of 5 µM and 1% (v/v) respectively for DPH and methanol. Steady-state light scattering and fluorescence were measured with a RF-5000 Shimadzu fluorimeter. Light scattering was measured at 90° with the excitation and emission monochromators set at 360 nm. Samples containing DPH were excited at 360 nm, and the emission was recorded between 380 and 550 nm. The sample temperature was controlled using a Haake F3-C bath, and the temperature change rate was 0.2 °C/min (Haake PG 20 controller). No hysteresis was observed for the light scattering and fluorescence intensities when the sample was heated and subsequently cooled.37,39 The iodine laser temperature-jump (ILTJ) setup has been described in detail previously.31,32 The iodine laser emits photons at 1315 nm which are absorbed by rotationalvibrational states of water leading to the heating of the solution.32 The heating achieved was of ca. 1 °C and was quite homogeneous for a thickness up to 3 mm.31 The heating time was 2.4 µs and was independent of the solution composition. The long-time limit for measuring relaxation times with this system was 1.5 s and was determined by the back-cooling rate of the solution.32 The relaxation processes were measured by light scattering at 330-380 nm (320 nm cutoff and UG-11 Schott filters) at a 90° light scattering angle. The relaxation kinetics when DPH was used as a fluorescence probe were obtained by exciting the sample with light between 330 and 380 nm (320 nm cutoff and UG-11 Schott filters) and measuring the excitation above 400 nm (400 nm cutoff filter). The small amplitude for the change in the fluorescence intensity required the use of a high-intensity excitation beam, and the kinetics were acquired at a high resolution for the voltage (0.2 mV). Under these conditions some bleaching of the DPH could be observed within 5-10 s. This bleaching was completely reversible when the excitation beam was removed. To avoid

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significant bleaching, a mechanical shutter was employed between the excitation beam and the sample. This shutter was opened immediately (1-3 s) before the laser was triggered. It is important to note that no bleaching was observed during the collection time (e200 ms) as ascertained by control experiments in which the laser was not triggered but the DPH emission was collected. Four to six single experiments from thermostated samples ((0.1 °C) were averaged for each relaxation trace, and the errors of the recovered relaxation times were typically 15%. The SF experiments were performed with an Applied Photophysics SX.18MV system. The light scattering detection was collected using the fluorescence setup of the equipment (90° detection angle) by exciting the sample at 360 nm. The stock solutions in 5 mL syringes were contained in a chamber kept at constant temperature (Haake C25 bath). A 1:1 mixing ratio was employed and the volume after mixing consisted of 200 µL. Between four and six data acquisitions were averaged for each experiment. The initiation of mixing and the acquisition as well as the analysis of the data were performed with the Applied Photophysics SX.18MV software (Acorn Risc PC).

Results Micellar Characterization by Steady-State Light Scattering and Fluorescence Studies. The micellization of PEO-PPO-PEO block-copolymers can be observed by measuring the light scattering intensity of the solution,23,28,33-35 since this parameter is proportional to the particle size, as well as refractive index differences between the aggregate and the solvent, and the particle concentration. The light scattering intensity of L64 solutions in the presence and absence of salt increased monotonically when the temperature was raised (Figure 1A). The cmt and CP values were determined from the light scattering curves as previously reported for L64.28 The micellization occurred at lower temperatures with an increase in the L64 concentration leading to a displacement of the light scattering curve to lower temperatures. In the presence of NaCl the micellization of L64 was also induced at lower temperatures (Figure 1A). Consequently, the addition of NaCl is equivalent to raising either the temperature or the L64 concentration of the block-copolymer solution.10 In the case of NaCl addition the cmt and CP are depressed. This result contrasts the effect observed with the increase of the L64 concentration where only the cmt is significantly depressed.28 The cmt and CP values for a 0.625% solution of L64 decrease from 37.0 ( 0.3 and 59.8 ( 1.5 °C in the absence of salt28 to 31.2 ( 0.329 and 52 ( 1.5 °C in the presence of 0.5 M NaCl and 25.3 ( 0.329 and 45.2 ( 1.5 °C in the presence of 1.0 M NaCl. 1,6-Diphenyl-1,3,5-hexatriene (DPH) is a probe molecule that has been frequently employed to study the fluidity of membranes by following changes of the DPH fluorescence anisotropy.36 An important property of DPH is that it does not fluoresce in water, and for this reason the emission intensity measured is derived solely from the membrane solubilized probe.37 A similar behavior was expected when using DPH fluorescence with block-copolymers. The change in the absorption spectra of DPH in the presence of block-copolymers has been previously employed to determine cmt values for these block-copolymer solutions.9 The absorbance for DPH increased as the temperature of the solution was raised, suggesting that, when micellization occurred, the DPH was progressively solubilized in the block-copolymer micelles.9 In the case of L64 (2.5% (w/v)), we observed that the absorbance of DPH increased from 0.15 at 25.5 °C to 0.4, 0.45, and 0.48 respectively at 31.5,

Figure 1. (A) Dependence with temperature of the steadystate light scattering intensity of pure L64 aqueous solutions ((0) 0.625% w/v and (O) 1.25% w/v) and of L64 aqueous solutions in the presence of 0.5 M NaCl ((4) 0.625% w/v). Arrow “a” indicates schematically the change due to the temperature increases in ILTJ experiments, whereas arrows “b” and “c” indicate changes for stopped-flow (salt jump) experiments. (B) Dependence with temperature of light scattering (0) and DPH fluorescence intensities (O, 5 µM DPH/ 1% v/v methanol) of a 2.5% (w/v) L64 solution. The intensities were normalized in the region before the DPH emission levels off. Arrow “d” shows schematically the change due to temperature increases in the ITLJ experiments.

32.5, and 33.5 °C. Above 33.5 °C the absorbances were constant within 0.01 absorbance units. Above the cmt the fluorescence of DPH increased when the temperature for a L64 solution was raised (Figure 1B), but it leveled off at about 40 °C (T - cmt ) 8 °C). In contrast, for the light scattering measurements the intensities still increased for higher temperatures. The slight intensity decrease observed in the plateau region for the DPH fluorescence at high temperatures was probably due to the decrease of the emission quantum yield of DPH. Such an effect was previously described in homogeneous solution38 and qualitatively also occurs in liquid membranes.37,39 Principles of Time-Resolved Measurements. The micellization dynamics were studied by iodine laser temperature-jump (ILTJ) and stopped-flow experiments (SF). In the ILTJ experiment the solution is heated for a short period of time when the laser pulse is absorbed by water (shortest possible heating time ca. 100 ps).32 As a consequence of the fast laser heating, the system is displaced from equilibrium, and the relaxation toward the new equilibrium at the higher temperature is measured. In the case of L64 solutions, heating always led to an overall increase in light scattering intensity as indicated schematically by the arrow “a” in Figure 1A. It is worth noting that the temperature increase in the ILTJ experiments was ca. 1 °C, which constituted a small perturbation of the system and therefore allows for the application of relaxation equations to calculate rate constants. (Arrow “a” in Figure 1A was exaggerated

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for clarity purposes.) The SF experiments are based on the mixing within 0.9 ms of two solutions with different compositions. The relevant conceptual difference in our experimental setup between ILTJ and SF measurements is that for the former only reversible processes are studied, whereas for the SF technique both reversible and irreversible reactions can be investigated. In the SF experiments with L64 one injection syringe contained L64 dissolved in water, and the second syringe contained a NaCl solution. Consequently, the final solution after mixing carried half of the initial L64 and NaCl concentrations. These measurements correspond to salt-jump experiments in which an overall increase of the light scattering intensity was observed. Two different types of salt-jump experiments were performed. At temperatures above the cmt of the initial L64 solution (arrow “c”, Figure 1A) the dynamics for the aggregation between two states that contain micelles were studied, whereas for temperatures below the cmt of the initial L64 solution (arrow “b”, Figure 1A) the aggregation behavior from a solution that initially only contains unimers was investigated. The latter experimental conditions cannot be achieved when ILTJ experiments are employed. In addition, the amplitudes for the ILTJ experiments are much smaller than those for the relaxation kinetics in the SF experiments because in the latter the perturbations are larger. DPH was previously employed using the ILTJ technique to measure dynamic phenomena in vesicles.37,39 In these experiments the DPH fluorescence, which is only detectable for the molecules solubilized in the vesicles, was continuously monitored in times from nanoseconds to seconds. The temperature jump induced by the laser pulse created the nonequilibrium condition for which the relaxation dynamics were measured. The same principle was employed in this study to follow the dynamics of L64 micellization. Although the photophysics of DPH is complex,40,41 the important feature for the experiments with L64 is that the fluorescence intensity increases when DPH is removed from water and is micellized in a hydrophobic environment. From the steady-state fluorescence experiments it is apparent that ILTJ experiments in the region where the fluorescence intensity changes with temperature will lead to an overall increase of the fluorescence intensity (arrow “d”, Figure 1B; the magnitude of the temperature jump has been exaggerated for clarity purposes). Iodine Laser Temperature-Jump Experiments. ILTJ experiments with light scattering detection at ∼360 nm revealed three relaxation processes.28 The first two relaxation times (τ1 and τ2) were observed at temperatures close to the cmt. At higher temperatures the first and third (τ3) relaxation processes were observed, whereas at temperatures close to the CP only the third process was detected. The first and second relaxation processes have opposite amplitudes (Figure 2A), and there was no further relaxation observed at times shorter or longer than those shown in Figure 2A,B. The overall change of the light scattering intensity after the first and second relaxation processes were completed was always positive. This result is consistent with the intensity increase observed in steady-state light scattering measurements. The first two relaxation processes were adequately fitted to monoexponential functions. The lifetime for the first process (τ1) was shortened as the temperature was raised and leveled off at higher temperatures.28 At each temperature,

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Figure 2. (A) ILTJ signal of 2.5% (w/v) L64 in water from light scattering detection at 35 °C (split time base). (B) ILTJ signal of 2.5% (w/v) L64 in water in the presence of 5 µM DPH and 1% (v/v) methanol from fluorescence detection at 35 °C (split time base; a shorter time base was used for the initial kinetics). The spike at short times comes from the iodine laser and corresponds to the time (t ) 2.4 µs) during which the sample was heated.

shorter τ1 values were measured when the concentration of L64 was increased (Figure 3A). This trend was also observed at high temperatures where the relaxation values level off for each L64 concentration. The amplitude for the first relaxation process decreased as the temperature was raised.28 The same trend was observed at all L64 concentrations studied, and the first relaxation process vanishes at temperatures between 16 and 19 °C above the cmt. The second relaxation process for L64 solutions was observed for a narrower temperature range above the cmt than the first process (T - cmt e 8 °C for 0.625% w/w and T - cmt e 11 °C for 5% w/w). The amplitude for this second process was negative, indicating that the size and/or number density of micelles decreased. The relaxation times τ2 were shortened, and the amplitudes decreased when the temperature was raised. The same trend was observed at all L64 concentrations investigated. At a constant temperature the values for τ2 decreased as the block-copolymer concentration was raised (Figure 3B). The values for the relaxation times corresponding to the second relaxation process were also determined for P84 and P104. These block-copolymers have the same relative content of PEO and PPO but have higher nominal molecular weights than L64. For all blockcopolymers the second relaxation process had a negative amplitude. The onset of the second relaxation process occurs at the cmt for L64 but is only observed at progressively higher temperatures for P84 and P104. The temperature range for which the second process was observed narrowed as the size of the blockcopolymer was increased. In addition, the second relaxation process was observed at all L64 concentrations studied, but it is only apparent for P84 and P104 concentrations that are respectively equal to or greater than 2% and 7.5%. The values for the relaxation time of the second process at a constant molar concentration of the block-copolymers (5% (w/v), 7.24% (w/v) and 10.17% (w/v) respectively for L64, P84, and P104) were lengthened substantially as the size of the Pluronic

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Figure 3. Relaxation lifetimes τ1 (A), τ2 (B), and τ3 (C) obtained from ILTJ experiments with light scattering detection at 0.625% (O), 1.25% (b), 2.5% (∆), and 5% (9) L64 (w/v) in aqueous solution as a function of temperature.

Figure 4. Temperature dependence of the second relaxation time (τ2) measured in ILTJ experiments using light scattering detection for aqueous solutions of L64 ((4) 5% w/v), P84 ((b) 7.24% w/v), and P104 ((0) 10.17% w/v) at the same molar concentrations.

increased (Figure 4). In addition, the dependence of the τ2 values on temperature was different for each blockcopolymer. In the case of L64, a decrease for the τ2 values was observed as the temperature was raised, whereas the opposite was measured for P104. P84 shows an intermediate behavior in which the τ2 values initially decreased and then increased at higher temperatures.

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The third relaxation process had a positive amplitude, and it was observed between 11 and 15 °C below the cloud point (Figure 3C). The onset of the third process occurs at lower temperatures for higher L64 concentrations. At the low temperature end the relaxation lifetimes were fairly constant, but they increased above 55 °C. This onset for the temperature dependence of τ3 was independent of the L64 concentration. The amplitude for this process at 2.5% (w/v) L64 increased as the temperature was raised, and close to the CP it decreased slightly.28 A similar behavior was observed at the other L64 concentrations studied. At a constant temperature the lifetimes were shortened when the block-copolymer concentration was increased. The L64 solutions for the ILTJ experiments using DPH fluorescence contained methanol which was used to properly solubilize the probe.37,39 The final methanol concentration was 1% (v/v), and it did not influence any of the three relaxation processes detected by ILTJ experiments with light scattering detection. Only one relaxation process could be detected when monitoring the changes in the DPH fluorescence (Figure 2B). This process was observable between 33 and 38 °C, and the relaxation lifetimes measured for the DPH fluorescence were the same, within a 15% margin, as those obtained from light scattering experiments for the second relaxation process. Addition of sodium dodecyl sulfate (SDS), which is a negatively charged surfactant, to L64 solutions leads to the formation of mixed micelles.42,43 These mixed micelles are negatively charged, and repulsion will play a role for any bimolecular process involving two micelles. For this reason the relaxations of a 0.625% (w/v) L64 solution in the presence of SDS were investigated. It is important to note that the SDS concentrations employed are well below the cmc of the surfactant, and no pure SDS micelles were formed. In the presence of surfactants all three processes were first-order. The relaxation lifetimes for the first process decreased, whereas the lifetimes for the second and third processes increased when SDS was added. The observed effects in the presence of SDS were reversed when NaCl was also added to screen electrostatic interactions in solution. Stopped-Flow Experiments. A preliminary report showed that SF is a useful technique to study the aggregation dynamics of L64. No time-resolved relaxation signal could be observed when the L64 solution in one injection syringe (initial solution) was mixed with water from the second syringe, although the L64 concentration after mixing (final solution) was halved, and an overall decrease of the light scattering intensity was observed as expected from steady-state light scattering measurements.29 This result suggests that the breakdown of micelles due to dilution, when the solution was abruptly exposed to a nonequilibrium situation, was much faster than 0.9 ms. It is important to note that this fast breakup does not constitute a measure of the lifetime of a micelle in a nonperturbed situation. The fact that no relaxation process was detected in the dilution experiment ensures that the signals observed in the salt-jump experiments were due to processes different from dilution. In the salt-jump experiments, the initial L64 concentration was always halved and the NaCl concentration was increased from 0 M to 0.5 or 1 M. For experiments performed below the cmt of the initial L64 solution, the relaxation dynamics studied correspond to a transition

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Figure 5. Dependence on the initial L64 concentration (i.e., concentration in the syringe; final concentrations are halved) for the three relaxation lifetimes ((0) τ1, [NaCl]final ) 1 M; (b) τ2, [NaCl]final ) 0.5 M; and (4) τ3, [NaCl]final ) 0.5 M) measured in salt-jump experiments. The temperatures corresponded to T - cmt values of the initial L64 solutions of -5 °C (τ1), 3 °C (τ2), and 9 °C (τ3). The values for τ1 correspond to the major component (