A Thermotropic Phase Transition in Polyelectrolyte ... - ACS Publications

Jul 28, 1998 - The overall results are consistent with a thermotropic transition within the PACA/CTAB/HDPC complexes from a high-temperature normal ...
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Langmuir 1998, 14, 4758-4764

A Thermotropic Phase Transition in Polyelectrolyte-Surfactant Complexes As Characterized by Deuterium NMR Peter M. Macdonald* and Vladimir Strashko Department of Chemistry and Erindale College, University of Toronto, 3359 Mississauga Road North, Mississauga, Ontario, Canada L5L 1C6 Received September 22, 1997. In Final Form: May 13, 1998 Deuterium nuclear magnetic resonance (2H NMR) spectroscopy of mixed cationic plus zwitterionic surfactant micelles exposed to the anionic polyelectrolyte poly(acrylic acid) (PACA) has been used to detect a thermotropic transition undergone in the phase-separated stoichiometric cation:anion complexes so formed. The deuteron labels were located on the quaternary methyls of either the cationic surfactant cetyltrimethylammonium bromide (CTAB-γ-d9) or the zwitterionic surfactant hexadecylphosphocholine (HDPC-γ-d6). The 2H NMR spectra of CTAB-γ-d9 consisted of a superposition of two Pake doublet spectral components, indicating the presence of two surfactant populations undergoing different types of anisotropic motional averaging. With decreasing temperature, and over a narrow temperature range, the Pake doublet with the larger quadrupolar splitting increased in intensity at the expense of that with the smaller quadrupolar splitting, demonstrating the occurrence of a highly cooperative thermotropic transition between two states represented by the two Pake doublets. This thermotropic transition was manifested by CTABγ-d9 but not by HDPC-γ-d6, attesting to its origin in the electrostatic interactions between the cationic and anionic species. The midpoint temperature of this transition decreased with increasing CTAB:HDPC ratio of the surfactant micelles. The 2H NMR T1 relaxation time of both Pake doublet spectral components increased with increasing temperature but was a factor of 2 shorter in the Pake doublet with the larger quadrupolar splitting, indicating that the molecular motions contributing to relaxation were fast on the 2H NMR time scale but hindered in the low-temperature versus high-temperature states. The overall results are consistent with a thermotropic transition within the PACA/CTAB/HDPC complexes from a high-temperature normal hexagonal (HI) liquid crystalline phase to a low-temperature lamellar (LR) liquid crystalline phase.

Introduction Interactions between polyelectrolytes and oppositely charged surfactants are mediated by a combination of electrostatic and hydrophobic forces. When the surfactant is above its critical micellar concentration, the mutual attraction between polyelectrolyte and surfactant is so strong that irreversible phase separation commonly results.1,2 If the surface charge of the surfactant micelle is reduced by mixing in uncharged or zwitterionic surfactants, a strategy introduced by Dubin and Oteri,3 then the electrostatic attraction may be attenuated to the point that the phase separation becomes reversible. Exploiting this innovation permitted Dubin and co-workers to define the factors critical to the formation of polyelectrolytesurfactant complexes.3-12 These were found to include * To whom correspondence should be sent. Tel.: 905-828-3805. Fax: 905-828-5425. E-mail: [email protected]. (1) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1983; p 203. (2) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1983; p 171. (3) Dubin, P.; Oteri, R. J. Colloid Interface Sci. 1983, 95, 453. (4) Dubin, P.; Davis, D. Macromolecules 1984, 17, 1294. (5) Dubin, P.; Davis, D. Colloids Surf. 1985, 13, 113. (6) Dubin, P.; Rigsbee, D.; McQuigg, D. J. Colloid Interface Sci. 1985, 105, 509. (7) Dubin, P.; Rigsbee, D.; Gan, L.-M.; Fallon, M. Macromolecules 1988, 21, 2555. (8) Dubin, P.; The´, S.; McQuigg, D.; Chew, C.; Gan, L.-M. Langmuir 1989, 5, 89. (9) Dubin, P.; Vea, M.; Fallom, M.; The´, S.; Rigsbee, D.; Gan, L.-M. Langmuir 1990, 6, 1422. (10) Dubin, P.; The´, S.; McQuigg, D.; Gan, L.-M.; Chew, C. Macromolecules 1990, 23, 2500.

the micelle’s surface charge, the solution’s ionic strength, and the polyelectrolyte’s molecular weight. Despite this impressive progress, little has been established regarding the structure of the phase-separated polyelectrolyte-surfactant complexes themselves. It is generally accepted that entire surfactant micelles are complexed by the polyelectrolyte, that the polyelectrolyte undergoes a contraction upon association with surfactant, and that the complexes contain stoichiometric amounts of positive and negative charge. Fluorescence spectroscopy of pyrene solubilized within the phase-separated complexes indicates that the surfactant counterions are quantitatively replaced by polyelectrolyte.13 Separate studies14 reveal that pyrene within the phase-separated material experiences a less open and less hydrated environment than that of normal micelles, although the degree of difference depends on the detailed structure of the polyelectrolyte. However, there is a paucity of information regarding the details of molecular organization and dynamics within the phase-separated complexes and the factors controlling the ease with which they are resolubilized. Much of the difficulty with polyelectrolyte-surfactant complexes arises from the fact that they are insoluble semisolids and, therefore, refractive to investigation by solution techniques. Solid-state NMR spectroscopy, however, is ideally suited to the study of such systems. (11) Xia, J.; Zhang, H.; Rigsbee, D.; Dubin, P.; Shaikh, T. Macromolecules 1993, 26, 2759. (12) Li, Y.; Xia, J.; Dubin, P. Macromolecules 1994, 27, 7049. (13) Abuin, E. B.; Scaiano, J. C. J. Am. Chem. Soc. 1984, 106, 6274. (14) Ananthapadmanabhan, K. P.; Leung, P. S.; Goddard, E. D. Colloids Surf. 1985, 13, 63.

S0743-7463(97)01051-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/28/1998

2H

NMR Proof of a Thermotropic Phase Transition

Figure 1. Chemical structures of the surfactants and polyelectrolytes employed in this investigation. HDPC, hexadecylphosphocholine; CTAB, cetyltrimethylammonium bromide; PSSS, poly(sodium styrene sulfonate); PACA, poly(sodium acrylate). For PSSS and PACA, the degree of polymerization was approximately 300.

Deuterium NMR is particularly advantageous because deuteron labels may be introduced, readily and specifically, at virtually any location within a chosen molecule, while the deuterium NMR spectrum is often simple to interpret in terms of the conformation and dynamics of the molecule or molecular segment to which the deuterons are attached. Recently, we reported the use of deuterium NMR to probe surfactant dynamics in polyelectrolyte-surfactant complexes formed between anionic polyelectrolytes and mixed cationic and zwitterionic surfactant micelles.15 It was found that the complexes formed with a flexible polyelectrolyte, such as poly(acrylic acid), were rather compact compared to the looser complexes formed with a stiff polyelectrolyte, such as poly(sodium styrenesulfonate). That study revealed, in addition, that the deuterium NMR spectrum from deuterated surfactants within such complexes consisted of several overlapping subspectra, indicative of a heterogeneous distribution of surfactant between different dynamic and/or conformational populations. In this report we present evidence that these deuterium NMR subspectra reveal the presence of a highly cooperative thermotropic phase transition undergone by the surfactants in polyelectrolyte-surfactant complexes. Experimental Section Materials. Cetyltrimethylammonium bromide (CTAB) was purchased from Aldrich (Milwaukee, WI). CTAB deuterated in the quaternary methyls (CTAB-γ-d9), hexadecylphosphocholine (HDPC), and HDPC deuterated in the quaternary methyls (HDPC-γ-d6) were synthesized as described previously.16,17 Poly(acrylic acid sodium salt) (PACA, 40 wt % solution in water, average MW 30 000) and poly(sodium 4-styrenesulfonate) (PSSS, dry powder, average MW 70 000) were purchased from Aldrich (Milwaukee, WI). The structures of each of these surfactants and polyelectrolytes are shown in Figure 1. Sample Preparation. Surfactant solutions of the appropriate composition and concentration were prepared in doubly distilled H2O for 2H NMR measurements. The total surfactant concentration was maintained constant at 10 mM while the ratio CTAB:HDPC was varied. This concentration is well above the critical micelle concentration for either CTAB (0.9 mM)18 or HDPC (32 µM).19 Polyelectrolytes were added as aliqouts from aqueous stock solutions of established concentration in amounts sufficient (15) Macdonald, P. M.; Tang, A. Langmuir 1997, 13, 2259. (16) Macdonald, P. M.; Staring, D.; Yue, Y. Langmuir 1993, 9, 381. (17) Semchyschyn, D.; Carbone, M.; Macdonald, P. M. Langmuir 1996, 12, 253. (18) Mandal, A.; Nair, B. J. Phys. Chem. 1991, 95, 9008. (19) Macdonald, P.; Rydall, J.; Kuebler, S.; Winnik, F. Langmuir 1991, 7, 2602.

Langmuir, Vol. 14, No. 17, 1998 4759 to yield a stoichiometric cation/anion ratio in the sample globally. The resulting phase-separated polyelectrolyte-surfactant complexes were isolated by low-speed centrifugation and then transferred to an NMR tube for measurement. NMR Spectroscopy. Deuterium NMR spectra were obtained at 45.98 MHz using a Chemagnetics CMX 300 NMR spectrometer employing a quadrupole echo excitation scheme as described previously.16,17 Typically, 1200 acquisitions were signal averaged (total acquisition time of 20 min) using a delay of 40 µs between the pair of pulses in the quadrupole echo sequence, with a recycle delay of 500 ms between repetitions. Deuterium longitudinal (T1) relaxation times were measured using a standard inversion recovery method with a quadrupole echo sequence replacing the usual final readout pulse. In variable-temperature measurements, the temperature was controlled to within (1 °C, and the samples were equilibrated at the desired temperature for 10 min prior to signal acquisition. Deuterium NMR spectra were simulated using a computer program based on the tiling approach of Grant and co-workers20 and permitting optimization of the quadrupolar splitting, the line width parameter T2, and the total integrated spectral area.

Results and Discussion Compact versus Loose Complexes with Flexible versus Stiff Polyelectrolytes. In micellar solution, deuterated surfactants yield 2H NMR spectra consisting of a single, sharp isotropic resonance line, due to the rapid isotropic tumbling of the surfactant micelles. In phaseseparated polyelectrolyte-surfactant complexes, however, micellar tumbling is severely curtailed and the resonance line broadens, as reported previously.15-17 If the residual motions are predominantly isotropic in nature, then the resulting 2H NMR spectrum consists of a single broad resonance line. If the residual motions are predominantly anisotropic in nature, then the resulting 2H NMR spectrum consists of a so-called Pake doublet line shape. Figure 2 shows examples of both isotropic and anisotropic 2H NMR spectra obtained from CTAB:HDPC (50: 50) micelles complexed with different polyelectrolytes. The left-hand column of spectra were obtained with PACA, while the right-hand column of spectra were obtained with PSSS. The top row of spectra were obtained with CTABγ-d9, and the bottom row with HDPC-γ-d6. When the polyelectrolyte-surfactant complexes are formed with PSSS, the 2H NMR spectrum consists of a broad isotropic resonance, whether one observes CTABγ-d9 or HDPC-γ-d6. In the CTAB-γ-d6 spectrum, there is a hint of a Pake doublet spectral component, as indicated by the arrows in the figure. Overall, the spectra in the presence of PSSS are characteristic of hindered isotropic molecular motion. When the polyelectrolyte-surfactant complexes are formed with PACA, the 2H NMR spectrum of CTAB-γ-d9 consists of two overlapping Pake doublets plus a narrow isotropic resonance. For HDPC-γ-d6, the 2H NMR spectrum consists of a single Pake doublet superimposed on an isotropic resonance. The frequency separation between the two maxima of the Pake doublet line shape is called the quadrupolar splitting. The quadrupolar splittings measured here are far smaller than the values expected for static deuterons or for methyl deuterons undergoing rapid methyl rotations. Hence, the 2H NMR spectra indicate that the surfactants in the PACA-surfactant complexes experience rapid anisotropic segmental and/or whole molecular motions. It follows that isotropic tumbling motions, for example, of entire surfactant micelles are slowed to the point that only anisotropic motional averaging influences the 2H NMR spectra. Moreover, the (20) Alderman, D. W.; Solum, M. S.; Grant, D. M. J. Chem. Phys. 1986, 84, 3717.

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Figure 2. 2H NMR spectra of phase-separated CTAB:HDPC 50:50 micelles complexed with either PACA (left-hand column) or PSSS (right-hand column). In the top two spectra, the deuteron labeled species was CTAB-γ-d9, while in the bottom two spectra the deuteron labeled species was HDPC-γ-d6. All spectra were acquired at 23 °C.

multiple spectral components with different quadrupolar splittings indicate the coexistence of different populations of surfactants experiencing different anisotropic motions. One explanation for the different results obtained with PACA versus PSSS, proposed previously,15 is that PACA, a flexible polyelectrolyte, forms complexes that are more compact than those formed with PSSS, a stiff polyelectrolyte. This notion is supported by Monte Carlo simulations of polyelectrolyte-surfactant interactions for different chain flexibilities.21 2H NMR Detects a Thermotropic Transition Undergone by CTAB in Complexes with PACA. To obtain a better understanding of the origin of the several overlapping subspectral components in the 2H NMR spectra of CTAB-γ-d9 and HDPC-γ-d6, we obtained 2H NMR spectra as a function of temperature. Figure 3 show temperature-dependent 2H NMR spectra for CTAB-γ-d9 within CTAB:HDPC (50:50) micelles complexed with a stoichiometric cation:anion equivalent of PACA. The top row shows experimental spectra, while the bottom row shows the corresponding spectral simulations. At the highest temperature, the spectrum for CTABγ-d9 consists of a single narrow Pake doublet with a quadrupolar splitting of approximately 1.5 kHz. As the temperature decreases, two new components appear in the spectrum. One is a small isotropic resonance, and the other is a wide Pake doublet with a quadrupolar splitting of approximately 7.0 kHz. With decreasing temperature, these latter two components increase in intensity at the expense of the Pake doublet with the narrower quadrupolar splitting. At the lowest temperature, the wide Pake doublet constitutes over 90% of the spectral intensity. These 2H NMR spectra of CTAB-γ-d9 are consistent with a temperature-dependent transition undergone by the cationic surfactant within the phase-separated PACA(21) Wallin, T.; Linse, P. Langmuir 1996, 12, 305.

Macdonald and Strashko

surfactant complexes from a high-temperature state, characterized by a Pake doublet with a quadrupolar splitting of approximately 1.5 kHz, to a low-temperature state, characterized by a Pake doublet with a quadrupolar splitting of approximately 7.0 kHz. At intermediate temperatures, these two states coexist in domains sufficiently large that exchange of surfactant between domains is slow relative to the time scale defined by the difference between their quadrupolar splittings. The computer-simulated 2H NMR spectra in the bottom row of Figure 3, corresponding to the experimental spectra in the top row, were produced by assuming that the experimental spectra consist of superimposed spectral components corresponding to populations of surfactants in slow exchange with one another on the 2H NMR time scale. The simulations reproduce the quadrupolar splitting and T2 line width of the individual spectral components, as well as their relative contribution to the overall spectral intensity, and generally show an excellent correspondence with the experimental spectral line shapes. T1 relaxation time measurements, to be discussed later, demonstrate that the relaxation delay employed in acquiring these spectra (500 ms) is at least 10 times longer than the longest T1 relaxation time observed. T2 relaxation time measurements (data not shown) indicate little difference in T2 between the various observed spectral components. Consequently, the spectral intensity (i.e., the integrated area under the Pake doublet line shape) of a given subspectrum allows one to quantify the amount of the various surfactant populations present at a given temperature. Figure 4 shows temperature-dependent experimental (top row) and simulated (bottom row) 2H NMR spectra of HDPC-γ-d6 within phase-separated PACA/CTAB/HDPC complexes. At all temperatures, the 2H NMR spectrum of HDPC-γ-d6 consists of a superposition of a narrow isotropic resonance and a Pake doublet with a quadrupolar splitting of approximately 1.2 kHz. At different temperatures, the proportion of these two components shifts only marginally. The 2H NMR spectra of HDPC-γ-d6 suggest that HDPC is not an active participant in the thermotropic transition undergone by CTAB. The fact that HDPC-γ-d6 produces a Pake doublet 2H NMR line shape indicates that it experiences anisotropic motional averaging and so must be trapped within the phase-separated PACA-surfactant complexes. Consequently, the thermotropic effects displayed by CTAB can be attributed to specific electrostatic interactions with the polyelectrolyte. The temperature dependence of the spectral intensity contributed by the three CTAB-γ-d9 spectral components (the wide, the narrow, and the isotropic) in surfactant micelles complexed with PACA is shown on the right in Figure 5 for the three CTAB:HDPC ratios investigated here: 20:80 (top panel), 50:50 (middle panel), and 80:20 (bottom panel). In all cases there is a temperaturedependent transition from a narrow Pake doublet to a wide Pake doublet with decreasing temperature. Only in the case of CTAB:HDPC 80:20 does the isotropic resonance contribute more than 20% of the total spectral intensity. The thermotropic transition occurs over a narrow temperature range, indicating that it is highly cooperative, involving a simultaneous change of state by many molecules. The Transition Temperature Depends on the CTAB:HDPC Ratio. Although similar changes are observed in the 2H NMR spectra of CTAB-γ-d9 for all CTAB:HDPC compositions, the main events occur at

2H

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Figure 3. Experimental and simulated temperature-dependent 2H NMR spectra of CTAB-γ-d9 in phase-separated CTAB:HDPC 50:50 micelles complexed with PACA. In the top row are experimental spectra, and in the bottom row are corresponding computersimulated spectra.

Figure 4. Experimental and simulated temperature-dependent 2H NMR spectra of HDPC-γ-d6 in phase-separated CTAB:HDPC 50:50 micelles complexed with PACA. In the top row are experimental spectra, and in the bottom row are corresponding computer simulated spectra.

different temperatures. For instance, as shown in the top left in Figure 5, the temperature at which the narrow Pake doublet decreases by 50% in intensity shifts to progressively lower values with increasing CTAB content. Likewise, the temperature at which the wide Pake doublet grows to constitute 50% of the spectral intensity shifts to lower values with increasing CTAB content, as shown in

the bottom left panel of Figure 5. As for the isotropic spectral component, the temperature at which it makes its maximum contribution also shifts to lower values with increasing CTAB content, as shown in the middle left panel of Figure 5. Note that this isotropic resonance accounts for nearly 70% of the spectral intensity at 10 °C for the CTAB:HDPC 80:20 micelles versus a maximum contribu-

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Figure 6. Temperature dependence of the intensities of the various spectral components in the 2H NMR spectra of CTAB: HDPC-γ-d6 micelles phase-separated by complexation with PACA. The intensities of the individual spectral components were obtained by line shape simulations such as those shown in Figure 4. Closed symbols correspond to the Pake doublet, and the open symbols correspond to the isotropic resonance. The three CTAB:HDPC ratios are represented as follows: 20: 80 (triangles), 50:50 (squares), and 80:20 (circles).

Figure 5. Temperature dependence of the intensities of the various spectral components in the 2H NMR spectra of CTABγ-d9:HDPC micelles phase-separated by complexation with PACA. The intensities of the individual spectral components were obtained by line shape simulations such as those shown in Figure 3. The relative intensity of a given spectral component is simply its percentage proportion of the total spectral intensity under given conditions. The three panels on the right, from top to bottom, compare the following CTAB:HDPC ratios: 20:80 (triangles), 50:50 (squares), and 80:20 (circles). Open symbols correspond to the wide Pake doublet, closed symbols correspond to the narrow Pake doublet, and gray symbols correspond to the isotropic resonance. The three panels on the left, from top to bottom, compare the temperature dependence of the narrow Pake doublet (closed symbols), the isotropic resonance (gray symbols), and the wide Pake doublet (open symbols) for the different CTAB:HDPC ratios, with symbols as above.

tion of about 16% and 8% for micelles containing 50 and 20 mol % CTAB, respectively. HDPC-γ-d6 Is Less Able To Sense the Thermotropic Transition. The dramatic temperature effects on the 2H NMR spectra of CTAB-γ-d9 are decidedly muted in the case of HDPC-γ-d6. In Figure 6, the spectral intensities contributed by the two HDPC-γ-d6 spectral components, the Pake doublet and the isotropic resonance, are plotted as a function of temperature for the three CTAB:HDPC compositions investigated: 80:20, 50:50, and 20:80. The main spectral component is always the Pake doublet, and its intensity increases with lower temperature, except in the case of CTAB:HDPC 20:80. Simultaneously, the intensity of the isotropic component decreases, except in the case of CTAB:HDPC 20:80. Interestingly, the midpoint temperature of these relatively small intensity changes shifts to higher values with

increasing CTAB content, precisely the opposite of the observation with CTAB-γ-d9. However, one cannot conclude from these findings that HDPC does not participate in the thermotropic transition evidently undergone by CTAB. It may be that the deuteron labels in HDPC-γ-d6 are simply too far removed from the site of the CTAB/ PACA interaction and that a different location such as along the HDPC alkyl chain, might prove more advantageous. 2 H NMR Quadrupolar Splittings of CTAB-γ-d9 and HDPC-γ-d6 in Different Domains. The quadrupolar splittings measured for all Pake doublet spectral components, for both CTAB-γ-d9 and HDPC-γ-d6, for all CTAB: HDPC ratios, and for all measuring temperatures are shown in Figure 7. For HDPC-γ-d6, there is a slight increase in the observed quadrupolar splitting with decreasing temperature, while at any one temperature the measured quadrupolar splitting increases slightly with increasing CTAB content of the initial micelles. Likewise for the narrow Pake doublet of CTAB-γ-d9, one observes a slight increase in quadrupolar splitting with decreasing temperature and/or increasing CTAB content. For the wide Pake doublet of CTAB-γ-d9, there is a pronounced increase in quadrupolar splitting with decreasing temperature and/or increasing CTAB content. In relative rather than absolute terms, the changes in quadrupolar splitting with temperature and CTAB content are, in fact, rather similar in all three situations. The temperature effects on the quadrupolar splittings are attributable to enhanced molecular ordering at lower temperatures. The effect of CTAB content may be due to some ordering effect of bound polyelectrolyte, since the overall micellar surface density of polyelectrolyte must increase with increasing CTAB content in a 1:1 stoichiometric complex such as studied here. It cannot be due to a specific conformational response to surface electrostatics, such as that undergone

2H

NMR Proof of a Thermotropic Phase Transition

Figure 7. Temperature dependence of the quadrupolar splittings of the various Pake doublet spectral components in the 2H NMR spectra of CTAB:HDPC micelles phase-separated by complexation with PACA. Closed symbols correspond to the wide Pake doublet in the case of CTAB-γ-d9 deuterium labels. Open symbols correspond to the narrow Pake doublet in the case of CTAB-γ-d9 deuterium labels. Gray symbols correspond to the Pake doublet in the case of HDPC-γ-d6 deuterium labels. The three CTAB:HDPC ratios are represented as follows: 20: 80 (triangles), 50:50 (squares), and 80:20 (circles).

by phosphocholine-containing amphiphiles,22 since the presence of a zwitterionic molecular segment is a prerequisite that CTAB manifestly lacks. 2 H NMR T1 Relaxation Times of CTAB-γ-d9 in Different Domains. The temperature dependence of the T1 relaxation time of the various spectral components in the 2H NMR spectrum of CTAB-γ-d9 contained within CTAB:HDPC micelles phase-separated with PACA is shown in Figure 8 for all CTAB:HDPC ratios investigated here. In all instances, a single T1 relaxation time was sufficient to describe the dependence of the spectral intensity of a particular spectral component on the delay time in the inversion recovery sequence. For the narrow Pake doublet of CTAB-γ-d9, as well as the isotropic spectral component, all T1 relaxation times fall within the range of 30-70 ms, and T1 always increases with increasing temperature. This indicates that the molecular motions contributing to the T1 relaxation fall in the fast motion regime relative to the characteristic time scale of 10-5-10-6 s for deuterons at 7.0 T. Typically these would be expected to correspond to internal conformational flexing and bond rotations, such as methyl rotations, having correlation times on the order of picoseconds. The T1 relaxation times appear to be virtually independent of the CTAB:HDPC ratio. When the T1 data are plotted in the Arrhenius format, as in Figure 8, one arrives at an activation energy of 19 kJ mol-1, again regardless of the CTAB:HDPC content of the initial micelles. Comparable activation energies are reported from 2H NMR T1 relaxation time studies of the choline methyls of the membrane lipid dipalmitoylphosphatidylcholine at temperatures above its gel-to-liquidcrystalline phase transition temperature.23 (22) Macdonald, P. M. Acc. Chem. Res. 1997, 30, 196. (23) Gally, H.-U.; Niederberger, W.; Seelig, J. Biochemistry 1975, 14, 3647.

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Figure 8. Arrhenius plot showing the temperature dependence of the 2H NMR T1 relaxation time of CTAB-γ-d9 in various CTAB:HDPC micelles phase-separated by complexation with PACA. Open symbols correspond to the wide Pake doublet, closed symbols correspond to the narrow Pake doublet, and gray symbols correspond to the isotropic resonance. The three CTAB:HDPC ratios are represented as follows: 20:80 (triangles), 50:50 (squares), and 80:20 (circles). The narrow Pake doublet, grouping together all CTAB:HDPC ratios, yields an activation energy of 19 kJ mol-1, with R2 ) 0.94. The wide Pake doublet, grouping together all CTAB:HDPC ratios, yields an activation energy of 18 kJ mol-1, with R2 ) 0.74.

For the wide Pake doublet of CTAB-γ-d9, all T1 relaxation times fall within the range of 15-30 ms, T1 always increases with increasing temperature, and similar values are obtained regardless of the CTAB:HDPC content of the initial micelles. Thus, the T1 relaxation time in the CTAB population giving rise to the wide Pake doublet is significantly shorter than that of the CTAB population giving rise to the narrow Pake doublet. One interpretation of this difference is that it arises due to additional constraints experienced by CTAB in the low-temperature versus high-temperature phases, possibly connected with a tighter packing of surfactants. Nevertheless, the Arrhenius plot in the bottom panel of Figure 8 indicates an activation energy of 18 kJ mol-1, a value little different from that measured for the narrow Pake doublet spectral component. Concerning the Nature of the Observed Thermotropic Transition. The pseudo-three-component phase diagram for systems of PACA and cationic surfactants of the alkyltrimethylammonium bromide type contains a teardrop-shaped two-phase region anchored in the water corner.24 Within this region, charge neutralized surfactant-polyelectrolyte complexes coexist with excess surfactant and water. When the polyelectrolyte is PACA, the interactions with the cationic surfactant are strong, and bromine and sodium ions tend to be excluded from within the phase-separated complexes. So the PACA/ CTAB/HDPC complexes investigated here are highly concentrated, containing a minimal amount of water. Surfactants in general, and CTAB in particular, can exist in various lyotropic liquid crystalline phases. Typically, with decreasing water content, the phase state progresses from a soluble micellar (L1) phase to a normal (24) Thalberg, K.; Lindman, B.; Bergfeldt, K. Langmuir 1991, 7, 2893.

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Figure 9. Schematic of possible rotational motions undergone by CTAB molecular segments and expected to influence the observed 2H NMR quadrupolar splitting of CTAB-γ-d9.

hexagonal (HI) liquid crystalline phase to a lamellar (LR) liquid crystalline phase.25 It is also common that, in the phase diagram of such amphiphile/water systems, a cubic liquid crystalline phase exists at the boundaries between the major phases.25 The details of the phase boundary positions depend sensitively on temperature as well as composition. These surfactant and surfactant-polyelectrolyte properties suggest that similar, albeit thermotropic, phase transitions might occur in the PACA/CTAB/HDPC systems. The 2H NMR results reported here support this notion. Specifically, the 2H NMR quadrupolar splitting reflects the details of the motions experienced by the molecule, or the molecular segment, to which the deuteron is attached. For the case of rapid anisotropic motional averaging, the measured quadrupolar splitting, ∆ν, is a function of the angle θ between the C-D bond vector and the axis of motional averaging, as quantified in eq 1, where

∆ν ) ∆ν0 < |1/2(3 cos2 θ - 1)| > Sf

(1)

∆ν0 is the static quadrupolar splitting (125 kHz for aliphatic deuterons) and the angular brackets indicate that one is measuring a time and ensemble average. Sf is an order parameter (0 e Sf e 1.0) quantifying the degree of off-axis wobble of the C-D bond. An order parameter equal to zero implies essentially no preferred motional axis. To illustrate the utility of eq 1, consider Figure 9, in which the structure of the headgroup region of CTAB is shown schematically. Imagine that this molecule is embedded in a lamellar LR liquid crystalline phase, and consider the heirarchy of modes of anisotropic motional averaging it would experience. At all temperatures measured here, the quaternary methyl groups should undergo rapid continuous rotational averaging about the N-Cγ bond axis, as illustrated. Assuming tetrahedral bond angles and an order parameter equal to 1.0, eq 1 indicates that such methyl rotations will reduce the observed quadrupolar splitting from the static value of 125 kHz to a motionally averaged value of 41.6 kHz. The reduction by a factor of 1/3 arises from using θ ) 70.5°, (25) Lindblom, G.; Rilfors, L. Biochim. Biophys. Acta 1989, 988, 221.

which corresponds to the geometry of the Cγ-D bond relative to the axis of motional averaging, the N-Cγ bond. A higher activation energy would be associated with rotations about the CR-N bond axis, i.e., rotation of the entire quaternary nitrogen headgroup, but this motion should occur readily at the temperatures employed here. The relevant angle for eq 1 is now that between the CR-N bond axis of rotation and the N-Cγ bond direction, so that again θ ) 70.5°, yielding a further reduction of the quadrupolar splitting by a factor of 1/3, to a value of 13.85 kHz. In a liquid crystalline lamellar architecture, the next most prevalent motion likely to influence the quadrupolar splittings of the quaternary methyl deuterons is rotation of the entire surfactant about its long molecular axis. Using θ ) 35.25° in eq 1, i.e., the angle between the CR-N bond direction and the surfactant’s long molecular axis, yields a predicted quadrupolar splitting of 6.93 kHz, which corresponds closely with the values observed at low temperatures with CTAB-γ-d9 in the PACA/CTAB/ HDPC complexes. In a normal hexagonal (HI) liquid crystalline phase, there exists the additional possibility for the surfactants to diffuse around the central axis of the rods composing that phase. Here, θ ) 90°, corresponding to the angle between the surfactant’s long molecular axis and the cylindrical rod axis. Consequently, this mode of motional averaging yields an additional reduction by a factor of 1/2 in the expected quadrupolar splitting, to a value on the order of 3.46 kHz. (The sign of the quadrupolar splitting cannot be determined from the measurements performed here; only its absolute value can be determined.) Note that there is every reason to expect a concomitant reduction in the order parameter Sf upon converting from a lamellar to a hexagonal liquid crystalline phase.26 Using Sf ) 0.67, as opposed to 1.0, brings the predicted quadrupolar splittings into agreement with the values observed for the high-temperature Pake doublet of CTABγ-d9 in PACA/CTAB/HDPC complexes. In a cubic liquid crystalline phase, the surfactants experience isotropic motional averaging due to the high local radius of curvature characteristic of the aggregate structure of such phases. Consequently, the quadrupolar splitting falls to zero, leaving only a narrow isotropic resonance line in the spectrum. This is the feature characteristic of the spectral component observed here at intermediate temperatures with CTAB-γ-d9 in PACA/ CTAB/HDPC complexes. Taken as a whole, these observations indicate that, in phase-separated PACA/CTAB/HDPC complexes, one may induce a thermotropic transition from a high-temperature hexagonal HI liquid crystalline phase, to an intermediate temperature cubic liquid crystalline phase, to a lowtemperature lamellar LR liquid crystalline phase. Further measurements, using independent techniques such as freeze fracture electron microscopy (FFEM) and X-ray diffraction, need to be performed to confirm these conclusions. We note that FFEM has already been applied to demonstrate the presence of layered and rodlike structures in various polyelectrolyte-ionic surfactant pairs.27 LA971051L (26) Lafleur, M.; Cullis, P. R.; Fine, B.; Bloom, M. Biochemistry 1990, 29, 8325. (27) Harada, A.; Nozakura, S. Polym. Bull. 1984, 11, 175.