J. Phys. Chem. 1992,96, 6071-6075 (7) Sharkov, A. V.; Pakulev, A. V.; Chekalin, S. V.; Matveetz, Y.A. Biochim. Biophys. Acta 1985,808, 94. (8) Polland, H.J.; Franz, M. A.; Zinth, W.; Kaiser, W.; Kolling, E.; Oesterhelt, D. Biophys. J. 1986, 49, 651. (9) Petrich, J. W.;Breton, J.; Martin, J. L.; Antonetti, A. Chem. Phys. Lrtt. 1987, 137, 369. (10) Dobler, J.; Zinth, W.; Kaiser, W.; Oesterhelt, D. Chem. Phys. Letr.
1988, 144, 215. (11) Mathies, R. A.; Brito Cruz, C. H.; Pollard, W. T.; Shank, C. V. Science 1988,240, 777. (12) Atkinson, G. H.; Brack, T. L.; Blanchard, D.; Rumbles, G. Chem. Phys. 1989, 131, 1. (13) van den Berg, R.; Jang, D.-J.; Bitting, H. C.; El-Sayed, M. A. Biophys. J . 1990,58, 135. (14) b i g , S.J.; Reid, P. J.; Mathies, R. A. J. Phys. Chem. 1991,95,6372. (15) Polland, H.-J.; Franz, M. A.; Zinth, W.; Kaiser, W.; Hegemann, P.; Oesterhelt, D. Biophys. J, 1985, 47, 55.
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(16) Blanck, A,: Oesterhelt. D. EMBOJ. 1987. 6. 265. (17) Kandori, H.; Yoshihara, K.; Tomioka, H.; Sisabe, H. Chem. Phys. Leu. 1991, 187, 579. 118) Tomioka. H.: Takahashi. T.; Kamo, N.; Kobatake, Y. Biochim. Biophys. Acta 1686, 884, 578. (19) Duschl, A.; McCloskey, M. A.; Lanyi, J. J . B i d . Chem. 1988, 263, 17016. (20) Kandori, H.; Kemnitz, K.; Yoshihara, K. J. Phys. Chem., submitted for publication. (21) Kandori, H.; Shichida, Y.; Yoshizawa, T. Biophys. J . 1989,515,453. (22) Metzler, D. E.; Harris, C. M. Vision Res. 1978, 18, 1417. (23) Birge, R. R.; Einterz, C. M.; Knapp, H. M.; Murray, L. P. Biophys. J. 1988, 53, 367. (24) Steiner, M.; Oesterhelt, D. EMBO J . 1982, 2, 1379. (25) Tittor, J.; Oesterhelt, D.; Maurer, R.; Desel, H.; Uhl, R. Biophys. J . 1987, 52, 999. (26) Oesterhelt, D.; Hegemann, P.; Tittor, J. EMBO J . 1985, 4, 2351.
ESR Spectroscopy of AOT Reverse Micelles. Location of Cation Guests and Comparison with Perfluorinated Ionomers Stacey A. Lossia,+ Stephen G. Flore: Sreehari Nimmala, Hongwei Li? and Shulamith Schlick* Department of Chemistry, University of Detroit Mercy, Detroit, Michigan 4821 9 (Received: January 23, 1992; In Final Form: March 26, 1992)
The objective of the present study was to assess to what extent reverse micelles (RM) based on sodium bis(2-ethyl-1-hexyl) sulfosuccinate(AOT)/water/kooctanemimic locations of cations and dynamical processes in perfluorinated ionomers containing pendant sulfonic acid groups (PFSA). ESR spectra of Cuz+and V02+ in the reverse micelles were measured at 120 and 300 K, respectively, and compared with results obtained in the ionomers. The location of cations in RM depends on the water content, expressed as w = [water]/[AOT]. The radius of the water pool is obtained from w, using r = 1 . 5 ~ .For w = 2 ESR spectra of Cut+ indicate ligation to the sulfonic groups of the surfactant. For w in the range 5-20 the cation appears to be located both near the sulfonic groups (site 1) and in a water environment (site 2). Reverse micelles with large water content corresponding to a water pool diameter of =90 A, and swollen PFSA with water pools of -40 A, stabilize the same Cu2+site. In both systems the cations are in a water environment, site 2. The existence of two cation sites in the intermediate w range, and the larger water pool size in RM that stabilizes the cupric cation in a water environment only, suggest a broad distribution of micelle sizes for a given w value. The dynamical properties of the cations in the same environment, based on the rotational correlation time T~ of VOz+ at 300 K, are different, most likely because of the connectivity of the water pools in the ionomers and the existence of separated droplets in the RM, even at high values of w.
Introduction Ionomers are amphiphilic polymers that contain about 10 mol % ionic groups located on the main chain, on pendant chains, or at the chain ends. Although the ionomers do not dissolve in water and other polar solvents, they are capable of absorbing large quantities of these solvents. The presence of both hydrophilic and hydrophobic groups in the same polymer chain leads to a complex morphology and to separation into polar and nonpolar domains. The existence and extent of phase separation in ionomers in the presence of various polar solvents have been extensively studied by scattering and spectroscopic methods and by mechanical relaxation.'.* Perfluorinated ionomers containing sulfonic acid groups on randomly distributed pendant chains (PFSA) have attracted considerable attention due to their outstanding mechanical, thermal, and chemical stability and to their technological importance as separation membranes in electrochemicalproce~ses.l-~ Studies on PFSA have been motivated by the desire to understand and ultimately to control the ionomer properties. Water as a swelling solvent has a very special role in the process of phase separation into ionic and organic regions in many ionomers and particularly in PFSA. Our recent I9F NMR studies have indicated that in the case of PFSA with long and short Undergraduate Research Fellow. 'Graduate Research Fellow. To whom corresp&den& should be addressed.
0022-3654/92/2096-607 1$03.00/0
pendant chains, Nafion made by DuPont (I) and Dow membranes (11), respectively, the phase separation is essentially complete for water as swelling solvent. NMR results also suggest that, unlike water, alcohols and other polar solvents penetrate into the organic domains; the result is dynamic averaging of the chemical shift anisotropy in some polymer chains on the time scale of the NMR experiment.4 -CF&
F2CF-
\
OC F2CFOCF2CF2S03H
I
CF3 I: Nafion membrane, acid form (long pendant chains) -CFtCF&F-
\
OCF~CF~SOJH
11: Dow membrane, acid form' (shortpendant chains) The model that appears to explain many properties of PFSA swollen by water is the two-phase model, which assumes the presence of water pools where the free charges are located and nonpolar domains that include mainly the organic backbone. The pendant sulfonic groups separate the polar domains from the organic domains? The presence of some side chains in the ionic domains has also been suggested, but this assumption has not been verified by our recent NMR results on swollen membranes. Extensive studies by small-angle X-ray and neutron scattering
Q 1992 American Chemical Society
6012 The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
(SAXSand SANS, respectively) suggest that the water pools in Nafion swollen by water have a diameter of a30-50 A, depending on the degree of s ~ e l l i n g The . ~ ~ rapid ~ cation diffusion in these ionomers has been interpreted as evidence that the water pools are interconnected by channels; a width of a 1 0 A for the channels has been proposed.z The diffusion of oxygen in the membranes, which is considerably more rapid in the PFSA than in Teflon, also suggests connectivity of the polar domains.’ The dimensions of the water pools in swollen PFSA are in the same range as the water pools in the reverse micelle system (RM) based on sodium bis(2-ethyl- 1-hexy1)sulfosuccinate (AOT, III), CH3
I
CH2COO(CH2)4CHCH2CH3
I
Na’SO3-CHCOO(CH2)4CHCH2CH3
I
CH3
111: Aerosol OT (AOT)
Lossia et al. TABLE I: Composition of the AOT/Water/Isooetaw Reverse Micelles W
2 5 10 15 20 30
r
6%
3.0 7.5 15.0 22.5 30.0 45.0
W’
3.7 x 104 8.5 X lo4 1.7 x 10-3
2.7 x 10-3 3.6 x 10-3 5.4
x 10-3
probe/ micelle4 0.001 0.01
0.08 0.29 0.68 2.30
“The number of water molecules per cation probe is 5560, fixed for all micelles.
perhaps better control of oxygen pressure, are needed in order to clarify the micellar effect. The location of a series of nitroxide probes differing in structure and charge in RM based on AOT/water/IO has been studied as a function of w and temperat~re.~’It has been concluded that positively charged probes, and probes with a long hydrocarbon tail, intercalate in the surfactant layer, while a neutral probe resides in the water phase. The rotation rate of the micelles as a function of w and the effect of this rotation on the ESR spectrum of the probe have also been discussed in this study. The cupric probe in w/o microemulsions has been studied by ESR, with focus on the glass vs ice formation as a function of the cooling rate.28 The results indicated that the micelle structure is preserved even at relatively low cooling rates, in agreement with other ESR, electron ” m p y , and scattering measurements.29,M This study did not include microemulsions based on AOT. To the best of our knowledge, no systematic investigation of the paramagnetic metal cations Cu2+ and V 0 2 + in AOT RM as a function of w or temperature has been published. The results we obtained provided some understanding not only on the nature of the water pools in the PFSA compared to RM but also on the structure of the RM as a function of the water content.
water, and isooctane (IO) or another hydrocarbon as the u0il*.8,9 Both systems, the ternary reveme micelle system and the ionomcrs, consist of three major components: the water microphase, the organic part, and the sulfonic group that separate the two regions. In the isotropic region of the phase diagram for AOT/water/IO, the radius r of the water pool has been correlated with the molar ratio w = [water]/[AOT] by the relation r = 1.5w, which has been deduced from SAXS and SANS experiments.I0 A similar relation has been suggested by fluorescence studies.liJ2 The ability to prepare a series of well-defined water pools in this ternary system is attractive when attempting to use the reverse micelles as model compounds for the more complex PFSA ionomers. This comparison is extremely important also when considering the diffusion of ions and other guests in media that are heterogeneous on a microscopic scale and the possible location of reactants in a confined reaction medium. The objective of the present study was to assess to what extent the reverse micelles based on AOT/water/IO can mimic location of cations and dynamical processes in the PFSA ionomers. We Experimental Section chose to study by electron spin resonance (ESR) the properties Aerosol OT (AOT) from Fisher was purified first by dissolution of two paramagnetic cations, Cu2+and V02+, which we have in methanol, addition of activated charcoal (1/ 12 of the surfactant studied in the Nafion and Dow i o n o m e r ~ , ~ ~The ’ ~ -ESR ~ ~ paweight), and stirred overnight; the mixture was then filtered three rameters of the cupric ion, often measured around 100 K, are very times, dried at 330 K for 1-2 h, and then dried in vacuum at sensitive to the immediate environment and can be interpreted ambient temperature for about 48 h.3’ Purified AOT was dissolved in terms of cation location. V02+ normally gives a resolved ESR in IO to a concentration of 0.4 M. spectrum at ambient temperature and can be used for the study The 0.01 M vanadyl solution was prepared by bubbling nitrogen of dynamics in the water pools of RM as a functioin of the water gas in deionized water, followed by addition of VOS04.2H20. To pool radius. Because the rate of reorientation is expected to be prevent oxidation, the vanadyl solution was kept under nitrogen sensitive to the size of the water we planned to measure and used to prepare the ternary RM mixture by adding to it the the value of the rotational comlation time rCin the reverse micelles amount of the 0.4 M AOT solution in IO to obtain the desired as a function of w and to compare with the value measured in the w value. A similar procedure was followed for the preparation PFSA, where the size of the water pool has also been deduced of RM containing Cu2+;63Cuenrichment (98%) was used for by scattering methods. maximum resolution in the ESR spectra. The probe concentration ESR studies of V 0 2 +in Nafion have suggested that the rotais expressed by the molar ratio w’ = [probe]/[AOT]. tional correlation time T, depends on the amount of water, and The composition of the reverse micelles for w values in the range in Nafion fully equilibrated with water at ambient temperature in Table I. T, is very close to that of the vanadyl probe in bulk ~ a t e r . ~ 2-30 ~ ~ is~ given ~ ESR spectra at X-band were measured with a Bruker 200D PFSA with short pendant chains show the same behavior, and SRC spectrometer operating at 9.7 GHz (empty cavity at ambient within experimental error a similar 7,value has been measured temperature) and 100 kHz magnetic field modulation, interfaced in these membranes.24 with a data acquisition system based on an IBM PC/XT and the Reverse micelles based on AOT/water/IO have been extensoftware EPRDAS (Mega Systems Solutions, Rochester, NY). sively studied by ESR spectrosoopyin an effort to deduce the effect Some spectra were plotted with the software SpectraCalc (Galactic of the value of w on the location and relaxation properties of Ind. Corp., Salem, NH). Samples were cooled using the Bruker paramagnetic probes, which were in most cases nitroxide free flow system 41 11VT. The microwave frequency was measured radicals. The potassium peroxylaminedisulfonateion (PADS, with the Hewlett-Packard 5342A frequency counter. Calibration Fremy’s salt), being negatively charged, is expected to be located of g values is based on a Cr3+in MgO standard, with g = 1.9796. in the water phase. It was reported that the line widths of this All spectra were recorded at a microwave power of 2 mW. probe are considerably narrower in the RM with w = 7 compared The correlation times of the vanadyl probe were calculated using to those in bulk water.25 These results have been explained by BASIC codes developed in our laboratory. assuming that the micellar structure isolates the probe and prevents the radical-radical interactions that contribute to the line width Results in bulk water. Another study of PADS reported brooder lines X-band ESR spectra of Cu2+ at 120 K in the RM for four in RM based on AOT/water/heptane with w in the range 2-20.% values of w are shown in Figure 1. The full spectrum is shown These contradictory results suggest that more experiments, and
ESR of AOT Reverse Micelles
I
I
I
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 6073
v
2700. I
2750 3000 3250 3500 Figure 1. X-band ESR spectra at 120 K of Cu2+in the ternary system 2500
AOT/water/IO for the indicated values of w = [water]/[AOT]. The 'stick" diagrams for the parallel quartets of sites 1 and 2 are indicated. The position of the C P standard is also given.
for w = 30, and vertically expanded spectra of the parallel region are shown for w = 2, 10, 20, and 30. The perpendicular components in all spectra are very similar, but the parallel components differ significantly. We measured gl1= 2.3579, g, = 2.074, and All = 0.0159 cm-I (site 1) for w = 2 and gll = 2.4081, g, = 2.082, and All = 0.0138 cm-I (site 2) for w = 30. The parallel quartets for the two sites are indicated in the stick diagrams. For w = 10 and 20 signals from Cuz+in both sites are detected; similar intensities for the two sites are measured for w = 10, but for w = 20 site 2 is dominant. ESR spectra corresponding to w = 5 are similar to those for w = 10, but the signal is weaker as expected for a lower water, and probe, content. Increasing the amount of water above w = 30 has no effect, and the ESR spectra are identical to those for w = 30. The parameters measured for site 1 are almost identical to those identified for a tetragonal cupric site with oxygen ligands from two sulfonic groups and four water molecules, in Nafion membranes swollen with an amount of water that is less than the amount required for full swelling of the membranes.13J4Similarly, site 2 was identified with a tetragonal cupric site ligated to oxygen ligands from water only and is dominant in Nafion swollen to equilibrium by water. The two cupric sites were detected in Nafion swollen by water at relative humidity 28-40%. Both water and the sulfonic groups are also present in the reverse micelles, and therefore it is logical to maintain a similar interpretation of the two sites defined in Figure 1. These results identify the location of the cation: In the smallest water pools (w = 2) the cation is near the surfactant head and ligated to the sulfonic groups. In micelles with w = 30 only cations in a water environment are detected. Micelles with w = 10 contain roughly equal populations of the probe in the two types of environment. ESR spectra of VOz+ in the RM have been measured at 300 K for w in the range 10-80. Representativespectra are given in Figure 2A for w = 10, 20, and 50. The spectra are dominated by the isotropically averaged spectrum. For w = 10, however, signals from a more rigid component are also detected, as indicated by the downward arrow; this result is in agreement with the existence of two Cuz+sites in RM with the same water content. For w 2 20 the signals represent an averaged spectrum, and the line widths were measured as a function of mI. The correlation time T~ was calculated from the amplitudes hi and peak-to-peak widths AHi for the hyperfine lines of the isotropic octet, with the assumption of a Lorentzian line shape and no overlap contribution to the line width. An average line intensity Ii was deduced from eq 1; the corrected peak-to-peak width AH: was calculated from
3140.
3500.
3860.
mI
4220.
Figure 2. (A) X-band ESR spectra at 300 K of V 0 2 + (0.01 M in water) in the ternary system AOT/water/IO for the indicated values of w = [water]/[AOT]. Downward arrow shows the signal from the immobilized V 0 2 + site. The position of the Cr3+standard is also given. (B) Curve fitting of the corrected line widths AH; for w = 30 to eq 3.
16.7
w= [Wa t e r l / [AOTI 33.3 50.0 66.7 83.3
100.0
2.00
= I
Reverse Micelles
1.60
.oo
4o
i
I 50
00
150
d
201
250
300
(8)
Figure 3. Correlation time ic at 300 K for V 0 2 + in RM as a function of the w values (0). Data for the cross-linked PAA swollen by water (from ref 21) are given as a function of pore diameter d (D). The values of 5, for V02+ in bulk water (m) and in Nafion swollen to equilibrium in water ( 0 ) are also shown and are not related to a d or w value.
eq 2 and fitted by a least-squares program to a polynomial in mI, as in eq 3.*'
In Figure 3B we present the data fitted in this way for w = 30. The coefficient A deduced from the fitting procedure cannot be used to calculate T ~ because , it contains contributions from unresolved proton superhy-perfinesplittings and spin rotation. The coefficient B can be used to calculate T, but contains the modulation of both the g and hyperfine tensors and is rather sensitive to errors. In most cases the coefficient C is used, since it depends only on the hyperfine tensor components, which are more accurately known than those of the g tensor. The T, values were calculated using a least-squares iterative program, as described in the and plotted as a function of w in Figure 3. Also given in Figure 3 are the T, values for the vanadyl probe in chemically cross-linked polyacrylamide (PAA) swollen by water, as a function of the pore diameter d, taken from our previous study.z1 We note that, in order to plot all data in a consistent way, the diameter d corresponds to the pore diameter in the cross-linked
Lossia et al.
6074 The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 SCHEME I: Location of Metal Cations in Reverse Micelles
w=2
5(w 20 to probe environment is water, and there is no reason to believe that Cu2+and V02+ are different. On the basis of these conclusions, we propose the locations of both cationic probes in RM as shown in Scheme I. We interpret the presence of two different cation environments in RM with 5 Iw I20 not as an indication of two possible sites in micelles with a given size defined by the water content but as evidence for a micelle size distribution. In support of this suggestion is the reduced population of Cu2+ in site 1 for w = 20 compared to that for w = 10, as seen in Figure 1. CompPriPoa of Reverse lwicelks and PFSA Ioaomers. Inspection of the results presented in Figure 3 indicates that the correlation time T, of V02+ for micelles with w > 30 (and d > 90 A) is practically constant and higher than the value in both Nafion and bulk water. Nafion membranes swollen by water and neutralized by 1%Cu2+are similar to micelles with w = 15 in terms of sulfonic group content.ls The dimension of the water pool is =40 A, measured form ESR data;22this value is in agreement with scattering results.' The higher f Cvalues in large micelles, compared to those in Nafion, are puzzling but in line with other studies. NMR relaxation studies for Na+ in reverse micelles indicate that even in large micelles (radius 73 A) the mobility of the cations is still highly reduced compared with bulk water,12in agreement with the results detected in this study. It is possible that in RM the cationic probes, although not ligated to the sulfonic acid groups, are close to the polar surfactant head. For this reason we suggest the location of the probe in the large micelles shown in Scheme I. The structure of RM at high w values is not very well understood.27 While self-diffusion measurements are consistent with closed water droplets, the increase in electrical conductivity in large micelles has been interpreted by assuming percolation of the droplets and formation of connective channels. It has been suggested, however, that the conductivity measurements can be
also explained by hopping of electrical charges across neighboring micelle^.^^?^^ The data presented in this study lend support to the description of isolated droplets in RM even at high water contents. In this respect it seems that the behavior of cations in the ternary RM system is different compared with that in PFSA ionomers. The low T, value for VOz+in ionomers, about the same as in bulk water, could be due to the connectivity of the water pools by the channels. The water swollen gels are an extreme case of water connectivity; the T, values for V02+ in this system (Figure 3 ) are similar to the value measured in bulk water even at relatively low pore diameters, -50 A. The T, is similar to that in the PFSA for a similar size of the water pool. The similarity between the RM and PFSA is reflected in the detection of the same sites for Cu2+in both systems. The sizes of the water pools in the two systems that stabilize one of the sites are however different. Site 2 of Cu2+in RM is the only site when w 1 30, corresponding to d 2 90 A, whereas in Nafion this site dominates at a water content corresponding to a water pool diameter of only 40 A. Site 1 was not detected exclusively even in Nafion containing a very small water content. In Nafion dried for 8 h in vacuum at 373 K two major sites for Cu2+have been detected: a site with tetrahedral symmetry (four ligands) and a site with trigonal-bipyramidal environment (five ligands).14 Gradual rehydration of the ionomer results in a gradual appearance of site 1 and the disappearance of the site with five ligands. We suggest that in RM the surfactant molecules can rearrange around the cation more easily and preserve an environment with six ligands. The chains in the PFSA are however less likely to have the mobility necessary to reorganize around the cation in the presence of a small water content. The difference in the dynamics of cations in RM and PFSA can be, to some extent, rationalized by the connectivity of the water pools in the ionomers and the existence of isolated micelles in RM. The structural difference, reflected in the stabilization of specific Cu2+sites for different diameters of the water pools in the two systems, is harder to explain. One possible explanation is a broad distribution of micelle sizes for a given w value not only in the intermediate region ( 5 Iw I20) but also at large water contents. Conclusions
The location of cations in RM depends on the w = [water]/ [AOT] ratio. For w = 2 ESR spectra of Cu2+indicate ligation to the sulfonic groups of the surfactant. For w in the range 5-20 the cation occupies two sites: near the sulfonic groups of the surfactant (site 1) and in a water environment (site 2). The results suggest that the two types of sites are due to a micelle size distribution, for a given value of w in this range. Reverse micelles with a water content corresponding to a water pool diameter of 90 A, and swollen PFSA with water pools of 4 0 A, stabilize the same Cu2+site. In both systems the cations are in a water environment, site 2. A broad distribution of micelle sizes for a given w value is suggested by the existence of two cation sites in the intermediate w range and the larger water pool size in RM that stabilizes the cupric cation in a water environment only. The dynamical properties of the cations in a water environment, based on the rotational correlation time T, of V02+ at 300 K, are different, most likely because of the connectivity of the water pools in the ionomers and the existence of separated droplets in the RM, even at high values of w. In very small micelles the surfactant molecules have the ability to rearrange and to maintain around the cation an octahedral symmetry with six ligands, even at very low water content. In dry Nafion, however, distorted Cu2+sites are detected with four and five ligands.
Acknowledgment. This research was supported by National Science Foundation Grant DMR-8718947 (Polymer Program) and by the donors of the Petroleum Research Fund, administered by the American Chemical Society. S.A.L. and S.G.F. were supported by the NSF Research Experience for Undergraduates
J. Phys. Chem. 1992,96,6075-6083 (REU) program. We thank DuPont and Dow Companies for donating the perfluorinated membranes. S.S.is grateful to the American Association of University Women (AAUW) for the 1991/ 1992 Founders’ Fellowship. We enjoyed illuminating discussions with M.P. Pileni and her colleagues. References and Notes (1) Ions in Polymers; Eisenberg, A,, Ed.; American Chemical Society: Washington, DC, 1980. (2) Stnrcrure und Properties of Ionomers; Pineri, M., Eisenberg, A., Eds.; Reidel: Dordrecht, 1987. (3) Perfluorinated Ionomer Membranes; Eisenberg, A.. Yeager, H. L., Eds.; ACS Symposium Series No. 180; American Chemical Society: Washington, DC, 1982. (4) Schlick, S.; Gebel, G.; Pineri, M.; Volino, F. Macromolecules 1991,
24, 3517.
(5) Ezzell, B. R.; Carl, W. P.; Mod, W. A. In Industrial Membrane Processes; White, R.E., Pintauro,P. N., Eds.;AIChE Symposium %riaNo. 248; American Institute of Chemical Engineers: New York, 1986; p 45. (6) Gierke, T. D.; Hsu, W. Y. In ref 3, Chapter 13. (7) Alonso-Amigo, M. G.; Schlick, S . J. Phys. Chem. 1989, 93, 7526. (8) Reuerse Micelles; Luisi, P. L., Straub, B. E., Eds.; Plenum Press: New York, 1984. (9) Srrucrure and Reuctiuity in Reverse Micelles; Pileni, M. P., Ed.; Elsevier: Amsterdam, 1989. (10) Pileni, M. P.; Zemb, T.; Petit, C. Chem. Phys. Letr. 1985, 118, 414. (111 Wonn. M.:Thomas. J. K.: Gritzcl. M. J. Am. Chem. Soc. 1976.98. 2391. ’We note that q 1 in’this paper should be corrected to r = 36.65ulk where u and g are the weight percentagu, of water and AOT, respectively.
6075
This expression is essentially the same as r = 1 . 5 ~ . (12) Wong, M.; Thomas, J. K.; Nowak, T. J . Am. Chem. Soc. 1977,99, 4730. (13) Alonso-Amigo, M. G.; Schlick, S . J. Phys. Chem. 1986, 90, 6353. (14) Schlick, S.;Alonso-Amigo, M. G. J . Chem. Soc., Furaduy Trans. 1 1987, 83, 3575. (15) Alonso-Amigo, M.0.;Schlick, S.Macromolecules 1989, 22, 2628. (16) Schlick, S.;Alonso-Amigo, M.G. Macromolecules 1989, 22, 2634. (17) Bednarek, J.; Schlick, S. J . Am. Chem. Soc. 1990, 112, 5019. (18) Schlick, S.; Alonso-Amigo, M. G.; Eaton, S.S.J. Phys. Chem. 1989, 93, 7906. (19) Bednarek, J.; Schlick, S. J , Am. Chem. Soc. 1991, 113, 3303. (20) Maiti, B.; Schlick, S.Chem. Muter. 1992, 4, 458. (21) Rex, G. C.; Schlick, S.Polymer, in press. (22) Barklie, R. C.; Girard, 0.;Braddell, 0. J . Phys. Chem. 1989, 93, 7906. (23) Martini, G.; Ottaviani, M. F.; Pedocchi, L.; Ristori, S.Mucromolecules 1989, 22, 1743. (24) Li, H.; Schlick, S.;Gebel, G.; Pineri, M.; Volino, F. To be published. (25) Okazaki, M.; Toriyama. K. J . Magn. Reson. 1988, 79, 158. (26) Kotake, Y.; Janzen, E. G. J. Phys. Chem. 1988,92, 3574. (27) Haering, G.; Luisi, P. L.; Hauser, H. J. Phys. Chem. 1988,92,3574. (28) Bruggeller, P. J . Phys. Chem. 1986, 90,1830. (29) Nakamura, H.;Baglioni, P.; Kevan, L.; Matsuo, T. J . Phys. Chem. 1991, 95, 1480. (30) Jahn, W.; Strey, R. J . Phys. Chem. 1988, 92, 2294. (31) Luisi, P. L.; Meier, P.; Imre, V. E.; Pande, A. In ref 8, p 335. (32) Chasteen, N. D.; Hanna, M. W. J . Phys. Chem. 1972, 76, 3951. (33) Langevin, D. In Strucrure and Reuctiuity in Reuerse Micelles; Pileni, M. P., Ed.; Elsevier: Amsterdam, 1989. (34) Safran, S. A.; Webman, I.; Grest, G. S.Phys. Rev. 1985, A32, 506.
Nltroxlde-Labeled Ruthenium(II)-Polypyrldyl Complexes as EPR Probes To Study Organized Systems. 1. Micellar Solutions and Miceiiization of Sodlum Alkyl Sulfates M. Francesca Ottaviani,+ Naresh D. Ghatli4t and Nicholas J. Turro*J Department of Chemistry, Columbia University, New York, New York 10027, and Department of Chemistry, University of Florence, 50121, Firenze, Italy (Received: January 28, 1992; In Final Form: April 3, 1992)
EPR probes that structurally resemble ruthenium(I1) trisphenanthroline complexes have been utilized to monitor the binding and dynamics of these complexes with different anionic detergents in aqueous solutions. The results and interpretation of these EPR experiments are compared with the results and interpretations of photophysical studies involving interactions of ruthenium(I1)-trisphenanthroline complexes and micelles. The EPR spectra have been evaluated in terms of both the hyperfine splitting (a polarity-sensitive parameter) and the rotational correlation time (a dynamics parameter). All experimentally recorded spectra could be successfully simulated as a single component or as the superposition of two components. Stronger binding of these probes is observed as the chain length of the detergent increases. For the same chain length, stronger interactions are observed for micelles containing a sulfate head group compared to a carboxylate group. The rotational diffusional coefficients obtained are found to correlate extremely well with the translational diffusion coefficients obtained in photophysical studies. Previously reported observations of the formation of premicellar aggregates at concentrations below the critical micelle concentration (cmc) have also been corroborated in this study.
Introduction Ruthenium-polypyridyl complexes have been extensively used for investigations of polyanionic microheterogeneous structures such as alkyl sulfate micelles,’ polymer solutions,2 DNA,’ and starburst dendrimers (SBD)? The most commonly used polypyridyl ligands are 2,2’-bipyridine (bpy) and 1,lO-phenanthroliine (phen). Previous studies have primarily involved photochemical measurements exploiting the well-understood luminescence properties of the Ru(I1) complexes;s both steady-state add time-resolved photoluminescence measurements using these complexes and different cationic quenchers have provided an insight into the cooperative phenomena occurring in these microheterogeneous systems, such as micellization6 or changes in surface morphology for the dendrimer system^.^ In experiments employing a probe to report on its environment it must always be considered that the probe may perturb its ‘Columbia University. *University of Florence.
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surroundings and therefore result in erroneous conclusions regarding the properties of the regions it is probing. The size of these probe molecules (about 18-Adiameter for Ru(phenhz’) is comparable with the size of some of the systems that have been studied (such as sodium alkyl sulfate micelles with seven or eight carbon atoms in the chain of the detergent or starburst dendrimers of generations 1 and 2). The oppositely charged nature of the probes (positive) and the environments (negative) ensures strong electrostatic attractive forces that will assist in binding; in the case of Ru(phen)32f and micelles, binding is enhanced by the hydrophobic forces.’ Hence, the interaction of the probe with the micellar or starburst surface may lead to a significant perturbation of the native system. Photophysical measurements have revealed several aspects of the interaction of Ru complexes with sulfate-terminateddetergents. For instance it has been found, from dynamic quenching experiments, that as the size of the micelle increases both the “unimolecular” quenching rate constants and the exit rates of these probes from the micelles decrease.“ Attempts to fit the quenching 0 1992 American Chemical Society
