Langmuir 1992,8, 1937-1942
1937
Electron Paramagnetic Resonance Lineshape Analysis of Small and Large Probes Introduced into Micellar Aqueous Solutions of Ammonium Pentadecafluorooctanoate Sandra Ristori and Giacomo Martini* Department of Chemistry, University of Florence, Via G . Capponi 9, 50121 Florence, Italy Received December 2, 1991. In Final Form: May 18, 1992
Aqueous micellar solutions of ammoniumperfluorooctanoate were studied by using EPR of paramagnetic probes with different characteristics. Small and large cationic probes (namely TempTMA+and CAT12) and long-chainneutral nitroxides (5-and 16-DXSA)were used to obtain structural and dynamicinformation. The analysis was mainly carried out in terms of the lineshapes and the hyperfine coupling constants of both experimental and simulated spectra. A large perturbing effect on the micellar system was observed with CAT12. TempTMA+and 5- and 16-DXSA gave details on the polarity and the motional properties of the micellar surface and in different regions of the micellar core, respectively. All of the long-chain nitroxides allowed us to further investigate the mutual solubility of perfluorinated and perprotiated surfactants.
Introduction Perfluorinated surfactants (PFS) have many properties in common with their perprotiated (PHS) analogues and a wide amount of literature exists on their physicochemical PFS form either micelles or lyotropic liquid crystals, depending on the concentration and the te.mperature.4-12 Although the formation of PFS micelles in water has been reported more than 30 years the information on the PFS micelle behavior is sparse and not so clear as for PHS micelles. However, because of the size of the fluorine atom, the fluorocarbon chain is less flexible than the hydrocarbon chain with the same carbon atom number?@ so that each CF2 roughly corresponds to 1.5 CH2 for the micelle f0rmation.l Moreover, in contrast with PHS, PFS are supposed to form nonspherical micelles whose exact shape is still the object of speculation.l5 As with other surfactants, PFS at high concentration turn from isotropic micellar systems to lamellar phases which are favored with respect to hexagonal or cubic 0nes.l' Electron paramagnetic resonance (EPR)spectroscopy has been used largely in the study of micellar and lamellar systems containing nitroxides as stable paramagnetic probes.16 Recently we carried out EPR studies on a perfluorinated polyether surfactant (PFPE) by using both (1) Shinoda, K.; Hatyo, M.; Hayashi, T. J.Phys. Chem. 1972, 76,909. (2) Asakawa, T.; Mouri, M.; Miyagishi,S.; Nishida, M. Langmuir 1989, 5, 343. (3) Hoffmann, H.; Kalus, J.; Thurn, H. Prog. Colloid PoZym. Sci. 1983, 262, 1043. (4) Fung, B. M.; Mamrosh, D. L.; ORear, E. A.; Frech, C. B.; Azfal, J. J. Phys. Chem. 1988,92,4405. (5) La Mesa, C.; Seeta, B. J. Phys. Chem. 1987,91,1450. (6) Guo, W.; Brown, T. A.; Fung, B. M. J. Phys. Chem. 1991,95,1829. (7) Mukerjee, P.; Gumkowski, M. J.; Chan, C. C.; Sharma, R. J. Phys. Chem. 1990,94,8832. (8) Tiddy, G. J. T. J. Chem. SOC.,Faraday Trans. 1 1972,68, 608. (9) Tiddy, G. J. T.; Wheeler, B. A. J. Colloid Interface Sci. 1974,47, 54. (10) Everise, E.; Tiddy, G. J. T.; Wheeler, B. A. J.Chem. SOC.Faraday Trans. 1 1976, 72,1747. (11) Fontell, K.; Lindman, B. J. Phys. Chem. 1983,87,3289. (12) Weber, H.; Hoffmann, H. Liq. Cryst. 1988, 3, 203. (13) Klevens, H. B.; Raison, M. J. Chim. Phys. 1954,51, 1. (14) Asakawa, T.; Mouri, M.; Miyagishi, S. Langmuir 1989, 5, 343. (15) Tiddy,G. J. T. InModern Trands of ColloidScience in Chemistry and Biology; Eicke, H. S., Ed.; Birkhauser: Basel, 1985; p 148. (16) Taupin, C.; Dvolaitaky, M. Insurfactant Solutions. New Methods
ofhuestigation; Zana, R., Ed.; Surfactant Science Series Vol. 22: Marcel Dekker: New York, 1987; p 359.
Q743-7463/92/2408-1937$03.00/0
small and large n i t r o ~ i d e s . ~This ~ J ~ has allowed us to establish some limits of application of the ESR technique to micellar solutions. In this paper we analyzed the ESR features of small and large nitroxides inserted in premicellar and micellar aqueous solutions of a typical perfluoroalkylcarboxylate, namely ammonium pentadecafluorooctanoate (NH4-PFO), which is known to give mic e l l e ~ ~ J Jand +~~ lamellar ~ h a s e 5 . lSince ~ ~ ~ micelles, ~ as rapidly fluctuating aggregates, can be considered as the precursors of liquid crystalline phases, we thought it necessary to discuss the EPR features in the micellar systems before approaching the lamellar-nematic phases both in the unoriented and oriented states, which will be discussed in a forthcoming paper.
Experimental Section Materials. Ammonium pentadecafluorooctanate,CF3(CF&COONH4, NHd-PFO, and n-perfluorooctane, CsFls, were purchased from Fluorochem, Ltd., Old Gossip, UK (purity >98%), and used as received. Solutions of the perfluorinated surfactant were prepared in to 1mol/L twice-distilledwater in the concentration range either by direct weighing or by dilution of more concentrated solutions. The nitroxides TempTMA+,CATlZ,B-DXSA,and 16-DXSA were used as purchased and without further treatment. TempTMA+and CAT12were dissolved in twice distilledwater in the appropriate amount to reach the desired concentration. For 5- and 16-DXSA,ethanol was used as the solvent; in this case, the solvent was accurately removed under a nitrogen flow before the addition of the surfactant aqueous solution into the probe tube. Measurements. Quartz cylindrical tubes of inner diameter -2 mm were used for bulk water and micellar solutions. EPR measurementswere performed on a Bruker Model 200D EPR spectrometer operating at the X-band (-9.5 GHz). Field calibration and magnetic parameter calculationwere carried out (17) Martini, G.; Ottaviani, M. F.; Ristori, S.; Lenti, D.; Sanguineti, A. Colloids Surf. 1990, 45,177. (18) Ristori, S.; Ottaviani, M. F.; Lenti, D.; Martini, G.Langmuir 1991,
-"--.
7 . )1QFiA
(19) Hoffmann, H.; Ulbricht, W. Z . Phys. Chem. (Miinich) 1977,106, 167. (20) Berr, S. S.; Jones, R. R. M. J. Phys. Chem. 1989, 93, 2555. (21) Burkitt, S. J.; Ottewill, R. H.; Hayter, J. B.; Ingram, B. T. Colloid Polym. Sci. 1987,265, 619. (22) Kunieda, H.; Shinoda, K. J. Phys. Chem. 1976,80, 2468. (23) Reizlin, K.; Hoffmann, H. Prog. Colloid Polym. Sci. 1984,69,83.
0 1992 American Chemical Society
Ristori and Martini
1938 Langmuir, Vol. 8,No. 8,1992 ( C H , ) & e - O
I---
4-trimelhyIammoni~2,2,6,6-lelramelhyl-l-piperidinybxy (TempTMA', Molecular Probe, Eugene, OR)
i
:1 6 5 1 4,4-dimelhyl-4-dodecylammoni~2,2,6,6-letramethyl-l -piperidinyloxy (CATl2, Molecular Probe)
= I
' 601
CH,-(CH,),-C-(CH2),-COOH '0 N '
-2
-3
0
-1 Log INH,-PFOI
m = 3,n = 12; 5-doxylstearic acid (5-DXSA) m = 14, n = 1; 16-doxylstearicacid (16-DXSA) (bdh lrom Sigma Chemicals, Munchen, Germany)
Figure 1. 14Nhyperfine coupling constant of TempTMA+ as a function of the logarithm of NH4-PFO concentration.
with the Aspect 2000 data handling system. Temperature was varied with the Bruker STlOO/7@I variable temperature assembly. The surface tension values were determined by using the DuNouy ring method. The same twice-distilled water as for the surfactant and probe solutions was used. Calculations. If not otherwise stated, all of the spectral simulations discussed in this work were performed by using the calculation procedure given by Schneider and Freed.24 This allowed us to obtain either the correlation times for the motion or the magnetic parameters of the probes.
Results and Discussion Figure 1 shows the variation a t 298 K of the 14N hyperfine coupling constant, AN,of a mol/L TempTMA+ solution as a function of the NH4-PFO concentration. Almost constant AN values of 1.68mT were obtained for mol/L; AN then [NHcPFO] increasing up to 2 X abruptly decreased to 1.645mT in the concentration range 3X to 0.1 mol/L. A further small decrease down to 1.635 mT was observed with increasing NH4-PFO concentration up to 1.0 mol/L. The AN decrease is a clear indication of a decreased polarity of the environment sensed by the -NO paramagnetic and this is often taken as a proof of micelle onset when observed in surfactant solution^.^^^^ Thus, from the AN data of Figure 1, a critical micellar concenmol/L was calculated. tration (cmc) of 2.5 (f0.2) X This value waa confirmed by surface tension measurements at 298 K (Figure 2). The surface tension decreased with the increase of the surfactant concentration down to values of 20-22 dyn/cm, significantly lower than those showed by usual hydrocarbon surfactants, with a sharp disconmol/L exactly. This tinuity at [NHd-PFOI = 2.5 X completely agrees with surface tension vs concentration plots previously reported for perfluorinated carboxylates and sulfonate^.^-^^^^*^^ The addition of TempTMA+ with resultant [surfactantl/[probel ratios R > 50 did not (24) Schneider, D. J.; Freed, J. H. In Biological Magnetic Resonance. Vol. 8. Spin Labeling: Theory and Applications; Berliner, L. J., Reuben, J., Eds.; Plenum Press: New York, 1989; p 1. (25) Janzen, E. G. Top. Stereochem. 1971,6, 117. (26) Yoshioka, H. J. Am. Chem. SOC.1979, 101, 28. (27) Ottaviani, M. F.; Baglioni, P.; Martini, G. J. Phys. Chem. 1983, 87, 3146. (28) Baglioni, P.; Ferroni, E.; Martini, G.; Ottaviani, M. F. J. Phys. Chem. 1984,88, 5187. (29) Baglioni, P.; Ottaviani, M. F.; Martini, G. J.Phys. Chem. 1986, 90,5878. (30) Mukerjee, P.; Mysels, K. J. In Colloidal Dispersion and Micellar Behauior; Mittal, K. L., Ed.; ACS Symposium Series No. 9; American Chemical Society: Washington, DC, 1975; p 239.
-3
I
1
-2
-1
L o g [NH,-PFOI
Figure 2. Behavior of the surface tension of NHd-PFO in water
as a function of the concentration: (A)pure NH4-PFO; (0) NHdPFO in the presence of 10"' mol/L CAT12.
introduce any appreciable change in the value of the y discontinuity. The calculated cmc of 2.5 X mol/L was in line with those reported for perfluorinated carboxylates and carboxylic acids, including perfluorooctanoate salts and perflurooctanoic acid.5JJ9*21-31*32For instance, very recently Guo et al.: using l9F NMR, obtained a cmc of 3.2 (f0.2) X mol/L for NH4-PFO in D20 at 298 K. A cmc of 3.3 X mol/L is also reported for NH4-PFO by Kunieda and ShinodaZ2from solubility data at the Krafft point (TK= 275.5 K) and a somewhat lower value of 9.6 X 10-3 mol/L is calculated by Burkitt (thesis quoted in ref 21) in NH4CUNH40H buffer solution at pH 8.8. The same AN trend was observed by varying the nitroxide concentrations in the range 2 X to 2 X mol/L with resultant R ratios from 50 to 20 000. On the basis of the discussion on the reliability of the EPR technique in the study of micellar systems reported in ref 18,the above finding ensured that TempTMA+was a good probe for the aggregational properties of NHd-PFO in solution. The EPR lineshape of TempTMA+ in PFO solution below cmc was typical of the free nitroxide in a (31) Hoffmann, H.; Ulbricht, W.; Tageeson, 2.2.Phys. Chem. (Miinich) 1978,113, 17. (32) Hoffmann, H.; Tagesson, 2.2.Phys. Chem. (Miinich) 1978,110, 113.
ESR of Ammonium Perfluorooctanoate Micelles
Langmuir, Vol. 8, No.8,1992 1939
Figure 3. Experimental (full line) and computed (dashed line) EPR spectra of moVL TempTMA+ in 0.4 mol/L aqueous solution of NH4-PFO (2' = 298 K).
pure aqueous environment (AN = 1.68 mT in water17). The correlation time for the motion was calculated to be 2 X lo-" 8, exactly as in water.17 This ruled out the interactions of the nitroxide with PFO- monomer anion or with premicellar aggregates. After the cmc was reached, the lineshape changed (Figure 3) and it closely resembled that given by the same nitroxide in perfluorinated polyether (PFPE) solution at a concentration well above its cmc,17which is attributed to the interaction between the positive -N(CH& moiety of the probe and the negative surface of locally ordered, anisotropic PFPE micelles. The same lineshape is also observed in several oriented systems such as in, for instance, Tempamine-doped DPPC bilayers oriented in a magnetic field.33 Exactly as with PFPE, the computer simulation was only possible by taking into account a much faster motion along a u axis directed along the fluorocarbon chain than in its perpendicular plane, thus assuming a microscopic order in the system, which might be defined in terms of an order parameterM
b)
d A Figure 4. Sketch of the structure of TempTMA+-PFO micelle (a) and of CAT12-PFO micelle (b).
A
S = 1/2(3CX,2... - 1) = (All' - A,')/(Az - A,) where ai is the time averaged director cosines of the u axis with respect to the principal axes and the S parameter reflects the molecular order in the immediate environment of the -N-O bond only.u The best fit of the computed spectrum with the experimental one was obtained with the following parameters: g,, = 2.0098
gyy= 2.0075
A,, = A, = 0.68 mT All'= 2.617 mT
= 9 x lo-"
A,, = 3.59 mT A,' = 1.162 mT
S = 0.5 (7)
gzz = 2.0037
T2,0= 0.15 mT 8
N =D p , = 5
Physically, the above assumption meant a strong interaction between the -N+(CH& group of TempTMA+ and hydrated -COO-of the PFO anion. It should be pointed out that the correlation time value used in this simulation did not truly reflect the reorientational dynamics of the system. In fact, the effective coupling constants A,' and All', which took into account the order parameter with respect to the u axis gave rise to a smaller magnetic anisotropy to be mediated by the tumbling motion and hence to a somewhat shorter T value. The actual structure of the aggregate could be represented as sketched in Figure 4a), where the partially ordered arrangement of the nitroxide -N-O groups accounted for the relatively high (33) Meirovitch, E.J . Phys. Chem. 1983,87, 845. (34)Freed,J.H. InSpinLabeling. TheoryandApplicatione;Berliner, L. J., Ed.; Academic Press: New York, 1976; Vol. 1, p 85.
& Figure 5. Experimental (full lines) and computed (dashedlines) EPR spectra of lo-"moVL CAT12 in NH4-PFO aqueoussolutions at various surfactant concentration(T= 298 K): (a) [N&-PFO] = 0; r = 6 X 1O-l1s; T2,o = 0.07 mT; (b) [NK-PFO] = 5 X 10-3 moVL, broad component, w, = 2 X 108 Hz, P = 0.75; (c) [NH4mol/L, w e d = 4 X 107 Hz 7 = 3.7 x 10-10 PFO] = 2 X 8, rgz= 2.5 X 10-lOs; T2,o= 0.16 mT; (d) [NHI:Pyfi = 0.1 mol/L; ( 7 ) = 3 X 10-lO s; T*,o= 0.08 mT.
motional anisotropy in a fashion similar to that reported for Tempo-stearamide, for which very similar spectra are observed.3s EPR spectra with almost the same AN (1.64 mT) as with TempTMA+were given by the positive, bulky CAT12 probe when used with surfactant concentration higher than (4-6) X mol/L, i.e. above the cmc of pure Nh-PFO. The CAT12 probe itself is able to form micelles at a cmc of 7.1 x 10-3 mol/L at 295 K,M which is a higher concentration than those used in this work (2 X to 4 x mol/L). However, more complex spectral shapes were observed near or below the surfactant cmc. Figure 5 shows the (35) Schreier-Mucillo, S.;Marsh, D.; Schneider, H.; Smith, J. C. P.
Chem. Phys. Lipids 1973, I O , 11.
(36) Fox, K.K. Tram. Faraday SOC. 1971,67,2802.
Ristori and Martini
1940 Langmuir, Vol. 8, No. 8,1992 EPR spectra of a 5 X 10-5 mol/L CAT12 water solution in bulk water (spectrum a), in solution below and near cmc of pure NH4-PFO (spectra b and c, respectively) and well above cmc (spectrum d). Spectrum b was clearly due to the superposition of absorptions arising from CAT12 in different chemical and magnetic situations with different motional properties. In particular, at least the following three cases can be recognized from the spectra reported in Figure 5: (i) CAT12 molecules free to move in a pure aqueous environment which were responsible for spectrum a and, in part, for the narrow triplets of spectrum b; (ii) CAT12 molecules magnetically diluted in the NH4-PFO micelles and responsible for spectrum d; (iii)magnetically interacting CAT12 molecules responsible for the broad spectrum c and for the broad absorption underlying spectrum b. The computation of spectra responsible of species i and ii was straightforward and good fittings were achieved with the following parameters:
A
n
Species i g,, = 2.0080
A,, = 0.68 mT
T2,0= 0.07 mT
g, = 2.072
g,, = 2.0035
A,, = 3.57 mT
A, = 0.82 mT T,,
= T,, = T,, = 6 X lo-" s
Species ii gxx= 2.0089
A,, = 0.67 mT
g, = 2.0073
g,, = 2.0036
A,, = 0.79 mT
A,, = 3.48 mT
T2,0= 0.08 mT N = D,,/Dl = 1.5
(7)=
3X
lo-''
In spite of the presence of a long chain, the diffusional reorientation of the -NO group of species i, i.e. CAT12 in bulk water, was well reproduced without anisotropy. A small anisotropy was necessary for the spectrum ii simulation, as shown by the anisotropic parameter N = 1.5 used in the calculation. N for spectrum ii was smaller than N = 5 used for TempTMA+ in the same micelles (see above). No order parameter was necessary to account for the CAT12 spectrum in the micellar phase. The flexibility of the Cl2 chain in CAT12 prevented the partial ordering suggested for TempTMA+(Figure 4b). Moreover, because of steric constraint, a weaker interaction of the bulky nitroxide with the hydrated negative micelle surface contributed to a more isotropic motion with a higher timeaverage of the magnetic anisotropy. Although spectra c and d were due to the same species ii, spectrum c was broader than spectrum d because of Heisenberg spinexchange with a characteristic exchange frequency wexch. It was reproduced with the same magnetic and motional parameters as species ii with an added Wexc 4 X lo7Hz. This meant that shorter electron-electron distances occurred: the NH4-PFO micelles did therefore contain more than one CAT12 molecule per aggregate. Finally, spectrum b was calculated as the superposition of the species i and the exchange broadened species ii (Welch = 2 X lo8 Hz), with relative contributions defined through an adjustable parameter PIi.e. the fractions of each absorption ranging from 0 to 1,which were 0.25 and
& Figure 6. EPR spectra of CAT12 at different concentrationsin a 2 X 10-3 mol/L aqueous solution of NHd-PFO. 0.75 for species i and ii, respectively. Both Wex& and P indicated the presence of molecular assemblies of NH4PFO in which CAT12 was relatively concentrated, at a concentration lower than the NH4-PFO cmc. As a proof, surface tension measurements of NH4-PFO solutions containing CAT12 gave very low y values even at [NHp PFOI < cmc. This is shown in Figure 2, where the surface tensions of NHcPFO in the presence of lo4 mol& CAT12 are also reported. We did not try to find the shifted value of cmc as a consequence of the addition of the large spin probe, because of its dependence on the probe concentration. We were indeed more interested to only show that the presence of an added long-chain probe strongly affected the surface behavior of NHd-PFO than to find new cmc values. The relative fractions of the species contributing to spectra like the signal 5b depended on both the surfactant and probe concentration, in the sense that at fixed [CAT121 the fraction of the broad, unstructured signal decreased with [NHd-PFOI and at fixed [NHd-PFOI this fraction increasedwith the increase of [CATlB]. The lack of linewidth variation for one set of peaks was a demonstration that the line broadening of the other set was indeed due to Heisenberg spin-exchange rather than chemical exchange of the two different species. This is shown, for instance, in Figure 6 which shows the EPR spectra of the to 4 X lo4 mol/ probe in the concentration range 2 X mol/L NHd-PFO solution. L dissolved in a 2 X The above finding sheds more light on the interpretation of the EPR data, particularly when the general behavior of the perfluorinated surfactant in the presence of perprotiated Surfactants or of counterions with hydrocarbon chains was taken into account. It is widely demonstrated in the literature that mixtures of PHS and PFS do not show ideal b e h a ~ i o r . Mukerjee ~ ~ ~ ~ ~ and My~els,~O in particular, explain the anomalous features of PHS and PFS mixtures or of partially fluorinated surfactants in terms of mutual phobicity of the hydrocarbon and fluorocarbon chains. This leads to a very scarce tendency to form mixed micelles. On this basis and from conduc(37) Shinoda, K.;Nomura, T.J. Phys. Chem. 1980,84, 365.
ESR of Ammonium Perfluorooctanoate Micelles tivity and relaxation data on PFS anions with varying alkylammonium counterions, Hoffmann and co-workers3l932 suggest that only electrostatic attractive forces are expected between micelle surface and counterions which strongly interact with each other to give a cohesive surface layer. The hydrophobic part of the counterions remains in contact with water without being inserted in the micelle interior. This model fits the spin exchange effects well if CAT12 and NH4+ are considered to be in competition with each other in the micelle, whose negative surface is increasingly covered with NH4+ ions as the R ratio increases. Let us now discuss the results obtained with doxylstearic acids. A large amount of literature exists on the use of DXSA nitroxides for the study of dynamic and structural characteristics of ordered systems such as bi- and m u l t i b i l a y e r ~ membrane~,38-~3 ,~~~ and lyo- and thermothropic liquid ~ r y s t a land s ~in~dispersed ~ ~ ~ ~systems such as micelles*~~~ and vesicles.4g51 The two probes 5- and 16-DXSA nitroxides were chosen in order to have the NO paramagnetic unit in different regions, when inserted in an ordered aggregate. Both the nitroxides used in this work had a relatively low water solubility. Large differences were observed in the lineshapes by passing from bulk water to micellar solution. Figures 7 and 8 show the EPR spectra of 5- and 16-DXSA in pure water and in 0.4 mol/L PFO solution as compared with the calculated ones. Both nitroxides moved with an almost isotropic motion ( T = 8 X lo-" and 5 X 10-l' s, respectively) in pure water. An isotropic, although slower ( T = 2.3 X 10-lO s), motion was retained by 16DXSA when in the micelles, whereas the 5-DXSA spectrum in the micelles could be only fitted by assuming a small anisotropy in the rotational diffusion coefficients (N= 21, with ( T ) = 1.1 X s. The longer ( 7 ) of 5-DXSA with respect to 16-DXSA might be produced by the doxyl groups in a different location inside the micellar aggregate, for instance, a more hindered region, i.e. nearer to the surface, for 5-DXSA, and a more fluid region, i.e. inside the hydrophobic core, for 16-DXSA. Moreover, the A Nof 16DXSA was almost constant a t 1.585-1.580 mT up to [NH4PFO] 2 X mol/L, then it slowly decreased at 1.561.55 mT at [NHd-PFO] 1 0.1 mol/L. The same trend was given by 5-DXSA. The A NValues of both 5- and 16-DXSA in the N&-PFO solutions below the cmc can be interpreted as for TempTMA+ on the basis of no appreciable interactions between probe and NHd-PFO monomer or premicellar aggregates. However, at the onset of the micelle formation, the A N decrease was not as large as would be expected from -NO groups inserted in decidedly more hydrophobic environments as the two supposed regions
Langmuir, Vol. 8, No. 8, 1992 1941
a Figure 7. Experimental(fulllines) and computed (dashedlines) EPR spectra at 298 K of 5-DXSA (a)in bulk water (concentration 2 X 10-6mol/L),T = 8 X lo-" s, T2,o = 0.05 mT; (b) in a 0.4 mol/L mol/L, ( T ) aqueous solution of NH4-PFO, [5-DXSA] = 5 X = 1.1 X lo+' a, Tz.0 = 0.1 mT.
-
~
&!!E., Figure 8. Experimental (full lines) and experimental (dashed lines) EPR spectra at 298 K of 18DXSA (a) in bulk water ([16= 0.05 mT; (b) DXSA] = 2 X 10-6 mol/L), 7 = 5 X 10-11 a, TZ,O in a 0.4 mol/L aqueous solution of NHrPFO, [l&DXSA] = 5 X 10-6 mol/L, T = 2.3 X 10-lo a, TZ,O = 0.05 mT.
~~
(38) Hemminga, M. Chem. Phys. Lipids 1983,32, 323. (39) Meirovitch, E.; Freed, J. H. J.Phys. Chem. 1980,84,3281. (40)Freed,J. H. InRotationul Dynamics of Small and Macromolecules; Dorfmtiller, Th., Pecora, R., Eds.; Lecture Notes in Physics, Vol. 293, Springer Verlag: Berlin, 1987; p 89. (41) Fung, L. W.-M.; Johnson, M. E. In Current Topics in Bioenergetics; Lee, C. P., Ed.; Academic Press: New York, 1984; Vol. 13, p 107. (42) Volwerk, J. J.; Griffith, 0. H. Magn. Reson. Rev. 1988, 13, 135. (43) Hubbell, W. L.; McConnell, H. M. J. Am. Chem. SOC.1971,93, 314. (44) Lasic, D. D. J. Phys. 1983, 44,737. (45) Broido, M. S.; Meirovitch, E. J . Phys. Chem. 1983,87, 1635. (46) Wikander, G.; Johansson, L. B.-A. Langmuir 1989,5, 728. (47) Bales, B. L.; Kevan, L. J. Phys. Chem. 1982,86,3836. (48) Stuhne-Sekalec, L.; Stanacev, N. Z. Chem. Phys. Lipids 1988,48, 1. (49) Lohman, W.; Kiefer, B.; Schmehl, W. 2.Naturforsch. 1986,41c, 799. (50) Ma, L. D.; Magin, R. L.; Bacic, G.; Dunn, F. Biochim. Biophys. Acta 1989,978, 283. (51) Bonosi, F.;Gabrielli,G.;Margheri, E.; Martini, G. Langmuir 1990, 6, 1769.
of localization are. The A N values in NH4-PFO micelles were also higher than in SDS micelles26 for which a penetration of water molecules is suggested. Water penetration has also been proved by Miiller and Birkhahn,52%~ing19FNMR in micellesof two fluorine-labeled soaps. The fact that the -NO group of the positively charged TempTMA+ and CAT12probes which reside near the negative surface micelle in the water side of the water/ micelle interface gave A N variations larger than or comparable with those of 5- and 16-DXSA is a further source of ambiguity. The actual situation might therefore be represented by the following two different cases: (i) the doxyl units were not localized inside the micelles, but on the water-PFO (52) Muller, N.; Birkhahn, R. H. J. Phys. Chem. 1968, 72,583. (53) Muller, N.; Birkhahn, R. H. J . Phys. Chem. 1967, 71,957.
1942 Langmuir, Vol. 8, No.8,1992 surface layers; (ii) the doxyl units were actually localized inside the expected regions of the micelles but they maintained at least a fraction of their hydration sphere, thus sensing an environment only slightly less polar than in pure water. Situation i contrasts with the large differences in the motional properties of the doxyls in water and in the micellar solution and, even further, with the large differences in the motional rate between 5- and 16-DXSA which only agrees for an environment with restricted and fluid motions, respectively. The values of the correlation times calculated did reflect significant differences in the microviscosity around the -NO groups which cannot be explained by assuming a surface location. Moreover, if the -NO groups of 5- and 1SDXSA were located at the surface this would imply a bent structure for 18DXSA with resultant mobility opposite to the observed mobility. On the other hand, situation ii seemed to contrast with the asserted very scarce mutual solubility (see above).3O However the presence of a few molecules of water for each DXSA molecule into the NHrPFO micelles could be enough to allow the doxyl penetration into the micelle network. As a proof, preliminary runs carried out by electron spin echo (ESE) spectroscopy54 confirmed (54) To be published.
Ristori and Martini the presence of water molecules (as DzO) bonded to 5DXSA and 16-DXSAinto micellar and lamellar phases of NH4-PFO.
Conclusion The results reported in this work on aqueous solution of a perfluorinated surfactant show that the use of EPR of spin probes to study the microenvironment of micellar systems represents a simple yet valuable method for obtaining a considerableamount of information provided that the probes do not significantlyalter the system under study. With NH4-PF0, as previously published for PFPE, this information is both comparable and complementary to those obtained with other methods when the small and positively charged TempTMA+and the neutral long-chain doxyl nitroxides are used. The charged long-chain CAT12 acts as strongly perturbing molecule. Thus, its use could be useful in proving the peculiar mutual behavior of perfluorinated and perprotiated compounds but without giving valuable information on the pure PFS micelles. Particular care must therefore be devoted to the choice of the paramagnetic probe.
Acknowledgment. Thanks are due to National Council of Researche (CNR), Italian Minister0 dell'universita' (MURST), and Montefluos SpA for financial support.
