J. Phys. Chem. 1984,88, 1720-1725
1720
Magnetically Aligned Surfactants. An Electron Spin Resonance Study B. J. Forrest*+and J. Mattai Chemistry Department, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3 (Received: January 31, 1983; In Final Form: July 19, 1983)
ESR spectra for various nitroxide spin probes are consistent with cylindrical micelle and disk-shaped micelle structures for magnetically aligning lyotropic liquid crystals based on potassium laurate, sodium decyl sulfate, or decylammonium chloride. Amphiphile rotation about the cylinder long axes results in additional averaging of the hyperfine splitting tensor. Oscillation of the micelles about the director of the mesophase, while rapid on the ZHNMR time scale, is slow on the ESR time scale. Line shapes for disk micelle systems based on various surfactants indicate changes in the average size and/or size distribution of the micellar units.
Introduction Certain lyotropic liquid crystals spontaneously align when subjected to an applied magnetic field. The amphiphile chains reorient such that their long axes are, on the average, perpendicular to the external field.’ If the micelle shape is cylindrical, the cylinder long axes are therefore parallel to the field direction but may oscillate about it. If, however, the micelles are of bilayer type, such as disk-shaped micelles or fragments of a lamellar structure, the bilayer normals are, on the average, perpendicular to the field. These two classes of surfactant systems have been designated type I C M and type 11 DM, where types I and I1 indicate positive and negative diamagnetic anisotropy, respectively, and the designation C M and DM denote cylindrical micelles and disk-shaped micelles.2 Type I DM systems also exist in which the bilayer normals and the hydrocarbon chains are parallel to the external field.3 Their preparation has been accomplished by the inclusion of an aromatic amphiphile into the disk micelle bilayers. Thus, it is possible to prepare bilayer structures whose magnetic susceptibility anisotropy may be manipulated to produce alignment either parallel or perpendicular to the applied magnetic field. Spin-labeled fatty acid derivatives intercalate into bilayer structures such that the nitrogen 2 p r orbital which contains the unpaired electron is nearly parallel to the chain long axis.44 By contrast, the steroid spin probe, cholestane, is incorporated such that this orbital is nearly perpendicular to the bilayer normal.es The structures of disk micelles, cylindrical micelles, and the fatty acid and cholestane spin probes are shown in Figure 1. Use of fatty acid and steroid spin probes allows qualitative monitoring of the average orientation of the orbital containing the unpaired electron. The orientation of the director with respect to the field is most easily determined via ZHN M R of samples prepared with DzO, while the orientation of the chain long axes and the interface may be followed by ESR of the appropriately doped lyotropic liquid crystal^.^ As well, since the alignment time but not the degree of alignment of the micelle units varies inversely as the square of the applied magnetic field strength, the rates of alignment and realignment upon rotation at field strengths of 0.33 T are slow enough to allow a complete rotational p r ~ f i l e . ~ - ’ ~ On the basis of structural models of the different classes of liquid-crystal systems? it is possible to predict the orientation of the z axis of the nitroxide spin probe in an aligned sample, Le. whether parallel or perpendicular to the field or perhaps randomly distributed in a plane which is parallel or perpendicular to the field. Therefore, it may be determined whether the proposed structures are consistent with the predicted ESR spectra. The structures of these types of lyotropic liquid crystals have been implied from their relationship to lamellar and hexagonal phases and from a small number of low-angle X-ray diffraction experiments. There is agreement that type I CM lyotropics based on decyl sulfates or laurate are composed of finite cylindrical TNSERC University Research Fellow.
0022-3654/84/2088- 1720$01.50/0
micelles of bilayer thickness whose undetermined length exceeds 500 (For a diagramatic representation of this system, see ref 9, Figure 1.) However, no type I DM and only one type I1 DM phase based on the amphiphile sodium decyl sulfate have been studied by the low-angle X-ray techniques. One studyi5proposed the presence of disk-shaped micelles of bilayer thickness greater than 1000 A in diameter with an average intermicelle distance of 80-140 A. A second report13 again proposed the presence of disk micelles but suggested a disk diameter of only approximately 70 A. It is most probable that the former interpretation is correct since very small micelles might be expected to tumble isotropically and would certainly be expected to exhibit very large “edge” effects.I6 It has previously been shown that disk micelles may be prepared in equilibrium with a lamellar phase and cylindrical micelles in equilibrium with a hexagonal phase.16i8 Since the finite micelles oscillate about the director rapidly on the ZHN M R time scale, much lower order parameters are obtained for the deuteriumlabeled hydrocarbon chains of the finite micelles.16 It is shown here that this oscillation of the whole micelles is slow on the ESR time scale, with very similar order parameters being observed in the aligned micelle phases and the parent nonaligning systems. A.93’3s14
Experimental Section Cholestane, potassium 4-doxyllaurate (4-KL), potassium 6doxyllaurate (6-KL), potassium 8-doxyllaurate (8-KL), and potassium 10-doxyllaurate (1 0-KL) were synthesized from the
(1) B. J. Forrest and L. W. Reeves, Chem. Rev., 81, 1 (1981). (2) K. Radley, L. W. Reeves, and A. S . Tracey, J . Phys. Chem., 80, 174 (1976). (3) B. J. Forrest, L. W. Reeves, and C. J. Robinson, J . Phys. Chem., 85, 3244 (1981). (4) S . P. Van, G. B. Birrell, and 0. H. Griffith, J . Mugn. Reson., 15,444 (1974). (5) P . C. Jost and 0. H. Griffith, Methods Enzymol. 49, 369 (1978). (6) J. Seelig and H. Limacher, Mol. Cryst. Liq. Cryst., 25, 105 (1974). (7) I. C. P. Smith in “Electron Spin Resonance, Elementary Theory and Applications”, McGraw-Hill, New York, 1974, p 483. (8) B. J. Gaffney and S.Chen, Methods Membr. Biol., 8, 291 (1977). (9) B. J. Forrest and L. W. Reeves, Chem. Phys. Lipids, 32, 73 (1983). (10) F. Y. Fujiwara and L. W. Reeves, Can. J . Chem., 56, 2178 (1978). (1 1) F. M. Leslie, G. R. Luckhurst, and H. J. Smith, Chem. Phys. Lett., 19, 345 (1973). (12) R. Casini, S.Faetti, M. Martinelli, and S . Santucci, J. Magn. Reson., 26, 201 (1977). (13) J . Charvolin, A. M. Levelut, and E. T. Samulski, J . Phys., Lett. (Orsay, Fr.) 40, L-587 (1979). (14) A. M. Figueiredo Neto and L. Q.Amaral, Mol. Cryst. Liq. Cryst., 74, 109 (1981). (15) L. Q. Amaral, C. A. Pimentel, M. R. Tavares, and J. A. Vanin, J . Chem. Phys., 71, 2940 (1979). (16) B. .I.Forrest and L. W. Reeves, Mol. Cryst. Liq. Cryst., 58, 233 (1980). (17) F. Y.Fujiwara and L. W. Reeves, J. Phys. Chem., 84, 653 (1980). (18) D. M. Chen, F. Y.Fujiwara, and L. W. Reeves, Can. J. Chem., 55, 2396 (1977).
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 9, 1984 1721
Magnetically Aligned Surfactants
I
D
IV
Ill
t
A,,=
32G
Ayy= 6 G
OK . Ax, = 6G
Ayf---
=6G
I
I I
' / I
3
6-doxyllaurate
C ho I e stane
Figure 1. (I) Representation of a disk micelle oriented such that the bilayer normal and local director are perpendicular to the applied field (type I1 behavior). For a type I disk-micelle system the bilayer normals align along the magnetic field. (11) Representation of a section of a cylindrical micelle aligned such that the director (symmetry axis) is parallel to the external field (type I behavior). The external field is along the z axis of the laboratory frame of reference (see Figure 2). (111) Structure of potassium 6-doxy1 laurate. The orbital containing the unpaired electron is nearly parallel to the molecular long axis. (IV) Structure of the cholestane spin probe. The orbital containing the unpaired electron is nearly perpendicular to the molecular
long axis. corresponding ketones as described p r e v i ~ u s l y . ~Sodium ~ ' ~ ~ ~decyl ~ sulfate (SDS), decylammonium chloride (DACl), potassium laurate (KL), and potassium (hepty1oxy)benzoate (KHB) were prepared and purified as previously d e s ~ r i b e d . ~ ~ 'The ~ * ~ratio * of labeled amphiphile to nonlabeled amphiphile was in the range of 1:200-1:2000 to ensure solubility of the spin probe in the micelles and to minimize the possibility of spin label-spin label interactions. The standard composition of the samples were by weight as follows: (A) KL:KC1:D20 = 680:47:1300 (type I CM), (B) KL:KCl:n-decanol:D,O = 680:47:140:1300 (type 11 DM), (C) KL:KCl:n-decanol:KHB:DzO = 436:47:140:282:1300 (type I DM), (D) SDS:n-decanol:D,O = 756:151:1200 (type I CM), (E) SDS:n-decanol:Na2SO4:D,O = 756:153:43:1200 (type I1 DM), (F) DACl:NH4Cl:D20 = 740:80:1200 (type I1 DM). In certain cases, more or less of the aqueous component was used in order to study the effect of dilution upon the liquid crystals and to probe the lamellar or hexagonal phases which are formed at lower water contents. For ESR experiments, the samples were contained in capillaries to minimize dielectric loss. These in turn were mounted in 5-mm N M R tubes. ESR spectra were recorded on a Varian E109-B Century Series ESR spectrometer at 22 OC. Spectra were recorded immediately upon placing the sample in the field, followed by alignment without spinning in a magnetic field of 0.7 T in the ESR spectrometer unless otherwise specified, for several hours or until no further change occurred in the spectral shape and hyperfine splitting. Samples were not routinely spun during the alignment process in the iron magnets since spinning of a type I C M system results in a planar distribution of directors, while spinning of a type I1 DM system leads to no spectral changes on sample rotation. Reorientation of the mesophase directors at 0.33 T is sufficiently slow to allow the observation of the complete angular dependence of the ESR spectra. In certain cases, the samples were prealigned with spinning in a magnetic field of 1.54 T or at 8.45 T in a superconducting solenoid where the magnetic (19) J. F. W. Keana, S. D. Keana, and D. Beatham, J. Am. Chem. SOC., 89, 3055 (1967). (20) W. L. Hubbell and H. M. McConnell, J. Am. Chem. SOC.,93, 314 (1 97 1). (21) L. W. Reeves and A. S. Tracey, J. Am. Chem. Soc., 97, 5729 (1975).
field is parallel to the tube long axis. ESR order parameters for the chain-labeled species describe the time-average excursions of the chain long axes. These were calculated from the aligned spectra before rotation (0' orientation) for micelles which align with their hydrocarbon chains perpendicular to the applied field by using the equation8,22 S=
Txx+ Tyy+ Tzz - 3To Tzz
-M T X X +
(1) Tyy)
where To is the observed splitting before sample rotation. For micelles which align such that the hydrocarbon chains align parallel to the field, the value upon rotation by 90' was used. In the case of the cholestane spin probe, the nitroxide 2pr orbital is very nearly orthogonal to the long molecular axis; the equivalent expression is TxX
+
S = Tyy
-
Tyy
+ T z z - 3To
M T X X
+ Tzz)
(2)
In the case of decylammonium chloride bilayers, the ESR spectral line shapes more closely resembled a polycrystalline powder even though complete alignment was indicated by deuterium NMR. Therefore, order parameters were calculated from the values of 2Tlland 2T,.27 An order parameter of unity would be expected for perfect order. Deuterium N M R spectra were recorded on a Nicolet 360 N B N M R spectrometer.
Results The ESR spectrum of 4-KL aligned in a potassium laurate type I1 DM bilayer is shown in Figure 2A along with that obtained by rotation by 90' about the tube long axis. The hyperfine splitting at 0' is 12.45 G and increases to a maximum upon rotation (see Table I), indicating an initial alignment such that the chain long axes are perpendicular to the applied field. The general shape of the spectra at all angles is unchanged by the (22) S. Schreier-Muccillo, D. Marsh, H. Dugas, H. Schneider, and I. C. P. Smith, Chem. Phys. Lipids, 10, 11 (1973).
1722
The Journal of Physical Chemistry, Vol. 88, No. 9, 1984
Forrest and Mattai
I 2
4
G CLIAIN
8
L 2
I 0 1 2
L
G CHAIN
POS1710N
8
1
0
1
2
POSITION
Figure 3. (A) Order parameter vs. position of the spin probe for potassium laurate type I1 DM systems (composition B) as determined by *H NMR (0)and ESR (0). The filled point is that obtained for the cholestane spin label. (B) As in (A) for the type I DM system of composition
C. Figure 2. (A) ESR spectrum of 4-KL in a potassium laurate type I1 DM system of composition B after alignment and following rotation by 90° about they axis of the laboratory frame. The 2p7r orbital containing the unpaired electron is approximately colinear with the bilayer normals of the individual micelles. Hyperfine splittings were measured as the distance between the zero-point crossings of the low- and midfield resonances. (B) ESR spectra of 6-KL in a potassium laurate type I1 DM system of composition B. (C) Laboratory frame of reference in which the z axis is along the magnetic field of a conventional electromagnet and they axis is coincident with the tube long axis. The diagram shows the alignment of the bilayer normals of a type I1 DM phase in the xy plane perpendicular to the field. Due to wall effects the preference is for alignment along the x axis. (D) Change in orientation of the bilayer normals of a type I1 DM system upon rotation of the tube by 90" about they axis.
A
A . Type I1 DM
orientation label
4-KL 6-KL 8-KL 10-KL
cholestane
0"
15"
30"
45"
60"
75"
90"
12.45 13.80 15.22 15.50 17.16
12.88 14.12 15.25 15.49 16.89
13.79 14.65 15.23 15.36 16.40
14.73 15.12 15.23 15.26 15.67
15.36 15.34 15.23 15.20 15.00
15.61 15.49 15.23 15.06 14.22
15.66 15.50 15.23 15.05 14.04
B. T y p e l D M
orientation label
4-KL 6-KL 8-KL
10-KL cholestane
0" 17.15 16.20 15.30 15.00 11.66
15" 16.93 16.04 15.28 15.00 12.24 C.
30"
45"
16.38 15.44 15.76 15.31 15.25 15.23 15.05 15.18 13.62 15.26 TypelCM
60"
75"
90"
14.09 14.54 15.08 15.29 16.51
12.65 13.70 15.05 15.31 17.20
12.64 13.41 15.05 15.39 17.35
75"
90"
orientation label
0"
15"
30"
45"
60"
cholestane 17.74 17.45 16.79 16.00 15.20 14.71 14.55 addition of approximately 15% more of the aqueous component. The aligned ESR spectra are fairly broad, and upon rotation a second high-field line becomes apparent, corresponding to chain axes which are parallel to the field as well as the presence of others more perpendicular to the field. These are not interconverting rapidly on the ESR time scale. On initial alignment the bilayer normals are distributed in a plane perpendicular to the magnetic field. If the sample is aligned in the absence of spinning, the preferred angle in this plane is with the bilayer normals also perpendicular to the tube along axis.'" This glass interface effect is desirable since it allows observation of the hyperfine splitting produced when the director is parallel to the external field for type I1 samples. Prior to alignment, the spectra are quite similar to
90'
Aligned
'0
" J
TABLE I: Hyperfine Splittings (Gauss) for Various Spin Probes
in Potassium Laurate Systems
Rotated
I
4
9oo
Figure 4. (A) ESR spectra of cholestane in a potassium laurate type I1 DM system (composition B). The orientation of the bilayer normals for the 0 and 90° spectra are given in Figure 2C,D. (B) ESR spectra of cholestane in a sodium decyl sulfate type I CM system of composition D. (C) The initial alignment of the cylinder long axes of type I CM systems is along the magnetic field of the ESR magnet. (D) After sample rotation by 90°, the cylinder long axes of the type I CM systems are along the x axis. The individual amphiphile long axes are now distributed in the yz plane at all angles with respect to the field.
that of a nonorienting lamellar phase formed at lower water contents. When a type I1 DM phase is aligned in a superconducting solenoid, the magnetic field is along the tube long axis and the directors align in a plane perpendicular to this axis. When the sample is placed in the ESR magnet, a spectral pattern results which is invariant to rotation about the tube long axis. When the doxy1 group is placed further down the hydrocarbon chains, the spectra become progressively sharper and the 0' hyperfine splitting increases as the label position approaches the more mobile region in the center of the bilayer. The spectra for the 6-doxy1 label at 0' and one rotated by 90' are shown in Figure 2B. The effect of sample spinning during alignment was also examined for this sample. Prealignment with spinning in a magnetic field of 1.5 T was followed by ESR spectral determination. The hyperfine splittings were independent of rotation angle and identical with those for the 0' orientation of a sample aligned without spinning. Such behavior illustrates that the spinning of the sample is sufficient to overcome glass-wall interface effects and produce alignment of the director along the tube long axis instead of in a plane perpendicular to the magnetic field. The ESR order parameters for the potassium laurate type I1 DM bilayers are shown in Figure 3 A along with those determined by deuterium NMR, while the hyperfine splittings for all orientations are given in Table I. The behavior of the cholestane spin label was also investigated in this system. The spectra of a nonspinning aligned sample and one rotated by 90' are shown in Figure 4. A hyperfine splitting of 17.16 G is obtained for the aligned sample, while a splitting of '/*(33.5 6.3) = 19.9 G would be expected for this spin probe if rapidly rotating about its long axis, with this axis perfectly
+
1"he Journal of Physical Chemistry, Vol. 88, No. 9, 1984
Magnetically Aligned Surfactants
1723
TABLE 11: Hyperfine Splittings (Gauss) for 4-KL and Cholestane Spi:: Probes in Various Surfactant Systems A . SDS Type I1 DM
orientation label
4-KL
0"
15"
30"
45"
60"
12.40 12.99
14.04 15.22 15.92 16.09 15.44 14.79 B. DACl Type I1 DM
cholestane 16.81 16.60
75"
90"
16.35 16.35 14.10 13.81
orientation label
0"
15"
cholestane 15.85 15.75
30"
45"
60"
15.70 15.50 15.39
75"
90"
15.30 15.30
C. SDS Type I CM
orientation label
4-KL
cholestane
0"
15"
12.44 12.65 17.03 16.95
30"
45"
60"
13.38 14.34 15.09 16.58 16.13 15.60
Figure 5. ESR spectra of 4-KL in decylammonium chloride type I1 DM bilayers (composition F). The orientation of the bilayer normals is given in Figure 2C,D. Compare with Figure 2A.
75'
90'
15.43 15.38 15.25 15.06
aligned perpendicular to the applied field. Rotation by 90" clearly gives rise to a "doubling" of the low- and high-field peaks, the outer ones associated with micelles whose normals are still perpendicular to the field and the inner ones whose normals are now parallel to the field. This is another example of the liquid crystal-glass interface effect. Substitution of a sufficient quantity of the aromatic amphiphile, KHB, results presumably in a disk-shaped micelle which aligns without spinning such that the director is parallel to the magnetic field. Thus, it is to be expected that the aligned hyperfine splitting of the chain-labeled laurates will be large and decrease on rotation by 90'. The hyperfine splittings for all spin probes are given in Table I. The aligned splittings decrease as the depth of the label in the bilayer increases, once again illustrating a fluidity gradient. The order profiles derived from the ESR data and those from the *H N M R data are shown in Figure 3B. The spectra for the cholestane label in this system has the opposite behavior as in the type I1 D M laurate system; i.e., the hyperfine splitting increases upon rotation to a maximum value of 17.35 G at 90". Other supposed type I1 DM systems were investigated by using the 4-KL probe. It must be remembered that, in these cases, the chemical nature of the head group and the chain length of the reporter do not match those of the host amphiphile. However, these spin labels do intercalate into the bilayer and can therefore be used to determine whether the molecular long axis is, on the average, more parallel or more perpendicular to the field. For 4-KL incorporated in the SDS type I1 DM system (composition E), spectra similar in shape to the laurate type I1 D M system were obtained. The hyperfine splitting was a minimum at 0' and increased to-a maximum at 90'. The complete rotational dependence is shown in Table 11. In addition, the cholestane spin probe showed behavior similar to that for the laurate type I1 DM system, the peaks being sharp at the 0" orientation with the hyperfine splitting its maximum value of 16.83 G. Rotation by 90' resulted in a doubling of the low- and high-field extrema, allowing observation of both parallel and perpendicular orientations due to the liquid crystal-glass interface effects upon alignment. Also investigated was the ternary system of decylammonium chloride, ammonium chloride, and water. The spectra of 4-KL in this bilayer are shown in Figure 5 for the aligned sample and after 90" rotation. These spectra are similar in appearance to those which have been observed for nitroxide-labeled fatty acids in phospholipid liposomes.22Distinct spectral features are resolved corresponding to chain long axes nearly parallel and more nearly perpendicular to the applied magnetic field both for the initial aligned spectrum and in the spectrum rotated by 90" in spite of the fact that 2H N M R indicates homogeneous alignment. However, in contrast to the ESR spectrum of this probe in the preceding lamellar phase, the relative intensities of the peaks associated with 2Tlland 2T, change upon rotation, the features associated with the parallel direction becoming much more
prominent on 90" rotation. The cholestane spectra are again similar to those obtained for multilamellar liposomes and in the prealigning lamellar region of the decylammonium chloride system, but in the aligning region of the phase diagram the spectra are sensitive to tube rotation, the hyperfine splittings being minimum after 90' rotation. Order parameters from these spectra may be expected to be larger than those calculated from single orientations on the ESR time scale:2 but it was found that the ESR order parameters do not change upon dilution from the lamellar phase through the aligning liquid-crystal region, being essentially constant at 0.43. In the isotropic region, the doubling of the ESR signals is not observed since tumbling of the micellar units is unhindered. A preliminary study has appeared which investigated the behavior of chain-labeled laurates in a laurate type I C M mesophase.lg It showed that this system having a cylindrical structure is consistent with the ESR spectral data. In order to extend these studies, this system was further studied by using the cholestane spin probe. As well, both chain-labeled laurates and cholestane were incorporated into a sodium decyl sulfate type I C M mesophase (composition D) to confirm that the results are qualitatively similar for different type I CM systems. The 4-doxyllaurate in SDS type I CM behaved as it had in the laurate type I CM system. On alignment, the hyperfine splitting achieved a minimum value of 12.13 G and upon rotation increased with noticeable broadening, especially of the high-field resonance (see Figure 3, ref 19). The hyperfine splittings for all rotation angles are given in Table 11. The cholestane spin probe whose 2pa orbital containing the unpaired electron is nearly orthogonal to the molecular long axis was also investigated in type I C M systems. The aligned spectra and those obtained upon 90' rotation are shown in Figure 4. In the laurate system (composition A), the hyperfine splitting is at its maximum of 17.40 G upon alignment and is reduced to 14.55 G on 90" rotation. Similar results were obtained for the decyl sulfate liquid crystal. The hyperfine splittings are given in Table 11.
Discussion Type IZ DM Bilayers. As has been shown in Tables I and I1 and Figures 1, 2, 4, and 5, the chain-labeled probes orient such that the chain long axes are preferentially perpendicular to the field direction, as is the director (symmetry axis of the mesophase). There is, however, for the 4-labeled species broadening of the lowand high-field spectral components for the laurate and decyl sulfate systems and an even more pronounced effect for the decylammonium mesophase. In this case, the spectra are more similar to those expected for a polycrystalline powder in that both parallel and perpendicular components are resolved. However, the relative intensities of the two components are not those of an isotropic three-dimensional distribution. Spectral broadening and/or resolution of two components can be caused by a tilt angle of the chains or a distribution or a spread angle, but it is difficult to distinguish between these.23 It is certain that these effects are not caused by incomplete or misalignment since the 2H N M R spectra are characteristic of extremely well-aligned liquid crystals. As well, these effects are not caused by wall effects since they (23) H . Schindler and J. Seelig, J . Chem. Phys., 59, 1841 (1973)
1724 The Journal of Physical Chemistry, Vol. 88, No. 9, 1984
occur before rotation of the type I1 DM systems (Figures lA, 4A, and 6A) as well as after alignment while spinning about the tube long axis. Spectra which most closely resemble those in the present work have been reported by Meirovitch and Freed24for well-oriented multilayers of dipalmitoylphosphatidylcholine in the lamellar liquid-crystalline La( 1) phase. Satisfactory line-shape fits were achieved using a two-dimensional distribution of local directors interpreted as a cooperative distortion wave persisting for time scales long with respect to ESR time scales. For small disk type fragments of a lamellar phase, this motion would not be distinguishable from micelle oscillation. Since the N M R and ESR experiments are sensitive to motions on different time scales, motions occurring at a rate of 105-107SKIwill be averaged out on the N M R time scale but not on the ESR time The spin-labeled amphiphiles experience a static distribution of orientations on the ESR time scale but an average orientation on the N M R time scale. On the 2H N M R time scale, motions occur which slowly decrease the observed chain order parameters in a linear fashion as the lamellar phase of decylammonium chloride bilayers is diluted with water. In addition, a more rapid decrease in order is found with dilution in the aligning micelle r e g i ~ n . ~ * . Since ~*,~~ the ESR order parameters for the decylammonium chloride system are unchanged from the lamellar region, through the aligning micelle region, the motions responsible for the decrease in the 2H N M R order parameters must be fast on the N M R time scale but slow on the ESR time scale. These motions include wavelike undulations of the lamellae and oscillations of the micelles about the director in the type I1 DM r e g i ~ n . ’ - ~ The ~ >NMR ~ ~ experiment sees an average orientation of the bilayer normals as being perpendicular to the external field. The ESR experiment will see a static distribution of a large number of orientations of spin probes if the micelles themselves are oscillating at a rate slower than lo7 s-1.
If the micelles were quite small, an alternative explanation could involve “edge effects”. The chain long axes of amphiphiles at the edges of disk micelles or on the hemispherical caps of cylindrical micelles would be found at all angles with respect to the applied field. The relative number of chains at the edges will increase as the size of the micelle decreases. Using lateral diffusion coefficients determined for various amphiphiles in liquid crystals and model membra ne^,^*^'-^^ one can show that lateral diffusion around the edges of disk micelles is not expected to be fast on the ESR time scale. For example, with a diffusion coefficient of 6 X cm2/s, an amphiphile could be expected to diffuse only in s. Only a very slight a distance of approximately 8 i-% spectral broadening would be expected. For disk micelles, this effect would be very low until the micelles became quite small in diameter. The intermicelle distance would then approach the magnitude of the disk diameter, leading to overall isotropic rotation of the micelles. Therefore, it can be stated that micelle oscillation is slow on the ESR time scale but rapid on the N M R time scale. It is interesting to note that a correlation time of 13 p s obtained from T 1 ,experments of type I C M liquid crystals has been associated (24) (25) (26) (27) (28) (1976). (29) (1983). (30)
E. Meirovitch and J. H. Freed, J . Phys. Chem., 84, 3281 (1980). R. P. Mason and C. F. Polnaszek, Biochemistry, 17, 1758 (1978). B. J. Gaffney and H. M. McConnell, J . Magn. Reson., 16, 1 (1974). A. Seelig and J. Seelig, Biochemistry, 13, 4839 (1974). F. Y. Fujiwara and L. W. Reeves, J . Am. Chem. Soc., 98, 6790
B. J. Forrest and L. W. Reeves, Mol. Cryst. Liq. Cryst., 90, 323
B. J. Forrest, L. Hecker de Carvalho, and L. W. Reeves in “Biomolecular Stereodynamics”, Vol. 11, R. H. Sarma, Ed., Adenine Press, New York 1981, pp 89-98. (31) J. Charvolin and P. Rigny, J . Chem. Phys., 58, 3999 (1973). (32) J. Charvolin and P. Rigny, Mol. Cryst. Liq. Cryst., 15, 21 1 (1971). (33) J. Charvolin and P. Rigny, Chem. Phys. Lett., 18, 515 (1973). (34) R. T. Roberts, Nature (London), 242, 348 (1973). (35) T. Bull and B. Lindman, Mol. Cryst. Liq. Cryst., 28, 155 (1974). (36) 0.Soderman, G. Lindblom, L. B. A. Johansson, and K . Fontell, Mol. Cryst. Liq. Cryst., 59, 121 (1980).
Forrest and Mattai with oscillations of the micelle units.37 It appears that oscillations of disk micelles take place on a similar time scale. The difference in spectral shape between the decylammonium type I1 DM system and the laurate and decyl sulfate systems probably is due to micelle size differences of the mesophase or due to differences in size distributions for the various systems. Large micelles oscillating through small angles would be seen as more perfectly oriented on the ESR time scale. A distribution of micelle sizes would have the effect of increasing the instantaneous angular distribution of the spin probes with respect to the field, as would smaller homogeneously sized micelles undergoing larger angular excursions on a time scale sloer than s. Therefore, it is concluded that the decylammonium chloride surfactant system is composed of smaller micelles and/or of a greater size distribution than for the laurate or decyl sulfate bilayers. As the spin probe is placed more deeply into the membrane, the spectra become sharper since rapid trans-gauche isomerizations of the hydrocarbon chain contribute to the averaging of the T tensor. Together, the results for the chain-labeled and cholestane spin probes yield information concerning the structure of the micelle units since the orientation of the director is known. For example, if the type I1 systems in which the director aligns perpendicular to the magnetic field were composed of cylindrical micelles, a cylindrical distribution of chain axes about the field would be seen. Rotation of 90’ would result in a decrease of the hyperfine splitting to a minimum, while the opposite behavior is opposite behavior is observed, i.e. a maximum, indicating chain long axes now parallel to the applied field. Thus, the ESR results are inconsistent with a cylindrical structure for the type I1 DM systems but are entirely consistent with the building blocks being composed of disklike micelles which align such that the bilayer normal is perpendicular to the applied field. For the cholestane probe oriented in a disk bilayer whose normal is perpendicular to the magnetic field, rapid rotation about the steroid long axis averages T,, and T,,, and thus a large splitting approaching 19.9 G would be expected. Upon rotation, due to wall effects superposition of spectra would be expected. Micelles which had initially aligned with their normals along the tube long axis (no change on rotation and therefore a large splitting) as well as those whose bilayer normals were initially orthogonal to this axis due to the liquid crystal-glass interface effect are predicted. This behavior is illustrated in Figure 4. The order parameters of the laurate type I1 DM system are compared with the 2H N M R results in Figure 3A. At the 4-position, the ESR order parameter is higher than that reported by N M R (0.30 vs. 0.18) but falls off rapidly as the depth in the membrane increases. Spin-label order parameters have been criticized due to the bulkiness of the reporter Recently, Taylor and measured 2H N M R order parameters for the 4- and 6-positions of a 5-doxy1 fatty acid in lecithin bilayers. The order parameter of 0.21 when compared with the 2H N M R order parameter of 0.47 in the absence of the doxy1 group proves the perturbation of the spin-label group. The ESR parameter of this molecule was which is 0.34; Le., for the same perturbed chain SEsR> SNMR, compatible with time scale arguments40 and perhaps time dependent tilting of chains. Therefore, the ESR order parameters measured here are meant as a qualitative guide. Type I Disk Micelle Bilayers. Spectra of chain-labeled laurates in the laurate type I DM system are essentially opposite to those of the type I1 DM system. The maximum splitting occurs at the 0’ orientation, indicating that the chain long axes align parallel to the applied field instead of perpendicular to it. Rotation by (37) M. I. Burgar, R. Blinc, M. M. Pintar, and L. W. Reeves, Mol. Crysf. Liq. Cryst., 84, 245 (1982). (38) J. Seelig and W. Niederberger, Biochemistry, 13, 1585 (1974). (39) M. G. Taylor and I. C. P. Smith, Biochim. Biophys. Acta, 599, 140 (1980). (40) R. P. Mason and C. F. Polnaszek, Biochemistry, 17, 1758 (1978). (41) M. G. Taylor and I. C. P. Smith, Biochemistry, 20, 5252 (1981). (42) J. Seelig, Q. Reu. Biophys., 10, 353 (1977). (43) D. Marsh, Biochemistry, 19, 1632 (1980).
Magnetically Aligned Surfactants
90° now results in a minimum splitting. For the cholestane spin label, a minimum hyperfine splitting is observed upon alignment, which increases to a maximum of 17.35 G upon rotation by 90°,indicating that the magnetic field is initially parallel to the steroid long axis, becoming perpendicular to it when rotated. Upon rotation, however, the observed spectrum differs somewhat from those of rotated type I1 DM systems in that superimposed spectra of labels whose steroid long axes are parallel and perpendicular to the field are not observed. This is because the initial alignment of the type I DM micelles is such that the bilayer normals are on the average parallel to the applied field. There exists only one parallel direction. For type I1 DM micelles, alignment of the director perpendicular to the field allows the bilayer normals to orient in a plane. Rotation of this type of sample leads to a distribution. Invoking the same type of argument as for the type 11 DM bilayers, one can state that the ESR spectral data are entirely consistent with the presence of disklike micelles whose bilayer normals align parallel to an applied magnetic field. This preferred direction of alignment stems from the overcoming of the diamagnetic anisotropy of the hydrocarbon chains by ring currents of the aromatic surfactant, KHB.) The ESR order parameters for the laurate chain and cholestane are shown in Figure 3B. This figure bears great similarity to that for the same reporter groups present in the laurate type I1 DM system. Chemically, the micelles are much the same in composition, with only a small percentage of the laurate molecules being replaced by potassium (hepty1oxy)benzoate in the type I case. Very minor differences in the ESR order parameters are observed between the two systems, with the type I micelles being slightly more ordered at all positions. The variance may be due to slight differences in average chain length in the mixed micelles or small changes in head-group orientation,' since oscillation of the micelles is slow on the ESR time scale, and therefore does not contribute to lowering of the order parameters. The effect of diluting the type I DM system was investigated over a range of water contents, and the order parameter for the 4-doxy1 label essentially constant at 0.34 A 0.02. Type Z CM Systems. The potassium laurate cylindrical micelle system has been the subject of a preliminary investigationg employing doxyllaurate spin labels. The results showed that this mesophase is made up of cylindrical micelles of finite length. A second system composed of sodium decyl sulfate, n-decanol, and D 2 0 which is also proposed to be composed of cylindrical micelles has been examined with both potassium 4-doxyllaurate and cholestane spin probes. The results for the chain-labeled species are very similar to those previously obtained for laurate cylindrical micelles. The cylinder long axes orient parallel to the applied magnetic, and thus the chain long axes are preferentially perpendicular to the field, resulting in a low initial splitting. Rotation by 90°,however, produces severe broadening, especially of the high-field resonance. This is caused by the presence of the chain long axes in a plane which contains the magnetic field and the averaging of the angles made by the nitroxide 2p7r orbitals with the magnetic field. The averaging is caused by rotation of
The Journal of Physical Chemistry, Vol. 88, No. 9, 1984 1725 the amphiphiles around the cylinder and/or rotation of the cylinders about their long axes, which produces some additional averaging of the T tensor; Le., there exists a dynamic distribution of the chain long axes6 and not a static one as would be the case for a type I1 disk micelle system which had been aligned without spinning. The behavior of the cholestane spin probe more clearly illustrates the additional averaging resulting from the combination of amphiphile rotation about the cylinder long axis and the rotation of the cylindrical micelle itself about the axis. Upon initial alignment, a larger low-field splitting of 17.03 G was found for the decyl sulfate type I C M system. Rotation by 90° gave a lower splitting of 15.08 G, which is close to the value expected for isotropic motion. However, the line widths of the rotated spectrum were much greater; Le., the peak to peak width of the high-field resonance increased from 3.0 G to approximately 5 . 0 G. In this configuration, T,, and T,, are completely averaged by rapid rotation about the steroid long axis43while averaging with TYY is incomplete since rotation about the cylinder long axis is a slower process.
Conclusions It has been shown possible to differentiate via ESR spectroscopy between various types of spontaneously aligning lyotropic liquid crystals. In all cases studied, the spin-label data for chain-labeled amphiphiles and the cholestane spin probe indicate that the type I and type I1 DM systems are composed of disklike micelles which orient such that the bilayer normals are on the average parallel and perpendicular to the magnetic field, respectively. As well, ESR studies of a second type I C M mesophase has been shown to be compatible with a structure composed of cylindrical micelles which orient such that the director is parallel to the applied field. Secondly, oscillation of the micelle units is slow on the ESR time scale but fast on the NMR time scale. Essentially, constant order was found for the lamellar phase and the aligning diskbilayer phase of the decylammonium chloride mesophase. As well, dilution had little effect on the ESR order parameters for disk micelles of potassium laurate. Therefore, the rate of micelle oscillation must be approximately 105-107 s-l. Thirdly, the diffusion of amphiphiles around the cylinder long axes of the type I C M systems results in additional averaging of the hyperfine splitting tensor, and therefore the arrangement of the nitroxide 2pa orbitals must be thought of as a dynamic distribution rather than a static one, even on the ESR time scale. Future saturation transfer experiments may offer additional information. Acknowledgment. The financial support of the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. Nuclear magnetic resonance spectra were obtained through the Atlantic Region Magnetic Resonance Centre, Halifax, Nova Scotia, funded by a major installation grant from NSERC. Registry No. KL, 10124-65-9; SDS, 142-87-0; DACI, 143-09-9; 4KL, 85145-55-7; 6-KL, 85145-56-8; 8-KL, 85145-57-9; IO-KL, 8514558-0; cholestane, 481-21-0.
