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ESR studies of aqueous cholesteric lyotropic liquid crystals
Chemistry and Physics of Lipids, 35 (1984) 1-10 Elsevier Scientific Publishers Ireland Ltd.
1
ESR STUDIES OF AQUEOUS CHOLESTERIC LYOTROPIC LIQUID CRYSTALS
BRUCE J. FORREST and JAIRAJH MATTAI Chemistry Department, Dalhousie University, Halifax, Nova Scotia, B3H 4J3 (Canada) Received August 22nd, 1983
accepted January 30th, 1984
Cholesteric aqueous lyotropic liquid crystals have been prepared by the addition of optically active guest molecules to Type II disk micelles which orient in magnetic fields. ESR spectra obtained when the helical axis is perpendicular to the field direction are sensitive to the nematic-cholesteric transition when the overall pitch is quite long, indicating the presence of regions of cholestericity in a nematic matrix. Once an equilibrium twisted structure is formed, further changes in the pitch of the helix could not be detected by the spin probe. The results support an irregular distribution of micelles in the untwisted mesophase. Keywords: micelle; ESR; cholesteric; liquid crystal.
Introduction Cholesteric aqueous lyotropic liquid crystals which orient in applied magnetic fields are of relatively recent origin [ 1 - 6 ] . "l'hese micellar systems may be formed in essentially two ways. The first is by the addition of optically active molecules to nematic mesophases [ 1 - 3 ] , while the second involves the use o f pure optically resolved amphiphiles [ 4 - 6 ] . We have previously reported ESR of disk micelle phases o f negative diamagnetic anisotropy (Type II DM) which align in an external magnetic field such that the bilayer normals are perpendicular to the field [7]. The addition of chiral species to these disk micelles results in a cholesteric phase of relatively long pitch which aligns with the helical or screw axis along the field [1,5]. As far as we are aware, the use of ESR as an investigative tool for aligning aqueous lyotropic cholesterics has not been reported. In the present work, the spin probes, cholestane and potassium 4-doxyllaurate (4-KL) have been incorporated into a disk micelle phase composed of sodium decylsulfate (NaDS), n-decanol, sodium sulfate and water. Cholestericity has been induced by the introduction of small quantities of guest molecules, viz. the alkaloids brucine and cinchonine as well as (--)-tartaric acid. Despite the limitation of the dimethyloxazolidinyl nitroxides as quantitative probes o f order in liquid crystals and membranes [ 8 - 1 2 ] , certain qualitative features such as the orientation or the distribution of the orientations of the 2prr orbital containing the unpaired electron are easily discerned [ 13 ]. In the case of the parent nematic disk micelle phase, this orbital for the cholestane spin probe is perpendicular to the bilayer normal while for 4-KL it is parallel to this 0009-3084/84/$03.00 © 1984 Elsevier Scientific Publishers Ireland Ltd. Published and Printed in Ireland
axis [14]. By examining the hyperfine splittings as a function of sample rotation angle, it is shown that cholestericity can indeed be detected by ESR at very low overall induced twists. The results indicate the presence of an emulsion of cholesteric particles floating in a nematic matrix [I 5]. The hyperfine splittings changes in a roughly linear fashion with the concentration of the chiral doping agent before reaching a limiting value beyond which no further ESR spectral changes were observed. Experimental Cholestane and 4-KL were synthesized as previously described [7,16], while NaDS was prepared by the method of Reeves and Tracey [17]. The ratio of spin label to non-labelled amphiphile was kept at approx. 1:6000 to ensure their solubility in the micelle and to minimize spin probe-spin probe interactions. The composition of the Type II disk miceUe system by weight was as follows. NaDS/Na2SO4/n-decanol/D20/H20 (pH 1.8) = 756:43 : 153:600:540. To this fixed composition, increasing quantities of the chiral molecules brucine, cinchonine or (--)-tartaric acid were added. Samples were introduced into 1.6 mm capillaries which were mounted, in turn, in 5 mm NMR tubes. ESR spectra were recorded on a Varian E109-B spectrometer at 25°C. Spectra were recorded immediately upon placing the sample in the cavity and then allowed to align in a magnetic field of 0.7 T for 24 h to ensure no further changes would occur in the hyperfine splittings or spectral shape. Spectra of the aligned sample (0 ° orientation) and after 90 ° rotation about the tube long axis (y-axis) were recorded. In two cases, with the cholestane spin probe, the samples were aligned with or without spinning in magnetic fields of 1.41 and 8.45 T, respectively. In the former case, the magnetic field of the electromagnet is perpendicular to the long axis of the sample tube, while in the latter, the magnetic field of the superconducting solenoid is colinear with tube long axis. Results
Cholestane spin probe For this spin probe, the orbital containing the unpaired electron defines the molecular based zz-axis, as the N - O bond axis does the xx-axis. The molecular long axis is then the yy-axis (Fig. 1). Cholestane intercalates into the bilayer disk micelle such that its molecular long axis is nearly parallel to the bilayer normal. The principal values of the hyperfine splitting tensor are Axx = 6.3 G, Ayy = 5.8 G andAzz= 33.5 G [18]. Figure 2 shows the ESR spectrum of an aligned disk micelle phase in the absence of any chiral guest molecule. (Admittedly cholestane itself is optically active, but its extremely low concentration in the bilayer coupled with its low twisting power renders the perturbation negligible) [1,19]. Three fairly sharp peaks are
Ayy = 5.8 G A
dk//
O > ~ . p - - O - - - > Axx
D
-"
T
=63G
Azz = 33.5G
~I I
Ho )
: Ti
I
I I
I i I
I
I i
Cholestane
Fig. 1. The structure of cholestane spin label and the molecular based coordinate system. Also shown is the intercalation of cholestane into a disk micelle.
observed for the 0 ° orientation, with a hyperffme splitting of 16.6 G as measured from the low to midfield zero point crossings. A splitting of approx. 19.9 G would be expected for a perfectly ordered spin probe rotating rapidly about its long axis. Upon rotation by 90 ° about the tube long axis (y-axis of the laboratory frame, Fig. 3), a lower splitting of 13.4 G is observed, together with a broadening of the low and high field resonances. Type II disk micelles align in an applied magnetic field such that the bilayer normals are on the average, perpendicular to the field which is defined by the laboratory z-axis. In the absence of sample spinning, the
10G Fig. 2. ESR spectrum of cholest~me in NaDS disk micelles after alignment (A) and after rotation by 90 ° about the laboratory ),-axis (B).
4 Y _-
_ He
_
_.),
x
Fig. 3. Laboratory based frame of reference in which the magnetic field direction defines the z-axis and the tube long axis is along the y-axis. preferred direction within the xy-plane is along x-axis although there does exist a distribution of a small number of micelles of differing alignment [20]. Therefore after rotation by 90 ° most bilayer normals are parallel to the field direction and thus the field is almost exclusively parallel to the molecular long axis, orthogonal to the orbital containing the unpaired electron. Also shown in Fig. 4 are spectra for the same micelle system containing increasing quantities of brucine. The spectra for the 0 ° orientation are virtually identical for all concentrations and a representative spectrum is shown in Fig. 4A. However, spectral changes occur in the 90 °
Fig. 4. A: E S R spectra of disk micelles containing 0.05 m o l % brucine before rotation (0° orientation). (Note the similarity to Fig. 2A.) B: the same sample after rotation (90 ° orientation). B - G are spectra after 90 ° rotation and contain the following increasing quantities of brucine. B, 0.05 tool%; C, 0.08 tool%; D, 0.10 tool%; E, 0.14 mol%; F, 0.33 tool%; G, 0.66 tool%.
orientation as cholestericity is induced, (Fig. 4 ( B - G ) ) as a result of a change in the distribution of the angle which the directors and thus the orbital containing the unpaired electron makes with the applied field. Upon initial alignment of a cholesteric disk phase, the bilayer normals are all perpendicular to the applied field and the screw axis is along the field or z-axis. However for a cholesteric phase, the directors undergo 360 ° rotations within the x y plane i.e. there is now a uniform distribution in this plane (Fig. 5). As brucine is added to the micelles, the ESR spectra upon 90 ° rotation show increasingly more intensity in the spectral extremes, i.e. the low field extreme of the low field resonance and the high field extreme of the high field resonance. The result is a continual increase in the measured hyperfine splitting until approx. 0.34 mol% added brucine, after which the value remains constant (Fig. 6) and no further changes in spectral shapes occur (Fig. 4). The twisting power of brucine and other optically active guest molecules towards disk micelle systems has been previously investigated by polarizing microscopy [1,5]. Cholesteric textures were observed at least as low as 0.113 mol% of brucine corresponding to a helical twist of l i p = 34.5 cm -1 where p is the pitch of the helix [1 ]. In addition the pitch decreased linearly with increasing concentrations of the optically active guest. Similar results were obtained by the use of cinchonine or (--)-tartaric acid as the chiral guest species. These observations are shown in Figs. 7 and 8. In Fig. 6, it is known that the ability of the chiral agents to induce cholestericity follows the order brucine > cinch0nine >,> tartaric acid, in agreement with optical studies [1,5]. This sequence is a result of a combination of twisting power and partitioning between bilayer and aqueous components [ 1,5]. In order to reconfirm that the helical axis aligns along the applied field, an experiment was performed in which the sample containing 0.66 mol% brucine was prealigned in a superconducting solenoid. In this case the field is along the tube long axis. When the sample is placed in the ESR spectrometer, a limiting A
B
Y
y
Fig. 5. Distribution of the local directors or disk bilayer normals after rotation. A: in the absence of a chiral guest, directors are predominantly along the field direction. B: in a fully cholesteric system the directors are uniformly in a cylindrical distribution hn the yz plane. The screw axis is along the x-axis. For the cholestane spin probe the orbital containing the unpaired electron is perpendicular to the director, while for the 4-KL label, it is colinear with the directors.
t
I
O0 []
r-I-
9o °
~o. is
14./ '0
~-~
~.~
13.~
12
o
J
b
i
~
m01e% guest omphiphile Fig. 6. Hyperfine splitting (G) of cholestane spin label vs. concentration of optically active component for the 0 ° orientation (uppermost horizontal line) and the 90 ~ orientation (lower curves) for brucine (a), cinchonine (o) and (--)-tartaric acid (~). The filled points refer to disk miceUes containing no optically active guest molecules.
Fig. 7. ESR spectra of cholestane in NaDS disk micelles (90 ° orientation) containing the following increasing quantities of cinchonine. A, 0.09 mol%; B, 0.17 reel%; C, 0.38 mol%; D, 0.59 mol%.
A
Fig. 8. ESR spectra of cholestane in NaDS disk micelles (90 ° orientation) containing the following increasing quantities of (--)-tartaric acid. A, 0.34 mol%; B, 1,00 mol%; C, 1.90 tool%; D, 3.30 mol%.
cholesteric spectrum with a splitting of 15.44 G which was invariant to sample rotation about the tube long axis i.e. the directors are now distributed uniformly in the xz plane since the screw axis is along the y-axis. In this way the screw axis may be manipulated to be along the y-axis or as previously, along the x-axis by alignment in an iron magnet followed by 90 ° rotation about the y-axis. The spinning of fully twisted samples in an iron magnet was found to be sufficient to overcome the helical packing of the twisted disk micelles. A sample containing 0.66 mol% brucine spun in a magnetic field of 1.41 T yielded an ESR hyperfine splitting of 16.6 G. In addition, the splitting was invariant upon rotation about the laboratory y-axis, and the lineshape was identical to those of samples aligned in iron magnets before rotation. The effect of spinning is sufficient to overcome the forces favouring helical packing and to align the micelles, even though twisted, such that their bilayer normals are along the spinning y-axis.
Doxyl laurate spin probe For this chain labeled nitroxide, the nitrogen 2prr orbital containing the unpaired electron (which defines the molecular based zz-axis) is co-linear with the molecular long axis. The N - O bond direction fixes the xx-axis. This amphiphile intercalates into the bilayer disk micelle such that the zz-axis is preferentially along the bilayer normal i.e. initially perpendicular to the applied magnetic field, and predominantly along the x laboratory axis (Fig. 3) [7,20]. Therefore in the absence of cholestericity, a low hyperfine splitting should be observed for the 0 ° orientation while a larger splitting should be found after 90 ° sample rotation
about the tube long axis (y-axis of the laboratory frame). The experimental values are 13.0 G and 17.2 G for the 0 ° and 90 ° orientations, respectively, in agreement with previous results [7]. The magnitude of the difference in hyperfine splittings is not as large as has been reported for some lamellar liquid crystals [ 14] because of oscillation of the micelles as whole bodies and/or co-operative distortion waves of the local directors [7,21 ] which result in spectral broadening. As the concentration of brucine is increased, the hyperfine splitting for the 0 ° orientation remains constant at 13.0 G through to the highest concentration studied (1.32 mol%). In the absence of a chiral solubilisate, the hyperfine splitting upon 90 ° rotation is 17.2 G, as previously stated. The addition of 0.34 mol% brucine lowers this value to 14.6 G. It remains constant as the concentration of brucine is increased to 0.66 mol% and finally to 1.32 mol%. Therefore the chain labeled nitroxide indicates the presence of a completely helical packing of micelles at the same concentration as did the cholestane label (0.32 mol%). For the 0 ° orientation, the bilayer normals are uniformly distributed in the xy plane and therefore the 2pTr orbital containing the unpaired electron (zz-axis) is preferentially perpendicular to the laboratory z-axis. The result is a low hyperfine splitting. Discussion What is being observed here by ESR, is the distribution of the directors [14]. For a helical arrangement, a uniform two-dimensional distribution from 0 ° to 360 ° would be expected, and indeed is observed after 0.34 mol% brucine is added. Once completely formed in this distribution the ESR method is insensitive to tighter twisting. Before the plateau in Fig. 6 is reached, it must be concluded that the entire sample is not 'cholesteric'. It has been proposed that the origin of the macroscopic twist is an induced twist in each planar bilayer micelle caused by short range steric interactions and that these twisted propeller shaped micelles pack most efficiently with an orientational helical long range order [5]. At low twists, what may be being seen by ESR spectroscopy is an emulsion of twisted micelles in a planar micelle nematic matrix. As more brucine is added, twist is induced into more of the planar miceUes, leading to a system of completely twisted disks, i.e. below the plateau region of Fig. 6, some of the spin probes reside in planar micelles, while some reside in very slightly twisted micelles which are beginning to pack with helical order. However, assuming a uniform distribution of the chiral molecules among the individual micelles, simple calculations involving estimates of the number of amphiphiles per micelle predict the presence of several optically active guests in each micelle even at the lowest concentrations studied [5]. Therefore all micelles would be expected to be twisted and packed into helices. The ESR result would then be expected to be a splitting corresponding to the plateau value of Fig. 6. A much more plausible explanation involves the presence of an irregular distribution of micellar and aqueous regions in the mesophase. Low angle X-ray
diffraction studies of NaDS Type II disk micelles showed evidence that this system is not in the strictest sense, nematic [22], but that some areas composed of aggregates of these platelets are present with little water between them. The twist induced into a micelle can only be translated into macroscopic cholesteric behaviour through an intermicellar force field [5]. Such forces are highly distance dependent [1,5,23]. Therefore certain regions characterized by high micelle density will experience much greater intermicellar forces responsible for transmitting helical order and pack in a cholesteric fashion. Other regions characterized by larger intermicelle distances will not be affected to nearly the same extent in spite of the fact that all individual micelles may be twisted to the same small degree. Thus, a superposition of cholesteric and non cholesteric regions are reported by ESR. As the individual micelles are increasingly distorted by increasing quantities of chiral additivies, the more water rich areas will begin to pack in a helical manner, eventually leading to a limiting hyperfine splitting when all spin probes are arranged in a cylindrical distribution. Conclusions
The transition from a Type II disk micelle phase to a cholesteric phase induced by the incorporation of optically active guest molecules is characterized by the growth of regions of helical micellar packing in a nematic matrix. Since the macroscopic cholesteric behaviour is translated through a distance dependent intermicellar force field, the onset of helical packing is associated with regions of high micelle density. Although all micelles should be twisted to the same degree, the communication of this twist is much weaker in water rich areas. As the concentration of the optically active species is increased, all micelles begin to assemble along a screw axis. The ESR spin probe method is then insensitive to further reductions in the pitch. Acknowledgements
The financial support of the Natural Sciences and Engineenng Research Council of Canada is gratefully acknowledged. Sample alignment at 8.45 T was made possible through the use of the facilities of the Atlantic Region Magnetic Resonance Centre, Halifax, Canada. References.
1 K. Radley and A. Saupe, Mol. Phys., 35 (1978) 1405. 2 P. Diehl and A.S. Tracey, FEBS Lett., 59 (1975) 131. 3 L.J. Yu and A. Saupe, J. Am. Chem. Soc., 102 (1980) 4879. 4 B.J. Forrest, L.W. Reeves, M.R. Vist, C. Rodger and M.E.M. Helene, J. Am. Chem. Soc., 103 (1981) 690.
i0 5 P.S. CoveUo, B.J. Forrest, M.E.M. Helene, L.W. Reeves and M. Vist, J. Phys. Chem., 87 (1983) 176. 6 M. Acimis and L.W. Reeves, Can. J. Chem., 58 (1980) 1533. 7 B.J. Forrest and J. Mattai, J. Phys. Chem., in press. 8 J. Seelig and W. Niederberger, Biochemistry, 13 (1974) 1585. 9 M.G. Taylor and I.C.P. Smith, Biochim. Biophys. Acta, 599 (1980) 140. 10 R.P. Mason and C.F. Polnaszek, Biochemistry, 17 (1978) 1758. 11 M.G. Taylor and I.C.P. Smith, Biochemistry, 20 (1981) 5252. 12 J. Seelig, Quart. Rev. Biophys., 10 (1977) 353. 13 B.J. Forrest and L.W. Reeves, Chem. Phys. Lipids, 32 (1983) 73. 14 J. Seelig, in: L.J. Berliner (Ed.), Spin Labeling. Theory and Applications, Academic Press, New York, 1976, p. 373. 15 P.G. deGennes, in: The Physics of Liquid Crystals, Clarendon Press, Oxford, 1974, p. 253. 16 J. F.W. Keana, S.D. Keana and D. Beatham, J. Am. Chem. Soc., 89 (1967) 3055. 17 L.W. Reeves and A.S. Tracey, J. Am. Chem. Soc., 97 (1975) 5729. 18 B.J. Gaffney and S. Chen, Methods Membr. Biol., 8 (1977) 291. 19 G.R. Luckhurst and H.J. Smith, Mol. Cryst. Liq. Cryst., 20 (1973) 319. 20 F.Y. Fujiwara and L.W. Reeves, Can. J. Chem., 56 (1978) 2178. 21 E. Meirovitch and J.H. Freed, J. Phys. Chem., 84 (1980) 20. 22 L.Q. Amaral and M.R. Tavares, Mol. Cryst. Liq. Cryst., 56 (1980) 203. 23 J.N. Israelachvili, Faraday Discuss. Chem. Soc., 65 (1978) 20.
1
ESR STUDIES OF AQUEOUS CHOLESTERIC LYOTROPIC LIQUID CRYSTALS
BRUCE J. FORREST and JAIRAJH MATTAI Chemistry Department, Dalhousie University, Halifax, Nova Scotia, B3H 4J3 (Canada) Received August 22nd, 1983
accepted January 30th, 1984
Cholesteric aqueous lyotropic liquid crystals have been prepared by the addition of optically active guest molecules to Type II disk micelles which orient in magnetic fields. ESR spectra obtained when the helical axis is perpendicular to the field direction are sensitive to the nematic-cholesteric transition when the overall pitch is quite long, indicating the presence of regions of cholestericity in a nematic matrix. Once an equilibrium twisted structure is formed, further changes in the pitch of the helix could not be detected by the spin probe. The results support an irregular distribution of micelles in the untwisted mesophase. Keywords: micelle; ESR; cholesteric; liquid crystal.
Introduction Cholesteric aqueous lyotropic liquid crystals which orient in applied magnetic fields are of relatively recent origin [ 1 - 6 ] . "l'hese micellar systems may be formed in essentially two ways. The first is by the addition of optically active molecules to nematic mesophases [ 1 - 3 ] , while the second involves the use o f pure optically resolved amphiphiles [ 4 - 6 ] . We have previously reported ESR of disk micelle phases o f negative diamagnetic anisotropy (Type II DM) which align in an external magnetic field such that the bilayer normals are perpendicular to the field [7]. The addition of chiral species to these disk micelles results in a cholesteric phase of relatively long pitch which aligns with the helical or screw axis along the field [1,5]. As far as we are aware, the use of ESR as an investigative tool for aligning aqueous lyotropic cholesterics has not been reported. In the present work, the spin probes, cholestane and potassium 4-doxyllaurate (4-KL) have been incorporated into a disk micelle phase composed of sodium decylsulfate (NaDS), n-decanol, sodium sulfate and water. Cholestericity has been induced by the introduction of small quantities of guest molecules, viz. the alkaloids brucine and cinchonine as well as (--)-tartaric acid. Despite the limitation of the dimethyloxazolidinyl nitroxides as quantitative probes o f order in liquid crystals and membranes [ 8 - 1 2 ] , certain qualitative features such as the orientation or the distribution of the orientations of the 2prr orbital containing the unpaired electron are easily discerned [ 13 ]. In the case of the parent nematic disk micelle phase, this orbital for the cholestane spin probe is perpendicular to the bilayer normal while for 4-KL it is parallel to this 0009-3084/84/$03.00 © 1984 Elsevier Scientific Publishers Ireland Ltd. Published and Printed in Ireland
axis [14]. By examining the hyperfine splittings as a function of sample rotation angle, it is shown that cholestericity can indeed be detected by ESR at very low overall induced twists. The results indicate the presence of an emulsion of cholesteric particles floating in a nematic matrix [I 5]. The hyperfine splittings changes in a roughly linear fashion with the concentration of the chiral doping agent before reaching a limiting value beyond which no further ESR spectral changes were observed. Experimental Cholestane and 4-KL were synthesized as previously described [7,16], while NaDS was prepared by the method of Reeves and Tracey [17]. The ratio of spin label to non-labelled amphiphile was kept at approx. 1:6000 to ensure their solubility in the micelle and to minimize spin probe-spin probe interactions. The composition of the Type II disk miceUe system by weight was as follows. NaDS/Na2SO4/n-decanol/D20/H20 (pH 1.8) = 756:43 : 153:600:540. To this fixed composition, increasing quantities of the chiral molecules brucine, cinchonine or (--)-tartaric acid were added. Samples were introduced into 1.6 mm capillaries which were mounted, in turn, in 5 mm NMR tubes. ESR spectra were recorded on a Varian E109-B spectrometer at 25°C. Spectra were recorded immediately upon placing the sample in the cavity and then allowed to align in a magnetic field of 0.7 T for 24 h to ensure no further changes would occur in the hyperfine splittings or spectral shape. Spectra of the aligned sample (0 ° orientation) and after 90 ° rotation about the tube long axis (y-axis) were recorded. In two cases, with the cholestane spin probe, the samples were aligned with or without spinning in magnetic fields of 1.41 and 8.45 T, respectively. In the former case, the magnetic field of the electromagnet is perpendicular to the long axis of the sample tube, while in the latter, the magnetic field of the superconducting solenoid is colinear with tube long axis. Results
Cholestane spin probe For this spin probe, the orbital containing the unpaired electron defines the molecular based zz-axis, as the N - O bond axis does the xx-axis. The molecular long axis is then the yy-axis (Fig. 1). Cholestane intercalates into the bilayer disk micelle such that its molecular long axis is nearly parallel to the bilayer normal. The principal values of the hyperfine splitting tensor are Axx = 6.3 G, Ayy = 5.8 G andAzz= 33.5 G [18]. Figure 2 shows the ESR spectrum of an aligned disk micelle phase in the absence of any chiral guest molecule. (Admittedly cholestane itself is optically active, but its extremely low concentration in the bilayer coupled with its low twisting power renders the perturbation negligible) [1,19]. Three fairly sharp peaks are
Ayy = 5.8 G A
dk//
O > ~ . p - - O - - - > Axx
D
-"
T
=63G
Azz = 33.5G
~I I
Ho )
: Ti
I
I I
I i I
I
I i
Cholestane
Fig. 1. The structure of cholestane spin label and the molecular based coordinate system. Also shown is the intercalation of cholestane into a disk micelle.
observed for the 0 ° orientation, with a hyperffme splitting of 16.6 G as measured from the low to midfield zero point crossings. A splitting of approx. 19.9 G would be expected for a perfectly ordered spin probe rotating rapidly about its long axis. Upon rotation by 90 ° about the tube long axis (y-axis of the laboratory frame, Fig. 3), a lower splitting of 13.4 G is observed, together with a broadening of the low and high field resonances. Type II disk micelles align in an applied magnetic field such that the bilayer normals are on the average, perpendicular to the field which is defined by the laboratory z-axis. In the absence of sample spinning, the
10G Fig. 2. ESR spectrum of cholest~me in NaDS disk micelles after alignment (A) and after rotation by 90 ° about the laboratory ),-axis (B).
4 Y _-
_ He
_
_.),
x
Fig. 3. Laboratory based frame of reference in which the magnetic field direction defines the z-axis and the tube long axis is along the y-axis. preferred direction within the xy-plane is along x-axis although there does exist a distribution of a small number of micelles of differing alignment [20]. Therefore after rotation by 90 ° most bilayer normals are parallel to the field direction and thus the field is almost exclusively parallel to the molecular long axis, orthogonal to the orbital containing the unpaired electron. Also shown in Fig. 4 are spectra for the same micelle system containing increasing quantities of brucine. The spectra for the 0 ° orientation are virtually identical for all concentrations and a representative spectrum is shown in Fig. 4A. However, spectral changes occur in the 90 °
Fig. 4. A: E S R spectra of disk micelles containing 0.05 m o l % brucine before rotation (0° orientation). (Note the similarity to Fig. 2A.) B: the same sample after rotation (90 ° orientation). B - G are spectra after 90 ° rotation and contain the following increasing quantities of brucine. B, 0.05 tool%; C, 0.08 tool%; D, 0.10 tool%; E, 0.14 mol%; F, 0.33 tool%; G, 0.66 tool%.
orientation as cholestericity is induced, (Fig. 4 ( B - G ) ) as a result of a change in the distribution of the angle which the directors and thus the orbital containing the unpaired electron makes with the applied field. Upon initial alignment of a cholesteric disk phase, the bilayer normals are all perpendicular to the applied field and the screw axis is along the field or z-axis. However for a cholesteric phase, the directors undergo 360 ° rotations within the x y plane i.e. there is now a uniform distribution in this plane (Fig. 5). As brucine is added to the micelles, the ESR spectra upon 90 ° rotation show increasingly more intensity in the spectral extremes, i.e. the low field extreme of the low field resonance and the high field extreme of the high field resonance. The result is a continual increase in the measured hyperfine splitting until approx. 0.34 mol% added brucine, after which the value remains constant (Fig. 6) and no further changes in spectral shapes occur (Fig. 4). The twisting power of brucine and other optically active guest molecules towards disk micelle systems has been previously investigated by polarizing microscopy [1,5]. Cholesteric textures were observed at least as low as 0.113 mol% of brucine corresponding to a helical twist of l i p = 34.5 cm -1 where p is the pitch of the helix [1 ]. In addition the pitch decreased linearly with increasing concentrations of the optically active guest. Similar results were obtained by the use of cinchonine or (--)-tartaric acid as the chiral guest species. These observations are shown in Figs. 7 and 8. In Fig. 6, it is known that the ability of the chiral agents to induce cholestericity follows the order brucine > cinch0nine >,> tartaric acid, in agreement with optical studies [1,5]. This sequence is a result of a combination of twisting power and partitioning between bilayer and aqueous components [ 1,5]. In order to reconfirm that the helical axis aligns along the applied field, an experiment was performed in which the sample containing 0.66 mol% brucine was prealigned in a superconducting solenoid. In this case the field is along the tube long axis. When the sample is placed in the ESR spectrometer, a limiting A
B
Y
y
Fig. 5. Distribution of the local directors or disk bilayer normals after rotation. A: in the absence of a chiral guest, directors are predominantly along the field direction. B: in a fully cholesteric system the directors are uniformly in a cylindrical distribution hn the yz plane. The screw axis is along the x-axis. For the cholestane spin probe the orbital containing the unpaired electron is perpendicular to the director, while for the 4-KL label, it is colinear with the directors.
t
I
O0 []
r-I-
9o °
~o. is
14./ '0
~-~
~.~
13.~
12
o
J
b
i
~
m01e% guest omphiphile Fig. 6. Hyperfine splitting (G) of cholestane spin label vs. concentration of optically active component for the 0 ° orientation (uppermost horizontal line) and the 90 ~ orientation (lower curves) for brucine (a), cinchonine (o) and (--)-tartaric acid (~). The filled points refer to disk miceUes containing no optically active guest molecules.
Fig. 7. ESR spectra of cholestane in NaDS disk micelles (90 ° orientation) containing the following increasing quantities of cinchonine. A, 0.09 mol%; B, 0.17 reel%; C, 0.38 mol%; D, 0.59 mol%.
A
Fig. 8. ESR spectra of cholestane in NaDS disk micelles (90 ° orientation) containing the following increasing quantities of (--)-tartaric acid. A, 0.34 mol%; B, 1,00 mol%; C, 1.90 tool%; D, 3.30 mol%.
cholesteric spectrum with a splitting of 15.44 G which was invariant to sample rotation about the tube long axis i.e. the directors are now distributed uniformly in the xz plane since the screw axis is along the y-axis. In this way the screw axis may be manipulated to be along the y-axis or as previously, along the x-axis by alignment in an iron magnet followed by 90 ° rotation about the y-axis. The spinning of fully twisted samples in an iron magnet was found to be sufficient to overcome the helical packing of the twisted disk micelles. A sample containing 0.66 mol% brucine spun in a magnetic field of 1.41 T yielded an ESR hyperfine splitting of 16.6 G. In addition, the splitting was invariant upon rotation about the laboratory y-axis, and the lineshape was identical to those of samples aligned in iron magnets before rotation. The effect of spinning is sufficient to overcome the forces favouring helical packing and to align the micelles, even though twisted, such that their bilayer normals are along the spinning y-axis.
Doxyl laurate spin probe For this chain labeled nitroxide, the nitrogen 2prr orbital containing the unpaired electron (which defines the molecular based zz-axis) is co-linear with the molecular long axis. The N - O bond direction fixes the xx-axis. This amphiphile intercalates into the bilayer disk micelle such that the zz-axis is preferentially along the bilayer normal i.e. initially perpendicular to the applied magnetic field, and predominantly along the x laboratory axis (Fig. 3) [7,20]. Therefore in the absence of cholestericity, a low hyperfine splitting should be observed for the 0 ° orientation while a larger splitting should be found after 90 ° sample rotation
about the tube long axis (y-axis of the laboratory frame). The experimental values are 13.0 G and 17.2 G for the 0 ° and 90 ° orientations, respectively, in agreement with previous results [7]. The magnitude of the difference in hyperfine splittings is not as large as has been reported for some lamellar liquid crystals [ 14] because of oscillation of the micelles as whole bodies and/or co-operative distortion waves of the local directors [7,21 ] which result in spectral broadening. As the concentration of brucine is increased, the hyperfine splitting for the 0 ° orientation remains constant at 13.0 G through to the highest concentration studied (1.32 mol%). In the absence of a chiral solubilisate, the hyperfine splitting upon 90 ° rotation is 17.2 G, as previously stated. The addition of 0.34 mol% brucine lowers this value to 14.6 G. It remains constant as the concentration of brucine is increased to 0.66 mol% and finally to 1.32 mol%. Therefore the chain labeled nitroxide indicates the presence of a completely helical packing of micelles at the same concentration as did the cholestane label (0.32 mol%). For the 0 ° orientation, the bilayer normals are uniformly distributed in the xy plane and therefore the 2pTr orbital containing the unpaired electron (zz-axis) is preferentially perpendicular to the laboratory z-axis. The result is a low hyperfine splitting. Discussion What is being observed here by ESR, is the distribution of the directors [14]. For a helical arrangement, a uniform two-dimensional distribution from 0 ° to 360 ° would be expected, and indeed is observed after 0.34 mol% brucine is added. Once completely formed in this distribution the ESR method is insensitive to tighter twisting. Before the plateau in Fig. 6 is reached, it must be concluded that the entire sample is not 'cholesteric'. It has been proposed that the origin of the macroscopic twist is an induced twist in each planar bilayer micelle caused by short range steric interactions and that these twisted propeller shaped micelles pack most efficiently with an orientational helical long range order [5]. At low twists, what may be being seen by ESR spectroscopy is an emulsion of twisted micelles in a planar micelle nematic matrix. As more brucine is added, twist is induced into more of the planar miceUes, leading to a system of completely twisted disks, i.e. below the plateau region of Fig. 6, some of the spin probes reside in planar micelles, while some reside in very slightly twisted micelles which are beginning to pack with helical order. However, assuming a uniform distribution of the chiral molecules among the individual micelles, simple calculations involving estimates of the number of amphiphiles per micelle predict the presence of several optically active guests in each micelle even at the lowest concentrations studied [5]. Therefore all micelles would be expected to be twisted and packed into helices. The ESR result would then be expected to be a splitting corresponding to the plateau value of Fig. 6. A much more plausible explanation involves the presence of an irregular distribution of micellar and aqueous regions in the mesophase. Low angle X-ray
diffraction studies of NaDS Type II disk micelles showed evidence that this system is not in the strictest sense, nematic [22], but that some areas composed of aggregates of these platelets are present with little water between them. The twist induced into a micelle can only be translated into macroscopic cholesteric behaviour through an intermicellar force field [5]. Such forces are highly distance dependent [1,5,23]. Therefore certain regions characterized by high micelle density will experience much greater intermicellar forces responsible for transmitting helical order and pack in a cholesteric fashion. Other regions characterized by larger intermicelle distances will not be affected to nearly the same extent in spite of the fact that all individual micelles may be twisted to the same small degree. Thus, a superposition of cholesteric and non cholesteric regions are reported by ESR. As the individual micelles are increasingly distorted by increasing quantities of chiral additivies, the more water rich areas will begin to pack in a helical manner, eventually leading to a limiting hyperfine splitting when all spin probes are arranged in a cylindrical distribution. Conclusions
The transition from a Type II disk micelle phase to a cholesteric phase induced by the incorporation of optically active guest molecules is characterized by the growth of regions of helical micellar packing in a nematic matrix. Since the macroscopic cholesteric behaviour is translated through a distance dependent intermicellar force field, the onset of helical packing is associated with regions of high micelle density. Although all micelles should be twisted to the same degree, the communication of this twist is much weaker in water rich areas. As the concentration of the optically active species is increased, all micelles begin to assemble along a screw axis. The ESR spin probe method is then insensitive to further reductions in the pitch. Acknowledgements
The financial support of the Natural Sciences and Engineenng Research Council of Canada is gratefully acknowledged. Sample alignment at 8.45 T was made possible through the use of the facilities of the Atlantic Region Magnetic Resonance Centre, Halifax, Canada. References.
1 K. Radley and A. Saupe, Mol. Phys., 35 (1978) 1405. 2 P. Diehl and A.S. Tracey, FEBS Lett., 59 (1975) 131. 3 L.J. Yu and A. Saupe, J. Am. Chem. Soc., 102 (1980) 4879. 4 B.J. Forrest, L.W. Reeves, M.R. Vist, C. Rodger and M.E.M. Helene, J. Am. Chem. Soc., 103 (1981) 690.
i0 5 P.S. CoveUo, B.J. Forrest, M.E.M. Helene, L.W. Reeves and M. Vist, J. Phys. Chem., 87 (1983) 176. 6 M. Acimis and L.W. Reeves, Can. J. Chem., 58 (1980) 1533. 7 B.J. Forrest and J. Mattai, J. Phys. Chem., in press. 8 J. Seelig and W. Niederberger, Biochemistry, 13 (1974) 1585. 9 M.G. Taylor and I.C.P. Smith, Biochim. Biophys. Acta, 599 (1980) 140. 10 R.P. Mason and C.F. Polnaszek, Biochemistry, 17 (1978) 1758. 11 M.G. Taylor and I.C.P. Smith, Biochemistry, 20 (1981) 5252. 12 J. Seelig, Quart. Rev. Biophys., 10 (1977) 353. 13 B.J. Forrest and L.W. Reeves, Chem. Phys. Lipids, 32 (1983) 73. 14 J. Seelig, in: L.J. Berliner (Ed.), Spin Labeling. Theory and Applications, Academic Press, New York, 1976, p. 373. 15 P.G. deGennes, in: The Physics of Liquid Crystals, Clarendon Press, Oxford, 1974, p. 253. 16 J. F.W. Keana, S.D. Keana and D. Beatham, J. Am. Chem. Soc., 89 (1967) 3055. 17 L.W. Reeves and A.S. Tracey, J. Am. Chem. Soc., 97 (1975) 5729. 18 B.J. Gaffney and S. Chen, Methods Membr. Biol., 8 (1977) 291. 19 G.R. Luckhurst and H.J. Smith, Mol. Cryst. Liq. Cryst., 20 (1973) 319. 20 F.Y. Fujiwara and L.W. Reeves, Can. J. Chem., 56 (1978) 2178. 21 E. Meirovitch and J.H. Freed, J. Phys. Chem., 84 (1980) 20. 22 L.Q. Amaral and M.R. Tavares, Mol. Cryst. Liq. Cryst., 56 (1980) 203. 23 J.N. Israelachvili, Faraday Discuss. Chem. Soc., 65 (1978) 20.