3802
J. Phys. Chem. 1983,87,3802-3804
Lyotropic Liquid Crystal with a Tetrahedral Orientation Pattern in a Magnetic Field Ulf Henrlksson' and Tomas Klason Department of Physical Chemistry, The Royal Institute of Technology, S-100 44 Stockholm 70, Sweden (Received June 6, 1983)
The hexagonal phase in the system water-hexaethylene glycol dodecyl ether (CI2EO6)has been studied by 2H NMR. The NMR spectrum from a liquid crystal formed in the magnetic field clearly shows that it is macroscopically oriented and from the rotational pattern it is shown that it contains four director orientations which are tetrahedrally oriented with respect to each other. This behavior is discussed in relation to possible structures of the cubic liquid crystalline phase which is stable at somewhat higher temperature.
Introduction Aqueous surfactant systems form a variety of liquid crystalline phases consisting of aggregates of surfactant molecules.'P2 The common lamellar, hexagonal, and cubic phases have directional order between the different aggregates and translational order in one, two, and three dimensions, respectively. These phases usually do not orient spontaneously in magnetic fields since they are very viscous and the orientating force caused by the anisotropy of the diamagnetic susceptibility is too small to overcome the frictional force. However, they can in many cases be oriented between glass plate^.^ It is sometimes possible to obtain a macroscopic orientation relative to the field if the liquid crystal is formed from an isotropic phase by slow cooling in a magnetic field.4 In some aqueous ionic surfactant systems the occurrence of lyotropic nematic liquid crystalline phases has been These phases are built up by rod-shaped or disk-shaped aggregates with directional order but no translational order. The viscosity of the lyotropic nematic phases is usually low and they therefore orient spontaneously in magnetic fields. Whether the aggregates orient with their symmetry axes parallel or perpendicular to the magnetic field depends on the sign of the anisotropy of the diamagnetic susceptibility. In this communication we report 2H NMR investigations of the orientation of a hexagonal lyotropic liquid crystal formed in the presence of a magnetic field. The sample contained D20 and the nonionic surfactant hexaethylene glycol dodecyl ether (C12E06)with the molar ratio 15.4:l. The phase equilibria in this system have been studied by Clunie et a1.8 and by Mitchell et al.9 The phase diagram is reproduced in Figure 1. The interaction between water and the ethylene oxide segments in the liquid crystalline phases in this system has previously been studied by 2H NMR.1° (1)P.Ekwall, Adu. Liq. Cryst., 1 , l(1971). (2)G. J. T. Tiddy, Phys. Rep., 57, 1 (1980). (3)J. J. de Vries and H. J. C. Berendsen, Nature (London),221, 1139 (1969). (4)A. Johansson and T. Drakenberg, Mol. Cryst. Liq. Cryst., 14, 23 (1971). (5) K. D. Lawson and T. J. Flautt, J. Am. Chem. Soc., 89,5489(1967). (6) B. J. Forrest and L. W. Reeves, Chem. Rev., 81,1 (1981). (7)N. Boden, K. McMullen, and M. C. Holmes, "Magnetic Resonance in Colloid and Interface Science", J. P. Fraissard and H. A. Resing, Eds. Reidel, Dordrecht, The Netherlands, 1980,pp 667-73. (8)J. S. Clunie, J. F. Goodman, and P. C. Symons, Trans. Faraday Soc., 65,287 (1969). (9)D. J. Mitchell, G.J. T. Tiddy, L. Waring, T. Bostok, and M. P. McDonald, J. Chem. Soc., Faraday Trans. I , 79,975 (1983). (10)T. Klason and U. Henriksson, 'Proceedings from the International Symposium on Surfactants in Solutions, Lund, 1982,"K. L. Mittal and B. Lindman, Eds., in press.
0022-3654/83/2087-3802$0 1.50/0
Experimental Section Hexaethylene glycol dodecyl ether was obtained from Nikko Co., Tokyo. The 2H NMR spectra were recorded a t 13.8 MHz and 2.1 T with a Bruker CXP-100 spectrometer. The sample was prepared in a 10-mm NMR tube and it was mixed by heating above the melting point of the lamellar phase. The sample was equilibrated a t room temperature in the isotropic phase region V1 outside the magnet for several days. It was placed in the magnet for several hours and then cooled to +7 "C with the rate 2O/h.
-
Results and Discussion Due to the quadrupole interaction a 2H NMR spectrum from a liquid crystal consists of two equally intense lines centered around the Larmor frequency. The frequency separation between the two lines depends on the orientation of the director with respect to the magnetic field and it is for an axially symmetric field gradient given byll
NOD,) = 2 ( u Q S @ i ) ( O D L ) (
(1)
The time average S = D#(OMD)is the order parameter which describes the average molecular orientation with respect to the director. VQ = (3/4)(e2Qq/h)where e2Qq/h is the quadrupole coupling constant, ODL is the an le between the director and the magnetic field, and Df$(ODL) is a Wigner rotation matrix element for a transformation from the director coordinate system to the laboratory system given by D#(ODL)
= (1/2)(3 cos2 ODL - 1)
(2)
The observable quadrupole splittings therefore fall in the range 0 IA I2luQSI. Figure 2a shows the 'H spectrum from a liquid crystal formed outside the magnetic field. This is a typical "powder spectrum" consisting of a superposition of doublets for all equally probable orientations. The separation between the two central peaks is" Ap
=
l'QsI
(3)
The spectrum in Figure 2b was obtained from a liquid crystal which had been formed in the magnetic field of the NMR spectrometer as described above. Figure 2c-k shows spectra for different rotations of the sample around the axis of the sample tube which is perpendicular to the magnetic field. It is evident from these spectra that the sample is macroscopically oriented and that it contains several different director orientations. For the interpretation of these spectra it is convenient to first consider (11) H. Wennerstrom, G.Lindblom, and B. Lindman, Chem. Scr., 6 , 97 (1974).
0 1983 American
Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 20, 1983 3803
Letters
a)
25
0
5'0
t
75
"t
i
@
= oo
=
b)
"t
@
C)
"t
@ = 144'
GzEOs
540
Figure 1. Phase diagram for the system C12E06-H20 (from ref 9; vertical axis: temperature in OC; horizontal axis: composition in wt % C,,EO,): (L,) lamellar phase, (HI) hexagonal phase, (V,) cubic phase. t marks the composition of the studied sample.
1 kHr
i. -f
I, I i
-
i
I
q= 144' \
Figure 3. Different director orientations in the tetrahedral orientation model described in the text. The axis of rotation is along the y axis. The model is symmetric with respect to the xz plane.
_
Flgure 2. H , NMR spectra at +7 OC from a sample with the mole ratio D,0C,g06 = 15.4:l.a: spectrum from a sample equilibrated outside the magnetic field. b-k: spectra from oriented sample at different rotation angles.
spectrum 2d, which was obtained after the sample had been rotated 54'. This spectrum exhibits a doublet of narrow lines. A comparison with the powder spectrum in Figure 2a shows that the doublet separation is IY$~. According to eq 1 and 2 this gives the following possible orientations: ODL = 35.3', ODL = 90°, and ODL = 144.7'. It should be noted that the former angle is the complement angle to the "magic angle" 54.7O. When the sample is rotated 9' further (Figure 2e), the doublet splits into three doublets: one with a larger, one with a smaller, and one with almost unchanged separation. This shows that all three director orientations mentioned above must be present in the sample. When the sample is rotated 144' (Figure 29, all three doublets collapse to a singlet at the Larmor frequency showing that ODL = 54.7' for all directors in this case. Considering the fact that the magic angle is half of the tetrahedral angle, these observations can be explained by the model shown in Figure 3. It is assumed that crystallites with four different director orientations exist in the sample and that these directors are arranged tetrahedrally with respect to each other. The directors are oriented relative to the magnetic field as shown in Figure 3a. When the sample tube is rotated around its axis, this model gives rise to the rotation pattern shown in Figure 4. For the two directors in the nz plane the rotation in eq 2 varies between +1 and matrix element Dg)(ODL) -1/2 while for the other two directors D#(ODL) varies between 0 and -112. As is seen in Figure 4 there is good
1
>
"
"
"
"
I
"
"
900
180°
Angle of rotation Flgure 4. Rotational pattern for the 'H quadrupole splittings. The full lines represent the pattern predicted by the tetrahedral orientation model. The ordinate has been scaled so that 1.0 corresponds to an orientation parallel to the magnetic field. The signs of the experimental points have been chosen in accordance with the model.
agreement between the experimental quadrupole splittings and the rotation pattern predicted by the tetrahedral model described above. It is known from X-ray diffraction that the hexagonal phase studied in this work consists of long, rod-shaped aggregates of surfactant molecules arranged in a two-dimensional hexagonal l a t t i ~ e . It ~ *is~hard to find any reason why a hexagonal phase which is oriented in a magnetic field should have orientations that are not parallel or perpendicular to the field. However, when the sample is slowly cooled from room temperature, it passes the region where the cubic phase is stable (Figure 1)and the expla-
3804
J. Phys. Chem.
1983,87,3804-3805
nation to the unexpected orientational pattern obtained in this work must probably be sought in the structure of this cubic phase. If the hexagonal phase is formed from a cubic phase which is oriented with respect to the magnetic field, it is possible that hexagonal crystallites will be oriented along the different directions that already are present in the cubic phase. To our knowledge no structural information is available about the cubic phase in the present system. The tetrahedral orientation pattern found in this work might indicate that it is built up by rods with their axes tetrahedrally arranged with respect to each other. Such structures have been proposed for cubic phases in other amphiphile-water systems. A structure with two interwoven but not connected networks of rods of finite length tetrahedrally joined four by four at each end (space group Pn3m) has been suggested by Rand et al. for the cubic phase which is stable at low pH in the system pho~phatidylethanolamine-water.~~-'~This
structure is the same as in ice VII. Recently, Longley and McInto~h'~ have proposed a similar structure for the cubic phase in the glycerol monooleate system (space group Pn3 or Pn3m). A structure with long, unconnected rods arranged tetrahedrally in a body-centered cubic lattice has been proposed by Saludjian and Reiss-Husson.16 This structure has, however, been criticized by Luzzati et al.17 Registry No. Hexaethylene glycol dodecyl ether, 3055-96-7. (12)A. Tardieu, T. Gulii-Krzywicki, F. hiss-Husson, V. Luzzati, and R. P. Rand, 'Abstracts from the British Biophysical Society Meeting on Biophysical Studies of Cell Membranes, Birmingham, 1969". (13)A. Tardieu, Thesis, University Paris Sud, Paris, France, 1972. (14)R. P. Rand, D. 0. Tinker, and P. G. Fast, Chem. Phys. Lipids, 6,333 (1971). (15)W.Longley and J. McIntosh, Nature (London),303,612(1983). (16)P. Sdudjian and F. Reiss-Husson, Proc. Natl. Acad. Sci. U.S.A., 77,6991 (1980). (17)V.Luzzati, A. Tardieu, and T. Gulik-Krzyvicki,Proc. Natl. Acad. Sci. U.S.A., 78,4683 (1981).
CH Stretching Overtone Spectra of Nltrobenzene and Its Deuterated Derivatives. Assignment of the Ortho CH Kathleen M. Gough and Bryan R. Henry' Chemistry Department, University of Mannoba, Winnipeg, Manitoba, Canada R3T 2N2 (Received: June 6, 1983)
The liquid-phase CH and CD stretching overtone spectra of C6H5NO2,C6D5NO2,and a mixture of C6D5NO2, C6HD4NO2, and C6H2D3N02 (H's are in the ortho positions) are reported. The spectra of C8H5NO2,as reported previously, and of C6D5N02are doublets. The higher frequency peak is assigned to the CH (CD) bonds in the ortho positions by comparison with spectra of the partially deuterated species.
positions. The anticipated effect of the nitro substituent is a decrease in bond length and a shift in the overtone spectra to higher frequencies. We suggested that this effect would be most pronounced at the ortho positions; hence, the assignment of the higher frequency peak to the CH bonds in the 2- and 6-positions seemed justifiable. The anharmonicity constant for the higher frequency peak was significantly smaller than that for the lower frequency The local-mode parameters, w and X,are affected by bond peak. This difference was interpreted as the result of length/strength and by steric factors, respectively. increased steric hindrance at the ortho position due to the For substituted benzenes, the electron-donating or nitro group. Finally, the ratio of the peak areas at Au = -withdrawing properties of the substituent result in 3 was 3:2 for low- to high-frequency peaks. changes in aryl CH bond strength and length. Similarly However, as one referee pointed out, these three obto the previous observations of Katayama and co-~orkers,~ servations (relative frequency, anharmonicity, and intenwe observed a shift in the position of the overtone peak sity) do not provide conclusive evidence for our chosen maxima relative to benzene which corresponded roughly assignment. The correlation with uI was only fair. An to the value of uI,the inductive part of the Hammett u . ~ adequate general theory for the intensities of peaks in However, in only one case were inequivalent hydrogens local-mode overtone spectra does not exist as yet. In some resolved. The overtones of nitrobenzene were doublets for notable cases, it has been clearly demonstrated that relaAv = 3-6. For the lower frequency peak in nitrobenzene, tive peak intensities do not correspond to the number of the frequency shift from benzene, AD, followed the same CH oscillators of a given type. For example, in cyclocorrelation with uI as the unresolved peaks for the other hexane, the equatorial peaks are about twice the intensity compounds. Since uI is empirically derived for the meta of the axial peaks.5 In the normal alkanes, the methyl and para positions only, we concluded that this peak was peaks are more intense than the.methylene peaks on a due to excitation of the CH bonds in the 3-, 4-,and 5per-hydrogen We have completed studies on the gas-phase spectra of toluene and the xylenesF9 and a series In a recent paper,l we reported the results of a study of the CH stretching overtone spectra for a series of liquidphase substituted benzenes. These higher overtones are described by the local-mode model2 which pictures the various CH bonds as uncoupled anharmonic oscillators. The energies of the overtone spectral peaks fit the equation A E ~ -(cm-') ~ = wu + X u 2 (1)
(1)K. M. Gough and B. R. Henry, J. Phys. Chem., in press. (2)B. R. Henry, Vib. Spectra Struct., 10,269 (1981). (3)Y. Mizugai and M. Katayama, J. Am. Chem. Soc. 102,6424(1980); Y. Mizugai, M. Katayama, and N. Nakagawa, ibid., 103, 5061 (1981). (4)R.W. Taft, "Steric Effects in Organic Chemistry", M. S. Newman, Ed., Wiley, New York, 1956,pp 594-7.
0022-3654/83/2087-3804$0 1.5010
(5)H. L.Fang and R. L. Swofford, J. Chem. Phys., 73,2607 (1980). (6)W. R. A. Greenlay and B. R. Henry, J. Chem. Phys., 69,82 (1978). (7)J. S.Wong and C. B. Moore, J. Chem. Phys., 77,603 (1982). ( 8 ) K.M.Gough and B. R. Henry, submitted for publication.
0 1983 American Chemical Society
