The Journal of
Physical Chemistry
0 Copyright 1993 by the American Chemical Society
VOLUME 97, NUMBER 47, NOVEMBER 25, 1993
LETTERS Birefringence Dispersion and Inversion in Lyotropic Liquid Crystals 0. Santin Filho and L. Q. Amaral' Instituto de Fiica, Universidade de Siio Paulo, C.P. 20516, Siio Paulo, Brazil Received: August 12, 1993; In Final Form: October 7, 1993"
The birefringence of a nematic cylindrical lyomesophase has been measured by polarized interferometry and optical microscopy in the wavelength interval 300-800 nm. An inversion of the optical signal is observed a t 387.5 nm; An > 0 for smaller A values and An < 0 for larger A values. The experimental results are well fitted to a theoretical curve with two dispersion terms, due to ultraviolet and infrared absorption bands, of respectively positive and negative signals.
Introduction Several methods for determining the dispersion of the birefringence (An) as a function of wavelength (A) for thermotropic liquid crystals were developed.14 Comparison among these methods was also d i ~ c u s s e d . ~ ~ ~ The study of optical properties of lyotropic liquid crystals is mainly restricted to direct microscopic observations7J' to identify and classify different mesophases9J0 and to study defects.11J2 Determination of An as a function of composition and temperature13J4 is carried out usually in a single-wavelength value, from Abbe refractometers or microscopic measurements. In this paper we investigate the behavior of the birefringence of an ordered N, lyomesophase as function of A, measured by the interferometric method and by polarized optical microscopy, and analyze the results in terms of polarizabilities. To our knowledge, the interferometric method has been already applied to thermotropic nematicsl-2 but not to the lyotropic N, nematics. Experimental Results Sodium dodecyl sulfate (Merck, P.A.) was recrystallized three times from absolute ethanol. Water and decanol- 1 (Riedel-deHaen) were triply distilled in an all-glass apparatus. The samples were prepared with the following weight percent composition:ls SLS (25.00%)/DeOH-l (4.47%)/H20 (70.53%). The formation of cylindrical N, mesophase was checked by visual inspection of *Abstract published in Advance ACS Absrwcrs, November 15, 1993.
the texture of the film formed at the test tube surface under flow, between crossed polarizers, by optical texture observations,9 and by X-ray diffraction.l6 The samples were transferred to flat capillaries 400 pm thick (Vitrodynamics) and sealed with Teflon ribbon and Parafilm. The alignment was carried out in a 2.3-T magnetic field (Varian XL-100) for 1-2 days, with direction of 80parallel to the flat surface of the capillary. The samples retained their orientation for several hours after removal from the field, as checked systematically by visual inspection between crossed polarizers during the experiments. Measurements were performed at 25 OC.
The absorption spectra with polarized light were obtained in a Beckman DU-7 spectrophotometer, equipped with two adjustable polarizers. Figure 1 shows the experimental setup, with the flat surface of the sample perpendicular to the direction of the propagation of light and with its optical axis forming 45O with the optical axes of the crossed polarizers. The background spectrum was carried out with the same geometry, but using a nonaligned sample between the crossed polarizers. The interference spectrum obtained is shown in Figure 2. When a polarized monochromatic light wave propagates from the polarizer to a homogeneous aligned phase with its polarization axis forming 45O with the director h, the ordinary and extraordinary rays in the outgoing light will have experienced a path
0022-3654/93/2097-12107%04.00/0 0 1993 American Chemical Society
Letters
12108 The Journal of Physical Chemistry, Vol. 97, No. 47, 1993 I
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Figure 1. Geometrical arrangement of the experiment. P = polarizer, A = analyzer, and A is the long (optical) axis of the sample. Note that LAP = 90' and LPA = 45'. In the micelle, the hydrocarbon chains are perpendicular to fi (see text).
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300.0
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Figure 3. Birefringence An as a function of wavelength A. Experimental points are obtained by interferometry and optical microscopy. Theoretical curve is obtained by fitting to eq 4.
Y
visualized by the darkening of the center of the optical field of the microscope. The rotation angle can be easily read in the appropriate scale of the compensator, and from it the path difference and IAnl are obtained. It is important to note that, for lyotropic N, phases, the values of An are negative in this range of wavelengths.8 This was checked by observation of the uniaxial cross in conoscopic geometry. Figure 3 shows the values of An as function of X as obtained by both methods. The result shows an inversion of the optical signal at 387.5 nm. For smaller values of X, An > 0, while for higher values An < 0.
Discussion
A (nm) Figure 2. Transmission spectrum obtained in the spectrophotometer. Sample is oriented with its optical axis forming 45' with the optical axes
Birefringence is caused by the anisotropic polarizabilities of liquid crystals and can be expressed in terms of the molecular properties by18-24
of the crossed polarizers. differenceL7
r = 4Anl (1) where d is the sample thickness and An = ne- nois the birefringence (ne is the refractive index for the extraordinary and no for the ordinary rays). When the two rays leave the sample, they will recombine and the polarization of the resultant will depend on the path difference r:17
r = KX/2 ( K = integer) (2) For K even, the outgoing ray will have the same polarization as the incoming ray, and the light will be absorbed by the analyzer. For K odd, the outgoing ray will have polarization perpendicular to the incoming ray, and the light will be transmitted by the analyzer. Figure 2 shows the alternate "maxima" and "minima" of transmission due to the modulation on r. Because An is about 103 times smaller for lyotropics than for thermotropics, only two maxima are observed, even with samples 10 times thicker (and this is the limit for having well-oriented samples). That is probably the reason why this method had not been applied yet to lyonematics. The signal of An and the K scale were obtained by comparison of the results at 536.5 nm with results from optical microscopy. The determination of An X X by optical polarized microscopy was carried out in a Carl Zeiss universal microscope, equipped with an Ehringhaus compensator and a monochromator, with orthoscopic illumination. The path difference, for each wavelength, between the ordinary and extraordinary components of light is compensated by changing (by rotation) the thickness of the calcite plate of the compensator. The compensation can be
with G ( T ) = gNZScfii* --fi*). The summation is over the absorption bands (both electronic and vibrational) at wavlengths A*, gis a proportionality constant, N is the number of molecules per unit volume, 2 is the number of active electrons, S is the order parameter, and cfil* - fi*)is the differential oscillator strength (proportional to dipole matrix elements for IR vibrations). This equation holds for X values not too near X*. According to eq 3, we see that if A >> A*, An depends only on A* and will be X independent, and if X fi*. For discotic nematic lyomesophases that have the paraffinic chains parallel to the optical axis, the reverse signal occurs. It is important to note that it was not necessary to take into account the strong absorption of the stretching mode of water (A* c 2900 nm), which indicates that the water has an average isotropic structure. For thermotropic rodlike liquid ~rystals20,23~~~ birefringence is due essentially to the electronic transitions in the UV region, for which in general A,* > fi*, although positive and negative birefringence contributions due to UV and IR resonance absorption bands have been reported for thermotropic compounds.20 In general, thermotropic rodlike mesophases have An > 0 while N, lyonematics have An < 0 in the visible region as a consequence of the IR absorption band of lyonematics, as opposed to the dominant UV absorption of thermotropics. The UV band a t 233 nm for this lyonematic phase could be ascribed to an "effective" value related to absorption bands of
C-C, C-0, 0-H,and SO4 radi~als.2~728This value could be checked independently by spectrophotometric measurements in the vacuum-ultraviolet region. Finally, the two novel results here reported, namely, the inversion of the optical signal near the UV region and the increase of [An(in the IR region (due to a characteristic IR absorption band), are general properties of lyonematics and may open new applications for these liquid crystals.
Acknowledgment. 0. Santin Filho has a doctorate fellowship from CAPES. We thank Prof. Hernan Chaimovich and Dr. J. Atilio Vanin for the use of equipment and helpful discussions. References and Notes (1) Kuczynski, W.; Stryla, B. Mol. Cryst. Liq. Cryst. 1976, 31, 267. (2) Kuczynski, W.; Pieranski, P.; Wojciechowski, K.; Stryla, B. Mol. Cryst. Liq. Cryst. 1977, 34, 203. (3) Riviere, D.; Levy, Y.; Imbert, C. Opt. Commun. 1978, 25, 206. (4) Wu, S. T.; Efron, U.; Hess, L. V. D. Appl. Opt. 1984, 23, 3911. (5) Laurent, M.; Journeaux, R. Mol. Cryst. Liq. Cryst. 1976, 36, 171. (6) Akahane, T.; Hashimoto, T.; Tako, T. Jpn. J. Appl. Phys. 1980,19, 1419. (7) Ekwall, P. Ado. Liq. Cryst. 1975, I , 1. (8) Yu, L. J.; Saupe, A. J. Am. Chem. SOC.1980, 102, 4879. (9) Bittencourt, D. R. S.; Amaral, L. Q. Liq. Cryst. 1989, 3, 283. (10) Fujiwara, F.; Reeves, L. W.; Suzuki. M.; Vanin, J. A. In Solution Chemistry of Surfactants; Mittal, K. L.,Ed.; Plenum: New York, 1979. (11) Meyer, R. Philos. Mag. 1973, 27, 405. (12) Williams, C.; Bouligand, Y. J. Phys. (Paris) 1974, 35, 589. (13) Bartolino, R.; Chiaranza, T.; Meuti, M.; Compagnoni, R.Phys. Reu. A 1982, 26, 11 16. (14) Meuti, M.; Barbero, G.; Bartolino, R.; Chiaranza, T.; Simoni, F. Nuovo Cimento 1984, 30, 30. (15) Amaral, L. Q.; Marcondes-Helene, M. E. J. Phys. Chem. 1988.92, 6094. (16) Amaral, L. Q. J. Appl. Crysrallogr. 1989, 22, 519. (17) Born, M.; Wolf, E. Principles of Optics, 5th ed.; Pergamon: Oxford, 1975. (18) Charney, E.; Halford, R. S. J . Chem. Phys. 1958, 29,221. (19) Vuks, M. F. Opt. Spektrosk. 1966, 60, 644. (20) Wu, S. T. Phys. Reu. A 1986, 33, 1270. (21) Wu, S. T.; Finkenzeller, U.; Reiffenrath, V. J. Appl. Phys. 1989,65, 4372. (22) Wu, S. T.; Wu, C. S. J. Appl. Phys. 1989,66, 5297. (23) Wu, S. T. J. Appl. Phys. 1991, 69, 2080. (24) Quina, F. H. In Chemical and Biological Generation of Excited Stares; Adam, W., Cilento, G., Eds.;Academic Press: New York, 1982. (25) Hui, Y. W.; Labes, M. M. J. Phys. Chem. 1987, 91,6066. (26) Wu, S. T.; Ramos, E.; Finkenzeller, U. J. Appl. Phys. 1990,68, 78. (27) Waggoner, W. H.; Chambers, M. E. Talmta 1960,5, 121. (28) Rao, C. N. R. Ultra-Violet and Visible Spectroscopy Chemical Application, 2nd ed.; Butterworths: London, 1967.
