J. Phys. Chem. 1985,89, 3737-3739 estimation of the rates of decomposition of excited radicals by chlorine atom loss.
Acknowlednment. This research has been supported by US. Department ofEnergy, Office of Energy Research, under Contract
3737
No. DE-AT03-76ER-70126. Registry No. CH2=CHC1, 75-01-4; '*Cl, 14158-34-0; CC1F3, 7572-9: HI, 10034-85-2: H, 1333-74-0; CH,39C1CHCI, 9691 5-22-9; CH2CHC139C1,96915-23-0; chlorine, 22537-i5-1.
Temperature and Concentration Range of the Biaxial Nematic Lyomesophase in the Mixture Potassium Laurate/l-Decanol/D,O A. M. Figueiredo Neto,*+ L. LiGbett,* and Y. Galerne* Laboratoire de Physique des Solides,t Batiment 510, UniversitZ Paris-Sud, 91405 Orsay, France (Received: August 6, 1984; In Final Form: March 14, 1985)
Two different planes of the three-dimensional phase diagram of the lyotropic mixture potassium lawate/ 1-decanol/D20 are investigated by optical microscopy, conoscopy, and X-ray diffraction techniques. For appropriate concentrations, the temperature range of the biaxial nematic phase goes up to 15 OC.
Introduction Mixtures of amphiphilic molecules and water may give nematic lyomesophases under proper temperature-concentration conditions.' From symmetry considerations? two uniaxial and one biaxial (NBX)3nematic phases are expected. Depending on whether the direc_tor (Z) orients parallel or perpendicular to the magnetic field (H), these uniaxial phases have been classified4 as calamitic (N,) and discotic (ND), respectively. The three nematic phases ND, NBX,and N, present a recent interest from both theoreticalZ and e ~ p e r i m e n t a l ~points - ~ of view. In 1980 Yu and Saupe published3 a phase diagram of the lyotropic mixture potassium laurate/ldecanol (at 6.24 wt %)/DzO where the different phases are identified by conoscopic and N M R measurements. Their phase diagram presents reentrant N, and NBX phases. On the other hand, Hendrikx and mworkers5 studied the phase diagram of the same mixture at a slightly different 6.27 wt % of 1-decanol (texture analysis, neutron, and X-ray measurements). They observed some differences by comparing to the diagram of ref 3: the NBX phase and the reentrant character of the N, phase were not observed. The discrepancies between these phase diagrams are important and seem not to be explained simply by the difference in the alcohol concentration. Moreover, the reentrant behavior of the N, and NBX phases was not observed in recent laser conoscopic measurements7 performed on a mixture which had to present this feature according to the ref 3 phase diagram. In this paper we present a detailed study of the potassium lawate/ 1-decanol/D20 phase diagram in two different planes of the concentration-temperature space a t a constant 1-decanol concentration (6.24 wt %) and at a fixed ratio between the potassium laurate (KL) and l-decanol (DeOH) concentrations, R = [KL]/[DeOH] = 4, in which larger temperature ranges of the NBXphase are found. The nematic phases are determined by crossing the results from three experimental techniques: optical microscopy (by observing the textures), conoscopy (by measuring the order parameter in the uniaxial and biaxial phases), and X-ray diffraction (by identifying the m i c r m p i c structure of the phases). Experimental Section Mixtures. The ldecanol is from Fluka (p.p.a. >99%), the D 2 0 is from CEA Saclay, and the potassium laurate is synthesized and recrystallized in the laboratory from commercial lauric acid (Fluka +On leave from the "Instituto de Fisica, Universidade de Sao Paulo, CP 20516, CEP 05508, Sao Paulo (SP) Brazil" CNPq financial support. 'Laboratoire associd au C.N.R.S. (LA No. 2).
0022-3654/85/2089-3737$01.50/0
p.p.a >99%). Its clarification point is 395 OC as described in ref 8. Special care is taken to avoid the contact of the KL with humid atmosphere. If not, the hydration of the amphiphilic molecules could increase the water concentration in the mixture and shift the transition temperatures in an uncontrolled manner. Mixtures are reported in previously well-cleaned glass tubes by carefully weighing each compound at a 0.005% accuracy. The sealed tube is then shaken in an electric vibrator and centrifuged for some minutes. This procedure is repeated several times until the mixture is homogeneous. It is then stored in a temperature-controlled stage a t about 25 O C . Optical Microscopy. Samples are sealed in flat microslides from Vitro Dynamics Inc. with inside dimensions 100-Fm thickness, 1-mm width, and 3-cm length, and placed in a temperaturecontrolled stage (of 1 O C accuracy). A polarized light microscope is used to observe the sample textures (orthoplan, Pol Leitz). Such an observation is an easy way to determine the temperature of the phase transitions, except in the case of the N,-NBx transition which can just be denoted by the occurrence of a faint veil. Conoscopy.' Samples are sealed in a glass cell of 1-mm thickness (from Hellma). The cell is placed in a servocontrolled thermostat (of 0.02 OC accuracy) which is itself held in a horizontal magnetic field of about 5 kG. The orientation of the sample (in the ND phase at the beginning of each experiment) is achieved by repeated rotations of the cell around the vertical direction in the magnetic field. The conoscopy is made with a He-Ne laser beam converging in the sample with a half-angle aperture of 50°. It allows one to determine unambiguously the macroscopic symmetry of each phase and to find their phase transition with a good temperature resolution (0.02 OC). This method always gave transition temperatures consistent with those found by the direct optical observation. (1) Radley, K.;Reeves, L. W.; Tracey, A. S. J. Phys. Chem. 1976,80, 174. (2) Freiser, M . J. Phys. Rev. Lett. 1970, 24, 1041. Alben, R. Phys. Reu. Lett. 1973, 30, 788. Rabin, Y.; Mullen, W. E.; Gelbart, W. M. Mol. Cryst. Liq. Cryst. 1982, 89, 67. Chen, Z. Y.; Deutch, J. M. J. Chem. Phys. 1984, 80, 2151. (3) Yu,L. J.; Saupe, A. Phys. Reu. Lett. 1980,45, 1000. (4) Hendrikx, Y.; Charvolin, J. J . Phys. (Lees. Ulis, Fr.) 1981, 42, 1427. (5) Hendrikx, Y.; Charvolin, J.; Rawiso, M.; Litbert, L.; Holmes, M. C. J . Phys. Chem. 1983,87, 3991. (6) Figueiredo Neto, A. M.; Galerne, Y.; Levelut, A. M.; Litbert, L. J. Phys., Lett., in press. (7) Galerne, Y.; Marcerou, J. P. Phys. Reu. Lett. 1983, 51, 2109. (8) Baum, E.; Demus, D.; Sackmann, H. Wiss. Z. Univ. Hulle 1970, 19, 37.
0 1985 American Chemical Society
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The Journal of Physical Chemistry, Vol. 89, No. 17, 1985
,el,
Figueiredo Net0 et al.
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Figure 1. Phase diagram of the mixture potassium laurate/l-decanol (at 6.24 wt %)/DzO: ISO,isotropic phase; ND, N,, NBX,nematic discotic, calamitic, and biaxial phases, respectively.
X-ray Diffraction. Samples are sealed in glass capillaries of 1.5” diameter placed in a temperature-controlled stage (of 0.1 OC accuracy). A monochromatic Laue camera9 with a magnetic field of about 10 kG is used. Synchrotron radiation from LURE ( L a b o r a t o i r e pour l’utilisation d u Rayonnement ElectromagnBtique, Orsay, France) is also used in high-resolution (and high-intensity) experiments, particularly, for the NBX phase. The X-ray diffraction measurements were mainly used as a microscopic tool to identify the lyotropic phases.6 It is however too slow of a technique to be systematically used for measuring the temperatures of the phase transitions. Each time we did we found transition temperatures consistent with the optical methods6 Observations and Results 1. Optical Microscopy Observations. The isotropic to uniaxial-nematic transition temperatures are determined by observing the textures under a polarizing microscope. These first-order phase transitions are clearly denoted by a ‘schlieren” texture (characteristic of the nematic phases) in a sample previously isotropic, on varying temperature. The transitions between the uniaxial and the biaxial phase are more difficult to observe optically. The ND (or N,) N B X phase transitions can be determined by looking at a domain of the uniaxial sample in homeotropic orientation. At the uniaxial to biaxial phase transitions, one observes in these regions the appearance of bright irregular domains.1° This texture can be explained as follows: initially in the uniaxial phase, the sample has its symmetry axis oriented (homeotropically); when the NBX phase is reached, this axis remains oriented but the two new axes (orthogonal to the first one) are not aligned in a preferred orientation, making inhomogeneities in the sample texture. 2. Conoscopic Observations. This technique determines whether the phase is optically uniaxial (positive or negative) or biaxial.’ Initially, the discotic nematic phase is homeotropically aligned (with the help of a magnetic field). The typical conoscopic cross is observed with at least two rings. As the temperature increases at the ND NBX phase transition, the cross splits. The positions of the interference fringes are measured at equilibrium and injected in a least-squares fit which yields both the refraction index differences. As expected, one of the index differences vanishes in the uniaxial phases, while all the three indices are different in the biaxial phase. 3. X-ray Diffraction. This technique identifies the microscopic structure of the different phases: discotic, calamitic, or biaxial. The method consists in obtaining two orthogonal views of the reciprocal structure in the nematic phases of a well-oriented sample. The reciprocal structure of the uniaxial phases may be schematized as a hollow circular cylinder parallel to the director with intense ends in the ND phase” and intense edges6 in the N, phase. The reciprocal structure of the NBX phase is a hollow barrel of elliptic section with the long axis of the ellipsis parallel to the magnetic field.6
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(9) Comes, R.; Lambert, M.; Guinier, A. Acta Crystallorg., Sec. A : Ctyst. Phys., Dqfr., Theor. Gen. Crystallogr. 1970, 26, 244. (10) Saupe, A.; Boonbrahm, P.; Yu,L. J. J . Chim. Phys. Phys.-Chim. Biol. 1983, 80, 7. ( 1 1) Charvolin, J.; Levelut, A. M.; Samulsky, E. T J . Phys., Lett. 1979, 40, L-587.
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Figure 2. Phase diagram of the mixture potassium laurate/ l-decanol/DzO at a fixed ratio R = [LK]/[DeOH] = 4 ISO,isotropic phase; ND, N,, NBX,nematic discotic, calamitic, and biaxial phases, respectively;
POL, polyphasic region. 4 . The Phase Diagrams. ( a ) At 6.24 wt % of 1-Decanol. Figure 1 shows the phase diagram at 6.24 wt % of 1-decanol. The transition temperatures are obtained by conoscopy and optical microscopy, and the different phases are identified by X-ray diffraction measurements.I0 As indicated above, the transition temperatures were always found consistent when measured by the different methods we have used (mainly optical microscopy and conoscopy) though the samples bere conditioned in different cells. The error of measurement of the phase diagram, in temperature and concentration, including the errors when preparing the samples, can directly be appreciated on the reproducibility of the data in Figure 1 (6T 1 K). By comparing this phase diagram with the one previously published by Yu and S a ~ p eone , ~ observes a similar topology with an important shift of the transition temperatures toward the smaller potassium laurate concentrations as also denoted by Hendrikx et aL5 The reentrant behavior of the NE, and N, phases is not observed at 6.24 wt % of l-decanol.12 The biaxial temperature range remains small (about 1.5 “C) between the ND and N, domains. The nematic domain does not exist for the potassium laurate concentration less than 24.2 wt %. Near the potassium laurate concentration equal to 24.95 wt % the phase diagram has a very sensitive region where small concentration fluctuations may cause an irreprcducible uniaxial to biaxial phase transition. The differences between our phase diagram and the previously published ones3J may be due to the nature of the chemicals used to prepare the mixture. In particular, the KL can be hydrated” introducing then an excess of water in the mixture. This effect could explain the shift of the region IS0 ND IS0 in our phase diagram to the small KL concentration region in comparison with the diagram of ref 3. ( b ) At a Fixed Ratio R = 4 . From a theoretical* and experimenta13point of view, it is known that the NBX phase can be found between the ND and N, phases. Looking for large temperature ranges of the NBXphase, it is interesting to study the phase diagram in a plane nearly tangent to the N, domain. Such a plane
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(12) We have observed the reentrant behavior of the NBxand N, phases for different decanol concentrations. (13) We have checked this point by finding again the same transition temperatures when using dried KL previously hydrated.
J . Phys. Chem. 1985,89, 3739-3747 corresponds approximately to the ratio R = [KL]/[DeOH] = 4, since we have verified that for lower R (
