J. Phys. Chem. 1990, 94, 3 186-3 188
3186
Bend and Splay Elastic Constants In Two Nematic Lyomesophases wtth Sodkm Decyl Sutfate and Potassium Laurate A. V. A. Pinto* Departamento de Flsica da Universidade Federal de Santa Catarina, 88049- Florian6polis, SC, Brazil
and L. Q. Amaral Instituto de F h c a da Universidade de Siio Paulo, Caixa Postal 20.516, 0[498-S50 Paula, s p , Brazil (Received: August I , 1988; In Final Form: April 14, 1989)
Nematic lyomesophases in the systems sodium decyl sulfate (SDS)/water/decanol/Na2S04 and potassium laurate (KL)/water/KCI, which form respectively Nd (discotic) and N, (cylindrical) phases, have been studied. Measurements of the critical magnetic field for the Freederickzs transition allowed determination of kii/Xa as a function of temperature; xa is the diamagnetic anisotropy and kii is an elastic constant (splay k l l for N, and bend k33for Nd phases). The results might indicate that micelles behave as flexible objects.
Introduction In a lyotropic nematic system the surfactant molecules aggregate to form anisotropic micelles. In the nematic case the micelles pack with long-range orientational ordering but spatial ordering is short range. Originally the nematic lyotropic systems were classified as type I, with positive diamagnetic anisotropy, Ax > 0, and type 11, with negative diamagnetic anisotropy, A x < 0.',2 Nematic uniaxial phases are formed by disk-shaped micelle^,^^^ denoted as Nd (discotic) or by rodlike micelles,4 denoted as N, (cylindrical or calamitic). Yu and Saupe5 have shown that both uniaxial and biaxial nematic phases exist. Few measurements of elastic constants have been reported on nematic lyotropic phases; the most complete were those6 measured in Nd phases with decylammonium chloride (DAC), giving values comparable to those obtained in thermotropics. In this paper we report studies in two lyonematic mixtures: a Nd phase with sodium decyl sulfate (SDS), water, decanol, and sodium sulfate, and a N, phase with potassium laurate (KL), water, and potassium chloride. Measurements of the critical magnetic field' H, = (r/ d ) ( k i i / x a ) l for / z a Freederickzs transition on an aligned sample film of thickness d allow determination of kii/xa,where xa = xi, - xI is the diamagnetic anisotropy and k,, is the elastic constant (splay k , , for N, phase and bend k j 3 for Nd phase). The main problem in this type of measurement is to achieve the required condition of "strong anchoring". We therefore discuss in some detail the cell used to obtain perfectly parallel walls and to assure such condition. The degree of anchoring was checked by conoscopic and textural observation of the samples and by the constancy* of the product H,d. Measurements of other systems (N, phase with SDS) and of other elastic constants, requiring different geometries, were tried,9 but the basic condition of strong anchoring was obtained only for splay in the N,-KL system and bend in the Nd-SDS system. ( I ) Radley, K.; Reeves, L.W.; Tracey, A. S. J. Phys. Chem. 1976,80, 174. (2) Fujiwara, F.; Reeves, L. W.; Suzuki, M.; Vanin, J. A. In Solution Chemistry ofSurfactants; Mittal, K. L., Ed.; Plenum: New York, 1979; Vol. 1, pp 63-79. (3) Amaral, L. Q.; Pimentel, C. A.; Tavares, M. R.; Vanin, J. A. J . Chem. Phys. 1979, 71, 2940. (4) Charvolin, J.; Levelut, A. M.; Samulski, E. T. J . Phys. Lett. 1979, 40,
L-587. (5) Yu, L. J.; Saupe, A. Phys. Rev. Lefr. 1980, 45, 1000. (6) Haven, T.; Armitage, D.; Saupe, A. J. Chem. Phys. 1981, 75, 352. (7) de Gennes, P. G. The Physics ojLiquid Crystals; Clarendon Press: Oxford, UK, 1974. (8) Frank, F. C. Discuss. Faraday SOC.1958,-25, 19. (9) Pinto, A. V. A . Ph.D. Thesis, lnstituto de Fisica, Universidade de SBo Paulo, 1984.
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Experimental Section Sample Preparation. Sodium decyl sulfate and potassium laurate were prepared as described in the The composition of the two systems studied were (a) SDS (37), 1decanol (6), sodium sulfate (6), water (51), (weight percent), Nd phase at room temperature; (b) KL (34.9, potassium chloride (3), water (62.5), (weight percent), N, phase at room temperature. In both cases, the components were carefully mixed to form a homogeneous phase by using a centrifuge and mechanical agitation. The samples were enclosed in specially developed glass cells (as described below) for measurements. Before the first use, the cells were cleaned with a strong detergent and rinsed well with distilled water. In the Nd phase the surface alignment is vertical and a pseudoisotropic nematic film forms, but if a magnetic field is not used it takes many hours to establish uniform alignment. In the N, phase, uniform alignment can be partially established by flow when the cell is filled and the application of a magnetic field will, after many hours, produce a uniform planar alignment. The KL-N, system, upon heating, transforms to a hexagonal p h a ~ e ; ~change . ' ~ in texture may start9 at 40 O C . The SDS-Nd system may present hydrolysis at high temperatures" and shows coexistence with an apparently hexagonal phase (texture observation9) at 60 OC. On cooling below room temperature, both systems enter into coagel phases?*12 For these reasons, the nematic domain was studied with Freederickzs transition only by heating from room temperature (about 22 "C) up to 40 O C ; hydrolysis of SDS above this temperature may occur to an appreciable extent, particularly because of the very long time of measurements.l' During the experiment the cells were placed in an oven whose temperature was controlled by circulating air; the temperature was constant to better than f0.2 OC. The absolute temperature error was estimated to be approximately f l OC. Glass Sample Cell Description. The glass cells used two rectangular pieces, 15 X 20 mm, of optically plane glass, separated by two tungsten wires in the horizontal direction; epoxyI3 was applied to the two edges after the wires. Three different diameters of wire were used, 0.13 1 f 1%, 0.15 1 f 1%, 0.21 2 f 1% mm, to make cells with different sample thicknesses. Two needles, which had been previously sectioned along the cylindrical axis, are then glued with epoxy on the free edges of the glass plates, in the vertical direction. A syringe is used to clean and/or to fill (IO) Figueiredo Neto, A. M.;Amaral, L. Q. Mol. Crysf.Liq. Cryst. 1983, 95, 129. ( I I ) Hcxhapfel, A.; Boidart, M.; Laurent, M. Mol. Cryst. Liq. Cryst. 1981, 75, 201.
L. Q. J. Appl. Crystallogr. 1984, 17, 476. (13) Araldite code o AV-138 and o HV-998. ( 1 2) Amaral,
0 1990 American Chemical Society
Elastic Constants in Lyomesophases
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 3187
TABLE I: Bend Elastic Constant for SDS-N,, Phase' H,, kG T, OC H,d, G c m k , , / x , dyn 803 & 3% 24.0 f 0.2 4.20 f 1.5% 89.0 f 3% 24.8 25.0 25.8 26.0 27.0 28.6 29.8 30.7 32.7 34.7 36.7 38.5
4.00 3.90 3.70 3.65 3.40 3.05 2.85 2.15 2.60 2.50 2.45 2.45
84.4 82.7 78.4 77.4 72.1 64.7 60.4 58.3 55.1 53.0 51.9 51.9
729 693 623 607 526 424 370 344 308 285 273 273
25.0 27.0 30.7 34.7
6.30 5.50 4.50 4.00
82.5 72.0 59.0 52.4
690 526 352 278
8.40
d, mm 0.212 f 1%
7.20
-$
N
Xu 4.80
2
3.60
2.40
0.131 f 1%
22.5
23.0 24.0 25.0 26.0
8.10 7.70 7.40 7.20
12.2 11.6 11.2 10.9
25.0
30.0
27.5
32.5
35.0
37.5
TEMPERATURE ("C) Figure 1. Bend elasticity vs temperature: SDS system (Nd).
"Sample thickness of d = 0.212 mm and d = 0.131 mm.
TABLE 11: Splay Elastic Constant for KL-N, Phase' T, "C H,, kG H,d, G-cm kll/X,, dyn 22.0 f 0.2 8.60 f 1.5% 13.0 f 3% 171 f 3%
6.00
*
0
"-- I
0.151 f 1%
'Sample thickness d = 0.151 mm.
,
I
I
1.68
d, m m
152 137 126 112
I
N
1.58
'
I
6
-6
1.44
H0
' 1.32
x*
the glass cells. To close the cells tight fitting Teflon caps are placed in the needles. Measurement Procedure. The optical setup to monitor the deformations caused by the magnetic field used a HeNe laser, and three aluminum mirrors to reflect conveniently the light beam in order to pass perpendicular to the glass cell (placed between the poles of a magnet) and afterwards to reach a light detector, connected to a XY recorder, which also received the signal of a gaussmeter. Two polarizers are placed in the light path and adjusted so that, in zero field, there is complete e~tinction.'~The magnetic field is switched on and changed in steps of 100 G with a waiting time of 30-60 min. If after this time, no intensity change is detected, the field is stepped again. We continue this until a change in light intensity is detected; this determines the critical field. The selected dwell time was not sufficient near the critical field where the relaxation time becomes very long. To reduce this error we averaged the data taken for increasing and decreasing fields; reproducibility was obtained within 3% error.
Results and Discussion Tables I and I1 give the results of the measurements. For both, the SDS and KL systems, several samples were measured and the data were reproducible within experimental error (3%). It should be noted that glass cells with different thicknesses were used and the product Hcd is independent of the film thickness, as it should be according to theory.* The "strong anchoring" condition for the Nc-KL system subsisted, however, only up to 26 OC. At higher temperatures surface alignment was no longer perfect. Figures 1 and 2 show the ratios k 3 3 / ~and a k l l / X afor Nd-SDS and N,-KL systems, respectively, as a function of temperature. The solid curves are least-squares fits to the experimental points; the best fit to both curves was a third-order polynomial. It should be remarked that we plotted results as a function of T and not of reduced temperature for two reasons: there is no N-I transition at higher temperatures and the two systems are not comparable even on a reduced temperature scale. It is however to be noted that, on approaching the nematic to hexagonal transition, the curves have a behavior similar to that found in ( I 4) Johansen, A. Manual of Petrographic Methods; Hafner: New York, 1968; p 345.
1.20 1.08 1 21.0
I
I
I
I
I
22.0
23.0
24.0
25.0
26.0
i 3
TEMPERATURE PC) Figure 2. Splay elasticity vs temperature: KL system (N,)
thermotropics and other lyotropic phases at the nematic to isotropic transition.6 The present results agree well with earlier measurements of the elastic constants on the DAC system.6 In particular, DAC data of k33/Xaon mixture l 6 is also well fitted by a polynomial of third order as a function of T; mixture 2, however, requires a fit as a function of ( T - TNI),which is a suitable reduced scale for both mixtures to fall on the same curve.6 The nematic range in DAC mixture 1 (35-55 "C) is nearer to the temperature range here studied (the range in DAC mixture 2 is shifted by about 10 OC upwards). Another point to be remarked is that both bend for Nd-SDS and splay for N,-KL could be fitted by polynomials of same order and agree with the type of behavior observed6 for bend in NdDAC, but not for splay in N,-DAC. The more pronounced divergence of bend for Nd-SDS occurs because the transition to coagel phase is just below room temperature'* in SDS and at a lower temperatureg in Nc-KL. A recent mea~urementl~ of k j 3 / x ain a Nc phase of the system KL/water/decanol at 25 OC gave a value (3.0 f 0.3) X lo2 dyn. This value compares reasonably well with our value (126 dyn) for k l l / X a ,since it is expected that kg3> k l l . The order of magnitudes measured for kii/X,, together with leads to elastic constants measured orders of magnitude for xa,16J7 for lyonematics of the same order as for thermotropics ( lo6 dyn). Considering the hard-rod approach1*one would expect for the product L4D ( D is diameter and L length of the rod) values of the same order of magnitude for thermotropics and lyotropics, since the product pZkT( p is density and kT thermal energy) may differ only by a factor 2. X-ray diffraction r e s ~ l t sgive ~ ? ~infor(15) Kroin, T.; Figueiredo Neto, A. M. Phys. Reu. 1987, A36, 2987. (16) Kumar, S.; Litster, J. D.; Rosenblatt, C. Phys. Reu. 1983, ,428, 1890. (17) Stefanov, M.; Saupe, A. Mol. Cryst. Liq. Cryst. 1984, 108, 309. (18) Straley, J. P. Phys. Rev. 1973, A7, 720.
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J . Phys. Chem. 1990, 94, 3188-3192
mation on D values, but there is controversy on the interpretation of data regarding L values. Using accepted D values one arrives at the rather unlikely result of L < D. A wav to eive meaning to such result is to Drowse semiflexibleIg long ro& f i r N, phase; (and large semiflex'ible disks for Nd phases). X-ray diffraction result^^^^ show that the diffraction associated with distances in the L direction is much weaker and broader than diffraction associated with distances in the D direction. So an alternative to the more generally accepted view (19) Khokhlov. A. R.; Semenov. A. N. Physica 1982, IIZA, 605.
of small micelles with small anisotropies would be a picture of semiflexible micelles made up either of large defective micelles or of interconnected small micelles. Such a picture is in agreement with the alternative interoretation of X-rav data.20*21 Acknowledgment. Thanks are due to Dr. T. R. Taylor for discussions. (20) Amaral, L. Q.; Helene, M. E. M.; Bittencourt, D. R.; Itri, R. J . Phys. Chem. 1987, 91, 5949. (21) Amaral, L. Q.; Marcondes Helene, M. E. J . Phys. Chem. 1988, 92, 6094.
Radlolytic Generation of Radical Cations in Xenon Matrices. Tetramethylcyciopropane Radical Cation and Its Transformations X.-Z. Qin and A. D. Trifunac* Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: August 8, 1989; In Final Form: October 30, 1989)
Radiolytic generation of radical cations in xenon matrices containing electron scavengers is illustrated by studying the 1,1,2,2-tetramethylcyclopropaneradical cation. Dilute and concentrated solutions of tetramethylcyclopropane in xenon without electron scavengers and neat tetramethylcyclopropaneyielded neutral radicals upon y-irradiation. Speculationon the mechanisms of radical formation is presented. The radical species observed in the y-irradiation of neat tetramethylcyclopropane appears to be identical with the paramagnetic species observed in CF2CICFCI2above 120 K, suggesting that a neutral radical rather than the ring-opened distonic radical cation is observed in the CF2CICFC12matrix.
Introduction
Formed by one-electron loss from the corresponding neutral compounds, radical cations are important chemical intermediates in the chemistry induced by ionizing or photoionizing radiation and in various electron- and charge-transfer processes. Radical cations are usually highly reactive and short-lived. The development of matrix-isolation methods in the past decade has allowed EPR spectroscopy to be used to study these intermediates.' The principle of these methods is to trap radical cations in different matrix cages separated by a large number of matrix atoms or molecules at low temperatures, hence slowing down and/or preventing their reactions. By use of the method pioneered by Shida and Kat0 in 1979,2 many organic radical cations have been studied by EPR. In this method, radical cations are generated by y-irradiation of a halocarbon solvent containing a small amount of substrate.la< The temperature range for halocarbon solvents is from 2 to 160 K. However, these matrices often show strong interaction with the substrate radical cations.lb&The chemistry as well as the structure of radical cations can be dependent on such matrix interaction. In 1982, a more inert neon matrix was employed for the EPR investigation of small radical cations by Knight.Id The technique involves photoionization of substrates in the gas phase during the process of deposition on a neon matrix at 4 K. The temperature range for neon is, however, limited (2-10 K). In halocarbon matrices the substrate radical cations are formed by hole transfer from the solvent cation to the substrate, while in the neon matrix the substrate molecules are probably directly photoionized at the site of deposition. Due to the large ionization potential difference between the rare gases (IPnmn= 21.6 eV) and most organic compounds (IPS 11 eV), the exothermicity of the hole-transfer process in a rare gas matrix may cause rearrangement or frag( I ) (a) Shida, T.; Haselbach, E.; Bally, T. Arc. Chem. Res. 1984, 17, 180. (b) Symons, M. C. R. Chem. SOC.Rev. 1984, 393. (c) Shiotani, M. Magn. Reson. Rev. 1987, 12, 333. (d) Knight, L. B. Acc. Chem. Res. 1986, 19, 313. (2) Shida, T.; Kato, K. Chem. Phys. Lerr. 1979, 68, 106.
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mentation of substrate radical cation^.^ At this time, only a few transition-metal carbonyl cations have been generated by y-irradiation and studied by EPR in the krypton m a t r i ~ . ~ The heaviest rare gas, xenon, has the smallest ionization potential of 12.1 eV. It is thus expected that the fragmentation of radical cations during the hole-transfer process in this matrix should be least likely. Surprisingly, no radical cation study in xenon has appeared in the literature so far. Here, we report the first EPR study of a radical cation in xenon. The 1,1,2,2-tetramethylcyclopropane (TMCP) radical cation was generated by y-irradiation of a xenon matrix containing the parent compound and a small amount of electron scavengers. A mechanism of its formation in xenon is proposed. The radical cation of TMCP has been studied in several freon mat rice^.^.^ Both the ring-closed and ring-opened radical cation of this compound have been reported in a CF2CICFC12m a t r i ~ . ~ The ring-closed cation (1) is characterized by an elongated one-electron bond between the two gem-dimethyl-substituted carbon atoms, while the ring-opened cation (2) is suggested as an orthogonal distonic species (CH3)2CCH2C+(CH3)2 with the spin confined on one dimethyl-substituted carbon.
1
2
(3) (a) Bally, T.; Roth, K.; Straub, R. Helv. Chim. Acra 1989, 72, 73. (b) Bally, T.;Haselbach, E.; Nitsche, S.; Roth, K. Tetrahedron 1986, 42, 6325. (4) (a) Morton, J. R.; Preston, K. F.; Strach, S . J.; Adrian, F. J.; Jette, A. N. J . Chem. Phys. 1979, 70, 2889. (b) Morton, J. R.; Preston, K. F. Organometallics 1984, 3, 1386. (5) Qin, X.-2.: Snow, L. D.; Williams, F. J . Am. Chem. SOC.1984, 106, 7640.
0 1990 American Chemical Societv
