J . Phys. Chem. 1987, 91, 5949-5953
5949
New Nematic Lyomesophase of Sodium Dodecyl Sulfate L. Q.AmaraI,* M. E. M. Helene, D. R. Bittencourt, and R. Itri Instituto de Fisica da Universidade de Siio Paulo. C.P. 20516, Siio Paulo, Brasil (Received: February 18, 1987)
The system sodium dodecyl sulfate (SLS)/decanol/water was investigated in search of a nematic phase. A N, phase (director n' parallel to magnetic field) was found at 23 OC with weight composition 25.00% SLS/70.53% H20/4.47% decanol; the transition to the isotropic phase occurs at 32 OC. Characterization of this N, phase was made by optical microscopy and X-ray diffraction; two diffraction peaks appear in the direction perpendicular to n', at 6' values of 64 and 32 A. These two diffraction bands are also present in the isotropic phase at 38 OC. Weak fourth and sixth orders are also detectable in this N, phase. Changes in the intensity ratio between the peaks for different nematic phases are accounted for by a model for the electron density as a function of the ratio between bilayer thickness and average intermicellar distance. A model of aggregates of cylindrical micelles is proposed for N, phases and is able to account also for biaxial phases.
Introduction Micelles of sodium dodecyl (lauryl) sulfate (SLS) are among the most investigated ones in aqueous isotropic micellar solutions. Besides extensive investigation in dilute solutions (near the cmc), the size, shape, and interactions of micelles have been studied by X-ray,' neutron,* and light ~ c a t t e r i n g in ~ , ~more concentrated solutions. Theoretical models have been worked out to account for micellar growth3v5 under changes in temperature and salt concentration and for intermicellar interaction^.^^^^^ On the other hand, in the liquid crystalline state formed at higher concentrations, lyotropic mesophases with long-range positional order obtained with the binary SLS/water system have also been investigated.' Lyomesophases obtained with ternary systems (addition of lipophilic substance) have been investigated for various amphiphiles, and some work on the ternary SLS/ water/decanol system has been reported.8 In recent years nematic (N) lyomesophases, formed usually by addition of alcohol and/or salt to amphiphile/water systems, have received increased attention. Two uniaxial phases have been characterized by NMR9 and X-ray diffraction,I0J1 and a biaxial phaseI2 has been observed between them, of considerable theoretical interest.I3 The original discovery of N phased4 was made ~
(1) Reiss-Husson, F.; Luzzati, V. J. Phys. Chem. 1964,68,3504; J . Colloid Interface Sci. 1966, 21, 534. (2) Hayter, J. B.; Penfold, J. J . Chem. Soc., Faraday Trans. I 1981, 77, 1851. Bendedouch, D.; Chen, S. H.; Koehler, W. C.; Lein, J. S.J . Chem. Phys. 1982, 76, 5022. Triolo, R.; Caponetti, E.; Graziano, V. J . Phys. Chem. 1985,89, 5743. (3) Missel, P. J.; Mazer, N. A.; Benedek, G. B.; Young, C. Y.; Carey, M. C. J. Phys. Chem. 1980, 84, 1044. Missel, P. J.; Mazer, N . A,; Benedek, G. B.; Carey, M. C. J . Phys. Chem. 1983,87, 1264. (4) Corti, M.; Degiorgio, V. Ann. Phys. 1978,3, 303; J. Phys. Chem. 1981, 85, 711. (5) Ikeda, S. J . Phys. Chem. 1984,88, 2144. ( 6 ) Hayter, J. 8.; Penfold, J. Mol. Phys. 1981, 42, 109. (7) Luzzati, V.; Husson, F. J . Cell Biol. 1962, 12, 207. Luzzati, V. In Biological Membranes; Chapman, D., Ed.;Academic: New York, 1968; pp 71-123. (8) Ekwall, P. In Advances in Liquid Crystals; Academic: New York, 1975; Vol. I, pp 1-142. Maciejewska, D.; Khan, A.; Lindman, B. Colloid Polym. Sci. 1986, 264, 909. (9) Radley, K.; Reeves, L. W.; Tracey, A. S. J. Phys. Chem. 1976,80, 174. Forrest, B. J.; Reeves, L. W. Chem. Reu. 1981, 81, 1. (101 Amaral. L. 0.:Pimentel. C. A,: Tavares. M. R.: Vanin. J. A. J. Chem. Phjs. 1979, 71, 2940. Figueiredo Neto, A. M.; Amaral, L. Q. Mol. Crysr. Lio. Crvst. 1981. 74. 109. '(llfCharvolin, J:; Levelut, A. M.; Samulski, E. T. J. Phys. Lett. 1979, 40, L-587. Hendrikx, Y.; Charvolin, J. J. Phys. (Les Ulis, Fr.) 1981,42, 1427. (12) Yu, L. J.; Saupe, A. Phys. Reu. Lett. 1980, 45, 1000. Bartolino, R.; Chiaranza, T.; Meuti, M.; Compagnari, R. Phys. Rev. A 1982, 26, 1116. Meuti, M.; Chiaranza, T.; Bartolino, R. Solid State Commun. 1983, 48, 751. Galeme, Y.; Marcerou, J. P. Phys. Rev. Lett. 1983,51,2109. Lacerda Santos, M. B.; Galerne, Y.; Durand, G. Phys. Reu. Lett. 1984, 53, 787. (13) Freiser, M. J. Phys. Rev. Lett. 1970,24, 1041. Alben, R. Phys. Rev. Lett. 1973, 30, 778. Straley, J. P. Phys. Reu. A 1974, 10, 1881. Rabin, Y.; McMullen, W. E.; Gelbart, W. M. Mol. Cryst. Liq. Cryst. 1982, 89, 67. Chen, Z. Y.; Deutch, J. M. J . Chem. Phys. 1984, 80, 2151. (14) Lawson, K. D.; Flautt, T. J. J. Am. Chem. SOC.1967, 89, 5489. Black, P. J.; Lawson, K. D.; Flautt, T. J. J . Chem. Phys. 1969, 50, 542.
in systems with sodium decyl sulfate (SDS); it was then mentioned that SLS did not lead to N phases. Since then many other systems forming N phases have been discovered and studied, those obtained from SDS and potassium laurate (KL) are among the most investigated systems. To our knowledge, no N phase obtained with SLS has so far been reported. In view of the recent interest on the isotropic-nematic transition13and of the controversy regarding size and shape of micelles in the N p h a ~ e s , ' ~it" was considered a worthwhile effort to search for a N phase obtained from SLS. The interest in such a phase in twofold: on the one hand, it would allow integration of information gathered independently from micelles in isotropic solutions and in lyotropic phases, and on the other hand, it would allow cross-comparison with KL and SDS, regarding the influence of polar heads and hydrocarbon tails in the micelles. An investigation of the system SLS/water/decanol was therefore performed and is presented here. A short abstract with preliminary results has been reported elsewhere.18 Experimental Section Commercial SLS (Merck, 99%), deionized water, and BDH decanol were used. Samples with several compositions (0.05% accuracy) were homogenized as usual9 by agitation and centrifugation and visually observed between crossed polarizers, in order to reach a nematic phase. Analysis of obtained phases was made by polarized optical microscopy (OM) and small-angle X-ray diffraction (XD). Samples were sealed in 0.2-mm-wide flat microslides for OM and in 1-mm glass capillaries for XD and could be oriented in a permanent magnet of 2 kG. A Wild (Orthoplan-pol) microscope equipped with crossed polarizers and photomicrograph facilities was used. XD photographs were obtained in transmission geometryI0J1 with a small-angle spin-hole facility and with a Laue camera, both using Cu K a radiation; the capillary-was in a vertical position, and the magnet could be placed with H horizontal and perpendicular to the X-ray beam. The sample temperature could be raised by heating the air in its surroundings (precision 2 "C). Results After several attempts, an homogeneous N phase was obtained at room temperature (23 f 2 "C) with weight composition 25.00% S L S / 7 0 . 5 3 % H20/4.47% decanol. This N phase showed a (151 Hendrikx. J.: Charvolin. J.: Rawiso. M.: Liebert. L.: Holmes. M. C. J. Phys Chem 1983,87, 3991. Holmes, M. C ; Charvolin, J. J . Phys.'Chem 1984.88. 810 (16) Amaral, L. Q. Mol. Cryst. Liq. Cryst. 1985, 124, 225. (17) Figueiredo Neto, A. M.; Galeme, Y.; Levelut, A. M.; Liebert, L. J . Phys., Lett. 1985, 46, L-499. (18) Amaral, L. Q.;Helene, M. E. M.; Bittencourt, D. R.; Itri, R. Presented at the 1 Ith International Liquid Crystal Conference, Berkeley, CA, June 30-July 4, 1986; Abstract M-121-LY.
0022-3654187 12091-5949%01.50/0 .~ , 0 1987 American Chemical Society I
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5950 The Journal of Physical Chemistry, Vol. 91, No. 23, 1987
a
Amaral et al.
b
Figure 1. Photomicrographsunder crossed polarizers of the+N, phase at 23 OC, with residual magnetic orientation r f parallel to the flat surface: (a) r f in the plane of the figure, in the vertical direction; (b) H in the plane of the figure, at 45O of (a).
transition to the isotropic I phase at 32 OC. Samples with the above composition were then analyzed by OM and XD. OM results in the N phase at 23 OC showed the typical Schlieren texture of nematic phases.lg The texture did not evolve to the pseudoisotropic pattern typical of Ndphases (disk micelles with axis and director Z perpendicular to disk plane), which align with Zprpendicular to the surface. When oriented, in a magnetic field H parallel to the flat microslide surface, the sample showed the typical planar texture of Figure 1, obtained when 2 is parallel to the surface. This result is typical of N, phases (cylindrical micelles with axis parallel to Z)." Surface orientation occurs in both cases with bilayers perpendicular to the surface. Small-angle XD photographs for samples in the N phase at 23 OC as well as for samples in the I phase at 38 "C are shown in Figure 2. XD results clearly show the existence of two diffraction maxima of comparable intensities with spacing ratio 1:2. Such maxima occur for s-' values of 64 f 2 and 32 f 1 A (s = 2 sin 8/X, with 28 the scattering angle and X the wavelength). In the N phase not subjected to a magnetic field (geometry Go) (Figure 2a), these two maxima have the same preferential direction, determined by surfaceorientation. The figure corresponds to a sample just after preparation. With time, surface orientation becomes more pronounced, and the peaks appear in the horizontal equator. Capillary surface orientation, giving XD preferentially in the horizontal equator, is typical for both N, and Ndphases.1° The strong XD peaks correspond to the distance between adjacent bilayers, and such an orientation is obtained for both N, phases with Z parallel to the capillary axis (and surface) and Ndphases with Z perpendicular to the surface (and capillary axis). In the I phase (Figure 2b) the two maxima are rotated isotropically but at essentially the %ames position. In the oriented N phase with H horizontal and perpendicular to both the capillary axis and X-ray beam (geometry G,), the two maxima concentrate in the vertical direction (Figure 2c). If we remove the magnet and rotate the capillary 90" ar-und its axis (geometry GI,,residual magnetic orientation with H parallel to the X-ray beam), the two maxima appear isotropically (Figure 2d). If we rotate the capillary another 90°, the result for G, was again obtained, evidencing the existence of good residual orientation (within 1 day). These combined results for Go,G,, and GI,define unambiguous~y,10J1*20 and independently from Oh$, the phase a,s N,, wiLh ZllH. No other possibility (N, with Z l H , Ndwith ZllH or Z l H ) would give this c">ination of results; such a definition is possible because when n ' l H, surface orientagon will define the direction of Z in the plane perpendicular to H. Magnetic orientation is therefore of the type expected for hydrEarbon chains, which align with the chain axisgerpendicular to H (N, phases with ZllH, Ndphases with ZlH).*l1 As a final check, XD was taken in G, but with the capillary rotating around its axis. In such conditions a phase with ZlH would have Z parallel to the capillary axis and a good orientation; the result was diffuse and almost isotropic rings, with slight
preferential direction in the horizontal equator, giving furthzr support to the charactrization of the phase as N,, with ZllH. The two strong diffraction maxima thus appear in the direction perpendicular to n'; no characteristic distance in the 2 direction can be detected. The presence of a second order (in the direction perpendicular to Z) in XD, and particularly for a N, phase, is quite unusual. In order to verify the possible presence of more than two orders of diffraction in this N, phase, XD was also taken with a Laue camera, with a smaller sample to film distance and overexposition of the two main maxima. In such conditions the fourth and sixth orders appear very weak (not possible to be shown in a reproduction) at s-' values of 16 and 10.6 A. No sign of the third and fifth orders could be detected.
Discussion There are two aspects to discuss: the N domain in the ternary system as compared to SDS and KL and the unusual XD results. N Domain. Figure 3 shows the phase diagram of the ternary SLS/water/decanol system, with this N, phase and other liquid crystalline phases previously obtained! It is seen that the N, phase occurs in between isotropic and hexagonal regions. It should be first remarked that the N domain occurs for SLS/H20 concentrations much lower than the ones reported hi~torically~*'~ for N phases in ternary and quaternary systems. Considering ternary amphiphile/water/decanol systems, N phases occur for SLS/H20 concentrations considerably lower than for SDS,11*'5*'9 but only a little lower than for KL systems studied more recently;12 thus, the N domain shifts to lower amphiphile/water ratios with increasing chain length. The ratio SLS/decanol is, however, larger than for KL, being almost the same as for SDS. Another important point to notice is that this ternary SLS system can be seen as a binary system with 26.17 wt % SLS/73.83 wt % H20, to which decanol has been added. Thus, the SLS concentration of the binary system that originates the N phase by addition of decanol is quite close to the concentration where the sphere-rod transition was proposed to occur in the isotropic binary system' (0.25 for 27 "C and 0.15 for 70 "C). It seems, therefore, that it is in the vicinity of the sphere-rod transition of the binary isotropic phase that conditions are settled for entrance in N phases through alcohol addition. This point is being further investigated.2 X D Results. Lyotropic N phases of amphiphiles with hydrocarbon chains are believed to be built of cylindrical (N,) or discotic (Nd)micelles. XD results10i1 lJ5 usually give only one diffraction band in each characteristic direction, interpreted as the average distance between micelles in that direction. Higher orders of diffraction in the ratio 1:2:3:4 ...are characteristic of a lamellar type of structure and totally unexpected for a N, phase with cylindrical symmetry. This SLS phase has the+typical symmetry of cylindrical micelles with the$xis parallel to H (discotic micelles have the axis perpendicular to H), and yet higher orders of lamellar type are present.
(19) Yu, L. J.; Saupe, A. J . Am. Chem. SOC.1980,102,4879.
(20) Bittencourt, D. R. Ph.D. Thesis, University of S6o Paulo, Brasil, 1986.
(21) Itri, R. Master Dissertation, University of S6o Paulo, Brasil, 1986.
New Nematic Lyomesophase of SLS .
The Journal of Physical Chemistry, Vol. 91, No. 23, 1987 5951
Figure 2. X-ray diffraction for the N, phase at 23 OC and for the I phase at 38 OC (the darker background is the shadow of a filter): (a) N phase not subjected to fi (geometry Go);(b) I phase,(c) N phase with fi present in the plane of the figure, in the horizontal direction (geometry GJ; (d) N phase with residual magnetic orientation ( H parallel to X-ray beam) (geometry Gll).
The appearance of a second-order peak has been obtained previously17 (for the two uniaxial and one biaxial phases of the ternary diluted KL system) with synchrotron radiation, when its intensity I2 was much weaker than the intensity Zlof the first peak. The obtention of a second order in KL has been attributed17 to a local “pseudolamellar” ordering of aggregates of statistically similar biaxial objects. The obtention of a weak second order in a specific system (that furthermore contained ferrofluid as an additional component) could, however, be due to a particular form of the micellar structure factor and/or of the interference function, so that other peaks of cylindrical symmetry would be particularly depressed. In the case of the SLS phase here reported the two orders have comparable intensities and orders as high as fourth and sixth are present. It thus seems clear that a unidimensional modulation exists in the direction perpendicular to the bilayer also in N, phases. This is a real puzzle, and it is not at all clear how such a lamellar type of structure could give origin to the different symmetries of N phases, since XD corresponds to a statistical average over the whole exposition time.
A columnar phase of oblate micelles is not a solution, since it would appear to XD essentially as a Nd phase (repetition distance in the Z direction). Biaxial micelles (in the form of bricks) with disorder around an axis to simulate17cylindrical symmetry do not seem to be a solution either; such a form is not l i k e l ~ l -for ~ ~a ~ ~ micelle, and it has not been shown explicitly that such a proposal would account for basic properties of N, phases (type of surface and magnetic orientation, precursor of hexagonal phases, etc.). Before trying to solve this puzzle, let us first consider the question of relative strength of higher orders in different N phases. We shall concentrate first on this point, which has not been discussed yet. The measured intensity I(3) is a function2of the single particle structure factor P(3)of the micellar unit and of the interparticle structure factor S(3) between micelles. Admitting, as usua1,15 that the observed peaks correspond essentially to peaks in S(9, it is (22) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Furuduy Trans. 2 1976, 72, 1525. Cabane, B.; Duplessix, R.; Zemb, T. J . Phys. (Les Ulis, Fr.) 1985, 46, 2161.
5952
The Journal of Physical Chemistry, Vol. 91, No. 23, 1987 DECANOL
/
Amaral et al. TABLE I: Calculated I , Measured d , p = / / d , and R Values for Different N Pbrses N phase I, A d, A P R SDS 32.9 38’ 0.87 0.53 KLl 32.4 43.36 0.75 0.00
\
KL2 SLS
32.4 38.0
48.7c 64
0.66 0.59
1.24 8.39
‘References 10 and 11. bReference25. eReference 17.
/
E Figure 3. Ternary-phase diagram of the system SLS/water/decanol at room temperature, showing the N, phase reported in this work. The isotropic (L,and Lz),lamellar (D), and hexagonal (E) regions correspond to previous work* on this system. L,
possible to understand changes in Iz/Ilby analysis of R = p(Z2)/p(Z1). The intensity ratio Iz/I1 will differ from R because of experimental ?-dependent factors, thermal fluctuations, and decay of higher order peaks in S(a. Comparative analysis of R values for different systems will be, however, meaningful if all N phases have equivalent degrees of short-range order. There is no a priori reason to expect strong variations in the degree of short-range order for different N phases. The result here obtained in the I phase of the ternary SLS system shows that the short-range order is very similar in N and I phases. Thus, for different N phases are expected to be due differences in 12/11 essentially to differences in R and not in the form of decay of S(3. Let us consider a simple unidimensional step model23for the electron density in the direction perpendicular to the bilayer. It has been shownz3 that, taking into account only the contrast between electron densities of polar heads (pl) and of an average water-paraffin environment (po), the structure factor P(s)oscillates with period 1 / 1 , 1 being the distance between centers of polar heads in a bilayer. The structure factors of order n will be p(n/d), with d = sl-lthe lamellar repetition distance. From such a model one gets
F, =
2(pi
- POW 7rn
sin
nrr cos n r p d
and
R =
(
-)2(cos
7)’
with r the polar head thickness and p = lfd. Thus, odd orders of diffraction will be zero for p = lfZ,while for this p value even orders have maxima. The ratio R goes to and 3 / 4 and diverges for p = l f 2 , when the zero for p = first-order peak disappears. The variable p was estimated from calculatedz4 I values and measured d values for N phases of SDS,lo.llKL1 (more concentratedz5),KL2 (more diluted”), and SLS; the upper limit far I values, corresponding to extended chains, has been considered. Results are shown in Table I; R values are given assuming r
