6094
J . Phys. Chem. 1988, 92, 6094-6098 data (dynamic and static) are generally consistent with the PFG-NMR results up to the vicinity of the cloud point. Longrange concentration fluctuations arise in the Cl2E8 and C12E7 solutions of higher concentration from the presence of discrete micellar clusters, the nature of which is not yet fully clear. However, the radii determined from light scattering are of similar magnitude to those from self-diffusion (taking into account the influence of polydispersity which enters differently depending on the technique employed).
stantially smaller than at low temperatures and behave in a manner more similar to CI2E8(and C12E7)except that the average micellar size is much larger (see Table I). Earlier there has been some disagreement as to whether or not micellar growth occurs as a function of temperature. The present results show that a general answer cannot be given to this question: surfactants having short ethylene oxide chains for a fixed methylene chain length behave very differently from those having longer ones (>CI2E7). With CI2E5,the results of N M R line-width measurements for the methylene group are also equivocal with regard to micellar size/shape at the higher end of the accessible temperature scale. Particular emphasis has been placed on the possible role in light-scattering measurements of critical concentration fluctuations and an eventual masking of sizelshape changes of the particles. It is found here that the light-scattering
Acknowledgment. We are grateful to the Swedish Natural Science Research Council for financial assistance, and to Peter Stilbs for contributing to the maintenance costs of the JEOL apparatus. Registry No. CI2E5,3055-95-6; C&,
3055-97-8; CI2E8,3055-98-9.
Nematic Domain in the Sodium Lauryl Sulfate/Water/DecanoI System L. Q. AmaraI* and M. E. Marcondes Helene Instituto de F h a da Uniuersidade de Siio Paulo, C.P. 20516, Siio Paul0 S.P., Brazil (Received: June 18, 1987; In Final Form: March 22, 1988)
The nematic (N) domain in the ternary system sodium dodecyl sulfate/water/decanol was investigated with optical microscopy, with varying concentrations and temperature. Either the water/amphiphile (M,) or the decanol/amphiphile ( M d )relative molar ratio was kept constant. The phase diagram for M , = 45.2 water molec$s/amphiphile molecule shows a sharp separation between N, phases (cylindrical micelles with director n' 11 magnetic field H) and Nd phases (discotic micelles with n' I H) at Md = 0.38. Phase diagrams as a function of M , present also N, phases for Md < 0.38 and Nd phases for Md > 0.38. Nd phases have a higher stability regarding water addition. The transition N,-Nd was followed by successive additions of the three components and occurred always at Md = 0.38 i 0.01 for several values of M,. Only in some samples, which changed from N, to Nd through ageing and/or special thermal treatment, the Nd-N, transition occurred through temperature variation. The ternary phase diagram at room temperature is presented; the N domain falls between isotropic, hexagonal, and lamellar regions previously determined for this system. The parameters Md and M , corresponding to the Nc-Nd transition for ternary systems with potassium laurate and sodium decyl sulfate (obtained from published phase diagrams) are compared with values for sodium lauryl sulfate. hfd values are very close for the three systems, while there is a marked variation of M , with amphiphile. The significance of the parameter Md for the N,-Nd transition is discussed. The results indicate a change in micellar form at this transition.
Introduction Micelles of sodium dodecyl (lauryl) sulfate (SLS) are among the more frequently investigated in aqueous isotropic micellar solutions.' Lyotropic mesophases with long-range positional order have been investigated in the binary SLS/water system2 and also in the ternary SLS/water/decanol ~ y s t e m . ~ Nematic (N) lyomesophases4have been extensively studied over the last decade, particularly those obtained with sodium decyl sulfate (SDS) and potassium laurate (KL) in ternary systems with water and decanol. Only recently5 a nematic lyomesophase of type Ns (cylindrical micelles with director n' parallel to magnetic field H) has been obtained for the first time with SLS, at a particular weight composition of the ternary system (25.00% SLS/70.53% H20/4.47% decanol). (1) Corti, M.; De Giorgio, V. J. Phys. Chem. 1981, 85, 71 1. Hayter, J. B.; Penfold, J. J. Chem. SOC.,Faraday Trans. 1 1981, 77, 1851. Missel, P. J.; Mazer, N. A.; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1983, 87, 1264. Cabane, B.; Duplessix, R.; Zemb, T. J. Phys. (Les Ulis, Fr.) 1985, 46,
In this paper we report an investigation of the N domain in this ternary system with varying concentrations and temperature. Our aim was not simply to produce one more phase diagram for a new system but to achieve a deeper understanding of the physicochemical conditions governing the appearance of N phases. We focused first on obtaining a? Nd phase (discotic micelles with director n' perpendicular to H) and subsequently in investigating some regions of the tridimensional space corresponding to changes in concentrations and temperature. The traditional investigation is done by fixing the weight percentage of one of the three components and studying the temperature variation for several weight concentrations of the other two components. Thus Yu and Saupe studied the systems SDS/D20/decano16and KL/H20/decano17 by fixing the decanol concentration. Hendrikx and Charvolin s t ~ d i e dthe ~ , SDS ~ system along two lines: f i n g the water concentration and f i n g the ratio R of amphiphile to decanol concentration, aiming to disentangle the effects of adding decanol and water to the micellar aggregates. They were, however, unable to find the biaxial Nbxphase reportedlo
2161.
(2) Reiss-Husson, F.; Luzzati, V. J. Phys. Chem. 1964, 68, 3504. J . Colloid Interface Sci. 1966, 21, 534. (3) Ekwall, P. Adu. Liq. Cryst. 1975, 1 , 1-142. Maciejewska, D.; Khan, A.; Lindman, B. Colloid Polym. Sci. 1986, 264, 909. (4) Radley, K.; Reeves, L. W.; Tracey, A. S . J. Phys. Chem. 1976,80, 174. Forrest, B. J.; Reeves, L. W. Chem. Reu. 1981, 1, 1. ( 5 ) Amaral, L. Q.; Helene, M. E. M.; Bittencourt, D. R.; Itri, R. J. Phys. Chem. 1987, 91, 5949.
0022-3654/88/2092-6094$01.50/0
(6)Yu, L. J.; Saupe, A. J. Am. Chem. SOC.1980, 102, 4879. (7) Yu,L. J.; Saupe, A. Phys. Rev. Lett. 1980, 45, 1000. (8) Hendrikx, Y.; Charvolin, J. J. Phys. (12s Ulis, Fr.) 1981, 42, 1427. (9) Hendrikx, J.; Charvolin, J.; Rawiso, M.; Liebert, L.; Holmes, M. C. J . Phys. Chem. 1983,87, 3991. (10) Bartolino, R.; Chiaranza, T.; Meuti, M.; Campagnari, R. Phys. Reu. A 1982, 26, 1116.
0 1988 American Chemical Society
SLS/Water/Decanol: Nematic Phases by Bartolino et al. in the SDS system studied at a fixed water concentration. Figueiredo Net0 et al. investigated" the KL system both at a fixed decanol concentration and at a fixed value of R , observing an enhancement of the biaxial range with the last procedure. In general the phase diagrams show high sensitivity to the purity of the components and are not easily reproducible, particularly in ternary systems. These difficulties have precluded a clear understanding of the physicochemical processes involved in the phase transitions of these systems. We decided to focus on the SLS system because it has been extensively studied in the isotropic I phase and it allows cross comparison with SDS and KL systems extensively studied in N phases. The first N, phase reported5 was obtained by a preliminary search in the ternary SLS system, which was studied by X-ray diffraction in several compositions.5~'2~13 In this investigation of phase diagrams as a function of temperature we chose to keep constant either the water/amphiphile ratio or the decanol/amphiphile ratio. The phase diagrams are presented as a function of the variable ratio, expressed in terms of relative molar ratio (number of water or decanol molecules per amphiphile molecule). This allows us to gain new insights on the important variables conditioning the nematic domain and particularly the Nc-Nd transition (although this work is not focused on the biaxial phase that may exist between Nd and N, phases).
Materials and Methods Sodium dodecyl sulfate (Merck 99%) and distilled 1-decanol (Merck) were used to prepare the mesophases. The water was distilled and deionized. The procedure for preparation of the liquid crystals followed standard procedures given in the l i t e r a t ~ r e .The ~ mesophases were prepared by weighing the appropriate amounts of components into small test tubes (accuracy of 0.05%). The mixtures were alternately stirred and centrifuged until the sample was seen to be homogeneous under crossed polarizers. Careful inspection of nematic phases in test tubes under crossed polarizers allows identifi~ation'~ of Nd and (Nc, Nbx)phases; characteristic textures are observed in the N film at the tube surface under flow. The identification has been consistently checked by optical microscopy characterization of the phases. For studies with polarized optical microscopy (OM) samples were usually sealed in 0.2-mm-thick flat microslides (Vitrodynamics) and examined with three microscopes in both orthoscopic and conoscopic geometries. Changes in the sample temperature were made by placing the slide in a temperature-controlled stage (accuracy of 0.1 OC for heating/cooling runs and 0.7 OC for fixed temperature). The velocity for increasing and decreasing the temperature in the stage was generally about 0.25 OC/min. Characterization of the samples by O M was made according to standard te~tures.'~-'' Surface alignment produces a homeotropic layer for N d phases (pseudoisotropic texture) and a planar layer (birefringent texture) for N, phases. The usual OM orthoscopic and conoscopic observations do not discriminate between N, and Nbxphases, so our characterization is always made with consideration of this restriction. This work is not focused on the biaxial phase. N d phases could be distinguished from isotropic I phases by clear birefringent regions at the edges of the microslides and also by conoscopic observations. X-ray diffraction (XD) was used to characterize some of the samples, sealed in I-mm glass capillaries. A fine collimator Laue camera with Cu K a radiation and 10-cm source-to-film distance (1 1) Figueiredo Neto, A. M.; Liebert, L.; Galerne, Y. J . Phys. Chem. 1985, 89, 3737. (12) Itri, R. Master Dissertation, University of SHo Paulo, Brasil, 1986. (13) Bittencourt, D. R. Ph.D. Thesis, University of SBo Paulo, Brasil, 1986. (14) Figueiredo Neto, A. M., private communication. (15) Lasic, D. D.; Marcondes Helene, M. E.; Reeves, L. W.; Szarka, M. Croat. Chem. Acta 1984, 57(1), 129. (16) Radley, K.; Saupe, A. Mol. Cryst. Liq. Cryst. 1978, 44, 227. (17) Holmes, M. C.; Boden, V.; Radley, K. Mol. Cryst. Liq. Cryst. 1983, 100, 93.
.
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 6095 40
30
01 0
--1--
I
1
1
I 0.2
0.1
0.3
0.4
0.5
Md Figure 1. Phase diagram as a function of the parameter Md, for a fixed value M , = 45.2. The arrows indicate Md values for the samples investigated outside the N domain.
was used; a permanent magnet of 2 kG could be placed with k horizontal and perpendicular to both the capillary (in vertical position) and the X-ray beam. The composition of the samples has been expressed both as weight percent (in the conventional ternary phase diagram) and as a function of the relative molar ratio Mx of substance X (X = w for water and X = d for decanol) in relation to amphiphile (A), given by
M , = (wt % of X/wt % of A)(WA/Wx) where WA and W, are the molecular weights of A and X; R is the ratio of amphiphile to decanol weight concentration, so that Md
= (l/R)(WA/Wd)
Phase Diagrams. A systematic search was initially made at room temperature by keeping the H,O/SLS concentration of the N, phase5 constant and studying several samples of the system, with varying quantities of decanol addition, until a Nd phase was found. Conoscopic observations evidenced that this phase was optically uniaxial positive. Figure 1 shows the results obtained by varying the temperature for several samples, with a fixed value M , = 45.2 water molecules/amphiphile molecule, and increasing the quantity of decanol. Samples in 0.2-mm flat capillaries were studied by O M with a rate of temperature change of 0.25 OC/min. The phase diagram is expressed as a function of the molar ratio Md, giving the number of decanol molecules per amphiphile molecule. Some of the samples have been studied at room temperature by XD; the characterization of N, and Nd phases was made by both OM and XD. The results in magnetically oriented samples showed the t y p i ~ a l ' *symmetries ~'~ of N, and Nd phases. A detailed analysis of XD results was presented e l s e ~ h e r e . ~ This J ~ ~ phase '~ diagram has a rather striking result of a sharp separation of N, and Nd phases. After the definition of the N, and Nd domains, as a function of Md, samples with fixed Md values were studied by addition of water and characterized by observation of flow texture in test tubes under crossed polarizers. Results are shown in Figure 2. For a given value of Md the system was either N, or Nd; the transition Nc-Nd could not be induced easily by changes in M , or temperature. The transition for increasing temperatures was to isotropic phases for high M , values a n d to polyphasic regions for low M , values. The transition from N phases to the lower temperature phases coagel and coagel* was studied in some detail. For the experimental conditions of Figures 1 and 2, a single transition at 10 OC (18) Amaral, L. Q.; Pimentel, C. A,; Tavares, M. R.; Vanin, J. A. J. Chem. Phys. 1979, 71, 2940. (19) Charvolin, J.; Levelut, A. M.; Samulski, E. T. J. Phys. Lett. 1979, 40, L-587.
Amaral and Helene
6096 The Journal of Physical Chemistry, Vol, 92, No. 21, 1988 DECANOL
?
40
45
55
50
.
/
Figure 2. Phase diagrams as a function of the parameter Mw, for three fixed values of the parameter Md. The points refer to characterized phases (eventually transitions): (a) Md = 0.400; (b) Md = 0.375; (c) Md = 0.328.
TABLE I: Values of M , and Mdfor Which the N,-Nd Transition Was Obtainedo M w
40.3 0.5 40.8 42.0 f 1.3 42.1 43.0 f 0.2 44.5 45.2
0
30
20
Mw
Md 0.376 f 0.005 0.373 f 0.008 0.376 f 0.012 0.389 f 0.003 0.384 f 0.002 0.376 0.018 0.376 f 0.008
*
"The errors refer to the mass added to the test tube to induce the transition. occurred for N, phases while two transitions occurred for Nd phases; the temperature of the first transition increased with increasing values of Md. This type of behavior was quite similar to that observed in Nd phases20 of the quaternary SDS/water/ decanol/salt system and in N, phases13 of the ternary SDS system. However, for samples in test tubes, a transition could be induced for both N, and Nd phases if the test tubes remained in a stable temperature of 16 "C for about 15 min. The N, phase subsisted at this temperature only for very high values of M,. N, and Nd samples were studied by XD to further investigate this lower temperature phase. The typical XD pattern of a lamellar coagel phase with extended carbon chains was observed in both samples after cooling to crystallization and subsequent reheating to 18 O C . The pattern was very similar to that obtained for Nd20and NcI3coagel phases of SDS, but with different repetition distance. In a subsequent series of experiments the transition Nc-Nd was followed by successive additions of the three components. The values of Md and M , where the transition was observed to occur are shown in Table I; errors refer to the mass added to the sample to induce the transition. The constancy of the k f d value associated with the transition, independent of M,, is striking. All the results obtained at room temperature have been mapped in the conventional ternary phase diagram (in weight percent
-
(20) Amaral, L. Q. J . Appl. Crystallogr. 1984, 17, 476.
SLS
Figure 3. Ternary phase diagram at room temperature: (a) N domain in relation to previously' obtained phase diagram showing isotropic (L, and L2), lamellar (D), and hexagonal (E) regions; (b) amplification of the N domain, showing phases obtained with several compositions.
composition) shown in Figure 3. The nematic domain falls between isotropic, hexagonal, and lamellar regions previously3 determined for the SLS ternary system; there is a very good agreement between the N-I transitions and the limiting line of the isotropic domain. Search for a Nd-N, Transition with Temperature. A considerable effort was made in searching for a composition where the Nd-N, transition would occur by temperature variation. Several samples were prepared with k f d in the vicinity of 0.38, but they were either N, or Nd when analyzed by OM at the usual speeds of heating or cooling. It should be remarked that standard systems showed their expected transitions under such speeds. However, it was noticed that some samples that were N, when prepared at a room temperature higher than 22 O C turned to Nd in the test tubes (and also in 1-mm cylindrical capillaries) with ageing (on the order of months) under some circumstances: rehomogenization from the coagel phase and/or long permanence (on the order of days) at a room temperature of about 18-19 "C. Such aged Nd samples were analyzed by OM in flat capillaries of 0.05, 0.2, and 0.4 mm and in cylindrical capillaries of 1 mm; in some cases a Nd-N, transition was then observed by temperature variation, occurring at a temperature in the interval between 20 and 23 OC. Typically, several white points appeared on the homeotropic Nd texture and started to grow, leading eventually to a planar texture over the whole sample. It was, however, a very slow transition, and in the thinner capillaries of 0.05 mm there could occur only a localized change of homeotropic to planar texture (as displayed in Figure 4), which could be assigned to a (Nd + N,) mixture in equilibrium (a Nd Nbxmixture is of course not ruled out). In many cases the mixture, believed to be (Nd N,), was reproducibly cooled to a Nd phase or heated to another polyphasic mixture (usually Nd I). In other cases a sample that remained Ndin the heating stage, upon slow and quick heating and cooling, could show the transition to N, phase out of the heating stage, at room temperature, under heat of the microscope light over the whole sample; out of the microscope it returned to the original state. Figure 5 shows examples of such light-induced transitions. Eventually some samples analyzed in the heating stage showed
+
+
+
SLS/Water/Decanol: Nematic Phases
The Journal of Physical Chemisfry. Vol. 92. No. 21, 1988 6097 ~~~
~~~~~~
i Figurr
I
room tcmpcraturc in :I (1 2 - m r 11at m i crosliilc. The N,,-N, boundary r e m i n c d stable i n a microscope with a wcaikcr lamp. The vertical dimension reprcnentr approximately the total width of the capillary (2 mm).
I,
Figure 5. OM of samples at rmm temperature, which displayed N,-N, transitions under a strong microscope light: (a) sample i n a I-mm cylindrical capillary (Ndshows a sharp characteristic line at the capillary axis); (b) sample in a 0.05-mm flat microslide (between the black Nd texture and the clear N, texture there is a region of intermediate texture).
the mixture only in the small part of the capillary that received the microscope light. After being formed, such (N, + Nd) mixtures have sometimes remained unalterated a t room temperature for several hours, particularly for samples in the thinner capillaries. In the thicker capillaries there was a tendency for the growth of one phase over the other. It seems therefore that surface anchoring has an effect in stabilizing the (N, + Nd) mixture. Figure 6 shows a Nd-N, (or Nb.) boundary in a 0.2-mm capillary that stayed stable for more than I h until the sample was removed from the orthoscopic microscope with micrograph facilities to a conoscopic microscope with a stronger lamp. It turned
6.
Oh4 01 \.IIII~IC
;I(
then quickly to the isotropic phase. This aged sample displayed the N,-l transition at 23 "C reproducibly (a temperature considerably lower than the original one of Figure 1) but achieved the Nd phase only eventually. On the other hand, samples that were Ndwhen prepared at a r m m temperature 2 2 2 OC did not evolve to a N, phase upon ageing and/or heating. When Md was very near 0.38, such Nd samples showed transitions to other polyphasic textures (and became cloudy in the test tubes) a t temperatures between 25 and 28 OC. All these observations indicate a nontrivial behavior of the Nd-N, transition in this system and the importance of physicochemical interactions. It should be noticed that the ageing effect cannot be due to a loss of water or decanol in the sample, since neither would produce an increase of Mdvalue, and that the form of heat exchange seems to be important in the Nd-N, transition. The ageing effect may be related to chemical reactions in the system (possibly hydrolysis of the alkyl sulfate to decanol and SOI2-).Hydrolysis would increase the parameter Md and thus promote the Nc-Nd transition. Furthermore, the new ions may act in a way that is equivalent to addition of salt, which also promotes the N,-Nd transition.' However, only the aged and thermally treated samples display the Nd-N, transition with changes in temperature; this effect cannot be associated simply with an irreversible decomposition by hydrolysis (particularly because hydrolysis is enhanced by temperature increase). There is therefore an indication that the Nd-N, transition with temperature may occur only for a very particular composition reached through ageing (and possibly hydrolysis) and involves other temperature-dependent physicochemical processes. The ageing effects (common to all lyomesophases") need further systematic investigation, particularly regarding the conditions of occurrence of the Nd-N, transition (in this as well as in other systems). It should be remarked that this SLS phase diagram displays the characteristic of an asymmetry that allows the N d phase to invade the N, domain at lower temperatures. This type of behavior is stronger in this SLS system but is also present in the KL'." and SDS6.9phase diagrams. These observations may be understood in terms of the model of aggregates (cylindrical micelles with lateral positional correlation) proposeds for the N, (and Nbr) phases, from the analysis of X D results in this SLS system, showing higher orders of unidimensional positional correlation also for N, phases. The transition to Nd phases may be due to physicochemical interactions transforming the plane of parallel cylinders into a defective lamella. This transformation could occur either along an intermediate Nbx phase (in which the planes become parallel) or in a first-order Nc-Nd transition. It is not possible at present to decide between (21) Blum. F. D.:Franrer. E. 1.: Rose. K. D.;Bryant, R. G.; Miller. W. G. Longmuir 1987, 3. 448.
6098 The Journal of Physical Chemistry, Vol. 92, No. 21, I988 TABLE 11: Range of Parameters R , Md,and M , Corresponding to the Transition N,-Nd for Ternary Systems Amphiphile/Water/DecsnoP amphiphile SDS6.8-10
KL7.1 I SLS (this work)
R 5.45 (-0.5) 4.0 (+0.2) 4.8 i 0.1
Md
M w
0.30 (+0.03) 0.38 (-0.02) 0.38 f 0.01
21 f 1 31 f 2 43 f 2
For SDS and KL, typical values and dispersions have been obtained by analysis of the published phase diagrams.
these two possibilities in this SLS system.
Discussion The results obtained and the sharpness of the N,-Nd transition as a function of the variable Md show that this is a very important parameter in this transition. The usual idea that the Nd --* NC transition is induced by increasing the amphiphile concentration may be rather misleading. In a ternary system, an increase in weight percent of amphiphileat a fixed weight percent Of decanol (as in the work of YU and SauPe6") Or at a fixed weight Percent of water (as in the works of Hendrikx et al.'s9 and Bartolino et a].'') corresponds to a decrease of the molar ratio Md. It would seem that such decrease induces the transition. The Nd domain extends to higher Of M w than the Nc domain, regarding the N-I transition, as shown in Figure 2. This regarding addition shows that the Nd phase has a higher of water. But the Nd-N, transition could not be induced easily by a change only in M,. The Of R$ Md* and M w corresponding .to the Nc-Nd transition for this system with SLS and for the systems with KL and SDS are shown in Table 11. These parameters were obtained by analysis of the phase diagrams reported previously for SDS6x8-" and K L . ' X ~ ~ This table reveals the significance of the variables Md and M , to the N,-Nd transition. The closeness of Md values for these systems and the marked variation of M , with the amphiphile should be emphasized. We believe that the pertinent variables for a clearer understanding of the effects of decanol and water in nematic lyomesophases are M d and M , and not the weight concentrations. The ratio M , increases on increase of chain length of the amphiphi1e9but an influence Of heads may exist' The KL chain length is effectively intermediate between SDS and SLS chain length;, because onebf the carbons of KL belongs to the polar head. There is some indication that hfd varies with chain length, but it is not possible to decide if h f d values differ in these systems: SLS and KL values are eoual: the SDS value is somewhat lower. but its dispersion is towaid higher values for SDS and lower for KL. The fixed R value used by Hendrikx et al.'q9 in the study of the SDS system corresponded to lower values of Md than those obtained from the work of Bartolino et a1.I' on the same system;
Amaral and Helene this might explain why Hendrikx et al. were unable to find the biaxial phase reported by Bartolino et al. It should be considered also that the amphiphiles might have different degrees of purity, which could mask partially the results. It would be important to obtain results with the systems potassium decanoate/water/decanol and of KL and SLS systems with dodecanol to define a systematic behavior for these parameters. We believe that it is the ratio of decanol/amphiphile molecules within the micelle that essentially defines whether the phase is N, or Nd. This ratio is probably related to the curvature of the hydrocarbon core/water interface. As observed experimentallyz2 and discussed recently in a theoretical of the I-N, transition, the cosurfactant molecules are preferentially partitioned into the lower curvature micellar regions. An increase in M~could stabilize a micelle with larger regions of lower curvature, in the form of disks. Our results favor strongly the more conventional idea that micelles are indeed changing their form at the transition N,-Nd, as a function of physicochemical interactions of intra- and intermicellar nature. The more recent proposal24 that all lyotropic N phases have the same biaxial micellar format, subject only to order-disorder transition,25 is not supported by our results. The more consistent picture would be to consider changes in micellar form induced by physicochemical interactions that may be temperature dependent (temperature-induced changes in chain length should also be consideredz6) coupled with order-&sorder effects of steric nature, The possibility of mixtures of rods and disks has hen considered both theoretically27 and experimentally,z8 but it should be remarked that the underlying hypothesis of changes in relative concentration of rods/disks implies a parallel physicochemical process of trans.ormation of one form into the other.
Acknowledgment. We thank R. Itri for help in preparation of some samples, Dr. A. M. Figueiredo Net0 for the availability of the temperature-controlled stage, and Dr. D.Svisero for use of microscopes of the Instituto de GeociEncias in some of the observations. The technical assistance of S. A. Silva is also acknowledged. Registry No. SDS, 151-21-3; 1-decanol, 112-30-1. (22) Hendrikx, Y.; Charvolin, J.; Rawiso, M. J . Colloid Interface Sci. 1984,100,597. Alpdrine, S.; Hendrikx, Y.; Charvolin, J. J . Phys. Lett. 1985,
.-,
dr; I - &,. 77 I
(23) Gelbart, W. M.; McMullen, W. E.; Ben-Shaul, A. J . Phys. (fes Ulis, Fr.) 1985, 46, 1137. McMullen, W. E.; Gelbart, W. M.; Ben-Shaul, A. J . Phys. 1985382, 5616. (24) Figueiredo Neto, A. M.; Galerne, Y . ;Levelut, A. M.; Liebert, L. J . phys, L ~ 1985, ~ 46, ~ L-499, , (25) Alben. R. R. Phvs. Reu. Lett. 1973. 30. 718. Stralev. J. P. Phvs. Rev. A 1974, 10, 1881. ' (26) Oliveira, M. J.; Figueiredo Neto, A. M. Phys. Rev. A 1986, 34, 3481. (27) Rabin, Y.; McMullen, W. E.; Gelbart, W. M. Mol. Cryst. Liq. Cryst. 1982. 89,67, (28) Rosenblatt, C. J . Phys. (Les Ulis, Fr.) 1986, 47, 1097.
