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Study of 1,2-Octanediol as Cosurfactant of Sodium Dodecyl Sulfate† I. Jime´nez, G. Montalvo, and E. Rodenas* Departamento de Quı´mica Fı´sica, Universidad de Alcala´ , E-28871 Alcala´ de Henares (Madrid), Spain Received March 10, 2000. In Final Form: June 13, 2000 The phase diagram of the sodium docecyl sulfate (SDS)/1,2-octanediol/water system at 25.0 ( 0.1 °C is given in this paper. The system shows an isotropic normal micellar phase at high water concentration, and two liquid crystalline phases characterized as the hexagonal phase and the lamellar phase. The diol behaves as a short-chain alkanol, except that the extended lamellar phase is stabilized at a very low SDS content. The lamellar phase is also formed by 1,2-octanediol with water. The rheology technique was applied to characterize the phases. The lamellar phase can be divided into four different zones according to the flow behavior. Some of the samples close to the boundary show structural transitions induced by shearing. Both liquid crystalline phases are very elastic, but they show very different relaxation times. The surfactant concentration affects size, stiffness, and the dynamic properties of the cylindrical units in the hexagonal phase.
I. Introduction The role of alcohols as cosurfactants in the ternary phase diagram is well-known. There are a lot of results in the literature concerning monofunctional alcohols for which the influence of the alcohol chain length,1,2 the temperature,3-5 and the surfactant counterion6,7 have been studied. There are not many papers that study diols or polyols. We have already reported8 the results obtained with the cetyltrimethylammonium bromide (CTAB)/1,6-hexanediol/ water system which show that 1,6-hexanediol behaves like a short-chain alcohol such as propanol. The phase diagram only has one clear and large isotropic low-viscous region, the L phase, and one hexagonal liquid crystal phase, H1. The L phase extends from the water-rich corner to the alcohol-rich corner, similar to results obtained with other monofunctional short-chain alcohols. The lamellar phase, LR, does not appear. These results indicate that this alcohol is solubilized in the micellar surface because of the steric difficulties in solubilizing in the micellar core.9 We have also studied other phase diagrams with more complex diols such as low weight poly(propylene glycol) (PPG) (Mw ) 425 and 1000). In these cases these watersoluble polymers behave like medium-chain alcohols,10,11 such as 1-butanol with the surfactants sodium dodecyl sulfate (SDS) and CTAB. This means that PPG is solubilized with the hydrophobic group between the surfactant chains and the polar group in the micellar † Part of the Special Issue “Colloid Science Matured, Four Colloid Scientists Turn 60 at the Millennium”.
(1) Fontell, K.; Khan, A.; Lindstro¨m. B.; Maciejewska, D.; PuangNgern, S. Colloid Polym. Sci. 1991, 269, 727. (2) Gamboa, C.; Olea, A.; Rios, H.; Henriquez, M. Langmuir 1992, 8, 23. (3) Durga Prasad, Ch.; Singh, H. N. Colloids Surf. 1990, 50, 37. (4) Sasaki, M.; Imae, T.; Ikeda, S. Langmuir 1989, 5, 211. (5) Makhloufi, R.; Cressely, R. Colloid Polym. Sci 1992, 270, 1035. (6) Ikeda, S. Colloid Polym. Sci. 1991, 269, 49. (7) Wa¨rnheim, T.; Jo¨nsson, A.; Sjo¨berg, M Prog. Colloid Polym. Sci. 1990, 82, 271. (8) Can˜adas, O.; Valiente, M.; Rodenas, E. J. Colloid Interface Sci. 1998, 203, 294. (9) Hoiland, H.; Blokhus, A. M. In The Structure, Dynamics and Equilibrium Properties of Colloidal Systems; Bloor, D. M., Wyn-Jones, E., Eds.; Kluwer Academic Publishers: Boston, MA, 1990. (10) Sierra, M. L.; Rodenas, E. Langmuir 1994, 10, 4440. (11) Sierra, M. L.; Rodenas, E. Langmuir 1996, 12, 573.
surface. With both surfactants the lamellar phase is formed. We have also reported the results for CTAB with glycerol and glyceraldehide as a cosurfactant,12 showing there is only a small interaction between the surfactant and the cosurfactants. Both systems exhibit only the L and the H1 phases. No lamellar phase LR was found even though it is the most common liquid crystalline phase.13 In this paper we study the properties of 1,2-octanediol as a cosurfactant of SDS. We are interested in the effects of glycol on the phase behavior. The system has been characterized by optical microscopy between crossed polarizers and rheology. And the formation of micellar aggregates has been characterized from conductivity and fluorescence quenching measurements. II. Materials and Methods Sodium dodecyl sulfate (SDS) (Sigma) and 1,2-octanediol (Sigma) were used as supplied. The quencher, N-cetylpyridinium chloride (CPyCl) (Merck) and the probe, pyrene (Merck), were recrystallized from methanol/diethyl ether mixtures. The phase diagram was constructed using macroscopic samples prepared by weighing the components in Pyrex glass tubes. Samples were homogenized by shaking in a Heidolph REAX 2000 vibrator, heating, and storing at 25.0 ( 0.1 °C. They were subsequently checked visually through crossed polarizers. The liquid crystalline behavior was investigated with a Laborlux S Leitz optical microscope between crossed polarizers coupled to a Yashica 108 Multiprogram camera, with a Leitz Periplan 10×/ 18 TL 160 mm lens for photography of the samples. Each phase was identified by comparing its textures with photomicrographs from literature.14 Specific conductivities were measured with a Crison 525 conductometer. The solution flask containing conductivity (cell constant 0.100 and 1.192 cm-1) was immersed in a water bath at 25.0 ( 0.1 °C. The fluorescence steady-state quenching measurements were carried out using a Perkin-Elmer spectrofluorimeter, model LS5B, at 25 ( 0.1 °C, using pyrene as the probe and N(12) Corte´s, A. B.; Valiente, M.; Rodenas, E. Langmuir 1999, 15, 6658. (13) Bleasdale, T. A.; Tiddy, G. J. T. The Structure, Dynamics and Equilibrium Properties of Colloidal Systems; Bloor, D. M., Wyn-Jones, E., Eds.; Kluwer Academic Publishers: Holland, 1990; p 397. (14) Rosevear, F. B. J. Am. Oil Chem. Soc. 1954, 628, 31. Rosevear, F. B. J. Am. Oil Chem. Soc. 1968, 581, 19.
10.1021/la0003589 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/13/2000
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Figure 1. Phase diagram for the SDS/1,2-octanediol/water system at 25.0 ( 0.1 °C. The four zones in the lamellar phase correspond to rheological conclusions. cetylpyridinium chloride as the quencher. The fluorescence emission was integrated over 69 s, and the data reported are mean values from several measurements. Micellar aggregation numbers were determined from the fluorescence intensity in the presence and absence of the quencher at 375 nm, which corresponds to the first peak of the emission spectrum, the one that is the least sensitive to the experimental conditions. The quencher can be considered to be located in the same environment as the surfactant molecules, and the concentration was so small that it can be safely assumed that it does not change the micelle structure. The rheological study was performed using a Carri-Med CSL 100 control stress rheometer, with a cone and plate configuration (40 mm‚1° and 20 mm‚2°) at 25.0 ( 0.1 °C. Two different experiments were carried out: steady flow (shear stress sweep mode) and oscillation. In the oscillation experiments, the storage modulus, G′, and the loss modulus, G", were measured as a function of stress to obtain the linear viscoelastic region. Once this was established, the oscillation measurements were carried out as a function of frequency at a constant stress.
III. Results and Discussion III. 1. Phase Diagram. Figure 1 shows the phase diagram for the ternary SDS/1,2-octanediol/water system. This system exhibits three uniphasic phases: two reduced regions, the L1 and H1 phases, and an extended LR phase region. The micellar phase, located near the water-rich corner, is small in size with respect to the binary SDS/ water system. This means that the 1,2-octanediol destabilizes it. The opposite effect is described for the 1,6hexanediol,8 where the alcohol groups are located in the extreme carbons. Otherwise the lamellar liquid crystal phase is very wide; it extends between a 3-57 wt % of SDS and a 13-84 wt % of alcohol, which means that the glycol stabilizes it. The samples of this phase are macroscopically turbid, but they show a typical mosaic texture with maltese crosses as observed with a microscope under crossed polarizers. The extended lamellar phase is one of the system characteristics. It is related to the behavior of 1,2-octanediol/water binary mixtures, which shows the lamellar phase formation contained in a multiphasic phase region, with alcohol crystals at high alcohol content or with the L1 phase at low alcohol content. Figure 2 shows the typical lamellar structure for this binary system, which is similar to the lamellar phase of the ternary system. The hexagonal phase is stabilized by the glycol, and it appears at lower concentrations than in the binary system
Figure 2. Polarizer optical microphotograph of the samples % SDS /% 1,2-octanediol ) 0/ 80 (w/w) showing the birrefringent crosses, LR phase. Total magnification x 100.
(Figure 1). This phase has a geometric texture described as fanlike when the samples are observed with a microscope between crossed polarizers.14 The lamellar phase is separated from the hexagonal phase by a biphasic region, in which both of them appear, Figure 3. The H1 phase shows a different texture like a cotton cloud when the SDS concentration is higher. III. 2. The L1 Phase Properties. III. 2. 1. Conductivity and Fluorescence. The critical micelle concentration (cmc) and the micellar ionization degree (R) for mixed SDS/1,2-octanediol micelles in water has been obtained from the specific conductivity values against SDS concentration, for the samples with fixed SDS/alcohol ratios and high water content. The micellar ionization degrees (R) were obtained as the ratio of the slopes of the conductivity curves versus SDS concentration, above and below the cmc. The R and cmc values are given in Table 1. The cosurfactant content seems to increase the micellar ionization degree. The cmc values decrease with the diol concentration. These results are similar to the other cosurfactants, such as the SDS/PPG11 and SDS/mediumchain alcohols (1-butanol, 1-hexanol)15 systems where the cosurfactants are solubilized near the micellar surface. This behavior is described for the 1,6-hexanediol in the presence of CTAB8 as well. Otherwise 1-octanol produces a decrease in the micellar ionization degree of SDS micelles.15 Fluorescence steady-state measurements have been used to determine aggregation numbers of the L1 phase (15) Rodenas, E.; Pe´rez-Benito, E. Langmuir 1991, 7, 232.
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Figure 3. Polarizer optical microphotograph of the sample % SDS/% 1,2-octanediol ) 50/10 (w/w) showing two phases, LR and H1. Total magnification is 100×. Table 1. cmc and the Micellar Ionization Degree r for Mixed SDS/1,2-Octanediol Micelles in Water [SDS]/[glycol], mM
cmc/mM
R
∞ 3 2 1 0.5 0.33
7.00-8.00 6.29 5.20 5.00 4.22 3.08
0.27 0.37 0.62 0.42 0.49 0.59
samples. The theoretical treatment used was taken from literature.16 It considers the probe and the quencher are distributed into the micelles according to Poisson statistics, with the quencher average occupation number (n˜ ) as the number of quencher molecules per micelle given by n˜ ) [Q]N/[Dn], where [Q] and [Dn] represent the quencher and micellized surfactant concentration, respectively, and N is the aggregation number. According to Poisson’s statistical law, the ratio of the fluorescence intensities in the absence and in the presence of the quencher (I0/I) is related to the quencher average aggregation number by the following expression
n˜ ) ln(I0/I) ) [Q]N/[Dn] so that a plot of ln(I0/I) against [Q] should give a straight line from which the aggregation numbers of the micelles can be obtained. The results fit the treatment, and the aggregation numbers for the micelles with a ratio [SDS]/ [alcohol] ) 1 are given in Table 2. The aggregation numbers increase with the surfactant concentration, and it is smaller than those obtained for SDS micelles. This means that, at low alcohol and surfactant concentrations, 1,2octanediol is solubilized in the micellar surface, like short(16) (a) Infelta, P. P. Chem. Phys. Lett. 1979, 88, 61. (b) Yekta, A.; Aigawa, M. N.; Turro, N. J. Chem. Phys Lett. 1979, 63, 543. (c) Tachiya, M. J. Phys. Chem. 1983, 78, 5282.
Table 2. Aggregation Number N and the Ratio II/IIIII as a Function of the Surfactant Concentration, at [SDS]/ [1,2-octanediol] ) 1 [SDS]/mM
N ( error
II/ IIII
8.33 16.67 33.33 50.00
22 ( 2 32 ( 2 51 ( 1 45 ( 3
1.13 1.06 1.03 1.03
chain alcohols, producing smaller micelles.17 The same effect is found for the 1,6-hexanediol.8 The ratio of intensities II/IIII, where II and IIII correspond to the fluorescence intensities of the first and the third peak of the pyrene fluorescence band, is near 1. This means that the polarity of the environment where the probe is solubilized is similar to 1-butanol (Table 2). Similar results were obtained for mixed micelles SDS/alcohol when the alcohol was located in the micellar surface.17 III. 2. 2. Rheological Study. Two different rheological behaviors of the L1 phase samples have been observed as the flow curves show in Figure 4. The samples of the binary SDS/water axis behave like Newtonian fluids, and the viscosity is independent of the shear rate. (The initial drop of the curve is due to the inertia of the system due to the low viscosity of the samples.) Similar results have been obtained with the SDS/PPG/water11 system and with similar viscosity values, from 7 × 10-3 to 9 × 10-3 Pa‚s. This behavior is consistent with the presence of small spherical micelles. The viscosity value increases with the addition of SDS, which could be related to an increase in the micellar size. The presence of glycol strongly affects the rheological behavior. The samples become non-Newtonian fluids, and they show the so-called shear-thinning behavior. At low shear rates, these samples with glycol behave like Newtonian fluids with a constant viscosity value inde(17) Rodenas, E.; Pe´rez, E. J. Phys. Chem. 1991, 95, 9496.
1,2-Octanediol as Cosurfactant of SDS
Figure 4. Viscosity as a function of the shear rate in samples of the L1 phase.
pendent of the shear rate, η0, which ranges from 0.1 to 0.45 Pa‚s. Above some critical shear rate γ˚ c the viscosity decreases and the samples become pseudoplastic. The η0 values are 2 orders of magnitude larger than ones in the spherical micelles without glycol. The viscosity η0 also increases with the surfactant concentration. The shear-thinning behavior is similar to systems of CTAB in the presence of different alcohols such as benzyl alcohol,18 glyceraldehide,12 and glycerol.12 The micelles are nonspherical, and this behavior is consistent with the formation of cylindrical micelles. Micelles are completely in a random position due to the thermal motion, but by increasing the shear rate, a progressive alignment of the cylindrical micelles along the streamlines takes place. The size of the Newtonian zone is expressed by the critical shear rate where the viscosity starts to decrease, γ˚ c. This value has been obtained as the intercept between the line of the plateau and the straight line with the negative slope at the higher shear rate. The γ˚ c seems to decrease as the viscosity of the samples η0 increases. As η0 could be related to the micellar size, greater η0 values correspond to a bigger micelle size. And γ˚ c seems to be related to the flexibility of the micelles, where smaller γ˚ c corresponds to micelles which are easier to orientate in the flow direction. This means that it is easier to deform and orientate these cylindrical micelles as their size increase.12 III. 3. The Lamellar Phase: Rheological Study. The lamellar phase behaves like a pseudoplastic fluid, and the viscosity decreases with the shear rate. The viscosity values are higher than the viscosities of the micellar phase samples. The flow behavior is shown in Figure 5, and the curves do not fit any empirical model throughout the range of shear rate. Similar curves have been obtained for the LR of the CTAB/benzyl alcohol/ water18 system. Some of the samples that are located near the L1 phase show some anomalies in the flow curve, Figure 6. At low shear rate, the samples behave like a pseudoplastic fluid, but the viscosity increases and reaches the maximum close to 100 s-1 and later on it decreases. This phenomenon is perfectly reproducible at different experimental conditions. It may be due to changes in the liquid crystal structure at the shear rate of the maximum, which has also been reported previously by different authors.19,21 This effect can be explained by considering the lamellar phase to be built up of open and piled bilayers at a low shear (18) Montalvo, G.; Valiente, M.; Rodenas, E. Langmuir 1996, 12, 5202.
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Figure 5. Viscosity as a function of the shear rate in samples of the LR and the H1 phases. The straight line fitting curve was obtained by means of the Cross equation.
Figure 6. Induced structure by shearing in the LR phase. Plot shows viscosity as a function of the shear rate.
rate. This system could have very high water penetration in agreement with similar systems such as SDS/benzyl alcohol/water20a or SDS/pentanol/water.20b Thus these open bilayers with structural defects can close easier to form vesicles, and this vesicle formation increases the viscosity. As the shear rate continues to increase, these vesicles can deform and orientate along the streamlines, decreasing the viscosity.19a,b,e,f Other authors19d explain that the stress may induce a significant proliferation of defects which are associated to a liquid crystal phase transition. The structural change has been seen with the microscope under crossed polarizers, and for the samples that have been taken under shearing for 2 h, two different structures appear together, Figure 7. The samples with anomalous flow behavior, due to the structural change under shearing, are marked by crosses in Figure 1. In different lamellar systems,19a,b,d,e the induced structures under shearing are described at a maximum shear (19) (a) Diat, O.; Roux, D.; Nallet, F. J. Phys. II 1993, 3 (9), 1427. (b) Roux, D.; Nallet, F.; Diat, O. Europhys. Lett. 1993, 24 (1), 53. (c) Valdes, M.; Manero, O.; Soltero, J. F. A.; Puig, J. E. J. Colloid Interface Sci. 1993, 160, 59. (d) Franco, J. M.; Mun˜oz, J.; Gallegos, C. Langmuir 1995, 11, 669. (e) La¨uger, J.; Weigel, R.; Berger, K.; Hiltrop, K.; Richtering, W. J. Colloid Interface Sci. 1996, 181, 521. (f) Zipfel, J.; Lindner, P.; Richtering, W. Prog. Colloid Polym. Sci. 1998, 110, 139. (20) (a) Guo, R.; Tianquing, L.; Weili, Y. Langmuir 1999, 15, 624. (b) Guo, R.; Compo, M. E.; Friberg, S. E. J. Dispersion Sci. Technol. 1996, 17 (5), 493. (21) Montalvo, G.; Rodenas, E.; Valiente, M. J. Colloid Interface Sci. 1998, 202, 232.
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Figure 7. Polarizer optical microphotograph of the sample % SDS/% 1,2-octanediol ) 20/27 (w/w) showing two phases induced by shearing. Total magnification is 100×.
rate close to 100 s-1, which is the same shear rate as in the SDS/1,2-octanediol/water system. But we have also reported this structural change at a shear rate near 1015 s-1, for the diluted C12E4/benzyl alcohol/water system with viscosity values that are 2 orders of magnitude smaller. Other authors have observed the phenomenon in diluted lamellar phases19c-f at a shear rate of 10 s-1 as well. This means that this structural change under shearing can occur in samples with lower viscosity but it happens at lower shear rate. It is difficult to find any relation between the viscosity values and the sample composition although the viscosity increases with both the surfactant and the alcohol content. To compare the viscosity of the samples with different compositions, the viscosity at a shear rate of 10 s-1 was analyzed and different zones, which correspond to a same range of viscosity, were delimited into the LR region of the phase diagram by dashed lines (Figure 1). The samples in zone 1 have a viscosity near 1 Pa‚s; zone 2 is located in the middle of LR and the viscosity ranges 3-6 Pa‚s; zone 3 is located near the alcohol-rich corner, and all the samples have nearly the same viscosity, 7-9 Pa‚s. These viscosity values are slightly higher than the viscosity in zone 2. Zone 4 is located near the surfactant-rich corner, and the samples are the most viscous, 10-33 Pa‚s. (All of these viscosity data are at the shear rate of 10 s-1.) The linear viscoelastic properties of the lamellar samples have also been studied by oscillatory experiments, and they are given in Figure 8. In all the measured samples, the storage modulus, G′, is greater than the loss modulus, G′′, for the whole frequency range. This is in agreement with an important elastic contribution in the lamellar structure. Both moduli are almost independent of the angular frequency, and there is no indication of a crossover of either modulus. This means that the system has no finite structural relaxation time. Similar results were obtained in other systems such as CTAB/benzyl alcohol/
Figure 8. Complex viscosity modulus (η*), elastic or storage modulus (G′), and viscous or loss modulus (G′′) as a function of the angular oscillatory frequency in the LR phase. Sample was % SDS/% 1,2-octanediol ) 25/35. (σ ) 15 Pa.)
water,18 AOT/water,22a or the lamellar phase formed by densely packed vesicles.22b,c This behavior has been related to a yield stress value, but it has not been detected in the SDS/1,2-octanediol/water system. The storage modulus has the same values as for the CTAB/benzyl alcohol/water system.18 III. 4. The Hexagonal Phase: Rheological Study. We have studied samples at a fixed 5 wt % of 1,2-octanediol. These samples show a shear-thinning behavior as shown in Figure 5. At a low shear rate the viscosity seems to reach a constant value η0, but the viscosity decreases (22) (a) Robles-Va´squez, O.; Corona-Galva´n, S.; Soltero, J. F. A.; Puig, J. E.; Tripode, S. B.; Valle´s, E.; Manero, O. J. Colloid Interface Sci. 1993, 160, 65. (b) Hoffmann, H.; Munkert, U.; Thunig, C.; Valiente, M. J. Colloid Interface Sci. 1994, 163, 217. (c) Gradzielski, M.; Hoffmann, H.; Panitz, J.-C.; Wokaun, A. J. Colloid Interface Sci. 1995, 169, 103.
1,2-Octanediol as Cosurfactant of SDS
Figure 9. Zero-shear-rate viscosity as a function of the surfactant concentration for the heagonal phase.
throughout the range of shear rates. Samples of the binary system SDS/water without glycol show the same flow behavior, but samples with more than 50 wt % surfactant are very viscous and we could not obtain the flow curve. The same behavior was obtained for the CTAB/benzyl alcohol/water system18 as well. But the viscosity is higher in the SDS/1,2-octanediol/water system, as the binary SDS/water11 system shows higher viscosity than the binary CTAB/water10 system with the same amount of surfactant. The results follow the Cross empirical equation, which fits curves with two plateaus separated by a power-law region
η - η∞ ) 1 + (K‚γ˚ )m η0 - η zero-shear-rate viscosity,η0, and infinite-shear-rate viscosity, η∞, are the two plateaus of viscosity at low and high shear rate, respectively, K is a constant parameter, and m is the rate index which fits the results. We used the Cross model as a tool to analyze the data. If we look carefully at the flow curve, there are no well-defined plateaus, but the error by applying the Cross method is smaller than 5% in all of the hexagonal samples. The fits are good as can be seen in Figure 5, and the treatment allows the results to be discussed. For the hexagonal samples, the η∞ is nearly zero and the rate index m is 1. The constant parameter K has an average value of 300 in all the samples. Consequently, all of these parameters are independent of the composition, while the η0 values increase steadily with the surfactant concentration in agreement with Figure 9. The highest viscosity corresponds to the samples near the boundary phase in the rich-surfactant corner. Contrary to that, the η0 value is nearly independent of the glycol concentration and the sample without glycol, at 40% of SDS, has almost the same zero-shear-rate viscosity as the sample with 5 wt % of glycol. The glycol seems to have a smaller effect on the viscosity with respect to the micellar phase, where the presence of 1,2-octanediol introduces changes in the micellar shape that strongly affect the viscosity. In addition, it is interesting to point out that at the same alcohol concentration (5 wt %), the viscosity drastically increases as the phase changes from the cylindrical micellar aggregates to the cylindrical hexagonal unit when adding SDS, from 10-1 to 105 Pa‚s. The viscoelastic properties of the hexagonal phase can be seen in Figure 10. The data were collected for two hexagonal samples in the limits of the phase, % SDS/%
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Figure 10. Complex viscosity modulus (η*), elastic or storage modulus (G′), and viscous or loss modulus (G′′) as a function of the angular oscillatory frequency in the H1 phase. Sample was % SDS/% 1,2-octanediol ) 25/5. (σ ) 40 Pa.)
diol ) 25/5 and 55/5. The complex viscosity modulus has a plateau at low frequencies, ηωf0, and the value decreases enormously at higher angular frequency. The storage G′ and loss G′′ moduli increase with the frequency, reaching a constant value or plateau, which is related to the hexagonal structural net.23 The results do not fit the Maxwell model at low angular frequencies, whereas they are common in other surfactant systems.18 The relaxation times, τ, were calculated by the inverse value of the angular frequency at G′′(ω) ) G′(ω), and above this frequency, the elastic contributions are higher than the viscous contributions. Both relaxation time and plateau ηωf0 increase with the surfactant concentration. The relaxation time goes from 67 to 159 s and the plateau ηωf0 goes from 205 to 6832 kPa‚s, in samples with 25% and 55% (w/w) of SDS, respectively. The relaxation times obtained in the SDS/1,2-octanediol/water system are 2 decades larger than the ones obtained for a sample of the same composition (25% surfactant/5% cosurfactant) in the CTAB/benzyl alcohol/water system.18 The ηωf0 is 10 times larger in the sample with SDS with respect to the sample with CTAB. In conclusion, the surfactant affects size and stiffness of the cylindrical units in the hexagonal phase. The SDS content affects the dynamic properties too, while the surfactant concentration does not have any influence in a system similar as CTAB/benzyl alcohol/water.18 IV. Summary From all of these results we can conclude that the presence of this glycol destabilizes the micellar phase with respect to the binary SDS/water system, while 1,6hexanediol stabilizes it. But the 1,2-octanediol stabilizes both liquid crystalline phases, the lamellar and the hexagonal phases. Otherwise, at very low surfactant and alcohol concentrations, both conductivity and fluorescence studies in the L1 phase show that 1,2-octanediol behaves like a shortchain alcohol and like 1,6-hexanediol. This is due to the fact that the glycol is located between the surfactant chains in the micellar interface. At higher SDS and 1,2-octanediol concentrations, the behavior of glycol seems to be different from short-chain alcohols as it stabilizes the cylindrical micelles. This means that the glycol has a stronger effect than the surfactant on the micellar shape. (23) Ferry, J. D. Viscoelastic Properties of Polymers; John Wiley & Sons Inc.: New York.
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The lamellar phase appears at very small amounts of SDS as the binary 1,2-octanediol/water system forms this phase as well. The flow behavior of the lamellar samples is pseudoplastic, and they are very elastic. Some of the samples show an anomalous flow behavior explained by structural transitions induced by shearing. The hexagonal samples behave like shear-thinning fluids. From the oscillatory measurements we obtained the relaxation time, but the samples do not fit the Maxwell treatment. In this phase, the surfactant affects both the
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structural and the dynamic properties of the cylindrical units in the hexagonal phase. Acknowledgment. This work has been financially supported by the CICYT through the PB95-0322-C02-01 program. G. Montalvo thanks Ministerio de Educacio´n y Cultura of the Spanish Government for a fellowship. The authors thank Mrs. M. Heijnen for linguistic assistance with the preparation of the paper. LA0003589
