Langmuir 1996, 12, 3143-3150
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A Novel Iridescent Gel Phase of Surfactant and Order-Disorder Phase Separation Phenomena Takamasa Yamamoto,† Naoki Satoh,† Tomohiro Onda,‡ and Kaoru Tsujii*,† Institute for Fundamental Research, Kao Corporation, 2606 Akabane, Ichikai-machi, Haga-gun, Tochigi, 321-34, Japan Received November 27, 1995. In Final Form: March 25, 1996X A novel iridescent gel phase of surfactant has been found in a ternary mixture of a double-chain surfactant (triethanolammonium dihexadecylphosphate), water, and ethanol. The structure of the iridescent gel phase has been studied mainly by X-ray diffraction and ultraviolet and visible light reflection techniques and determined to be lamellar, having the spacing distance of a submicrometer. The color appearance of the solutions results from diffraction of visible light by the lamellar structure of bilayer membranes. It is particularly interesting to note in this system that the iridescent color can be changed with temperature (0-20 °C), ethanol concentration (15-48 wt %), and surfactant concentration (1.0-2.0 wt %) as well. The iridescent color begins to shift to the blue side at a certain critical temperature when the temperature is elevated. Turbidity of the solution also starts to increase at the same critical temperature. When these critical temperatures are connected as a function of surfactant concentration, a curve similar to that of the lower critical solution temperature type phase diagram is obtained. In the two-phase region of this diagram, the iridescent color changes with temperature, owing to the concentration change of the condensed (lamellar) phase, and does not change at constant temperature even when the surfactant concentration is altered. These novel phenomena in the present iridescent surfactant systems can be ascribed to the phase separation into a lamellar structure and disordered bilayers of the surfactant gel phase. This order-disorder phase transition is substantiated by the freeze-fracture electron microscopic technique. This new type of phase separation phenomenon gives us a challenging fundamental problem on the interactions between colloidal particles.
Introduction One of the most interesting recent topics in the field of colloid and surface science1-12 is the iridescent phenomena of some surfactants and surfactant mixtures in dilute (1-2 wt %) aqueous solutions. The iridescent color appears by Bragg reflection of visible light from the periodic lamellar structure of bilayer membranes having a spacing distance of a submicrometer.5 Most of the iridescent surfactant systems reported so far are lamellar liquid-crystalline phases. In recent years, however, the iridescent gel phases of surfactant have been also observed in aqueous solutions of alkyldimethylamine oxides,7 dioctadecyldimethylammonium chloride,11 and sodium propyl octadecyl sulfomaleate.12 The present paper deals with a novel iridescent system of surfactant gel phase composed of a ternary mixture of triethanolammonium dihexadecylphosphate (DHP), water, and ethanol. Three characteristic aspects † Current address: Tokyo Research Center, Kao Corporation, 2-1-3 Bunka, Sumida-Ku, Tokyo 131, Japan. ‡ Current address: Imaging and Recording Research Laboratories, Kao Corporation, 2606 Akabane, Ichikai-machi, Haga-gun, Tochigi, 321-34, Japan. X Abstract published in Advance ACS Abstracts, June 1, 1996.
(1) Tsujii, K.; Satoh, N. In Organized Solutions. Surfactants in Science and Technology; Friberg, S. E., Lindman, B., Eds.; Marcel Dekker, Inc.: New York, 1992; p 341. (2) Lasson, K.; Krog, N. Chem. Phys. Lipids 1973, 10, 177. (3) Nagai, M.; Ohnishi, M. J. Soc. Cosmet. Chem. Jpn. 1984, 18 (1), 19. (4) Suzuki, Y.; Tsutsumi, H. Yukagaku 1984, 33 (11), 48. (5) Satoh, N.; Tsujii, K. J. Phys. Chem. 1987, 91, 6629. (6) Thunig, C.; Hoffmann, H.; Platz, G. Prog. Colloid Polym. Sci. 1989, 79, 297. (7) Imae, T.; Sasaki, M.; Ikeda, S. J. Colloid Interface Sci. 1989, 131, 601. (8) Platz, G.; Thunig, C.; Hoffman, H. Prog. Colloid Polym. Sci. 1990, 83, 167. (9) Strey, R.; Schomacker, R.; Nallet, F.; Roux, D.; Olsson, U. J. Chem. Soc., Faraday Trans. 1 1990, 86, 2253. (10) Naitoh, K.; Ishii, Y.; Tsujii, K. J. Phys. Chem. 1991, 95, 7915. (11) Satoh, N.; Tsujii, K. Langmuir 1992, 8, 581. (12) Berlepsch, H.; Strey, R. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 1403.
S0743-7463(95)01069-9 CCC: $12.00
in this system are (1) the iridescent color can be changed by the three parameters of temperature, surfactant, and ethanol concentrations, (2) ethanol is an essential component to obtain the iridescent solutions, and (3) color appears only in the gel phase of the surfactant. Experimental Section Dihexadecylphosphate was a kind gift from Mr. Shinji Yano of Wakayama Research Laboratories, Kao Corporation, and was synthesized by essentially the same procedures reported previously.13 The crude sample was recrystallized five times from ethanol and checked by acid value measurement to be 107.2 mg/g (102.6 mg/g theoretical). Triethanolammonium dihexadecylphosphate [DHP: (n-C16H33O)2P(dO)O-+HN(CH2CH2OH)3] was prepared by neutralization of the acid-type agent with triethanolamine and recrystallized three times from ethanol. The final DHP sample was determined by elemental analysis to contain 65.0% C (65.6% theoretical), 11.9% H (11.9%), 2.0% N (2.0%), 4.4% P (4.4%) and showed a melting temperature of 6768 °C. Ethanol, triethanolamine, and distilled water were purchased from Kanto Chemical Co. and guaranteed to be reagent grade. Ternary sample mixtures were completely dissolved at high temperature (∼70 °C) until homogeneous, and then the resulting solution was allowed to stand for at least 1 day at room temperature. Turbid solutions so obtained were cooled down to the desired temperatures very slowly (1-3 °C/h) in order to prepare the iridescent solutions. Slower cooling was necessary to obtain the iridescence for the sample solutions with lower concentration of ethanol. Light reflection spectra of iridescent DHP solutions were measured by a spectro-multichannel photodetector (Ohtsuka Electronics, type MCPD-110A) and detected at a normal direction against incident white light. The intensity of reflected light from the sample solutions was calibrated by that from a TiO2 white board. The sample solution in a capped quartz cell was removed from the temperature-controlled water bath, and spectral measurements were taken immediately. Small angle X-ray diffraction experiments were carried out to elucidate the structure of the surfactant gel phase at higher (13) Kurosaki, T.; Nishigawa, N.; Matsunaga, A. Yukagaku 1990, 39 (5), 346.
© 1996 American Chemical Society
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Figure 1. Color photographs of iridescent DHP solutions as functions of temperature and surfactant concentrations. Ethanol concentration was kept constant at 24 wt %. concentrations. An X-ray diffractometer equipped with a rotating copper anode (Rigaku, Rotaflex FR) was employed. The specimens were put into a sealed glass capillary. The temperature was kept constant at 5 or 20 °C. The tube voltage and current were 50 kV and 70 mA, respectively. An imaging plate was utilized to detect the diffracted X-rays. An exposure time of 5-100 h was necessary depending upon the surfactant concentration. Differential scanning calorimetry (DSC) was performed with a Seiko Electronics type-200 DSC apparatus. Sixty milligrams of the sample solution was put in a silver cell and the cell was sealed. The sample cells were cooled down to 0 °C and then heated at the rate of 0.5 °C/min. The turbidity measurements
were done with the spectro-multichannel photodetector described above at a wavelength of 450 nm. Freeze-fracture transmission electron microscopy was carried out with the following procedures. Freeze-fracturing and replication was performed in a freeze-fracture apparatus (JEOL JFD-1010). A small amount of the sample solutions was placed on a copper disk and was quickly frozen, being immersed in liquid nitrogen equilibrated with solid state nitrogen (T below -196 °C). Aqueous sample solutions are believed to be frozen in an amorphous state containing no crystalline ice in this procedure. After the frozen sample was fractured at -120 °C, the fractured surface was evaporated by platinum/carbon vapor under an angle of 30° on the rotating disk and was reinforced with carbon
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Figure 2. Color photographs of iridescent DHP solutions as functions of temperature and ethanol concentrations. DHP concentration was kept constant at 1.2 wt %. evaporation under the same conditions. The replica was separated in ethanol and mounted on a copper grid. The replicas were observed by use of a transmission electron microscope (TEM) (JEOL JEM-9010).
Results Iridescent Regions in Ternary Mixtures of DHP/ Water/Ethanol. Figure 1 shows the color photographs of iridescent DHP solutions as functions of temperature and surfactant concentrations. As already reported, in lamellar liquid-crystalline phases1-6,8-11 the iridescent color changes from red to blue with increasing concentra-
tion of surfactant. It is particularly interesting to note in this system that the color can also be changed by temperature. Similar photographs of DHP solutions as functions of temperature and ethanol concentrations are shown in Figure 2. The iridescent color changes are somewhat complicated with respect to the ethanol concentrations. It moves to the red side first and returns to the blue side again as the concentration increases. In both Figures 1 and 2, the solutions show the blue shift in color with elevating temperature and finally become colorless with some turbidity. These colorless and turbid solutions are shown to reflect ultraviolet light and are
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Figure 3. Phase diagrams of the iridescent-colored region of DHP solutions in three-component systems of DHP/water/ethanol at 5, 10, and 15 °C.
Figure 4. DSC thermograms of 1.2 wt % DHP solutions with various ethanol concentrations. Endothermic peaks correspond to the gel-micellar phase transitions.
suddenly transformed to clear transparent solutions with further increase of temperature. The phase diagrams of the iridescent-colored region at various temperatures are given in Figure 3. One can see easily from the figure that the iridescent region becomes smaller and smaller when temperature increases. Phase Behaviors of Iridescent DHP Solutions. Figure 4 exhibits the DSC thermograms of 1.2 wt % DHP solutions at various ethanol concentrations. A clear endothermic peak was observed in each thermogram. At these phase transition temperatures (Tc’s), the samples turned to clear transparent solutions from turbid (UVreflecting) states, being accompanied by a dramatic decrease in viscoelasticity. In addition, the solutions lost any optical anisotropy at the transition temperatures. It can be clearly seen from Figures 1, 2, and 4 that the iridescent color appears below the phase transition temperature. This transition is undoubtedly assigned to be a gel-micellar phase transition, since this surfactant shows a gel-liquid-crystalline phase transition in pure water. The presence of ethanol in the solutions may result in the transformation of the liquid-crystalline phase to the micellar one. The phase diagram of 1.2 wt % DHP solutions is shown in Figure 5 as functions of temperature and ethanol concentrations. The diagram can be divided into six regions: the iridescent gel phase showing no color change with temperature (region A); the iridescent gel phase showing color change with temperature (region B); a colorless and turbid gel phase (region C); a clear transparent micellar solution (region I); two more regions located to the left of the dotted line. The boundary between regions A and B was determined by reflection spectra
Figure 5. Phase diagram of 1.2 wt % DHP solution as functions of temperature and ethanol concentrations. Tc represents the gel-micellar phase transition temperatures. Iridescent color does not change with temperature in region A and changes in region B. The colorless and turbid gel phase is in region C. In region I, the solutions are clear, transparent, and optically isotropic. Regions to the left of the dotted line show no iridescent color. In the left region, Tc represents the gel-liquid-crystalline phase transition temperatures. Optical anisotropy was observed in regions A, B, and C.
measurements, as shown later. The solutions in regions A, B, and C are optically anisotropic, but the micellar phase (region I) is isotropic. Color Changes Depending on Temperature and Surfactant Concentrations. The reflection spectra of the iridescent gel phase of DHP are shown in Figure 6 at three different temperatures. The spectra at 5 °C are quite normal and just similar to those reported in liquidcrystalline phases.5,11 At 20 °C, on the other hand, all the spectra are gathered together at the same position in the UV region, even when the surfactant concentration is changed. The spacing distance d of the periodic structures can be calculated from the wavelength at each maximum in reflection spectra utilizing the Bragg equation5 and is plotted against temperature in Figure 7. One can see from the figure that the spacing distance d does not change in the lower temperature range, but starts to shift to the blue side at a certain critical temperature. This inflection point gives the boundary between regions A and B in Figure 5. As mentioned previously, the iridescent colored solutions become turbid and colorless with increasing temperature. Figure 8 shows the results of turbidity measurements of the DHP solutions as a function of temperature. The turbidity of the solutions starts to
A Novel Iridescent Gel Phase of Surfactant
Figure 6. Reflection spectra of the iridescent DHP gel phase as a function of surfactant concentration. Spectra at 5 °C (a), 15 °C (b), and 20 °C (c).
Figure 7. Spacing distance d of the iridescent DHP gel phase plotted against temperature at various DHP concentrations and an ethanol concentration of 24 wt %.
increase at a certain temperature and suddently diminishes again for micellar solutions at the phase transition point (Tc). It is worth noting that the temperatures at which the turbidity starts to increase coincide very well with the starting points of the blue shift shown in Figure 7. Figure 9 shows spacing distances d plotted against (1 - c)/c, where c is weight fraction of the surfactant. It can be seen from Figure 9 that the iridescent color (i.e., d) changes with the concentration of surfactant in the range of higher concentrations and lower temperatures. At higher temperatures, however, the color does not change in a certain dilute concentration range. Discussion Structure of Iridescent DHP Gel Phase. The structures of iridescent solutions elucidated so far are lamellar ones. When the lamellar structure is formed uniformly in whole space of the solution, the spacing distance d can be related to the weight fraction of
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Figure 8. Turbidity at 450 nm of 1.2 wt % DHP solutions as a function of temperature in the presence of 24 wt % ethanol. Tc indicates the phase transition temperature observed by DSC.
Figure 9. Spacing distance d of the iridescent DHP gel phase plotted against (1 - c)/c (c, weight fraction of DHP) at various temperatures. Ethanol concentration is 24 wt %. Five data points at higher concentration region (both at 5 and 10 °C) were obtained by small angle X-ray diffraction experiments, and other points at lower concentrations were from the maximum wavelength of the light reflection spectra.
surfactant c as5,14
d)
(
)
1 - c F1 + 1 d1 c F2
(1)
where F1 and F2 are the densities of the surfactant and water layers, respectively, and d1 is the thickness of a unit layer of the lamellar leaflet. In order to elucidate the structure of the DHP gel phase, small angle X-ray diffraction measurements have been done for solutions of higher concentration than that of the iridescent solutions. Figure 10 shows the X-ray diffraction patterns at 30 wt % DHP. Sharp first-order and clearly distinguishable higher-order peaks are observed. We can, of course, calculate the spacing distance d (10.1 nm at 30 wt %) from (14) Luzzati, P.; Mustacchi, H.; Skoulios, A.; Husson, F. Acta Crystallogr. 1960, 13, 660.
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Figure 10. Small angle X-ray diffraction patterns of 30 wt % DHP in 24 wt % ethanol aqueous solutions at 5 °C (a) and 20 °C (b).
these X-ray data. Furthermore, these diffraction patterns correspond to the spacing with an interrelation of 1:1/2:1/3:..., which indicates a lamellar structure of the gel phase at both 5 and 20 °C. The spacing distances d obtained from both X-ray diffraction and reflection spectra of visible light are plotted against (1 - c)/c in Figure 9. As expected from eq 1, this plot gives us a good straight line at the temperatures below 10 °C. This result means that the structure of the iridescent gel phase is also lamellar at lower temperatures. At higher temperatures, however, the plot deviates from the above equation and has an inflection point at a specific concentration. Similar inflection was observed in the d vs (1 - c)/c plot of iridescent gel phase of dioctadecyldimethylammonium chloride at nearly the same point as that in Figure 9.11 These abnormal phenomena will be discussed in the next section. Phase Separation in the DHP Gel Phase. As mentioned previously, the plot of d against (1 - c)/c deviates from a straight line in the iridescent gel phase of DHP at higher temperature range (Figure 9). This result strongly suggests that some kind of phase separations occurs in the iridescent DHP gel phase, because any homogeneous lamellar structures in the whole space of the solutions must obey eq 1. It seems quite reasonable that if the phase separation takes place, the turbidity increases, the spacing distance d (i.e., the concentration of the iridescent phase) changes with temperature, and no color change (constant spacing distance d) is observed with changing concentration of the surfactant. Concerning the above, it is very interesting to note that the temperatures at which the turbidity starts to increase are coincident with those of the beginning of the color change, as pointed out in the previous section. Both temperatures are plotted against the surfactant concentrations in Figure 11, and a lower critical solution temperature (LCST) type phase diagram is obtained. Phase separation between microdomains of order and disorder phases may occur, and the separation into two bulk phases is not observed. The viscoelastic nature, having a yield value of this system, must prevent the solution from bulk separation. An interesting question is, What kind of phase separation occurs in our iridescent DHP gel phase? Figure 12 shows the reflection spectra observed against temperature at the surfactant concentration of 0.8 wt %, which is very close to the critical point in the phase diagram of Figure 11. The wavelength of the reflection peak remains constant until the temperature reaches the critical tem-
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Figure 11. Phase diagram of DHP gel phase as functions of temperature and surfactant concentration in an ethanol/water mixture (24/76 by weight). Temperatures at which iridescent color (2) and turbidity (b) start to change and the reflection peak (4) appears on a spectrum are plotted against surfactant concentrations. This curve can be regarded as an LCST-type phase diagram. Gradual phase transition takes place from ordered lamellar to disordered bilayer state in the shaded region. The positions denoted by an × are the compositions where the freeze-fracture TEM is observed.
Figure 12. Reflection spectra of 0.8 wt % DHP solutions as a function of temperature in an ethanol/water mixture (24/76 by weight). Small peaks observed in UV region are due to second-order diffraction.
perature (∼12 °C) and then moves to shorter wavelength with increasing temperature. It must be noticed that only one reflection peak can be observed even after the temperature elevates into the two-phase separation region. This fact indicates that the dilute phase being equilibrated with the concentrated iridescent lamellar phase is a disordered one. In order to elucidate the above, the freeze-fracture electon micrograph of the sample was observed at the positions denoted by an × in the phase diagram of Figure 11. The replica TEM images are shown in Figure 13. Figure 13 beautifully shows the ordered (a) and disordered (c) structures of bilayer membranes as well as the intermediate state (b) between them. These results seem reasonable, since the disordered lamellar15 or the disordered bilayer (sponge) phases8,9,16-18 are known (15) Dubois, M.; Zemb, Th. Langmuir 1991, 7, 1352. (16) Miller, C. A.; Ghosh, O.; Benton, W. J. Colloid Surf. 1986, 19, 197. (17) Bassereau, P.; Marignan, J.; Porte, G. J. Phys. (Paris) 1987, 48, 673. (18) Hoffmann, H.; Thunig, C.; Munkert, U. Langmuir 1992, 8, 2629.
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Figure 13. Freeze-fracture electron micrograph of the structures of 1.8 wt % DHP (a), 0.8 wt % DHP (b), and 0.4 wt % DHP (c) solutions. These solutions at 5 °C were instantly frozen by immersing them in liquid nitrogen equilibrated with solid nitrogen.
in the liquid-crystalline state. The boundary between the lamellar and the disordered phase in the lower temperature region below the LCST boundary was determined from Figure 14. Peak areas of light reflection normalized by the surfactant concentration are plotted against the
DHP concentrations in Figure 14. The normalized reflection area starts to decrease at about 1 wt % of the surfactant indicating some looseness of the ordered structure and disappears completely at 0.4 wt %. We can imagine that the ordered bilayer (lamellar) phase in the
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Figure 14. Peak area of light reflection spectra of 5 °C normalized by the surfactant concentration plotted against DHP concentrations.
higher concentration region transforms to a disordered phase continuously in a higher-order transition manner. Order-Disorder Phase Equilibria in Colloidal Systems. Phase separation phenomena in colloidal systems have been already well-known in monodispersed latices and other spherical colloidal systems.19-22 In these systems, the solution separates into an ordered crystalline (iridescent) phase and a disordered milky white (random dispersion) phase. Two kinds of theory have been proposed for these phase separation phenomena of monodis(19) Kose, A.; Hachisu, S. J. Colloid Interface Sci. 1974, 46, 460. (20) Hachisu, S.; Kobayashi, Y. J. Colloid Interface Sci. 1974, 46, 470.
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persed spherical colloids. One is the Alder transition theory,19,20,23,24 and the other is an electrostatic attractive interaction theory between colloidal particles.21,22 The Alder transition theory is essentially the same idea as that of the Onsager theory describing the phase separation of needle-shaped tobacco mosaic virus.25,26 The ordering of part of the particles allows more space for the remainder to gain the configurational entropy. Our iridescent surfactant gel phase provides one more additional example of phase separation in the plate-shaped colloidal systems. Phase separation phenomena have never been observed in the iridescent lamellar liquid-crystalline phase. Why the phase separation occurs only in the system of solid bilayer dispersions (gel phase) and not in the liquid bilayer systems (liquid-crystalline phase) is of profound interest. This question may offer, we believe, one of the most fundamental problems concerning the interactions between colloidal particles. Acknowledgment. The authors express their sincere thanks to Professor Toyoichi Tanaka of MIT for helpful and exciting discussions. They also thank Dr. Junryo Mino, the head of R&D division of Kao Corp., for his permission to publish this paper. LA9510691 (21) Ise, N. Angew. Chem., Int. Ed. Engl. 1986, 25, 323. (22) Sogami, I.; Ise, N. J. Chem. Phys. 1984, 81, 6320. (23) Alder, B. J.; Wainright, T. E. Phys. Rev. 1962, 127, 359. (24) Alder, B. J.; Hoover, H. G.; Young, D. A. J. Chem. Phys. 1968, 49, 3688. (25) Onsager, L. Phys. Rev. 1942, 62, 558. (26) Onsager, L. Ann. N.Y. Acad. Sci. 1949, 51, 627.
