Langmuir 1992,8, 581-584
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Iridesdent Solutions Resulting from Periodic Structure of Bilayer Membranes. 2t Naoki Satoh and Kaoru Tsujii* Institute for Fundamental Research, Kao Corporation, 2606 Akabane, Ichikai-machi, Haga-gun, Tochigi 321-34, J a p a n Received J u n e 24, 1991. I n Final Form: October 1, 1991 Iridescent aqueous solutionsof dioctadecyldimethylammonium chloride (DOAC)have been newly found in the concentration range of 1-2 w t 96. The structure of the solutions has been studied mainly by X-ray diffraction and ultraviolet and visible light reflection techniques and determined to be the lamellar liquid crystalline phase having the space distance of a submicrometer. The color appearance of the solutions results from diffraction of visible light by lamellar liquid crystalline structure of bilayer membranes. The mechanism of color appearance in DOAC solutions is the same as that of aqueous solutions of alkenylsuccinic acid, the structure of which is already elucidated by us. It is interesting to note, however, that the iridescent liquid crystalline phase of DOAC is understood to be thermodynamicallymetastable state, because of the following observations: (1)the iridescent-colored solutions can be obtained only by the sophisticated special procedures and (2)the color of the solutions disappears easily by weak agitations such as gentle shaking and/or stirring, and never recovers. It is worth noting, furthermore, in this DOAC system that the dramatic blue shift in the reflection spectra can be observed when the iridescent liquid crystal transforms into the gel phase on cooling. This blue-shift phenomenon might be due to the phase separation into iridescent gel and water phases on gelation processes. Introduction
The iridescent phenomena of some kinds of hydrophobic surfactant and surfactant mixtures have been found The aprecently in their dilute aqueous pearance of iridescent color in the surfactant solutions can be explained by Bragg reflection of visible light from the periodic lamellar structure of bilayer membranes having a spacing distance of a s~bmicrometer.~The present work deals with the new iridescent system of dioctadecyldimethylammoniumchloride (DOAC) and water. This new system has two novel aspects. The first one is the stability of the iridescent phase. The iridescent structureof DOAC is quite sensitive to and easily destroyed by the weak agitations such as gentle shaking and/or stirring in contrast with the case of alkenylsuccinic acid.4 These results indicate that the iridescent liquid crystalline phase of DOAC is not thermodynamically stable, and so a sophisticated special technique is necessary to obtain the iridescent-colored solutions of DOAC. The method involves the quick freezing and thawing after making homogeneous solutions by ultrasonication. This may be the reason why such a popular surfactant, DOAC, has not yet been found to show the iridescent phenomena. The second aspect is the presence of an iridescent gel phase. The DOAC-water system is also known to have a thermodynamically stable gel phaseg which does not show iridescent p h e n ~ m e n a . ~The J ~ iridescent color of DOAC solutions shows the dramatic blue shift as the gelation + Part 1 is ref 4. (1)Lasson, K.; Krog, N. Chem. Phys. Lipids 1973,IO, 177. (2) Nagai, M.;Ohnishi, M. J. SOC.Cosmet. Chem. Jpn. 1984,18 (l), 19. (3)Suzuki, Y.;Tsutsumi, H. Yukagaku 1984,33(ll),48. (4)Part 1: Satoh, N.; Tsujii, K. J. Phys. Chem. 1987,91,6629. (5)Thunig, C.;Hoffmann, H.; Platz, G. Progr. Colloid Polym. Sci. 1989,79,297. (6)Imae, T.; Sasaki, M.; Ikeda, S. J.Colloid Interface Sci. 1989,131, 601. (7)Platz, G.;Thunig, C.; Hoffmann, H. Progr. Colloid Polym. Sci. 1990, 83,167. (8) Strey, R.; Schomacker, R.; Nallet, F.;Roux, D.; Olsson, U. J.Chem. SOC.,Faraday Trans. 1 1990,86,2253. (9) Kodama, M.; Seki, S. Hyoumen 1984,22,61. (10)Kunieda, H.; Shinoda, K. J. Phys. Chem. 1978,82,1710.
proceeds from the liquid crystalline phase a t higher temperature than the phase transition point. The iridescent gel of DOAC is also a new phase realized only through the iridescent liquid crystalline phase. Experimental Section
The DOAC sample used in this work was purchased from Sherex Ltd., and purified by recrystallization three times from acetone. The distribution of hydrocarbon chain length was checked by gas chromatography, and the purity of dioctadecyl compound was determined to be more than 92% Iridescent solutionswere made by the followingspecial procedures: (1)the appropriate amount of DOAC was dispersed in pure water at 80 "C by shaking, (2)ultrasonicationtreatment with 100-W power was done for 1 min, which made the above turbid solution homogeneous translucent and less viscous, and (3) the solution was frozen quickly in dry ice-acetone, and thawed gently at 70 "C in water bath. Iridescentcolor becomes more and more vivid and uniform by the repeated process of ultrasonication,freezing, and thawing. Reflection spectra of the iridescent DOAC solutions were measured by a spectro multichannel photodetector (Ohtsuka electronics type MCPD-110),and detected at normal direction against irradiating white light. The intensity of reflected light from the sample solutions was calibrated by that from a Ti02 white board. The temperature of the sample solutions was kept constant at 60 "C during all of the experiments. The change of reflection spectraduring the gelation process from the iridescent liquid crystalline phase was observed intermittently on the samplestransferredfrom 60 to 20 O C atmosphere. The iridescent gel phase was observed to be maintained unchanged for several weeks in a supercooled state at 20 "C. Small-angle X-ray diffraction experiments were carried out using an X-ray diffractometerequipped with a rotating copper anode (Rigaku,Rotaflex RU200 type). The specimens were put into a sealed glass capillary. The temperature was kept constant at 60 and 20 "C for the measurements of liquid crystalline and gel phases, respectively. Tube voltage and current were 59 kV and 50 mA, respectively. The camera used in the experiments was a Rigaku small-angle scattering camera with a two pinhole slits system. Diffraction images were recorded on the Fuji industrialX-rayfilms. An exposuretime of 6-100 h was necessary depending upon the surfactant concentration (10-40 w t %) of the sample solutions.
0743-7463/92/2408-0581$03.00/00 1992 American Chemical Society
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582 Langmuir, Vol. 8, No. 2, 1992
Satoh and Tsujii
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1
Figure 1. Photographs of iridescent DOAC solutions of various concentrations from 1.0 wt 96 (left) to 2.2 wt % (right).
1.6Wt%
I 400
600
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Wavelength Inm
Figure 2. Reflection spectra of visible light from the iridescent-
colored solutions of Figure 1.
Results Iridescent Phenomena in Liquid Crystalline Phase. Figure 1 shows the color photographs of the iridescent DOAC solutions of various concentrations. The color of the solutions changes from pearly red to transparent thick blue with increasing concentration of the surfactant from 1.2 to 2.2 w t %. The reflection spectra of the above iridescentsolutionsare shown in Figure 2. The wavelength of maximum reflection,,,A, changes from 660 to 440 nm with increasing concentration of the surfactant from 1.2 to 1.8 wt %. The reflection peak becomes more intense and sharper at shorter wavelength. This result strongly ' suggests that the ordering of the solutions becomes higher with increasing concentration of the surfactant. The appearance of the solutions changes from turbid and violet to colorless and transparent via colorless and turbid with decreasingconcentrationfrom 1.2 to 0 w t % . The solutions are colorless and translucent above 2.2 wt %. One series of anisotropic X-ray diffraction patterns on the equatorial line was obtained from all DOAC solutions of 10-40 w t % . These X-ray patterns correspond to the spacing with an interrelation of k1/2:l/3..., which indicates a lamellar structure of the solutions. For example, Bragg reflections as far as fifth order were observed from the solution of 20 wt % , indicating an interplanar distance, d , of 15.3 nm. Strong anisotropic Bragg reflections of the above suggest that lamellar leaflets are oriented parallel to the wall of a glass capillary. The diffraction pattern of 10 wt % solution was more broadened and less anisotropic than that of solutions with higher concentration than
10 wt % . All observations described above are quite similar to those of another iridescent solution of alkenylsuccinic acids.4 Change of Reflection Spectra on Gelation. The iridescent DOAC solutions changed to a gel state by lowering the solution temperature below ca. 40 "C. The color of the solutions shows dramatic blue shift on gelation, and the color of the gels is stable for several weeks at room temperature. Figure 3 shows the color photographs of the gelation process of the 1.2 w t % of DOAC iridescent solution, as the sample was transferred from a water bath at 60 "C to a stage at room temperature (20 "C). Blue shift occurred from the bottom of the sample bottle which was cooled more quickly by the stage. The change of reflection spectra on gelation of 1.5 wt % iridescent solution is shown in Figure 4a. The reflection peak a t 490 nm diminished, and a new peak was alternatively developed at 360 nm. The new peak observed after gelation is more broadened and less intense than the peak before gelation,which indicates that the gel structure is less regular than the liquid crystalline structure. The difference of maximum wavelength , A, between before and after gelation becomes small as the concentration of surfactant increases. Figure 4b shows the change of reflection spectra of the 2.4 w t % solution. The Amax of the liquid crystallinephase shifts only 10nm to the shorter one, and the decay of the reflection peak of the gel phase is more drastic than the case of 1.5 wt %. In the 2.8 wt % solution, the blue shift does not occur any more. Only the decay of the reflection peak of the liquid crystalline phase was observed, and the reflection from the gel phase does not appear. X-ray diffraction measurements were performed for the concentrated DOAC gels of 10-40 wt %. Diffraction patterns from the 20,30, and 40 w t % gels show that the gel phase also consists of a lamellar structure with a shorter lattice constant than that of the corresponding liquid crystalline phase. The diffraction intensity from the gel phase is weaker than that of the liquid crystal. No Bragg reflection was observed from the 10 wt % gel sample in spite of the exposure of 100 h.
Discussion Structure of Metastable Iridescent Liquid Crystalline Phase. As mentioned previously the structure of concentrated DOAC solutions is determined by X-ray diffraction to be a lamellar liquid crystal. The next problem is to check whether the same lamellar structure is maintained even in the dilute iridescent solutions or
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Periodic Structure of Bilayer Membranes
n -.-
Figure 3. Photographs of color change in the gelation process: (left) before gelation, (middle) during gelation, and (right) after gelation. The gelation was completed in a few minutes.
not. When the lamellar structure is formed uniformly in the whole space of the solution, the spacing distance, d, can be related to the weight fraction of surfactant, c, as4J1
where p1 and p2 are the densities of surfactant and water layer, respectively, and dl the thickness of a unit layer of the lamellar leaflet. The data of the interplanar spacing, d, in the region of dilute solutions were estimated from the results of reflection spectra (Am=), using Bragg's equation 2nd sin 6 = Am, where n is the refractive index of water and 6 = 90°. The data of concentrated solutions were, of course, obtained from X-ray diffraction. The calculated distance, d, in the liquid crystalline phase is plotted against (1- c)/c in Figure 5. Both plots obtained from the dilute and concentrated solution regions give the identical single straight line with a y intercept of 3.3 nm. It can be concluded from these results that the iridescent dilute solutions of DOAC also consist of lamellar liquid crystals having extremely long spacing distances and the iridescent color appears by the interference of reflected light by single bilayer membranes of DOAC. The lamellar structure can be simply swelled by water up to ca. 100 times the surfactant by weight. This fact stronglysuggests that only the repulsive interaction is dominant between two lamellar leaflets to maintain the periodic structure. This mechanism of color appearance and swellingbehavior of DOAC is exactly the same as that of alkenylsuccinic acid (ASA) solution^.^ There are two possible mechanisms to explain such a long-range repulsive interaction. The first one is the DLVO theory which is applied by Wennerstrom to parallel plates in a salt-free system,12and ~~~~
~
~
~~
(11)Luzzati, P.; Mustacchi, H.; Skoulios, A.; Husson, F. Acta CrystallOgF. 1960, 13, 660. (12) Engstrom, S.; Wennerstrom, H. J . Phys. Chem. 1978, 82, 2711.
n
c"
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300
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Wavelength Inm
250
300
350
400
450
Wavelength /nm
Figure4. Changeof reflection spectrumin visible andUV regions during gelation process: (a) 1.5 wt % and (b) 2.4 wt %.
the second one is the undulation stabilization theory.l3 Wennerstrom's theory seems more reasonablebecause the small amount of ionic surface active compound is essential
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584 Langmuir, Vol. 8, No. 2, 1992
500
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20
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Il-CI/C
Figure 5. Interplanar spacing, d , against (1 - c ) / c calculated
from reflection spectra of light for the liquid crystalline phase at 60 "C (0)and the gel phase at 20 "C (@), and from X-ray diffraction for the liquid crystalline (*) and the gel phase (0). to obtain the iridescent solutions even in the systems composed of the nonionics as the main surfa~tants.~J* This conclusion is also supported by the fact that the iridescent structure is destroyed by addition of the electrolytes. The stability of the iridescent phase of DOAC is completelydifferent from that of alkenylsuccinic acid. The color of ASA solutions recovers from the turbid state made from the iridescent solution by stirring, and becomes more and more bright by standing in a water bath at above the coagel-liquid crystalline phase transition temperature. Figure 6 shows the recovering process of color in the 1.5 wt % aqueous solution of ASA, the color of which is extinguished by stirring. On the contrary, the iridescent color of DOAC solutions easily disappears by a gentle shaking and/or stirring and never recovers by standing the sample solutions in a water bath. The iridescent DOAC solution changes from the bright colored state to the bluewhite one via turbid appearance by the treatment of gentle shaking. Kunieda and Shinoda reported that the aqueous solutions of DOAC prepared by shaking in a ampules were separated into two phases in the dilute concentration region where we found iridescent phenomena, and formed the homogeneous liquid crystalline phase at higher concentrations above 3.7 wt % .lo The phase-separated bluewhite solution must be the thermodynamically stable one, and the iridescent homogeneous liquid crystal having the same structure as that at higher concentrations is considered to be a metastable state realized by the quick freezing method of the solutions. The freezing process of water seems to segregate the colloidal particles to arrange into a layered form. K.T. found that the ultrafine boehmite colloids formed a beautiful honeycomb or slitlike (13)Helfrich, W.2.Naturforsch. 1978, 33a, 305. (14)Naitoh, K.;Ishii, Y.; Tsujii, K. J. Phys. Chem. 1991, 95,7915.
550 600 Wavelength Inm
650
Figure 6. Recovering process of iridescent color of 1.5 wt % ASA solutions from extinguished state made by stirring. The
numbers written in the figure denote the time in seconds after stopping the stirring.
layered structure constructed with thin walls of boehmite particles by the freezing technique similar to the above,l5 although the mechanism was not clear. The (Iridescent) Gel Phase. The spacing distance, d, against (1- c ) / c plot for the gel phase is shown also in Figure 5. Contrary to the case of liquid crystalline phases, the above plot for gel phases has an inflection point at about 2.8 wt % In the higher concentration range than the break point (2.8 wt % 1, the d vs (1- c ) / c plot is almost parallel to the plot of the liquid crystalline phase, which means that the gel phases also consist of a lamellar structure constructed with thinner (2.5 nm) membranes. The shorter interplanar distance, d, than that of the liquid crystalline phase can be, therefore, interpreted by the decrease of the thickness of a unit (bilayer) membrane in the homogeneous lamellar phase. The plot of d against (1 - c ) / c in the region of the iridescent dilute gels gives also a straight line, which indicates that the structure of the gel is still lamellar. The extrapolated value of they intercept is about 57 nm. One can imagine two possible structure models from these results. If we assume the gel is still maintained as a homogeneous phase, the unit structure of the lamellar changesfrom a single (bilayer) membrane to a considerably larger (57 nm) unit. If we consider the unit thickness of the lamellar structure remains unchanged from the higher concentration ranges, the gel phase must be separated out in dilute concentration ranges. Although we do not have any more evidence to decide the true structure between two possibilities of the above, the latter model seems to be more realistic, since the solution of the binary separated phase is thermodynamically stable in the liquid crystalline state as mentioned previously.
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Registry No. DOAC, 107-64-2. (15)Fukasawa, J.; Tsujii, K. J. Colloid Interface Sci. 1988,125, 155.
