J . Phys. Chem. 1991, 95, 7915-7918
7915
Iridescent Phenomena and Polymerization Behaviors of Amphiphiiic Monomers in Lamellar Liquid Crystalline Phase Kouichi NaitohJ Yasuo Ishii,+ and Kaoru Tsujii*it Wakayama Research Laboratories, Kao Corporation, 1334 Minato, Wakayama 640, Japan, and Institute for Fundamental Research, Kao Corporation, 2606 Akabane, Ichikai-machi, Haga-gun, Tochigi 321 -34, Japan (Received: February 13, 1991)
The amphiphilic monomer n-dodecyl glyceryl itaconate (DGI) has been found to show iridescent color in dilute (1-2 wt %) aqueous solutions in the presence of a small amount of ionic surface active impurities. The results of reflection spectrum measurement of light indicate that the iridescent solutions consist of lamellar liquid crystalline phases having submicrometer spacing distances. This structure of the solutions is the same as those of other iridescent surfactant solutions known so far. The iridescent color results from the interference of light reflected from the periodic structure of bilayer membranes. The DGI molecules can be polymerized inside the planar bilayer membranes by using H202 as an initiator of photopolymerization. The lamellar structure itself of the solutions has been kept unchanged by this polymerization. The iridescent color, however, shows the remarkable red shift during the polymerization processes. This red-shift phenomenon is interpreted as showing that the spacing distance between bilayer membranes becomes larger in polymerization processes, since the area of bilayer membranes is reduced by polymerizationowing to the smaller occupied area of polymerized DGI molecules than of monomeric ones. Copolymerizationof DGI with acrylamide and methylenebis(acry1amide) has been also done to immobilize the iridescent structure inside the hydrogels.
was checked by IH, I3C, and COSY two-dimensional N M R techniques and element analysis. Dodecyl itaconate was neutralized by sodium hydroxide and N,N-dimethylbenzylamine in methanol at 50 O C to obtain the corresponding salts of the acid. Aqueous solutions of DGI and the mixtures with another surfactant were made by dissolving the appropriate amount of surfactants in water at the temperature of about 55 O C (higher than the gel-liquid crystalline phase transition temperature, ca. 43 "C) and kept standing for several hours at the same temperature to obtain the clear solutions. The freeze-thaw method was useful to obtain the full brightness of color of the solutions.' Polymerization of DGI in the iridescent solutions was carried out by the photopolymerization technique, using H202as an initiator. A high-pressure mercury lamp (450 W) was utilized as a UV light source. Every solution of monomer and initiater was bubbled by nitrogen gas prior to polymerization reactions. The monomer solutions (ca. 10 g) containing the initiater were put in sample tubes and kept standing for 1-2 h at 50 OC and then irradiated by UV light. The temperature was kept constant at about 50 O C by UV irradiation during the polymerization processes. The sample tubes were rotated a t 15 rpm to obtain homogeneous irradiation to the sample solutions. The conversion of DGI monomer to polymers was calculated from the reduction of UV absorption at 220 nm due to the double bond of the moExperimental Section nomer. Copolymerization of DGI with acrylamide and methyThe sample of n-dodecyl glyceryl itaconate [abbreviated DGI; lenebis(acry1amide) was made in the same manner as that of the n-CI2H2~0COCH2C(=CH2)COOCH2CH(0H)CH2OH] was homopolymerization described above. synthesized by the following reactions. n-Dodecanol was reacted Reflection spectra of the iridescent solutions were measured with itaconic acid anhydride, first at 110 "C for 20 min and then by a spectro multichannel photodetector (Ohtsuka Electronics at 130 OC for 30 min." n-Dodecanol and itaconic acid anhydride Type PCMD-I IO). The temperature of the sample solutions was were purchased from Tokyo Kasei Ltd. The crude sample of kept constant in a water bath. dodecyl itaconate was recrystallized once from n-hexane and twice The density of aqueous solutions of DGI was determined by from ethanol. Dodecyl glyceryl itaconate was prepared by the Gay-Lussac pycnometer a t 50 OC. Distilled water was used as reaction of dodecyl itaconate with glycidol in toluene, using a small standard for the density (0.9881 g/cm3 at 50 "C). The density amount of NJVdimethylbenzylamine as a catalyst.I2 The reaction mixture was stirred at about 105 OC for 1 h. After the reaction ( I ) Lasson, K.;Krog, N. Chem. Phys. Lipids 1973, 10, 177. was finished and the reactants cooled had to room temperature, (2) Nagai, M.;Onishi, M. J . SOC.Cosmer. Chem. Jpn. 1984, / 8 , 19. n-hexane was added. The crude sample was precipitated out from (3) Suzuki, Y.; Tsutsumi, H. Yukagaku 1984, 33,48. the reaction mixture in a refrigerator at -18 OC and was purified (4) Satoh, N.;Tsujii, K.J . Phys. Chem. 1987, 91. 6629. ( 5 ) Fendler, J. H.; Tundo, P. Acc. Chem. Res. 1984, 17, 3. by recrystallization twice from hexane/acetone mixed solvent. The (6) Bader, H.; Dorn, K.; Hupfer, B.; Ringsdorf, H.Ado. Polym. Sd. 1985, recrystallized sample was finally purified by a silica gel column 64, I . (Wakogel C-200, 50-cm length), using a chloroform/methanol (7) Breton, M.J. Macromol. Sci., Rev. Macromol. Chem. 1981, C21, 61. mixture (10/1 by volume) as an eluent. The final reaction product (8) Fukuda, K.; Nakahara, H. Kagaku Sohsetsu 1983, No. 40, 82.
Introduction The iridescent phenomena in dilute aqueous solutions of surfactants and surfactant mixtures are one of the most interesting current topics in the field of surfactant chemistry.'+ The solution structure of alkenylsuccinic acid has been studied to elucidate the mechanism of color appearance and determined to be a lamellar liquid crystalline phase having a submicrometer spacing distance! The iridescent color appears by the interference of reflected light from the bilayer membranes of the amphiphilic molecules. This paper deals with a novel iridescent surfactant system that consists of a polymerizable amphiphile. This system is of interest from the viewpoint not only of iridescent phenomena but also of the new category of organized polymerization. Organized polymerization reaction of amphiphilic monomers has been extensively investigated in vesicles of spherical shape?" in Langmuir-Blodgett films of dry and even in lamellar liquid crystals of nonaqueous phase.1° However, polymerization behaviors in planar bilayer membranes separated each other so long distances (submicrometer) have never been reported. Such reactions are a new category of organized polymerization, since the reaction must occur inside the single bilayer membrane. It is also interesting to study the effect of polymerization on the iridescent structure of solutions.
'*Institute Wakayama Research Laboratories. for Fundamental Research. 0022-3654/91/2095-7915$02.50/0
(9) Shibazaki. Y. Hyomen 1986, 24, 634. (IO) Friberg, S.E.; Yu,B.;Campbell, G. A. J . Folym. Sei. 1990,28,3575. ( 1 1 ) Huber, W. F.; Lutton, E. S. J . Am. Chem. Soc. 1957, 78, 3919. ( I 2) Kubota, H. Kobunrhi Kako 1973, Special Issue 9 (Epoxy Resins), 18.
0 1991 American Chemical Society
7916 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
Naitoh et al.
TABLE I: Additives and Their Amounts Necessary for Color Appearance in DGI Solutions
f”
concn/wt % in DGI
additive N,N-dimethylbenzylammonium dodecyl itaconate
sodium dodecyl itaconate sodium octadecanoate sodium dodecyl sulfate
0.2-2.0 0.2-2.0 -1.5
0.3-0.8
dodecyltrimethylammonium chloride
0.1-0.5 -1.5
n-butylamine
~~
300
A00
500
600
700
1000
PO0
BOO
Wave length/nm
Figure 2. Reflection spectra of DGI solutions of 1.0, 1.2, and 1.4 wt % in a wide range of wavelengths.
L
300
400
500
600
700
800
900
1000
1100
Wave length/nm Figure 1. Reflection spectra of DGI solutions as a function of concentration. The numbers within the figure denote the concentration of DGI in weight percent.
of the surfactant layer in aqueous solutions is estimated to be 1.018 g/cm3 from the results of density measurements.
Results Iridescent Solutions of MI Monomer. Solution behaviors of DGI monomer depend highly upon the degree of purification of the sample. When the sample is purified by just recrystallization, the solutions are maintained as a single phase up to about 15 wt % DGI, and the iridescent solutions can be obtained in the concentration range 1-2 wt %. The color of the solution changes from red to blue with increasing concentration of DGI. When the sample is extremely purified by silica gel column, however, a liquid crystalline phase containing ca. 35 wt % water is separated out even in the dilute aqueous solutions of DGI, and no iridescent phenomenon can be observed. One may expect from the above results that some impurities present in the recrystallized sample play an important role to give the iridescent color to DGI solutions. We can imagine that such impurities are ionic surface active ones, because some nonionic surfactants containing a small amount of ionic agents show similar iridescent phenomena.2 We added some kinds of ionic surface active substances to the DGI sample purified extremely by silica gel and checked the effect of these compounds on the solution behaviors of DGI. As is expected, the aqueous solutions of extremely purified DGI sample show the iridescent color by addition of the above ionic compounds. The added compounds and their amounts necessary for color appearance in DGI solutions are listed in Table I. Among these additives, N,N-dimethylbenzylammonium dodecyl itaconate is the best one to obtain the clear solutions and is the only possible ionic impurity remaining in the process of sample preparations. Reflection Spectra of Iridescent Solutions. Figure 1 shows the reflection spectra of light as a function of DGI concentrations. The DGI sample contains 0.5 wt 9% N,N-dimethylbenzylammonium dodecyl itaconate. As one can see from the figure, the wavelength at the maximum of the reflection spectrum shifts to shorter numbers with increasing concentration of DGI. It is worth noting that the reflection spectrum has a sharp maximum even in infrared and ultraviolet regions of light. The reflection spectra of relatively dilute solutions of DGI in a wide range of wavelengths are shown in Figure 2. Two or three
1
350
I
400
500
450
Wave length/nm
Figure 3. Changing profiles of the reflection spectrum of 2.5 wt % DGI
solution during polymerization processes. The numbers within the figure indicate UV irradiation (polymerization) time in minutes.
0
5
10
15
Irradiation t h e l m in Figure 4. Time-conversion curve of the DGI polymerization reaction in 2.5 wt 7% solution. reflection peaks can be observed from one solution of DGI. The reflection peaks with shorter wavelength correspond to higher order diffraction of light from the same periodic structure. The appearance of higher order reflection peaks indicates that the regularity of the structure in the iridescent solutions of DGI is very good. Polymerization of DGI in Iridescent Solutions. When polymerization starts in the iridescent solutions of DGI monomer, the color exhibits a dramatic red shift. Figure 3 shows the change of reflection spectrum during polymerization of a 2.5 wt 9% DGI solution containing 0.15 wt 5% H202.The color of the solution can be maintained up to the polymerization time of 18 min. However, when the polymerization proceeds to more extent, the color disappears, and finally the polymer precipitates out. The time-conversion curve of DGI polymerization is shown in Figure
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7917
Iridescent Phenomena of Amphiphilic Monomers
TABLE 11: Copolymerization Conditions of DGI with Acrylamide and Methylenebis(acry1amide)
reagent DGlO acrylamide
concn/wt % 1.90-2.86 4.29 0.476 0.143
methylenebis(acry1amide) H202
ODGl contains 0.5 wt % N,N-dimethylbenzylammonium dodecyl itaconate.
1 /c
Figure 7. Interplanar spacing, d, plotted against 1 / c .
300
500
400 Wave length/nm
Figure 5. Reflection spectra of DGI solutions containing 4.29 wt W acrylamide, 0.476 wt % methylenebis(acrylamide), and 0.143 wt % H202 as a function of DGI concentration shown in the figure.
I
1
40
50
I
l/C Figure 8. Plots of interplanar spacing, d, against l/c for the solutions
of DGI before (0)and after ( 0 )polymerization. The data are taken from Figures 5 and 6.
400
500
600
700
Wave length/nm Figure 6. Reflection spectra of DGI solutions after polymerization is completed. Original monomer solutions are those shown in Figure 5 . 4. One can understand from the figure that the polymer precipitation occurs when the conversion to polymers exceeds about 70%. The molecular weight of the polymer precipitated out from the solution was determined by gel permeation chromatography to be about 100000. Copolymerization of DGI with acrylamide and methylenebis(acrylamide) was done under the conditions shown in Table 11. Figure 5 shows the reflection spectra of DGI monomer solutions containing comonomers and initiater. Comonomers and initiater give essentially no effect on the structure of iridescent DGI solutions. The spectra in Figure 5 are changed to those as shown in Figure 6, after polymerization is complete. Although the reflection peaks are broadened by polymerization, the iridescent color is maintained in the hydrogel of acrylamide. Discussion Structure of Iridescent Solutions of DGI Monomer.
The structure of iridescent solutions of alkenylsuccinic acid has been already elucidated by mainly UV and X-ray diffraction techn i q u e ~ .The ~ conclusion of the above study is that the solutions consist of lamellar liquid crystalline phases having submicrometer
spacing distances. The diffraction phenomenon of reflected light from the bilayer membranes stacked periodically results in the appearance of interference color in the solutions. Let us check first whether the structure of DGI solutions is the same as that of alkenylsuccinic acid or not. When the lamellar structure is uniformly formed in whole space of the solutions, the interplanar spacing, d, can be related to the weight fraction of surfactant, c, as49I3
d = l(1
- C ) P I / C P Z + lldi
(1)
where p I and p 2 are the density of the surfactant and water layer, respectively, and d l is the thickness of the surfactant layer. As the weight fraction of DGI,c, is much smaller than unity, eq 1 can be rewritten as
d = Pldl/P2C (2) The plot of interplanar distance, d, against I / c is shown in Figure 7 and gives a very good straight line. The spacing distance, d , was estimated from the wavelength of maximum reflection of spectra A, in Figure 1 by using Bragg's equation, 2nd sin 0 = mAmax,where n is the refractive index of water, 0 = 90°, m = 1. The above result strongly indicates that the iridescent solutions of DGI monomer consist of the lamellar liquid crystalline phase, in the same manner as the case of alkenylsuccinic acid. This conclusion is confirmed by the result shown in Figure 2. As can be seen from the figure, a series of A,, has an interrelation of (13) Luzzati. P. V.; Muslacchi, H.; Skoulios, A,; Husson. F. Acta Cryslallogr. 1960, 13, 660.
J . Phys. Chem. 1991, 95, 7918-7925
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Polymerization
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very good straight lines, which means that the lamellar structure remains undestroyed after polymerization even in the hydrogel of acrylamide. We can estimate the thickness of bilayer membranes, d l , from the slope of the straight line of Figure 8 since p I / p 2 is already determined to be 1.03 (see the last paragraph under Experimental Section). The occupied area of one molecule at the surface of the bilayers, s, is also calculated by the equation14
-e-
s = 2Mw/dlPINA
Figure 9. Schematic illustration for the polymerization process of DGI.
TABLE III: Values of d , and s before and after Polymerization before polymerization after polymerization
dllnm
s/nm2
3.47
0.356 0.264
4.69
1:1/2:1/3, which also indicates the lamellar structure of the solutions. Red Shift in Reflection Spectra during Polymerization Procesxs of DCI. As mentioned previously, the reflection spectra of iridescent DGI solutions show a dramatic red shift during polymerization processes. The structure of iridescent solutions after polymerization needs to be known to understand the above red-shift phenomenon. The plots of d against l / c before and after polymerization of DGI solutions are made from the data of Figures 5 and 6 and are shown in Figure 8. Both plots are shown to be
(3)
where M, and NA are the molecular weight of DGI and Avogadro’s number, respectively. The additive, N,N-dimethylbenzylammonium dodecyl itaconate, is neglected in the calculation of the molecular weight of DGI, since the amount of additive is quite small (0.5 wt %). The calculated dl and s of DGI solutions before and after polymerization are listed in Table 111. The occupied area of one molecule is remarkably reduced more than 25% after polymerization is complete. One may understand the reason red shift occurs in polymerization processes from the above result. The interplanar distance, d, must be expanded during polymerization of DGI to keep the lamellar structure uniform in the whole space of the solutions, because the bilayer membranes shrink in their surface area by the polymerization. Figure 9 shows the schematic illustration of the polymerization process mentioned above. Expanded distance between bilayer membranes, of course, results in the red shift of the reflection spectrum of light. Registry No. DGI (homopolymer), 135585-35-2;dodecyl glyceryl itaconate, 135585-34-1;n-dodecanol, 112-53-8;itaconic acid anhydride, 21 70-03-8; dodecyl itaconate, 23405-96-1; glycidol, 556-52-5; DGI with acrylamide and methylenebis(acrylamide), 135585-36-3; N,N-dimethylbenzylammonium dodecyl itaconate, 135598-32-2;sodium dodecyl itaconate, 135598-33-3;sodium octadecanoate, 822-16-2;sodium dodecyl sulfate, 151-21-3;dodecyltrimethylammoniumchloride, 11 2-00-5; n-butylamine, 109-73-9. (14) Luzzati, P. V. Biol. Membr. 1968, 1, 71.
Reduction Potentials of Vesicle-Bound Vioiogens Yabin Lei and James K. Hunt* Department of Chemical and Biological Sciences, Oregon Graduate Institute of Science and Technology, 19600 Northwest von Neumann Drive, Beaverton, Oregon 97006- I999 (Received: November 9, 1990)
Thermodynamic reduction potentials have been determined by using spectroelectrochemicaland cyclic voltammetric methods for a homologous series of amphiphilic viologens (N-methyl-N’-alkyl-4,4’-bipyridinium ions, C,MV2+) in a variety of media, including dihexadecyl phosphate (DHP), dioctadecyldimethylammonium,and phosphatidylcholine small unilamellar vesicles. In general, potentials for both one-electron steps, Le., C,MV2+ + e- C,MV+ and C,MV+ + e- C,MVo, were insensitive to the alkyl chain length, which was varied over the range n = 6-20. The single exception was a large decrease (- 100 mV) in the first reduction potential for DHP-bound viologens when the chain length was increased from n = 10 to n = 12; this effect was attributed to a change in binding topography. The magnitudes of the reduction potentials were highly dependent upon the vesicle charge; the pattern observed indicated that interfacial electrostatic interactionsbetween the surfactant headgroups and bipyridinium rings were the dominant factors determining the potentials. As discussed in the text, the data allow resolution of several heretofore puzzling observations concerning viologen reactivities in microphase suspensions.
-
Introduction
Viologens (N,N’-dialkyl-4,4’-bipyridinium ions) have found widespread application as redox mediators in synthesis, photoconversion, and molecular electronics.’ Many of these applications utilize microphase-separated media, which can have profound effects upon the thermodynamic properties and attendant reactivity of the viologens. For example, viologens have been shown to
* Author
to whom correspondence should be
addressed.
0022-365419112095-7918$02.50/0
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catalyze chemical and photochemical debromination of organic dibromides in an ethyl acetate-water two-phase system under conditions where these reactions did not occur to any appreciable extent in homogeneous solution.’ The simultaneous presence of (1) Compilations of recent literature can be found in refs 2-4. (2) Bockman, T. M.; Kochi, J. K. J . Org. Chem. 1990,55,4127. ( 3 ) Hurst, J. K. In Kinetics and Catalysis in Microheterogemus Systems; Gritzel, M., Kalyanasundaram, K., Eds.; Surfactant Science Series; Dekkcr: New York, 1990; pp 183-226.
0 199 1 American Chemical Society
