Langmuir 1995,11, 4661-4664
4661
Structure of Networks Formed in Concentrated Solutions of Nonionic Surfactant Studied by the Pulsed-Gradient Spin-Echo Method Tadashi &to,*
Nobuya Taguchi, Toshiaki Terao, and Tsutomu Seimiya
Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, Minamiohsawa, Hachioji, Tokyo 192-03, Japan Received June 23, 1995. In Final Form: August 25, 1995@ Surfactant self-diffusion coefficients (D) have been measured on concentrated micellar solutions (> 10 wt %) of nonionic surfactant (C16E7)and liquid crystal (cubic and hexagonal) phases by using the pulsedgradient spin-echo method. In the lower temperature range, the self-diffision coefficient in the micellar phase is much smaller than that in the liquid crystal phases where the lateral diffusion is dominant. Also, the activation energy for the self-diffusionprocesses calculated from the temperature dependence of D is much larger than those in the liquid crystal phases and depends on concentration and temperature only slightly. These results confirm the validity of our diffusion model previously reported which takes into account intermicellar migration of surfactant molecules at the entanglement point. As the temperature is raised above about 45 “C (the lower critical solution temperature is 51 “C at about 1 wt %), however, the activation energy decreases rapidly toward the value for the liquid crystal phases. Above about 60 “C, the self-diffision coefficient depends on concentration only slightly and its absolute value coincides with that expected from the temperature dependence of D in the cubic phase. From these results and the structure of the cubic phase, it is inferred that cross-links of wormlike micelles exist above about 45 “C and that the extent of cross linking increases with increasing temperature. Relations with phase behaviors are also discussed.
Introduction In recent years, dynamical properties of semidilute (or concentrated) solutions of wormlike micelles have been studied extensively from the viewpoints of viscoelasti~ity,l-~~ * ,p~i ~n,n~a~b i l i t y , self-diffision ~ ~ , ~ ~ ~ , ~ of solubilized probe,15-18surfactant s e l f - d i f f u s i ~ n , and ~ ~ - the ~~ scattering vector dependence of dynamic light scattering.zz-z5 In particular, much attention has been paid Abstract published in Advance A C S Abstracts, November 1, 1995. (1)Hoffmann, H. Proceedings of the ACS Symposium; American Chemical Society: Washington, DC, 1994 and references therein. (2)Hoffmann, H.; Rauscher, A.; Gradzielski, M.; Schulz, S. F. Langmuir 1992, 8, 2140. (3) Shikata,T.;Hirata, H.;Kotaka,T. Langmuir 1987,3,1081,1988, 4, 354, 1989, 5, 398. (4) Shikata, T.; Pearson, D. Langmuir 1994, 10, 4027. ( 5 ) Cates, M. E.; Candau, S. J. J . Phys. Condens. Matter ISSO, 2, 6869 and references therein. (6) Kern, F.; Zana, R.; Candau, S. J. Langmuir 1991, 7, 1344. (7) Appell, J.; Porte, G.; Khatory, A,; Kern, F.; Candau, S. J . Phys. II France, 1992,2, 1045. (8) Khatory, A.; Kern, F.; Lequeux, Appell, J.; Porte, G.; Morie, N.; Ott, A.; Urbach, W. Langmuir 1993, 9, 933. (9) Khatory, A.;Lequeux, F.;Kern, F.;Candau, S. J.Langmuire 1993, 9, 1456. (10)Berret, J.; Appell, J.; Porte, G. Langmuir 1993, 9, 2851. (11)Drye, T. J.; Cates, M. E. J . Chem. Phys. 1992,96, 1367. (12) Turner,M. S.; Marques,C.; Cates, M. E.Langmuir 1993,9,695. (13)Imae, T. Proceedings ofthe ACS Symposium; American Chemical Society: Washington, DC, 1994 and references therein. (14)(a) Imae, T.; Hashimoto, K.; Ikeda, S. Colloid Polym. Sci. 1990, 268, 460. (b) Hashimoto, K.; Imae, T. Langmuir 1991, 7, 1734. (c) Hashimoto, K.; Imae, T.; Nakazawa, K. Colloid Polym. Sci. 1992,270, 249. (15)Ott, A.; Urbach, W.; Langevin, D.; Schrutenberger, P.; Scartazzini, R.; Luisi, P. L.J. Phys. Condens. Matter 1990,2, 5907. (16)Bouchaud, J. P.; Ott, A,; Langevin, D.; Urbach, W. J. Phys. II 1991, I, 1465. (17) Nemoto, N.; Yamamura, T.; Osaki, K.; Shikata, T. Lunggmuir 1991, 7, 2607. (18) Nemoto, N.; Kuwahara, M. Colloid Polym. Sci. 1994,272, 846. (19) Kato, T.; Terao, T.; Tsukada, M.; Seimiya, T. J . Phys. Chem. 1993,97,3910. (20) Kato, T.; Terao, T.; Seimiya, T. Langmuir 1994, 10, 4468. (21) Monduzzi,M.;Olsson,U.;Soderman,0.Langmuir 1993, 9,2914. (22) Brown, W.; Johansson, K.; Almgren, M. J . Phys. Chem. 1989, 93, 5888. @
to the network structure including cross-links formed by fusion of wormlike micelles.1~7-11~20~z1 Such a structure is called “transient cross link”,7“multiconnectednetwork”,8 “living network”,ll or “bicontinuous micellar solutionsHz1 and is distinguished from the “entangled network or “unsaturatednetWork”ll where two wormlike micellesjust contact each other at the entanglement point without cross links. In the previous s t ~ d y we , ~have ~ ~ measured ~ ~ the surfactant self-diffusion coefficient (D)on semidilute solutions ofanonionic surfactants C16E7, C14E6, and C14E726 by using the pulsed-gradient spin-echo (PGSE) method. In the concentrated region ( > 10 wt %), the self-diffusion coefficient increases with increasing concentration (c)and follows the power law (D cZl3)which can be explained by a simple model taking into account the intermicellar migration at the entanglement point. If wormlike micelles are “multiconnected”, surfactant molecules can diffuse over a large distance without “intermicellar” migration (or hopping). In this case, the observeddiffusion coefficient is dominated by the lateral diffusion alone. Such a situation is really encountered in liquid crystal phases. Especially, the cubic phase resembles the “multiconnected network” although the regularity disappears in the latter case. In the present work, we have performed measurements of the self-diffusion coefficient of C16E7 in the cubic and hexagonal phases in order to compare the results with those in the micellar phase. In addition, we have extended the concentration and temperature ranges for measurements in the micellar phase in order to discuss the change in the network structure.
-
Materials.
Experimental Section was purchased from Nikko Chemicals,Inc.,
i n crystalline form and used without further purification. (23) Nemoto, N.; Kuwahara, M. Langmuir 1993, 9, 419. (24) Koike, A,;Yamamura,T.;Nemoto, N. Colloid Polym. Sci. 1994,
272, 955. (25) Imae, T. J . Phys. Chem. 1989, 93, 6720. (26) In this paper, n-alkyl polyoxyethylene surfactants, CnHzn+l(OCzHd),OH, are denoted as C,E,.
0743-7463/95/2411-4661$09.00/00 1995 American Chemical Society
Kato et al.
4662 Langmuir, Vol. 11, No. 12, 1995 70 60
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Figure 1. Partial phase diagram of C ~ ~ E T - D system: Z O LI, micellar solution; W + L1, coexisting liquid phases; HI, hexagonal phase; VI,cubic phase; La,lamellar phase. The dots indicate PGSE measurements in the present study. Deuteriumoxide purchasedfrom ISOTEC,Inc. (99.9%),was used after being degassed by bubbling of nitrogen to avoid oxidation of the ethylene oxide group of surfactants. Determination of Phase Diagram. The boundary between the isotropic phase (micellar and cubic phases) and anisotropic phase (hexagonaland lamellar phases) was determined by using a polarizingmicroscope (OlympusBHSP)with a Metller FP82HT hot stage. In order to distinguish the cubic phase from the micellar phase, small angle X-rayscattering measurementswere made by using an apparatus (MAC Science)constructedfrom an X-ray generator (SRAMXP18,18 kW),incident monochromator (WISi multilayer crystal), Kratky slit, and imaging plate (DIP 200).27 We measured also 2H NMR spectra of D2O in the hexagonal phase and observed quadrupole splittings.28 PGSE. PGSE was measured at 399.6MHz with a JEOL JNMEX400 Fourier transform NMR spectrometer. The absolute magnitude of the gradient was calibrated against the value of the diffusion coefficient of pure water at 25 "C, 2.30 x m2 s-l, reported by Mills.29 The diffusional attenuation of the echo amplitude is given by30
MIM, = exp[- y2G2Dd2(A- 613)l
:W_Phas,e,
2.9
60
CubCphaSe pure liquid
(1)
where MIMo is the ratio of echo amplitudes in the presence and absence of the gradient, y is the gyromagnetic ratio, G is the magnitude of gradient, 6 is the duration of each gradient pulse, and A is the time between the two gradient pulses (corresponds to the diffusion time). In our experiments, G was kept constant at about 2.2 T m-l and 6 was varied under the constant A value (120-300 ms). Details of the measurements have been already r e p ~ r t e d The . ~ ~measurements ~~~ were made above about T,15 K (TC, the lower critical solution temperature) where entanglement of wormlike micelles is expected from the light scattering s t ~ d i e s . ' ~ , ~ ~
Results Phase Diagram. Figure 1 shows a partial phase diagram of the C16E7-DzO system. Although the phase diagram of this system has not been reported even for a HzO solution, main features of it are similar to those of homologous systems.32 The dots in the figure indicate PGSE measurements in the present study. (27)Yoshida, H.; Kato, T.; Murata, T.; Sakamoto, K. Mem. Fac. Technol., Tokyo Metrop. Univ., in press. (28) At a certain concentration in the hexagonal phase in Figure 1, we have observed two doublets whose relative intensities depend on temnerature - - --.= -.-- _-.. 129) Mills, R. J. Phys. Chem. 1973,77,685. ~ -_ . , (30) Stejskal, E1. 0.; Tanner, J. E. c.1. Chem. Phys. 1966,42,288. (31)Kato, T.; Anzai, S.; Seimiya, T. J. Phys. Chem. 1990,94, 7255. (32) Mitchell, D. J.;Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. F'. J . Chem. Sot., Faraday Trans. 1 1983,;'9, 975.
, ,
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Figure 2. Logarithm of surfactant self-diffusioncoefficientvs reciprocal temperature. The concentrations for the micellar solutions are indicated in the figure (in wt %). The surfactant concentrations are 50%(45 and 50 "C)and 57%(35,40,and 45 "C)for the cubic phase and 40% (35,45, and 45 "C), 50% (35 and 40 "C),57% (25 "C),and 60% (30 "C)for the hexagonal phase. DOis the diffusion coefficient along the axis of the rod forming the cubic phase calculated from eq 739and the data in the cubic phase.
Temperature Dependence of Self-Diffusion Coefficient and Activation Energy for Self-Diffusion Processes. In Figure 2, the logarithm ofthe self-diffision coefficient of is plotted against the reciprocal temperature. In the hexagonal phase, the direction of each rod is expected to be random and so the lateral diffusion coefficient along the axis of the rod may be three times the observed D value. It can be seen from the figure that in the lower temperature range the self-diffision coefficient in the micellar phase is much lower than those in the liquid crystal phases. As the temperature increases, the self-diffusioncoefficient in the micellar phase rapidly increases. Above about 60 "C, the self-diffision coefficient depends on concentration only slightly and falls into the line obtained by extrapolating the plot in the cubic phase to higher temperatures. Figure 2 demonstrates also that the slopes for the hexagonal and cubic phases and the pure liquid of surfactant are almost the same. On the other hand, the slopes for the micellar solutions are much steeper than those for other three phases. From these plots, we have obtained the activation energy for the self-diffusion processes by using the equation
where R is the gas constant. When the log D- 1IT plot is not linear, the derivative in eq 2 has been obtained by fitting the data to a third or forth order equation. Figure 3 shows the temperature dependence of the activation energy thus obtained. It should be noted that the-activation energies in the hexagonal phase and in the pure liquid are nearly equal to that in the cubic phase as can be seen from Figure 2. Figure 3 demonstrates that the activation energy in the micellar phase is much larger than that in the cubic phase below about 45 "C. As the temperature is raised above about 45 "C, however, the activation energy decreases rapidly and approaches the value for the cubic phase.
Discussion Summaryof Our Previous Results. In the previous we have proposed a diffusion model taking into
Langmuir, Vol. 11, No. 12, 1995 4663
Networks Formed in Surfactant Solutions 200
1
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0
w
501
0 ~ ~ ' " ~ ~ " ' " " ~ ' ' ' ' ~ ' ~ ' 30 40 50 60 70 Temperature / 'c
Figure 3. Temperature dependence of the activation energy for self-diffusion processes. The value for the cubic phase is indicated by the broad line whose width indicates an experimental error.
account the intermicellar migration of surfactant molecules. In this model, it is assumed that (1)a surfactant molecule diffuses in a micelle along its contour during the time tmig and the migrates to adjacent micelles at the entanglement point, (2)t d gsatisfies the condition R,2lDL
