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Langmuir 1997, 13, 5054-5055
Thermoresponsive Viscoelasticity of Sodium 1-Oxo-1-[4-(tridecafluorohexyl)phenyl]-2-hexanesulfonate Aqueous Solutions Kazuhiko Tobita,† Hideki Sakai,†,‡ Yukishige Kondo,‡,§ Norio Yoshino,‡,§ Makio Iwahashi,| Nobuyuki Momozawa,†,‡ and Masahiko Abe*,†,‡ Faculty of Science and Technology, Science University of Tokyo, 2641, Yamazaki, Noda, Chiba 278, Japan, Institute of Colloid and Interface Science, Science University of Tokyo, 1-3, Kagurazaka, Shinjuku, Tokyo 162, Japan, Faculty of Engineering, Science University of Tokyo, 1-3, Kagurazaka, Shinjuku, Tokyo 162, Japan, and Department of Chemistry, School of Science, Kitasato University, Sagamihara, Kanagawa 228, Japan Received April 3, 1997. In Final Form: June 26, 1997X The thermal viscosity behavior or temperature-dependent viscosity of an aqueous solution of sodium 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2-hexanesulfonate (FC6-HC4), a fluoro-hybrid type surfactant, which has a fluorocarbon chain and a hydrocarbon chain in a molecule, was examined. The FC6-HC4 solution showed viscoelastic behavior at concentrations around 10 wt %. Further study on the temperature dependence of the viscosity of a 10 wt % solution revealed that the viscosity increased about 102 times, showing a maximum as temperature rose from 6 to 36 °C, and then decreased about 10-4 times with further rising at in temperatures up to 64 °C. Moreover, tan δ, a parameter for viscoelasticity, exhibited a minimum at 36 °C. The phenomenon found in this work was reversible with respect to temperature change. In other words, an aqueous solution of FC6-HC4 was found to show thermoresponsive viscoelastic behavior.
We have so far synthesized fluoro-hybrid type surfactants with a fluorocarbon chain and a hydrocarbon chain in a molecure1 and reported their solution properties such as their micelle-forming concentrations and Krafft points.2-4 These investigations revealed that the fluorohybrid type surfactants have many excellent properties such as the ability to simultaneously emulsify both hydrocarbon oil and fluorocarbon oil. We have further found that a concentrated aqueous solution of sodium 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2-hexanesulfonate (FC6-HC4), one of the fluoro-hybrid type surfactants, exhibited unusual viscoelastic behavior.5 Thus, the viscosity of an aqueous FC6-HC4 solution showed a maximum at a concentration around 10 wt % and decreased thereafter as the concentration increased, instead of rising monotonically. We report here the results of a study on the temperature dependence of the viscoelasticity of an aqueous FC6-HC4 solution. A stress-controlling type rheometer (Carri-Med CLS2100, TA Instruments) was employed to measure flow characteristics and dynamic viscoelasticity. A cone-plate with a diameter of 4 cm and an angle of 2° was adopted so that the same shear rate is uniformly given to the sample in the gap. Viscosity measurement was conducted in such a way that shear stress is increased from zero to * To whom all correspondence should be addressed. Telephone: 81-471-24-8650. Fax: 81-471-24-8650. E-mail: abemasa@koura01. ci.noda.sut.ac.jp. † Faculty of Science and Technology, Science University of Tokyo. ‡ Institute of Colloid and Interface Science, Science University of Tokyo. § Faculty of Engineering, Science University of Tokyo. | Kitasato University. X Abstract published in Advance ACS Abstracts, August 15, 1997. (1) Yoshino, N.; Hamano, K.; Omiya, Y.; Kondo, Y.; Ito, A.; Abe, M. Langmuir 1995, 11, 466. (2) Ito, A.; Kamogawa, K.; Sakai, H.; Hamano, K.; Kondo, Y.; Yoshino, N.; Abe, M. J. Jpn. Oil Chem. Soc. 1996, 45, 479. (3) Ito, A.; Kamogawa, K.; Sakai, H.; Hamano, H.; Kondo, Y.; Yoshino, N.; Abe, M.; Langmuir 1997, 13, 2935. (4) Ito, A.; Kamogawa, K.; Sakai, H.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir 1996, 12, 5768. (5) Abe, M.; Tobita, K.; Sakai, H.; Kondo, Y.; Yoshino, N.; Kasahara, Y.; Matsuzawa, H.; Iwahashi, M.; Momozawa, N.; Nishiyama, K.; Langmuir 1997, 13, 2932.
S0743-7463(97)00347-8 CCC: $14.00
Figure 1. Relationship between viscosity and concentration for an aqueous FC6-HC4 solution at 25 °C.
1.0 Pa within 30 s and the shear stress vs shear rate curve obtained is approximated by the Bingham equation (eq 1):
(shear stress) ) A + B × (shear rate)
(1)
The slope B in the above linear equation gives viscosity, and the intercept A means yield value. All measurements were carried out at temperatures between 4 and 64 °C. Temperature was controlled within (0.1 °C with a Peltier element. Dynamic viscoelasticity measurement was performed in the linear range of percent strain after finding a linear range for each sample. The parameters for dynamic viscoelasticity were determined at a fixed angular frequency of 1.0 Hz. The solvent used was distilled water for injection (Ohtsuka Pharmaceutical Co., Ltd.). Figure 1 shows the relation between viscosity and concentration (25 °C) for an aqueous FC6-HC4 solution. A maximum appeared in the viscosity at a surfactant concentration of about 10 wt %. A similar viscosity behavior has been reported for an m-cresol solution of hydroxypropylcellulose (HPC), a liquid crystal-forming polymer,6 but not for an aqueous surfactant solution. Although the viscosity lowering for the polymer solution at high concentrations was ascribed to the formation of (6) Baek, S. G.; Magda, J. J.; Larson, R. G.; Hudson, S. D.; J. Rheol., 1994, 38, 1473.
© 1997 American Chemical Society
Thermoresponsive Viscoelasticity
Figure 2. Temperature dependence of the viscosity of an aqueous FC6-HC4 solution. (b):10 wt %; (9) 20 wt %; (2) 30 wt %.
a liquid crystal, this explanation cannot be true for an aqueous FC6-HC4 solution because no liquid crystal formation was observed at concentrations around 10 wt %. Liquid crystal formation for this surfactant was found to start at a much higher concentration of 30 wt % with a polarization microscope. The relation between viscosity and temperature is shown in Figure 2 for an aqueous FC6-HC4 solution. The viscosity reduced with rising temperature at the surfactant concentrations 20 and 30 wt %, while it increased from about 0.9 Pa‚s to about 90 Pa‚s (102 times) as the temperature rose from 6 to 36°C to show a maximum and then decreased to about 0.009 Pa‚s at 64 °C (10-4 times) with further temperature rise when the surfactant concentration was 10 wt %. We call such a temperaturedependent viscosity change thermoresponsive viscoelasticity. Thermoresponsive viscoelasticity was observed with this surfactant at least in the concentration range 7-15 wt %, and the value of the maximum viscosity and the temperature at which the maximum viscosity appears were dependent on the surfactant concentration. Visual observations revealed that the temperatures at which a minimum appeared on the curve in the figure, 6 °C for the 10 wt % solution and 22 °C for the 20 wt % solution, correspond to the temperatures at which the surfactant deposits. No surfactant deposition was found in the 30 wt % solution in the temperature range used in this work because the surfactant forms a lamellar liquid crystal at this concentration. Moreover, thermoresponsive viscoelasticity was hardly observed with aqueous solutions of monohydrocarbon chain type surfactants such as 1-pentanesulfonic acid sodium salt and sodium dodecyl sulfate, monofluorocarbon chain type surfactants such as perfluoroheptanoate, and the other fluoro-hybrid type surfactants such as sodium 1-[4-(nonafluorobutyl)phenyl]1-oxo-2-octanesulfonate (FC4-HC6). Namely, thermoresponsive viscoelastic behavior is specific for FC6-HC4 and observed only with its aqueous solutions in a certain concentration range. Dynamic viscoelasticity measurement was performed exclusively on the aqueous 10 wt % FC6-HC4 solution that exhibited the thermoresponsive viscoelastic behavior at a fixed angular frequency (1.0 Hz) while the temperature of the solution was varied. Although both the storage modulus (G′) and the loss modulus (G′′) changed, showing an upward convex curve with increasing temperature, they did not necessarily exhibit a peak at 36 °C (Figure 3). That is, a temperature of 36 °C is strictly the temperature at which the ratio of the elastic component and the viscous component or tan δ () G′′/G′) becomes a (7) Kapnistos, M.; Hinrichs, A.; Vlassopoulos, D.; Anastasiadis, S. H.; Stammer, A.; Wolf, B. A. Macromolecules 1996, 29, 7155.
Langmuir, Vol. 13, No. 19, 1997 5055
Figure 3. Temperature dependence of G′, G′′, and tan δ for an aqueous FC6-HC4 solution. (b) G′; (9) G′′; (2) tan δ; frequency ) 1.0 Hz.
Figure 4. Temperature dependence of T1 and D for aqueous FC6-HC4 solution ([) H2O; (1) 1 wt %; (b) 10 wt %; (9) 20 wt %.
minimum. Such a behavior of G′, G′′, and tan δ would have been commonly observed with polymer melts or concentrated polymer solutions.7 The spin-lattice relaxation time (T1) and self-diffusion coefficient (D) of water were determined through 1H-NMR measurement in order to seek a main cause for viscoelastic body formation of a 10 wt % FC6-HC4 solution (Figure 4). The sample for the measurement was placed in a 5 mm i.d. NMR tube. The air inside the tube was replaced by argon to remove the oxygen dissolved in the sample, and the tube was sealed after an inner tube containing D2O was inserted in it. The apparatus used was a JEOL EX400 MHz NMR, and the magnetic field strength employed was 160 G cm-1 ()1.6 T m-1). The spin-lattice relaxation time, T1, for the proton increased with rising temperature and decreased with increasing surfactant concentration, both in a monotonous way. Similarly, the self-diffusion coefficient of water, D, increased monotonically with rising temperature and decreased monotonically with increased surfactant concentration. Namely, no unusual behavior in these parameters was found either at 10 wt % FC6HC4 concentration or at 36 °C. This suggests that the hydration structure of water molecules does not contribute much to the viscoelastic body formation in solutions of the hybrid type surfactant, judging from the NMR or ultramicroregional observations. Hence, the unusual thermoresponsive viscoelastic behavior of the hybrid type surfactant would be linked closely to its chemical structure, especially the formation of some network structure between micelles. We will further examine this point. LA970347O
