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Langmuir 1997, 13, 2932-2934
Anomalous Viscoelasticity of Concentrated Solutions with a Fluorinated Hybrid Surfactant Masahiko Abe,*,†,‡ Kazuhiko Tobita,† Hideki Sakai,†,‡ Yukishige Kondo,‡,§ Norio Yoshino,‡,§ Yasutoshi Kasahara,| Hideo Matsuzawa,| Makio Iwahashi,| Nobuyuki Momozawa,† and Katsuhiro Nishiyama†,‡ 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 October 21, 1996. In Final Form: March 17, 1997X
Flow (rheological) properties and dynamic viscoelasticity of concentrated solutions of a hybrid surfactant containing a fluorocarbon chain and hydrocarbon chain within a single molecule were analyzed. The results indicated that the viscosity of the hybrid surfactant solution is not proportional to the increase in its concentration, and it showed an anomalously large value around a 10 wt % solution. It was also indicated that the surfactant solution in this range of concentration behaves like a viscoelastic substance with a characteristic of rubber like elasticity.
We have recently synthesized a series of surfactant molecules containing a fluorocarbon chain and a hydrocarbon chain within a single molecule,1 and their solution properties have been investigated from a viewpoint of interfacial chemistry;2-4 some of them can emulsify a ternary component system of hydrocarbon/water/perfluoropolyether oil and float a surfactant aqueous solution on a hydrocarbon substance such as benzene. At present, a concentrated solution in water (around 10 wt %) of sodium 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2-hexanesulfonate (FC6-HC4), one of these surfactants, was found to demonstrate exceptionally anomalous viscoelasticity. The purpose of this paper is to discuss the concentration dependence of this phenomenon. The flow properties and dynamic viscoelasticity were measured using a stress-controlled rheometer (Carri-Med CSL2100, TA-instruments, Co). A double concentric cylinder system for measuring flow properties and a cone plate sensor with a 2° cone angle and 6 cm diameter were used for measuring dynamic viscoelasticity. The shear stress was continuously altered in a range of 0 -50 Pa at 25 ( 0.1 °C while measuring the flow properties. The dynamic viscoelasticity was analyzed within a range of strain which was established to have linear relationship with complex modulus of each sample. The angular frequency was 1-100 rad/s during the analysis. Distilled water for injection, Japanese Pharmacopoeia, from Ohtsuka Pharmacy Co., Tokyo, was used as water supply. * To whom all correspondence shoud 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 Interfacial Science, Science University of Tokyo. § Faculty of Engineering, Science University of Tokyo. | Department of Chemistry, School of Science, Kitasato University. X Abstract published in Advance ACS Abstracts, May 1, 1997. (1) Yoshino, N.; Hamano, K.; Omiya, Y.; Kondo, Y.; Ito, A.; Abe, M. Langmuir 1995, 11, 466. (2) Ito, A.; Komogawa, K.; Sakai, H.; Hamano, K.; Kondo, Y.; Yoshino, N.; Abe, M. J. Jpn. Oil Chem. Soc. 1996, 45, 479. (3) Ito, A.; Sakai, H.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir 1996, 12, 5768. (4) Ito, A.; Komogawa, K.; Sakai, H.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir, in press.
S0743-7463(96)01023-2 CCC: $14.00
Figure 1 shows the change when containers with 1, 10, 20, and 30 wt % solutions of FC6-HC4 were tilted from a upright position. The surface of all the solutions, 1, 20, and 30 wt %, but 10 wt % inclined immediately after the containers were tiled. The 10 wt % solution took approximately 5 s before the surface inclined. It should be noted that the diluted solution (10 wt % solution) from 20 or 30 wt % also showed the same behavior mentioned above. This indicates that the 10 wt % solution is of high viscosity. The 10 wt % solution was also found to have a characteristic of rubberlike elasticity when a glass rod was pulled from the solution (Figure 2). However, 1, 20, or 30 wt % solutions did not demonstrate the same behavior. We then analyzed the flow properties of 1, 10, 20, and 30 wt % solutions. The viscosity defined as the incline (the shear stress/shear rate) around zero shear rate was largest with the 10 wt % solution (Figure 3). This diagram also indicates the following: (1) The rheological property of the 1 wt % solution had characteristics similar to Newtonian fluid from the fact that the viscosity was hardly dependent upon the shear rate and did not show any sign of hysteresis. (2) The flow curve of 10 wt % solution showed inconsistency at a high shear rate and also showed hysteresis. (3) The rheological property of the 20 wt % solution had characteristics similar to Newtonian fluid from the fact that the viscosity was hardly dependent upon the shear rate and did not show any sign of hysteresis. (4) The viscosity of 30 wt % solution decreased proportional to the shear rate increase but did not show hysteresis. The flow curve of 10 wt % solution shown in Figure 3 is expressed as the correlation between the viscosity and shear rate, which is shown in Figure 4. The viscosity maintained a constant value of approximately 12 Pa‚s until shear rate reached to ca. 20 s-1 , then rapidly decreased to 10-2 order (curve a). When the shear rate was reduced, hysteresis appeared (curve b). When another measurement was applied to the sample to which the same amount of shear rate had been applied before, it followed Visc.-Shear rate curve b. Thus, the viscosity of a 10 wt % solution irreversibly changed when a shear stress was © 1997 American Chemical Society
Anomalous Viscoelasticity
Langmuir, Vol. 13, No. 11, 1997 2933
Figure 4. Correlation between viscosity and shear rate for 10% aqueous solutions of the hybrid surfactant (FC6-HC4) at 25 °C.
Figure 1. Change in the surfaces of 1, 10, 20, and 30% aqueous solutions of the hybrid surfactant (FC6-HC4) when containers are tilted from a upright position.
Figure 5. Dependence of storage modulus on angle frequency for aqueous solutions of the hybrid surfactant (FC6-HC4) at 25 °C.
Figure 2. Rubber-like behavior of the 10% solution of the hybrid surfactant (FC6-HC4) when a glass rod is pulled from the solution.
Figure 3. Flow curves for 1, 10, 20, and 30% aqueous solutions of the hybrid surfactant (FC6-HC4) at 25 °C.
applied. When a small shear stress which does not decrease the viscosity was applied, it returned to the original curve (the flat area of curve a). Generally, viscosity of a single-linked hydrocarbon or
a single-linked fluorocarbon surfactant tends to monotonically increase with an increase in the surfactant concentration. However, it is a unique characteristic of the hybrid surfactant that the viscosity reaches the maximum value at a certain concentration. Figure 5 shows the dependency of angle frequency on storage modulus G′. A 1 wt % solution was not measured, because it hardly had any elasticity. The following are the results and discussion of the experiments: (1) A 10 wt % solution showed significant angular frequency dependency from the low- to high-frequency regions and showed high storage modulus in a highfrequency region. (2) The storage modulus of the 20 wt % solution had some angular frequency dependency, but the values were relatively low. (3) G′ in the 30 wt % solution hardly showed angular frequency dependency. However, the storage modulus was relatively high. Generally, it is the characteristic of concentrated suspension that the storage modulus does not show angular frequency dependency, and this is assumed to be caused by the relaxation of a temporary internal formation such as a network formation consisting of dispersed molecules.5 This suggests that the same phenomenon occurred in the 30 wt % solution in the hybrid surfactant shown in (3). The result in (2) suggests that there is hardly any formation of aggregates in the 20 wt % solution. The results in (1), however, suggests that the 10 wt % solution also has some formation of aggregates which is different from those observed at the 30 wt % solution.6 Figure 6 shows the correlation between the loss tangent (tan δ ) loss modulus/storage modulus) and the concen(5) Matsumoto, T. Nihon Reoroji Gakkaishi 1986, 14, 167. (6) Kaneko, T.; Hayashi, T.; Amari, T. J. Jpn. Soc. Colour Mater. 1995, 68, 673.
2934 Langmuir, Vol. 13, No. 11, 1997
Figure 6. Relationship between tan δ (loss modulus/storage modulus) and the concentration of the hybrid surfactant (FC6HC4) at 100.0 rad/s angle frequency (25 °C).
tration of the hybrid surfactant solution at 100 rad/s. tan δ becomes the minimum value around 10 wt %. This indicates that the 10 wt % solution not only has elasticity but also has viscosity as well. The 30 wt % solution also has a low tan δ value; however, as shown in Figure 5, the G′ value at 30 wt % solution is smaller than that at 10 wt %. Thus the 10 wt % solution is considered to be more viscoelastic than the 30 wt % solution. Additionally, a lamellar liquid crystalline phase is formed at 30 wt % solution. In order to investigate the reason why this hybrid surfactant (FC6-HC4) shows a viscoelasticity when its concentration becomes around 10 wt %, we have to consider the structural change of the water molecule and of aggregating condition of the surfactant. We tried to measure the structural change of water molecule with increasing concentration of surfactant by NMR. The selfdiffusion coefficient (D) of water was obtained by the pulsed gradient spin echo technique of the NMR method and the 1 H-NMR spin-lattice relaxation time (T1) of water was measured by the inversion-recovery method using a 400 MHz NMR spectrometer ((JEOL EX-400) at 30 °C (Figure 7). However, both D and T1 decreased monotonously with
Abe et al.
Figure 7. The 1H-NMR spin-lattice relaxation time (T1) and the self-diffusion coefficient (D) of water as a function of the hybrid surfactant (FC6-HC4) at 30 °C.
increasing its concentration. This means that the motion of water molecules is not restricted around 10 wt % solution. We have not measured the structural change of its aggregating condition for the time being. Regarding the reduction of viscosity with increasing solute concentration, Baek et al.7 have reported that the formation of liquidcrystalline (a water-soluble polymer, hydroxypropylcellulose) results in reduction of the viscosity. In fact, the hybrid surfactant (FC6-HC4) used here did form a liquidcrystalline at 20 wt % and high concentration solution but did not form it at 10 wt %. However, for this hybrid surfactant, the internal structure of micelle varies depending upon the increase of the concentration in the diluted condition.2,4 These results alone may not be sufficient to support the following statement. However, we believe that the surface orientation change due to the structural change of the micelle caused by the increase of the concentration is the main cause for the abnormal viscoelasticity in a solution with high concentration. Further studies such as small angle X-ray diffraction measurements are in progress and the results will be published in the near future. LA961023N (7) Baek, S. G.; Magda, J. J.; Larson, R. G.; Hudson, S. D. J. Rheol. 1994, 38, 1473.
