Langmuir 1998, 14, 4753-4757
4753
Temperature-Induced Critical Phenomenon of Hybrid Surfactant As Revealed by Viscosity Measurements Kazuhiko Tobita,† Hideki Sakai,†,¨‡ Yukishige Kondo,‡,§ Norio Yoshino,‡,§ Keiji Kamogawa,‡ Nobuyuki Momozawa,†,‡ and Masahiko Abe*,†,‡ Faculty of Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278-8510, Japan, Institute of Colloid and Interface Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan, and Faculty of Engineering, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan Received January 26, 1998. In Final Form: May 18, 1998 We have recently found that sodium 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2-hexanesulfonate (FC6HC4), one of the synthesized hybrid surfactants that have both fluorocarbon chain and hydrocarbon chain in the molecule, exhibits a thermoresponsive viscoelastic behavior in aqueous solution. The aim of this study is to examine the viscosity behavior in solution of synthesized surfactants with different chemical structures (different lengths of fluorocarbon and hydrocarbon chains) as a function of temperature and to analyze the data obtained by using a theory of probability. The study reveals the importance of the chemical structure in which the numbers of carbon atoms are 6 and 4 in the fluorocarbon and hydrocarbon chains, respectively, for the hybrid surfactant to show the thermoresponsive viscoelastic behavior. This behavior is also found to be observed during the process where the size of assemblies of surfactant molecules decreases with rising temperature and when the probability of collision is highest between small and large molecular assemblies.
1. Introduction In recent years, investigations have increasingly been conducted on fluorohybrid surfactants with both fluorocarbon chain and hydrocarbon chain in the molecule.1-5 For example, Guo et al.1,2 have synthesized a series of anionic hybrid surfactants, CmF2m+1CH(OSO3Na)CnH2n+1 (m ) 6-9, n ) 1-9), and studied their solution properties. These compounds have disadvantages, however, in that they easily absorb moisture and slowly undergo hydrolysis in the air. We have succeeded in synthesizing new hydrolysis-free anionic hybrid surfactants by combining a fluorocarbon chain and hydrocarbon chain via a phenyl group and investigated the Kraft point, surface tension lowering ability, and molecular assembly forming concentration for aqueous solutions of these synthesized compounds.4-6 The surfactants are found to possess the excellent property of emulsifying both hydrocarbon and fluorocarbon oils at one time. In addition, one of these hybrid surfactants, sodium 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2-hexanesulfonate (FC6-HC4), exhibits a peculiar behavior in solution that the viscosity at 25 °C shows a maximum at about 10 wt % and decreases thereafter, instead of increasing monotonically with increasing * To whom all correspondence should be addressed. Tel: 81471-24-8650. Fax: 81-471-24-8650. E-mail:
[email protected]. ac.jp. † Faculty of Science and Technology. ‡ Institute of Colloid and Interface Science. § Faculty of Engineering. (1) Guo, W.; Li, Z.; Fung, B. M.; O’Rear, E. A.; Harwell, J. H. J. Phys. Chem. 1992, 96, 6738. (2) Guo, W.; Fung, B. M.; O’Rear, E. A. J. Phys. Chem. 1995, 96, 10068. (3) Inoue, H.; Arai, S.; Kakuta, Y.; Taki, M.; Masuda, H.; Moronuki, N.; Yamada, M. Mem. Fac. Technol., Tokyo Metrop. Univ. 1992, 42, 4511. (4) Yoshino, N.; Hamano, K.; Omiya, T.; Kondo, Y.; Ito, A.; Abe, M. Langmuir 1995, 11, 466. (5) Ito, A.; Sakai, H.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir 1996, 12, 5769. (6) Ito, A.; Kamogawa, K.; Sakai, H.; Hamano, K.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir 1997, 13, 2935.
concentration.7 Moreover, the viscosity of 10 wt % surfactant solution as a function of temperature gives a maximum at about 36 °C, and this anomalous viscosity behavior of the solution is reversible with respect to temperature change.8 In this study, we have synthesized double-chain surfactants with different chemical structures, sodium 1-oxo1-[4-(tridecafluorohexyl)phenyl]-2-butanesulfonate (FC6HC2), sodium 1-[4-(nonafluorobutyl)phenyl]-1-oxo-2hexanesulfonate (FC4-HC6), and sodium 1-(4-hexylphenyl)1-oxo-2-hexanesulfonate (HC6-HC4), measured their temperature-dependent viscosity behavior in solution, and analyzed the data using a new analytical equation derived on the basis of the assumption that the anomalous viscosity is caused by changes in the size of molecular assemblies of surfactant molecules, to gain a better understanding of the thermoresponsive viscosity behavior in solution of the hybrid surfactant (FC6-HC4) with a rather low molecular weight. 2. Experimental Section 2.1. Materials. Sodium 1-oxo-1-[4-(fluoroalkyl)phenyl]-2alkanesulfonates (FCm-HCn: (m,n) ) (6,4), (6,2), and (4,6)) (Chart 1a) were synthesized according to the method described in a previous paper.4 HC6-HC4 (Chart 1b), which has a structure similar to that of the hybrid surfactant and whose hydrophobic chains are both hydrocarbon groups, was synthesized by the method described in an earlier paper.9 2.2. Measurements. Rheology. A stress-control-type rheometer (Carri-Med CLS2 100, TA-Instruments) was used to measure the viscosity and flow characteristics of solutions. A cone-plate with a diameter of 4 cm and an angle of 2° was employed so that all parts of the sample in the gap experience the same shear rate. After stress (up to 1.0 Pa) was given to the sample within 30 s, the shear stress-shear rate curve obtained (7) Abe, M.; Tobita, K.; Sakai, H.; Kondo, Y.; Yoshino, N.; Kasahara, Y.; Matsuzawa, H.; Iwahashi, M.; Momozawa, N.; Nishiyama, K. Langmuir 1997, 13, 2932. (8) Tobita, K.; Sakai, H.; Kondo, Y.; Yoshino, N.; Iwahashi, M.; Momozawa, N.; Abe, M. Langmuir 1997, 13, 5054. (9) Hamasaki, M.; Kondo, Y.; Tobita, K.; Sakai, H.; Abe, M.; Yoshino, N. J. Jpn. Oil Chem. Soc. To be published.
S0743-7463(98)00100-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/14/1998
4754 Langmuir, Vol. 14, No. 17, 1998
Tobita et al.
Chart 1. Molecular Structure of (a) Sodium 1-Oxo-1-[4-(fluoroalkyl)phenyl]-2-alkanesulfonate and (b) Sodium 1-(4-Hexylphenyl)-1-oxo-2-hexanesulfonate
was approximated by the Bingham equation (eq 1):
(shear stress) ) C + η (shear rate)
(1)
where η is the viscosity and C is the yield value. The measurements were made at temperatures between 6 and 64 °C, and the temperature was controlled within 0.1 °C by means of a Peltier element. Electron Microscopy. Observations of molecular assemblies of FC6-HC4 were conducted by transmission electron microscopy. A sample solution at 20 °C was rapidly cooled and frozen in liquid nitrogen and then fractured at -120 °C. The fractured sample surface was coated successively with platinum in about 5 nm thickness and at 45° and carbon in about 15 nm thickness and at 90°, respectively, by means of sputtering (Freeze Replica Preparing Apparatus, FR-7000A, Hitachi, Ltd.). The metal replica thus prepared was observed with a transmission electron microscope (model H-7500, Hitachi, Ltd.) at an acceleration voltage of 120 kV.
Figure 1. Temperature dependence of viscosity for 10 wt % aqueous FC6-HC2 solution (O), 10 wt % aqueous FC4-HC6 solution (4), 10 wt % aqueous HC6-HC4 solution (0), and 10 wt % aqueous FC6-HC4 solution (b).
3. Results As reported in the previous paper,8 the viscosity of aqueous 10 wt % solution of FC6-HC4 increases about 100 times and shows a maximum value of 90.9 Pa‚s when the temperature rises from 6 to 36 °C and then decreases with further rise in temperature to give a value about one ten thousandth of the maximum at 64 °C. This viscosity change with temperature is able to be reversed on the same curve if the temperature was lowered, demonstrating the reversibility of the phenomenon. To examine this peculiar thermoresponsive viscoelasticity of FC6-HC4 in detail, the relation between the viscosity of 10 wt % solution and temperature is investigated for FC6-HC2 with the identical fluorocarbon chain but a shorter hydrocarbon chain, FC4-HC6 with a shorter fluorocarbon chain and a longer hydrocarbon chain, and HC6-HC4 with two different hydrocarbon chains (Figure 1). For comparison, the data for FC6-HC4 solution in the previous paper8 are also shown in the figures (solid circles). Since the FC6-HC2 solution becomes turbid below 32 °C, the shear stress-shear rate curve is not able to be approximated by eq 1, and hence, the viscosity values at a shear rate of 10 s-1 are given by a dash-dotted line in Figure 1. The viscosity decreases monotonically with increasing temperature above 34 °C and exhibits no anomalous behavior. No peculiar viscosity behavior is also observed with solutions of either FC4-HC6 or HC6-HC4 as shown in Figure 1. Solutions of single hydrocarbon chain surfactants, single fluorocarbon chain surfactants, and their mixtures exhibit no such peculiar viscosity behavior as reported in the earlier paper.8 We have so far failed to
Figure 2. Electron micrograph from a freeze-fracture method of 10 wt % aqueous FC6-HC4 solution at 20 °C.
synthesize double fluorocarbon chain surfactant due to a very slow rate of the reaction. Hence, we assume at present that the structure of surfactant should be of double chain type, with one fluorocarbon chain and one hydrocarbon chain, having six carbon atoms in the former and four carbon atoms in the latter, in order that the surfactant exhibits the thermoresponsive viscoelastic behavior mentioned in the previous paper.8 Figure 2 is a typical transmission electron micrograph of a freeze fracture replica of molecular assemblies in 10 wt % solution of FC6-HC4. Small molecular assemblies of about 80 nm in size are clearly observed, in addition to large assemblies of about
Thermoresponsive Viscoelastic Behavior
Langmuir, Vol. 14, No. 17, 1998 4755
0) is the parameter characterizing a rate of degradation of large molecular assemblies. The polynomial in eq 2 is denoted by D, which is approximated only by two terms
D ) A exp
(
)
(
)
T - Tcrit T - Tcrit + B exp KA KB
(3)
where A, B, KA, and KB are new parameters. Thereby, eq 2 can be written as
NL0 NL ) 1+D
(4)
Since the total volume should be kept constant when large molecular assemblies change into small molecular assemblies, if the number concentration of small molecular assemblies is NS, then we can have
(NL0 - NL)VL ) NSVS
(5)
and hence NS is given from eqs 3-5 as Figure 3. (upper) A model in which large molecular assemblies ML change into small molecular assemblies MS around a critical temperature Tcrit. (lower) The number concentration NL of large molecular assemblies ML as a function of temperature T.
500 nm in size. It is a striking fact that molecular assemblies of FC6-HC4 in solution have a nearly spherical shape, instead of thread- or rodlike shape that has been reported to be necessary for molecular assemblies to show a notable viscoelastic behavior.10-12 It cannot be proved that each large molecular assembly shown in Figure 2 is one large micelle, but it seems likely that each is an aggregation of small particles such as micelles. 4. Discussion 4.1. Theoretical Approach. On the basis of a probability theory, we will discuss the viscosity of solution containing various sizes of molecular assemblies. A model is proposed in which molecular assemblies ML with a large volume VL change into those assemblies MS with a small volume VS around a critical temperature Tcrit as shown in the upper part of Figure 3. The transmission electron micrograph shown in Figure 2 corresponds to be near the border between ML and ML + MS regions in Figure 3, in other words, at the temperature where large molecular assemblies begin to break down. Let the number concentration of large molecular assemblies be NL at an arbitrary temperature T. If the temperature dependence of NL is assumed to be represented by the solid line in Figure 3, it can be expressed in the following equation with Cn values as the expansion coefficients
NL
NL )
∞
1+
NS ) NL0
D VL 1 + D VS
(6)
An assumption is made here that the viscosity of surfactant solution is dependent on the friction emerging from collisions between molecular assemblies. The probability of collisions between large and small molecular assemblies, ML and MS, is apparently proportional to the product of NLNS and VL and VS, that is, NLNSVLVS, where the factor NLNS is introduced from a standpoint of combinatorial mathematics. Similarly, the probability of collision between large assemblies, ML and ML, and that between small assemblies, MS and MS, are proportional to NLC2VL2 and NSC2VS2, respectively. Here, NLC2 ) NL!/[2!(NL - 2)!] = NL2/2 and NSC2 ) NS!/[2!(NS - 2)!] = NS2/2. Thus, we obtain the following probabilities of collision:
NLNSVLVS
between ML and MS
/2NL2VL2
between ML and ML
1
1
/2NS2VS2
between MS and MS
(7)
If the viscosities arising from the collisions between ML and MS, ML and ML, and MS and MS are denoted by ηLS, ηLL, and ηSS, respectively, then the viscosity of the entire system will be given by
η)
ηLS NLNSVLVS + ηLL 1/2NL2VL2 + ηSS 1/2NS2VS2 NLNSVLVS + 1/2NL2VL2 + 1/2NS2VS2
(8)
0
(
∑ Cn exp n n)1
)
T - TCrit K
(2)
where NL0 is the number concentration of large molecular assemblies at a sufficiently low temperature and K (K > (10) Imae, T.; Kamita, R.; Ikeda, S. J. Colloid Interface Sci. 1984, 99, 300. (11) Imae, T.; Kamita, R.; Ikeda, S. J. Colloid Interface Sci. 1985, 108, 215. (12) Hashimoto, K.; Imae, T. Langmuir 1991, 7, 1734.
Substituting eqs 4 and 6 into eq 8 and rearranging yields
η)
2ηLSD + ηLL + ηSSD2 (1 + D)2
(9)
Figure 4 shows the result of calculations using the above equation for (a) ηLS . ηLL, ηSS and (b) ηLS , ηLL, ηSS. The viscosity exhibits a maximum at the critical temperature Tcrit when ηLS . ηLL, ηSS, while it decreases monotonically with increasing temperature if ηLS , ηLL,ηSS. In other words, the viscosity of the system shows a maximum when the friction caused by collision between
4756 Langmuir, Vol. 14, No. 17, 1998
Tobita et al. Table 1. Calculated Parameters of Mixed Model of Particle Sizes
Figure 4. Simulation curves of viscosity obtained from eq 9 as a function of temperature: (a) ηLS . ηLL, ηSS; (b) ηLS , ηLL, ηSS.
Figure 5. Temperature dependence of viscosity for 10 wt % aqueous FC6-HC4 solution: b, experimental data; s, theoretical curve. The dash-dotted line (a) represents ηLL ) ηSS ) 0 in eq 9, and the short-dotted line (b) represents ηLS ) 0 in eq 9.
large and small molecular assemblies is higher than that caused by collision between large assemblies or small assemblies. 4.2. Comparison with Experimental Results. In Figure 5 are shown the data of viscosity measurements on 10 wt % FC6-HC4 solutions (closed circles). The solid line in this figure represents the theoretical curve fitted to the experimental data by means of the least-squares method using eqs 3 and 9. Curve a is obtained by putting ηLL ) ηSS ) 0 in eq 9, and hence, it is the curve for the 2ηLSD/(1 + D)2 term alone (that is, the viscosity due to collisions between ML and MS). Similarly, curve b is for the (ηLL+ ηSSD2)/(1 + D)2 term, that is, the sum of the viscosity arising from collisions between ML and ML and that arising from collisions between MS and MS. Clearly, the solid line obtained using eqs 3 and 9 is in good agreement with the experimental data. The calculated values for the parameters that appear in eqs 3 and 9 are shown in Table 1.
parameters
FC6-HC4/H2O
parameters
FC6-HC4/H2O
ηLS/Pa‚s ηLL/Pa‚s ηSS/Pa‚s Tcrit/°C
183 0.502 0.00580 36.8
KA/°C KB/°C A B
1.80 6.96 0.910 0.0565
The value of ηSL is much larger than those of ηLL and ηSS, supporting the relation ηLS . ηLL, ηSS. These results strongly suggest that the peculiar thermoresponsive viscoelastic behavior of aqueous FC6-HC4 solution is due to the presence of molecular assemblies of different sizes. Thus, large assemblies (ML) formed in the low-temperature region change gradually into small assemblies (MS) as temperature rises and mixtures of molecular assemblies of different sizes give rise to the peculiar thermoresponsive viscoelastic behavior of the surfactant solution. Now, let us consider the reason FC6-HC4 solution exhibits such anomalous thermoresponsive viscoelastic behavior. As mentioned before, the chemical structure for surfactant to show the thermoresponsive viscoelastic behavior is such that the molecule is of a hybrid double chain type, with one fluorocarbon chain and one hydrocarbon chain, the former of which has six carbon atoms and the latter of which four carbon atoms. We have reported in a previous paper6 that FC6-HC4 forms small molecular assemblies (micelles) at low concentrations with its more hydrophobic fluorocarbon chains directing toward the micelle core and its less hydrophobic hydrocarbon chains orienting near the bulk phase. Moreover, a study by the pyrene probe method revealed that the micropolarity of the interior for hybrid surfactant micelles is different from that for ordinary surfactant micelles in that it shows no decrease at the cmc but starts decreasing at a concentration 10 times as high as the cmc. In other words, the hydrophobicity (consisting of hydrocarbon chains) where pyrene molecules are solubilized increases when the number of micelles increases considerably. There are at least two cases that cause change in the hydrophobic environment for pyrene. One is the situation in which those hydrocarbon chains solubilizing pyrene as well as fluorocarbon chains are incorporated into the core of micelles as a consequence of change in the structure of micelles produced by increased surfactant concentrations. The other is the case where those small micelles formed at low surfactant concentrations with their hydrocarbon chains orienting near the bulk phase approach to each other so that the hydrocarbon chains of neighboring micelles are tangled with each other to strengthen the hydrophobic interaction, thereby enlarging the hydrophobicity around the pyrene probes. The 10 wt % concentration of FC6-HC4 used in this work is about 1000 times as high as the cmc for the hybrid surfactant obtained in the earlier report.4 At such high concentrations, strongly hydrophobic fluorocarbon chains are forced to be located at the site far away from the bulk water, i.e., the micelle core, and a large number of small (compact) micelles are formed with their hydrocarbon chains inhabiting outside the benzene rings (in the vicinity of the bulk). As the number of small micelles increases, the frequency of intermicellar collisions also increases to make it easy for small micelles to form large aggregates shown in Figure 2 through the hydrophobic interaction between the intertwined hydrocarbon chains inhabiting near the micelle surfaces. However, in the case of FC6HC2, the hydrophobic interaction between hydrocarbon chains is not strong enough to form large molecular assemblies because of the short hydrocarbon chain length
Thermoresponsive Viscoelastic Behavior
even though the fluorocarbon chains direct toward the micelle core as in the case of FC6-HC4. Namely, hydrocarbon chains of hybrid surfactant play an important role in forming large molecular assemblies in solution, and if they are located in the vicinity of micelle surface, they are quite likely to actively participate in the hydrophobic intermicellar interaction, thereby forming large molecular assemblies through gathering of small molecular assemblies. As to another hybrid surfactant FC4-HC6, this cannot form micelles with its hydrocarbon chains locating near the bulk phase in view of the hydrophobicity balance between the two kinds of hydrophobic chains and, instead, forms lamellar liquid crystals at a lower concentration (1.2 wt %)8 than the concentrations where FC6-HC4 forms lamellar liquid crystals. As a consequence, the surfactant cannot form large molecular assemblies. Furthermore, double hydrocarbon chain type surfactants are unlikely to form compact micelles even though either of the hydrocarbon chains are located near the micelle surface. From what has been discussed so far, the necessary conditions for double chain hybrid type surfactant to exhibit the thermoresponsive viscoelastic behavior are suggested as follows. Compact micelles of the surfactant are formed with its fluorocarbon chains being incorporated into the micelle core and then the micelles gather to make
Langmuir, Vol. 14, No. 17, 1998 4757
large molecular assemblies through the hydrophobic interaction between the hydrocarbon chains located near the micelle surface as the surfactant concentration increases. After these conditions are satisfied, the viscoelastic behavior is observed in the process of degradation of the large assemblies when temperature is raised. 5. Conclusions The viscosity of aqueous FC6-HC4 solution exhibits a peculiar behavior, showing a maximum at the critical temperature Tcrit. This peculiar viscosity behavior has not been observed with solutions of either FC6-HC2 having the identical fluorocarbon chain and a shorter hydrocarbon chain, FC4-HC6 having a shorter fluorocarbon chain and a longer hydrocarbon chain, or HC6-HC4 having no fluorocarbon chain but two hydrocarbon chains. The main reasons for FC6-HC4 solution to show the thermoresponsive viscoelastic behavior have been suggested as follows. Large molecular assemblies of the surfactant formed at low temperatures are degraded to small molecular assemblies as temperature is raised and the anomalous viscosity emerges in the process of assembly degradation when the probability of collision is highest between large and small molecular assemblies. LA9801007
