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Langmuir 1996, 12, 5768-5772
Micellar Solution Properties of Fluorocarbon-Hydrocarbon Hybrid Surfactants Atsushi Ito,† Hideki Sakai,†,§ Yukishige Kondo,‡,§ Norio Yoshino,‡,§ and Masahiko Abe*,†,§ Faculty of Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278, Japan, and Faculty of Engineering and Institute of Colloid and Interface Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku, Tokyo 162, Japan Received March 13, 1996. In Final Form: June 11, 1996X Solution properties of sodium 1-oxo-1-[(4-fluoroalkyl)phenyl]-2-alkanesulfonates, hybrid surfactants containing a fluorocarbon chain and a hydrocarbon chain in the molecule, have been measured in terms of Krafft point, surface tension, n-octane/water and water/perfluoro-n-hexane interfacial tension, and aggregation number of micelles. The Krafft point, the area occupied by a molecule at the air/water and octane/water interfaces, and the aggregation number of micelles increase with an increase of fluorocarbon and/or hydrocarbon chain length, while critical micelle concentration (cmc), surface tension, and octane/ water interfacial tension at the cmc decrease with an increase of fluorocarbon and/or hydrocarbon chain length. These surfactants are found to exhibit lower surface tension and hydrocarbon/water and fluorocarbon/ water interfacial tension at the same time and make it possible to float a surfactant aqueous solution on a hydrocarbon substance such as benzene.
Introduction Most surfactants used in practical applications are mixtures, because the solution properties of mixtures of surfactants are well-known to be superior to those of the individual surfactants involved.1 Recently, fluorocarbon surfactants containing a fluoroalkyl chain as the hydrophobic group have been synthesized with the development of fluorine chemistry and found to exhibit peculiar abilities such as thermal and chemical stability, high surface activity, high surface modification ability, and low critical micelle concentration (cmc).2,3 In solutions containing mixtures of hydrocarbon surfactants with fluorocarbon surfactants, the tendency to form aggregates is substantially different from those in solutions of mixtures of homogeneous surfactants: extremely heterogeneous micelles are formed.4-12 Recently, hybrid surfactants, composed of a fluorocarbon chain and a hydrocarbon chain in the same molecule, have gathered the attention of researchers. For instance, a * 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. ‡ Faculty of Engineering, Science University of Tokyo. § Institute of Colloid and Interface Science, Science University of Tokyo. X Abstract published in Advance ACS Abstracts, August 1, 1996. (1) Ogino, K.; Abe, M. Mixed Surfactant Systems; Marcel Dekker: New York, 1993. (2) Matsuda, K.; Abe, M.; Ogino, K.; Yoshino, N.; Sawada, H. Shikizai Kyokaishi 1994, 67, 88. (3) Yoshino, N.; Morita, M.; Ito, A.; Abe, M. J. Fluorine Chem. 1995, 70, 187. (4) Carlfors, J.; Stilbs, P. J. Phys. Chem. 1984, 88, 4410. (5) Asakawa, T.; Miyagishi, S.; Nishida, M. J. Colloid Interface Sci. 1985, 104, 279. (6) Muto, Y.; Esumi, K.; Meguro, K.; Zana, R. J. Colloid Interface Sci. 1987, 120, 162. (7) Kalyanasundaram, K. Langmuir 1988, 4, 942. (8) Burkitt, S. J.; Ingram, B. T.; Ottewill, R. H. Prog. Colloid Polym. Sci. 1988, 76, 247. (9) Matsuki, H.; Ikeda, N.; Aratono, M.; Kaneshina, S.; Motomura, K., J. Colloid Interface Sci. 1992, 150, 331. (10) Guo, W.; Guzman, E. K.; Heavin, S. D.; Li, Z.; Fung, B. M.; Christian, S. D. Langmuir 1992, 8, 2368. (11) Abe, M.; Yamaguchi, T.; Shibata, Y.; Uchiyama, H.; Ogino, K.; Yoshino, N.; Christian, S. D. Colloids Surf. 1992, 67, 29. (12) Kamogawa, K.; Tajima, K. J. Phys. Chem. 1993, 97, 9506.
S0743-7463(96)00240-5 CCC: $12.00
new series of hybrid surfactants was synthesized and its solution properties were investigated by Guo et al.13,14 and Inoue et al.15 Moreover Kunitake et al.16 prepared the hybrid amphiphiles which form bilayer membranes. More recently, we have synthesized new hybrid surfactants, and reported these surfactants can emulsify a ternary-component system of hydrocarbon/water/perfluoropolyether oil.17 Understanding both the properties and structure of micelles containing fluorocarbon and hydrocarbon chains in a molecule is also of great theoretical interest in its own right. In this paper, we will report the effect of fluorocarbon and hydrocarbon chain length on micellar and adsorption properties of the hybrid surfactants. Experimental Section Materials. Hybrid surfactants, sodium 1-oxo-1-[(4-fluoroalkyl)phenyl]-2-alkanesulfonates, CmF2m+1C6H4COCH(SO3Na)CnH2n+1 (C6H4 ) p-phenylene, m ) 4, 6, n ) 2, 4, 6; FCm-HCn), were synthesized and purified as in our previous paper.17 n-Octane and perfluoro-n-hexane were purchased of reagent grade from Tokyo Kasei Kogyo Co., Ltd., Tokyo, Japan. They were used without further purification. Water used in this experiment was distilled water for injection, Japanese Pharmacopoeia, obtained from Otsuka Pharmacy Co., Ltd., Tokyo, Japan. Measurements. Krafft point measurements of surfactant solutions were carried out with a high-sensitivity differential scanning calorimeter (DSC-8240, Rigaku Co., Ltd., Tokyo, Japan) by placing the surfactants containing a given amount of water in a high-pressure crucible and heating from -20 to 120 °C at a heating rate of 1.0 °C‚min-1. The surface tension and interfacial tension between n-octane and surfactant aqueous solution were measured using a Wilhelmy type surface tensiometer (CBVP-A3, Kyowa Scientific Co., Ltd., Tokyo, Japan) with a platinum plate. (13) Guo, W.; Li, Z.; Fung, B. M.; O’Rear, E. A.; Harwell, J. H. J. Phys. Chem. 1992, 96, 6738. (14) Guo, W.; Fung, B. M.; O’Rear, E. A. J. Phys. Chem. 1992, 96, 10068. (15) Inoue, H.; Arai, S.; Kakuta, Y.; Taki, M.; Masuda, H.; Moronuki, N.; Yamada, M., Mem. Fac. Technol,. Tokyo Metrop. Univ. 1992, 42, 4511. (16) Kunitake, T.; Tawaki, S.; Nakashima, N. Bull. Chem. Soc. Jpn. 1983, 56, 3235. (17) Yoshino, N.; Hamano, K.; Omiya, T.; Kondo, Y.; Ito, A.; Abe, M. Langmuir 1995, 11, 466.
© 1996 American Chemical Society
Fluorocarbon-Hydrocarbon Hybrid Surfactants
Langmuir, Vol. 12, No. 24, 1996 5769
The interfacial tension measurements between surfactant solution and perfluoro-n-hexane were performed with a spinning drop interfacial tensiometer (University of Texas at Austin, Model 300). The interfacial tensions were obtained from use of Vonnegut’s equation18
1 λ ) ∆Fω2r30 4
(1)
where γ is the interfacial tension, ∆F the density difference between the two phases, ω the angular velocity, and r0 the cylindrical radius. The density of each phase was measured with a digital density meter (Kyoto Electronics Co., Model DA210). The aggregation number of micelles was measured with a submicron particle analyzer (4700-type, Malvern Instruments Ltd., Worcestershire, U.K.). The optical source of the lightscattering apparatus was an argon-ion laser operating at 488.0 nm with an output power of 5 W maximum (Inova-90, Coherent Co., Palo Alto, CA). The average scattered intensity at a scattering angle of 90° was measured to determine the aggregation number of micelles (static light-scattering method). Before measurement, the aqueous solution of surfactant used as a sample was passed three times through a membrane filter with a 0.1 µm pore size (cellulose nitrate, Toyo Roshi Co., Ltd., Tokyo) for optical purification. The refractive index measurements required to determine the aggregation number of micelles were obtained with a differential refractometer (RM-102, Otsuka Electronics Co., Ltd., Osaka, Japan). The reduced intensity of scattered light, R, is given19 by
( )
R ) φb
n20I
(2)
n2bI0
where I0 and I are the measured intensities of incident and scattered light and n0 and nb are the refractive indices of water and benzene, respectively. The calibration constant of the apparatus for benzene, φb, was determined by the value of the reduced intensity of light scattered from benzene,20 3.259 × 10-5cm-1. Light scattered from a dilute micellar solution at the given concentration, c, is described by Debye’s equation:21
K(c - c0) 1 ) + 2B2(c - c0) R - R0 M
(3)
where M is the average molecular weight of micelles and B2 is the second Virial coefficient. R0 is the reduced scattering intensity for the solution at the cmc, and c0 is practically equal to the reduced scattering intensity of water. K is the optical constant derived from eq 4:
K)
4π2n20 dn λ4NA dc
( )
2
(4)
where (dn/dc) is the specific refractive index increment of a solution, NA is Avogadro’s number, and λ is the wavelength of incident light. The aggregation number is calculated with the following equation:
N)
M Mm
(5)
where Mm is the molecular weight of the surfactant.
Results and Discussion
Figure 1. Relationship between cmc and chain length of FCmHCn at 25 °C (a) Fluorocarbon chain length is constant. (b) Hydrocarbon chain length is constant. Table 1. Krafft Points, cmc, and, γcmc of FCm-HCn
FC4-HC2 FC4-HC4 FC4-HC6 FC6-HC2 FC6-HC4 FC6-HC6
Krafft point (°C)
cmc (mol‚L-1)
γcmc (mM‚m-1)
