Ind. Eng. Chem. Res. 2000, 39, 2697-2703
2697
Fluorocarbon Hybrid Surfactants Characterization of Admicelles and Its Solubilization Masahiko Abe,*,†,‡ Akihiro Saeki,† Keiji Kamogawa,‡ Hideki Sakai,†,‡ Yukishige Kondo,‡,§ Norio Yoshino,‡,§ Hirotaka Uchiyama,| and Jeffery H. Harwell| 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-ku, Tokyo 162-8601, Japan, Faculty of Engineering, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan, and School of Chemical Engineering and Material Science, The University of Oklahoma, 100 East Boyd, Norman, Oklahoma 73019
Admicelle formation of the fluorocarbon hybrid surfactant 1-oxo-1-[4-(fluoroalkyl)phenyl]-2alkanesulfonate (FCm-HCn) on the aluminum oxide particle surface has been studied by the adsorption isotherm, ζ-potential of the particle surface, degree of counterion dissociation, contact angle, and pyrene fluorescence probe measurement. Solution properties of FCm-HCn have been compared with a dialkyl hydrocarbon surfactant, sodium 1-4-alkylphenyl-1-oxo-2-alkanesulfonate (HCm-HCn). Adsorption isotherm study and ζ-potential measurement indicate that the fluorocarbon hybrid surfactant can adsorb on the aluminum oxide surface at lower concentrations than the hydrocarbon surfactant. Furthermore, the total amount of surfactant adsorbed on the aluminum oxide surface is higher than that of the hydrocarbon surfactant. A pyrene fluorescence probe study shows that pyrene molecules are solubilized near the hydrocarbon chain of the surfactant molecules, indicating that the hydrocarbon domain exists in the fluorocarbon hybrid surfactant admicelle. No pyrene excimer formation is observed in the hybrid surfactant admicelle but in the hydrocarbon surfactant, implying the hybrid surfactant molecules are packed tightly in the admicelle. In addition, the admicelle of the hybrid surfactant is capable of solubilizing both 2-naphthol and 1-trifluoromethyl-2-naphthol and the total amount of solubilized solute increases with increasing surfactant concentration. Introduction Surfactant molecules consist of two parts, hydrophobic and hydrophilic groups within the molecules. The surfactant forms molecular aggregates such as micelles and vesicles. In addition, it is well-known that surfactant molecules adsorb onto a solid/liquid interface as monoor bilayers.1-5 Of the variety of surfactant functionalities, solubilization by the molecular aggregates is of great importance in many industrial fields, such as cosmetics, detergents, and so forth. Among the molecular aggregates, the phenomena associated with solubilization in surfactant adsorption layers (admicelle) have great potential for new application in environmental areas where organic toxic substances are solubilized in the hydrophobic domain of the aggregate and removed from an aqueous stream.6,7 Recently, a fluorocarbon hybrid-type new surfactant, 1-oxo-1-[4-(fluoroalky)phenyl]-2-alkanesulfonate (Figure 1a and b) was introduced to improve surfactant functionality.8 This novel new surfactant has two characteristic hydrophobic groups, hydrocarbons and fluorocarbons, in the molecules. Generally, despite a superior capability of reducing surface tension, a small number of fluorocarbon surfactants shows effective interfacial * To whom correspondence should be addressed. † 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. | The University of Oklahoma.
Figure 1. Chemical structure of sodium 1-oxo-1-[4-(fluoroalkyl)phenyl]-2-alkanesulfonate) (FCm-HCn) and sodium 1-(4-alkylphenyl)-1-oxo-2-alkanesulfonate (HCm-HCn).
reduction properties.8 However, the fluorocarbon hybrid surfactant appears to show not only a superior surface tension reduction property but also a remarkable interfacial tension reduction. Because of low interfacial tension, the fluorocarbon hybrid surfactant enables one to emulsify water, hydrocarbon oil, and fluorocarbon oil simultaneously.8 This unique surfactant is expected to have a unique adsorption property as a result of both hydrocarbon and fluorocarbon alkyl groups in the molecules as hydrophobic groups. Having two such characteristic hydrophobic groups, one can expect that this novel surfactant will be able to solubilize hydrocarbon oil as well as fluorocarbon oil in their hydrophobic domain.
10.1021/ie990832+ CCC: $19.00 © 2000 American Chemical Society Published on Web 06/20/2000
2698
Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000
This present paper describes adsorption characteristics of the hybrid surfactant onto a solid/liquid surface. We also report its solubilization behavior of organic solute in the admicelle. Experimental Section Surfactant. Sodium 1-oxo-1-[4-(fluoroalkyl)phenyl]2-alkanesulfonate (FCm-HCn) was used in this study. The methods for synthesis and purification were described in a previous paper.9 The chemical structures of these surfactants are described in Figure 1. The fluorocarbon (m) and hydrocarbon (n) chain lengths of the hybrid surfactants used in this study were [m, n] ) [6, 4] and [4, 6], respectively. The critical micelle concentrations (CMCs) of the surfactants were FC6HC4 0.23 mM, FC4-HC6 1.2 mM, HC6-HC4 3.1 mM, and HC4-HC6 6.6 mM. The surface tensions of each surfactant at its CMC were FC6-HC4 20.4 mN/m, FC4-HC6 18. 9 mN/m, HC6-HC4 36.1 mN/m, and HC4-HC6 34.9 mN/m. Sodium 1-4-alkylphenyl-1-oxo-2-alkanesulfonate (HCm-HCn) was used as a reference. As a typical monoalkyl anionic surfactant, sodium dodecysulfate (Nihon Surfactant Industries) was also used. An Al2O3 particle (Nihon Aerosil, Aluminum Oxide C, particle size 13 nm) was used as the adsorbent. The specific surface area of alumina oxide measured by the BET method was 100 ( 15 m2/g. Pyrene used as a fluorescence probe and 2-naphthol as a solute were obtained from Wako Pure Chemical and used without further purification. Water used in this study was distilled water for injection JP (Japanese Pharmacopoeia) from Ohtsuka Pharmacy Co., Tokyo. Adsorption Isotherm. Various concentrations of surfactant solution (pH ) 4.9-9.8) were prepared and the aluminum oxide particles, 0.33 g, were added. The solutions with aluminum oxide were shaken for 4 days at 30 °C in an incubator. The aluminum oxide was centrifuged (Kokusan Co. H-11N, 1 h, 4000 rpm) and supernatant was analyzed by using a total organic carbon analyzer (Shimadzu TOC-5000). The adsorption isotherm was prepared by using its concentration difference between pre- and postadsorption experiment. ζ-Potential Measurement. Various surfactant solutions, 10 mL, were prepared, into which aluminum oxide fine particles, 0.0010 g, were added. The mixed solutions were shaken for a day at 30 °C in the incubator. The ζ-potential of the aluminum oxide particle was measured by using the laser doppler method (Particle Sizing System, model NICOMP 370). Degree of Counterion Dissociation Measurement. The degree of counterion dissociation was determined by using a pH meter (Toa Denpa Kogyo, model IM-40S) with a Na-glass electrode (TOA, model NA115B) at 30 °C. A saturated KCl-calomel electrode was used as a reference. Contact Angle Measurement. A plate type of the aluminum oxide was used for the contact angle measurement. Various concentrations of surfactant solutions were prepared and dropped on the aluminum oxide plates. The contact angles of the solutions were measured immediately after dropping (Kyowa Scientific, Tokyo Model CA-A). Fluorescence Spectrum Measurement. First, 0.1 mM pyrene chloroform solution was placed in the test tube and chloroform was then removed by evaporation. A known concentration of surfactant solution and
Figure 2. Adsorption isotherm of FCm-HCn and HCm-HCn on aluminum oxide particles at 30 °C.
aluminum oxide particles are placed in the test tubes and shaken for a day at 30 °C. Fluorescence spectrum measurement was performed between 335 and 600 nm at an excitation wavelength of 335 nm (Shimadzu, model RF-5000). Total Amount of Solubilized Organic Solute in an Admicelle. A series of surfactant solutions containing an organic solute and aluminum oxide (0.083 g) were prepared. The solutions were mixed at 30 °C for a day and the aluminum oxide was removed by the centrifuge. The concentration of organic solute solubilized in the admicelle was then determined by HPLC (Toyo Soda Co., model HLC-803D). Result and Discussion Adsorption Isotherm of Fluorocarbon Hybrid Surfactants. Figure 2 shows the adsorption isotherms of the hybrid surfactants onto the aluminum oxide surface at 30 °C. Both figures show the comparison between the fluorocarbon hybrid surfactants and the hydrocarbon surfactants. Figure 2a,b shows the adsorption isotherm for the FC6-HC4/HC6-HC4 system and for the FC4-HC6/HC4-HC6 system, respectively. The equilibrium concentration used in the figure is equivalent to the concentration of the supernatant after the adsorption. The total amount of surfactant adsorbed onto the aluminum oxide increases with increasing equilibrium concentration of the surfactants. Especially, the adsorptions of both hybrid surfactants (FC6-HC4 and FC4-HC6) are much higher than hydrocarbon surfactants at low equilibrium concentrations. It is wellknown that anionic surfactant adsorption reaches a maximum and the surface of aluminum is saturated with the surfactant above its critical micelle concentration.10,11 However, the adsorption of the hybrid surfactants increases at even higher concentrations. Figure 3 depicts the ζ-potential of the aluminum oxide surface as a function of equilibrium concentration of the surfactants. The ζ-potential for all surfactants changes from positive to negative values as the equilibrium concentration increases. At higher concentrations, the ζ-potential reaches a constant value and those for hybrid surfactants are much lower (-34 mV) than those for hydrocarbon surfactants (-26 mV). The equilibrium concentrations of the hybrid surfactants at the ζpotential equal to zero are about an order of magnitude lower than those of hydrocarbon surfactants. The results
Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000 2699 Table 1. Degree of Dissociation of Hybrid Surfactants (FC Type, HC Type)
Figure 3. Relationship between the ζ-potential of aluminum oxide particles and equilibrium concentration of FCm-HCn and HCm-HCn at 30 °C.
are consistent with the fact that both hybrid surfactants adsorb on the aluminum oxide surface at lower concentrations than hydrocarbon surfactants. According to the fact that the adsorption of the hybrid surfactants continues to increase, even above the cmc, and shows a constant ζ-potential value, it is indicated that the hybrid surfactant can form multiple layers on the aluminum surface and the surfactant molecules orient in a denser fashion than hydrocarbon surfactant molecules. Now, the total amount of adsorption and ζ-potential are plotted as a function of equilibrium concentration of the surfactants in Figure 4. Parts a-e of Figure 4 show two hybrid surfactants, two hydrocarbon surfactants, and SDS, respectively. As mentioned above, the adsorption of the hybrid surfactants reaches maximum and the ζ-potential remains constant in the same concentration region for that of the hydrocarbon surfactants shown in Figure 4c-e. However, the adsorption of both hybrid surfactants continues to increase above the concentration where the ζ-potential shows constant values. Therefore, the adsorption mechanism of the hybrid surfactant in a high concentration region seems to be different from that of hydrocarbon surfactants such as HCm-HCn and SDS. Figure 5 shows the concentration of sodium ion as a function of surfactant concentration. The slope of each line determines the degree of counterion dissociation and the values are summarized in Table 1. The degrees of counterion dissociation for both of the fluorocarbon hybrid surfactants are smaller than those of the hydrocarbon surfactants. The electrostatic repulsion force between adsorbed hybrid surfactant molecules on the aluminum oxide surface will be smaller than that for the hydrocarbon surfactant
FC type
degree of dissociation
HC type
degree of dissociation
FC6-HC4 FC4-HC6
0.34 0.50
HC6-HC4 HC4-HC6
0.47 0.65
molecules as a result of lower counterion dissociation. Therefore, the hybrid surfactant can adsorb tightly and the total amount of adsorption is higher than that of the hydrocarbon surfactants. This also explains why the ζ-potential of the aluminum oxide surface with the hybrid surfactant is negatively higher than that of hydrocarbon surfactants. Contact angles of the surfactant solution on the aluminum oxide plate are plotted as a function of surfactant concentration in Figure 6a,b. Both contact angles of the fluorocarbon and hydrocarbon surfactants decrease with concentration and FCm-HCn surfactants show much lower contact angles than those of HCm-HCn over the concentration range. This may be attributed to the fact that the affinity of a fluorocarbon hybrid surfactant to an aluminum oxide surface is relatively high and the surfactants can adsorb at lower concentrations. Pyrene Fluorescence Probe Study. Of the five peaks of the fluorescence spectrum of pyrene molecules, the ratio of the intensities of the first (375 nm) to the third (386 nm) peaks, I1/I3, is well-known to be almost proportional to the polarity in the region near the pyrene molecule solubilized in the aggregates.12 The ratio of I1/I3 decreases with an increase in the hydrophobic environment. Therefore, the micropolarity in the surfactant aggregates such as micelles, vesicles, and liposomes is monitored by measuring the ratio of I1/I3. To confirm that the pyrene molecules are solubilized in an admicelle on the aluminum oxide surface, the experiment is performed below the cmc. Figure 7 is a plot of the I1/I3 ratio of pyrene as a function of the equilibrium concentration of FC6-HC4 and the ratio to cmc. Without aluminum oxide particles, the I1/I3 ratio is constant, indicating no micelle exists in the solution. As shown in the figure, the I1/I3 ratio decreases with an increase in the surfactant equilibrium concentration in the presence of aluminum oxide particles. Pyrene molecules are now solubilized in the admicelle and the hydrophobic environment around the pyrene molecules increases with surfactant concentration. The same trend is observed for a hydrocarbon surfactant (HC6-HC4 system). To compare the micropolarities of the inside membranes, the I1/I3 ratios for the FC6-HC4 and HC6-HC4 systems are compared as a function of surfactant amount adsorbed on the alumina particles (Figure 8). The I1/I3 ratio for the HC6-HC4 system is slightly lower than that for the FC6-HC4 system, indicating that the polarity inside the membrane for the fluorocarbon hybrid surfactant is higher than a normal hydrocarbon surfactant. Figure 9 shows pyrene fluorescence spectra of surfactant/aluminum oxide solution for the FC6-HC4 and HC6-HC4 systems at 30 °C. Typical five peaks in pyrene fluorescence spectra are shown for the FC6-HC4 system, while the HC6-HC4 system shows an additional peak around 470 nm. This is due to an excimer formation of pyrene molecules. This excimer formation of pyrene was not observed in a wide range of pyrene concentration in the FC6-HC4 system. The excimer
2700
Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000
Figure 4. Adsorption isotherm and ζ-potential of aluminum oxide particles at 30 °C: (a) FC6-HC4; (b) FC4-HC6; (c) HC6-HC4; (d) HC4-HC6; (e) SDS.
formation is dependent on microfluidity around pyrene molecules. As pyrene is solubilized in the surfactant
membrane on an aluminum oxide surface, we can consider, on one hand, that the excimer formation is a
Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000 2701
Figure 7. I1/I3 ratio of pyrene fluorescence in an FC6-HC4 aqueous solution with and without aluminum oxide particles. Figure 5. Activity of sodium ion as a function of surfactant concentration at 30 °C.
Figure 8. I1/I3 ratio of pyrene fluorescence as a function of the total amount of surfactant adsorption.
Figure 9. Pyrene fluorescence spectra in a surfactant-aluminum oxide solution. (Equilibrium concentration is below the cmc at 30 °C.)
Figure 6. Relationship between contact angle and surfactant concentration.
result of higher fluidity inside the membrane of the hydrocarbon surfactant. On the other hand, the fluorocarbon hybrid surfactant seems to have a lower microfluidity in the membrane, indicating the packing condition of the fluorocarbon surfactant is more rigid
than that of the hydrocarbon surfactant. Because the degree of counterion dissociation for the fluorocarbon hybrid surfactant is lower than that of the hydrocarbon surfactant, the packing of fluorocarbon surfactants in the membrane can be tighter on the aluminum oxide surface. The first peak of the pyrene monomer fluorescence spectrum is closely related to the polarity of the solvent.12 In other words, the wavelength of the first peak
2702
Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000
Figure 10. Relationship between the wavelength of I1 of pyrene fluorescence spectra and equilibrium concentration of FC6-HC4.
Figure 12. Total amount of 1-trifluoromethyl-2-naphthol solubilized in the FC6-HC4 admicelle as a function of equilibrium concentration.
surfactant admicelle has a hydrocarbon domain capable of solubilizing hydrocarbon organic solute. A solubilization study for fluorinated organic solutes by the hybrid surfactant is also studied and shown in Figure 12. 1-Trifluoromethyl-2-naphthol is selected in this study as a model compound for the fluorinated organic solutes. The solubilization of 1-trifluoromethyl2-naphthol is observed and increases with increasing surfactant concentration. Finally, the admicelle of the fluorocarbon hybrid surfactant is capable of solubilizing both the hydrocarbon and fluorocarbon organic solutes. It should be mentioned that the admicelle solubilization by the hybrid surfactant is applicable to the removal of organic solute from a contaminated aqueous solution. Figure 11. Total amount of 2-naphthol solubilized in the FC6-HC4 admicelle as a function of equilibrium concentration.
depends on the polarity of the solvent. Figure 10 shows the wavelength of the first peak (I1) as a function of the equilibrium concentration of the FC6-HC4 surfactant. The first peak is kept constant and independent of the concentration. The first peak of pyrene molecules is near 372 nm in hexane and 369 nm in perfluorohexane. The first peak of pyrene in the fluorocarbon surfactant is somewhat closer to hexane than to perfluorohexane. Therefore, it is possible to infer that the pyrene molecules are solubilized near the hydrocarbon chain portion of the hybrid surfactant molecules. These results indicate the hydrocarbon domain may exist in the fluorocarbon hybrid surfactant admicelle. Solubilization of Organic Solute in a Surfactant Admicellle. 2-Naphthol is one of the carcinogenic organic solutes that needs to be removed from wastewater.6,7 Figure 11 shows the total amount of 2-naphthol solubilized in the FC6-HC4 admicelles as a function of equilibrium surfactant concentration. When the equilibrium concentration increases, both adsorption of the surfactant onto aluminum oxide and solubilization of 2-naphthol in the admicelle increase. A pyrene solubilization study indicates the fluorocarbon hybrid
Conclusion An adsorption isotherm study and ζ-potential measurement indicate that the fluorocarbon hybrid surfactant can adsorb onto aluminum oxide at lower concentrations compared to the typical hydrocarbon surfactant. Furthermore, the total amount of surfactant adsorbed on the aluminum oxide surface is higher than that of the hydrocarbon surfactant. The pyrene fluorescence probe study shows that pyrene molecules are solubilized near the hydrocarbon chain of the surfactant molecules, indicating that the hydrocarbon domain may exist in the fluorocarbon hybrid surfactant admicelle. This admicelle is capable of solubilizing both 2-naphthol and 1-trifluoromethyl-2-naphthol. The total amount of solubilized solute increases with increasing surfactant concentration. Finally, the admicelle of the fluorocarbon hybrid surfactant can be applied to the separation field in the removal of organic compounds from a contaminated aqueous stream. Literature Cited (1) Somasundaran, P.; Fuerstenau, D. W. J. Phys. Chem. 1966, 79, 90. (2) Goujon, G.; Cases, J. M.; Mutaftschiev, B. J. Colloid Interface Sci. 1976, 56, 587.
Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000 2703 (3) Clunie, J. S.; Ingram, B. T. In Adsorption from Solution at Solid Liquid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: New York, 1983; p 105. (4) Schwuger, M. J. In Anionic Surfactants; Lucassen-Reynders, E. H., Ed.; Marcel Dekker: New York, 1981; p 11. (5) Dobias, B. In Coagulation and Flocculation; Dobias, B., Ed.; Marcel Dekker: New York, 1993; p 47. (6) Esumi, K.; Motoba, M.; Yamanaka, Y. Langmuir 1996, 12, 2130. (7) Esumi, K.; Uda, S.; Goino, M.; Ishidzuki, K.; Suhara, T.; Fukui, H.; Koide, Y. Langmuir 1997, 13, 2803. (8) Abe, M.; Yoshino, N. J. Jpn. Oil Chem. Soc. 1996, 45, 73.
(9) Abe, M.; Yoshino, N. J. Jpn. Oil Chem. Soc. 1996, 45, 991. (10) Fujie, M. J. Jpn. Oil Chem. Soc. 1996, 45, 181. (11) Bitting, D.; Harwell, J. H. Langmuir 1987, 3, 500. (12) Terada, H.; Yoshimura, T. Liposome; Springer-Verlag: Tokyo, 1992; p 136.
Received for review November 12, 1999 Revised manuscript received February 23, 2000 Accepted February 26, 2000 IE990832+
