Surface Chemical and Solution Properties of Fluorinated Silicon

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Langmuir 1998, 14, 2055-2060

2055

Surface Chemical and Solution Properties of Fluorinated Silicon Oligomers with Carboxylic Acid Groups Junpei Nakagawa,† Keiji Kamogawa,‡ Hideki Sakai,†,‡ Tokuzo Kawase,§ Hideo Sawada,| Jiradej Manosroi,⊥ Aranya Manosroi,⊥ and Masahiko Abe*,†,‡ Faculty of Science and Technology, Science University of Tokyo, Yamazaki 2641 Noda, Chiba 278, Japan, Institute of Colloid and Interface Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku, Tokyo 162, Japan, Faculty of Household Sciences, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi, Osaka 558, Japan, Department of Chemical Engineering, Nara Technical College, 22 Yada, Yamatokoriyama, Nara 639-11, Japan, and Faculty of Pharmacy, Chiang Mai University, Chiang Mai 50200, Thailand Received November 3, 1997. In Final Form: February 2, 1998

Syntheses were performed of copolymers of acrylic acid and trimethylvinylsilane with a fluoroalkyl group on both terminals (fluorinated silicon oligomers with carboxylic acid groups; RF-(CH2CHCOOH)x(CH2CHSiMe2)y-RF; RF ) C3F7, C2F5(CF2OCFCF3), C2F5(CF2OCFCF3)2). Surface tension and static lightscattering measurements were conducted to investigate the solution properties of the oligomers synthesized, and the effects were examined of RF, the CH2CH(COOH) segment, and the copolymerization ratio of the oligomers on their solution properties. These fluorinated silicon oligomers were found to have an excellent ability to lower the surface tension of water (the lowest surface tension attained: 17.4 [mN/m]) and form molecular assemblies of a colloidal dimension that is larger in size than ordinary surfactant micelles above a certain concentration. Introduction of RF into the silicon oligomers or increase in the chain length of the introduced RF reduced the concentration at which molecular assemblies were formed. Increase in the number of CH2CH(COOH) segments had no effect on the surface tension lowering ability while it caused a rise in the concentration of molecular assembly formation. Moreover, an increase in the number ratio of CH2CH(COOH) segments to CH2CH(SiMe3)) segments was found to diminish the surface tension lowering ability and reduce the concentration at which formation of molecular assemblies started.

1. Introduction It is known that polymer surfactants are in general superior as dispersing agents and flocculants but inferior in other surface active properties to low-molecular-weight surfactants.1 Introduction of fluorine atoms into polymer surfactants is expected to raise their surface tension lowering ability even though it may cause a decrease in their solubility in water.2 We have synthesized acrylic acid oligomers with a fluoroalkyl group (RF) introduced onto each of the terminals and examined their surface chemical properties.3 Thus, we have found that these oligomers are highly soluble in polar solvents such as water and methanol and reduce the surface tension of water to 20 mN/m. We have further synthesized copolymers of acrylic acid and trimethylvinylsilane (fluorinated silicon oligomers with carboxylic acid groups; RF-(CH2CHCOOH)x-(CH2CHSiMe3)y-RF) to give oil solubility to the RF-acrylic acid oligomers.4,5 These newly synthesized oligomers have * To whom all correspondence should be addressed at Faculty of Science and Technology, Science University of Tokyo. E-mail: [email protected]. † Faculty of Science and Technology, Science University of Tokyo. ‡ Institute of Colloid and Interface Science, Science University of Tokyo. § Osaka City University. | Nara Technical College. ⊥ Chiang Mai University. (1) Tanizaki, Y. J. Jpn. Oil Chem. Soc. 1985, 34, 973. (2) Yoshida, T., Ogaki, T., Shindo, S., Yamanaka, I., Eds. Surfactant Handbook; Kogaku-Tosho: 1991; p 31. (3) Sawada, H.; Minoshima, Y.; Gong, Y.-F.; Matsumoto, T.; Kosugi, M.; Migita, T. J. Jpn. Oil Chem. Soc. 1992, 41, 649.

been found to be easily soluble in not only polar solvents such as water and methanol but also aromatic solvents such as benzene, toluene, and xylene6 and to possess an ability to prevent HIV virus from proliferating, that is, an excellent anti-AIDS activity.7-9 The anti-AIDS activity of the fluorinated silicon oligomers is equivalent to or even higher than that of dextran sulfate, which has so far drawn public attention as a polymeric anti-AIDS medicine, and these oligomers are expected to be a chemically stable anti-AIDS medicine with low side effects.8 We presume that this new function of the fluorinated silicon oligomers is closely related to their surface chemical characteristics. In this study, we examine the surface chemical properties of fluorinated silicon oligomers (RF-CH2CHCOOH)x(CH2CHSiMe3)y-RF; RF ) C3F7, C2F5(CF2OCFCF3), C2F5(CF2OCFCF3)2) such as their ability to lower the surface tension of water and the critical assembly concentration (cac) and the effects of their chemical structure such as RF, the number of CH2CH(COOH) segments, and copolymer compositions in the oligomers on their solution properties. (4) Sawada, H.; Ohashi, A.; Oue, M.; Abe, M.; Mitani, M.; Nakajima, H.; Nishida, M.; Moriya, Y. J. Jpn. Oil Chem. Soc. 1994, 43, 1097. (5) Sawada, H.; Itoh, N.; Kawase, T.; Mitani, M.; Nakajima, H.; Nishida, M.; Moriya, Y. Langmuir 1994, 10 , 994. (6) Sawada, H.; Gong, Y.-F.; Minoshima, Y.; Matsumoto, T.; Nakayama, M.; Kosugi, M.; Migita, T. J. Chem. Soc., Chem. Commun. 1992, 537. (7) Baba, M.; Kira, T.; Shigeta, S.; Matsumoto, T.; Sawada, H. J. Acquired Immune Defic. Syndr. 1994, 7, 24. (8) Sawada, H.; Ohashi, A.; Oue, M.; Baba, M.; Abe, M.; Mitani, M.; Nakajima, H. J. Fluorine Chem. 1995, 75, 121. (9) Sawada, H.; Tanba, K.; Itoh, N.; Hosoi, C.; Oue, M.; Baba, M.; Kawase, T.; Mitani, M.; Nakajima, H. J. Fluorine Chem. 1996, 77, 51.

S0743-7463(97)01200-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/21/1998

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Scheme 1

Table 2. Minimum Surface Tensions of Aqueous Fluorinated Silicon Oligomer Solutions at 30 °C

Table 1. Fluoroalkyl Groups, Number Average Molecular Weights, Polydispersity, CH2CH(COOH) Segment Numbers, CH2CH(SiMe3) Segment Numbers, and Copolymerization Ratios of Fluorinated Oligomers Containing Carboxylic Acid Groups sample

RF

x

y

x:y

M hn (M h w/M h n)

PAA R(16-1) RF(13-2) RF(66-0) RF(85-1) RFO(19-6) RFO(34-7) RFO(95-6) RFO(25-9) RFO(42-3) RFO(156-0) RFO2(167-0)

none none C3F7 C3F7 C3F7 C2F5(CF2OCFCF3) C2F5(CF2OCFCF3) C2F5(CF2OCFCF3) C2F5(CF2OCFCF3) C2F5(CF2OCFCF3) C2F5(CF2OCFCF3) C2F5(CF2OCFCF3)1

28 16 13 66 85 19 34 95 25 42 156 167

0 1 2 0 1 6 7 6 9 3 0 0

100:0 92.2:7.8 87.5:12.5 100:0 98.6:1.4 76.0:24.0 82.3:17.7 94.0:6.0 74.0:26.0 74.0:26.0 100:0 100:0

2000 (-) 1250 (2.76) 1480 (1.45) 5100 (1.47) 6600 (1.94) 2510 (1.25) 3770 (1.55) 8000 (1.76) 3300 (1.41) 3300 (1.41) 12000 (1.54) 12970 (1.90)

2. Experimental Section 2.1. Materials. The fluorinated silicon oligomers used in this investigation were copolymers of acrylic acid and trimethylvinylsilane with a fluoroalkyl group (RF) at both terminals. The details of their syntheses were described in a previous paper.4 The outline of the reactions is shown in Scheme 1. In Table 1 are shown the fluoroalkyl group (RF), the copolymerization ratio, the number average molecular weight, the polydispersity (Mw/Mn), and the mean segment number for the oligomers synthesized.4 The following abbreviations were employed to denote the oligomer compositions. When RF was C3F7 and the numbers of CH2CH(COOH) and CH2CH(SiMe3) segments were 13 and 2, 85 and 1, and 66 and 0, respectively, the oligomers were called RF(13-2), RF(85-1), and RF(66-0), respectively. Likewise, if RF was C2F5(CF2OCFCF3) with an ether bond and the numbers of the two segments were 19 and 6, 34 and 7, 95 and 6, 25 and 9, 42 and 3, and 156 and 0, respectively, the oligomers were then designated as RFO(19-6), RFO(34-7), RFO(95-6), RFO(25-9), RFO(42-3), and RFO(156-0), respectively, and if RF was C2F5(CF2OCFCF3)2 with two ether bonds and the numbers of the two segments were 167 and 0, the oligomer was denoted by RFO2(167-0). The oligomer with no RF at either of the terminals was abbreviated to R(16-1) when the numbers of the two segments were 16 and 1, respectively. For comparison, poly(acrylic acid) ((CH2COOH)x) (Mn ) 2000) (Aldrich Chemical Co.), which is similar in structure to the fluorinated silicon oligomers and is a polyelectrolyte without RF and a CH2CHSiMe3 segment, was used. The polyelectrolyte is hereafter abbreviated to PAA. The water used was distilled water for injection (Ohtsuka Pharmaceutical Co., Ltd.). 2.2. Methods. 2.2.1. Surface Tension Measurement. Surface tension measurements were made with a Wilhelmy surface tension balance (Kyowa Interface Science Co., Ltd., Type A3) using a platinum plate at 30 °C. 2.2.2. Static Light Scattering Measurement. Static light scattering measurements were carried out at a scattering angle of 90° with a light scattering measuring apparatus (Malvern, 4700-Submicron Particle Analyzer) equipped with an argon ion laser (488 nm) as the light source at 30 °C. The strength of scattered light was measured on dust-free aqueous oligomer solutions of different concentrations. The removal of dusts was done by passing the solution through a membrane filter of pore size 0.4 µm (Nomura Microscience Co., Ltd.). Benzene for UV absorption spectrum measurement was used as the standard substance.

sample

γ/mN‚m-1

RF(13-2) RF(85-1) RF(66-0) RFO(19-6) RFO(34-7) RFO(95-6) RFO(42-3) RFO(25-9) RFO(156-0) RFO2(167-0) PAA R(16-1)

22.6 22.5 19.2 19.6 20.3 19.9 19.9 21.6 17.5 17.4 32.3 31.7

2.2.3. Measurement of pH. Measurements of the pH of aqueous oligomer solutions were performed with a digital pH meter (Corning, M130) at 30 °C. 2.2.4. Viscosity Measurements. A falling ball type viscometer (Haake Microviscometer) was used to measure the viscosity of aqueous oligomer solutions at 30 °C. Aqueous oligomer solution at a given concentration was sucked into a syringe (volume 0.3 mL), and the syringe was positioned in the viscometer after complete removal of bubbles from the solution by pulling a metal ball (diameter 3.06 mm) up and down in it with a magnet. After the solution was allowed to stand for 10 min until it attained a constant temperature (30 °C), the falling time of the ball (t in seconds) through the solution was measured. The dynamic viscosity (n) was calculated according to the following equation:

n ) K(d1 - d2)t where K is a constant (predetermined using a value of 0.7967 as the viscosity of water), d1 is the density of the ball (7.8), and d2 is the density of the solution (1.0).

3. Results and Discussion 3.1. Surface Tension. First, the concentration dependence of the surface tension of oligomer solutions was examined. Parts A and B of Figure 1 show the surface tension data for RFO(156-0) and RFO2(167-0) and for R(161), PAA, and RF(13-2), respectively. The surface tension for solutions of the RF-oligomers with an ether bond at both terminals decreased with increasing concentration and leveled off above a certain concentration, as shown in Figure 1A. This leveled-off surface tension value was defined as the minimum surface tension (γ). The γ values for all of the oligomers are shown in Table 2. In general, the γ values for aqueous solutions of polymer surfactants are higher than those of low-molecular-weight surfactants.10 On the other hand, aqueous solutions of the oligomers were found to have low γ values, for example, 17.4 mN/m for RFO2(167-0) and 17.5-22.6 mN/m for the other RF-oligomers. This demonstrates that these oligomers possess an excellent ability to reduce the surface tension of water, which is comparable to that for lowmolecular-weight fluorinated surfactants.11 As is clealy seen in Figure 1B, the surface tension of aqueous solutions of PAA and R(16-1), both of which have no RF at the terminals, did not level off in the concentration range studied though it decreased as the concentration increased. This would be due probably to the difficulty with which their hydrophobic groups are favorably oriented at the air/water interface.12-16 In fact, the (10) Tanizaki, Y. J. Jpn. Oil Chem. Soc. 1985, 34, 973. (11) Ono, H.; Otoshi, Y. J. Jpn. Oil Chem. Soc. 1985, 34, 1035. (12) Rfos, H. E.; Rojas, J. S.; Gamboa, I. C.; Barraza, R. G. J. Colloid Interface Sci. 1993, 156, 388. (13) Ishiguro, S.; Hartnett, J. P. Int. Commun. Heat Mass Transfer 1992, 19, 285.

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Figure 1. Relationship between surface tension and concentration. (A) RFO(156-0) and RFO2(167-0) at 30 °C: (O) RFO(156-0); (4) RFO2(167-0). (B) PAA, R(16-1), and RF(13-2) at 30 °C: (O) PAA; (4) R(16-1); (0) RF(13-2). (C) RF(13-2) and RF(85-1) at 30 °C: (O) RF(13-2); (0) RF(85-1). (D) RFO(19-6), RFO(34-7), and RFO(95-6) at 30 °C: (O) RF(19-6); (4) RFO(34-7); (b) RFO(95-6).

solution of such oligomers possessing an RF at the terminals and almost the same molecular weight as R(161) and PPA exhibited a constant surface tension above a certain concentration (critical assembly concentration). The minimum surface tension values of R(16-1) and PAA were 31.1 and 32.3 mN/m, respectively, in the concentration range studied in this work, showing that their surface tension reducing ability is lower than that of RFO(156-0) and other RF-oligomers. This suggests that the excellent surface tension lowering ability of fluorinated silicon oligomers arises from the presence of RF at their terminals. Meanwhile, it is reported that fluorocarbon surfactants preferentially adsorb at the air/water interface from their mixtures with hydrocarbon surfactants in solutions, since the energy of adsorption is higher for the fluorocarbon group (CF2) (1300 cal/mol) than for the hydrocarbon group (CH2) (900 cal/mol).17 It is also reported that low-molecular-weight hydrocarbon surfactants can hardly reduce the surface tension of water to around 20 mN/m.18 In other words, the fluorinated silicon oligomers can appreciably lower the surface tension of water because (14) Sato, T.; Okayama, T. J. Appl. Polym. Sci. 1992, 46, 641. (15) Sokotowski, A.; Burczyk, B.; Rainer, H.; Holzbauer, H.; Herbest, M. Colloids Surf. 1991, 57, 307. (16) Anton, P.; Laschewsky, A. Makromol. Chem. 1993, 194, 1. (17) Davies, K. Proceedings of the 2nd International Congress on Surface Activity; Butterworth: London, 1957. (18) Yoshida, T., Ogaki, T., Shindo, S., Yamanaka, I., Eds. Surfactant Handbook; Kogaku-Tosho: 1991; pp 69-116.

RF moieties are favorably oriented at the air/water interface. Next, the effects were examined of the chemical structure of the oligomers on the surface tension of water. In parts C and D of Figure 1 are respectively shown the surface tension-concentration curves for RF(13-2) and RF(85-1), and for RFO(19-6), RFO(34-7), and RFO(95-6). The oligomers in each figure have the same RF and nearly the identical number of CH2CH(SiMe3) segments but a different number of CH2CH(COOH) segments. All of the oligomers exhibited a remarkable surface tension lowering ability. Table 2 shows the values of minimum surface tension for the oligomers, indicating that the minimum surface tension is almost independent of the number of CH2CH(COOH) segments. If the oligomers are assumed to adsorb with their molecules stretched at the air/water interface, then shorter oligomers would have a closer orientation of RF groups, thereby giving a lower minimum surface tension. However, since the minimum surface tension is independent of the number of CH2CH(COOH) segments, as seen in Table 2, the oligomers are supposed to adsorb at the air/water interface with their principal chains except RF groups away from the interface or forming loops exposed to the air. Next, viscosity measurements were performed on oligomer solutions to check the above argument. A typical example is shown in Figure 2, where the viscosity of RFO(156-0) solution is plotted as a function of oligomer concentration.

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Figure 2. Effects of the concentration of RFO(156-0) oligomer on viscosity and reduced viscosity at 30 °C.

Figure 3. Relationship between surface tension and concentration for RFO(25-9), RFO(34-7), and RFO(42-3) at 30 °C: (b) RFO(25-9); (0) RFO(34-7); (O) RFO(42-3).

The reduced viscosity decreased monotonically with increasing oligomer concentration up to about 0.1%, beyond which it remained nearly unchanged. The viscosity itself became nearly constant above a certain oligomer concentration, corresponding to the concentration behavior of the reduced viscosity. Such concentration dependence of solution viscosity is often observed for polyelectrolytes in water and is said to be produced by a change in the structure of polyelectrolyte molecules in solution from a linear form to a coil (loop). Hence, it is suggested that the oligomers similarly take a coil structure above a certain concentration. Figure 3 shows the surface tension-concentration curves of the oligomers (RFO(25-9), RFO(34-7), and RFO(42-3)) which have the same RF, nearly the same molecular weight, and different numbers of CH2CH(COOH) and CH2CH(SiMe3) segments. In Table 2 are shown the minimum surface tension values obtained for the oligomers, indicating an increasing tendency of the surface tension value with increasing ratio of the number of CH2CH(SiMe3) segments to that of CH2CH(COOH) segments. The lowest minimum surface tension value was given by the oligomer RFO2(167-0) with no CH2CH(SiMe3) segment. On the other hand, RFO(259), having the largest number of CH2CH(SiMe3) segments exhibited the highest minimum surface tension value among the oligomers with C2F5(CF3OCFCF3) as RF, suggesting the prevention of favorable orientations at the air/water interface for RF groups due to the steric hindrance of CH2CH(SiMe3) segments.

Nakagawa et al.

When a comparison was made in terms of the surface tension lowering ability between the oligomers RF(13-2), RF(85-1), and RF(66-0), all of which have C3F7 as RF, and the oligomers RFO(19-6), RFO(34-7), RFO(95-6), RFO(25-9), RFO(42-3), and RFO(156-0), all of which have C2F5(CF2OCFCF3) as RF, the latter oligomer group was found to be higher in ability than the former one. This would be caused by the difference in the orientations at the air/ water interface between C3F7 groups which are so rigid and hardly bendable19 that only the terminal CF3 groups can orient to the interface and C2F5(CF2OCFCF3) groups which are flexible enough20 to allow CFCF3 groups to orient to the interface, in addition to the terminal C2F5 groups, because these RF groups have an ether bond. Meanwhile, the effect of RF chain length was examined on the surface tension of solutions of the oligomers RFO(156-0) and RFO2(167-0), having nealy the same number ratio of CH2CH(COOH) segments to CH2CH(SiMe3) segments but a different CF2OCFCF3 chain length. The surface tension was independent of the length of the RF chain. This would indicate that the number of CF3 groups that can orient to the air/water interface remains unchanged even if the length of the CF2OCFCF3 chain changes. From the findings mentioned so far, it is concluded that the order of the ability to lower the surface tension of water for RF is as follows: C2F5(CF2OCFCF3)2 = C2F5(CF2OCFCF3) > C3F7. 3.2. Concentration for Molecular Assembly Formation. Static light scattering measurements were conducted to confirm the formation of molecular assemblies in solutions of the fluorinated silicon oligomers. Figure 4A shows the concentration dependence of the strength of scattered light from the oligomer solution for RFO(19-6), RFO(34-7), and RFO(95-6). For all of these oligomers, the ratio of the strength of the scattered light from the oligomer solution to that from benzene (standard substance) was small and almost constant at low oligomer concentrations and it increased abruptly above a certain oligomer concentration. This suggests formation of molecular assemblies of oligomers (larger in size than ordinary surfactant micelles23) above this concentration because the strength of scattered light depends on the number and size of particles in the medium.21-23 A similar tendency was found for the other oligomers though the concentration at which the strength of scattered light suddenly increases differed from one to another. The concentration at which the strength of scattered light suddenly rises is defined as the critical assembly concentration (cac), and the values of the cac are shown in Table 3 for all of the oligomers synthesized in this work. The cac was determined as the intersecting point of the two straight lines drawn by the east squares method based on the data obtained below and above the oligomer concentration at which the strength of scattered light exhibits a sudden increase. Next, the effects of the chemical structure of oligomers on the cac were examined. Thus, the effect was first examined of the number of CH2CH(COOH) segments on the cac of the oligomers RF(13-2) and RF(85-1), and RFO(19-6), RFO(34-7), and RFO(95-6), which have the same RF, nearly the same number of CH2CH(SiMe3) segments, and (19) Fluorinated Polymers; The Society of Polymer Science: Kyouritsu-Shuppan, 1990; pp 2-4. (20) Yukawa, Y. Streiwieser’s Organic Chemistry; Hirokawa-Shorten: 1985; Vol. 1, p 243 (in Japanese). (21) Gonz, T. J. Colloid Interface Sci. 1992, 153, 73. (22) Reddy, N. K.; Fordham, P. J.; Attwood, D.; Booth, C. J. Chem. Soc., Faraday Trans. 1990, 8, 1569. (23) Nakagawa, J.; Kamogawa, K.; Sakai, H.; Momozawa, N.; Kawase, T.; Sawada, H.; Sano, Y.; Abe, M. Langmuir 1998, 8, 2061.

Silicon Oligomers with Carboxylic Acid Groups

Figure 4. Relationship between relative scattered light intensity (I/IC6H6) and concentration. (A) RFO(19-6), RFO(34-7), and RFO(95-6) at 30 °C: (O) RFO(19-6); (4) RFO(34-7); (0) RFO(95-6). (B) RFO(156-0) and RFO(167-0) at 30 °C: (O) RFO(156-0); (0) RFO(167-0). Table 3. Critical Assembly Concentrations (cac’s) of Aqueous Fluorinated Silicon Oligomer Solutions at 30 °C sample

cac/mol‚L-1

RF(13-2) RF(85-1) RF(66-0) RFO(19-6) RFO(34-7) RFO(95-6) RFO(42-3) RFO(25-9) RFO(156-0) RFO2(167-0) PAA R(16-1)

3.46 × 10-6 5.90 × 10-6 9.80 × 10-4 2.97 × 10-6 3.89 × 10-6 1.57 × 10-5 5.40 × 10-6 8.46 × 10-7 1.91 × 10-5 2.64 × 10-6 1.27 × 10-3 5.55 × 10-5

a different number of CH2CH(COOH) segments (Table 3), revealing that the cac shifts upward with increasing number of CH2CH(COOH) segments. This would be brought about by the increased electrostatic repulsion between the dissociated carboxylic acid groups resulting from an increase in the number of CH2CH(COOH) segments, which suppresses the molecular assembly formation of oligomers. In this connection, it is worth citing the facts that an increase in the number of ionic heads attached to a hydrophobic tail group increases the cmc of low-molecular-weight ionic surfactants due to the increased electrostatic repulsion24 and an increase in the ionic blocks in polymeric surfactants also increases their cmc.25 These reports support the findings in this work. (24) Patrickios, C. S. J. Phys. Chem. 1995, 99, 17437.

Langmuir, Vol. 14, No. 8, 1998 2059

The dissociation of the carboxylic acid groups of oligomers was confirmed by a decrease in the pH of the oligomer solution with increasing concentration. Additionally, a rough coincidence between the cac and the concentration at which the solution viscosity levels off (Figure 2) for RFO(156-0) suggests that the oligomers form molecular assemblies of a looplike structure. The effect was then examined of RF chain length on the cac of the oligomers RFO(156-0) and RFO2(167-0), which have nearly the same numbers of CH2 CH(COOH) and CH2CH(SiMe3) segments and a different chain lengh of RF with an ether bond (Figure 4B and Table 3), showing that the cac is lower for the oligomer with the longer RF chain than for the other. As the length of the RF chain with an ether bond increases, the number of hydrophobic CF3 groups as well as that of ether bonds increases. However, the introduction of an ether bond can only be expected to produce a small increase in the hydrophilicity of oligomers, since the ether bond alone has merely a low hydrophilicity26 and the neighboring fluorine atoms would cause a decrease in the electron density of the lone pairs in the oxygen atom due to their strong electron attractivity.23 These considerations suggest that a decrease observed in the cac of the oligomer caused by an increase in the length of the RF chain is ascribed to the increased hydrophobicity of oligomer molecules, thereby strengthening the hydrophobic interaction between them, the driving force for molecular assembly formation. This suggestion seems to be supported by the finding that the cac of R(16-1) with no RF group is higher (5.55 × 10-5 mol/L) than that of RF(13-2). In addition, the effect was examined of the coplymerization ratio on the cac of the oligomers RFO(25-9), RFO(34-7), and RFO(42-3), which have the same RF, nearly the same molecular weight, and different copolymerization ratios (Table 3), showing an upward shift of cac with an increase in the proportion of CH2CH(COOH) segment (x). This would be brought about by an increase in the electrostatic repulsion between the carboxylic acid groups and a decrease in the hydrophobic interaction with increasing proportion of CH2CH(COOH) segment because the former interaction suppresses molecular assembly formation while the latter interation enhances it. From what has been discussed so far, the following mechanism is proposed for the molecular assembly formation of fluorinated silicon oligomers in aqueous solution. While molecular assembly formation is prevented by the electrostatic repulsion between the dissociated carboxylic acid groups at low oligomer concentrations, molecular assemblies can be formed at high oligomer concentrations, where the intermolecular distance decreases and the van der Waals forces increase, thereby making it possible for the promotive force to exceed the suppressive force (electrostatic repulsion). Meanwhile, the determination of the degree of dissociation of the carboxylic acid groups on the basis of the pH measurements showed that the degree of dissociation decreases with oligomer concentration and levels off above the cac. This would give a plausible basis to the mechanism of molecular assembly formation mentioned above. 4. Conclusions The effects were examined of the number of CH2CH(COOH) segments and the copolymer compositions on the ability to reduce the surface tension of water and the (25) Tsunashima, R. Kobunshi 1996, 45, 482. (26) Kitahara, A.; Tamai, Y.; Hayano, S.; Hara, I. Surfactantss Properties, Applications, Chemical Ecologies: Kodan-sha: 1986; p 23.

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critical concentration for molecular assembly formation (cac) for copolymers of acrylic acid and trimethylvinylsilane with a fluoroalkyl group at their terminals (fluorinated silicon oligomers with carboxylic acid groups), and the following results were obtained. (1) An increase in the number of CH2CH(COOH) segments has practically no effect on the ability of the oligomer to reduce the surface tension of water while it makes an upward shift in the cac. (2) The oligomers that have an RF group at both terminals lower the surface tension of water more remarkably than those oligomers having no RF group and form molecular assemblies at extremely low concentrations.

Nakagawa et al.

(3) While increase in the length of the RF chain from C2F5(CF2OCFCF3) to C2F5(CF2OCFCF3)2 hardly affects the ability of the oligomer to reduce the surface tension of water, it causes a decrease in the cac. The order of the ability to reduce the surface tension of water among RF groups is as follows: C2F5(CF2OCFCF3)2 = C2F5(CF2OCFCF3) > C3F7. (4) An increase in the proportion of CH2CH(SiMe3) segments (y) produces a decrease in the ability of the oligomer to reduce the surface tension of water and brings about a downward shift in the cac. LA9712000