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Micelle Formation and Surface Adsorption of N-(1,1-Dihydroperfluoroalkyl)-N,N,N-trimethylammonium Chloride Takaharu Yamabe,† Yoshikiyo Moroi,*,† Yutaka Abe,‡ and Toshio Takahasi‡ Graduate School of Sciences, Kyushu University-Ropponmatsu, Fukuoka 810-8560, Japan, and Material Science Research Center, Lion Corporation, Tokyo 132-0035, Japan Received May 31, 2000. In Final Form: September 5, 2000 The critical micelle concentrations (cmc’s) of fluorinated cationic amphiphiles, N-(1,1-dihyroperfluorooctyl)N,N,N-trimethyammonium chloride and N-(1,1-dihydroperfluorodecyl)-N,N,N-trimethyammonium chloride, were determined from the specific conductivity change versus the concentration. The cmc values for the former have a shallow minimum around 298 K, while the latter cmc’s monotonically increased with temperature over the temperature range from 288.2 to 328.2 K. The contribution of the CF2 group to the standard Gibbs energy change of micellization was found to be 1.7 times as large as that of the CH2 group. The surface tension was measured against the concentration at the temperatures of 288.2, 298.2, 308.2, and 318.2 K and then analyzed to give the surface excess concentration, from which the relationship between the surface pressure (π) and the molecular surface area (A) at the air/water interface was obtained. The adsorbed membrane was in a liquid state up to the concentration of cmc for both amphiphiles at all the temperatures above, judging from the π-A curve. From the curves, the longer amphiphile was found more closely packed just below the cmc and more surface-active at larger molecular surface area. The entropy change for the surface adsorption decreased from positive to negative values with increasing concentration for the shorter homologue, while the longer homologue had only the negative change with much steeper decrease with increasing concentration.
Introduction Fluorinated surfactants are more surface-active than their corresponding hydrogenated surfactants in such respects as critical micelle concentration (cmc) and interfacial tension. Thus, their solution and interfacial properties have long been a matter of interest from both theoretical and practical viewpoints.1 However, reports on fluorinated surfactants are much less in number compared with those of conventional hydrogenated surfactants as for their dilute solutions, although basic reports are several in number.2-10 This is partly because the small gradient of refractive index with concentration peculiar to fluorinated compounds greatly limits the utility of lightscattering methods, which are most useful for size determination of their aggregates. Moreover, their low cmc value for a longer fluorinated chain caused the operational difficulties of dealing with very dilute solutions without experimental errors and influences of contamina* To whom correspondence should be addressed. Tel, 81-92-726-4742; fax, 81-92-726-4842; e-mail, moroiscc@ mbox.nc.kyushu-u.ac.jp. † Kyushu University. ‡ Lion Corporation. (1) Kissa, E. Fluorinated Surfactants; Dekker: New York, 1993. (2) Kunieda, H.; Shinoda, K. J. Phys. Chem. 1976, 80, 2468. (3) Guittard, F.; Taffin de Givenchy, E.; Cambon, A. J. Colloid Interface Sci. 1996, 177, 101. (4) Mukerjee, P.; Handa, T. J. Phys. Chem. 1981, 85, 2298. (5) Mukerjee, P.; Korematsu, K.; Okawauchi, M.; Sugihara, G. J. Phys. Chem. 1985, 89, 5308. (6) Tomasic, V.; Chittofrati, A.; Kallay, N. Colloids Surf., A 1995, 104, 95. (7) Mukerjee, P.; Gumkowski, M. J.; Chan, C. C.; Sharma, R. J. Phys. Chem. 1990, 94, 8832. (8) Fisicaro, E.; Pelizzetti, E.; Bongiovanni, R.; Borgarello, E. Colloids Surf. 1990, 48, 259. (9) Tamori, K.; Esumi, K.; Meguro, K.; Hoffmann, H. J. Colloid Interface Sci. 1991, 147, 33. (10) Krafft, M. P.; Giulieri, F.; Riess, J. G. Colloids Surf., A 1994, 84, 113.
tion. What makes matters worse is the low reproducibility of solution properties for longer fluorinated surfactants.11 Therefore, most fluorinated ionic surfactants for investigation have been those of alkyl chains with less than 11 carbon atoms. Even if a surfactant ion has a longer fluorocarbon, it is mostly anionic.12 In other words, very little is known about fluorinated cationic surfactants, which is due to difficulty in preparing the stable cationic ones. In this study then, three homologous fluorinated surfactants, N-(1,1-dihydorperfluorooctyl)-N,N,N-trimethylammonium chloride (C8-TAC), N-(1,1-dihydroperfluorodecyl)-N,N,N-trimethylammonium chloride (C10TAC), and N-(1,1-dihydroperfluorododecyl)-N,N,N-trimethylammonium chloride (C12-TAC), were prepared in order to study their physicochemical properties in their aqueous solutions. Unfortunately, however, the present surfactants are not as stable in aqueous solution as expected; 2% of C8-TAC decomposes 4 days after preparation of the aqueous solution and 4% after 10 days. Therefore, the solution properties had to be measured as soon as possible after the solution preparation. Fortunately, any minimum was not observed for surface tension-concentration curves, which is an indication of the absence of surface-active impurities coming from the decomposition. This is mentioned later more in detail. As for C12-TAC, the solution properties could not be observed with reproducibility because the drifts of electric conductivity and surface tension of the solution with time made it difficult to measure them precisely. In some cases, the drift continued for more than 10 days, and the observed results were not reliable at all, as is easily expected. (11) Furuya, H.; Moroi, Y.; Kaibara, K. J. Phys. Chem. 1996, 100, 17249. (12) Moroi, Y.; Tacke’uchi, M.; Yoshida, N.; Yamauchi, A. J. Colloid Interface Sci. 1998, 197, 221.
10.1021/la000749n CCC: $19.00 © 2000 American Chemical Society Published on Web 11/14/2000
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Experimental Section Preparation of Amphiphiles. Alkylperfluoroalkanoates (Exfluor Res.), 2.0 M dimethylamine solution in methanol (Aldrich), and other reagents (Kanto Chemicals) were used without further purification. 1H NMR spectra were obtained with a JEOL 300 MHz spectrometer for purity check of the amphiphiles. N-(1,1-Dihydroperfluorooctyl)-N,N,N-trimethylammonium Chloride (1). Methyl chloride was introduced into N-(1,1-dihydroperfluorooctyl)-N,N-dimethylamine (1.8 g, 4.2 mmol) dissolved in acetonitrile (50 mL) and stirred with NaHCO3 (100 mg) in an autoclave at 3 kg/cm2.13 After 4 days stirring the solution was dried by evaporation of the solvent, and then the white crystalline solid was purified by recrystallization from ethanol/acetone solution to afford 1.1 g of 1(55%). 1H NMR (CD3OD) δ: 4.80(CH2, 2H, t, J ) 16 Hz), 3.61(CH3, 9H, s). N-(1,1-Dihydroperfluorodecyl)-N,N,N-trimethylammonium Chloride (2). N-(1,1-Dihydroperfluorodecyl)-N,N-dimethylamine (1 g, 1.9 mmol) was methylated by the same method as 1 to afford 0.4 g of 2 (36%). 1H NMR (CD3OD) δ: 4.67 (CH2, 2H, t, J ) 16 Hz), 3.47 (CH3, 9H, s). N-(1,1-Dihydroperfluorododecyl)-N,N-dimethylamine (3). Dimethylamine (30%) (50 mL) was titrated with methylperfluorododecanoate (10 g, 15.9 mmol) and stirred for 1 day. The solution with solid was dried by vacuum evaporation to produce a yellow solid. The solid dissolved in diethyl ether was then titrated into LiAlH4 (1 g) dispersed in diethyl ether at -10 °C. After 3 days stirring at room temperature, water (1 mL), 15% NaOH aqueous solution (1 mL), and water (3 mL) were added stepwise to the diethyl solution cooled by iced water. The white solid was filtrated and washed with fresh diethyl ether. The diethyl ether solution was dried with anhydrous MgSO4 and then evaporated. The yellow solid thus obtained was distilled (80 °C, 0.5 mmHg) to afford 2.7 g of 3(27%). 1H NMR (CDCl3) δ: 3.00 (CH2, 2H, t, J ) 16 Hz), 2.44 (CH3, 6H, s). N-(1,1-Dihydroperfluoroododecyl)-N,N,N-trimethylammonium Chloride (4). 3 (2.0 g, 3.3 mmol) was methylated by the same method as 1 to afford 1.1 g of 4 (51%). 1H NMR (CD3OD) δ: 4.65 (CH2, 2H, t, J ) 16 Hz), 3.45(CH3, 9H, s). The surfactants were further purified by repeated recrystallization from mixed solvent: ethanol-acetone for C8-TAC (1), ethanol-cyclohexane for C10-TAC (2), and methanol-acetone for C12-TAC (4). Their purity was checked by elemental analysis and NMR spectroscopy. The observed and calculated results (in parentheses) were in satisfactory agreement with regard to weight percentage: C 27.43 (27.66), H 2.34 (2.32), N 2.89 (2.93) for C8-TAC; C 26.78 (27.03), H 1.96 (1.92), N 2.35 (2.42) for C10TAC; C 26.68 (27.03), H 1.96 (1.92), N 2.35 (2.34) for C12-TAC. As for the NMR spectroscopy, there was no peak except the objective compound, which indicates a high purity of more than 99%. Water used was distilled twice from alkaline permanganate solution. Electric Conductivity. A conductivity electrode (TOA Electronics Ltd. CG-511B) was set in a test tube filled with a 3 mL surfactant solution of ca. twice the cmc. After a certain volume of the solution was withdrawn, the same volume of water was introduced into it with a microsyringe. This process was repeated until the cmc value could be precisely determined. The conductivity was measured by a conductivity meter (TOA Electronics Ltd. CM-60S), where the temperature was controlled within (0.01 K. Surface Tension. The surface tension was determined by the drop volume method under atmospheric pressure in a water bath, where the bath temperature was thermostated within (0.01 K. After equilibration of the apparatus with the thermostat, ca. 80% of total drop volume was produced and left standing for 15 min for further adsorption equilibrium. Then, the total volume was produced and determined. The precision was within (0.07 mN m-1.
Results and Discussion Specific conductivity changes of C8-TAC are given in Figure 1. The specific conductivities change more slowly (13) Matsui, K.; Kikuchi, Y.; Sugimoto, K.; Suzuki, N. JP 1986, 86, 207-362.
Figure 1. Changes of specific conductivity with total surfactant concentration. Table 1. Cmc Change with Temperature of Surfactants cmc/10-3 mol dm-3 temp/K
C8-TAC
C10-TAC
288.15 298.15 308.15 318.15 328.15
13.8 13.6 13.7 14.4 15.7
1.41 1.48 1.68 1.90 2.10
at higher temperatures over the concentration range around the cmc, suggesting that micelles of smaller aggregation number are formed at higher temperatures. The above statement is also the case for C10-TAC. As for C12-TAC, the period of time for conductivity to reach the equilibrium was quite long, say a few days above the cmc, and the cmc was roughly estimated to be 0.16 mmol dm-3 at 298.2 K by a break of the conductivity change. On the other hand, the former two took less than 10 min to reach a stable conductivity. Instability of the conductivity for C12-TAC above the cmc might result from polydispersity of the molecular aggregates of quite larger size. Such large aggregate formation is often the case for long fluorocarbon amphiphiles.11 The molecular aggregates change their size among themselves through transfer of monomeric solute in the bulk. If the chemical potential of a monomer in equilibrium with one aggregate is nearly equal to that in equilibrium with the other aggregate of different aggregation number, it should take a very long time to reach a perfect equilibrium of the aggregates system. This means that the time to reach the equilibrium above the cmc depends on the history of solution preparation. This is the case for C12-TAC. The cmc change with temperature is summarized in Table 1. The cmc values for C8-TAC have a shallow minimum around 298 K, while those for C10TAC monotonically increased with temperature over the temperature range from 288.2 to 328.2 K. The results suggest that the negative enthalpy change contributes more to micellization than the entropy change for fluorinated surfactants.12 The logarithms of cmc’s are plotted against the number of carbon atoms for three homologous surfactants in Figure 2, where those of hydrocarbon homologues with the same headgroup, trimethylammonium chloride, are also given.14 The Gibbs energy contribution to micellization per CF2 group is 1.7 times as large as that per CH2 group from the slope of the plots,
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Figure 2. Dependence of ln cmc on the number of carbon atoms of the alkyl chain for alkyltrimethylammonium chloride at 298.15 K.
Figure 4. Changes of surface tension with concentration for C10-TAC.
of the surface layer, and nis is the surface concentration of component i per unit area. Equation 1 does not agree with the phase rule. Therefore, two other intensive variables should be deleted: chemical potentials of the two components of both sides of the interface, air (1) and water (2) in the present case. Thus, eq 1 becomes
dγ ) -ss dT + τd dP -
Γi dµi ∑ i)3
(2)
where ss, τd, and Γi are, respectively, the excess entropy per unit area at interface or surface, the thickness between the two dividing surfaces, and the surface excess of component i per unit area. As for a 1-1 electrolyte that dissociates completely in the solution, eq 2 is written as
dγ ) -ss dT + τd dP - Γ+ dµ+ - Γ- dµ-
Figure 3. Changes of surface tension with concentration for C8-TAC.
where the degree of counterion binding to micelle is assumed to be the same. The value 1.7 is larger than the previous ones, 1.312 and 1.5,8 which is probably due to more closely packed aggregation in the present micelles. The surface tensions (γ) versus the concentrations are plotted for C8-TAC and C10-TAC in Figures 3 and 4, respectively. The absence of a minimum around the cmc indicates the high purity of the amphiphiles. The cmc values determined by the surface tension are in good agreement with those by conductivities within experimental errors (Table 1). The surface tension (γ) is a function of temperature T, pressure P, and chemical potential µi of component i (the Gibbs-Duhem equation for an interface)15,16
dγ ) -s dT + τ dP -
∑i ni
s
dµi
(1)
where s is the entropy of the surface layer, τ is the width
(3)
where the subscripts + and - indicate positively and negatively charged dissociated ions, respectively. In this case, the degrees of freedom become three from two phases (air and solution) and three components (surfactant, water, and air) as is consistent with eq 2. At constant temperature and pressure the surface excess can be obtained as follows
Γ ) -(1/2RT)(∂γ/∂ ln a()T,P
(4)
where the following activity coefficient f( is used:17
f( ) A′xI/(1 + 5 × 10-8B′xI) + 0.16I
(5)
The parameters A′ and B′ are tabulated in the ref 17. The surface tensions versus the logarithm of activity below the cmc are shown in Figures 5 and 6 for C8-TAC and C10-TAC, respectively. The γ-ln a( curves were analyzed by second-order curve fitting, and the slope was used to evaluate the surface excess as is given by eq 4. The surface excess thus obtained is illustrated in Figures 7 and 8 for (14) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; NSRDS-NBS 36, 1971. (15) Motomura, K.; Aratono, M. Langmuir, 1987, 3, 304. (16) Moroi, Y. Micelles: Theoretical and Applied Aspects; Plenum Press: New York, 1992; Chapter 8. (17) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions, 2nd ed.; Butterworth: London, 1959; Chapter 9.
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Figure 5. Second-order curve fittings of surface tension versus activity below the cmc for C8-TAC.
Figure 7. Changes of surface excess with concentration for C8-TAC.
Figure 6. Second-order curve fittings of surface tension versus activity below the cmc for C10-TAC.
Figure 8. Changes of surface excess with concentration for C10-TAC.
C8-TAC and C10-TAC, respectively, where the points correspond to the concentrations used for calculation. From the surface excess the molecular surface area (A) at the surface can be calculated by the following relation
Table 2. Surface Properties of Surfactants
A ) 1/NAΓ
C8-TAC
(6)
where NA is the Avogadro’s number. The results are given in Table 2 and in Figure 9. From Figure 9, the longer amphiphile was found to be more closely packed just below the cmc and more surface-active at larger molecular surface area. At the same time, the adsorbed surfactants are quite similar to the expanded liquid film at air/water interface.18 The chemical potentials are also a function of temperature, pressure, and concentration
dµ( ) -s( dT + v( dP + (∂µ(/∂C()T,P dC(
surfactant
(7)
where s and v are the partial molar entropy and volume, (18) Defay, R.; Prigogine, I.; Bellemans, A.; Everett, D. H. Surface Tension and Adsorption; Longmans: London, 1966; Chapter 6.
C10-TAC
temp K
γcmc mN m-1
Γmax 10-6 mol m-2
Amin nm2 molecule-1
288.15 298.15 308.15 318.15 288.15 298.15 308.15 318.15
19.9 19.9 21.1 21.5 19.2 19.5 18.7 19.8
4.15 4.19 4.02 3.67 5.02 4.78 4.62 4.62
0.400 0.396 0.413 0.453 0.331 0.348 0.359 0.359
respectively. After introducing eq 7 into eq 3, the following equation results:
∆s ) ss - Γ+s+ - Γ-s) -(∂γ/∂T)P,conc
(8)
Application of eq 8 enables us to evaluate ∆s from Figures 10 and 11. The γ-T curves seem to be linear over the observed temperature range. The γ values of C8-TAC
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Figure 9. Changes of surface pressure with molecular surface area at 288.15 K.
Figure 10. Changes of surface tension with temperature for C8-TAC.
definitely decrease with increasing temperature at lower concentrations, while they increase with increasing temperature at higher concentrations. On the other hand, the γ values of C10-TAC definitely increase with increasing temperature over all concentration ranges. The ∆sconcentration relations thus obtained are shown in Figure 12. The ∆s values of both surfactants decrease with increasing concentration. The entropy change for the surface adsorption decreased from positive to negative values with increasing concentration for the shorter homologue, while the longer homologue had only the negative change with much steeper decrease with increasing concentration. Such steeper decrease was not observed for the corresponding hydrocarbon surfactant with the same headgroup.19 As is clear from Figures 7 and 8, the entropy decrease is closely connected with increase in surface adsorption of the amphiphiles. In other words, (19) Aratono, M.; Okamoto, T.; Motomura, K. Bull. Chem. Soc. Jpn. 1981, 60, 2361.
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Figure 11. Changes of surface tension with temperature for C10-TAC.
Figure 12. Entropy changes for surface adsorption with concentration.
the surface adsorption from the bulk solution brings about larger negative entropy change. Larger negative entropy change was also observed for micellization of fluorinated surfactant.12 Therefore, larger negative entropy change due to molecular gathering escaping from aqueous bulk is characteristic of fluorinated amphiphiles. This means that the surface formed by both C8-TAC and C10-TAC surfactants becomes more ordered or more restricted in molecular level with increasing adsorption. In addition, the ordered aggregation at the surface layer becomes more prominent over a small concentration range for longer homologues than for shorter homologues. Acknowledgment. This work is supported by a Grantin-Aid for Scientific Research No. 10554040 from the Ministry of Education, Science, and Culture of Japan, which is gratefully acknowledged. LA000749N
