Langmuir 1991, 7,51-55
51
Surface Tensions and Critical Micelle Concentrations in Mixed Solutions of Potassium Perfluorononanoylalaninate and Potassium Acylalaninates Shigeyoshi Miyagishi,* Masanobu Higashide, Tsuyoshi Asakawa, and Morie Nishida Department of Chemistry and Chemical Engineering, Faculty of Technology, Kanazawa University, Kodatsuno, Kanazawa 920, Japan Received September 11, 1989.I n Final Form: July 2, 1990 Surface tensions of mixed solutions of potassium perfluorononanoylalaninate (PfnA1a)-potassium lauroylalaninate (LauAla) and mixed solutions of PfnAla-potassium myristoylalaninate (MyrAla) were measured. A curve of critical micelle Concentration (cmc) vs composition for each system agreed with the curve calculated by assuming perfect phase separation in the micelles. That is, two types of micelles were formed and each of them was composed of only one type of surfactant. In the systems richer in PfnAla as compared with the system in which the cmc exhibited a maximum value, the surface tensions at the cmcs were equal to that of PfnAla. In these systems, adsorbed films at the cmcs were composed of only PfnAla. Such a tendency was more remarkable in the PfnAla-LauAla systems than in the PfnAla-MyrAla systems.
Introduction Since long-chain perfluorocarbons are only partly miscible with long hydrocarbons in some cases,' aqueous mixed solutions of hydrocarbons and fluorocarbon surfactants sometimes give two types of micelles, one rich in a fluorocarbon surfactant and the other rich in a hydrocarbon ~urfactant.~*3 The immiscibility of perfluorocarbon and hydrocarbon surfactants had been pointed out by Mukerjee and Yang.2 The condition of the immiscibility was then discussed in more detail by Shinoda and N ~ m u r a . ~ The group of Miyagishi et al. has investigated the composition and concentration of the two types of micelles, the immiscible region, its temperature dependence, the conformation of the hydrocarbon chain in the micelle, etc."' In the mixed solutions of these surfactants, there is a maximum on the curve for the first critical micelle concentration (cmc) vs total composition. The micelles rich in the hydrocarbon surfactant are formed a t the first cmc in the region of total composition richer in hydrocarbon surfactant when compared to the composition having the maximum cmc. The micelles rich in the perfluorocarbon surfactant are formed a t the first cmc in the compositionsricher in the perfluorocarbon surfactant than the composition having the maximum cmc. The micelles rich in the other surfactant, as compared to the surfactant formed a t the first cmc, begin to be formed when the total concentration of the surfactant mixture reaches the second cmc with two types of micelles existing in the concentrations above the second cmc. The total composition region in which the second cmcs are observed becomes wider as the total concentration increases. The region of the micelle phase separation in the micelle composition becomes narrower with increasing temperature and disappears at the upper critical temperature (for example, the upper (1) Hildebrand,J. H.;Prausnitz,J. M.;Scott,R. L. RegularandRelated Solutions; Van Nostrand Reinhold New York, 1970; Chapter 10. (2) Mukerjee, P.; Yang, A. Y. S. J. Phys. Chem. 1976, BO, 1388. (3) Shinoda, K.; Nomura, T. J . Phys. Chem. 1980, 84, 365. (4) Aaakawa, T.; Miyagishi, S.;Nishida, M. J. Colloid Interface Sci. 1985, 104, 279. (5) Asakawa, T.;Johten, K.; Miyagishi,S.;Nishida,M. Langmuir 1985, I, 347. (6) Asnkawa, T.; Miyagishi, S.;Nishida, M. Langmuir 1987, 3, 821. (7) Asakawa, T.;Johten, K.; Miyagishi,S.;Nishida, M. Langmuir 1988, 4, 136.
0743-7463/91/2407-0051$02.50/0
critical temperature is expected to be about 60 "C in the system lithium perfluorooctanesulfonate-lithium tetrade~ylsulfate~). In the systenms so far investigated, each micelle always contained a small amount of the other component in addition to the major component (partially miscible, that is, w12 was less than m for the regular solution theory). In the present paper, the w12 systems that equal almost m , where two surfactants are virtually immiscible in the micelles, are reported. Immiscibility is important as an oil repellent and fire extinguishingagent property. This is especially important since these agents play their role on the liquid's surface; therefore, quantitative estimation of these compositions in an adsorbed film is important to study. In the present study, surface tensions were measured as a function of surfactant concentration, since the compositions of the adsorbed films could be estimated from the surface tension vs concentration curves.*-lO In the present investigation, both perfluorocarbon and hydrocarbon surfactants, each of which had the same hydrophilic groups, were used to rule out any effects due to differences in hydrophilic groups.
Experimental Section Materials. Lauroylalanine and myristoylalanine were synthesized and purified by a previously described procedure.llJ2 Perfluorononanoylalaninewas synthesized by the reaction of methylperfluorononanoate with alanine in methanol in the presence of sodium hydroxide and then recrystallized from an ethanol-chloroform mixture (mp 180 "C). The reaction temperature was kept at 0 "C to prevent decompositionof the product into perfluorononanoic acid. Methylperfluorononanoate was prepared from perfluorononanoic acid (Asahi Glass Co., AG119)and purified by fractional distillation. Each derivativewas dissolved in potassium hydroxide solution containing 90 mM potassium chloride. The concentration of potassium hydroxide was 10 mM more than the concentration of the derivative. Procedure. Surface tension was measured at 30 "C by a drop weight method. A computer program to determine surface (8).Aratono,M.; Uryu, S.;Hayashi, Y.; Motomura, K.; Matuura, R. J. Collocd Interface Sci. 1982, 93, 162. (9) Ikeda, S. Bull. Chem. SOC.J p n . 1977,50, 1403. (10) Funasaki, N.; Hasa, S. J. Phys. Chem. 1980, 84, 736. (11) Miyagishi, S.; Nishida, M. J. Colloid Interface Sci. 1978,65,380. (12) Miyagishi, S.;Ishibai, Y.; Asakawa, T.; Nishida, M. J. Colloid Interface Sci. 1985, 103, 164.
0 1991 American Chemical Society
Miyagishi et al.
52 Langmuir, Vol. 7,No. 1, 1991 80 y2
4 CI
c
\ 3
V
I ’ W
I 0
IC
1
L
:
-5
-6
-4
-3
-2
-1
1
y 2
Figure 4. Comparison of experimental data of cmcs ( 0 )with
calculated values. Solid line and dotted line correspond to w12 = m and 2, respectively, in the LauAla-PfnAla mixed systems.
c
log
I 0.5
Figure 1. Surface tensions in LauAla-PfnAla mixed systems. 80 y2
1
1
1
0.65 0.50
0.40 0.30 0.20
Io
0 10
L
I
:
-5
-6
-3
-4
-2
-1
Figure 2. Surface tensions in MyrAla-PfnAla mixed systems.
,
ZI 0
0.5
1
Figure 5. Comparison of experimental data of cmcs ( 0 )with
calculated values. Solid line and dotted line correspond to w1z = m and 2, respectively, in the MyrAla-PfnAla mixed systems.
c
log
0.5 y2
1 1
y 2
Figure 3. Surface tensions at cmcs: ( 0 )LauAla-PfnAla systems; ( 0 )MyrAla-PfnAla systems.
tension was kindly provided by Ramesh Babu.I3 The time required t o attain equilibrium was about 4 h in a dilute solution of the perfluorocarbon surfactant. However, the time was only about 1h in the more concentrated solutions and in the solutions containing hydrocarbon surfactants. The experimental error was 0.04 mN/m.
Results and Discussion Surface Tension and Critical Micelle Concentration (cmc). The experimental data at different mole fractions of a hydrocarbon surfactant (y2) in each system are given in Figures 1-5. The surface tensions in the potassium perfluorononanoylalaninate (PfnA1a)-potas(13)Ramesh Babu, S.J. Colloid Interface Sci. 1987, 115, 551.
sium lauroylalaninate (LauAla)systems are plotted against the logarithm of the concentration of the surfactant mixture (C)in Figure 1. The curves tend to favor largely the PfnAlaside; that is,this figure indicates that the surface tension of the mixed systems is significantly depressed by a small addition of PfnAla. Finally the surface tension reached 15mN/m. This value is one of the lowest surface tensions of the perfluorocarbon surfactant solutions. The break points on the curves correspond to the cmcs. Some systems rich in LauAla had two break points on their corresponding curves. As can been seen from a later discussion, the micelles composed of LauAla are present in the concentrations between the first and second break points and in the concentration region above the second break point, where the micelles composed of LauAla and the micelles composed of PfnAla coexist, respectively. In the system with one break point, the micelles of PfnAla begin forming at the cmc. As shown in Figure 3, each system in the range of y2 < 0.85 had one break point, and the surface tension at the break point (cmc) was equal to the value at the cmc of pure PfnAla. When y2 > 0.85 the surface tension at the break point increased rapidly. A more detailed discussion concerningthe adsorption state is given in the later section. In the systems so far reported by several investigators, the fluorocarbon surfactants were partially miscible with the hydrocarbon surfactants in the micelles.24 Most of the systems discussed were based on a regular solution approximation ~ i e w p o i n t .The ~ same approximation was used in our experiments. The concentrations of the first component ( C d and the second component (C2,) are expressed as follows (see eqs 4 to 6) in the presence of added salt (C,)and when In C1 = -K,, In C, + constant
Surface Tensions in Mixed Solutions and In C2 = -Kgz In C,
Langmuir, Vol. 7, No. 1, 1991 53
+ constant
cmc = C,,
+ C,,
(3)
where Xi is the mole fraction of the i component in a micelle and Cj is the cmc in a system composed of the i component. The experimental values of the cmc were always much larger than the values calculated in the case where the interaction parameter w12 equals 2 and were larger than the values for w12 that equaled 9. The latter value suggested virtual nonmixing of the two surfactants in the micelles (XI > 0.999). On the other hand, for the case of absolutely no mixing of the micelles, their formula can be modified. The chemical potential of the first component in the water phase is p1 = pIo
+ RT In C,,
(4) and the chemical potential in the mixed micelle phase is
+
klm = plmo RT In f i x ,
+ RT K,, In (20007ru2/DRTC’) (5)
In the system composed of only the first component, the chemical potential in a micelle is
+
= plmo RTK,, In (20007ra12/DRTC)
(6) When a perfect phase separation happens in the micelle systems (w12 = m), each micelle consists of only a single component and the chemical potential in the micelle is given by p,,
+ RTK,, In (2000m12/DRTC’)
plm = plmo
(7)
Since p1 = plm, combining eqs 4 and 7 gives
+
In C,, = In (cmc y,) = (plmo- p l o ) / R T K,,In
(20007ra12/DRTC‘)( 8 ) In a micelle solution containing only the first component In C, = (plmo- p l o ) / R T+ K,, In (2000m~,~/DRTC) (9) As C = C, C1 and C’ = C, + cmc, we obtain eq 10 from eqs 8 and 9
+
c, e,+ e,
)
c, c,+ c,
)
cmc=-(y1 C,+ cmc
KI,
Similarly cmc=-(y, C,+ cmc
KE,
Here y is the mole fraction of a component in a mixed system and the subscripts 1 and 2 are the perfluorocarbon and hydrocarbon surfactants, respectively. When C, >> cmc, eqs 10 and 1 1 become cmc = cmc,/y,
(12)
cmc = cmc2/yz (13) In the present case, eqs 12 and 13 can be applicable since C, is 0.1 mol/dm3 and is much larger than the cmcs. The cmcs are shown with their values calculated from eqs 12 and 13 in Figure 4. The experimental cmcs are in good agreement with the calculated values. The calculated cmcs
under the condition of critical mixing (w12 = 2 ) are also plotted. The experimental values were fairly large compared to the values of the critical mixing. These results indicate that PfnAla is virtually immiscible with LauAll in the micelles. Therefore, it may be concluded that at the cmcs, virtually pure PfnAla micelles or pure LauAla micelles are formed. This does not always mean ideal nonmixing because some curves of surface tension versus the logarithm of the surfactant concentration have a small minimum. However, this result may be the first case of a virtually perfect phase separation in micelles when taking into account the following fact: The micelle compositions determined by Motomura’s method14from Figures 4 and 5 suggested perfect nonmixing and the cmc values were larger as compared to the values in the case of w12 = 9. The surface tensions were significantly decreased in the PfnAla-potassium myristoylalaninate (MyrAla) systems compared to those in the PfnAla-LauAla systems since the surface tensions in the MyrAla solutions were more depressed than thosein the LauAla solutions. The surface tensions at the cmcs in the mole fraction range of MyrAla below 0.4 were equal to that of pure PfnAla (Figure 3). This range was smaller when compared with the PfnAlaLauAla system (0-0.79). The cmcs increased in these ranges with an increasing mole fraction (yz) of the hydrocarbon surfactant and reached a maximum in the corresponding systems, respectively. For a larger mole fraction of MyrAla, the cmcs decreased and, in addition, the second cmcs were found. As can be seen from Figure 5, the measured cmcs agreed with the calculated values of perfect nonmixing. Therefore, it is concluded that a virtually perfect phase separation in the micelles occurs in the PfnAla-MyrAla system. On the other hand, although lithium hexadecyl sulfate (LHS) has a larger hydrophobic alkyl chain than LauAla and MyrAla, the systems composed of LHS and lithium perfluorooctanesulfonate (LiFOS), which had the same perfluorocarbon chain as PfnAla, did not exhibit perfect phase separation.61’ In these systems, two types of micelles were formed but they only partially contained the other surfactant in addition to the major component. LHS and LiFOS are lithium salts of strong acids that are different from each other. In the present study all the hydrophilic parts of the surfactants are potassium salts of alanine, which is a weak acid. The perfect phase separation of micelles may result from weak acidity of the hydrophilic groups or the introduction of identical hydrophilic groups into the two surfactants. In either event, the results indicate the significance of a hydrophilic group for micellar phase separation. Adsorbed Film. The surface excess densities in the PfnAla-LauAla and -MyrAla systems are shown as a mole function of each hydrocarbon surfactant (yz) in Figures 6 and 7 . The surface excess density of PfnAla was much larger than that of LauAla. The surface excess density increased as yz decreased and approached the surface density of PfnAla. This tendency became significant as the total surfactant concentration increased. At concentrations above 0.125 mmol/dm3 and in the range of yz below 0.5,the surface excessdensities became nearly equal to that of PfnAla. This result suggests that the adsorbed film is remarkably rich in PfnAla. In order to confirm this point, the composition of the adsorbed film was calculated. Several methods have been proposed to determine the surface compo~ition.~-~O According to these methods, the mole fraction of a hydrocarbon surfactant (14) Motomura, K.; Yamanaka, M.; Aratono, M. Colloid Polym. Sci. 1984,262, 948.
Miyagishi et al.
54 Langmuir, Vol. 7, No. I , 1991
0 ~~
0
0.5
1
y2
Figure 6. Total surface densities in LauAla-PfnAla mixed systems at constant total concentration: (1)0.335; (2)0.123;(3) 0.0454; (4) 0.0249; (5) 0.0167 mmol/dm3.
5
L N
€ 3
\
I
- 1
L 0
0.5
1
y2
Figure 7. Total surface densities in MyrAla-PfnAla mixed systems at constant total concentration: (1)0.335;(2) 0.0827; (3) 0.0304; (4) 0.0167; (5) 0.00917. 1 1
N
X
I
0.5 I I
I
I I
I
/
/
/
0
0.5
1
y2
Figure 8. Dependence of surface composition ( x 2 ) on bulk composition (yz)in LauAla-PfnAla mixed systems at constant surface tension: (1) r = 68;(2) 56; (3)40 mN/m; (4)r at cmc. in an adsorbed film (xz) is given by eq 14 in the presence of a large excess of added salt by neglecting the surface excess density of the co-ion and the concentration variation of the counterion. The calculated results are shown in x2
0.5
1
y 2
~
= Y z - CYIY2/C)(C/Y2)T,P,r
(14)
Figure 8. As was expected, the adsorbed film was remarkably rich in PfnAla. Addition of a small amount of PfnAla resulted in a reduction of the LauAla fraction in the adsorbed film. This result does not always mean that the adsorption of PfnAla inhibits adsorption of
Figure 9. Dependence of surface composition ( X Z ) on bulk composition Cy2) in MyrAla-PfnAla mixed systems at constant surface tension: (1) r = 68; (2) 56; (3) 40 mN/m; (4)r at cmc. LauAla. As can be seen from Figure 6, the total surface excess density increased with the addition of PfnAla. However, in the systems with higher concentrations and/ or higher contents of PfnAla, increasing adsorption of PfnAla prevented the adsorption of LauAla. In the systems where the micelles of PfnAla were formed a t the first cmcs, all the surfaces were occupied by only PfnAla a t the cmcs. The surface tensions a t the cmcs in these systems were very close or quite similar to that of PfnAla (Figure 3). The surface excess density of MyrAla was larger than that of LauAla because of the stronger surface activity of the former. Therefore, the surface excess density of MyrAla did not significantly differ from that of PfnAla. The density, however, was less. The surface excess densities in the mixed systems of PfnAla-MyrAla are plotted as a function of y2 in Figure 7. A decrease in the MyrAla content resulted in a nearly linear increase in the surface excess density a t low concentrations. The surface excess densities a t concentrations near the cmc were similar to that of PfnAla in the range of y2 between 0 and 0.5. The compositions of the adsorbed film are shown in Figure 9. The adsorbed films were rich in PfnAla. However, the hydrocarbon surfactant was considerably richer in these systems than in the PfnAla-LauAla systems. Athough PfnAla might interact more repulsively with MyrAla than with LauAla, apparently PfnAla was more miscible with MyrAla in the adsorbed film than with LauAla. That is, the apparent miscible region of yz was from 0.4 to 1.0 in the PfnAla-MyrAla systems and from 0.79 to 1.0 in the PfnAla-LauAla systems. In any event, it may be concluded that a less surface active hydrocarbon surfactant is better in order to cover the water surface with perfluorocarbon surfactant molecules by a smaller addition of a perfluorocarbon surfactant. On the other hand, x~ values in Figures 8 and 9 are apparent values and do not always suggest that the two surfactants are miscible in the absorbed films in a range of larger yz. The values in a region of higher hydrocarbon surfactant content are possibly averages between two separated films (a film of PfnAla and a film of LauAla or MyrAla). However phase separation in an adsorbed film is not necessarily a condition for phase separation in micelles, because Motomura’s group pointed out that the surface and micelle compositions are similar in some cases but not in others.15 His group did not discuss the point in detail. Our results indicated that a region of phase separation in an adsorbed film increased with increasing (15)Ikeda, N.;Shiota, E.; Aratono, M.; Motomura, K. BulZ. Chem. SOC.Jpn. 1989,62,410. Matsuki, H.; Ando, N.; Aratono, M.; Motomura, K. Bull. Chem. SOC.J p n . 1989,62, 2507.
Surface Tensions in Mixed Solutions surfactant concentration. Since at (and above) cmc, equilibrium is established between the adsorbed film phase and micelle phase, composition in the adsorbed film phase is determined by that of the micelle phase. In the present case, by taking into account the fact that phase separation is observed in the micelle phase at cmc and the adsorbed film below cmc, we may conclude that phase separation occurs in the adsorbed film over all y2 values at cmc and that x2 is a value representing a mixture of the two separated phases. While the acyl amino acid surfactant can be a model of acyl peptide surfactants which are used as fire extinguishingagents, introduction of a perfluorinated acyl group may make the peptide surfactants more resistant to fire. Addition of such perfluorinated acyl peptide surfactants may result in enhancement of the action in fire extinguishing agents.
Langmuir, Vol. 7, No. 1, 1991 55
Conclusions In mixed solutions of PfnAla-LauAla and PfnAlaMyrAla, two types of micelles, each of which was composed of only one type of surfactant, were observed. PfnAla adsorbed preferentially on air-water interfaces, and especially adsorbed films were composed of only PfnAla in they2 value range of 0-0.4of LauAla and0-0.79 of MyrAla, respectively. That is, surface tensions at cmc were equal to that of PfnAla in these regions of y2. In richer regions of y2, LauAla or MyrAla existed together with PfnAla in the adsorbed films. Registry No. K(PfnAla), 130904-80-2; K(LauAla),7640241-0;K(MyrAla),130904-81-3.
