Langmuir 1992,8, 2368-2375
2368
Art i d e s Mixed Surfactant Systems of Sodium Perfluorooctanoate with Nonionic, Zwitterionic, and Cationic Hydrocarbon Surfactants W. Guo, E. K. Guzman, S. D. Heavin, Z. Li, B. M. Fung,’ and S. D. Christian Department of Chemistry and Institute for Applied Surfactant Research, University of Oklahoma, Norman, Oklahoma 73019-0370 Received May 4, 1992. In Final Form: July 6, 1992
Mixed surfactant systemsof sodium perfluorooctanoate(SPFO) with N-triethoxylatedheptanylamide (HEA8-3),with 3-(decyldimethy1ammonino)-l-propanesulfonate (DEDIAP),and with octyltrimethylammonium bromide (OCTAB) have been studied by the use of surface tension and ‘9F and ‘HNMR. The results show that only one type of mixed micelle is formed for the mixed system of SPFO/HEA8-3as well as the system of SPFO/DEDIAP. Pseudo phase diagrams for both mixed systems are calculated from the dependenceof the criticalmicelleconcentzations(cmc’s)on the mole fractionof the fluorocarboncomponent, and they show large negative deviation from ideal behavior. For the mixed systems of SPFO/OCTAB, the cmc’s and surface tensions are reduced tremendously upon mixing, and the NMR studies combined with centrifugation results suggest the mixed surfactants coacervate to form large aggregates. Introduction It is well-known that the micellizationof two surfactants with identical head groups and different alkyl chains usually exhibits ideal mixing and the micellization of mixtures of fluorocarbon surfactants and hydrocarbon surfactants with similarly charged head groups exhibits positive deviation from ideality.”’ The latter is due to the mutual phobicity between the fluorocarbon and hydrocarbon chains. This positive deviation has also been observed in mixtures of liquid fluorocarbons and liquid hydrocarbons8 and mixtures of a nonionic fluorocarbon surfactant and a nonionic hydrocarbon s ~ r f a c t a n t .On ~ the other hand, the micellization of mixtures of a fluorocarbon surfactant and a hydrocarbon surfactant with differently charged head groups usually shows negative deviation from ideal mixing behavior.1° The major factor responsible for the negative deviation is the electrostatic stabilization effect which overcomes the phobicity between the chains. A comprehensive review of mixed hydrocarbon-fluorocarbon surfactant systemshas been given by Funasaki.ll Most reports in the literature on the mixing behavior of fluorocarbon (FC) surfactants and hydrocarbon (HC) ~
~
(1)Lange, H. Kolloid Z.1953, 131, 96. (2)Clint, J. H. J. Chem. SOC., Faraday Trans 1 1976, 71, 1327. (3)Funasaki, N. J. Colloid Interface Sci. 1978,67,384. Funasaki, N.; Hada, S. J. Phys. Chem. 1979,83, 2471. (4)Mukerjee, P.; Mysels, K. J. ACS Symp. Ser. 1975, No. 9, 239. Mukerjee, P.; Yang, A. Y. S. J.Phys. Chem. 1976,80, 1388. (5)Shinoda, K.;Nomura, T. J. Phys. Chem. 1980,84, 365. (6)Mysels, K.J. J. Colloid Interface Sci. 1978, 66, 331. (7)Funasaki, N.; Hada, S. J.Phys. Chem. 1980,84,736.Funasaki, N.; Hada, S. J. Colloid Interface Sci. 1980, 73, 425. (8)Hildebrand, J. H.; Scott, R. L. Regular Solutions; Prentice-Hall: Englewood Cliffs, NJ, 1962;Chapter 8. Hildebrand, J. H.; Scott, R. L. The Solubility of Nonelectrolytes;Dower Publications: New York, 1964; p 136. Hildebrand, J. H.; Scott, R. L. Regular and Related Solutions; Van Nostrand Reinhold New York, 1970; p 154. (9)Funasaki, N.;Hada, S. J.Phys. Chem. 1983,87, 342. (10)Rubingh, D. N. Solution Chemistry of Surfactants; K. L., Ed.; Plenum Press: New York, 1979;Vol. 1, p 337. (11)Funasaki, N. In Mixed Surfactant Systems; eds. Ogino, K., Abe, M., Eds.; Marcel Dekker: New York, in press.
0743-7463/92/2408-2368$03.00/0
surfactants have been mainly concerned with anionicanionic mixtures. There have been a few investigations of anionic-nonionic mixtures such as sodium perfluorooctanoate (SPFO) or lithium perfluorooctanesulfonate (LiFOS) mixed with polyoxyethylenated glycol alkyl esters12J3or polyoxyethylenated alkylphen01s.l~ These nonionic surfactants usually have large head groups and long (OCHzCH3, chains ( n = 9-20). More recently, the mixture of SPFO with N-alkanoyl-N-methylglucamine, which has a shorter hydrophilic chain, has been studied by Sugihara et al.15 For most of these anionic/nonionic mixed surfactant systems, the dependence of the critical micelle concentration (cmc) on the mixing ratio shows negative deviation from ideal mixing behavior which has been commonly ascribed to the electrostatic stabilization due to interactions between the head groups. Zwitterionic surfactants are also called amphoteric surfactanta. Betaine (R~RZR~N+R’COZ-, where R1, Rz,R3, and R’ are hydrocarbon chains, with R1 being the longest chain) and sulfobetaine (RIRZR~N+R’SO~-) are the most commonly used zwitterionic surfactants. Negative deviation from ideal mixing behavior in the mixtures of betaine or sulfobetaine with various other hydrocarbon surfactants has been studied by Rosen and Zhu.’6 Mixed micelles were formed in these mixtures, and the interaction of betaine with the second surfactant increases in the order nonionic < cationic cmc) (11) phaee than in the micelle phase. This is similar to the phase diagram reported by Sugihara et al. for other where Cmo is the monomer concentration, 6a , is the mixtures of anionic FC surfactants and nonionic HC observed chemical shift at concentration c, is the surfactants.1S Because the surfactant molecules are in monomer chemical shift which is directly obtained from rapid exchange between monomer and micellar state, there the experimental chemical shift below the cmc, and 6~ is is no unambiguous technique readily availableto determine the micelle chemical shift which can be obtained by the experimental values of XFMdirectly. However, the extrapolating the 6~ - l / c curve to c = ( l / c = 0). If the predicted cmc curve (Figure 1,solid line) obtained by using change of monomer concentration above the cmc is eqs 3-8 well agrees with the experimental cmc data, which considered to be negligible and Cmo = C,, where C, is the is a good indication that this model is applicable to the cmc value, eq 11 becomes mixed systems under investigation. By using the values of YF and YH obtained from this ,6 = 6,i + (6mo- 6,,)Cm/C (12) modeling, the activity of each component within the micelle When aok is plotted against l/c, it is independent of can be calculated concentration for c < C,. For c > Cm, Bobchanges linearly with l/c. All the plots of 6~ versus l / c and 6~ versus l / c aH = Y H ( ~ -X F M ) (9) for SPFO/HEA8-3 system and other mixed FC/HC QF YPXFM (10) surfactant systems exhibit these characteristics, and the details are discussed elsewhere.30 According to eq 12, the The results are plotted as a dashed curve in Figure 2, and intersection point of the two linear segmentsyields l/Cm, the activities for ideal mixing (YH = YF = 1) are plotted and an extrapolation of the linear segment at high as a solid line in the same graph. It is seen that both Q H concentrations to l / c = 0 yields ami. Values of Cm thus and OF show significant deviation from ideal behavior. obtained are plotted against XFin Figure 1, and values of However, up in a HC-rich region ( Q H being close to unity) (ami - amo) are plotted in Figure 3. Since the terminal CF3 is much smaller than U H in the FC-rich region (QF being and CH3 groups are most affected by the interior of close to unity). The main reason for these mixing micelles, the ar-CFz and a-CHz groups by the head group interactions, only these molecular segments will be con(36)Yoda, K.;Tamori, K.;Eeumi, K.;Meguro, K.J.Colloid Interface Sci. 1989, 131, 282. sidered. The FC chain is lees polarizable than water; its
I
---
--.
~
Guo et ai.
2372 Langmuir, Vol. 8,No. 10,1992
3 a0
-0.12 02
OA
(16
0s
OD
1.0
02
QA
QB
08
1.0
Figure 3. Values of (b,i - bmo) for (a) (e)CF3, (e)(u-CF~, (b) (A)CHs, and (e)(r-CH2groups plotted as a function of the total mole fraction of FC component in the SPFO/HEA8-3 mixed system.
effect on the neighboring nuclei is to cause their chemical shift to decrease. For the CF3 and the CH3 groups, (6mj -),6 is always negative; the monotonic decrease of (6mi - 6mo) with increasing XF (Figure 3) corresponds to an increasingmole fraction of the FC componentin the mixed micelles. Sincethe head group of the PFO-ion is negatively charged, its presence increases the shielding of the neighboring a-CH2 group in HEA8-3. Thus, the dependence of (ami - 6mJ for the a-CH2 group on X F is similar to that for the CH3 group (Figure 3b). On the other hand, the changes in the chemical shift of the a-CF2 group show a rather different characteristic; (ami - 6mo) for the a-CH2 group is positive for X F < 0.6 and negative for X F > 0.6 (Figure 3a). This indicates that for small XF, the environment for the a-CF2 groups in the mixed micelle is less shielded compared with the monomer phase, because when the mixed micelle is HC-rich, the anionic head groups in the FC surfactant are separated by the nonionic counterparts in the HC surfactants. Mixed System of Anionic SPFO/Zwittarionic DEDIAP. The cmc values of this system with different mole ) determined by surfacetension fractionsof SPFO ( X F were and NMR measurements, and the results are shown in Figure 4. The negative deviation from ideality is more pronounced compared with the mixed system SPFO/ HEA8-3. This is because there is a positive charge in the tetraalkylammonium group of the z~tterionicsurfactant, DEDIAP, attached to the hydrocarbon chain. Therefore, the interaction between the two components in the mixed micelle is dominated by a strong Coulombic attraction between the carboxylate head group in SPFO and the positively-charged tetraalkylammonium group in DEDI-
AP.
The theoretical treatment applied to this system is the same as that discussed above; eqs 1-8 are used, and IH is taken to be unity again, assuming that the effect of ionic strength on the cmc of the DEDIAP is negligible, The calculated values of cmcT are plotted against X F (solid line) and XFM(dashed line) as shown in Figure 4, with j3 = -9.5 and 6 = 14.7from least-squares calculations. The more negative value of 6 compared to the SPFO/HEA8-3 mixture corresponds to increased attractive interactions between the two types of surfactant head groups upon micellization. The increase of the positive 6 corresponds
CI
m
Y
m
02
06
1.0
0
XF Figure 4. Values of cmc for the SPFO/DEDIAP mixed system obtained from different experiments: 0,the cmc obtained from surface tension measurement; +, the cmc obtained from lH NMR measurement; X, the cmc obtained from 19FNMR measurement. The solid curve is the calculated cmc plotted against the X F ,the dashed curve is the calculated cmc plotted against the XFM.
to larger mutual phobicity and difference in partial molar volume between the FC/HC chains, probably because the hydrocarbon chain in DEDIAP is longer than that in HEA8-3. However, the effect of j3 is more important so that the overall interaction free energy (expressedby the component activities as discussed later) is more favorable for the formation of mixed micelles. There are two loops in the pseudo phase diagram. The loop on the right-hand side is similar to that in Figure 1, indicating that the fluorocarbon component is richer in the monomer phase, relative to the micelle phase, for large values of X F . The deviation ( X F- XFM)is larger than that for the SPFO/ HEA8-3 mixed system because there is now a stronger Coulombic attraction between the head groups of the two components. The loop on the left-hand side (XF< -0.3, HC-rich range) of Figure 4 is much more prominent than that in Figure 1. It corresponds to a significant deviation from ideal mixing behavior, indicating that the micelle phase is richer in SPFO than the monomer phase. In this region, the surface of the hydrocarbon-rich micelle is mainly composed of DEDIAP, which has a zwitterionic
Mixed Surfactant Systems
Langmuir, Vol. 8, No. 10, 1992 2373
00-
4.m 4.04-1
1
4.06-
4.w -
1 a0
------l 4.10
02
0.4
0.6
a8
OB
1.0
OA
02
a6
08
1.0
XF Figure 5. Values of (S,i - Smo) for (a) (A)CF3, (+) a-CF2, (b) (A)CH3, and (+) a-CH2 groups plotted as a function of the total mole fraction of FC component in the SPFO/HEA8-3 mixed system. head group. Although it is electrically neutral as a whole, there is still Coulombic repulsion between the tetraalkylammonium groups because they are part of the hydrophilic group and may be packed closeto each other on the micelle surface. Therefore, comparedto the HEA8-3-richmicelle, the DEDIAP-rich micelle is energetically less favorable. The insertion of SPFO into these micelles would create a Coulombic attraction between the carboxylate group and the tetraalkylammonium group. Therefore, in a DEDIAP-rich region, the SPFO component is enriched in the micellar phase compared to the monomer phase due to its tendency to the stabilize the host micelle. The activities of both components are calculated from eqs 9 and 10 and plotted in the form of U F versm U H in Figure 2 as a dotted line. The plot is similar to that for the SPFOIHEAS-3 mixed system, but the negative deviation is much more pronounced due to favorable Coulombic interaction between the head groups. The values of (ami - amo) for the a-CF2 and CF3 groups in the mixed micelles are plotted as a function of XFin Figure 5a; those for the a-CH2 and CH3 groups are plotted in Figure 5b. The results show a trend similar to that for the case of the SPFO/HEA8-3 mixed system (Figure 3). Basically, the values of (ami - am0) for the CF3 and CH3 groups are fairly similar in the two mixed systems, indicating the environments in the interior of the mixed micellesare similar. By comparisonof the phase diagrams, it is noticed that for the same XFvalue in the FC-rich region, the mole fraction of the FC componentin the mixed micelle is smaller in the SPFO/DEDIAP mixed system. This corresponds well with the less negative value of (ami - am0) for the CF3 group. On the other hand, for XFI0.9, (ami - 6mo) for the a-CF2 group is always positive (nearly zero at XF= 0.9) and numerically larger than that in the SPFO/HEA8-3mixed system. This is due to the stronger Coulombicinteraction between the carboxylategroup and the tetraalkylammonium group. The relatively small change in (ami - 6), for the a-CHz group might be related to the fact that the head group of DEDIAP is very bulky and prevents close interaction between the a-CH2 group and the neighboring environment. Aggregationin the Mixed System of Anionic SPFO/ Cationic OCTAB Mixed Systems. Because of the large variation in the cmc values, for clarity the cmc's are plotted
h
$
1
1000
v
E
m
i
E
I
i
m m
c
9
10.0 '
" 03 .
'
'0.4 '
D
'
0.6 '
m
0.8
1.0
XF
Figure 6. Values of cmc for the SPFO/OCTAB mixed system obtained from surface tension experiments.
in a logarithmic scale in Figure 6. The numerical values are given elsewhere.30 In the range of 0.25 I XF I0.80, the cmc is between 1.2 and 1.7 mM, and Tcmc is about 14-15 dyn/cm. Such low values of cmc and surface tension are close to those of nonionic fluorocarbon surfactants. This is due to a large reduction of the Coulombicrepulsion between ionic head groups of the same charge by the insertion of oppositely charged head groups. Therefore, the coverage of surfactant molecules at the solution-air interface is much larger than that for either pure component, and the surface absorption reaches saturation at very low concentration (near the cmc).20 Similarly, it is very likely that the mixed micelles formed in this range compriseequimolar mixtures of anionic SPFO and cationic OCTAB so as to minimize the Coulombic repulsion between the head groups. Beyond this range of mixing ratio, the cmc is strongly dependent on the XF.Because the mixed surfactant solution is richer in OCTAB for XF < 0.25 and richer in SPFO for XF> 0.80,the dependence of the cmc on XFis rather similar to the case for the SPFO/ DEDIAP mixed systems in similar ranges of XF as displayed in Figure 4. Unfortunately, numerical calculations could not be made because of the difficulty in evaluating the activity coefficient of either component. A t concentrations much higher than the cmc, the mixed
2374 Langmuir, Vol. 8,No.10, 1992
Guo et al.
Table I. Chemical Shifts vs Concentration in the Mixed Systems of SPFO-OCTAB chemical shift (ppm) concentration (mmol/L) monomer aggregate 0.3 0.53 0.79 0.98 1.97 3.48 4.30 4.53 5.51
h
-81.575 -81.552 -81.570 -81.571 -81.569 -81.548 -81.637 -81.642 -81.733 -81.723
6.00
I ~ " ' I J " ' 1 " " I " ' ' I ' ' ~ ~ I ' ~ ~
80
SI
im
iio
la0
PP
Figure 7. leFspectra of SPFO/OCTAB mixed systems with Xp = 0.50 for the total concentration of (a) 1.6, (b) 2.0, (c) 4.1, (d) 7.1, (e) 8.8, (f) 9.2, and (g) 11.2 mM.
f
f e L Jc. LkL b
i
JLJd
a --._
_.____-
iJ"U
- - ~-----~--I-_-. ~ - -------TT----T---r---85 -90 .96 .lo0 .lo5 -110 -115 -120
A
-Ji----A
?ry3 aCH8
L juI d
L--
"
PI"
Figure 8. leFspectra of SPFO in SPFO/OCTAB mixed systems with (a) Xp 0.8'5 mM, (b) Xp 0.8,lO mM, (c) Xp = 0.5,5 mM, (d) Xp = 0.5, 10 mM, (e) XF = 0.25, 5 mM, and (0 XF= 0.25, 10 mM.
system of SPFO/OCTAB is further complicated by coacervation of the mixed surfactants,which is evidenced by changes in the spectral characteristics in the lgFNMR. The l9FNMR spectra of mixtures of Xp = 0.5 at different concentrations are shown in Figure 7. For comparison, the l9Fspectra of mixtures at 5 and 10 mM for XF= 0.25, 0.5, and 0.80 are shown in Figure 8. For all Xp values, there ia a small concentrationrange above the cmc in which the lgFspectra exhibit similar characteristics as those in normal micellar solutions (Figure 8). The boundary of this concentration range is about 4-5 times the cmc. It ia likely that normal-sizemicellesexist in this concentration range, similarto the SDS/OCTABmixed systemsreported
-83.596 -83.673 -83.674 -83.614 -83.607
3.2
B-CHZ
2.8
2.4
2.0
1.6
1.2
0 8 PPm
Figure 9. lH spectra of OCTAB in SPFO/OCTAB mixed system with (a) pure OCTAB, below cmc, (b) XF = 0.25,5 mM, (c) Xp = 0.25, 10 mM, (d) Xp = 0.5, 5 mM, (e) X F = 0.5, 10 mM, and (f) XF = 0.80, 5 mM.
by Barker et al.22When the totalsurfactant concentration is higher than the boundary value, all peaks in the 19F spectra are broadened abruptly. For XFI0.5, and a total surfactant concentration of 10 mM, the signal of the CF3 group shows two separate peaks, the chemical shifts of which do not change significantly with concentration (Table I). The peak with a less negative chemical shift is assigned to be the signal of SPFO in the monomer. The second peak is more shielded and it is attributed to SPFO in an aggregated state. By the w e of line-shapeanaly~is,3~ the average mean lifetime of the surfactant molecule in such a mixed system is found to be (5-7)X s.% Since the exchange rate between monomers and normal-size micelles is very fast, with a mean lifetime lo+ to we suggestthat the significant increase in the mean lifetime is mainly cawed by the existence of larger and more ordered aggregates,or possibly a separate phase. The CF2 groups do not show a distinct second set of peaks, probably because they are too broad and overlap too extensively to be distinguishable. The lH spectra of the SPFO/OCTAB mixtures are shown in Figure 9, spectra b-f. The peaks are also much broader than those of normal micellar solutions, especially for mixtures with higher values of Xp and higher concentrations. Since the 'H spectrum at 5 (37) Williams, K. C.; Brown, T.L. J. Am. Chem. SOC.1966,88,4134. (38) Guzman, E. M.S.Theeia, University of Oklahoma,1989. (39) Anianseon, E. A.; W d ; S. N.; Almeqen, M.; Hoffmann, H.;
Kielmann, J.; Ulbricht, W.;Zana, R.; Lang, J.; Tondre,C. J. Phye. Chem. 1976, 80, 905.
Langmuir, Vol. 8, No. 10, 1992 2375
Mixed Surfactant Systems
J----JK JL-dJYJL-JK JL-JYl
did not show any appreciablechange in the peak intensities or chemical shiftafter centrifugation and no sedimentation was observed. These results suggest that in the present case the aggregatesformed in the SPFO/OCTABmixtures may be much larger than normal micelles and exist as a separate phase. After centrifugation at very high force (14000g),most of the large aggregates sedimented from the aqueous phase and the lgFspectra became similar to that of a normal micellar solution. This situation is similar to that of a nonionic FC surfactant,which also forms large aggregates that can be separated by sedimentation.=
Conclusion When the anionic fluorocarbon surfactant (SPFO) is mixed with a nonionic hydrocarbon surfactant (HEA8-3) or with a zwitterionic hydrocarbonsurfactant (DEDIAP), mixed micelles containing both types of surfactants are formed. Both mixed systems shownegative deviation from ideality. Pseudo phase diagrams have been calculated from the cmc dependence on the mole fraction of the FC component by the use of a modified regular solution theory combined with the phase separation model. Changes in the 19Fchemical shifts of the FC component and the 'H chemical shifts of the HC component are consistent with features of the phase diagram. For the anionic SPFO/ cationic OCTAB mixed system, the cmc is reduced tremendously upon mixing. In addition to the normal mixed micelles, large aggregates appear at higher surfactant concentrations. The exchange rate between the monomer state and the aggregate state is slow, and the aggregates can be separated by centrifugation. This indicates that the aggregatesin the mixed system probably form a separate phase rather than normal micelles.
JL.X
Figure 10. leFspectra of SPFO in SPFO/OCTAB mixed systems with Xp = 0.5,lO m M after centrifugation,with the centrifugal force being (a) Og, (b)19OOg, (c) 4500g, (d) 7700g, (e) 11600g, (0 13300g, and (g) 14000g. mM for XF = 0.8 is already too broad to resolve, the spectrum for 10 mM is not shown. The unusually broad peaks are indicative of slow exchange. To further examine the formation of aggregates, several solutions with XF = 0.5 and a concentration of 10 m M were centrifuged for 15 min at different speeds, and the l9F NMR spectra of the supernatant were recorded after the centrifugation (Figure 10). The results show that the intensity of the aggregatesignal decreaseswith the increase of the centrifugal force. At the highest centrifugal force used, the CFa peak for the aggregate species almost vanished, and the peaks became much sharper (Figure log). After the centrifugation of the SPFO/OCTABmixed solutions, a gel-like viscoa fluid was found in the bottom of the centrifugal tube. In contrast, the lgFNMR spectra of Solutions made up of SPFO/DEDIAP (with XF= 0.50)
Acknowledgment. We gratefully acknowledge the assistance of industrial sponsorsof the Institutefor Applied Surfactant Research, including E. I. du Pont de Nemours & Co, Kerr-McGee Corporation, SandozChemicals Corp., and Union Carbide Corporation. Registry No, SPFO,335-95-5; HEA8-3, 143266-94-8; DED I M , 15163-36-7;OCTAB, 2083-68-3.
