Langmuir 1989,5, 343-348
343
Probe Methods for the Second cmc of Fluorocarbon and Hydrocarbon Surfactant Mixtures Tsuyoshi Asakawa,* Mikio Mouri, Shigeyoshi Miyagishi, and Morie Nishida Department of Chemistry and Chemical Engineering, Faculty of Technology, Kanazawa University, Kanazawa 920, J a p a n Received J u n e 1, 1988. I n Final Form: October 11, 1988 Probe methods were used to study fluorocarbon and hydrocarbon surfactant mixtures such as lithium perfluorooctanesulfonate (LiF0S)-lithium dodecyl sulfate (LiDS) and LiFOS-lithium tetradecyl sulfate (LiTS). The micellar microenvironment was estimated by the fluorescence intensities of l-anilinonaphthalene-8-sulfonate (ANS),auramine, and 13/11 of pyrene. Both the fluorescence behavior and the low solubility of pyrene as to the fluorocarbon surfactant suggested that the LiFOS micelles would form loose "mini" micelles. Second cmc values were determined by the fluorescence probe methods and 'H and 13C NMR chemical shifts, which were interpreted by the coexistence of two kinds of mixed micelles. Introduction Fluorescence probe analysis has been an important method for the study of microscopic information as to the nature of micelle aggregates in aqueous solution.'-1° The precise structure of micelles and the nature of solubilization sites for probes have been the top issues. Hydrophobic fluorescence probes exhibit different fluorescence characteristics depending on the properties of the solubilizing medium. Such behavior as a function of surfactant concentration has been used to determine the critical micelle concentrations (cmc v a l ~ e s ) For . ~ example, fluorescence probes such as 1-anilinonaphthalene-8-sulfonate and pyrene are sensitive to the polarity of the solubilizing medium. Few studies for micellar characteristics have been reported for micelles composed of fluorocarbon surfactant and mixed micelles.11-'8 Significant differences in micellar properties between fluorocarbon and hydrocarbon surfactant are expected due to the difference in nature of the hydrophobic moiety. The micellar splitting of micelles was proposed for the nonideal mixtures of fluorocarbon and hydrocarbon surfactant^.'^ If the splitting of micelles would occur, some microscopic changes in micelle aggregates would be expected corresponding to the appearance of another type of mixed micelles. A probe method would (1) Nakajima, A. Bull. Chem. SOC.Jpn. 1971, 44, 3272. (2) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977,99, 2039. (3) Ji-Sun, K.; Chang-Kew, K.; Pill-Soon, S.; Keun-Moo, L. J. Colloid Interface Sci. 1981, 80, 294. (4) Lianos, P.; Lang, J.; Strazielle, C.; Zana, R. J . Phys. Chem. 1982, 86, 1019. ( 5 ) Lianos, P.; Lang, J.; Zana, R. J. Phys. Chem. 1982, 86, 4809. (6) Schore, N. E.; Turro, N. J. J. Am. Chem. SOC.1974,96, 306. (7) Shinitzky, M.; Dianoux, A. C.; Gitler, C.; Weber, G. Biochemistry 1971, IO, 2106. (8) Gratzel, M.; Thomas, J. K. J . Am. Chem. SOC.1973, 95, 6885. (9) Ananthapadmanabhan, K. P.; Goddara, E. D.; Turro, N. J.; Kuo, P. L. Langmuir 1985, I, 352. (10) Turro, N. J.; Lee, P. C. C. J . Phys. Chem. 1982,86, 3367. (11) Carlfors, J.; Stilbs, P. J . Phys. Chem. 1984, 88, 4410. (12) Treiner, C.; Bocquet, J. F.; Pommier, C. J. Phys. Chem. 1986,90, 3052. (13) Treiner, C.; Khodja, A. A.; Fromon, M. Langmuir 1987, 3, 729. (14) Muto, Y.; Esumi, K.; Meguro, K.; Zana, R. J. Colloid Interface Sci. 1987, 120, 162. (15) Hoffmann, H.; Kalus, J.; Thurn, H. Colloid Polym. Sci. 1983,261, 1043. (16) Hoffmann, H.; Ulbricht, W.; Tagesson, B. Z. Phys. Chem. 1978, 113, 17. (17) Burkitt, S. J.; Ottewill, R. H.; Hayter, J. B.; Ingram, B. T. Colloid Polym. Sci. 1987, 265, 619. (18) Burkitt. S . J.: Ottewill. R. H.: Havter. J. B.: Inaram, - B. T. Colloid Polym. Sci. 1987, 265, 628. (19) Mukerjee, P.; Yang, A. Y. S. J . Phys. Chem. 1976,80, 1388.
be a useful method to evaluate such changes in the micelle characteristics. However, it is necessary to take into account the solubilizing behavior of a surfactant micelle. NMR chemical shifts reflect the local electronic environment in a micelle aggregate. 13C NMR spectroscopy of surfactant solutions can provide information on molecular conformation on the basis of the high resolution and a wide chemical shift The concentration dependence of the chemical shift was used to deduce quantitative information on the micelle aggregation number and qualitative information on the conformational change of an alkyl chain. The 'H NMR chemical shift has been utilized to obtain information on the environment of a solubilizate in the micelle aggregaksF3a We expected that the chemical shift change might be caused by the interaction between a fluorocarbon and a hydrocarbon chain. It is expected that such an influence could be derived from lone electron pairs on a fluorine atom. In this paper, the changes in the micellar microenvironment by a mixing of fluorocarbon and hydrocarbon surfactants were estimated by the fluorescence probe method and the 'H NMR and 13C NMR chemical shift methods. Experimental Section Materials. Lithium perfluorooctanesulfonate (C8Fl7SO3Li, LiFOS) and lithium alkyl sulfates (CI2H2&3O4Li, LiDS; C14H29S04Li,LiTS) were prepared by the same procedures as reported (ANS, p r e v i ~ u s l y . ~Pyrene, ~ * ~ ~ 1-anilinonaphthalene-8-sulfonate Wako Pure Chemical Ind., Ltd.), and auramine Cl6Hl6N2O3S, (C17H21N3HCl, Kanto Chemical Co., Inc.) were used as received. The other reagents were of guaranteed grade. Fluorescence Measurements. The fluorescence characteristic of pyrene was determined by using a Hitachi MPF-4 fluorescence spectrophotometer. The emission spectra of pyrene were obtained by exciting the samples at 335 nm (Ex slit width 5 nm, Em slit width 1nm). The spectra were used to determine the ratio &/I1 of the intensities of the third (I3)and first (Il)vibronic peaks of monomeric pyrene solubilized in micelle aggregate. The emission spectra of ANS and auramine were recorded on a Hitachi 204-S (20) Persson, B.-0.; Drakenberg, T.; Lindman, B. J. Phys. Chem. 1976, 80, 2124. (21) de Weerd, R. J. E. M.; de Haan, J. W.; van de Ven, J. M.; Achten, M.; Buck, H. M. J. Phys. Chem. 1982,86, 2523. (22) Chang, D. L.; Rosano, H. L.; Woodward, A. E. Langmuir 1985, I , 669. (23) Fox, K. K.; Robb, I. D.; Smith, R. J. Chem. Soc., Faraday Trans. 1 1972, 68, 445. (24) Fendler, E. J.; Day, C. L.; Fendler, J. H. J. Phys. Chem. 1972, 76, M -1A---.
(25) Asakawa, T.; Miyagishi, S.;Nishida, M. J. Colloid Interface Sci.
1985, 104, 279. (26) Asakawa, T.; Johten, K.; Miyagishi, S.; Nishida, M. Langmuir 1985, I, 347.
0~43-7463/89/240~-0343$01.50/0 0 1989 American Chemical Society
Asakawa et al.
344 Langmuir, Vol. 5, No. 2, 1989 4
3
I
-
\
2
0
20
Total
40
60
Conc.(mM)
Figure 1. Fluorescence intensity of ANS vs the total concentration with fixed overall compositions for the LiFOS-LiDS system: (0)a = 1 (LiFOS); ( 0 )a = 0 (LiDS);(A)a = 0.3;(W) cy = 0.55; (0) cy = 0.75.
fluorescence spectrophotometer at the peak, 490 and 500 nm, with excitation at 385 and 440 nm, respectively. The probe concentrations were prepared as 1 X 104-1 X 10-3(surfactant)M pyrene, M auramine, respectively. All M ANS, and 1 X 6.32 X solutions were measured without deaeration at 25 "C. Solubilization of Pyrene. To prepare the saturated pyrene solutions, excess crystalline pyrene was added to micellar aqueous solutions,and the solutions were stirred at 40 "C. Then the sample solutions were allowed to equilibrate for 2 days at 25 "C. They were filtered by a membrane filter (0.22-pm pore size). After an aliquot of the filtrate was diluted with methanol, the concentrations of pyrene were determined. Absorbance was measured at 334 nm (6 = 5.0 X lo4 M-l cm-' ). NMR Chemical Shifts. NMR spectra were obtained with a JEOL FXlOOS spectrometer by using freshly prepared surfactant solutions in D20 (99.75%,Merk). The external reference for *H NMR was a D,O solution of 2.5% sodium 2,2-dimethyl2-silapentane-5-sulfonatecontained in a coaxial capillary inside a 5-mm tube. The external reference for I3C NMR was a CC14 solution of 10% tetramethylsilane contained in a coaxial capillary inside a 10-mm tube. All chemical shifts are referred to an external reference signal. The behavior of hydrocarbon surfactantscould be observed selectively in the mixed solutions, because the signal of fluorocarbon surfactants did not appear in these NMR spectra. The signals of a fluorocarbon surfactant split into many bands owing to the coupling of carbon and fluorine nucleus. Thus its intensities were very weak.
Results and Discussion Fluorescence Probe. We investigated the dependence of fluorescence intensity on surfactant concentration in detail and indicated the utility of a probe method. Three fluorescence probes, ANS, pyrene, and auramine, were used to evaluate the microenvironment in the micelle aggregates. It is well-known that the fluorescence intensity of ANS exhibits a dependence on the solvent polarity. This property was used to determine the cmc of a surf a ~ t a n t . ~ 'The variation of ANS fluorescence intensity is given in Figure 1 as a function of the total surfactant concentration. The intensity in water (Io) was used as a standard. As to a single surfactant system, the fluorescence intensity linearly increased above the cmc with increase in hydrocarbon surfactant concentrations (LiDS, cmc 8.6 mM), while the intensity hardly increased with increase (27) Abuin, E. B.; Lissi, E. A. J. Colloid Interface Sci. 1983, 95, 198.
in fluorocarbon surfactant concentration even if above the cmc (LiFOS, cmc 7.1 mM). The position of maximum fluorescence peak (Amm) remained almost constant in the LiFOS system (fluorocarbon micelles) even if the concentration increased above the cmc, while it decreased from 510 nm (Amm in water) to 487 nm (Amm in LiDS solution) as the LiDS concentration increased. The results suggest that the ANS probe was not incorporated into LiFOS micelles or that the LiFOS micelles have far less hydrophobic circumstance around the micelle-solubilized ANS probe than LiDS ones. It may result from the fact that the fluorocarbon chains have a low solubility of organics compared with the hydrocarbon chains. The fluorocarbon chains have a higher hydrophobicity than the hydrocarbon chains as judged from a low water solubility and much lower cmc of a fluorocarbon surfactant. Nevertheless, an appreciable water contact of the fluorocarbon chain would exist at the solubilization site of ANS in the LiFOS micelle because the LiFOS micelle was considered to form "loose" micelles in contrast to the typical hydrocarbon surfactants. As pointed out in previous reports, 19F NMR and fluorescence quenching results also indicated that SPFO micelles possessed a small, loose, and open structure.l0VBD The fluorocarbon chains are certainly more bulky than the hydrocarbon chains and tend to make its chains much more rigid. The head-group area per surfactant on the micellar surface is given by dividing the total surface by the aggregation number. The head-group area per fluorocarbon surfactant was larger than that of hydrocarbon surfactant due to the small aggregation number and the bulkiness. Thus, the large distance between the hydrophilic groups led to water penetration into the fluorocarbon micellar palisade layer. In the LiFOS-LiDS mixed system, abrupt increases in fluorescence intensity were observed above the mixture cmc in the region of a = 0-0.45 (mole fraction of LiFOS), as expected. On the other hand, in the region of a = 0.48-0.75, the fluorescence intensity was not so much increased above the mixture cmc, but it was abruptly increased far above the mixture cmc. The fluorescence behavior could be explained as follows. The LiDS-rich micelles first appear in the high mole fraction of LiDS ( a = 0-0.45, the left side of the cusp of the mixture cmc curve in Figure 8). On the other hand, in the high mole fraction of LiFOS ( a = 0.45-0.75, the right side of the cusp of the mixture cmc curve in Figure 8), the LiFOS-rich micelles first appear at the mixture cmc. The fluorescence intensity was not so much increased because the LiFOS-rich micelles also form loose micelles as described above. As the concentration increased far above the mixture cmc, a second type of mixed micelle rich in LiDS would appear. That is to say, in all cases, the abrupt increase in fluorescence intensity is considered to correspond to the formation of hydrocarbon-rich micelles. The inflection point corresponds to the second cmc, i.e., it is understandable as the transition from one type to two types of mixed micelles. Similar behavior was observed by a gel filtration for LiFOS-LiTS mixed system.30 It has been shown that the ratio of the fluorescence intensity of the third and first vibronic peaks of pyrene is a sensitive parameter characterizing the polarity of the probe's environment.'p2 The dependence of 13/11 on the surfactant concentrations above the mixture cmc is illustrated in Figure 2. The increase in the value of 13/11 is an indication of the solubilization into a more hydrophobic (28) Muller, N.; Simsohn, H. J. Phys. Chem. 1971, 75, 942. (29) Ulmius, J.; Lindman, B. J. Phys. Chem. 1981, 85, 4131. (30! Asakawa, T.;Miyagishi, S.; Nishida, M. Langmuir 1987, 3, 821.
Langmuir, Vol. 5, No. 2, 1989 345
Second cmc by Probe Methods
I
60-
0.8
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.-c
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-
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60
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a
s -
Conc.(mM)
0
tration with fixed overall compositions for the LiFOS-LiDS CY = 0.5; ( 0 )CY = 0.7; system: (0)CY = 0 (LiDS);(A) a = 0.3;(0) (A)a = 1 (LiFOS).
environment. The solubility of pyrene in 60 mM LiFOS solution was 6.0 X lo4 M and was larger than that in water (8.0 X lo-’ M). Therefore, pyrene would be surely solubilized in the LiFOS micelles, although LiFOS micelles have a low solubility for pyrene in contrast to the hydrocarbon micelles. Nevertheless, the value of 13/I , in LiFOS micelles indicated the same value in water. 13/11in water has a value of about 0.52, in hydrocarbon solvent about 1.54, and in fluorocarbon solvent about 1.75 (in our experiments). Therefore, the solubilization site of pyrene in the LiFOS micelles is probably hydrated with an appreciable water contact. The polarity of the mixed micelles was intermediate between the two single surfactant micelles in all cases. Significant variation in the fluorescence behavior was observed in the region of a = 0.5-0.8. At a = 0.7 (mixture cmc 8.8 mM), the value of 13/11was small around the mixture cmc at which the LiFOS-rich micelles first appeared. As the concentration increased, the value of 13/11also increased, probably according to the incorporation of LiDS surfactant into the LiFOS-rich micelles. The inflection point was observed in a rather high concentration, which was present near the second cmc by ANS fluorescence data. It has been observed that the pyrene 13/11is quite dependent on the nature of a surfactant head group.2 Thus, the observed 13/11could also be interpreted as a measure of the compactness of the head-group structure and the extent of water penetration into micellar aggregates. The dependence of the fluorescence intensity of auramine on solvent has been shown to be sensitive to the viscosity of the probe’s environment and not to the pol a r i t ~ . ~That ~ ” ~is, the quantum yield decreased by the energy dissipation due to intramolecular rotational diffusion of the probe. The changes of fluorescence intensity reflect the changes of the effective microviscosity felt by auramine in its microenvir~nment.~l>~~ The dependence of fluorescence intensity on the surfactant concentration above the cmc is shown in Figure 3. The microviscosity in the solubilization site of auramine is smaller in LiFOS micelles than in LiDS micelles but depends on the surfactant concentration. The probe would be solubilized in the anionic micellar surface due to the cationic group of the probe. The LiFOS micelles would form a loose micellar surface due to a small aggregation number. The motion of hydrophilic group would not be so much restricted as LiDS micelles. Thus, the microviscosity felt by auramine in LiFOS micelles would be smaller than in LiDS micelles. (31) Kobayashi, Y.; Nishimura, M. J . Biochem. (Tokyo) 1972, 71,275. (32) Oster, G.; Nishijima, Y. J . Am. Chem. SOC.1956, 78, 1581.
60
40
20
Figure 2. Pyrene fluorescence ratio &/I1 vs the total concen-
T o t a l Conc.(mM )
Figure 3. Fluorescence intensity of auramine vs the total concentration with fixed overall compositions for the LiFOS-LiDS system: (0)CY = 0 (LiDS);(A) CY = 0.3;(0)CY = 0.5; ( 0 )CY = 0.7; (A)CY = 1 (LiFOS).
0
20
Total
40
Conc ( m M )
Figure 4. Solubility of pyrene for the total concentration with CY fixed overall compositions for the LiFOS-LiDS system: (0) = 0 (LiDS); (A) CY = 0.2; (0) a = 0.4; ( 0 )CY = 0.6; (A)CY = 0.7; (w)
CY
= 1 (LiFOS).
The information felt by auramine could be interpreted as a measure of the viscosity of the micellar surface. In the mixed systems, inflection points were present in the region of a = 0.454.75. The microviscosity of LiFOS micelle was increased by the addition of LiDS molecule. These inflection points were also assigned to the second cmc values. Solubilization. Pyrene is a water-insoluble compound and can be solubilized into the hydrophobic region of micelle. The solubilization method has been attempted to determine the cmc of a surfactant. But the solubilized amount of pyrene was too little to evaluate the cmc of a fluorocarbon surfactant. On the basis of this characteristic of pyrene solubilization, we could detect the formation of a hydrocarbon-rich micelle in the fluorocarbon and hydrocarbon surfactant mixed system. Figure 4 shows the concentration of the pyrene solubilized by the addition of LiDS, LiFOS, and their mixtures. In high mole fraction of LiFOS, the solubilized amount was abruptly increased far above the mixture cmc. These inflection points correspond to the formation of LiDS-rich micelles, that is to say, the second cmc. The observed second cmc values were in a similar tendency as those determined by the fluorescence probe methods (see Figure 8). The solubilization of pyrene into micelles might not change seriously the monomer-micelle equilibrium.
Asakawa et al.
346 Langmuir, Vol. 5, No. 2, 1989
:I .=
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-
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56
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55
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8
;
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0
50
100
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Figure 5. 'HNMR chemical shift vs the total concentration with fixed overall compositions ( a = 0.35) for the LiFOS-LiDS system: ( 0 )a-methylene;(M) long-chain methylene; (A)terminal methyl.
As described above, the probe methods in this paper can be used to detect the formation of the hydrocarbon-rich micelle in the LiFOS-LiDS system. These methods were not suitable to determine the second cmc for a mixed system such as LiFOS-LiTS. In this case, the cmc of the hydrocarbon surfactant (LiTS cmc 2.3 mM) is considerably lower than that of a fluorocarbon surfactant (LiFOS cmc 7.1 mM). Thus, the LiTS-rich micelle first forms at the mixture cmc in a wide mole fraction region. Therefore, the first mixture cmc only can be determined by these probe methods. NMR Chemical Shifts. The dissolution characteristics of surfactants are often determined by chemical shift changes in micellar solution^.^*^^ These mixed micelles also were studied by 'H NMR chemical shifts, which are shown in Figure 5 . The a-methylene signal of a LiDS molecule was shifted upfield with an increase in a surfactant concentration, while the chemical shift of its central methylene chain was constant up to a certain concentration but began to shift upfield at a high total concentration. However, the chemical shifts of the terminal methyl group remained constant. The local electronic environment of the methylene chain was affected by the incorporation of a fluorocarbon chain. Generally, the chemical shift changes are caused by the changes in polarity and/or conformation. The change in microenvironment of the methylene chain incorporating into fluorocarbon-rich micelles is expected to be different from that into hydrocarbon-rich micelles. Thus, the second cmcs corresponding to the formation of fluorocarbon rich micelles were detected by the variation of the chemical shift as a function of total concentration. It is known that 13C NMR studies of surfactant solutions can provide the information on the conformations of surfactant molecules.20-22The signals of a hydrocarbon surfactant only appear in NMR spectra. The signals of a fluorocarbon surfactant split into many bands owing to the coupling between carbon and fluorine nucleus. Its intensities were very wehk. Thus, the behaviors of hydrocarbon chains can be selectively evaluated even if in the fluorocarbon and hydrocarbon mixed systems. As shown in Figure 6, each signal was shifted downfield by the increase of total concentration except for the amethylene carbon. This phenomenon has been interpreted by a steric effect, the so-called Downfield shifts (33) Grant, D. M.; Cheney. B. J . A m . Chem. Soc 1967, 89, 5315.
r u
f 0
d
-
( 1 1 1 1 .
IO 0 IO 0 M o l e F r a c t i o n of L i F O S
IO
Figure 7. NMR chemical shift vs mole fraction of LiFOS0.3 M LiFOS-LiDS; (0) LiDS and LiFOS-LiTS systems: (0) 0.3 M LiFOS-LiTS; ( 0 )LiDS single system. The concentrations of LiDS are 0.3(1- a ) M. For example, at a = 0.2 in a pure LiDS system, the LiDS concentration is 0.3(1- 0.2) = 0.24 M. When the data are plotted at a = 0.2, both the single LiDS and LiFOS-LiDS mixed systems contain LiDS in identical concentration. are indicative of an increase in the trans conformer, while upfield shifts are indicative of an increase in the gauche conformer for the corresponding carbons. In this mole fraction ( a = 0.351, LiDS-rich micelles first form at the cmc, and the incorporation of LiFOS into LiDS-rich micelles is gradually increased by the increase in the total concentration. The energy difference between trans and gauche conformations for fluorocarbons is larger than that for hydrocarbon chains.34 The increase in population of the trans conformer is probably caused by the incorporation of a rigid fluorocarbon chain. Thus, the incorporation of LiFOS molecules induced a more extended configuration of the chain in the micelle core, which leads to downfield shifts. These downfield shifts by the incorporation of LiFOS are displayed in Figure 7. Weerd et al. reported I3C NMR chemical shifts with respect to a mixed surfactant system containing different chain lengths.21 In such a case, an increase in the gauche conformer was observed for the longer methylene chain near a micelle core. They suggested that such rearrangements of methylene chains were caused by increased distances between the long methylene chains in the mixing process of the surfactant having different chain lengths. Such behavior was not observed for the LiFOS-LiDS mixed systems even if the surfactants have different chain lengths. The reason is as follows: Hoffmann et al. reported that the radius of micellar aggregate of LiFOS was determined to be 17 A by small-angle neutron-scattering measurement^.'^ That is, the radius for LiFOS (carbon chain length 8) micelles (17 A) was approximately identical (34) Fontell. K.; Lindman, B. J . Phys. Chem. 1983, 87, 3289
Langmuir, Vol. 5, No. 2, 1989 347
Second cmc by Probe Methods
C
0 .c
2
c
C
: 0
20-
0.2
V
Mole
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Figure 9. Micellar pseudophase diagram for the mixed LiFOS-
u 0.2
0.4
0.4
0.6
0.8
1.0
Mole Fraction of L i F O S Figure 8. Micellar pseudophase diagram for the mixed LiFOSLiDS system: (e) second cmc by ANS; (A)pyrene 13/11;(W) auramine; (0) pyrene solubility; (A)'HNMR chemical shift; (0) I3C NMR chemical shift; (-) calculated second cmc (XF = 0.882, XH = 0.250, X u = 0.450, CAz = 11.1); (- - -) mixture cmc curve?6
with that of sodium dodecyl sulfate (carbon chain length 12) micelles (16 A), which is four carbon atoms longer. In other words, fluorocarbon chains must be in an all-trans conformation in the micellar interior and have an ordered structure compared to hydrocarbon micelles. Therefore, both LiFOS and LiDS have similar chain lengths. The incorporation of a fluorocarbon chain might cause the ordered structure in the formation of a micelle rather than change the distance between the long methylene chains. In Figure 7 , not only the effect of mixing LiFOS with hydrocarbon micelles but also the effect of surfactant concentration in a pure component system (LiDS) was shown for comparison. The variation of LiDS concentration in a pure surfactant system was expressed as the variation of the mole fraction ( a ) ,that is to say, 0.3(1 - a) M. In such a plot, the effect on a chemical shift by the LiDS concentration in a LiFOS-LiDS mixed system could be easily compared with that of the pure LiDS system. In the pure component (LiDS) system, slight downfield shifts were observed by the increase in concentration (from a = 1 to 0 as shown in Figure 7), which had been explained in terms of a steric effect.33 Such behavior was already reported with prior studies of sodium octanoate, n-nonylammonium, and dihexanonylphosphatidylcholine.35-37 In mixed systems, the downfield shifts were also observed for all methylene carbon by incorporation of LiFOS. These downfield shifts did not result from the effect of surfactant concentration as judged from the results of the pure component system. The downfield shift by the formation of micelle was reported to be largest in the middle of the chain while relatively small at either end.22 In our study, the shift was not necessary largest in the middle of the chain by the incorporation of fluorocarbon into the hydrocarbon surfactant micelles. The change was largest a t the a-methylene in both 'H and 13C N M R chemical (35) Drakenberg, T.; Lindman, B. J. Colloid Interface Sci. 1973, 44, 184. (36) Perrson, B.-0.; Drakenberg, T.; Lindman, B. J. Phys. Chem. 1976, 80, 2124. (37) Burno, R. A,; Roberts, M. F.; Dluhy, R.; Mendelsohn, R. J. Am. Chem. Soc. 1982, 104, 430.
LiTS system: (w) second cmc by 13C NMR chemical shift; (-) calculated second cmc (X,= 0.944, XH = 0.205, X u = 0.875, C a = 7.5); (- - -) mixture cmc curve.26 shifts. This suggested that the surface area per micellar surfactant increased owing mainly to the immiscibility (a mutual phobicity of fluorocarbons and hydrocarbons due to the weak interactions of fluorocarbons and hydrocarbons as compared to their self-interactions) of fluorocarbon and hydrocarbon chains and the bulky fluorocarbon chain. These effects resulted in the largest change of microenvironment a t the micellar surface. Chenney and Grant estimated the shift change between the gauche and trans conformer of carbon chains to be 4.8 ppm.= On the basis of this shift, the percent change of gauche to trans upon addition of the fluorocarbon surfactant was calculated to be in the range 14-20% for the central carbon chains. Thus, this behavior corresponds to a more extended configuration of the hydrocarbon chain by the rigid fluorocarbon chain in the micellar core, maximizing the hydrophobic interaction between the hydrocarbon chains. As described above, second cmc values were determined by the fluorescence probe methods and 'H and 13CNMR chemical shifts. The obtained second cmc values of LiFOS-LiDS and LiFOS-LiTS mixed systems are summarized in Figures 8 and 9, respectively. If the splitting of micelles occurs, the second cmc values are given by material balances.38 cmc2 = XF-CY CAZ XAZ
-XH
cmc2 = CY-XH
CAZ
where X u and CAZare the composition and concentration under the azeotropic condition, XF and XH are the mole fraction of fluorocarbon surfactant in fluorocarbon-rich and hydrocarbon-rich micelles, respectively, and a is the mole fraction of fluorocarbon surfactant in the mixture. The larger value of cmc2 in eq 1and 2 leads to the second cmc. The values of XF, XH, XAZ, and CAZ were obtained from the prediction of the group contribution method.26 The calculated second cmc exhibited a similar tendency to the measured one. Thus, the second inflection beyond the mixture cmc could be interpreted by the formation of another type of mixed micelles. In Figures 8 and 9, the fluorocarbon-rich micelle first appears at the right side of the cusp of the mixture cmc curve. The hydrocarbon-rich (38) Asakawa, T.; Johten, K.; Miyagishi, S.; Nishida, M. Langmuir 1988, 4, 136.
348
Langmuir 1989, 5 , 348-352
micelle first appears a t the left side of the cusp of the mixture cmc curve. Both fluorocarbon-rich and hydrocarbon-rich micelles, which are mutually saturated each other, coexist within the second cmc curves. These second cmcs were also observed by self-diffusion measurements of the Fourier transform NMR pulsed-gradient spin-echo method (FT-PGSE NMR).39
micellar systems. The significant changes in the fluorescence and NMR spectra were observed above the mixture cmc. The abrupt changes far above the mixture cmc were assigned to the formation of another type of mixed micelles, that is, the second cmc. Above the second cmc, both fluorocarbon-rich and hydrocarbon-rich mixed micelles coexisted.
Conclusion The use of probes has been investigated to determine the second cmc and to examine the microenvironment of
Acknowledgment. We are grateful to Dainippon Ink Chemical Industry Co., Ltd., for providing the fluorocarbon surfactant. Registry No. LiFOS, 29457-72-5; LiDS, 2044-56-6; LiTS, 52886-14-3;ANS, 82-76-8;auramine, 2465-27-2; pyrene, 129-00-0.
(39) Asakawa, T. et al., unpublished data.
Influence of Pre- and Postdeposited Au on Coadsorbed CO on Ru(001) Kyoichi Sawabe, Chikashi Egawa, Tetsuya Aruga, and Yasuhiro Iwasawa" Department of Chemistry, Faculty of Science, T h e University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received August 25, 1988. I n Final Form: November 18, 1988 The geometrical and electronic effects of pre- and postdeposited Au on the adsorption of CO on Ru(001) have been examined by means of AES, TDS, UPS, XPS, and LEED. It is shown that Au grows via the Stranski-Krastanov mechanism on the clean Ru(001) surface held at 670 K as previously reported. Submonolayer coverages of Au form two-dimensional (2D) islands at around 670 K, and annealing this surface to 1000 K causes the nearly atomical dispersion of Au atoms over the surface as characterized by sharpening of Au 5d features in UPS. The 2D islands of Au suppress the CO uptake without any noticeable change in the shape of CO TD spectra, whereas highly dispersed Au atoms effectively prevent the formation of long-range order of CO, hence resulting in remarkable broadening of CO TD peaks. The postdeposition of Au on the CO-saturated surface results in extensive crowding of CO, which is characterized by unusually large binding energies of CO 5 u / 1 ~and 4u molecular orbitals and considerable lowering of CO desorption temperatures. The effects of postdeposited Au are explained in terms of increased CO-CO interaction and weakened CO-metal bonds due to the enhanced crowding by Au. Thusthe influence of pre- and postdeposited Au on coadsorbed CO on Ru(001) is well ascribed to the geometrical (ensemble) effect. There is no indication of the electronic effect of Au in the present system.
Introduction Recently, bimetallic surfaces prepared on single crystals have been extensively investigated toward a goal of the full understanding of the mechanism of enhancement of catalytic activity and/or selectivity in bimetallic catalysts relative to their individual components. Several models have been hypothesized to explain the drastic change of catalytic properties in bimetallic systems. First, the alloying may cause an electronic modification of either or both of the component metals (ligand effect). Second, the additive metal may geometrically block the formation of active sites or ensembles which are required for a reaction to occur (ensemble effect). Third, each component may promote different reaction steps or cooperatively enhance the reaction and, thus, act synergistically with each other (synergistic effect). Among many possible combinations for bimetallic systems, the combination of group Ib/group VI11 metals has been most widely investigated by using well-defined single-crystal s u r f a ~ e s . l - The ~ group Ib metals have a dl0s1 (1) Christmann, K.; Ertl, G.; Shimizu, H. J . Catal. 1980, 61, 397. (2) Vickermann, J. C.; Christmann, K.; Ertl, G. J . Catal. 1981, 71, 175. (3) Vickermann, J. C.; Christmann, K.; Ertl, G.; Heimann, P.; Himpsel, F J.; Eastmann, D. E Surf. Sci. 1983. 134. 367.
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configuration and hence exhibit only very weak interaction with molecules like CO and Hz.Therefore, the addition of group Ib metals such as Cu and Au onto transition-metal surfaces is expected to result only in blocking of adsorption sites for those molecules. Nevertheless, epitaxial and alloyed Au/Pt(lll) surfaces have been reportedg to show the enhancement in activity and selectivity for cyclohexane dehydrogenation to form benzene. It was suggested that Au serves to weaken the chemisorption bond of benzene and hence reduce the selfpoisoning by the adsorbed product. The same A u / P t ( l l l ) sample also exhibited the enhancement in activity for hexane conversion, whereas the deposition of Au onto Pt(100) did not show any promotion effects, suggesting the structure sensitivity of alloy catalysis for this reaction.1° It has also been demonstrated5 (4) Houston,
J. E.; Peden, C. H. F.; Feibelman, P. J. Phys. Reu. Lett.
1986, 56, 375.
(5) Peden, C. H. F.; Goodman, D. W. J. Catal. 1986, 100, 520. (6) Gocdmann, D. W.; Yaks, J. T., Jr.; Peden, C. H. F. Surf. Sci. 1985, 164, 417. (7) Rocker, G.; Tochihara, H.; Martin, R. M.; Metiu, H. Surf. Scz. 1987, 187 . - ., m ---. (8) Harendt, C.; Christmann, K.; Hirschwald, W.; Vickermann, J. C. Surf. Sci. 1986, 165, 413. (9) Sachtler, J. W. A.; Somorjai, G. A. J . Catal. 1984, 89, 35.
6 1989 American Chemical Society
