Langmuir 1988,4 , 136-140
136
The short component is probably due to an aggregate species by analogy to the pure TTPa film. This species makes up only 7% of the fluorescent population and thus does not noticeably red-shift the fluorescence and absorption spectra. While a good fit may be obtained with a two-component decay for measurements involving up to 20 OOO counts, the fit becomes increasingly inadequate a t higher counts. It is suspected that the decay is, in fact, made up of a distribution of 1ifeth1es.l~ The distribution is currently being analyzed by using a program for measurements at 200 OOO counts and will be described in a forthcoming paper. The TTPa monolayer was found to be stable a t least over a period of 2 weeks in air a t room temperature. Futhermore, no bleaching of the pigment was seen under irradiation by either light source. (13)(a) James, D.R.; Liu, Y.4.; de Mayo, P.; Ware, W. R. Chem. Phys. Lett. 1985, 120, 460-465. (b) James, D.R.; Ware, W. R. Chem. Phys. Lett. 1986, 126,7-11. (14)Siemiarczuk, A.; McIntosh, A. R.; Ho, T.-F.; Stillman, M. J.; Roach, K. R.; Weedon, A. C.; Bolton, J. R.; Connolly,J. S. J. Am. Chem. SOC.1983,105, 7224-7230.
The results of measurements of the same mixture at the air-water interface are also included in Table I. The photophysical situation appears to be close to that in the LB film. This is an important observation since it shows that the transfer of the monolayer film from the air-water interface to the quartz slide has little effect on the nature of the film. Conclusions
It was found that TTPa aggregation in LB films could be reduced by dilution in DOPC. The resulting monomeric TTPa fluoresced with a reproducible lifetime of 10.7 f 0.2 ns, which we offer as a standard. The reduced aggregation was also apparent in the absorption and fluorescence spectra as a blue shift away from the earlier reported aggregate spectrum toward that of TTPa in methylene chloride. The authors will make available, on request, samples of TTPa and/or certified LB films of the standard system. Registry No. TTPa, 93082-03-2;DOPC, 10015-85-7;quartz, 14808-60-7.
Monomer-Micellar Equilibrium of Fluorocarbon and Hydrocarbon Surfactant Solutions by Ultrafiltration Tsuyoshi Asakawa,*t Kazuhiro Johten, Shigeyoshi Miyagishi,*t and Morie Nishida*t Department of Industrial Chemistry, Faculty of Technology, Kanazawa University, Kanazawa 920, Japan Received February 20, 1987. I n Final Form: August 3, 1987 An ultrafiltration method was used to study fluorocarbon and hydrocarbon surfactant mixtures such as lithium perfluorooctanesulfonate (LiF0S)-lithium alkyl sulfate, having a different chain length of 10 (LiDeS), 12 (LiDS), or 14 (LiTS), and sodium perfluorooctanoate (SPFO)-sodiumdodecyl sulfate (SDS). The method was useful to reveal the monomer-micelle equilibrium beyond the mixture cmc. The azeotropic point was observed close to the maximum of the mixture cmc curve for the LiFOS-LiTS and LiFOS-LiDS systems, respectively. This result was interpreted by the occurrence of micelle demixing. Introduction
Recently, we have used a group contribution model to predict critical micelle concentrations of binary surfactant mixed systems.' The method was also applied to quantitative investigation of the mixing of fluorocarbon and hydrocarbon surfactants. The immiscibility has been worthy of remark in the viewpoint of technical interest such as oil repellent and fire-extinguishing properties.2 The coexistence of two kinds of mixed micelles was predicted by Mukerjee et al.3*4 The micelle demixing is quite plausible, but it has not been proven. A more direct experimental method is needed to resolve the issue. Several experimental data for this subject were presently available.'-lo The monomer compositions were also predicted, but they have not been determined by a direct experimental method. If the demixing of micelles would occur, the concentrations of monomers must be constant according to the pseudo-phase-separation approximation of micellization under the constant temperature and pressure Department of Chemistry and Chemical Engineering, Faculty of Technology, Kanazawa University, 2-40-20Kodatsuno, Kanazawa 920,Japan.
0743-7463/88/2404-0136$01.50/0
conditions. The monomer concentrations were not verified because of the experimental difficulty in direct measurement of monomer concentrations. Thus, the monomer needs to be separated from the micellar solution by an appropriate method. An ultrafiltration is effective at removing dissolved high molecular weight organics from water.11-14 Surfactant monomer passes through an ultrafiltration membrane with (1)Asakawa, T.; Johten, K.; Miyagishi, S.; Nishida, M. Langmuir 1985, 1, 347. (2)Shinoda, K.;Nomura, T. J. Phys. Chem. 1980, 84, 365. (3)Mukerjee, P.;Mysels, K. J. ACS Symp. Ser. 1975, 9, 239. (4)Mukerjee, P.;Yang,A. Y. S. J. Phys. Chem. 1976,80, 1388. (5)Mysels, K.J. J. Colloid Interface Sci. 1978, 66, 331. 1982,59, 573. (6)Mukerjee, P.J. Am. Oil Chem. SOC. (7) Funasaki, N.; Hada, S. J. Phys. Chem. 1980, 84, 736. (8)Funasaki, N.;Hada, S. J. Phys. Chem. 1983, 87, 342. (9) Carlfors, J.; Stilbs, P. J. Phys. Chem. 1984, 88, 4410.
(10)Asakawa, T.;Miyagbhi, S.; Nishida, M. J. Colloid Interface Sci.
1985, 104, 279.
(11)Osborne-Lee, I. W.; Schechter, R. S.; Wade, W. H. J. Colloid interface Sci. 1983, 94, 179. (12)Osborne-Lee, I. W.: Schechter, R. S.; Wade, W. H.; Baraket, Y.
J. Colloid Interface Sci. 1986, 108, 60. (13)Scott, H. J. Phys. Chem. 1964, 68, 3612. (14)Warr, G. G.;Grieser, F.; Healy, T. W. J. Phys. Chem. 1983, 87, 1220.
0 1988 American Chemical Society
Langmuir, Vol. 4, No. 1, 1988 137
Monomer Compositions by Ultrafiltration pores small enough to block micelle passage. Osborne-Lee e t al. have measured cmc, monomer composition, and micelle composition for binary mixtures by a n ultrafiltration method."J2 They have shown that a regular solution theory gave good fits t~ the mixture cmc data but failed t o predict the correct micellar compositions. Kamrath and Franses calculated concentrations of monomer, micelle, and counterion in binary surfactant mixtures by both a pseudo-phase-separation model and a mass action mode1.15J6 Nagarajan also estimated them by a molecular theory.17 However, there are few experimental results to verify the predicted monomer and micelle concentration beyond the mixture cmc. In this paper, the micelle compositions of fluorocarbon and hydrocarbon surfactant mixtures were measured as a function of the composition of the monomeric surfactant in equilibrium with the micellar phase. We will reveal the micelle demixing region in such systems as a function of alkyl chain length. Experimental Section Materials. Lithium perfluorowtanesulfonate (LiFOS),sodium perfluormtanoate (SPFO),and alkyl sulfate salt were prepared by the same procedures as reported The other reagents were of guaranteed grade. Ultrafiltration. The membranes used in the present study were obtained from Amicon Corp. A membrane, YC-05, can exclude molecules with molecular weight greater than 500 and was only suitablefor these investigated surfactant systems. The membrane had a diameter of 62 mm. The fitrations were carried out in a low-pressurecell (UHP-62,Toyoroghi Co.) with a capacity of 200 mL at 25 O C . The cell was equipped with a magnetic stir bar suspended above the membrane along with a magnetic stirrer to provide for adequate mixing in the cell. The effluent was collected by putting nitrogen pressure (3.0 kg/cm2)on a mother liquor. Sampling was done after 10% of the mother liquor was flowed. The concentrations of the filtrand and filtrate were determined by HPLC or iaotachophoresis. All experiments were performed with solutions containing 10 mM LiCl or NaC1. Under these conditions, there was no evidence of concentrationpolarization or hindrance of the transport of monomeric surfactant through the membrane. HPLC Analysis. A Model BIP-1 liquid chromatograph (Japan Spectroscopic Co., Ltd.) equipped with a Model RID-300 differential refractometer and a ChromatopacC-R3A (Shimadzu Co.) data module were used in the present study. The chromatographic column (150 X 4.6 mm id) was packed with Finepak SIL C18S(5 pm, spherically shaped ODS/silica, Japan SpectroscopicCo., Ltd.). The used surfactants were separated efficiently with acetonitrile-water (5:4, v/v) eluents containing 10 mM tetra-n-butylammonium bromide. Aqueous surfactant solutions were injected into the HPLC column by using a sample loop injector (Reodyne). Isotachophoresis. Isotachophorograms were recorded on a Shimadzu IP-2A equipped with a potential gradient detector. The capillary tube consisted of a main column (150 X 0.5 mm) and a precolumn (40 X 1.0 mm i.d.) and was thermostated at 15 "C. The leading electrolyte solution was prepared as follows. An aqueous solution containing 8.33 mM histidine monohydrochloride, 12.5 mM histidine, and 8.33 mM calcium chloride was mixed with acetonitrile (l:l,v/v). Then the solution was degassed ~ mM aqueous in vacuo. The terminating electrolytesolution w a 10 sodium octanoate solution. The migration current was 200 pA after 250 PA for 7 min.
Results and Discussion Ultrafiltration experiments were performed for fluorocarbon and hydrocarbon surfactants mixtures as a function (15) Kamrath, R. F.; Franses, E. I. Znd. Eng. Chem. Fundam. 1983, 22, 230.
(16) Kamrath, R. F.;Franses, E. I. J. Phys. Chem. 1984, 88,1642. (17) Nagarajan, R. Langmuir 1985, I , 331.
/-E lk LAt E
c
0
20
,
,
40
60
Concentration(rn M )
Figure 1. Filtrate concentration of single systems by ultrafilLiDS, ( 0 )SPFO, (A)LiDeS. tration: (0)LiFOS, (A)LiTS, (0)
- I I
0
10
20
T o t a l Conc. (rn M ) Figure 2. Variation of monomer concentrationsfor the equimolar LiFOS-LiTS system: ( 0 )LiTS, (A)LiFOS, (W) total monomer concentration.
of the surfactant concentration. All solutions contained such sufficient electrolyte, as the effect of the diffusion potential on the transport of a dispersed surfactant through the membrane was negligible." Under these conditions, the ultrafiltration of surfactant solution below the cmc produced a filtrate and a filtrand with identical surfactant concentration. Above the cmc, the concentrations in the filtrate became constant and were equal to the cmc of the surfactant. That is, Figure 1 shows that the cmc equals a saturated concentration of the surfactant monomer. Thus, we can regard the filtrate concentration as the concentration of monomer which is in equilibrium with the micelle. Figure 2 shows the results of ultrafiltration for the LiFOS-LiTS mixed system. The size of the mixed micelle was not smaller than that of two pure component micelles as judged by the results of the gel filtration of the LiFOS-LiTS system.18 As was expected, the mixed micelles did not pass through the membrane as well as the pure component micelles. The selectivity in the transport through the membrane was not detected between the fluorocarbon and hydrocarbon surfactants. Moreover, it was shown that no micelles permeated the membrane by the dye tagging technique.13 That is, the micelle-solubilized (water-insoluble) dye was not detected in the filtrates. Therefore, the concentrations in the filtrate represented those of the monomeric surfactants. Then the composition of the mixed micelle was determined by material balances. It can be seen from Figure 2 that the concentration of monomeric LiTS remains constant beyond the mixture
Asakawa et al.
138 Langmuir, Vol. 4, No. 1, 1988
I
IU h
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E
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c
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o
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I
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Figure 3. Plots of monomeric composition as a function of total concentrationwith fmed overall compitions for the LiFOS-LiTS system. I
E
-
v
L
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,.
v
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Conc. ( m M )
Figure 5. Variation of monomer concentrationsfor the equimolar LiFOS-LiDS system: ( 0 )LiDS, (A)LiFOS, (W) total monomer concentration.
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0.4
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0.8
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Figure 4. Micellar. pseudo-phase diagram for the mixed LiFOS-LiTS system: ( 0 ) second cmc by ultrafiltration, (-) calculated, X F = 0.950, X H = 0.216,Xu = 0.84,CAz = 4.6,( - - - ) mixture cmc curve.
cmc, while that of monomeric LiFOS is nearly equal to the total concentration of LiFOS up to 5 mM or above. Furthermore, the total concentration of LiTS a t the mixture cmc nearly agrees with the cmc of pure LiTS solution. These results mean that the micelles rich in LiTS are formed a t the cmc. These results correspond to the almost complete demixing in the micelle phase in this region. As the total concentration increased, the monomer curves exhibited a second inflection point and attained the plateau of the saturated monomer concentrations. The second inflection point was also observed for the LiFOS-LiTS mixtures by the conductance experiment. This concentration corresponds to the second cmc; i.e., it can be appreciated by the transition from one type to two types of mixed micelles. Similar results were obtained in the region of 0.4-0.7 mole fraction of LiFOS. The monomer composition is plotted as a function of total concentration with a given mole fraction in Figure 3. Each curve represents a relation between the total concentration and monomer composition with a fixed composition of the surfactant mixture. Below the cmc, the total concentration is identical with that of monomeric surfactant. Above the cmc, the monomer composition curve approaches Xs = 0.84 (monomer composition) at high total concentration. When the given mole fraction is equal to 0.84,the monomer composition remained constant even if beyond the mixture cmc. This condition corresponds to the so-called azeotropy. The occurrence of azeotropy indicates some nonideality in the mixed micellar system and has not been directly revealed only from the cmc data in the mixed systems.
, I
,
0
10
Total
20
Conc. ( m M )
Figure 6. Plots of monomeric compositionas a function of total concentrationwith fixed overall compositions for the LiFOS-LiDS system.
In Figure 4,the variation of the second cmc was compared with the calculated one. From material balances, the calculated value is given by the following equations:'O
XF - XAZ
CMC2 = X F - a
CAZ
where X, 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 1 and 2 leads to the second cmc. The values of XAZand CAZwere obtained from the ultrafiltration experiment. The phase separation region, the values of XF and XH,were obtained from the prediction of the group contribution method.' 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. This conclusion has been confirmed by the gel filtration experiment.18 Figure 5 shows the variation of monomer concentrations as a function of total concentration for the LiFOS-LiDS system. The monomer Concentration broke a t the mixture cmc, which was identical with the cmc of the conductance experiment. At this mixing ratio (a = 0.5),LiFOS-rich micelles first appeared a t the mixture cmc. The concentration of monomeric LiFOS decreased beyond the mixture cmc, while that of LiDS increased up to a certain con(18) Asakawa, T.; Miyagishi, s.; Nishida, M. Langrnuir 1987,3, 821.
Monomer Compositions by Ultrafiltration
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Langmuir, Vol. 4, No. 1, 1988 139
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0.8
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Figure 7. Micellar pseudo-phase diagram for the mixed LiFOS-LIDS system: ( 0 )second cmc by ultrafiltration, (-) calculated, XF = 0.910, X H 0.250,X u = 0.43,CM = 8.4, (---) mixture cmc curve.
0
%L 0.2 0.6 0.8 1.0 0.4
Monomer Mole Fraction LiFOS
Figure 9. Variation of monomer concentration with monomer composition for the LiFOS-LiTS system. The line is computed from the group contribution method.
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0
I 0
20
Total
40
60
Conc. ( m M )
Figure 10. Variation of monomer concentration with monomer composition for the LiFOS-LiDS system. The line is computed from the group contribution method.
Figure 8. Variation of monomer concentrationsfor the equimolar SPFO-SDS system: ( 0 )SDS, (A)SPFO, (H) total monomer concentration. centration. A t higher total concentration, the monomer concentration remained constant, similar to the LiFOSLiTS system. Beyond the second cmc, the fixed monomeric composition was 0.43. The second cmc was observed in the region of 0.4-0.55 mole fraction of LiFOS. In Figure 6, each curve indicates the monomer composition as a function of total concentration with a fixed overall composition of the LiFOS-LiDS mixed system. The azeotropic mixture was also observed when the mole fraction was 0.43. The second cmc curve was obtained in a manner similar to the manner in which the cmc curve was obtained in the LiFOS-LiTS system (Figure 7). We could not determine the exact region of phase separation because of the uncertainty in the second cmc data. Thus we used the predicted values from group contribution method for the phase separation region. In Figure 8, the variation of monomer concentrations is plotted against the total concentration for the SPFOSDS system. The total monomer concentration curve has two inflection points. Such a phenomenon was encountered in the conductance curve. Mukerjee and Yang have suggested that the second inflection point is the evidence that a second type of mixed micelle rich in SPFO appears and that two types of micelles coexist a t concentrations beyond the second inflection point? Kamrath and Franses suggested that the second inflection point could be due simply to the abrupt maximum in the SPFO monomer concentration and not necessarily to micelle demixing.15 However, our result indicated that the SPFO monomer concentration still increased with the total concentration beyond the second inflection point. It is worth noting that
02
04
06
0.8
IO
Monomer Mole Fraction LiFOS
Figure 11. Variation of monomer concentration with monomer composition for the LiFOS-LiDeS system. The line is computed from the group contribution method.
the monomer concentration of SPFO was nearly equal to the applied concentration even if beyond the mixture cmc. This means that pure SDS micelles appear at the cmc and that all of the applied SPFO monomers exist as monomers. The SPFO monomers began gradually to incorporate into the SDS micelles after the total concentration increased to about the second inflection point. The micelle composition varied as the total concentration increased; therefore, the monomer composition and concentration varied because they were in equilibrium with the micelles. When the total concentration reached 60 mM, the micelle composition was 0.185 (SDS-richmicelles). It does not yet reach the micelle demixing region ( X H = 0.304 - XF = 0.883)judged from the prediction of the group contribution method. But this observation seems not to be definitive
Langmuir 1988,4, 140-144
140
.-0 0.6
-
0.4
-
c L
LL
-0 E,
8
i
0
A
0 0
0
:
Figure 12. Variation of micelle composition with monomer composition: ( 0 )LiFOS-LiTS, (A)LiFOS-LiDS, (W) LiFOSLiDeS. The l i e is computed from the group contribution method. evidence to resolve the issue of the coexistence of two kinds of mixed micelles or not. In Figures 9-12, the monomer concentration (see Figures 9-11) and micellar pseudo-phase composition (see Figure 12) are plotted versus the composition in the monomer phase for mixtures of LiFOS with LiTS, LiDS, and LiDeS,
respectively. The predicted curves from the group contribution method are shown for comparison. We have already proposed that the mixture cmc can be successfully given by the group contribution method.’ An important feature of the group method is to be able to describe the series of mixed systems containing the same functional groups by the use of the same interaction parameters. In contrast, the regular solution theory cannot make a prior prediction of the micelle composition even for the series of mixed systems containing the same functional groups. In conclusion, ultrafiltration is a useful method to indicate the variation of monomer concentrations and compositions beyond the mixture cmc. The micelle demixing region was increased by the increase of the alkyl chain length as expected. The fixed monomer composition, which corresponds to the azeotropic condition, was interpreted by the occurrence of micelle demixing. Acknowledgment. We are grateful to Dainippon Ink Chemical Industry Co., Ltd., for providing the fluorocarbon surfactant. Registry No. LiFOS, 29457-72-5; SPFO, 335-95-5; SDS, 151-21-3; LiDeS, 2044-55-5; LiDS, 2044-56-6; LiTS, 52886-14-3.
Ellipsometric Observation of the Adsorption of Sodium Dodecyl Sulfate Gregory J. Besio,t Robert K. Prud’homme, and Jay B. Benziger* Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544 Received March 16, 1987. In Final Form: July 14, 1987 In this paper we report the observation of the formation of surface micelles, or hemimicelles, using ellipsometry. SDS is adsorbed onto a platinum electrode from solutions with concentrations below the critical micelle concentration (crnc). Formation of the surface micelles is initiated by applying a potential at the electrode, inducing local concentrations above the critical hemimicelle concentration (chmc) near the electrode surface. The thickness of the adsorbed layer, as measured by ellipsometry, based on a film refractive index of 1.44,at low potentials is 10-15 A, which corresponds to the extended dimension of the SDS molecule. At higher potentials, transitions to films 2-3 times the initial film thickness are observed. Introduction In solution, surfactants spontaneously aggregate, as a result of their unique hydrophilic/ hydrophobic nature. The aggregation is a function of the solution concentration of the surfactant. At low concentrations, typically lesa than 1 mM, the surfactant molecules exist in solution as monomers and dimers. Above a certain transition concentration, known as the critical micelle concentration or cmc, the surfactant will form micelles. As the surfactant solution concentration increases further, the number of micelles in the solution increases until a second transition occurs in which the aggregation number of the micelles and the shape of the micelles change. Surfactant solutions in contact with solid surfaces exhibit similar transitions to their pure solution anal~gs.l-~ A solid surface in contact with a dilute surfactant solution will have individually adsorbed surfactant molecules. As the solution concentration is increased, surface aggregation +Currentaddress: General Electric CRD, PO Box 8, Building K-1, 4B33, Schenectady, New York 12301
will occur in parallel to the solution aggregation. At a concentration of approximately 0.1 cmc, surface aggregates termed “hemimicelles”will begin to form. Hemimicelles are the adsorbed equivalent of micelles. Above the critical hemimicelle concentration (chmc),5-7 the surface coverage rapidly increases until the entire surface is coated with a surfactant bilayer, a t which point the surface is saturated, and no further appreciable adsorption will occur. The (1) Schamehorn, J. F.;Schecter, R. S.;Wade, W. H. J. Colloid Interface Sci. 1982,85(2),463. (2) Schamehorn, J. F.; Schecter, R. S.;Wade, W. H. J. Colloid Interface Sci. 1982,85(2),479. (3) Nunn, C. C.;Schecter, R. S.;Wade, W. H. ACS Symposium on Chemistry and Enchanced Oil Recovery, Atlanta, GA, 1981. (4) Nunn, C. C.;Schecter, R. S.;Wade, W. H. J . Colloid Interface Sci. 1981, 80(2), 598. (5) Harwell, J. H.; Schecter, R. S.; Wade, W. H. AIChe. J. 1985,31(3), Helfferich, F. G.; Schectr, R. S. AIChe. J. 1982,28(3), 415. Harwell, J. H.; 448. .
(6) Lim, H. K.;Fernandez, M. E.; Schamehorn, J. F.; Nunn, C. C.; Wade, W. H.; Schecter, R. S.,to be published. (7) Harwell, J. H.; Hoskins, J. C.; Schecter, R. S.; Wade, W. H. Langmuir 1985, 1, 251.
0 1988 American Chemical Society 0~43-~463/8S/2404-0~40$01.50/0
