Langmuir 1997, 13, 4569-4573
4569
Micellar Immiscibility of Lithium 1,1,2,2-Tetrahydroheptadecafluorodecyl Sulfate and Lithium Tetradecyl Sulfate Mixture Tsuyoshi Asakawa,* Kouji Amada, and Shigeyoshi Miyagishi Department of Chemistry and Chemical Engineering, Faculty of Engineering, Kanazawa University, Kanazawa 920, Japan Received February 10, 1997. In Final Form: May 19, 1997X We examined the immiscibility of fluorocarbon and hydrocarbon surfactants having the same head groups, i.e., lithium 1,1,2,2-tetrahydroheptadecafluorodecyl sulfate (LiHFDeS) and lithium tetradecyl sulfate (LiTS). The micellar pseudophase diagram was determined by conductivity and fluorescence quenching methods. The coexistence of two kinds of micelles has been demonstrated by the fluorescence quenching of pyrene with a fluorocarbon quencher. Pyrene is located in LiTS-rich micelles, and its fluorescence is quenched by micelle-solubilized quenchers. However, the pyrene fluorescence is hardly quenched by a fluorocarbon quencher when a second type of micelle rich in fluorocarbon surfactant appears. The collision probability between the fluorocarbon quencher and pyrene will be small within the lifetime of the excited pyrene because the fluorocarbon quencher and pyrene are separately solubilized in LiTS-rich and LiHFDeS micelles, respectively. The effect of addition of salt on the depression of quenching was also examined.
Introduction The solution properties of surfactants are mainly determined by their molecular structure and the nature of the hydrophobic chain. Surfactant mixtures show various behaviors depending on the kinds of surfactants. Especially, mixtures of fluorocarbon and hydrocarbon surfactants have been reported by several investigators from the viewpoints of both practical application and solution theory.1-7 Pairs of anionic-anionic surfactants which have different head groups were mainly investigated to reveal a phenomenon of microscopic demixing. It is necessary to demonstrate the less miscible nature of fluorocarbon and hydrocarbon chains in a micelle by using surfactant mixtures having the same head groups. Tanford indicated that the methylene group closest to the head group is not hydrophobic at all.8 For small globular micelles one or two methylene groups may be outside the micelle core. The methylene groups close to the head group may hardly interact with other hydrophobic chains. The variation of miscibility caused by such effects of hydrophobic chains has not been examined. The coexistence of two kinds of mixed micelles was proposed for fluorocarbon and hydrocarbon surfactant mixtures by the simulation of mixture cmc curves with several solution models.1-6 The plausible model of separated fluorocarbon-rich and hydrocarbon-rich micelles under certain conditions was verified by several experimental methods.9,10 The microscopic aspect of demixing micelles should be investigated by an appropriate experimental method. * E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, August 1, 1997. (1) Mukerjee, P.; Yang, A. Y. S. J. Phys. Chem. 1976, 80, 1388. (2) Shinoda, K.; Nomura, T. J. Phys. Chem. 1980, 84, 365. (3) Funasaki, N.; Hada, S. J. Phys. Chem. 1980, 84, 736. (4) Kamrath, R. F.; Franses, E. I. Ind. Eng. Chem. Fundam. 1983, 22, 230. (5) Asakawa, T.; Johten, K.; Miyagishi, S.; Nishida, M. Langmuir 1985, 1, 347. (6) Hoffman, H.; Possnecker, G. Langmuir 1994, 10, 381. (7) Asakawa, T.; Hisamatsu, H.; Miyagishi, S. Langmuir 1995, 11, 478. (8) Tanford, C. The Hydrophobic Effect; Wiley-Interscience Publication: New York, 1980. (9) Carfors, J.; Stilbs, P. J. Phys. Chem. 1984, 88, 4410. (10) Asakawa, T.; Miyagishi, S.; Nishida, M. Langmuir 1987, 3, 821.
S0743-7463(97)00131-5 CCC: $14.00
Recently, we indicated that the method of pyrene fluorescence quenching by a fluorocarbon quencher will be useful to verify the coexistence of two kinds of mixed micelles.11 Pyrene is almost completely solubilized in hydrocarbon-rich micelles, and its fluorescence is quenched by not only cetylpyridinium chloride (CPC) but also (1,1,2,2-tetrahydroheptadecafluorodecyl)pyridinium chloride (HFDePC) quenchers. CPC quenches the fluorescence of pyrene, while HFDePC hardly quenches it in fluorocarbon and hydrocarbon surfactant mixtures because HFDePC would be solubilized in fluorocarbon micelles. Since pyrene tends to partition almost completely into hydrocarbon micelles with exit rates smaller than their fluorescence decay rates, a collision probability between pyrene and HFDePC would be small within the lifetime of the excited pyrene. That is, the depression of quenching by only HFDePC will be ascribed to the separated solubilization of pyrene and HFDePC into two kinds of micelles. In this paper, we report aqueous solution properties of a new surfactant, lithium 1,1,2,2-tetrahydroheptadecafluorodecyl sulfate (LiHFDeS), which has the same head group as the hydrocarbon surfactant lithium tetradecyl sulfate (LiTS). Since LiHFDeS has two methylene groups closest to the head group, we could demonstrate the less miscible nature of fluorocarbon and hydrocarbon chains in a micelle core. We also intend to verify experimentally the coexistence of two kinds of mixed micelles using the pyrene fluorescence quenching method. Experimental Section Materials. Lithium tridecyl sulfate (C13H27SO4Li, LiTrS) and lithium tetradecyl sulfate (C14H29SO4Li, LiTS) were prepared by the procedures reported previously.5 LiHFDeS was synthesized from 1,1,2,2-tetrahydroheptadecafluorodecanol (PCR Inc.) by sulfation with chlorosulfonic acid in diethyl ether.12 The lithium salt of the product was washed with diethyl ether many times. After the surfactant was extracted with ethanol, it was recrystallized from a carbon tetrachloride-ethanol (2:1) mixture. Quenchers, CPC, and HFDePC were the same reagents as reported previously.7 The other reagents were of guaranteed grade. (11) Asakawa, T.; Hisamatsu, H.; Miyagishi, S. Langmuir 1996, 12, 1204. (12) Asakawa, T.; Hashikawa, M.; Amada, K.; Miyagishi, S. Langmuir 1995, 11, 2376.
© 1997 American Chemical Society
4570 Langmuir, Vol. 13, No. 17, 1997
Figure 1. Surface tensions of aqueous surfactant solutions: (O) LiTS, (4) LiTrS, (b) LiHFDeS. Measurements. Conductivity measurements were carried out using a conductivity meter, Model CM-20S (TOA Electronics Ltd.). Surface tensions were measured by the Wilhelmy technique (Kyowa Kagaku surface tension meter, Model A-3). Steadystate fluorescence spectra of pyrene were obtained by exciting the samples at 335 nm (Hitachi F-3010 spectrometer, excitation slit width 5 nm, emission slit width 1.5 nm). The spectra were used to determine the ratios (I1/I3) of the fluorescence intensities of the first (I1, 373 nm) and third (I3, 384 nm) vibronic peaks of monomeric pyrene solubilized in micellar solutions. The fluorescence quenching ratios (I/I0) were calculated by using the fluorescence intensities at 384 nm in the absence of quencher (I0) and in the presence of quencher (I). All experiments were performed at 25 °C.
Asakawa et al.
Figure 2. Specific conductivity vs surfactant concentration in aqueous solution: (O) LiTS, (b) LiHFDeS, (4) equimolar LiHFDeS-LiTS mixtures at constant mole fraction (R ) 0.5).
Results and Discussion The surface tension method was used to check the cmc and purity of the new fluorocarbon surfactant LiHFDeS. Figure 1 shows the surface tensions of LiTS, LiTrS, and LiHFDeS against the logarithm of surfactant concentration in aqueous solutions. Their surface tension curves did not show minima around their cmc’s. The absence of 1,1,2,2-tetrahydroheptadecafluorodecanol in the LiHFDeS sample was also confirmed by HPLC analysis. The cmc’s of LiTS, LiTrS, and LiHFDeS were 2.3, 4.2, and 2.9 mM, respectively. The cmc of LiHFDeS corresponds to that of LiTS. That is, the hydrophobicity of eight CF2 groups corresponds to that of twelve CH2 groups. The hydrophobicity of fluorocarbon was about 1.5 times that of hydrocarbon judging from the cmc’s of surfactants. The surface tensions of LiHFDeS and LiTS above their cmc’s were 21.0 and 41.0 mN/m, respectively; that is, LiHFDeS is more surface active than LiTS. The minimum area per molecule at the solution-air interface was calculated from the equation of the Gibbs adsorption isotherm. The value of LiHFDeS (0.46 nm2/molecule) was similar to that of LiTS (0.48 nm2/molecule) even if the fluorocarbon chain was bulky. The compactness in the adsorbed film seems to depend on the cross sectional area of the sulfate head group. The conductivity method was also used to measure the cmc’s of pure and mixture surfactant systems. Figure 2 shows the conductivity curves of LiTS, LiHFDeS, and their equimolar mixture at fixed composition R ) 0.5 (R: mole fraction of LiHFDeS). The cmc’s of LiTS and LiHFDeS were 2.2 and 2.6 mM, respectively. Their mixture cmc’s were also determined by the conductivity method. The mixture cmc of equimolar LiHFDeS-LiTS (R ) 0.5) was significantly higher than those of the single surfactant systems. This demonstrates the immiscibility of LiHFDeS and LiTS in the micelles. We evaluated the immiscibility of LiHFDeS-LiTS systems by the mixture cmc curves in detail. Figure 3
Figure 3. Cmc’s of mixtures of LiHFDeS and LiTS. The plotted points are experimental values. The solid line and the dotted line are the cmc curve and the micellar composition curve predicted by regular solution theory, respectively.
shows the measured conductance cmc values in comparison with the calculated ones using a regular solution model according to Shinoda et al.2 The mixture cmc curve was well fitted when β ) 2.5 (β: interaction parameter), which predicts that the pseudophase separation region occurs between XH ) 0.144 and XF ) 0.856 (XH, mole fraction of LiHFDeS in LiTS-rich micelle; XF, mole fraction of LiHFDeS in LiHFDeS-rich micelle). That is, the separation region of the LiHFDeS-LiTS system (that is to say, FC8C2-C12C2 mixture in abbreviation) was expanded in comparison with those of the C8F17COONH4-C12H25SO4NH4 and C8F17SO3Li-C12H25SO4Li systems; i.e., for FC8C12 mixtures, β ) 2.2 means that the separation region occurs between XH ) 0.25 and XF ) 0.75 with mole fraction of fluorocarbon surfactant.2,5 The introduction of the two methylene chains closest to the head group distinctly induced the immiscibility of fluorocarbon and hydrocarbon chains in a micelle core. We are able to demonstrate the less miscible nature of fluorocarbon and hydrocarbon chains in a micelle composed of two surfactants having the same head groups. Next, we focused on the microscopic aspect of demixing micelles by fluorescence probe methods. The fluorescence intensity ratio of the first and third vibronic peaks of pyrene has been known to be sensitive to solvent polarity. The decrease in the value I1/I3 is an indication of solubilization into a more hydrophobic environment. Under our experimental conditions, the values of I1/I3 in water, hexane, and perfluorohexane were 1.85, 0.65, and 0.57, respectively. The value of I1/I3 ) 1.65 in LiHFDeS
Micellar Immiscibility of Anionic Surfactant Mixture
Figure 4. Pyrene fluorescence ratios I1/I3 vs total surfactant concentration with constant mole fraction for LiHFDeS-LiTS systems. Pyrene concentration was fixed at 1.0 × 10-7 M: (O) R ) 0 (LiTS), (4) R ) 0.5, (0) R ) 0.7, (]) R ) 0.9.
micelles was close to that of water. LiHFDeS micelle has a low solubilization power toward pyrene. If pyrene was solubilized, the pyrene in LiHFDeS micelles would be hydrated with an appreciable water contact. Figure 4 shows the dependence of I1/I3 on total surfactant concentration of LiHFDeS-LiTS mixtures with constant mole fraction. The abrupt increase in the value of I1/I3 was observed along with the micelle formation for LiTS single surfactant and LiTS-rich mixtures. The I1/I3 values in the region of R ) 0-0.5 were similar to those of LiTS pure micelle. However, the I1/I3 values increased with increasing mole fraction of LiHFDeS (R) and indicated values intermediate between those of the two single-surfactant micelle systems. The LiTS-rich micelle first appears at the mixture cmc in the region of R ) 0-0.5, while the LiHFDeS micelle first appears at the mixture cmc in the region of R ) 0.6-1.0. The appreciable decrease in the value of I1/I3 at the higher concentration of R ) 0.7 would be ascribed to the appearance of second type of LiTS-rich micelle. The pair of probes, pyrene and CPC, have been well investigated by many investigators.13-16 Pyrene is localized almost completely into micelles due to extremely low solubility toward water.17 Cationic CPC also tends to partition almost completely into micelles due to attractive electrostatic interactions toward anionic micelles. The exit rates of pyrene are smaller than the fluorescence decay rates.18 Therefore, micelle-solubilized pyrene molecules are expected to remain within host micelles during a fluorescence time scale. The average occupancy number of pyrene is kept small (about 0.1), and thus a micelle contains zero or one probe. The average occupancy number of CPC quencher is more than 1 for the purpose of effective collisions toward pyrene. The pair of pyrene and CPC can fulfill these conditions described above, and thus intramicellar quenching is predominant. A quencher having a fluorocarbon chain was introduced to this system in order to clarify the microscopic aspect of demixing micelles. Since the quenching of pyrene emission is due to the pyridinium group of CPC, HFDePC (13) Almgren, M. Adv. Colloid Interface Sci. 1992, 41, 9. (14) Hashimoto, S.; Thomas, J. K. J. Colloid Interface Sci. 1984, 102, 152. (15) Sapre, A.; Rao, K. S.; Rao, K. N. J. Phys. Chem. 1980, 84, 2281. (16) Malliaris, A.; Lang, J.; Zana, R. J. Chem. Soc., Faraday Trans. 1 1986, 82, 109. (17) Schwarz, F. P. J. Chem. Eng. Data 1977, 22, 273. (18) Singer, L. A. In Solution Behavior of Surfactants; Mittal, K. L., Fendler, E. J., Eds.; Plenum Press: New York, 1982; Vol. 1. (19) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951.
Langmuir, Vol. 13, No. 17, 1997 4571
Figure 5. Fluorescence quenching ratios of pyrene in fixed 10 mM total surfactant concentrations as a function of quencher concentration. Pyrene concentration was fixed at 1.0 × 10-5 M: (O) CPC, (b) HFDePC in 10 mM LiTS, (4) CPC, (2) HFDePC in 10 mM LiHFDeS-LiTS (R ) 0.5).
will also have quenching ability. Figure 5 shows the fluorescence quenching of pyrene solubilized in pure LiTS micelles and mixed micelles as a function of HFDePC quencher concentration in comparison to CPC. The fluorescence of pyrene was effectively quenched by HFDePC with increasing its concentrations similarly to CPC. HFDePC is solubilized in LiTS micelles despite the immiscibility of fluorocarbon and hydrocarbon chains because of the attractive electrostatic interactions between the cationic quencher and anionic surfactants. There is no significant difference in quenching ability between CPC and HFDePC quenchers. The small difference of quenching ability between HFDePC and CPC will come from the difference in the diffusion time of quenchers in micelles, because the intramicellar quenching would be a diffusioncontrolled encounter between pyrene and quenchers. The diffusion of HFDePC will be slightly restricted compared with CPC due to the bulky and rigid fluorocarbon chain. The difference will also come from the immiscibility between fluorocarbon chain and pyrene. That is, micelles solubilizing pyrene have less solubilization ability toward HFDePC having a fluorocarbon chain compared with LiTS pure micelles. The effects of nonrandom distribution and/ or segregation of HFDePC among LiTS micelles are unsolved problems. The micellar aggregation numbers of LiTS micelles were 53 and 47 by using CPC and HFDePC, respectively, if we evaluated by the usual plot of ln(I/I0) versus quencher concentration.18 In spite of a smaller value compared with the expected aggregation number, the fluorescence quenching method will be useful to our purposes of clarifying nonideal behavior of mixed micelles. The fluorescence of pyrene was effectively quenched by CPC in not only pure LiTS micelles but also mixed micelles of LiHFDeS-LiTS. In contrast with CPC, HFDePC hardly quenched pyrene emission with increasing quencher concentrations only in the mixed micelles as shown in Figure 5. Both pyrene and CPC are localized almost completely into LiTS-rich micelles. Under such conditions, the quenching by CPC is considered to be effective both in pure and mixed micelles. However, HFDePC tends to partition into LiHFDeS-rich micelles due to the immiscibility of fluorocarbon and hydrocarbon chains when LiTS-rich and LiHFDeS-rich micelles are both present in aqueous solutions. Therefore, the collision probability between pyrene (in LiTS-rich micelles) and HFDePC (in LiHFDeS-rich micelles) would be inhibited within the lifetime of the excited pyrene. The intramicellar quenching is predominant in this system, and the intermicellar
4572 Langmuir, Vol. 13, No. 17, 1997
Figure 6. Effect of additives on fluorescence quenching ratios in fixed 10 mM LiTS systems. Pyrene and quencher (HFDePC) concentrations were fixed at 1.0 × 10-5 M and 2.5 × 10-4 M, respectively: (O) LiTrS, (4) LiCl, (b) LiHFDeS.
exchange of quencher not only via the bulk but also via micellar collision can be neglected.20,21 That is, the depression of quenching only by HFDePC only in mixed micelles could be ascribed to the coexistence of LiTS-rich and LiHFDeS-rich micelles. This behavior is in accordance with the demixing micelles predicted by the regular solution model.2-4 There is almost no quenching in spite of more than two HFDePC molecules per micelle in average occupancy. This no-quenching behavior indicates the immiscibility of two surfactants in micelles and the formation of almost pure LiTS micelles even with binary surfactant systems. Next, we reconfirmed the quenching behavior in mixtures by some additives. Figure 6 shows the effect of additives, LiCl, LiTrS, and LiHFDeS, toward the fluorescence quenching ratios of pyrene in a fixed 10 mM LiTS system. The ratios I/I0 increased with increasing LiTS concentration. The concentration of micelle aggregates increases by the addition of LiTrS with ideal mixing in micelles. Therefore, the decreases in the average occupancy numbers of pyrene and quencher led to the decrease in the collision probability between them. The ratios I/I0 were almost constant up to 10 mM in the case of LiCl. This suggests that the micelle aggregation number was almost kept constant at these low salt concentrations. In contrast, the ratio I/I0 abruptly increased with increasing LiHFDeS concentration and remained almost constant at higher LiHFDeS concentrations. The quenching ratio was affected by the addition of LiHFDeS surfactant. If perfect demixing occurs, almost no change in the ratio I/I0 should be expected up to about 2 mM considering the cmc of LiHFDeS (2.6 mM). The abrupt increase in I/I0 suggests that LiTS micelles will solubilize the added LiHFDeS molecules. The diffusion of HFDePC quencher must be slightly restricted in LiHFDeS-solubilized LiTS micelles in comparison to pure LiTS micelles probably due to the formation of LiHFDeS-HFDePC complex. When the LiTS micelles solubilize LiHFDeS molecules up to the limit, the second type of micelles rich in LiHFDeS can begin to appear by the further addition of LiHFDeS. Under such conditions, the collision probability between the separately solubilized pyrene and HFDePC will be suppressed depending on the immiscibility of fluorocarbon and hydrocarbon species. Figure 7 shows LiHFDeS concentration dependence of I/I0 using HFDePC quencher at various LiTS concentra(20) Tachiya, M. J. Chem. Phys. 1982, 76, 340. (21) Dederen, J.; Auweraer, M. V.; Schryver, F. C. D. Chem. Phys. Lett. 1979, 68, 451.
Asakawa et al.
Figure 7. Variation of fluorescence quenching ratios by the addition of LiHFDeS at fixed LiTS concentrations. Pyrene and quencher concentrations were fixed at 1.0 × 10-5 M and 2.5 × 10-4 M, respectively: (O) CPC, (b) HFDePC in 5 mM LiTS, (4) CPC, (2) HFDePC in 7 mM LiTS, (0) CPC, (9) HFDePC in 20 mM LiTS.
Figure 8. Micellar pseudophase diagram of LiHFDeS-LiTS. The dotted line is the calculated second cmc curve (XH ) 0.144, XF ) 0.856, XAZ ) 0.46, CAZ ) 3.7 mM).7 The dashed line is the calculated second cmc curve for perfect demixing of micelles: (O) mixture cmc’s by conductivity method, (4) second cmc’s by surface tension method, (2) second cmc’s by fluorescence quenching method.
tions in comparison to CPC quencher. The fluorescence of pyrene was effectively quenched by CPC in all systems. The ratios I/I0 increased with increasing LiTS concentration due to the increase in the concentration of micelle aggregates. The ratio I/I0 slightly increased by the addition of LiHFDeS. This suggests that the concentration of LiTS-rich micelle aggregates would slightly increase along with the solubilization of LiHFDeS, and/or the solubilized LiHFDeS would slightly affect the diffusion of CPC molecules. The depression of fluorescence quenching only by HFDePC was also observed by the addition of LiHFDeS. The ratio I/I0 abruptly increased with the addition of LiHFDeS, became nearly 1, and remained almost constant at higher LiHFDeS concentrations. The inflection points of curves of I/I0 near plateau regions could be assigned to a second cmc, at which two kinds of mixed micelles appear in solution because the constant almost lack of quenching corresponds to the separated solubilization of pyrene and HFDePC. The inflection points hardly depend on LiTS concentrations, i.e., those are about 1.5 mM LiHFDeS. Figure 8 shows the second cmc’s determined by surface tension and the fluorescence quenching method with comparison to calculated ones. The experimentally determined second cmc’s were close to the calculated curve
Micellar Immiscibility of Anionic Surfactant Mixture
Figure 9. Fluorescence quenching ratios vs total surfactant concentration with constant mole fraction for LiHFDeS-LiTS systems. Pyrene and quencher (HFDePC) concentrations were fixed at 1.0 × 10-7 M and 2.5 × 10-4 M, respectively: (O) R ) 0 (LiTS), (4) R ) 0.1, (0) R ) 0.2, (]) R ) 0.3.
of a perfect demixing. The LiTS-rich micelle first appears at the mixture cmc in the region of R ) 0-0.46. As the total surfactant concentration increases with a certain fixed mole fraction, the monomeric LiHFDeS increases until the second cmc, at which the LiHFDeS-rich micelle appears. Both LiHFDeS-rich and LiTS-rich micelles, which are mutually saturated, coexist above the second cmc. As shown in Figure 8 (dotted line), the experimental second cmc’s cannot be simulated if the compositions of splitting micelles are XH ) 0.144 and XF ) 0.856 according to the prediction of the regular solution model. The probe, pyrene, may disturb the micellar property and/or micellar immiscibility. Therefore, the concentration of pyrene was lowered to 10-7 M, which is lower than the solubility in water. We can also examine the fluorescence quenching behavior below and above the cmc. Figure 9 shows the fluorescence quenching for LiHFDeSLiTS mixtures with constant mole fraction as a function of total surfactant concentration. The ratio I/I0 in water was rather high reflecting the low collision probability between pyrene and HFDePC. There was no specific interaction between pyrene and HFDePC molecules. However, the ratio I/I0 abruptly decreased with the addition of surfactant below the cmc. The complex of LiTS and HFDePC would occur, and pyrene would interact with premicelles composed of LiTS and HFDePC. Above the cmc, the ratio I/I0 slightly increased with increasing the total surfactant concentration for the LiTS single system. The slight increase is due to the increase in concentration of micelle aggregates. In contrast, the ratio I/I0 abruptly increased with increasing the total surfactant concentration for LiHFDeS-LiTS mixtures. At higher concentrations of R ) 0.2 and 0.3, the ratio I/I0 became almost 1; that is, the HFDePC quencher hardly quenched the pyrene emission. This behavior cannot be explained by the increase in concentration of micelle aggregates judging from the large difference in I/I0 values between R ) 0 and R ) 0.3. This behavior is due to the coexistence of LiHFDeS-rich (solubilized HFDePC quencher) and LiTSrich (solubilized pyrene) micelles as described above. The inflection points of curves of I/I0 (at almost no quenching) decreased with increasing from R ) 0.2 to R ) 0.3. This decrease correspond to the decrease in second cmc with increasing R (Figure 8). We further investigated the effect of the addition of salt on the depression of quenching in order to get information on the effect of counterion concentration on micellar immiscibility. Figure 10 shows the fluorescence quenching ratios of pyrene in fixed 10 mM LiHFDeS-LiTS as a
Langmuir, Vol. 13, No. 17, 1997 4573
Figure 10. Effect of addition of LiCl toward fluorescence quenching ratios in fixed 10 mM LiHFDeS-LiTS systems. Pyrene and quencher concentrations were fixed at 1.0 × 10-5 M and 2.5 × 10-4 M, respectively: (O) CPC, (b) HFDePC in 10 mM LiTS, (4) CPC, (2) HFDePC in 10 mM LiHFDeS-LiTS (R ) 0.02), (0) CPC, (9) HFDePC in 10 mM LiHFDeS-LiTS (R ) 0.2).
function of LiCl concentration. CPC quencher effectively quenched the pyrene emission both in the LiTS single system and LiHFDeS-LiTS mixtures. The slight decrease in the ratio I/I0 with increasing LiCl concentration is due to the increase in micellar aggregation numbers. But the micellar aggregation numbers of LiTS-rich micelles would be scarcely affected by the solubilization of LiHFDeS. For R ) 0.02 LiHFDeS-LiTS mixtures, the ratio I/I0 using HFDePC remained almost constant with sufficient addition of LiCl. Sufficient addition of LiCl will reduce the monomeric LiHFDeS concentration. This suggests that the concentrations of monomeric LiHFDeS will not affect the quenching of pyrene emission. But the diffusion of HFDePC quencher was slightly restricted in LiHFDeSsolubilized LiTS micelles in comparison to CPC quencher. For R ) 0.2 LiHFDeS-LiTS mixtures, the ratio I/I0 using HFDePC remained almost 1 with sufficient addition of LiCl. The micellar immiscibility was scarcely affected by the counterion concentration. The addition of LiCl suppresses the electrostatic repulsion between head groups, leading to more ordered packing of hydrophobic chains in the micelles. The almost demixing behavior was hardly changed by the compactness in micelle surface. In conclusion, we have presented direct experimental evidence of the coexistence of LiHFDeS-rich and LiTSrich micelles. LiTS-rich micelles first appear at the mixture cmc in the LiTS-rich region. As the total surfactant concentrations increase with a certain fixed mole fraction, the LiTS-rich micelles can solubilize LiHFDeS molecules to a certain extent judging from the following results. The regular solution model predicted that the separation region will be 0.144-0.856 (mole fraction of LiHFDeS) by fitting the mixture cmc curve. The fluorescence quenching method indicated the abrupt depression of quenching by the slight addition of LiHFDeS using HFDePC (see Figure 6). At the second cmc, LiHFDeS-rich micelles will appear. The LiHFDeS-rich micelles can solubilize all of HFDePC quencher judging from the almost lack of quenching using HFDePC. The introduction of the two methylene chains closest to the head group distinctly demonstrated the immiscibility of fluorocarbon and hydrocarbon chains within the micelle core. Acknowledgment. We gratefully acknowledge that this work was supported in part by the Asahi Glass Foundation. LA970131M
