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Langmuir 1999, 15, 3464-3468
Demicellization of Sodium Perfluorooctanoate and Dodecyl Sulfate Mixtures Revealed by Pyrene Fluorescence Quenching Tsuyoshi Asakawa* and Shigeyoshi Miyagishi Department of Chemistry and Chemical Engineering, Faculty of Engineering, Kanazawa University, Kanazawa 920-8667, Japan Received October 27, 1998. In Final Form: March 2, 1999 The behavior of fluorocarbon and hydrocarbon quenchers toward pyrene fluorescence was informative as to not only the distribution of quenchers among micelles but also the microscopic micelle demixing. The slight differences of quenching extent between fluorocarbon and hydrocarbon quenchers were observed to come from the deviation from Poisson distribution of fluorocarbon quenchers among hydrocarbon micelles. The intermicellar statistical distribution of quenchers could be influenced by the low affinity between fluorocarbon and hydrocarbon chains. The lost quenching of pyrene was observed due to the separated solubilization of pyrene and fluorocarbon quencher into two types of mixed micelles. However, the differences of quenching extent between fluorocarbon and hydrocarbon quenchers became small for sodium perfluorooctanoate-dodecyl sulfate mixtures at high concentration and temperature. The mutual miscibility in a micelle increased due to the increase of the micelle size under high surfactant concentrations and the heat of mixing at high temperature. The pyrene fluorescence quenching method using fluorocarbon quencher was effective way to indicate the demicellization phenomenon.
Introduction Mixtures of fluorocarbon and hydrocarbon surfactants show unusual characteristics of micellization in aqueous solutions.1-4 A peculiar phenomenon demicellization, has been proposed for two types of micelles to coexist.5-7 There are only few examples of this behavior, probably due to experimental difficulties. Recently, we have demonstrated that the method of pyrene fluorescence quenching using a fluorocarbon quencher is useful to verify the coexistence of two types of micelles.8 Pyrene and fluorocarbon quencher tend to be separately solubilized into two types of micelles, i.e., hydrocarbon-rich and fluorocarbon-rich micelles, respectively. The small collision probability between pyrene and fluorocarbon quencher within the lifetime of excited pyrene result in lost quenching of pyrene fluorescence. The fluorescence behavior of pyrene can be used to reveal the microscopic aspects of intramicellar events such as demicellization. Fluorescence quenching in micelles is supposed to be diffusion controlled for the pair of pyrene and cetylpyridinium quencher.9-12 The quenching occurs when pyrene * To whom correspondence should be addressed.
[email protected].
E-mail:
(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) Holland, P. H.; Rubingh, D. N. Mixed Surfactant Systems; ACS Symposium Series 501; American Chemical Society: Washington, DC, 1992 and references therein. (4) Kissa, E., Fluorinated Surfactants; Marcel Dekker Inc.: New York, 1994. (5) Mysels, K. J. J. Colloid Interface Sci. 1978, 66, 331. (6) Funasaki, N.; Hada, S. J. Colloid Interface Sci. 1980, 73, 425. (7) Lake, M. J. Colloid Interface Sci. 1983, 91, 496. (8) Asakawa, T.; Hisamatsu, H.; Miyagishi, S. Langmuir 1996, 12, 1204. (9) Almgren, M. Adv. Colloid Interface Sci. 1992, 41, 9. (10) Hashimoto, S.; Thomas, J. K. J. Colloid Interface Sci. 1984, 102, 152. (11) Spare, A.; Rao, K. S.; Rao, K. N. J. Phys. Chem. 1980, 84, 2281. (12) Malliaris, A.; Lang, J.; Zana, R. J. Chem. Soc., Faraday Trans 1 1986, 82, 109.
and quenchers encounter each other. The lifetime of excited pyrene is shorter than the time scale of solubilizate exchange. The distribution of quenchers among micelles only influences the extent of quenching because micelles act as cages. Under such conditions, the distribution has been usually treated as Poisson statistics. Recently, Barzykin reported the statistical distribution model of probes among micelles with a pairwise interaction between probes.13 The distribution was altered by interactions between probes, yielding the differences from the Poisson distribution. In this paper, the effect of fluorocarbon chain toward the distribution among micelles was examined in aqueous micelle solutions of hydrocarbon surfactants. Then, the behavior of pyrene fluorescence quenching was investigated in fluorocarbon and hydrocarbon surfactant mixtures. The demicellization phenomenon can be expected for sodium perfluorooctanoate-dodecyl sulfate mixtures not only at high surfactant concentration but also at high temperature. Experimental Section Materials. Fluorocarbon and hydrocarbon surfactants were prepared by the same procedures as reported previously.14 Abbreviations for the surfactants are as follows: sodium perfluorooctanoate (SPFO), C7F15COONa; lithium perfluorooctanesulfonate (LiFOS), C8F17SO3Li; sodium decyl sulfate (SDeS), C10H21SO4Na; sodium dodecyl sulfate (SDS), C12H25SO4Na; lithium dodecyl sulfate (LiDS), C12H25SO4Li. Pyrene (Aldrich) was used as received. Cationic fluorocarbon quenchers were synthesized from the corresponding 1,1,2,2-tetrahydroperfluoroalkyl iodide (PCR Inc., FL) as reported previously.15 Abbreviations for the quenchers are as follows: HFOPC, [C6F13CH2CH2NC5H5]+Cl-; HFDePC, [C8F17CH2CH2NC5H5]+Cl-; HFDPC, [C10F21CH2CH2NC5H5]+Cl-. Cetylpyridinium chloride (CPC) was obtained from Tokyo Kasei Kogyo Co., Ltd. and recrystallized (13) Barzykin, A. V. Chem. Phys. 1992, 161, 63 (14) Asakawa, T.; Johten, K.; Miyagishi, S.; Nishida, M. Langmuir 1985, 1, 347. (15) Asakawa, T.; Hisamatsu, H.; Miyagishi, S. Langmuir 1995, 11, 478.
10.1021/la981513a CCC: $18.00 © 1999 American Chemical Society Published on Web 04/24/1999
Demicellization of SPFO-SDS Mixtures
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Figure 1. Fluorescence intensity ratio I1/I3 of pyrene in aqueous solutions of quenchers: (b) HFOPC; (2) HFDePC; (9) HFDPC; (O) CPC.
Figure 2. Fluorescence intensity ratio I0/I of pyrene in aqueous solutions of quenchers: (b) HFOPC; (2) HFDePC; (9) HFDPC; (O) CPC.
three times from an acetone-ethanol mixture. The other reagents were of guaranteed grade. Measurements. Pyrene was dissolved in water by stirring overnight at 25 °C. Then, surfactant was accurately added to part of a 3 × 10-7 M pyrene aqueous solution. Then, quencher was accurately added to it just before fluorescence measurements. Steady-state fluorescence spectra of pyrene were recorded with a Hitachi F-3010 spectrometer using a thermostated cell. The typical pyrene monomer fluorescence spectra were observed by exciting at 335 nm (excitation band-pass 5 nm, emission bandpass 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. The fluorescence intensity ratios (I/I0) were calculated by using the fluorescence intensities at 384 nm in the absence of quencher (I0) and in the presence of it (I).
concentration effect of pyrene and quencher into micelles. The difference in the I0/I value between CPC and HFDePC would be due to the low affinity between pyrene and fluorocarbon chain. The slight but steady increase in the I0/I value for HFOPC was observed because cmc is higher than 20 mM. The slight change in the I0/I value for HFDPC is due to the low solubility toward water. The pair of pyrene and CPC quencher have been used to determine micelle aggregation numbers.19 Both pyrene and quencher are completely incorporated into the micelles with no intermicellar transfer during the time of fluorescence quenching. The average occupancy number of pyrene is kept small, and thus most micelles do not contain pyrene, but single occupancy dominates among the occupied micelles. The quenching in a micelle containing both an excited pyrene and quencher is much faster than the fluorescence decay, so that unquenched fluorescence is observed only from micelles without quenchers.19 The distribution of quencher among micelles is known to obey Poisson statistics with noninteracting quenchers. Therefore, the fluorescence intensity ratio I/I0 relates to the probability for micelles containing no quencher.
Results and Discussion The solubility of pyrene toward water is extremely low.16 The monomer emission of pyrene was only observed, suggesting a homogeneous solution in water. The fluorescence intensity ratio of the first and third vibronic peaks of pyrene is well-known to be sensitive to solvent polarity.17 The decrease in the I1/I3 value is an indication of solubilization into a more hydrophobic environment of micelles. The pyrene concentration was kept low enough in order to minimize the disturbance of micellar characteristics by the incorporation of pyrene. Figure 1 shows the variation of I1/I3 values as a function of quencher concentration. The slight decreases in I1/I3 values were observed below cmc of quenchers. The I1/I3 values of CPC significantly decreased due to the formation of micelles. The I1/I3 values of HFDePC were considerably high compared with those of CPC probably because the HFDePC micelle has a low solubilization power toward pyrene.18 Fluorocarbon quenchers have similar quenching abilities toward pyrene emission, because the quenching is due to the pyridinium group. It can be presumed that pyrene is susceptible for quenching by a diffusioncontrolled encounter with a fluorocarbon quencher despite the low affinity between pyrene and fluorocarbon chain.8 Figure 2 shows the variation of fluorescence intensity ratio I0/I as a function of quencher concentration. I0 and I are fluorescence intensities in the absence and presence of quencher, respectively. Such Stern-Volmer plots gave sharp changes in fluorescence intensities near the cmc. The effective quenching above the cmc comes from the (16) Schwarz, F. P. J. Chem. Eng. Data 1977, 22, 273. (17) Kalyanasundaram, K. Langmuir 1988, 4, 942. (18) Almgren, M.; Wang, K.; Asakawa, T. Langmuir 1997, 13, 4535.
(
I/I0 ) P(0) ) exp(-nav) ) exp -
nagg[Q] C - cmc
)
(1)
where nav is the average number of quenchers per micelle, nagg, [Q], and (C - cmc) are micelle aggregation number, quencher concentration, and micelle concentration, respectively. The fluorescence intensity ratios I/I0 for 20 mM SDS decreased with increasing concentration of quenchers as shown in Figure 3. The I/I0 values for CPC were well fitted by using eq 1 when cmc ) 8.1 mM and nagg ) 65. However, the I/I0 values for fluorocarbon quencher deviated from the calculated curve at high quencher concentrations. The cationic fluorocarbon quencher are completely incorporated into micelle due to the attractive electrostatic interaction toward anionic micelles, judging from the similar quenching ability of HFOPC despite its high cmc. The average occupancy number of pyrene gave no differences in quenching ratios under such a low occupancy number, which was less than 0.1. The deviation from the calculated curve slightly increased with increasing chain length of the fluorocarbon. Such behavior was confirmed by many experimental results in alkyl sulfate salt micelles. The deviation could come from the low affinity between hydrocarbon and fluorocarbon chains. (19) Grieser, F.; Drummond C. J. J. Phys. Chem. 1988, 92, 5580 and references therein.
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Figure 3. Fluorescence quenching of pyrene using various quenchers in 20 mM SDSaqueous solution. The solid line was calculated using eq 1. Key: (b) HFOPC; (2) HFDePC; (9) HFDPC, (O) CPC.
Asakawa and Miyagishi
Figure 5. Deviation from Poisson distribution of quenchers among SDS micelles. The I/I0 values were measured in 20 mM SDS 0.1 M NaCl as a function of quencher concentrations. The average number of quencher was calculated by using cmc ) 1.7mM and nagg ) 75. The lines were calculated using eq 2 when R ) 0 (solid line), R ) 0.1 (dotted line) and R ) 0.3 (dotdashed line). (b) HFOPC; (2) HFDePC; (9) HFDPC; (O) CPC.
Figure 4. Distribution of quenchers among micelles for nav ) 1. Poisson distribution: (-O-) R ) 0; (--b--) R ) 0.1; (- -2- -) R ) 0.2; (- -9- -) R ) 0.3.
The immiscibility will induce the segregation of fluorocarbon quenchers in hydrocarbon micelles. The distribution of quencher among micelles could be affected by such nonideal characteristics. Barzykin introduced the statistical distribution of quenchers among micelles in pairwise interaction of quenchers.13 The equilibrium intermicellar distribution is given by
P(n) ) [nav(1 - Rnav)]n (1 + R)n(n-1)/2 exp[-nav(1 - Rnav/2)] n! (2) where P(n) is the probability for a micelle containing n quenchers and R denotes the interaction parameter between quenchers. The eq 2 reduces to the Poisson distribution for R f 0. Figure 4 shows the variation of calculated P(n) with a for nav ) 1. The Poisson distribution illustrates the P(0) value is identical to the P(1) value for nav ) 1 without interactions between quenchers (R ) 0). However, the probabilities for micelles containing one quencher, P(1), decreased with increasing the values, while those containing no quencher, P(0), increased. The increase in P(0) values are due to the slight increase in probabily for micelles containing more than four quenchers. Such a segregation will result in a slight depression of fluorescence quenching for fluorocarbon quenchers. Similar quenching behavior was observed for alkyl sulfate micelles in the absence and presence of salt with various surfactants and salts concentrations. No mono-
Figure 6. Fluorescence quenching of pyrene in SDS and SPFO-SDS equimolar mixture: (b) HFDePC; (O) CPC in SDS; (2) HFDePC; (4) CPC in SPFO-SDS.
meric surfactants would affect the quenching behavior. The I/I0 values were measured in 20 mM SDS 0.1 M NaCl as a function of quencher concentrations. The average number of quencher (nav) was calculated by using experimental cmc and micelle aggregation number. The observed I/I0 values can be considered to be identical with the P(0) values under these experimental conditions as to pyrene fluorescence quenching in micelles. Figure 5 shows the variation of I/I0 values for SDS micelles as a function of average number of quencher (nav). The I/I0 values slightly increased with increase of the chain length of fluorocarbon quencher. The deviation from the calculation curve with Poisson distribution was simulated by eq 2. The slight depression of fluorescence quenching only for fluorocarbon quenchers can be ascribed to the slight increase in the probability for micelles containing no quencher. Next, we applied the HFDePC quencher to SPFO-SDS mixtures. Figure 6 shows the variation of fluorescence intensity ratio I/I0 as a function of total surfactant concentration. The quenching was very effective near cmc due to the concentration effect of quenchers into few micelles. The I/I0 values increased with increasing surfactant concentrations due to the increase in number of micelles. The slight differences between CPC and HFDePC are observed for pure SDS systems in accordance with Figure 3. In contrast, significant differences between CPC and HFDePC are observed for SPFO-SDS equimolar mixtures. The significant depression of quenching around
Demicellization of SPFO-SDS Mixtures
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Scheme 1. Plausible Micelle Pseudophase Diagram for Typical Binary Mixturesa
a Mixture cmc curves were simulated by using experimental values.21, 22 Second cmc curves (dashed straight line) for SPFOSDS were reported by Sugihara et al.;21 the other second cmc curves were given by material balances.15
40-55 mM suggests the coexistence of SPFO-rich and SDS-rich micelles. HFDePC tends to partition into SPFOrich micelles due to the low affinity between fluorocarbon and hydrocarbon chains when SDS-rich micelles are present in aqueous solutions. Therefore, the collision probability between pyrene in SDS-rich micelles and HFDePC in SPFO-rich micelles was inhibited within the lifetime of the excited pyrene. However, the I/I0 values for mixtures abruptly decreased with increasing total surfactant concentration above 60 mM as shown in Figure 6. This change can be ascribed for the demicellization. As the total surfactant concentration increased further beyond 60 mM, one type of micelles can disappear due to the formation of large mixed micelles having a large solubilization capacity. The quenching of pyrene by HFDePC will be effective if only pyrene and quencher are solubilized in a same micelle. Therefore, the I/I0 values using HFDePC become close to those using CPC at high surfactant concentrations (60-100 mM). The quenching behavior using CPC and HFDePC in one type of micelles became similar to that in pure SDS systems. The slight differences between CPC and HFDePC would come from the segregation of fluorocarbon quencher among micelles. Solutions of fluorocarbon and hydrocarbon liquids have large positive deviations from Raoult’s law. The critical solution temperature is 22.65 °C at 0.37 mole fraction of perfluorohexane for n-C6F14-n-C6H14 system.20 The upper critical solution temperature increases regularly with the number of carbon atoms. The heat of mixing increased with increase in the molecular size, i.e., the chain length of surfactants. The effect of chain length of surfactant toward limited solubility in micelles can be examined by using this developed quenching method. Scheme 1 illustrates the experimental cmc and/or predicted second cmc curves for typical surfactant mixtures with different chain lengths.15,21 The coexistence of two types of micelles have been predicted in the higher concentration regions as shown in Scheme 1. Sufficient additions of NaCl toward SPFO-SDeS mixtures resulted in similar mixture cmc values with LiFOS-LiDS mixtures. Thus, we can investigate the relation between the quenching behavior and the chain length of surfactant with suppression of monomeric surfactant. Figure 7 shows the variation of I/I0 in LiFOS-LiDS and SPFO-SDeS (0.2 M NaCl) equimolar mixtures. The differences between CPC and HFDePC in (20) Bedford, R. G.; Dunlap, R. D. J. Am. Chem. Soc. 1958, 80, 282. (21) Sugihara, G.; Nakamura, D.; Okawauchi, M.; Sakai, S.; Kuriyama, K.; Tanaka, M.; Ikawa, Y. Fukuoka Univ. Sci. Rep. 1987, 17, 31. (22) Asakawa, T.; Miyagishi, S.; Nishida, M. J. Colloid Interface Sci. 1985, 104, 279.
Figure 7. Fluorescence quenching of pyrene in surfactant equimolar mixtures: (b) HFDePC; (O) CPC in SPFO-SDeS in 0.2 M NaCl; (2) HFDePC; (4) CPC in LiFOS-LiDS.
Figure 8. Effect of temperature on fluorescence intensity in 50 mM SPFO-SDS equimolar mixture: (b) 0.5 mM HFDePC; (O) 0.5 mM CPC.
LiFOS-LiDS mixtures were larger than those in SPFOSDeS mixtures. This suggests the coexistence of two types of mixed micelles with low miscibility toward each other as shown in Scheme 1. The differences between CPC and HFDePC were small for SPFO-SDeS mixtures, suggesting rather miscible mixed micelles. The mutual immiscibility between fluorocarbon and hydrocarbon species significantly increased with increasing chain length of surfactants, judging from the lost quenching for LiFOSLiDS. The differences in quenching extent between CPC and HFDePC increased with increasing in chain length of surfactants as shown in Figures 6 and 7 (SPFO-SDeS < SPFO-SDS < LiFOS-LiDS). The immiscible regions increased with increasing in chain length of surfactants as shown in Scheme 1. Moreover, Scheme 1 shows the coexistence of two types of micelles between 33 and 60 mM in SPFO-SDS equimolar mixtures in accord with the significant depression of quenching around 40-55 mM. As the total surfactants concentrations increased further, one type of mixed micelles would disappear due to the large solubilization capacity of the other type of mixed micelle along with micelle growth. That would correspond to the effective quenching in a micelle beyond 60 mM for SPFO-SDS equimolar mixtures. The increase in temperature will induce the fusion of two types of micelles judging from the analogy with fluorocarbon and hydrocarbon liquid mixtures as mentioned above. Figure 8 shows the variation of fluorescence intensity in SPFO-SDS equimolar mixture as a function of temperature. The differences between CPC and HFDePC increased with decreasing temperature, while
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those significantly decreased with increasing temperature beyond 40 °C. The miscibility of fluorocarbon and hydrocarbon surfactants in micelles increased with increasing temperature with similar to fluorocarbon and hydrocarbon liquid mixtures. The inflection point near 40 °C seems to correspond to the transition from two types of micelles to one type of mixed micelles. The mutual miscibility in a micelle increased due to the increase in heat of mixing at
Asakawa and Miyagishi
a higher temperature. The pyrene fluorescence quenching method using fluorocarbon quencher will be effective to indicate such a demicellization phenomenon. Acknowledgment. The authors thank Mrs. Ohya for assistance. LA981513A
