2376
Langmuir 1995,11, 2376-2379
Effect of Urea on Micelle Formation of Fluorocarbon Surfactants Tsuyoshi Asakawa," Michiyo Hashikawa, Kouji Amada, and Shigeyoshi Miyagishi Department of Chemistry and Chemical Engineering, Faculty of Technology, Kanazawa University, Kanazawa 920, Japan Received October 7, 1994. I n Final Form: January 17, 1995@ The effect of urea on micelle formation of fluorocarbon surfactants was investigated by measuring conductivity cmc and fluorescence intensity of probes (l-anilinonaphthalene-8-sulfonate, auramine, and pyrene). We examined various fluorocarbon surfactants which consist of different ionic head groups, i.e., lithium perfluorononanoate (LiPFN),diethylammonium perfluorononanoate (DEAPFN),lithium perfluorooctanesulfonate(LiFOS),and lH,lH,W,W-perfluorodecyl sulfate (LiHFDeS). Significant differences do exist between fluorocarbonand hydrocarbon surfactants; the cmc's ofthe fluorocarbonsslightly decreased with the addition of urea, while those of the hydrocarbons increased as reported previously. The slight decreases in cmc's of fluorocarbon surfactants were confirmed by comparison with LiDS and LiHFDeS which contain the same ionic head group. The counterion dissociation degree of micelles always increased with the addition of urea. The micropolarities of fluorocarbon micelles were almost identical with the polarity of water, and were independent of urea concentration. The microviscosities of DEAPFN micelles decreased with urea concentration. These results suggest the significant differences in micellar characteristics between fluorocarbon and hydrocarbon surfactants.
Introduction The use of urea as a denaturant of proteins is well known.l,2 The presence of urea and its derivatives modifies aqueous solution properties. However, the mechanism of urea action is not understood sufficiently. Two different mechanisms have been proposed to explain urea action on aqueous solution^.^-^ One is that urea acts as a water structure breaker (indirect mechanism). The other is that urea participates with the solvation of hydrophobic chains in water by replacing some water molecules in the hydration shell of the solute (direct mechanism). Since Bruning and Holtzer first demonstrated that urea disrupts micelles,6 many investigators reported that critical micelle concentrations (cmc's) of ionic and nonionic surfactants significantly increase with the addition of urea in aqueous solutions.'-1° Hydrophobic interactions play an important role in the formation of micelles. It has been generally accepted that urea acts as a breaker of hydrophobic interaction^.^ Recently, a computer simulation of urea action in aqueous solution showed negligible influence of urea on the water structure, while urea weakens the water-water interaction by replacing several water molecules from an apolar solvation ~ h e 1 1 . ~ ~ ~ Baglioni et al. indicated that urea slightly decreased the micropolarity of the micellar surface and increased the microviscosity of the micellar surface by monitoring the ESR of nitroxide radicals in sodium dodecyl sulfate and dodecyltrimethylammonium bromide aqueous solutions.ll These results were interpreted as a direct mechanism of urea action. Briganti et al. presented systematic experimental and theoretical investigations Abstract published in Advance ACS Abstracts, June 1, 1995. (1)Franks, F. In Water, A Comprehensive Treatise; Plenum Press: New York, 1978;Vol. 4. (2)Enea, 0.; Jolicoeur, C. J. J. Phys. Chem. 1982,86, 3870. (3)Franks, H. S.;Franks, F. J. J. Chem. Phys. 1966,48, 4746. (4)Kuharski, R.A.;Rossky, P. J. J.Am. Chem. SOC.1984,106,5786. ( 5 ) Kuharski, R.A.; Rossky, P. J. J.Am. Chem. SOC. 1984,106,5794. (6)Bruning, W.; Holtzer, A. J. Am. Chem. SOC. 1961,83, 4865. (7)Mukerjee, P.;Ray, A. J. Phys. Chem. 1963,67,190. (8)Schick, M. J. J. Phys. Chem. 1964,68, 3585. (9)Emerson, M. F.; Holtzer, A. J. Phys. Chem. 1967,71,3320. (lO)Tanford, C. In The Hydrophobic Effect, 2nd ed.; WileyInterscience: New York, 1980. @
of the effect of urea on micellization and micellar groWth.12 They found that the cmc increased, micellar size decreased, and the sphere to rod shape transition temperature shifted to higher values. They indicated a useful theoretical approach for understanding the delicate balance of hydrophobic and hydrophilic interactions and the intermicellar and intramicellar interactions in such systems. Missel et al. suggested that the reduction in the micellar radius was attributed to a lowering of the interfacial tension between hydrocarbon core and water in the presence of urea.13 The hydrophobiceffect decreased about 9% per mole of added urea. There are many experimental results which suggest that urea has various effects on the properties of micellar solutions as described above. In this paper, we focused on the effect of urea on fluorocarbon micelles to obtain information on the characteristics of fluorocarbonmicelles. The cmc's of various surfactants were measured by the conductivity method in aqueous urea solution. The microenvironment of micelles was evaluated by the fluorescence probe method.14 We considered a systematic experimental study of the effect of urea on fluorocarbon micelles which consist of different ionic head groups. Moreover, a new anionic fluorocarbon surfactant (LiHDeS) was prepared to confirm the urea action toward the fluorocarbon chain in comparison with lithium dodecyl sulfate (LiDS).
Experimental Section Materials. Lithium perfluorononanoate (CsFl&OOLi, LiPFN),diethylammoniumperfluorononanoate (CsF17COONHz(CzH&,DEAPFN),lithium perfluorooctanesulfonate (C~F17S03Li, LiFOS), lithium dodecyl sulfate (ClzH&04Li, LiDS), and diethylammonium tetradecyl sulfate ( C ~ ~ H Z ~ S O ~ ~ H Z ( C Z H & , (11)Baglioni, P.; Rivava-Minten, E.; Dei, L.; Ferroni, E. J. Phys. Chem. 1990,94,8218. (12)Briganti, G.; Puwada, S.; Blankschtein, D. J.Phys. Chem. 1991, 95,8989. (13)Missel, P. J.; Mazer, N. A.; Carey, M. C.; Benedek, G. B. In Solution Behavior of Surfactants; Mittal, K. L., Fendler, E. J., Eds.; Plenum Press: New York, 1982;Vol. 1. (14)Miyagishi, S.;Asakawa, T.; Nishida, M. J. Colloid Interface Scz. 1967,115,199.
0743-746319512411-2376$09.00/00 1995 American Chemical Society
Effect of Urea on Fluorocarbon Micelles
Langmuir, Vol. 11, No. 7, 1995 2377
Table 1. Effect of Urea on Cmc's of Fluorocarbon and Hydrocarbon Surfactants at 25 "C surfactant u r e a cmdmM SdS1
C~HzsS04NHz(CzHs)z (DEATS)
0 3 6 8 0 n
a
CsF17COOLi (LiPFN)
CsF17COONHz(CzH& (DEAPFN)
C~F17S03Li(LiFOS)
6 8 0 3 6 8 0 3 6 8 0 n
a
CsFi7CH2CHzS04Li (LiHFDeS)
6 0 6 8
8.7 10.0 11.5 12.4 1.0 1.5 2.1 2.8 11.3 9.5 9.3 10.0 3.0 3.3 3.4 3.4 7.1 5.8 5.2 2.4 2.5 2.2 2.4
0.37 0.48 0.66 0.77 0.40 0.49 0.63 0.61 0.57 0.70 0.71 0.81 0.07 0.51 0.46 0.64 0.63 0.77 0.90 0.46 0.82 0.85 0.84
DEATS) were prepared by the same procedures as reported previ0us1y.l~Lithium lH,lH,,W,W-perfluorodecylsulfate (c817CH&H2SOdLi, LiHFDeS) was synthesized from lH,lH,W,Wperfluorodecanol (PCR Inc.) as follows. After sulfonation of perfluorodecanol with chlorosulfonic acid in diethyl ether, viscous precipitates were obtained. The precipitates were washed with diethyl ether several times. After the surfactants were extracted with ethanol, a methanol solution of surfactants was placed on TOYOPAK ODs-M (TOSOH Co.) to remove traces of perfluorodecanol. The surface tensions of aqueous solutions of the obtained LiHFDeS did not show minima around the cmc. 1-Anilinonaphthalene-8-sulfonate(C16H1&03S, ANS, Wako Pure Chemical Ind., Ltd.), auramine ( C I ~ H Z ~ N ~ Kanto HC~, Chemical Co., Inc.), and pyrene (C16H10, Wako Pure Chemical Ind., Ltd.) were used as received. Urea (Wako Pure Chemical Ind., Ltd.), 1,3-dimethylurea,and 1,1,3,3-tetramethylurea(Tokyo Kasei Kogyo Co., Ltd.) were used as received. 1,3-Dimethylurea was reagent grade and the other reagents were of guaranteed grade. Measurements. Conductivity measurements of aqueous surfactant solutions were carried out using conductivity meter Model CM-2OE (TOAElectronics, Ltd.). Fluorescence spectra of probes (ANS, pyrene, auramine) were recorded on a Hitachi F-3010 fluorescence spectrometer. The probes were prepared as 6.3 x M auramine under similar MANS and 1.0 x experimental conditions as reported previously.16 The fluorescence emission spectra of pyrene (1.0 x M) were obtained by exciting the samples at 335 nm (excitation slit width 5 nm, emission slit width 1.5nm). The spectra were used to determine the ratios (13/11)of the fluorescence intensities of the third (13) and first (ZI) vibronic peaks of monomeric pyrene solubilized in micelle aggregates. All experiments were performed at 25 "C.
Results and Discussion The cmc's of various fluorocarbon and hydrocarbon surfactants were determined at 25 "Cby the conductivity method. The effects of urea on the cmc's are summarized in Table 1. It was found that the change in slope of conductivity at the cmc became more gradual with increasing urea concentration. When SIand Sz were used to denote the slope of the conductivity versus surfactant concentration curves below and above the cmc, respectively, the SdS1ratio was used as the index of counterion dissociation degree of micelles (Table 1). Here we find that the SdS1 ratios of LiPFN, LiFOS, and LiHFDeS (15)Asakawa, T.; Johten, K.; Miyagishi, S.; Nishida, M. Langmuir 1985, 1, 347.
(16)Asakawa, T.; Fukita, T.; Miyagishi, S. Langmuir 1991,7,2112.
s -
E
O1.0
0 5 10 Urea concentrati oYM Figure 1. Dependence of cmc/cmco on urea concentration for fluorocarbon and hydrocarbon surfactants. cmco is cmc of aqueous surfactant solution in the absence of urea: (0)LiPFN, (A) DEAPFN, (0) LiFOS, (0) LiHDeS, ( 0 )LiDS, (A)DEATS.
(LiPFN, 0.57;LiFOS, 0.63;LiHFDeS, 0.46) are larger than that of LiDS in the absence of urea (LiDS, 0.37). This suggests that the dissociation degree of the lithium ion of fluorocarbon micelles is larger than that of hydrocarbon micelles. However, the SdS1 ratio of DEAPFN is much smaller than that of LiPFN. This suggests preferential binding of the diethylammonium ion to perfluorononanoate micelles, similar to Hoffmann's r e ~ u 1 t . lThe ~ SdS1 ratios always increased with the addition of urea. Especially, urea raises the SdS, ratio of DEAF'FN about nine times that of DEAPFN in water. The counterion dissociation degree of DEAPFN micelles becomes close to that ofLiPFN micelles by the addition of urea. The cmc's of LiDS and DEATS increased about 5%and 20%per mole of added urea, respectively. Traditionally, the observed behavior of hydrocarbon micelles could be explained in terms of the weakening of hydrophobic interactions by urea which acts as a water structure breaker. Urea tends to stabilize the surfactant monomer since urea enhances the solubility of hydrocarbons in water. However, the effect of urea on the cmc's of fluorocarbon surfactants was different from that of hydrocarbon surfactants. Figure 1 obviously displays the experimental cmc dependence of various surfactants as a function of urea concentration; cmco denotes the cmc of surfactant in the absence of urea. The cmdcmco ratios showed significant differencesbetween hydrocarbon and fluorocarbon surfactants. The cmc's of fluorocarbon surfactants slightly decreased with the addition of urea. The slight increase in the cmc of DEAPFN could be ascribed to the abnormally low counterion dissociation degree of micelles in the absence of urea. Urea enhanced the repulsive force between the ionic head groups of DEAPFN at the micellar surface judged from the large increase of SdS1 ratio. The large increase in electrostatic repulsion between ionic head groups would be unfavorable to micelle formation. The decrease in the cmc of the fluorocarbon surfactant was confirmed by comparison with surfactants which consist of the same ionic head group, i.e., LiDS and LiHFDeS. Thus, the behavior would not result from a specific interaction between urea and ionic head group. We could conclude that urea has t h e opposite effect on micelle formation of fluorocarbon surfactants. It should be pointed out here that the microscopic characteristics of micelle formation of fluorocarbon surfactants must be different from that of hydrocarbon surfactants. We investigated the microscopic differences between the fluorocarbon and hydrocarbon micelles by using -(17)Hoffmann, H.; Ulbricht, W.; Tagesson, B. 2.Phys. Chem. 1978, 113, 17.
Asakawa et al.
2378 Langmuir, Vol. 11, No. 7, 1995
10
0
5
5 10 Urea concentration/M Figure 2. Dependence of ANS fluorescenceintensity on urea concentration at constant surfactant concentration. ZOis the fluorescenceintensity without surfactant: Ex 385 nm; Em 490 nm, 6.3 x MANS; (0)20 mM LiPFN, (A) 10mM DEAPFN, ( 0 )20 mM LiDS, (A)10 mM DEATS.
5
c
0
fluorescenceprobe methods. The probe methods were used to evaluate the micellar characteristics according to the knowledge of solubilization behavior. Figure 2 shows the dependence of A N S fluorescence intensity on urea concentration at constant surfactant concentration. The fluorescence intensity in the absence of surfactant (lo) was used as a standard. The fluorescence intensity of ANS is known to depend on the solvent polarity. The fluorescence intensities in hydrocarbon surfactants were about three to five times of that in water because the hydrocarbon micelles create the hydrophobic site. On the other hand, the fluorescence intensities in LiPFN and DEAPFN hardly increased with fluorocarbon surfactant concentrations even if above their cmc’s. This suggests that the fluorocarbon micelles have far less hydrophobic character around the micelle-solubilized A N S probe. Appreciable water contact of the fluorocarbon chain would exist at the solubilization site of A N S in the fluorocarbon micelles because the fluorocarbon surfactants form “loose” micelles in contrast to the hydrocarbon micelles.18 The remarkable feature of the fluorocarbon micelles did not change with the addition of urea as shown in Figure 2. On the other hand, the fluorescence intensities in the hydrocarbon micelles decreased with the addition of urea, which indicates the increase in the micropolarities of hydrocarbon micelles. This suggests that urea lowered the size of hydrocarbon micelles with water penetration. The dependence of the fluorescence intensity of auramine on urea concentration is shown in Figure 3. The fluorescence intensity has been used as the index of viscosity of the microenvironment. l4 The fluorescence was used as a intensity in the absence of surfactant (IO) standard. The microviscosity in the solubilization site of auramine is lower in LiPFN micelles than in hydrocarbon micelles and slightly depends on the urea concentration. The microviscosities sensed by auramine in LiPFN, LiDS, and DEATS were about two, three, and four times that in water, respectively. However, the microviscosity of DEAPFN was much larger than others probably because of the abnormally low counterion dissociation degree of micelles. The close packing in the DEAPFN micellar surface might be the cause of the large microviscosity of DEAPFN micelles. The microviscosity of DEAPFN abruptly decreased with the addition of urea and became close to that of LiPFN in 6 M urea. This behavior could be ascribed to the increase in the counterion dissociation degree of micelles by the addition of urea. The effects of urea on the micellar micropolarity and microviscosity are summarized in Table 2. The fluores(18)Turro, N. J.; Lee, P. C. J . Phys. Chem. 1982, 86, 3367.
I
5 10 Urea concentration/M Figure 3. Dependence of auramine fluorescenceintensity on urea concentrationat constant surfactant concentration. ZOis the fluorescenceintensity without surfactant: Ex 440 nm, Em 500 nm, 1.0 x M auramine; ( 0 )20 mM LiPFN, ( A ) 10 mM DEAPFN, ( 0 )20 mM LiDS, (A)10 mM DEATS. 0
Table 2. Effect of Urea on Micellar Micropolarity and Microviscosity surfactant u r e a ANS ZfIo pyrene 13/11 auramine JZ, 20 mh4 LiDS 10 mM DEATS 20 mM LiPFN
10mMDEAPFN
0 8 0 8 0 8 0 8
2.7 1.5 4.7 1.6 1.0 1.0
1.0 1.0
0.80 0.75 0.85 0.79 0.58 0.61 0.64 0.58
2.9 3.6 4.3 4.9 2.0 2.4
11.8 3.2
cence intensity ratio of the third and first vibronic peaks of pyrene has been known to be sensitive to solvent polarity. The increase in the value ofI3/I1is an indication of the solubilization into a more hydrophobic environment. The solubilization site of pyrene was proved to be in the palisade 1 a ~ e r . Under l~ our experimental conditions, the values ofIdZ1in water, hexane, and perfluorohexane were 0.52, 1.54, and 1.75, respectively. The values of 13/11 in fluorocarbon micelles were close to that of water. That is, the solubilization site of pyrene in fluorocarbon micelles is probably hydrated with appreciable water contact. The values of 13/11in hydrocarbon micelles slightly decreased with the addition of urea in comparison with I l l 0 of ANS. The micropolarity sensed by A N S in the micellar surface of hydrocarbon surfactants increased with the addition of urea, while that sensed by pyrene in the palisade layer slightly increased with the addition of urea. The formation of interamide hydrogen bonds between urea and proteins has been questioned.20 We further examined the effects of urea derivatives on the cmc’s as shown in Figures 4 and 5. Figure 4 shows the dependence of cmc’s on 173-dimethylureaconcentration. In general, the cmds increased with the addition of dimethylurea, similar to dodecyltrimethylammonium b r ~ m i d e while ,~ the cmc of LiPFN decreased with the addition of dimethylurea by a curve similar to that in Figure 1. Figure 5 shows the dependence of cmc’s on 1,1,3,3-tetramethylurea concentration. The order of effectiveness in micelle formation is tetramethylurea > dimethylurea urea. Tetramethylurea would lower the dielectric constant of aqueous solution resulting in the larger influence toward the cmc, while the effect of urea and dimethylurea may (19) Kalyanasundaram, K.;Thomas, J. K. J . Am. Chem. Soc. 1977, 99.2039. (20) Jencks, W. P. In Catalysis in Chemistry and Enzymology; McGraw-Hill: New York, 1969.
Langmuir, Vol. 11, No. 7, 1995 2379
Effect of Urea on Fluorocarbon Micelles
1
r
E
\
! 0
0 0.5 1 .o 1,3-di methylurea Conc./M Figure 4. Dependence of cmc’s on dimethylureaconcentration for fluorocarbonand hydrocarbon surfactants: (0) LiPFN, (A) DEAPFN, ( 0 )LiDS, (A)DEATS.
bl
1 1 / / 1 1 1 1 1 1 1 0 0.5 1 .o
1,1,3,3-tetramethyIurea Conc./M Figure 5. Dependence of cmc’s on tetramethylurea concentration for fluorocarbon and hydrocarbon surfactants: (0) LiPFN, (A) DEAPFN, ( 0 )LiDS, (A)DEATS. be less because of the rather small changes in the dielectric constant. We have to consider the factor of contributions to micellar stability by penetrating additives. Tetramethylurea may provide some stabilization due to solubilization into the micelles. Such an effect might induce the decreases in cmc’s for LiPFN and DEAF’FN. We have no direct experiments that demonstrate the mechanism of urea action on fluorocarbon micelle formation. However, we may have found here that urea may act as a direct mechanism rather than an indirect
mechanism. The fluorocarbon chains have a higher hydrophobicity compared with the hydrocarbon chains as judged from a low water solubility and much lower cmc’s of fluorocarbon surfactants.21 The partial molar volume changes of fluorocarbon surfactants are larger than those of hydrocarbon surfactants with micelle formation.22 The change in the contact area of the fluorocarbon chain to water would be larger than that of hydrocarbon with micelle formation.23 Therefore, the extent of the structural reorganization of water around the fluorocarbon chain would be large with a resultant gain in entropy as to the micelle formation. If urea acts as a water structure breaker around fluorocarbon chain (indirect mechanism), the cmc’s of fluorocarbon surfactants should increase with the addition of urea. Thus, a direct mechanism might be suitable for the effect of urea on the fluorocarbon micelles; that is, urea participates with the solvation ofhydrophobic solutes by replacing some water molecules in the hydration shell of the solutes. The fluorocarbon micelles would be stabilized by replacing some water molecules with urea, because the fluorocarbon micelles form looser packing of ionic head groups with the water penetration into the micellar palisade layer. However, both the theoretical approach and more direct experimental methods are necessary to solve the mechanism of urea action toward fluorocarbon chains. Although we cannot mention that the water structure around the fluorocarbon chain may be different from that of the hydrocarbon chain, the characteristics of fluorocarbon micelles are much different from those of hydrocarbon micelles. Conclusion The cmc’s of hydrocarbon surfactants are well known to increase with the addition of urea, while the cmc’s of fluorocarbon surfactants slightly decrease with the addition of urea. The opposite effect on fluorocarbon micelles cannot be ascribed to the specific interactions between urea and ionic head group in comparison with LiDS and LiHFDeS. The meaningful urea action would derive from the characteristics of fluorocarbon micelles, that is, the looser packing micelles with the water penetration revealed by the fluorescence probe method. The effect of water structure around fluorocarbon chain toward the micelle formation will be a problem to solve in the future. LA9407875 (21) Shinoda, K.; Soda, T. J . Phys. Chem. 1963, 67, 2072. (22) Shinoda,K.; Hato, M.; Hayashi, T. J.Phys. Chem. 1972,76,909. (23) Desnoyers,J. E.; Lisi, R. D.; Ostiguy, C.; Perron, G. In Solution Chemistry ofSurfuctunts;Mittal, K. L., Ed.; Plenum Press: New York, 1979: Vol. 1.
