J. Phys. Chem. 1989, 93, 3710-3713
3710
salinity in the three-phase region increase with decreasing interfacial tension. As can be seen in Figures 1, 5, and 11, in the three-phase region, as ywwdecreases, the droplet size and the phase volume of the microemulsion decrease. However, the droplet size and the phase volume of the microemulsions in two-phase regions (in the vicinity of the three-phase region) increase in spite of the decreasing yww. A fundamental thermodynamic equation indicates that the interfacial tension (y) is a change in the free energy of the interface (AQ) with an increase in the unit interfacial area (.4,4):34 AGs/AA = y
(12)
As can be seen in this equation, the decreasing y results in in-
creasing interfacial area (AA). It can be postulated that the increasing interfacial area is caused by the increased number of microemulsion droplets or by the increased swelling of the microemulsion droplet due to increasing adsorption of decane, of course without any aggregation. In the three-phase region, the decrease of yew may result in the increase of the number of microemulsion droplets, while in these two-phase regions (in the vicinity of the three-phase region) it may bring about the swelling of microemulsion droplets. The effect of the interfacial tension on the microemulsion formation in the two-phase regions would be distinctly different from that in the three-phase region. Mechanism of Phase Transitions in Multiphase Microemulsion System. As mentioned previously, some experimental data show drastic changes at the characteristic salinities (SIand SI,);the structural transition of the multiphase microemulsion may occur (34) Adamson, A. W. Physical Chemistry of Surfaces; Wiley-Interscience: New York, 1982.
at these salinities. Furthermore, in Figures 4, 6, 8, and 10, the data within Z, and ZII, where two different types of microemulsions coexist, would be related to the mechanism of the phase transition in the multiphase microemulsion system, because the data in these zones show conspicuous changes. In ZI,growing of the middle-phase microemulsion and breaking of the lower phase microemulsion will take place. This is because within this zone, with increasing salinity, the surfactant (SOS) molecules move from the lower phase to the middle phase, as shown in Figure 8; meanwhile, the moisture content in the lower phase microemulsion increases, but that in the middle phase microemulsion decreases (Figure 10). The sizes of droplets in both the lower phase and the middle phase microemulsion decrease (Figure 6). On the other hand, the middle phase microemulsion will begin to break up and the upper phase microemulsion will begin to grow with an increase in salinity in ZII. Thus, Z, and ZIIwill be the transitional stages in the multiphase microemulsion system. The phase transition with increasing salinity in the multiphase microemulsion system will consequently take place through these transitional stages, because the dissociation of anionic surfactants may be changed continuously and the hydrophilic-lipophilic balance of the surfactant may also be changed continuously with increasing salinity. Acknowledgment. This research was supported by Japan National Oil Corp. and Saneyoshi Scholarship Foundation. We thank Dr. S. Qutubuddin, Case Western Reserve University, for discussions concerning this paper. Registry No. NHA, 111-27-3; SOS, 142-31-4; NaC1, 7647-14-5; n-decane. 124-18-5.
High-pressure Studies of a Fluorescence Probe for the Critical Micelle Concentration in Sodium Dodecyl Sulfate Kimihiko Hara,* Hideo Suzuki, Department of Chemistry, Faculty of Science, Kyoto University, Sakyo- ku, Kyoto 606, Japan
and Noboru Takisawa Department of Chemistry, Faculty of Science and Engineering, Saga University, Saga 840, Japan (Received: August 5, 1988; In Final Form: October 11, 1988)
The utility of the fluorescence intensity ratio of vibronic bands of pyrene as a probe for determining the pressure effect on critical micelle concentration (cmc) and on the micropolarity of micelle interiors was investigated at high pressures up to 400 MPa for a typical anionic surfactant (sodium dodecyl sulfate). The effect of the probe on the transition from the singly dispersed state to the micellar state was also examined. The resulting cmc’s agree well with those from the conductivity method, exhibiting a maximum around 100 MPa. In the concentration region of so-called singly dispersed nonmicellar state, an anomalous decrease in the intensity ratio, observed with increasing pressure, may be explained by some pressure-induced aggregation.
Introduction
The fluorescence intensity of the (0-0)vibronic band in pyrene monomer molecules is dependent on the polarity of the environment,’ and the behavior of the band is analogous to that of the “Ham” band observed originally in benzenes2 By making use of this fact, it has been found that it is possible to use pyrene as a probe molecule for detecting the polarity in micelle interiors, termed here “ m i ~ r o p o l a r i t y ” . ~Especially, ~~ this technique is ( 1 ) Nakajima, A. Bull. Chem. Sac. Jpn. 1971, 44, 3272. (2) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience:
London, 1970. (3) Kalyanasundram, K.; Thomas, J. K. J . Am. Chem. Sac. 1977,99,2039. (4) Turro, N . J.; Kuo, P. L.;Somasundaran, P.; Wong, K. J. Phys. Chem. 1986, 90, 288.
0022-3654/89/2093-37 10$01.50/0
expected to be quite useful for the study of nonionic micelle^.^ In this application, however, it is important to ensure the absence of any influence of the probe molecule on the micelle properties. This fluorescence probe method is also a valuable technique for studying the pressure effect on the microproperties of micelles. The intensity ratio of the first vibrational peak at 373 nm (Ii) to the third one at 384 nm (I,) in pyrene molecules is used as a parameter characterizing the micropolarity of the probe’s environment. In this paper the intensity ratio [ , / I 3 was investigated at various surfactant concentrations, and on this basis, the critical micelle (5) Ananthapadmanabhan, K. P.; Goddard, E. D.; Turro, N. J.: Kuo, P. L. Langmuir 1985, 1 , 352.
0 1989 American Chemical Society
Cmc in Sodium Dodecyl Sulfate at High Pressures
I
'a
d
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3711
h
I
iI
1.21
1
Figure 1. Variation of the 11/13ratio as a function of SDS concentration at normal pressure. ([Py] = 2.0 X 10" mol/kg, 0;10 X 10" mol/kg, 0 ; 50 X 10" mol/kg, A).
concentration (cmc) of a typical anionic surfactant, sodium dodecyl sulfate (SDS),was determined at high pressures. The results were analyzed in comparison with those obtained from electrical conductivity t e c h n i q ~ e . ~ - ~
200
1 00
0
3 00
Pressure
400
/ M Pa
Figure 2. Variation of the 11/13ratio against pressure at various SDS concentrations. (HzO, 0 ; [SDS] = 6.0 X lo-' mol/kg, 0 ; 7.0 X lo-' mol/kg, A; 9.0 X lo-' mol/kg, 0; 8.0 X lo-' mol/kg, 0;8.5 X mol/kg, 0). mol/kg, V; 10.0 X
Experimental Section The surfactant sodium dodecyl sulfate (SDS, ClzHzsSO4Na), purchased from BDH Chemicals, Ltd., was employed without further purification. The probe molecule pyrene (Nakarai Chemicals, Ltd.) was purified by sublimation. Water employed as solvent was chromatographic grade, and ethanol was spectroscopic grade. All solutions were degassed by repeated freezepumpthaw cycles just before measurement. They showed no impurity emission in the spectral region of interest at the sensitivity levels used. Steady-state fluorescence spectra were recorded by exciting at 335 nm. Measurements at high pressures up to 400 MPa were carried out at 303 K. The high-pressure cell, emission equipment, and method of analyzing the data have been described elsewhere.1°
Results Figure 1 shows the dependence of the relative intensity ratio, 11/13,in the monomer emission of pyrene as a function of SDS surfactant concentration at atmospheric pressure. They track the expected cmc behavior of SDS. For the accurate determination of 11/13values at high pressures, the peak shift with pressure was taken into consideration. It was also confirmed that analogous results were also obtained when area ratio was used instead of intensity ratio. At lower concentrations of SDS, less than around 7.0 X mol/kg, the ratio exhibits a constant value of ca. 1.89, which corresponds to the value of pyrene dissolved in aqueous phase. As SDS begins to form micelles at ca. 7.0 X mol/kg, the ratio decreases sharply, indicating the solubilization of the probe into a more hydrophobic environment. After the formation of micelles, the ratio keeps constant again with a value of 1.3 1. First of all, the influence of the probe molecule on micellar formation was examined at several probe concentrations. At concentrations of pyrene higher than 50 X 10" mol/kg, the shoulder region of the transition becomes sluggish, as depicted in Figure 1. While at dilute concentrations of less than ca. 10 X lod mol/kg, the probe molecule seems to have no significant influence on every transition curve studied. Therefore, every measurement at high pressure hereafter was made at a concentration of 10 X 10" mol/kg. Moreover, at this concentration, excimer fluorescence of pyrene is not observed at all. ( 6 ) Hamann, S. D. Rev. Ph s. Chem. Jpn. 1978, 48, 60. (7) Osugi, J.; Sato, M.; I f u h , N. Reu. Phys. Chem. Jpn. 1965, 35, 32. (8) Brun, T. S.; Hailand, H.; Vikingstad, E. J . Colloid Interface Sci. 1978, 63, 89.
(9) Taniguchi, Y.;Suzuki, K. Reu. Phys. Chem. Jpn. 1979, 49, 91. (10) Hara, K.; Arase, T.; Osugi, J. J . Am. Chem. SOC.1984, 106, 1968.
,.il
,
0
'
L
5
6
7
[SDS]/
8
9
1011
12
IO 'mol/kg
Figure 3. Variation of the 1,/13ratio as a function of SDS concentration at various pressures. (0,0.1 MPa; D, 100 MPa; A, 200 MPa; V, 300 MPa; 0, 400 MPa).
TABLE I: Determination of Cmc of SDS by Fluorescence Probe Method at High Pressures in Comparison with That from the Conductivity Method
cmc( I)," pressure, MPa 0.1 50 100 150 200 250 300
mol/kg 6.7 8.0 8.0 8.2 1.5
cmc(2); lo-) mol/kg 8.0 8.8 9.3 9.0
9.0
cmc(1it.),c lo-' mol/kg 8.2 9.1 9.5 9.4 9.1
-8.5 -7.0
'Present data from initiation point 1. bPresent data from terminal point 2. 'Data from ref 6. Figure 2 shows the pressure dependence of 11/13ratio at various SDS concentrations. Figure 3 shows the dependence of 1,/13ratio as a function of SDS concentration a t various pressures. The constant value of the ratio at the concentration region lower than cmc decreases with increasing pressure. Both figures indicate the characteristic transition behavior of forming and dissolving of micelles. The cmc's at high pressures were determined from Figure 3. The results obtained from two points, Le., initial break-off point (1) and final point (2) of the transition, which are indicated by arrows in Figure 1, are listed in Table I, together with the literature data from the electrical conductivity method.6 It was found that the data obtained from the point 2 are consistent with those obtained from the conductivity method. At pressures higher than
Hara et al.
3712 The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 I
"f
3
11
1
0
6si
s
IO0
200
300
P r e s s u r e / MPa
Figure 4. Pressure dependence of the cmc with pressure ( 0 ,present data; 0,data from ref 6 ) .
r
:
I
*
100
200 Press u re
300
4 00 0-
/ M Pa
Figure 5. Variation of the Z1/Z3 ratio as a function of pressure in the
concentration well below the cmc (H20, 0; [SDS] = 1.0 X 10" mol/kg,
12
0
0; 2.0 X 10" mol/kg, A).
250 MPa, it was difficult to determine accurate cmc values. The cmc determined at normal pressure is 8.1 X mol/kg, which is in excellent agreement with the literature value (8.2 X mol/kg)." The results obtained from point 2 and from the conductivity method change almost in parallel with each other against pressure (cf. Figure 4). It should be noted here that they clearly show a maximum at the pressure of ea. 100 MPa, which has been also reported p r e v i o ~ s l y . ~However, -~ there is another report that no maximum is observed by the absorption method using naphthalene as probe.12 This contradiction seems to be due to the effect of high probe concentration. From Figures 2 and 3, in addition, we can find some other interesting results: (a) In dispersed nonmicellar state, the Z,/Z3 ratio is nearly equal to the value in pure water, and it decreases with pressure. (b) Whereas in the micellar state, it decreases to a value nearly equal to ethanol, and it remains almost constant with pressure. (c) Even in concentrations well below the cmc, mol/kg, for example, as seen in Figure 2 for the case of 6.0 X the ratio decreases rather remarkably at pressures higher than 200 MPa. NOsuch behavior has been reported by conductivity method. To examine further this unexpected behavior in nonmicellar region, the measurements at some concentrations much lower than cmc were also carried out, the results of which are shown in Figure 5 . A slight dependence on the pyrene concentration was also observed, although it was not plotted in this figure. Discussion It has been suggested that the perturbation in vibronic band of pyrene is more dependent on solvent dipole moment ( D ) than
o-3-3-O-
I00
200 Pressure
300
/ MPa
400
Figure 7. Pressure dependence of the Zl/Z3 ratio in water and in ethanol (0, H20: 0, ethanol).
on bulk solvent dielectric constant According to the plots of 1 1 / 1 3 values against both properties in a series of solvents, however, it is found that the correlation with bulk dielectric constant is much better. Figure 6 is an example of the plot of the intensity ratio against dielectric constant for a series of protic solvents. The Z,/13 values in various solvents and the data of solvent dielectric constants and dipole moments were taken from ref 3. The present data in ethanol and in water are also included. As seen in this figure, the ratio increases almost linearly up to around 1.6 with increasing the bulk solvent dielectric constant up to ca. 50. But it is likely to reach a maximum, and then it decreases with further increase in dielectric constant. This fact explains the aforementioned experimental result that the intensity ratio in pure water decreases with pressure, in spite of the fact that the dielectric constant of water increases with pressure.13 In Figure 7 is shown the change of 11/13values for pure water with pressure, together with those for ethanol for comparison. As for water it decreases with pressure, whereas in the case of ethanol, it slightly increases with pressure. This result for ethanol is consistent with the increase of dielectric constant with pressure.I4 Here, the 11/13value in micellar state (cf. Figure 2) has a value of about 1.31, the value of which just corresponds to that of ethanol. Thus, it means that the micropolarity in the SDS micellar state measured from a pyrene probe is equal to that of ethanol. This may imply that the probe molecule is not located in a deep inner hydrophobic part of micelles. As a matter of fact, the position of the second or third carbon from polar head group has
___
(1 1) Elworthy, P. H.; Mysels, K. J. J . Colloid Interface Sci. 1966, 21, 331. (12) Rodriguez, S . ; Offen, H. W. J . Phys. Chem. 1977.81, 47.
(13) Srinvasan, (14) Srinvasan,
K. R.;Kay, R. L. J . Chem. Phys. 1974, 60, 3645. K. R.; Kay, R. L. J . Solution Chem. 1975, 4, 31 1
J . Phys. Chem. 1989, 93, 3713-3720
been proposed as the location of the probe molecule, based on the chemical shift in ‘H N M R measurement^.'^ On the other hand, there has been a suggestion that water can penetrate into micelles.16 If this is the case, the amount of the penetration is expected to be pressure dependent. In our results, however, the Z,/Z3 ratio for micellar state was almost constant within our experimental errors. This may indicate that the water penetration is constant and limited in the neighborhood of the polar head groups. But this aspect needs to be examined further. The pressure dependence of the cmc, which is shown in Figure 7, is connected with the change in the partial molal volume in the micelle formation by the following equation:”
rm vs,
vm
vs
AVm = - where and are the partial molal volumes of the surfactant in the micellar and singly dispersed states, respectively. p is the number ratio of counterion to surfactant ion in micelles. By neglecting the variation of p with pressure, the value of Armat atmospheric pressure, calculated from initial slope of the pressure dependence (Figure 4), is +13 cm3/mol, when a This plot was satisfactorily value of 0.73 is adopted for represented as a polynomial of third degree in pressure. The appearance of a maximum appeared in the plot of cmc against pressure indicates that A r mchanges its sign from positive to negative. The positive volume change of micellization has been attributed to the elimination of hydrocarbon-water contact accompanied with micellization.s But why does it change its sign with pressure? Is it simply because the micellar state is much (15) Zachariasse, K. A.; Kozankiewicz, B.; Kiihnle, W. Surfactanf in Solution; Mittal, K. L., Fendler, F. J., Eds.; Plenum: New York, 1984; Vol. I , p 565. (16) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Sysfems; Academic Press: New York, 1975. (17) Tuddenham, R. F.; Alexander, A. E. J . Phys. Chem. 1962.66, 1839.
3713
more compressible than the singly dispersed state, as has been suggested?I8 The unexpected decrease of the apparent micropolarity in nonmicellar region as seen in Figure 5 seems to have something to do with this problem. In other words, the decrease in micropolarity of the singly dispersed state can be caused by some pressure-induced aggregation around the probe molecule due to hydrophobic interaction, which might be called microemulsion. This process is expected to be accompanied with an increase in the Vsvalue with pressure. In fact, a negative value of apparent molal compressibility for the single dispersed state has been remay predominantly conported.8 Therefore, rather than tribute to the change in the sign of Avm. In the results of the present work, the cmc values defined from the terminal point 2 agreed well with those from conductivity method, although in the paper of Turro et aL4 the inflection point was taken as the cmc. The Occurrence of the substantial transition region in the [‘/I3 value in the fluorescence technique may suggest the preliminary appearance of the aggregates with lower aggregation number, as has been called “premicelle” state,lg prior to the formation of ordinary stable large micelles. The higher the pressure is, the broader the transition becomes. It would also be closely correlated with the discussion mentioned above. From the point of view of a high-pressure study not only for the cmc determination but also for the investigation of various microproperties of micelle interiors, we can conclude that the fluorescence probe technique is a useful and reliable method.
vs
v,,,
Acknowledgment. We thank Prof. Y. Taniguchi of Ritsumeikan University for the help and cooperation throughout this work. Registry No. SDS, 151-21-3; pyrene, 129-00-0. ( 1 8) Kaneshina, S.; Tanaka, M.; Tomida, T. J . Colloid Inferface Sci. 1974,
48, 450.
(19) Somasundaran, P.; Ananthapadmanabhan, K. P.; Ivanov, I. B. J. Colloid Interface Sci. 1984, 99, 128.
Surface-Enhanced Resonance Raman Scattering from Langmuir-Blodgett Monolayers: Surface Coverage-Intensity Relationships Jae-Ho Kim, Therese M. Cotton,* R. A. Uphaus, Department of Chemistry, University of Nebraska-Lincoln,
Lincoln, Nebraska 68588-0304
and D. Mobius Max-Planck Institut fur biophysikalische Chemie, 0 - 3 4 0 0 Gottingen-Nikolausberg, FRG (Received: August 16, 1988)
The relationship between surface coverage and surface-enhanced resonance Raman scattering (SERRS) was examined for a cyanine dye (S-120) dispersed in Langmuir-Blodgett monolayers containing arachidic acid and methyl arachidate as an inert diluent. The monolayers were supported on Ag island films that were vacuum-deposited on glass slides. The optical properties (absorbance/transmittance) and excitation profiles of the SERRS intensities were also measured as a function of concentration. The maximal SERRS enhancement was observed at 500 nm or close to the Ag plasmon resonance of the films. The results show that SERRS intensities for various vibrational modes of the dye maximize at submonolayer dye coverage. The observed nonlinear coverage behavior is attributed to changes in the dielectric properties and/or dipole-dipole coupling between the dye molecules as a function of surface coverage.
Introduction The enormous sensitivity resulting from the surface-enhanced Raman effect suggests that it should provide a powerful technique for surface analysis. However, as indicated by the considerable numbers of experimental and theoretical papers that have appeared during the past 10 years, there are many uncontrolled or poorly defined variables that can profoundly affect the magnitude of the 0022-3654/89/2093-37 13$01.50/0
enhancement. These include the optical properties of the metal substrate (Ag has been the most frequently studied), the morphology of the substrate, the surface potential of the metal, the excitation wavelength used to produce Raman scattering, the metal-adsorbate interactions (chemisorption vs physisorption), the distance between the adsorbate and the metal surface, and the optical properties of the medium surrounding the metal and 0 1989 American Chemical Society
