10814
J. Phys. Chem. 1994, 98, 10814-10818
Effects of /I-Cyclodextridsurfactant Complex Formation on the Surfactant Monomer -Micelle Exchange Rate in Aqueous Solutions of Sodium Perfluorooctanoate and P-Cyclodextrin D. J. Jobet and R. E. Verrall Department of Chemistry, University of Saskatchewan Saskatoon, Saskatchewan, Canada S7N OW0
E. Junquera and E. Aicart’ Departamento de Quimica Fisica, Facultad de Ciencias Quimicas, Universidad Complutense de Madrid, 28040-Madrid, Spain Received: February 7, 1994; In Final Form: May 24, 1994@
Solutions of sodium perfluorooctanoate (SPFO) and P-cyclodextrin @-CD) were studied using ultrasonic relaxation (0.8- 190 MHz), speed of sound, and electrical conductivity techniques. The observed relaxation processes are believed to be due to the surfactant monomer-micelle exchange process and the molecular dynamics of the P-cyclodextridsurfactant complex. The surfactant monomer-micelle exchange process was analyzed using the Aniansson model after the sound absorption due to the presence of the cyclodextrid surfactant complex was substracted from the total sound absorption. The resulting data show a single relaxation process indicating that the kinetics of the monomer-micelle exchange process for the SPFO micelles is relatively unaffected by the presence of p-CD or the 1:1 inclusion complex. Any contribution of counterion binding to p-CD or of a fast counterion exchange process appears to affect only the micelle distribution and not the mean aggregation number, the exit rate for the monomer, or the average volume change associated with the exchange. Furthermore, the presence of the guest molecule, SPFO, appears to induce ultrasonic relaxation processes which are related to the P-cyclodextrin structure and are normally absent in solutions containing only P-CD.
Introduction We have recently reported’ the results of ultrasonic studies of micellar and nonmicellar solutions of decyltrimethylammonium bromide (DTAB) containing P-cyclodextrin @-CD). From this study it was found that the relaxation process due to the monomer-micelle exchange was unaffected by the presence of the 1:1 cyclodextridsurfactant complex. However, in order to analyze the data using the theory of Aniansson and Wall,* it was necessary to substract from the total absorption that absorption due to the 1:l complex. Since the two processes can be treated separately, it can be assumed that they are either kinetically independent or that the formation kinetics for the 1:1 complex is much slower such that the concentration of the free surfactant can be accounted for by simple chemical equilibrium considerations. Perfluorinated surfactants, of which sodium perflurooctanoate (SPFO) is the most commonly studied, also form complexes with cycl~dextrins.~-~ In general, these surfactants are more hydrophobic than their hydrocarbon analogues and as a result have lower critical micelle concentrations (cmc). However, some studies indicate the aggregation number (n) for SPFO micelles is not that much different than its hydrocarbon a n a l o g ~ e forming , ~ ~ ~ small hydrated “mini” micelles.* It has also been reported that micellar solutions containing mixtures of perfluorinated and hydrocarbon surfactants show a behavior that indicates the mixing of the two surfactants is not
* Address correspondence to: E. Aicart, Departamento de Quimica Ffsica, Facultad de Ciencias Qufmicas, Universidad Complutense de Madrid, 28040-Madrid, Spain. t Present address: AECL Research, Research Chemistq Branch, Whiteshell Laboratories, Pinawa, Manitoba, Canada ROE 1LO. Abstract published in Advance ACS Abstracts, September 15, 1994. @
0022-365419412098-10814$04.5010
giving rise to the possible formation of independent micellar systems.‘O Studies of the complexes formed between perfluorinated surfactants and p-CD3-’ suggest that perfluorinated surfactants form strong complexes with p-CD, due mainly to a large crosssectional diameter of the fluorocarbon chain such that it fits snugly into the p-CD torus.3 However, the values of the formation constants calculated for these complexes differ by several orders of magnitude, depending on the method used to obtain them. C o n d ~ c t i v i t yand ~ ~ 19F ~ ~ ~NMR3 studies of these systems suggest that the association constant for the 1:1 complex is larger than that for the analogous hydrocarbon surfactant, sodium octanoate, while a more recent velocity of sound study6 suggests that they are more comparable. It has also been suggested that the 1:l complex may associate with the Na+ to form a (1:l):l ~ o m p l e x . ~ In this paper, we extend our studies of the cyclodextrid surfactant interactions to include the sodium perfluorooctanoate (SPFO) and p-CD system. Only a limited number of ultrasonic relaxation studies of the monomer-micelle exchange process for SPFO micelles have been The most recent study by Kat0 et al.14 investigated the monomer-micelle exchange process for both SPFO and cesium perfluorooctanoate(CsPFO) micelles. They observed a decrease in both the ratio of the exit rate for the monomer to the mean aggregation number (k-In) and the volume change (AV) when the counterion was changed from Na+ to Cs+. Therefore, it appears that the nature of the counterion has a strong effect on the micelle exchange kinetics for perfluorinated surfactants. Given the sensitivity of the monomer-micelle exchange kinetics to the counterion in perfluorinated surfactant systems, the formation of a (1: 1):1 complex may have an effect on the monomer-micelle exchange process in the /3-CDISPFO system. 0 1994 American Chemical Society
Monomer-Micelle Exchange
J. Phys. Chem., Vol. 98, No. 42, 1994 10815
1000 * A
f.1
t 1
M SPFO
M SPFO SPFO SPFO SPFO
0 0 0 0 0 0 . 0 2 0 M C1 M m m P m 0.084 M C1 AAAAA 0.134 M C1
I
I
10
100
I
I
f (MHz) Figure 1. A plot of df versus frequency for several concentrations of the 1:1 complex. Experimental Section Materials. p-CD (Aldrich, '99%) was used without further purification. The surfactant, SPFO (PCR Reasearch, 99%), was twice recrystallized from an acetone/methanol mixture (90/10 v/v) and dried under vacuum. All solutions were prepared using Super Q millipore water. Ultrasonic Absorption Measurements. Ultrasonic absorption measurements were made at 25.00 "C using a previously described a p p a r a t ~ s .Both ~ ~ ~the ~ ~cylindrical resonator (0.8-5 MHz) and the pulsed (10-190 MHz) methods were employed. Sound velocity measurements were made using a MAPCO Nunsonic (Model 6080) sound velocimeter which operates on the principle of the "sing around" method. Although the temperature stability of the bath was f 0 . 0 1 "C, localized heating may have occurred during the cylindrical resonator measurements. However, due to the low voltage applied to the crystal (10 V), this effect was minimal. The ultrasonic data were fitted to the expression: j
'd = EA,/{ 1
+ vfr)2} +B
i= I
where a is the absorption coefficient at frequency f, j is the number of relaxations, and B is the background or classical sound absorption for the solution (typically 23 x lo-'' Np s2 cm-l for water). The computer program for fitting the data was based on the Marquardt algorithm and fitted the data by minimizing the x2 value. A plot of the residuals was also produced to assist in the determination of the number of relaxation processes. Conductivity Measurements. Conductance measurements were made at 25.00 i 0.01 "C using a Wayne Kerr Precision Component Analyzer 6425 and an immersion conductivity cell having a cell constant of 1.1573 cm-'. Measurements were made using a titrametric dilution method. Results and Discussion Figure 1 shows the ultrasonic relaxation spectra obtained for 1:l complexes of SPFO with @-CD. As with our previous study,' the presence of a strong relaxation spectrum for the 1:l P-CDlsurfactant complex is unexpected as there are no micelles
1
10
o
n
I
100
,
f (MHz) Figure 2. A plot of'd versus frequency for solutions where the C ~ C D remained fixed at 0.020 M and the concentration of SPFO varied between 0.042 and 0.138 M. and therefore no monomer-micelle exchange process. Furthermore, unlike substituted fi-cyclodextrins,'2 p-CD has only a weak ultrasonic absorption in the frequency range studied here.'8s'9 Therefore, it seems unlikely that the large sound absorptions observed here, especially at high concentrations of the 1:1 complex, are due to the presence of p-CD alone. As with the P-CDIDTAB system, the presence of a relaxation spectrum for the 1:1 complex must result from the interactions between the surfactant and the cyclodextrin. As we indicated previously,' we believe the ultrasonic absorption found for aqueous solutions of the 1:l complex is due to movement in the cyclodextrin structure induced during the initial stage of the formation of the complex. Figure 2 shows the variation in the ultrasonic relaxation spectrum obtained when the concentration of p-CD (Cg-c~)is held constant at 0.020 M and the concentration of SPFO (CJ is varied between 0.042 and 0.138 M. These spectra show very little sound absorption in the 8-20 MHz range, indicating that little contribution occurs from the 1:1 complex. Therefore, we have assumed that the variation in the ultrasonic absorption spectrum observed here is due largely to changes in the micelle concentration and hence the micelle-monomer exchange process. Changes in the micelle concentration are also indicated by the decrease in absorption in the 0.8-8 MHz range, where the relaxation due to the monomer-micelle process is expected to occur. Figure 3 shows the effect of adding p-CD to a solution containing 0.14 M SPFO. As the concentration of p C D increases, the sound absorption in the 0.8-8 MHz range starts to decrease while it increases slightly in the 8-60 MHz range. As noted above, these overall changes are due to the formation of the 1:l complex, which increases in concentration at the expense of the SPFO micelle concentration, as more p-CD is added to the system. Figure 4 shows a plot of the molar conductance (A) for SPFO plotted against increasing p-CD concentration for a fixed concentration of SPFO. As with the P-CDIDTAB system, the variation in A is not smooth; A decreases when all the surfactant that forms SPFO micelles is removed via formation of the 1:1 complex. This decrease appears because beyond this point, the
Jobe et al.
10816 J. Phys. Chem., Vol. 98, No. 42, 1994 a/f2 x 1017
(NP
s2 cm-’) M SPFO
0
+
0.020 M
II-CD
0 . M.~@-CD M SPFO + 0 . 0.8_ M SPFO + 0.020 M p-CD M SPFO + 0.020 M p-CD ~
1000
M 8-CD M p-CD M p-CD M p-CD M p-CD M p-CD
‘0 01
M SPFO
+
0.020
M 4-CD
100
1004
l0
I
I
1
10
100
shown for comparison.
000000.134 o3xo0.100 AMAA0.071 xxxxx0.041
M M M M
SPFO SPFO SPFO SPFO
45
35
10
100
1
Figure 3. A plot of Cry versus frequency for solutions where the SPFO concentration remained fied at 0.14 M and the Cp-c~varied between 0.030 and 0.14 M. The relaxation curve when C ~ C=D0.0 M is also
3
1
f (MHz) I
1
f (MHz)
55
1
Figure 5. A plot of Crf versus frequency for solutions where C, > 0 and sound absorption from the 1:l complex has been subtracted. that the absence of the initial increase in A with the initial addition of p-CD is due to the rapid binding by the 1:1 P-CDI SPFO complex of the Na+ upon its release from the ~ u r f a c e . ~ This view is supported by the decrease in Na+ activities found when p-CD is added to nonmicellar SPFO and SDS solutions. The p-CDISDS system undergoes a 1.5% decrease in Na+ activity upon the addition of 4.0 x m ~ l d m -of~p-CD to a 5.0 x m ~ l - d m -solution ~ of SDS. At the same concentrations of surfactant and p-CD, the P-CDISPFO system undergoes a 4.8% decrease. Assuming the decrease in Na+ activity is due to the formation of the (1:l):l complex, this indicates that the formation constant for the @-CD/SPFO)/Na+ complex may be at least a factor of 3 stronger than the @-CDI SDS)/Naf complex. While DTAB forms large micelles, SPFO forms small “mini” micelles, and thee “loose” weakly associated aggregates may also be affecting the Na+ activity. Alternatively, Junquera et a1.6 have reported that, with respect to the cmc for SPFO in the absence of P C D , the SPFO monomer concentration decreases with the addition or small amounts of P C D . This decrease in the monomer concentration could also be contributing to the decrease in the Na+ activity. Figure 5 shows the ultrasonic relaxation data obtained when the sound absorption due to the 1:1 complex is substracted from the data for the mixed system. The parameters (fr,A, and B ) that result from fitting this data to eq 1 using j = 1 can be found in Table 1. In order to obtain the “background” sound absorption, the sound absorption data for the 1:l complex were fitted to eq 1. The resulting fit parameters were then used to calculate the contribution from the complex to the measured df for each of the frequencies in the mixed spectra. These were then subtracted to obtain the sound absorption data shown in Figure 5. As can be seen from Table 1, and x2 values for the fit of the background corrected data is larger than the ideal value of 1.OO, due to the rather large uncertainty in d ’ arising from the subtraction process. This larger uncertainty is a factor that had to be considered when evaluating the uncertainty in the parameters derived from treating the sound absorption data of the monomer-micelle exchange process using the Aniansson and Wall model. As with the DTABIP-CD study,’ the ultrasonic absorption data obtained here indicate the SPFO micelles break down to form 1: 1 complexes. The decrease in the ultrasonic absorption in the 0.8-8 MHz range as the concentration of p-CD increases (Figure 2) is a strong indication of this. Since the p-CD
1 0 2 4 6 8 10 [p-CD] x 10‘ (mol dm-3)
Figure 4. A plot of A versus C ~ Cfor D solutions containing 0.041, 0.071, 0.100, and 0.134 M SPFO. addition of more p-CD results in the removal of monomeric SPFO from solution decreasing the conductivity of solution. One noticeable difference between the conductance data obtained here and the conductivity measurements made for the P-CDIDTAB system is the absence of an increase in A during the initial additions of 0-CD. The increase in the solution conductivity for the @-CDIDTABsystem’ was believed to be due to the release of Br- from the micelle surface due to the breakdown of the DTAB micelles and the subsequent formation of the 1:1 complex. It might be argued that the absence of this increase in the fl-CDISPFO system may reflect the smaller molar conductance of the Na+ relative to the Br-, therefore, making a smaller contribution to the total conductance of the solution. However, Reinsborough et al. have investigated the B-CDI SPF03 and p-CDl(SDS) sodium dodecyl sulfate systems20*21 and have offered an alternative interpretation. Their studies suggest
Monomer-Micelle Exchange
J. Phys. Chem., Vol. 98, No. 42, 1994 10817
TABLE 1: Ultrasonic Relaxation Best Fit Parameters after Sound Absorption from the 1:l Complex Has Been Subtracted CpCD(mol-dm-‘) C, (mopdm-’) Cf(mol-dm-’) C, (mol-dm-’) A (Np s2 cm-2) x 10’’ fi (MHz) B (Np s2 cm-I) x loi7 x2 u (ms-’) 0.020 0.080 0.020 0.050
0.020 0.030
0.020
0.060 0.134 0.083 0.134 0.109 0.134 0.138
0.030 0.046 0.030 0.042 0.030 0.037 0.030
0.014 0.026 0.046 0.054 0.063 0.074 0.092
1553 1207 1548 2017 2269 2607 2863
concentration is so low, the decrease in CY in. this / range ’ is almost due solely to the decrease in the ultrasonic relaxation frequency for the monomer-micelle exchange process, which results from a decrease in the concentration of micelles due to the formation of the 1:1 complex. In order to quantify the relationship between the formation of the 1:1 complex, the change in the micelle concentration and the shift in the ultrasonic relaxation frequency, the Aniansson and Wall2 theory was used. This theory requires a knowledge of the concentration of “free” surfactant (Cf) that is able to exchange with the micelles, which for simple aqueous surfactant systems is approximated by the cmc. For the aqueous SPFO system, Kat0 et al.14 obtained the concentration of free surfactant using the expression:
where P is the fraction of counterion dissociated from the micelle and z is the charge on the counterion. For the mixed P-CDl surfactant systems, this concentration must be modified somewhat, as there is a fraction of the surfactant that is complexed and is considered unexchangeable with the micelle. Furthermore, there appears to be a much larger fraction of the 1:2 SPFOIP-CD complex in these system^;^,^ therefore, the concentration of “free” SPFO cannot be approximated by the cmc (0.032 M) in the absence of P-CD as was the case for DTAB.’ Recently, Junquera et a1.6 have studied the effects of P-CD on the velocity of sound ( u ) for micellar SPFO solutions and predicted the following relationship between Cf and the apparent critical micelle concentration (cmc*) in the mixed system:
Cf = cmc* - CP&A
(3)
where the parameter A is the average stoichiometry for the p-CDISPFO complex, Le., the average number of p-CD molecules per SPFO molecule in a complex. The cmc* is found by extrapolating to the concentration of SPFO where u undergoes an inflection for a plot of u versus the concentration of SPFO at a fixed concentration of p-CD. Using the data reported by Junquera et al., the values of cmc* were estimated for the p-CD concentrations studied here. Using these interpolated values, A = 1.2, and eq 3, the values of Cf were calculated and are shown in Table 1, along with the total SPFO concentration and the concentration of surfactant contributing to micelles (C,), where C, = Ct - cmc*. Using these C, and Cf values, as well as the values of fr given in Table 1, the Aniansson and Wall theory was used to determine the micelle kinetic parameters k-/u2 and k-In, where 2 n . = l/z = k-/c?
+ (k-/n)(C,/cmc)
(4)
and t is the relaxation time for the exchange of a surfactant monomer with the micelle. On the basis of the kinetic model proposed by Aniansson and Wall, and from Teubner,22 the expression to analyze the relaxation amplitudes is
0.74 1.01 1.22 1.25 1.27 1.31 1.37
29 38 32 36 31 33 34
kax = (Af~)/2= [(nA,2cmc)/(2RT~,)]r/(l
3.7 3.2 2.9 4.4 5.2 2.9
3.6
1499 1498 1497
1500 1503 1489 1479
+ r)= yAv2
(5) where pmaxis the sound absorption maximum per wavelength, Po is the adiabatic compressibility of the solvent, AV is the average volume change associated with the exchange, r = (02/n)(C&f), and R and Thave their usual meaning. Therefore, from a plot of pmaxversus y, the average volume change associated with the exchange process can be evaluated. Since s t ~ d i e s have ~ ~ - shown ~ ~ that the kinetics for the inclusion of the surfactant into the CD cavity is much slower than that the rate of inclusion for the surfactant monomer-micelle exchange rate, it is assumed that the fraction of complexed SPFO is unavailable to exchange with the micelle, and hence the “cmc” is assumed to be Cf. Figure 6 shows a plot of 2x5 versus C,/Cf using the data given in Table 1. A least-squares fit of the data points yields k-/n = (1.3 k 0.4) x IO6 s-l and k-/u2 = (5.4f 0.8) x lo6 s-l, from the slope and intercept, respectively. The solid line through the points in Figure 1 is the best-fit line according to the least-squares analysis. The kinetic parameters for the pure SPFO are found by Kat0 et al.14 to be k-/n = 1.15 x IO6 s-l and k-/a2 = 0.93 x lo6 s-l, respectively. The first one is in good agreement with our k-In result, but the value for k-lu2 found here is larger than the value found in the absence of P-CD14 and the difference between these values is outside the standard deviation for the fit. Therefore, statistically it appears that the ratio k-/u2increases in the presence of P C D . However, the values offr are dependent on the success of the “background” subtraction procedure described earlier and this may in part be contributing to the larger intercept. Since the slope (k-In) for the two studies are similar, it is reasonable to assume that the exit rate ( E )of the monomer from the micelle and the mean micelle aggregation number are unchanged by the presence of P-CD. Therefore, the larger value for k-/u2is due to a decrease in the variance for the micelle size distribution (a2).This was not observed for the P-CDIDTAB system. The mean aggregation number for SPFO is found to vary, depending on the method used in the determination. Luminescence indicate a low number, ranging between 7 and 10, while small angle neutron scattering26indicates n is larger, ranging between 27 and 31. For the sake of comparison with the data of Kat0 et al.,14the value of n = 27 will be used. This value yields k- = 3.5 x lo7 s-I for SPFO in the presence of p-CD and, from the intercept, u = 2.5. This value is slightly lower than the value of 5.8 found by Kat0 et al.14 One explanation for this “sharpening” of the micelle size distribution may be the binding of the counterion by the 1:l complex. The removal of the counterion by the complex may affect the fraction of counterion bound to the surface and hence the micelle size di~tribution.~~ But the uncertainty of the “background” substration procedure could also be an explanation for the difference in u for the SPFO system in the absence and in the presence of p-CD. In either case as since Kat0 et al. have not reported any uncertainty values of their u data, it is difficult to say whether there is agreement between the u value obtained here and that obtained from their study.
Jobe et al.
10818 J. Phys. Chem., Vol. 98, No. 42, 1994 2 n f , x 10
-6
Kat0 et al.14 observed no effect of the fast counterion exchange process on the monomer-micelle exchange rate. The similarity in the ultrasonic properties observed here with those found by Kat0 et al.I4 in the absence of p-CD suggests that counterion exchange is also not important here either. Therefore, unlike the micelle size distribution, it appears that neither fast counterion exchange or the binding of the counterion by the complex observed by Reinsborough et aL3,*0has an effect on the kinetic rate constants for the exchange process, or the average volume change associated with the exchange.
(s-l)
9-
s
8-
Conclusions
Sj
0
J
4 ! . # "
0
'
I
1
,
#
2
1
I
,
,
3
,
,
!
.
,
4
Cm/Cf Figure 6. A plot of 2nfr versus C&mc for solutions where C,
> 0.
i
Acknowledgment. Funding from the National Sciences and Engineering Research Council of Canada is gratefully acknowledged. E.J. wishes to thank the M.E.C. (Spain) for financial assistance in the form of a travel grant.
2.5j
2.0
This ultrasonic absorption study of the P-CDISPFO system indicates that the kinetics of the monomer-micelle exchange process for the SPFO micelles is relatively unaffected by the presence of p-CD or the 1:l complex. The occurrence of counterion binding to the P-CDlsurfactant complex or of fast counterion exchange processes appears to affect only the micelle size distribution and not the mean aggregation number or the exit rate for the monomer.
1
References and Notes (1) Jobe. D. J.: Verrall. R. E.: Junauera., E.:, Aicart. E. J . Phvs. Chem. I
0.5
i
0.0 0.0
1993,97, 1243.
0.4
0.8
y
x 10'
1.2
1.6
mol2 cm-6
Figure 7. A plot of pmaxversus y for solutions where C,
> 0.
Figure 7 shows a plot of pmaxversus y and from the slope, a value of AV = 13 f 2 ~m~mmo1-l was obtained. This value is also in agreement with the value of AV = 11 cm3mol-' found by Kat0 et al. for SPFO in the absence of P-CD.14 The observation that AV is relatively unaffected by the presence of the SPFOIP-CD complex indicates the water molecules expelled from the cyclodextrin cavity upon the formation of the complex are not involved in the monomer exchange process. These results also support the view suggested earlier' that the monomer-micelle exchange process is kinetically independent of the formation of the 1:1 complex; therefore, the surfactant lost to complex formation can be accounted for by a simple equilibrium model. This AV value is larger than that obtained for sodium octanoate, which indicates perfluorinated hydrocarbons are more hydrophillic than their hydrocarbon analogues, as the volume change is thought to be proportional to the number of hydrophobically hydrated water molecules that are returned to the solution upon the return of a surfactant monomer to the micelle.28 The values of the amplitude relaxation A, of the P-CDISPFO system (Table 1) are greater than the corresponding amplitudes of the P-CDIDTAB system (Table II of ref l), which is also consistent with the lower AV value (5.6 cm3mol-') for the latter system.
I
'
(21 Aniansson. E. A. G.: Wall. S. N. Chemical and Bioloaical Appiicutions of Rejawation Spectrometry; Wyn-Jones, E., Ed.; D. Riidel Publishing: Dordrecht, Holland, 1975; pp 223-238. (3) Palepu, R.; Reinsborough, V. C. Can. J. Chem. 1989, 65, 1550. (4) Palepu, R.; Richardson, J. E.; Reinsborough, V. C. Langmuir 1989, 5, 218. (5) Guo, W.; Fung, B. M.; Christian, S. D. Langmuir 1992, 8, 446. (6) Junquera, E.; Tardajos, G.; Aicart, E. Langmuir 1993, 9, 1213. (7) Saint, Aman, E.; Serve, D. J. Colloid Interface Sci. 1990,138, 365. ( 8 ) Jobe, D. J.; Skalski, B. D.; Verrall, R. E. Langmuir 1993,9,2814. (9) Turro, N. J.; Lee, P. C. C. J. Phys. Chem. 1982, 86, 3367. (10) Reinsborough, V. C.; Schultz, T. D.; Xiang, X. Aust. J . Chem. 1990, 43, 11. (11) Mukerjee, P.; Yang, A. Y. S.J . Phys. Chem. 1976, 80, 1388. (12) Hoffmann, H.; Ulbricht, W. Z. Phys. Chem. 1977, 181, 167. (13) Gettins, J.; Jobling, P. L.; Walsh, M. F.; Wyn-Jones, E. J . Chem. SOC.,Faraday Trans. 2 1980, 76, 794. (14) Kato, S.; Harada, S.; Nakashima, H.; Nomura, H. J. Colloid Interface Sci. 1992, 150, 305. (15) Rassing, J.; Sams,P. J.; Wyn-Jones, E. Faraday Trans. 1978, 1247. (16) Jobe, D. J.; Verrall, R. E.; Reinsborough, V. C. Can. J. Chem. 1990, 68, 2131. (17) Verrall, R. E.; Nomura, H. J . Solution Chem. 1977, 6, 1. (18) Kato, S.; Nomura, H.; Miyahara, Y . J. Phys. Chem. 1985, 89, 5417. (19) Rauh, S.;Knoche, W. J. Chem. SOC.,Faraday Trans. 1 1985, 81, 2551. (20) MacPherson, Y. E.; Palepu, R.; Reinsborough, V. C. J. Inclusion Phenom. Mol. Recognit. Chem. 1990, 9, 137. (21) Palepu, R.; Reinsborough, V. C. Can. J . Chem. 1988, 66, 325. (22) Teubner, M. J. Phys. Chem. 1979, 83, 1979. (23) Okubo, T.; Maeda, Y.; Kitano, H. J . Phys. Chem. 1989, 93,3721. (24) Turro, N. J.; Okubo, T.; Chung, C.-J. J. Am. Chem. SOC. 1982, 104, 1789. (25) Hersey, A,; Robinson, B. H. J . Chem. SOC.,Faraday Trans. 1 1986, 80, 2039. (26) Berr, S.; Jones, R. R. M.; Johnson,J. S., Jr. J . Phys. Chem. 1992, 96, 5611. (27) Berr, S.; Jones, R. R. M. J. Phys. Chem. 1989, 93, 2555. (28) Sugihara, G.; Mukejee, P. J. Phys. Chem. 1981, 85, 1612.
