Langmuir 1994,10, 51-56
51
A Study of Fluorocarbon-Hydrocarbon Surface Active Agent Mixtures by NMR Spectroscopy R. M. Clapperton and R. H. Ottewill* School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.
B. T. Ingram Newcastle Technical Centre, Procter and Gamble Limited, Newcastle-upon-Tyne NE12 9TS, U.K. Received March 24, 1993. I n Final Form: October 4, 199P Binary mixtures of ammonium perfluorooctanoate with ammonium decanoate have been examined by a combination of 1H and 19F NMR spectroscopy over a range of fluorocarbon:hydrocarbonratios. From chemical shift data obtained on the mixtures it has been possible to elucidate the micelle composition as a function of total surface active agent concentration. The experimental results can be described approximately by a regular solution model, and show nonideal, unfavored mixing of the hydrocarbon and fluorocarbon species in the micelle. Introduction An understanding of how mixed systems of fluorocarbon and hydrocarbon surface active agents behave has been developing over recent years. It has become apparent that mixing of fluorocarbon tailgroups with hydrocarbon tailgroups in the micelle is nonideal and unfavored. The degree of nonideality increases with the length of the hydrocarbon tai1112 which is consistent with the behavior of mixed fluorocarbon-hydrocarbon liquid systems? Several earlier studies looked at the properties of the surface active agent mixture a t the aidwater interface.&l2 More recently mixed fluorocarbon-hydrocarbon micelles have been studied directly by the techniques of small-angle neutron scattering (SANS),13fluorescence quenching,14 and NMR.2 Most experimental techniques available for the study of mixed surface active agent systems monitor the combined contribution of the two components to the overall physical properties of the system. NMR offers the advantage, through the application of both I9F and lH spectroscopy, of enabling the fluorocarbon and hydrocarbon surface active agent to be observed independently in the same mixed system. This allows the concentration
* Address correspondence to Professor R. H. Ottewill, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 LTS, U.K. a Abstract published in Advance ACS Abstracts,
December 15,
1993. (1) Shinoda, K.; Nomura, T. J. Phys. Chem. 1980,84,365. (2) Aeakawa, T.; Miyagishi, S.; Nishida, N. J. Colloid Interface Sci. 19AK. -104. 279. - - I ----I
(3) Scott, R.L. J. Phys. Chem. 1958, 62, 136. (4) Burkitt, S. J.; Ingram, B. T.; Ottewill, R. H. Rog. Colloid Polym. Sci. 1988, 76, 247. (5) F u n d , N.; Hada, S. J. Phys. Chem. 1983,87, 342. (61 Smith.1. H.:WwiU.R.H.SurfaceActiue&ents:SCISymposium; - Giety of C h e m i h Industry: London, 1979; p ?I. (7) Klevens, H. B.; Raison, M. J. J. Chin. Phys. Phys.-Chim. Biol. 1914,51, 1. (8) Funaeaki,N.; Hada, S. J. Colloid Interface Sci. 1980, 73,425. (9) Guo-Xi, Z.; Bu-Yao, Z. Colloid Polym. Sci. 1983,261, 89. (10) Ueno, M. In Colloid and Interface Science; Kerker, M., Ed.; Academic Preee, New York, 1977; Vol. 2, p 141. (11) Funaeaki, N.; Hada, S. J. Phys. Chem. 1980,84,736. (12) F u n d i ,N.; Hada, S. Chem. Lett. 1979, 717. (13) Burkitt, S. J.; Ottewill,R. H.; Hayter, J. B.; Ingram,B. T. Colloid Polym. Sci. 1987,266,628. (14) Muto, Y.; Esuim, K.; Meguro, K.; h a , R. J. Colloid Interface Sci. 1987, 120, 162. '
0743-7463/94/2410-0051$04.50/0
at which each species first enters the micellar state to be determined and the change in micelle composition with total surface active agent concentration to be calculated. Both these factors can be used to express the extent of nonideality of mixing in terms of the micelle interaction parameter, Bm. The determination of the micelle composition enables the monomer concentration of fluorocarbon and hydrocarbon components above the critical micelle concentration (cmc) to be calculated as a function of total surface active agent concentration. The systems studied in this work were binary mixtures of ammonium perfluorooctanoate (APFO) and ammonium decanoate (AmDec);these were buffered at pH 9.0 in NH4OH-NH4C1 solutions at an ionic strength of 0.1. The air/ water interfacial properties of both systems have been reported previously,4 as has a SANS study of APFOAmDec mixtures.13 An NMR study of these systems complements this earlier work, providing a link between the surface and micellar properties. Experimental Section Materials. Decanoic acid was Fluka puriss grade material with an estimated gas-liquid chromatography (GLC) purity of >99%. The acid was converted to the ammonium salt by neutralizationwith aqueousammoniasolution, and then buffered by addition of ammonium chloride. The ammonium perfluorooctanoate was Rimar material. The buffer pH and ionic strength in the mixed systems (see earlier') was the same as in the single surface active agent systems, pH 9.0, ionic strength 0.1.
All water used was doubly distilled from an all-Pyrex still. Deuterium oxide (99% DzO)was obtained from Aldrich. NMR Measurements. lgFNMR spectrawere determinedat 20 "C using an FX 90 Q Fourier transform spectrometer. The 1H spectrawere obtainedusing a JNM GX 270 FT spectrometer. All samples were examined in aqueous buffer solutions which contained 10% by volume of DzO as a reference material; these conditions corresponded to those used for small-angle neutron scatteringstudies. The 19F chemical shift data are expressed in parta per million displaced relative to CFCb. The 1H chemical shift data are expressed in hertz displaced relative to the methylene group (Cz)adjacent to the carboxylate headgroup of the hydrocarbon molecule. This negates the effect of an unavoidable systematicerror in the chemical shift of the proton peaks relativeto the reference compound (DzO). It was assumed that, being adjacent to the headgroup of the surfactant molecule
0 1994 American Chemical Society
Clapperton et al.
52 Langmuir, Vol. 10,No. 1, 1994
and thus maintaininga high degree of water contact, the resonance frequency of these protons changed very little on micellization. The convention used for numbering the carbon atoms is illustrated below:
I
H-Cs-H
I H-C,-H I
H-CE-H
I
H-CB-H
I
H-C,-H
I I H-Q-H
H-Ce-H
F-&F
I
F-CI-F
I I F-CB-F I F-C,-F I
F-Cs-F
On the basis of the phase behavior of liquid fluorocarbon-hydrocarbon mixture^,^ two conditions are believed to exist for micellar solutions of the surface active agent mixtures. Condition I. The first is where complete mixing in the micelle of the two components is possible but not necessarily ideal. At any particular concentration above the mixed cmc, all micelles present will be of the same composition, x . If the pure state cmc of the two components is different and/or if mixing is nonideal, x will differ from the total solution composition a at the mixed cmc. However, for all mixed systems, x must tend to a as the total concentration (CT) far exceeds the cmc, assuming the monomer concentration remains small. The solution and micellar compositions,a and x , respectively,expressed as mole fractions are given by
F-Ce-F
(3)
I
F
I
(4)
H-C1o-H
I
H
Theory The physicochemical environment around a surface active agent molecule is quite different when the molecule exists as a single monomer in an aqueous solution as opposed to being one of an associated unit of many moleculeswithin a micelle. As a consequence,the tailgroup protons of a hydrocarbon surface active agent experience a small upfield NMR shift on micellization, while fluorocarbon nuclei experience a downfield shift on micellization. Generally, the exchange rate of a surface active agent molecule between monomer and micellar states is several orders of magnitude greater than the NMR relaxation rate,16 and consequently a single resonance signal is obtained in the spectrum, reflecting the time-averaged environment of the molecule. For both single and mixed systems, the general expression for the observed chemical shift of the surface active agent molecules is given by
where CFc and C p are the total solution concentrations of hydrocarbon and fluorocarbon. C z c and ::C are the micellar concentrations of each. For a given a and CT, the observed 19Fand lH chemical shifts (f&, 60&) are given by FC 6M o: bd
= xCm ( o6 mFC o-
FC)
+
FC
(5)
CT
I t can be assumed that above the cmc, : : 6 and:;6 are constant and equal to 6 r b a n d below the cmc. Experimentally, : : 6 and do not vary between the single and mixed systems. Two unknowns exist in eqs 5 and 6 (a) : : 6 and 6Lc. These depend upon x and so vary with CT. (b) Cg and CFz. Unlike for single systems, these should also vary wlth CT. (1) 6obad = (cmo/cT)6mo + (Cmi/cT)ami vs Ctc -'gives 6:C and Plotting &:6 vs C!,? and where 6mo and 6,i are the monomer and micelle chemical : : 6 for a micellar of composition a, since at infinite concentration (CT-~ 0) micelle composition ( x ) is shifts and CT, Cmo, and Cmi are the total surface active equivalent to solution composition (a). agent concentration, the monomer concentration, and the By studying a range of solution compositions (a),it is micelle concentration. Single Systems. For single systems, given that the possible to establish a relationship between and monomer concentration is approximately constant above and x . One can then determine the micelle compothe cmc, eq 1 can be rewritten for CT 1 cmc as sition as a function of total mixture concentration for a given value of a. 6obsd = (cmc/CT)(amo- ami) + ami (2) The followinggeneral equations for surface active agent mixtures are first combined with eqs 3-6: Hence, a plot of 6obsd vs CT-' is linear above the cmc (see Figures 1 and 2). Extrapolation of CT-~ 0 gives ami, (7) while is equivalent to a0wbelow the cmc. Mixed Systems. In mixtures of fluorocarbon-hydrocarbon surface active agents, the polarities of the two tailgroups differ significantly; hence, where close contact exists, such as in mixed micelles, the chemical shift of Solving simultaneously to eliminate CLt and :C : gives each is affected by the presence of the other. Thus, hydrocarbon in the mixed micelle should cause an upfield shift of 19Fpeaks relative to the pure fluorocarbon micelle. Conversely, lH peaks should display a downfield shift in mixed micelles relative to the pure hydrocarbon micelle.
-'
-
-
(15) Nakagawa, T.; Tokiwa, F. In Surface and Colloid Science; MatijeviE, E., Ed.; John Wiley and Sons: New York, 1969; Vol. 9, p 69.
and :;6 can be determined As described above, experimentally as a function of x . Hence, putting
Surface Actiue Agent Mixtures
Langmuir, Vol. 10, No. 1, 1994 53
E
'I" 6
1.5
0
40
80
120
160
200
I
I
10
20
240
I
1.0
I
I
vo
30
0.5 PPm
0 0
I
IO
60
[Concentrationlmol dm'3]-1
[Conccntration/mol
Figure 1. Chemicalshift,relative to monomer, against reciprocal concentrationof APFO A,Cz; 0 ,C,; 0, CS. Inset: lQFspectrum of APFO.
Figure 2. Chemical shift,relative to monomer,againstreciprocal concentration of AmDec: 0, C3; 0 , C&; A, C~O.Inset: 'H spectrum of AmDec. I
I
I
2
and inserting into eq 9 gives
fHC(X))
6th
+ fHCW(10)
are known, eq 10 For any mixture where and can be solved numerically to give a value for x . Having determined x as a function of CT, it is then possible to determine the fluorocarbon and hydrocarbon monomer (and hence micelle) concentrations as a function of CT. (11)
Condition 11. The second possible condition for mixed systems is where two kinds of mixed micelles coexist in equilibrium, one fluorocarbon-rich,the other hydrocarbonrich (independent micellization). This is analogous to partially miscible liquid mixtures. This is not straightforward to identify by NMR as the observed chemical shift is a function of three different environments. In this case, following the mathematical approach described above, too many variables exist to enable the micelle chemical shift dependence on micelle composition to be established. For the present system, however, small-angle neutron scattering results indicated that independent micellization does not occur in the APFO-AmDec system examined.13 Results Examples of the NMR spectra obtained for APFO and AmDec are illustrated in Figures 1 and 2. The assignment of each peak to specific groups of protons or fluorine nuclei within each moleculeis given. Although the chemical shift of each peak varied with total surface active agent concentration, both for the single species and for the mixtures, the general pattern of peaks for each surface active agent remained the same. The chemical shift dependence on concentration for solutions of APFO and AmDec as a function of concentration is given in Figures 1 and 2. All these curves display the characteristic features expected for a single-component
0
I 40
I
I
80
120
I 160
Concentrationlmol
I 200
240 I
I
280
dm-31-1
Figure 3. 1QFchemical shift, relative to monomer, against reciprocal concentrationof APFO, for an APFO-AmDec mixture with a = 0.56; A, Cz; O! Cy; 0, Cs. Inset: lH chemical shift, relativeto monomer, agcunst reciprocal concentrationof AmDec, for the same mixture: 0, c3; A, c10. system:16-20a constant value below the cmc (6J, and a linear dependence on the reciprocal of concentration above the cmc, tending to the micelle chemical shift as CT-~ 0. For APFO and AmDec, data have been collected for a number of groups along the tail. I t shows an increase in the chemical shift change on micellization with increasing distance of the group from the headgroup; this can largely be explained by a greater reduction in the timeaveraged water contact for groups far removed from the headgroup.21 The accuracy of determination of the cmc is greatest where the difference between the monomer shift and the micelle shift is maximal. The values obtained (APFO, 8.2 X 10-3 M; AmDec, 5.7 X M) are similar to those obtained previously from surface tension measurements4 (APFO, 9.6 X 10-3 M; AmDec, 4.1 X M). The chemical shift dependence on reciprocal concentration for APFO-AmDec mixtures is different from that obtained on the single-componentsystems, being nonlinear above the cmc. This reflects a change in micelle composition with total surface active agent concentration. Figure 3 shows 19F and 1H chemical shifts as a function of concentration for an CY = 0.56 mixture, and Figure 4 shows similar results for an CY = 0.89 mixture. These are typical of the observations made on the mixed systems in that the deviation from linearity above the cmc followed a similar pattern for the chemical shifts of all groups in the tails. As with the single systems, the data for the terminal perfluoromethyl (Cs) and methyl groups (Clo) show the
-
~~
(16) Muller, N.;Simsohn, H. J. Phys. Chem. 1971, 75,942. (17) Muller, N.;Johnson, T. W. J. Phys. Chem. 1969, 73,2042. (18) Henriksson, N.;Odberg,L. J. Colloidhterface Sci. 1974,46,212. (19) Muller, N.;Birkhahn, R. H. J. Phys. Chem. 1967, 71,957. (20) Muller, N.;Birkhahn, R. H. J. Phys. Chem. 1968, 72,583. (21) Ulmius, J.; Lindman, B. J . Phys. Chem. 1981, 85, 4131.
Clapperton et al.
54 Langmuir, Vol. 10, No. 1, 1994
1
3
IL
I
Table 1. APFO-AmDec Mixtures
1
n
l
a
lH cmc/ (moldm3)
0 0.20 0.40 0.56 0.72 0.89 1.00
0
40
80
120
200
160
[Concentrationlmal
290
280
dm-31-1
Figure 4. lgF chemical shift, relative to monomer, against reciprocal concentration of APFO, for an APFO-AmDec mixture with a = 0.89 A, C2; 0 , C,; 0,CS. Inset: 'H chemical shift, relative to monomer, againstreciprocal concentrationof AmDec, for the same mixture: 0, Cs; 0, Cd-cg; A, 0. 3
L 2 . $
-m 'B UI
1
U
0 40
80
120
160
200
290
[Concentrationlmol dn1-~1-'
Figure 5. lSF chemical shift, relative to monomer, against reciprocal concentration of APFO for various mixtures: a = 0, 0;a = 0.56, A;a = 0.72,O; a = 0.89,O; arrows, intercept values.
t
I
I
I
10
20
I
I
30
90
I 50
I
1
60
[Concentrationlmol dm'3]-1
Figure 6. 1H chemical shift, relative to monomer, against reciprocal concentration of AmDec for various mixtures: a = 0.56, 0; a = 0.89, A; a = 1.0, 0;arrows, intercept values.
greatest displacement from monomer chemicalshift values, and consequently this has been chosen for the determination of the mixed cmc values and the micelle chemical HC Jmi). FC shifts (ami, Figures 5 and 6 show the 19Fand 'H chemical shifts and the dependence on the reciprocal of the total concentration for a range of APFO-AmDec mixtures. It can be seen from these figures that the monomer chemical shift is identical to that observed for the single surface active agent systems. This indicates negligible contact between the
lgF cmc/ (moldm") 8.2 X 1 P
1.9 X
5.0 X 5.7 x 10-2
1.8 X 3.2 X 5.0 X
:6 80.58 80.58 80.58 80.58 80.58 80.58
65 83.21 82.99 82.73 82.57 82.31 81.94
,6no
6,iHC
353.71 353.71 353.71
357.80 356.00 354.90
353.71 353.71
349.49 347.61
molecules in solution in the monomeric state. The effect of hydrocarbon in the micelle on 19Fmicellar shifts is to cause an upfield shift, while fluorocarbon produces a downfield shift on lH micellar shifts. In both cases 6,i , , ,a is reduced. By applying a best fit analysis to the data, and extrapolating to 1/cT = 0, values for 6mi were obtained and are listed in Table 1. In Figure 5 it can be observed that, over some sections, i.e., just above the cmc, the curves are reasonably linear. Extrapolation of these curves to 1/cT = 0,for a = 0.56 and 0.72, gives a ami of ca. 83, essentially identical with the chemical shift for pure APFO. This can be taken as evidence that for these systems the micelles are almost exclusively fluorocarbon molecules. In the case of a = 0.89, however, the curve was linear at high concentrations but then changed gradient and decreased more rapidly. The linear extrapolated value of 6mi at 1/cT = 0 was 81.9 ppm; the nonlinear extrapolation gave an association concentration of 5.5 X 1V mol dms. However, the data shown in Figure 6, for CY = 0.56 and 0.89, suggest from the change in ami observed (Table 1)that some hydrocarbon chains are incorporated into the predominantly fluorocarbon micelles. Moreover, for some peaks a downfield lH shift was observed at the cmc in contrast to the upfield shift observed with AmDec alone. The cmc values obtained from both the lH and lgF spectra for each mixture were found to be identical within the limits of experimental error. This is evidence for formation of a mixed micelle at this cmc, and in Figure 7 the cmc variation with solution composition a is compared with that expected for ideal mixing. Over the complete range of a,the experimental cmc exceeds the ideal value, indicating an aversion to mixingin the micelle. Figures 8 and 9 shows the lH and 19Fmicelle chemical shift dependence on micelle composition. As a good approximation, the plots fit two linear sections,intersecting at approximately a = 0.7. This enables linear expressions and f H c ( x ) to be obtained, thus simplifying the for FC(x) solution of eq 10. Application of this equation to the data for the a = 0.89 and a = 0.56 mixtures gives the micelle composition, x , as a function of the reciprocal of the total surface active agent concentration (Figure 10). As is 'to be expected from cmc data, the micelle composition/reciprocal concentration profile displays a large deviation from ideality, with mixing in the micelle unfavored. In a previous study,13the micelle composition for a number of mixed APFO-AmDec solutions was determined by the technique of SANS contrast matching. For each solution composition a,the micelle composition x was measured at a single concentration. The values obtained for the a = 0.56 and a = 0.89 solutions by this technique are included in Figure 10 and show excellent agreement with the NMR data. Given the micelle composition as a function of total surface active agent concentration and applying Rubingh's regular solution model for nonideal micellar mixing,22 it is possible to determine the micelle interaction parameter, &, from the following equations:
Surface Active Agent Mixtures
Langmuir, Vol. 10, No. 1, 1994 55
‘
o.o+
--0
1 0
I
I
I
I
0.2
I
I
O
~
0.8
0.6
0.9
I
1.0
LI
Figure 7. Critical micelle concentrations,obtained from NMR measurements, as a functionof a for various mixtures: - - -.ideal mixing curve.
I
I
0
I 0.2
I
I 0.4
I
I
1
0.6
I 0.8
l
l
1 .o
Micelle Composition, x
Figure 9. l9F chemical shift, relative to monomer, as a function of micelle composition, x: A, Cz;0 , C,; 0,C8.
-U
0
0.2
0.4
0.6
0.8
1 .o
Micelle Composition, x
Figure 8. ‘Hchemical shift, relative to monomer, as a function of micelle composition, x : A, c3; 0, C&e; 0,CIO. (A’- CT) f [(A’- C d 2 + ~CYCTA’]~’~ (14) 2 A‘ where A’ = f&2* - flC1*. C1* and C2* are the pure state cmc’s of the two components, and f1 and f2 are the activity coefficients of the two components defined by23 X =
[Concentration/mol d ~ n - ~ l - l
Figure 10. Composition of the mixed micellar species,x , against the reciprocal of the total surface agent concentration: 0 and 0, values obtained from N M R measurements at a = 0.56 and 0.89, respectively; m, results from small-angle neutron scattering - - -,curves calculated on the basis of ideal mixing; e~periments;~~ -, curve calculated using Bm = 1.6; ---,curve calculated using & = 2.2.
8, describes the molecular interactions of the mixture and is proportional to the excess free energy of micellar mixing given by
GE = 8,RT ~ ( 1 x-)
(16) The only Om value to fit all the micellar composition data for the a = 0.56 mixture was 1.6 f 0.1 (Figure 10). This agrees well with values obtained in previous work for other mixed systems where the surface active agents have similar tailgroup lengths, Le., 8, = 1.75 for C,F1&OONaCloH2+3OdNa1 and 1.8 for the same mixture.2 For the a = 0.89 mixture, a small variation in x produces a large variation in the calculated value of; @ , however, a best fit value of 2.2 f 0.4 was obtained as shown in Figure 10. The above method for calculating 8, is considered to be more accurate than methods employing the mixed cmc values (as determined by NMR), since it can be shown that small errors in the cmc determination can be (22) Rubingh, D. N. In Solution Chemistry of Surfactants; Mittal, K. L., W.; Plenum Preaa: New York, 1979. (23) Hddebrand, J. H.; Prauenitz, J. M.; Scott, R. L. Regular and Related Solutions; Van Noatrand Reinhold: New York, 1970.
translated into a large error in 8, by the calculations. In addition, the 8, value calculated from micelle composition data applies to a wide range of micelle composition values ( x = 0.05 0.45 for a = 0.56). At the actual cmc of the mixtures, the micelles have been shown to be almost exclusively fluorocarbon. Figure 11 shows the monomer concentrations of APFO and AmDec as a function of the total surface active agent concentration in the system. For the two pure components the monomer concentration above the cmc remains constant; however, the transition to the micellar state appears to be somewhat sharper for AmDec than for APFO. In the case of AmDec at a = 0.56 the monomer concentration increases smoothly as the total concentration of the mixture is increased. At the same a value the concentration of APFO monomer increases to a concentration which is above that found with APFO alone. This is somewhat surprising but is an experimental observation and is outside the limits of experimental error. However, it is clear as shown in Figure 10 that the micelles formed just above the cmc are predominantly fluorocarbon but do contain a smallamount of AmDec. As further micelles are formed, the monomer concentration of APFO decreases
-
56 Langmuir, Vol. 10, No. 1, 1994
0.ou 0
i 0.02
0 Total Concentrationlmol d ~ n - ~
Figure 11. Monomer concentrationas a function of totalsurface active agent for APFO, AmDec, and APFO-AmDec mixtures: (a) APFO, -, a = 0.0; a = 0.56;- - -,a = 0.89;(b) AmDec, -, a = 1.0; a = 0.56; - - -, a = 0.89. -e-,
-e-,
slowly as the monomer concentration of AmDec increases. It is of interest that for a mixture close to this composition (a = 0.50) a lower surface tension was observed at the aidwater interface (15.2mN m-l) at the cmc compared with that obtained with APFO alone (16.6mN m-l). It was also observed by neutron scattering13in this region of a values that the micellar weight (-39 OOO) was much higher than the micellar weight of the single species (-17 600 for APFO and -12 600 for AmDec). At a = 0.89, with AmDec the predominant species, small maxima are observed in the monomer concentrations for both species although they occur at slightly different total concentrations. At the higher total concentrations both monomer concentrations appear to remain essentially constant, suggesting above a total concentration of ca. 0.12 mol dm-3 a constant micelle composition. This is also consistent with the values of micelle composition shown in Figure 10. Conclusions The results of this study and a previous studyz4show that for mixed systems of surface active agents, which do
Clapperton et al.
not display independent micellization, combined '9F and 1H NMR analysis is able to provide extensive information about the systems. For example, it enables the composition of mixed micelles to be probed over a wide range of concentrations above the cmc and to be compared with that obtained by other techniques such as SANS. Application of the regular solution model for nonideal micellar mixing to the data obtained with the APFOAmDec system indicates that a single value of om (1.6f 0.1)can be applied over the composition range from x = 0.05 to x = 0.56. However, over the range of x values between 0.8 and 0.9 the value of & was estimated to be 2.2 f 0.4. Thus, it was not possible to fit the results over the whole range of compositions with a single value of &, suggestingthat for this hydrogen-fluorocarbon system the nonideality is not fully accounted for by the regular solution model. An interesting point from this and previouswork4J3 is the indication that mixing nonideality is more significant in hydrocarbon-rich micelles than in fluorocarbon-rich micelles; j3,,,is larger for x values between 0.80 and 0.90 than at x values between 0.05 and 0.56. A point possibly connected with these observations is that SANS indicated a change in micelle shape for x values greater than 0.7.'3 It is also in this region that 6zc and 13: show much more dependence on micelle composition, also suggesting an asymmetry in nonideal behavior over the micelle composition range. An important contribution from the present study has been an evaluation of the monomer concentrations of APFO and AmDec both below and above the cmc. It should now be possible to utilize these concentration profiles to estimate the surface composition of mixed interfacial films under similar conditions.26 Moreover, recent developments of the neutron reflectivity technique have shown this to be a valuable method for estimating the surface concentration of APFO and AmDec directly, thus providing a comparison with information obtained from surface tension/concentration profiles.% Acknowledgment. The authors acknowledge with thanks support of this work by the Science and Engineering Research Council, U.K., and Procter and Gamble Limited, Newcastle-upon-Tyne, U.K. (24) Clapperton, R. M.; Ingram, B. T.;Ottewill, R. H.; Rennie, A. R. In Mixed Surfactant System; Holland, P. M., Rubingh, D. N., Eds.; ACS Symposium Series;American Chemical Society: Washington,DC,
1992; Vol. 501, p 268. (25) Clapperton, R. M.; Ingram,B. T.;Ottewill,R. H. To be published. (26) Simister, E. A.; Lee, E. M.; Lu, J. R.; Thomas,R. K.; Ottewill, R. H.; Rennie, A. R.; Penfold, J. J. Chem. SOC.,Faraday Trona. 1992,88, 3033.
