Langmuir 1994,10, 311-380
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Noncritical Behavior near the Cloud Point in Perfluorinated Ionic Micellar Solutions Zhi-Jim Yu and Ronald D. Neuman* Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849 Received November 9,1992. In Final Form: October 1 5 , 1 9 9 9
The solution properties of a noncritical cloud point system, namely, sodium perfluorooctanoate/H20/ tetrapropylammonium bromide, were investigated by dynamic and static light scattering. Critical-type behavior is observed when the temperature approaches the minimum cloud point (Td) from the one-phase region: However, for other sodium perfluorooctanoate solutions, but of lower concentration than that correspondingto the minimum cloud-pointtemperature, the apparent hydrodynamic radius may decrease as the temperature approaches the cloud point. It is thus shown that a cloud-point system may not always exhibit critical-type behavior as many authors believe in the literature.
Introduction It is commonly observed that aqueous solutions of nonionic surfactants become very turbid at a well-defined when temperature, which is known as the cloud point (Td), heated.' Recently, cloud-point behavior also has been found for several aqueous solutions of ionic surfactants." Although much attention has been given to cloud-point systems over the past decade, many solution behaviors of cloud-pointsystems still remain incompletely understood. To date, all known cloud-point systemsundergo marked changes in solution properties when the temperature approaches the cloud point. For example, the forwardscattered intensity (I,) of light, X-rays, or neutrons and the apparent hydrodynamic radius (Rh) measured by dynamic light scattering diverge with increasisg temperature for micellar solutions. This anomalous behavior in solutionproperties has led to a long-standing debate. Some authors interpret the increase in R h or I, in terms of micellar growth, Le., the micellar size increases as Td is approached,1+5-7 or the formation of micellar clusters (micellar association)with minimum globule micelles8or even with decreased micellar size? while others believe that such solution behaviors arise from critical concentration fluctuations.10-12 Even for noncritical cloud-point sy~tems, many authors attribute the increase in R h or 1, with temperature to an onset of critical beha~i0r.ll-l~The To whom correepondence should be sent.
Abs~a~publishedin Aduance ACSAbstracts, January 16,1994. (1) Nakagawa,T. InNonionic Surfoctants, Surfactant ScienceSeries; Schick. M. J.. Ed.: Marcel Dekker: New York. 1967: Vol. 1...D 558. .. ( ~ d p p d , ' &PO&, G.d. PhyS. Lett. 1983; 4, LASS. (3) Yu, Z.-J.; Xu, G. J. Phys. Chem. 1989,93,7441. Zemb, T. N.; Drifford, M. J. Phys. Chem. 1990,94, (4) Wnrr, G.0.; 3086. (5) Cebula, D. J.; Otbwill,R. H. Colloid Polym. 1982,260, 1118. (6) Ravey, J . 4 . J. Colloid Interface Sci. 1988,94, 289. (7) Brown,W.; J o h n , R.; Stilbe, P.; Lmdman, B.J. Phye. Chem. 198&87,4548: (8) Richbrmg, W.H.; Burchard, W.; Finkelmann, H. J. Phys. Chem. 1988, 92,6032. (9) Cu"ins,P. G.; Hayter, J. B.;Penfold;J.; Staplee,E. Chem.Phys. Lett. 1887,188,436. (10) Hernnann. K. W.: Bruehmiier, J. G.; Courchene, W. L. J. Phys. Chem. 1966, 70, 2909. (11) Corti, M.;Degiorgio, V. J. Phys. Chem. 1981,85,1442. (12) Corti, M.; Minero C.; Degiorgio, V. J. Phys. Chem. 1984,88,909. (13) Hayter, J. B.;Zulauf, M. Colloid Pol m. Sci. 1982,260, 1023. (141 Triolo, R.; Msgid, L.; Johnson, J. S.;&dd, H.R.J. Phys. Chem. 1982,86,3689. (15) Dorehow, R. B.;Bunton, C. A.; Nicoli, D. F. In Surfactants in Solution; Mittal, K. L., Bothorel, P., Ede.; Plenum Press: New York, 1986; VOL 4, p 263. (16) Magid, L. J. In Nonionic Surfactants, Surfactant Science Series; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; Vol. 23, p 677. 0
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evidence for this view, however, is based only on investigations of micellar systems which meet the requirements for a critical system. Hence, the following question arises: can one apply the conclusion drawn from the study of criticalsystemstoexplain the micellar solution behaviors of noncritical cloud-point systems? Another issue associated with cloud-point systems is the question of nonuniversality of the critical exponents. Corti et al.leJ9 reported that some nonionic surfactant systems display nonuniversal criticalbehavior: The critical exponents y and v which describe the divergence in the osmotic compressibility and the correlation length, respectively,are much lower than what are expected on the basis of the 3D-Isingmodel,or smaller than those predicted from mode-mode coupling theory. These workers suggested that three-dimensional models which exhibit continuously varying critical exponents might be required for micellar solutions. This finding generated much theoretical interest.20*21Recently, however, it has been suggestedthat the nonuniveraalityof the criticalexponents is due to experimental limitations in obtaining the critical c o m p a e i t i ~ n . ~We ~ *therefore ~~ believe that an investigation of the power-law behavior in a noncritical cloud-point system would be instructive. In a systematic investigation of the solution behaviors of anionic perfluorinated surfactants, which have many peculiar properties such as much higher surface activity and micellization ability than that of their hydrocarbon analogues, we found a unique phenomenon that such solutions possess a cloud point in the presence of tetraalkylammonium bromide (with alkyl group longer than ethyl) but without the anomalous behavior of an increase in I, and Rh with temperature approaching the cloud point.2s Thus, we want to emphasize in this paper that critical-type behavior is not a general phenomenon in cloud-point systems. Instead of ageneral R h increase,Rh may decrease when the temperature approaches the cloud point. We also want to show that for some cloud-point systems which display critical-typebehavior, the exponents describing the power lawa may well deviate from the theoretical predictions for critical systems. (17) Dietler, G.; Cannell, D. S. Phye. Rev. Lett. 1988, 60, 1852. (18) Corti, M.; Degiorgio, V.; Zulauf, M. Phys. Rev. Lett. 1982, 48, 1617. (19) Corti, M.; DegiorpiO, V. Phys. Reo. Lett. 1986,55, 2005. (20) Fieher, M. E. Phys. Reu. Lett. 1986,57, 1911. (21) Bagnule, C.; Bervillier, C. Phys. Rev. Lett. 1987,58,435. (22) Robledo,A.;Martinez-Mekler,G.;Varea,C.Europhys.Lett. 1991, 16, 405. (23) Yu, Z.-J.; Neuman, R. D. Manwcript in preparation.
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378 Langmuir, Vol. 10, No. 2, 1994
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0.2 [ProflBrI (MI
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Figure 1. Cloud-point temperature as a function of ProdNBr concentration at constant CTFNa concentration of 25 mM. Experimental Materials and Methods Perfluorooctanoic acid and tetrapropylammonium bromide (ProcNBr), which were commercial research grade products (Aldrich), were recrystallized 3 times from mixed solvents of benzene and acetone and dried under vacuum for 6 h. Sodium perfluorooctanoate (CTFNa) was prepared by mixing equimolar amounts of perfluorooctanoicacid and sodiumhydroxide. Highpurity water from a Millipore reverse osmosis/Super-Qsystem waa subsequently doubly distilled with the first distillation being from alkalinepermanganate. Dynamic and staticlight scattering measurements and the sample preparation procedure are described elsewhere.% The sample temperature was controlled to *0.02
OC.
10
0
Although no cloud point has been observed previously for perfluorinated ionic surfactant systems, we discovered for the first time that such systems can exhibit cloudpoint phenomenon. Figure 1shows the cloud point as a function of the organic counterion concentration at 25 mM CTFNa for the sodium perfluorooctanoate/HzO/ tetrapropylammonium bromide system. The cloud-point temperature (Td)was determined by heating a solution to a temperature above which the solution became slightly turbid. Below Td the solution is homogeneous,while above Td it is in a two-phase regime. Since dilute solutions of pure tetrapropylammonium perfluorooctanoate do not show cloud-point behavior, the system presented here is a mixture of C7FNa with excess of ProdNBr. It can be seen from Figure 1that with increasing [ProdNBr] the cloud point decreases at low [ProrNBrl and increases at high [ProdNBr], having a minimum cloudpoint temperature at 0.19 M Pro4NBr. The minimumcloud point temperature was checked by tracing the change in scattered intensity (I) of the solution by the static lightscattering technique, and it was found to be 42.73 "C beyond which I was unstable and irreversible. The surfactant concentration of the two coexisting phases at a temperature slightly above the minimum cloudpoint temperature was examined through chemical analysis. It was found that one phase is very dilute and the other phase is surfactant enriched, [CyFNaI being as high as 3 M. Figure 2 illustrates the cloud-point temperature as a function of [C7FNal at 0.19 M ProrNBr. This phase behavior suggests that it is similar to that of a binary mixture of tetrabutylammonium tetradecyl sulfate and water,26i.e., one of the two coexisting phases is surfactant (24)Yu,2.-J.; Neuman, R. D.Langmuir 1992,8, 2074. (26)Yu,2.-J.; Xu,G.J. Phys. Chem. 1989, 93, 7441.
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[WNaI ") Figure 2. Cloud-point temperature as a function of CTFNa concentration at constant ProrNBr concentration of 0.19 M.
8 60
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Results and Discussion
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Temperature (T) Figure 3. Apparent hydrodynamic radius (Rdas a function of temperature for different C7FNa concentrations at 0.19 M ProrNBr: ( 0 )26 m M (+) 20 mM, ( 0 )15 m M (A)10 mM CTFNa. The arrows indicate the cloud points. dominated. Therefore, it may be concluded that the minimum cloud point in Figure 1 is far from a critical point. Figure 3 shows the dynamic light-scattering results for various [CTFNa] at 0.19 M Pro4NBr in the temperature range from 15 "C to the respective cloud-point temperatures. The apparent diffusion coefficient (0) was measured by dynamic light scattering at scattering anglesfrom 30" to 90". The apparent hydrodynamic radius ( R h ) was deduced from D by the Stokes-Einstein relation
kBTl(6~vD) (1) where k g is the Boltzmann constant, T is the absolute temperature, and v is the viscosity of the solution at the critical micelle concentration (cmc) which is 0.336 mM C7FNa.23 The viscosity9 was measured with a Ubbelohde viscometer with the temperature controlled to within fO.06 OC. It can be seen from Figure 3 that R h decreases with increasingtemperature at lower surfactant concentrations. At high surfactant concentration of 25 mM, R h increases with increasing temperature until the cloud point is reached. For the intermediate surfactant concentration of 20 mM, R h decreases initially, passes through a minimum, and increases again a t high temperature. There are two possibilities for the increase of R h with temperature. One is an increase in micellar size, which is, however, in contradiction with the micellar behavior at low surfactant concentrations as seen in Figure 3. The Rh
Solution Properties of a Cloud Point System
Langmuir, Vol. 10, No. 2, 1994 379
r
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30 40 Temperature ("C)
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Figure 4. Relative Viscosity of the mixed solution of 25 mM C7FNa and 0.19 M Pro4NBr as a function of temperature.
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'. t
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Figure 5. Correlation length (and osmotic compressibility KT of the mixed 25 mM C?FNa/O.l9M ProdNBr solution plotted as a function of reduced temperature t = (Td - m/Td: ( 0 )S: (+) KT.
other possibility is an increase in intermicellar interactions rather than micellar size. Since the sensitivity of viscosity to intermicellar interactions is different from that of dynamic light scattering, we measured the viscosity of the micellar solutions. Figure 4 shows the relationship between relative viscosity and temperature for the 25 mM C7FNa solution. It is seen that the relative viscosity decreases with increasing temperature over the whole temperature range. Clearly, the micellar size in this solution decreases with increasing temperature. It is, therefore, concluded that the increase of apparent hydrodynamic radius of the micelles with increasing temperature is due to an increase in intermicellar interactions. Figure 5 shows the static correlation length (0 and the osmotic compressibility ( K T )in the temperature range from 20 to 42.64 OC as a function of the reduced temperature (e) where c = ( T d - T)/Td. {was obtained by fitting the scattered intensity, I, measured at different scattering angles (19)from 30 to 90°, to the Ornstein-Zernike relation I = Zd(l+q") (2) where q = ( 4 ~ n I bsin2(19/2) ) is the scattering vector, n is the refractive index of the solvent, A,I is the wavelength of the incident beam in vacuo, IO is proportional to KT according to the relation I,, = AC2(dn/dc)2k,TKT (3) where A is an instrumental constant, C (=c - cmc) is the
0
20
8
,
40 Rg
60
80
(nm)
Figure 6. Apparent hydrodynamic radius R h versus apparent radius of gyration R,: ( 0 ) experimental data for mixed Pro4NBr/C,FNa solutions (15-25 mMC7FNa at constant 0.19 M ProdNBr) at T 5 25 "C;(-) theoretical predictions for spheres, oblate ellipsoids, and rigid rods whose smallest geometrical parameter is 1.Ck1.7 nm. total micellar concentration, c is the surfactant concentration, and dnldc is the refractive index increment which we assume does not change much over the temperature range investigated. It may be seen that both KT and { increase linearly with decreasinge in the low c range when plotted on a logarithmic basis as shown in Figure 5. KT and {in that temperature range can then be described by the power laws: KT a c y and { 0: e-", where y and v are constants. A least-squares fit of the log KT vs log e and log { vs log e curves from 36 to 42.64 "C to the power laws yields y = 0.14 f 0.02 and v = 0.09 f 0.01. Assuming that the effects of micellar clustering and critical type behavior are negligible at temperatures far from a cloud point, the apparent radius of gyration (R,) can be deduced from the Debye equation
where Re and Re" are the Rayleigh ratio at c and cmc, respectively, M, is the apparent molecular weight of the micelles, and K is the optical constant. R, is determined from the ratio of the slope to the intercept of the measured linear curve obtained by plotting 1/(Re- Reo)versus sin2 012, where 6 varies from 30 to 90". Figure 6 shows the relationship between Rh and R, for mixed solutions of 15 - 25 mM C7FNa and 0.19 M Pro4NBr and the theoretical predictions of Rh versus R, for geometric models of spheres, rigid rods, and oblate ellipsoids. The formulas employed for the theoretical curves are the same as those in ref 26. The cross-sectional radii of the rods and the semiminor axis of the oblate ellipsoids are estimated as 1.0-1.7 nm which comprises the fully extended length of the surfactant molecule (ca. 1.0 nm) plus a counterion binding layer of 0-0.7 nm. From Figure 6 it is seen that the experimental data range between the theoretical curves for rigid rods and oblate ellipsoids. Considering the large Rh or R, values for the experimental data, the deviation from the theoretical curves for rigid rods may be interpreted as being due to the semiflexibility of rodlike micelles.27*= (26) Sande, W.V. D.;Persoons, A. J. Phys. Chem. 1986,89,404. (27) Yu,2.d.; Zhao, G.-X. J. Colloid Interface Sci. 1989, 130, 421.
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with increasingtemperature for cloud-point systems. Our Our resutls show severalunique features for a noncritical resulta clearlyshow that critical concentrationfluctuations cloud-point system: First, the apparent hydrodynamic may not always have a significant effect (if they do at all) radius of the micelles may increase with increasing on the light-scatteringproperties of noncritical cloud-point temperature until the cloud point is reached. However, as demonstrated by the viscosity measurements, the systems. In other words, the critical behaviors which are inherent in critical systems are not an inherent characincrease in R h is not due to micellar growth. Instead, the teristic of cloud-point systems. It should not come as a increase in R h (or 0 is attributed to intermicellar intersurprise to observe critical behaviors in a critical cloudactions. point system, but one must certainly wonder why the Secondly,a power-law dependent of rand KT on reduced conclusions drawn from study of critical systems in the temperature exists. However, the power-law exponents are much smaller than those of critical systems where y past were also considered to be applicable to noncritical cloud-pointsystems. Caution must therefore be employed = 1 and v = 0.5 from mean-field theory or y = 1.24 and Y = 0.63 from the 3D-Isingmodel. Also,the observed ratio before one attributes an increase in R h (or IJ to critical concentration fluctuations for noncritical cloud-point y/v is only 1.6,which is greatly different than the ratio of systems. approximately 2 obtained from the scaling laws for a critical-point system. Nonuniversal critical exponents in Finally, the occurrence of phase separation at the cloud critical systems have been reported in some binary nonionic point is not necessarily related to the formation of micellar surfactant systemsl8JQand in a quaternary microemulsion clusters. The increase in R h and I8for the mixed solutions of 20 and 25 mM IC7FNaI at constant 0.19 M ProrNEh system.29Carefulreinvestigationsperformed to check the original findings did not confirm the earlier resulta but near the respective cloud-point temperatures may be instead indicated universal critical e ~ p o n e n t s . ~The ~ * ~ ~ *interpreted ~ as a formation of clusters of rodlike micelles. deviationsfrom scaling-lawbehavior in the present system However, the dimensions of such rodlike micellar clusters may indicate a difference between intermicellar interacin the mixed solutions of 10 and 15 mM [CvFNa], if the tions in noncritical cloud-point systems and critical micellar clusters exist at all, must decrease with the concentration fluctuations in critical systems. temperature approaching the cloud point as indicated by Thirdly, the most striking solution behavior is that R h the observed decrease of R h and I#. Therefore, the phase measured by dynamic light scattering decreases when the separation is more likely induced by changesin the solution temperature approaches Td for the 10 and 15 mM C7FNa behaviors of the surfactant and tetrapropylammonium solutions, which is in violent contrast with earlier surbromide molecules or the individual micelles rather than factant studies that R h (or dynamic correlation length) of being induced by micellar clustering. micellar solutionsdetermined by light scattering increases Acknowledgment. This research was supported by (28) Porte,G.; Appell, J.; Poggl, Y. J. Phys.Chem. 1980,84,3105. the Office of Basic Energy Sciences, Division of Chemical (29) Bellocq, A. M.; Honorat, P.; Roux, D.J. Phys. (Paris) 1986,46, Sciences, Department of Energy under Grant No. DE743. FG05-85ER13357. (30)Wolcoxon, J. P.;Kaler, E.W. J. Chem. Phys. 1987,86, 4684.
