J . Phys. Chem. 1990, 94, 3086-3092
3086
Liquid-Liquid Phase Separation in Cationic Micellar Solutions Gregory G. Warr,*?+** Thomas N. Zemb,+ and Maurice Driffordt Dgpartement de Physico- Chimie, Centre d'Etudes NuclPaires de Saclay, 91 I91 Gif-sur- Yvette Cedex, France, and Department of Physical Chemistry, The University of Sydney, Sydney, NS W ,2006, Australia (Receiued: July I I , 1989; In Final Form: November 1 . 1989)
We have investigated the phase equilibria and liquid isotropic phase of solutions of the cationic surfactant dodecyltributylammonium bromide in water and D20.The extremely hygroscopic solid surfactant forms an isotropic liquid phase up to at least 92 wt % in water and undergoes a phase separation on warming onto two liquid isotropic phases, with a critical point at 46 wt % surfactant and 48 O C in D 2 0 . Small-angle neutron scattering spectra were measured on solutions with compositions between 1 and 80 wt % surfactant and at temperatures from 20 to 60 O C in order to determine micellar aggregation numbers and charges and interfacial areas per molecule of the surfactant. Intermicellar monomer concentrations are also determined from scattering spectra by a novel method, and these are used to explain the range and nature of the concentration-dependent intermicellar interactions which give rise to phase separation.
Introduction
Aqueous solutions of nonionic surfactants are well-known for their complex phase behavior and particularly for their propensity to undergo clouding and liquid-liquid phase separation at elevated temperatures.'-3 Certain zwitterionic surfactants," as well as some ionic surfactants in the presence of high concentrations of added electrolyte,salso exhibit such phase separations. All such solutions have critical compositions less than or equal to about 1 wt 5% surfactant. To date, however, no binary mixtures of ionic surfactant and water have been observed to exhibit phase separation phenomena upon heating. In this paper we report the existence of such behavior in solutions of dodecyltributylammonium bromide in water and D 2 0 and detail an investigation of the structure of the isotropic solution phase. In nonionic surfactants (by which we mean here poly(oxyethylene) alkyl ethers), there is continued debate over the roles of critical concentration fluctuations and micellar growth as mechanisms for phase separation. Evidence for both small, spherical and large, polydisperse (cylindrical) micelles has been presented in apparent contradiction.'"I0 Kjellander," and more recently Goldstein,12have argued on theoretical grounds that the extraordinarily low surfactant volume fractions at the critical point, +cr,t, are inconsistent with spherical micelles, but they also point out that growth of cylindrical micelles in the absence of attractions between them is insufficient to produce the experimentally observed consolute boundaries. Thus, a rapprochement does seem to be emerging in which micelle growth and intermicellar attractions are seen to be related.'3v'4 The microscopic origins of phase separation are still elusive: Several workers have suggested that the ethylene oxide oligomers which form the hydrophilic parts of the micelles become dehydrated upon heating so that repulsive hydration interaction between micelles give way to van der Waals a t t r a c t i ~ n . ~ *This ' ~ is then manifested macroscopically as phase separation. Direct surface force measurements, however, indicate the existence of attractive hydration interactions at elevated temperatures,I6 due to the specific nature of the amphiphilic hydration of the methylene and ether oxygen units in the poly(oxyethy1ene) chain. Another related suggestion is that changes in poly(oxyethy1ene) chain conformations with temperature are the source of the changing micelle-micelle and micelle-water interactions which cause phase separation." Ionic micelles in concentrated electrolyte solution which phase separate on heating (e.g., hexadecyltrimethylammonium bromide/NaCI03) are believed to be long, flexible cylinders or "giant micelles" and have been the subject of some study, although not to the same depth as the n o n i o n i ~ s . ~ ~A11 ' ~ . ionic ' ~ micellar systems * T o whom correspondence should be addressed. 'Centre d'Etudes Nucltaires de Saclay. 'The University of Sydney.
0022-3654/90/2094-3086$02.50/0
exhibiting lower critical solution temperatures known so far contain large amounts of added electrolyte (>2 M), thus screening the repulsive electrostatic effects which usually stabilize the solutions. Dodecyltributylammonium bromide (CI2NBu3Br)in aqueous solution phase separates upon heating to form two isotropic liquid phases in equilibrium. This is a departure from the behavior of the related dodecyltrimethyl-, triethyl-, and tripropylammonium surfactants, which all remain stable with increasing temperature. As was previously reported,20even in dilute solutions a long-range attractive potential must be invoked to describe elastic and dynamic light scattering results on this system. We describe below the properties and phase equilibria of C12NBu3Brin aqueous solution and present the results of a small-angle neutron scattering (SANS) study of the isotropic solution phase. Experimental Section
Dodecyltributylammonium bromide (C12NBu3Br)was prepared from I -bromododecane and tri-n-butylamine by refluxing for 24 h in ethanol. The crude product was recovered by rotary evaporation of the solvent and then recrystallized three times from acetone/ether. CI2NBu3Bris extremely hygroscopic and forms an isotropic liquid at room temperature with a small percentage of uptaken water. The solid product was stored under nitrogen when not in use. Analysis by HPLC showed that the product contained a small amount of isomeric impurity, probably C,,NBu,Br, and trace amounts of unreacted starting products but that it was >99% pure. The level of impurity was too low to be detected by ' H NMR. ( I ) Triolo, R.; Magid, L. J.; Johnson, J. S., Jr.; Child, H. R. J. Phys. Chem. 1982,86, 3689.
( 2 ) Mitchell, D. J.; Eddy, G. J. T.; Waring, L.; Bostock, T.; MacDonald, J. J . Chem. Soc., Faraday Trans. I 1983, 79, 975. (3) Schick, M. J. Nonionic Surjacfanfs;Surfactant Science Series, Vol. 1; Marcel Dekker: New York, 1967. (4) Lang, J. C.;Morgan, R. C. J. Chem. Phys. 1980, 73, 5849. ( 5 ) Porte, G.; Appell, J. J. Phys. L e f f .1983, 44, L-689. (6) Zulauf, M.; Weckstrom, K.; Hayter, J. B.; Corti, M.; DiGiorgio, V. J. Phys. Chem. 1985,89, 341 1. (7) Lum Wan. J. A.; Warr, G. G.; White, L. R.; Grieser, F. Colloid Polym. Sci. 1987, 265, 528.
(8) Cebula, D. J.; Ottewill, R. H. Colloid Polym. Sci. 1982, 260, 1 1 18. (9) Ravey, J.-C. J. Colloid Interface Sri. 1983, 94, 289. (IO) Lofroth, J.-E.; Almgren, M. In Surfacranfs in Solufion; Mittal, K. L , Lindman, B., Eds.: Plenum: New York, 1984; Vol. I , D 267. ( 1 I ) Kjellander, R. J. Chem. Soc., Faraday Trans. 2 '1982, 78, 2025. (12) Goldstein, R. E. J. Chem. Phys. 1986, 84, 3367. (13) Zana, R.; Weill, C. J. Phys., Lett. 1985, 46, L-953. (14) Strey, R.; Pakusch, A. In Surfacfanfs in Solution; Mittal, K. L., Bothorel, P., Eds.; Plenum: New York, 1986; Vol. 4, p 465. (15) Hayter, J. B.; Zulauf, M. Colloid Polym. Sci. 1982, 260, 1023. (16) Claesson, P.;Kjellander, R.; Stenius, P.; Christenson, H. G.J . Chem. SOC.,Faraday Trans. I 1986, 89, 2735. (17) Karlstrom, G. J. Phys. Chem. 1985, 89, 4962. (18) Imae, T.;Sasaki, M.; Abe, A.; Ikeda, S . Langmuir 1988, 4 , 414. (19) Porte, G. J. Phys. Chem. 1983,87, 3541. (20) Drifford, M.; Belloni, L.; Dubois, M. J. Colloid Interface Sci. 1987, 118, 50.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 3087
Phase Separation in Cationic Micellar Solutions
80 w t % C,, NBu, Br
-
E 0.3 -
0
V
0 e
6ot F40i
-
0
,
20
,
,
I
,
,
I
80
60
40
wt % dodecyltributylammonium
0
0
25°C 40°C 60°C
0
,A
s
,
0 O 0
.-u)
P
20
A
100
bromide
Figure 1. Partial phase diagrams of C12NBu3Brin D20(-) and H20 (---): concentration (wt %) vs temperature ("C). The dotted line shows the approximate location of the hydrated crystalline phase. C,, NBu,Er, T
8 wt % C,, NBu,Br
=
25°C
0 25'C A 0
40'C 60°C
. .... ... .I. a
0.200
q
.*,
. . H a
0.400
II.
..
(A-? 0.200
Figure 2. Small-angle neutron scattering spectra of 8 wt % ' solutions of C12NBu3Brin D20at 25, 40, and 60 "C.
Conductivity measurements used to determine the critical micelle concentration (cmc) were made on a Tacussel type CD-810 conductivity bridge with platinized platinum electrodes, and all solutions were thermostated to 20 OC. The phase boundary was determined by repeated warming and cooling of surfactant solutions in H 2 0 or DzO and noting the appearance or disappearance of turbidity. Above the phase boundary all samples separated into two clear, isotropic liquid phases after a few minutes and were slow to remix on cooling. SANS measurements on solutions of CI2NBu3Brin D 2 0 were carried out at the PACE line at the Laboratoire Leon Brillouin, CEN Saclay. Two sample-to-detector distances were employed, both using neutrons of wavelength 3.97 A. The results were placed on an absolute scale by calibration with the incoherent scattering spectra of water and vanadium and were combined to yield a single spectrum for each temperature and concentration.
Results and Discussion Phase Equilibria. Partial binary-phase diagrams for CI2NBu3Brin HzOand D 2 0 are shown in Figure 1. In contrast with the typical phase behavior of nonionic surfactant/water systems, these mixtures are extremely simple, being devoid of liquid crystalline mesophases between 20 and 100 OC. The hydrated crystals exist only above 93 wt % surfactant, and no melting transition could reliably be observed due to the extreme hygroscopicity of the compound. A second major difference between this system and other phase-separating aqueous micellar solutions is the shape of the consolute boundary. Note particularly the critical volume fraction, compared with the usual which is at 46 wt % surfactant in DzO, value of around 1 wt % for micellar systems. The location of the critical point is unambiguous here, despite the flatness of the lower boundary of the two-phase domain: Extrapolation of SANS spectra to zero wave vector as a function of concentration and temperature shows a clear maximum in scattered intensity at 46
a
40.00
35.00
30.00
0.400 ( i - 1 )
1:
1
C,, NBu,Er, T
=
47'C
o :
25.00
D
20.00
E
c
m
x
15.00 10.00 5.00 0 00
0.000
0.4'00
0.200
q
Figure 4. SANS spectra of C,2NBu3Brat (a) 25 OC and (b) 47 OC, in D20: 1 wt %, at 20 O C (0); 8 wt % (V);17 wt % (0); 34 wt % (A);47 wt % (0);65 wt % (+); 80 wt % (m).
wt %, which increases sharply as 48 "C is approached tending to diverge at the critical point (see e.g. ref 14). The two-phase domain is also extremely wide, so that phase separation leaves two liquids in equilibrium with compositions of near 8 and 80 wt % C12NBu3Br.The cmc at 20 O C in H,O is 4.8 X mol (0.21 wt %) by conductivity. On the dilute side of the two-phase domain there is therefore a micellar solution. This is in equilibrium with a surfactant-rich phase, also containing long-range structure, as can be seen from the SANS spectra (Figures 2 and 3). The SANS spectra at 8 and 80 wt % surfactant are practically independent of temperature over the temperature range studied. This is not so for intermediate compositions: those which lie below the two-phase domain. Figure 4 shows the experimental spectra obtained at 25 and 47 OC. There is a clear increase in the scattered
3088
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990
Warr et al.
intensity at low q as the phase boundary is approached. Intermicellar Monomer Concentrations. I n high volume fraction micellar solutions such as the present system, there is little known about the solution structure and in particular about the equilibrium monomer concentration of surfactant. Even in dilute solution, until the recent advent of pulsed gradient FT-NMR, equilibrium monomer concentrations had only been satisfactorily determined for a limited number of ~ y s t e m s . ~ ' -Recently, ~~ however, a larger number of surfactant solutions have been inv e ~ t i g a t e d yielding ~ ~ s ~ ~ monomer concentrations that are in good agreement with t h e ~ r y . ~ ~Unfortunately, .~' at high surfactant concentrations most methods are subject to interference. I n particular, monomer concentrations derived from self-diffusion coefficients depend on micellar morphology, hydration, and intermicellar collisions, as well as on monomer c o n c e n t r a t i ~ n . ~ ~ - ~ ~ The absolute scattering intensity of a micellar solution depends 70 0 on the difference in scattering length densities, or contrast, between Figure 5. Monomer concentrations (mol dm-)) of C,,NBu,Br as deterthe dispersed micelles and the solvent. This may be exploited to mined from the invariant of the scattering spectra a s a function of the obtain monomer surfactant concentrations as follows. For an total amphiphile concentration (wt %) and temperature ( " C ) . arbitrary system containing a mixture of two media, 1 and 2, of differing scattering length densities p I and p2, the total scattered TABLE I: Monomer Concentrations, Molecular Areas, and Surface intensity is given by31 Potentials of C12NBulBrSolutions in D,O at 25 "C / -
[C,*NBu3Br] in
mol
where Vis the total volume of the sample, r is a position vector, and q is the wave vector. The exponential describes the interference between scattered waves in the solution. For known particle geometries and spatial distributions this integral may be performed and the scattering law calculated. The second moment of the scattered intensity, or invariant q*, is the total scattered intensity for the sample and is given by integration of eq I , thus
wt %
[monomer], mol dm-3
dm-3
K-I,
sin,
1.05" 0.024 ((5.0 i 2.0) x 10-3) 0.0074 8.01 0.184 0.04 f 0.01 0.062 0.132 16.7 0.384 0.07 f 0.01 0.270 34.3 0.789 0.16 f 0.02 46.8 1.077 0.24 f 0.03 0.356 0.515 0.31 f 0.05 65.3 1.500 80.1 1.843 0.59 f 0.10 0.543 (100.0) (2.23) (2.23)
A 43 15 12
8 6 5 4
e*lo/ kT 6.2 5.3 4.9 4.0 3.7 3.5 2.9
Z, A-l 0.001 0.011 0.016 0.043 0.060 0.112
" A t 20 OC.
where the scattering length densities have been identified with the internal (micellar) and solvent domains of the solution, and aintis the volume fraction of micelles in solution. q* depends only on the contrast and on the volume fractions of the two domains. It is independent of the interference effects which determine the shape of the I ( q ) curve; cf. Figure 4. The invariant as calculated in eq 2 arises from the contrast between micelles and solvent, so the appropriate experimental I(q) for integration is the scattered intensity with background noise, incoherent scattering, and isotropic solvent scattering all subtracted as part of the base line. This also includes the scattering by dissolved monomeric surfactant, which is constant over the q range investigated here, and is thus accounted for in the normal process of background subtraction. Scattering length densities for the (micellized) surfactant and for D 2 0 were calculated from known data, yielding -3.2 X lo9 (21) Kale, K. M.; Cussler, E. L.; Evans, D. F. J . Phys. Chem. 1980, 84, 593. (22) Kale, K. M.; Cussler, E. L.; Evans, D. F. J . Solution Chem. 1982, 1 1.. 581. . ... ~ (23) Cutler, S. G.;Meares, P.; Hall, D. G. J . Chem. Soc., Faraday Trans. I 1978, 74, 1758. (241 Lindman. B.; Nilsson. P.-G.; Stilbs, P.; Wennerstrom, H. Pure Appl. Chem.. 1984, 28 1 . (25) Jansson, M.; Stilbs. P. J . Phys. Chem. 1985, 89, 4868. (26) Gunnarsson, G.; Jonsson, B.; Wennerstrom, H . J . Phys. Chem. 1980, 84,.31i4. (27) Gunnarsson, G.;Jonuon, B.; Wennerstrom, H. In Solution Chemisrry of Surfactants; Mittal, K. L., Fendler, E. J., Eds.; Plenum: New York, 1982; Vol. I , p 317. (28) Jansson, M.; Warr, G. G. Submitted for publication in J . Colloid Interface Sci. (29) Feaucompr6, B.; Lindman, B. J . Phys. Chem. 1987, 91, 383. (30) Jonsson, B.; Wennerstrom, H.;Nilsson, P.-G.; Linse, P. Colloid Polym. Sci. 1986, 264, 77. (31) Kostorz, G. I n Neutron Scattering, Kostorz, G.,Ed.; Treatise on Materials Science and Technology, Vol. I ; Academic Press: New York. 1979; Vol. 15. (32) Porod, G. In Small-Angle X-ray Scattering, Glatter, O., Kratky, O., Eds.; Academic: New York, 1982.
and +6.34 X 1OIo respectively. The scattering length density of the solvent is then simply the weighted mean of the scattering length densities of D 2 0 and C12NBu3Br,viz. Psolv
=
PD10%2O + PC12NBu,Br@monomer @ D ~ O *monomer
+
where * D ~ O + *monomer
+
*Pint
= 1
In this way using the experimental invariant, we solved eqs 2 and 3 uniquely for pWlvand ai,,,and hence obtained the monomer concentration of C12NBu3Brin the micellar solution. Monomer concentrations derived in this way are shown in Figure 5 as a function of total surfactant concentration and temperature and are listed in Table I. A similar calculation for sodium octanoate micelles yielded an approximately constant value for the monomer concentration, which is acceptable within the range of experimental ~ncertainty.~~ It can be seen from Figure 5 that the monomer concentration increases smoothly with total surfactant content but that it is almost independent of temperature over the entire range investigated. Note also that the packing fraction of micelles (Table I) remains below 0.63-the random close-packing volume fraction for spheres. Contrary to expectations, the micelles are able to remain spheroidal at least up to 80 wt % solution. This increase in monomer concentration is at variance with experimental and theoretical evidence for several other ionic surfactant systems, where the monomer concentration decreases above the cmc but the unbound counterion concentration increase^.^'-^' We have, however, confirmed the results in the dilute solution regime from self-diffusioncoefficientsmeasured by pulsed gradient FT-NMR.'* Decreasing intermicellar monomer concentrations seem to be a general property of ionic surfactant solutions, with CI2NBu3Br the exception. All systems reported with this behavior, however, (33) Zemb, T. N . ThCse d'Etat, 1985
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 3089
Phase Separation in Cationic Micellar Solutions 2.0
r
I
on the basis of dynamic light scattering experiments.20 Recall that typical values for ionic surfactants such as SDS and DTAB are near to 63 A2 per molecule.37 From a consideration of the interfacial energies of micellization, the optimal aggregation number is related to the interfacial molecular area, A , by38 [dACo/dA]a=Ao = 0 where AGO is the free energy of micellization of the surfactant. This equation is satisfied when attractive (surface tension) and repulsive (electrostatic, steric) forces balance each other. We ascribe the large A. obtained for ClzNBu3Brcompared with other ionic surfactants to steric interactions between bulky butyl chains surrounding the quaternary nitrogen. We are currently investigating the conformation of these alkyl chains by NMR.39 The electrostatic contribution to the free energy of micellization depends on the spacing between ionic head roups at the micelle surface, and hence on Ao. A value of 120 per molecule corresponds to a surface charge density of 13.3 MCcm-2 for the surface of an “undressed” micelle (no condensed counter ion^),^^ whereas the usual value is 63 A2 per molecule, or 25.3 pC cm-2. Application of the Poisson-Boltzmann equation to these micelles shows a significantly lower surface potential for C12NBu3Brand also less adsorbed counterion in the diffuse part of the double layer. This is consistent with conductivity results which indicate a lower degree of counterion association for C12NBu3Brnear the cmc than is observed for DTAB.37 As will be seen below, analysis of SANS spectra also shows that Cl,NBu3Br micelles have very high fractional charges in dilute solution. Micellar Size and Interactions. SANS spectra of micellar solutions of CI2NBu3Brwere modeled by using the hypernetted chain (HNC) integral equation40 of Hayter et al., using the rescaled mean spherical approximation (RMSA)!’ Micelles were assumed to be monodisperse spheres, in which case the intra- and intermicellar interference terms (eq 1) can be separated. The scattered intensity is then given by
i2
Jconcentration (Molar;
Figure 6. (a) Conductivity of CI2NBu3Brat 20 OC in H 2 0 , showing the cmc as the break point in the curve. (b) Molar conductivity, A, of C12NBu3Brvs [C12NBu3Br]1/2, showing the curvature in the plot in the
cmc region.
are classical surfactants with sharp cmcs and aggregation numbers which vary only a small amount with total surfactant concentration from their expected spherical values observed at the cmc (55 for SDS34and dodecyltrimethylammonium bromide (DTAB);35 17 for sodium ~ c t a n o a t e ~ ~Theoretical ). models that predict decreasing monomer concentrations above the cmc are also constructed for fixed aggregation numbers (and abrupt cmcs), but different behavior can arise in systems where aggregation is more gradual, giving rise to diffuse cmcs and a variation of the aggregation number with concentration. The molar conductivity of aqueous C12NBu3Br(Figure 6) near the cmc exhibits considerable curvature, rather than the sharp break delineating micellar and nonmicellar domains, indicating a gradual onset of micelle formation. Increasing monomer concentrations above the cmc and the broad cmc region itself are thus both manifestations of weakly cooperative micellization. Surfactant Head Group Areas. The total internal interfacial area, Z, delimited by changes in the scattering length density in a micellar solution or in any heterogeneous medium can be obtained from the limiting (large q ) value of 941(q),in the region where interference effects have been damped to near This region of the spectrum was available to us from scattering spectra obtained between 1 and 65 wt % C12NBu3Brsolutions, but at 80 wt % surfactant a clear limiting value could not be obtained due to the position of the scattering peak at large 9. From Z and the monomer surfactant concentration, the area per molecule, Ao, at the micelle/solution interface was obtained. Within experimental uncertainty we found a value of 120 15 if2per molecule for CIzNBu3Br,almost independent of both temperature and composition, in D,O. This is in good agreement with a value of 150 f 5 if2per molecule estimated from geometrical considerations
*
(34) Huisman, H. F. Proc. K.Ned. Akad. Wet. 1964,1367,361, 376,388, 407. (35) Guveli, D. E.; Kayes, J. B.; Davis, S. S. J . Colloid Interface Sci. 1979, 72, 130. (36) Hayter, J . B.; Zemb, T. N. Chem. Phys. Letr. 1982, 93. 91
where VmOn is the monomer surfactant volume, Naggthe micelle aggregation number, cT the total surfactant concentration, cmon the equilibrium monomer concentration, ro the micelle radius, Pi(q) the particle form factor due to intramicelle interference, and Sii(q) the structure factor arising from intermicelle interference. The interaction potential used to model Sii(q) was a one-component macrofluid description of the electrostatic interactions:’ with an additional attractive intermicellar potential of the Yukawa form
where Vois the contact potential, d the decay length of the interaction, and r the separation between micelle centers. d was always set to be 6 A, in order to approximate the fully extended length of a butyl group and hence estimate the range of an intermolecular potential of steric or hydrophobic origins. This form of attractive potential is exactly that used by Hayter et al.6*15 for nonionic micellar solutions over a range of concentrations and temperatures. They found that a short-range attractive potential (37) Brady, J. E.; Evans, D. F.; Warr, G. G.; Grieser, F.; Ninham. B. W. J . Phys. Chem. 1986, 90, 1853. (38) Evans, D. F.; Mitchell, D. J.; Ninham, B. W. J . Phys. Chem. 1984. 88, 6344. (39) Chachatv, C.; Jansson. M.; Li. P.; Warr, G. G. Manuscript in
preparation. . (40) Belloni, L. Chem. Phys. 1985, 99, 43. (41) Hayter, J. B.; Penfold, J . Mol. Phys. 1981, 42, 109
3090 The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 6.0
(a) 1 w l K N-23
Warr et al.
(b) 8 wt %
2-15
N-45
5.0
2-28
4.0
3.0 2.0 1.0 0
0.0
0.1
(c) 18 wt w N-402-20 u -8
8.0
I
n
1
0.1
0.3
0.2
:
%=-$I
(d) 34 vvt % N-40
2-12
- &.-0
6.0
--- u,
..... u,
4.0
--
2
5
2.0
0.1
0.2
0.3
0.4
q(I-1)
Figure 7. Calculated and experimental SANS spectra at 25 'C and (a) 1 wt %. (b) 8 wt %, (c) 16 wt %, and (d) 34 w t % CI2NBu3Brin D20.
( d = 3-5 A) best described the scattering from micelles of a variety of poly(oxyethy1ene) alkyl ethers. Various mean field potentials have been employed previously to describe interactions in theoretical models of surfactant clouding phenomena;"*'2 however, the detailed functional form of Ua,,only slightly affects the shape of calculated I(q) curves.42 The remaining adjustable parameters in the system are thus the micelle aggregation number, Naa, the micellar charge, Z,and Uo;the range of the electrostatic potential, K - I , is determined by the equilibrium monomer concentration. (The nominal micellar hydration is coupled to Z so we have fixed it at six molecules of D , O / C , , N B U ~ B ~ . ~To~ )describe the small-angle scattering on an absolute scale, these three parameters constitute the minimum possible number in the presence of critical fluctuations. Naggis the major factor in determining both peak position on the q axis (from the first maximum in Sii(q))and its intensity (eq 4), highlighting the importance of absolute scaling of scattering spectra. Z and Uocontrol the peak position to a lesser extent but mainly affect the shape of the I ( 9 ) curve, in particular the zero-angle scattered intensity, I(0). Adequate fits were obtained in dilute solution allowing for the approximately 15% uncertainty in the measured i n t e n ~ i t i e s . ~ ~ Excellent fits were obtained for micellar solutions at low volume fractions. At higher micelle concentrations, as seen in Figure 7c,d shape fluctuations become significant and the calculated peak is at higher angle than the observed one. This is probably due to changes in the micelle morphology from monodisperse spheres to polydisperse spheroids with increasing concentration and may also be due to the inability of the HNC equation to describe strongly attractive interactions such as those occurring near the critical point. In these solutions the model predicted much more highly ordered spectra than were observed, having narrower scattering peaks. (An example is shown in Figure 7d.) To improve the fit, additional parameters involving mass or shape polydispersity could be introduced, but no information would be gained by following this line. In dilute solutions (18 wt %), the calculated spectra were almost independent of Uo.At higher surfactant concentrations Uoaffects peak height and I(0) significantly. An example is shown in Figure (42) Walrand, S.; Belloni, L.;Drifford, M. Europhys. Lett. 19817.4, 591. (43) Derian, P.-J.:Belloni, L.; Drifford, M. J . Chem. Phys. 1987.86, 5708.
7d, where the effect is particularly dramatic as the net interaction is changed from repulsive (U, < e\ko/k7')to attractive (Vo> e\ko/k7").Note however that the sensitivity of scattered intensity to Nagsprevented us from accurately modeling peak position and intensity for this and higher concentration samples. There are several indirect methods of determining the micelle size, however. For a rule-of-thumb estimate of Nagswe have assumed that the position of the maximum in Z(q), qman,is equivalent to the first peak in the intermicellar structure factor, Sii(q).qmaxthen corresponds to a Bragg spacing and may be used to calculate the intermicellar center-to-center distance, D* = 2n/q,,.This in turn leads directly to the micellar concentration and to an estimate of Nag% from Nags = {[surfactant] - [monomer])/(micelles] In those samples where critical scattering is significant and the peak was displaced or obscured completely, Naggcould not be determined. For cases where I(0) is small, this approach gives a reasonable approximation to the micelle aggregation number (see Table 11). The micelles increase in aggregation number over most of the lower part of the concentration range and at higher concentrations exhibit an apparent decrease. A model-independent way of examining the size of the scattering units is realized via the mean chord of the scattering spectrum, e,. This is defined as follows32
e, = (*/4*)&"
dq
(6)
and is the weight average of the length of all chords drawn through the scattering unit. For spheres of radius ro this reduces to E , = 3r0/2. Mean chords for C,,NBu3Br solutions and calculated ro assuming spherical micelles are shown in Table 11. These values are averages of results at several temperatures, with a standard deviation of f l A. These results enable us to dismiss the possibility that long, anisotropic micelles exist in this system. Indeed, the largest micellar radii approximate the value expected for spheres4s (44) Neubauer, G.; Hoffmann, H.; Kalus, J.; Schwandner, D. Chem. Phys. 1986, 110, 247. (45) Tanford, C . The Hydrophobic Effect; Academic Press: New York, 1969.
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 3091
Phase Separation in Cationic Micellar Solutions
TABLE I!: Micelle Size Parameters Determined from SANS Spectra at 25 and 44 O C N w
T, O C
wt %CC,,NBu3Br
a
b
Z
20 25
1.05 8.01 16.7 34.3 46.8 65.3 80. I 8.01 46.8 65.3 80.1
16 34 53 57 59 44 24 34
24 45 40
15 28 20
44
45
28
UOlkT 0 0-2 5-8
ro = 2t',/3
r~ = 4.519min
31.0 30.3
21.0 20.7 18.3 13.7
0- 2
28.7 22.1 13.2 29.7 30.0 23.6 13.4
19.2 18.0 16.3 17.1 17.1 14.2 11.3 18.0 16.0 14.2
t'C
9.0 19.8 20.0 15.7 9.0
11.0
From qmar.*From complete fit.
if allowance is made for the bulky tributylammonium head group. A space-filling molecular model predicts that C,,NBu,Br with a fully extended tail has length between 19 and 22 A, depending on the conformation of the butyl moieties. A check on the derived ro is achieved using the position of the first minimum in a plot of 941(q)vs 9. For spherical micelles this minimum lies at 9 = 4.5/r0 (from the first zero of the form factor for spheres). Radii obtained in this way agree well with the mean chord result (Table 11). The decreasing ro values obtained from both methods are also consistent with the decrease in Nags for C12NBu3Brmicelles observed at high surfactant concentrations. The small mean chord values observed suggest spherical or spheroidal micelles at all concentrations and temperatures above the cmc in the liquid isotropic region of the phase diagram. Other morphologies for the surfactant aggregates such as bicontinuous phases are unlikely due to the small mean chord values, and inverted structures at high surfactant contents can be precluded on the grounds of the low contrast observed in the system. High solubility of the surfactant monomer in water is a requisite for the observed invariant. A RMSA model of water droplets in the 80 wt % surfactant case also gives a highly ordered spectrum, in qualitative disagreement with the observed result. Finally, we can calculate the surfactant packing parameter for these micelles.46 Geometric constraints on the packing of the alkyl tails of surfactant molecules in micelles of various shapes can be expressed in terms of a single parameter, u/Aol,where u is the volume of the surfactant alkyl tail and I is the radius of the hydrocarbon core of the micelle. The packing parameter predicts that micelles will be spherical when u/Aol S 1 /3 and cylindrical or oblate when 1/3 C c/AoI S I . For a dodecyl chain I = 17 A and u = 350 A3, giving as a criterion for spherical micelles A. 2 63 A,, which is easily satisfied by CI2NBu3Br,for which A. = 120 f 15 A2. Even if we allow the extreme case where all of the butyl groups form part of the h drophobic "core" of the micelles, this gives I = 20 A, u i= 800 and A. 2 120 A,. The packing parameter is thus consistent with small, spherical micelles, as we found from more direct analysis of the SANS spectra. The critical volume fraction ad,= 0.46 in CI2NBu3Brsolutions is anomalously high, not only in comparison with observed values in nonionic micellar systems but also for theoretical predictions for spherical micelles. Various models predict values for a,, of the order of 0.0512and 0.08-0.171' for spheres interacting through an attractive potential only, with the exact value depending on the range and depth of the attractive well. In the C12NBu3Br system the intermicellar interaction potential contains a repulsive, electrostatic component as well as the attractive one and goes from net repulsive to net attractive as concentration increases. This is a result of the increasing intermicellar monomer concentrations which screen the electrostatic repulsion between micelles. Table I shows the variation of the Debye screening length, K-I (calculated from the monomer surfactant concentration), with total concentration. The effective range of the electrostatic repulsion becomes equal to the steric barrier (butyl chain length) in the region of
i3,
(46) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1916, 72, 1525.
the observed critical point, so we would expect attractive interactions to dominate in this region. The reason for the temperature dependence of the total interaction potential is not so clear. As pointed out by Claesson et a1.,I6van der Waals attractive interactions alone cannot account for the change with temperature of the attractive interactions necessary to explain phase separation: Over the temperature range 25-50 OC there is only a 10%increase in the Hamaker constant for alkane/water/alkane interactions. Some other kind of force must therefore be operating. In simple binary fluids exhibiting similar critical phenomena (e.g., ethanolamine/water), hydrogen-bonding interactions have been identified with the necessary temperature-dependent interaction^.^' Recent experimental measurements on forces between hydrophobic surfaces have identified long-range attractions between alkane layers separated by water. The energies of these interactions far exceed those typical of dispersion forces.48 The hydrophobic effect45is a central phenomenon in surfactant self-assembly; however, it is unusually only considered in light of alkyl chain transfer out of water into the aggregated state or in terms of protein folding. "Hydrophobic hydration" due to water structuring around alkyl groups is known to be at the heart of self-assembly processes and also occurs from nonaggregating hydrophobic solutes. The hydrophobic hydration of tetraalkylammonium salts, for example, is well-d~cumented.~~ Ion pairs formed by tetraalkylammonium bromides are attributed to entropic causes, the hydration of a single ion pair being favored over that of two hydrophobic species.50 In a like manner an entropic attraction between relatively hydrophobic -NBu3+ groups at the surface of micelles should be favored when electrostatic repulsions are screened. We contend that the hydrophobic hydration of the n-butyl groups surrounding the charged ammonium group of CI2NBu3Bris the source of the temperature-dependent attractive interaction observed in the SANS spectra. This, combined with the self-screening behavior of the surfactant monomer, leads to a liquid-liquid coexistence phenomenon at elevated temperatures. The nature of the water and its hydrogen-bond structure in surfactant solutions are currently being investigated by Raman spectroscopy.51 Conclusions The phase separation behavior of micellar C12NBu3Brsolutions into two isotropic liquids is a result of two effects. The first is the weakly cooperative nature of the micellization of C12NBu3Br, which is manifested as a diffuse cmc region, changing aggregation numbers with surfactant concentration and particularly by increasing monomer concentrations above the cmc. Second is the hydrophobic nature of the n-butyl groups surrounding the charged (47) Goldstein, R. E.; Walker, J. S. J . Chem. Phys. 1983, 78, 1492. (48) Pashley, R. M.; McGuiggan, P. M.;Ninham, B. W.; Evans, D. F. Science (Wushingron, D.C.)1985, 229, 1088. (49) Green, J. L.; Lacey, A. R.; Sceats, M. G. J. Phys., Colloq. 1981.48, CI-53. (50) Evans, D. F.; Kay, R. L. J. Phys. Chem. 1966, 70, 366. ( 5 1 ) Ashburner, P. H.; Lacey, A. R.; Sceats, M. G.; Warr, G. G. Manuscript in preparation.
J . Phys. Chem. 1990, 94, 3092-3098
3092
ammonium head group of the surfactant. This seems to favor a water structure driven attraction between micelles. The attractive component of the intermicellar potential becomes dominant at high surfactant concentrations, where monomeric surfactant acts as an background electrolyte, screening electrostatic repulsions between micelles. Competition between the attractive and respulsive components of the intermicellar potential leads to
a high critical volume fraction of micelles. Acknowledgment. We acknowledge the help of M. Dubois and P. Lixon with the analysis of the surfactants, that of I. Barnes and L. Auvray with the SANS experiments, and also L. Belloni for his assistance with the theoretical modeling of these micellar solutions.
Photosensitized Electron-Transfer Reactions and H, Evolution in Organized Microheterogeneous Environments: Separation of Ground-State Xanthene-Bipyridinium Complexes by SiOz Colloids I. Willner,* Y. Eichen, and E. Joselevich Department of Organic Chemistry and Fritz Haber Center for Molecular Dynamics Research, The Hebrew University of Jerusalem, Jerusalem 91 904, Israel (Received: July 13, 1989; In Final Form: October 25, 1989)
Rose bengal, Rb2-, forms a ground-state complex with N,N'-dimethyl-4,4'-bipyridinium, MV2+,with an association constant of K, = I1000 i 1 100 M-I. Static electron-transfer quenching of excited Rb2- occurs in the complex structure, but charge separation is eliminated due to rapid back electron transfer in the encounter cage complex of photoproducts. In the presence of added Si02colloid particles the [Rb2--MV2+] complex is separated through the selective association of MV2+to the negatively charged colloid interface. Upon illumination of a solution that includes Rb2-, MV2+,and the sacrificial electron donor triethanolamine (TEOA) in the presence of SiO, colloid, the photosensitized formation of MV" proceeds effectively, 4 = 0. I . Mechanistic studies reveal that TEOA reduces excited Rb2- in the primary electron-transfer process. The intermediate photoproducts, TEOA" and RV3-, are stabilized against back-electron-transfer reactions by means of electrostatic interactions with the S O 2 interface, leading to the electrical repulsion of RV3- from the colloid interface. The control of the recombination process of the intermediate photoproducts leads to the subsequent effective reduction of MV2+. A xanthene dye-bipyridinium BMV2+,K, = I7000 i 3400 complex is also formed between eosin, Eo2-, and N,N'-dibenzyl-3,3'-dimethyl-4,4'-bipyridinium, M-I. The complex is separated by a SiO, colloid that is immobilized with Pd metal catalyst sites. Separation of the complex allows charge separation and subsequent H2 evolution (or hydrogenation of ethylene) upon illumination of the microheterogeneous assembly in the presence of TEOA. Mechanistic studies show that the SiO, colloid controls the photoinduced electron-transfer process, and stabilization of the intermediate photoproducts against the back-electron-transfer process is achieved.
Photosensitized electron-transfer reactions (eq 1 ) provide a general route for the conversion of light energy into chemical potential.'-3 Numerous examples of the subsequent utilization A
+ D 2A- + D+
(1)
of the redox products in H2 e v ~ l u t i o n , C ~ -0~2 hydrogenation of unsaturated substrates,l03'l regeneration of natural
cofactors and subsequent biotransformation^,'^-^^ and NO< fixationf5have been reported in recent years. The photosensitized electron-transfer process from the light absorbent, S , to an electron acceptor, A, is limited by the electron-transfer quenching efficiency and separation yield of the primary encounter cage complex of photoproducts (eq 2). Subsequently, the back-electron-transfer process of the separated photoproducts (eq 3) degrades the energy S*
( I ) Energy Resources Through Photochemistry and Catalysis; Gratzel, M., Ed.; Academic Press: New York, 1983. (2) Photochemical Conuersion and Storage of Solar Energy: Connolly. J . S . , Ed.; Academic Press: New York, 1981. (3) (a) Bard, A. J . Science (Washington, D.C.)1980, 207, 1380. (b) Gratzel. M. Arc. Chem. Res. 1981, 14, 376. (c) Willner, 1.; SteinbergerWillner, B. Int. J . Hydrogen Energy 1988, 13, 593. (4) Photogeneration of Hydrogen; Harriman, A,, West, M. E., Eds.; Academic Press: London, 1983. ( 5 ) (a) Sutin, N.; Creutz, C. Pure Appl. Chem. 1980,52, 2717. (b) Keller, P.; Moradpour, A.; Amouyal, E. J . Chem. Soc., Faraday Trans. I 1982, 78, 3331. (6) (a) Gratzel, M.; Kalyanasundaram, K.; Kiwi, J . Struct. Bonding (Berlin) 1982, 49, 37. (b) Kirch, M.; Lehn, J.-M.; Sauvage, J.-P. Helo. Chim. Acta 1979,62, 1345. (c) Krishnan, C. V.; Sutin, N. J . Am. Chem. SOC.1981, 103, 2141. (7) (a) Hawecker, J.; Lehn, J.-M.; Ziessel, R. Helo. Chim. Acta 1986, 69, 1990. (b) Lehn, J:M.; Ziessel, R. Proc. Natl. Acad. Sei. U.S.A. 1982, 79, 701.
(8) (a) Maidan, R.; Willner, 1. J . A m . Chem. SOC.1986, 108, 8100. (b) Maidan, R.; Willner, 1. J . Am. Chem. Soc. 1986, 108, 1080. (c) Willner, I.: Maidan, R.; Mandler, D.; Diirr. H.; Dorr, G.; Zengerle, K. J . Am. Chem. SOC. 1987, 109. 6080. (9) (a) Mandler, D.; Willner, I. J . Am. Chem. SOC.1987, 109, 7884. (b) Mandler, D.; Willner, I. J . Chem. Soc., Perkin Trans. 2 1988, 997. (IO) Degani, Y.; Willner. 1. J . Chem. Soc., Perkin Trans. 2 1986, 37.
+ A ---k,
[S+.-A-]
4,
--.
S+ + A-
(2)
S+ + AS+A (3) stored by the photoproducts and limits the yield of usable redox products. Various organized microenvironments composed of micelles,'618 liposome^,'^*^^ polyelectrolytes,2' charged ~ o l l o i d s , ~ ~ , ~ ~ &b
( I I ) Mandler, D.; Willner, I. J . Phys. Chem. 1987, 91, 3600. (12) Mandler, D.; Willner, 1. J . Chem. SOC.,Chem. Commun. 1986, 851. ( I 3) Mandler, D.; Willner, 1. J . Chem. SOC.,Perkin Trans. 2 1986, 805. (14) Willner, 1.; Mandler, D. Enzyme Microb. Technol. 1989, 11, 467. ( 1 5 ) Lapidot, N.; Riklin, A.; Willner, I. J . A m . Chem. SOC.1989, 1 1 1 ,
1883. (16) (a) Fendler, J . H. Chem. Reo. 1987, 87, 877. (b) Matsuo, T.; Takama, K.; Tsutsui, Y.; Nishizima, T. J. Coord. Chem. Reo. 1980, 10, 195. ( I 7) (a) Kalyanasundaram, K. Chem. SOC.Reo. 1978, 7,453. (b) Turro, N. J.; Gratzel, M.; Braun, A. M. Angew. Chem., Int. Ed. Engl. 1980, 19, 675. (c) Thomas, J. K. Chem. Reu. 1980, 80, 283. (18) (a) Kurihara, K.; Fendler. J. H.; Ravet, I.; Nagy, J . J . Mol. Catal. 1986, 34, 325. (b) Meyer, M.; Wallberg, L.: Kurihara, K.; Fendler, J. H. J . Chem. SOC.,Chem. Commun. 1984, 90. (19) Kurihara, K.; Sukigara, M.; Toyoshima, Y. Biochem. Biophys. Res. Commun. 1979, 88, 320. (20) (a) Sudo, Y.; Toda, F. Nature (London) 1979, 279, 807. (b) Ford, W. E.;Otvos, J. W.; Calvin, M. Nature (London) 1978, 274, 507. (c) Ford, W . E.: Otvos, J. W.; Calvin. M. Proc. Natl. Acad. Sci. U.S.A.1979, 76, 3590.
0022-3654/90/2094-3092$02.50/0 C 1990 American Chemical Society
