J. Phys. Chem. 1991,95, 5335-5344 (dodecane/DBSA mole ratio = 1.5). In the five-component (toluene/butanol SDS water NaCI) microemulsion system study by Olsson et al. bicontinuous solutions were observed above a NaCI/SDS mole ratio of 1.7 and toluene/SDS mole ratio of 12. A quantitative comparison between these two systems is not feasible due mainly to the presence of butanol in the SDS microemulsion. But, from the amount of electrostatic screening agent (acid or salt) required to produce a low mean curvature of the surfactant film (i.e., a bicontinuous microem~lsion~~) it is apparent that HCI is an effective screening agent, possibly more so than NaCl since bicontinuous microemulsions are formed at lower oil content. However, also the high surfactant concentrations (12-16%) used in the DBSA microemulsion (compared to 6.8% SDS) would favor formation of bicontinuous microemulsions with less oil. In summary, the microstructural changes documented here for an acidic microemulsion by the multicomponent self-diffusion NMR method are analogous to changes known for ionic systems with salt as well as for nonionic systems with temperature. As
5335
concluded by other workers, dynamic processes involving aggregate merging have been invoked to explain the transitions effected by oil or acid addition to oil-in-water micelles. Aggregate merging is inferred from the magnitude of D(oi1) relative to D(DBSA). Addition of oil to oil-in-water micelles causes greater confinement of oil while addition of HCI causes an increase in the rate of aggregate merging. One virtue of using acidic microemulsions for studying microstructural transitions of aggregates is that they are four-component systems since mineral acids have better solubility properties than salts. Continued investigation of these novel acidic microemulsions, by N M R and other physical techniques, will enrich the knowledge of microstructural changes common to many types of microemulsions. Acknowledgment. The author is grateful to Stefan0 Carminati for determining the phase diagram and to Ulf Olsson for valuable comments concerning the manuscript. Registry No. DBSA,27176-87-0; HCl, 7647-01-0; dodecane, 11240-3.
Surfactant-Counterlon Dynamics in Pyridinium Octyi Phosphate Lyotropic Mesophases. A Nuclear Relaxation Study Claude Cbacbaty* and Tbierry Bredel Dspartement de Physique GGnGrale, DSMfDPHGfSCMfBP 121, C.E.N. de'saclay, 91 191 Gif-sur- Yvette Cedex, France (Received: June 11, 1990) At increasing water concentrations,the pyridinium octyl phosphate (OCTP) forms successivelylamellar, hexagonal mesophases and isotropic micellar solutions. The conformational and dynamical properties of OCTP in the lyotropic mesophases have been investigated by multinuclear resonance and relaxation at several magnetic field strengths. The molecular and C-H bonds order parameters of the octyl phosphate anion have been obtained from the IlP chemical shift anisotropy, 13C-31P and '3C-1H dipolar splittings. The order parameters of C-H bonds as well as the dynamics of the conformational changes have been derived from the fits of both the splittings and longitudinal relaxation rates, assuming a model of trans + gauche isomerizationsabout the C-C bonds. The probabilities of the trans rotamer along the chain are found to be very high, exceeding generally 0.8. No significant differences are observed between the HI and L, phases. In both phases the octyl phosphate anion behaves as a quite rigid molecule undergoing a very anisotropic reorientation with 10 < D,/D, < 20. Similar experiments have been done on a perdeuterated octanoic probe diluted in the mesophases. The 2H longitudinal relaxation rates being independent of the frequency between 13 and 46 MHz, it is concluded that there is no significant contribution of collective motions to the longitudinal relaxation under our experimental conditions. The I'P relaxation enhanced by the pyridinium protons shows that this cation and the octyl phosphate form a tight ion pair diffusing at about the same rate in the lamellar phase. In both phases, the pyridinium ion keeps however a large reorientational freedom evidenced by short correlation times and small order parameters.
Introduction Although reported some 30 years ago,' the aggregation of sodium and potassium alkyl phosphates in aqueous solutions has been the subject of a few studies.'-5 A possible reason is that the solubility of these alkaline salts decreases rapidly with the chain length while the Krafft point of micellar solutions is shifted above room temperature for chains longer than C10. In spite of these limitations the alkyl phosphates have some interesting properties which have motivated further studies using NMR6-l0 and small-angle neutron scattering:" the effect of the charge on (1) Cooper, R. S.J. Am. Oil. Chem. Soc. 1%3,40,462. (2) Tahara, T.; Satake, I.; Matuura, R. Bull. Chem. Soc. Jpn. 1969, 42, 2101. ( 3 ) Nakagaki, H.; Handa, T. Bull. Chem. Soc. Jpn. 1975.48.630. (4) Rumyantacya,N. M.;Smyalkoskaya, E. N.; Gusev, A. 0 . ; Khar'kov, S.N.J . Appl. Chem. USSR 1979,52.2094. ( 5 ) Arakawa, J.; Pethica, B. A. J. Colloid Interface. Sci. 1980, 75, 441. (6) Chevalier, Y.; Chachaty, C. Coli. Polym. Sci. 1984, 262, 489. (7) Perly. B.; Quaegebeur, J.-P.; Chachaty, C.; Gallot. B. Mol. Cryst. Liq. Cryst. 1985. 128, 287. (8) Chevalier, Y.; Chachaty, C. J. Phys. Chem. 1985, 89, 875. (9) Chevalier, Y.; Chachaty, C. J . Am. Chem. Soc. 1985, 107, 1102. (10) Chachaty, C.; Quaegebeur, J.-P.; Caniparoli. J.-Ph.; Korb, J.-P. J. Phys. Chem. 1986, 90,1 1 15.
0022-3654 191/2095-5335$02.50 f 0
micellar size and shape," the complexing properties toward divalents ions? and the ability to form lyotropic mesopham for chain length as low as C4.'Jo Above six carbons, the complete study of the phase diagram as a function of the composition and temperature is restricted by the high melting point of the mesophase which corresponds to a partial decomposition of the surfactant. Replacing the alkaline cations by organic ones like pyridinium increases the surfactant solubility at least for chain length shorter than C10 and results in a lowering of the Krafft point below room temperature as well as of the melting point of the mesopham.l2J3 It has been observed in the case of pyridinium octyl phosphate (pyridine/octyl dihydrogen phosphate 1: 1 molar ratio) that the properties of the micellar solutions are not essentially different from the corresponding sodium salt.I2 For pyridinium octyl phosphate (OCTP) concentration larger than 2 M, the aqueous solution gives rise to lyotropic liquid crystals whose phase diagram ( 1 1 ) Chevalier, Y.; Belloni, L.; Hayter, J. B.; Zemb, T. J. Phys. 1985,86, 749. (12) Chachaty, C.; AhlnBs, T.; Lindstrom, B.; Nery, H.; Tistchenko, A. M. J . Colloid. Interface Sci. 1988, 122, 406. (13) Chachaty, C.; Bredel, Th.; Tistchenko, A. M.; Caniparoli, J.-Ph. U9. Crysf.1988, 3, 815.
0 1991 American Chemical Society
5336 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 360 350
Chachaty and Bredel 01
L
1
- 4 340
b 320 310
i 4
lb
I
I! 11
0
20
40
60
80
100
xw Figure 1. Phase diagram of the OCTP/water system: I, isotropic; HI, hexagonal; L,, lamellar.
has been delineated by 31PNMR as reported here. Such a system provides an opportunity for studying the influence of the phase on the ordering and dynamical behavior of both the surfactant anion and cation. The interaction between these two components in the lamellar phase has been the subject of a recent theoretical and experimental work." The aim of the present study is to provide a thorough description of the ordering and dynamical behavior of the three components of the system, namely water, pyridinium cation, and octyl phosphate anion. One of the main points of this work is to make a detailed investigation on the chain dynamics in the lamellar phase by means of computer calculations of C-H bond order parameters and of "C relaxation times, giving access to the rotamer populations and dynamics of conformational changes. The principles of the calculation of relaxation rates have been described in a recent paper.I5 A quite similar approach has been reported by for the chain dynamics in thermotropic liquid crystals. Comparison will be done with the lyotropic hexagonal, thermotropic lamellar,I3 and isotropic (micellar)" phases to examine the effect of intermolecular constraints on the conformations and dynamics of OCTP. This work is completed by experiments on a diluted perdeuterated octanoic acid probe to extend the frequency range of relaxation measurements. Materials and Methods The octyl dihydrogen phosphate was prepared by addition of phosphorus oxychloride to n-octyl alcohol according to the method of Gamrath et a1.'* The pyridinium octyl phosphate was formed by addition of pyridine or deuterated pyridine to octyl dihydrogen phosphate in the 1:l ratio. After addition of water ( H 2 0or D20), the samples were heated overnight at 60-80 O C . As deuterated octyl dihydrogen phosphate is not available, some 'H experiments have been done on perdeuterated octanoic acid (from Merck Sharp and Dohme), in 5% w/w solution in the mesophases. The homogeneity of the liquid crystalline phases was controlled by observation under polarized light. The 2H, 13C, 0,and NMR experiments were performed with Bruker WH90 (Bo = 2.15 T), MSL300 (Bo = 7.05 T), and WMSOO (Bo = 11.75 T) spectrometers. In order to obtain well-oriented spectra, the samples were heated up to the melting point and oooled down in the magnet. The longitudinal relaxation rates were determined by inversion recovery with proton-gated decoupling and delays >5TI. The IH-13C dipolar couplings were measured by heteronuclear &resolved experiments with spectral widths of 12000 and 2000 Hz in the F1 and F2 domains and time increments of 80 ms. Typically 256 series of 128 scans with 6-s delays between the pulses were necessary to obtain a good signal-to-noise ratio. These experiments were performed at 75.5 MHz and all splittings were resolved. (14) Korb, J.-P.; Bredel, Th.; Chachaty. C.; Van der Maarel, J. R. C. J . Chem. Phys. 1990, 93, 1964. (IS) Caniparoli, J.-Ph.;Grassi. A.; Chachaty, C. Mol. Phys. 1988,63,419. (1 6 ) Dong, R. Y.J. Chem. Phys. 1988.88, 3962. (17) Dong, R. Y.;Richards, G. M.J. Chem. Phys. 1989, 91. 7276. (18) Gamrath, H. R.; Hatton, R. E.;Weesner, W.E. Ind. Eng. Chem. 1954,46, 208.
o~
2
IL
H
10
m
Figure 2. I'P spectra (121.6 MHz) of the OCTP/H20 mesophases.
The
spectrum of the hexagonal phase has been simulated for a Gaussian line shape and a random orientation distribution of the director in planes parallel to the field of the superconducting magnet.
The theoretical calculations of splitting and relaxation rates were done in the interactive mode on a IBM 3090 computer by means of programs written in APL2 language. Results and Discussion 1. Phase Diagram and Director Orientation. The phase diagram (Figure 1) has been obtained by 31PNMR (Figure 2) as a function of the weight fraction of OCTP in the OCTP/H20 system. The nature of the phase was deduced from the sign and magnitude of the 31Pchemical shift anisotropy A u (CSA) and confirmed by X-ray scattering. In hexagonal (HI) and lamellar (La)phases, the chemical shielding tensor is axially symmetric about the director of the mesophase, Le., the mean direction of the rod axes (HI) or the perpendicular to the surfactant bilayers (La).For an angle 8 between the director and the water/surfactant interface, the CSA is
A U = y2(3 COS' 8 - I)(un- uI) all and u1 being the motionally averaged principal values of the chemical shielding tensor given below. As 8 = 0 or 90° for the lamellar and hexagonal phases, respectively, the La HI transition results in a reduction by a factor of 2 and a change in the sign of Au, at least if the molecular ordering is nearly the same in the two phases. This is actually the case and for OCTP weight fractions 0.48 < X, < 0.75 (HI) and 0.75 < X, < 1 (La) the distance between the peak and the shoulder of the ,IP spectra of randomly oriented samples yields +7.2 < A u < +7.5 ppm and -14 < A d -13.5 ppm (Figure 2). A similar behavior has been reported for lower homologues (C4 and C6 phosphates), the nature of the phase being confirmed by X-ray scattering.' Placed in the field of a superconducting magnet at room temperature, the 31Pspectrum of a lamellar sample corresponds to a spherical distribution of director orientations. Under similar conditions the spectrum of the hexagonal phase is indicative of a random distribution in planes parallel to the magnetic field and then to the axis of the NMR tube, as confirmed by computer simulations (Figure 2). Afterwards, placing the sample in the field of an electromagnet, one obtains a spectrum invariant with the rotation of the NMR tube. The shape of this spectrum approaches that observed for a spherical distribution of director orientations. This means that, in the high field strength of a superconducting magnet, the local directors are coplanar with the axis of the cylindrical NMR tube.
-
The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 5337
Surfactant-Counterion Dynamics in OCTP Mesophases
0.01 -0.01 -0.02 -0.03 -0.04
HI
-0.05 -0.06
c . . . . . . . . , . . , , , . , . , . i 3W
310
305
315
320
T ( K )
0.04 0.02
'
0
300
-
Figure 3. 2Hand I7O spectra of deuterated pyridinium and IH2l7Oin the hexagonal OCTP/H20 phase. On warming the liquid cristalline samples up to the transition with the isotropic phase and slowly cooling them in the magnetic field of a superconducting magnet, the shoulder of the hexagonal and the peak of the lamellar 31Pspectrum are replaced by a symmetric narrow line. This is characteristic of the alignment of the director parallel or perpendicular to Bo,Le., to the axis of the tube, for the HI and L, phases, respectively. On removing the samples from the field, this orientation remains unchanged during several weeks in the case of the HI phase whereas the La phase disorients in a few minutes. The same behavior is observed for the chemical shift anisotropy of the pyridine I3C as well as for the 2H (water and pyridinium) and water I7O. For both the latter nuclei, the spectrum does not depend of the sign of the quadrupolar coupling and no large differences should be observed in the splitting of the lines between the two oriented phases. This is clearly the case for the 2H but not for I7O. In the lamellar phase, the water I7O line width is indeed very broad and only the central line of the quintet (ml = mI = transition) is observed. On the other hand, both these nuclei give very well resolved spectra in the hexagonal phase (Figure 3). Most of the experiments reported here have been performed on aligned samples with X, = 0.6 (HI phase) and X, = 0.85 (Laphase). 2. Molecular Ordering and Dynamics of the Components. pvridinirnn Cation. The order parameters and diffusion coefficients of the pyridinium cation have been mainly provided by the deuteron quadrupolar splittings and longitudinal relaxation rates. For quadrupolar nuclei like 2H and I'O, the relationship between the splittings and order parameters is
-
3xi 41(21- 1)
S22) sin2 8 cos 2a
+
310
320
330
340
350
T(K)
2000 Hz
A u ~=
La
. .
S33 sin2 p cos 2y
1 + -(SI1 - ,!?2z)(1 + 3
cos2 6 ) cos 2a cos 27 - 2 cos 8 sin 2a sin 2y
it
(2)
where a,8, and y are the Euler angles between the frames of the quadrupolar coupling tensor and of the molecular ordering tensor whose eigenvalues are SII,Szz,S33.x, is the quadrupolar coupling constant of the nucleus of spin I, equal to the xzzprincipal value of the quadrupolar tensor whose asymmetry parameter is 7) = (x,
Figure 4. Temperature dependence of the pyridinium order parameters in the hexagonal and lamellar phases.
- X,,!/X,,.
$ is the angle between Bo and the director. For pyridine the A1, A2, A3 principal axes of the ordering tensor correspond to the C2symmetry axis, to the direction perpendicular to AI in the molecular plane, and to the axis perpendicular to the ring, respectively. For C-H bonds the Euler angles are a = 58.3O, /3 = 90°, y = 0 (Hl,H5);a = 123O, f l = 9 0 , y = 0 (H2,H4);and (Y = 180°, /3 = !No, y = 0 (H3). The deuteron coupling constants and asymmetry parameter are taken from ref 19. The temperature dependence of the order parameters obtained by solving a system derived from eq 2 are given in Figure 4. The signs of the order parameters, not provided by the deuteron splittings, have been derived from the chemical shift anisotropies of carbons (Figure 5 ) . The principal values of the I3C shielding tensor of the pyridinium cation are unknown. However, the solid-state NMR of aromatic compounds shows that the largest one is in the +lo0 to +120 ppm range along the direction perpendicular to the ring." From the signs and values of the order parameters (Table I), it is seen that the molecular axis A3 tends to be aligned perpendicular to the surfactant-water interface. For a large fraction of the positive charge localized on the N H group, the Coulombic interaction with octyl phosphate anions would likely orient the C2symmetry axis (AI) toward the surfactant-water interface. This is not the case and INDO calculationsindeed show that the positive charge density is quite delocalized over the pyridinium ring with, however, a slight excess at the proton of the imino group. This is possibly the cause of the asymmetry of the ordering tensor (Sll - S22)/S33 which is positive and in the 0.20-0.27 or 0.12-0.13 range for the lamellar and hexagonal phases, respectively. The pyridinium ordering tensor being asymmetric, the same is expected for the diffusion tensor. The diffusion coefficients have therefore been obtained from the deuteron longitudinal relaxation rates by using expressions given by Woessner et aL2I for an asymmetric ellipsoid tumbling in an isotropic medium. Calculations using the expressions given by Freedz2 for an axially symmetric tensor indeed show that, for small order parameters, with absolute values less than 0.1-0.2, the longitudinal relaxation rates are within experimental errors, the same as in isotropic P.;Jonas Pedersen, E. 1.Magn. Reson. 1981, 44, 101. (20) Mehring. H.The Principles of High Resolurion NMR in Solids; Spnnger Verlag: New York, 1982. (21) Woessner, D.E.J . Chem. Phys. 1962, 37, 647. (22) Frecd, J. H.J . Chem. Phys. 1977,66.4183. (19) Jacobsen, J.
5338 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991
Chachaty and Bredel
TABLE II: C d o r a r r t i d d Dpunical Parameters of OCTP in Mesopbra and in MieclLr solution Lyotropic Lamellar (X, = 0.85) P-
PT lO-"W2(G
~~~~~~~~~~~~~~~~~~~~~~~~~~
1.0
0.90 0.90 0.90 0.90 0.90 0.80
T), s-'
3
3
4
4
4
4
p 2 = 0 . 3 4 5 , ~ , = 3 . 51~0 - ' o ~ , ~ , , = 2 . 210-"s ~ P-
Thermotropic Lamellar o ~ - ~ C , - ~ ~ - ~ ~ - ~ ~ - ~ ~ - ~ ~
PT 10-"W2(G+T),s-'
1.0
0.80 0.80 0.80 3
3
3
0.90 0.85 3
3
0.80 3
& = 0 . 3 3 0 , ~ ~ = 3 1. 0 ~- 1 0 ~ , ~ o = 2 lo-" .0~ s Lyotropic Hexagonal (X, = 0.6) P-
~
PT 10-"W2(G +T), s-'
~
1.0
0.87 3
0.90 0.90 0.85 0.85 0.75 3
3
3
4 = -0.185, T1 = 2.0 x lO-"S, 0.5 M P-
-
Figure 5. "C spectra (75.5 MHz) of randomly oriented OCTP/H10 mesophases.
position AY*, kHz A Y ~kHz , ( s-l (TI-l)clk, s-I Lamellar Phase (4 = 90°, Y = 13.6 MHz, T = 300 K) 3
13 2,4
6.336 9.089 9.650
6.330 9.157 9.588
128.2 79.0 77.8
SI1 = -0.04;Sa = -0.07; S y y = +0.11; Dll' 8.5 ol1= 7.0 x 107; D ] , = 2.0 x 109
121.4 88.8 77.2 X
10';
Hexagonal Phase (4 = Oo, Y = 77 MHz, T = 300 K) 3
1s 2,4
7.659 9.330 9.687
7.657 9.348 9.67 1
19.80 16.50 15.75
SII= +0.026; Sll = +0.034; Sly= -0.061; Dll = 4.4 DZl = 8.8 X IO'; Dyy= 5.7 X IO9
23.13
15.15 14.90 X
lo9;
D in radians s-l.
medium. This approximation is most likely valid when the ordering and diffusion tensors are both non axially symmetric. The pyridinium diffusion tensor is found to be very asymmetric with DZ2 0.8.42 (41) Korb, J.-P.; Winterhalter, M.; McConnell, H. M. J . Chem. Phys. 1984,80, 1059.
CO -cl-c2-c3-C4-c5
-c6-c7
0.70 0.95 0.80
PT
W2(G+T),
s-'
0.85
0.68
3 x 1o'O
r1=5.5x 10-10s,r,,=4.0x lo-" s,
p2=&a-0.26,
si1 =-0.115. &2
= -0.145
Hexagonal Phase, 300 K ~~~~~~~~~~~~~~~~~~~~~~~
0.82
PT
W2( G +T) ,s-'
T~ = 4.0 x
p2 =
0.95
0.90 0.90 0.80
4 x 1010 lo-'' S,T, = 2.7 x lo-" S,
= -0.142 Si1 = +0.062,$2 = +0.080
As the spectral resolution is very good (Figure 13), the experiments on octanoic acid gave us the opportunity of testing the validity of our approach by accurately fitting the deuteron quadrupolar splittings and relaxation rates (Figure 14). For the splittings, the quality of the fits has been substantially improved by introducing in eq 3 1 the value of SII - SZ2derived from the asymmetry parameter (ca.0.12) of the effective inertial tensor, computed by means of the parameters of Table IV. This table shows that the ordering and dynamics of the octanoic acid probe in the OCTP/H20 lamellar and hexagonal phases are not essentially different from those of the surfactant molecules: the time scales of the internal and overall motions are comparable, the latter being very anisotropic, whereas the probabilities of the trans rotamer as well as lS331are slightly smaller. Moreover, as for the octyl phosphate anion, the reorientation of octanoic acid is faster in the hexagonal than in the lamellar phase. Taking the parameters obtained from the octanoic probe in lamellar phase, it seemed to us worthwhile to examine the effect of the molecular ordering on the longitudinal relaxation rates of a flexible molecule, in the absence of ODF contribution and in extreme narrowing conditions. The profiles of (TI-I)lH along the aliphatic chain, calculated for different values of P2 (Figure 15), show that this effect is hardly perceptible below 0.4 and is rapidly (42) Chachaty, C.; Caniparoli, J.-Ph.; Faure, A,; Tistchenko, A. M. J . Phys. Chem. 1988,92,6330. (43) Lawenstein. A.; Igner, D.; Zehavi, U.; Zimmerman, H.; Luckhurst, G . Liq. Cryst. 1990, 7, 451.
J. Phys. Chem. 1991,95, 5344-5352
5344
1
2
3
4
5
6
DEUTERON N'
Figure 15. Computed dependence of the deuteron longitudinalrelaxation on the molecular ordering for Lorentzian spectral densities. The parameters of these calculations are given in Table IV for the lamellar phase. attenuated with the distance to the polar head for larger values of this order parameter. Conclusion
This work reports the dynamical and conformational behavior of an anionic surfactant with organic counterion in hexagonal and lyotropic mesophases. In these phases the surfactant behaves as
a quite rigid entity undergoing a very anisotropic motion about a privileged axis nearly coincident with that of the all-trans conformer. The analysis of the 13C-'H splittings as well as the 'F relaxation reveals unexpectedly high probabilities for the trans rotamer about the C-C and 0-C bonds, without striking differences between the hexagonal and lamellar phases. The enhancement of the probabilities of the most elongated conformers mmt likely results from confinement effects due to intermolecular interactions, which we did not attempt to analyze. They indeed not only correspond to the potential of mean torque as in nonionic surfactant^^^ but also to the Coulombic interactions between the polar heads. Owing possibly to the moderate ordering of the surfactant (&, zz -2F2hcx= 0.34-0.37), there is no evidence of an ODF contribution to the I F or 2H longitudinal relaxation in the 2-1 I-T range of magnetic field strengths used here. Calculations based on experimental data show that, for Lorentzian spectral densities, the influence of the molecular ordering on the longitudinal relaxation at the usual NMR frequencies becomes very small below F2 = 0.4 in a lamellar phase. This observation could be helpful to introduce some simplifications in the calculations of relaxation rates in liquid crystals.
Acknowledgment. We are greatly indebted to Dr. B. Gallot (Laboratoire des Materiaux Organiques, CNRS, Vernaison) for X-ray experiments as well as to Drs. J.-P.Korb (Laboratoire de Physique de la MatiEre CondensEe, Ecole Polytechnique, Palaiseau) and J. R. C. van der Maarel (Gorlaeus Laboratories, University of Leiden) for stimulating discussions. Registry NO. OCPT, 117826-62-7.
Effect of Alcohols on the Phase Behavior of Microemulsions M.Kablweit,* R. Strey, and G . Busse Max-Planck-Institut fir Biophysikalische Chemie, Postfach 2841, 0-3400, Catringen, FRG (Received: November 9, 1990)
In the literature, medium-chain alcohols are frequently considered as cosurfactants that act mainly on the properties of the amphiphilic monolayer at the water/oil interface in microemulsions. In this paper it is suggested that alcohols should rather be considered as cosolvents that distribute between the aqueous and the oil-rich bulk phases, and the interfacial layer, thereby decreasing the effective hydrophilicity of the amphiphile as well as the effective hydrophobicity of the oil. This is supported by the fact that alcohols adsorb rather weakly at the waterloil interface in amphiphile-free H2D-oil mixtures. Experiments on the effect of nonionic amphiphiles on the mutual solubility between water and alcohols, furthermore, give no indication for the formation of (inverse) micelles in the alcohol-rich phase. Finally, it is demonstrated that the effect of alcohols on the phase behavior and, accordingly, on the interfacial tension uabbetween the water-rich and the oil-rich phases depends on the position of the experimental temperature Tapwith respect to the mean temperature T of the three-phase body of the ternary water-ail-amphiphile mixture. With nonionic amphiphiles, Orb increases upon the addition of alcohol if Tap > but decreases (at first) if Tcxp< With ionic amphiphiles, one finds the reverse.
r,
r.
I. Introduction Consider a mixture of water (A), an oil (B), and a nonionic amphiphile (C). Within a welldefmed temperature interval AT, such a mixture may separate into three coexisting liquid phases, a water-rich (a), an oil-rich (b), and an amphiphile-rich (c) phase. At the mean temperature T of AT one finds a maximum of the mutual solubility between water and oil, and a minimum of the interfacial tension crab between phases a and b, both properties being the basis for applying such mixtures in research and industry. The separation of the mixture into three phases arises from the interplay between the miscibility gaps of the three corresponding binary mixtures A-B, B-C, and A-C. Because the features of these phase diagrams, in particular, those of the two latter mixtures, depend sensitively on the chemical nature of the oil (that is, its hydrophobicity), as well as that of the amphiphile (that is,
the hydrophilicity of its head group in relation to the hydrophobicity of its tail), this also holds for the dependences of T,AT, and b,b on the natures of the oil and the amphiphile. As a consequence, all three properties change systematically as one varies either the oil or the amphiphile within a homologous series. The qualitative rules of this dependence were summarized in a recently published review article.' Consider now the effect of an added fourth component. The Gibbs-Duhem relation between the variations of the field variables in the bulk phases 0 = s6T + &6fii Sp = Q (1.1) states that, at constant external pressure, the chemical potentials ( I ) Kahlweit, hi.; Strey, R.;Busse, G . J . Phys. Chem. 1990, 91, 3881.
0022-3654191 /2Q95-5344$Q2.5Q/Q0 1991 American Chemical Society
