5762
J. Phys. Chem. 1986, 90, 5762-5766
Microstructure of Microemulsions of the System H,O-n -Tetradecane-&E, F. Lichterfeld, T. Schmeling, and R. Strey* Max- Planck- Institut fuer biophysikalische Chemie, 0-3400 Goettingen, West Germany (Received: September 16, 1985; In Final Form: July 1 1 , 1986)
In this paper we present small-angle X-ray spectra of the homogeneous and the lamellar phase in the system H20-n-tetradecane-C12E5,measured along well-defined paths through the phase prism. The investigations are based on the features of the phase behavior of such systems reported earlier. The variation of the single sharp peak in the lamellar phase with composition is typical for one-dimensional swelling. For homogeneous microemulsions adjacent to the body of heterogeneous phases we found a single broad scattering maximum, the position and intensity of which varies systematically with composition. The variation of the corresponding Bragg spacing is found to be inconsistent with a layered, lamellar-like structure. NMR-self-diffusion work on the same system ruled out closed droplet structures at comparable water and oil volume fractions; instead bicontinuity was established. Our results are consistent with a disordered bicontinuous interspersion of water- and oil-rich domains with practically all amphiphile concentrated at the well-defined internal interface.
continuous region^^^-^' of water and oil. In all these models the I. Introduction amphiphile is assumed to form a (monomolecular) layer separating Microemulsions are macroscopically homogeneous solutions of water and oil domains. water (+salt), oil(s), and amphiphile (nonionic and ionic). On One reason for the unclear situation lies in the fact that quite a submicroscopical scale, however, the solutions are heterogeneous, a number of experiments were, unfortunately, performed without separated into water-rich and oil-rich domains, the amphiphile detailed knowledge of the phase behavior of the system. We have, being concentrated at the interface. The microstructure of these therefore, drawing vertical sections through the phase thermodynamically stable interspersions is still unresolved; in prism which help to find well-defined paths along which the particular, a consistent theory of microstructure and phase bemicrostructure may be studied by various methods. For the present havior is still lacking. The investigations are greatly facilitated study we have chosen H20-n-tetradecane-ClZE5. We have if the number of components is kept as small as possible. For this supplemented the existing phase diagrams of Kunieda and Shireason, we have studied’” the phase behavior of ternary systems noda2*by the above-mentioned vertical sections (section 111). We H,O-oil-nonionic amphiphile with n-alkyl polyglycol ethers (CiE,) have performed small-angle X-ray measurements along well-deas amphiphiles. As we have shown, the efficiency of the amfined paths, in particular either varying the amphiphile concenphiphile with respect to solubilization of both H 2 0 and oil varies tration at constant water-oil ratio or varying the water-oil ratio systematically with its amphiphilicity and with temperature: the at constant amphiphile concentration (section IV). In the maximum solubilization capacity of each amphiphile is reached meantime NMR-self-diffusionz9 measurements have been perat that temperature at which the three-phase body touches the formed along the latter path from which significant conclusions surface of the body of heterogeneous phases. This temperature, about the connectivity of the microstructure have been drawn. as well as the minimum amount of amphiphile needed to prepare NMR-self-diffusion and conductivity measurements have been a homogeneous solution of equal masses of H 2 0 and oil, varies successfully applied by Ninham and Evans and co-workers to the systematically with the amphiphilicity, Le., i and j in CiEj, and ternary H20-oil-didodecyldimethylammonium bromide system with the hydrophobicity of the oil, e.g., its carbon number in a in order to clarify the microstructure in these system^.'^,^^*^^ We homologous series. However, the question how the amphiphile note, however, that their systems were not close to the corremanages to keep comparable amounts of water and oil in hosponding three-phase bodies, even though quite large homogeneous mogeneous solution is still unanswered. An introduction to the one-phase regions were observed. Accordingly, a qualitatively present state of understanding of microemulsion microstructure different behavior is to be expected. may be found in a number of recent papers7-10 and references therein. 11. Experimental Section Various models have been proposed: the microstructure has 1 . Materials. The nonionic amphiphile C12E5(n-alkyl polybeen pictured as submicroscopical droplets of either water in oil ~ ~ , ~ ~glycol ether) was purchased from Nikko Chemicals, Tokyo, and r a n d ~ m , ~ * ~or~ ~, ’ r d e r e dbior vice is judged to be >98% pure from gas chromatography. The ntetradecane >99% was purchased from EGA Chemie, Steinheim. (1) For a review see: Kahlweit, M.; Strey, R. Angew. Chem. 1985,24,654. The water was twice quartz-distilled. The specimens were weighed (2) Kahlweit, M.; Lessner, E.; Strey, R. J . Phys. Chem. 1983,87, 5032.
(3) Kahlweit, M.; Lessner, E.; Strey, R. J . Phys. Chem. 1984, 88, 1937. (4) Kahlweit, M.; Strey, R.; Haase, D. J. Phys. Chem. 1985, 89, 163. (5) Kahlweit, M.; Strey, R.; Firman, P.; Haase, D. Longmuir 1985,1,281. (6) Kahlweit, M.; Strey, R.; Firman, P. J . Phys. Chem. 1986, 90, 671. (7) Kaler, E. W.; Davis, H. T.; Scriven, L. E. J . Chem. Phys. 1983, 5685. ( 8 ) Kotlarchyk, M.; Chen, S. H.; Huang, J. S.; Kim, M. W. Phys. Rev. Lett. 1984, 53, 941. (9) Auvray, L.; Cotton, J. P.; Ober, R.; Taupin, C. J . Phys. Chem. 1984, 88, 4586. (10) Chen, S. J.; Evans, D. F.; Ninham, B. W.; Mitchell, D. J.; Blum, F. D.; Pickup, S. J. Phys. Chem. 1986, 90, 842. (11) See e.g.: Microemulsions; Robb, I. D., Ed.; Plenum: New York, 1982. Micellization, Solubilization, and Microemulsions; Mittal, K. W., Ed.; Plenum: New York, 1977. (12) Schulman, J. H.; Hoar, T. P. Nature 1943, 152, 102. (13) Schulamn, J. H.; Matalon, R.; Cohen, M. Discuss. Faraday SOC. 1951, 11, 117. (14) Lagues, M.; Ober, R.; Taupin, C. J. Phys. Lett. 1978, 39, L-487. (15) Baker, R. C.; Florence, A. T.; Ottewill, R. H.; Tadros, Th. F. J . Colloid Interface Sci. 1984, 100, 332. (16) Saito, H.; Shinoda, K. J. Colloid Interface Sei. 1979, 32, 647.
(17) Shinoda, K.; Friberg, S. Adu. Colloid Interface Sci. 1975, 4, 281. (18) Huh, C. J. Colloid Interface Sci. 1979, 71, 408. (19) Shinoda, K. Prog. Colloid Polym. Sci. 1983, 68, 1. (20) Friberg, S.; Lapczynska, F.; Gillberg, G. J . Colloid Interface Sci. 1976, 56, 19. (21) Talmon, Y.; Prager, S. J. Chem. Phys. 1978, 69, 2984. (22) Jouffray, J.; Levinson, P.; De Gennes, P. G. J . Phys. 1982,43, 1241. (23) De Gennes, P. G.; Taupin, C. J. Phys. Chem. 1982, 86, 2294. (24) Kaler, E. W.; Bennet, K. E.; Davis, H. T.; Scriven, L. E. J. Chem. Phys. 1983, 79, 5613. (25) Scriven, L. E. Nature 1976, 263, 123. (26) De Geyer, A,; Tabony, J. Chem. Phys. Lett. 1985, 113, 83. (27) Auvray, L.; Cotton, J. P.; Ober,R.; Taupin, C. J . Phys. 1984,45, 913. (28) Kunieda, H.; Shinoda, K. J . Dispersion Sci. Technol. 1982, 3, 233. (29) Olsson, U.; Shinoda, K.; Lindman, B. J . Phys. Chem. 1986,90,4083. (30) Chen, S . J.; Evans, D. F.; Ninham, B. W. J . Phys. Chem. 1984,88, 1631. (31) Blum, F. D.; Pickup, S.; Ninham, B.; Chen, S. J.; Evans, D. F. J . Phys. Chem. 1985,89, 711.
0022-3654/86/2090-5762$01.50/0 0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5763
Microstructure of the System H20-n-Tetradecane-C12E5 in and investigated at the same day. The compositions along path D (see below) were prepared by mixing two stock solutions each containing a volume fraction +s = 0.15 of C12E5 in either pure H 2 0 or n-tetradecane. 2. Small-Angle X-ray Scattering (SAXS). Small-angle X-ray spectra were measured by applying a Kratky camera, Type KA, No. 1268, Paar, Graz, with a position-sensitive proportional counter TEC, Model 205, and electronics including a multichannel analyzer 7100 and time to pulse height converter, EG + G, Ortec, Oak Ridge, using CuKa radiation (A = 1.54 A). The specimen were sealed in glass capillaries of 1 mm outer diameter and 0.01 mm wall thickness. All parts of the camera were evacuated to reduce background scattering. The typical measuring time was 1 h and the temperature stability 70.02 K. All measured intensities are given in relative units. When several spectra are shown on one figure, they were measured under the same experimental settings. 3. Data Treatment. From the raw scattering curve the averaged scattered intensity between q = 0.39 and 0.41 A-' as background was subtracted. The scattering curves were desmeared and smoothed by a method similar to that of V ~ n k . We ~ ~note that in view of the predicted and observed 4"' decay of the scattering curves (see below) the particular choice of the background has a negligible influence on the spectra shown below. For spectra showing a distinct maximum at qmaxthe corresponding Bragg spacing D,, = 2.rr/qmaxwas evaluated. According to scattering theory32the tail of the scattering curves I ( q ) is proportional to the specific internal surface S/ V of a microscopically inhomogeneous two-phase medium, irrespective of its particular microstructure S / V = ~ + , + ~ 4 4 I ( q ) / Q for large q
(1)
The division by the "invariant"
C12ES
4 7 8 OC
H2O
Tetradecane
60 9 ["CI
t 20
n 0
20
LO
60
80
wt %
H,O/Tetradecane 50 / 50
100 C12ES
eliminates all system-dependent factors. Thus, knowing the respective volume fractions aWand a0of the water (1)- and oil (2)-rich domains, the specific internal surface is obtained from scattering experiment. 111. The Phase Diagram
The top diagram in Figure 1 shows three isothermal sections through the phase prism of H2n-tetradecane-C12E5as published by Kunieda and Shinoda.28 It shows the change of shape of the threephase triangle with temperature as well as the Schreinemaker grooves' at the top of the triangles. The central diagram in Figure 1 shows the vertical section A through the phase prism erected on the center line of the Gibbs triangle. It was determined by starting at high C12E5 concentrations and successive dilution with equal masses of H 2 0 and n-tetradecane. It shows the boundary of the two-phase regions (2@) and the three-phase body (3+) as well as the lyotropic mesophases (nomenclature according to T i d d ~ at ~ ~high ) C12E5 concentrations. The texture of the mesophases was determined under a polarizing microscope with hot stage using crossed POlarizers. We found the mesophases to be, in general, surrounded by narrow regions of coexistence with the neighboring phase. Due to the difficulty of determining the exact position of the boundaries, the data points show the first occurrence of anisotropy. In particular, the tip of the L, phase pointing toward the 3+ region is rather a coexistence region between L, of higher C12E5 content and the isotropic phase (1+). Between the lamellar phase (La) and the two-phase regions (2+) one finds isotropic channels (1 +) from high C12E5 concentrations down toward the three-phase region. The extent of the lamellar phase permits determination of the lamellar spacing as a function of the composition (Figure 1 (top): section A at constant H20/oil ratio, section B at constant (32) Glatter, 0.;Kratky, 0. Small Angle X-ray Scattering, Academic: New York, 1982. ( 3 3 ) Tiddy, G. T. J. Phys. Rep. 1980, 57, 1.
0 H2°/C12E5
85Al14.6
20
40
60 wt%
80
100
Tetradecane/C12E, 81.7118.3
Figure 1. Upper: Gibbs triangle of ternary system H20-n-tetradecane-C12E,. Shape of three-phase triangle (3+) at different temperatures and approximate extent of the lamellar mesophase La (see text). A, B, C, D sections through phase prism. Center: Section A shows the observed phases as function of temperature. Note the extent of the onephase (19) region. Lower: Section D shows the observed phases as function of temperature. Note the one-phase channel extending from the water to the oil side.
H20/C12E5ratio, and section C at constant oil/C12E5ratio). These signals may then be compared with those in the homogeneous channels which are easily obtained from the same specimen by simply adjusting the temperature. The lower diagram in Figure 1 shows the vertical section (D) through the phase prism erected on a line parallel to the H20-oil side at a C12E5 volume fraction as= 0.15 in the groove near the three-phase region A. It was determined by mixing the two stock solutions described in section 11. By repeated measurements of the temperature at which the mixture became either clear or turbid, the data points could be determined to within a few hundreds of a degree, except for the two-phase region at low temperatures on the water-rich side which showed considerable
5764
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
Lichterfeld et al.
1Lo @ -022
’0
120
-
028
I>
5
i: 1
I(q1
[ a u l 100 -
8
i\
80-
o
150
A
path A path B path C
i IAI 1
I 60 LO
0 100 & l
20 0
0
002
OOL
006
-c
008
010
012
50
q[ P I
-
Figure 2. X-ray spectra at different surfactant volume fractions @# along path A in the lamellar phase (full symbols), in the isotropic liquid (1 @) above the lamellar phase (open symbols) (see Figure 1 (center)).
undercooling. The section D shows a narrow homogeneous (1 0) channel from the water-rich to the oil-rich side which ascends temperaturewise with increasing oil concentration. It permits one to study the microstructure of the homogeneous solution continuously as it changes from an o/w to a w/o “emulsion” at constant volume fraction of the amphiphile.
IV. Results Lamellar Phase (La). First, we studied the small-angle X-ray scattering (SAXS) of the lamellar phase. The structure of this phase is known to be a sandwichlike sequence of alternating water and oil layers separated by the a m p h i ~ h i l e . ~ ~Essentially ,~~-~~ only a single sharp peak is observed. Figure 2 shows some of the X-ray spectra along section A, Le., at constant H20/oil ratio with increasing volume fraction 0,of the amphiphile at constant temperature. Within experimental error, these spectra are independent of temperature. The shift of the peak with increasing H20/oil content can be attributed to a one-dimensional swelling found in many systems.37 In that case the variation of the peak positions should depend only on the inverse volume fraction of the amphiphile where the volume of the amphiphile molecule u, = Mp;INA-l is calculated from the molecular weight M and density p, of the amphiphile. a, is the effective head group area in the lamellar phase; for C12Ej is was found3s to be a, = 42.9 A2. Figure 3 shows the Bragg spacing along paths A (constant H20/oil ratio), B (constant H20/C12E5ratio), and C (constant oil/CI2E5ratio) vs. 0,and in the binary system H20/C12E5(0, = 0). It shows that the spacing is indeed independent of the H20/oil ratio, since the experimental data along all three paths coincide. The full line, evaluated from eq 3 with a, = 42.9 A2, describes the data well except for systematic deviations at low 0,. These deviations are to be expected, since with decreasing 0,the stability of the lamellar phase should also decrease (as monitored, e.g., by the decreasing viscosity) so that distortions like dislocat i o n ~and ~ ~curved defects38 are likely to appear. Isotropic Phase. If one remains in section A (Figure 1, center) and raises the temperature, one enters the homogeneous channel below the upper two-phase region. As can be seen from Figure 2, the maxima decrease in height, become broader, and move toward smaller angles. With decreasing amphiphile volume (34) 866. (35) 1543. (36) 189. (37)
Spegt, P. A.; Skoulios, A. E.; Luzatti, V. Acta Crystallogr. 1961, 14, Ekwall, P.; Mandell, L.;
Fontell, K. Acta Chem. Srand. 1968, 22,
Mocharafieh, N.; Friberg, S . ; Larsen, D. Mol. Liq. Cryst. 1979,53,
Ekwall, P. In Aduances in Liquid Crystals; Brown, G . H . , Ed.; Academic: 1975; Vol. 1. (38) Paz, L.; Di Meglio, J. M.;Dvolaitzky, M.;Ober,R.; Taupin, C. J . Phys. Chem. 1984, 88, 3415.
02
0
04
0.6
0.8
1.0
Qs
Figure 3. Bragg spacing as function of surfactant volume fraction as. Full line calculated from eq 3.
1
\
10-1 0
f, ‘
‘
~
-,. / 0 ~
- ~IA’I
os = 0.15
002 O O L 006 008 01
Figure 4. X-ray spectra at different water to oil ratios at constant surfactant volume fraction @, = 0.15 . Note the characteristic 44 decay for
large 4 . fraction the broad peak increases in height and moves toward smaller angles. In this respect the data show the same behavior as small-angle neutron scattering (SANS) spectra in the D20n-decane-AOT system8 and D20-toluene-n-butanol-SDS-NaCl.g Figure 4, finally, shows the spectra along section D, Le., at constant volume fraction of the amphiphile with varying water-oil ratio. Again, the single peak is observed; however, it appears not as pronounced due to the logarithmic ordinate chosen in order to demonstrate the q-4 decay at large q. The peak decreases as one moves from the center to either side, its position being shifted toward larger scattering angles, Le., smaller Bragg spacings. In this respect the data show the same behavior as SANS spectra in H20-p-xylene-n-hexanol-sodium dodecylben~enesulfonate.~~ The variation of the peak height with changing water to oil ratio resembles that found in the H,O-toluene-n-butanol-SDS-NaCl system from SAXS27and SANS.26 V. Discussion The spectra show two predominant features, the q4 decay for large q and the broad scattering peak at intermediate q. The q4 dependence indicates a well-defined internal surface32 at which there is a jump in electron density, in our case from the ~~
(39) Cebula, D. J.; Ottewill, R. M.; Ralston, J.; Pusey, P. N. J . Chem. Soc., Faraday Trans. 1 1981, 77, 2585. (40) Strey, R., unpublished results.
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5765
Microstructure of the System H20-n-Tetradecane-C12E5 151
I
’
’
’
’
7
1
I
0.2
0.6
0.1
0.8
1
$0 +
1
0,
Figure 5. Specific internal surface of microemulsions at constant volume fraction of surfactant as= 0.15 as function of oil/water ratio, defined as @a/(@w + a,,). Full line: total interface of a monomolecular amphiphile layer assuming the amphiphile head group area to be the same as in the lamellar phase.
hydrocarbon to the water-rich domains. While for evaluation of the electron density difference absolute intensity measurements are required, for evaluation of the specific internal surface, according to eq 1, relative intensity suffices. The results for constant surfactant volume fraction 0,= 0.15 are shown in Figure 5. From eq 2 it is obvious that the contributions to the invariant Q as q 0 and also because of Z q4 as q become small. This is fortunate since at low q one is limited by primary beam and at high q the counting statistics becomes poor. Nevertheless, this introduces some uncertainty, so that we estimate the error in Sf V determined from the scattering curves to be of the order of *IO%. The full line is calculated assuming all amphiphile molecules to be concentrated at the interface between water- and oil-rich domains with the same amphiphile head group area as in the lamellar phase. The observed peaks of the homogeneous solutions (Figures 2 and 4) indicate the existence of a characteristic length, the broadness of the peak being a measure for disorder. The results shown in Figure 2 in connection with the phase diagram (Figure 1, center) suggest to conceive the isotropic solution as a “molten” lamellar phase. A (distorted) lamellar-like arrangement of H20 and oil layers separated by the amphiphile could explain the observed single broad peak. In that case, however, one would expect the peak position to vary with the volume fraction 0,of the amphiphile according to eq 3 which, at constant 0, (path D), would predict a constant position contrary to the experiment. If one assumes either w/o or ofw droplets, (microemulsions in the sense of Schulman12),one would expect the Bragg spacing to be a measure for the mean distance between the droplets in An some distorted cubic8 or p a r a c r y ~ t a l l i n e arrangement. ~~*~~ elementary derivation, assuming for instance a body-centered arrangement, gives for the Bragg spacing
-
-
\
loot
-2%-
-
/
I ’
= 0.15 0
/
\
0)
(4) where the volume fraction 0 d of the droplets includes part of the amphiphile. Since in the case of C12ESthe hydrophobic and hydrophilic portions of the molecule are almost equal, it appears reasonable to add half of the volume fraction of the amphiphile, a,, to the volume fractions of either oil or water. At constant a,, as in section D, eq 4 predicts an increase of the Bragg spacing with increasing oil or water content as @ p d 2 / 3 , with the difficulty as to which value of ad to assume for zero oil or water content. If one sets ad = @s/2,one finds the two broken lines in Figure 6 , the left one for o/w, the right one for w/o emulsions. Apart from the quantitative difference, these curves seem to describe the experimental curve in parts at least qualitatively which may have led a number of authors to the conclusion that, in particular, ~~~~
(41) Hosemann, R.; Bagchi, S. N. Direct Analysis of Diffraction by Matter; North-Holland: Amsterdam, 1962. (42) Ishii, Y . ;Matwnoka, H.; Is, N . Berg. Bunsensges. Phys. Chem. 1986, 90, 50.
0
QS L
0.15
0.2
--
06
01
08
1
00
@a @w +
Figure 6. Bragg spacings obtained from maxima of X-ray spectra ( 0 ) . Broken lines calculated from eq 4 assuming either droplets of water in oil (right) or vice versa (left). Dash-dotted line calculated from eq 3 assuming a lamellar like structure. Full line calculated from q 5.
their SANS results seem to confirm the droplet mode18.39also for comparable water and oil volume fractions. We believe, however, this agreement to be fortuitous. One reason is the difficulty to visualize the inversion from o/w to w/o droplets (which experimentally has never been observedz4), a difficulty not occurring in so-called bicontinuous models. The second reason is that N M R self-diffusion measurementsZ9yield high diffusion coefficients for both oil and water, a result which is incompatible with closed droplet structures. In addition, a steep increase of the electrical conductivity-with a trace of salt added-on the oil-rich side is observed,40indicating the existence of conductive channels when 0, is increased above 0.15. The existence of a peak is difficult to explain by the random bicontinuous structure as the Voronoi model proposed, e.g., by Talmon and Prager?’ since such a structure would not show any scattering maximum at all. The periodic bicontinuous structure proposed by Scriven,z5on the other hand, should lead to a number of sharp reflections instead of a single maximum. Due to the sidedness’ of the amphiphilic molecule, however, the oil and water domains have to be arranged in an alternating structure, e.g., like in a sponge with the empty space for oil and the solid material for water, and would give some quasi-periodicity. The scattering experiment monitors due to the alternating nature the typical repeat distance, that is, the sum of the average “diameters” of a water and an oil domain and two amphiphile interfaces. In random two-phase media this “diameter” is closely related to the correlation length43or persistence lengthZZ
a = c0.,QWV/S where the constant c may have-depending on the model-values of 4,43 6,22or 5.82.21 In view of the fact that we do observe a scattering peak and the microemulsion structure is apparently less random than p r e v i o ~ s l y ~ * ~ ~ * ~ ~we cannot expect any of the values to be quantitatively correct. Thus, we can only assume with the proportionality constant determined by the geometrical configurations, and with 0,’and 0,’being the volume fractions of oil and water, respectively, each increased by half of 0,.The full line of Figure 6 shows that the variation of the peak position is indeed correctly described. Furthermore, the scattering intensity is predicted43 to vary as I
-
a3
(6)
Figure 7 shows that this is also approximately the case. In spite of the apparent success of the bicontinuous model to describe at least semiquantitatively the variation of spectra with (43) Debye, P.; Anderson, Jr., H. R.; Brumberger, H. J . Appl. Phys. 1957, 28, 619.
J . Phys. Chem. 1986, 90, 5166-5110
5766
100
1
'
k I
/ i
hi
/
01 0
"
02
-"
"
"
06
08
OL
'
has been shown4$to explain the phase behavior of these microemulsion systems. The inclusion of gradient terms into the free energy functional leads to straightforward e x ~ l a n a t i o nof~ the ~ single broad scattering peak observed for a variety of microemulsion It remains a major task to combine the ~ ~ ,curvaturelo ~~ of the interface current ideas on f l e ~ i b i l i t yand and the fractal properties of disordered media47with the observed phase behavior. In order to achieve this, additional data from NMR, neutron and light scattering, relaxation kinetic, dielectric, and viscosity measurements along the described paths are required. This work is in progress.
1
00
@of%
Figure 7. Intensities at qmar(see Figure 4). Full line calculated from
eq 6 . composition, a number of questions remain open. One is the experimentally verified fact that the position of the three-phase body and thus of the groove on the temperature scale depends systematically on both the nature of the oil and the amphiphile (see, e.g., Figures 4 and 5 in ref 6), and that, accordingly, the high mutual solubility between oil and water in the groove is restricted to a rather narrow temperature interval, as can also be seen on Figure 1. There is strong evidence that the phase behavior is governed by the proximity of tricritical point^.^,^ A free energy functional without gradient terms of the phenomenological Landau theory"
Acknowledgment. This work was carried out in the laboratory of Prof. M. Kahlweit, to whom we are indebted for his support and helpful discussions. We, further, thank B. Faulhaber for the help with the phase diagram determinations. We also thank Dr. U. Wuerz for introducing us to the X-ray scattering technique and Prof. R. Hosemann for helpful discussions. Also the constructive criticism of an anonymous reviewer is gratefully acknowledged . Registry No. CI2ES,3055-95-6; tetradecane, 629-59-4. (44) Griffith, R. B. J. Chem. Phys. 1974, 60, 195. (45) Kleinert, H. J. J. Chem. Phys. 1986, 84, 964. (46) Teubner, M., to be published. (47) Hughes, B. D., Ninham, B. W., Eds. The Mathematics and Physics of Disordered Media; Springer Verlag: Heidelberg, West Germany, 1983.
Small-Angle Neutron Scatterhrg from Hexadecyftrimethylammonium Bromide Micelles in Aqueous Solutlons Stuart S. Berr,*t E. Caponetti,*#James S. Johnson, Jr.,I Richard R. M. Jones:Il
and Linda J. Magidt
Department of Chemistry, Wake Forest University, Winston-Salem, North Carolina 27109, Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996- 1600, Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, and 3M Company, Industrial and Consumer Sector Research Laboratory, 3M Center, St. Paul, Minnesota 55144 (Received: November 12. 1985: In Final Form: March 10, 1986)
Small-angle neutron scattering measurements have been carried out at 50 OC as a function of external contrast (different ratios of H20to D20)on 0.12 M solutions of hexadecyltrimethylammonium bromide, with the methyls of the ammonium group protiated and deuterated. The results indicate that most of the surfactant hydrocarbon is in a micellar core penetrated by little, if any, solvent. The micelles are described adequately by a dispersion of monodisperse prolate ellipsoids. There is a significant effect on the size of the aggregates by the isotopic composition of the solvent.
We present here small-angle neutron scattering patterns from aqueous solutions of a cationic surfactant, hexadecyltrimethylammonium bromide (or cetyltrimethylammonium bromide (CTAB)). The effects of varying the external contrast in scattering power' between solute and solvent were investigated by measurements as a function of H20/Dz0. We also measured patterns from the surfactant with the methyl groups of the ammonium head group protiated (CTAB-per H) and deuterated (CTAB-D9), in order to test models with results from different contrasts in the shell region. *Oak Ridge Associated Universities Fellow at Oak Ridge National Laboratory, 1984-1 986. Wake Forest University. *University of Tennessee. Permanent affiliation: Instituto Chimica Fisica, University of Palermo, Palermo, Italy. A Oak Ridge National Laboratory (Research sponsored by US. Department of Energy under Contract DE-AC05-84021400 with Martin Marietta Energy Systems, Inc.) 11 3M Company.
0022-3654/86/2090-S766$01 S O / O
There is no longer serious dispute about the formation of sizable aggregates (micelles) by ionic amphiphiles in aqueous solutions at some concentrations and temperatures. Details of the structure of the micelles, however, continue to be debated. Results of experiments with probes solubilized in micelles and of effects of micelles on reaction rates have been interpreted to indicate substantially more hydrocarbon-water contact than would occur if the hydrocarbon is distributed in a compact core, in proximity to solvent in a shell comprised also of ionic head groups and complexed counterions. Rough micellar surfaces and transient water-fied cavities, among other irregularities, have been inferred from such measurementse2 These views have been challenged on the basis of interphase-theory predictions of configurations3 and on other grounds. (1) Kostorz, G.; Lovesey, S . W. Trearise Mater. Sci. Technol. 1979, 15, 230-231. (2) Menger, F. M. Narure (London) 1985, 313, 603. (3) Dill, K. A.; Flory, P. J. Proc. Narl. Acad. Sci. U.S.A. 1981, 78, 676.
0 1986 American Chemical Society
