J. Phys. Chem. 1989, 93, 4855-4861 exchange rate from the N M R experiment with jumping or hopping rates in clays determined by other techniques. The median of recently reported values for the diffusivity in clays of varying hydration is IO-" cm2 s-lS4l From this value and an average distance between exchange sites of 1 1.5 A, one obtains a value of 0.8 X 1O's-I for the exchange rate. This compares favorably with the value of 2.2 X lo3 s-l obtained from the line width. Thus the N M R spectra, like the ESR spectra, are sensitive to water content and/or location. At least two and possibly three types of ion motion can be identified from the combined ESR and N M R studies. The ESR spectra identify the very rapid bulk solutionlike movement. The N M R spectra identify ion motion at sheet or edge sites, which is considerably slower. Finally, the N M R spectra indicate an interlayer ion motion of intermediate rate. There are consequences of this diffusion to the migration of environmentally contaminated metals in clays. Our results lead to predictions about ion mobility in clays that is independent of the anion or of its solubility. Most workers conclude that migration of metal ions takes place by a process dependent upon the solubility of the metal salt. In the case where the metal ion is adsorbed on the clay, ion mobility does not require the model of solid dissolution for migration. In fact, the process does not require an aqueous
4855
environment. We calculate that cadmium, for example, would move 5 cm/year based on the diffusion coefficient. By this scheme for areas of high cadmium concentration such as the Foundry Cove in New the cadmium would have moved 150 cm since the original contamination. This study demonstrates that '13Cd MAS N M R provides chemical shift and line widths that identify different components in ion-exchanged clays. Additionally, dynamics and paramagnetic effects rather than chemical shift dispersion account for the line widths. Ion exchange does not take place between the two environments on the N M R time scale. These N M R results are consistent with and provide some description of the distinct kinds of metal ion environments.
Acknowledgment. We gratefully acknowledge the partial support of this research from the National Science Foundation via awards CHE82-07445 (P.D.E.) (the N S F R I F in N M R Spectroscopy at the University of South Carolina) and CHE8306580 (P.D.E.) and from the National Institutes of Health (GM26295 (P.D.E.)). Further, S.B. acknowledges the New York State Health Research Council via Award 20-053 and USC for partial support for his sabbatical leave. Registry No. Cd, 7440-43-9; montmorillonite, 1318-93-0; hectorite, 12173-47-6; kaolinite, 1318-74-7; alginic acid, 9005-32-7.
(41) (a) Ege, D.; Ghosh, K.; White, J. R.; Equey, J. F.; Bard, A. J. J. Am. Chem. SOC.1985, 107, 5644. (b) Habti, A,; Keravis, D.; Levitz, P.; Van Damme, H. J. Chem. SOC.,Faraday Trans. 2 1984,80, 67.
(42) Bank, S.; Bank, J. F.; Marchetti, P. S.; Ellis, P. D. J. Enuiron. Qual. 1989, 18, 25.
Microemulsifying Polar Oils Klaus R. Wormuth and Eric W. Kaler* Department of Chemical Engineering, BF- 10, University of Washington, Seattle, Washington 981 95 (Received: July 26, 1988; In Final Form: January 4 , 1989)
The phase behavior and microstructure of C12E6/etheroil/water mixtures have been examined systematically as a function of increasing oil polarity. As the ratio of ether linkages to methylene groups of the ether oils is increased, the oils become more water soluble and more polar (less hydrophobic). When ethylene glycol dibutyl ether is replaced with the more polar ethylene glycol diethyl ether in CI2E6/etheroil/water mixtures, the three-phase region shrinks and disappears: the system bypasses a tricritical point. Simultaneously, the liquid crystalline region retreats to higher surfactant concentration, and light and X-ray scattering measurements indicate that the microstructuredecreases dramatically in size. According to small-angle X-ray scattering results, CI2E6/ethyleneglycol diethyl ether/water mixtures retain an interface between oillike and waterlike domains. However, since the ether is highly soluble in the water and interface domains, the microstructure is diffuse.
Introduction Many surfactants organize into microstructures and thus promote the solubilization of liquids as incompatible as oil and water. A mixture of oil and water made thermodynamically stable by an amphiphile is called a microemulsion.I4 Microemulsions are differentiated from normal solutions by the presence of microstructure induced by topological ordering of oil and water domains by the amphiphile. All amphiphiles are surface active since they contain both polar water-loving (hydrophilic) and nonpolar oil-loving (hydrophobic) moieties. Low molecular weight amphiphiles (e.g., alcohols) cluster on a bimolecular to multimolecular level and can promote small-scale and short-lived topological but only a subclass of amphiphiles called (1) Microemulsions: Structure and Dynamics; Friberg, S . E., Bothorel, P., Eds.; Chemical Rubber: Boca Raton, FL, 1987. (2) Bellocq, A. M.; Biais, J.; Bothorel, P.; Clin, B.; Fourche, G.; Lalanne, P.; Lemaire, B.; Lemanceau, B.; Roux, D. Adu. Colloid Interface Sci. 1984, 20, 167. (3) Friberg, S. E. Prog. Colloid Polym. Sci. 1983, 68, 41. (4) Danielsson, I.; Lindman, B. Colloids Surf. 1981, 3, 391.
0022-3654/89/2093-4855$01.50/0
surfactants have the distinctive ability to form micelles and liquid crystals and thus promote microemulsion microstructures. Surfactants contain both moieties of high hydrophilicity and high hydrophobicity; thus, they are highly amphiphilic.* Microstructure cannot be inferred from phase diagrams alone since phase behavior patterns of weakly structured solutions, micellar systems, and microemulsions are qualitatively similar.+l2 However, the ( 5 ) Bellocq, A. M.; Bourbon, D.; Lemanceau, B. J. Dispersion Sci. Technol. 1982, 2, 27. (6) Kilpatrick, P. K.; Davis, H. T.; Scriven, L. E.; Miller, W. G. J. Colloid Interface Sci. 1987, 118, 270. (7) Bodet, J.-F.; Davis, H. T.; Scriven, L. E.; Miller, W. G. Langmuir 1988, 4, 455. (8) Laughlin, R. G. In Aduances in Liquid Crystals; Brown, G . H., Ed.; Academic: London, 1978; Vol. 3, p 41. (9) Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1985, 24, 654. (10) Friberg, S. E. Prog. Colloid Polym. Sci. 1983, 68, 41. (1 1) Lindman, B.; Stilbs, P. In Microemulsions; Friberg, S. E., Bothorel, P., Eds.; Chemical Rubber: Cleveland, 1978. (12) Rushforth, D. S.; Sanchez-Rubio, M.; Santos-Vidals, L. M.; Wormuth, K. R.; Kaler, E. W.; Cuevas, R.; Puig, J. E. J . Phys. Chem. 1986, 90, 6668.
0 1989 American Chemical Society
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presence of liquid crystalline phases often indicates that the nearby isotropic phases contain micellar or microemulsion microstructure.I3 From many studies, microemulsion phase behavior and microstructure depend critically on the chemical structure of each of the ingredients.14-16 In this work, we explore a new type of nonionic surfactant/oil/water system containing oils that are more polar and less hydrophobic than the alkanes usually used. Many present and promising applications exist for microemulsions with polar oils." Polar oils are often toxic solvents, and dispersion in a water-rich microemulsion reduces the solvent concentration and thus mitigates environmental hazards. A microemulsion incorporating a polar oil may provide for high solubility of large polar molecules such as dyes, drugs, and pesticides. Indeed, solvent dyes exhibit high solubility in the core and palisade layer of microemulsion droplets containing polar oils; these microemulsion inks may find application in ink-jet printing.18 Nonetheless, it is not clear that all surfactant/polar oil/water mixtures contain the microstructure characteristic of microemulsions, particularly if the oil polarity is high. Despite a paucity of experiments on polar oils in microemulsions, there are some clues about how increasing oil polarity affects phase behavior and microstructure. For example, as the chain length of the alcohol decreases in mixtures of ionic surfactant, alcohol, and water, ordered liquid crystalline phases melt to isotropic mixed micellar phases.lg Similarly, the extent of the liquid crystalline phase in the phase diagram of sodium dodecyl sulfate/pentanol/water grows with addition of long-chain esters, whereas short-chain esters dissolve the liquid crystals.20 Kahlweit et al. clearly demonstrate that nonionic surfactant/alkane/water systems tend toward a tricritical point when the hydrophobicity of the oil is reduced.21 Also, gradually replacing isooctane with the more hydrophilic hexanol drives a five-component mixture toward a tricritical point; a tricritical line has been mapped as a function of salt and temperature.22 Experimental and theoretical evidence indicates that penetration of alcohols and less hydrophobic alkanes such as cyclohexane into surfactant tails sterically constrains the curvature of the interface between oil and water microdomains to favor water-in-oil structure^.^^ Nonetheless, these studies do not directly examine microemulsions with polar oils. How phase behavior evolves with increasing oil polarity and whether the mixtures retain the distinct oil and water domains characteristic of microemulsions are still open questions. Here, we present a systematic study of the effect of oil polarity on the phase behavior and microstructure of three-component nonionic surfactant/polar oil/water systems. The polar oils studied are ethers; inserting oxygen atoms into alkane chains increases oil polarity. For simplicity, we define oil polarity as the opposite of oil hydrophobicity; thus, polar oils are more soluble in water than alkanes. As oil polarity is increased, the three-phase region shrinks and disappears by passing a tricritical point. Simultaneously, the liquid crystalline region retreats to higher surfactant concentration. Microstructure size and ordering, as measured by light and X-ray scattering, decrease dramatically as the oil becomes more polar, and thus, a fundamental relationship between phase behavior, microstructure, and critical points unfolds. (13) Lichterfeld, F.; Schmeling, T.; Strey, R. J . Phys. Chem. 1986, 90, 5762. (14) Wormuth, K . R.; Kaler, E. W. J . Phys. Chem. 1987, 91, 61 1. (15) Stilbs, P.; Rapacki, K.; Lindman, B. J . Colloid Interface Sci. 1983, 95, 583. (16) Lang, J.; Rueff, R.; Dinh-Cao, M. J . Colloid InferfaceSci. 1984, 101, 184.
(17) Gillberg, G . In Emulsions and Emulsion Technology; Lissant, K. J., Ed.; Dekker: New York, 1984; Part 111, p 1. (18) Wormuth. K. R.; Cadwell, L. A,; Kaler, E. W., manuscript in preparation. (19) Ekwall, P. In Advances in Liquid Crysfals;Brown, G.H . , Ed.; Academic: London, 1975: Vol. I , p 1 (20) Friberg, S. E.; Gan-Zuo, L. J . SOC.Cosmer. Chem. 1983, 34, 73. (21) Kahlweit, M.; Strey, R.; Firman, P. J . Phys. Chem. 1986, 90, 671. (22) Kunieda, H. J . Colloid Interface Sci. 1988, 122, 138. (23) Evans, D. F.: Mitchell, D. J.; Ninham, B. W. J . Phys. Chem. 1986, 90, 2817.
Wormuth and Kaler Experimental Section The shorthand notation used for ethers and ethoxylated alcohols is
CjOCj = CH3(CH2)j-IO(CH2)j-iCH3 CjOC2OCj = CH,(CH2)j-10CH2CH20(CH2)i-ICH3 CjEj = CH3(CH,)j-,O(CH2CH20),H All purchased chemicals were of high purity and were used as received. C4E1(2-butoxyethanol), C4Ez (2- [2-n-butoxyethoxy]ethanol), toluene, anisole, i-C30C3(isopropyl ether), c4oc4 (butyl ether), CsOCs (pentyl ether), cyclohexane, octane, dodecane, propyl bromide (all 99% pure), c6oc6 (hexyl ether; 98%), and C 2 0 C 2 0 C 2(ethylene glycol diethyl ether; 95%) were purchased from Aldrich Chemical Co. C4E3(triethylene glycol mono-n-butyl ether, 97%) and C40C20C4(ethylene glycol dibutyl ether; 98%) were purchased from American Tokyo Kasei Inc. C 3 0 C 4 0 C 3 (1,4-bis[n-propoxy]butane;98%) was purchased from Parish Chemical Co. Sodium metal, ethylene glycol, ethyl ether, and all salts used were 99% pure from J. T. Baker Chemical Co. The surfactant c&6 (hexaethylene glycol mono-n-dodecyl ether; 98%) was purchased from Nikko Chemical Co. Ltd. The water was deionized and doubly distilled. C 3 0 C 2 0 C 3was prepared by a modified Williamson ether synthesis." A 16-g amount of sodium metal and excess ethylene glycol (200 mL) were charged to a 500-mL reactor, stirred, and heated. After the Na was consumed, propyl bromide (82 g) was added dropwise as the mixture refluxed. After reaction, the products were filtered to remove NaBr, extracted with saturated NaCl brine, and extracted with ethyl ether. The ethyl ether phase was dried with CaSO,, and the decanted liquid was rotary evaporated to remove ethyl ether. Distillation of the remaining liquid under vacuum yielded a C3El fraction. This C3E1fraction (35 g) was reacted again with excess Na metal (1 1 g) and propyl bromide (56 g) in isopropyl ether solvent (200 mL), and the same workup was repeated. The chemical structure of the product C30C20C3was confirmed by IH and I3C NMR. The purity was 97% by gas chromatography; the main impurity was C3EI,with a minor impurity of ethylene glycol. For the phase behavior studies, samples were prepared in tightly capped vials, shaken, and allowed to equilibrate in a constanttemperature bath at 10.05 "C. The liquid crystalline regions were determined by placing the sample between crossed polarizers illuminated by an unpolarized light source and visually observing bright spots (birefringence). The regions so determined include the isotropic solution-liquid crystal two-phase region as well as the liquid crystal phase. Compositions in the oil/water miscibility gaps were determined by measuring the water content in each phase by Karl-Fischer titration (Metrohm Titrator Model 633). The static light scattering apparatus consisted of a laser light source (2-W argon ion, Spectra Physics Model 165), a thermostated sample holder with refractive index matching bath, and a Thorn/EMI phototube detector. The scattered light was never depolarized, so multiple scattering effects were unimportant. Scattered intensity was converted to a Rayleigh ratio ( R B with ) use of benzene standard.25 For all samples, RBwas independent of the magnitude of the scattering vector q = [ 4 m / X ] sin [0/2] (1) where n is the refractive index of the sample, X is the wavelength of the light (4880 A), and 0 is the scattering angle. Samples were filtered by gravity through 0 . 4 5 - ~ mMillipore filters into 1.3cm-diameter cells. The small-angle X-ray scattering (SAXS) apparatus consisted of a Philips Model 3100 X-ray generator with a slit-collimated Kratky camera. The Cu K a radiation (A = 1.54 A) was filtered (24) Morrison, R. T.; Boyd, R. N. Organic Chemistry; Allyn and Bacon: Boston, 1973; p 556. (25) Hiemenz, P. C. Principles of Colloid and Surface Chemistry; Dekker: New York, 1986.
The Journal of Physical Chemistry, Vol. 93, No. 12, 1989 4857
Microemulsifying Polar Oils
1
c20c20c2 4
Toluene
4
I
T
I
Anisole iso-C30C3
c40c20c4 c4oc4 I
csocs
0
5
W,
I
10
.
WlC 15
20
wt% C12E6
c60c6
C ycio hexane
Octane Dodecane 40
Figure 1. Ranking of oil polarity from the partitioning of 25 wt % C,Ej in equal masses of oil and water ( T = 25 "C). C20C20C2 is the most
1
polar oil examined. with nickel and cobalt foils according to the method of Ross,26 and the entrance slit was varied between 25 and 200 pm to obtain a scattering vector (9) range of 0.005-0.3 A-1. The scattered radiation was detected with DuPont NDT-75 X-ray film, and the developed film was analyzed for optical density with a slit-collimated densitometer. Since the intensities were smeared by the collimation system, the optical densities were desmeared by the method of The scattering patterns were corrected for sample cell scattering and sample transmission and put on an absolute scale with a calibrated polymer standard. The sample cells were 1.O-mm thin-walled glass capillaries, and temperature was controlled to f O . l OC.
Results The relative polarity of the ether oil is determined simply by observing the partitioning of C4Ej 0'= 1-3) between oil and water at room t e m p e r a t ~ r e . C4Ej ~ ~ (25 wt %) was added to equal masses of oil and water, and the phase behavior is noted in Figure 1. All multiphase behavior fit four categories described by the notations -2 , 2 , 2, or 3.31 The 2 denotes a two-phase sample with the mass fraction of the aqueous phase larger (thus, C4Ej resides mostly in the lower aqueous phase). The 2 describes a two-phase sample with the mass fraction of the upper phase larger (C4E, prefers the oil phase). The 2 denotes a two-phase sample with the mass fractions of the upper and lower phases approximately equal (C4Ej partitions into both phases). The 3 describes a three-phase sample with most of the C4Ej residing in the middle phase. For some of the ether oils, 2, 2, and 2 were difficult to distinguish visually. If the phase volumes were measured within 10% of equal partitioning of C4Ej between oil and water, the system was considered 2. As oil polarity increases (or hydrophobicity decreases), a 2 2 (or 3) 2 phase progression results; as j of C4Ej decreases, the same phase progression results.30 Oil polarity is determined by constructing a grid of phase behavior as a function of oil type 3 transitions properly and j of C4Ej with all the 2 2 or 3 ordered (Figure 1). For alkanes, the number of carbon atoms specifies polar rank. However, for ether oils, the polar ranking is less sensitive to carbon number than to the number of oxygens: c6oc6 has the same rank as c4oc4 on the C4Ej scale, whereas C 4 0 C 2 0 C 4is more hydrophilic than CsOCs (Figure 1). Since C 2 0 C 2 0 C 2is totally miscible with water at 25 wt % C4Ej 0' =
-
-
- -
Ross, D. A. J . Opt. SOC.Am. Rev. Sei. Instrum. 1928, 16, 433. Vonk, C. G. J . Appl. Crystallogr. 1975, 8, 340. Vonk, C. G. J . Appl. Crystallogr. 1981, 14, 8. Vonk, C. G.J . Appl. Crystallogr. 1971, 4 , 340. Kahlweit, M.; Lessner, E.; Strey, R. J . Phys. Chem. 1983, 87, 5032. (31) Knickerbocker, B. M.; Pesheck, C. V.; Davis, H. T.; Scriven, L. E.
(26) (27) (28) (29) (30)
J . Phys. Chem. 1982, 86, 393.
0
5
10
15
20
wt% C12E6 Figure 2. Phase behavior of CI2E6in equal masses of oil and water: (1) oil is C40C, and (2) oil is i - C 3 0 C 3 .
2, 3), C 2 0 C 2 0 C 2is the most polar oil. With the polarity of the ether oils at hand, the mapping of the phase behavior of surfactant/ether oil/water mixtures is simplified. It is ~ e l l - k n o w n ~that ' * ~the ~ phase behavior of nonionic amphiphile (CjEj)/oil/water mixtures as a function of temperature and weight percent of amphiphile looks like a fish (Figure 2) when represented on a plane slice through the multiphase body at a fixed oil/water ratio. Three parameters describe the fish: the temperature range of the three-phase body (AT), the temperature (T,.), and the weight percent of amphiphile (w,) coordinates of the point (x) where the tail of the fish meets the body.33 Also, the lowest weight percent of surfactant at which liquid crystals form is denoted as wlc. The phase diagrams of CI2E6/i-C30C3/waterand C1,&/ C40C4/waterare similar to those observed with alkane oils (Figure 2). The increase in oil polarity lowers T, in i-c3oc3 mixtures compared to c4oc4 mixtures; however, w, is lower in c4oc4 than in i-C30C3. In addition, wlc is lower in c4oc4 than in i-C30C3. The phase diagrams of the ether oils C 4 0 C 2 0 C 4and C 3 0 c4oc3 in CI2E6and water are identical; T,, AT, and w, are the same, and wlcis only slightly different (Figure 3). Thus, the phase behavior is governed by the total number of carbon atoms and not the arrangement of carbon atoms between oxygen atoms. Increasing oil polarity (decreasing i) in the C12E6/ CjOC20Cj/watermixtures causes T, to sink and AT to shrink to zero (Figure 4). Mixtures of C30C20C3and C 2 0 C 2 0 C 2are used to achieve the intermediate values of i = 2.5 and 2.1. In samples with i = 2 at T = 7.5 OC, both the upper and lower phases in the 2 region near the one-phase region exhibit strong Tyndall scattering, a result consistent with the presence of a tricritical point between i = 2.1 and 2. As i decreases, the liquid crystalline region retreats to higher surfactant concentration (wlc > 20 wt % surfactant) between i = 4 and 3. Since ordered liquid crystalline phases no longer reside nearby, we suspect that the isotropic solutions in the one-phase region become less ordered as oil polarity increases. (32) Kahlweit, M.; Strey, R.; Firman, P.; Haase, D. Langmuir 1985, 1 , 281.
(33) Kahlweit, M.; Strey, R. J . Phys. Chem. 1987, 91, 1553.
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The Journal of Physical Chemistry, Vol. 93, No. 12, 1989 40
10
P P
-
0
1
I
I
I
2
3
4
5
i Figure 5. Rayleigh ratio (Re)from light scattering of samples at 17 wt % C12E,and T, as a function of oil polarity (i).
-
I
2
3.0 2.0
t!
2 1.0
:0.0
_I
-1.0
-2 0
0.0 20
0
10
5
15
05
1.0
1.5
2.0
2.5
a-I
20
M%C,,E6 Figure 3. Phase behavior of Ct2E6in equal masses of oil and water: (1) oil is CIOC40C3and (2) oil is C,0C20C,.
Figure 6. Small angle X-ray scattering curves on an absolute scale: logarithm of scattering intensity (cm-I) vs scattering vector q for samples at 17 wt % C12E,and T, as a function of oil polarity (i).
TABLE I: SAXS Parameters sample cm-l D,,,, A
40
45 5 252 83.7 23.1 7.7
i = 4 i = 3 i = 2.5 i = 2.1 i = 2
s/u,
290 25 1 208 160 116
A-'
0.0 12 0.016 0.024 0.035 0.054
invariant Q,A-3 0.01 14 0.0113 0.0101 0.0067 0.0058
i-4
C,0C20Ci/water was probed with static light scattering and SAXS. Samples were prepared in the one-phase tail of the C,2E6/CiOC20Ci/waterfish at 17 wt 70CI2E6and at the temperature T,. The Rayleigh ratio (&), which is q-independent, decreases by a factor of 40 with decreasing i in C,OCzOCimixtures (Figure 5). Plots of the SAXS absolute intensity vs q show a sharp maxand moves to higher q as imum that decreases in intensity (Imax) oil polarity ( i ) increases (Figure 6 and Table I). The Bragg spacing. a characteristic size
30
i-3
-
T (OC)20
i 2.5
Dim,, = 2s/qmax
-
was calculated from the scattering peak and decreases as i decreases (Table I). SAXS of samples with a distinct interface between regions of different electron densities follows Porod's law34
i 2.1 10
lim (441) = 4 lim (431) = 2a:(p0 - p,)2
i-2
0
0
(2)
Q--
I
I
5
10
I
15
(3)
Q--
with s / u the specific surface, po - pw the electronic contrast between oil and water, and I and I the desmeared and smeared scattering intensities, respectively. Normalizing by the invariant allows s / u to be obtained
20
Moh C,2E6 Figure 4. Phase behavior of CI2E6in equal masses of C,OCzoCi and water as a function of oil polarity (i). Mixtures of C30C20C3and C20C20C2give the intermediate values of i = 2.5 and 2.1, respectively.
To determine whether microemulsion microstructure exists in samples containing polar oils, the microstructure of CIZE6/
(4)
(34) Kratky, 0.; Glatter, 0. Small Angle X-Ray Scattering Academic: London, 1982.
The Journal of Physical Chemistry, Vol. 93, No. 12, 1989 4859
Microemulsifying Polar Oils
60
40
T ("C)
1
" 0
20
40
.
I
.
I
60
.
A
0
0
100
80
2
20
1
2
wt% 6o
60
40
20
80
100
c,oc20c,
I 60 40
2
T ("C) 1
100
2
20
0 0
60
20
40
60
Figure 8. Miscibility gaps for ethers and water: and (2) C20C20C2/water.
20
80
100
wt% c20c20c2
1
" 0
20
40
60
c20c20c, H2O + C2OC2OC2
80
100
(WtYO)
Figure 7. Phase behavior of CI2E6/C20C20C2/water as a function of temperature and ratio of oil to water for fixed CIZE6concentrations of (1) 7 wt %, (2) 1 1 wt %, and (3) 15 wt %.
where qj, and qj, are the volume fractions of the oil and water domains. As oil polarity increases, the specific surface ( s / u ) increases, and the change of s / u is inversely proportional to the change in the characteristic size D,,, (Table I). The previously discussed phase diagrams were determined from samples containing equal amounts of oil and water. To confirm that the three-phase region disappears in the C , & , / C20C20C2/watersystem, the phase behavior at varying ratios of oil to water and fixed surfactant concentration was examined (Figure 7). No three-phase region was found. However, as the amount of surfactant increases, the two-phase region, which is always skewed to the oil-rich side, narrows and forms a "waist" at 1 1 wt 7' % C&6, with a one-phase channel from the water side to the oil side appearing at 15 wt % C&6. The pure C20CzOC2/watermiscibility gap is skewed to the oil-rich side and is much narrower than the gap for C3@C20C3/water(Figure 8). Since the C20C20C2/water miscibility gap narrows with decreasing temperature, a lower critical point probably exists below 0 OC.
Discussion The results show clearly that increasing the polarity of ether oils drives the phase behavior past a tricritical point and simul-
(1)
C30C20C3/water
taneously reduces the size and ordering of the microstructure. SAXS shows that the mixture containing the most polar oil (CzOC20C2)still retains the distinct interface characteristic of microemulsions; however, the structure is small and diffuse. As oil polarity is decreased in alkane systems, T,, w,, and AT all decrease because the three-phase body shrinks in all dimensions.21 If AT becomes zero, the three-phase body disappears and a tricritical point is passed. (However, it is important to note that the actual tricritical point may not lie in the 50/50 oil/water plane, or even exist at atmospheric pressure.) In mixtures containing surfactant amphiphiles, a liquid crystalline region arises in the tail of the fish.21 The presence of the liquid crystalline region augurs the isotropic solutions nearby contain the microstructure required of microemulsions.13 Substitution of ether oils for alkane oils allows the phase behavior of surfactant/oil/water mixtures to be extended beyond that studied previously. Since ethers are more polar than alkanes, the phase behavior pattern of the fish occurs at lower temperatures. The number of oxygen atoms in the ether influences the oil polarity ranking more than the number of carbon atoms (Figure 1 ) . Also, phase behavior is indifferent to the arrangement of the carbon atoms between oxygens (Figure 3). Since ether oils with two oxygens are somewhat chemically similar to the ethoxylated head group of the alcohol, it is not surprising that 2 systems are favored. Increasing the polarity of the ether oil causes the three-phase region to shrink and vanish, and so, the phase behavior must pass a tricritical point (Figure 4). The phenomenological model of Kahlweit et al. predicts this result.21 Consider the three binary systems oil/water, water/surfactant, and oil/surfactant that form the basis for the ternary surfactant/oil/water phase diagram (see Figure 5 in ref 9). The upper critical point of lower miscibility gap of the oil/surfactant system (cp,) is connected to the lower critical point of the upper miscibility gap of the water/surfactant (cp,) system by a critical line (see Figure 11 in ref 9).3s Decreasing the temperature difference between cp, and cp, will stress the critical line and cause it to break. The break point is the (35) Kahlweit, M.; Strey, R.; Haase, D.J . Phys. Chem. 1985, 89, 163.
4860
The Journal of Physical Chemistry. Vol. 93, No. 12, 1989 2.5
I
/
O.O
t 5
/
O/
10
15
20
25
30
35
Tx Figure 9. Critical scaling of the width of the three-phaseregion AT with distance from the assumed tricritical point.
tricritical point: A three-phase region grows from the tricritical point as the temperature difference between cp, and cp, is decreased (see Figure 13 in ref 9). The opposite trend is observed when increasing the polarity of the oil: Oil and surfactant become more miscible, and cp, sinks to lower temperatures, and thus, the temperature difference between cp, and cp, is increased. Concurrently, the three-phase region in the ternary system also sinks to lower temperatures ( T , decreases), shrinks in width ( A T decreases), and eventually disappears through the tricritical point, healing the critical line as we observe (Figure 4). Theoretically, the same tricritical point could be achieved with an alkane oil, but highly polar alkanes are gases at 0.1 MPa.21 Alkane oils and water are always practically immiscible. However, as ether oils become more polar, the oil/water miscibility gap narrows significantly (Figure 8). The gap for the C20C20C2/watersystem also narrows with decreasing temperature, which suggests that a lower critical point exists below 0 O C (Figure 8). The lower critical point of the oil/water system (cp,,,) must rise along a critical line as it extends into the ternary diagram since surfactant mixes oil and water. Since C P ? / ~is probably at a higher temperature than cp,, perhaps the critical line connecting cp, and cp, in fact passes through cp,/,; in that case, the critical line could rise from cp, to intersect the oil/water plane at cp,/, and then continue up to cp,. On approach to the tricritical point, the width of the three-phase region should follow the prediction of critical scaling36
where Ttcpis the temperature of the tricritical point and B is a scale factor. If the tricritical point for the polar oil mixture is assumed close to 7.5 OC (Figure 4), scaling of AT holds only close to the tricritical point (between i = 2.1 and 2; Figure 9). The exact location of the tricritical point was not determined, and in fact, since the oil/water miscibility gap for the ether oils becomes skewed to the oil-rich side as oil polarity increases, the actual tricritical point probably does not lie in the 50/50 oil/water composition plane. As the three-phase region shrinks, the nearby liquid crystalline region retreats to higher surfactant concentration, suggesting that the neighboring one-phase mixtures become less ordered. Indeed, both light and X-ray scattering confirm that the microstructure becomes small and diffuse as oil polarity is increased. All scattering samples were prepared in the one-phase region in the tail of the fish at 50/50 oil/water by weight and 17 wt % surfactant, (36) Griffiths, R. B. J . Chem. Phys. 1974,60, 195. Creek, J. L.; Knobler, 1981, 74, 3489.
C. M.; Scott, R. L. J . Chem. Phys.
Wormuth and Kaler and thus, any structures are likely to be bicontinu~us.~’ As oil polarity is increased, both absolute static light scattering and SAXS decrease by a factor of approximately 50 (Figures 5 and 6 and Table I). Changes in interactions between oil and water domains probably do not account for such a large change in light scattering, and so the size of oil and water microdomains must decrease. The light scattering is q independent, indicating no contributions from large characteristic lengths and no contributions from critical ~ c a t t e r i n g . ~ ~ The SAXS experiment measures an average distance in a sample; the distances probed in our experiment ranged from 10 ,, may be related to the sum of the to 600 A. The dimension D average oil plus water domain sizes (including two interfaces).13 Indeed, Jahn and Strey have demonstrated that the domain sizes found from freeze-fracture micrographs of carefully prepared samples are close to those found from SAXS.39 All of our SAXS curves show one fairly sharp intensity maximum (Figure 6), which rules out random bicontinuous models of the microstructure@and also rules out ordered periodic models of bicontinuous s t r ~ c t u r e s . ~ ~ Indeed, our data fit well with a model of disordered lamellae,42 which is comparable to a thermodynamic Commonly used models of polydisperse interacting spherical structuresu do not fit our data. As oil polarity increases, the scattering intensity at the peak decreases dramatically and qmaxmoves to higher q (Figure 6). D,,, for the system with the most polar oil is only 116 A and is comparable in size to the length of two fully extended C,2E6 molecules (40 A each) or micellar dimension^.^^ In addition to the scattering maximum, all of our samples show Porod’s law behavior and thus contain an interface. Since the overall composition of the samples by volume remains essentially constant with changing oil polarity (4%= 0.158, 4,, = 0.455, 4w = 0.387) and s / v increases by a factor of 4.5 (Table I), the area per headgroup of the surfactant must also increase by a factor of 4.5 as the oil becomes more polar. Freer motion of the surfactant at the interface or penetration of oil and water at the interface both would increase s/v and make the interface more diffuse and perhaps thicker. (The present SAXS spectra do not allow a quantitative estimate of the interfacial thickness.) The substantial solubility of polar oil in water (18 wt % at T = 7 “ C for C20C20C2;Figure 8) would allow oil to permeate the interface and water domains. As oil polarity increases, all evidence clearly points to a substantial decrease in the size and order of the microstructure in surfactant/polar oil/water mixtures. The polar oil permeates all of the microdomains, expanding the interface. This decrease in microstructure is found as the phase behavior bypasses a tricritical point. Increasing oil polarity beyond that studied here would likely dissolve microemulsion microstructure completely and produce instead a weakly structured solution lacking distinct oil and water domains.
Conclusions This study uncovers a fundamental relationship between phase behavior and microstructure in microemulsions through a systematic investigation of nonionic surfactant/ether oil/water systems. As oil polarity is increased (oil hydrophobicity is decreased), the three-phase region vanishes and thus passes by a tricritical point, the nearby liquid crystalline phase retreats to higher surfactant concentration, and the microstructure decreases in size and order. Increasing oil polarity beyond that studied here (37) Kahlweit, M.; Strey, R.; Haase, D.; Kunieda, H.; Schmeling, T.; Faulhaber, B.; Borkovec, M.; Eicke, H.-F.; Busse, G.;Eggers, F.; Funck, T. H.; Richmann, H.; Magid, L.; Siiderman, 0. Stilbs, P.;Winkler, J.; Dittrich, A,; Jahn, W. J . Colloid Interface Sci. 1987, 118, 436. (38) Cazabat, A. M.; Langevin, D.; Meunier, J.; Pounchelon, A. Adu. Colloid Interface Sci. 1982, 16, 175. (39) Jahn, W.; Strey, R. J . Phys. Chem. 1988, 92, 2294. (40) Talmon, Y.; Prager, S. J . Chem. Phys. 1978, 69, 2984. (41) Scriven, L. E. Nature 1976, 263, 123. (42) Vonk, C. G.;Billman, J. F.; Kaler, E. W. J . Chem. Phys. 1988, 88, 3970. (43) Teubner, M.; Strey, R. J . Chem. Phys. 1987, 87, 3195. (44) Vrij, A. J . Chem. Phys. 1979, 71, 3267. (45) Wilcoxon, J. P.; Kaler, E. W. J . Chem. Phys. 1987, 86, 4684.
J . Phys. Chem. 1989, 93, 4861-4867 will likely cause microstructure to vanish; the resulting solution having no distinct oil and water domains could not be considered a topologically ordered microemulsion.
Acknowledgment. We sincerely thank J. P. Canselier for assistance with the synthesis of C3OC2OC3 and J. F. Billman for assistance in obtaining the SAXS results. We acknowledge
4861
discussions with A. Sporer, M. Kahlweit, and R. Strey. This work was supported by the IBM Corp. Registry No. C20C20C2,629-14-1; I'-c,Oc3, 108-20-3; C40C20C4, 112-48-1;c4oc4, 142-96-1; csocs, 693-65-2; c6oc6, 112-58-3; Cl2E6, 3055-96-7; C4E2, 112-34-5;C4E3, 143-22-6; C3OC4OC3, 91 179-75-8; C30C20C3,18854-56-3; C4E,, 11 1-76-2; toluene, 108-88-3; anisole, 100-66-3;cyclohexane, 110-82-7; octane, 11 1-65-9; decane, 124-18-5.
Hydrocarbon Chain Conformation of Bipolar Surfactants in Micelles. A Magnetic Field Dependent 13C and 14N NMR Spin-Lattice Relaxation and Nuclear Overhauser Effect Study of N ,"-1 ,PO-Eicosanediyibis(triethyiammonium bromide) Tuck C. Wong,*.+vtK. Ikeda,l K. Meguro,O 0. Soderman,f U. OIsson,*and B. Lindmanf Physical Chemistry 1 , Chemical Center, University of Lund, Lund, Sweden, and Department of Applied Chemistry, Institute of Colloid and Interface Science, Science University of Tokyo, Tokyo 162, Japan (Received: July 27, 1988; In Final Form: November 23, 1988)
A field-dependent I3C and I4N spin-lattice relaxation and nuclear Overhauser effect study has been performed on an aqueous micellar system of an a,w-bifunctional surfactant, N,N'- 1,2O-eicosanediylbis(triethylammonium bromide). The I3C relaxation rates and NOESwere analyzed on the basis of the "two-step" model, and the fast correlation time and order parameter for each carbon segment on the hydrocarbon chain and an overall slow correlation time for the whole micelles were obtained. The almost flat order parameter profile for the hydrocarbon chain suggests that the surfactant chains adopt a predominantly stretched form in micelles. This is in contradiction to the conclusion drawn from the more indirect chemical relaxation results for similar systems. The slow correlation time, which primarily describes the time scale for the tumbling of the micelles, is found to be about 2-3 ns and rather concentration independent,indicating rather small micellar aggregates. Several important properties of this micellar system, such as the small micellar size (low aggregation number) and the peculiarly low capacity for solubilizing hydrophobic substances, can be explained by the conformational properties of the surfactant molecules in the micelles.
Introduction There have been a number of studies of the physical properties of a,o-bifunctional (or bolaform) surfactants in solution.'-1° The ionic a,w-type surfactants usually exhibit significantly different properties in several respects from those of "normal" surfactants with a single head group on a single hydrocarbon chain. The salient differences are as follows: First, the propensity of micelle formation of these a,w-type surfactants is generally lower;5*6the size of the micelles formed is relatively small;6s10and the cmc of these surfactants is generally higher than that of single-head surfactants of comparable chain length.4j6 Second, these a,w-type surfactancts have been shown to adopt a folded (or wicketlike) conformation at the air-water interface!,' Third, it was suggested in several studies that these surfactants may also exist predominantly in a folded conformation in However, with regard to the last point, there has been no study that provides direct evidence on the conformation of these surfactants in micellar solutions. There have been several studies of the aliphatic chain conformation of several bifunctional surfactants in liquid crystalline phases."-14 The knowledge of the principal conformation these surfactants adopt in micelles is very important because it may provide fundamental explanations for some important properties of these micelles, most notably, the small micellar size, the phase behavior, and the extremely low capacity for solubilizing hydrophobic s ~ b s t a n c e s . ~ ~ It has been demonstrated16 that, for isotropic solutions containing molecular aggregates where there is no static dipolar or quadrupolar interaction, analysis of multifield N M R relaxation rates and nuclear Overhauser effect (NOE) provides direct in-
'
On leave from the Department of Chemistry, University of Missouri, Columbia, MO 6521 1. *University of Lund. Science University of Tokyo.
formation on the conformation of the surfactant molecules via the determination of the order parameters of the various segments of the molecules. We have, therefore, undertaken a multifield I3C spin-lattice relaxation rate and N O E study of the aqueous micellar solutions formed by a cationic a,w-type surfactant, N,N'- 1,20-eicosanediylbis(triethylammonium bromide) (Cz0(NEt3)*Br2,hereafter referred to as C&t6), the cmc, self-diffusion, and the phase diagram of which have recently been investigated in this 1aborat0ry.I~I4N spin-lattice and spin-spin relaxation rates (1) Elworthy, P. H. J . Pharm. Pharmacol. 1959, 11, 557. (2) Elworthy, P. H. J . Pharm. Pharmacol. 1959,11, 624. (3) Ueno, M.; Hikota, T.; Mitama, T.; Meguro, K.J. Am. Oil Chem. Soc. 1972,49,250. Ueno, M.; Yamamoto, S.; Meguro, K.J. Am. Oil Chem. Soc. 1974, 51, 373. (4) Menger, F. M.; Wrenn, S. J. Phys. Chem. 1974, 78, 1387. ( 5 ) Johnson, J. R.; Fleming, R. J . Phys. Chem. 1975, 79, 2327. (6) (a) Yiv, S.; Kale, K.M.; Lang, J.; Zana, R. J . Phys. Chem. 1976, 80, 2651. (b) Yiv, S.; Zana, R. J. Colloid Interface Sci. 1980, 77,449. (c) Zana, R.; Yiv, S.; Kale, K. M. J . Colloid Interface Sci. 1980, 77, 456. (7) Meguro, K.; Ikeda, K.;Otsuji, A.; Taya, M.; Yasuda, M.; Esumi, K. J . Colloid Interface Sci. 1987, 118, 372. (8) Zana, R.; Muto, Y.; Esumi, K.;Meguro, K.J . Colloid Interface Sci.
1988, 123, 502. (9) Cipiciani, A,; Fracassini, M. C.; Germani, R.; Savelli, G.;Bunton, C. A. J . Chem. SOC.,Perkin Trans. 2 1987, 547. (10) McKenzie, D. C.; Bunton, C. A,; Nicoli, D. F.; Savelli, G. J . Phys. Chem. 1987, 91, 5709. (11) Gallot, B. R. Mol. Cryst. Liq. Cryst. 1971, 13, 323. (12) Seelig, J.; Limacher, H.; Bader, P. J. Am. Chem. SOC.1972, 94,6364. (13) Forrest, B. J.; de Carvalho, L. H.; Reeves, L. W.; Rodger, C. J. Am. Chem. SOC.1981, 103, 245. (14) Gutman, H.; Luz, Z.; Charvolin, J.; Loewenstein, A. Liq. 1987, - Cryst. . 2, 739. (15) Ikeda, K.; Khan, A,; Meguro, K.; Lindman, B., to be submitted for
publication.
(16) See, for example: Lindman, B.; Saerman, 0.;Wennerstrom, H. In Surfactant Solutions, New Methods of Itwestigation; Zana, R., Ed.; Dekker: New York. 1987.
0022-365418912093-4861$01.50/0 0 1989 American Chemical Societv
