Langmuir 1995,11, 4728-4734
4728
Phase Behavior and Microstructure of Polymerizable Microemulsions K. M. Lusvardi, K.-V. Schubert, and E. W. Kaler* Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received July 5, 1995. I n Final Form: September 14, 1995@ The phase behavior of polymerizable alkyl methacrylates (C&fA, k = 1,4,and 6)in aqueous solutions containing mixtures of the cationic surfactants dodecyltrimethylammonium bromide (DTAB) and didodecyldimethylammonium bromide (DDAB) is systematically studied. The added degree of freedom afforded by mixing the surfactants allows location of a region in the water-rich corner of the phase diagram where a homogeneous microemulsionphase forms for three alkyl methacrylates of differinghydrophobicity. Microstructural studies using small angle neutron scattering (SANS) demonstrate that oil-in-water microemulsions form and show that the shape of the aggregates depends on the DTAB:DDAB ratio and the oil content.
Introduction Mixtures of surfactants with water and oil display complex phase behavior. The phase behavior of nonionic surfactants (C), in ternary mixtures with water (A) and oil (B) is by now ~ell-known.l-~ The phase rule requires that at constant pressure a ternary system has three independent variables. As shown by Kahlweit, the three most convenient variables for study of phase behavior are the weight percentage of the oil, a = B/(A B), in the mixture of oil and water, the weight percentage of surfactant in the total mixture, y = C/(A B C), and the temperature, T. The phase behavior of nonionic surfactants in water depends strongly on temperature, and consequently with nonionic surfactants it is possible to observe rich phase behavior in water/oillsurfactant mixtures over the entire range of surfactant concentrations ( y ) as a function of temperature. This phase behavior is most easily discussed when it is represented as an upright phase prism with T as the ordinate and the Gibbs triangle A-B-C as the base. Erecting a vertical section through the phase prism at constant water-to-oil ratio ( a= 50 wt %) results in a two-dimensional phase diagram in the shape of a “ f i ~ h ” .The ~ “fish tail” describes a one-phase homogeneous microemulsion region and the “fish body” represents a three-phase region (where a microemulsion phase is in equilibrium with both an excess water and an excess oil phase). The extent of this “fish”and its location in temperature provides key information about the particular system of water, oil, and surfactant. However, if the nonionic surfactant is replaced with an ionic surfactant, the phase behavior is almost insensitive to temperature. Such phase behavior is then limited to only two variables, a and y , and can be represented on just a Gibbs triangle. To increase the usefulness of ionic surfactants, they must be mixed with other amphiphiles or surfactants to give an additional degree of freedom in phase space.
+
+ +
* To whom correspondence should be addressed: FAX, (302)8314466; email,
[email protected]. Abstract published in Advance A C S Abstracts, November 1, 1995. (1)Kahlweit, M.; Strey, R.; Busse, G. Phys. Rev. E 1993,47,4197. (2) Shinoda, K.;Saito, H. J. Colloid Interface Sci. 1968,26, 70. (3) Shinoda, K.;Friberg, S. A d a Colloid Interface Sci. 1976,4,281. (4)Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1985,24, 654. (5) Kahlweit, M.; Strey, R.; Schomacker, R. In Reactions i n Compartmentalized Liquids; Knoche, W., Schomacker, R., Eds.; SpringerVerlag: Berlin, 1989. @
0743-7463/95/2411-4728$09.00/0
Microemulsions have a range of applications, but one of the more interesting is their use to produce nanosized latex particles.‘j Here, we investigate the phase behavior of polymerizable alkyl methacrylates [H(CHdkOC(O)C(CHz)CHB, denoted as CkMA] in aqueous solutions containing mixtures of dodecyltrimethylammonium bromide and didodecyldimethylammonium bromide (DTAB and DDAB) at 60 “C. Microemulsion regions in ternary mixtures of water, polymerizable oil, and single-tailed surfactant DTAB were observed on the water-rich side of the phase diagram.7*sHowever, when DTAB is replaced with its double-tailed analogue DDAB, a microemulsion region on the oil-rich side of the phase diagram forms for a variety of a l k a n e ~ . ~ -Inl ~addition, mixing DTAB and DDAB in a 3:l ratio with water and methyl methacrylate leads to a microemulsion phase that extends from the water-rich to the oil-rich side of the diagram.16 In this study, we systematically map out the phase behavior of three alkyl methacrylates using the DTAB: DDAB (C and D) ratio as the necessary third degree of freedom. In analogy with the phase prism known for systems containing nonionic surfactants,l E = D/(C D) is now the ordinate ofthe phase prism and Gibbs triangles (A-B-C or A-B-D) are the base and top (see Figure la). Phase behavior may be studied conveniently either with increasing overall surfactant concentration (y ) as function of6 at constant oil-to-water ratio (a)(plane 1in Figure la) or with increasing a as function of c at constant y (plane 2 in Figure la).
+
(6) Candau, F.In Polymerization in Organized Media; Paleos, C. M., Ed.; Gordon and Breach Science Publishers: Philadelphia, PA, 1992; p 215. (7) PBrez-Luna, V. H.; Puig, J. E.; Castano, V. M.; Rodriguez, B. E.; Murthy, A. K.; Kaler, E. W. Langmuir 1990,6,1040. (8)Rodriguez-Guadarrama, L. A.;Mendizabal, E.; Puig, J. E.; Kaler, E. W. J. Appl. Polym. Sci. 1993,48,775. (9)Chen, S.J.; Evans, D. F.; Ninham, B. W. J. Phys. Chem. 1984, 88, 1631. (10)Blum, F.D.; Pickup, S.; Ninham, B.; Chen, S. J.; Evans, D. F. J. Phys. Chem. 1986,89,711. (11)Fontell, K.;Ceglie, A.; Lindman, B.; Ninham, B. Acta Chem. Scand. A 1986,40,247. (12) Evans, D. F.;Mitchell, D. J.; Ninham, B. W. J. Phys. Chem. 1986,90,2817. (13) Zemb, T.N.; Hyde, S. T.; Derian, P.; Barnes, I. S.; Ninham, B. W. J. Phys. Chem. 1987,91, 3814. (14)Barnes, I. S.; Hyde, S. T.; Ninham, B. W.; Derian, P.; Drifford, M.; Zemb, T. N. J. Phys. Chem. 1988,92,2286. (15)Strom, P.; Anderson, D. M. Langmuir 1992,8, 691. (16)BlBger, F.;Murthy, A. K.; Pla, F.; Kaler, E. W. Macromolecules 1994,27,2559.
0 1995 American Chemical Society
Langmuir, Vol. 11,No. 12, 1995 4729
Phase Behavior of Microemulsions
a
b
D
riane
I
- Plane 2
A
oil
B
I 1 I 0 /
t
4 I \
E
\
0
r+
A
-a
B
H20
oil
-a
Figure 1. (a) The phase prism for a quaternary system at constant temperature with E, a concentration variable, as the ordinate. Plane 1and plane 2 represent vertical sections through the prism at constant a and constant y , respectively. (b) A schematic phase prism that shows the progression of the one-phase microemulsion region as a function of mixed-surfactant composition, E.
Figure l b is a schematic of the phase prism based on the known limits.7,8J0-12J5J6In systems with DTAB as the surfactant ( E = 0, bottom of Figure lb), a one-phase region exists on the water-rich side and extends to fairly low oil concentrations. The triangle on top of Figure l b is a schematic of the same system where DDAB replaces DTAB ( E = 100). Here, the one-phase region occurs on the oil-rich side of the triangle at relativelylow surfactant and water concentrations. Therefore one might expect that at some E and y, these one-phase regions connect and a one-phase channel extends from the water-rich side of the phase diagram to the oil-rich side (Figure l b center). To determine the extent of the one-phase region and its location in E and y, a vertical section through the phase prism at a = 50%(equal amounts of oil and water) can be made. This two-dimensional phase map provides useful information about how to make a microemulsion in a particular system of water, polymerizable oil, and surfactant. The polymerizable oils investigated, methyl methacrylate (CIMA), n-butyl methacrylate (C4MA), and n-hexyl methacrylate (CSM..A), exhibit a wide range of water solubilities. ClMA, a fairly hydrophilic oil, is soluble up to 1.5 wt % in water, while c6MA is virtually insoluble in water.17 Additional insight into the phase behavior of these multicomponent mixtures can be had by erecting a vertical section (plane 2) through the phase prism perpendicular to plane 1. On this plane there is a narrow channel of macroscopically homogeneous solutions that extends from the water-rich to the oil-rich side of the phase diagram. Following this "one-phasechannel"allows study of these solutions close to the body of heterogeneous phases as compositions change by varying a and E at a constant surfactant concentration y. To investigatequantitatively the evolution of microstructure as a fixnction of composition, we performed small-angleneutron scattering (SANS) experiments in the homogeneous microemulsion region at relatively low oil concentrations and at different DTAB: DDAB ratios for all three alkyl methacrylates. This allows (17) Vinyl and Diene Monomers, High Polymers Series; WileyInterscience: New York, 1970; Vol. XXN.
us to compare how the hydrophobicity of the oil, the oil content, and the DTAB:DDAB ratio effect the microstructure.
Experimental Section DTAB and DDAB (from TCI America with 98%purity) were recrystallizedthree times from acetone containing a small amount of anhydrous ethanol and dried in a vacuum oven at T = 35 "C for 24 h. Methyl methacrylate (ClMA),butyl methacrylate ((24MA), and hexyl methacrylate (C6M.A)(all from ScientificPolymer Products, 99%) were used as received. The inhibitor (hydroquinone methyl ester) was not removed from the oils before sample preparation to avoid oligomer formation, since small amounts of oligomers are known to reduce the extent of the onephase region.l* Deuterium oxide (CambridgeIsotope Laboratory, 98%deuterated)was used as received. Water was twice distilled and deionized. Phase behavior studies were carried out as previously described.19 Phase diagrams were determined at T = 60.0 f 0.1 "C and the accuracy of the phase boundaries shown for a, E, and y are within f0.4 wt %. Here, in terms of the masses mi,a = moil/(moil+ mwater), E = ~ D D A B ~ D D A +B~ D T A B ) and , y =~ D D A B ~DTA$(~DDA ~ DBT A B moil mwater). The presence of lamellar liquid crystallke phases was determined by observing the sample between crossed polarizers. For some samples, phase separation times were long (days), and so the nature of the coexisting phases was not established. These areas are presented as hatched or shaded regions in the phase diagrams. The samples made for SANS studies contained D20 instead of HzO, and this substitution caused slight shifts in phase boundaries. Nonetheless, the samples investigated were homogeneous before and after each scattering experiment. Small-angle scattering experiments were performed on the NG-7 spectrometer at the Cold Neutron Research Facility of the National Institute of Standards and Technology (NIST) in Gaithersburg, MD. The neutron wavelength was A = 5 with MA = 0.15, and the magnitude of the scattering vector q ranged from 0.02 to 0.22 (q = 4n/A sin(8/2),where 8 is the scattering angle). The samples were held in quartz cells with 2 mm path lengths and maintained at 60.0 f 0.1 "C. The scattering spectra were corrected for background, detector sensitivity, solvent and empty cell scattering, and sample transmission. The spectra
+
+,
+
+
A
(18) Gan, L. M.; Chew, C. H.; Friberg, S. E. J. Macromol. Sci. Chem. 1983, A19,739.
(19) Schubert, K.-V.; Strey, R. J. Chem. Phys 1991,95,8532.
Lusvardi et al.
4730 Langmuir, Vol. 11, No. 12, 1995 were then radially averaged and placed on an absolute scale using standards provided by the neutron facility. The error associated with each intensity data point reflects the counting statistics of the 2-D detector.
Theory The observed scattered intensity from a dispersion of interacting particles is given by20-22 I ( q ) = np(lF(q)lz)+ nPl(F(q))l2[S(q)11+ B
(1)
where F ( q )is the single particle amplitude factor, S(q)is the interparticle structure factor, and npis the particle number density. The structure factor depends only on the potential of interaction and reflects the spatial arrangement of the micelles. The averaged square and the squared average of F ( q ) reflect micelle size, shape, and the intermicellar distribution of scattering centers. B is a constant term that represents the residual incoherent scattering, which is mainly due to the hydrogen present in the hydrogenated oils and surfactants in the sample. For a solution of monodisperse spheres, (IF(q)I2)l(F(q))I2 is identical with zero, thus eq 1 simplifies to
(2) For asymmetrical or polydisperse particles, an approximate expression can be derived assuming that there is no correlation of particle orientation or size with position.20B21 This “decoupling approximation” allows the application of the structure factor expressions for spherical particles to describe dispersions of polydisperse or elongated particles. The particles are redefined as equivalent spheres for the calculation ofS(q) and eq 1can be rewritten in a form similar to that of eq 2 as
where
and
Several particle geometries were tested in calculating model spectra, but only a suspension of prolate ellipsoidal particles produces model spectra that match those measured. A core-and-shell geometry was used to calculate the model intensity. The hydrocarbon core is assumed to contain the surfactant tails and solubilized oil while the surrounding shell contains the surfactant head group ions and associated counterions, as well as the water molecules that hydrate these ions (1 and 4, respectively).20 The solvent contains the remaining water, any oil that is soluble in the aqueous phase,17 dissociated counterions, and unaggregated surfactant molecules. We have previously 23 calculated the concentrations of aggregated and unaggregated surfactant molecules and here assume that the addition of oil does not affect these values. (20)Hayter, J . B.; Penfold, J. Colloid Polym. Sci. 1983,261,1022. (21)Kotlarchyk, M.;Chen, S.-H. J. Chem. Phys 1983,79, 2461. (22)Hayter, J. B. In Physics of Amphiphiles: Micelles, Vesicles, Microemulsions; Degiorgio, V., Corti, M., Eds.; North-Holland: New York, 1985. (23)Lusvardi, K. M.; Full, A. P.; Kaler, E. W. Langmuir 1995,11, 487.
The prolate ellipsoidal droplet is described by the radius ofthe major axis, Rt,maj,the radius of the minor axis, Rt,min, and the fraction of associated counterions, 6. These three parameters along with the incoherent scattering background, B, are adjusted to determine the model intensity. The first three parameters, together with molecular volume^,^^^^^ completely specify the core-and-shell geometry of the elliptical microemulsion droplet. The known volume fraction of the dispersed phase, 4, and the microemulsion droplet dimensions determine the number density of microemulsion droplets, np= 3@/4~~Rt,min~Rt,maj. The surfactant aggregation number, Nagg,and the oil aggregation number, Nail, are computed from mass balances assuming that aggregated surfactant and oil molecules are distributed equally among microemulsion droplets (i.e., the microemulsion droplets have identical compositions). Once the micelle dimensions have been specified, the scattering amplitude, F(q),for an ellipsoidal particle with a core-and-shell distribution of scattering length densities and the structure factor, S(q), for the ionic mixedsurfactant microemulsion droplets are calculated as previously described.23
Results Phase Behavior. Shown in Figure 2 are vertical sections through the phase prism for which the oil-towater ratio (a)is held constant at 50 wt %. The phase behavior is presented in terms of increasing overall surfactant concentration ( y ) as a function of the DDAB fraction in the surfactant mixture ( E ) . Parts a-c of Figure 2 show the progression for the oils ClMA, C4MA, and CgMA, respectively. The shape of these phase diagrams is similar to the “tailregion” of the “fish diagram” observed in water, oil, nonionic surfactant system^,^ but there is no three-phase region (“fish body”) in these mixtures. For CIMA, a large homogeneous microemulsion phase dominates the phase diagram at overall surfactant concentrations ( y ) greater than 10wt % (Figure 2a). Below the one-phase region and at low surfactant concentrations, a water-rich microemulsion phase forms in equilibrium with an excess oil phase (denoted as 2). A gradual transition from 2to 2 (an oil-rich microemulsion phase in equilibrium with an excess water phase) occurs at low y with increasing E , and phase separation occurs rapidly. This type of phase transition can be observed in nonionic ~ ~ ! ~ ~with comsystems past a tricritical p ~ i n t . Samples positions above those in the homogeneous microemulsion phase contain a precipitate and an isotropic phase. No lamellar liquid crystalline phases were observed. When CIMA is replaced by the more hydrophobic oil, C4MA,the phase diagram becomes more complex (Figure 2b). There is alamellar liquid crystalline phase (La),which extends to fairly low surfactant concentrations, embedded in the homogeneous microemulsion phase. The La region on the diagram includes both the pure Laphase and the coexistence regions where this phase is in equilibrium with an excess phase. As for C1MA, there is a 2 region at low y and E . However at high E , there is no transition to 2, but instead, stable opaque emulsions form. Little or no phase separation of these emulsions was observed over a period of days. At compositionsabove the lamellar phase and above the upper one-phase boundary, there is a multiphase area containing a precipitate along with a (24)Berr, S.S.;Coleman, M. J.; Jones, R. R. M.; Johnson, J. S., Jr. J. Phys. Chem. 1986,90,6492. (25)Wormuth, K. R.;Kaler, E. W. J. Phys. Chem. 1989,93,4855. Strey, R.; Aratono, M.; Busse, G.; Jen, J.;Schubert, (26)Kahlweit, M.; K.-V. J. Chem. Phys 1991,95,2842.
Langmuir, Vol. 11,No. 12, 1995 4731
Phase Behavior of Microemulsions H20 - CkMA - DTAB - DDAB T = 60" C, a = 50 wt %
a
H2O - CkMA - DTAB - DDAB
a
T=60"C,y= 12wt%
80 emulsion
ClMA
b
b
0
.
80-
.--
,
'
,
'
I
.
,
.
- -- -
C 80
-
60
70-
n
60 -
v
a0
h
a0
z w
40
w
v
50 -
40m , .
.-
0
20
10
30
y(wt %)
0
10
20
30
40
50
a (wt %)
Figure2. Vertical sectionsthrough the phase prisms ofwater, polymerizable oils, and surfactant mixtures at a = 50 wt % showing the progression of phase behavior with increasing hydrophobicity of oils (a) CIMA, (b) C4MA, and (c) c6MA. The dashed line at y = 12 wt % indicates the vertical section of constant surfactant concentration shown in Figure 3.
Figure 3. Vertical sectionsthrough the phase prisms ofwater, polymerizable oils, and surfactant mixtures at y = 12 wt % showing the progression of phase behavior with increasing hydrophobicity of oils (a) ClMA, (b) C4MA, and (c) c6MA. The crosses represent compositions of the samples for SANS measurements.
microemulsion phase and an excess phase, as shown by the dashes in Figure 2b. For c6M.A (Figure 2c), the embedded lamellar liquid crystalline phase grows even larger and the multiphase precipitate regions connect. These characteristics completely dominate the phase diagram in this area and the one-phase region retracts to higher y. Again, at high E , a turbid, stable emulsion forms. Additional insight into the phase behavior of multicomponent mixtures is provided by erecting a vertical section through the phase prism perpendicular to the ( E , y ) plane, i.e., plane 2 in Figure la. Figure 3 shows the phase diagrams for all three systems in the same sequence as in Figure 2. The overall surfactant concentration, y , is set at 12wt % to intersect the one-phase microemulsion region for all three oils. A one-phase homogeneous "channel" spans the phase diagram from a = 0% to 50%for all three oils. For thelonger chain oils, the channel narrows as the L a phase continues to high oil ratios and the 2 region extends to higher E and lower a. The L a phase intersects the onephase region for C4M.A and c6MA (Figure 21, and thereby produces a small one phase "wedge" that extends to a = 37 wt % (for CJIA) and a = 41 wt % (for C6M.A) at E values around 70 wt % (Figure 3b and 3c). The upper boundary marks an emulsion, or multiphase region, where phase separation occurs extremely slowly and phase identification is difficult. The shaded region, at low oil concentrations, describes samples that are slightlyturbid and, over
time, separate to a clear phase with foam on top. As the DDAB fraction ( E ) increases, this region gives way to clear, viscous birefringent gels that are solidlike at low oil content. In this case the upper boundary (dashed line) cannot be determined because the solution becomes too viscous to stir. The lower boundary for all three systems marks an o/w microemulsion phase in equilibrium with an excess oil phase (2). Microstructure. The phase diagrams show that there is a region at fairly low oil concentrations in which all three oils form one-phase microemulsions for the same values of a,y, and E (Figure 4). The size and shape of the unpolymerized microemulsion droplets in this region were determined using SANS. For all samples, T = 60 "C, and the overall surfactant concentration is held constant at 12%,while both the DTAB:DDAB ratio and oil concentration are varied systematically. The SANS spectra are presented on absolute scale and the solid lines are model fits. Figure 5 is an example of the scattering curves from samples in which a (oil concentration)is varied at constant E (DDAB fraction). With increasing a,the position of maximum intensity shifts to lower q concurrently with an increase in the maximum intensity. Modeling the spectra as prolate ellipsoids suggests (Table 1)that the microemulsion droplets grow with increasing oil concentration. For C&lA and CJW, the droplets swell uniformly as both&- andRt,,aj increase (i.e., the axial ratio remains relatively constant). However, CJMA droplets grow only
4732 Langmuir, Vol. 11,No. 12, 1995
Lusvardi et al.
Table 1. Parametersfrom SANS Spectra Analysisa compositionb
fitted parameters
calculated parameters
oil
(wt%)
(wt%)
?r
6
B
ClMA ClMA ClMA
3 5 7
30 30 30
42 49 57
22 22 22
0.81 0.81 0.81
0.41 0.30 0.25
A 1.9 2.3 2.6
c 4 M A c 4 M A c 4 M A
5 7 9
25 25 25
50 56 62
28 32 36
0.79 0.80 0.82
0.38 0.44 0.42
1.8 1.8 1.7
CSMA
3 5 7
30 30 30
50 54 61
27 31 35
0.83 0.83 0.83
0.43 0.47 0.52
1.9 1.8 1.8
CSMA CSMA
a
E
nP (X
Nagg
Noil
NagglNoil
x
111 114 118
48 105 170
2.31 1.09 0.69
4.6 4.8 5.1
1.2 0.9 0.7
185 241 307
159 291 480
1.16 0.82 0.64
3.6 5.0 6.0
1.3 1.0 0.8
177 233 309
79 173 300
2.24 1.35 1.03
1.9 3.0 4.3
10-6A-3) 2.0 2.0 1.9
a Modeling results assuming a core-and-shell scattering length density profile and prolate ellipsoidal microemulsion droplets. T = 60 “C. a = moid(moi1 + mwater). = ~ D D A B / ( ~ D D A +B~ D T A B ) . &,ma is the shell radius of the major axis. Rt,min is the shell radius of the minor axis. 6 is the fraction of associated counterions. B is the background intensity. A is the axial ratio. npis the number density of microemulsion droplets. N a g g is the number of surfactant molecules per microemulsion droplet. Nail is the number of oil molecules per microemulsion droplet. x = (CNpb(1mod - Iexdd7)2/(Npts - N p a r + = ( ~ D D A B+ ~ D T A B M ~ D D +A~ DB T A B+ moil + mwater) = E%.
H20 - CkMA - DTAB - DDAB
20
T=6O0C,y=12wt%
f+&E
0.00
0
10
0.10
T=60°C y = 12 wt %
0.15
0.20
q (I/h
20
a (wt %)
Figure 4. The shaded region shows the area in which all three alkyl methacrylates overlap in a homogeneous microemulsion region. The crosses represent compositions of the samples in which SANS experiments for the three oils are compared a t the same position in the phase diagram. T=60°C ~=30wt%
60 n
E
Y v H
.-5 6
0.05
= 35%
40
v)
2
20
Figure 6. SANS spectra of ClMA microemulsions showing the effect of increasing DDAB concentration in the surfactant mixture ( E ) . Symbols represent the measured intensity and the lines are the model intensity.
increase in E therefore causes a decrease in the droplet number density (n,) since the overall surfactant concentration is constant. A comparison of the spectra from the microemulsions containing different oils at the same position in the phase diagram is shown in Figure 7. Data fitting shows (Table 3) that the ClMA droplets are the smallest and most ellipsoidal. In addition, as the hydrophobicity of the oil increases, the microemulsion droplets grow and become less elongated.
Discussion 0 0.00
0.05
0.10
0.15
0.20
q t1/&
Figure 5. SANS spectra of CSMA microemulsions showing the effect of increasing oil concentration(a). Symbolsrepresent the measured intensity, and the lines are the model intensity.
ia the R t , m a j direction and thus become more ellipsoidal with increasing oil concentration. Figure 6 shows a set of spectra from samples at fked a as a function of E . With increasing E , again q m m shifts to smaller q and the maximum intensity increases. For the three different oils, the modeling suggests (Table 2) that the droplets grow more elongated and show increased counterion association (6)with higher DDAB fractions in the surfactant mixture. Both the surfactant and oil aggregation numbers (Naggand Nail) increase, so an
Phase Behavior. Microemulsions containing equal amounts of water and oil form at fairly low overall surfactant concentrations ( y ) over a large range of DTAB: DDAB ratios ( E ) for the three different oils at T = 60 “C (Figure 2). However, the phase diagrams become more complicated as lamellar mesophases (La) that are embedded in the homogeneous microemulsion phase appear along with emulsion and multiphase regions as the chain length of the oil increases. Additionally, the “tail”moves to higher DDAB concentrations ( E ) with increasing oil chain length. For example, at y = 12%, the one-phase boundary is crossed at E = 40%, 48%, and 55%for CJWA, C4MA, and CSMA, respectively. This suggests that larger fractions of the more lipophilic surfactant, DDAB, are required to solubilize more hydrophobic oils. DDAB, which
Phase Behavior of Microemulsions
Langmuir, Vol. 11, No. 12, 1995 4733 Table 2. Parameters from SANS Spectra Analysig
compositionb a (wt%)
oil
‘(r
n;,
a-3)
B
A
Nagg
No11
NagglNoll
x
1.36 3.30 3.38
2.1 2.3 2.4
2.2 2.0 1.7
102 114 128
94 105 123
1.09 1.09 1.04
3.8 .. 5.1 6.0
1.0 0.9
263 291 323
0.85 0.83 0.82
4.8 5.0 5.0
150 158 173
1.39 1.39 1.35
3.0 3.0 3.0
44 49 54
22 22 22
20 25 30
50 56 59
32 32 33
0.78 0.80 0.82
1.41 1.44 1.41
1.6 1.7 1.8
0.8
223 241 264
20 25 30
48 51 54
31 31 31
0.79 0.80 0.83
1.53 1.44 1.47
1.6 1.7 1.8
1.1 1.0 1.0
208 219 233
ClMA ClMA
5 5 5
c 4 M A c 4 M A c 4 M A
7 7 7
CSMA CSMA CSMA
5 5 5
a*b
?g
8 0.79 0.81 0.84
(wi%) 25 30 35
ClMA
calculated parameters
fitted parameters (X
Refer to Table 1 for definition of parameters. --
I
0.00
T=60°C
0.05
0.10
0.15
I
0.20
9 (l/A)
Figure 7. SANS spectraof microemulsions at the same position in the phase diagram showing the effectof the hydrophobicity of the oil. Symbols represent the measured intensity and the
lines are the model intensity.
is orders of magnitude more efficient than DTAB,27 increases the surfactant mixture efficiency as E increases. In mixtures of nonionic surfactants, oil, and water 28 and mixtures of single and double-tailed anionic surfactants with oil and brine 29 lamellar phases appear in the onephase “tail”region and grow larger only as the efficiency of the surfactant increases. In this case, the observation of more complex phase diagrams with increasing oil chain length is a consequence of the increased efficiency of the surfactant mixture needed to microemulsify the oil. Ternary phase diagrams of pure DDAB, alkanes or styrene, and water11J2J5contain lamellar, emulsion, and complicated multiphase areas as well. In addition, at low oil concentrations in DDAB, water, and dodecane mixtures,” solidlike gels form comparable to those found in the L, region at low a (oil) and high E (DDAB fractions) in Figure 3. Microstructure. Pure DTAB micelles in water are nearly spherical, but they grow and become increasingly ellipsoidal as DDAB is added.23 However, adding oil to mixed DDAB-DTAB micelles releases the constraint that the surfactant tails must meet at the center ofthe micelle.30 Therefore, model SANS spectra corresponding to several particle geometries were compared to the experimental spectra. In contrast to the case of oil-in-water microemulsions ofonly DTAB, water, and styrene, which contain monodisperse spherical droplets,31only prolate ellipsoids (27) Weers, J. G.; Scheuing, D. R. J . Colloid Interface Sci. 1991,145, 563. (281 Kahlweit, M.;Strey, R.; Firman, P. J. Phys. Chem. 1986, 90, 671. (29)Kahlweit, M.J . Phys. Chem. 1996,99, 1281. (30)Nagarajan, R.; Ruckenstein, E. Langmuir 1991, 7,2934. (31)Full, A. P.;Kaler, E. W. Langmuir 1994, 10,2929.
with a core-and-shell scattering length density profile captured the shape of the experimental scattering curves reported here. A model of polydisperse spheres was tested by averaging the scattering amplitude for monodisperse spheres over a Schultz distribution of sizes, but the fit of this model to the data at both low q and for q > 0.10 A-1 was poor. A model of monodisperse oblate ellipsoids was tested and also gave poor fits to the data. We did not attempt to model the microstructure as bicontinuous rather than discrete droplets because the oil volume fractions we investigated are very low, the DDAB concentration in the surfactant mixture is small (the effective surfactant packing parameter ranges from 0.40 to O.4Ei2’), and the microemulsion viscosities are low. In addition, the scattering spectra closely resemble those of mixed DTAB-DDAB micellar solutions.23 As a first iteration in the fitting procedure, the oil solubility in the aqueous phase was allowed to vary, giving a fifth adjustable parameter. However, the optimum fitted value of this parameter was equal to the solubility of the oil in water>7for all three oils. Thus, this value was fixed and the experimental curves were fit with only four adjustable parameters. The model fits begin to deviate from the experimental data at low values of q for samples with large oil concentrations and high DDAB fractions. At these compositions, the center-to-center distance between the droplets begins to approach twice their semimajor axes. Clearly the “decoupling” approximation, which assumes that there is no correlation between orientation and position, is inappropriate when the rotation of ellipsoidal droplets is hindered by the proximity of other droplets. At present there is no model available to describe the scattering spectrum from a dispersion of highly anisometric interacting particles. Combining phase behavior results with SANS modeling allows some rationalization of macroscopic observations in light of the microstructural trends. For example, as the DDAB concentration in the microemulsion droplets increases, counterion condensation increases. This would decrease the effective area of each surfactant headgroup, thereby reducing the curvature of the surfactant film and yielding larger and more ellipsoidal droplets (Table 2).As E continues to increase, the microemulsion gives way to a lamellar phase (Figure 3), suggesting that the curvature of the droplets gradually decreases until eventually flat, infinite bilayers are more favorable. In addition, as the oil concentration increases (at relatively low and constant E ) the phase boundary from 1 to 2 is approached (Figure 3). SANS results (Table 1) suggest that the droplets grow larger and the mole ratio of surfactant to oil (Nag&Voil) decreases. Once a limiting ratio is reached (for a constant DTAB:DDAB ratio), the droplets can no longer accom-
Lusvardi et al.
4734 Langmuir, Vol. 11, No. 12, 1995 Table 3. Parameters from SANS Spectra Analysie compositionb
fitted parameters
a
E
oil
(wt%)
(wt%)
(A)
ClMA
5 5 5
25 25 25
7 7 7
30 30 30
c 4 M A
CSMA
ClMA c 4 M n
CSMA a,b Refer
Rt,maj
calculated parameters nP
Rt,min
(A)
6
B
A
Nagg
Noil
NaggJNo~
x
44 50 51
22 28 31
0.79 0.79 0.80
0.36 0.38 0.44
2.1 1.8 1.7
2.2 1.2 1.0
102 185 219
94 159 158
1.09 1.16 1.39
3.8 3.6 3.0
57 59 61
22 33 35
0.81 0.82 0.83
0.25 0.41 0.52
2.6 1.8 1.8
1.9
0.8 0.8
118 264 309
170 323 300
0.69 0.82 1.03
5.1 5.0 4.3
(X
10-6A-3)
to Table 1 for definition of parameters.
modate more oil, and the microemulsion exists in equilibrium with an excess oil phase (2). For comparison, recall that the structure of microemulsions made with a nonionic surfactant changes gradually from a dispersion of isolated, highly curved oil droplets in water at low a to a water and oil bicontinuous spongelike structure (zero curvature) at a = 50 wt %, with an appropriate increase in t e m p e r a t ~ r e .Our ~ SANS experiments suggest that the droplet curvature decreases with increasing DDAB fractions ( E , which is analogous to temperature in nonionic systems) and droplet size increases with increasing oil concentrations (a). This delicate interplay between oil content and DDAB fraction allows an analogous continuous transition from discrete oil droplets to a bicontinuous structure of oil and water along the homogeneous “channel” (Figure 3). The scattering from samples in the oil-in-water microemulsion region also provides a link between the hydrophobicity of the oil and the phase behavior. Comparing the SANS results of the three oils at equivalent a,y, and E (Figure 4 and Table 3)suggests that droplet size increases and elongation decreases with increasing oil hydrophobicity. The solubility of ClMA in water is 1.5 wt %, therefore at a = 5%, approximately 30% of the oil is dispersed in the continuous water domains. The microemulsion droplets that form in this case are small and closely resemble the elongated empty mixed DTABDDAB micelles.23 Table 3 shows that the surfactant to oil mole ratio increases with increasing hydrophobicity (i.e., chain length) of the oil. This suggests that the onephase oil-in-water microemulsion region should shrink at a constant overall surfactant concentration as the oil chain length of CkMA increases since more surfactant molecules are required per oil molecule. Figure 3 shows
exactly this progression; for example, at E = 0% more than 10%of ClMA can be solubilized, but only 5%of C4MA and 3%of C6MA can be solubilized under the same conditions.
Conclusions Mixing a single-tailed cationic surfactant with its double-tailed analogue alters the curvature of the surfactant film in a microemulsion and allows solubilization of oils that differ in hydrophobicity. Vertical sections through the phase prism at an equal oil to water ratio provide information about the extent of the one-phase microemulsion region and its location in E and y . Vertical sections at a constant overall surfactant concentration ( y = 12%)show a one-phase homogeneouschannel that spans the phase diagram from a = 0% to 50%for all three oils. Microstructural studies in this region at low oil concentrations demonstrate that an oil-in-water microemulsion is present. A decoupling model and the assumption that the microemulsion droplets are prolate ellipsoids allow a good description ofthe scattering patterns. SANS modeling suggests that droplet curvature decreases with increasing DDAB fractions and droplet size increases with increasing oil concentrations. Acknowledgment. We acknowledge the support of the National Institute of Standards and Technology, US. Department of Commerce,for providing the facilities used for experiments performed there. Some of this material is based upon activities supported by the National Science Foundation under Agreement No. DMR-9122444. The assistance of Charlie Glinka and John Barker is gratefully acknowledged. Financial support was provided by the Delaware Research Partnership. LA950970D
