J . Phys. Chem. 1988, 92, 768-773
768
geometry determined by the internal interactions of atoms forming the fragment even if the fragments are subject to structural limitations of the lattice (even if the SiOAl angles in the unprotonated 9SiOAlf fragments attain different values). The difference in the S i 0 and A10 bond lengths in various fragments does not seem m, while the SiOAl angles are likely to to exceed 0.02 X differ at most by about 15'. Likewise, other characteristics calculated for the 3SiOXAlf fragments with sterical limitations are very close to those obtained for identical fragments without sterical limitations (cf. Tables I and 11). It is obvious that structural limitations influencing individual fragments in the lattice (and depending primarily on the number of T atoms in the zeolite windows in which the fragment is situated) affect namely those parameters whose deviation from their optimal values will lead to the smallest increase in the overall energy of the system. For zeolites such a parameter is the SiOT angle in the 9 S i O T f fragments, since the ~ a l c u l a t e d ~amount ~'~~'~ of energy required to deform this angle from its minimum energy value to 120' or 180' does not exceed 20 kJ/mol. Relaxation of the zeolite geometry associated with isomorphous substitution of A1 for Si has recently been explained" by local deformation of the SiOAl bridges (even protonated ones). However, in view
of the fact that deformation of the SiOAl angle in the 9SiOAlf fragments is energetically more advantageous than in the 3SiOHAlf fragment^,^ it appears likely that deformation of the unprotonated 3SiOAlf bridges will mainly take place upon substitution of A1 for Si. Similarly, a change in the geometry of the 3SiOAlf fragment originating from ion coordination to the 0 atom is likely to be compensated by deformation of the SiOT angles of adjoining skeletal 0 atoms to which no ion is bonded. The variations found for the geometry characteristics of the structurally different types of the bridging OH groups can cause relatively small changes in the dissociation energy of these groups (by about 40 kJ/mol) and, consequently, small changes in their acid strength. On the other hand, the geometry variations of the same order of magnitude were found to be responsible for the observed differences in the vibrational frequencies of the bridging OH groups.'O Moreover, these results may be helpful in interpreting the structural characteristics derived from X-ray measurements; e.g., they support the that the TO bond lengths from X-ray data can be used for estimation of the population of individual types of OH groups in H forms of zeolites. Registry No. H ' ,
12408-02-5; Li',
17341-24-1; Na+, 17341-25-2.
Curvature and Geometric Constraints as Determinants of Microemulslon Structure: Evidence from Fluorescence Anisotropy Measurements Vicki Chen, Gregory G. Warr, D. Fennel1 Evans,* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455
and Frank G. Prendergast Department of Pharmacology, Maya Foundation, Rochester, Minnesota 55905 (Received: April 15, 1987; In Final Form: July 9, 1987)
Steady-state anisotropy measurements using an amphiphilic fluorescence probe, (trimethy1amino)diphenylhexatriene (TMA-DPH), and an oil-soluble probe, diphenylhexatriene(DPH), are reported for three-component microemulsions, employing didodecyldimethylammonium bromide (DDAB) as the surfactant, simple alkanes, and water. The anisotropies of TMA-DPH are almost constant when oil is added to the microemulsions but decrease upon addition of water. The results are interpreted in terms of a structural model based on geometric packing constraints of surfactant-coatedcylinders and spheres. The anisotropy shows local changes at the surfactant-water-oil interface which in turn can be related to global structure.
Introduction An unsolved question in colloid science concerns the relationship between microemulsion structure and measurable physical properties. This issue was implicit in the original work by Schulman four decades ago and still constitutes a major challenge:] it has persisted because microemulsions are inherently complex systems. At present, the most thoroughly characterized systems consist of four or five components, utilize surfactants or cosurfactants which are soluble in oil or water, and possess labile structures which rearrange in response to small variations in composition or temperature. In an attempt to circumvent some of these difficulties, we have characterized a simple, three-component microemulsion system. It employs as surfactant the didodecyldimethylammonium halides which are only sparingly soluble in either oil or water. Consequently, the oil-water interfacial area is directly related to surfactant concentration, and in the absence of cosurfactant, oil specificity becomes evident. From conductance, viscosity, and
N M R diffusion coefficient measurements, and observations on oil and counterion specificities, we have made the case that microemulsion structure can be directly related to curvature of the surfactant film at the oil-water interface.2 This in turn can be understood in terms of a delicate balance between headgroup repulsion and oil penetration into the surfactant chains. When combined with a recognition of the extraordinarily strong attractive force which operates on water-in-oil systems, the interfacial tension between the microemulsion and oil can be directly related to the ~tructure.~ In this paper, we employ fluorescence anisotropy measurements using a cationic amphiphilic probe in order to characterize in more detail the properties of the surfactant-oil-water interface. We interpret changes in anisotropy in terms of changes in local interfacial curvature and describe in more detail a model for microemulsions which relates curvature and structure. (2) Evans, D. F.; Mitchell, D.J.; Ninham, B. W. J . Phys. Chem. 1986,90,
2817. (1)
Prince, L. M., Ed. Microemulsions; Academic: New York, 1977.
(3) Allen, M.; Evans, D.F.; Mitchell, D.J.; Ninham, B. W. J . Phys. Chem. 1987, 91, 2320.
0022-3654/88/2092-0768$01.5Q/Q0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 169
Structure of Microemulsions
Experimental Section The fluorescent probe molecules diphenylhexatriene (DPH) and (trimethy1amino)diphenylhexatrienep-toluenesulfonate (TMADPH) were purchased from Molecular Probes Inc. (Eugene, OR) and were used without further purification. A stock solution of 1 X lo4 M probe in tetrahydrofuran and acetonitrile, respectively, was prepared, and sufficient aliquots of stock solution were dried onto glass vials to yield approximately 5 X 10" M probe concentration in the final solution. Microemulsion components of surfactants, alkanes, and water were then added, and the mixture was sonicated to distribute the probe. Molar ratios of fluorophore Didodecyldimethylammonium to surfactant were ,
2
0.21
4-
.-2
0.20
2
0.19
0.16 0.15
4 20
I
30
40
50 60 WT. % DECANE
70
EO
Figure 6. Steady-state anisotropies for TMA-DPH in decane, DDAB, and water microemulsions along constant water-to-surfactant ratios (H,O/DDAB is on a molar basis).
aqueous phase could serve as possible quenchers, although it has been observed that both iodide and p-toluenesulfonated salts of TMA-DPH were reported to show similar lifetime behavior.5,6 We examined the maximum range over which the fluorescence lifetimes can vary by looking at microemulsion systems that vary from those with no halide counterions and low water content to those with significant halide counterion concentrations and with large water contents. The results are given in Figure 5 . DDAAc (didodecyldimethylammonium acetate) forms microemulsions with cyclohexane at very low water concentrations and yields, as expected, the longest lifetime.13 DDAB microemulsions with large amounts of water yield the lowest lifetimes which are almost constant down to 5 wt % water content. Below 5% water there is an increase in the lifetimes which can be attributed to decreasing accessibility of TMA-DPH to the polar solvent.6 It is assumed for TMA-DPH steady-state anisotropy measurements that the fluorophore does not diffuse along the interface on the time scale of its lifetime as this would cause additional depolarization of the emission. The resulting decrease in apparent anisotropy values would invalidate comparison with measurements taken when the fluorophore remains anchored (at shorter lifetimes). Variation in lifetimes for TMA-DPH therefore becomes significant when they are on the order of the times in which the fluorophore can diffuse (between the time of excitation and emission) beyond the conelike region shown in Figure 4. If the time scale of the experiment is considered to be 6 ns (2 lifetimes) and the diffusion coefficient of the fluorophore is approximately that of DDAB (3 X lo-' cm2/s),s the lateral motion of the probe is on the order of 4 A or about half the diameter of a surfactant h e a d g r ~ u p . ' ~Thus, the anisotropy data for these various mi(13) Chen, V . , unpublished data.
Chen et al.
772 The Journal of Physical Chemistry, Vol. 92, No. 3, 1988
0'24 0.23
2
e
0.21 o.22 0.20
.-5 C
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5
H20lDDAB = 10.7
t
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I
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016
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I
0.14
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I
30
40
50
70
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WT.% HEXANE Figure 7. Steady-state anisotropies for TMA-DPH in hexane, DDAB,
and water microemulsions along constant water-to-surfactant ratios. (H,O/DDAB is on a molar basis.) ANISOTROPY AloNo CONSTANT
0.25
H201DDAB RATIO
0.24
0.23
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. 0
H20lDDAB HPOIDDAB HPOIDDAB HZO/ODAB H20/DDAB
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72
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9
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0.16 0.15 0
10
20
30
40 50 WO/DDAB
60
70
80
90
Figure 10. Steady-state anisotropies with TMA-DPH in DDAB, water, and alkane microemulsions. (H20/DDAB is on a molar basis.) Figure 8. Steady-state anisotropies for TMA-DPH in cyclohexane,
DDAB, and water microemulsions along constant water-to-surfactant ratios. (H,O/DDAB is on a molar basis.) croemulsion systems are thus comparable for our analysis. Steady-State Anisotropy. We first examined the anisotropy behavior of TMA-DPH at fixed water to surfactant ratios (w/s) along oil dilution paths for decane, hexane, and cyclohexane (see Figures 6-8). Over the bulk of the one-phase region, the anisotropy remains relatively constant, but as the compositions approach the oil corner, the anisotropies decrease slightly. In contrast, measurements along constant s/o for DDAB microemulsions give anisotropies which decrease substantially with increasing water. For a given oil, the measurements were taken at several s/o ratios. Figures 9 and 10 show anisotropy values which are almost independent of the oil volume fraction and suggest a single curve when plotted against molar water to surfactant ratio (w/s). Because each curve consists of data from widely varying w/o ratios, substantial scatter about the curve is expected. At low w/s ratios, the anisotropy is highest for all of the oils. The anisotropy decrease with added water is not linear but falls rapidly at first and finally reaches a plateau. The onset on the plateau region occurs at the lowest w/s for the most highly penetrating oil and is not readily discernible for tetradecane.2 In general, the curves lie at progressively lower anisotropy values as alkane length decreases, the exception being with the highly (14) Cussler, E. L . Diffusion: Mass Transfer in Fluid Systems; Cambridge University: New York, 1984; Chapter 2.
penetrating, nonlinear cyclohexane. In DDAB microemulsions, two features with TMA-DPH are noted: (1) The fluidity of the surfactant interface ranges from quite rigid (rss= 0.25) to very fluid (rss= 0.15) compared with lipid bilayers. This is striking since high interfacial fluidity has been proposed as a necessary condition to favor formation of micoremulsions over rigid, ordered structures (liquid crystals).2s15 (2) Changes in anisotropy vary with H,O/DDAB ratios but not with varying oil content at fixed H20/DDAB ratios. The ordering of the H,O/DDAB curves in Figure 9 suggests oil uptake decreases as the alkane becomes longer. this is consistent with evidence from bilayer studies and from other observations on other properties of these microemulsions.2,1 The independence of rss in the oil dilution experiments (Figures 6-8) indicates that oil uptake remains almost constant at each fixed H20/DDAB ratio. The curvature of the o/w interface is not changing along dilution lines. In terms of the geometric model, addition of water results in the interface becoming more planar, Le., ueff is decreasing. Since the anisotropy is larger for the less penetrating oils, Figure 10, one predicts that a decrease in ueff upon addition of water should result in an increase in anisotropy. This is contrary to what is observed (see Figures 9 and 10) and suggests that changes in headgroup interaction which lead to larger areas per molecule, ao, must play an important role. DDAAc microemulsions employing the more highly hydrated chloride counterion give lower anisotropy than the equivalent bromide.13 The more highly repulsive headgroups (larger ao) thus make a major contribution (15) deGennes,
P.G.; Taupin, C. J . Phys. Chem. 1982, 86, 2294
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 773
Structure of Microemulsions ANISOTROPY MEASUREMENTS WITH DPH PROBE
007
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I
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o
*
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DPH are thus sensitive to the mobility of the oil molecules. Our data were taken as oil dilution pathways along constant w/s from the high surfactant boundary toward the oil corner (see Figure 11). The anisotropy behavior of both decane and cyclohexane shows a plateau at low oil content and a sharp decrease above some critical oil fraction. This behavior suggests that, below this critical value, almost all of the probe, and hence the oil, is embedded in the surfactant chains. At still lower weight fraction of oil, the surfactant chains must intermesh to take up the voids between adjoining surfactant coated water conduits. As oil content increases, unhindered oil begins to appear as evidenced by the sudden decrease in DPH anisotropy. This is important in view of the attractive forces acting on cylinders of water in free oil as detailed earlier.
I
30
40
50
60
70
wr%OIL Figure 11. Steady-state anisotropies with DPH in DDAB, water, and alkane microemulsions along constant water to surfactant ratios. (H,O/DDAB = 2.9 for cyclohexane and H,O/DDAB = 25.7 for decane by mole.)
to the decreasing anisotropy or increasing fluidity of the interface upon addition of water. We have described what local changes are experienced by the interface as detected by anisotropy measurements. The dependence of anisotropy on water to surfactant ratio provides experimental evidence that curvature remains constant along lines of fixed volume to surface area as predicted by the geometric model. Water dilution of fixed s/o ratios thus sweeps across changing curvature, and the anisotropy changes in a corresponding way. It should be noted that these anisotropy measurements do not measure curvature via probe diffusion over the surface of the interface as in experiments of Di Meglio et al. but measure localized properties of the interface.16 A different type of information on microemulsions comes from using dipehnylhexatriene (DPH), the hydrophobic analogue of TMA-DPH. DPH can reside in both bulk oil and surfactant tails; thus its mobility is much greater than TMA-DPH which is tethered to the interface.” The anisotropy measurements with (16) DiMeglio, J. M.; Pay, L.; Dvolaitzky, M., Taupin, C. J . Phys. Chem. 1984,88, 6036.
(17) Prendergast, F. G.; Enge1;L. W. Biochemistry 1981, 20, 7338.
Conclusions The anisotropy of the amphiphilic fluoerscence probe, TMADPH, has been determined in the DDAB three-component microemulsions as a function of oil and composition. Along s/w lines in the triangular phase diagram, the anisotropy is almost constant, but it decreases as one moves along water dilution lines toward the water corner. Since this probe is located in the surfactantoil-water interface, it samples the steric constraints imposed by local curvature. The anisotropy of the oil-soluble probe, DPH, is much smaller. A model based on the packing constraints is used to define the phase region where conduits and spheres can exist. When combined with the experimentally determined minimum and maximum water lines, phase diagrams which approximate those observed experimentally are obtained. The major discrepancy occurs along the minimum water (AB) phase boundary and is discussed in terms of the large attractive forces associated with water-filled conduits in an oil continuum. This model permits local curvature to be related to global structure. The changes in anisotropy with composition and oil reflect the local interfacial curvature and thus substantiate the model described above. Acknowledgment. V. C. thanks Hercules Corpration for support as a predoctoral fellow. D.F.E. acknowledges support by the US.Army (Contract No. DDAG 29-85-K-0169). Registry No. TMA-DPH, 71316-28-4; DPH, 1720-32-7; DDAB, 3282-73-3; DDAAc, 16613-01-7; DDAOH, 23381-53-5; decane, 12418-5; cyclohexane, 110-82-7.
