J . Phys. Chem. 1986, 90, 2817-2825
2817
FEATURE ARTICLE Oil, Water, and Surfactant: Properties and Conjectured Structure of Simple Microemulsions D. F. Evans,*t D. J. Mitchell,$ and B. W. Ninhamt* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, and Department of Applied Mathematics, Research School of Physical Sciences, Institute for Advanced Studies, Australian National University, Canberra 2601, Australia (Received: July 26, 1985; In Final Form: January 9, 1986)
Phase diagrams and physical properties of three-component ionic microemulsions are reviewed. It is argued that microstructure is set by curvature arising from a balance between repulsive head group forces and opposing forces due to oil uptake in surfactant hydrocarbon tails, together with an overriding constraint set by geometric packing. The picture which emerges is consistent with theories of surfactant-water aggregation and can be generalized to include multicomponent systems.
1. Introduction This paper reviews some properties of a class of mixtures formed from ionic amphiphiles, water, and oils categorized by the loosely defined term microemulsions. Microemulsions have wide industrial and household use.’ They show great promise for new materials development by polymerization and for controlled drug release through cubic liquid crystal phases. Further, and an additional stimulus to current interest, the mathematics and physics of random and microstructured matter is a growth area of modern physical sciences, for which consumer market microstructured fluids provide well-defined reproducible prototypes. Microstructure is the central issue, to which thermodynamics, or the usual scaling arguments which beset statistical mechanics, can contribute as little as they do to an understanding of the mechanism in phase transitions. As remarked by McQuarrie,* all pair distributions look alike. They oscillate and decay and are all boring. So too might it be said for phase diagrams. However, on the other side of the coin, sophisticated statistical mechanical modeling is impossible, not just because the force laws are not yet known and no one has yet the faintest idea of how to deal properly with the vexed question of concentration units, so-called “cratic” contributions to the free energy, but because microemulsions are often bicontinuous3 and there is not even the beginnings of a usable mathematical theory for surfaces of nonzero curvature. In the circumstances all one can do is to try to give an impressionistic evocation of the picture, still blurred, tantalizingly close to focus, but one hopes nonetheless usable. We deal initially with ternary systems formed from double-chained cationic amphiphiles, water, and alkanes or alkenes. In so doing, our intent is to show that the microstructure and extent of parts of the ternary phase diagram can be understood in terms of elementary concepts that derive from and link directly to theories of surfactant-water aggregation. Our systems have been deliberately chosen.e7 The surfactants are virtually insoluble in both water and the oils and are therefore constrained to reside at the oil-water interface. This circumstance makes the unraveling of microstructure relatively easy. Ionic amphiphiles are used because head group and interaggregate interactions can now be modeled reasonably well due to recent progress8 in ionic micellar self-assembly and in direct measurement of forces. The cosurfactants often employed in microemulsion studies partition between oil and water and vastly University of Minnesota. *Australian National University.
0022-3654/86/2090-2817$01.50/0
complicate interpretation. Hence, in the first instance we dispense with them. Once the principles which govern self-assembly and microstructure in ternary systems are identified, design rules can be formulated which allow microstructure to be tuned up on demand. This can be illustrated with different oils, counterions, cosurfactants, and mixtures. Further, reasons for the existence of ultralow surface tension at two- and three-phase boundaries can be identified. At a pragmatic level the story that seems to be emerging is simple. Microstructure is set by interfacial curvature. This curvature is prescribed by a balance between head group forces set by the electrostatic double layer and opposing hydrocarbon tail interactions set by the oil, taken together with geometric constraints set by volume ratios of components. At another level the story is complex in that those interactions between interfacial regions which together with entropic factors determine two- and three-phase regions are very subtle. (1) D. Tabor, J. Colloid Znrerface Sci., 75,240 (1980). This paper, on clay tablets in cuneiform (ca.700 B.C.), was retrieved by George Smith along with the rest of Ashurbanipal’s library at Nineveh last century following Layard‘s initial discoveries. It resides in the Ashmolean Museum at Oxford. It is probably the first to deal with oil-watersurfactaht mixtures. The first known usage is that of the Australian sawfly larva (ca.IO8 B.C.). This primitive and highly successful insect has a caterpillar (larval) stage which encapsulates poisonous terpenes and other secondary compounds of the eucalyptus leaves on which it feeds in a diverticulum which contains thb oils in a microemulsion. The microemulsion is used to great effect in repelling predators. The first human usage, apart from associated Europehn liqueurs, is that in Mother Gardner’s recipe for washing woolens, still purchasable in its city of origin, Melbourne, and still more effective than products of present-day competitors. (It is made of soap, methylated spirits, water, and a dash of eucalyptus oil.) (2) D. A. McQuarrie, private communication. (3) L. E. Scriven, Nafure (London),263, 123 (1976). The first suggwtion that emulsions can be bicontinuous was apparently made by G. H. A. Clowes, J . Phys. Chem., 20, 407 (1916). (4) S. J. Chen, D. F. Evans, and B. W. Ninham, J . Phys. Chem., 88, 1631 (1984). .
(5jF. D. Blum, S.Pickup, B. W. Ninham, S. J. Chen, and D. F. Evans, J . Phys. Chem., 89, 711 (1985). (6) B. W. Ninham, S.J. Chen, and D. F. Evans, J . Phys. Chem., 88,5855 (1984). ,--(7) S. J. Chen, D. F. Evans, B. W. Ninham, D. J. Mitchell, F. D. Blum, and S.Pickup, J . Phys. Chem., 89, 711 (1985). (8) D. J. Mitchell and B. W. Ninham, J. Chem. SOC., Faraday Trans. 2, 77, 601 (1981); D. F. Evans, D. J. Mitchell, and B. W. Ninham, J . Phys. Chem., 88, 6344 (1984). (9) K. Fontell, A. de Ceglia, B. Lindmann, and B. W. Ninham, Acta I
Scand. Chern., in press.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 13, 1986
2818
The outline of this essay is as follows. Section 2 is concerned with considerations required for a complete description. In section 3 observations and phase diagrams on ternary systems are collected together and related to properties of binary surfactant-water systems. In section 4 packing and curvature considerations are related to microstructure. In section 5 these ideas are illustrated. It is shown how microstructure can be altered systematically by mixed surfactants, oils, cosurfactants, and other factors. The state of play is summarized in section 6. We make no distinction between swollen micelles or microemulsions because there is The term microemulsion is taken to mean any clear thermodynamically stable (reversible to temperature cycles) phase comprising at least surfactant, oil, and solvent which may be in equilibrium with excess oil or solvent or both. In fact, it will probably emerge that even such a characterization is too restrictive. The difference between microemulsions and emulsions is probably tenuous, and it is possible to form self-microemulsions from oil and water alone, Le., from weakly dissociated oils which through dissociation form their own "surfactant"; indeed such a system is probably the basis for mineral flotation processes. However, the word microemulsion has come into widespread acceptance. Strictly speaking, too, as noted by Becher in his well-known book on emulsions, the term surfactant, an abbreviation for surface active agent, is an abomination in the sight of emulsion chemists. But amphiphile is worse. 2. Why Microemulsions Form and Ultralow Surface Tension: Theoretical Considerations At normal room temperature and at atmospheric pressure water and o i r a r e mutually insoluble (immiscible) and will form a two-phase system with a large interfacial tension (-50 dyn/cm). Addition of a small amount of surfactant will reduce the interfacial tension because the surfactant is adsorbed at the interface. The addition of further surfactant will reduce the interfacial tensions rapidly until the surfactant either forms large aggregates in one phase, possibly solubilizing some of the other phase, or forms a third phase. At equilibrium one of these situations must occur before the interfacial tension becomes negative. This is because, were the interfacial tension to become negative, the interfacial area would increase until enough surfactant was taken up by the interface to reduce the interfacial tension to zero. The system might, for example, form a dispersion of oil drops in water or water drops in oil or both or form a lamellar phase (alternate layers of water and oil with surfactant at the interfaces and in solution). If the free energy consisted of bulk free energy plus surface free energy terms only (G = GB + Gs), all these possibilities would have the same free energy because the total free energy is the same for each in the absence of interactions. Minimum free energy would be achieved when the total interfacial area was such that the interfacial tension vanished. Formation of Lamellar Phase. But this argument ignores interactions. In practice, higher order terms appear in the free energy. Consider, for example, a lamellar phase in equilibrium with bulk phase. Interactions will occur across the layers and G = GB Gs + Gint. For given numbers of oil, water, and surfactant molecules a lamellar phase is typified by specifying the thickness of either the oil or water layer. (The interactions will produce uniform thicknesses for each type of layer.) If the interaction across either layer is attractive, oil or water will be squeezed out until a potential minimum occurs. On the other hand, if the interactions are repulsive, the layers will expand to take up all available oil or water. At the same time the layers will thicken or thin to obtain the value for the interfacial area which make the Gibbs free energy G a minimum, Le., dG/dA = (d/dA)(Gs + G,,J =. 0. Thus, if we add enough surfactant to make the interfacial tension of a single interface vanish, the system would rather form
+
(10) K. Shinoda and S.Friberg, Adv. Colloid Interface Sei., 4.28 1 (1979; K. Shinoda, J . Phys. Chem.,submitted for publication. (1 1) Becher, P., Ed., Emulsion Theory and Practice,Reinhold, New York, 1965, p 2.
Evans et al. a lamellar phase of finite layer thicknesses. If the interactions are purely repulsive (so that all the oil and water are taken up), as we decrease the surfactant concentration, the layer thicknesses will increase, tending to infinity. But in this limit, dG/dA corresponds to the interfacial tension of an isolated interface. So the lamellar phase exists only when the concentration of surfactant is sufficient to make the interfacial tension of a single interface negative. However, in fact, there are long-range attractive van der Waals interactions across the layers. Thus, a lamellar phase consisting approximately of isolated interfaces of zero tension could lower its free energy by squeezing out oil or water. Therefore, we expect to see a lamellar phase in equilibrium with excess oil or water or both. This would occur at a lower chemical potential of surfactant than that required to make the interfacial tension at the onset of an isolated interface vanish. The magnitude of the interfacial tension at the onset of the lamellar phase is essentially the free energy of interaction per unit area. Nonplanar Interface. A similar story holds for water drops in oil (or vice versa). Attractive interactions between the drops will tend to squeeze out excess oil whereas repulsive interactions or translational entropy will tend to cause the system to take up all available oil. Repulsive interactions within the drops (or unfavorable curvature energy) will tend to expand the drops to take up all water available, whereas attractive interactions will tend to contract the drops, squeezing out excess water. One would expect in general curvature energy to favor one kind of drop (oil-in-water or water-in-oil), and if indeed either form of drop occurs, this favored kind should occur at optimal size (minimum curvature energy) and the positive interfacial tension at onset of the dispersion should be essentially the curvature of the free energy of the drop (per unit area). The phase which first forms is that one among the competitors which at onset has the lowest surfactant chemical potential. Ultralow Surface Tension. It is sometimes claimed in the literature that microemulsions form because of the low interfacial tension achieved by the surfactant (plus cosurfactant plus salt in a more general situation). This argument puts the cart before the horse. The low interfacial tension occurs because the system forms a microemulsion with its low curvature and weak interactions rather than small highly curved structures or structures with strongly attractive interactions. To design a system capable of achieving ultralow surface tensions is to design a system which will not form highly curved structures (e.g., small micelles). For this one needs weak head group interactions or double chains.* On the other hand, to avoid strongly attractive interactions, the head group interactions should not be too weak. To determine the phase diagram in general we need to calculate the free energy for all possible structures (water drops, oil drops, lamellar phase, cylinders, bicontinuous phases, etc.) and find what structure has the minimum free energy.8 These free energy calculations can be carried out analytically for a lamellar phase of a surfactant-water system at the level of straightforward double-layer theory. Even the hexagonal phase can be handled, given enough energy and faith in the Poisson-Boltzmann equation and provided one is prepared to ignore specific counterion effects. But with oil as a third component there is an additional oil-specific contribution to the free energy which makes these possible phases almost inaccessible to first principle calculations. To make matters worse, the (liquid) clear microemulsion phase is impossible. It used to be thought that droplet models (oil-inwater) applied for some systems (SDS-butanol-toluene-watersalt) which exhibited ultralow surface tension. The evidence, adduced from QELS experiments to determine the size of such presumed droplets, is inadmissible, or at least not proven. Application of theories of self-assembly (Appendix I) indicates that while the trends for surface tension and ultralow surface tension at the oil-microemulsion interface are well-described, the actual radii of droplets are much too low (curvature is too high). In fact, the data can only be explained if the mean curvature is much lower. That situation is achievable only with a bicontinuous phase. Since no mathematical description of such surfaces exists, sta-
Feature Article
The Journal of Physical Chemistry, Vol. 90, No. 13. 1986 2819 Wt 32
x)
7
1
I
Water
% of
34
36
I
38
40
I
I
42
Figure 1. Partial phase diagrams for alkanes, water, and DDAB components in wt %. Taken from ref 5. tistical mechanics is not really useful at this stage in delineating microstructure or phase boundaries. In the circumstances we are reduced to experiment. 3. Observations on Ternary S y s t e m 3.1. Carionic Systems. We first consider the system didodecyldimethylammonium bromide (DDAB)-alkane-~ater.~~~ Partial phase diagrams at 25 OC are given in Figure 1 for hexane through tetradecane. A more complete phase diagram for dodecane has been determined by Fontel19and will be reported in detail elsewhere. Beyond the shaded regions are hexagonal, cubic liquid crystalline, and lamellar phases. Near the high surfactant corner there is an additional lamellar phase, and a three-phase region exists between zero water content and the line AB. Below the boundaries of the k phase (onephase region) at low surfactant content there is a three-phase region and a spontaneous emulsion region, white, opaque, and nonbirefringent, comprising extremely fine emulsion droplets. For dodecane the emulsion will not break and, with excess water or oil, is in equilibrium with excess solvent. For other oils the onset of the spontaneous emulsion can be temperature dependent. The surfactant has a solubility less than I% by weight in water and in all the oils and must therefore reside at the oil-water interface. In the regions bounded by ABD the systems form single clear microemulsion (L,) phases. The first observation is that there is a high and systematic degree of oil specificity. The boundary AB at which the L2phase first forms on addition of water to an oil surfactant mixture varies from 6% for hexane to about 26%for tetradecane. Note that 26%water corresponds to the maximum packing fraction of 74% which would be taken up were the L, phase spheres of surfactant surrounding oil droplets in a face-centered cubic array. For tetradecane the L, phase does not extend to the oil corner so that at very low surfactant and water it cannot form the reverse spherical droplets (water-in-oil) formed by all the other oils. 3.2. Conducriuiry. Along the onset line AB all the microemulsions are conducting (Figure 2). On continued addition of water at constant oil/surfactant ratio the conductivity goes to zero abruptly. It changes by up to 8 orders of magnitude. Below the line DD' (and still in the L, phase) the solutions are still clear and are evidently disconnected water droplets. Tetradecane exhibits no such "anomalous" behavior. The conductivity behavior is the inverse of that usually observed for microemulsions containing alcohol as cosurfactant (and soluble surfactant). This percolation-like transition occurs along a line of constant water-to-surfactant ratio for all the oils. The conductivity decreases along the line AB as whatever structure is involved moves toward the reversed water-in-oil droplets existing at extremely high oil content. 3.3. viscosity. Similar data exist for viscosity (Figure 3). T h e viscosities are extremely high at low water content (e.& 70 times that of bulk hexane at 6%water and 01s 0.4). One can infer
6 I
11
I
I
15
I
I
I
I
1
25
1
I
I
1
35
WtK of Water i n Microemulsion Figure 2. Typical conductance data for alkane-DDAB-water m i c w emulsions. Units of spsilic conductance are $ T I cm-'. Conductance at the plateau region is that of the bulk oil. Taken from ref 4. Note that tetradecane (closed circlcs) shows no percolation behavior.
,
263[
3,2
,
WI% of Water
,
,
3:
,
4; ,46
22-
D
h.
18
14
-
10
-
- 34
c
- 30 0
octane
tetrodecone 10
I
I
14
I
s/0=0.250-
- 22
5/0=0.293
2-s/o=0.343 I
- 26
decane
6-
I
18
I
I
22
I
4
I l l
26
30
Wt% of Water Figure 3. Characteristic viscosity behavior for alkanes (ref 4). Again note that tetradecane microemulsions have qualitatively different prop des.
that there is a high degree of connected water channels that permeate the surfactant41 mixture. 3.4. NMR. Diffusion NMR studiess show that all the systems, including tetradecane, are bicontinuous in water and oil up to the line DD' and thereafter are oil continuous only. For tetradecane the line DD' is not reached before phase separation occurs. 3.5. Alkenes and Cyclohexane. Phase diagrams and conductivity data for alkenes are given in ref 6. Note that the onset line (AB) is displaced significantly for I-alkenes and there is a higher degree of oil specificity. Of special interest is the singlephase region (Figure 4) and conductivity data (Figure 5 ) for cyclohexane' and 1-hexane: matched by the viscosity,' which will be discussed below.
2820 The Journal of Physical Chemistry, Vol. 90, No. 13, 1986
Evans et al.
DDA B
Water
10
30
50
70
90
Cyclohexane
Figure 4. Partial phase diagram for cyclohexane, water, and DDAB in wt %. TABLE I: Correlation between Microemulsion Properties and Chain-Melting Temperatures
T,," OC
_______
surfactant 2C12N+Br 2CI4N+ 2CI6Nt CI8CIONt CIsCl*N+ C,SCMN+ 2ClSN+
sonicated sample b 16 28
b b 21 >50
lamellar phase 16 31 44 22 29 38 >50
hexane
water,d % decane tetradecane
6 4 c 4
12 6
5 I c
22 15
c
C
8 10 c
13 16 c
C
c
T, denotes a chain-melting temperature in either sonicated or lamellar phase from differential scanning calorimetry. bNo peak, T, < 5 OC. c N o L2 phase forms. "o/(o + s) = 20%.
3.6. Chain Length. Table I lists a comparison at 25 OC of minimum water content required to form an L2 phase with different oils for several surfactants.12 It can be inferred that microemulsion formation requires that the surfactant chains be fluid, or close to the fluid state, in a (curved) configuration to admit oil uptake. A similar correlation exists between chain-melting temperatures and the capacity to form microemulsions for double-chained anionic surfactants. 3.7. Counterion Effects. No L2 phase at 25 O C forms with (C16)2(Me)2NBr.With acetate as the counterion, a microemulsion does form. For DDAB, exchange to C1- or I- induces qualitative changes in the phase diagram;13 for example, with C1- microemulsions form easily only with highly penetrating oils and the single-phase region is much narrower than for Br-. This shows clearly the difficulties which confront theoretical modeling. 4. Curvature and Packing 4.1. Preliminary Remarks. To see how some sense might be made of these data, we consider first surfactant-water systems. Theories of micellar aggregates reduce at lowest level8 to the statement that the structures which form are determined by the surfactant parameter v/aolc. Here u is the volume of the hydrocarbon tail region per surfactant (v = 27.4 + 26.9n A3),I, is an optimal chain length, usually (except for vesicless) about 80% of the fully extended chain length ( I , = 1.5 1.26n A) where n is close to the number of carbon atoms per chain, and in a zeroth-order theory a, is the area per head group in a bilayer configuration. The rules are as follows: u/aol, C spherical micelles; C u/aolc C polydisperse rod-shaped or globular micelles; 1 / 2 C v/aolc C 1, single-walled vesicles or bilayers (liposomes); u/aolc > 1, reverse structures. (These simple rules have been shown to embrace the older HLB characteri~ation.'~)For
+
(a) The authors are indebted to Prof. T. Kunitake for providing this data prior to publication. (b) A. M. Cazabat, D. Langevin, J. Meunier, and (12)
A. Pouchelon, Adu. Colloid Interface Sci., 16, 175 (1982). ( 1 3) V.Chen, D. F. Evans, and B. W. Ninham, manuscript in preparation. (14) P. Becher, J . Dispersion Sci. Technol., 5(1), 81 (1984).
0
8
16
24
Wt % ti20 in cyclohexane microemulsion Figure 5. Conductivity behavior across the one-phase region for cyclohexane micr~emulsions.~ 1-Hexeneis similar.6 Note that conductivity increases prior to subsequent decrease.
DDAB (X-ray data) v = 700 A3,I, = 11.5 A, a, = 68 A2, and u / a l = 0.82, a lamellar phase forms. The surfactant parameter can be changed for ionic surfactants by addition of salt. This lowers head group area and increases v/al. Thus, a transition from globules to rodlike micelles is induced for SDS. The reason such rules generally work is that for unbranched amphiphiles u and 1 scale and head group areas usually span a fairly narrow range (40-65 A'). In the~ries'~-''head group areas and the surface free energy of an aggregate can be determined by balancing electrostatic repulsion against other opposing head group forces-chain packing, exposed hydrophobic regions, hard-core repulsion-which collectively prescribe curvature. These are virtually independent of ~ a l t . ' ~ -Subtleties '~ beyond the zeroth-order theory are accommodated by the Poisson-Boltzmann description of electrostatic free energies provided differences in hydrated counterion size are taken into The story for vesicles is more complicated, involving a complex interplay between chain stiffness and curvature energy.s The operation of these rules can be illustrated by several examples. 1. Curvature can be controlled by changing head group area through addition of salt. An example is the transition of SDS from globular micelles to rods with added salt. 2. Curvature can also be altered by varying chain volume. This can be done most dramatically by admixing single-chained surfactants to a (cloudy) lamellar phase of the corresponding double-chained surfactant in water; e.g., DDAB (v/al = 0.82) forms a lamellar phase. Dodecyltrimethylammonium bromide (DTAB) (u/al= has almost the same head group area and the same chain length. On progressive addition of this single-chained surfactant, the mixed system has (o/al),ff = (XDDAB(o.82) XDTAB(1/3))/(XDDAB + XDTAB) where X are mole fractions. At (v/al)eff= 0.6 the system clears to form vesicles. At (u/al),ff 0.5 a clear extremely viscous phase (cylindrical micelles) forms, decreasing in viscosity as (u/al),ff decreases further. With such experiments, with or without salt, the single-chained surfactant must be above its cmc (at the equivalent salt concentration at which vesicles form) before absorbing into the double-chained region. 3. While DDABr or DDACl or longer chain analogues form lamellar phases, different counterions set different head group areas. Thus, hydroxide, acetate, and a range of carboxylates are strongly hydrated, sit far from the head group surface, and increase head group repulsion. These surfactants form spontaneous vesi c l e ~ ~ above * - ~ ~their chain-melting temperatures.
+
-
(15) D. F. Evans and B. W. Ninham, J . Phys. Chem., 87, 5025 (1983). (16) D. F. Evans, D. J. Mitchell, and B. W. Ninham, J . Phys. Chem.,88, 6344 (1984). (17) D. J. Mitchell and B. W. Ninham, J. Phys. Chem., 87, 2996 (1983). (18) B. W. Ninham, D. F. Evans, and G. J. Wei, J . Phys. Chem., 8 7 , 5 0 2 0 (1983).
The Journal of Physical Chemistry, Vol. 90, No. 13, 1986 2821
Feature Article TABLE II: Characteristics of Reverse Microemulsion Dropletsa
TABLE 111: Characteristics of Primitive Lattices
sc
c = (4Ly3 n = c/21/2
a = 0.8660v'/'
a = 0.7937V'l'
~~
hexane
1.33
octane decane dodecane
1.86 3.0 3.39
41 57 92 104
2.23
1.63 1.49 1.38 1.36
2.03 1.84 1.80
BCC
FCC
b =c =
c = (2v)'/3 n = (3c)'12/2 =
"vial, = 0.82.
4. A similar phenomenon is observed with anionic surfactants?2 Thus, for example, Texas-1 Na+ forms a lamellar phase. Upon ion exchange of Na+ for the large tetraethanolamine cation, the system forms spontaneous vesicles. 5. The rules sometimes appear not to apply.15 Usually this can be traced to the effect of interactions, coupled with chain stiffness.* An interesting example is the spontaneous vesicle-tomicelle transition observed with DDAOH or with acetate and other carboxylates with increasing surfactant c ~ n c e n t r a t i o n . ~ ~ In a corresponding zeroth-order theory of microemulsion droplets, provided the chains are sufficiently fluid and oil penetration is ignored, the condition for dropletss is again vial =Z 1. That condition is satisfied by almost any double-chained surfactant. 4.2. Oil Penetration. Work on alkane uptake on black lipid films suggests that these ideas can be taken f ~ r t h e r . ~The ~ ?rule ~~ here is that when alkane length is very much less than surfactant chain length I , the oil penetrates strongly and swells the bilayer by a factor of 2, at almost constant head group area. By contrast, when the oil chain length is equal to or exceeds I, the bilayer does 11.5 A from not swell. Our model surfactant DDAB (CI2,1, X-ray data) is fluid at room temperature, and the alkanes, hexane through hexadecane, span the range from highly penetrating oils to those which do not absorb into the surfactant tail region. Assume to first approximation that head group area is constant. For hexane, the chain volume should swell by a factor of up to 2, ueff/u = 2, i.e., reverse structures, while for tetradecane, ueff/u 5 1, Le., structures of normal curvature are expected. The effect of oil penetration can be exhibited by considering the phase diagrams of Figure 1. Below the percolation threshold in conductivity (DD'), the microemulsions are disconnected water droplets in a narrow region prior to phase separation. The surfactant, being insoluble in both water and in the oils, resides at the water-oil interface. Along the line DD', $,/4, is constant where 4, and 4, are water and surfactant volume fractions, respectively. Let V, and v, be the volumes of surfactant and water per droplet. Then, if R , is the radius of the water sphere and N, the number of surfactant molecules per sphere 4a V, -RW3 = V,, h R W 2= Nsao = -ao 3 U
-
These equations determine the droplet radii. Using a. = 68 A2, 1, = 12.5 A, and v = 700 A3, from lamellar phase data, oil uptake can be estimated as follows: let 1 be the length of the surfactant tail in oil. Then 4?r NsUeff = - [ ( R , + - Rw31 (3) 3
o3
(19) Y. Talmon, D. F. Evans, and B. W. Ninham, Science, 221, 1047 (1983). (20) S. Hashimoto, J. K. Thomas, D. F. Evans, S. Mukherjee, and B. W. Ninham, J. Colloid Interface Sei., 95, 594 (1983). (21) B. Kachar, D. F. Evans, and B. W. Ninham, J . Colloid Interface Sci., 100, 287 (1984). (22) J. Brady, D. F. Evans, B. W. Ninham, and B. Kachar, J . Am. Chem. SOC.,106, 4279 (1984). (23) J. E. Brady, D. F. Evans, F. Griesser, G. G. Warr, and B. W. Ninham, J . Am. Chem. Soc., submitted for publication. (24) R. Fettiplace, Biochim. Biophys. Acta, 513, 1 (1978); J. Requena, D. F. Billett, and D. A. Haydon, Proc. R. SOC.London, A , 347, 141 (1975); J. Requena and D. A. Haydon, Proc. R. SOC.London, A , 347, 161 (1975). (25) D. W. R. Gruen and D. A. Haydon, Pure Appl. Chem., 52, 1229 (1980); Chem. Phys. Lipids, 30, 105 (1982).
where ueff is the effective volume of the surfactant chains plus oil taken up. Hence (4)
Estimates require a knowledge of the chain length in the presence of oil. Table I1 lists values assuming I = I, (a lower bound) and 1 = 1- (upper bound, chains fully stretched). Note that the trend and orders of magnitude of oil uptake are as expected. 4.3. Packing of Reverse Micelles and Gels. Some useful insights into microemulsion structure can be found by considering packing properties of reverse micelles. Such a model is useful in those parts of the phase diagram where available oil is less than the maximum amount that can be taken up. Assume that all available oil is taken up in the surfactant tails; e.g., for 1-hexene, choose (40 &)/4, = ueff/v < 2 . The system then behaves to a good approximation as reversed micelles. Suppose that the system forms equal-sized spheres of water arranged on average on a lattice. In eq 2 6,is now replaced by ( 1 - 4,) and u by veff At constant 4s/$o (or constant ueff), on addition of water the spheres will increase in size4at (nearly) constant head group area. The surfactant tails will rearrange to fill up the remaining space. There is a maximum water uptake characteristic of reversed the surfactant tails (still micellar systems. At a certain containing the strongly adsorbed oil) become fully stretched and can no longer touch, and an unfavorable vacuum forms. The structure must collapse. Let us determine this maximum water content. Let d,,, be the maximum distance of any part of the surfactant region from the center of the nearest water sphere. The characteristics of simple cubic, face-centered cubic, and bodycentered cubic lattices are given in Table 111, where Vis the volume per micelle (volume of the primitive cell), c the cube size, n the nearest-neighbor distance (center to center), and d,,, the distance from the center of any micelle. Now R , and V are the same functions of 4, regardless of lattice, viz.
+
+,
- =4,1 - 4,
a0Rw 9
4 = -4a - Rw3
3Vdf
,
3
u
(5)
and R , and V are monotonic functions of 4,. For a given ueff/al there is a maximum volume function of water that can pack with $,(BCC) > 4,(FCC) > &(SC). For a given lattice the maximum & is given by d,,, = R, + I, = a V J 3where a is given in Table I11 and is characteristic for each structure. Hence
I,
=
L
Y
~
-/ R~ , =
[ ~ 4 ( 4 ~ / 3 ) / 4 , -) ]1]R, /~
(6)
In the limit that all oil is taken up by the surfactant, and assuming none is squeezed out with water increase, the boundaries which determine existence of such phases are plotted in Figure 6. For a lattice of reversed cylindrical water micelles, the corresponding equation is
where a' = (?r/2)lI2= 1.2533
square lattice
= ( ( 2 ~ ) ~ / ~=/ 1.0996 3 ~ / ~ ) hexagonal lattice
(9)
Evans et al.
2822 The Journal of Physical Chemistry, Vol. 90, No. 13, 1986
/j
S
90
HlXIGWAt
Flgure 6. Allowed regions of packing far reversed micelles for various lattices. The calculation assumes all oil is taken up in the chains. Below the curves reversed micellar phases are not allowed. A simple experiment bears out these predictions: Choose an oil which is taken up strongly by the surfactant, e&, I-hexene, and take +,/& < 1. All the oil is immediately solubilized in the surfactant tail region. Take typically utm/u= 1.2, 1.4, ... and add water (cf. Figure 6 and Table 11). At low water content a clear freely flowing solution of reverse micelles appears. On further addition of water the clear solution becomes successively more rigid, becoming an extremely solid clear gel at 35% water content. Above $, = 35% no more water is taken up and the gel breaks to form a two-phase system as expected. 4.4. Close Packing of Spheres and Cylinders. Further bounds on allowable structures can be established by considering close packing for reversed spheres and cylinders in different lattices. Let +be the maximum volume fraction of spheres which can pack. For simple cubic (SC), BCC. and FCC lattices this is @ = */6 = 0.5236, 31/zu/8= 0.6802, and 2%/6 = 0.7405, respectively. The volume of a sphere of water plus surfactant plus adsorbed oil is u = (4*/3)(RW+ I ) l . Hence
SO I C C
51%
sc
Figure 7. Boundaries allowed by packing for reversed spheres or cylindrical micraemulsions in different lattice configurations. (Calculations are done for ulol = 0.82.) These are geometrically allowed Structures below the curves. Hexane L,phase region is superimposed.
C.
c.
But from eq 2, &/& = 3u/aR,, whence 3u
4,/4w
$v$w)l/3
Figure 8. Geometry of disturbed sphere in BCC packing configuration. Taken from ref 26 and 27. - 11
(11)
For hexagonal or cubic packing of inverted cylinders the corresponding inequalities are 2u 9,/& ;1(+/$")'/211 (12) where @ = * / 4 = 0.7854 ( S Q ) and ~ / 2 ( 3 l /=~0.9070 ) (hexagonal). The boundaries allowed are drawn in Figure 7 with vlal = I . Below the curves labeled for each lattice structure spheres or cylinders are geometrically allowed structures. Superimposed on the figure are the L, regions for hexane and dodecane. Note that while over most parts of the L,-phase region cylinders are allowed, spheres are not permitted over a wide region. 4.5. The Model of Lissanf. Other bounds can be placed on structures allowed by geometry which bear on hexagonal or cubic liquid crystal to lamellar transitions. Our concern is not with such phases here but only with the L, phase. It can be seen that while packing constraints above do allow hexagonal phase packing over most of the region, spherical packing is not allowed at high s/o ratios. Given that experiment demands a biwntinuous phase, that the conductivity decreases with increasing water content, and that
apart from nonpenetrating oils like tetradecane (which appear to have normal (average) curvature) all the systems are reverse phases, the most elementary visualization is as follows: the one-phase system at the onset line AB is a random network of connected conduits whose curvature is set by a balance of forces between bead group forces and those due to opposing oil penetration imposed by the oils. With increasing water a t constant o/s the conduits expand in diameter, at constant curvature, and a t sufficient water break off to form disconnected spheres. That picture is far too simple. Even if it were so, changes in effective counterion concentration are quite large over a dilution path (CD), and one expects some change in bead group area, therefore in oil uptake, and curvature along such a path. One way to visualize these structural changes has already k e n given by Li~sant'~.~' in his seminal work on high internal-phase emulsion pacfing. To see how structural changes can come about, (26) K. J. Lissant, J . Colloidol Inrerfoee Sa'.. 22, 462 (1966). (27) Emulrionr ond Emulsion Technology, Part 1. K. J. Lissant, Ed.. Marcel Dekker. New York. 1974. (28) James Morris end Pax Brittanica. Heowns Command and Fnrcwell rhe Trumpefs,Penguin Books, London. 1979. The quotation is from the last volume, Chapter I . p 22.
Feature Article
The Journal of Physical Chemistry. Vol. 90,No. 13. 1986 2823
consider oil and water, with no surfactant, and imagine first that oil is to be regarded as an internal phase. We begin with Figure 8 which shows a sphere of this internal phase surrounded by the Wigner-Seitz cell for a BCC packing configuration. In Figure 8a the sphere just touches the hexagonal faces of the tessellating figure and is in a close-packed situation. As the volume of internal phase expands, it will flatten against the hexagonal faces and begin to form necks or a bicontinuous structure. This continues until the boundaries of the flattened sphere touch the squares, and further necks appear. The process continues until in Figure 8c the continuity in conducting path allowed to the water (external phase) is disrupted. The argument can be extended by considering oil plus surfactant as the internal phase. The surfactant tails must be associated with oil, and their head gmup with water. In the proceps of expanding the internal phase imagine that the surfactant redisposes itself with head group confined to the water region and with appropriate curvature. The resulting structure is a bicontinuous phase. The Lzphase will form initially once curvature constraints set by head group repulsion vs. oil penetration are met. 4.6. Geometry of Distorted Spheres. This geometric argument can be made quantitative by model calculations. Consider again BCC packing which allows the highest internal phase. The cube has side c. The Wigner-Seitz cell (tessellating figure) is a tetrakaidecahedron (TKDH) which has these characteristics: there are 6 squares at distance c / 2 from the center, 8 hexagons at distance 3I/’c/4, 24 vertices at distance 5%/4, and 36 edges at distances 3c/4(2’/’), of length c/2(2’/’). A circumscribing sphere will have radius 5I/’c/4, and an inscribed sphere will have radius] c / 2 where it touches the squares (Figure 8b) and 3I/’c/4 where it touches the hexagons (Figure 8a). Figure 8c corresponds to the limit of bicontinuity. The volume of the TKDH is 2 1 2 , and the volume of the sphere is ( 4 r / 3 ) ( 3 ( 3 ’ / ’ ) / 6 4 ) 2 , so that y ( T ) / V = ( 3 ’ / ’ / 4 ) ~= 0.68 which is close packing. Now consider a sphere of radius b, initially circumscribing, and allow the radius to shrink. We wish to work out the volume of external phase (water), and its area, as the sphere shrinks. As b reduces, c / 2 < b < 5 ’ / L / 4 , the volume cut off from each of the square faces is
and from the hexagonal faces
d.
c.
Figure 9. Gwmctry of distorted sphere in FCC packing urnfiguration. Taken from ref 26 and 27.
microemulsion) can exist at extremely low water content provided other constraints are met. Similarly, the total exposed area of water (unshaded regions) is A, = * [ ( a
+ 4(3112))b~- 24621;
c/2
< b < 5%/4
As the sphere of radius b shrinks further, 3‘/%/4 contributions from the square faces disappear.
r‘
V, = - [ 1 2
(16)
< b < c/2,
+ r ( E ( b / ~ ) ’ - 4(3’”)(b/~)’ + 3’/’/4)]
A, = 4(3l/’)cb
- 12rb‘.
3‘/*c/4
< b < c/2
(17)
At the point b = 4 2 (Figure Sc), the volume fraction of water is 0.0605. We expect a change in connectivity at this point. The corresponding geometry for the face-centered cubic lattice is illustrated in Figure 9. The tessellating figure is a rhomboidal dodecahedron (RDH). Again suppose the unit cube has side c. The Wigner-Seitz cell has 12 faces made up of identical rhombuses. There are six vertices a distance c / 2 from the center and six at a distance 3’/’c/4. The face centers of the 12 faces are ’ ) the center, and the length of the edges is distant ~ / 2 ( 2 ~ /from 3’/’c/8. The inscribed sphere (close packing of undistorted spheres) has radius ~ / 2 ~ /and * , the RDH has volume $14. Again we imagine a circumscribed sphere of radius b which contracts ’ . the from b = 11’/’c/8 (limit of bicontinuity) to b = ~ / 2 ~ / As sphere shrinks, the external phase left is
The volume of water left inside is
vw= VTKDH- (vsPi,, - ~ V S-Q8 2
= 211
v d
+ r [ l a ( b / c ) ’ - (6 + 4(3’’’))(b/c)’ + (%+ 3 1 / z / 4 ) ] J
(15) At the limit of bicontinuity (b = 3 ~ / 4 ( 2 ~ / ’ )&(min) ), = 0.0055.
It is clear then that a bicontinuous structure (geometrical model (29) C.-U. H-nn,G. Klar. and M. Kahlweit, 1. ColloldItce,fmScl.. 82, 6 (1981): M. Kahlweit, J. Colloid Inferfme Sci., 90, I97 (1982): M. Kahlwcit. E. Lcssncr,and R. S t y , J. Phyr. Chem.. 8l. 5032 (1983): J. P h p . Chrm.. 88, 1937 (1984). (30) P.G. de Genncs and C. Taupin. J. Phys. Chem., 84,2294 (1982). (31) Y. Talmon and S. Prager, Norwe (London), 267, 333 (1977); J. Chcm. Phys.. 69.2984 (1978); J. Chem. Phys.. 16, 1535 (1982). (32) K. E. Bennett. J. C. Hatfield. H. T.Davis,C. W. Macasko, and L. E. Scrircn. (1. D. Robb. Ed.), Plenum. New Yak, 1982. p 65. (33)’G. Gunnarsson, B.lonsan, and H. Wennersnh, 1. P h p . Chem.. 84,
_.._ (34) J. N. Israclachvili. D.I. Mitchell, and B. W. Ninham 1. Chem.Sa., I l l d , r\. o,* m ““ l.
Faraday Tmm. 2.12, 1525 (1976). (35) B. R. Vijapndran and T. P. Bush, 1. corroid~tcerfmeSci..69,383
,.
,,a,a, ,I
I,.
(36) D. F. Evans, S. Mukhcrjre. D.1. Mitchell, and B. W. Ninham, J. Co/ioid Inferface Sci., 93. 184 (1983). (37) D.J. Mitchell, B. W. Ninham. and D. F. Evans,1. Colloid Inferface Sei., 101.29 (1984).
and the corresponding area of the external phase is A , = 4rb2 - 24rb(b
- ~ / 2 ~ / ’ ) ,l / z 3 / ’ C b / c < l11/z/8 (19)
At the limit of bicontinuity, where our conjectured microemulsion phase can first form, b/c = 1 l’/’c/S and &(min) = 0.0275. (The curvature of these model systems is appropriately negative.) 4.7. Geometry and Structure. The upshot of such considerations is as follows: If geometry were the sole determinant of structure and if the microemulsions were monodisperse, then formation of a connected bicontinuous phase is possible in a BCC configuration at water contents as low as &, = 0.0055 and for the FCC lattice at & = 0.0275. At and below 94% internal phase (surfactant plus oil) the FCC lattice is a more efficient packing, and above 94%. the BCC is preferred. Lissant’6J’ has argued further and in fact demonstrated for emulsions that around 96.4% a further change in the rate of change of physical properties occurs. Data for the highly penetrating oils cyclohexane and I-hexene (Figures 4 and 5 ) show a dramatic change in both conductivity and viscosity between 96 and 93 vol W internal phase, consistent with this hypothesis. Further support for our hypothesis can be drawn from perusal of Table 1of ref 4 which lists volume fraction of components with various oils along the initial line AB at which
2824
The Journal of Physical Chemistry, Vol. 90, No. 13, I986
the microemulsions form a one-phase system. If surfactant plus oil is viewed as an internal phase, then the internal phase at onset varies from about 98.5% volume cyclohexane, 94% hexane, 91% octane, 87% decane, 85% dodecane, to 74% for tetradecane. Taken together with Table 11, we see that there is a pattern. The oils oppose head group repulsion of the surfactants and set allowed curvature. Once that condition is met, the structure is imposed by geometry, which demands, for our systems, a bicontinuous structure. Highly penetrating oils clearly require more highly curved surfaces and therefore less water content for formation. For tetradecane (little oil uptake) the system is close to zero curvature on formation. As one traverses a dilution path at constant o/s ratio, the curvature of the bicontinuous phase is too low to satisfy the energetic requirements imposed on the surfactant by head group repulsion, and oil penetration and disconnected inverse spheres form at a point schematically equivalent to Figure 8c. The observation of spontaneous emulsion phases represents an interesting development. With a surfactant that is insoluble in oil or water, at low surfactant concentration the interfacial area required can be accommodated only by large droplets. These are stabilized by interactions between opposing near-planar charged surfaces or multilamellae swollen with oil and water rather than by curvature. Whatever theories are invoked to explain this phenomenon, it seems that there is no longer any necessity to demand a clear distinction between microemulsions and emulsions. Phases lamellar or otherwise containing high oil fractions are probably stabilized by electrostatic interactions across the oil. 5. Induced Curvature Changes The story is embryonic and not terribly satisfactory. Nonetheless, it does admit a number of tests. (a) Counterion dependence is observed and expected, just as for surfactant-water systems. In hydroxide or acetate form, DDA will not form microemulsions, at least with the weakly penetrating oils. These surfactants form spontaneous v e ~ i c l e s rather ~ ~ - ~than ~ lamellar phase in water; i.e., head group repulsion is much larger, and the oils are not taken up in these systems. Again, this correlates with the behavior of corresponding surfactant-water systems. (b) Mixtures of single- and double-chained surfactants will have reduced inverse curvature in the presence of oils. Suppose a microemulsion is made with the alkanes from a mixture of DDAB and dodecyltrimethylammonium bromide at (u/al),ff C 0.6 (cf. section 4.1). For such values of the surfactant parameter this system forms vesicles. One expects this system to admit less oil penetration and therefore to form an L2 phase only at higher water contents than for DDAB alone. Indeed this is so. For the strongly penetrating oils hexane-decane, the onset line occurs around 26% water fraction, rather than 6-1 3%. The conductivity data for such systems behave as would be expected with reduced inverse curvature (cf. ref 7). (c) Addition of a water-insoluble alcohol like dodecanol (which changes head group area little and acts to increase (u/al),ff) admits more oil penetration, which again is reflected in phase diagrams and physical proper tie^.^ (d) With mixed oils, one expects, e.g., for a tetradecane/hexane mixture to induce more (reverse) curvature, which again is reflected in measured parameters.’ (e) Addition of salt to a system with net (inverse) curvature cannot induce ultralow surface tension at the oil-microemulsion interface. This is because addition of salt to such systems will obviously result in a higher curvature, all other factors being equal, except of course for tetradecane. It can be produced if attractive interactions following addition of salt can squeeze out enough oil and give rise to a net positive curvature. With a highly penetrating oil this is impossible. To produce ultralow surface tension it is necessary to begin with a microemulsion of net positive curvature, in our case, for example, by changing (u/uOer, through admixing single-chained surfactants and using an oil which penetrates little. The curvature of the bicontinuous phase can then be reduced appropriately by addition
Evans et al. of salt. The same principles apply to other ionic microemulsions which use cosurfactant. Finally, we remark that the same principles apply to nonionic surfactants.8 The extreme sensitivity to temperature, which correlates with the cloud point behavior of these surfactants in water, has caused some confusion in the literature. This has recently been explored by direct force measurement^^^ between bilayers adsorbed on mica with poly(ethy1ene oxide) head groups interacting across water. The forces change from repulsive to attractive at precisely the cloud point temperature. These results were determined first by NMR m e a ~ u r e m e n t s . ~ ~
6. Conclusions James Morris, in his marvelous trilogyZ8on the British Empire, had this to say on the Jubilee celebration for Queen Victoria. “If to the Queen herself all the myriad peoples of the Empire really did seem one, to the outsider their unity seemed less apparent. Part of the Jubilee jamboree was to give the Empire a new sense of cohesion but it was like wishing reason upon the ocean, so enormous was the span of that association, and so unimaginable its contrasts and contradictions.” Microemulsions really are a lot like the British Empire, elusive, full of contradiction, and yet with an underlying thread of unity. KahlweitZ9and Shinoda and Friberg’O have separately pieced together that unity, the one through an admirable thermodynamic approach and the other through common sense. And de Gennes30 and Talmon and Prager3’ have come to the problem from the physicists’ viewpoint, different but not less valid, as have Scriven and Davis and their colleagues, who have come closest to microstructure. We have attempted to gain some insights from a simpler, less global, pragmatic, and probably in the end more British way of thought. By using a surfactant insoluble in oil and water, which must therefore reside at the oil-water interface, whose chain length is such that it allows systematic changes in curvature from extreme inverse to normal, one comes to a viewpoint. It is that curvature, set by a balance of head group forces, and those due to oil penetration, both quantifiable and systematic, together with constraints forced by geometry (incompressibility), that pretty much set the background. It couples sensibly with what we know of surfactant-water aggregation. The claim is preposterous. But so was the British Empire. And so are microemulsions. Appendix I We here account for our assertion that droplet models fail to account for ultralow surface tensions induced by salt in ionic microemulsions. To do so, we appeal to recent theories of ionic micellar ~ e l f - a s s e r n b l y . ~ ~ These - ’ ~ * ~go ~ somewhat further than earlier in that surface contributions to the free energies of an aggregate can be quantified. A consistent and quantitative account of cmcs, aggregation numbers, and ion binding parameters emerges from that theory which indeed holds up to at least 170 OC.15 One expects the theory to give a reasonable description of interfacial tensions at the ionic micellar solution-oil interface also. The results of an inordinately tedious40 calculationr6give for spherical micelles (38) P. Claeson, H. Christenson, and R. Kjellander, J . Chem. SOC.,Faraday Trans. 2, submitted for publication. (39) P. G. Nilson, H. Wennerstrom, and B. Lindman, J . Chem. Scr., 25, 67 (1 9 8 9 , and paper cited therein. (40) So tedious, indeed, that misprints for which two of the authors (D. J.M., B.W.N.) are jointly famous render the theory incomprehensible. (The other (D.F.E.) is incapable of either spelling or proofreading.) Reference 16 is wrong-for the curvature energy by a factor of 2 due to an incomplete partial derivative. Reference 14 uses this incorrect expression, but this here makes little difference to the results. In reference 15 eq A.15 from which eq A1 is derived is correct subject to occasional incorrect signs. The correct answer is
2825
J. Phys. Chem. 1986,90, 2825-2829
"[ 2
(1
+ $)"'-1 1 1 (A.l)
where s = 4?re2/wakT, z is the dielectric constant of water, a is the head group area, R is the radius of the micelle, e is the unit charge, k is Boltzmann's constant, T is the temperature, K = (8~n&/zkT)'/~,and 4 = (cmc salt concentration). If we take the SDS-NaC1-oil interface and assume no or little oil uptake in the micelles, the calculated interfacial tensions are y = 5.4, 4.6, 3.4, 2.3, and 1 dyn/cm at salt concentrations of 0, 0.01, 0.03, 0.1, and 0.3 M, respectively. The value 4.6 is in good agreement with a measured value y 5 for the SDS-O.1 M NaC1-heptane interface."*3s There is an extreme paucity of data on this and similar well-defined systems (in fact, only one data point and no aggregation numbers). Even though the numbers will change a little due to attractive or repulsive interactions and entropic contributions, theory seems to be on the right track for this
+
-
problem. The predicted dependence of y on salt concentration in the presence of excess oil is extremely weak (logarithmic) in accord with observation. Thus, if we take a presumed oil-in-water 500 8, and head group area 50 A2, we "droplet" size of R have y = 0.3, 0.2, and 0.1 dyn/cm at concentrations of 0.1, 1, and 3 M salt, respectively. For SDS-alcohol-water-oil microemulsions a typical "drop" size is R = 100 8, at 0.6 M NaC1.I2 The predicted value of y is 1 dyn/cm, much larger than the experimental value of y 0.1, This disagreement is worse at higher salt concentrations which give ultralow surface tension. The discrepancy between theory and experiment seems to be real. It is resolved if one admits that the interpretation of scattering data (from which structure is deduced) depends on the model assumed. There can be very real difficulties in interpreting QELS experiments for micellar and microemulsion system^.^^,^' If these microemulsions are bicontinuous rather than droplets, with then the possibility of very low overall curvature, the problem no longer exists.
-
-
--
SPECTROSCOPY AND STRUCTURE Matrix Isolation Investigation of the Complexes of the Molecular Hatogens with Cyclopropane and Its Derivatives Bruce S. Ault Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: September 30, 1985; In Final Form: February 28, 1986)
Single- and twin-jet deposition techniques have been employed in conjunction with matrix isolation to investigate the reaction products arising from the codeposition of cyclopropane and its derivatives with CI2,CIF, and Br,. In each case, the infrared spectra indicated formation of an isolated 1:l complex with perturbed modes of both the acid and base subunits detected. For example, in the C1F.c-C3H6complex the stretching mode of the complexed ClF subunit shifted 29 cm-I to lower energy, while vl0 and v l l of cyclopropane each shifted approximately 16 cm-'. The spectra suggested a mode of interaction and coordination that was comparable to that deduced for the analogous hydrogen halide complexes, namely interaction with the midpoint of one of the carbon-carbon bonds, with the halogen lying in the plane of the three-membered ring. The halogens were noted to perturb the cyclopropane subunit to a degree equal to or greater than that caused by the hydrogen halides, while ClF did not perturb the base subunit substantially more than did the nonpolar C12 and Br2 molecules.
Introduction Numerous studies]-" in the past two decades have shown that the isomerization of cyclopropane to propene is catalyzed by both Lewis and Bronsted acids, notably HCI, HBr, BC13, and BBr3. In order to more thoroughly understand this catalytic action, and to characterize the interaction between cyclopropane and the hydrogen halides, the 1:l complexes of cyclopropane with the hydrogen halides have been studied extensively in recent Ross, R. A.; Stimson, V. R.J . Chem. SOC.1962, 1602. (2) Maccoll, A.; Ross, R.A. J . Am. Chem. Soe. 1965, 84, 4997. (3) Stimson, V. R.; Taylor, E. C. Aust. J . Chem. 1976, 29, 2557. (4) Lewis, D. K.; Bosch, H. N.; Hossenlopp, J. M. J . Phys. Chem. 1982, 86, 803. (5) Truscott, C. E.; Ault, B. S. J. Phys. Chem. 1984, 88, 2323. (6) Barnes, A. J.; Paulson, S. L. Chem. Phys. Lert. 1983, 99, 326. (7) Buxton, L. W.; Aldrich, P. D.; Shea, J. A.; Legon, A. C.; Flygare, W. H. J . Chem. Phys. 1981, 75, 2681. (1)
0022-3654/86/2090-2825$01.50/0
In these complexes, the cyclopropane unit serves as an electron donor or base, forming a hydrogen bond to the hydrogen halide so that the hydrogen halide lies in the plane of the C3ring, bonded to the midpoint of one of the carbon-carbon bonds. The molecular halogens are well-known Lewis acids and form charge-transfer complexes with a large range of bases, particularly for I, and Br2.9-I' One might, then, anticipate a substantial interaction between cyclopropane and the molecular halogens, yet the dark ~~'~ reaction of c-C3H6with Br2 at -78 "coccurs very ~ l o w l y , ' and no reaction has been reported between c-C3H6and Clz. However, Aleksanyan and Gevorkyan have presented preliminary evidence (8) Legon, A. C. J . Phys. Chem. 1983, 87, 2064. (9) Benesi, H. A.; Hildebrand, K. 0. Acra Chem. SOC.1949, 71, 2703. (IO) Mulliken, R. s. J . Phys. Chem.1952, 56, 801. (11) Mulliken, R.S.; Person, W. B. J. Am. Chem. SOC. 1969, 91, 3409. (12) Deno, N. C.; Lincoln, D. N. J . Am. Chem. SOC.1966, 88, 5357. (13) Skell, P. S.; Day, J. C.;Shea, K. J. J . Am. Chem. SOC.1976,98, 1195.
0 1986 American Chemical Society
