Langmuir 1994,10, 2972-2976
2972
Effect of Acrylamide and Dodecane on the Phase Behavior of Monoolein S. Puwada,t S. B. Qadri, J. Naciri, and B. R. Ratna* Center for Bio I Molecular Science and Engineering, Naval Research Laboratory, Washington, D. C. 20375 Received February 18, 1994. I n Final Form: June 10, 1994@ Lipids when dissolved in water have been shown to self-assemble and form a broad spectrum of complex microstructures which include isotropic micellar and microemulsion phases and liquid crystalline lamellar, hexagonal, and bicontinuous cubic phases. The phase behavior of the monoolein-water system as a function of a polymerizable monomer (acrylamide)and an oil (dodecane)was studied. It is found that with increasing acrylamide concentration, the cubic phases of monoolein are destabilized and replaced by the lamellar phase. This effect was then reversed by adding dodecane to the system. The observed behavior is explained using the surfactant parameter a = h l . It is found that acrylamide interacts with the monoolein head group and increases the head group area a (smaller a),while dodecane partitions between the hydrophobic tails and increases the values of vll and a, thus reversing the effects of acrylamide.
Introduction A variety of surfactants and lipids, when dissolved in water, are known to exhibit thermodynamically stable bicontinuous cubic phases.' These phases are described as bicontinuous because both the hydrophobic and hydrophilic regions are simultaneously continuous.2 The structures of these phases have been described in terms of infinite periodic minimal surfaces (IPMS).3 These saddle-shaped surfaces have zero mean curvature and negative Gaussian curvature. Two types of bicontinuous cubic phases have been observed in lyotropic systems, depending on whether the polarlapolar surface is curved toward the apolar part (positive curvature) or toward the polar part (negative curvature). The former are known as the normal or type I cubic phases with the minimal surface being located a t the middle of the aqueous region. In the latter case, the minimal surface lies a t the midplane of the lipid bilayer, and they are referred to as the inverse or type I1 bicontinuous cubic phases. The existence of cubic symmetry in these phases and the continuity of both the lipid and aqueous regions have been e ~ t a b l i s h e dSo .~ far, bicontinuous cubic phases exhibiting double diamond (Pn3m),primitive (Im3m),and gyroid ( l a 3 4 symmetries5 have been found. Cubic phases exhibit certain unique characteristics that include (i) a large interfacial area between the two disconnected regions, (ii) uniform nanopores whose size
* To whom correspondence should be addressed. Also at the Dept. of Biochemistry, Georgetown University Medical Center, Washington, DC 20007. Also at the Dept. of Chemistry a n d Biochemistry, University of Colorado, Boulder, CO 80309. Abstract published inAduanceACSAbstracts, August 15,1994. (1)Lindblom, G.; Rilfors, L. Biochim. Biophys. Acta 1989,988,221. (2)(a) Luzzati, V.;Tardieu,A.; Gulik-Kryzwicki,T.; Rivas, E.; ReissHusson, F. Nature 1968,220, 485. (b) Lindblom, G.; Larsson, K.; Johansson, L.; Fontell, K.; Forsen, S. J.Am. Chem. SOC.1979,215,701. (c) Longley, W.; McIntosh, T. J. Nature 1983,303,612. (d) Charvolin, J. J.Phys. Colloq. 1985,C3,173. (3)(a) Scriven, L. E.Nature 1976,263,123.(b) Larsson, K.; Fontell, K.; Krog, N. Chem. Phys. Lipids 1980,27,321. (c) Anderson, D. M.; Davis, H. T.; Scriven, L. E.; Nitsche, J. C . C. Adu. Chem. Phys. 1990, 77,337. (4)(a) Luzzati, V.;Spegt, P. A.Nature 1967,215,701. (b) Larsson, K.Nature 1976,304,664.(c) Mariani, P.; Luzzati, V.; Delacroix, H. J. Mol. Biol. 1988,204,165. (5)(a) Schoen, A. H. NASA Tech. Rep. 1970, No. 05541. (b) International Tables for X-ray Crystallography; The Kynoch Press: Birmington, U.K., 1968. @
can be precisely controlled, and (iii) multiconnectivity of pores. These properties make the cubic phase a desirable matrix for a number of technological applications, such as catalysis, bioremediation, controlled release: nanoparticle synthesis,' and ultrafiltration. However,for most applications, one has to find a way of retaining the cubic structure under various environmental conditions. Polymerization of one or more components comprising the cubic phase has been shown to be a possible way of achieving both chemical and mechanical ~ t a b i l i t y . ~ , ~ Polymerization of the cubic phase can be achieved either by incorporating a polymerizable group within the lipid structure or by introducing polymerizable monomers as solutes in the hydrophobic or hydrophilic regions. However, any such additions are bound to alter the region over which the cubic phase exists in the temperaturecomposition phase space. The phase behavior of lipidwater systems has been phenomenologically examined by Israelachvililo and Hyde'l in terms of a surfactant parameter, a = vlal, where v and 1 are the volume and length of the lipid tail and a is the cross sectional area of the head group in a given solvent environment. For a > 1,the interface curves toward the aqueous phase (Figure la), while a 1, the interface curves toward the lipid tails. The tail volume v and length 1 depend on the chemical structure of the tail, temperature, and the presence of any hydrophobicadditives. The cross sectional area of the head group a0 is sensitive to temperature, counterion concentration, degree of hydration, and the presence of hydrophilic additives. Polar solutes that bind to the head groups will increase head-head repulsions and lower the tendency to form inverted structures (Figure lb). On the other hand, nonpolar solutes that partition into the hydrophobic tail region will increase the lateral (6)Wyatt, D. M.;Dorschel, D. Pharm. Technol. 1992,Oct, 116. Engstrom, S. Lipid Technol. 1990,2,42. Puwada, S.;Qadri, S. B.; Naciri, J.; Ratna, B. R. J.Phys. Chem. 1993,97,11103. (7)Puwada, S.;Baral, S.; Chow, G. M.; Qadri, S. B.; Ratna, B. R. J.Am. Chem. SOC. 1994,116,2135. (8)(a)Anderson, D. M.; Strom, P. In Polymer Association Structures; El-Nokaly, M. A., Ed.;ACS Symposium Series 384;American Chemical Society: Washington, DC, 1989;p 204. (b) Strom, P.;'Anderson, D. M. Langmuir 1992,8,691.(c)Anderson, D. M.; Strom, P. PhysicaA 1991, 176,151. (d) Strom, P. J. Colloid Interface Sci. 1992,154, 184. (9)Laversanne, R. Macromolecules 1992,25,489. (10)Israelachvili, J. N.; Marcelja, S.;Horn, R. Q.Reu. Biophys. 1980, 13,121. (11)Hyde, S.T. J. Phys. Colloq. 1990,C7,209.
0743-746319412410-2972$04.50/00 1994 American Chemical Society
Langmuir, Vol. 10,No. 9,1994 2973
Phase Behavior of Monoolein
(a) a > 1 (b) a = 1 (c) a > 1 Figure 1. Schematic showingthe effect of polar and nonpolar
additives on the preferred curvature at the lipid-water interface. The surfactant parameter a = v/aZ,where v and Z are the volume and length of the lipid tail and a is the cross sectionalarea of the head group. (a)Theinterface curves toward the aqueous phase (top) for a > 1. (b) When a polar solute binds to the head group, it increases the effectivecross sectional area, thus decreasing a. When a = 1,the interface is flat. (c) Hydrophobicsolutes partition into the tail region, increasing the value of u/Z (hence a) and the tendency to form inverted phases. pressure in this region12and enhance the tendency to form inverted nonlamellar phases (Figure IC).In this paper, we show that these two counteracting effects can be used to stabilize the bicontinuous cubic phase. Monoolein, a monoglyceride with an unsaturated oleic acid tail attached to a glycerol head group, exhibits inverse cubic phases over a broad range of temperature and lipid/ water ratios.13J4 We have studied the phase behavior of monoolein as a function of acrylamide concentration. Acrylamide is a polar monomer that can be polymerized by free radical polymerization. We find that acrylamide binds to the glycerol head group and decreases the range of stability of the cubic phase. This effect was counteracted and the cubic phase regained by the addition of dodecane (an oil which partitions into the hydrophobic tail region).
Experimental Section Materials. Monoolein with a reported purity of 99% was obtainedfromNuchek Prep and usedwithout further purification. Acrylamide and dodecanewere obtained from Sigma Chemical Co. Double-distilled water that had been fed through a Milli-q system was used to prepare all samples. Sample Preparation. Samples were prepared by weighing out known amounts of the dry lipid into microcentrifuge tubes and hydrating with an aqueous solution containing a known weightpercentageof acrylamide. The sampleswere then weighed again to determine accurately the weight percentages of the different components and centrifuged at 3000 rpm until the sample appeared homogeneous. Samples containing dodecane were prepared by first mixing dodecane with the dry lipid and then adding the aqueous acrylamide solution. In this paper, acrylamideand dodecane concentrations are defined as a weight percent of the aqueous solution and of the lipid, respectively. Lipid concentrations are defined as the ratio of the combined weights of monoolein and dodecane to the total weight of the sample. Optical Microscopy. Samples,taken between a microscope slide and a cover slip, were examined under a polarizing microscopefortheir textures. Toprevent homeotropicalignment of the lamellar phase, the slides were roughened by coating with polymethylmethacrylate or PMMA. While the cubic and L:! phases were optically isotropic, both lamellar and hexagonal phases exhibited strong birefringence. "he lamellar phase was identified by its characteristic oily streaks,uniaxial crosses, and focal conics,and the hexagonal phase was identifiedby the fanlike textures.15 The optically isotropic cubic and Lp phases were differentiated by the enhanced viscosity of the cubic phase and by the shape of the air bubbles in the sample, which appear ~~
(12)Gruner, S.M.;Tate, M. W.; Kirk, G. L.; So, P. T. C.; Turner, D. C.; Keane, D. T.; Tilcock, C. P. S.; Cullis, P. R. Biochemistry 1988,27, 2853. (13)(a) Lutton, E. S. J. Am. Oil Chem. SOC.1965,42, 1068. (b) Larsson, IC;Fontell, K.; Krog, N. Chem. Phys. Lipids 1980,27,321. (14)Hyde, S.T.; Andersson, S.;Ericsson,B.; Larsson, K, 2.KristuZZogr. 1984,168,213. (15)Rosevear, F. B. J. Am. Chem. SOC.1964,31,628.
Figure 2. Polarized optical micrographof a contact preparation
of monooleinwith water. The lipid was taken between an optical slide and a cover slip, and the water was allowed to diffuse through the lipid, thus creating a concentration gradient in the sample. The photograph shows the various phases observed: At the two extremes are the optically isotropic Lz (left top) and the cubic (right) phases, corresponding to low and high water percentages, respectively. The two phases are differentiated by the shape of the air bubble, which appears spherical in the Lzphase and faceted in the cubicphase. Thebirefringent region in the middle is characteristic of the lamellar La phase. faceted in a cubic phase and spherical in an L2 phase (see Figure 2). The excess water phases could be identified by the turbidity of the sample. Small-AngleX-rayScattering(SAXS).SAXSstudies were conducted at the National Synchrotron Light Source using monochromatic(0.15498 nm) focusedX-rays. The sampleswere sealed in vacuum-tight holders between two Mica windows, and the diffractionpatterns were recordedusing an imageplate (Fuji MedicalSystems)kept at a distance of about 1mfromthe sample. The exposure times were typically less than 3 min. We could unambiguouslyindexsix reflectionsin the doublediamondphase and three to four prominent reflections in the gyroid phase in most of the samples. The unit cell size calculated from these reflections is accurate to within 1%.
Results and Discussions Effect of Acrylamide. Figure 3a shows the phase behavior of monoolein in pure water, in the temperature range of 20-100 "C. At very high lipid concentrations (>95 wt %), a fluid isotropic phase (L2) is observed a t all temperatures. As lipid concentration is decreased, the sample exhibits a fluid lamellar (La) phase below 57 "C. The lamellar phase then transforms into a bicontinuous cubic phase as lipid concentrations are further lowered to between -75 and 60 wt %. Below 60 wt % lipid, the cubic phase coexists with excess water. SAXS data show that the region marked as cubic in the figure actually consists of two cubic phases, a double diamond phase at low lipid contents and a gyroid phase at higher lipid contents. At temperatures above 90 "C, an inverted hexagonal phase (HII)is present. Figure 3b show the effect of adding 10%acrylamide to the aqueous phase of the sample. With the addition of acrylamide,the lamellar phase is stabilized, accompanied by a decrease in the range of stability of the nonlamellar phases. The inverted hexagonal phase is absent, and the cubic phases are present only in a small region of the phase diagram between 50 and 90 "C. However, the excess water boundary is shifted to lower lipid concentrations. Below 50 "C, a region consisting of coexisting cubic and lamellar phases can be observed under the microscope.
Puvvada et al.
2974 Langmuir, Vol. 10, No. 9, 1994 ~
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Figure 3. Phase diagram of monoolein (a) in pure water, (b) in 10% acrylamide solution, (c) in 20% acrylamide solution, and (d) in 10%acrylamide solution in the presence of 2% dodecane. The various phases shown include a lamellar phase (La), a bicontinuous cubic phase (C), an inverse hexagonal phase (HII),and an isotropic micellar phase Lz. The lines are only guides to the eye and do not represent true phase boundaries. The cubic phase samples whose structures were determined from SAXS are shown by numbers 1 (gyroid)and 2 (double diamond) in the phase diagram. Table 1. Observed Reflections and Unit Cell Sizes for 60 wt % Lipid Samples Containing Different wt % of Acrylamide at 22.5 "C 0 wt % acrylamide 1wt % acrylamide 2 wt % acrylamide 3.1 wt % acrylamide 3.9 wt % acrylamide 10 wt % acrylamide hkl 6 (A) d ( A ) hkl 6(A) d ( A ) hkl d(A) d (A) hkl d(A) hkl 6(A) d ( A ) hkl 6(A) d ( A ) 99.7 Da 99.4 D 99.0 D 99.4 D 98.9 D 98.7 D
D 110 D 111 D 200 D 211 D 220 D 221 a D: double diamond phase. G gyroid phase.
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73.5 60.0 52.0 42.3
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With increasing temperature, the optically isotropic cubic phase region is found to develop from the birefringent lamellar region. The coexisting lamellar phase is present in samples containing more than 5 wt % acrylamide, and the temperature a t which it completely disappears increases with increasing acrylamide concentration. The lamellar phase, however, could not be identified in the SAXS measurements (see below) for acrylamide concentrations between 5 and 10 wt %, where all observed reflections could be indexed to the gyroid cubic phase. This is probably due to the relatively small fraction of the sample being in the lamellar phase. It should be mentioned here that transition temperatures were reproducible upon repeated heating and cooling and did not show any indication that the acrylamide is polymerizing a t these temperatures. Figure 3c shows the effect of increasing the acrylamide concentration to 20%. At this higher acrylamide content, the cubic phase is absent and the lamellar phase is now stable over the entire temperature-concentration plane. However, it is interesting to note that the range of the Lz phase region is unaffected by the addition of acrylamide. Table 1and Figure 4 show small-angle X-ray scattering (SAXS) data and the corresponding unit cell size as a
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Acrylamide Figure 4. Variation in unit cell size in the cubic phase as a function of acrylamide concentration at 42.4 "C (0)and 22.5 "C (a,.). The squares (circles)denote the gyroid (doublediamond) phase. Only the 0% acrylamide sample at 42.4 "C is in the excess water region. Wt.%
function of acrylamide content for a sample containing 60 wt % monoolein a t 22.5 and 42.4 "C. Only the sample
Langmuir, Vol. 10, No. 9, 1994 2975
Phase Behavior of Monoolein
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Figure 6. Plots of the lipid volume fraction 41( 0 )and the area per lipid molecule a (A) evaluated using eqs 1and 2 as a function of acrylamide concentration for samples containing -60 wt % lipid at 22.5 "C. The lines represent linear fits to the data. containing no acrylamide a t 42.4 "C is in coexistence with excess water. At both temperatures, the unit cell size increases with increasing acrylamide concentration. At 42.4 "C (open circles), the samples exhibit a double diamond cubic phase for all acrylamide concentrations studied. The 22.5 "C data (filled symbols) show a transition from double diamond (circles)to gyroid (squares) at around 4 w t % acrylamide. Both double diamond (unit cell of 104.4A)and gyroid (unitcell of 164.4A)cubic phases are present at 4 wt % acrylamide. The ratio of the two unit cell dimensions (1.57) a t the transition is in very good agreement with the theoretical ratio for this transition.14 The measured unit cell sizes and the symmetry of the phase are used to evaluate the lipid volume fraction and the area per lipid molecule a at the lipid-water interface in the context ofthe IPMS descriptions of the cubic phase. From geometrical and topological relations for the IPMS surfaces, expressions for the lipid volume fraction 41 and the area per lipid molecule a(1)a t a distance I from the minimal surface have been derived16 and are given by
and
where d is the unit cell size, x is the Euler characteristic ( x =~ -2, x p = -4, and XG = -a), u is the dimensionless area (UD = 3.091, up = 2.345, and UG = 1.919), and the subscripts D, P, and G refer to the double diamond, primitive, and gyroid phases. The distance 1 can be assumed to be constant and equal to 17.3 as reported by Chung and (2afTkey.l' The volume u ofthe lipid molecule is estimated to be 630 A3from the measured lipid density of approximately 0.94 g/mL and the monoolein molecular weight of 356.5. The lipid volume fraction 41 and the area per lipid molecule a evaluated using eqs 1 and 2 are plotted as a function of acrylamide concentration at 22.5 "C for samples containing -60 wt % lipid in Figure 5. The head group area a, which can be fitted to a straight line, increases
A
(16)(a) Anderson, D.;Wennerstrom, H.; Olsson, U.J.Phys. Chem. 1989,93,4243.(b)Turner, D.C.; Wang, Z. G.; Gruner, 5.M.; Mannock, D. A.; McElhaney, R. N. J.Phys. ZI Fr.1992,2,2039. (17)Chung, H.; Caffrey, M. Biophys. J. 1994,66,377.
with increasing acrylamide content. As a result, headhead repulsions are enhanced, setting up a strain that can be relieved by a phase transition to a structure with a larger aredvolume. The observed phase transition from double diamond to gyroid at 4% acrylamide is consistent with this since the G surface has a n aredvolume which is 2.2% larger than that for the D surface.14 The calculated value of 41 is found to decrease with increased acrylamide concentration in both cubic phases. For the double diamond cubic phase (Pn3m)containing no acrylamide, the value of 41 = 0.62 (calculated from eq 1 using the measured unit cell size) is in agreement with the expected value of 0.615, for a 60 wt % lipid sample and a measured lipid density of 0.94. With increasing acrylamide concentration, 41 of the Pn3m phase decreases to avalue of 0.59 at 4 wt % acrylamide. At this concentration, the Pn3m phase is found to coexist with a gyroid (Ia3d) cubic phase with a lipid volume fraction of 0.615. Above 4 wt % acrylamide only the Ia3d phase is present and its lipid fraction decreases to a value of 0.586 a t 10 w t % acrylamide. This behavior is consistent with the presence of coexisting double diamond and gyroid cubic phases below 4 wt % acrylamide and coexisting gyroid and lamellar phases above 4 wt %. The fraction of the sample that is in the gyroid phase below 4 wt % acrylamide is very small and therefore is not observed in the SAXS measurements. The fraction, however, increases with increasing acrylamide content until a t 4 wt % acrylamide the gyroid phase can be detected in the SAXS measurement. Beyond 4 wt % acrylamide, the decrease in 41of the gyroid phase with increasing acrylamide suggests that the coexisting lamellar phase has a larger 41. At even higher acrylamide contents of 20 wt %, the lamellar phase becomes the predominant phase (see Figure 312). Effect of Dodecane. The results presented in the previous section showthat both gyroid and double diamond cubic phases are destabilized by the addition of acrylamide due to an increase in head group area a . In this section we show that the destabilizing effect on the cubic structure can be reversed by proportionately increasing the value of vll through the addition of dodecane. Figure 3d shows the phase diagram of samples containing 2% dodecane (by weight of monoolein) and 10 w t % acrylamide (by weight of the aqueous phase). In comparison with Figure 3b (10% acrylamide, no oil), the most prominent change is a decrease in the range of stability ofthe lamellar phase. In the absence of dodecane, the lamellar phase is stable up to 80 "C and lipid concentration above 72%. With the addition of 2% oil, the range of stability of the lamellar phase has been reduced to temperatures below 70 "C and lipid concentration above 77%. In other words, the lamellar phase (which was stabilized by the addition of acrylamide) is partially destabilized by adding a small amount of dodecane. The addition of dodecane also shifts the excess water boundary to higher lipid concentrations. We have carried out SAXS studies on 70 and 60 wt % lipid plus dodecane samples containing 10 wt % acrylamide in the aqueous phase and varying fractions of dodecane, a t 22.5 and 40 "C. The data for the two lipid concentrations are very similar, and we use the data for the 60 wt % samples to illustrate the effect of dodecane on the phase behavior. Table 2 shows the effect of dodecane on the unit cell size. For a 60 wt % lipid sample, in the absence of both acrylamide and dodecane, it was possible to index unambiguously six reflections to the double diamond (Pn3m) cubic structure. With the addition of 10 wt % acrylamide, a phase transformation to a gyroid (Ia3d) phase was observed. This effect could be reversed by the addition of dodecane. Figure 6 shows the unit cell size as a function of added dodecane for a sample containing 10% acrylamide and 60% lipid plus dodecane. At 22.5 "C the
2976 Langmuir, Vol. 10,No. 9,1994
Puvvada et al.
Table 2. Observed Reflections and Unit Cell Sizes aa a Function of Dodecane Concentration for Samples Containing 60 wt % Lipid and 10 wt % Acrylamide at 22.4 "C 10 wt % acrylamide 10 wt % acrylamide 10 wt % acrylamide 10 wt % acrylamide 10 w t % acrylamide 0 wt % acrylamide 7.7 wt % dodecane 10.5 wt % dodecane 4.9 wt % dodecane 2 wt % dodecane 0 wt % dodecane 0 wt % dodecane hkl d ( A ) d ( A ) hkl d ( A ) d ( A ) hkl d(A) d ( A ) hkl d ( A ) d ( A ) hkl d ( A ) d ( A ) hkl 6(A) d (A) 110 70.5 99.7 D" 211 69.7 170.7 Gb 110 80.2 113.4 D 110 72.7 102.8 D 110 65.5 92.6 D 110 55.7 78.8 D 111 200 211 220 221 (I
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65.5 113.4 56.0 112.0 46.0 112.7
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Figure 6. Unit cell size as a function of added dodecane for a sample containing 10% acrylamide and 60% lipid plus dodecane at 22.5 "C (0,W) and 40 "C (0). The squares (circles)
denote the gyroid (doublediamond)phase. All samples except that containing no dodecane are in the excess water region of the phase diagram.
cubic phase transforms from one exhibiting a gyroid symmetry (unit cell size = 154.6 A) to one exhibiting a double diamond symmetry (unit cell size = 100.8A) upon the addition of 2 wt % dodecane. The ratio between the two unit cells dimensions, 1.53,is close to the theoretical ratio of 1.57 for this transition. Above 2 wt % dodecane, the sample continues to exhibit a double diamond cubic phase, with decreasing unit cell size. It should be pointed out here that all samples with dodecane concentration 2 2 wt % are in coexistence with excess water with the line of equilibrium between the cubic and the excess water phases being shifted to higher lipid concentrations. This shift in the excess water phase boundary (see Figure 3b,d) could be responsible for the decrease in the unit cell size observed with increasing dodecane concentration. Through the use of eqs 1and 2 as before, values of @le and a were evaluated as a function of dodecane for the 60 wt % samples. In this case @k is an efective volume fraction and includes both the volume of the lipid and the volume of dodecane. The effective chain volume u is assumed to increase linearly with dodecane as v = vlip noilvooilrwhere veil is the molecular volume of dodecane and nail is the number of moles of dodecane per mole of lipid. The calculated @le and a using the unit cell size data a t 22.5 "C are plotted in Figure 7. The value of @le in the double diamond cubic phase increases from 0.56 with 2%dodecane to 0.64 at 10% dodecane, conforming to the shift in the excess water phase boundary to higher lipid concentrations.ls However, the head group area a appears to be insensitive to the addition of dodecane. Thus, dodecane effectively increases the value of the surfactant parameter a,via chain volume u , and thus reverses the effect of acrylamide.
+
(18)Since the amount of dodecane is known, &,the true lipid fraction, could be estimated and was found to vary from 0.55 at 2 w t % dodecane to 0.61 at 10 wt % dodecane.
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Figure 7. Plots of the lipid volume fraction +le (0,W) and the area per lipid molecule a (A)evaluated using eqs 1and 2 as a function of added dodecane for 10 wt % acrylamide samples containing -60 wt % lipid plus dodecane at 22.5 "C. The lines are linear fits to the data. The hollow and filled squares represent the double diamond and gyroid cubic phases, respectively. Above 2 w t % dodecane,the double diamond cubic phase coexists with excess water (see also Figure 3d). Concluding Remarks In this paper, we report the phase behavior of the monoolein-water system as a function of acrylamide and dodecane. Both optical microscopy and small-angle X-ray scattering studies show that the lamellar phase is stabilized by the addition of acrylamide. The observed data can be rationalized by a n interaction between the monoolein head group and acrylamide, which results in a n increase in the cross sectional area a of the head group. The increased head group size decreases the value of the surfactant paramter a = u/uZ and stabilizes the lamellar phase. The effect, however, could be reversed by the addition of dodecane which will partition to the tail region because of its hydrophobicity. This will increase the effective tail volume v and decrease the tail length 1 due to the increased disorder of the tails. Both these effects will increase the value of a,thus reversing the effect of acrylamide. The optical microscopy data clearly show that the lamellar phase is destabilized by the addition of dodecane. The SAXS studies show that the double diamond to gyroid phase transition observed upon the addition of acrylamide is reversed by adding dodecane. Preliminary studies also show that acrylamide could be polymerized inside the cubic phase. These studies indicate that, although the cubic structure is retained, the unit cell size decreases. Studies to characterize the structure of the polymerized phases are in progress. Acknowledgment. This work was supported by the Office of Naval Research. Part of the research was carried out a t the National Synchrotron Light Source, Brookhaven National Laboratory.
