J . Phys. Chem. 1987, 91, 5944-5948
5944
Phase Dlagrams of Dloleoylphosphatldylchollne with Formamide, Methylformamide, and Dimethylformamlde Bjorn A. BergenstAbl* and Per Stenius Institute for Surface Chemistry, 11486 Stockholm, Sweden (Received: January 28, 1987; In Final Form: June 19, 1987)
The phase diagrams of dioleoylphosphatidylcholine (“dioleoyllecithin”, DOPC) with water, formamide, methylformamide, and dimethylformamide have been investigated in the temperature range 0-250 OC. DOPC swells extensively with water and formamide to form lamellar phases, but its solubility in either solvent is very low. DOPC dissolves to some extent in methylformamide, and the formation of the lamellar phase is limited to a very narrow concentration range. With dimethylformamide no lamellar phase is formed. The melting point of the crystalline DOPC phases in equilibrium with dimethylformamide is higher than for the other solvents. In all systems, at low solvent concentrations, cubic and reversed hexagonal liquid crystalline phases are formed. The differences in the phase equilibria can be rationalized in terms of solvation energies and solvophobic interactions between hydrocarbon chains and the solvents. It is concluded that the ability of the solvent to participate in hydrogen bonding appears to correlate with the occurrence of repulsive solvation forces between lecithin surfaces.
Introduction The ability of lecithins and a number of other phospholipids to spontaneously swell with water to form bilayer structures is of essential importance for the way these molecules function in biological and technical systems. The nature of the interactions leading to this swelling has therefore recently been the subject of e~perimentall-~ as well as theoretical5-* investigations. A question of considerable interest is to what extent the rather unusual hydrogen-bonding properties of water determine the repulsive forces between the lipid bilayers. In this paper we report an attempt to elucidate this question by exchanging the water for solvents with systematically decreasing hydrogen-bonding capacity: formamide, methylformamide, and dimethylformamide. The interaction between lecithin bilayers in water involves a strongly repulsive force that keeps the stacked bilayers in the lamellar phase apart. This interaction was first observed by Le Neveu et a1.I and termed the “hydration force”. The studies were further developed by Parsegian et a1.,2who noted the importance of this repulsive interaction for the stability of biological colloids. The close connection between these interactions, phase equilibria, and lamellar phase swelling was pointed out by Jonsson5 and, in particular, for lecithin systems by Guldbrand et aL6 The nature of the repulsive interaction is still unclear. The lecithin molecule is zwitterionic, and the bilayers are not expected to carry any net charge. In agreement with this, the swelling shows a comparatively small sensitivity to ele~trolyte.~Indeed, it has been observed by Evans et a1.I0 that a lamellar phase is formed by lecithin dissolved in pure molten ethylammonium nitrate. Electrostatic effects due to ionic charges therefore cannot give a complete explanation of the hydration force. In concentrated electrolyte solutions the effects appear to be connected to the hydration series of cations rather than to diffuse layer theory. This is clearly illustrated in the series of direct measurements of the forces between lecithin bilayers adsorbed on mica surfaces immersed in electrolyte solutions that have recently been reported by Marra and I ~ r a e l a c h v i l i . ~ ~ ~ (1) Le Neveu, D. M.; Rand, R. P.; Parsegian, V. A,; Ginzell, D. Eiophys. J . 1977, 18, 209. ( 2 ) Parsegian, V. A,; Fuller, N.; Rand, R. P. Proc. Natl. Acad. Sci. U.S.A. 1979, 76, 2750. (3) Marra, J.; Israelachvili, J. Biochemistry 1985, 24, 4600. (4) Marra, J. J . Colloid Interface Sei. 1986, 109, 11. (5) JBnsson, B. Dissertation, Lund University, Lund, Sweden, 1981. ( 6 ) Guldbrand, L.; Jonsson, B.; Wennerstrom, H. J. Colloid Interface Sci. 1982, 89, 532. (7) Marcelja, S.; Radic, N. Chem. Phys. Letr. 1976, 42, 129. (8) Kjellander, R.; Marcelja, S. Chem. Scr. 1985, 25, 117. (9) Gottlieb, M. H.; Eanes, E. D. Biophys. J . 1972, 12, 1533. (10) Evans, D. F.; Kaler, E. W.; Benton, W. J. J . Phys. Chem. 1983,87, 533.
Marcelja and Radic’ suggested that structural effects could be responsible for the repulsion. Later, Jonsson and Wennerstrom” have pointed out that image charge effects due to the stacking of water and amphiphile layers with different dielectric constants could explain the repulsive interaction. This approach has been further discussed by Marcelja and Kjellander.8 From these experimental and theoretical approaches it is clear that further understanding of the interaction requires a better knowledge of how it varies with the properties of the solvent. Friberg et al.I23I3showed that ethylene glycol and other short diols were able to form lamellar liquid crystalline phases together with lecithin. Later, it was shown in our laboratoryL4that the swelling of the lamellar phase formed by ethylene glycol/lecithin is due to a “solvation” force of the same order magnitude as the hydration force in the water/lecithin lamellar phase. Direct measurements of repulsive forces between mica surfaces immersed in ethylene glycol were performed by Christenson and Horn.Is Their results suggest that the Occurrence of polar surface/polar solvent interactions leading to “solvation” forces may be quite general; the nature of these interactions is still difficult to analyze. In this paper we report an attempt to elucidate the problem by changing the hydrogen-bonding properties of the solvent systematically. For this purpose, formamide, methylformamide, and dimethylformamide constitute a suitable series: they have largely similar dipole moments, but their ability to form hydrogen bonds changes systematically from formamide to dimethylformamide.
Materials and Methods Chemicals. Synthetic 1,2-dioleoyl-sn-glycero-3-phosphocholine (“dioleoyllecithin”, DOPC) of 99% purity was purchased from Avanti Polar Lipids Inc., Birmingham, AL. The D O E was dried for 3 days in a high-vacuum freeze-drier equipped with a liquid nitrogen cooling trap. Formamide (99% ACS reagent), methylformamide (99% GC), and dimethylformamide (anhydrous, 99% Gold Label) were from Aldrich Chemicals. Special care was taken to avoid the influence of traces of humidity from the air on the mixed samples. The samples were always larger than 100 mg. To ensure proper mixing and (1 1) Jonsson, B.; Wennerstrom, H. J . Chem. SOC.,Furaduy Tram. 2 1983, 79, 19. (12) Moucharafieh, N.; Friberg, S . E. Mol. Cryst. Liq. Cryst. 1979, 49, 23 1 (13) El Nobly, M. A,; Ford, L. D.; Friberg, S . E. J . Colloid Interface Sci. 1981. 84. 228. (14) Persson, P. K. T.; Bergenstlhl, B. A. Eiophys. J . 1985, 47, 743. (15) Christenson, H. R.; Horn, R. G. J . colloid Interface Sci. 1985, 103, 50.
0022-3654187 I209 1-5944%01.50/0 , 0 1987 American Chemical Society I
,
The Journal of Physical Chemistry, Vol. 91, No. 23, 1987 5945
Dioleoyllecithin in Formamides
200
1
200 -
50 Water
0.5
Dioleoyllecithin [VIVI
Figure 1. Phase diagram for 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and water. All concentrations are given as volume/volume.
equilibration, they were mixed through centrifugation back and forth in flame-sealed test tubes several times a day for about 1 week. Opened samples were sealed with Parafilm and stored over dried silica gel for the time of investigation (usually less than a week). X-ray Investigations. X-ray diffraction was recorded with a low-angle Kiessig camera equipped with a position-sensitive electronic detector (PSD 100, Tennelec Inc, Oak Ridge, TN). The X-ray source was Ni-filtered Cu K a radiation. Measurements were made in the range 0.12-1.7 deg. About 1-min exposures were required to obtain good diffraction patterns. The detector was repeatedly calibrated with crystalline sodium octanoate (repeat distance 23 A). The samples were contained in freshly prepared, flame-sealed glass capillaries. Microscopy. The texture of the samples was observed in a polarizing microscope equipped with an electrically heated "Koeffler" table. To characterize the phase transitions roughly without prolonged heating, the temperature was usually raised at a rate of 2-5 OC/min. For transitions involving the cubic phase, several heating-cooling cycles in the temperature range around the transition temperature were performed in order to get good observations of the cubic "crystallites". Examples of observations during these cycles are given in Figure 6. Because of the high boiling points of the formamides, it was not necessary to use sealed samples in these studies. The upper melting point of the lamellar phase formed in the water/DOPC system was determined visually by observing the phases when they were heated in flame-sealed glass capillaries.
Results The phase diagram of the water/DOPC system is shown in Figure 1. The lamellar phase is formed in an extensive concentration range (1040% water). With more solvent a dispersion of the lamellar phase in almost pure water is easily formed. The solubility of the DOPC in water is probably very low (of the order mol/dm3 16). As expected for this unsaturated lipid, no gel phase is observed. The maximum swelling of the lamellar phase was determined by X-ray diffraction. Figure 5 shows the repeat distance, the thickness of the lipid and the solvent layers, and the area per molecule of DOPC in the lamellae. The repeat distances range from 50 to 64 A. They agree well with previous results" which indicate distances between 49 and 64 A. In spite of this agreement, estimates of the maximum uptake of water by the lamellar phase vary. Lis et a1.I' give the value 54% water, while Gutman et a1.I8 report 45% water. (16) Tanford, C . In The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley: New York, 1976. (17) Lis, L. J.; McAlister, M.; Fuller, N.; Rand, R. P.; Parsegian, V. A. Biophys. J . 1982, 37, 657. (18) Gutman, H.; Arvidson, G.;Fontell, K.; Lindblom, G. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; p 143.
1
Formamide
0.5
Dioleoyllecithin
[v / V I Figure 2. Phase diagram for 1,2-dioleo)'-sn-g1ycero-3-phosphocho1ine
(DOPC) and formamide. Below 10% water a complex mixture of crystalline and liquid crystalline phases is formed, probably similar in character to the mixtures observed in the same composition range in the egg lecithin/water system investigated by Luzzati et al.19 A cubic phase is formed at temperatures above 75 OC; it is stable up to 125 OC. At higher temperatures (up to about 230 "C) a reversed hexagonal phase is formed. This phase beh3,vior agrees well with previous reports on the phase behavior of purified soybean lecithinZoand egg l e ~ i t h i n . ' ~ ~ ~ ' Both the cubic and the hexagonal phases are identified through their texture in the polarizing microscope. The texture of the reversed hexagonal phase (Figure 6a) is fanlike and agrees well with the description by R o s e ~ e a r . ~The ~ ! ~observations ~ of the cubic phases are more complicated because the pl we is optically isotropic. However, under favorable conditions the refractive index of the cubic liquid crystals is different enough for the surrounding solvent to make direct observation in nonpolarized light possible (Figure 6b in this paper and Figure 3c in ref 20). In less favorable cases, the cubic liquid crystals can only be observed against an anisotropic background (Figure 6c). If the system is allowed to pass several times over the phase boundaries (particularly the upper one), the size of the liquid crystalline domains increases and it then becomes easier to make good observations of the samples. The texture described above, the isotropic character, the macroscopic stiffness of the samples, the location in the phase diagram (according to general knowledge about the phase equilibria in surfactant/water systems, cubic phases are expected to m u r above the melting of the crystalline phase in the concentration range between the lamellar and the reversed hexagonal phase24),and the similarity to previously investigated lecithin all support the interpretation of the observations in the polarizing microscope. The phase diagram for the system formamide/DOPC is given in Figure 2. The phase equilibria resemble those in the water/DOPC system. The lamellar phase occurs in an extensive concentration range and is stable at least up to 65% of formamide. The results from the X-ray investigations are given in Figure 5b. The maximum solvent layer thickness is 64 A, which is larger than for water. The area per head group is fairly constant (74 A2). At low solvent concentrations (0-10% formamide), a mixture of the lamellar phase and crystalline DOPC (below 40 "C) as well as other liquid crystalline phases is formed. A cubic phase occurs from the temperature range 90-120 to 175-210 "C. A reversed (19) Luzzati, V.; Gulick-Krzywicki, T.; Tardieu, A. Narure (London) 1968, 218, 1031. (20) Bergenstahl, B. A.; Fontell, K. Prog. Colloid Polym. Sci. 1983, 68, 48.
(21) (22) (23) (24)
Small, D. M. Lipid Res. 1967, 8, 551. Rosevear, F. B. J. Am. Oil Chem. SOC.1954, 31, 628. Rosevear, F. B. J . Soc. Cosmer. Chem. 1968, 19, 581. Fontell, K. Mol. Cryst. Liq. Crysr. 1981, 63, 59.
5946 The Journal of Physical Chemistry, Vol. 91, No. 23, 1987 TtCl 200
150
100 ,
~,
,
,
,
Cubic Lamellar
50
Crystalline Methylformamide
0.5
Dioleoyiiecithin (v / V I
Figure 3. Phase diagram for 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and methylformamide. Dispersion of liquid crystal in a solution phase
2ool 150-
100.
Is0 tropic solution
I
50.
Dime thylformamide
Dioleoyiiecithin
(V/Vl Dispersion of crystalllne phase in DMF
Figure 4. Phase diagram for 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and dimethylformamide.
hexagonal phase is formed at solvent concentrations around 5% in the temperature range 175-220 "C. Figure 3 shows the phase diagram for the methylformamide/ DOPC system. A lamellar phase is formed in this system, too, but the swelling of the phase is very limited. The X-ray diffraction studies indicate that the single-phase region is confined to a narrow solvent concentration range around 13% methylformamide (Figure 5c). The repeat distance is about 41.0 %, at the lowest and about 41.5 %, at the highest methylformamide concentration. Observations in the polarizing microscope are consistent with the occurrence of a lamellar phase. When excess solvent is added, fragments of the lamellar phase can be observed until the methylformamide concentration reaches about 40%. Above this concentration the system consists of a solution of DOPC in methylformamide. The solubility limit is given as a dotted line in Figure 3. It is based on observations in the microscope. As the temperature increases, the two-phase area between the lamellar phase and isotropic solution becomes narrower. The highest temperature of existence of the lamellar phase is about 110 "C. Above this temperature a reverse hexagonal phase occurs. This phase is able to incorporate about as much methylformamide as the lamellar phase, but it is stable to considerably lower methylformamide concentrations. The hexagonal phase melts around 230 "C. In the temperature range 80-1 10 OC and at methylformamide concentrations lower than those in the lamellar phase, a cubic phase is observed. Figure 6b illustrates the appearance of a cubic DOPC sample containing 6.4% methylformamide in the polarizing microscope at 95 "C. In the temperature range 40-80 "C a
Bergenstdhl and Stenius mixture of lamellar phase and crystalline DOPC is observed. At temperatures above the stability domain of the cubic phase, the texture is typical of a reverse hexagonal phase (Figure 6a). Figure 4 shows the phase diagram for the dimethylformamide/DOPC system. The Krafft temperature, Le., the lowest temperature at which the formation of liquid crystalline phases or solutions occurs, is considerably higher than in the other systems (=40 "C, compared to well below 0 OC for the other solvents). The melting points of the crystalline phase are indicated by a dotted line in Figure 4 (the lower one). Below this line there is a mixture of crystalline DOPC and almost pure solvent. No lamellar phase is observed in this system. A cubic phase appears in the range 40-70 OC. At higher temperatures a reversed hexagonal phase occurs in a rather broad concentration range. Its melting point is about 220 OC. Figure 6c shows the use of the anisotropic hexagonal phase as a background to the isotropic cubic phase. The sample was first heated to 160 O C where the hexagonal phase is formed. Then the sample was cooled to 60 OC. DOPC dissolves in dimethylformamide, but the isotropic solution area is limited to much higher temperatures and to lower lecithin concentrations than with methylformamide. The solubility curve, as it appears from observations of macroscopic samples, is indicated roughly in Figure 4.
Discussion The striking resemblance between the water and the formamide systems (Figures 1 and 2) suggests that the DOPC interacts with formamide much in the same way as with water. Three major differences are observed when the solvent is changed to dimethylformamide (Figure 4): the lowest temperature at which liquids crystalline phases occur (the Krafft temperature) is considerably raised, there is no lamellar phase, and the DOPC is more soluble in the solvent above the melting point of the crystalline phase. Methylformamide (Figure 3) is intermediate between these two solvents: the Krafft temperature is low, and the region of existence of the lamellar phase is very narrow. However, one difference is that the solubility of DOPC in the solvent is quite large. As starting point for a discussion on these differences, let us consider the possible contributions to the free energy of formation of a DOPC/solvent phase from crystalline DOPC and the pure solvent. We write the free energy as the sum of several contributions:
The first contribution, AGmixing,is the free energy of mixing, which is of course always negative and includes excluded-volume effects and the surfactant self-association which is extensive at most concentrations. Secondly, there is a positive contribution due to the transfer of hydrocarbon chains from a hydrocarbon environment into the solvent, AGhc/solv.According to the current picture of the factors governing the phase equilibria in aqueous systems, the low solubility of hydrocarbon in water ("hydrophobic interaction") is the main driving force for micellization and the formation of liquid crystalline aggregates. This contribution to the free energy of the formation will increase with increased exposed hydrocarbon solvent interface. Hence, its magnitude will follow the sequence molecularly disperse solution >> micellar solution >> lamellar phase > reversed hexagonal or cubic phase. The third term, AGmelting, is the positive contribution from the melting of crystalline DOPC. This term is of course independent of the solvent and will decrease with increasing temperature. Its significance in the DOPC system is obvious from the high melting point of the pure compound, about 200 OC. The last contribution, AGsolvation, is the solvation of the polar head group. Comparison of the magnitudes of this strongly negative term is the actual aim of this study. AGsolvation will also include contributions from the repulsive interactions between the head groups in the aggregates and long-range interactions between
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a, DOPC-water; b, DOPC-formamide; c, DOPC-methylformamide. Lower scale: (+) The fundamental repeat distance (Bragg spacing), d ( 0 )the lipid bilayer thickness, dl; (0)the solvent layer thickness, 4. Upper scale: The area per molecule in the bilayers (calculated assuming constant densities of the lecithin and the solvents independent of the mixing, and the density of DOPC has been estimated to 1 g/mL). Several measurements were made in two-phase systems (lamellar phase dispersed in solvent), for instance the point with 50% DOPC and 50% water and all points with more than 13% of methylformamide. In these cases calculations of bilayer thicknesses or molecular areas are meaningless. A few samples with very small amounts of formamide or methylformamide show two spacings and are partly crystalline. The phase structures in these samples were not determined. TABLE I: Physical Properties of Solvents' solvent diel const diwle moment water 78 1.85 formamide 109 3.73 meth ylformamide 1 82c,c 3.83 dimethylformamide 37 3.82 ethylene glycol 37 2.28
donor no.'
accepter. no.*
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55 40
72.1 58.2
27
16
35.2d 47.7
egg lecithin
Y.
mJ/m2
bD. OC 100 220 180 156 198
fD.
OC
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nnh 1.33 1.4472 1.4319 1.4305 1.4318
n."Pa.s 1 .oo 2.93 1.69 0.82d 19.9
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'Except when specifically noted, all data are from: Handbook of Chemistry and Physics, 58th ed.; CRC: Cleveland, OH, 1977. bFrom ref 25. CFromref 26. dFrom ref 27. eFrom ref 28. fFrom ref 29. 8From ref 30. hRefractive index. 'Viscosity. the aggregates. These factors limit the growth of micellar aggregates and determine the stability and the swelling ability of the different liquid crystalline s t r u ~ t u r e s . * , ~ ~ ~ J ~ Qualitatively, the differences and equalities between the phase diagrams shown in Figures 1-4 may be explained in terms of these interactions. The water and formamide systems are largely similar: the solubility of DOPC in both solvents is low, the lamellar phase swells extensively (on a molar basis, the maximum swelling is about 30 mol of H203* and at least 35 mol of formamide per mole of DOPC), a cubic phase occurs at low temperatures and low solvent concentrations, and a reverse hexagonal phase occurs a t higher temperatures in the same concentration range. The insolubility of DOPC in both solvents and the swelling of the lamellar phase indicate that the balance between "solvophobic" forces (that lead to aggregation) and long-range repulsion between the aggregates (that lead to the swelling) is rather similar in water and formamide. The refractive indexes of lecithin and formamide are rather similar (see Table I), which indicates that the van der Waals attraction
between the layers (which determines the upper limit of the swelling of the lamellar phase) in formamide will be smaller than in water (cf. ref 14). The reduced swelling of the lamellar phase in methylformamide could be due either to the large solubility of DOPC in methylformamide or to a reduced repulsion between the lamellar bilayers. With dimethylformamide, the lamellar phase disappears completely. The solubility of DOPC in this solvent is considerably lower than in methylformamide, although one would expect the interactions between hydrocarbon and dimethylformamide to be considerably more favorable. We suggest that the reduced solubility and the more limited tendency to form liquid crystalline aggregates (the increased Krafft point) are due to a weaker solvation of the polar group. The relatively high solubility of DOPC in methylformamide would then be due to a combination of a weaker solvophobic effect than in formamide (which should promote the transition from lamellar phase into a solution phase) combined with a stronger solvation of the polar choline group than
5948 The Journal of Physical Chemistry, Vol. 91, No.'23, 1987
Bergenstlhl and Stenius forces between DOPC aggregates. Some general characteristics of these sofvents are given in Table I. The polarity of the molecules is fundamentally due to the bond between the electronegative carbonyl group and the electropositive nitrogen. This bond remains largely unchanged in the solvent series, as is indicated by the almost constant dipole moment. The most important difference between the solvents is their ability to accept electrons and, hence, to form hydrogen bonds. In formamide, there are two hydrogens that are able to participate in hydrogen bonding, in methylformamide there is one, and in dimethylformamidethere is virtually none. A quantitative comparison of these differences is given by the donor and acceptor numbers of the compounds according to G ~ t m a n ?also ~ given in Table I. These data in combination with our results suggest that the ability to form hydrogen bonds is an impo'itant characteristic of solvents that solvate lecithin bilayers in such a way that repulsive interactions are created. The reduced association in the solvents is indicated by the decreasing viscosities, boiling points, freezing points, and surface tensions in the series (Table I). The dielectric constant varies strongly in the solvent series. This is so because it is closely related not only to the dipole moment but also to the molecular association induced by hydrogen bonding in the solvent. The dielectric constant of methylformamide is significantly larger than for formamide but much lower in dimethylformamide. However, the value for dimethylformamide, 37,is of the same order as the value for ethylene glycol. In the latter solvent repulsive forces between lecithin bilayers occur and the phase equilibria develop just as in water.12-14 It is therefore not possible to correlate the observed changes in the phase equilibria directly to changes in the dielectric constant. The changes in the regions of existence of the cubic and reversed hexagonal phases with the solvent in this series are rather limited. In these phases, interactions between the hydrocarbon moieties will be of primary importance in the balance that gives rise to certain structures. That the diagrams are largely similar suggests that only minor amounts of the solvents penetrate the hydrocarbon parts of the reversed hexagonal or the cubic phases. In a comparison of micelle formation in water and hydrazine31 considerable differences are observed at low temperatures, but the behavior in both solvents becomes more similar as temperature increases. A similar interpretation of the present results would suggest that the similarities of the phase diagrams above 100 "C indicate that the solvation properties of the solvents at these temperatures are quite similar. Finally, we note that the formation of liquid crystalline phases in a non-hydrogen-bonding solvent, dimethylformamide, is also an interesting phenomenon in its own right.
Acknowledgment. We gratefully acknowledge that it was Dr.
V. A. Parsegian who suggested the use of formamides as a gradually changing solvent series and Dr. P. K.T. Persson who
Figure 6. ,Microscopic texture of samples between crossed polarizers (magnification60X): (a, top) The reversed hexagonal phase. The Sample contains 6.4% (v/v) methylformamide; temperature 160 OC. (b, middle) The cubic phase in nonpolarized light; 6.4% methylformamide, temperature 95 O C . (c, bottom) The occurrence of cut& phase seen against an optically anisotropic background of reversed hexagonal phase. The sample contains 37%dimethylformamideand was first heated to 150 O C and then cdoied to 60 OC. This sample is located in a two-phase area (solution-liquid crystalline phase), but the amount of solution is small an&" can 'be seen in this micrograph.
suggested the use of formamide as an interesting solvent analogous to water. We also thank Dr. K.Fontell for his highly appreciated advice with regard to the choice of reference compounds in the X-ray experiments. Registry NO. D O E , 4235-95-4; FA, 75-12-7; MFA, 123-39-7;DMF, 68-1 2-2. ~~
(25) Gutman, V. The Donor and Acceptor Approach to Molecular Interactions; Plenum: New York, 1978. (26) Ray, A. Nature (London) 1971, 231, 313. (27) Eberling, C. L:In Kirk Othmer Encyklopedia of Chemical Technology 3rd ed.; Wiley-Interscience: New York, 1979; Vol. 11, pp 259-268. (28) Leader, G. R.; Gormley, J. F. J . Am. Chem. SOC.1951, 73, 5732. in dimethylformamide (which especially should favor the melting (29) French, C. M.; Glover, K. H. Trans. Faraday SOC.1955,51, 1418. (30) Cherry, R. J.; Chapman, D.J. Mol. Biol. 1965, 40, 19. of the crystalline phase, but also will contribute to increase the (31) Ramadan, M. S.; Evans, D.F.; Lumry, R. J. Phys. Chem. 1983,87, solubility dirdly). Thus, the solvents formamide, methylform4538. amide, and dimethylformamide represent a strong, an interme(32) This value is difficult to determineaccuratelv. The results of ref 17 solvation diate. and a weak abilitv to solvate DOPC and to- create .. . _ - _ _ - . _ _ _ _indicate 50 and those of ref 18 35 mol of H20/molrof DOPC. ~
I
