J. Phys. Chem. 1983, 87, 533-535
533
Liquid Crystals in a Fused Salt: ,6,y-Distearoylphosphotidylcholine in N-Ethylammonium Nitrate D.
F. Evans,’ E. W. Kaler,
Department of Chemlcal Englneerlng and Meterlels Science, Unlverslty of Mlnnesota. Mlnneapolls, Minnesota 55455
and W. J. Benton Department of Chemical Engineerlng, Rlce Unlversity, Houston, Texas 7725 1 (RecelveO: October 25, 1982; In Flnal Form: December 17, 1982)
The formation of liquid crystals of P,y-distearoylphosphotidylcholine(DSPC) in N-ethylammonium nitrate, a low-melting fused salt, has been documented with polarizing microscopy and small-angle X-ray scattering. L, transitions with increasing temperature are present in analogy with the behavior of DSPC The Lr P, in water. For a 1:l (by weight) mixture the d spacing for the L, phase is 63 8, and the surfactant head group area is 76 A2.
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Introduction Our experiments demonstrate the formation of lyotropic smectic phases in N-ethylammonium nitrate (EAN), a low melting fused salt. While liquid crystals have been extensively studied in aqueous solutions’ and characterized in other polar solvents? our observation appears to be the first report of liquid crystal formation in a completely ionic medium. N-Ethylammonium nitrate has three hydrogen bond sites per solvent molecule and thus possesses the ability to form a three-dimensional structure in the liquid state. The transfer of rare gases and hydrocarbons from a reference solvent like cyclohexane to EAN is accompanied by negative enthalpy and entropy changes similar to those observed in water, a result consistent with the structuring of this fused salt around hydrocarbon chain^.^ This suggests the possibility of “hydrophobic”-likeinteractions with nonpolar groups and amphiphiles. Surfactants aggregate to form micelles in EAN, although the critical micelle concentrations (cmc) are seven to ten times larger than those reported in aqueous solution4and the micelles are considerably smaller. The second virial coefficients for micellar solutions are close to those predicted for hard spheres and reflect the almost complete suppression of Coulombic forces as might be expected in a medium which is 11.7 M in ions.5 It would appear that EAN provides an especially useful new solvent for studying amphiphile aggregation and colloidal interactions in liquids. Experimental Section &y-Distearoylphosphotidylcholine(DSPC) was used as received from Calbiochem-Behring Corp. and was stored under refrigeration. N-Ethylammonium nitrate (EAN) was prepared by adding 3 M nitric acid to a cooled 25% aqueous solution of ethylamine so that a slight excess of amine remained. Most of the water was removed with a (1)Tiddy, G. T. J. Phys. Rep. 1980,57, 1. (2) (a) Moucharafieh, N.; Friberg, S. E. Mol.Cryst. Liq. Cryst. Lett. 1979, 49, 231-8. (b) Larsson, K. J. Sci. Food. Agric. 1978, 29, 909. ( c ) Larsen, D. W.; Friberg, S. E.; Christenson, H. J. Am. Chem. SOC.1980, 102,6565. (d) Nokaly, M. F.; Ford, D. L.; Fribert, S. E.; Larsen, D. W. J. Colloid.Interface Sci. 1981, 84,228. (e) Gan-zuo, L.; El-Nokaly, M.; Friberg, S. E. Mol.Cryst. Liq. Cryst. Lett. 1982, 72,183-8. (0Larsen, D.W.; Rananavare, S. E.; El-Nokaly, M.; Friberg, S. E. In press. (3)Evans, D.F.;Chen, S.-H.; Schriver, G. W.; Arnett, E. M. J.Am. Chem. SOC.1981, 103, 481. (4) Evans, D.F.; Yamauchi, A.; Roman, R.; Casassa, E. Z. J. Colloid Interface Sci. 1982, 88, 89. ( 5 ) Evans, D.F.; Yamauchi, A.; Wei, G. J.; Bloomfield, V. A. J.Phys. Chem., submitted for publication.
rotary evaporator; final drying was achieved with a lyophilizer. The anhydrous fused salt was then dissolved in acetonitrile, mixed with activated charcoal, filtered, recrystalized, and redried in the lyophilizer. Weighted portions of DSPC and EAN were combined in Teflon-capped test tubes and homogenized with successive heating and cooling cycles, vortex mixing, centrifugation, and ultrasound. Optical studies were conducted with a polarizing microscope (Optiphot-Po1,Nikon) equipped with a hot stage attachment (Mettler FP52/ FP5). Thin layers (-20 pm) of the sample were formed between microscope slide and coverslip. Typically, a mixture was heated to 80 “C or above and allowed to anneal for 24 h and then observed over a temperature range from 25 to 100 “C. Small-angle X-ray patterns were obtained with a flatplate Warhaus f i b camera by using Cu Ka radiation and Kodak No-screen X-ray film. Samples were loaded in 1-mm diameter glass capillary tubes and equilibrated at the temperature of interest for at least 24 h before beginning measurements. Results Two transitions were observed by polarizing microscopy in a 1:l mixture by weight (11mol % ) of DSPC-EAN at 56.7 and at 57.3 “C. From 57.3 to 100 “C (the highest temperature studied) a smectic A texture consisting of “oily streaks” interspersed with planar or pseudo-isotropic regions was observed (Figure 1, T = 57.6 “C). The “oily streak” regions are tilted bilayers6and the dark regions are where the bilayers are perpendicular to the optic axis. Below 56.7 “C the “oily streaks” take a rigid angular appearance and appear as bundles of sticks (Figure 2, T = 56.1 “C). As the temperature of the mixture is lowered and passes through 56.7 “C an increase in birefringence is observed which spreads rapidly across the field as the new texture of rigid “oily streaks” slowly forms. Diffraction patterns were measured for the sample at 90,85,75, and 55 “C. The patterns obtained at 75 “C or higher are identical. They show two reflections in the ratio 1:1/2as expected for a lamallar phase (L,) in addition to a very faint, broad reflection corresponding to a spacing of -4 A. The d spacing is 63 A. The L, phase at 55 “C produced four low-angle reflections in the ratio 1:1/2:’/3:’/4 as well as sharp reflection corresponding to a 4.1-A spacing. The L8, d spacing is 59 A. The surface area per DSPC (6) Asher, S. A.; Pershan, P. S. Biophys. J. 1979,27,393-422.
0022-3654/83/2087-0533$01.50/00 1983 American Chemical Society
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The Journal of Physical Chemisfry, Vol. 87, No. 4, 1983
Letters
Flgure 1. Micrograph of &ydistearoylphosphotidylcholine-N-ethylammonium nitrate 1: 1 mixture through crossed polarizers, T = 57.6 OC.
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b
4
r
I 3
I
! 1 I I
-re
2. Micrograph of /3,ydistearoylphosphotidylcholine-N-ethylammonium nitrate 1:1 mixture through crossed polarizers, T = 56.1 OC.
molecule in the bilayer can be calculated by using the partial molar volumes of DSPC (0.958 cm3/g, ref 7) and EAN (0.820cm3/g, ref4)- The surface areas are 76 A2 in the L, phase and 81 A2 in the L , phase.
Discussion The transitional behavior of DSPC-EAN mixtures is analogous to the well-known order-disorder or gel-liquid crystal transition found in lecithin-water mixtures.s-10
(7) Tardieu, A.; Luzzati, V.; Roman, F. C. J. Mol. Biol. 1973, 75, 711-22.
254,6068-78.
(8)Janiak, M.J.; Small, D. M.; Shipley, G. G. J. Biol. Chem., 1979,
The Journal of
Letters
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The phase transitions observed with increasing temperaP, La. The P, phase ture have been designated L, is an intermediate between the L, and La phases where the bilayers are in a rippled arrangements7 The P, phase changes to L, phase at higher temperature. For DSPCHzO, the reported transitions are 51.5 "C for the L, P, and 54.9 "C for the P, La.B For the DSPC-EAN mixture the transition temperatures are increased to 56.7 and 57.3 "C, thus the P, phase is present over only a 0.6 "C temperature range. The ratios of the X-ray reflections and the appearance of a sharp reflection at 4.1 A for the L, phase are analogous to the behavior of DSPC in water.7 However, the d spacings measured in EAN mixtures are slightly smaller than those reported for DSPC in water7 (64 A, L,). Also, the surface areas per lecithin molecule are much higher in EAN than those in water (52 A2 for L, phase).7 A similar difference in surfactant head group area I observed in micellar solutions. From light-scattering measurements, the aggregation numbers for tetradecyl- and hexadecylpyridinium surfactanta are 17 f 1and 26 f 2 in EAN.6 For
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(9) Mabrey, S.;Sturtevant, J. M. Methods Memb. Biol. 1978, 9, 237-74. (10) Chapman, D. In 'Ordered Fluida and Liquid Crystale";Porter, R. S.; Johnson, J. F. Ed.; American Chemical Society: Washington, DC, 1967; Adv. Chem. Ser. No. 63, pp 157-66.
Physical Chemistry, Vol. 87, No. 4, 1983 535
spherical micelles, these correspond to surface areas of 100
A2,which are considerably larger than the corresponding values of 50-60 A2 observed in aqueous solution.
The paucity of information presently available makes it difficult to account in detail for differences in the lecithin d spacing and head group areas in EAN and water. However, the following factors are likely to be important. (1)The interfacial tension for oil-fused salt (20 dyn cm-l)I1 is considerably lower than that observed for oil-water (50 dyn cm-I). This must be a major factor in determining the amount of hydrocarbon exposed to the solvent. (2) The interaction between adjacent zwitterionic head groups within a given bilayer will be very different in part because of the ionic character and larger size of EAN. (3) The interaction between the two bilayers may be significantly arising from modified. For example, hydration the zwitterionic head group-dipolar solvent interaction in water will be very different in a completely ionic solvent in which double layer forces are greatly attenuated. These factors will be explored in forthcoming publications. Registry No. DSPC, 4539-70-2; EAN, 22113-86-6. (11) Mukherjee, S.;Evans,D. F., unpublished data. (12) Pashley, R. M. J. Colloid Interface Sci. 1981, BO, 153. (13) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 2, 169. (14) Lis, L.J.; McAlister, M.; Fuller, N.; Rend, R. P.; Parsegian, V. A. Biophys. J. 1982, 37, 657.
