Formation of a Liquid Crystalline Phase in the Water− Sodium

Langmuir 1998, 14, 3691-3697

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Formation of a Liquid Crystalline Phase in the Water-Sodium Taurodeoxycholate System Håkan Edlund,† Ali Khan,*,‡ and Camillo La Mesa§ Department of Chemistry and Process Technology, Chemistry, Mid Sweden University, S-851 70 Sundsvall, Sweden, Physical Chemistry I, Centre for Chemistry and Chemical Engineering, P.O. Box 124, University of Lund, S-221 00 Lund, Sweden, and Dipartimento di Chimica, Universita` degli Studi “La Sapienza”, p.le Aldo Moro 5, 00185 Roma, Italy Received November 3, 1997. In Final Form: March 26, 1998 The binary phase equilibria for the system sodium taurodeoxycholate-water have been studied. The system forms a liquid crystalline phase in addition to the previously known isotropic solution phase at 22 °C. 2H (water) NMR quadrupole splitting, SAXS data in combination with the polarizing microscopic texture observed for the liquid crystal indicate that the liquid crystalline phase consists of a hexagonaltype aggregate structure. A metastable liquid crystal, probably with lamellar-type structure, also appears to exist prior to the formation of the stable hexagonal phase. A micellar growth is measured with the NMR self-diffusion method for the isotropic solution phase. Electrical conductance experiments are used to determine the liquid crystal-solution thermal transition.

Introduction Investigations on the polymorphic behavior of bile salts have received much attention, in view of the possible links between phase structure and dismetabolic diseases.1 Studies on the lyotropic organization in mixtures of bile salts and surfactants, or lipids, have been reported.2-5 The formation of gels and fibers is well documented,6,7 but no evidence has, however, been presented on the occurrence of liquid crystalline behavior in water-bile acid salt systems. Previous investigations have dealt with systems containing water and alkali metal cholanates, their mono-, di-, and trihydroxy derivatives, as well as some glyco and tauro conjugated salts. The phase behavior for most water-bile acid salts mixtures is well established from several independent investigations.8-11 Because of their very peculiar molecular structure with one or more OH groups on the same polar side of the steroidal skeleton, bile acid salts do not have the unequivocal separation into polar and nonpolar regions which most surfactants and lipids have. Also the bile acid salts do not have flexible alkyl chains. The molecular structure of sodium taurodeoxycholate is drawn in Figure 1. The numbers 3 and 12 indicate the location of OH groups in the steroidal nucleus. In most instances (with the exception of cheno bile acids) hydroxyl * Corresponding author. Telephone: +46 46 2223247. Fax: +46 46 2224413. E-mail: [email protected]. † Mid Sweden University. ‡ University of Lund. § Universita ` degli Studi “La Sapienza”. (1) Small, D. M. J. Colloid Interface Sci. 1977, 58, 581. (2) Ulmius, J.; Lindblom, G.; Wennerstro¨m, H.; Johansson, L. B. Å.; Fontell, K.; So¨derman, O. Biochemistry 1982, 21, 1553. (3) La Mesa, C.; Khan, A.; Fontell, K.; Lindman, B. J. Colloid Interface Sci. 1985, 103, 373. (4) Sva¨rd, M.; Schurtenberger, P.; Fontell, K.; Jo¨nsson, B.; Lindman, B. J. Phys. Chem. 1988, 92, 2261. (5) Swanson-Vethamuthu, M.; Almgren, M.; Bergenståhl, B.; Mukhtar, E. J. Colloid Interface Sci. 1996, 178, 538. (6) Rich, A.; Blow, D. M. Nature 1958, 182, 423. (7) Blow, D. M.; Rich, A. J. Am. Chem. Soc. 1960, 82, 3566. (8) Ekwall, P. J. Colloid Sci. 1954, 6, suppl. I, 1. (9) Fontell, K. Kolloid Z. Z. Polym. 1971, 244, 246. (10) Fontell, K. Kolloid Z. Z. Polym. 1971, 246, 710. (11) Carey, M. C.; Small, D. M. Arch. Intern. Med. 1972, 132, 506.

Figure 1. Molecular structure of sodium taurodeoxycholate, NaTDC, 3R,12R-dihydroxy-5β-cholan-24oic-acid-N-[2-sulfoethyl]amine sodium salt. Both OH groups are facing in the same direction, giving rise to one polar and one nonpolar side to the steroidal nucleus.

groups linked to the steroidal nucleus of bile acid salts are facing toward the same direction. Thus, from a geometrical point of view, these molecules can be considered to have planar structures with polar and apolar sides, respectively. Their packing gives supramolecular aggregates different from proper micelles and is expected to play a significant role in mesophase formation.12,13 Here, we present a preliminary study on the binary phase behavior of the water-sodium taurodeoxycholate system at 22 °C. Single phases and phase microstructure are characterized by combining visual observation, optical polarizing microscopy, small-angle X-ray diffraction, 2H NMR quadrupole splitting, NMR self-diffusion, and electrical conductance techniques. The present contribution is a part of a more extensive work on the phase behavior and phase structure of some taurine-conjugated bile acid salts, including the thermal stability of phases and the role of added sterols on the phase equilibria of the systems.14 Experimental Section Materials. Sodium taurodeoxycholate, referred to as NaTDC, 99% nominal purity, was purchased from Aldrich. It was purified (12) Luzzati, V.; Mustacchi, H.; Skoulios, A. E.; Husson, F. Acta Crystallogr. 1960, 13, 660. (13) Fontell, K. Mol. Cryst. Liq. Cryst. 1982, 63, 59. (14) Khan, A.; La Meas, C. Manuscript in preparation.

S0743-7463(97)01197-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/28/1998

3692 Langmuir, Vol. 14, No. 13, 1998 by dissolution in hot ethanol, filtered, and precipitated upon addition of cold acetone. The fine white powder was dried under vacuum (10-3 mmHg) at 70 °C for 1 day. Analytical grade ethanol and acetone, Fluka, were used as received. Isotopic enriched deuterium oxide, 99.8 atom % 2H, was purchased from Cambridge Isotopes Laboratories, Maine. Doubly distilled, filtered, and deionized water (conductivity κ of about 10-6 Ω, at 25 °C) was used. Sample Preparations. The samples were prepared by weight, in tightly sealed or flame-sealed glass tubes. The contents of the samples were mixed by repeated centrifugation at low speed (3000-5000 rpm) several times, heated in an air oven at 50 °C for 30 min, and centrifuged again to avoid bubble formation. After thorough mixing, the samples were allowed to stand at room temperature for several days for equilibration. Methods. Preliminary visual observations were performed between crossed polars. The optical textures of surfactant samples were examined with Optical Axioplan (Zeiss) and Ceti polarizing optical microscopes. The microscopes are equipped with a hot stage, operating between 10 and 65 °C with an accuracy of (0.5 °C. 2H is a quadrupole nucleus with the spin quantum number I ) 1. When deuterium is present in an anisotropic hexagonal or lamellar liquid crystalline phase, the 2H NMR spectrum is split to a doublet. The 2H NMR spectra were recorded at a resonance frequency of 41.45 MHz by a JEOL FX 270 superconducting pulsed FT-NMR spectrometer. A control unit was used to regulate the air-flow temperature at 22 ( 0.5 °C. Samples were equilibrated at 22 °C for a couple of weeks before being transferred into the preheated NMR probe. They were kept in the probe for 15 min before the spectra were recorded. The procedure was repeated a few times within a period of six months. Analysis of the 2H NMR spectra indicates that the samples attained equilibrium within 3-4 weeks. 1H NMR self-diffusion measurements were performed on the same spectrometer by using the Fourier transform pulsed fieldgradient spin-echo (FT-PGSE) method at 22 °C. Details on the apparatus setup and data acquisition, etc., are described elsewhere.15,16 Small-angle X-ray scattering (SAXS) experiments were performed by a Kratky compact system equipped with a positionsensitive detector, containing 1024 channels of width 51.3 µm.17 A Cu KR radiation of wavelength 1.54 Å was used, and the sample to detector distance was 277 mm. The samples for the SAXS measurements were placed into a paste holder between thin mica windows and the temperature in the X-ray camera was controlled by a Peltier element. During the measurements, the space between the sample and the detector was kept under vacuum, to minimize air scattering. Electrical conductance was measured by an Amel conductivity meter, model 134, equipped with a platinum-coated open cell (cell constant of 1.001 cm-1).18 The lyotropic mesophase samples were put into Pyrex glass tubes (15 mm i.d.) and gently heated to 40 °C, up to the formation of a homogeneous solutions phase. The cell was immersed in the liquid and allowed to equilibrate overnight at room temperature. Teflon holders were used to lock the cell position in the measuring tube. The samples were located in a Haake thermostatic unit, and the temperature was increased at 0.03-0.05 °C min-1. A similar procedure was used in the cooling mode. The temperature was measured by a Heto PT-CB II thermometer, with an accuracy of (0.005 °C.

Results Phase Behavior. The phase behavior of the system water-NaTDC was investigated by examining the samples between crossed polars and with optical polarizing microscopy, X-ray diffraction, NMR, and electrical conductance methods. The phase boundaries from the above methods are superimposable, within the limits set up by the experimental accuracy. (15) Stilbs, P. Prog. Nucl. Magn. Reson. 1987, 19, 1. (16) Regev, O.; Kang, K.; Khan, A. J. Phys. Chem. 1994, 98, 6619. (17) Singh, M. A.; Ghosh, S. S.; Shannon, R. F., Jr. J. Appl. Crystallogr. 1993, 26, 787.

Edlund et al.

Figure 2. Phase diagram of the binary water-sodium taurodeoxycholate system at 22 °C. 2Φ denotes a two-phase region.

Investigation in isothermal and isoplethal mode allows us to define the widths of the two-phase regions, both solution-liquid crystal and solid-liquid crystal. In the latter case the accuracy of the phase boundaries is low and thermal hysteresis is large. The isothermal phase diagram of the system waterNaTDC is presented in Figure 2, at 22 °C. The solution region extends up to about 26 wt % NaTDC and is followed by a complex two-phase region, indicated as 2Φ in the phase diagram. There are significant differences within the biphasic region. From 26 wt % NaTDC, a solution region in equilibrium with spherulitic crystals was observed, and at concentrations above 29 wt % NaTDC a metastable liquid crystalline phase coexists with the solution. The macroscopic appearance of some selected samples in the complex two-phase region is shown in Figure 3. All samples were heated to yield a single solution phase and then allowed to stand at room temperature. On cooling, the samples indicated by the capital letters E, F, G, and H in Figure 3a form an upper mesophase within a few hours at the air-solution interface, where the bile salt molecules have been concentrated. After some time the formation of spherulitic crystals (or pseudocrystals) is observed on the sample surface. With time the crystals tend to sink to the vial bottom, as can be seen in Figure 3b. The crystals look like spherulitic units of nearly constant size arranged to form maltese crosses. These maltese crosses are one of the typical textures for a lamellar liquid crystalline phase. The behavior of maltese crosses in a solution phase was observed as early as 1954.19 The behavior of surfactant-water mixtures reported in Figure 3 is reproducible. The volume of the mesophase increases from sample E to sample H. Sample H, which is close to the single liquid crystalline phase, contains a very small quantity of the isotropic solution. The content of the mesophase increases in the order sample A > sample B > sample C, and only a small quantity of the mesophase is detected in sample A, which lies near the single solution phase. However, similar treatment of samples prepared in the single solution phase region does not give rise to the formation of any mesophase within the observation time (several months). The kinetics of phase separation in the complex twophase regions is slow. For instance, when a sample of nominal concentration of 27.6 wt % NaTDC is heated for the formation of an isotropic solution phase and, then, allowed to equilibrate at room temperature, an upper viscoelastic phase is formed in the test tube. This phase is transformed into a biphasic solid-solution system within several hours. The above properties are unusual compared to those of systems forming liquid crystalline phases20 and imply, presumably, an epitaxial relationship between structures (18) La Mesa, C.; Sesta, B. J. Phys. Chem. 1987, 91, 1450. (19) Rosevear, F. B. J. Am. Oil Chem. Soc. 1968, 19, 581.

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Figure 3. (a, top) Optical appearance of different samples on increasing the concentration of bile salt. In samples A-G, the amount of NaTDC is increasing from 26 to 31 wt %. In samples D and E, spherulitic crystals are formed on the surface of a “liquid crystalline” phase and sink to the bottom. The liquid crystalline sample indicated by the letter H contains 41.0 wt % NaTDC. (b, bottom) Sample D (28.3 wt % NaTDC) against the polarized light, kept at 22 °C for several hours. Note the crystal shape at the bottom of the test tube and the nucleation of crystals into the upper liquid crystalline phase.

of the different aggregates in solution, solid, and liquid crystalline state.21 The range of existence of the single liquid crystalline phase is between about 37 and 60 wt % NaTDC. Hydrated crystals, in equilibrium with liquid crystalline material, (20) Laughlin, R. G. In Surfactants; Tadros, Th., Ed.; Academic Press: New York, 1984; p 53. (21) Ranc¸ on, Y.; Charvolin, J. J. Phys. Chem. 1988, 92, 2646.

are observed at concentrations higher than 60 wt % NaTDC. Polarizing Microscopy. The anisotropic liquid crystalline samples were observed by optical polarizing microscopy. The samples exhibit nongeometrical striated textures, one of the general textures observed for hexagonal liquid crystalline phases.19 The micrograph recorded in the hexagonal phase at 40.0 wt % NaTDC, at room temperature, is shown in Figure 4.

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Figure 4. Nongeometrical striated microscopic texture of a hexagonal liquid crystalline sample (40 wt % NaTDC): crossed polarizers, ca. 150×, 22 °C.

Figure 6. Slit-smeared small-angle X-ray scattering patterns obtained in (a) 40.0 wt % NaTDC and (b) 43.1 wt % NaTDC, at 22 °C. The positions with respect to the first most intense peak, q1, are shown on the upper X-axis. Bold arrows indicate the position of the reflections discussed in the text; weak arrows indicate reflections not completely understood. Table 1. Experimental SAXS Findings for Some Selected Samples in the Liquid Crystalline Phase of the Sodium Taurodeoxycholate-Water System at 22 °C

2H

Figure 5. NMR spectra recorded in the liquid crystalline phase at 22 °C.

For NaTDC concentrations between 70 and 74 wt % a dark background (indicating the occurrence of an isotropic phase) is observed between crossed polars; however, attempts to separate the phase from floating crystals were unsuccessful. This is indirect evidence of the very high viscosity of the isotropic phase. 2 H NMR Quadrupole Splittings. We have observed a quadrupolar splitting in the 2H NMR spectra for the liquid crystalline phase. Some 2H NMR spectra recorded in the liquid crystalline phase are reported in Figure 5. The ∆ values, measured at the peak-to-peak distance in hertz, range between 100 and 400 Hz and are very small compared to those of other systems.22 Moreover, the 2H NMR spectra recorded, even after allowing the liquid crystalline samples to equilibrate for several months, did not develop into powder-type anisotropic spectra. (22) Khan, A. In Nuclear Magnetic Resonance. A Specialist Periodic Report; Webb, G. A., Ed.; Royal Society of Chemistry: London, 1995; Vol. 24, p 514 and references therein.

%NaTDC

d (Å)

rHC (Å)

q1 (Å-1)

q2 (Å-1)

q3 (Å-1)

40.0 41.8 43.1 44.7

51.5 52.3 51.0 50.3

20.3 21.1 20.9 20.9

0.122 0.120 0.123 0.125

0.210 0.207 0.213 0.217

0.231 0.227 0.232 0.236

Small-Angle X-ray Scattering. Two SAXS diffraction patterns, obtained from samples in the anisotropic liquid crystalline phase, are presented in parts a (40.0 wt % NaTDC) and b (43.1 wt % NaTDC), respectively, of Figure 6. The thin arrows in the two figures indicate very weak reflections, or broad bumps, which are neither indexed nor discussed here. In Table 1 X-ray diffraction data are reported for a few samples in the lyotropic liquid crystalline phase. For a sample containing 48.5 wt % NaTDC, no small angle diffraction patterns were observed, although anisotropic textures were observed in the polarizing microscope. The scattering functions for the samples in Figure 6 are dominated by strong correlation peaks at q1 ) 0.122 Å-1 and q1 ) 0.123 Å-1, respectively. Weaker reflections are observed at higher q values. The relative positions of the three Bragg peaks are 1:x3:1.89, for all samples investigated between 40 and 48 wt % in the liquid crystalline region. The three Bragg peaks are close to the 1:x3:2 relationship typical for hexagonal liquid crystals

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Figure 7. Electrical conductance in msiemens (mS), as a function of temperature in °C, for a sample containing 46.4 wt % NaTDC. Notations: E, liquid crystalline phase; L1, solution phase; 2Φ is the two-phase region.

Figure 8. 1H NMR line widths, ν1/2, measured for the methylene proton at carbon 26 of the bile acid salt shown as a function of the concentration of bile salt in the isotropic micellar solution phase at 22 °C.

(i.e., hexagonal arrangement of cylindrical or helical assemblies).23,24 However, the hexagonal structure is not unequivocally ascertained from the location of the above diffraction peaks. Assuming that the hexagonal phase is made of long cylindrical aggregates, the radius of a rod, rHC, can be calculated from the equation

in equilibrium conditions are superimposable with those obtained in scanning mode. Further information on the phase behavior close to the transition region can be obtained by combining electrical conductance with other findings. When κ values in Figure 7 decrease, the apparent turbidity of the samples increases. The combination of such effects implies that the correlation length for composition fluctuations increases close to the phase boundaries. Conversely, the orientational order of the lyotropic domains, controlling ion motion, decreases. The two effects coexist up to complete disappearance of the two-phase region. 1 H NMR Line Width. For 1H NMR in micellar solutions, there are a number of factors that determine the line width, and the theory can be complex.27 In this study, we are interested in investigating the possibility of a micellar growth and of a sphere-to-rod micellar transition as a function of surfactant concentrations. Such changes are expected to be manifested in the change of the sharp 1H line for the small spherical micelles (ν1/2 ≈ 5-10 Hz) to a broad one for large nonspherical micelles (ν1/2 > 10 Hz).28 This is due to the fact that as the surfactant aggregates increase in size, it takes a longer time for them to rotate as well as for a surfactant molecule to diffuse along the curved micelle surface. The long correlation time for large aggregates will make the 1H NMR tranverse relaxation rate rapid. The proton NMR line widths ν1/2 measured at half-height of the resonance signal for the methylene proton at carbon 26 of the bile acid salt are shown in Figure 8 as a function of the concentration of bile salt. The results indicate a growth of micelles at a rather low concentration, and the growth slowly increases with increased NaTDC concentration. This finding is similar to that reported for the system sodium deoxycholate-water3 but unlike that for the sodium cholate-water system, in which no micellar growth is measured in the isotropic micellar solution phase.5 1H NMR Self-Diffusion. The decay of proton signals after perturbation by a pulsed field gradient obeys the equation29

rHC ) dx(2ΦHC/πx3)

(1)

where d is the value of the first-order Bragg spacing and ΦHC is the volume fraction occupied by the rods. From the radius of the cylindrical aggregates the area per polar head group SA is obtained by the equation

SA ) 2VHC/rHC

(2)

where VHC is the volume of the surfactant molecule. The aggregate radii and other lattice parameters for some samples are given in Table 1. Electrical Conductance. This technique was used in isoplethal mode, to determine liquid crystallinesolution and solid-liquid crystalline phase transitions. In the latter case the uncertainty in the location of the transition temperature is high and is sensitive to heating rates. Conversely, the liquid crystalline-solution transition temperature can be easily defined from experiments. A typical experimental run is shown in Figure 7, where electrical conductance values κ are reported as a function of temperature. The hysteresis on cooling is significant. This is not unusual in the transition between the lyotropic mesophase and the solution phase or between the solid phase and the solution phase,25 as well as in close proximity to the Krafft point (i.e. the micellar solution-disperse solid phase transition).18 Hysteresis depends on the phase structure and geometry,26 on cooling rates, applied magnetic fields, etc. To rule out the occurrence of spurious (kinetic) effects on the observed behavior, additional measurements were performed by allowing the samples to equilibrate at each temperature for 1 h. The electrical conductance values (23) Fontell, K. Mol. Cryst. Liq. Cryst. 1981, 63, 59. (24) Fontell, K. In Liquid Crystals and Plastic Crystals; Gray, G. W., Winsor, P. A., Eds.; Ellis Horwood: Chichester, 1974; Vol. 2, p 80. (25) Franc¸ ois, J.; Skoulios, A. Kolloid Z. Z. Polym. 1967, 219, 144. (26) Muzzalupo, R.; Ranieri, G. A.; Terenzi, M.; La Mesa, C. Ber. Bunsen-Ges. Phys. Chem. 1995, 99, 515.0.0.

(27) Ulmius, J.; Wennerstro¨m, H. J. Magn. Reson. 1977, 28, 309. (28) Khan, A.; Lindman, B.; Shinoda, K. J. Colloid Interface Sci. 1989, 128, 396. (29) Lindman, B.; Wennerstro¨m, H.; So¨derman, O. In Surfactant Solutions. New Methods of Investigation; Zana, R., Ed.; Marcel Dekker, New York, 1986; Chapter 6, p 295.

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In[A°/AG] ) (γδG)2[∆-δ/3]D

Edlund et al.

(3)

where AG and A° are the echo signal amplitudes with and without applied gradient pulses, respectively, γ is the gyromagnetic ratio of the proton, and δ and G are the width and intensity of the field gradient pulse, respectively. The diffusion time, i.e., the effective time during which the spin diffusion is measured, is given by ∆, and D is the measured self-diffusion coefficient. Equation 3 is a simplified version of the original equation, taking into account the amount of nuclei in a certain chemical environment, as well as their transverse relaxation times and diffusivity, respectively.30 The self-diffusion coefficients of water, DHDO, and surfactant, Dbile, measured in the micellar solution phase are plotted in Figure 9 as a function of bile salt concentration at 22 °C. The accuracy of the D values for both the components is between 1 and 2%. In the solution phase the water self-diffusion coefficient decreases monotonically on increasing the amount of NaTDC. As to the self-diffusion behavior of the bile acid salt, we have observed a variation of about 1 order of magnitude in the self-diffusion coefficient for the investigated concentration range. This indicates that the micellar growth is continuous. For instance, at concentrations close to 2 and 16 wt % NaTDC, the surfactant self-diffusion values are about 9 × 10-11 and 1 × 10-11 m2 s-1, respectively. The corresponding hydrodynamic radii increase from about 35 Å at 2 wt % to 315 Å at 16 wt % surfactant calculated from the self-diffusion data and not taking into account obstruction of the aggregates. Discussion Information from the above results allows us to make some preliminary comments about the phase behavior of the binary water-NaTDC mixtures. Optical polarizing microscopy, deuterium quadrupole splitting, and smallangle X-ray diffraction data indicate the occurrence of a liquid crystalline phase in the phase diagram. The observed phase structure is of hexagonal symmetry; perhaps, the behavior is quite distinct from that of n-alkyl chain surfactants, such as sodium dodecyl sulfate or dodecyltrimethylammonium bromide.31,32 The region of existence of the this mesophase is sensitive to temperature and extends up to 35 °C. This is a small temperature range compared to those for most ionic surfactant systems that form liquid crystalline phases. Such low thermal stability could be one of the reasons why the existence of a liquid crystalline phase in waterNaTDC mixtures has not been previously reported. In addition, the puzzling behavior close to the phase boundaries, with formation of spherulitic crystals on the sample surface, could have led other authors not to investigate the whole system in more detail. A remarkable effect peculiar to this system is the unusual behavior between the solution region and the liquid crystalline phase. To our knowledge, such a complex phase behavior, with occurrence of a metastable lyotropic phase, has not been reported previously. Furthermore, the kinetics of phase separation in this particular region is very slow, up to some days. 2 H NMR quadrupole splitting values are much smaller than those in typical and normal liquid crystalline phases.33,34 This can be ascribed to the very small bond (30) Stejskal, G. O.; Tanner, A. J. Chem. Phys. 1965, 42, 288. (31) Kekicheff, P.; Tiddy, G. J. T. J. Phys. 1987, 48, 1571. (32) McGrath, K. M. Langmuir 1995, 11, 1835. (33) Tiddy, G. J. T. Phys. Rep. 1980, 57 (1), 1.

Figure 9. Self-diffusion coefficient in m2 s-1 of water, DHDO, and bile salt, Dbile, in the micellar solution phase versus concentration of NaTDC (wt %) at 22 °C.

order parameters for the binding of heavy water at interfaces. The spectral profiles of deuterium quadrupole splittings do not show any powder pattern behavior, and the observed line shapes resemble more those observed in ribbon, or rippled, mesophases.35-37 This indicates that fast exchange conditions are not fulfilled or that the interfacial curvature is not uniform. Because of their molecular structure (Figure 1), bile acid salts cannot form conventional spherical micelles, or any liquid crystalline phase, without invoking special arrangements of the single steroidal units into aggregates. The occurrence of tubular helical assemblies, with polar heads facing inward, has been proposed for the aggregates formed by some bile acid salts in dilute systems.38-42 It can be expected, thus, that the liquid crystalline phase has a supramolecular organization somehow related to those reported for the corresponding solution phases. Some type of arrangement of the steroidal nuclei makes the formation of a mesophase possible, and at present, we do not know why such a phase is formed in NaTDC and (34) Kang, C.; Khan, A. J. Colloid Interface Sci. 1993, 156, 218. (35) Chidichimo, G.; Golemme, A.; Doane, J. W.; Westerman, P. W. J. Chem. Phys. 1985, 82, 536. (36) Chidichimo, G.; La Mesa, C.; Ranieri, G. A.; Terenzi, M. Mol. Cryst. Liq. Cryst. 1987, 150b, 221. (37) Henriksson, U.; Blackmore E. S.; Tiddy, G. J. T.; So¨derman, O. J. Phys. Chem. 1992, 96, 3894. (38) Murata, Y.; Sugihara, G.; Fukushima, K.; Tanaka, M.; Matsushita, K. J. Phys. Chem. 1982, 86, 4690. (39) Murata, Y.; Sugihara, G.; Nishikido, N.; Tanaka, M. In Solution Behaviour of Surfactants: Theoretical and Applied Aspects; Mittal, K. L., Fendler, E. J., Eds.; Plenum: New York, 1982; Vol. 1, p 611. (40) Giglio, E.; Loreti, S.; Pavel, N. V. J. Phys. Chem. 1988, 92, 2858. (41) Briganti, G.; D’Archivio, A. A.; Galantini, L.; Giglio, E. Langmuir 1996, 12, 1180. (42) D’Archivio, A. A.; Galantini, L.; Gavuzzo, E.; Giglio, E.; Scaramuzza, L. Langmuir 1996, 12, 4660.

Water-Sodium Taurodeoxycholate System

has not been observed in the structurally related sodium deoxycholate-water system, where the formation of stabilizing hydrogen bonds between adjacent OH and COO- groups favors the formation of helical structures in gels and fibers.6,7 Small-angle X-ray scattering data indicate an unusual structural arrangement of NaTDC compared to other liquid crystal-forming surfactants. The diffraction patterns reported in Figure 6 have repeat distances very similar to each other, and the d spacings are nearly constant in the whole concentration range that we have investigated, as indicated in Table 1. In all cases the relative positions of the three main peaks correspond to the 1:x3:1.89 relationships, which are slightly different from those expected for “ideal” hexagonal mesophases (1: x3:2). The relative intensities between the three main reflections are the same in all investigated samples. In the diffraction patterns three further reflections are observed, indicated as q4, q5, and q6. These broad, weak reflections are not fully understood at present, and studies are in progress to clarify such aspects. The results indicate an unusual structural arrangement of NaTDC ions compared to those for other liquid crystalforming surfactants. If we assume the occurrence of an helical arrangement of bile salt anions to form hexagonal aggregates, it is possible that the pitch of such a structure changes slightly with composition. If the mutual arrangement of helical pitches is accepted, the most reasonable assignment for the observed hexagonal mesophase is of the reverse kind, the F phase in Ekwall’s notation.43 The d spacings in Table 1 are reasonably consistent with such an hypothesis. In fact, they fit more reasonably into a reverse hexagonal phase, because of the volume fraction constraint and of the bile salt molecular structure. The assignment is preliminary, and further measurements are required to check its validity. Electrical conductance investigations on lyotropic mesophases were used to determine sol-gel transitions25 or the orientational effects played by applied magnetic fields.44,45 They do not give any direct information on the lyotropic mesophase. We can safely assume, in this regard, that the conductive behavior of bile acid salts is mostly due to sodium ions. Support comes from studies at low surfactant content, indicating that the limiting mobility of bile acid anions is low compared to that of alkali ions46,47 Aggregation reduces the contribution due to taurodeoxycholate ions, and the effect is even more significant if sodium ions are partly bound to bile salt aggregates. (43) Ekwall, P. In Advances in Liquid Crystals; Brown, G. H., Ed.; Academic Press: New York, 1975; Vol. 1, p 1. (44) Photinos, P. J.; Saupe, A. J. Chem. Phys. 1985, 84, 517.

Langmuir, Vol. 14, No. 13, 1998 3697 Table 2. Slope of Electrical Conductance Data, in msiemens °C-1, in the Liquid Crystalline Phase, in Solution, above the Phase Transition Temperature, and in Undercooled Solution, for a Sample of 45.8 wt % NaTDC phase

slope (mS °C-1)

liquid crystalline phase micellar solution undercooled solution

0.414 0.417 0.418

In the liquid crystalline phase the motion of the sodium ion is comparable in magnitude with that observed in the corresponding homogeneous solution (Figure 7). In particular, κ values increase with temperature, the slope of κ(T) is nearly the same in the different phases, and heating-cooling cycles are immaterial (Table 2). Such a behavior implies very close activation energies to ion motion in the two phases and, presumably, a similar reciprocal arrangement of polar/apolar domains. In other words, the chemical environment sensed by migrating sodium ions is similar in mesophases and solution phases. Conclusions The data presented here indicate the occurrence of a hexagonal liquid crystalline phase in the binary system composed of water and NaTDC. This is an unexpected result, which contradicts the generally accepted statement that “Conjugated and free bile salts form micelles but not liquid crystals”.1 The preliminary assignment reported here is based on several independent techniques and gives support to the existence of a mesophase with an arrangement of some hexagonal packing. Further work is in progress to clarify still open questions and the occurrence of mesophases in other taurine derivatives of some bile acid salts. Acknowledgment. Financial support was granted from Mid-Sweden University, Sundsvall, for H.E. C.L.M. and A.K. acknowledge, respectively, the Department of Physical Chemistry I, Lund University, and Department of Chemistry, University of Rome, for supporting them with a Visiting Professorship. The work presented here is part of a Cost Action Research Project on Physical Chemistry at Interfaces, under the auspices of EC. The project is partly financed by TFR (the Swedish Research Council for Engineering Science). LA971197K (45) Photinos, P. J.; Saupe, A. J. Chem. Phys. 1986, 85, 7467. (46) Sesta, B.; La Mesa, C.; Bonincontro, A.; Cametti, C.; Di Biasio, A. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 798. (47) Sesta, B.; D’Aprano, A.; Princi, A.; Filippi, C.; Iammarino, M. J. Phys. Chem. 1992, 96, 9545.