J. Phys. Chem. 1991, 95,445-451
445
Temperature Dependence of Molecular Conformation and Liquid Structure of cls -9-Octadecenoic Acid Makio Iwahashi,* Yoshimi Yamaguchi, Department of Chemistry, School of Hygienic Sciences, Kitasato University, Sagamihara, Kanagawa- ken, Japan 228
Tadashi Kato, Department of Chemistry, Faculty of Sciences, Tokyo Metropolitan University, Setagaya- ku, Tokyo, Japan 158
Teruo Horiuchi, Lion Corporation, Sumida- ku, Tokyo, Japan 130
Ikuko Sakurai, Institute of Physical and Chemical Research, Wako, Saitama-ken, Japan 351 -01
and Masao Suzuki Nippon Oil and Fats Co. Ltd., Ohama, Amagasaki, Japan 660 (Received: March 2, 1990; In Final Form: July 2, 1990)
The temperature dependence of molecular conformation and liquid structure for cis-9-octadecenoic acid in the neat state has been studied by means of differential scanning calorimetry (DSC) and by measurement of the density, viscosity, self-diffusion coefficient, infrared spectrum, fluorescence polarization of DPH dissolved in the acid, ESR order parameter for four kinds of spin probe incorporated in the acid, 'Hand 13CNMR relaxation times T,and T2,and X-ray diffraction. On the basis of the experimental results, it is concluded that the acid has three kinds of liquid structure depending on temperature: ( I ) The first structure, in temperatures from the melting point to 30 "C, consists of clusters of a quasi-smectic liquid crystal; the clusters are randomly orientated and their sizes are smaller than the wavelength of visible light. (2) The second one, in the temperature range between 30 and ca. 55 OC, presumably consists of clusters having a less ordered structure. (3) The third one appears to be an isotropic liquid above ca. 55 "C.
Introduction
Experimental Section
Unsaturated fatty acids are widely distributed in biological tissues as a main constitutent of biomembranes and play an important role in membrane functions such as flexibility, fluidity, and material transfer. The functions seem to depend mainly on the property as a liquid or a liquid crystal of the acid rather than that as a solid. Concerning the liquid state of the fatty acid, however, only a little is known about the molecular order of structure and physicochemical properties for the liquid of the fatty acid, even for cis-9-octadecenoic acid (oleic acid: the most u " o n naturally occurring unsaturated fatty acid). The liquid state of the acid has hitherto been believed to be merely isotropic irrespective of temperature or pressure. Recently an interesting preheating effect (hysteresis) on the crystallization temperature of the cis-9-octadecenoic acid (as will be shown in Figures 1 and 2) has been observed by using an extra-purc sample of the acid:'S2 The liquid of the acid appears to be not always isotropic but to have a structure depending on temperature. In the present study we investigated the influence of temperature on the molecular conformation and the liquid structure for cis9-octadecenoic acid in its pure liquid state in comparison with that for tetradecane, by measuring density p, viscosity 7,crystallization temperature T,, self-diffusion coefficient D , infrared spectrum, fluoresence polarization P, ESR order parameter S, 'Hand I3C NMR spin-lattice relaxation time T,, and X-ray diffraction.
Materials. Samples of cis-9-octadecenoic acid (purity greater than 99.9%;'-3 Nippon Oil and Fats Co. Ltd.) and tetradecane (99.9%; Kanto Kagaku Co. Ltd.) were used without further purification. The purity for the samples of the acid and the tetradecane was confirmed by gas-liquid chromatography (Shimazu GC-14A with a capillary column of SP-25604 for the acid and Hitachi 663-50 with a column of SE30 for the tetradecane, respectively). A sample of 1,6-diphenyl-l,3,5-hexatriene(DPH; the purest grade one) from Wako Junyaku Co. Ltd. was purified by recrystallization twice from acetone and was used as a fluorescence probe. The following ESR spin probes from Sigma Chemical Co. were used without further purification: S-(N-oxy-4,4-dimethyl3-oxazolidinyl)octadecanoic acid (5NS), 7-(N-oxy-4,4-dimethyl-3-oxazolidinyl)octadecanoicacid (7NS), 1 2-(N-oxy-4,4dimethyl-3-oxazolidiny1)octadecanoicacid (12NS), and 16-(Noxy-4,4-dimethyl-3-oxazolidinyl)octadecanoicacid ( 16NS). The samples for ESR and N M R measurements were prepared after being fully purged with argon gas. DSC Measurements. Melting point and crystallization temperature, T,, for the cis-9-octadecenoic acid were determined with a differential scanning calorimeter, DSC (Rigaku Denki Co., Model PTC- 1OA), using pure samples of mercury, D20, carbon tetrabromide, and indium as standard materials for temperature calibration of the calorimeter. Density, p. The density, p, of the acid in the temperature range 20-40 f 0.01 "C was measured with a vibrational-type densimeter
( 1 ) Suzuki, M.; Ogaki, T.; Sato, K. J . Am. Oil Chem. Soc. 1985,62,1600. (2) Kobayashi. M.: Kaneko, F.;Sato, K.: Suzuki, M. J. Phys. Chem. 1986,
(3) Sato, K.; Suzuki, M. J . Am. Oil Chem. SOC.1986, 63, 1356. (4) Suzuki, M.; Sato, K.; Yoshimoto, N.; Tanaka, S.; Kobayashi, M. J . Am. Oil Chem. SOC.1988,65, 1942.
90, 6371.
0022-365419 112095-0445$02.50/0 0 1991 American Chemical Society
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The Journal of Physical Chemistry, Vol. 95, No. I , 1991
(Anton Paar Co., Model DMA602) and that in another temperature range, 40-90 f 0.01 "C, with a Gay-Lussac type pycnometer. Degassed pure water and mercury purified by distillation were used for calibrating the densimeter and the pycnometer, respectively. The obtained p values were used for evaluation of viscosity. Viscosity, 7. The viscosity, 7, of the acid was measured with an Ostwald capillary viscometer in the temperature range 20-90 "C with an accuracy of f0.05 OC. Pure water was used for calibrating the viscometer. Infrared Spectrum. The infrared spectrum for the liquid sample of the acid was measured with a Perkin-Elmer infrared spectrometer (Model 983) equipped with a heating device (Hitachi Model 1 RH-2) in the temperature range 26-1 I O "C. SelfDiffusion Coefficient, D. The self-diffusion coefficient, D, for the sample was obtained by pulsed-gradient FT-NMR method.s.6 All measurements were made on protons at 99.6 MHz, with internal D 2 0 lock in the temperature range 19-90 f 0.5 "C on a NMR spectrometer (JEOL FX-100). Each sample was placed in a 1mm-diameter tube inserted in a 5-mm-diameter tube containing D 2 0 (for NMR lock). NMR Spin-Lattice Relaxation Time T I . The N M R spinlattice relaxation time, T I ,for IH and 13Cof the acid was obtained by inversion recovery method' employing a 180-~-90" pulse sequence. T , for ' H was measured with a 400-MHz NMR spectrometer (JNM-GSX 400) in the temperature range 25-91.2 "C, and T I for I3C was measured with a 90-MHz N M R spectrometer (JEOL FX-90Q) in the temperature range 32-91.2 "C. The sample was placed in a 5-mm-diameter tube with a I-mmdiameter tube containing D20. Determination of the Crystallinity C in the Liquid State. Crystallinity, defined as C = fs'/(l +fs9,8q9 of cis-9-octadecenoic acid in the temperature range -I 3-25 f 0.5 "C was determined with a BULKA N M R spectrometer (Model Minispec P20i). f is a correction f a c t ~ r ,I ~the . ~signal height at 70 ps after the 90" pulse in the magnetization decay curve, and s ' a signal height exceeding I at a certain dead time after the 90" pulse. ESR Measurements. ESR spectra were recorded in the temperature range of 20-135 OC on a ESR spectrometer (Japan Electron Optics Laboratory Model JES-FE) with IWkHz, 0.63-G modulation. The concentration of the spin probe was kept in the order of 1 O4 M. The apparent order parameter S was established from the spectra. The quantity of S was defined aslo
where A, and A, are the parallel and vertical components of the hyperfine splitting. The principal values of the hyperfine tensor A,,, A,, and A,, are assumed to be 6.3, 5.8, and 33.6 G,respectively." The aN and aN' are defined as
and uN'
= (AP
+ 2AV)/3
(3)
respectively. Fluorescence Measurements. The steady-state fluorescence polarization P for DPH (3.34 X IOd mol/L) dissolved in the cis-9-octadecenoic acid was measured in the temperature range ( 5 ) Iwahashi, M.; Ohbu, Y.; Kato, T.; Yamaguchi, Y. Bull. Chem. SOC. Jpn. 1986, 59, 3771. (6) Stejskal, E. 0.;Tanner, J. E. J. Chem. Phys. 1965,42, 288. Stejskal, E. 0. J . Chem. Phys. 1968, 49, 1768. (7) Farrar. T. C.: Becker, E. D. Pulse and Fourier Transform NMR; Academic Press: New York, 1971; Chapter 2. (8) van Putte, K.; van den Enden, J. J . Phys. E : Sci. Insfrum. 1973, 6, 910. (9) van Putte, K.; van den Enden, J. J. Am. Oil Chem. SOC.1974.51, 316. (IO) Hubbell, W. L.: McConnell, H.M. J. Am. Chem. Soc. 1971,93, 314. ( 1 1 ) Gaffney, B. J.; McConnell, H. M. J . Magn. Reson. 1974, 16, 1.
Iwahashi et al.
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15-72 f 0.05 OC with a fluorescence spectrometer (Hitachi Model 850). Polarizers (Hitachi Model 650-0155) were used to polarize both exciting and emitting beams. The fluorescence polarization, P, was calculated according to (4)
where Ivvand IVH are intensities of the vertical and horizontal components excited by the vertically polarized incident beam, respectively. G is a factor1*used for correcting the polarization in the instrument. X-ray Diffraction. X-ray diffraction patterns for the liquid of cis-9-octadecenoic acid were obtained by use of an X-ray diffraction measuring system,I3 in which the direction of an incident X-ray beam was vertical and reflections from the sample were observed on a film set horizontally. The camera length was 64.7 mm. Ni-filtered Cu K a radiation (A = 1.541A) was used. The applied voltage and filament current for the X-ray tube were 37.5 kV and 20 mA, respectively. The sample of 1.6 mm in thickness was sealed in a cell with mica windows of 25-30 pm in thickness. Results and Discussion DSC Studies. Figure 1 shows a typical DSC performance for a I . 1-mg sample of cis-9-octadecenoic acid, in which each holding time at a pre-heating temperature Tp was 15 min; both heating and cooling rates were 2 "C/min. Endothermic peaks indicate the melting point and exothermic peaks the crystallization temperature Tc. The melting point observed reveals that the crystal used for the present experiment is a-type modifi~ation.l-~ Figure 2a shows the Tc vs Tp relationship for the acid (circles) and a 2.0-mg tetradecane sample (triangles), in which open circles exhibit results obtained from Figure 1 and closed circles results obtained individually for seven samples weighing from 0.99 to 1.19 mg. They are in fair agreement with one another. The Tc value for the acid depends obviously on Tp: It remains constant (about 10.4 "C) up to Tp = 30 O C , decreases sharply in the Tp range of 30 to ca. 50 "C, and then becomes again (12) Sakurai, 1.; Sakurai, T.; Seto, T.; Iwayanagi, S. Chem. Phys. Lipids 1983, 32, 1. (13) Azumi, T.; McGlynn, S . P. J. Chem. Phys. 1962, 37, 2413.
Liquid Structure of cis-9-Octadecenoic Acid 12.0
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The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 447
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Figure 2. (a) Relationships between crystallization temperature and preheating temperature Tp for a small amount of samples of cis-9-octadecenoic acid (circles) and for a 2.0-mg sample of tetradecane (triangles). Open circles are obtained independently (batchwise) for samples of 0.99-1 .I9 mg; closed circles are obtained continuously, for the 1.IO-mg sample of the acid shown in Figure I . (b) Relationship between crystallization temperature and preheating temperature Tpfor large samples (4.95 mg) of cis-9-octadecenoicacid. Circles denote 15-min holding time at the preheating temperature and squares 60-min holding time.
constant (4.7 "C) above Tp = ca. 50 O C . On the other hand, the Tc vs Tp relationship for a larger amount of sample (4.95 mg) of the acid (open circles in Figure 2b) kept at various preheating temperatures ( Tp) for 15 min is distinct from that for the smaller amount samples: the Tc value remains constant up to Tp = 70 OC, decreases sharply in the range of 70-80 O C , and then becomes constant above 80 "C. However, another large amount of sample (open squares in Figure 2b) kept at the same preheating temperatures for 60 min shows an almost similar profile to that for the small amount one. That is, the Tp values corresponding to the two break points in the Tc-Tp relationship were always ca. 30 and 50 O C , even for the large amount sample after an ample holding time at a preheating temperature. Apparently, the preheating and the storage period for the acid influence its crystallization temperature: This hysteresis phenomenon suggests the presence of three different liquid structures in the temperature ranges below 30 O C , between 30 and ca. 50 O C , and above ca. 50 O C . On cooling these structures are supercooled more or less and change its crystallization temperature, depending on the preheating and cooling conditions. On the other hand, Tc of tetradecane was not influenced by the preheating and cooling conditions. The distinct difference between the acid and tetradecane presumably results from the different mobilities for those molecules in their liquids; namely, the tetradecane should have a higher mobility than the cis-9octadecenoic acid. In fact, the diffusion coefficient, D, obtained by NMR measurement, of tetradecane is about 20 times larger m2/s for than that of the cis-9-octadecenoic acid: 6.00 X
Figure 3. Temperature dependence of the apparent hydrodynamic radius a.
tetradecane and 3.24 X lo-" mz/s for cis-9-octadecenoic acid at 25 O C , respectively. Consequently, even if tetradecane had various liquid structures depending on temperature, the arrangements of its molecules would easily change, through the cooling process, to other equilibrium structures corresponding to its present temperature because of the high mobility of the molecules. Thus the preheating effect on the Tc would not be observed for the tetradecane. On the contrary, owing to the extremely low mobility for the acid molecules, three liquid structures would hardly alter under an ordinary cooling condition with DSC and affect its crystallization temperature. Density, Viscosity, Self-Diffusion Coefficient, and Hydrodynamic Radius. In Table I, data of the density, p, viscosity, 7, and self-diffusion coefficient, D, for cis-9-octadecenoic acid measured at various temperatures are summarized. We have shown in a previous paperS that the apparent hydrodynamic radius a, obtained from 7 and D by using the Stokes-Einstein formula a = kT/(4soD) (5) under a slip boundary condition,'"'' reflects the conformation of a long-chained molecule that migrates in its liquid. In this equation, k is the Boltzmann constant and T the absolute temperature. Figure 3 shows the a vs T relationship obtained for the cis-9octadecenoic acid. The radius a increases with increasing temperature up to 30 "C,then becomes almost constant in the range 30-55 O C , and increases again above 55 O C : Two break points also exist at 30 and 5 5 OC which correspond obviously to the T p values at the two break points in the Tc vs Tpcurve. Consequently, these results on a further emphasize the existence of three distinct structures in liquid cis-9-octadecenoicacid in the three temperature ranges. (14) Sutherland, G. B. B. M. Philos. Mag. 1905, 9, 781. (15) Li, J. C. M.; Chang, P. J . Chem. Phys. 1955, 23, 518. (16) Hu,C. M.; Zwaning, R.J . Chem. Phys. 1974, 60, 4354. (17) Espinosa, P. J.; de la Torre, J. G . J . Phys. Chem. 1987, 91, 3612.
448
The Journal of Physical Chemistry, Vol. 95, No. 1, 1991
Iwahashi et al.
100
90
lo
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0 5 1 5 20 25 Temperature/ 'C
30
Figure 4. Temperature dependence of crystallinity in the liquid of 9-octadecenoic acid.
cis-
In order to get more convincing evidence for the existence of the structures in liquid cis-9-octadecenoic acid, we measured its infrared spectrum, fluorescence polarization, ESR order parameter, crystallinity ('H NMR spinspin relaxation time T2), and IH or I3C NMR spin-lattice relaxation time T I . Infrared Spectra. The infrared spectra for the sample indicate that the hydrogen bonding between the carboxyl groups of two cis-9-octadecenoic acid molecules is conserved under the temperature range from 23 to 103 O C ; namely, the strong absorption band at 1710 cm-' due to the C=O stretching vibration and the relatively broad band at 920 cm-' due to the O H out-of-plane bending vibration of the carboxyl group, which are assigned to the hydrogen-bonded dimer, remain completely unaltered throughout the temperature range. Thus, two cis-9-octadecenoic acid molecules are always dimerized in the liquid by hydrogen bonding of the carboxyl groups. Crystallinity and Fluorescence Polarization. Figure 4 shows the temperature dependence of the crystallinity, C, for cis-9-octadecenoic acid determined by pulsed NMR measurement. Ap parently, a crystallike property still remains beyond the melting point of the acid. This crystallinity determination is based on the fact that transversal magnetization decays for solidified fat much faster than for fluid oil: The spinspin relaxation time T2obtained for the solidified fat is in general about IO ps, while that for fluid oil is I00 ms.'* In the present study, we defined that the T2value for solidified cis-9-octadecenoic acid at -1 3 "Ccorresponded to 100% crystallinity and that for the liquid acid at 25 OC corresponded to 0% crystallinity. Figure 5 shows the temperature dependence of fluorescence polarization, P, for DPH dissolved in cis-9-octadecenoic acid. The P value, being relatively high in the vicinity of the melting point, decreases steeply with increasing temperature up to 30 OC and then gradually decreases. The relatively high P value in the low-temperature range indicates that rotational movement of the rodlike DPH molecule is apparently restricted by a high local viscosity of the liquid surrounding the DPH molecule. The high viscosity corresponds to the high crystallinity in the same temperature range in Figure 4: cis-9-Octadecenoic acid seems to possess a crystallike property such as a liquid crystal in the temperature range. I t is assumed at the first sight that small crystallites (nuclei) remain in the melt just after the fusion of the crystal and that they contribute to the crystallinity, the high local viscosity, and the high crystallization temperature (the ease of the crystallization) on cooling. However, each P value in Figure 5 was obtained after keeping the sample at a desired temperature for 1 h; furthermore, P values for a special sample (triangles), which has been liquefied from the gas phase by vacuum distillation (1 70 "C, 0.5mmHg), agreed well with those for the sample (open circles) prepared by melting the crystalline acid just before the measurement within an ex(18) van Putte, K. Minispec Application Nore 5; Bruker Analytische Messtechnik GMBH: Silberstreifen.
Figure 5. Temperature dependence of fluorescence polarization P for DPH dissolved in cis-9-octadecenoicacid. Open circles denote the sample molten just before the measurement; closed circles, the supercooled one; triangles, the special one that has never solidified since its preparation by vacuum distillation (170 "C, 0.5 mmHg).
perimental error. In addition, the P value for a sample (closed circle) supercooled below the melting point is also on the same line. The P values even for the molten samples obtained immediately after the fusion of crystal are also in equilibrium; the crystallinity and the relatively high freezing point for cis-9-octadecenoic acid in the range between its melting point and 30 "Care not attributed to the crystallites assumed above. The second break point, which should be observed around 55 "C, is ambiguous because of the scatter of Pvalue at a temperature higher than 50 O C . The scatter is due to the extremely low quantum yield for the DPH fluorescence at a high temperature. Andrich and VanderkooiI9 studied DPH fluorescence polarization in phospholipid vesicles and found the two discontinuities in the temperature vs polarization curve; the profile near the second discontinuity at higher temperature is similar to our polarization result. They insisted that the region before the first discontinuity at lower temperature refers to a paracrystalline state, the region after the second discontinuity at higher temperature to a molten (disordered) state, and the region between the two discontinuities to a phase-transition region. The last region seems, however, to be a liquid-crystal-like region rather than the phase transition because the region is too broad to be regarded as a simple melting. 'HNMR and 13CNMR Spin-Lattice Relaxation Time and ESR Order Parameter. Figure 6 shows the temperature dependence of the 'HNMR spin-lattice relaxation time TI. Each curve for every proton bends discontinuously at ca. 55 O C , indicating that a specific change takes place in the liquid at the temperature. TI for IH N M R gives us much and complicated information of the molecular level, such as interactions with other molecules surrounding the object molecule; hence, TI does not directly indicate the fragmental movement in the acid molecule. On the contrary, T I for I3C NMR is related to segmental motion in the molecule: specifically rotational tumbling and to a lesser extent translational and internal motion.M Then, we measured I3C NMR spin-lattice relaxation time TI for the liquid sample of cis-9-octadecenoic acid. The I3Cchemical shift assignments for the acid were made by using NMR data by Batchelor et aL2' Figure 7 shows the obtained temperature-dependence curve of TI for I3C; each curve also bends similarly at ca. 55 "C. I3C spin-lattice relaxation of a protonated carbon is overwhelmingly dominated by dipole-dipole interactions with the attached protons.22 Consequently, TI is related to the number of directly (19) Andrich, M. P.; Vanderkooi, J. M. Biochemistry 1976, 15, 1257. (20) Freeman, R.; Hill, H. D. W.J. C h m . Phys. 1970, 53,4103. (21) Batchelor, J. G.; Cushley, R. J.; Rrestegard, J. H. J . Org. Cbem. 1974, 39, 1698. (22) Kuhlmann, K. F.; Grant, D. M.; Harris, R. K. J . Cbem. Pbys. 1970, 52. 3439.
Liquid Structure of cis-9-Octadecenoic Acid 11 I
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bonded hydrogen N a n d the effective correlation time T , for the rotational reorientation. Thus, TIis given in terms of N and 1/ T , : * ~
where h is Planck’s constant, yc and y H are the gyromagnetic ratios of 13Cand ‘H,respectively, rCHis the C-H distance usually (23)
Hertz, H.G. Prog. Nucl. Magn. Reson. Spectrosc. 1967, 3,
159.
Figure 8. Temperature dependence of reciprocal of the effective correlation time, r, for the rotational reorientation of the segments in the cis-octadecenoic acid molecule. 0 denotes the carbon at the 2nd position;
e, at the 3rd one; 0, at the 8th one; +, at the 9th one; 0 , at the 10th one; 0, at the 16th one; A,at the 17th one; $I, at the 18th one. The upper illustration is a schematic drawing for the fragmental movements of a dimer of cis-9-octadecenoicacid.
about 0.109 nm, and 1/rCrepresents the degree of rotational reorientation, that is, the degree of the rotational movement for a fragment. Figure 8 shows the 1/rCvs T relationships that have also break points a t ca. 55 OC,indicating that the rotational movement of each fragmentary moiety increases more steeply above ca. 55 OC. At any temperature, the rotational movement for the segment at the second position attached to the dimerized carboxylic group is considerably restricted, while that for other fragment increases toward the end of the hydrocarbon except the carbons at the ninth
Iwahashi et al.
450 The Journal of Physical Chemistry, Vol. 95, No. I , 1991 I
,
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and tenth positions: The fragmental movements for the carbon atoms at the ninth and tenth positions (cis double bond) are as nearly equal as that at the third position. The upper illustration in Figure 8 is a schematic drawing for the fragmental movements of the dimerized molecules. This dynamical molecular conformation resembles that in the a-form crystal2 of the cis-9-octadecenoic acid. Figure 9 shows the relationships between the ESR order parameter, S,and inverse temperature 1 / T for the samples that are spin-labeled at various positions. Each relationship also shows a sharp break point around 55 OC regardless of its position labeled with a doxy1 radical: The local viscosity decreases steeply above 55 O C . These NMR and ESR results suggest that a discontinuous change takes place in some physical properties of the liquid of cis-9-octadecenoic acid around 55 "C: The liquid structure or the molecular conformation for cis-9-octadecenoic acid changes obviously into a more highly disordered one, namely, an isotropic structure, above 55 OC. Consequently, it is concluded that cis-9-octadecenoic acid has most likely three kinds of liquid structures depending on temperature: (1) the first structure, presumably a liquid crystal, in a temperature range from the melting point up to 30 OC;(2) the second one, a less ordered liquid crystal, in a temperature range of 30 to ca. 55 O C ; and (3) an isotropic liquid above ca. 55 OC. However, liquid crystals are known generally to be opaque owing to the fluctuation in long-range orientational order, while the sample used for the present study is always perfectly transparent without any colors. Further, visual observation of the sample under a polarization microscope did not show any optical anisotropy at room temperature, in spite of the much experimental evidence suggesting the possibility of the structured liquids. This discrepancy can be elucidated by the following assumption: The liquid of cis-Poctadecenoic acid consists of very small clusters of the liquid crystal whose dimensions are extremely small compared with the wavelength of visible light; the clusters having a structure are randomly oriented. Hence, the overall macroscopic anisotropy would not be observed. In order to confirm the presence of such clusters, we carried out an X-ray diffraction measurement at 22 OC. Figure 10 is the X-ray diffraction pattern obtained for the liquid sample of cis9-octadecenoic acid. A diffuse ring with a strong intensity at a wide angle and also a diffuse one with weak intensity at a small angle were observed. These rings suggest the presence of the clusters having a structure, Le., the cybotactic clusters?e27 which (24) Stewart, G.W.;Morrow, R. M. Phys. Reu. 1927,30,232.
.-..-"__I
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-_I_-.
Figure 10. X-ray diffraction pattern for the liquid sample of cis-9-octadecenoic acid at 22 OC. Exposure period was I .5 h. The radii for the large diffuse ring and for the small ring are 22.76 and 5.78 mm, respectively, which were obtained by means of microphotometric observation for the pattern.
Figure 11. Arrangement proposed for the dimers of cis-9atadecenoic acid molecules in the clusters taking the structure of a quasi-smectic liquid crystal.
may consist of 10-1OOO molecules. Microphotometric observation of this photograph (Figure 10) along the direction across the center of the rings gave the spacings corresponding to the two diffuse rings obtained from the peaks of the intensity curve, Le., a long spacing of 17.3 A and a short spacing of 4.58 A. Larsson** also observed a similar X-ray scattering for the monoglyceride-water system above the "melting point" of the lamellar liquid crystalline phase. The phase above the melting . point is called the L2 phase.29 From the obtained long spacing value, which is longer compared to that of cis-9-octadecenoic acid, he suggested that the ordered structure is held even above the melting point and that the "melting" of the lamellar liquid crystalline phase means a reduction in size of the ordered domains. The long spacing of 17.3 A obtained in our experiment, however, seems too small, because two molecules of cis-Pactadecenoic acid (25) Morrow, R. M. Phys. Reu. 1928, 31, IO. (26) Jirgensons, B.; Straumanis, M. E. A Short Textbook of Colloid Chemistry; Pergamon Press Limited: London, 1962; Chapter 15. (27) Kawamura, Y.;Okano, K. Jpn. J . Appl. Phys. 1983.22, 1749. (28) Larsson, K. J . Colloid Interface Sci. 1979, 72, 152. (29) Ekwall, P. Aduunces in Liquid Crysfuls;Academic Press: New York, 1975; Vol. I.
J. Phys. Chem. 1991, 95, 451-457 always form a dimer in its pure liquid as clarified by the IR spectrum measurements: The acid dimer should have a larger value of at least twice'J that obtained for the long spacing. The incomprehensive value for the long spacing can be explained, as indicated in Figure 1 1 , by a unique arrangement proposed for the dimerized molecules in the clusters. The dimerized acid molecules in the clusters arrange longitudinally and alternately: The dimerized carboxyl groups in one dimer and the methyl groups at the terminal of the neighboring dimers arc aligned alternately in a same lateral plane. Such alignment in the longitudinal direction for the acid molecules is the same as that for dodecanoic acid molecules in A-form crysta1.30*3' The structure somewhat resembles the smectic liquid crystal. Thus we call it a quasi-smectic liquid crystal structure. If dimerized acid molecules take this structure, segments near the end of the acid molecule, i.e., methyl and methylene groups near the 18th position, can move significantly, because the movement of the carboxy groups in the neighbor dimer is restricted as shown in the upper illustration in Figure 8. The lateral distance among neighboring dimers in this structure would increase with an increase in temperature. Consequently, it is reasonable for the hydrodynamic radius for the acid molecule (30) Lomer, T. R. Acta Cryslallogr. 1963, 16, 984. (31) Goto, M.; Ashida, E. Bull. Chem. Soc. Jpn. 1978, 51, 70.
45 1
to increase with increasing temperature (see Figure 3). In addition, when the molecules are cooled, they are liable to assemble into a crystal, because their arrangement does not so apart from the crystal structure. Therefore, the relatively large local viscosity and the high crystallization temperature of cis-9-octadecenoic acid in the temperature range between the melting point and 30 OC are well explained by the presence of the quasi-smectic liquid crystal structure. A consecutive rise in temperature would lead the aligned acid molecules to a more disordered structure and finally to an isotropic one. Acknowledgment. We express our deepest thanks to Professor Hiroshi Kobayashi for his helpful discussion and encouragement throughout the present experiment, Dr. Midori Goto, National Chemical Laboratory for Industry, for her helpful discussion about X-ray diffraction results, Dr. Yasuaki Kawamura, the Institute of Physical and Chemical, for his valuable discussion about liquid crystals, Dr. Michihiro Yamaguchi, the Research Center of Shiseido Cosmetic Co., for his help in the 'HNMR measurement, and Dr. Hideyo Matsuzawa, Mr. Yoshio Ogura, Mr. Toshiyuki Kato, Miss Mugiko Iwasaki, Mr. Kazuo Saito, and Mr. Yoshinori Hayashi for their help in the DSC, density, viscosity, and polarization measurements. This work was supported in part by a Grant-in-Aid for Scientific Research (No. 02640356) from the Japanese Ministry of Education.
Chemical Reactions in Microemuisions: Probing the Local Dielectric Number of the Dispersed Water Reinhard Schomacker Max- Planck-Institut fur biophysikalische Chemie, Postfach 2841, 0-3400 Gottingen, FRG (Received: March 6, 1990; In Final Form: June 25, 1990)
The experiments described in this paper were performed for answering the question how to distinguish between a microemulsion and a weakly structured solution. The local dielectric number of water dispersed in ternary mixtures of water, oil, and amphiphile is probed by studying two chemical reactions sensitive for this physical property. The kinetics of the decomposition of murexide and the pK, value of Thymol blue are used as chemical probes. Two hydrophilic probes are chosen to ensure that the probe is located in the aqueous domains and not at the interface or in the nonpolar domains. If the probe studied in the ternary mixture experiences an environment similar to bulk water, the mixture is classified as a microemulsion. If the dielectric number of the environment of the probe is much lower than that of water, the ternary mixture is classified as a weakly structured solution. A borderline can be drawn at that concentration of water needed to hydrate the head groups of the amphiphile molecules. The presence of microstructure in the weakly structured solution can also be shown by studying a probe reaction. The complexation reaction of NiZ+with the hydrophobic ligand pyridin-2-azodimethylaniline(PADA) is a sensitive probe for the presence of hydrophobic aggregates in a solution.
Introduction
In a recently published review article, Kahlweit and co-workers' suggested microemulsions to be defined as stable colloidal dispersions of either water or oil droplets sufficiently large for the dispersed solute to exhibit the properties of a bulk phase. The size of the droplets appears to be inversely proportional to the square root of the interfacial tension between the water-rich and the oil-rich phase in the presence of a saturated monolayer of the amphiphile. Because the interfacial tension decreases with increasing efficiency of the detergents, and depends, furthermore, sensitively on temperature and the chemical nature of the oil as well as on the brine concentration, the transition from weakly structure homogeneous solutions to microemulsions in the above narrower sense is gradual. This raises the question how to determine the borderline experimentally. In this paper we suggest ( I ) Kahlweit, M.; Strey, R.; Busse, G. J . Phys. Chem. 1990, 94, 3881.
0022-3654/91/2095-045 1$02.50/0
studying chemical reactions in the dispersed phase for probing its properties. For water in oil dispersions (w/o) this can be done by measuring the kinetics of the decomposition of murexide in the presence of an acid, which permits determination of the effective relative dielectric permittivity (dielectric number) in the water domains as it depends on temperature, the efficiency of the amphiphile, and the carbon number of the surrounding oil phase. Experiments show that the phase behavior of ternary mixtures of water oil and nonionic nonionic amphiphiles depends sensitively on the nature of the oil and the amphiphile and on the temperaturee2 In Figure 1 (top) a schematic phase prism summarizes the general patterns of the phase behavior of such mixtures. This phase prism is discussed on the basis of two vertical sections through the prism. Figure 1 (middle) shows a section at a constant (2) (a) Kahlweit, M.; Strey, R.; Firman, P.; Haase, D.; Jen, J.; Schomacker, R. Langmuir 1988, 4, 499. (b) Kahlweit, M.;Strey, R.; Schomacker, R.; Haase, D. Langmuir 1989, 5, 305.
0 1991 American Chemical Society
