J. Phys. Chem. 1995,99, 4335-4338
4335
Pyridinium Ring Orientation in a Lyotropic Liquid Crystal of a Dodecylpyridinium Iodide- Water System As Studied by 2HNMR Masataka Tansho"9' and Shoichi Ikedas Department of Chemistry, Faculty of Science, Nagoya University, Nagoya 464-01, Japan Hiroshi Ohki5 and Ryuichi Ikeda Department of Chemistry, University of Tsukuba, Tsukuba 305, Japan Received: September 2, 1994; In Final Form: January 3, 1995@
The temperature dependence of the 2H NMR spectra in the deuterated pyridinium head group was measured in a lyotropic liquid crystal of an 85 wt % dodecylpyridinium iodide-water system. It was shown from the quadrupole splittings of three nonequivalent 2H sites that "the motionally averaged inclination angle" of the pyridine ring increases by ca. 6" on heating from 24 to 40 "C.
Introduction N-Dodecylpyridinium iodide ( C I ~ H ~ ~ C ~abbreviated H ~ N I ) to DPI has been intensively studied owing to its marked properties as an organic semiconductor',2and thermotropic liquid c r y ~ t a l . ~ Recently, it has been reported that DPI micelles undergo a saltinduced sphere-rod transition around room temperatures4 We can expect that DPI forms a unique system among many kinds of amphiphiles which exhibit lyotropic liquid-crystal phases with water because its head group is more bulky than those of other analogous amphiphilic systems so far studied, i.e., sodium laurate5s6and dodecyl ~ u l f a t e ,and ~ alkyltrimethylammonium halide5.7s8and dodecyldimethylamine ~ x i d e . ~ . ~ *H NMR has been a useful technique for studying polymorphism and molecular dynamics in lyotropic liquid crystal^,^-'^ in which most studies so far reported are on deuterated alkyl chains and 2H20 in solvents, while only a few measurements of deuterized head groups have been carried ~ u t . ' ~ From . ' ~ 2H NMR measurement of the deuterated pyridine ring, we can expect that resonance signals of *H nuclei bonded to three kinds of nonequivalent carbons on a ring,I5 namely, C(2) (or C(6)), C(3) (or C(5)), and C(4), are distinguished, and from these results the head group orientation is estimated. In the present study, we measure differential thermal analysis (DTA) and 2H NMR to detect the presence of a uniform lyotropic phase and to get information about the dynamic structure around the head group in this phase.
Experimental Section
I
I
I
I
10
20
30
40
1
50°C
Figure 1. Differential thermal analysis (DTA) thermograms of the 85 wt % dodecylpyridinium iodide- water system observed around room temperature. The temperature was scanned at ca. 0.5 K min-I. The cooling run shown by the double arrow was scanned at ca. 2 K min-I.
at an interval of 5 wt % were prepared at ca. 95 "C in a sealed glass ampule. Samples of DPI in the liquid-crystal phase were observed under an Olympus BH polarizing microscope. DTA was performed with a homemade apparatus.I6 2HNMR spectra were recorded using a Bruker MSL-300 spectrometer at a Larmor frequency of 46.073 MHz, applying the quadrupole echo pulse sequenceI7 with the pulse interval of 30 ps and the 90O-pulse width of 7.8 ps. The pulse sequence time was set at 2.0 s, and ca. 1000 signals were accumulated. The sample temperature was controlled by a Bruker VT-1000 controller.
Results and Discussion
Dodecylpyridinium-d5 iodide (DPI-d5) was prepared by refluxing a mixture of distilled 1-iodododecane (Aldrich 98%) and pyridine-& (Aldrich, 99 atom % 2H) with a molar ratio of 1:1.1 for 3 h at ca. 100 "C3 and recrystallizing four times from ethanol at ca. 0 "C. The purity of the resulting yellow crystals was evaluated by chemical analysis for nondeuterated DPI synthesized by the same procedure. Anal. Calcd: H, 8.06; C, 54.40; N, 3.73; I, 33.81. Found: H, 8.19; C, 54.16; N, 3.67; I, 33.73. Specimens with various mixing ratios of DPI with water + Present address: National Institute for Research in Inorganic Materials, Tsukuba 305, Japan. Present address: Department of Materials Science and Engineering, Faculty of Engineering, Fukui 910, Japan. 5 Present address: Department of Chemistry, Faculty of Science, Hiroshima University, Higashi-Hiroshima 724, Japan. Abstract published in Advance ACS Abstracts, March 1, 1995. @
u . d
Polarizing Microscope Observation. The present observations revealed that the DPI-water system forms a colloidal dispersion around 50 wt %, mixtures of colloid and liquid crystal between 55 and 80 wt %, and a uniform liquid crystal at ca. 85 wt %. Since a wide range of liquid-crystal phase over ca. 16 "C was obtained at 85 wt %, the 85 wt % sample (with a molar ratio of DPI to water of 1:3.7) was investigated by 2H NMR measurement. (The 100 wt % system was reported to have liquid-crystal phases in a range of only 4-5 O C 3 ) This system is expected to form a hexagonal phase because a fanlike or angular texture was observed under polarizing micro~cope.~ Differential Thermal Analysis (DTA). From the DTA thermograms shown in Figure 1 together with the microscope observations, it was clearly shown that a uniform lyotropic liquid-crystal phase was formed between 24 f 1 and 40 f 1
0022-365419512099-4335$09.00/0 0 1995 American Chemical Society
Tansho et al.
4336 J. Phys. Chem., Vol. 99, No. 12, 1995
2
8
5 4
2 '
:
0 20
-10
0 -10 kHz
lo
0 kHz
-10
Figure 2. 2H NMR spectra of the 85 wt % dodecylpyridinium-ds iodide-water system observed at 46.073 MHz.
"C. Above 40 "C, this phase separated into two phases: a colloid and a liquid-crystal phase. The fact that the heat anomaly observed at 24 "C was always accompanied by a long tail on the high-temperature side up to ca. 40 "C on both heating and cooling runs suggests that DPI molecules gradually acquire motional freedoms with rising temperature above the phasetransition temperature of Tc = 24 "C. The liquid-crystal phase could persist in the low-temperature range below 20 "C over several months by a rapid cooling across T,. When the mixture was cooled gradually, however, a mixture of solid and colloidal solution was formed in most cases. 2HNMR. Below T,, we observed two kinds of *H Nh4R signals with peak-to-peak widths of ca. 120 kHz and less than 2 kHz, which are assignable to 2H in the solid state and colloidal solution, respectively. Upon heating to T,, at first, several signals other than the narrow peak line observed below T, were obtained. When the temperature was kept at T, for 150 min, the narrow component disappeared and a stable signal with three components was obtained as shown in Figure 2. We can see from the spectra observed above Tc the presence of three nonequivalent 2H sites having almost axially symmetric electric field gradients (efgs) with the intensity ratio 2:2:1. The axially symmetric signal observed implies that dodecylpyridinium ions in this phase undergo uniaxial rotation rather than diaxial motions, which was reported in the ribbon phase of the potassium palmitate-benzyl alcohol-water systemlo and the cesium tetradecanoate-water system.'' The spectra shown in Figure 2 have line shapes a little deformed from the Pake doublet which is usually observed in liquid crystals when the direction of the efg principal axis is distributed isotropically.6 The observed deformation is attributable to the molecular alignment by the applied magnetic field and has been observed in many cases in the hexagonal phase but not in the lamellar phase.6 The splitting widths of the three components were determined within f0.005 kHz, and their temperature dependencies are shown in Figure 3. The width of the weak component showed a marked decrease upon heating, while those of the other two
30
40
50
tPC Figure 3. Temperature dependences of the quadrupole splitting vq of three nonequivalent ZHsites in a pyridinium ring of the 85 wt % dodecylpyridinium-ds iodide-water system. Values of *Hbonded to C(4) in the ring are given by 0, and the others by 0. Arrows indicate the transition temperatures determined by differential thermal analysis (DTA).
strong ones increased gradually with temperature. The difference in width and also the temperature gradient of the two strong components decreased slowly with increasing temperature. Above 40 "C, we observed a single sharp line appearing at the center assignable to the signal in the isotropic colloidal phase. The temperature dependence of quadrupole splittings shown in Figure 3 was unexplainable by only the acceleration of the head group motional fluctuation but was attributed mainly to the temperature-dependent orientation of the pyridinium ring-plane with respect to the principal ordering axis of the pyridinium ring. The weakest signal in the three components can be assigned to 2H bonded to C(4) (abbreviated to 2H(4)), while the other strong signals can be assigned to 2H(2) and 2H(3). The fact that the splitting of 2H(4) is the smallest indicates that the pyridinium-ring axial rotation about its C2-axis, which is expected to reduce the splitting in 2H(2) and 2H(3) more than in 2H(4), is still slow and the motion about the C2-axis is not axially symmetric in this phase. Here, two typical cases A and B for the pyridinium orientation shown in Figure 4 are discussed, namely, the pyridinium ring being perpendicular to the plane formed by the pyridinium C2-axis and the principal ordering axis of pyridinium ring (case A) or parallel to the plane (case B). According to D ~ a n e , ' * .the ' ~ 2H splitting width vq is given by
vq = 3/4vQL{s,,[(3/2 cos2,L? - '/J
(s,,
+ 1/2vLsin2,L? cos 2a1+
- s~?)['/~ sin2 B cos 2 a
+ 1/6qL(1+
cos2p> cos 2 a cos 2y - 1/3qLcos p sin 2 a sin 2yl) (1) where Y Q and ~ vL are the quadrupole coupling constant (e2QqL/ h ) and the asymmetry parameter, respectively, measured in a rigid lattice where all orientational motions are frozen. S,, and ,S - ,S, are the degree of the order of the long axis and a measure of asymmetry in fluctuation of this axis, respectively. a, @, and y are the Euler angles which give the orientation of the efg principal axis at a 2Hnucleus with respect to the principal molecular axis. If one assumes the planar structure of the pyridinium ring, one obtains the following equation by substituting a = y =
J. Phys. Chem., Vol. 99, No. 12, I995 4337
Dodecylpyridinium Iodide-Water System
CaseA
CaseB
Figure 4. Models of head-group orientation. A: The pyridinium ring is perpendicular to the plane made by the principal ordering axis and the pyridinium C2-axis. B: The ring is parallel to the plane. The angle between the pyridinium C2-axis and the principal ordering axis is shown by
e.
1
I
I
0
1 2vq(4)/(vq(2)+yq(3)) Figure 6. Observed and calculated *Hquadrupole splittings vq at three sites H(2), H(3), and H(4) on a pyridinium ring plotted against 2vq(4)/(vq(2) ~ ~ ( 3 )Observed ). values for 2H-C(4)and *H-C(2), C(3) are given by 0 and 0,respectively. Calculated values for cases A and B are shown by solid and broken lines, respectively.
+
Case A l-
pldegree 0
Q
Q
40
Case B l -
30
I-@
20
0 O0
1
T
30
50
40
pldegree Figure 5. Calculated angle-dependence factors for three nonequivalent sites H(2), H(3), and H(4) on a pyridinium ring as a function of "inclination angle" e for models A and B given in Figure 4.
tPC Figure 7. Temperature dependences of "inclination angle" e evaluated The arrows mean the transition temperatures for cases A (0)and B (0). observed by differential thermal analysis (DTA).
x12 into eq 1
"the motionally averaged inclination angle" of the pyridinium C2-axis with respect to the principal ordering axis is given by e as shown in Figure 4, we obtain /3 = for 2H(4) and, for 2H(2) and 2H(3), the values of /3 as functions of e by assuming the structure given above for the pyridine molecule. Here, we assume that the pyridinium ring undergoes a rapid 180O-flip motion and not axial rotation about the C2-axis; this motion can readily occur accompanied by no structural disorder. Under this assumption, values of vq for 2H(2) and *H(3) are expressed as the averaged vq of the two equivalent positions superimposed by the 180O-flip motion. We calculated the angle-dependence factor { [3/2c0s2/3 - l/2] - '/2qL sin2/3} in eq 3 as a function of e for both cases A and B using the reported values20 of qL = 0.03, 0.03, and 0.01 for 2H(2), 2H(3), and 2H(4), respectively, and plotted that data in Figure 5. To compare these theoretical results with the experimental data, the observed splitting widths vq shown in Figure 3 and the calculated vq derived from values in Figure 5 are plotted against 2vq(4)l(vq(2) vq(3)) as shown in Figure 6. We can see from this figure that the case A model
vq = 3 / 4 v $ { ~ z z [ ( 3cos2 / 2 p - '/J - 1/27L sin2 /?I
(s, - SJ.-'/~
sin2B
+
+ I / , g L ( 1 + cos2PI]> (2)
Since ,S - S,, < 0.1 in most cases, we approximately have
vq = 3/4v$~z,{[3/2cos2 ,B - 1 / 2 ~- 1/27Lsin2 p>
(3)
by assuming S, - Syy = 0. Since it has been reported that qL = 0.01-0.03,20 the contribution to vq from the last term in eq 3, which might be different in cases A and B is very small. Thus we can expect that eq 3 is approximately equally applicable to both cases. The detailed structure of a free pyridine molecule has been determined from the rotational spectra in the gaseous state providing that the C(4)-H bond forms angles of 57.56' and 62.16' with the C(3)-H and C(2)-H bonds, respectively.21If
+
4338 J. Phys. Chem., Vol. 99, No. 12, 1995 is more probable than the case B one, because the absolute magnitude of the difference Ivq(2) - vq(3)1in case A, which is about twice that in case B, is closer to the observed one. This result is consistent with the crystal structure of butylpyridinium chloride having a staggered conformation of the pyridinium ring.22 For case A, we evaluated the e values in this phase by fitting the theoretical values of 2vq(4)/(vq(2) vq(3)) to the experimental data given in Figure 6. The values of e = 43" and 49" at 24 and 40 "C,respectively, were obtained from the temperature dependence of @ shown in Figure 7. These angles are larger than the expected angles of 35.3" [=(180" - 109.47")/ 21, which is calculated by assuming the trans-zigzag structure of the alkyl chain. This suggests that the chain rotation is no longer uniaxial but that fluctuation of the rotation axis such as precessional motion takes place in this phase and this fluctuation amplitude increases with temperature.
+
References and Notes Fudali, D.; Zyczkowska, T. Rocz. Chem. 1968, 42, 1733. Ciechanowics, M. Rocz. Chem. 1974, 48, 1113. Knight, G. A.; Shaw, B. D. J. Chem. Soc. 1938, 682. Ikeda, S.; Fujio, K. Colloid Polym. Sci. 1992, 265, 1009. ( 5 ) Ekwall, P. Advances in Liquid Crystals; Brown, G. H., Ed.; Academic Press: New York, 1975; Vol. 1, pp 1-142. (6) Kilpatrick, P. K.; Bogard, M. A. Lungmuir 1988, 4 , 790. (1) (2) (3) (4)
Tansho et al. (7) Khan, A.; Fontell, K.; Lindblom, G . J. Phys. Chem. 1982,86, 383. ( 8 ) Henriksson, U.; Blackmore, E. S.; Tiddy, G.J. T.; Soderman, 0. J. Phys. Chem. 1992, 96, 3894. (9) Lawson, K. D.: Flautt, T. J. J. Phys. Chem. 1968, 72, 2066. (10) Pope, J. M.: Doane, J. W. J. Chem. Phys. 1987, 87, 3201. (11) Blackburn, J. C.; Kilpatrick, P. K. Lungmuir 1992, 8, 1679. (12) Abdolall, K.; Burnell, E. E.; Valic, M. I. Chem. Phys. Lipids 1977, 20, 115. Delikatny, E. J.; Burnell, E. E. Mol. Phys. 1989, 67, 757. (13) Gally, H.-U.; Niederberger, W.: Seelig, J. Biochemistry 1975, 14, 3647. (14) Auger, M.; Carrier, D.: Smith, I. C. P.; Jarrell, H. C. J. Am. Chem. SOC.1990, 112, 1373. (15) Bak, B.; Hansen-Nygaard, L.; Rastrup-Andersen, J. J. Mol. Spectrosc. 1958, 2, 361. (16) Prabhumirashi, L. S.; Ikeda, R.; Nakamura, D. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 1142. (17) Davis, J. H.; Jeffrey, K. R.; Bloom, M.; Valic, M. I.; Higgs, T. P. Chem. Phys. Lett. 1976, 42, 390. (18) Pope, J. M.; Dubro, D.; Doane, J. W.; Westerman, P. W. J. Am. Chem. SOC.1986, 108, 5426. (19) Doane, J. W. Magnetic Resonance of Phase Transitions; Owens, F. J., Poole, C. P., Farach, H. A., Eds.; Academic Press: New York, 1979; pp 171-246. (20) Ambrosetti, R.; Catalano, D.; Forte, C.; Veracini, C. A. Z. Naturforsch. 1986, 41a, 431. (21) Sbrensen, G. 0.;Mahler, L.; Rastrup-Andersen, N. J. Mol. Struct. 1974, 20, 119. (22) Ward. D. L.: Rheinebarger, R. R.; Popov, A. I. Acta Crystallogr. 1986, C42, 1771. JP942393S
