Langmuir 1986, 2, 739-743
739
Polarized Fourier Transform Infrared Spectra and Molecular Orientation of a Water-Dioctadecyldimethylammonium Chloride System in the Coagel and Gel Phases Takeshi Kawai, Junzo Umemura, and Tohru Takenaka" Institute for Chemical Research, Kyoto University, Uji,Kyoto-fu 611, Japan
Michiko Kodama, Yoshiko Ogawa, and SyOz6 Seki Department of Chemistry, Faculty of Science, Kwansei Gakuin University, Nishinomiya, Hyogo 662, Japan Received March 25, 1986. I n Final Form: June 25, 1986 Polarized Fourier transform infrared spectra have been recorded on two types of oriented crystallites (A and B) of the 13 wt 70 water-dioctadecyldimethylammonium chloride (DODAC) system in the two thermodynamically stable states, i.e., the coagel and gel phases, which appear respectively below and above the gel-liquid crystalline transition temperature. Crystallites A and B are distinguished by the difference in the relative orientation of the methylene chain with respect to the hydrophilic N+(CH3)2group of the molecule in both coagel and gel phases. Furthermore, it is found that the trans-zigzag planes of the methylene chains have a tendency to orient perpendicular to the direction of crystal growth in the coagel phase, while they are almost freely rotated in the gel phase. On the other hand, the CN'C plane of the hydrophilic group orients parallel (perpendicular) to the direction of crystal growth in both coagel and gel phases of crystallite A (crystallite B). The hydrophilic group in the gel phase reveals no accordance with the generally accepted concept that the hydrophilic group is in a partially fused state in the gel phase. Furthermore, it is discovered that there is only one type (I) of oriented bound water in the coagel phase, while there are two types (I and 11) of oriented waters in the gel phase.
Introduction The thermotropic phase transition such as coagel- and gel-liquid crystalline transitions is one of the characteristic features of binary systems of water and amphiphilic compounds. In the case of the binary system of water and lipid which is a major component of biological membranes, its thermotropic transition behavior is very important in connection with the biological function of membranes. Accordingly, many physicochemical investigations have been undertaken to examine the structural aspects accompanying phase transitions of such systems.l However, the role of the water molecule in these phase transitions is not necessarily well understood. In a previous paper,2 we reported a Fourier transform infrared (FT-IR) study on the phase transitions of the water-dioctadecyldimethylammonium chloride (DODAC; (CH3(CH2)17)2N+(CHJ2C1-) system which exhibits the similar thermotropic behavior as that of the water-lipid systems. There we observed drastic changes in number, frequency, and intensity of the water and DODAC bands upon the coagel-gel and gel-liquid crystalline phase transitions. The presence of bound water was also noticed in the coagel and gel phases. These features were different from the case of the water-octadecyltrimethylammonium chloride (ODAC; CH3(CH2)17N+(CH3)3C1having a single hydrocarbon chain) system where bound water was found only in the coagel phase.3 In this paper, we prepared a n orientd sample of the water (13 wt %)-DODAC system and recorded its polarized infrared spectra to investigate the molecular orien(1) Lee, A. G . Biochim.Biophys. Acta 1977, 472, 237. (2) Umemura, J.; Kawai, T.; Takenaka, T.; Kodama, M.; Ogawa, Y.; Seki, S. Mol. Cryst. Liq. Cryst. 1984, 112, 293. (3) Kawai, T.; Umemura, J.; Takenaka, T.; Kodama, M.; Seki, S. J. Colloid Interface Sci. 1985, 103, 56.
0743-7463/86/2402-0739$01.50/0
tation of the water and DODAC molecules in the coagel and gel phases.
Experimental Section The DODAC sample is the same as that described in the previous paper.2 Water used in this experiment is a mixture of doubly distilled H20 (0.96 mol fraction) and D20 (0.04 mol fraction) purchased from Merck, Darmstadt. Therefore, it contains ca. 92 mol % H20, 7.7 mol % HDO, and 0.2 mol % D20. The mixture of DODAC and the water was sandwiched between two CaF2 windows and was assembled in a Harrick DLC-M25 demountable liquid cell. The water content of the sample was determined to be 13 & 1 wt % by referring the infrared spectroscopic data to the phase as described previou~ly.~~~ The sample was first heated to ca. 65 "C in an electric oven, and then the heater of the oven was cut off, the sample being allowed to stand for ca. 16 h with a gradual temperature decrease. The sample thus obtained is made up of many oriented crystallites as shown in the polarization micrograph of Figure 1. A glance at Figure 1 and inspections of the crystallites by polarized infrared spectra revealed that the crystallites were classified into two groups which had different molecular orientations (crystallites A and B). For polarized infrared measurements, we selected a small area of each crystallite of ca. 0.5-mm diameter and the other part was covered by a black paper. The polarized infrared spectra of each crystallite were recorded in the coagel phase below 35 "C and gel phase between 35 and 44 O C 7 on a Nicolet 6000C FT-IR spectrophotometer equipped with a MCT detector and a wire-grid polarizer. The resolution was 4 cm-' and interferograms were accumulated 1000 times. The direction of the electric vector of the incident beam was set parallel and perpendicular to the direction of crystal growth. (4)Kodama, M.; Kuwabara, M.; Seki, S. Thermochim. Acta 1981,50, 81. (5) Kodama, M.; Kuwabara, M.; Seki, S. Mol. Cryst. Liq. Cryst. 1981, 64, 277. (6) Kodama, M.; Kuwabara, M.; Seki, S. In Thermal Analysis; Proceedings of the 7-th ICTA; Heyden & Son: New York, 1982; Vol. 2, p 822. (7) There was no intermediate state3between the coagel and gel phases a t this water content.
0 1986 American Chemical Society
Kawai et al.
740 Langmuir, Vol. 2, No. 6, 1986
Table 1. Major Infrared Absorption Bands of the 13 wt 90 Water-DODAC Sample in the Coagel and Gel Phases magel gel dichroism dichroism crydtallite frequency, crystallite frequency. em-' A B Em-' A B ~
, Figure 1. Polarizatinn microphotograph of a 13 wt 90 waterDODAC sample consisting of small crystallites in the magel phase. Crystallites A and R exhibit different types of molecular orientation. Arrows indicate the directions of crystal growth in respective crystallites.
3443 3370
II I
I II
3247 3032 3020 2977 2955 2917 2871 2852 1623 1491 1473
I
II
II I II
I /I II
I I I I I
I I
1421
II
I
II 1I
I
3489 3405 335' 3241 3042 3019 2979 2953 2920 2870 2851 1633 1489 1470 1467 1423
assignment*
II
I
I
I1
v.(H*O) I v.(H*O) I
II
I
u.(CHdN+)) u.(CH,(N+))
I I
II II I I
II 1I
stretehing; 6, bending.
WAVENUMBER /
C M '
Figure 2. Polarized infrared spectra of a 13wt % waterDODAC sample (crystallite A) in the coagel phase at 27 O C . The electric vectors are parallel (- - -) and perpendicular (-1 to the direction of crystal growth. Results The results of the spectral measurements of the two crystallites in the coagel and gel phases will be described separately. Crystallite A. Figure 2 represents the polarized infrared spectra of crystallite A in the coagel phase at 27 O C . Most of the observed frequencies of the H,O and DODAC bands coincide with those of the coagel spectrum of an unoriented sample? These frequencies and dichroism are summarized in Table I, together with their assignments. Almost all bands show respective clear dichroism. The assignments of the 1491- and 1421-cm-' bands to the asymmetric and symmetric bending bands, respectively, of the methyl group attached t o the N+ atom have been made by referring to infrared spectra of analogous pounds.p10 Figure 3 represents the polarized infrared spectra of crystallite A in the gel phase at 43 'C. Unlike the coagel phase, there is no dichroism for the antisymmetric and symmetric CH, stretching hands a t 2920 and 2851 cm-', respectively, in Figure 3. However, the symmetric CH, bending band of the N+ (CH& group a t 1423 cm-' shows the dichroism similar to that in the coagel spectra of Figure (8) Nerdel, F.; Lehmann, W. Chem. Be,. 1959,92,2460. (9) Takenaks, T. Nippan Kogoku Zosshi 1961,82,1309. (10) Van Senden, K.G. Reel: J . R . Neth. Chem. Soe. 1965.84,1459.
WAVENUMBER / cm-'
Figure 3. Polarized infrared spectra of a 13wt % waterDODAC sample (crystallite A) in the gel phase at 43 O C . The electric vectors are parallel (---) and perpendicular (-1 to the direction of crystal growth.
2, indicating that the hydrophilic part of DODAC still keeps its orientation in the gel phase. Figure 3 gives a different feature in the OH stretching vibration region of water from that in the coagel phase. There are four bands with dichroism in the region 3500-3200 cm-', instead of three in the coagel phase. Figure 4 shows the polarized infrared spectra in the OD stretching region of crystallite A in the coagel(27 "C) and gel (43 "C) phases. Although the noise level is not small because of the small amount (1 w t %) of HDO present in the sample, it is apparent that the coagel spectra give one broad band at ca. 2516 cm-' without clear dichroism, while the gel spectra give two peaks around 2528 and 2490 cm-' without and with dichroism, respectively. Crystallite B. Figure 5 represents the polarized infrared spectra of crystallite B in the coagel phase at 29 "C. Comparison between Figure 5 and Figure 2 (crystallite A) makes the following points clear. First, the dichroism of the bands due to the methylene chains, such as the antisymmetric and symmetric CH, stretching bands a t 2917 and 2852 cm-', respectively, and the CH, scissoring band a t 1473 cm-' is practically the same in both crystallites A and B (see Table I). Second, the bands due to the methyl group attached to the NC atom, like asymmetric and symmetric bending bands at 1491 and 1421 cm-', respec-
FTIR and Molecular Orientation of a H,O-DODAC System
Langmuir, Vol. 2, No. 6, 1986 741
Lo 0
WAVENUMBER / cm-'
Figure 6. Polarized infrared spectra of a 13 wt % water-DODAC sample (crystallite B) in the gel phase at 42 "C. The electric vectors are parallel (- - -) and perpendicular (-) to the direction of crystal growth.
I
\\
in
m
90.
2620
2575
25sO
2525
29f5
25b0
29 J
WAVENUMBER /cm+
Figure 4. Polarized infrared spectra in the OD stretching region of a 13 wt % water-DODAC sample (crystallite A) in the coagel phase at 27 "C (bottom) and in the gel phase at 43 "C (top). The sample contains 1wt % HDO. The electric vectors are parallel (- - -) and perpendicular (-) to the direction of crystal growth.
l,*l
1
1; 'I
1.00
'
3600
I
COAGEL
I I'
x 3.3
11
3400 I
I
3200 I
3000 I
' - \
WAVENUMBER I cm-'
Figure 5. Polarized infrared spectra of a 13 w t % watel-DODAC sample (crystallite B) in the coagel phase at 29 "C. The electric vectors are parallel (- - -) and perpendicular (-) to the direction of crystal growth.
tively, exhibit opposite dichroism in crystallites A and B. Third, the bands due to water, like the OH stretching bands a t 3443 and 3370 cm-' and the HOH bending band a t 1623 cm-', also show opposite dichroism in both crystallites. Figure 6 shows the polarized infrared spectra of crystallite B of the sample in the gel phase a t 42 "C. Similarly to the gel spectra of crystallite A in Figure 3, the bands due to the methylene group of crystallite B show almost no dichroism. However, the bands due to the methyl group in the hydrophilic part and those due t o water in Figure 6 exhibit the dichroism opposite to that of crystallite A in Figure 3.
Discussion Coagel Phase of Crystallite A. From the frequency (1473 cm-l) of the singlet band due to the CH, scissoring vibration of the unoriented DODAC sample in the coagel phase, we have concluded that the trans-zigzag planes of the methylene chains are parallel with each other.3 In Figure 2, the symmetric CH2 stretching band at 2852 cm-' and the CH2 scissoring band a t 1473 cm-' exhibit perpendicular dichroism to the direction of crystal growth. On the other hand, the CH2 antisymmetric stretching band a t 2917 cm-l shows parallel dichroism. The transition moments of these bands are perpendicular to the methylene chain axis, the former two bands being in the transzigzag plane while that of the latter band being perpendicular to the trans-zigzag plane. Therefore, it may be inferred that the trans-zigzag plane has a tendency to orient perpendicular to the direction of crystal growth and that the chain axis is perpendicular to the surface of CaF, windows. These results are schematically illustrated in Figure 7A(c), although the actual orientation is not so complete. The asymmetric and symmetric bending bands (1491 and 1421 cm-I) of the methyl group attached to the N+ atom exhibit perpendicular and parallel dichroism, respectively, in Figure 2. Since the transition moment of the symmetric bending band of (N+)CH3group of DODAC is within the CH3-N+-CH3 plane, the observed dichroism of this band indicates that the plane has a tendency to orient parallel to the direction of crystal growth (Figure 7A(c)). According to the thermal analysis of the sample with 13 wt % water most of the water molecules (11 wt %) are in a bound state between bilayers of the DODAC molecules in the coagel phase. Only remaining water (2 wt % ) is in the bulk free state coexisting with the hydrated crystal.P6 In Figure 2, the OH stretching bands of water appear a t 3443 and 3370 cm-' with parallel and perpendicular dichroism, respectively. Since the dichroism of the latter band is the same as that of the HOH bending band at 1623 cm-', this OH stretching band can be assigned to the symmetric mode, and consequently the former band a t 3443 cm-l is ascribed to the antisymmetric mode. The frequency separation of 73 cm-' for these two bands is typical for hydrates with symmetrically bonded H,O molecules." Since the transition moments of the symmetric OH stretching and HOH bending bands are parallel (11) Lutz, H.D.; Christian, H.J. Mol. Struct. 1982, 96,61.
742 Langmuir, Vol. 2, No. 6, 1986
Kawai et al.
COAGEL
H\C/H
+
GEL
H\c/H
I+-C*=N=CM{:
H/c\H
i // Direction of c r y s t a l g r o w t h
0
I\
H H
Figure 7. Schematic illustration of the orientations of methylene chains, N+(CH3)zgroup of DODAC, and waters in the coagel (left) and gel (right)phases for the two types of crystallites A (top) and B (bottom). This figure is just a rough sketch for an easy understanding of relative molecular orientations: the actual orientation is not so complete as this. to the bisector of the HOH angle, the perpendicular dichroism of these bands implies that the bisector is perpendicular to the direction of crystal growth (Figure 7A(c)). Note that the relative number of the water molecules to the DODAC molecule is not exact in Figure 7A(c). The 11wt % water of hydration corresponds to four molecules per one DODAC molecule. The small perpendicular band at 3247 cm-' is ascribed to the overtone of the HOH bending band a t 1623 cm-l., Since the amount of D 2 0 species in the sample is negligible as stated above, the OD stretching bands in Figure 4 practically arise from the HDO species in which the OD stretching mode is decoupled with the OH stretching mode. If the HDO molecules in the coagel phase are oriented like the H 2 0 molecules illustrated in Figure 7A(c), the transition moment of the OD stretching band may have parallel and perpendicular components to the direction of crystal growth with almost identical intensities. In fact, the broad OD streching band at 2516 cm-' has almost no dichroism as shown in Figure 4. Gel Phase of Crystallite A. There is no dichroism for the antisymmetric and symmetric CH2 stretching bands at 2920 and 2851 cm-', respectively, in the gel phase spectra of Figure 3. This fact is consistent with the feature expected for the hexagonal chain packing where the orientation of the trans-zigzag plane of the methylene chain is at random around the chain axis (Figure 7A(g)).2 The CHp scissoring band exhibits a small doubling2at 1470 and 1467 cm-'. The similar splitting of the CH, scissoring band has been found in the hexagonal or so-called "rotor" phase of n-alkanes.', The two components of the doublet have different dichroism from each other as seen in Figure 3. Therefore, the hexagonal phase with 3-cm-' splitting of the CH2 scissoring band seems to have certain quasi-stable positions of the trans-zigzag plane, although its stability (12) Casal, H. L.; Mantsch, H. H.; Cameron, D. G.; Snyder, R. G. J. Chem. Phys. 1982, 77, 2825.
may not be so large as compared with that in the usual orthorhombic packing which has more than 10-cm-' splitting of the scissoring band.13 The fact that the two components have almost identical intensities suggests that the CH2 scissoring band has no dichroism as a whole. On the other hand, the antisymmetric and symmetric CH, bending bands of the N+(CHJ2 group at 1489 and 1423 cm-', respectively, in the gel phase (Figure 3 and Table I) show the dichroism similar to those in the coagel spectra of Figure 2. Thus, it is obvious that the hydrophilic part of DODAC in this water content (13 wt %) is still in a fixed state. As was pointed out previously,2this finding also reveals no accordance with the generally accepted concept14J5that the polar group is in a partially fused state in the gel phase of amphiphile-water systems. In the gel phase spectra of Figure 3, three OH stretching bands of water appear at 3489,3405, and 3354 cm-l. The dichroism of the first and third bands is parallel to the direction of crystal growth and that of the second band is perpendicular. These results make a clear contrast to the coagel spectra where only two bands were observed at 3443 and 3370 cm-l with parallel and perpendicular dichroism, respectively. Since the dichroism of the hydrophilic band in the gel phase is almost unchanged from that in the coagel phase, it is reasonable to consider that the same type of oriented water as that found in the coagel phase (type I water) still persists in the gel phase (Figure 7A(g)). Then, the two bands at 3489 and 3405 cm-' (the separation 84 cm-') with the parallel and perpendicular dichroism can be ascribed to the antisymmetric and symmetric OH stretching bands of this type of water, respectively, corresponding to the 3443- and 3370-cm-' bands in the coagel phase. The frequency shifts of the two bands (13) Snyder, R. G. J . Mol. Spectrosc. 1964, 7, 116. (14) Vincent, J. M.; Skoulios, A. E. Acta Crystallogr. 1966,20,432,441, 447. (15) Winsor, P. A. In Liquid Crystals and Plastic Crystals; Gray, G. W., Winsor, P. A., Eds.; Ellis Horwood: Chichester, 1974; Vol. 1, p 199.
FTIR and Molecular Orientation of a H20-DODAC System Table 11. OH Stretching Frequencies of H20 and OD Stretching Frequencies of HDO in the Coagel and Gel Phases of 13 wt 9% Water (H,O:HDO = 12:l)-DODAC Sample
v,(H@) u,(H,O)
u,,(OH)~ 4OD) v,,(OH) / 4OD)
coagel type I 3443 3370 3407 2516 1.354
gel type I 3489 3405 3447 2528 1.363
type I1 3438b 3354 3396 2490 1.364
+
(u,(HzO) u,(H,0))/2. bEstimated from the separation of 84 cm-' between v,(H,O) and u,(H,O); refer to type I water.
from the coagel to gel phases amounts to +46 and +35 cm-l. This may indicate that the packing or the bonding of the water molecules in the gel phase is loosened by the coagel-gel phase transition which introduces another type of oriented water described below. The parallel band at 3354 cm-' is in the frequency region of the symmetric OH stretching vibration of water. However, there is no sign of the presence of the antisymmetric OH stretching vibration around 3438 cm-l expected from the frequency separation of ca. 84 cm-' between the antisymmetric and symmetric OH stretching bands. A possible reason for this is that the direction of the transition moment of the antisymmetric OH stretching band is perpendicular to the plane of the optical windows, as illustrated on the right and left sides of the DODAC molecule in Figure 7A(g) (type I1 water). This will be clarified in the next paragraph by the examination of the OD stretching vibration region in Figure 4. In the gel-phase spectra of Figure 4, the band a t 2528 cm-l without dichroism can be ascribed as that due to type I water, the corresponding coupled bands of H 2 0 species being 3489 and 3405 cm-' in Figure 3. If a HDO molecule is present as type I1 water, the transition moment of the OD stretching mode is parallel to the direction of crystal growth. The parallel dichroism of the 2490-cm-l band in Figure 4 is consistent with this postulate. In Table 11, the antisymmetric and symmetric OH stretching frequencies of H 2 0 , the average of these two frequencies (v,,(OH)), the OD stretching frequencies (Y(OD)) of HDO, and the ratio v,,(OH)/Y(OD) are summarized for the coagel phase (type I water) and the gel phase (type I and I1 waters). The ratios v,,(OH)/v(OD) range from 1.354 to 1.364 and are quite reasonable in light of the average value of 1.355 which has been obtained from a large collection of data on ices and hydrates.l&l* This fact also supports the appropriateness of the assignments of the water bands in this study. It is found from Table I1 that the OH and OD stretching frequencies of type I1 water
Langmuir, Vol. 2, No. 6, 1986 743 in the gel phase are lower than the corresponding values of type I water in the coagel and gel phases. This fact indicates that the hydrogen bondings in type I1 water are stronger than those in type I water. This is presumably due to the environmental difference of the water molecules such that type I water is concerned with the hydrogen bonds with the C1- ions,lg while type I1 water is exclusively concerned with the hydrogen bonds with surrounding waters. Crystallite B. As is apparent from polarized spectra in Figures 5 and 6, the dichroism of the methylene bands of crystallite B is the same as that in crystallite A, both in the coagel and in the gel phases. However, the dichroism of the N+-(CH& bands in the hydrophilic part and those of the water bands of crystallite B are just opposite to those of crystallite A. Thus, we can readily draw schematic illustrations of the orientations of each part of DODAC and of waters in crystallite B as shown in Figure 7B(c),(g). The trans-zigzag plane of the methylene chain orients in the same way as that in crystallite A (in the coagel phase the plane is perpendicular to the direction of crystal growth, while in the gel phase it is a t random around the chain axis), but the N-(CH,), groups and waters in crystallite B orient perpendicular to those in crystallite A both in the coagel and in the gel phases. Here it is to be noted that the directions of crystal growth of crystallites A and B are almost perpendicular to each other as seen in Figure 1. Therefore, it is concluded that the orientations of both water and the hydrophilic part in crystallite A are practically parallel to those in crystallite B. This may come from the fact that this part of the molecule in the first layer is directly attached to the window surface, and two-dimensional interactions govern the orientation of the first hydrophilic layer. The orientations of the methylene chains in crystallites A and B are practically perpendicular to each other. The difference of the relative orientation of the hydrophilic part to the methylene chain in crystallites A and B indicates that there are two different molecular conformations of the DODAC molecules. Since the exact conformations cannot be determined from the present experiment, the structural analysis by X-ray diffraction is desirable.
Acknowledgment. We are indebted to Dr. Kaoru Tsujii and Hiromichi Takahashi of Kao Co., Ltd., for their kind supply of the DODAC sample. Thanks are also due to Koichi Okano of Sanyo Kasei Kogyo Co., Ltd., for the GC-mass analysis of the sample. This research was partly supported by the Grant-in-Aid on Special Project Research for Organic Thin Films for Information Conversion from the Ministry of Education, Science and Culture, Japan. Registry No. DODAC, 107-64-2.
(16) Falk, M., private communication. (17) Umemura, J.; Birnbaum, G. I.; Bundle, D. R.; Murphy, W. F.; Bernstein, H. J.; Mantsch, H. H. Can. J . Chem. 1979, 57, 2640. (18) Novak, A. Struct. Bonding (Berlin) 1974, 18, 177.
(19) Taga, T.; Machida, K.; Kimura, N.; Hayashi, S.; Umemura, J.; Takenaka, T. Acta Crystallogr., Sect. C 1986, C42, 608.
