J. Phys. Chem. 1996, 100, 8487-8498
8487
Conformational Analysis of Nonionic Surfactants in Water by a Selective Monodeuteration Method. C-D Stretching Infrared Spectroscopy of r-Monodeuterododecyl-ω-hydroxytris(oxyethylene)s Sei Masatoki, Keiichi Ohno, Hiroshi Yoshida, and Hiroatsu Matsuura* Department of Chemistry, Faculty of Science, Hiroshima UniVersity, Kagamiyama, Higashi-Hiroshima 739, Japan ReceiVed: February 26, 1996X
The conformation of the alkyl chain in R-dodecyl-ω-hydroxytris(oxyethylene) (C12E3) in water has been studied by C-D stretching infrared spectroscopy. This conformational analysis is based on the fact that the wavenumbers of the isolated C-D stretching vibrations are sensitive to the conformation in the vicinity of the C-D bond. Infrared spectra were measured for five selectively monodeuterated species of C12E3, namely CH3(CH2)11-kCHD(CH2)k-1(OCH2CH2)3OH, where k ) 1, 2, 4, 6, and 8, in the lamellar (LR) and isotropic solution (L2) phases. The C-D stretching bands for each of the monodeuterated species were assigned to particular conformations of the specifically deuterated part of the chain. From the observed intensities of the C-D stretching bands, the fractions of the trans conformation around the dodecyl C-C bonds and the oxyethylene-adjoining O-C bond and the fractions of the consecutive trans conformations around the two adjoining bonds were evaluated. The conformational change at the phase transition from L2 to LR is not significant and only a small increase in the trans fraction is observed for the C-C bonds close to the alkyl/ oxyethylene interface. This implies that the conformational states in the L2 and LR phases in the vicinity of their boundary are substantially not different. In the LR phase, when the composition or the temperature approaches the region of the phase separation or transition, the trans fractions for the C-C bonds closer to the alkyl/oxyethylene interface and those closer to the chain terminal decrease significantly. These observations indicate that the conformational transformation from trans to gauche at these chain positions makes the lamellar structure less stable and leads eventually to the structural destruction. The fraction of the consecutive trans conformations may be used as a measure of the conformational order at particular positions of the chain. The present results show that the ordering is the highest in the middle of the chain in the LR and L2 phases. This vibrational spectroscopic observation, together with the previous NMR observations, indicates that the alkyl/oxyethylene interface is flexible with respect to the conformation and the orientation of the chain.
Introduction Self-organization of molecules for the creation of functional units is one of the fundamental processes in biological systems. The most important self-organized system is a biomembrane, which is composed of a protein-containing lipid bilayer as the middle component.1 The bilayer and many other self-organized structures constitute supramolecular systems,2,3 and a broad range of substances have been known to date to form supramolecular structures.4 The amphiphilic molecules such as surfactants and lipids self-assemble in water and other solvents to form various supramolecular aggregates with different shapes and sizes.5-10 The most typical phases of the supramolecular structures formed by these amphiphiles in the solvents are the lamellar phase, the hexagonal phase (normal and reversed), and the cubic phase (normal and reversed), which are liquidcrystalline phases characterized by long-range order. These amphiphilic molecules form in their isotropic solutions the aggregates lacking long-range correlations, such as spherical micelles, cylindrical micelles, and bilayers. In order to study the structure of supramolecular systems consisting of amphiphilic molecules, conformational properties of the molecular chain are important in relation to the phase behavior.11 Despite its fundamental importance, no direct experimental evidence has been reported of the conformational relevance to the phase structure.12 One of the most interesting X
Abstract published in AdVance ACS Abstracts, April 15, 1996.
S0022-3654(96)00580-1 CCC: $12.00
classes of amphiphilic molecules is a family of nonionic surfactants consisting of the hydrophilic moiety of the oxyethylene chain and the hydrophobic moiety of the alkyl chain.13,14 The systems of these surfactants and water exhibit diverse aggregate structures at different compositions and temperatures, giving a variety of characteristic phase diagrams.15 By changing the lengths of the alkyl and oxyethylene chains, systematic studies are possible of the structure of the aggregates, the conformation of the hydrophobic and hydrophilic moieties, and other relevant properties. In the present work, we have studied one of the typical nonionic surfactants, R-dodecyl-ω-hydroxytris(oxyethylene), CH3(CH2)11(OCH2CH2)3OH, conventionally abbreviated as C12E3, for clarifying the conformational properties and their implications in the phase behavior and the supramolecular structures. The system of C12E3-water exhibits phase behavior as shown in Figure 1.15 In the phase diagram, the liquid-crystalline lamellar phase, to be denoted as LR, is observed in concentration regions of approximately 45-80 wt % below about 50 °C. In more concentrated regions, the isotropic solution phase L2 occurs, which is a phase of liquid surfactant containing dissolved water, not fully miscible with water.15 In lower-concentration regions, the separated phases of W + LR and L2 + LR are observed, where W is a phase of water containing surfactant monomers. Using infrared spectroscopy, we have studied the conformation of C12E3 in the LR and L2 phases at the compositions and temperatures indicated in Figure 1. © 1996 American Chemical Society
8488 J. Phys. Chem., Vol. 100, No. 20, 1996
Masatoki et al. SCHEME 1
Figure 1. Phase diagram of the C12E3-water system.15 The compositions and temperatures at which the spectral measurements were performed are indicated by solid circles for the LR phase and open circles for the L2 phase.
Infrared spectroscopy, together with Raman spectroscopy, is one of the most widely used techniques for studying the structures of supramolecular systems. This spectroscopy has in fact been applied to a wide variety of amphiphilic colloidal systems, and various supramolecular phases have been characterized.16 Vibrational spectroscopy, which implies infrared and Raman spectroscopy, is particularly useful for investigating the conformation of chain molecules, since the spectra exhibit characteristic bands associated with particular conformational states of the chain.17 By using these marker bands, detailed conformational analysis is possible even for complex systems. We have studied by Raman spectroscopy the molecular conformation of a series of CnEm surfactants (n ) 1-16 and m ) 1-8) in the solid state and have clarified the relation between the conformation and the chain length.18-21 Vibrational spectroscopy has been applied extensively to aqueous solutions of CnEm and other related surfactants.22-34 In most of these studies, the molecular structure has been discussed in relation to the phase transitions in the surfactant-water systems. The phase and conformational transitions in lipids and biomembranes have also been studied by vibrational spectroscopy.35 In the present conformational analysis by infrared spectroscopy, we have employed selectively monodeuterated species of C12E3, in which just one of the hydrogen atoms in the alkyl chain is substituted by deuterium. Using various monodeuterated species with deuterium introduced into specific sites in the chain, we observed the stretching vibrations of the isolated C-D bond. In our previous studies,36,37 we have found that the wavenumbers of the isolated C-D stretching vibrations are sensitive to the conformation in the vicinity of the C-D bond and are correlated directly to the C-D bond lengths. This method is thus capable of determining the conformational state of the specifically deuterated part of the chain and eventually, after examining all possible monodeuterated species, the conformational state of whole of the chain. The previous authors38-40 have studied the isolated C-H stretching vibrations of monoprotonated deutero-n-alkanes, in which one of the hydrogen atoms is protium and any others have been replaced by deuterium. These are the compounds that are deuterated in reverse of the present monodeuteration. Since our compounds with deuterium introduced into any desired site in the chain are more easily prepared than the reverse compounds, the isolated C-D stretching vibrations have the potential of being used as conformational markers more extensively than the isolated C-H stretching vibrations. The present work is the first application of the selectively monodeuterated C-D stretch-
ing vibrations to the supramolecular system, aiming at establishing the conformational relevance to the formation of the supramolecular structure. The monodeuterated species of C12E3 we studied are CH3(CH2)10CHD(OCH2CH2)3OH (C12-1-d1-E3), CH3(CH2)9CHDCH2(OCH2CH2)3OH (C12-2-d1-E3), CH3(CH2)7CHD(CH2)3(OCH2CH2)3OH (C12-4-d1-E3), CH3(CH2)5CHD(CH2)5(OCH2CH2)3OH (C12-6-d1-E3), and CH3(CH2)3CHD(CH2)7(OCH2CH2)3OH (C12-8-d1-E3). In the following, after describing preparation of the samples, spectral measurements, and ab initio calculations on model compounds, we give the definition of the conformation of monodeuterated compounds with respect to the configuration. We then examine the relation between the C-D stretching wavenumbers and the associated conformations for the relevant model compounds. The relation obtained is applied to the assignment of the C-D stretching bands of the monodeuterated species of C12E3. After describing the procedure of the conformational analysis using the C-D stretching vibrations, we examine the conformational behavior of the alkyl chain with changing composition of the C12E3-water system and temperature and discuss its relevance to the phase behavior. Finally, we compare the conformation profiles obtained in the present work with the order parameters derived from NMR experiments. Experimental Section Materials. Five selectively monodeuterated species, C12-1d1-E3, C12-2-d1-E3, C12-4-d1-E3, C12-6-d1-E3, and C12-8-d1-E3, were synthesized from pertinent selectively monodeuterated 1-chloro- or 1-bromododecane and triethylene glycol by the Williamson method. The syntheses of monodeuterated 1-chloroand 1-bromododecanes are outlined in Scheme 1.41,42 Monodeuterated species of butyl methyl ether, i.e., CH3OCHD(CH2)2CH3 (butyl-1-d1 methyl ether) and CH3OCH2CHDCH2CH3 (butyl-2-d1 methyl ether), and of hexane, i.e., CH3(CH2)2CHDCH2CH3 (hexane-3-d1), were prepared according to the method shown in Scheme 2. The products were purified by
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J. Phys. Chem., Vol. 100, No. 20, 1996 8489
SCHEME 2
vacuum distillation and the purity was checked by gas chromatography to be better than 95%. Infrared Measurements. Aqueous solutions of the five monodeuterated species of C12E3 with compositions of 50, 60, 70, 80, 85, 90, 95, and 100 wt % were prepared for infrared measurements. Homogenization of the solutions was achieved by heating them at 50 °C for several days. The solution sample held between two KRS-5 plates with a lead spacer of 0.1 mm thickness was contained in a home-made variable-temperature cell made of a brass cylinder with a wound Nichrome wire for heating. The infrared spectra were measured at 26, 36, 45, 54, and 65 °C. The temperature was monitored by a copperconstantan thermocouple and was controlled within (0.5 °C during the spectral measurements. The compositions and temperatures at which the spectral measurements were performed are indicated in Figure 1. The spectra were measured at least three times at each of these points. The spectra of the monodeuterated C12E3 in aqueous solution were obtained by subtracting the solvent water absorption from the original recordings. The infrared spectra of monodeuterated model compounds, butyl-1-d1 methyl ether, butyl-2-d1 methyl ether, and hexane-3-d1, were measured for the neat liquid at 26 °C. The spectra were recorded on a Nicolet Impact 400 infrared spectrometer equipped with a DTGS detector by coadding 400 scans at a resolution of 1 cm-1. The observed spectral profiles of the C-D stretching vibrations were analyzed by fitting with resolved Lorentzian components.
Figure 2. Molecular models of possible conformational forms of the monodeuterated model compounds with the R-configuration: (a) CH3OCHD(CH2)2CH3, (b) CH3OCH2CHDCH2CH3, and (c) CH3(CH2)2CHDCH2CH3. Deuterium atoms are represented by solid circles.
Calculations Ab initio molecular orbital (MO) calculations were carried out on butyl methyl ether and hexane as model compounds for C12E3 in order to evaluate the lengths of the C-D bonds and the wavenumbers and infrared absorption intensities of the isolated C-D stretching vibrations for various conformations of CH3OCHD(CH2)2CH3, CH3OCH2CHDCH2CH3, and CH3(CH2)2CHDCH2CH3. The conformational forms considered are shown in Figure 2. The calculations were performed with the GAUSSIAN 92 program43 at the Computer Center of the Institute for Molecular Science, Okazaki, by the restricted Hartree-Fock method using the 6-31G** basis set. Results and Discussion Configurations and Conformations of Monodeuterated Compounds. In a molecule of selectively monodeuterated species of C12E3, each of the two hydrogen atoms bonded to a particular carbon atom in the dodecyl chain was replaced by deuterium in equal probability. This carbon atom is an asymmetric center giving two different configurations, R and S, in equal amount. In Figure 3, all possible conformational forms of a W-X-CHD-Y-Z molecule with the R- and S-configurations, consisting of trans (T), gauche+ (G+), or
Figure 3. Molecular models of all possible conformational forms of a W-X-CHD-Y-Z molecule with the R- and S-configurations. The ascending sequence of the chemical groups H < D < Y < X has been applied. Deuterium atoms are represented by solid circles.
gauche- (G-) conformation for each of the relevant single bonds,44,45 are illustrated for explaining the relation between the conformations with different configurations, where the ascending sequence of the chemical groups H < D < Y < X has been applied. It is shown, for example, that the TG+ form of the R-configuration and the TG- form of the S-configuration are the enantiomers of each other and are accordingly spectroscopically equivalent in that they give the same wavenumbers
8490 J. Phys. Chem., Vol. 100, No. 20, 1996
Masatoki et al.
TABLE 1: Conformations of a W-X-CHD-Y-Z Molecule with the R- and S-Configurationsa molecular conformationb
conformation of the C-D bond
R-configuration
S-configuration
W-X-C-Dc
D-C-Y-Zc
TT TG+ G-T G-G+ TGG-GG+T G+G+ G+G-
TT TGG+T G+GTG+ G+G+ G-T G-GG-G+
g g g g g g t t t
g g g g t t g g t
} } }
designationd WX/gg/YZ WX/gt/YZ WX/tg/YZ WX/tt/YZ
The ascending sequence of the chemical groups H < D < Y < X is applied. For the conformational forms, see Figure 3. The molecular conformations of the R- and S-configurations placed in the same row in the table are the enantiomers of each other and are spectroscopically equivalent in that they give the same vibrational wavenumbers. If X ) Y and W ) Z, the TG+ (R-configuration), TG- (S), G-T (R), and G+T (S) forms are equivalent, the TG- (R), TG+ (S), G+T (R), and G-T (S) forms are equivalent, and the G-G- (R), G+G+ (S), G+G+ (R), and G-G- (S) forms are equivalent. c Conformation of the C-D bond with respect to the molecular skeleton: t, trans; g, gauche (gauche+ or gauche-). d See text. a
b
TABLE 2: Calculated C-D Bond Lengths (r), C-D Stretching Diagonal Force Constants (k), C-D Stretching Wavenumbers (ν), Infrared Absorption Intensities of the C-D Stretching Vibrations (I), and Observed C-D Stretching Wavenumbers (νobsd) for the Model Compounds calculatedc ν/cm-1
I/km mol-1
CH3O-CHD-CH2-CH2CH3 1.0847 5.749 1.0847 5.745 1.0904 5.514 1.0909 5.485 1.0911 5.490 1.0917 5.457 1.0918 5.459
2377.4 2376.3 2323.4 2317.3 2317.2 2310.3 2310.5
34.83 34.55 47.14 48.85 38.97 42.40 42.37
OC/gt/CC OC/gt/CC OC/gg/CC OC/gg/CC OC/gg/CC OC/tg/CC OC/tg/CC
CH3O-CH2-CHD-CH2CH3 1.0862 5.703 1.0866 5.684 1.0870 5.677 1.0872 5.669 1.0873 5.668 1.0890 5.597 1.0891 5.595
2365.4 2361.4 2357.3 2357.5 2357.0 2341.2 2340.7
17.59 19.57 15.92 16.95 16.12 22.85 23.54
CC/gt/CC CC/tg/CC CC/tg/CC CC/gt/CC CC/gg/CC CC/gg/CC CC/gg/CC
CH3CH2-CH2-CHD-CH2CH3 1.0886 5.588 1.0886 5.587 1.0887 5.586 1.0888 5.581 1.0893 5.570 1.0896 5.561 1.0896 5.561
2340.3 2340.1 2339.9 2338.9 2335.5 2333.7 2333.6
24.49 25.95 25.91 27.29 21.38 22.12 22.05
mol conformna
conformn of the C-D bondb
G+G+T G+TT G-G-T TG-T G-TT TTT TG+T
CO/tg/CC CO/tg/CC CO/gt/CC CO/gt/CC CO/gg/CC CO/gg/CC CO/gg/CC
TTGTG-GTTT TTG+ TG-T TG+T TG+G+ TTGTG+T TG+G+ TG-GTTT TG-T TTG+
r/Å
k/mdyn Å-1
νobsd/cm-1
}
}
2163 2128
}
2111
}
2166
} }
} }
2153 2144
2153
2136
a For the conformational forms, see Figure 2. The R-configuration is assumed. b For the conformation designation, see text. c Calculated at the RHF/6-31G** level.
of normal vibrations. In Table 1, the equivalence relation between the conformations of the R- and S-configurations for a W-X-CHD-Y-Z molecule is shown. It is seen that the alternation of the configuration interchanges the two gauche conformations, G+ and G-. Although the monodeuterated compounds studied in this work actually consist of the two configurations, we will mention only the conformations of the R-configuration in subsequent discussions. In Table 1, the conformations of the C-D bond with respect to the molecular skeleton (W-X-C-D and D-C-Y-Z) are indicated, where no distinction is made between gauche+ and gauche-. This specification of the conformational state of the C-D bond is important, as discussed below, for the interpretation of the observed C-D stretching vibrations of the monodeuterated C12E3. We propose a designation of the conformational disposition of the C-D bond in the W-X-CHD-Y-Z structure; WX/gt/YZ, for example, means that the conformation of the W-X-C-D disposition is gauche (g) and the conformation of the D-C-Y-Z disposition is trans (t).
C-D Stretching Vibrations of Model Compounds. Our previous studies36,37 have shown that the wavenumbers of the isolated C-D stretching vibrations of monodeuterated alkyl chains depend on the local conformation in the vicinity of the C-D bond and that this conformational dependence of the C-D stretching wavenumbers is correlated directly to the C-D bond lengths calculated by the ab initio MO method. The C-D stretching bands of the selectively monodeuterated C12E3 can therefore be assigned to appropriate local conformations on the basis of the relation between the C-D bond lengths and the isolated C-D stretching wavenumbers. Since it is not practical to calculate the bond lengths for the C12E3 molecule itself consisting of many atoms, we have established such correlation for simple monodeuterated model compounds CH3OCHD(CH2)2CH3, CH3OCH2CHDCH2CH3, and CH3(CH2)2CHDCH2CH3. Table 2 gives the results of the ab initio MO calculations for possible conformational forms of the three model compounds with the R-configuration. The conformations containing the
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J. Phys. Chem., Vol. 100, No. 20, 1996 8491
Figure 4. Infrared spectra in the 2050-2250 cm-1 region of the monodeuterated model compounds and the monodeuterated species of C12E3 in the neat liquid state at 26 °C: (a) CH3OCHD(CH2)2CH3, (b) C12-1-d1-E3, (c) CH3OCH2CHDCH2CH3, (d) C12-2-d1-E3, (e) CH3(CH2)2CHDCH2CH3, and (f) C12-4-d1-E3. The smallest component for CH3(CH2)2CHDCH2CH3 (e) is ascribable to an overtone or combination vibration.
G+G- or G-G+ sequence were, however, not considered because of the high instability of the structure.45 The wavenumbers and the infrared absorption intensities of the isolated C-D stretching vibrations evaluated for the model compounds will be used for the spectral analysis of the monodeuterated C12E3. The results in Table 2 indicate that the shorter C-D bonds give higher isolated C-D stretching wavenumbers. This correlation is consistent with the corresponding correlation between the isolated C-H stretching wavenumbers and the C-H bond lengths discussed previously.39,40 It is evident from the calculated results that the C-D (or C-H) bond length depends significantly on the conformational disposition of the C-D (C-H) bond relative to the molecular skeleton. For CaH3OCbHDCcH2CdH2CeH3, the G+G+T and G+TT forms both have the Ca-O-Cb-D disposition with the trans conformation and the D-Cb-Cc-Cd disposition with the gauche conformation; this conformational disposition of the C-D bond is designated, in accordance with the nomenclature mentioned before, as CaO/ tg/CcCd or simply as CO/tg/CC as shown in Table 2. Similarly, the C-D bond for the G-G-T and TG-T forms is designated as CO/gt/CC and that for the G-TT, TTT, and TG+T forms as CO/gg/CC. The calculated results show that the C-D bond length increases in going from rCD(CO/tg/CC) to rCD(CO/gt/ CC) and to rCD(CO/gg/CC). For CH3OCH2CHDCH2CH3 it increases in order of rCD(OC/gt/CC) < rCD(OC/gg/CC) < rCD(OC/tg/CC) and for CH3(CH2)2CHDCH2CH3 in order of rCD(CC/gt/CC), rCD(CC/tg/CC) < rCD(CC/gg/CC). In accordance with the increase of the C-D bond length, the C-D stretching diagonal force constant and, in consequence, the C-D stretching wavenumber decrease (Table 2). The calculated wavenumbers for various conformations of the monodeuterated model compounds are well correlated to the observed wavenumbers, although the calculated values without scaling are always larger by 9-10% than the observed values. The correlation between the conformations and the C-D stretching wavenumbers for CH3OCHD(CH2)2CH3 is used for the assignment of the C-D stretching bands for C12-1-d1-E3, that for CH3OCH2CHDCH2CH3 is used for the assignment for C12-2-d1-E3, and that for CH3(CH2)2CHDCH2CH3 is used for the assignment for C12-4-d1-E3, C12-6-d1-E3, and C12-8-d1-E3. The infrared spectra in the C-D stretching region of the model compounds, in comparison with those of the relevant monodeuterated species of C12E3, are shown in Figure 4. It is seen that the spectral profiles of the model compounds and the surfactant species correspond well. Assignment of the C-D Stretching Bands of Monodeuterated C12E3. Figure 5 shows the infrared spectra in the
Figure 5. Infrared spectra in the 2050-2250 cm-1 region of 90 wt % aqueous solutions of the monodeuterated species of C12E3 at 26 °C: (a) C12-1-d1-E3, (b) C12-2-d1-E3, (c) C12-4-d1-E3, (d) C12-6-d1-E3, and (e) C12-8-d1-E3.
2050-2250 cm-1 region of 90 wt % aqueous solutions of C121-d1-E3 through C12-8-d1-E3 at 26 °C, Figure 6 shows the infrared spectra of aqueous solutions of C12-2-d1-E3 and C126-d1-E3 at 26 °C with different compositions, and Figure 7 shows the infrared spectra of 80 wt % aqueous solutions of C12-2-d1E3 and C12-6-d1-E3 at different temperatures. The observed spectral profiles of the C-D stretching vibrations are resolved into components by fitting with the Lorentzian function. The fits are unique once the number of the components involved is known, which has been actually known from the spectral analysis of the relevant model compounds based on theoretical calculations. The C-D stretching band profile for C12-1-d1-E3 is resolved into three component bands A, B, and C; band A is assigned, in accordance with the results for CH3OCHD(CH2)2CH3, to the G+G+ and G+T conformations, band B to the G-G-, TG-, and G-T conformations, and band C to the TT and TG+ conformations, respectively, of the R-configuration of the CH2OCHDCH2CH2 part. These assignments together with those for other monodeuterated species of C12E3 are shown in Table 3. The spectral profile for C12-2-d1-E3 is explained by three component bands D, E, and F. These bands are assigned, on the basis of the results for CH3OCH2CHDCH2CH3, to the TGand G-G- conformations (band D), the TT, TG+, and G-T conformations (band E), and the G+T and G+G+ conformations (band F) of the OCH2CHDCH2CH2 part. The C-D stretching profile for C12-4-d1-E3, C12-6-d1-E3, and C12-8-d1-E3 is resolved into two component bands G and H, which are interpreted, on the basis of the results for CH3(CH2)2CHDCH2CH3, to be
8492 J. Phys. Chem., Vol. 100, No. 20, 1996
Masatoki et al. C-C bonds and the oxyethylene-adjoining O-C bond in C12E3 are evaluated from the observed integrated intensities of the resolved bands, as measured as areas of the Lorentzian components, for the isolated C-D stretching vibrations and from the infrared intensities of these vibrations calculated for the relevant conformations of the monodeuterated model compounds (Table 2). Procedure of the conformational analysis is described below. The intensities of the C-D stretching bands, IA, IB, IC, ..., are given by the fractions, fxy, of the pertinent conformations xy for the W-X-CHD-Y-Z structure, where x, y ) T, G+, or G-, and by the infrared absorption intensities per molecular unit, Ixy, for these conformations. For C12-1-d1-E3 we have
IA ) a(fG+G+IG+G+ + fG+TIG+T) IB ) a(fG-G-IG-G- + fTG-ITG- + fG-TIG-T) IC ) a(fTTITT + fTG+ITG+) for C12-2-d1-E3
ID ) a′(fTG-ITG- + fG-G-IG-G-) IE ) a′(fTTITT + fTG+ITG+ + fG-TIG-T) Figure 6. Infrared spectra in the 2050-2250 cm-1 region of aqueous solutions at 26 °C of C12-2-d1-E3 (a-e) and C12-6-d1-E3 (f-j) with compositions of 60 (a and f), 70 (b and g), 80 (c and h), 90 (d and i), and 100 wt % (e and j).
IF ) a′(fG+TIG+T + fG+G+IG+G+) and for C12-4-d1-E3, C12-6-d1-E3, and C12-8-d1-E3
IG ) a′′(fTG-ITG- + fG+TIG+T + fG+G+IG+G+ + fG-G-IG-G-) IH ) a′′(fTTITT + fG-TIG-T + fTG+ITG+) where a, a′, and a′′ are appropriate constants. We have, on the other hand, the following equivalence relations and the possible ranges of the conformation fractions fxy. For the CH2OCHDCH2CH2 part of C12-1-d1-E3 and the OCH2CHDCH2CH2 part of C12-2-d1-E3, we have 0 e fTT e 1, 0 e fTG+ ) fTG- e 0.5, 0 e fG+T ) fG-T e 0.5, and 0 e fG+G+ ) fG-G- e 0.5. For the CH2CH2CHDCH2CH2 part of C12-4d1-E3, C12-6-d1-E3, and C12-8-d1-E3, we assume that fTG+ ) fG+T and fTG- ) fG-T in order to reduce the number of parameters to determine. This assumption is necessary because the observed spectra give only two component bands, G and H. We then have the relations for these species: 0 e fTT e 1, 0 e fTG+ ) fTG- ) fG+T ) fG-T e 0.25, and 0 e fG+G+ ) fG-G- e 0.5. We also have a general relation for any species
fTT + fTG+ + fTG- + fG+T + fG-T + fG+G+ + fG-G- ) 1
Figure 7. Infrared spectra in the 2050-2250 cm-1 region of 80 wt % aqueous solutions of C12-2-d1-E3 (a-e) and C12-6-d1-E3 (f-j) at temperatures of 65 (a and f), 54 (b and g), 45 (c and h), 36 (d and i), and 26 °C (e and j).
associated with the TG-, G+T, G+G+, and G-G- conformations (band G) and the TT, G-T, and TG+ conformations (band H) of the CH2CH2CHDCH2CH2 part. Procedure of Conformational Analysis of Monodeuterated C12E3. Populations of the conformational states of the dodecyl
where fG+G- and fG-G+ have been assumed to be zero, since the G+G- and G-G+ sequences have exceedingly higher energies than others.45 The experimental data available in this work still do not allow to determine all of the conformation fractions uniquely but give only possible ranges of their values. By using the observed area intensities of the C-D stretching bands for IA to IH and the calculated intensities for the model compounds (Table 2) for ITT, ITG+, etc., we have obtained the ranges of the conformation fractions fTT, fTG+, etc. for each of the C-C bonds in the dodecyl chain and for the oxyethylene-adjoining O-C bond. The trans fraction for the X-C bond in the W-X-CHDY-Z structure, fT(X-C), is given by fTT + fTG+ + fTG- and the trans fraction for the C-Y bond, fT(C-Y), is given by fTT + fG+T + fG-T. The analysis of C12-1-d1-E3 gives the trans fractions for the O-C1 bond (fT(O-C1)) and the C1-C2 bond (fT(C1-
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TABLE 3: Observed C-D Stretching Wavenumbers (νobsd) and Conformational Assignments for the Monodeuterated Species of C12E3 compound
banda
νobsdb/cm-1
νobsd/cm-1 for model compd
skeletal conformnc
conformn of the C-D bondd
C12-1-d1-E3
A B C D E F G H G H G H
2169 2132 2115 2172 2157 2144 2152 2136 2154 2136 2151 2138
2163e 2128e 2111e 2166f 2153f 2144f 2153g 2136g 2153g 2136g 2153g 2136g
G+G+, G+T G-G-, TG-, G-T TT, TG+ TG-, G-GTT, TG+, G-T G+T, G+G+ TG-, G+T, G+G+, G-GTT, G-T, TG+ TG-, G+T, G+G+, G-GTT, G-T, TG+ TG-, G+T, G+G+, G-GTT, G-T, TG+
CO/tg/CC CO/gt/CC, CO/gg/CC CO/gg/CC OC/gt/CC OC/gg/CC OC/tg/CC CC/gt/CC, CC/tg/CC CC/gg/CC CC/gt/CC, CC/tg/CC CC/gg/CC CC/gt/CC, CC/tg/CC CC/gg/CC
C12-2-d1-E3 C12-4-d1-E3 C12-6-d1-E3 C12-8-d1-E3
For the band designation, see Figure 5. b Observed wavenumbers for the neat liquid (100 wt %) at 26 °C. c The conformation of the skeletal CH2O-CHD-CH2CH2 part for bands A, B, and C, that of the OCH2-CHD-CH2CH2 part for bands D, E, and F, and that of the CH2CH2CHD-CH2CH2 part for bands G and H. The R-configuration is assumed. d For the conformation designation, see text. e CH3OCHD(CH2)2CH3. f CH3OCH2CHDCH2CH3. g CH3(CH2)2CHDCH2CH3. a
Figure 8. Composition dependence of the trans fractions fT for the respective bonds at different temperatures. Solid and open symbols are used respectively for the LR and L2 phases.
C2)) and the analysis of C12-2-d1-E3 gives the trans fractions for the C1-C2 bond (fT(C1-C2)) and the C2-C3 bond (fT(C2C3)) in the dodecyl chain -O-C1-C2-C3-C4-‚‚‚-C11-C12. The values for fT(C1-C2) obtained from the two analyses are in fact consistent with each other. In subsequent discussions, we use the result from the analysis of C12-2-d1-E3 for fT(C1C2), since this analysis gave smaller ranges of the fractions than the other. In each of the spectral analyses of C12-4-d1-E3, C126-d1-E3, and C12-8-d1-E3, we obtained the same values for the trans fractions for the Ca-Cb bond (fT(Ca-Cb)) and the Cb-Cc bond (fT(Cb-Cc)) in the CH2CaH2CbHDCcH2CH2 part of the molecule, because we have assumed that fTG+ ) fTG- ) fG+T ) fG-T for this part. Accordingly, the values obtained for fT(CaCb) or fT(Cb-Cc) are averages for the two adjoining C-C bonds. When using the observed intensities for evaluating the conformation fractions, we should consider a possibility that the absorption coefficients may be different in different phases. Since its effect, if any, is expected to be substantially the same for any of the C-D stretching vibrations in the same phase, we consider this effect on the relative intensities in the same
phase to be practically canceled. Another effect that might modify the band intensities in a liquid-crystalline phase compared to those in an isotropic liquid phase is possible alignment of molecular chains in the sample materials for infrared measurements. In the present analysis, we have assumed that the samples have no bulk alignment, although there is a possibility of the alignment at surfaces of the KRS-5 plates which are, however, not highly flat. The results, obtained on this assumption, that the conformation fractions are practically continuous at the phase boundaries might support indirectly this assumption, although the resultant conformation fractions should involve the contributions from possible changes of the conformational states. Conformational Behavior of the Alkyl Chain with Changing Composition and Temperature. The results of the trans fractions fT for the O-C1, C1-C2, C2-C3, C3-C4-C5, C5C6-C7, and C7-C8-C9 bonds in the dodecyl chain are displayed in Figures 8 and 9 from different viewpoints; while the former shows the dependence of fT on the composition of the C12E3water system at different temperatures, the latter shows the
8494 J. Phys. Chem., Vol. 100, No. 20, 1996
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Figure 9. Temperature dependence of the trans fractions fT for the respective bonds at different compositions. Solid and open symbols are used respectively for the LR and L2 phases.
dependence of fT on temperature at different compositions. As mentioned before, the conformation fractions obtained for the C3-C4-C5, C5-C6-C7, and C7-C8-C9 bonds are averages of the fractions for the two adjoining bonds. It is important to note that the errors in the fT values shown in Figures 8 and 9 involve systematic uncertainties resulting from the supposed ranges of the fxy values described in the preceding section. The total errors in the derived values of the conformation fractions are in fact due almost entirely to these uncertainties. The errors at different compositions and temperatures are, however, not independent of one another, because the supposed ranges of fxy are common in all analyses. This implies that the most probable value of fT is not necessarily in the middle of the evaluated range and that, if the most probable value for a particular composition and a temperature were determined to be, for example, at the bottom of the range, the corresponding most probable values for other conditions would also be practically at the bottom of the respective ranges. The significance of the apparent errors will be more important when we discuss in a later section the positional dependence of fTT along the chain. (a) Conformational Behavior with Changing Composition. On addition of water to the neat liquid of C12E3, the trans fractions fT for the C1-C2 and C2-C3 bonds decrease in the L2 phase at the first stage, but they increase when the composition approaches the phase boundary with LR. The conformational change of the C7-C8-C9 bonds, which are located closer to the chain terminal, is different from that of the C1-C2 and C2C3 bonds closer to the alkyl/oxyethylene interface. On addition of water to the neat liquid, the trans fractions for the C7-C8C9 bonds increase at first, but they decrease slightly when the water content is increased toward the phase transition to LR. The conformational behavior of the C3-C4-C5 and C5-C6-
C7 bonds in the L2 phase is intermediate between the behavior of the C2-C3 bond and of the C7-C8-C9 bonds, and the trans fractions for these bonds increase progressively with increasing water content. The conformational changes of the C-C bonds in the L2 phase at higher temperatures, where the phase separates directly to W + L2 with increasing water content, are similar to those at lower temperatures. The conformational change at the L2/LR phase transition is generally not significant; only a slight increase of the trans fraction is observed for the C1-C2 and C2-C3 bonds. In the LR phase, the trans fractions for the C-C bonds increase in general when the water content is increased. For the C1-C2 and C7-C8-C9 bonds, however, the trans fraction increases at first with addition of water, but it turns to a decrease with more water added. This trend of the conformational change is more significant at higher temperatures. For the O-C1 bond, the conformational change with the composition in the L2 phase is small and is almost constant at lower temperatures. The conformational behavior of this bond in the LR phase is similar to that of the C1-C2 bond, but the decrease of the trans fraction in the region closer to the phase separation is even more significant than the C1-C2 bond. The conformational change of the O-C1 bond at the L2/LR phase transition is not clearly observed. (b) Conformational Behavior with Changing Temperature. The conformational change at the L2/LR phase transition is practically not observed for any of the C-C and O-C bonds when the temperature is changed. More interesting conformational behavior is found within the respective phases. In the L2 phase, the trans fractions fT for the C-C bonds in the dodecyl chain decrease when the temperature is increased, except for the C1-C2 bond which exhibits a small increase of the trans fraction. It is noted that the decreasing rate of fT for the C2-
Conformation of Monodeuterated Nonionic Surfactants TABLE 4: Observed Enthalpy Differences (HG-HT) between the Gauche and Trans Conformations around the Dodecyl C-C Bonds and the Oxyethylene-Adjoining O-C Bond in C12E3 in the Neat Liquid bond
HG-HT/kcal mol-1
bond
HG-HT/kcal mol-1
O-C1 C1-C2 C2-C3
0.6 ( 0.2 -0.0 ( 0.2 1.8 ( 0.7
C3-C4-C5 C5-C6-C7 C7-C8-C9
0.8 ( 0.4 0.4 ( 0.2 0.7 ( 0.2
C3 bond is much higher than that for the other bonds. The trans fraction for the O-C1 bond decreases only slightly with increasing temperature. From the temperature dependence of the trans fractions fT for the C-C and O-C bonds in the neat liquid (100 wt %), we have evaluated, as shown in Table 4, the enthalpy differences between the gauche and trans conformations around the respective bonds.44 The results for the C3-C4-C5, C5-C6-C7, and C7-C8-C9 bonds indicate that the trans conformation is more stable than the gauche conformation by approximately 0.40.8 kcal mol-1, which is consistent with the enthalpy difference, 0.6 kcal mol-1, for the C-C bonds in liquid n-alkanes.46,47 For the C2-C3 bond, however, the enthalpy difference is larger than that for the C3-C4-C5, C5-C6-C7, and C7-C8-C9 bonds. The similar conformational stabilities of the trans and gauche conformations for the C1-C2 bond are in accord with the corresponding conformational stabilities for the C-C bond adjoining the O-C bond in dialkyl ethers in the liquid state.48 The enthalpy difference for the O-C1 bond is much smaller than the result for the O-C bond in simple dialkyl ethers in the liquid state, for which the trans conformation is more stable than the gauche conformation by about 1.1 kcal mol-1.49 The peculiar conformational stabilities of the bonds close to the alkyl/ oxyethylene interface may be associated with the aggregation of molecules in the neat liquid as suggested previously for CnEm surfactants.15,50 In the LR phase, the trans fractions for the C-C bonds generally decrease with increasing temperature, although the decrease is considerably small for the C3-C4-C5 and C5-C6C7 bonds. It is important to note that the trans fraction for the C1-C2 bond decreases most significantly among the C-C bonds studied when the temperature is raised. This conformational behavior is in striking contrast with the behavior of the same bond in the L2 phase, where the trans fraction increases with increasing temperature. The decrease of the trans fractions with increasing temperature in the LR phase is more prominent for the compositions with more water content. For the O-C1 bond, the decrease of the trans fraction is observed in the LR phase at 50 wt %. Chain Conformation and Phase Behavior. The important phase transition in the C12E3-water system is the one from L2 to LR when the water content is increased at constant temperature below about 50 °C or when the temperature is decreased at constant composition roughly between 70 and 80 wt % (Figure 1). In a composition region of 50-70 wt %, the LR phase changes with increasing temperature to the L2 phase with narrow region, which is actually thought to be the L3 phase,15,51 and eventually to W + L2. Other relevant phase transitions are those from LR to the separated phases of W + LR and L2 + LR with addition of water. The conformational change of the dodecyl chain at the phase transition from L2 to LR with increasing water content is not significant and only a small increase in the trans fraction is observed for the C1-C2 and C2-C3 bonds at lower temperatures (Figure 8). This implies that only the conformational states of the C-C bonds close to the alkyl/oxyethylene interface change to be more trans-populated, so that the alkyl chain may well be
J. Phys. Chem., Vol. 100, No. 20, 1996 8495 fitted to the lamellar structure. The L2/LR phase transition with changing temperature gives no substantial conformational changes (Figure 9). In addition, the conformation profiles, to be discussed later, are almost continuous across the phase boundary. It can be stated therefore that the conformational states of the alkyl chain in the L2 and LR phases in the vicinity of their boundary are substantially not different, although small conformational changes are observed for the alkyl/oxyethylene interface region. The conformational changes of the C-C and O-C bonds in the L2 phase with increasing water content or with increasing temperature depend on the position of the bond in the chain as described before. The process of these characteristic conformational changes should be correlated with the formation of varying aggregate structures in the L2 phase. In the LR phase, some of the bonds in the dodecyl chain exhibit notable conformational behavior with changing composition and temperature. On increasing temperature to approach the L2 phase, the trans fractions for the O-C1 and C1C2 bonds decrease significantly, particularly at lower surfactant concentrations. On the other hand, the decrease of the surfactant concentration toward the phase separation into W + LR or L2 + LR gives a decrease in the trans fraction for these bonds. These observations indicate that the conformational transformation from trans to gauche at the bonds close to the alkyl/ oxyethylene interface makes the lamellar structure less stable and leads eventually to the phase separation or transition. The conformational behavior of the C7-C8-C9 bonds located closer to the chain terminal is also peculiar in that their conformational changes with the composition and temperature are similar, though their magnitudes are smaller, to those observed for the bonds closer to the alkyl/oxyethylene interface. This implies that the conformational change from trans to gauche at the part closer to the chain terminal is also responsible for the destruction of the lamellar structure. Another remarkable observation is that the temperature dependence of the conformation of the C1-C2 bond in the LR phase is different from that in the L2 phase and of the corresponding C-C bond in simple dialkyl ethers. In the LR phase, the trans fraction for this bond decreases with increasing temperature, in contrast with the slight increase in other cases. This conformational behavior in the LR phase is in harmony with the behavior of other C-C bonds in the dodecyl chain and is possibly associated with the cooperative conformational rearrangement of the chain in the lamellar structure. Molecular conformation in other CnEm-water systems has been studied by Raman spectroscopy. Gaufre`s and co-workers26,28 have shown using a difference spectrum technique that there is no significant spectral changes at the transition between the isotropic micellar solution (L1) phase and the hexagonal (H1) phase in the C8E5-water system26 and at the transition between the L1 phase and the cubic (I1) phase in the C12E8-water system.28 They concluded therefore that there is no discontinuity in the conformational state of the molecular chain at these phase transitions. This conclusion is consistent, although they were looking at unresolved overall conformational state of the molecular chain, with our present results for the C12E3-water system that the conformational change of the dodecyl chain at the L2/LR phase transition is not significant. More significant conformational changes are observed within the region of the particular phases. The previous authors26,28 have shown that there are some spectral changes with temperature in the L1 phase and interpreted them to be associated with the conformational change of the hydrophilic oxyethylene moiety.
8496 J. Phys. Chem., Vol. 100, No. 20, 1996
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Figure 10. Positional dependence of the trans fractions fT at different compositions and temperatures. Solid and open symbols are used respectively for the LR and L2 phases. Carbon atoms are indicated by their numbers. The fT values calculated with the rotational-isomeric state model are shown by dots in comparison with the observed results for 100 wt %.
Figure 11. Positional dependence of the consecutive trans fractions fTT at different compositions and temperatures. Solid and open symbols are used respectively for the LR and L2 phases. Carbon atoms are indicated by their numbers.
The conformation of C12E5 in water and in the neat liquid has been studied by 13C NMR spectroscopy. Ahlna¨s et al.50 have shown for the alkyl chain of this surfactant that, when going from the neat liquid to the L1 phase or when lowering temperature in the L1 phase, the amount of the trans conformation increases. Since they studied the conformational state in the L1 phase with surfactant concentrations less than 34 wt %, their results are not directly compared with the present results for the L2 phase of C12E3 with higher concentrations. The temperature dependence of the amount of the trans conformation in the L1 phase of C12E5 is similar in general to that in the L2 phase of C12E3. Conformation Profiles and Order Parameters. Figure 10 shows the plots of the trans fractions fT as a function of chain position for various compositions and temperatures, and Figure 11 shows similar plots of the fractions, fTT, of the consecutive trans conformations around the two adjoining bonds; these plots
will be called the single trans conformation profiles and the consecutive trans conformation profiles, respectively. These results indicate that the fractions fTT have much larger apparent errors than fT. The errors are, however, systematic ones as mentioned before, so that if we make an assumption, for example, that fG+G+ ) fG-G- ) 0, we then have the fractions fTT for any bond at the bottom of the evaluated ranges. This consideration indicates that we can study overall trends of the positional dependence of the conformation fractions more precisely than one would expect simply by looking at the figures. Figures 10 and 11 show that the conformation profiles are not significantly different between the LR and L2 phases and for different compositions and temperatures. The single trans fraction fT is the highest (0.7-0.8) for the C2-C3 bond and is the lowest (0.35-0.5) for the C1-C2 bond among the C-C bonds studied. The observed consecutive trans fractions fTT in
Conformation of Monodeuterated Nonionic Surfactants the LR and L2 phases show their maximum at the C5-C6-C7 bonds which are located just in the middle of the dodecyl chain. These experimental observations may be compared with the conformation probabilities, equivalent to the conformation fractions, calculated with the rotational-isomeric state model.45 The calculations were performed on a dodecyl chain linked to an ether oxygen by using the standard values of the first-order conformational energies (Eσ), in units of kcal mol-1, of 0.9 for the O-C1 bond, -0.2 for the C1-C2 bond, and 0.5 for the C2C3 and other C-C bonds and of the second-order conformational energies (Eω), in the same units, of 0.34 for the C2-C3 bond and 2.2 for other C-C bonds.45 As shown in Figure 10, the calculated values of fT agree in general with the observed values, but appreciable differences are noted for the O-C1 and C2-C3 bonds. These results are apparently correlated to the peculiar conformational stabilities of these bonds as mentioned before. A previous study52 of X-ray measurements of the C12E3water and C12E4-water systems in the LR phase has indicated that these surfactants give half-bilayer thicknesses considerably less than expected from the all-trans dodecyl chain. This observation is consistent with the present finding that the dodecyl chain in this phase contains a considerable amount of the gauche conformation. These experimental results demonstrate that the alkyl chains in the lamellar structure are significantly disordered. Since the disorder of the alkyl chain originates from the conformational transformation from the trans state to the gauche state around the C-C bonds, the population of the consecutive trans conformations is correlated more closely to the order of the chain than the population of the single trans conformation. It may be reasonable then to use the fTT values to represent the conformational order of the alkyl chain. An important finding is that, although the long-range order in the system should be quite different between the LR and L2 phases, the consecutive trans conformation profiles are essentially the same for the two phases and there is no distinct discontinuity of the profiles at the phase transition. This result is consistent with the observation that the conformational changes with the composition and temperature are almost continuous at the phase transition as discussed before. Another remark is that the consecutive trans conformation profiles show the highest conformational order in the middle of the alkyl chain in the LR and L2 phases. This will be discussed later in comparison with the order parameters derived from NMR spectra. Figure 11 shows that the conformational order in the region of the LR phase closer to the phase separation into W + LR or L2 + LR decreases more rapidly toward the chain terminal. This is the same observation as made in the composition dependence of fT (Figure 8), indicating that the conformational change from trans to gauche near the chain terminal is responsible for the instabilization of the lamellar structure. The conformational order determined by infrared spectroscopy may be compared with the order parameters from NMR spectroscopy.53 Ward et al.54 and Schnepp and Schmidt55 have obtained the order parameters from the quadrupolar splittings in the 2H NMR spectra for the LR phase of the C12E4-water system and for the same phase of the C12E6C1-water system, respectively, where C12E6C1 represents CH3(CH2)11(OCH2CH2)6OCH3. The results of these studies indicate that the maximum ordering of the alkyl chain occurs at the third to the fifth carbon (C3 to C5) positions from the alkyl/oxyethylene interface for C12E4 and at the C2 to C4 positions for C12E6C1 and that the order decreases progressively toward the chain terminal. Ahlna¨s et al.,50 on the other hand, have studied 13C NMR relaxation of C12E4 in the neat liquid and of C12E5 in the neat
J. Phys. Chem., Vol. 100, No. 20, 1996 8497 liquid and in the L1 phase, and determined the order parameters in these phases on the basis of the two-step model.56 They obtained the results that the order is maximum at the C3 to C6 positions for either of the neat liquid and the L1 phase. The very low order was observed at the end of the alkyl chain. The 13C NMR spectrum of the L phase of C E showed that the R 12 5 lines due to the terminal methyl carbon and its neighbor carbon are very narrow.50 This observation is in agreement with the conformational fluctuation at the terminal part of the alkyl chain as suggested by the previous and present work. The present results of the consecutive trans conformation profiles in the LR and L2 phases are consistent with the order parameter profiles obtained from the NMR spectra. The important observation in the conformational behavior and the order parameters for the CnEm-water systems is that the chain ordering is the highest in the middle of the alkyl chain, but not at the hydrophobic/hydrophilic interface as observed for the LR phase of ionic surfactant systems,57 in which the electrostatic interactions should play an important role of ordering the interface region. For the nonionic surfactant systems, the observed behavior of the chain conformation and the ordering is consistent with the flexible structure at the alkyl/oxyethylene interface. It should be noticed here that the order parameters derived from NMR spectra are measures of the average bond orientation in all motions faster than about a microsecond, while vibrational spectroscopy (infrared and Raman) observes the trans-gauche conformational changes on a time scale of 10-14-10-13 s.58 A combination of the results of the order parameters and the conformational behavior should clarify overall dynamical properties of the molecular chains in the relevant phases. A study of 2H NMR spectroscopy on the present system is in fact now in progress. Conclusions Conformational analysis of the alkyl chain in the C12E3water system has been performed by C-D stretching infrared spectroscopy employing five selectively monodeuterated species of C12E3. The conformational change at the phase transition from L2 to LR is not significant and only a small increase in the trans fraction is observed for the C-C bonds close to the alkyl/ oxyethylene interface. This implies that the conformational states in the L2 and LR phases in the vicinity of their boundary are substantially not different. In the LR phase, when the composition or the temperature approaches the region of the phase separation or transition, the trans fractions for the C-C bonds closer to the alkyl/oxyethylene interface and those closer to the chain terminal decrease significantly. These observations indicate that the conformational transformation from trans to gauche at these chain positions makes the lamellar structure less stable and leads eventually to the structural destruction. The fraction of the consecutive trans conformations may be used as a measure of the conformational order at particular positions of the chain. The present results show that the ordering is the highest in the middle of the chain in the LR and L2 phases. This vibrational spectroscopic observation, together with the NMR observations, indicates that the alkyl/oxyethylene interface is flexible with respect to the conformation and the orientation of the chain. The flexibility at the interface is probably one of the important factors to characterize the aggregated supramolecular structures of the CnEm-water systems. References and Notes (1) Singer, S. J.; Nicolson, G. L. Science 1972, 175, 720-731.
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