Interdigitation of an Intercalated Surfactant Bilayer - American

J. Phys. Chem. B 2001, 105, 7639-7650

7639

Interdigitation of an Intercalated Surfactant Bilayer N. V. Venkataraman and S. Vasudevan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore - 560012 India ReceiVed: February 14, 2001; In Final Form: May 14, 2001

The cationic surfactant cetyltrimethylammonium (CTA) has been ion-exchange intercalated from an aqueous solution of the surfactant by a two-step process into the galleries of layered cadmium thiophosphate to give Cd0.83PS3(CTA)0.34. Two phases of the intercalated product are observed. One is a phase characterized by a lattice expansion of 26.5 Å, in which the methylene chains of the CTA ion adopt a tilted bilayer arrangement. This phase, when left in the reaction media, transforms to a phase characterized by a lattice expansion of 12.5 Å with no change in chemical stoichiometry. Using X-ray diffraction and orientation-dependent infrared spectroscopy, it has been possible to establish that the observed collapse of the interlayer spacing is a consequence of the interdigitation of the methylene “tails” of the intercalated CTA ion. Infrared, Raman, and 13 C NMR spectroscopies have been used to establish the conformation of the methylene chains in the intercalated normal bilayer and interdigitated bilayer phases of Cd0.83PS3(CTA)0.34 and the dispersion of the delocalized methylene wagging (ν3) and rocking-twisting (ν8 ) modes as a probe of the planarity of the chains. The results indicate that a majority of the methylene chains in the intercalated interdigitated bilayer adopt an all-trans planar conformation, whereas, in the normal bilayer phase of Cd0.83PS3(CTA)0.34, although a majority of the methylene units are in a trans configuration, the presence of a few gauche defects is sufficient to destroy the planarity of the methylene chain. Planarity of the methylene chains is the key factor for the absence or occurrence of interdigitation of the intercalated surfactant bilayer.

Introduction Changes in bilayer membrane morphology consistent with acyl chain interdigitation have been widely reported.1 Interdigitation demands a drastic alteration of lipid packing, in which the acyl chains of each monolayer extend across the bilayer mid-plane, effectively interdigitating into the opposing monolayer. As a result, the interdigitated membrane is much more closely packed than the normal bilayer, and consequently, membrane bilayer properties are significantly altered.2 Interdigitation of liposomal biomembranes can be induced by the presence of small organic molecules such as short-chain alcohols,3 certain peptides,4 and also anesthetics such as chlorpromazine5 and tetracaine.6 Although the biological significance of such structures have not been clearly established, it is believed that the increased permeability to hydrophobic solutes at the boundary between the normal bilayer and interdigitated regions has a regulatory role in biological systems.1,2,4 Intercalation of surfactants in a variety of layered solids has been reported,7-10 and the application of these systems as crystalline models of lipid membranes has been suggested in the literature.7,8 Recently, we reported that the cationic surfactant cetyl trimethylammonium bromide [CH3(CH2)15N(CH3)3Br] (CTAB) can be intercalated into layered cadmium thiophosphate, forming an intercalated bilayer.11 The host lattice cadmium thiophosphate is formed by the stacking of CdPS3 sheets, built from edge-sharing CdS6 and P2S6 polyhedra, with a van der Waals gap of 3.2 Å.12 Intercalation of the surfactant cation cetyl trimethylammonium (CTA) was effected by a two-step ionexchange process. In the first step, hydrated potassium ions are ion-exchange intercalated within the galleries of CdPS3, with * Corresponding author. E-mail: [email protected].

an equivalent loss of cadmium ions from the layer and a lattice expansion of 2.8 Å13,14 EDTA

CdPS3 + 2xK+(aq) 98 Cd1-xPS3K2x(H2O)y + xCd2+(aq) (x ) 0.17) In the second step, the intercalated hydrated potassium ions are quantitatively exchanged for the surfactant cation, CTA

Cd1-xPS3K2x(H2O)y + 2xCTA+(aq) f Cd(1-x)PS3(CTA)2x + 2xK+(aq)

(x ) 0.17)

The formation of Cd0.83PS3(CTA)0.34 occurs with a lattice dilation of 26.5 Å, compared to that of pristine CdPS3, with the CTA ions within the gallery forming a bilayer with the methylene chains tilted at an angle of 55° with respect to the interlamellar normal.11 The second step of the above reaction, in which CTA ions exchange with the interlamellar hydrated potassium ions, occurs fairly rapidly. We observed, however, that when the intercalated compound, Cd0.83PS3(CTA)0.34, is left in contact with the aqueous CTAB solution for longer periods of time, typically 48 h, a phase with a lattice expansion of 12.5 Å is obtained. Surprisingly, no change in the stoichiometry accompanies this transformation. Here we show that the structural change responsible for the collapse of the interlayer spacing is an interdigitation of the alkyl chains of the intercalated surfactant. This compound, Cd0.83PS3(CTA)0.34, therefore, presents an ideal system to probe the conformational changes that occur upon interdigitation of the alkyl chains in a bilayer. The host lattice, cadmium thiophosphate, has the advantage that it provides single crystals with the bilayer confined between rigid,

10.1021/jp0105802 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/18/2001

7640 J. Phys. Chem. B, Vol. 105, No. 32, 2001

Figure 1. X-ray diffraction pattern showing the 00l reflections of the potassium ion intercalate Cd0.83PS3K0.34(H2O) (dotted line) and the CTA ion intercalate Cd0.83PS3(CTA)0.34 at different stages of the reaction (solid line).

atomically smooth, inorganic surfaces. We have probed the conformation of the alkyl chains in the normal and interdigitated intercalated bilayer using orientation-dependent infrared vibrational spectroscopy, FT-Raman, and 13C NMR spectroscopy. Experimental Section Cadmium thiophosphate, CdPS3, was prepared from the elements following the procedure reported in the literature.15 Cadmium metal powder, phosphorus, and sulfur in stoichiometric amounts were sealed in quartz ampules at 10-6 Torr and heated at 650 °C for 2 weeks. Single crystals of CdPS3 were grown by chemical vapor transport using excess sulfur as a transporting agent. The charged end of the ampule was maintained at 650 °C and the cooler end at 600 °C. Plateletlike transparent crystals were obtained with a typical size 5 × 5 × 0.1 mm3. Cetyl trimethylammonium bromide (CTAB)(Loba Chem) was recrystallized from acetone. CdPS3(powder and crystals) was treated with a 4 M aqueous solution of potassium chloride in the presence of 0.1 M EDTA and 1 M K2CO3/KHCO3 as buffer to give Cd0.83PS3K0.34(H2O). The compound has a c-axis lattice spacing of 9.4 Å, corresponding to a lattice expansion of 2.8 Å compared to that of the pristine host, CdPS3. The interlamellar potassium ions in Cd0.83PS3K0.34(H2O) were further ion-exchanged with cetyl trimethylammonium (CTA) ions by refluxing in an aqueous solution of 0.05 M CTAB at 50 °C for 6 h to give Cd0.83PS3(CTA)0.34.11 Completion of ion-exchange was ascertained from the disappearance of the 00l reflection of Cd0.83PS3K0.34(H2O) in the X-ray diffraction pattern (Figure 1) and the appearance of a new series of 00l reflections with a lattice spacing of 33 Å, corresponding to the formulation Cd0.83PS3(CTA)0.34. This compound can be isolated and is stable. However, when allowed to remain in contact with the aqueous CTAB solution, a new set of 00l reflections corresponding to a interlayer lattice spacing of 19 Å begins to appear. X-ray diffraction recorded after 48 h

Venkataraman and Vasudevan showed a single phase with a lattice spacing of 19 Å. It was found that the stoichiometry of this phase, too, corresponds to Cd0.83PS3(CTA)0.34. Cadmium ion stoichiometry was established by atomic absorption spectroscopy (Perkin-Elmer 4381) and that of the intercalated CTA ion by CHN analysis. (Intercalate I 33 Å phase: Cd, 29.5%; C, 23.95%; N, 1.53%; H, 4.51%. Intercalate II 19 Å phase: Cd, 28.56%; C, 24.5%; N, 1.32%; H, 3.81%.) Phosphorus and sulfur stoichiometries were not estimated and assumed to be same as that of CdPS3. Powder X-ray diffractions were recorded on a Shimadzu XDD1 diffractometer using Cu KR radiation. Crystals were mounted flat on a sapphire disk, which occupied the same position as that of the regular sample holder on the X-ray goniometer. FTRaman spectra were recorded on Bruker IFS FT-Raman spectrometer, using a Nd:YAG (wavelength 1.064µm) laser as exciting radiation. All spectra were recorded at 4 cm-1 resolution with an unpolarized beam. Spectra of the powders were recorded using an aluminum sample holder. Raman spectra of crystals of Cd0.83PS3(CTA)0.34 were recorded for a 180° geometry; crystals were held between two thin glass slides, with a gold mirror placed at the back. The laser power was kept at 100 mW, and typically, ∼400 spectra were co-added to improve the signal-to-noise ratio. 13C CP-MAS NMR was carried out on a Bruker DSX-300 solid-state spectrometer at a Larmor frequency of 75.46 MHz with a contact time of 1 ms. The spectra were externally referenced to tetramethylsilane(TMS). Infrared spectra of crystals of Cd0.83PS3(CTA)0.34 were recorded in the spectral range 400-4000 cm-1 on a Bruker IFS55 spectrometer equipped with a polarizer accessory. The crystals were mounted on a hollow copper block and cooled using a CTI-Cryogenics closed-cycle cryostat. The sample temperature could be varied from 300 to 40 K. The cryostat was evacuated at 10-2Torr to prevent condensation on the crystals. The spectra for different orientations of the electric field vector, E, of the incident IR with respect to the C* axis of the crystals (the axis normal to the layers) were obtained using the arrangement described in ref 16. In this arrangement, the crystals are held in the sample block of the cryostat in such a way that the C* axis of the platelet-like crystal is at an angle of 45° with respect the propagation vector of the incident IR beam. From a measurement of the IR spectrum for two different angles of polarization of the electric field vector, E, the spectra for E ⊥ C* and E | C* could be recovered, and subsequently, the spectrum for any orientation, φ, of E with respect to C* could be calculated.16 Results X-ray Diffraction. The X-ray diffraction patterns of crystals, the surfactant intercalated Cd0.83PS3 (CTA)0.34, and the starting compounds Cd0.83PS3K0.34(H2O) are shown in Figure 1. The X-ray diffractogram recorded at different stages of the reaction is shown in Figure 1. The interlayer spacing of the intercalate Cd0.83PS3(CTA)0.34, as calculated from the 00l reflections in the X-ray diffractogram recorded after 1 hour, is 33 Å. This corresponds to a lattice expansion of 26.5 Å compared to that of CdPS3. If removed from the aqueous reaction media at this stage, this phase, with a lattice spacing of 33 Å, is stable for extended periods. This phase will henceforth be referred to in this section as the 33 Å phase. The intercalated methylene chains in this phase had been shown to adopt a tilted bilayer structure; the tilt angle as calculated form the observed lattice expansion and the length of the all-trans CTA ion, ∼22 Å,17 is 55° with respect to the interlamellar normal.11 As mentioned in the Experimental section, when the CTA intercalated compound was

Intercalated Surfactant Bilayer

J. Phys. Chem. B, Vol. 105, No. 32, 2001 7641

Figure 2. Possible structures of the alkyl chains of the intercalated surfactant in the 19 Å phase of Cd0.83PS3(CTA)0.34: (a) lateral tri-molecular layered arrangement and (b) tilted interdigitated bilayer arrangement.

allowed to remain in contact with CTAB solution for longer periods of time, typically 48 h, a new series of 00l reflections is observed in the X-ray diffraction. It may be seen from Figure 1 that by 48 h the transformation is complete, giving a single phase with a c-axis lattice spacing of 19 Å. This phase will henceforth be referred to in this section as the 19 Å phase. Chemical analysis shows that the chemical compositions and stoichiometries of the two phases of the CTA intercalated compound are identical. This suggests that the origin of the differing gallery heights of the two phases is a consequence of differences in the organization of the alkyl chains of the intercalated surfactant in the two phases. The lattice spacing of 19 Å corresponds to a lattice expansion of 12.5 Å compared to that of pristine host, CdPS3. Two possible arrangements of the intercalated surfactant cation that can account for the observed expansion of 12.5 Å for the 19 Å phase are shown in Figure 2. In the first panel (Figure 2a), the alkyl chains adopt a trimolecular, laterally layered arrangement in which large sections of the molecular axis of the chains are perpendicular to the interlamellar normal (parallel to the layers). The cross-sectional diameter of the alkyl chain is ∼4 Å,18 while that of the N+(CH3)3 “head” group is ∼4.8 Å.18 The arrangement shown in Figure 2a can therefore account for a lattice expansion of either ∼12 or ∼12.8 Å. The cross-sectional diameter of the methylene chains discounts the possibility of a lateral bilayer arrangement. The second possibility is a tilted interdigitated bilayer (Figure 2b), in which the alkyl chains of the opposing layers interpenetrate each other such that terminal methyl groups of one layer are close to the “head” group of the opposing layer. In this arrangement, the molecular axis is tilted with respect to the interlamellar normal. Assuming that the intercalated CTA ions are fully extended (length ≈ 22 Å17 ), the tilt angle as calculated from the lattice expansion is 55°. This incidentally is identical to the tilt angle of the normal bilayer arrangement observed in the 33 Å phase.11 Thus, a determination of the tilt angle can, in principle, be used to distinguish between the two possible arrangement of Figure 2. Vibrational Spectroscopy Vibrational spectroscopy has been extensively used for probing conformation in alkyl chain assemblies.19 Infrared and Raman spectroscopic studies of n-alkanes,20 n-alcohol,s21 and several other alkyl chain systems have led to detailed correlation of the spectra with structural features such as chain conformation, chain packing, and even specific conformational sequences. The position, line shape, and splitting of the methylene stretching and bending modes have been used to determine the conformation of methylene units in various phases of n-alkanes. The room-temperature IR and FT-Raman spectra of the 33 and 19 Å phases of Cd0.83PS3(CTA)0.34 in the 3200-500 cm-1 region are shown in panels a and b of Figure 3, respectively.

Figure 3. FT-IR and FT-Raman spectra of (a) the 33 Å phase and (b) the 19 Å phase of Cd0.83PS3(CTA)0.34.

TABLE 1: Infrared Frequencies and Assignments of the 33 and 19 Å Phases of the Intercalate Cd0.83PS3(CTA)0.34 Cd0.83PS3(CTA)0.34 33 Å phase (cm-1)

Cd0.83PS3(CTA)0.34 19 Å phase (cm-1)

3010 (m)

3017 (m)

2952 (sh) 2919 (s) 2850 (s) 1623 (s) 1485 (sh) 1467 (s) 1376 (s) 1350-1150 1000-700 967 (s) 911 (s) 721 (s)

2949 (sh) 2920 (s) 2847 (s) 1630 (s) 1483 (sh) 1464 (s) 1376 (s) 1350-1150 1000-700 964 (s) 906 (s) 721 (s)

assignment N+-(CH

3)3 C-H aysm stretch CH3 C-H asym stretch CH2 C-H antisym stretch CH2 C-H sym stretch H2O bending N+-CH3 C-H sym bend CH2 scissoring CH3 umbrella -(CH2)n- wagging modes -(CH2)n- rocking modes

C-N+ stretching CH2 rocking

The spectral features and their positions are similar for the two phases. The assignment of the vibrational bands of the 33 Å phase had been reported earlier,11 and a similar assignment would hold well for the 19 Å phase (Table 1). (These assignments are based on the assignments for crystalline CTAB.22) The conformationally sensitive vibrational modes of the two phases are discussed in greater detail in the subsequent sections. It may be pointed out that in neither the 33 or 19 Å phases is the methylene scissoring mode (∼1467 cm-1) or the rocking mode (720 cm-1) split into two components as in the


Venkataraman and Vasudevan

Figure 4. FT-IR spectra, in the C-H stretching region, recorded for two orientations of the electric field vector (E) of the incident IR beam with respect to the interlamellar normal (C*), E ⊥ C* (solid line) and E | C* (dotted line) for (a) the 33 Å phase and (c) the 19 Å phase of the intercalate Cd0.83PS3(CTA)0.34. The plot of the corresponding dichroic ratios (Iφ/Iφ+90) as a function of φ for the symmetric (dotted line) and the antisymmetric (solid line) modes for (b) the 33 Å and (d) the 19 Å phases. (φ is the angle the between E and C*).

case of crystalline CTAB.22 These modes are known to be sensitive to the packing arrangement in alkyl chain assemblies.23 In an orthorhombic arrangement, these modes are split due to lateral interchain interactions between contiguous CH2 groups of adjoining chains. In monoclinic and triclinic packing where there is only one chain per unit cell as well as in alkane chain assemblies of low density where lateral interchain interactions are weak, splitting of these modes is absent. The latter is the more likely a reason for the absence of splitting of the methylene scissoring and rocking modes in the 33 and 19 Å phases of Cd0.83PS3(CTA)0.34. In addition to the vibrational bands of the intercalated surfactant, bands due to water are observed in the spectra of both phases. A broad band at 3200 cm-1 (not shown in the spectra) and a sharper band at 1620 cm-1, due to the stretching and bending modes, respectively, of interlamellar water, suggest that the cationic “head” of the intercalated CTA ion may be partially hydrated. The relative intensity of the water bands with respect to the methylene stretching modes is almost identical in the two phases, indicating that the extent of hydration of the “head” group is similar in the 33 and 19 Å phases. Different batches of both the 33 and 19 Å phases showed minor variations in the relative intensities of the water vibrational bands with respect to those of the methylene stretching modes; there was, however, no change in the corresponding lattice spacing. C-H Stretch and the Tilt Angle. The C-H stretching region of the IR spectrum of alkyl chain assemblies is sensitive to the conformation of the methylene chains. For an all-trans

alkyl chain, as in the case of crystalline n-alkanes, the symmetric and antisymmetric stretching modes of the methylene [-(CH2)n] groups appear in the ranges of 2846-2849 and 2916-2918 cm-1, respectively.24 With increasing numbers of gauche conformers, as in the high-temperature disordered liquid phases of n-alkanes, the positions of these peaks shift to higher wavenumbers, typically, 2856-2858 and 2924-2928 cm-1 for the symmetric and antisymmetric stretching modes.25 Increased number of gauche conformers in the chain is also reflected in an increased line width for these bands. The IR spectra of the methylene stretching region in crystals of the 33 and 19 Å phases recorded at room temperature for the two orientations E ⊥ C* and E | C* are shown in panels a and c of Figure 4, respectively. The spectra of both the phases show two intense bands at identical values, 2850 and 2919 cm-1, corresponding to the symmetric and antisymmetric stretching modes of the methylene -(CH2)n- groups. These frequencies are slightly higher than those for the corresponding modes in the all-trans crystalline n-alkanes but are still considerably lower than the observed values in the disordered liquid phase of the n-alkanes. This suggests that a majority of the methylene units of the intercalated surfactant, in both the 33 and 19 Å phases, adopt the trans configuration; gauche conformers are present, but their population is small. We use the polarized IR measurements of the CH2 stretching region in the IR spectrum to determine the tilt angle. The tilt angle, θ, is the angle the molecular axis of an all-trans methylene

Intercalated Surfactant Bilayer chain makes with the interlamellar normal. The spectra of the methylene stretching region for the E ⊥ C* and E | C* orientations of the 33 and 19 Å phases are shown in Figure 4a,c. From the spectra for these two orientations, the spectrum for different angles, φ, of the electric field vector of the incident IR radiation, E, with respect to the interlamellar normal C*, may be calculated and the dichroic ratios Iφ/Iφ+90 for different vibrational modes computed;16 I(φ) is the intensity of the vibrational mode for an angle φ. The ratio Iφ/Iφ+90 for the methylene symmetric stretching mode (2850 cm-1) of the 33 and 19 Å phases have been plotted as a function of the angle φ in Figure 4b,d. The dichroic ratio for the symmetric C-H stretching mode peaks at 39° and 36° for the 33 and 19 Å phases, respectively (Figure 4b). The dichroic ratio of the methylene antisymmetric stretching mode also peaks at this value of φ for both the phases. The dichroic ratio for a vibrational mode will peak at a value of φ for which the electric field vector of the incident IR beam is parallel to the transition dipole moment of that mode. The IR frequencies of the methylene stretching modes indicate that the intercalated methylene chain in both the phases adopt an essentially all-trans conformation. For such a chain, the transition dipole moment of the methylene symmetric stretching modes (µs) are orthogonal to the molecular axis but lie in the molecular plane of the trans methylene chain, whereas those of the antisymmetric stretching modes (µas) are orthogonal to the molecular axis and also to the molecular plane. The fact that the dichroics for these two modes for both phases peak at approximately the same values of φ, the angle between the electric field vector E and the interlamellar normal C*, implies that the molecular plane of the all-trans methylene chain of the intercalated surfactant has no fixed orientation with respect to the C* axis.11 The transition dipole moment of the symmetric and antisymmetric stretching modes of the methylene groups lie in a plane perpendicular to the molecular axis, and the angle at which the dichroic ratio peaks is, therefore, the angle this plane makes with the interlamellar normal. The angle, φ, at which the dichroic ratio of the methylene stretching modes peak would be equal to (90° - θ), where θ is the tilt angle. From the plot of the dichroic ratio of the symmetric stretching mode as a function of φ (Figure 4b), the tilt angle of the intercalated methylene chains in the 33 and 19 Å phases are 51° and 54°, respectively, while the values determined from the dichroic ratio of the antisymmetric stretching mode are 50° and 55°. The tilt angle of the 19 Å phase, ∼55°, indicates that the alkyl chains of the intercalated surfactant adopts an interdigitated bilayer arrangement, as shown in Figure 2b, rather than the lateral trimolecular layered arrangement (Figure 2a). The tilt angle for the latter arrangement would be 90°. It is interesting to note that the tilt angle for interdigitated bilayer calculated from the observed X-ray lattice expansion and the length of a fully extended all-trans CTA ion ∼22 Å17 is 55°. This is not unexpected since methylene stretching mode frequencies indicate that a majority of the CH2 units are in a trans configuration and hence the methylene chain fully stretched in the interdigitated bilayer of the 19 Å phase. In the following section, we investigate the conformational differences in the methylene tail of the intercalated CTA ion in the normal bilayer (33 Å) and the interdigitated bilayer (19 Å) phases of Cd0.83PS3(CTA)0.34. Raman Spectrum. C-H Stretching Region. The Raman spectrum of alkyl chain assemblies is usually dominated by two prominent bands, centered at 2880 and 2850 cm-1, which are assigned to the methylene antisymmetric and symmetric stretch-


Figure 5. FT-Raman Spectrum in the (a) C-H stretching region and (b) C-C skeletal stretching region of the normal bilayer (dotted line) and interdigitated bialyer (solid line) of the intercalate Cd0.83PS3(CTA)0.34.

ing modes. The ratio of the intensities of the band at ∼2880 cm-1 to that at ∼2850 cm-1 (I2880/I2850) is known to be sensitive to both conformational disorder of the alkyl chains and their packing26 and has been widely used to characterize alkyl chain assemblies. The reported values of (I2880/I2850) vary from ∼2 as in crystalline n-alkanes to ∼0.7 in the corresponding liquidphase.27 It has been shown that the intensity of the methylene symmetric stretching mode, I2850, is sensitive to lateral interactions and hence to the packing arrangement.26 Difference in lateral packing is also reflected as changes in the line width of the 2850 cm-1 band. The intensity I2880, however, is unaffected by such interactions but is sensitive to the conformation of the methylene chain, decreasing in the liquid phase compared to in the solid phase. The intensity ratio I2880/I2850 is thus sensitive to both conformational order and lateral interactions. The FT-Raman spectrum of the normal bilayer and interdigitated bilayer phases of Cd0.83PS3(CTA)0.34 in the C-H stretching region is shown in Figure 5a. Peaks due to the antisymmetric and symmetric methylene stretching modes are observed at 2882 and 2850 cm-1. The ratio I2880/I2850 for the bilayer phase is 1.38, while that for interdigitated bilayer is 1.41. The observed difference in the intensity ratios suggests that the alkyl chains in the bilayer phase are more disordered than those in the interdigitated bilayer phase. Since I2850 is sensitive to the packing of the alkyl chains, the observed differences in the intensity ratio could also arise from the differences in the packing arrangement in the two phases. To distinguish the relative importance of these two contributions to the intensity ratio, I2880/I2850, it is necessary to examine a region of the spectra sensitive exclusively to conformation of the alkyl chains. C-C Skeletal Stretch. The C-C skeletal stretching region in the Raman spectra from 1050 to 1150 cm-1 has been shown to be sensitive to the relative amounts of gauche and trans conformers present in an alkyl chain. An all-trans alkyl chain normally shows two strong bands at 1130 and 1065 cm-1. The

7644 J. Phys. Chem. B, Vol. 105, No. 32, 2001 presence of a central band at 1080 cm-1 is characteristic of randomization (interruption of all-trans conformation) of the chain due to the presence of gauche conformers.27 The peak intensities of these bands have been reported to give quantitative information about the relative amount of gauche to trans conformers in the chain. The ratio of the peak heights of the band at 1130 cm-1 (I1130) to the band at approximately 1080 cm-1 (I1080) has been considered as a measure of the amount of gauche conformer present in the methylne chain.27 This ratio has, for example, been extensively used for studying the orderdisorder transitions in phospholipids.19 The Raman spectrum in the skeletal stretching region of the intercalates are shown in Figure 5b. For the intercalate bilayer phase, the intensity ratio I1135/I1088 is 1.25, while for the interdigitated bilayer phase, the value is 3.08, thus indicating an increased presence of gauche conformers in the bilayer phase compared to in the interdigitated bilayer. To identify the population of the specific conformational sequences containing gauche bonds, we have examined the methylene wagging region in the IR in greater detail. Localized Methylene Wagging Modes (IR). Methylene wagging modes in the IR spectrum of n-alkanes in the region 1300-1400 cm-1 are known to exhibit peaks with characteristic frequencies for different conformational sequences.28 These bands are specific to localized structures that contain a gauche bond. For example, a peak at 1341 cm-1 indicates a penultimate bond oriented such that the terminal methyl group is in gauche conformation relative to the methylene group, three carbon atoms away (end gauche-eg). A peak at 1354 cm-1 is due to adjacent gauche bonds (gg), and a peak at 1368 cm-1arises from a gauche-trans-gauche′ (g-t-g′) sequence or a kink. Since these are localized modes, the area under the peaks is proportional to the probability of occurrence of the specific bond sequence. The methyl umbrella deformation band, which appears at 1377 cm-1, is taken as an internal standard and the spectra in this region normalized with respect to this band. The relative population of specific conformational sequences containing a gauche configuration may then be obtained from the normalized intensities.29 The spectrum of the intercalated bilayer phases at 300 K, normalized with respect to 1377 cm-1 band, is shown in Figure 6a (filled circles). The spectrum shows bands at 1341, 1353, and 1368 cm-1, assignable to the eg, gg, and g-t-g′ + kink sequences, respectively, and is similar to that reported earlier.11 The spectrum can be fitted by the weighted sum of the individual sequences assuming a Lorenztian line-shape for each component.29 Such a fit (solid line), along with the individual components (dotted lines), is also shown in Figure 6a. The spectrum for the interdigitated bilayer phase recorded at 300 K is shown in Figure 6b. It may be seen that except for the peak appearing at 1341 cm-1, due to end gauche defects, features due to double gauche (gg) and g-t-g′ sequences are completely absent. The relative intensity of the end-gauche 1341 cm-1 band with respect to the 1377 cm-1 band, too, is less for the intercalated interdigitated bilayer compared to that for the normal bilayer (Figure 6a). This clearly indicates that at room temperature, the population of gauche conformers is higher in the normal bilayer phase than that of the interdigitated bilayer phase of Cd0.83PS3(CTA)0.34. It should be noted that single gauche conformers in the interior of the hydrocarbon chain cannot be specifically monitored in the FT-IR spectroscopic analysis. Specific conformational sequences can be identified, but the FT-IR analysis does not provide information on the location of such sequences within


Figure 6. FT-IR spectrum in the methylene localized wagging region of (a) normal bilayer phase and (b) the interdigitated bilayer phase of the intercalate Cd0.83PS3(CTA)0.34 at 300 K. Closed circles are the experimental data. The solid line is a fit to the experimental data, and the dotted lines are the individual Lorentian components.

the hydrocarbon chain. To do so, we have recorded the 13C NMR spectra of the intercalated normal bilayer and interdigitated bilayer phases of Cd0.83PS3(CTA)0.34. NMR Spectroscopy. NMR spectroscopy has been widely used in the study of surfactant systems such as micelles, liquid crystals, and microemulsions.30,31 13C NMR has also been used to probe the conformation and dynamics of surfactant molecules at interfaces.32,33 13C NMR Chemical shift differences have been used to characterize chain conformation as well as conformational heterogeneity.34 It has been shown that the degree of shielding of a carbon atom in a methylene chain depends on the relative population of trans and gauche conformers, with the trans conformer giving rise to a downfield shift.33 The 13C CP-MAS NMR spectra of the intercalated normal bilayer and interdigitated bilayer phases of Cd0.83PS3(CTA)0.34 are shown in Figure 7, along with the spectrum of crystalline CTAB. The numbering of the carbons and their assignments are also shown. The assignments are based on the reported values for CTAB.35 The spectrum of crystalline CTAB shows eight carbon resonances, which, in order of increasing downfield shift, can be assigned to C1, CN, C14, C4-C13, C2, C3, C15, and C16 carbon atoms, respectively. A comparison of the 13C NMR spectra of the intercalated compounds and pure CTAB shows that the resonance positions of most of the C atoms are not significantly shifted except for that of the C1 atom. In the intercalated compounds, C3 and C15 are not resolved as separate resonances; C14, too, is poorly resolved. The intercalated normal bilayer phase shows an additional resonance at 34 ppm. The C1 atom resonance of the intercalated compounds is considerably downfield-shifted compared to that of crystalline CTAB. This indicates an enhanced trans conformation for the C1 atom in both the normal bilayer and interdigitated bilayer phases of Cd0.83PS3(CTA)0.34. A major difference in the CPMAS 13C spectrum of pure CTAB and Cd0.83PS3(CTA)0.34 is



Figure 7. 13C CP-MAS NMR spectra of (a) crystalline CTAB and the (b) interdigitated bilayer phase and (c) normal bilayer phase of the intercalate Cd0.83PS3(CTA)0.34. The carbon atoms are numbered sequentially starting from the methylene carbon attached to the ′head′ N+-(CH3)3 group.

that the CN and C1 resonances are considerably narrower and more intense in the intercalated compounds (the CP-MAS spectra were recorded under identical conditions). This suggests decreased mobility of the R-carbon(C1) and N+(CH3)3 “head” group for the intercalated CTA compared to that for solid CTAB, probably as a consequence of coloumbic interactions with the negatively charged Cd0.83PS3 layers. The appearance of two peaks for the C4-C13 resonance in the intercalated normal bilayer and its absence both in crystalline CTAB and in the interdigitated bilayer phase of Cd0.83PS3(CTA)0.34 indicate greater conformational heterogeneity in the former. The downfield resonance (35.6 ppm, Figure 7) is assigned to methylene units in a trans configuration; its position is identical to that in crystalline CTAB and in the intercalated interdigitated bilayer. The upfield resonance (34 ppm, Figure 7) is assigned to methylene carbons in a gauche configuration. This assignment is similar to that of a recent report on the 13C NMR of surfactant molecules intercalated in clays, where an additional upfield resonance was observed. This had been assigned to a disordered conformation.34 The results of 13C NMR spectroscopy are in agreement with IR and Raman analysis which also showed that alkyl chains in the intercalated normal bilayer were more disorderedshigher number of gauche configurationscompared to those in the intercalated interdigitated bilayer. The 13C NMR also indicates that gauche configurations have no preferred location within the chain and can occur anywhere between C4 and C13 carbon atoms in the hexadecyl methylene tail of the intercalated CTA ion. To assess the impact of these gauche configurations on the planarity of the alkyl chains, we examine the vibrational modes in the infrared spectrum, which are delocalized over the length of the methylene chain. Progression Bands of the Methylene Chain. Vibrational spectra of long chain molecules have been interpreted on the basis of vibrational modes of an infinite polymethylene chain.36

Conformational order in alkyl chains causes a coupling of the methylene vibrational modes. The vibrational spectra of ordered alkyl chain systems have been analyzed based on dispersion curves by plotting the frequencies ν as a function of the phase angle φ between neighboring methylene units. These have been compared with the theoretical dispersion curves for methylene vibrational modes obtained by a normal coordinate analysis of an infinite polymethylene chain. For an infinite polymethylene chain, only vibrational modes at φ ) 0 or π are infrared and/or Raman active. In the case of a finite chain, in addition to the φ ) 0 or π mode, a series of bands, namely, progression bands, appear in the IR and/or Raman spectrum. These bands have been studied for n-alkanes,20 n-alcohols,21 fatty acid,37and the n-alkyl trimethylammonium bromides.22 The progression bands are usually weak in intensity; however, it has been pointed out that end group substitution usually enhances the intensity of these progression bands compared to that of the crystalline n-alkanes.38 Vibrational modes in an all-trans methylene chain are described through a coupled oscillator model for which the eigenvalues of the vibrational secular equation are given by20

4π2ν2 ) Ηo + 2ΣΗm(cos mφk) where Ηo and Ηm are the matrix elements of the secular determinant. The φ's are the phase difference between adjacent oscillators as given by

φk ) kπ(n + 1) [k ) 1, 2, 3, ...n] where n is the number of oscillators in the chain. The progression bands appearing in the spectrum are analyzed by assigning a k value after identifying the particular mode to which it belongs. When correctly assigned a smooth curve results from a plot of νk versus φk. The existence of progression bands and



Figure 8. FT-IR spectrum of crystalline CTAB showing the progression bands in (a) wagging-twisting (1350-1150 cm-1) and (b) rocking (1000700 cm-1) regions. Band belonging to different progression series are designated as w (wagging), t (twisting-rocking), and r (rocking-twisting), together with the appropriate k values. The frequencies of the progression bands and their assignment are given in Table 2.

the ensuing dispersion curves is illustrated using crystalline CTAB (n ) 15) as the example. Figure 8 shows the IR spectrum in the region of 1400-700 cm-1 for crystalline CTAB. Bands belonging to different vibrational mode progressions are identified by comparing with the values reported for n-alkanes20 and n-alcohols.21 These modes are marked sequentially (k ) 1, 2, 3, ...) in Figure 8. The observed band positions (Table 2) have been plotted as a function of k along with the theoretical dispersion curves for an infinite polymethylene chain36,39 (solid line) in Figure 9. The dispersion curve shown are for the ν3, ν4, ν7, and ν8 branches of the progression bands. The ν3 and ν4 arise from CH2 wagging and C-C stretching modes, respectively, of the methylene chain. In the ν7 and ν8 modes, coupling between the rocking and twisting modes occur. At φ ) 0, the ν7 modes is pure rocking and ν8 twisting. As the phase angle increases, the mixing of rocking and twisting occurs, and finally at φ ) π, the assignment is reversed; ν7 is pure twisting, and ν8 is pure rocking. For the simplest possible chain imaginable, identical harmonic oscillators with nearest-neighbor coupling, the integer k has a well-defined meaning; it indicates the number of antinodes in the standing wave associated with each vibration. Such a description would be approximately valid for solid CTAB and the intercalated CTA ion.

The progression bands for the interdigitated bilayer and normal bilayer are shown in Figure 10 and Figure 12. The quality of the spectra is not as good as that for the crystalline CTAB, and consequently, the analysis is restricted to the ν3 (wagging) and ν8 (twisting-rocking) modes since these show the strongest dispersion. The subsequent analysis is based on the intensity of the progression bands which are considered to be proportional to the number of chains in an all-trans planar state. It is thus assumed that a single gauche configuration in the chain is sufficient to destroy the coupling that produces the progression. The wagging (ν3) and twisting-rocking (ν8) progression bands for the interdigitated bilayer are shown in Figure 10. The spectrum was recorded at 300 K. The wagging progression bands appear in the 1400-1150 cm-1 region and the rocking modes in the 1000-700 cm-1 region of the IR spectrum. The frequencies and assignments are tabulated in Table 2. For the interdigitated bilayer, no significant change in the intensity of the bands was observed on lowering the temperature. The two intense bands at 910 and 965 cm-1 (Figure 10) may be assigned to the C-N+ stretching modes of the “head” group (Table 2). The dispersion curves for the ν3 and ν8 bands of the interdigitated bilayer phase are shown in Figure 11 and are similar to



TABLE 2: Peak Position of the Progression Bands of Crystalline CTAB and Normal Bilayer (33 Å) and Interdigitated Bilayer (19 Å) Phases of the Intercalate Cd0.83PS3(CTA)0.34a crystalline CTAB (cm-1)

Cd0.83PS3(CTA)0.34 (19 Å phase, 300K) (cm-1)

Cd0.83PS3(CTA)0.34 (33 Å phase, 40K) (cm-1)

1340 (w) 1320 (m) 1312 (vw) 1299 (m) 1281 (m) 1279 (sh) 1258 (sh) 1252 (m) 1242 (m) 1235 (m) 1223 (w) 1216 (w) 1207 (vw) 1200 (vw) 1192 (vw) 1177 (vw) 1165 (vw) 986 (w) 973 (m) 963 (s) 950 (s)

1341 (w) 1321 (w)

1321 (vw)

1296 (w)

1294 (w)

1274 (w)

1274 (w)

1254 (w)

1255 (w)

1238 (w) 1220 (w)

1239 (w) 1216 (w)

a

w-9 w-8 t-11 w-6 t-7 w-5 t-4 w-4 N+-CH3 rocking w-3 t-3 w-2 t-2 w-1 t-1 r-2

964 (s) 935 (vw)

918 (w) 910 (s) 887 (vw) 876 (m) 835 (m) 802 (m) 772 (m) 747 (m) 729 (s) 719 (s)

assignment

906 (s) 877 (w) 835 (w) 798 (w) 769 (w) 748 (w) 721 (s)

973 (w) 967 (s) 956 (s) 933 (vw) 911 (s) 889 (vw) 876 (w) 834 (vw) 796 (w) 747 (w) 721 (s)

C-N stretching

C-N stretching methyl rocking r-6 r-7 r-8 r-9 r-10 r-11 r-15

w, wagging; t, twisting-rocking; r, rocking-twisting.

that of crystalline CTAB (Figure 9). The solid line in Figure 11 is the dispersion curve calculated for an infinite polymethylene chain.36,39 The ν3 and ν8 progression bands for the intercalated normal bilayer phase of Cd0.83PS3(CTA)0.34 are shown in the second and third panel of Figure 12. At room temperature, the progression bands especially the ν8 series are difficult to observe. It is only at low temperature (40 K) that the progression bands are clearly defined and the k values associated with the ν3 and ν8 series assignable (Table 2). These have been plotted in Figure 11. It may be seen that dispersion curves for the ν3 and ν8 modes for the intercalated bilayer are identical to those for the interdigitated bilayer at 300 K. The localized wagging modes of the intercalated bilayer phase are shown in the first panel of Figure 12 for the corresponding temperatures. At room temperature and above, features due to the double gauche (1354 cm-1) and g-t-g′ + kink (1368 cm-1) defects (marked by arrows in Figure 12a) are clearly defined, whereas the progression bands are not. As the temperature is lowered, the population of the double gauche and “kink” g-t-g′ defect conformers decrease, which is expected since the gauche configuration is higher in energy than the trans. As these gauche conformers disappear, the number of intercalated alkyl chains having an “all-trans” planar conformation increases. This is reflected in the increase in the intensity of the progression bands due to such planar conformers at low temperature. It may be seen that at 40 K when the progression bands are easily identified, the defect bands, except for the end gauche, are almost absent. The analysis of the progression bands of the intercalated normal bilayer and the interdigitated bilayer phases of Cd0.83PS3(CTA)0.34 clearly brings out the difference between them.

Figure 9. Dispersion curves for the methylene wagging (ν3), C-C stretching (ν4), methylene twisting-rocking (ν7), and methylene rockingtwisting (ν8) branches of the progression bands for crystalline CTAB. The filled circles are the experimental values, and the solid line is the calculated dispersion curve for infinite polymethylene chain.36,39



Figure 10. FT-IR spectrum in the (a) methylene wagging and (b) rocking-twisting regions of the spectrum of the interdigitated bilayer phase of the intercalate Cd0.83PS3(CTA)0.34.The frequencies of the progression bands and their assignment are given in Table 2.

In the interdigitated bilayer, a majority of the alkyl chains are completely planar (all-trans conformation). In the normal bilayer, although a majority of the methylene units of the alkyl chain are in trans conformation (their methylene stretching frequencies are indicative of this), the presence of a few gauche defects at room temperatures is sufficient to destroy the planarity of the alkyl chains. These results also help us understand the stability of the normal bilayer phase of Cd0.83PS3(CTA)0.34 and as to why it, rather than the interdigitated bilayer phase, is formed initially. A planar all-trans alkyl chain would occupy a cylindrical volume in space with a cross-sectional diameter of ∼4 Å18 and length depending on the number of methylene units. In Cd0.83PS3(CTA)0.34, the calculated distance between methylene chains assuming that the CTA ions are grafted at regular intervals is 9.2 Å (calculated assuming that the in-layer lattice parameters of the intercalated Cd0.83PS3(CTA)0.34 is the same as that of the host CdPS3). A cylindrical volume with cross-sectional diameter 9.2 Å is theoretically available for each methylene chain of the intercalated surfactant bilayer. Density considerations would, therefore, suggest that the interdigitated rather than the normal intercalated bilayer should have been formed. The presence of a few (one per chain) gauche defects, either in the interior or termini, is, however, sufficient to make the chains nonplanar and consequently occupy a lager volume in space. This is shown schematically in Figure 13; the presence of a single g-t-g′ defect in an otherwise all-trans methylene chain effectively

Figure 11. Dispersion curves for the interdigitated bilayer (closed circles) and normal bilayer (open triangles) phases of intercalate Cd0.83PS3(CTA)0.34. Data for interdigitated bilayer were recorded at 300 K, while those of the normal bilayer phase were measured at 40 K.



Figure 12. FT-IR spectra at different temperatures of the normal bilayer phase of the intercalate Cd0.83PS3(CTA)0.34. (a) The localized methylene wagging modes (1400-1300 cm-1). The wagging modes of specific conformational sequences containing gauche configuration are marked. (b) The methylene wagging progression (ν3) (1350-1150 cm-1) and (c) the methylene rocking-twisting progression (ν8) (1000-700 cm-1). The frequencies of the progression bands and their assignment are given in Table 2.

Figure 13. Schematic representation of the methylene chain of a CTA ion in an all-trans conformation and with a single (g-t-g′) “kink” defect. The cross-sectional area of the chain is taken perpendicular to the chain axis.

Figure 14. Cartoon representation of the conformation of the intercalated surfactant molecule at 300 K in (a) the normal bilayer and (b) interdigitated bilayer phases of Cd0.83PS3(CTA)0.34. The population of the high energy conformers in the normal bilayer phase has been exaggerated.

7650 J. Phys. Chem. B, Vol. 105, No. 32, 2001 increases the cross-sectional diameter 2-fold from the value for an all-trans planar conformation. In the as-formed Cd0.83PS3(CTA)0.34, the vibrational spectra clearly indicate that at room temperature a majority of the intercalated methylene chains of the surfactant ion are nonplanar. It is only in the absence of the gauche defects that the chains can attain a planar conformation and occupy less volume of the interlamellar space for the interdigitated bilayer to be formed. A cartoon representation of the difference in the conformation of the intercalated surfactant in the normal and interdigitated bilayer phases of Cd0.83PS3(CTA)0.34 is shown in Figure 14. Conclusion Cetyltrimethylammonium ions have been ion-exchange intercalated by a two-step process into the galleries of cadmium thiophosphate to give Cd0.83PS3(CTA)0.34. In the as-formed intercalated compound, the lattice expansion is 26.5 Å, with the methylene chains of the CTA ion adopting a tilted bilayer arrangement. This phase, when left in the aqueous reaction media, eventually transforms to a phase characterized by a lattice spacing of 19 Å, corresponding to a lattice expansion of 12.5 Å compared to that of the pristine host. No change in chemical stoichiometry accompanies this transformation. Using X-ray diffraction and orientation-dependent infrared spectroscopy, it has been possible to establish that the observed collapse of the interlayer spacing is a consequence of the interdigitation of the methylene “tails” of the intercalated CTA ion. The two phases of Cd0.83PS3(CTA)0.34, therefore, correspond to an intercalated normal bilayer and an intercalated interdigitated bilayer. Interdigitation occurs with the value of the tilt angle, the angle between the molecular axis of the methylene chain and the interlamellar normal, identical to that in the normal bilayer phase, 55°. The frequencies of the conformationally sensitive methylene symmetric and antisymmetric stretching modes indicate that in both the normal and interdigitated intercalated bilayer phases of Cd0.83PS3(CTA)0.34, a majority of the methylene units are in a trans configuration. Raman spectroscopy, however, shows that the concentration of gauche conformers is lower in the intercalated interdigitated bilayer. The increased conformational heterogeneity of the methylene chains of the intercalated surfactant ion in the normal bilayer phase compared to that of the interdigitated bilayer phase is also seen in the 13C NMR. The planarity of the methylene chain of the intercalated surfactant was investigated by examining the progression bands due to the ν3 (wagging) and ν8 (twisting-rocking) delocalized modes of the methylene chain. For the methylene chains in the intercalated interdigitated bilayer, these modes are easily identified in the room-temperature infrared spectra, indicating that a majority of the methylene chains in the intercalated interdigitated bilayer adopt an all-trans planar conformation. The situation in the normal bilayer phase of Cd0.83PS3(CTA)0.34 is, however, quite different. Although a majority of the methylene units are in a trans configuration, the presence of a few gauche defects is sufficient to destroy the planarity of the chain and at room temperature. It is only at low temperature when the population

Venkataraman and Vasudevan of the high-energy gauche conformers decreases that the progression bands due to the all-trans planar conformation are seen in the spectrum. References and Notes (1) Slater, J. L.; Huang, C. Prog. Lipid Res. 1988, 27, 325. (2) Komatsu, H.; Okada, S. Biochim. Biophys. Acta 1995, 1237, 169; 1996, 1283, 73. (3) Mou, J.; Yang, J.; Haung, C.; Shao, Z. Biochemistry 1994, 33, 9981. (4) Janshoff, A.; Bong D. T.; Steinem, C.; Johnson J. E.; Ghadiri M. R. Biochemistry 1999, 38, 5328. (5) McIntosh, T, J.; McDaniel, R. V.; Simon, S. A. Biochim. Biophys. Acta 1983, 731, 109. (6) Maruyama, S.; Hata, T.; Matsuki, H.; Kaneshina, S. Biochim. Biophys. Acta 1997, 1325, 272. (7) Lagaly, G. Angew. Chem., Int. Ed. Engl. 1976, 15, 575. (8) Borja, M.; Dutta, P. K. J. Phys. Chem. 1992, 96, 5432. (9) Ogawa, M.; Kuroda, K. Bull. Chem. Soc. Jpn, 1997, 70, 2593. (10) Vaia, R, A.; Teukolsky, R, K.; Giannelis, E, P. Chem. Mater. 1996, 6, 1017. (11) Venkataraman, N. V.; Vasudevan, S. J. Phys. Chem. B 2001, 105, 1805. (12) Brec, R. Solid State Ionics 1986, 22, 3. (13) Clement, R.; Lagadic, I.; Leaustic, A.; Audiere, J. P.; Lomas, L. Chemical Physics of Intercalation II. NATO ASI Ser., Ser. B 1993, 315. (14) Jeevanandam, P; Vasudevan, S. Solid State Ionics, 1997, 104, 45. (15) Klingen, V. W.; Ott, R.; Hahn, H. Z. Anorg. Allg. Chem., 1973, 396, 271. (16) Arun, N.; Vasudevan, S.; Ramanathan, K. V. J. Am. Chem. Soc. 2000, 122, 6028. (17) Length estimated from Insight II molecular modeling system; Biosym Technologies: San Diego, CA, 1993. (18) Israelachvili, J. B Intermolecular and Surface forces; Academic Press: New York, 1985. (19) Wallach, D, F, H.; Verma, S. P.; Fookson, J. Biochim. Biophys. Acta 1979, 559, 153. (20) Snyder, R. G.; Schachtschnelder, J. H. Spectrochim. Acta 1963, 19, 85. (21) Tasumi, M.; Shimaanouchi, T.; Watanabe, A.; Goto, R. Spectrochim. Acta 1964, 20, 629. (22) Uno, T.; Machida, K.; Miyajima, K. Spectrochuim. Acta 1968, 24A, 1749. (23) Snyder, R. J. Mol. Spectrosc. 1961, 7, 116. (24) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A.J. Phys. Chem. 1984, 88, 334. (25) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (26) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta 1978, 34, 395. (27) Brown, K. G.; Bicknell-Brown, E.; Ladjadj. J. Phys. Chem. 1987, 91, 3436. (28) Snyder, R. G. J. Chem. Phys. 1967, 47, 1316. (29) Maroncelli, M.; Qi, S, P.; Strauss, H, L.; Snyder, R. G. J. Am. Chem. Soc. 1982, 104, 6237. (30) Chachaty, C. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 183. (31) Lindman, B.; Soderman, O.; Stilbs, P. In Surfactants in Solution; Mittal, K. L, Ed.; Plenum Press: New York, 1989; Vol. 7, p 1. (32) Gao, W.; Reven, L. Langmuir 1995, 11, 1860. (33) Soderlind, E.; Stilbs, P. Langmuir 1993, 9, 1678. (34) Wang, Li-Qiong; Liu, J.; Exarhos, G. J.; Flanigan, K, Y.; Bordia, R. J. Phys. Chem. B,2000, 104, 2810. (35) Williams, E.; Sears, B.; Allerhand, A.; Cordes, E. H. J. Am. Chem. Soc. 1973, 95, 4871. (36) Snyder, R. G. J. Mol. Spectrosc. 1960, 4, 411. (37) Kobayashi, M.; Kaneko, F.; Sato, K.; Suzuki, M. J. Phys. Chem. 1989, 93, 485. (38) Uno, T.; Machida, K.; Miyajima, K. Spectrochim. Acta 1968, 24A, 1741. (39) Tasumi, M.; Shimonauchi, T.; Miyazwa, Y. J. Mol. Spectrosc. 1962, 9, 261.

Interdigitation of an Intercalated Surfactant Bilayer - American

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