Langmuir 1997, 13, 3029-3034
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Induced Circular Dichroism of Isotropic and Magnetically-Oriented Chiral Nematic Suspensions of Cellulose Crystallites Xue Min Dong and Derek G. Gray* Paprican and Department of Chemistry, McGill University, Pulp and Paper Research Centre, Montreal, Quebec, Canada H3A 2A7 Received October 28, 1996. In Final Form: March 3, 1997X
Stable colloidal suspensions of rodlike cellulose crystallites, prepared by acid hydrolysis of cellulose fibers, form a chiral nematic phase above a critical concentration. The direction of the chiral nematic axis may be controlled by applying a strong magnetic field. In the presence of Congo red, a dye with strong affinity for cellulose, the suspensions show induced circular dichroism (ICD) at the dye absorption wavelengths, indicating that the dye molecules are in a chiral environment. The isotropic suspension shows a relatively weak positive ICD peak, while the chiral nematic phase shows a very strong negative ICD peak when viewed along the chiral nematic axis. The peak is weaker for unaligned chiral nematic samples and is very small when viewed at right angles to the chiral nematic axis. Thus the ICD in the chiral nematic phase results from the orientation of the dye molecules in a chiral nematic array. The ICD band intensity increases with chiral nematic pitch.
Introduction 1
Circular dichroism (CD) spectroscopy measures the difference in the absorption of left- and right-handed circularly polarized light as a function of wavelength as the light passes through an optical active medium. An apparent absorption may also result from the reflection of circularly polarized light by chiral nematic liquid crystalline phases.2 Cellulose is optically active but lacks chromophores in easily accessible regions of the spectra. It does absorb in the far UV, a region that is inaccessible to most CD spectrometers. However, instead of trying to measure the pure cellulose CD spectra in the far UV range, a CD signal may be induced by the association between a CD inactive chromophore and the chiral cellulose structure. This induced circular dichroism (ICD) may result from a covalent link between the cellulose backbone and the chromophore or by a strong specific physical interaction between the chromophore and the polymer chain. This technique has been widely used to study biopolymers such as proteins, nucleic acids, and polysaccharides.3 For polysaccharides, effective chromophores include dyes such as Congo red that interact strongly with the polymer chain. Ritcey and Gray4 found an induced CD signal for Congo red in aqueous solutions of cellulose oligomers with five and six glucose units (and for methylcellulose) but not for shorter units. Similar CD peaks were observed by Engle, Purdie, and Hyatt5 for an extended range of dyes and substrates. The CD peaks for the complexes between the cellulose oligomers or cellulosic polymers and the dyes were split into a negative peak at long wavelengths and a positive peak at short wavelengths. This is characteristic of the exciton splitting predicted for chromophores in a X
Abstract published in Advance ACS Abstracts, April 15, 1997.
(1) Harada, N. Circular Dichroic Spectroscopy: Exciton coupling in organic stereochemistry; University Science Books: Mill Valley, CA, 1983. (2) Chandrasekhar, S. Liquid Crystals; Cambridge University Press: Cambridge, 1977. (3) Hatano, M. In Induced Circular Dichroism in Biopolymer-dye systems; Okamura, S., Ed.; Advances in Polymer Science 77; SpringerVerlag: Berlin, 1986. (4) Ritcey, A. M.; Gray, D. G. Biopolymers 1988, 27, 479. (5) Engle, A. R.; Purdie, N.; Hyatt, J. A. Carbohydr. Res. 1994, 265, 181.
S0743-7463(96)01046-3 CCC: $14.00
helical relationship to each other. Presumably, the dyes associate with the chains in a helical complex. A second and quite distinct form of induced circular dichroism has been widely observed for anisotropic chromophore molecules dissolved in chiral nematic liquid crystalline media. This liquid crystalline induced circular dichroism (LCICD)6 results in very large CD peaks; it has been observed for Congo red in certain regenerated cellulose films,7 and for acridine orange in lyotropic chiral nematic solutions of acetylethylcellulose.8 In this case the CD signal appears to result from the orientation of the linearly dichroic dye molecules in a helicoidal arrangement. Stable colloidal suspensions of rodlike cellulose crystallites can be prepared by acid hydrolysis of cellulose fibers, and these suspensions form a chiral nematic phase above a critical concentration.9,10 Typically, the crystallites are of the order of 100 nm long and 5 nm wide and are composed of crystalline Cellulose I, stabilized by sulfate groups on their surface. The chiral nematic liquid crystalline phase forms at concentrations above about 5% by weight in saltfree suspensions. In this paper we investigate the induced circular dichroism observed when dyes adsorb on these colloidal particles, in both the isotropic and chiral nematic phases. A strong magnetic field was used to align the chiral nematic structure. This provides a novel way to measure the induced CD along and normal to the chiral nematic axis. Experimental Section (a) Materials. Suspensions of cellulose crystallites associated with sodium as counterions were prepared from filter paper and characterized as described previously.11,12 The aqueous suspensions, with a pH of 6.75, were used for subsequent experiments. Congo red (certified grade, Fisher Scientific) was recrystallized (6) Saeva, F. D.; Wysocki, J. J. J. Am. Chem. Soc. 1971, 93, 5928. (7) Ritcey, A. M.; Gray, D. G. Biopolymers 1988, 27, 479. (8) Guo, J. X.; Gray, D. G. Liq. Crys. 1995, 18, 571. (9) Revol, J.-F.; Bradford, H.; Giasson, J.; Marchessault, R. H.; Gray, D. G. Int. J. Biol. Macromol. 1992, 14, 170. (10) Revol, J. F.; Godbout, L.; Dong, X. M.; Gray, D. G.; Chanzy, H.; Maret, G. Liq. Cryst. 1994, 16, 127. (11) Dong, X. M.; Kimura, T.; Revol, J.-F.; Gray, D. G. Langmuir 1996, 12, 2076. (12) Dong, X. M.; Gray, D. G., Langmuir in press.
© 1997 American Chemical Society
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Figure 1. Schematic representation of magnetic field alignment for the anisotropic suspensions of cellulose. Parallel and perpendicular orientations give fingerprint and planar textures, respectively. three times from 50% aqueous ethanol, and its purity was checked by thin layer chromatography. Trypan blue (certified purity, Aldrich) was used without further purification. The structures of these dyes, which show a strong affinity for cellulose, are shown below.
(b) Sample Preparation and Instrument Measurement. Stock dye solutions were prepared in volumetric flasks with deionized water. Congo red-dyed anisotropic cellulose suspensions were prepared by mixing the desired amount of a chiral nematic suspension (weight percentage concentration ) 8.17%) with the proper amount of dye solution. The overall concentrations of dye and cellulose crystallites in the prepared samples were calculated from the added amount, the original concentrations, and the total weight. However, the addition of the dye solution generated a mixture of isotropic and chiral nematic phases, due to dilution and the increase in ionic strength due to dye and counterions. The biphasic suspensions were stored at room temperature for at least one day to allow equilibrium between the phases to be established. The chiral nematic phase was then carefully transferred into a quartz spectrophotometric cell with a short path length of 0.01 cm. In order to eliminate flow orientation and residual inhomogeneity in the sample, the dyed suspension was allowed to stand in the spectrophotometric cell for at least 24 h before CD measurement. The isotropic samples dyed with Congo red or trypan blue were prepared by mixing an isotropic cellulose suspension with the corresponding dye solution. The samples were placed into a 1.0 cm path length cell and shaken well before measurement. A 7.2 T magnetic field (generated by the magnet from a Varian XL300 NMR Spectrometer) was used to orient the anisotropic samples. The sample cell was placed inside the field in one of two directions, perpendicular or parallel, with respect to the direction of the magnetic field. Because of the negative diamagnetic susceptibility of cellulose, the individual crystallites tend to orient with their long axes at right angles to the field direction.10 In this case, the chiral nematic structure does not untwist in the field, but the orientation of the chiral nematic axis rotates so that it is parallel to the field. Thus, when the spectrometer cell is placed in the field for 17 h with the viewing direction at right angles to the field (i.e., with the cell windows parallel to the field), the chiral nematic phase reorientates to give a fingerprint texture, as shown on the left in Figure 1. If the cell is allowed to stand for 2 h with the viewing direction parallel
Dong and Gray
Figure 2. Circular Dichroism spectra of the isotropic cellulose suspension dyed with Congo red (solid line) and without dye (dotted line). Suspension concentration is 6.16% w/w, and the concentration of Congo red is 3.93 × 10-5 M. to the field, a planar texture is observed, with the chiral nematic axis at right angles to the window surfaces (on the right in Figure 1). The CD spectra for the well-aligned samples were run immediately after the magnetic field was removed. The CD spectra were recorded with a Jasco 500C spectropolarimeter. Baseline corrections were made by subtracting the spectrum of the appropriate blank suspension without added dye. The measurement conditions were selected to give the best signal/noise ratio. The anisotropic sample was checked for linear dichroism by mounting the sample cell on a rotating stage to permit the accumulation of CD spectra at different sample orientations. The magnetically oriented suspension with a planar texture was used for this measurement. The spectra were recorded from 0° to 180° at 30° intervals. UV-visible absorption spectra were recorded with a Pye Unicam SP 8-150 spectrophotometer. In order to estimate the linear dichroism of the anisotropic sample, a polarizer was placed in the light path in front of the sample to obtain plane polarized incident light. The anisotropic sample was mounted on a rotating stage, and the maximum absorption (at 525 nm) was measured at various sample orientations. The texture of the liquid crystal phase was recorded using a Nikon Microphot-FXA cross-polarized optical microscope equipped with a camera. The chiral nematic pitch of the mesophase was measured by optical microscopy as twice the line spacing of the fingerprint texture.
Results and Discussion (a) Isotropic Suspensions. The apparent circular dichroism spectra of two isotropic suspensions of cellulose crystallites are given in Figure 2. The dotted line in the figure is the ICD signal for the blank suspension, and the solid line is for the same suspension with 3.93 × 10-5 M Congo red. The concentration of Congo red in the dyed suspension sample was selected from a series of experiments to give the best signal/noise ratio: the signal became very noisy at low dye concentrations and saturated at high dye concentrations, giving a false zero signal. For the cellulose suspension without dye, no CD signal was expected. In fact a significant negative CD signal was observed. The intensity of this signal increased as the wavelength of the incident light decreased. Solutions of cellobiose and of hydroxypropyl cellulose showed no CD signal, so it does not appear to be due to glucose units or to the cellulose molecular backbone. It is possible that some chromophoric groups might be present on the crystallite surfaces or that preferential scattering occurs from the chiral particles. The source of this CD signal will require further investigation. The most interesting result from this experiment is that a positive CD band at around 525 nm is observed (Figure 2) for the isotropic suspension containing Congo red. The peak is superimposed on the negative signal from the
Cellulose Crystallites
Figure 3. UV-visible absorption spectra of Congo red in 0.01 M NaCl solution (a) and in 6.16% isotropic cellulose suspension (b). The ICD spectrum for the isotropic suspension containing Congo red, calculated from the difference of the spectra in Figure 2, is shown in part c.
Figure 4. UV-visible absorption spectrum (a) and ICD spectrum (corrected for dye-free suspension CD) (b) of an isotropic cellulose suspension dyed with trypan blue. The suspension concentration is 5.76% w/w, and the concentration of trypan blue is 2.13 × 10-5 M.
suspension. The UV-visible absorption spectra for Congo red in 0.01 M NaCl solution (Figure 3a) and in the cellulose suspension (Figure 3b) show that the characteristic absorption peak for Congo red (487 nm) is shifted toward longer wavelengths (525 nm), indicating a strong interaction between the chromophore and the cellulose. This peak is close to the CD signal that results when the spectra in Figure 2 are subtracted. This difference CD spectrum is shown in Figure 3c; the correspondence with the Congo red/cellulose absorption peak in Figure 3b confirms that the CD peak is due to the presence of Congo red in a chiral environment. The small difference in the maximum position is within experimental error. The observation of a positive induced CD signal due to association of a dye with cellulose was confirmed with a second dye, Trypan blue. The results are given in Figure 4. Again, an ICD peak was observed at the same wavelength as the absorption peak for the dye/cellulose combination.
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Congo red and trypan blue are achiral, CD inactive molecules, yet the dye-isotropic cellulose suspensions are clearly CD active. Figures 3c and 4b are thus the ICD signals corresponding to the π-π* excitation energy of the dye/cellulose complex. The nature of the chiral dye/cellulose complex in these dilute isotropic suspensions is not certain at present. In the earlier experiments for Congo red in aqueous solutions of cellulose oligomers or cellulosic polymers,5,7 the induced CD signals were split into a negative peak at long wavelengths and a positive peak at short wavelengths. However, no such splitting is observed for the ICD peaks for Congo red and Trypan blue in the isotropic cellulose suspensions. In the case of the suspensions of crystallites, the cellulose is in the form of crystalline aggregates, of dimensions of the order of 115 nm long and 7 nm wide. The mechanism by which the ICD signal is generated is an open question. Possibilities include (i) adsorption of the dye on the cellulose surface occuring in some preferential geometry that distorts the individual dye molecules into a chiral form, (ii) the elongated dye molecules adsorbing on the rod-shaped crystallites such that their mean orientation is at an angle (other than 0° or 90°) to the long axis of the crystallite, thus forming a chiral (helical) array of chromophores on the surface of each crystallite, or (iii) the dye molecules adsorbing parallel to the long axis (thus favoring hydrophobic interactions between sugar rings and aromatic rings in aqueous media5,13), but the crystallite itself is twisted with a given handedness, so that the long axes of the adsorbed dye molecules assume a helical relationship with each other. While there is evidence that microfibrils of an algal cellulose are twisted in a right-handed helix,14 it is not clear whether the crystallites from cotton are twisted. Thus, dilute isotropic suspensions of cellulose crystallites clearly show positive ICD peaks with Congo red and Trypan blue, but the source of the signal is not certain. (b) Anisotropic Suspensions. Suspensions of cellulose crystallites form a chiral nematic liquid crystalline phase when the concentration is higher than a certain critical value. The individual crystallites become oriented in a chiral helicoidal arrangement that persists over dimensions of the order of millimeters. Congo red is strongly adsorbed on cellulose. If this adsorption takes place in a specific orientation on the surface of the cellulose (e.g., if the long axis of the dye molecules is aligned with the cellulose chain), then the dye molecules might be expected to assume the helicoidal orientation characteristic of the chiral nematic state, and an induced CD signal should be detected. The ICD spectra for the chiral nematic suspensions differ from the results for the isotropic systems. In contrast to the positive apparent ellipticity of the isotropic samples, the anisotropic phase with a planar texture viewed along the chiral nematic axis shows a very strong negative CD band at 525 nm. When the initial total concentration of the cellulose suspension was fixed at 7.85% w/w, the ellipticity increased almost linearly with the dye concentration, as shown in Figure 5. Thus, Congo red is the limiting reagent in the formation of the dye/cellulose complex. If Beer’s law is applied in the optical measurement of the suspension samples, the molar ellipticity can be calculated from eq 1.1
[θ] )
θ LC
(1)
where θ is the measured ellipticity angle, L is the cell (13) Carroll, B.; Cheng, H. J. Phys. Chem. 1962, 66, 2585. (14) Hanley, S. J.; Revol, J.-F.; Gray, D. G. Cellulose, in press.
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Figure 5. ICD spectra of anisotropic cellulose suspensions containing different concentrations of Congo red. From top to bottom, the concentrations are 9.69 × 10-6 M, 1.91 × 10-5 M, 3.02 × 10-5 M, 3.84 × 10-5 M, 5.31 × 10-5 M, and 1.94 × 10-4 M. The initial total suspension concentration (as in biphasic sample) is fixed at 7.85% w/w.
Figure 6. CD spectra for an anisotropic suspension with a planar texture recorded at 30° intervals as the sample is rotated from 0° to 180°. The suspension concentration in the anisotropic phase is approximately 8% w/w, and the Congo red concentration is about 3.9 × 10-5 M.
length, and C is the concentration of Congo red. The average molar ellipticity for anisotropic suspensions, calculated from the results shown in Figure 5, is 3.36 × 106 deg‚mol-1‚cm-1. The molar ellipticity of the isotropic sample from the result in Figure 3c is 1.20 x 103 deg‚mol-1‚cm-1. (Note that the path length of the CD sample cell is different for isotropic suspensions and anisotropic suspensions.) The molar ellipticity of the anisotropic samples is thus more than three orders of magnitude larger than that of the isotropic sample and of opposite sign. The negative CD signal is so strong that it swamps any trace of the small positive ICD signal generated by the dye in the isotropic suspension (Figure 2). The CD spectra, measured as a function of the sample rotation angle for an anisotropic suspension with planar texture, are given in Figure 6. The sample was rotated about the spectrometer beam direction, so that the flat surfaces of the sample remained normal to the spectrometer beam during rotation. The eight spectra recorded at 30° intervals from 0° to 180° are effectively superimposed. This indicates the macroscopic linear dichroism in the sample is very small. It is important to distinguish between macroscopic and quasi-layer birefringence and dichroism in chiral nematic samples. In any layer, normal to the helicoidal axis, whose thickness is small compared to the helicoidal pitch, the individual rods are preferentially oriented, and thus the layer will display linear birefringence and dichroism. (The de Vries model for the optical properties of chiral nematics is based on the
Dong and Gray
Figure 7. ICD spectra of a single anisotropic suspension with two different liquid crystalline textures obtained by magnetic field alignment (see Figure 1). The spectrometer beam passes normal to a fingerprint texture to give spectrum a and passes normal to a planar texture to give spectrum b. The sample is the same as in Figure 6.
helicoidal arrangement of birefringent layers.15) If the layer is much thicker than the pitch, then the preferential orientation follows a helicoidal arrangement, and the birefringence and dichroism of the individual quasi-layers approximately cancel out when viewed along the helicoidal axis, resulting in the almost complete absence of linear birefringence and dichroism for planar textures. The absence of linear dichroism was also checked by measuring its absorption spectrum with a UV-visible spectrophotometer equipped with a linear polarizer. At the maximum absorption wavelength (525 nm), the apparent absorption from the anisotropic suspension dyed with Congo red was found to be independent of the rotation angle of the sample relative to the polarizer. This result also suggests that the linear dichroism in the suspension used for the CD study is negligible and that the ICD signal results from the chiral nematic order. The strong ICD signal from these suspensions is analogous to that observed in many other liquid crystal/dye systems. For chiral nematic liquid crystals, different texture patterns can be observed with the polarizing optical microscope because of the difference in the direction of chiral nematic ordering relative to an observer. When the helicoidal axis of the sample is normal to the substrate surface, a planar texture is observed. If the helicoidal axis of the sample is parallel to the substrate surface, a fingerprint texture will be seen. The observed texture depends on the boundary conditions, container shape and external field, and sample history. Usually, when a very thin quartz cell is used as the container, the observed texture will be planar, since in this case the long axes of the cellulose crystallites align parallel to the quartz window surfaces. Because of the negative diamagnetic susceptibility of cellulose, a magnetic field can be used to realign the chiral nematic director10,16 as shown in Figure 1. Very well aligned fingerprint and planar textures of the anisotropic suspensions were obtained in this way, as observed by optical microscopy. The ICD spectra of samples of the chiral nematic suspension containing Congo red were measured both along the chiral nematic axis (where the spectropolarimeter beam passes normal to a sample with planar texture) and at right angles to the chiral nematic axis (where the beam passes normal to a sample with fingerprint texture). The CD spectra are given in Figure 7. These two spectra are from the same sample run under exactly the same conditions, except that (15) deVries, H. D. Acta Crystallogr. 1951, 4, 219. (16) Krigbaum, W. R. In Polymer Liquid Crystals; Ciferri, A., Krigbaum, W. R., Meyer, R. B., Eds.; Academic Press: New York, 1982; Chapter 10.
Cellulose Crystallites
the direction of the chiral nematic director relative to the spectropolarimeter beam differs by 90°. The planar texture shows a very strong negative ellipticity, about -1200 mdeg. The fingerprint texture, however, has only a small ellipticity, about -50 mdeg. The difference in ICD signal between these two orientation directions is truly significant. We assume that the linear dye molecules are aligned along the cellulose crystallites, and so the orientation of the major axis of the dye molecules mirrors that of the cellulose crystallites. The crystallites in these chiral nematic suspensions are oriented in a helicoidal array, with a pitch of the order of 70 µm for the textures whose ICD is shown in Figure 7. Thus, the dye molecules are also aligned in a helicoidal supramolecular array of the same handness, with their long axes normal to the helicoidal axis. This arrangement of dye chromophores causes a differential absorption of left- and right-circularly polarized light when viewed along the helicoidal (chiral nematic) axis.17 Viewed in this direction, the chromophores of neighboring dye molecules are all rotated at a slight angle in the same sense, relative to their neighbors. However, when viewed normal to the chiral nematic axis, the induced CD peak size is very much smaller, indicating much less differential absorption. The chirality of the helicoidal arrangement is not apparent when viewed from this angle. The source of the large ICD signal clearly must relate to the helicoidal supramolecular orientation of the dye molecules in the chiral nematic phase rather than to any induced molecular chirality or other local effect, which would be independent of macroscopic director orientation. The small signal observed for the fingerprint texture may be due to imperfect alignment of the chiral nematic axis parallel to the walls (especially near the walls, where a planar texture is more stable, or near defects in the texture, where the chiral nematic axis changes directions or is discontinuous). Since a thin (0.01 cm) spectrophotometric cell was used for the anisotropic sample measurement, the stable texture of the anisotropic suspension is planar, due to surface alignment. The fingerprint texture produced by the magnetic field will be unstable and will reorient to a stable planar texture in the absence of the field. This reorientation process was observed by optical microscopy with crossed polarizers. Photomicrographs of the changes in liquid crystalline texture with time are shown in Figure 8. Immediately after the removal of the magnetic field, the fingerprint texture appears very well aligned, with a clear, regular periodicity (Figure 8a). Thereafter, the fingerprint texture begins to reorient, changing fastest at the edge of the sample and at defects in the texture. After 3.5 h, the periodicity becomes less regular and an area of planar texture (green area) appears (Figure 8b). With increasing relaxation time, the texture becomes more and more planar (Figure 8c). Within a day, most of the fingerprint texture has been transformed to planar texture (Figure 8d). This reorientation of the fingerprint texture with time was also accompanied by a corresponding change in the ICD spectra of the sample. As shown in Figure 9, the apparent negative ellipticity increases with relaxation time. The initially rapid rate of change decreases with time; when a completely planar texture is finally formed, the ellipticity is the same as that shown in Figure 7b for the initially planar sample. If the ICD signal from the planar anisotropic phase is caused by the chiral nematic structure, then any change in this structure, such as a change in the chiral nematic (17) Saeva, F. D. In Liquid Crystals, the Fourth State of Matter; Saeva, F. D., Ed.; Marcel Dekker: New York, 1979; Chapter 6.
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Figure 8. Photomicrographs (crossed polars) of the liquid crystalline textures displayed by a suspension of cellulose crystallites at different relaxation times following the removal of the magnetic field that generated the initial fingerprint texture. (a) 0 h; (b) 3.5 h; (c) 7.5 h; (d) 24 h. The sample is the same as in Figure 6.
Figure 9. Change in ellipticity with time for an anisotropic suspension displaying fingerprint texture after the magnetic field was removed. The sample is the same as in Figure 6.
Figure 10. Plot of the apparent ellipticity against the chiral nematic pitch for the anisotropic suspensions of cellulose crystallites dyed with Congo red. The initial total suspension concentration is fixed at 7.50% w/w, and the Congo red concentration is 3.63 × 10-5 M. The pitch was changed by adding NaCl from 0 to 1.25 × 10-3 M to thesuspensions.
pitch, should have some effects on the ICD band intensity. A simple way to change the chiral nematic pitch of these suspensions of cellulose crystallites is to add salt.11 By adding small amounts of NaCl to the suspension, the magnitude of the ellipticity of the ICD was observed to increase with increasing chiral nematic pitch (Figure 10). This relationship is similar to that observed for an acetylethylcellulose liquid crystal system.8 It can be
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qualitatively explained by the theory of Chandrasekhar et al.2,18 (c) ICD and Handedness. The very large negative ICD band observed for the chiral nematic suspensions of cellulose is thus attributed to the helicoidal orientation of the adsorbed dye molecules. The sign of the peak depends on the handedness or the chiral nematic structure. The chiral nematic pitch of these suspensions is much longer than the wavelength of visible light, so it is not possible to determine the handedness directly from the handedness of circularly polarized light reflected by the sample. However, by examining the handedness of solid films dried down from chiral nematic cellulose suspensions by oblique sectioning and transmission electron microscopy19 or by chiroptical methods,20 it is clear that the cellulose crystallites are ordered in a left-handed supramolecular helicoidal arrangement. Assuming that the long axis of the dye molecules aligns with the long axis of the crystallites, the dye molecules must also assume a left-handed chiral nematic structure, and this corresponds to a negative ICD band. This is in agreement with observations on right- and left-handed mesophases of acetylethylcellulose, where left-handed chiral nematic solutions containing acridine orange give negative ICD bands and right-handed chiral nematic solutions give positive ICD bands.8 However, these results contrast with observations in cholesteryl ester systems, where righthanded helicoidal arrangements give negative ICD peaks and left-handed arrangements give positive peaks.3,6 In the isotropic phase, we observe a much weaker positive ICD band for these suspensions. This behavior contrasts with two other situations observed in cellulosebased systems. For cellulose derivatives that form lyotropic liquid crystalline solutions, there is often no ICD signal in the isotropic phase, even for dyes that give strong ICD bands in the chiral nematic phase.8 For cellulose (18) Ranganath, G. S.; Chandrasekhar, S.; Kini, U. D.; Suresh, K. A.; Romasesh, S. Chem. Phys. Lett. 1973, 19, 556. (19) Giasson, J. EÄ tudes Microscopiques d’Helicoı¨des de Syste`mes Cellulosiques in vitro. Ph.D. Thesis, McGill University, 1995. (20) Revol, J. F.; Godbout, L.; Gray, D. G. International Patent Application PCT/CA 95/00067-09.02.95, 1996.
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oligomers and methylcellulose in dilute solution with Congo red, the ICD peak has positive and negative contributions because of exciton splitting.7 As in the case of the cellulose crystallites, there is a strong interaction between dye and cellulose oligomers, but the concentration of dye molecules associated with the oligomers is apparently sufficient that the dye molecules are in a mutually chiral (possible helical) arrangement to generate the exciton splitting. The coverage of dye molecules on the surface of the cellulose crystallites is estimated (for a typical suspension concentration of 7.8% and a Congo red concentration of 5 × 10-5 M) to be around 0.3%, assuming complete adsorption, so exciton splitting due to chiral interactions between dye molecules on a crystallite is not expected at this low coverage. In an early model for the chiral nematic state, Straley21 pointed out that helically grooved rods can approach each other more closely at some angle that depends on the helical pitch and handedness of the grooves. Thus rods with grooves in a left-handed helix whose pitch is very much longer than the rod diameter will pack into a righthanded chiral nematic arrangement, and vice versa, to minimize the excluded volume of the system. It is tempting to try and relate the reversal of sign of the ICD band in isotropic and chiral nematic phases to a change from dye interactions with the (possibly) right-handed helical structure of individual rods in isotropic solution to dye orientation in the left-handed helicoidal structure of the chiral nematic phase, but this seems premature; the source of the ICD signal from the isotropic phase needs to be clearly identified. Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada and the Paprican graduate student program for support. X.M.D. thanks the Pall Corporation and the Government of Quebec for scholarships. LA9610462 (21) Straley, J. P. Phys. Rev. A 1976, 14, 1835.
