Effect of Counterions on Ordered Phase Formation in Suspensions of

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Effect of Counterions on Ordered Phase Formation in Suspensions of Charged Rodlike 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 July 22, 1996. In Final Form: February 10, 1997X Stable colloidal suspensions of cellulose crystallites with negatively charged sulfate groups on their surface were prepared by acid hydrolysis of filter paper. The suspensions, which were free of added electrolyte, formed chiral nematic ordered phases above a critical concentration. A sharp boundary was observed between coexisting chiral nematic and isotropic phases, enabling measurements to be made of the relative amounts of each phase as a function of total cellulose concentration. The isotropic-to-chiral nematic phase equilibrium was sensitive to the nature of the counterions present in the suspension. Samples were prepared with sodium, potassium, cesium, ammonium, tetramethylammonium, tetraethylammonium, tetrapropylammonium, tetrabutylammonium, trimethylammonium, and triethylammonium counterions. Suspensions with H+ counterions formed an ordered phase at the lowest concentrations of crystallites. For inorganic counterions, the critical concentration for ordered phase formation increases in the order H+ < Na+ < K+ < Cs+. For the organic counterions, the critical concentration in general increases with increasing counterion size, suggesting that the equilibrium is governed by a balance between hydrophobic attraction and steric repulsion forces. The nature of the counterions also influences other properties of the suspensions, such as their stability, the temperature dependence of the phase separation and of the chiral nematic pitch, and the redispersability of dried samples made from the suspensions.

Introduction Acid hydrolysis of cellulose under controlled conditions gives stable aqueous suspensions of cellulose crystallites, which behave as rodlike polyelectrolytes of colloidal dimensions; these suspensions form an ordered chiral nematic liquid crystalline phase when the suspension concentration is higher than a certain critical value.1,2 Theories for the ordered phase formation of rodlike polyelectrolytes have focused on repulsive steric and electrostatic interparticle forces.3-5 In these theories, the shape (axis ratio) of the rods governs the volume fraction of rods in the suspension that is necessary for ordered phase formation. For charged rods, the electrostatic contribution to the interparticle forces will depend on the ionic strength of the system. The phase separation of rodlike polyelectrolytes has been found to be strongly affected by axis ratio and ionic strength.6-9 However, the interparticle forces include not only the steric and electrostatic forces but also other contributions such as hydration forces, hydrophobic interactions, and hydrogen bonding. Any change in these forces should also induce a change in the phase separation. Not much attention has been paid to the effects of these forces on the phase separation of rodlike polyelectrolytes. We find that the easily observed formation of macroscopic ordered phases provides a sensitive indicator of these contributions. The counterions associated with polyelectrolytes play a very important role in these interparticle forces. X

Abstract published in Advance ACS Abstracts, March 15, 1997.

(1) Revol, J.-F.; Godbout, L.; Dong, X. M.; Gray, D. G.; Chanzy, H.; Maret, G. Liq. Cryst. 1994, 16, 127. (2) Revol, J.-F.; Bradford, H.; Giasson, J.; Marchessault, R. H.; Gray, D. G. Int. J. Biol. Macromol. 1992, 14, 170. (3) Odijk, T. Macromolecules 1986, 19, 2313. (4) Sato, T.; Teramoto, A. Physica A 1991, 176, 72. (5) Onsager, L. Ann. N.Y. Acad. Sci. 1949, 51, 627. (6) Dong, X. M.; Kimura, T.; Revol, J.-F.; Gray, D. G. Langmuir 1996, 12, 2076. (7) Strzelecka, T. E.; Rill, R. L. Macromolecules 1991, 24, 5124. (8) Inatomi, S.; Jinbo, Y.; Sato, T.; Teramoto, A. Macromolecules 1992, 25, 5013. (9) Fraden, S.; Maret, G.; Casper, D. L. D.; Meyer, R. B. Phys. Rev. Lett. 1989, 63, 2068.

S0743-7463(96)00724-X CCC: $14.00

Properties of polyelectrolytes such as osmotic pressure,10 hydration,11 and ion-exchange properties12 are counteriondependent. In this paper, the effects of different counterions on the phase separation and on the stability of ordered suspension of cellulose will be examined. The effect of counterions on the redispersion of dried samples made from these suspensions will also be described. Experimental Section Materials. Aqueous solutions of sodium hydroxide and potassium hydroxide (0.5 M) were prepared from reagent grade samples (BDH Inc.). Aqueous solutions of cesium hydroxide (25%), ammonium hydroxide (30%), tetramethylammonium hydroxide (25%), tetraethylammonium hydroxide (35%), tetrapropylammonium hydroxide (1.0 M), tetra-n-butylammonium hydroxide (40%), and trimethylamine (25%) were purchased from Aldrich Chemical Co. and used as ∼0.5 M solutions by proper dilution with deionized water. Triethylamine was freshly distilled and diluted to give an 0.5 M aqueous solution before use. Sample Preparation and Measurement. Suspensions of cellulose crystallites were prepared by hydrolyzing Whatman No. 1 filter paper with sulfuric acid to give rodlike particles with average length 115 nm and width 7 nm (measured by transmission electron microscopy), as described in a previous paper.6 As a result of the treatment, negatively charged acid sulfate groups were formed on the surface of the cellulose crystallites to give a surface charge density of 0.15 e/nm2, thus stabilizing the colloidal suspensions.13 Suspensions associated with other monovalent counterions were prepared by neutralizing this acidform suspension with the corresponding base until its pH was neutral. The neutralization process was monitored using a pH meter (Accumet 50, Fisher Scientific) under moderate stirring. The suspensions made with this procedure were labeled S-H, S-Na, S-K, S-Cs, S-NH4, S-TMA, S-TEA, S-TPA, S-TBA, S-TriMA, and S-Tri-EA, representing suspensions with H3O+, sodium, potassium, cesium, ammonium, tetramethylammonium, tetraethylammonium, tetrapropylammonium, tetra-n-butylammo(10) Chu, P.; Marinsky, J. A. J. Phys. Chem. 1967, 71, 4352. (11) Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531. (12) Anderson, C. F.; Record, M. T. Annu. Rev. Biophys. Chem. 1990, 19, 423. (13) Marchessault, R. H.; Morehead, F. F.; Koch, M. J. J. Colloid Sci. 1961, 16, 327.

© 1997 American Chemical Society

Charged Rodlike Cellulose Crystallites nium, trimethylamine, and triethylamine as the counterions of the sulfate groups, respectively. In order to isolate the effect of counterions, all the suspensions were prepared in deionized water without adding any other electrolytes. The phase separation experiments were carried out as described previously.6 The eleven different counterions associated with cellulose crystallites prepared above can be divided into two classes. One is the univalent inorganic counterions, H+, Na+, K+, and Cs+. The other is the univalent organic counterions, which can be further classified as strong basic cations, TMA+, TEA+, TPA+, and TBA+, and weak basic cations, NH4+, -Tri-MA+, and -Tri-EA+ (NH4+ is included with the organic series because it is not hydrated). The actual experiments were carried out in three separate series: alkali cation series, weak basic organic cation series, and strong basic organic cation series. A replicate acid-form suspension was included with each of the three series as a standard for comparison. The experimental conditions were carefully controlled and were the same for each series of samples during the experimental measurements. The suspension concentrations, as weight percentages, were measured by weighing the suspension and the corresponding dried sample. The apparent translational diffusion coefficients of the cellulose suspensions were measured by photon correlation spectroscopy (PCS) using a BI-2030-AT digital correlator (Brookhaven Instruments Corp.) and a vertically polarized 50 mW He-Ne laser (Spectra-Physics). The experiments were carried out at a scattering angle of 45° and a temperature of 25.0 ( 0.1 °C. All the suspension samples were measured at a fixed concentration of 0.25 ( 0.01%. Prior to the sample preparation, the suspensions were treated with an ultrasound disrupter 350 (Branson Sonic Power Co.) and filtered through a 0.2 µm filter to remove coagulated particles. Dust-free deionized water was obtained by passing fresh deionized water through a 0.2 µm filter twice before use. The chiral nematic pitch of the suspensions was determined with a polarized microscope (Nikon Microphot-FXA) equipped with a camera. The suspensions were sealed in flat capillaries (path length 0.4 mm, from Vitro Dynamics Inc.) and kept at room temperature until an equilibrium was achieved before measurements. For the measurement of pitch as a function of temperature, the capillary was placed on a hot stage (Mettler PF 52) mounted in the microscope beam and was heated at a rate of 1 °C/min to a final temperature of 65 °C. When the desired temperature was reached, the sample was stabilized at this temperature for at least 30 min before measurement. Infrared spectra of the thin films from S-Na and S-H suspensions were recorded using a FT-IR spectrometer (Bruker IFS 48) equipped with a A590 microscope. The thin films were cast from diluted suspensions and dried under vacuum at 35 °C for three days. The sample was placed on a stage of NaCl crystal under the microscope, and the spectrum was recorded with 2000 scans.

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Figure 1. Plot of the anisotropic phase volume fraction, φ, versus the total suspension concentration for cellulose suspensions with inorganic counterions as indicated in the figure.

Figure 2. Plot of the anisotropic phase volume fraction, φ, against suspension concentration for the suspensions indicated in the figure.

Results and Discussion

Figure 3. Plot of the anisotropic phase volume fraction, φ, against total suspension concentration for various tetraalkylammonium salt-form suspensions.

Effect of Counterions on Phase Separation. The phase separation of cellulose suspensions with the eleven different species of counterions was examined. Phase separation occurred in all the suspensions, whether the counterion was organic or inorganic, and the anisotropic phases all displayed chiral nematic properties. Above a critical concentration of around 5 wt %, the suspensions separated into an isotropic upper layer and an anisotropic lower layer, with a sharp boundary clearly visible between the two phases. Increasing the total concentration resulted in an increase in the relative proportion of the lower chiral nematic phase. In this paper, the phase separation will be characterized by a parameter φ, the ratio of the volume of the anisotropic phase to the total volume of the suspension. The plot of φ against total suspension concentration for the alkali cation series is given in Figure 1. For the weak basic and strong basic organic cation series, the plots of φ against the total

suspension concentration are given in Figures 2 and 3, respectively. The critical concentrations for ordered phase formation and the values for φ are clearly counteriondependent. In the inorganic counterion series, the phase diagrams for the acid-form suspension and the three salt-form suspensions have similar shapes (Figure 1). As observed previously,6 a slight curvature at higher total suspension concentrations is evident for the acid-form suspensions. The acid-form suspension yields the highest volume of anisotropic phase at any given total suspension concentration. Therefore, it has the lowest critical concentration and the greatest tendency for ordered phase formation. The suspension with cesium counterions has the highest value for the critical concentration required to form an anisotropic phase (5.9%). There is only a small difference between S-Na and S-K, with S-Na having a slightly lower critical concentration. In this series, the tendency for

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Table 1. Critical Concentrations for Anisotropic Phase Formation with Different Counterionsa series 1

series 2

series 3

crit conc crit conc crit conc suspension (%) suspension (%) suspension (%) S-H S-Na S-K S-Cs

4.9 5.3 5.4 5.9

S-H S-NH4 S-Tri-MA S-Tri-EA

4.9 5.2 5.5 6.0

S-H S-TMA S-TEA S-TPA S-TBA

4.7 5.2 5.5 6.0 5.5

a Values are from extrapolation to φ ) 0 of the lines in Figures 1-3.

ordered phase formation is in the order S-H > S-Na > S-K > S-Cs. For weakly basic organic counterions, the results are similar to those of the inorganic counterion series. The dependence of phase separation on the counterion species is clearly demonstrated (Figure 2). As with the above series, the acid-form suspension has a lower critical concentration than the salt-form suspensions for anisotropic phase formation. Within the three salt forms, the critical concentration increases from 5.2% for S-NH4 to ∼6.0% for S-Tri-EA. The tendency for the ordered phase formation is S-NH4 > S-Tri-MA > S-Tri-EA. For the strongly basic tetraalkylammonium salt suspensions, the tendency toward anisotropic phase formation decreases as the alkyl chain length of the counterions increases from methyl to propyl group (Figure 3). However, with tetra-n-butylammonium counterions, the volume fraction of the anisotropic phase of S-TBA is higher than that of S-TPA and very close to that of S-TEA. Thus, in this series of counterions, the tendency for anisotropic phase formation is in the sequence S-TMA > S-TEA = S-TBA > S-TPA. The approximate critical concentrations for the formation of anisotropic phases was estimated from the experimental data in Figures 1-3 by extrapolating the linear portions of the curves below φ ) 0.5 to φ ) 0. The results are listed in Table 1. The major source of error is the difficulty of reproducing exactly the hydrolysis conditions from batch to batch of the suspension. A different batch was used for each series of counterions. From the results of the above three series, it seems that the tendency for anisotropic phase formation of cellulose suspensions varies in a systematic fashion with the nature of the counterion in each series. The nature of the forces acting between charged anisotropic colloidal suspensions remains controversial and not well understood, and any interpretation of these effects is speculative at this stage. Factors Influencing the Phase Equilibrium. The phase separation of rodlike polyelectrolytes is determined by interparticle forces such as steric repulsion, electrostatic repulsion, hydration, and hydrophobic interactions. Any change in these forces will result in a change in the equilibrium between anisotropic and isotropic phases. If the repulsive force increases, the effect is equivalent to an increase in apparent excluded volume of the particles, and thus a decrease in the critical concentration for anisotropic phase formation is expected. If the repulsive force between particles decreases, the critical concentration will increase. For all the suspensions in the above three series, the source of cellulose, the conditions of sulfation, and all the experimental procedures were carefully kept constant; only the counterions changed. The properties of the counterions should thus be responsible for the observed differences in phase separation. Counterion properties such as ion size, dissociation constant, hydration number, and hydrophilic/hydrophobic balance are expected to be of primary importance.

Table 2. Apparent Diffusion Coefficients Measured by Photon Correlation Spectroscopy for Two Series of Cellulose Crystallite Suspensions with Sulfate Surface Groups and Inorganic (Series 1) and Organic (Series 3) Counterionsa counterion (series 1)

Dt (×10-8 cm2/s)

counterion (series 3)

Dt (×10-8 cm2/s)

H+ Na+ K+ Cs+

4.4 4.7 5.0 5.1

H+ TMA+ TEA+ TPA+ TBA+

3.9 5.4 5.1 4.6 4.0

a The values with H+ counterions in each series indicate the reproducibility of the preparations.

For the inorganic cation series, the hydration number and the hydrated ion size decrease with increasing atomic number in the order Na+ > K+ > Cs+.14 When these hydrated cations bind to the negatively charged particle surface, hydration force will be generated between particles. For hydrophilic colloidal suspensions, such as cellulose crystallites, the hydration force is repulsive and generally increases with the hydration number of the counterions,15 so the repulsive hydration force is expected to be in the order S-Na > S-K > S-Cs. In highly charged polyelectrolyte systems, the distribution of counterions is not uniform. Some of the counterions may condense on the surface of polyion as bound ions, and some are far away from the surface, behaving like free ions. The interactions between the cellulose crystallites in this work will obviously be sensitive to the distribution of counterions. According to the Manning model,16 for a specific polyion with fixed charge density, the condensed portion is predicted to be (1 - ξ), where ξ is a parameter dependent on the linear charge density of the polyion. It is not clear whether the Manning model of the polyelectrolyte as an infinite line charge is appropriate for our cellulose crystallites, which are much thicker than molecular polyelectrolyte chains. The model also predicts that ξ is constant for all univalent counterion species and thus does not directly predict the observed differences for Na+, K+, and Cs+ counterions. To try and find more direct evidence that the effective particle size of the polyelectrolyte is reduced as the atomic number of the inorganic counterions increases, translational diffusion coefficients (Dt) of suspensions with different counterions were measured with photon correlation spectroscopy.17,18 The results are presented in Table 2. It should be noted that the values listed above were measured on salt-free polyelectrolye solutions, where longrange interactions between the crystallites will introduce a cooperativity in diffusion; the values for Dt are not true single particle diffusion coefficients. Salt-free conditions were used for both phase separation and diffusion measurements, and the apparent diffusion coefficients hopefully reflect the relative differences in hydrodynamic dimensions of the crystallites with different counterions. For the suspensions with monovalent alkali metal counterions (series 1), the apparent translational diffusion coefficients of the particles increase in the order H+ < Na+ < K+ < Cs+ Thus the hydrodynamic volume of the particles decreases from H+ to Cs+, presumably because (14) Davies, C. W. Ion Association; Butterworths: London, 1962; p 150. (15) Israelachvili, J. N. Intermolecular & Surface Forces, 2nd ed.; Academic Press Inc.: San Diego, CA, 1992; p 260. (16) Manning, G. J. Chem. Phys. 1969, 51, 924. (17) Schumacher, G. A.; van de Ven, T. G. M. J. Chem. Soc., Faraday Trans. 1991, 87, 971. (18) Polverari, M.; van de Ven, T. G. M. Colloids Surf., A 1994, 86, 209.

Charged Rodlike Cellulose Crystallites

Figure 4. Sketch illustrating the hydrophobic interaction in suspensions of cellulose crystallites associated with organic counterions.

of decreasing hydration of the countercations directly associated with the cellulose particles. Cellulose crystallites prepared by the same method as those described in Table 2, but dispersed in 0.01 M NaCl solution, were apparently more mobile, giving a value of 8.2 × 10-8 cm2/s for Dt. Thus, the changes in mobility resulting from changing the counterions associated with the crystallites mirror the change in isotropic/anisotropic phase equilibrium observed in Figure 1, where the tendency to form an anisotropic phase followed the order S-H > S-Na > S-K > S-Cs. Qualitatively, both effects may be related to changes in effective hydrodynamic dimensions; the phase separation of rodlike polyelectrolytes is directly related to the effective size of the particles. As the particle size decreases, their excluded volume is reduced, and a higher suspension concentration is required to form an ordered phase. The critical concentration should thus increase with the atomic number of the counterions. This is observed for the inorganic counterion series in Figure 1. The results for the organic counterion series are different from those for the alkali cation series. The organic counterions are presumed to have little or no hydration. The dynamic light scattering results for the suspensions with tetraalkylammonium counterions (Table 2, series 3) show a decrease in translational diffusion coefficient from tetramethyl- to tetra-n-butylammonium counterions. This mirrors the increasing size of the cations, and presumable it also mirrors the change in dimensions of the cellulose crystallites with which the cations are associated. However, the phase separation data (Figure 3) show that factors other than size must be important, because the suspension with the smallest cation (tetramethylammonium) forms an ordered phase at the lowest concentration of crystallites. In this case, increasing counterion size leads to a decreasing tendency to form an anisotropic phase, in contrast to the case for the inorganic counterions. A possible explanation may rest on the fact that the organic counterions have alkyl chains, resulting in a strong hydrophobic interaction between the organic portions.19 In aqueous suspensions of cellulose crystallites, the electrostatic repulsive forces may thus be reduced by an attractive contribution stemming from the hydrophobic interaction of the alkyl chains of the counterions, as sketched in Figure 4. The hydrophobic attraction force will increase with increasing hydrophobicity of the counterions. This implies that the attractive contribution to the force between particles with different organic counterions will be S-TBA > S-TPA > S-TEA > S-TMA and S-Tri-EA > S-Tri-MA > S- NH4. Thus, for the suspensions (19) Israelachvili, J. N.; Pashley, R. M. Nature 1982, 300, 341.

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associated with organic counterions, the interparticle forces are affected by the counterions in opposing directions. Both the steric repulsive force and the hydrophobic attractive force will increase with the size of organic counterions. When the size of the counterion increases, the increased repulsive force will enhance the excluded volume of particles. At the same time, the increased attractive component of the force will reduce the excluded volume of the particles. The balance between these two effects determines the direction of the change in total interparticle forces and hence the balance between anisotropic and isotropic phases. In the suspensions associated with organic counterions, except for the tetra-n-butylammonium suspension, it appears that hydrophobic interactions play a predominant role in the phase separation. As the counterion changes from NH4+ to trimethylammonium+ (Tri-MA) and triethylammonium+ (Tri-EA), or from tetramethylammonium+ (TMA) to tetraethylammonium+ (TEA) and tetrapropylammonium+ (TPA), the hydrophobic-hydrophobic attraction between counterions on the surface of cellulose crystallites increases, weakening the existing interparticle repulsive force of the suspensions and causing an increase in the critical concentration for ordered phase formation. However, when the counterion size increases from tetrapropylammonium+ to tetra-n-butylammonium+, the increase in steric repulsion appears to outweigh the increase in hydrophobic attraction. As a consequence, a lower critical concentration for phase transition is obtained for the suspension with tetra-n-butylammonium+ counterions. Hydrophobic interaction plays a central role in many surface phenomena, in micelle formation, in biological membrane structures, etc. In existing theories for the phase separation of rodlike species, the hydrophobic attraction force has not been considered. These experiments suggest that an extension of the current theories to include hydrophobic interactions might be worthwhile. Preliminary experiments on the influence of the divalent counterions Ca2+ and Ba2+ on phase separation showed that phase separation is much more sensitive to these cations than to univalent counterions. Although anisotropic phases did form under certain conditions, often either a gel or an isotropic phase was produced. Effect of Time and Temperature on the Stability of Suspensions. Suspensions of cellulose crystallites are quite stable at room temperature, no matter what type of counterion is associated with the sulfate groups on the cellulose surface. For a biphasic sample with both isotropic and anisotropic phases, the anisotropic phase volume fraction does not change on standing for at least a month. Since the phase separation of rodlike polyelectrolytes is very sensitive to particle size, ionic strength, and counterion species, etc., any change in the suspension will be reflected in the change of volume fraction of the anisotropic phase, φ. Thus, φ turns out to be a very good parameter to characterize the stability of suspensions. The stability of suspensions at 60 °C was checked by recording the change in φ with time. Six suspensions with different counterions were sealed in separate glass tubes and stored in an oven at 60 °C for up to 9 days. The results are presented in Figure 5. All the suspensions in salt form show very good stability. Only slight changes (less than 5%) are observed after 9 days. However, the acid suspension shows instability at 60 °C. The fraction of ordered phase, φ, decreased from 0.6 to 0.15 within the first 6 days, after which it stabilized at φ ) 0.15. The change in stability of acid suspensions may be explained as follows. As mentioned earlier, the suspensions of

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Figure 5. Stability of cellulose suspensions with different species of counterions at 60 °C: (×) S-Na (9.46%); (0) S-K (9.29%); (]) S-NH (9.37%); (4) S-TMA (7.45%); (O) S-TBA (7.87%); (b) S-H (7.60%).

Figure 6. Volume fraction of anisotropic phase, φ, as a function of temperature for acid-form suspensions with concentrations as indicated in the figure. Samples were held at the indicated temperature for 3 days.

Table 3. Change in the pH of Acid-Form Suspensions after Heating at 60 °C for 3 Days conc (% w/w)

pH (before heating)

pH (after heating)

∆pH

∆[H+] (mM)

10.53 9.30 8.78 7.35 6.47 5.40

2.11 2.17 2.19 2.27 2.32 2.41

1.99 2.02 2.08 2.17 2.21 2.28

-0.12 -0.15 -0.11 -0.10 -0.11 -0.13

2.47 2.78 1.86 1.39 1.38 1.36

cellulose crystallites are stabilized by the charged sulfate groups (OSO3-) on the surface of cellulose. When the counterions of the sulfate groups are protons, an acidcatalyzed desulfation reaction will occur, resulting in replacement of the charged sulfate ester groups on the crystallite surface by hydroxyl groups. The drop in surface charge and increase in ionic strength would destabilize the suspensions, so that the anisotropic phase volume fraction φ will be reduced, as shown in Figure 5. The desulfation of the acid form in pure water would generate sulfuric acid, which would lead to a decrease in the pH of the medium. The pH of the suspensions before and after heating at 60 °C was measured, and the results are listed in Table 3. Seven samples with different concentrations were examined. All the pH values decreased, and the acid concentrations, defined as ∆[H+], increased after heating. This is in accord with the occurrence of a desulfation reaction at 60 °C. At low temperatures, where the reaction rate is slow, the suspension appears stable. For the neutral salt-form suspensions, the desulfation reaction does not occur because of the absence of an acid catalyst. The apparent leveling off in the volume fraction of the anisotropic phase with time observed for the acid form at 60 °C may be due to the gelation of the suspension, which occurs at high ionic strengths and low surface charge. The gelation prevents observation of any further changes in the isotropic/anisotropic equilibrium. The effect of temperature on phase separation was examined for samples of S-H and S-Na suspensions. The samples were kept in a water bath at the desired temperature for 3 days before the measurement. From results for the phase separation of S-H samples (Figures 6), it seems that if desulfation is responsible for the change in phase composition, then the desulfation reaction becomes significant around 40-50 °C. In contrast to the S-H samples, for S-Na samples (Figure 7), as the temperature increased from 23 to 60 °C, the volume fraction of the anisotropic phase decreased gradually but only very slightly. Chiral Interactions. Another important property of the chiral nematic anisotropic phase is the chiral twisting

Figure 7. Volume fraction of anisotropic phase, φ, as a function of temperature for S-Na suspensions with concentrations indicated in the figure.

interaction, which is characterized by the chiral nematic pitch, P. The effect of temperature and counterion on the chiral nematic pitch was measured for salt-form suspensions of S-Na, S-TEA, and S-TPA. S-H was not included because of its instability at high temperatures. Usually for lyotropic chiral nematic liquid crystals, the pitch is very sensitive to temperature.20-22 However, for these salt-form suspensions of cellulose crystallites, the chiral nematic pitch remains more or less the same as the temperature is increased from 25 to 65 °C. The dependence of pitch on temperature is presented in Figure 8. These suspensions show a much lower sensitivity of pitch to temperature than molecular and polymeric chiral nematic phases, mirroring their lower temperature dependence of phase composition. Presumably, this is due to the relative insensitivity of ionic and steric interactions to temperature. Since long range chiral interactions must be occurring to generate chiral nematic phases in these relatively dilute suspensions, we attempted to measure the change of chiral nematic pitch in the presence of small chiral molecules. L-, D-, and DL-alanine and L-, D-, and DL-phenylalanine were used for the test. Anisotropic suspensions (9.12 wt %) were diluted with 0.05 M solutions of each amino acid to give 7.6 wt % biphasic suspensions cotaining 8.4 × 10-3 M amino acid. No significant difference in chiral nematic pitch was found between the samples mixed with L- and D-molecules as compared to the suspension mixed with the corresponding DL-racemic compounds. Obviously, (20) Guo, J. X.; Gray, D. G. Macromolecules 1989, 22, 2086. (21) Guo, J. X.; Gray, D. G. Liq. Cryst. 1995, 18, 571. (22) Leder, L. B. Chem. Phys. Lett. 1970, 6, 285.

Charged Rodlike Cellulose Crystallites

Figure 8. Temperature dependence of the chiral nematic pitch for the anisotropic phase of some suspensions. The total suspension concentrations (not anisotropic phase concentrations) are listed on the figure.

whatever the source of the chiral interactions between the cellulose crystallites, it is not influenced by interactions with dilute chiral molecules. Redispersion of Dried Suspensions. The chiral nematic structure of these suspensions may be retained when the suspensions are prepared and dried under appropriate conditions.2 Films with unusual and useful optical properties are produced if the chiral nematic pitch is of the order of the wavelength of visible light.23 The utilization of such suspensions may depend on their stability and their redispersability, so the redispersability of dried film samples made from suspensions with different counterions was examined. Samples of all eleven suspensions described above were dried on a Teflon sheet in a vacuum oven at 35 °C for 24 h. At this relatively low temperature, desulfation of the acid form suspension is negligible and can be ignored. For salt-form samples, the dried films swelled immediately after the water was added. Within 2 h, these samples became gel-like. With brief ultrasound treatment, these gels turned into homogeneous suspensions and separated into two phases after a few hours of standing at room temperature. The dried S-H film, however, did not redisperse in water. Even with strong ultrasound treatment, no homogeneous suspension could be formed. The FT-IR spectra of two dried films, one from S-H suspension and one from S-Na suspension, are shown in Figure 9. A difference spectrum based on the assumption that the C-H stretches are the same for both samples is also presented. Several fairly intense bands appear in the difference spectrum of the hydrogen-bonding region. The frequencies of the four peaks are 3438, 3402, 3307, and 3253 cm-1. Liang and Marchessault have published work on hydrogen bonding in native cellulose.24 They (23) Revol, J.-F.; Godbout, L.; Gray, D. G. Patents pending. (24) Liang, C. Y.; Marchessault, R. H. J. Polym. Sci. 1959, 37, 385.

Langmuir, Vol. 13, No. 8, 1997 2409

Figure 9. FT-IR spectra of cellulose films dried from dilute suspensions of cellulose crystallites S-H (A) and S-Na (B). Part C represents the difference spectrum (spectrum A - spectrum B).

assigned the peaks at 3307 and 3402 cm-1 to intermolecular hydrogen bonding in cellulose crystallites. By comparison with the case for other sulfates, the peak at 3438 cm-1 may be assigned to the stretch of sulfate hydrogen bonds (S-O-H). The intermolecular hydrogen bonding generated from cellulose backbones is much stronger in the S-H film than that in the S-Na film, while the intramolecular hydrogen bonding (assigned by Liang and Marchessault to the major peak at 3350 cm-1) is large but similar for both samples. It seems that the extra intermolecular hydrogen bonding between crystallites in S-H films may be responsible for the sample’s nonredispersability. Conclusions The phase separation of rodlike suspensions of cellulose crystallites stabilized by surface sulfate groups depends strongly on the nature of their counterions. For inorganic counterions, the critical concentration for ordered phase formation increases in the order S-H < S-Na < S-K < S-Cs. For organic counterions, the critical concentration depends on the balance of hydrophobic attraction and steric repulsion. The nature of the counterions also influences other properties of the suspensions, such as their stability, the temperature dependence of the phase separation and of the chiral nematic pitch, and the redispersability of dried samples made from the suspensions. Acknowledgment. J.-F. Revol provided critical comments and useful suggestions. We thank the Natural Sciences and Engineering Research Council of Canada for support, M. Polverari for helping with the PCS instrument, and Soo Hang Wong for experimental assistance. X.M.D. thanks the Pall Corporation and the Government of Quebec for scholarships. LA960724H