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Langmuir 2003, 19, 24-29
Translational and Rotational Dynamics of Rodlike Cellulose Whiskers M. M. De Souza Lima,†,‡ J. T. Wong,§ M. Paillet,# R. Borsali,*,† and R. Pecora§ LCPO-CNRS-ENSCPB, Bordeaux University (UMR 5629), 16 Avenue Pey Berland, 33607 PESSAC Cedex, France, UEM-DFF, Department of Pharmacy and Pharmacology, Maringa´ State University, CEP 87020-900 Maringa´ , Parana´ , Brazil, Chemistry Department, Stanford University, Stanford, California, and CERMAV-CNRS, Joseph Fourier University, B. P. 53, F-38041 Grenoble Cedex, France Received May 20, 2002. In Final Form: September 17, 2002 The dynamic properties of suspensions of rodlike cellulose microcrystallites (whiskers) are investigated using polarized and depolarized dynamic light scattering (DLS, DDLS) and transient electric birefringence (TEB). The whiskers were prepared by controlled sulfuric acid hydrolysis from cotton and tunicate cellulose fibers. Translational (D) and rotational (Θ) diffusion coefficients in dilute suspensions were measured. For the cotton system, D ) 5.5 × 10-8 cm2/s, ΘDDLS ) 552 s-1, and ΘTEB ) 536 s-1. With the use of Broersma’s relations, the rotational and translational diffusion coefficients lead to values of the length L ) 255 nm and the cross-section diameter d ) 15 nm. For the tunicate-derived whiskers, D ) 1.5 × 10-8 cm2/s, ΘDDLS ) 11 s-1, and ΘTEB ) 14 s-1. These values and Broersma’s relations give L ) 1160 nm and d ) 16 nm. The DDLS and TEB techniques are in good agreement for the rotational diffusion coefficients, especially for the cotton whiskers. These results provide validation for using DDLS to study the dynamics of cellulose whisker systems in both simple suspensions and complex environments. Cellulose whiskers may thus be used as model rigid rods dispersible in aqueous suspensions the structure and dynamics of which may be readily studied by widely available techniques.
Introduction Cellulose whiskers are obtained from natural cellulose fibers such as cotton, wood, and tunicate (a marine animal from the Mediterranean sea).1,2 Starting from the raw material and after successive chemistry steps and, ultimately, controlled sulfuric acid hydrolysis, the cellulose whisker microcrystals are suspended in aqueous media.3-8 The crystallinity and dimensions of these crystallites depend on the origin of the cellulose fibers, as well as on details of the preparation method.9-13 For the cotton whiskers, electron microscopy gives averaged imensions of about 10 nm in diameter and 300 nm in length and about 15 nm in diameter and 2000 nm in length for the * To whom correspondence should be addressed. E-mail:
[email protected]. † Bordeaux University. ‡ Maringa ´ State University. § Stanford University. # Joseph Fourier University. (1) Ranby, B. G. Discuss. Faraday Soc. 1951, 11, 158. (2) Ott, E.; Spurlin, H. M. High Polymers - Cellulose and Cellulose Derivatives; Interscience: New York, 1963; Vols. I and II. (3) Marchessault, R. H.; Morehead, F. F.; Walter, N. M. Nature 1959, 184, 632. (4) Battista, O. A.; Smith, P. A. Ind. Eng. Chem. 1962, 54, 20. (5) Favier, V.; Chanzy, H.; Cavaille´, J. Y. Macromolecules 1995, 28, 6365. (6) Dong, X. M.; Kimura, T.; Revol, J. F.; Gray, D. Langmuir 1996, 12, 2076. (7) Ebeling, T.; Borsali, R.; Paillet, M.; Diat, O.; Cavaille´, J. Y.; Chanzy, H.; Dufresne, A. Langmuir 1999, 15 (19), 6123. (8) De Souza Lima, M. M.; Borsali, R. Langmuir 2002, 18, 992. (9) Gardner, K. H.; Blackwell, J. Biopolymers 1974, 13, 1975. (10) Battista, O. A. Microcrystalline Polymer Science; McGraw-Hill: New York, 1975. (11) Chanzy, H.; Imada, K.; Vuong R. Protoplasma 1978, 94, 299. (12) Chanzy, H. In Cellulose Sources and Exploitation; Kennedy, J. F., Philips, G. O., Williams, P. A., Eds.; Ellis Horwood: New York, 1990; p 3. (13) Neville, A. C. Biology of Fibrous Composites. Development Beyond the Cell Membrane; Cambridge University Press: Cambridge, U.K., 1993.
tunicate.5,8,14 These nonflocculating charged rod-shaped particles in aqueous suspensions display chiral nematic (cholesteric) order above a critical concentration.15-17 Recent work on such particles using small angle X-ray scattering (SAXS) under shear7 and light scattering18,8 emphasized, among other properties, the polyelectrolyte nature of these rodlike whiskers.8,19 These microcrystallites also have been added to polymer matrixes to form nanocomposites with improved mechanical properties. Examples include poly(β-hydroxyoctanoate),20 starch,21 and more recently poly(vinyl alcohol)- and poly(vinyl) acetate-based films.22 Because of their interesting properties, much attention has been given to industrial applications,5,21-24 among them optics and cosmetics. These whiskers also constitute an excellent model system for conducting well-controlled experiments on the dynamics and structure of rodlike particles in various complex environments. The extreme anisotropy (in shape and optical properties) that is characteristic of these rodlike whiskers makes them (14) Dufresne, A.; Kellerharls, M. B.; Witholt, B. Macromolecules 1999, 32, 7396. (15) Revol, J.-F.; Bradford, H.; Giasson, J.; Marchessault, R. H.; Gray, D. G. Int. J. Biol. Macromol. 1992, 14, 170. (16) Revol, J.-F.; Godbout, L.; Dong, X.-M.; Gray, D. G. Chanzy, H.; Maret, G. Liq. Cryst. 1994, 16, 127. (17) Araki, J.; Wada, M.; Kuga, S.; Okano, T. Langmuir 2000, 16, 2413. (18) Bica, C. I. D.; Borsali, R.; Geissler, E.; Rochas, C. Macromolecules 2001, 34, 5275. (19) Dong, X. M.; Kimura, T.; Revol, J. F.; Gray, D. Langmuir 1996, 12, 2076. (20) Dubief, D.; Samain, E.; Dufresne, A. Macromolecules 1999, 32, 5765. (21) Angles, M. N.; Dufresne, A. Macromolecules 2001, 34, 2921. (22) De Souza Lima, M. M.; Carlotti, S.; Ibarboure, E.; Borsali, R., in preparation, 2002. (23) Favier, V.; Canova, G. R.; Cavaille´, J. Y.; Chanzy, H.; Dufresne, A.; Gauthier, C. Polym. Adv. Technol. 1995, 6, 351. (24) Revol, J. F.; Godbout, L.; Gray, D. G. J. Pulp Pap. Sci. 1998, 24, 5, 146.
10.1021/la020475z CCC: $25.00 © 2003 American Chemical Society Published on Web 12/02/2002
Translational and Rotational Dynamics
Langmuir, Vol. 19, No. 1, 2003 25
good candidates for study by dynamic depolarized light scattering (DDLS), a technique that has been extensively applied to the study of relatively small molecules but has not been used much for macromolecules because of the relatively weak depolarized signals from most macromolecular systems. In light scattering experiments, the incident light beam is usually vertically polarized relative to the scattering plane. The scattered light often contains both vertical and horizontal (in the scattering plane) contributions, which can be separately measured using analyzing prisms.25,26 In DDLS, the time fluctuations of the horizontal (depolarized) component, which is very intense for the cellulose whiskers, are studied. In this paper, we use polarized dynamic light scattering (vertical component) (DLS) to obtain translational diffusion coefficients of cotton and tunicate whiskers in dilute suspensions and DDLS to obtain the rotational diffusion coefficients.27 We also use transient electric birefringence decay (TEB)28 to provide a complementary measure of the rotational diffusion coefficients for comparison with those obtained from DDLS.29 These methods characterize the whiskers in suspension, avoiding any artifacts that may occur when drying and placing them on surfaces, as is required in many imaging techniques. In addition to the characterization, we demonstrate that these rigid, rodlike systems are relatively easy to study in situ using wellestablished scattering and birefringence techniques. Experimental Section 1. Sample Preparation. Aqueous suspensions of cotton whiskers were prepared from filter paper Whatman no. 1 following the procedure of Dong et al.19,30 The suspensions of tunicate whiskers obtained from the tunics of Microscosmus fulcatus were prepared according to the procedure of Wise et al.31 The acid hydrolysis conditions were optimized by varying the time and the temperature of hydrolysis as previously described.8 The final aqueous suspensions of cotton and tunicate whiskers neither precipitate nor flocculate. Dilution to the desired concentrations was done by addition of deionized-distilled water to the initial aqueous suspensions. The samples were equilibrated for a few days prior to performing the dynamic light scattering and TEB experiments. 2. Dynamic Light Scattering. The polarized and depolarized dynamic light scattering measurements were carried out using an argon ion laser as the light source and an ALV 5000 autocorrelator to compute the scattered light electric field time autocorrelation functions. The relaxation times that characterize the suspension dynamical behavior were found using the standard data analysis program CONTIN32 in terms of a continuous distribution of exponential decay times. The intensity autocorrelation function is directly measured. The intensity autocorrelation function is related to the scattered electric field autocorrelation function. Assuming Gaussian statistics for the scattered field, the normalized second-order (intensity) autocorrelation function is related to the normalized first-order (electric field) autocorrelation function by the Siegert relation.33
g2(t) ) 1 + β|g1(t)|2
(1)
where β is a spatial coherence factor dependent upon the geometry of the detection system. Often g1(t) may be expressed as (25) Berne, B. J.; Pecora, R. Dynamic Light Scattering: With Applications to Chemistry, Biology and Physics; Dover: New York, 2000. (26) Pecora, R. J. Chem. Phys. 1968, 48, 4126. (27) Zero, K.; Pecora, R. In Dynamic Light Scattering; Pecora, R., Ed.; Plenum: New York, 1985; p 59. (28) Tsvetkov, V. N.; Andreeva, L. N. Adv. Polym. Sci. 1981, 39, 95. (29) Pecora, R. J. Chem. Phys. 1968, 50, 2650. (30) Dong, X. M.; Revol, J. F.; Gray, D. Cellulose 1998, 5, 19. (31) Wise, L. E.; Murphy, M.; d’Addiecco, A. A. Pap. Trade J. 1946, 122, 35. (32) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 213, 219. (33) Siegert, A. J. MIT Radiat. Lab. Rep. 1943 (no. 465).
g1(t) )
∫
∞
0
dΓ G(Γ) exp(-Γt)
(2)
where g1(t) is the Laplace transform of the decay rate distribution function G(Γ) that gives the relative intensity of light scattered with a frequency Γ. The form of g1(t) depends on the system under consideration. For a dilute solution of spherical, monodisperse, optically isotropic particles undergoing Brownian motion, G(Γ) may be represented by a single exponential:
g1(t) ) exp(-q2Dt)
(3)
where D is the translational diffusion coefficient and q, the magnitude of the scattering wavevector, is related to n, the refractive index of the medium, λ, the wavelength in a vacuum of the light, and θ, the scattering angle: q ) (4πn/λ)(sin θ/2). For rodlike particles suspended in a liquid, g1(t) is somewhat more complex. A short discussion of generalizations of eq 3 for rodlike particles and the depolarized component is given in the Results and Discussion section. All light scattering experiments were performed at 25 °C. 3. Transient Electrical Birefringence. The birefringence apparatus used in this work is similar to that used by Phalakornkul, Gast, and Pecora34 to study rotational dynamics of synthetic polymers. A few modifications to their apparatus and data acquisition procedures have been made. A Spectra-Physics series 2000 argon ion laser operating in the power-regulated mode at λ ) 488 nm is the light source. The head and neck of the electrode house a calibrated thermistor (encased in glass) used to monitor the temperature of the sample accurately. The sample cell is injected into a copper cylinder that has temperaturecontrolled water running through it to provide constant sample temperature. Thus, the temperature is constantly monitored by the thermistor and controlled to (0.1 °C by the temperature bath. All TEB experiments were performed at 20 °C, and the results were viscosity- and temperature-corrected to 25 °C to be compared to DLS and DDLS data. The amplification and data storage systems are also different from the ones used earlier by Phalakornkul et al.34 For instance, the resulting voltage is initially amplified (maximum gain ) 10) by an EG&G Ortec 9301 fast preamplifier. The amplified signal is then attenuated by an amplifier/discriminator. This signal is digitized by a Tektronix TDS 220 digitizing oscilloscope. The TDS 220 has an 8-bit vertical resolution, a 1 GS/s sample rate, and 2500 data points plot capability. The light intensity data, I(t), were converted to birefringence34,35 signals by
∆I(t) ) Iβ
[
sin2 β -
]
πl ∆n(t) - sin2 β λ0 2
sin β + Kex
(4)
where ∆I(t) is the relative change in the intensity due to the birefringence of the suspension, Iβ is the nonretarded scattering light intensity, and β, the angle between the analyzer and its crossed position, is adjusted during the experiment to achieve the optimal signal-to-noise ratio. ∆n(t) is the temporal change in the sample birefringence. Kex is the extinction ratio of the complete system. In the study of decaying birefringence signals, the time t starts after the field is turned off. When β ) 0, for a dilute monodisperse system of rods undergoing rotational diffusion with rotational diffusion coefficient Θ, eq 4 can be approximated by
I(t) ≈ I(0) exp(-12Θt)
(5)
When β is smaller than 1 but much larger than half of the steadystate retardation, the intensity is directly proportional to the birefringence (34) Phalakornkul, J. K.; Gast, A. P.; Pecora R. Macromolecules 1999, 32, 3122. (35) Elias, J.; Eden, D. Macromolecules 1981, 14, 410.
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Langmuir, Vol. 19, No. 1, 2003 I(t) ≈ I(0) exp(-6Θt)
De Souza Lima et al. (6)
For polydisperse systems, eqs 5 and 6 become sums of exponentials with each exponential decaying at a rate determined by the rotational diffusion coefficient of the species giving rise to it. Effective rotational diffusion coefficients were obtained from CONTIN analysis of the experimental decay data32 as was done in the dynamic light scattering experiments. The chosen distributions were reported in terms of the relaxation frequencies Γ. The TEB decay function is in fact proportional to the DDLS first-order time autocorrelation function evaluated at zero wavevector (q ) 0). Further details of the TEB apparatus and technique are given elsewhere.34 In TEB experiments, the pulse width must be chosen so that the slowest mode has time to “orient” while the electric field is on.36-38 The field strength is chosen as the lowest magnitude field that gives birefringence decays of good signal-to-noise quality. For the cotton system, the field strength used was 75 kV/m and the experiments were done with two pulse widths of 30 and 50 µs. For the tunicate system, the field strength is 100 kV/m and the pulse widths were 200 and 300 µs. The pulse frequency was 1 rectangular pulse per second. This pulse rate was slow enough to allow the system to return to its equilibrium state after each excitation by the short pulses.
Figure 1. Autocorrelation functions measured at the angles from top to bottom: 40°, 50°, 70°, and 120° for cotton whisker suspensions at a concentration of 0.1 wt % in IVV detection.
Results and Discussion For monodisperse cylindrically symmetric molecules or particles that are not too long,25,39,40 the polarized (IVV) and depolarized (IVH) light scattering first-order time correlation functions in dilute solution are related to the rotational (Θ) and translational (D) diffusion coefficients of the molecule by
4 〈N〉Raniso2 45 exp[-(q2D + 6Θ)t] (7)
IVV(q,t) ) 〈N〉Riso2 exp(-q2Dt) +
IVH(q,t) )
1 〈N〉Raniso2 exp[-(q2D + 6Θ)t] 15
Figure 2. Slow (b) and fast (O) relaxation modes for the cotton whiskers at concentration 0.2 wt % in IVV detection.
(8)
where 〈N〉 is the average number of particles in the scattering volume, Riso is the isotropic part of the polarizability tensor, and Raniso is the molecular optical anisotropy. IVV and IVH in eqs 7 and 8 are proportional to the measured normalized g1(t) in eq 1. For long particles (approximately qL > 5), more terms containing both the translational and rotational diffusion coefficients must be added to eqs 7 and 8. These extra terms usually relax faster than the leading terms given above.39,40 They could be important for the tunicate observed at large q; however, the results below indicate that ignoring these terms does not cause serious errors in the sizes derived for the whiskers. It also appears that the second term on the right-hand side of eq 7 is not important. We should note that polydispersity (different sized whiskers or whisker aggregates in the suspension) would add terms that appear like those in eqs 7 and 8 but with different rotational and translational diffusion coefficients. One of our objectives here is to determine the validity of ignoring many of these complications in the analysis of the data.8 Figure 1 shows the polarized DLS autocorrelation functions (IVV geometry) at several scattering angles for (36) Lewis, R. J.; Pecora, R.; Eden, D. Macromolecules 1986, 19, 134. (37) Matsumoto, M. J. Chem. Phys. 1992, 96, 4750. (38) Bowers, J. S.; Prud’homme, R. K. J. Chem. Phys. 1992, 96, 7135. (39) Aragon, S. A.; Pecora, R. J. Chem. Phys. 1985, 82, 5346. (40) Claire, K. C.; Pecora, R. J. Phys. Chem. B 1997, 101, 746.
Figure 3. Slow (b) and fast (O) relaxation mode for the tunicate whiskers at concentration 0.01 wt % in IVV detection.
suspensions of cotton whiskers at a concentration of 0.1 wt %. The solid lines are the best CONTIN fits.32 Two relaxation modes (which we call here fast and slow) are observed in the autocorrelation functions for both the cotton and tunicate whiskers. Figure 2 shows the variation of the extracted frequencies Γ according to eq 2 as a function of q2 for the cotton whiskers at a concentration of 0.2 wt %, and Figure 3 shows those for the tunicate whiskers at c ) 0.01 wt %. In both cases, the slow mode is the dominant mode, representing 90% or more of the scattered intensity. All relaxation modes observed in the polarized DLS are diffusive, that is, the relaxation frequencies vary as q2.
Translational and Rotational Dynamics
Figure 4. Translational diffusion coefficients D measured by DLS versus concentration C for the cotton whiskers: the slow (b) and fast (O) modes.
Figure 5. Translational diffusion coefficients D measured by DLS versus concentration C for the tunicate whiskers: the slow (b) and fast (O) modes.
The effective translational diffusion coefficients, Di ) Γi/q2 associated with these modes are plotted in Figures 4 and 5 as functions of the whisker concentrations. The values extrapolated to infinite dilution are shown in each case. In this range of concentration, the D associated with the slower mode (which we identify below with the diffusion of whiskers close to the average size) is independent of the concentration. The effective rotational diffusion coefficients for these rodlike whiskers were determined by DDLS (IVH component) and TEB. Figure 6 shows the DDLS autocorrelation functions at several scattering angles for the tunicate whiskers at a concentration of 0.073 wt %. For both the cotton and tunicate whiskers, CONTIN analysis of the autocorrelation functions shows two modes. As we found for polarized DLS, the slower mode represents 90% or more of the correlation function. Figures 7 and 8 show, respectively, the variation of the frequencies Γ as functions of q2 for the cotton and tunicate systems. Both relaxation frequencies approach positive, nonzero values as q goes to 0. According to eq 8, these extrapolated values give 6Θ. Effective translational diffusion coefficients may also be obtained from the slopes of the lines in Figures 7 and 8. These diffusion coefficients are essentially the same as those we obtained above from the polarized DLS autocorrelation functions. This illustrates that the depolarized signals for the whiskers are, unlike those for most polymers, intense enough in dilute suspensions to obtain relatively precise values of the translational diffusion coefficients.
Langmuir, Vol. 19, No. 1, 2003 27
Figure 6. Autocorrelation function measured at the angles from top to bottom: 60°, 90°, and 120° for tunicate whisker suspensions at concentration 0.073 wt % in IVH detection.
Figure 7. Slow (b) and fast (O) relaxation modes for the cotton whiskers at concentration 0.2 wt % in IVH detection.
Figure 8. Slow (b) and fast (O) relaxation modes for the tunicate whiskers at concentration 0.01 wt % in IVH detection.
The effective rotational diffusion coefficients measured by DDLS are plotted in Figure 9 for the cotton and in Figure 10 for the tunicate whiskers. The slow mode in each case is independent of the concentration in the range considered. TEB also shows two decay modes for each system. The rotational diffusion coefficients obtained from the dominant slow mode, as well as the relative slow mode amplitudes, are shown in Figure 11 for the cotton and in Figure 12 for the tunicate whiskers. Like the DLS results, the slow mode represents most of the decay.
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Langmuir, Vol. 19, No. 1, 2003
Figure 9. Slow (b) and fast (O) relaxation modes for the cotton whiskers at several concentrations, measured by DDLS.
De Souza Lima et al.
Figure 12. Rotational diffusion coefficient Θ (O) versus concentration for the tunicate whiskers measured by TEB and amplitude (b) of the slow mode obtained from CONTIN analysis. Table 1. Translational and Rotational Diffusion Coefficients Measured by Polarized and Depolarized DLS and TEB for the Cotton and Tunicate Whiskers and Dimensions Obtained from Them Using the Broersma Relationsa D (cm2/s) DLS
Θ (s-1) TEB
slow mode fast mode
Cotton Whiskers 5.5 × 10-8 552 536 4.6 × 10-7
slow mode fast mode
Tunicate Whiskers 1.5 × 10-8 11 14 1.8 × 10-7 467 687
a
Figure 10. Slow (b) and fast (O) relaxation modes for the tunicate whiskers at several concentrations, measured by DDLS.
Θ (s-1) DDLS
L (nm)
d (nm)
255
1160 230-270
15
16 16
Error estimates are (10% for the diffusion coefficients.
and the solvent viscosity, η. The translational diffusion coefficient is
D ) (kbT/(3πηL))[δ - (1/2)(γ| + γ⊥)]
(9)
with
δ ) ln(2L/d) γ| ) 0.807 + 0.15/δ + 13.5/δ2 - 37/δ3 + 22/δ4 γ⊥ ) -0.193 + 0.15/δ + 8.1/δ2 - 18/δ3 + 9/δ4 The rotational diffusion coefficient is given by
Θ ) (3kbT/(πηL3))(δ - ξ)
(10)
where Figure 11. Rotational diffusion coefficient Θ (O) versus concentration for the cotton whiskers measured by TEB and amplitude (b) of the slow mode obtained from CONTIN analysis.
Table 1 presents a comparison of the translational and rotational diffusion coefficients, v, determined experimentally by polarized DLS, DDLS, and TEB. We note the good agreement between the rotational diffusion coefficients obtained by TEB and DDLS for the slow mode. Broersma41-43 has given equations relating the translational and rotational diffusion coefficients of a long rigid rod in dilute suspension to the rod length, L, the ratio of the rod length to the cross-sectional diameter, d, (41) Broersma, S. J. J. Chem. Phys. 1960, 32, 1626. (42) Broersma, S. J. J. Chem. Phys. 1960, 32, 1632. (43) Broersma, S. J. J. Chem. Phys. 1981, 74, 6989.
ξ ) 1.14 + 0.2/δ + 16/δ2 - 63/δ3 + 62/δ4 The translational and rotational diffusion coefficients in Table 1 may be used with eqs 9 and 10 to find the whisker length and diameter. For the cotton whiskers, the best fit to the slow mode rotational and translational diffusion coefficients gives L ) 255 nm and d ) 15 nm. For the tunicate whisker, we find L ) 1160 nm and d ) 16 nm from the slow modes. The dimensions obtained from the slow modes correspond approximately to the average size of the whiskers as measured by transmission electron microscopy.5,8,14 The faster modes are likely due to sample polydispersity. They appear to correspond to relatively short whisker fragments that CONTIN resolves as a separate relaxation mode. In the case of the tunicate whiskers, dimensions obtained applying the Broersma relations to the fast mode give
Translational and Rotational Dynamics
L ) 230-270 nm and d ) 16 nm. For the cotton whiskers, the Broersma relations along with the fast mode translational diffusion coefficient give small L/d (between 1 and 2), a range in which the Broersma relations are no longer valid. It is not possible to simultaneously obtain dimensions that give both the translational and rotational diffusion coefficients of the fast modes for the cotton whiskers. Conclusions We have studied the translational and rotational dynamics of two rodlike systems in aqueous suspensions, cotton and tunicate whiskers having different dimensions. We have used polarized and depolarized dynamic light scattering and TEB to probe their dynamical behavior. In the range of investigated concentrations, we observed two relaxation times (fast and slow) in each technique. The slow mode corresponds to the motion (translation and rotation) of the average size whisker. The Broersma model dimensions are L ) 255 nm and d ) 15 nm for the cotton
Langmuir, Vol. 19, No. 1, 2003 29
whiskers (ratio L/d ) 17) and L ) 1160 nm and d ) 16 nm (ratio L/d ) 72.5) for the tunicate whiskers. The rotational diffusion coefficients extracted from DDLS and those from TEB are in good agreement. The whiskers are relatively easy to prepare, are dispersible in water, can be made in a variety of sizes, and can be used in composite liquids. The electric charge on the whiskers may also be varied so that polyelectrolyte effects may be studied. They, in addition, are amenable to study by readily available techniques. They constitute a versatile model system for investigating the structure and dynamics of rigid rodlike particles in suspensions. Acknowledgment. This work was supported by NATO Collaborative Linkage Grant 979044. M. Miriam de Souza Lima thanks CAPES-Brazil for financial support during her stay in France. R.B. acknowledges financial support from CNRS, Re´gion Aquitaine, and FEDER. LA020475Z
