Deuterium Nuclear Magnetic Resonance Study of the Nematic Phase

J. Phys. Chem. B 1999, 103, 5427-5433

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ARTICLES Deuterium Nuclear Magnetic Resonance Study of the Nematic Phase of Vanadium Pentoxide Aqueous Suspensions O. Pelletier, P. Sotta,* and P. Davidson Laboratoire de Physique des Solides, UniVersite´ Paris-Sud (CNRS UMR 8502), Baˆ t. 510, 91405 Orsay Cedex, France ReceiVed: NoVember 23, 1998; In Final Form: April 21, 1999

This article describes deuterium NMR experiments on nematic single domains of vanadium pentoxide suspensions in D2O. Despite V2O5 volume fractions as low as 0.7%, the spectra are well-resolved quadrupolar doublets, characteristic of a uniaxial symmetry. We show that a simple model for the behavior of water in these suspensions accounts for the value of the splitting and for its concentration dependence. In this way, we obtain information on the interaction between water molecules and V2O5 colloidal ribbons. Then, the solvent (D2O) is used as a probe to study the reorientation time of the nematic phase. We show that this technique is sensitive enough to distinguish between reorientation through a transient instability or through a homogeneous rotation. In this latter case, we measure the characteristic time of reorientation τ in a magnetic field, as a function of volume fraction and temperature. We show a very strong temperature dependence of τ, which contrasts with the athermal phase diagram of this system.

1. Introduction Mineral lyotropic liquid crystals are interesting systems because they may combine the properties of mesophases (anisotropy and fluidity) with those of mineral materials (electronic properties such as magnetism or enhanced conductivity). Also, they may provide valuable systems to test different models of liquid crystal ordering. Examples of such phases are still rather scarce,1 even though one of them, namely, vanadium pentoxide (V2O5) aqueous suspensions, was discovered long ago.2 More recently, these suspensions have been extensively studied,3-5 and their phase diagram as a function of temperature and concentration has already been characterized, essentially by optical and structural methods (small-angle X-ray scattering, SAXS). V2O5 suspensions are made of highly anisotropic and fairly rigid V2O5 ribbons dispersed in water. The average dimensions of these objects have been evaluated at room temperature using electron microscopy and SAXS. Their persistence length is roughly 300 nm, whereas their overall contour length is not precisely known but is probably not much larger than their persistence length. Their width is about 20 nm, and their thickness is 1 nm. This latter dimension is well-defined because it results from the structural arrangement of the VO5 pyramids which form the ribbons. In contrast, both contour length and width may be subject to some polydispersity. Due to acid dissociation of surface V-O-H groups in water, the ribbons bear a rather large linear electrical charge density of approximately 0.5 e/Å. At the V2O5 volume fractions used in most experiments, the pH of the suspensions lies around 3, which means that the lowest ionic strength achievable for this system * Author for correspondence (e-mail: [email protected]).

is still rather high (I ∼ 10-3 mol). Thus, electrostatic interactions are partially screened (the Debye length is ∼10 nm) and can be modeled by a repulsive hard core potential to a good approximation. Let us now describe the phase diagram of the system (Figure 1). At high dilutions, these dispersions form an isotropic phase. Then, on increasing the V2O5 concentration, the system undergoes a strongly first-order isotropic/nematic phase transition with a biphasic region extending over the range Φ ) 0.50.7% (where Φ is the V2O5 volume fraction). It should be noted that such low values of Φ at the transition are rather uncommon. Besides, the nematic order parameter S was measured by SAXS at the transition: S ) 0.75 ( 0.05. Both the position and width of the biphasic region and this value of S agree with the predictions of the Onsager theory of the nematic/isotropic transition, provided that electrostatic interactions are properly taken into account.6,7 This suggests that excluded volume is the dominant interaction and also that polydispersity as well as flexibility should be rather low. The fact that temperature has little influence on the phase boundaries is another strong hint that the description in terms of hard particles is appropriate. Despite their highly biaxial shape, V2O5 ribbons form a uniaxial nematic phase at these concentrations. Indeed, the average distance d between ribbons, determined by SAXS, follows d ∝ Φ-0.5, which is the typical behavior of nematic phases of rods. The average distance between ribbons being larger than their width, they can be thought of as rotating freely around their long axes. At a higher volume fraction (Φ ∼ 1-2%), the fluid nematic phase turns into a viscoelastic weak physical gel which still has a nematic organization. The mechanism of this sol/gel transition is not precisely known so far. When the volume fraction Φ exceeds 4%, the ribbons cannot rotate freely around

10.1021/jp9845165 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/15/1999

5428 J. Phys. Chem. B, Vol. 103, No. 26, 1999

Pelletier et al.

Figure 1. Phase diagram of the binary system V2O5/nH2O.

their long axes any more and they seem to organize in a biaxial nematic phase.8 Finally, on further drying, a hydrated powder (V2O5‚1.8H2O), called xerogel, is obtained. This powder exhibits a layered structure and is known as a classical intercalation compound.3 Other studies have focused on the behavior of this mineral nematic phase in a magnetic field.9 It is indeed possible to orient the suspensions in the concentration range where their viscosity is not too high (in a 2 T magnetic field, the alignment can be achieved for 0.7% < Φ < 1%). Single domains are easily grown within times ranging from minutes to hours, depending on the volume fraction. As already observed for other polymeric liquid crystals, hydrodynamic instabilities occur during the reorientation of these single domains in the magnetic field, resulting in the appearance of transient striped textures. More surprisingly, the anisotropy of the diamagnetic susceptibility is actually so small that, up to now, it has proved undetectable. The fact that the mesophase aligns in the magnetic field therefore illustrates the collective nature of the nematic ordering. Complementary to structural studies, deuterium NMR is used here to correlate phase transitions to changes in the dynamic properties at a microscopic scale. First, we show the influence of the nematic organization and symmetry on the solvent behavior (D2O). This gives information about the interactions between the ribbons and the solvent molecules. Then, water can be used as a very simple probe to study the behavior of the nematic phase in a magnetic field. The essential property used here is the possibility to produce nematic single domains in a magnetic field. Moreover, the reorientation time of the phase, upon a sudden change of magnetic field direction, varies drastically as a function of concentration and temperature. The opportunity to work with single domains of nematic suspensions of particles of well-characterized shape should allow one to draw meaningful comparisons with microscopic models. This article is organized as follows: the sample preparation and NMR experimental setups are described in section 2. In section 3, the orientation of water molecules at equilibrium is investigated and their interactions with the V2O5 ribbons are discussed. Section 4 describes the dynamics of reorientation of the nematic phase. Then, these experimental results are compared with the predictions of a microscopic model for the dynamics of a suspension of rigid rods.10 2. Sample and Experimentals 2.1. Sample Preparation. Concentrated samples were synthesized according to a well-documented procedure.5 At this stage, the V2O5 volume fraction is a few percent. The samples were then diluted 10-100 times in deuterated water (EurisoTop, 99.9% 2H), to obtain an almost complete deuteration of the solvent by an exchange process. Deuterated samples of the required concentrations were then prepared by slow evaporation. This simple process is well-suited to produce the fluid samples

Figure 2. Deuterium NMR spectrum obtained on deuterated water molecules, in a magnetically oriented nematic monodomain, at room temperature. The volume fraction is Φ ) 0.8%. A typical deuterium NMR spectrum in pure D2O is inserted for comparison.

used for these studies; 5-mm diameter NMR tubes were filled with samples and sealed. Volume fractions were determined with an accuracy better than 0.1% by weighing dried suspensions. We have performed experiments with samples within the narrow concentration range in which macroscopic alignment can be achieved with a 2 T magnetic field, i.e., Φ ) 0.7-1% (Φ is the V2O5 volume fraction). 2.2. NMR Experiments. Deuterium (D) NMR experiments were performed on a Bruker CXP spectrometer operating at 13 MHz together with a 2 T electromagnet. Temperature was controlled with a Bruker VT100 variable temperature unit. The signal-to-noise ratio is such that spectra can be acquired as single scans. Some experiments were also performed at 61.4 MHz in a magnetic field of 9.4 T on a Bruker MSL 400 spectrometer with a high-power solid-state probe. Complementary proton selfdiffusion coefficient measurements have been performed on the Bruker CXP spectrometer, using the standard pulsed field gradient technique, with a homemade gradient probe. The gradient magnitude was varied between 0.2 and 1.1 Tm-1. The basic concepts of D NMR in anisotropic fluids have been developed in numerous references.11 The dominant interaction for a deuterium nucleus is the electric quadrupolar interaction, which splits the resonance line into a doublet. This splitting gives the local order parameter of the phase, defined as 〈P2(cos θ)〉, where θ is the angle between the magnetic field and the quadrupolar tensor and brackets denote a temporal average. Also, the macroscopic ordering of a system may in principle be inferred from the distribution of splittings observed in the spectrum. The quadrupolar tensors in the D2O molecule are approximately along the O-D chemical bonds, with an interaction constant νQ of the order of 250 kHz. 3. Orientation of Water Molecules Figure 2 shows a typical D NMR spectrum obtained from a fluid nematic sample of volume fraction Φ ) 0.8%, at room temperature, in a 2-T magnetic field. The sample is a magnetically oriented single domain, with the director n (defined as the average direction of the long axes of the ribbons) lying parallel to the magnetic field B. Strikingly, we observe a wellresolved doublet with a splitting of about 100 Hz and no isotropic contribution (i.e., no central narrow line in the spectrum). Thus, quadrupolar interactions are not averaged to zero, which indicates that water molecules undergo anisotropic motions on the NMR time scale even at these high dilutions. In contrast, the spectrum of the isotropic phase only shows, as expected, a narrow, isotropic line. For each component of the doublet, the line width is not any larger than that observed with

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NMR Study of Nematic Phase of V2O5

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Figure 4. Deuterium NMR splitting obtained on water as a function of the molar concentration c defined as c ) n(V2O5)/n(H2O). The straight line corresponds to the fit of the data using eqs 2 and 3. Figure 3. Normalized deuterium NMR splitting obtained on water in an oriented nematic single domain, as a function of the angle Ω between the director n and the magnetic field B (insert). The curve corresponds to eq 1 in the text.

pure water (Figure 2, insert). Indeed, the observed width of all these NMR lines is limited by the experimental resolution. Despite the presence of about 1% paramagnetic impurities in the ribbons (VIV species), no paramagnetic broadening is observed. First, to clarify the orientation of the residual quadrupolar tensor with respect to the nematic director, let us examine the angular dependence of this splitting. In a uniaxial system, the measured splitting varies with the angle Ω between B and the local symmetry axis according to:

|3 cos 2Ω - 1|

∆ν(Ω) ) ∆ν(0)

2

(1)

The characteristic reorientation time of a single domain, after a sudden rotation in the field, is of the order of 500-5000 s, depending on temperature and concentration (see below). Therefore, it is possible to rotate the sample tube to an arbitrary angle Ω and to consider the director as fixed during the acquisition of a NMR spectrum. In this way, we have measured the splitting as a function of the angle Ω between the director n and the magnetic field B (Figure 3). These results reproduce perfectly the Ω variation in eq 1, which demonstrates that the symmetry axis for the residual quadrupolar interactions is the nematic director of the phase, n. All these observations mean that a well-defined order parameter is actually measured. All water molecules undergo fast exchange between bound states, in which they are sensitive to the macroscopic orientation of the particles in the phase, and free states, in which they diffuse through the phase within bulk liquid water. In other words, on the time scale probed by NMR, it is impossible to distinguish between two different types (bound and free) of water molecules. Besides, the symmetry property illustrated in Figure 3 does not necessarily imply that the ribbons rotate quickly around their long axes: water molecules exchange quickly between different sites, possibly located on different ribbons, which are equally distributed around the director axis n, given the overall uniaxial symmetry of the system. After considering the angular dependence of the splitting, let us now discuss its order of magnitude. The measured splitting at zero angle ∆ν(0) is given by an expression of the form:

∆ν(0) ) νQSPf

(2)

in which S is the nematic order parameter (defined from the orientational distribution function of the ribbons), f is the fraction of bound water, and P is the order parameter of the water

molecules with respect to the director axis, in the bound state. P is related to the average angle between the quadrupolar tensor of the bound water molecules and the director axis. As already mentioned in the Introduction, the order parameter S of the particles was measured independently by SAXS.5 It is already very high at the isotropic/nematic phase transition (S = 0.75) and does not change much over the concentration range investigated here. It is also reasonable to assume that the average number m of bound water molecules per V2O5 unit remains constant in this range. f may then be written:

f ) mc

(3)

with c being the molar fraction n(V2O5)/n(H2O). The problem here is that P and f cannot be measured independently in a simple way. Figure 4 shows the D NMR splitting versus the molar fraction c. The data are compatible with the linear behavior expressed by eqs 2 and 3. If the quadrupolar tensors of the water molecules were assumed to be aligned along the director (i.e., the long axis of ribbons), Figure 4 would then give on average 0.13 deuterium (i.e., 0.06D2O) bound to each V2O5 moiety. This number is clearly too small, as will be discussed in the next paragraph. In other words, the observed splitting (see Figure 2) is surprisingly small, even though the system is extremely diluted. Therefore, the angular averaging factor P must be considered. At this point, it is useful to recall what is known about the structure of water close to the ribbons. As mentioned above, a xerogel can be reversibly obtained by drying the solutions at room temperature. This xerogel contains about 1.8 water molecules per V2O5 unit in close interaction with the ribbons. The first water monolayer should be chemically bound to the surface of the oxide aggregates, whereas the next ones would rather be physically adsorbed. Spectroscopic studies of the xerogel have shown that the water molecules are not oriented at random.3,12 Two types of water molecules have been distinguished. For the first type, one of the HO bonds is perpendicular to the V2O5 ribbons, thus forming a hydrogen bond with an oxygen on the surface. Water molecules of the second type have their C2 axis perpendicular to the surface of the ribbons, with their oxygen atoms bound to the vanadium atoms located at the surface. In this latter case, free reorientation around the C2 symmetry axis induces an averaging of the quadrupolar tensor along this axis, with a reduction factor P2(R) with R ) 108°/2 ) 54° (108° is the angle between bonds in H2O). It turns out that this value is extremely close to the magic angle, which would result in an almost complete tensor averaging. To a lesser extent, the same type of reasoning would also apply for the first type of water molecules. Such considerations can explain why the observed splitting is indeed very


Pelletier et al. phase itself and not those of the water molecules. After being oriented for a long enough time in the magnetic field, the sample tube is rotated in such a way that the nematic director lies at an angle Ω0 with respect to the magnetic field. In the simplest case, the director comes back along B in a characteristic time determined by a balance between magnetic and viscous torque. The magnetic torque (per unit volume) is:18

1 Γmag ) - χaB2 sin 2Ω 2

(4)

where χa ) χ| - χ⊥ is the anisotropy of the diamagnetic susceptibility and Ω(t) is the instantaneous angle between B and n. The viscous torque is:18

Γvisc ) -γ1

Figure 5. Schematic representation of a water molecule undergoing fast exchange between different sites on the surface of the ribbons.

small, leading in turn to a very small estimate of the number of bound water molecules per V2O5 unit. Altogether, it is impossible to know whether the averaging process around the C2 axis results from jumps from one site to another on the surface or from the fast rotation of a bound water molecule itself. The behavior of water molecules, giving rise to the averaging process described above, is represented schematically in Figure 5. In an attempt to separate the two factors P and f in eq 2, we measured the self-diffusion coefficients of water molecules using the pulsed field gradient proton NMR technique. Indeed, the self-diffusion coefficient measured in this experiment is sensitive to f only. These measurements indicate that the number of hindered water molecules per V2O5 unit lies between 5 and 25 depending on concentration. Despite rather large experimental uncertainties, it is therefore clear that the actual number of bound water molecules is much larger than one, which confirms the above interpretation. Systems with such a well-defined order parameter for solvent molecules at so high dilutions are not common. A few other examples may be found in lyotropic systems: highly swollen lamellar phases,13 stretched polymer networks,14 polypeptide suspensions,15,16 or lyomesophases of xanthone derivatives.17 These studies dealt with systems of solvent volume fractions around 90%, but to the best of our knowledge, no splitting has ever been reported for solvent volume fractions larger than 99%. A reason such reports are so scarce is simply that extremely dilute mesophases are not common either. In the following section, we show that the observation of this splitting not only conveys information about water but also can be used to probe the behavior of the mesophase itself. 4. Reorientation Dynamics of the Nematic Director 4.1. Experimental Results. The observation of a wellresolved doublet on deuterated water molecules may be used to follow the evolution of the nematic texture in a magnetic field. For nematic phases, according to eq 1, the NMR splitting reflects directly the orientation of the particles with respect to the magnetic field. This property was used in Figure 3 in a static way, to show the uniaxial symmetry of the phase. We now use it to study the dynamic properties of the nematic phase. Note that the dynamic properties probed here are those of the nematic

∂Ω ∂t

(5)

where γ1 is the rotational viscosity which describes the reorientation of the nematic director around a perpendicular axis (in terms of Leslie-Ericksen coefficients: γ1 ) R3 - R2). From eqs 4 and 5, the resulting time evolution is then described by the following equation:

tan Ω ) tan Ω0 e-t/τ

(6)

The characteristic reorientation time is:

τ ) γ1/χaB2

(7)

The orientation angle Ω(t) is followed as a function of time by performing time-resolved measurements of the splitting, which is relatively easy because the time scales involved are large enough. Once the splitting for Ω ) 0 has been measured, the angle Ω(t) is related to the measured splitting ∆ν(t) by eq 1 and the curve shown in Figure 3. In Figure 6a, the splitting is plotted as a function of time for an initial rotation of 90°. As expected, the splitting recorded just after the sample has been rotated is one-half its asymptotic value reached when the director and magnetic field are parallel. Figure 6b shows the evolution of ln(tan Ω) as a function of time. If eq 6 were obeyed, then the experimental points should lie on a straight line, which is clearly not the case. In fact, for any initial angle Ω0 > 60°, the reorientation takes place through a transient hydrodynamic instability which creates “zigzag” domains as was observed by polarized light microscopy.9 This results in a distribution of nematic directors which is reflected in the increase of the half-height line width15,19 (Figure 6, insert). The theoretical description of this instability is rather involved since elastic terms also need to be considered.20-22 In principle, an analysis of the line shape21,22 as a function of time can give information about the nematic viscosities as well as the Frank elastic constants. Unfortunately, the splittings are too small compared to the line width for such an analysis to be performed. We have therefore performed the reorientation experiments starting from an angle Ω0 ) 45°. In this case, no instability occurs so that the director simply undergoes a global reorientation with a uniform texture all along the process. Thus, at each time, a well-resolved splitting is measured, which corresponds to a well-defined orientation. In this case, the data is welldescribed by eq 6 and the reorientation time τ can be measured fairly accurately (Figure 7a). Moreover, the line width now remains remarkably constant as a function of time (Figure 7b). Note also that we verified the B2 variation in eq 7 by measuring τ at two different values of the magnetic field (2 and 9.4 T).

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Figure 6. a: Splitting values measured as a function of time, in a reorientation experiment starting from Ω0 ) 90°. b: ln(tan Ω) versus time. The variation is not linear, and eq 6 is not obeyed. Insert: Halfheight line width as a function of time, in the same experiment.

As described in the previous paragraph, our choice of geometry and initial condition Ω0 ) 45° does prevent the appearance of any macroscopic flow, which allows us to measure τ1 ) γ1/χaB2 in a very simple way. In a next step, it would be very interesting to measure independently the anisotropy of the diamagnetic susceptibility χa. In principle, this quantity can be obtained from the shift of the center of the frequency spectrum when the angle Ω is varied. However, no such shift was observed within experimental resolution (about 5 Hz), even in a magnetic field of 9.4 T (i.e., at 61.4 MHz). This means that the macroscopic susceptibility anisotropy χa is too weak to be measured and has a value certainly not larger than roughly 5/(61.4 × 106) ≈ 10-7 (in units of µ0). Indeed, water molecules are sensitive to the overall (average) susceptibility of the phase, which remains weak due to dilution, even though ribbons may individually have an appreciable diamagnetic anisotropy. As a comparison, let us recall23 that the value of χa for a typical thermotropic nematic like MBBA is 1.2 × 10-4 µ0, while the order of magnitude of χa for nematic suspensions of tobacco mosaic virus (TMV) is 2 × 10-6 µ0. In two series of experiments, the reorientation time τ was measured as a function of concentration (Figure 8) and temperature (Figure 9). τ increases drastically on approaching the sol/gel transition within the nematic domain. The variation within the fluid nematic domain is not monotonic, and the inflections in the curve (Figure 8) are reproducible and far beyond error bars. Besides, at a given concentration, the reorientation time τ decreases dramatically as temperature is raised. 4.2. Discussion. We now discuss the concentration and temperature dependences of the reorientation time. We have already pointed out that the phase boundaries and the nematic

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Figure 7. a: ln(tan Ω) versus time in a reorientation experiment starting from Ω0 ) 45°. The line is a fit of the data by eq 6. b: Halfheight line width as a function of time, in the same experiment.

Figure 8. Evolution of the reorientation time τ as a function of volume fraction Φ. The temperature is T ) 20 °C. The line is a guide to the eye.

order parameter at the isotropic/nematic transition are welldescribed by hard rod fluid theories. It is therefore natural to compare our experimental results with the predictions of a hard rod model for the nematic viscosities. Most of these models were devised for cylindrical particles. In the concentration range considered here, V2O5 ribbons are known to rotate around their long axis so that we assimilate them to cylinders of effective diameter roughly equal to their width. The theoretical understanding of the dynamics of nematic suspensions is still partial. It is mostly based on the work of Doi and Edwards10 who considered suspensions of hard stiff rods of diameter d and length L in a medium of viscosity ηs. Within this framework, the reorientation time is given as:


Pelletier et al.

Figure 9. Log-log plot of the reorientation time τ as a function of temperature. The volume fraction is Φ ) 0.8%. The data were corrected for the temperature variation of water viscosity. The straight line is a fit by a power law.

τ) γ1 2

χaB

)

ν 2 ν 1 S (1 - S2)2 (8) πη L9 2+S ν* ln(L/d) - 0.8 s χ B2 a/p

( )

where χa/p ) χa/ν is the magnetic susceptibility anisotropy per ribbon, ν is the number of rods per unit volume, and ν* is that of the isotropic phase at the isotropic/nematic phase transition. Let us first discuss the concentration dependence of the reorientation time. S varies slowly as a function of ν so that, in the concentration range investigated here, eq 8 would only give a moderate monotonic variation as concentration increases. The reorientation time should increase slightly near the isotropic/ nematic transition and then remain roughly constant. This does not correspond to the observed experimental behavior. Instead, our data show a nonmonotonic variation, with a huge increase near the sol/gel transition. This is hardly surprising because the assumptions underlying the Doi and Edwards treatment are certainly not met near the sol/gel transition due to the onset of attractive interactions. It is nevertheless interesting to compare the value measured close to the isotropic/nematic transition to the prediction of the theory. Unfortunately, the L9 dependence of the reorientation time implies that any slight error in the value of L may induce a change of several orders of magnitude in the value of τ. At this point, we can use the additional information obtained from previous SAXS experiments in which we estimated the rotational diffusion constant D*r in the isotropic semidilute regime by monitoring the relaxation of the flowinduced orientation (in the nematic phase, Dr is related to γ1 by Dr ∼ νkbT/γ1). In this way, we found that D*r(φ ) 0.5%) ∼ 1 s-1. According to Doi and Edwards, the value of Dr in the nematic phase is related to that in the semidilute regime by:

(ν*ν ) (1 - S )

Dr(nematic) ) Dr*

2

2 -2

(9)

which gives Dr ) 2.7 s-1 in the nematic phase (φ ) 0.7%). It is interesting to note that this value would give a susceptibility anisotropy per ribbon χa/p ) (kbT)/(2DrτB2) ≈ 4 × 10-26 J/T2. This value is certainly small enough to explain why χa could not be measured. (As a comparison, χa/p for TMV particles23 is 1.6 × 10-24 J/T2.) Besides, the value of Dr in the semidilute isotropic regime is consistent with the Doi and

Edwards theory for rods of length L a few hundred nanometers and of diameter d ) 20 nm at a volume fraction Φ ) 0.5% in a solvent of viscosity ηs ) 10-3 SI. The situation is different when dealing with the temperature dependence of the reorientation. Indeed, we found a huge decrease of the reorientation time in the temperature range 300350 K. This behavior is reasonably described by a T-9.5 power law after being corrected for the temperature variation of water viscosity. Besides, we checked optically that the sample investigated did not have any defect and was still a single domain, so that the measured variation is an intrinsic property of the system. This behavior is particularly puzzling because the phase diagram is completely athermal. Indeed there is no explicit temperature dependence in eq 8, as expected for a hard core fluid. It seems reasonable to assume that the anisotropy of the diamagnetic susceptibility remains constant in the temperature range investigated (in fact, it might decrease slightly with temperature, which would lead to a moderate increase of the reorientation time). The order parameter S also remains constant (we did not measure any variation of the splitting value on increasing temperature). Thus, the only quantity in eq 8 which may contain a temperature dependence is L. Since τ is very sensitive to any change in L due to the L9 factor, it is tempting to explain our results by a temperature-driven variation of L. Dynamic aggregation, as observed in the case of PBLG,24 may be a mechanism that could account for the data. However, such an aggregation process has not been documented so far in our system. One could also consider the effect of a finite flexibility of the ribbons. If the length L involved in eq 8 were taken to be the persistence length Lp of the ribbons, then the observed temperature variation would not be so surprising. Indeed, the persistence length of a wormlike chain is classically written as Lp ) /(kbT) (where is the curvature energy of the chain which should be temperature-independent). Replacing L by the above expression for Lp leads to a temperature dependence of the reorientation time of the form τ ∼ T-9. The exponent -9.6 observed experimentally (Figure 9) is remarkably close to the theoretical value -9. However, if Lp was temperature-dependent, then the phase diagram should not be athermal since the volume fraction at the transition Φi scales such as 1/Lp. Thus, the origin of the discrepancy between the static and dynamic properties remains unclear. 5. Conclusion In the first part of this study, we have shown that the deuterium spectra of V2O5 suspensions in D2O are well-resolved narrow quadrupolar doublets that directly reflect the uniaxial symmetry of a nematic single domain. It is therefore possible and relatively easy to obtain information about the behavior of water in this nematic colloidal system. Obtaining the same information would probably require a much more complicated experimental approach in the case of isotropic colloidal dispersions. The high sensitivity of the NMR technique allows us to work with nematic samples of volume fractions just beyond that of the nematic/isotropic transition (i.e., around 1%). To our knowledge, reports of the observation of a quadrupolar splitting for solvent molecules in lyotropic mesophases at such high dilutions are very uncommon, which points out that the potential of this simple experimental technique has not been fully explored yet. It could be applied in particular to the study of field or shear-induced orientational ordering in colloidal suspensions. We chose to use D2O as a probe to investigate the reorientation of the nematic director after a sudden rotation in the magnetic field. Thus, we were able to study the collective reorientation

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time of the V2O5 nematic suspensions as a function of concentration and temperature. In particular we found a very strong temperature dependence. This experimental fact is very puzzling since the phase diagram of the suspensions is completely athermal in the temperature range considered in this study. Acknowledgment. We are deeply indebted to Prof. J. Livage for fruitful discussions. We also thank the referees for directing us toward refs 15 and 24. References and Notes (1) For a recent review, see: Davidson, P.; Batail, P.; Gabriel, J. C. P.; Livage, J.; Sanchez, C.; Bourgaux, C. Prog. Polym. Sci. 1997, 22, 913. (2) Zocher, H. Zeitschrift. Anorg. Allg. Chem. 1925, 147, 91. (3) Livage, J. Chem. Mater. 1991, 3, 578. (4) Davidson, P.; Garreau, A.; Livage, J. Liq. Cryst. 1994, 16, 905. (5) Davidson, P.; Bourgaux, C.; Schoutteten, L.; Sergot, P.; Williams, C.; Livage, J. J. Phys. II Fr. 1995, 5, 1577. (6) Onsager, L. Ann. N.Y. Acad. Sci. 1949, 51, 627. (7) Vroege, G. J.; Lekkerkerker, H. N. W. Rep. Prog. Phys. 1992, 55, 1241. (8) Pelletier, O.; Bourgaux, C.; Diat, O.; Davidson, P.; Livage, J. Submitted.

J. Phys. Chem. B, Vol. 103, No. 26, 1999 5433 (9) Commeinhes, X.; Davidson, P.; Bourgaux, C.; Livage, J. AdV. Mater. 1997, 9, 900. (10) Doi, M.; Edwards, S. F. The Theory of Polymer Dynamics; Oxford University Press: Oxford, 1986. (11) (a) Samulski, E. T. Polymer 1985, 26, 177 and references therein. (b) Seelig, J. Q. ReV. Biophys. 1977, 10, 353. (12) Vandenborre, M. T.; Prost, R.; Huard, E.; Livage, J. Mater. Res. Bull. 1983, 18, 1133. (13) Halle, B.; Quist, P. O. J. Phys. II 1994, 4, 1823. (14) Sotta, P.; Deloche, B.; Herz, J. Polymer 1988, 29, 1171. (15) Orwoll, R. D.; Vold, R. L. J. Am. Chem. Soc. 1971, 93, 5335. (16) Samulski, E. T.; Berendsen, H. J. C. J. Chem. Phys. 1972, 53, 3920. (17) Perahia, D.; Wachtel, E. J.; Luz, Z. Liq. Cryst. 1991, 9, 479. (18) De Gennes, P. G.; Prost, J. The Physics of Liquid Crystals, 2nd ed.; Oxford University Press: Oxford, 1993; Chapter 5. (19) Schwenk, N.; Spiess, H. W. J. Phys. II Fr. 1993, 3, 865. (20) Srajer, G.; Fraden, S.; Meyer, R. B. Phys. ReV. A 1989, 39, 4828 and references therein. (21) Casquilho, J. P.; Gonc¸ alves, L. N.; Martins, A. N. Liq. Cryst. 1996, 21, 651. (22) Esnault, P.; Casquilho, J. P.; Volino, F.; Martins, A. F.; Blumstein, A. Liq. Cryst. 1990, 7, 607. (23) Fraden, S.; Maret, G.; Caspar, D. L. D. Phys. ReV. E 1993, 48, 2816. (24) Wissenburg, P.; Odijk, T.; Kuil, M.; Mandel, M. Polymer 1992, 33, 5328.

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