Formation of Colloidal Nanocellulose Glasses and Gels - Langmuir

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The formation of colloidal nanocellulose glasses and gels Malin Nordenström, Andreas B. Fall, Gustav Nyström, and Lars Wågberg Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01832 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on August 31, 2017

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The formation of colloidal nanocellulose glasses and gels Malin Nordenström*†, Andreas Fall†§, Gustav Nyström‡, Lars Wågberg*† †

Department of Fibre and Polymer Technology and Wallenberg Wood Science Center, KTH

Royal Institute of Technology, Teknikringen 56, SE-100 44 Stockholm, Sweden ‡

Department of Health Science & Technology, ETH Zurich, Schmelzbergstrasse 9, 8092 Zurich,

Switzerland Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ABSTRACT Nanocellulose (NC) suspensions can form rigid volume-spanning arrested states (VASs) at very low volume fractions. The transition from a free-flowing dispersion to a VAS can be the result of either an increase in particle concentration or a reduction in interparticle repulsion. In this work, the concentration-induced transition has been studied with a special focus on the influence of particle aspect ratio and surface charge density, and an attempt is made to classify these VASs. The results show that for these types of systems, two general states can be identified: glasses and gels. These NC suspensions had threshold concentrations inversely proportional to the particle aspect ratio. This dependence indicates that the main reason for the

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transition is a mobility constraint which, together with the reversibility of the transition, classifies the VASs as colloidal glasses. If the interparticle repulsion is reduced, the glasses can transform into gels. Thus, depending on preparation route, either soft and reversible glasses or stiff and irreversible gels can be formed. INTRODUCTION Volume-spanning arrested states (VASs) of nanocellulose (NC) suspensions are soft materials that are rapidly growing in interest. They can be formed at very low volume fractions (φ), below φ = 0.00071, 2. These VASs have a very high optical transparency and are stable for long periods of time; months to years in sealed containers, according to our experience. VASs based on NC are mechanically rigid; the shear stiffness is one order of magnitude higher than that of agar or peptide gels with the same water content2, 3. The structuring or orientation of the particles in the VAS can be controlled by orienting the particles either before2, 4, 5 or after1, 6 the flow to nonflow transition. The particle-structured VAS can be utilized to prepare dry NC materials with controlled orientation and anisotropic material properties4. In addition, the well-defined particle network has been used as a template into which a polymer or monomer can be introduced, allowing the preparation of NC-reinforced composites with a well-defined, homogeneous or oriented distribution of the reinforcing NC particles1, 7. The general classification of colloidal arrested states is not straightforward, and over the years many definitions have been proposed8,

9, 10

. However, Tanaka et al11 have introduced two

different states for systems containing anisotropic nanoparticles: glasses and gels. Both classes show a solid-like character: not flowing in inverted cuvettes, with storage moduli much larger than loss moduli (G’>> G’’), and long relaxation times11. The glass is a VAS based on repulsive particle-particle interactions, the solid-like character being given by free volume/particle


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mobility constraints, often referred to as “caging”11, 12, 13. The liquid-glass transition is reversible, the suspension being transformed to a liquid of freely moving particles when subjected to dilution14,

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, a change in temperature16, a change in ionic strength11 or the application of

mechanical shear16. The particle distribution in the glasses is homogeneous at length scales greater than the dimensions of the particles, showing no features in the structure factor obtained by light/x-ray scattering on these length scales11. The scattered intensity during the liquid-glass transition increases only moderately, since the features do not increase in size11. Gels, on the other hand, are based on attractive interparticle interactions with long-lived particle bonds/joints11. The gelation is caused by aggregation of the particles into a percolated network, often forming a fractal structure11, 15. Depending on the nature of the interparticle interaction, the gelation can be either reversible or irreversible15. Both gels and glasses age with time. The aging of glasses is associated with slow rearrangements of particles which leads to a decrease in free energy17. For gels the aging process involves an increase in the number of particle–particle joints, and aggregates created can densify the structure. This is observed by light scattering as an increase in scattered intensity and, if the gel network is strengthened by aging, as an increase in the relaxation time of the structure. However, continued aggregation can also damage the gel network and even break it down, if the particle attraction is stronger than the strength of the gel network18. In this case, the relaxation time decreases and non-flow to flow transition may occur. Apart from repulsive glasses and attractive gels, there is a third state, where both attractive and repulsive interactions are significant but where the repulsive interactions dominate. Such intermediate systems are called attractive glasses11. Flow to non-flow transitions of NC suspensions have been induced both by increasing the particle volume fraction and by reducing the interparticle repulsion: The particle volume fraction


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has been increased by solvent evaporation2,

19, 20, 21, 22, 23

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or by osmosis20, and repulsive

electrostatic interactions have been reduced by increasing the ionic strength1, 4, by lowering the pH1, 2, or by solvent exchange to a low dielectric solvent5, 7, 24. In all cases, a volume-spanning arrested state with a clear elastic character is formed; the suspensions do not flow in inverted cuvettes1, 19 and rheological measurements show a storage modulus (G’) much larger than the loss modulus (G’’)1, 2, 21, 22, 23. Even though there has been a substantial amount of research on NC VASs, the discussions in the literature have so far not included their classification or the types of particle-particle interactions that exist within the different types of cellulosic networks, but such a classification is essential for optimizing the use of the inherent properties of NC in new materials and devices. Moreover, there has been no comparison of VASs based on NC, with different particle dimensions, different surface charge densities and different types of surface charge. To our knowledge, so far only Saito et al2 have compared two different types of VASs based on cellulose nanofibril (CNF) suspensions. They showed that by reducing the pH of a CNF suspension, where the particle concentration was already above the VAS threshold (0.4 wt%), from pH 8 to pH 2 the network was substantially strengthened. In rheological strain sweep measurements, the acid-induced VAS experienced brittle fracture whereas the original suspension transitioned to a flowing liquid. It was suggested that the interparticle interactions switched from repulsive to attractive when acid was added, but the classification of the VASs was not discussed at all. This transformation to an immobilized state following a pH change from 7 to 225, or by increasing the ionic strength4, was also exploited to make filaments with oriented fibrils from a dispersion with a concentration below the VAS threshold. However, a unifying classification scheme based on a common set of physical properties is still lacking. Such a classification should indeed have the potential to integrate research of different experimental NC


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systems, to lead to an increased fundamental structural understanding and to serve as a framework for new NC-based materials. In the present contribution, we compare six types of NCs, differing in dimensions, charge and types of charge, i.e. carboxyl groups or sulfate esters. The flow to non-flow transition following an increase in particle volume fraction is monitored by dynamic light scattering (DLS), and the threshold volume fraction for the transition is quantified by DLS as well as suspension flow tests in inverted cuvettes. A CNF and a CNC sample with the same type of surface charge and similar surface charge densities are compared in detail. The difference in aspect ratio between these two types of particle is more than 5-fold. The reversibility of the VAS formed, as well as the degree of aggregation of the individual particles during the transition, has been evaluated by dilution studies. Finally, the effect of turning off (adding acid) or reducing (adding salt) the interparticle repulsion within the VAS has been studied. The solvent conditions have also been changed back to repulsive to study the reversibility upon dilution of these aggregated particle networks, with or without mechanical agitation. EXPERIMENTAL SECTION Preparation of cellulose nanofibrils (CNF) For the preparation of CNF, two different types of pre-treatment were used: carboxymethylation for medium charged CNF (MC-CNF) and 2,2,6,6-tetramethylpiperidinyl-1oxyl (TEMPO)-mediated oxidation for lowly and highly charged CNF (LC-CNF and HC-CNF). Carboxymethylated CNF was produced at RISE Bioeconomy, Stockholm, Sweden, according to the method described by Wågberg et al.26 For the TEMPO-oxidation, a never-dried dissolving sulfite pulp, made from 60 % Norwegian spruce and 40 % Scots pine, was used (Domsjö Dissolving Pulp, Aditya Birla Domsjö AB,


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Sweden). TEMPO-mediated oxidation was carried out according to a previously described method27, where fibers (20 g dry weight) were suspended in a 2 L aqueous solution containing TEMPO (2 mmol) and NaBr (20 mmol). The pH of a 14 % NaClO solution was set to 10, and the oxidation was initiated by adding the NaClO solution to the fiber suspension. The volume of NaClO solution was 10 mL for LC-CNF and 30 mL for HC-CNF. Following the pre-treatment, the fibrils were liberated by mechanical disintegration using a high-pressure homogenizer, where the suspension was passed 6 times through a pair of chambers having inner diameters of 200 and 100 µm (Microfluidizer M-110EH, Microfluidics Corp., USA). The resulting gel was diluted and dispersed by probe-sonication, followed by centrifugation at 4100g. After this treatment, the CNF was well dispersed as characterized by AFM and DLS measurements. Preparation of cellulose nanocrystals (CNC) Carboxymethylated CNCs (MC-CNC) were prepared by HCl hydrolysis of carboxymethylated CNF as reported in the literature28,

29

. The CNC was washed by centrifugation (4100g) and

dialyzed against deionized water for 7-8 days (12-14 kDa MWCO). After dialysis, the pH was set to 9 using NaOH and the dispersion was sonicated for 10 minutes and centrifuged for 1 h (4100g). Sulfuric-acid-hydrolyzed CNCs were prepared from a dry dissolving pulp (Domsjö Dissolving Pulp, Aditya Birla Domsjö AB, Sweden) according to a previously established protocol30. After the CNC had been washed by centrifugation, the precipitant was collected and dialyzed against deionized water, followed by pH adjustment to pH 8, probe-sonication and centrifugation. Preparation of bacterial nanocellulose (BNC)


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Bacterial cellulose nanocrystals were prepared from commercially available coconut gel cubes (Chaokoh, Thai shop). The coconut cubes (with a size of about 1 cm3) were pre-treated by washing three times with 2 L of deionized water, followed by stirring in 2 L of NaOH solution (0.1 M) for 48 hours and finally washing with deionized water until the pH stabilized at around 7. The hydrolysis was performed by soaking 100 g of pre-treated coconut cubes in 40 % H2SO4 at 80 °C for 4 h, and quenching in a 10-fold amount of deionized water. The hydrolyzed material was washed twice with deionized water, collected by centrifugation and dialyzed for 5 days against deionized water (~14 kDa MWCO). After the dialysis, the suspension was first sonicated for 10 min, followed by centrifugation for 60 min at 4000g. Surface charge determination The

surface

charge

density was

determined

by polyelectrolyte

titration

against

polydiallyldimethylammonium chloride (PDADMAC), using a Stabino Particle Charge Mapping unit (Particle Metrix GmbH, Germany). The measurements were carried out at pH 9 and the charge density reported is the average from three titrations. Atomic Force Microscopy (AFM) The morphologies of the different particles were investigated using AFM (Multimode 8, Bruker, USA) in the tapping mode. Dilute particle dispersions with volume fractions between 7×10-6 and 7×10-5 (corresponding to 0.01-0.1 g/L, assuming the density of cellulose to be 1.5 g/cm3) were adsorbed for 1 minute onto PEI-coated silica substrates followed by rinsing with water (Milli-Q grade) and drying with nitrogen. An RTESPA tip (Bruker, USA) with 300 kHz resonance frequency, 40 N/m spring constant, and 8 nm tip radius (as specified by the producer) was used. Inverted cuvette test


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The NC suspension was poured into rectangular 1x1x4 cm cuvettes. The particle volume fraction was increased by evaporating water from the dispersions at 45 °C. In order to avoid skin formation on the surface of the dispersions, the evaporation rate was reduced by using perforated lids. Still, a vertical concentration gradient may have existed. Stirring of the samples during the evaporation could have avoided such a gradient, but stirring was not used as the intention was to study the undisturbed VAS formation. The VAS threshold volume fraction, at which the dispersion transformed from a free-flowing fluid to a non-flowing VAS, was determined by inverting the cuvettes. The same cuvettes were also used for the DLS measurements. Dynamic Light Scattering The hydrodynamic diameters and the translational diffusion coefficients of the particles were determined by dynamic light scattering (DLS) (Zetasizer ZEN3600, Malvern Instruments Ltd., U.K.). The measurements were conducted on dilute samples with volume fractions between 3×10-5 and 7×10-5 to ensure individualized particles and avoid interparticle interactions. The translational diffusion coefficient D obtained by the DLS measurements were used in combination with the particle diameter d, measured by AFM, to calculate the particle lengths L. In order to reduce the Debye length and thereby the effective particle size, a NaCl concentration of 1 mM was used. The particles were assumed to behave as rigid rods and L was calculated using the rod diffusion equation ݇ܶ ܿଵ ݀ ܿଶ ݀ ଶ ‫=ܦ‬ × ቆln ‫ ܮ‬− ln ݀ + ܿ଴ + + ଶ ቇ 3ߨߟ‫ܮ‬ ‫ܮ‬ ‫ܮ‬ where k is Boltzmann’s constant, T is the absolute temperature, η is the viscosity of the solvent and c0, c1 and c2 are constants (see de la Torre and Bloomfield31 for details). The average hydrodynamic diameters and the diffusion coefficients were determined from the intensity correlation function (ICF) using the cumulants method.


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DLS was also utilized for the determination of the VAS threshold volume fraction. The volume fraction was increased as described above for the inverted cuvette test. Measurements were conducted on samples with different particle volume fractions, and the appearance of nonergodicity was detected as a decrease in the ICF intercept at t = 01, 32, 33, 34. The shift in intercept was caused by an increase in the relaxation time above tend (the time at the end of each 10 s run), as the Malvern software algorithm (Zetasizer Software, Malvern Instruments Ltd., U.K.) shifts the curve so that ICF = 0 at tend. Each ICF curve is an average of 10 runs and each intercept value was obtained by averaging the values of 5 curves. For all DLS measurements, the measurement duration (10 s per run) and measurement position (4.65 mm from the cuvette wall) were kept constant. Dilution of arrested states The self-dispersibility of MC-CNF and MC-CNC was studied by diluting VASs with Milli-Q water to volume fractions within the semi-dilute regime (φ = 0.0011 for MC-CNF and φ = 0.0067 for MC-CNC). Water was placed on the surface and the sample cuvettes were slowly inverted once each day to evaluate the self-dispersion. NaCl or HCl was added to some of the MC-CNF and MC-CNC VASs, before the samples were diluted, in order respectively to reduce or remove the electrostatic interparticle repulsion. A drop of NaCl or HCl solution was placed on the VAS–air interface and the sample was allowed to equilibrate for 24 h. After equilibration, the concentrations were 100 mM NaCl and 10 mM HCl. The samples were thereafter washed, changing the washing liquid (Milli-Q water) once each day until the total concentration was ≤ 1mM NaCl for MC-CNF and MC-CNC or ≤ 3 µM HCl for MC-CNF. For MC-CNC with added HCl, the washing liquid was changed until the


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concentration was ≤ 60 µM HCl, after which the VASs started to break down and thereby hinder the washing liquid from being changed. RESULTS Nanocellulose characterization Six types of nanocellulose were investigated: TEMPO-oxidized CNF with low and high surface charge densities (LC-CNF and HC-CNF), carboxymethylated CNF and CNC with medium surface charge density (MC-CNF and MC-CNC), sulfonated CNC (S-CNC) and bacterial nanocellulose (BNC). Table 1 shows the particle dimensions (d and L), aspect ratios (a) and surface charge densities (σ). The table also includes the VAS threshold volume fractions (φVAS), obtained from inverted cuvette test, as well as the Debye length (1/ߢ = 0.304/ඥ‫ܥ‬௜௢௡ ) and concentration of counterions (Cion) at φVAS. AFM micrographs of individualized particles are shown in Figure 1. Table 1. The particle dimensions, surface charge density, VAS threshold volume fraction, Debye length and counterion concentration at the flowing to non-flowing transition for the different NCs used in the present investigation. LC-CNF MC-CNF HC-CNF BNC

MC-CNC S-CNC

da [nm]

3.3 (1.2)

2.9 (1.2)

3.3 (1.2)

6.7 (2.5) 5.0 (2.0)

7.0 (2.7)

L [µm]

3.9

1.7

0.86

0.52

0.31

0.20

a (L/d)

1180

586

261

78

62

29

σ [µmol/g] 170

490

990

60

360

350

1/κ [nm]

15

7.3

3.4

12

2.8

1.7

φVAS

0.0015

0.0023

0.0053

0.0073

0.022

0.060

Cion [mM]

0.4

1.7

7.9

0.7

12

32

a

The values in parentheses are the standard deviations based on >1000 data points.


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For MC-CNF and MC-CNC two batches with slightly different particle properties were used. The first batches, with properties presented in Table 1, were used for the determination of VAS threshold concentrations, while the second batches were used for dilution experiments. For the second batches, the surface charge density and aspect ratio were respectively 430 µmol/g and 525 for MC-CNF, and 300 µmol/g and 79 for MC-CNC.

Figure 1. AFM images of (A) LC-CNF, (B) MC-CNF, (C) HC-CNF, (D) BNC, (E) MC-CNC and (F) S-CNC. Transition from dispersion to volume-spanning arrested state The VAS threshold volume fraction was evaluated both by inverting cuvettes and with DLS. Figure 2a shows inverted cuvettes where a free-flowing sample has fallen to the bottom whereas a sample that has transformed to a VAS remains at the top of the cuvette. For DLS measurements of samples in the dispersed state, the ICF intercept (at t = 0) was approximately 0.9. However, when the particle volume fraction approached the VAS threshold,


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the ICF intercept decreased, as shown in Figure 2b, because the relaxation time exceeded the measurement time. It was found that an intercept value of 0.7 nearly always corresponded to a transition from dispersion to VAS for the samples and DLS equipment used in this study, and this has also been observed in previous work1.

Figure 2. (A) Inverted cuvette tests of MC-CNF and MC-CNC for the determination of the VAS threshold concentration. (B) ICF intercepts from DLS measurements for nanocellulose dispersions of different particle volume fraction. The DLS data for BNC and S-CNC do not follow the same trend as for the other samples; their intercept values are low (≤ 0.7) already in the dispersed state and no drop in the intercept was observed close to the threshold volume fractions for non-flowing characteristics determined with the inverted cuvettes. This may be due to the formation of slow moving structures consisting of liquid crystalline ordered domains (S-CNC) or aggregated particle clusters (BNC) at volume fractions far below the VAS threshold. For these two systems, therefore, the VAS threshold volume fraction was evaluated solely by the inverted cuvette test.


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The nanocellulose samples investigated had different aspect ratios ranging between 29 and 1180, as determined by combining AFM and DLS data, as shown in Table 1. Figure 3 shows that dispersions of particles with higher aspect ratios formed VASs at lower particle volume fractions. The VAS threshold volume fraction is well described by the inverse of the aspect ratio, as is shown by the solid line in Figure 3, which represents the function φVAS = 1.54×a-1 fitted to the experimental data, with R2 = 0.92.

Figure 3. Threshold volume fractions for the transition from a dispersed state to a VAS, determined by the inverted cuvette test. The solid line shows the relationship φVAS = 1.54×a-1, fitted to the experimental data. Dilution of volume-spanning arrested states The reversibility of the flow to non-flow transition was studied for two samples, MC-CNF and MC-CNC. These particles have the same types of charge and similar charge densities, but significantly different aspect ratios: 525 for MC-CNF and 79 for MC-CNC. Immediately after dilution, the samples consisted of two phases: the VAS and the dilution liquid. However, both samples formed a single homogeneous liquid phase a few days after dilution.


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After dilution, the ICF intercept was measured by DLS and the results are shown in Figure 4a, where the broken lines show the ICF intercepts of the dispersions before the volume fraction increase. For both samples, the intercept increased after dilution and stabilized after 8–10 days at a value slightly lower than that before the evaporation of water and re-dilution. When mechanical agitation (Ultra-Turrax, 6000 rpm, 3 min) was applied to the dispersion, intercept values similar to the original ones were obtained for both MC-CNF and MC-CNC (red star and green cross, respectively, in Figure 4a).

Figure 4. (A) ICF intercept of a diluted VAS of MC-CNF and MC-CNC, without and with mechanical agitation (Ultra-Turrax, UT). The broken lines represent the ICF intercepts of the original dispersions before evaporation of water and re-dilution. The data points at x = 0 were measured prior to dilution. (B) Ratio of hydrodynamic diameter after re-dilution (DH) to that before water evaporation (DH,org), measured by DLS at φ = 7×10-5. The dilution-induced self-dispersion was also studied by measuring the hydrodynamic diameter of the particles after dilution by removing aliquots of the sample at different times after dilution, further diluting the withdrawn aliquots to φ = 7×10-5 and measuring with DLS. The hydrodynamic diameters presented in Figure 4b show that after dilution the particle size is


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slightly larger (approximately 20 % for MC-CNF and 45 % for MC-CNC) than before the flow to non-flow transition. The size increase is probably caused by some irreversible aggregation occurring in the process of forming a VAS, as a consequence of the decreased electrostatic repulsion, and that this aggregation was more severe in the MC-CNC system. When the diluted samples were mechanically agitated, the hydrodynamic diameter decreased slightly for MC-CNC but was not significantly affected for MC-CNF. Influence of added salt/acid The reversibility of the flow to non-flow transition was also studied after removal or reduction of the electrostatic interparticle repulsion. VASs were created by evaporation of water and a drop of either HCl or NaCl was added to the VAS. The VASs were thereafter diluted with Milli-Q water and the ICF intercepts are presented in Figure 5. When HCl was added to MC-CNF, a very stiff VAS was formed and the particle mobility was substantially decreased, making DLS measurement impracticable. Therefore, there are not any intercept values for this sample presented in Figure 5. Comparison of Figure 5 with Figure 4a shows that the self-dispersion behavior is different when the electrostatic repulsion in the VAS has been reduced or removed, even if electrostatic interactions are restored by the dilution. The ICF intercepts remain considerably lower than the original values over the time scale of the experiment (5 weeks). When the MC-CNF VASs were diluted, both the samples with added NaCl and HCl remained non-flowing. The VASs with added HCl decreased slightly in volume while the one with NaCl swelled substantially, the volume increasing by approximately 40 %. The difference in behavior of the samples upon dilution may be caused by the different counterions to the CNF charges. For MC-CNC, both with added NaCl and with added HCl, the VASs started to break down a few


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days after dilution. After 5 weeks, the samples consisted of a small non-flowing phase and a flowing phase with a viscosity higher than that of the original dispersion.

Figure 5. ICF intercept of VASs of (A) MC-CNF with added NaCl and (B) MC-CNC with added NaCl or HCl, after dilution with Milli-Q water. The ICF intercepts of the original dispersions before the evaporation of water and addition of salt/acid are shown by the broken lines. How the self-dispersion of the VAS following the addition of NaCl or HCl was affected by the application of an additional force was also investigated. The diluted samples were agitated using Ultra-Turrax as described above and hydrodynamic diameters were measured after further dilution to φ = 7×10-5. The results of these experiments are presented in Table 2, together with data for the VASs without added NaCl or HCl, with and without mechanical agitation. For MCCNC without added NaCl or HCl, the hydrodynamic diameter decreased after agitation but remained greater than before the immobilization, whereas the size for MC-CNF did not change significantly. For all samples with added NaCl or HCl, the hydrodynamic diameters were much greater than in the original dispersions. In the case of the agitated samples of MC-CNF, however, similar sizes are obtained with and without added NaCl.


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Table 2. Hydrodynamic diameters (in nm) for MC-CNC and MC-CNF before the evaporation of water and after dilution of the VAS without and with mechanical agitation. DH,orga

DH,dilutedb DH,UTc

MC-CNF

385 (36)

466 (65)

494 (41)

MC-CNF +NaCl

-

-

494 (8)

MC-CNF +HCl

-

-

3560 (1360)

MC-CNC

81.7 (0.8) 114 (5)

101 (2)

MC-CNC +NaCl

-

-

123 (3)

MC-CNC +HCl

-

-

133 (2)

a

Hydrodynamic diameters before water evaporation, bafter dilution and cafter dilution and mechanical agitation. The values in parentheses are the standard deviations, based on 3 samples. It needs to be stressed that all the dilution experiments were also carried out with the first

batches of MC-CNF and MC-CNC, which were used for the VAS threshold concentration determinations. The results of these measurements can be found in SI. DISCUSSION Mechanisms behind the formation of VASs for different NCs It is known that highly anisotropic particles can form non-flowing VASs at very low volume fractions relative to the threshold volume fractions of hard spheres, φ = 0.5-0.635, due to their large effective volume fraction. If the particles are charged the effective volume is further increased by the surrounding ionic cloud which can be described by adding the Debye length to the effective radius of the particles. Suspensions of electrostatically stabilized rod-like NC particles form VASs by a concentration increase at volume fractions 1-2 orders of magnitude lower than that required for spherical


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particles. Only a few studies have been reported of the transition from flowing to non-flowing conditions of NC suspensions by simple particle concentration increase2,

19, 23

, especially

concerning the type of VAS which the dispersed particles are forming (gels or glasses). Nor has the mechanism responsible for the transition been established. To fill this gap we have investigated the mechanisms involved in the particle immobilization for different particle aspect ratios and different surface charge densities, as well as the properties of the VASs, i.e. the relaxation time and the self-dispersibility. For the NC particles investigated in this study, the VAS threshold concentration decreased with increasing aspect ratio, as has previously been reported for various types of anisotropic colloidal particles12,

18, 36

. For rod-like particles, three different concentration regimes with

different degrees of mobility constraint can be defined37. In the dilute regime (φ < a-2) particle collisions are rare, and the particles can rotate/translate in three dimensions. In the semi-dilute regime (a-2 < φ < a-1) there are contact points between the particles and particle translation is partially inhibited whereas particle rotation around its axis and vibration is still possible. At concentrations within the concentrated regime (φ > a-1), the particle mobility is severely constrained due to multiple contact points with neighboring particles and in this regime only rotation and vibration of free particle segments can occur. As shown in Figure 3, the VAS threshold concentration is inversely proportional to the particle aspect ratio, indicating that when the system reaches the more concentrated regime it becomes kinetically arrested and the particles are immobilized, apart from their vibrational and some rotational movements. The rotational movements are probably also restricted for the particles with the highest aspect ratios. The influence of surface charge density on the VAS threshold concentration may have different underlying causes: (a) too low a surface charge would be insufficient to electrostatically


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stabilize the system38, so that

particle aggregation can then take place with increasing

concentration and this can lead to gelation, i.e. the formation of a VAS based on attractive interparticle interactions, where the van der Waals interactions dominate over the electrostatic repulsion; (b) in the case of more highly charged particles, where the electrostatically induced repulsion is sufficient to keep the particles apart at low to medium concentrations, the counterions to the charges will significantly increase the ionic strength of the medium when water is evaporated, and an increase in particle concentration can be sufficient to reduce the particle–particle repulsion, and lead to gel formation; and (c) if the counterion concentration does not become sufficiently high to reduce the electrostatic stabilization significantly, the reduction in Debye length with increasing ionic strength will decrease the effective volume fraction and thus increase the threshold for the liquid–glass concentration, i.e. when the system forms a VAS due to particle mobility constraints. The results for the CNF samples in Table 1 show a clear increase in the VAS threshold concentration with increasing charge. Thus, as alternatives (a) and (b) concern reduced colloidal stability it is clear that this is not the major cause for the immobilization of the CNF colloid. This was expected since the increase in ionic strength emanating from the counterions to the CNF is far too small (0.4-7.9 mM Na+) to induce colloidal instability at the VAS threshold. Earlier studies have shown that CNF dispersions are stable in this ionic strength region39, 40, and our dispersions showed no sign of aggregation at these ionic strengths in the low concentration regime (see SI Figure S1). In the case of the CNC, on the other hand, it is possible that the concentration of counterions (12-32 mM) could affect the electrostatic stabilization, since this ionic strength is in a region where aggregation has been observed41, and where our systems approach the aggregation regime at low concentrations (see SI). The agreement of our results


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with the third alternative, however, suggests that the mobility constraints are more important and that a colloidal glass is formed as the concentration increases. It is also important to establish the factors that dominate in the mobility constraints for the NCs. As mentioned earlier, the effective volume of charged anisotropic particles is governed both by the aspect ratio, since high aspect ratio particles occupy larger volumes than low aspect ratio particles, and by the surface charge density and counterions to these charges, due to the compression of the surrounding ionic cloud with increasing counterion concentration following an increase in particle concentration. In Table 3, the effects of aspect ratio (row 3) and surface charge density (row 4) on the VAS threshold volume fraction are compared. The values in the second row are the, experimentally determined, relative VAS threshold volume fractions (φAVAS/φBVAS) for the pair of samples associated with each column. The concentration at which the particles reach the concentrated regime37 is proportional to a-1 and therefore the theoretical effect of aspect ratio on the relative VAS threshold volume fraction should scale as φAVAS/φBVAS = aB/aA. For charged particles the effective aspect ratio is proportional to the Debye length (κ-1), assuming the Debye length to be short compared to the particle length, giving φAVAS/φBVAS = κA/κB. The theoretical relative VAS threshold volume fractions are calculated using the aspect ratios and Debye lengths presented in Table 1. If only the CNF samples (columns 2 and 3) are compared, it is difficult to tell which effect predominates in the transition. In the comparison between MC-CNF and LC-CNF (column 2) the increase in threshold volume fraction predicted by the two ratios is greater than the experimentally observed ratio. We suggest that this is due to too low a colloidal stability in the case of the LC-CNF that starts to aggregate when the concentration increases, reducing the particle number concentration and increasing the volume fraction for the flow to non-flow transition. When HC-CNF is compared with MC-CNF (column


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3), both the aB/aA and the κA/κB ratios predict a threshold concentration increase very close to the observed value. When MC-CNC is compared with MC-CNF (column 4), the observed increase in the VAS threshold concentration is well predicted by the difference in aspect ratios. In contrast, the decrease in Debye length predicts a volume fraction increase only one fourth of that observed. Table 3. Comparisons between the experimentally determined relative VAS threshold volume fraction, aspect ratio and Debye length for the NC particles with different physical dimensions and different charge densities. Ratio

A=MC-CNF A=HC-CNF A=MC-CNC compared with compared with compared with B=LC-CNF B=MC-CNF B=MC-CNF

φAVAS/φBVAS 1.5

2.4

9.7

aB/aA

2.0

2.3

9.5

κA/κB

2.1

2.2

2.5

Classification of the VASs from dilution experiments Upon dilution, the MC-CNF and MC-CNC VASs reached a free-flowing condition within a few days without any mechanical agitation. DLS measurements (Figure 4) showed ICF intercepts similar to those for the original dispersions, demonstrating ergodic systems. The slight increase in particle size implies partly attractive interparticle interactions, but the reversibility of the transition suggests that repulsive interactions predominate. The results imply that upon dilution of the VAS, the larger network structures which cause the dynamic arrest are broken down, resulting in a dispersion of small aggregates. The mobility of the aggregates is substantially higher than the particle mobility in the VAS, which causes the ICF intercept to increase after dilution, but the hydrodynamic diameter will remain slightly larger than the


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original particle size. The reversibility of the transition, in combination with the aspect ratio dependence discussed above, indeed indicate that the VASs could be categorized as colloidal glasses. A rheological study by Saito et al2 also indicated stable systems and the formation of a colloidal glass when the concentration was increased, but the current important distinction between the VASs was not made2. Their CNF dispersions formed VASs around φ = 0.0027 and when a shear strain was applied a soft transition between the non-flowing and the flowing state was observed. The volume fraction when reaching the VAS can be predicted by φ = K×a-1, where K is a constant and where the best fit to our data gives K = 1.54. A comparison of this relation with the threshold volume fractions of nanocellulose particles reported in the literature shows a good fit, as shown in Figure 6. Thus, it seems that the relationship between VAS threshold volume fraction and particle aspect ratio found in this study can be generally applied to nanocellulose dispersions. However, for rod-like particles of other origins that also form VASs, such as inorganic boehmite rods12, 18, fd-viruses13 and single-wall carbon nanotubes42, the VAS threshold volume fractions do not necessarily follow the same trend. Clearly this is a system-specific trend and the difference in VAS threshold volume fraction of different particle types may be due to factors such as particle aggregation, and factors controlling this, or to the formation of ordered domains.


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Figure 6. The VAS threshold volume fraction as a function of the inverse aspect ratio (a-1) for various types of rod-particle dispersions. The circles are data from the current study and the broken line illustrates the volume fraction–aspect ratio relationship derived from our results. The squares are literature data for other nanocellulose dispersions reported by (1), (2) Shafiei-Sabet et al (2012 and 2013, respectively)21, 22, (3) Honorato-Rios et al19 and (4) Urena-Benavides et al23. The triangles are data for other rod-shaped particles: (5), (6) boehmite (Wierenga et al and Buining et al, respectively)12, 18, (7) fd-virus particles (Kang et al)13 and (8) single-wall carbon nanotubes (Hough et al)42. Our results also show that if the electrostatic repulsion is significantly reduced by adding either salt or acid, the particles can be irreversibly aggregated, forming a VAS which resists dilution. This indicates a transformation from a colloidal glass to a colloidal gel. When the NaCl/HClinduced MC-CNC VASs were washed, they partly broke down, probably as a result of the higher counterion concentration due to the higher particle volume fraction at the VAS transition. The greater difference in ionic strength between (a) the MC-CNC VAS and the washing liquid (MilliQ water), than between (b) the MC-CNF and Milli-Q, creates a higher osmotic pressure and


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increases the driving force for water to enter and dilute the VAS. This may explain the breakup of the MC-CNC VAS and not of the MC-CNF. Transitions between colloidal glasses and gels when the ionic strength is changed have been reported for various systems of charged anisotropic particles, such as laponite disks11 and graphene oxide sheets43. Interestingly, this type of transition has previously been observed for CNF, but not identified, in a rheological study by Saito et al2. Strain ramps showed a soft nonflowing to flowing transition for the pH 8 CNF VAS, which changed to a brittle fraction of the VAS after the pH had been reduced to 2. The addition of acid also caused the storage modulus to increase by about two orders of magnitude. Thus, both the current study and the study by Saito et al show that nanocellulose dispersions can form two types of VASs, glasses and gels, with distinctly different properties. For the glasses the transition is reversible, where the particles are easily redispersed, whereas the gels are much stiffer with irreversibly aggregated particles, providing VASs which resist water dilution/submerging. CONCLUSIONS Our results show a strong correlation between the VAS threshold volume fractions for NCs and the inverse particle aspect ratio, but not such a clear correlation to the surface charge density of the particles. This indicates that the main driving force for the transition from dispersion to VAS is the increased particle mobility constraint, meaning that the VAS can be classified as a colloidal glass. This classification is also supported by the reversibility of CNF and CNC VASs, showing a non-flow to flow transition as the samples are diluted. The flowing phase obtained after dilution is a suspension of slightly aggregated particles, indicating partly attractive interparticle interactions, especially for the CNCs investigated.


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Our results also show a clear change in the network properties (increased relaxation time) when the electrostatic repulsion is reduced by the addition of acid or salt. In all cases, after repulsion reduction, the particle aggregation is irreversible, showing an increase in particle/aggregate size after dilution and strong mechanical agitation. These VASs are based on attractive interparticle interactions and are therefore classified as colloidal gels. ASSOCIATED CONTENT Supporting Information Hydrodynamic diameters of MC-CNF and MC-CNC at different ionic strengths. DLS results from dilution of the first batches of MC-CNF and MC-CNC. AUTHOR INFORMATION Present addresses §

A.F.: RISE Bioeconomy, Box 5604, SE-114 86 Stockholm, Sweden.

ACKNOWLEDGMENT Lars Ödberg is gratefully acknowledged for valuable input and support. Christina Schütz is thanked for the preparation of BNC. RISE Bioeconomy is acknowledged for providing the carboxymethylated CNF. G.N. acknowledges funding from the Swiss National Science Foundation Ambizione Grant No. PZ00P2_168023/1. The Wallenberg Wood Science Center is acknowledged for financial support. REFERENCES (1)

(2)

Fall, A. B.; Lindstrom, S. B.; Sprakel, J.; Wagberg, L. A physical cross-linking process of cellulose nanofibril gels with shear-controlled fibril orientation. Soft Matter 2013, 9 (6), 1852-1863. doi: 10.1039/c2sm27223g. Saito, T.; Uematsu, T.; Kimura, S.; Enomae, T.; Isogai, A. Self-aligned integration of native cellulose nanofibrils towards producing diverse bulk materials. Soft Matter 2011, 7 (19), 8804-8809. doi: 10.1039/c1sm06050c.


25

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13) (14)

(15) (16)

(17)

Page 26 of 29

Luo, Z.; Wang, S.; Zhang, S. Fabrication of self-assembling d-form peptide nanofiber scaffold d-EAK16 for rapid hemostasis. Biomaterials 2011, 32 (8), 2013-2020. doi: 10.1016/j.biomaterials.2010.11.049. Hakansson, K. M. O.; Fall, A. B.; Lundell, F.; Yu, S.; Krywka, C.; Roth, S. V.; Santoro, G.; Kvick, M.; Wittberg, L. P.; Wagberg, L.; Soderberg, L. D. Hydrodynamic alignment and assembly of nanofibrils resulting in strong cellulose filaments. Nat. Commun. 2014, 5, 10. doi: 10.1038/ncomms5018. Walther, A.; Timonen, J. V. I.; Diez, I.; Laukkanen, A.; Ikkala, O. Multifunctional HighPerformance Biofibers Based on Wet-Extrusion of Renewable Native Cellulose Nanofibrils. Adv. Mater. 2011, 23 (26), 2924-2928. doi: 10.1002/adma.201100580. Sehaqui, H.; Mushi, N. E.; Morimune, S.; Salajkova, M.; Nishino, T.; Berglund, L. A. Cellulose Nanofiber Orientation in Nanopaper and Nanocomposites by Cold Drawing. ACS Appl. Mater. Interfaces 2012, 4 (2), 1043-1049. doi: 10.1021/am2016766. Capadona, J. R.; Van Den Berg, O.; Capadona, L. A.; Schroeter, M.; Rowan, S. J.; Tyler, D. J.; Weder, C. A versatile approach for the processing of polymer nanocomposites with self-assembled nanofibre templates. Nat. Nanotechnol. 2007, 2 (12), 765-769. doi: 10.1038/nnano.2007.379. Almdal, K.; Dyre, J.; Hvidt, S.; Kramer, O. Towards a phenomenological definition of the term ‘gel’. Polymer Gels and Networks 1993, 1 (1), 5-17. doi: 10.1016/09667822(93)90020-I. Rehage, G. Thermodynamische Eigenschaften von Gelen. Berichte der Bunsengesellschaft für physikalische Chemie 1977, 81 (10), 968-979. doi: 10.1002/bbpc.19770811015. Arnott, S.; Fulmer, A.; Scott, W. E.; Dea, I. C. M.; Moorhouse, R.; Rees, D. A. The agarose double helix and its function in agarose gel structure. Journal of Molecular Biology 1974, 90 (2), 269-284. doi: 10.1016/0022-2836(74)90372-6. Tanaka, H.; Meunier, J.; Bonn, D. Nonergodic states of charged colloidal suspensions: Repulsive and attractive glasses and gels. Physical Review E 2004, 69 (3), 031404. doi: 10.1103/PhysRevE.69.031404. Buining, P. A.; Philipse, A. P.; Lekkerkerker, H. N. W. Phase-behavior of aqueous dispersions of colloidal boehmite rods. Langmuir 1994, 10 (7), 2106-2114. doi: 10.1021/la00019a016. Kang, K.; Dhont, J. K. G. Structural arrest and texture dynamics in suspensions of charged colloidal rods. Soft Matter 2013, 9 (17), 4401-4411. doi: 10.1039/c3sm27754b. Ruzicka, B.; Zulian, L.; Zaccarelli, E.; Angelini, R.; Sztucki, M.; Moussaïd, A.; Ruocco, G. Competing Interactions in Arrested States of Colloidal Clays. Phys. Rev. Lett. 2010, 104 (8), 085701. doi: 10.1103/PhysRevLett.104.085701. Zaccarelli, E. Colloidal gels: equilibrium and non-equilibrium routes. J. Phys.: Condens. Matter 2007, 19 (32), 50. doi: 10.1088/0953-8984/19/32/323101. Appel, J.; Folker, B.; Sprakel, J. Mechanics at the glass-to-gel transition of thermoresponsive microgel suspensions. Soft Matter 2016, 12 (9), 2515-2522. doi: 10.1039/c5sm02940f. Zargar, R.; Nienhuis, B.; Schall, P.; Bonn, D. Direct Measurement of the Free Energy of Aging Hard Sphere Colloidal Glasses. Phys. Rev. Lett. 2013, 110 (25), 258301. doi: 10.1103/PhysRevLett.110.258301.


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Page 27 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(18)

(19)

(20) (21)

(22)

(23)

(24)

(25)

(26)

(27)

(28)

(29)

(30)

(31)

Wierenga, A.; Philipse, A. P.; Lekkerkerker, H. N. W.; Boger, D. V. Aqueous Dispersions of Colloidal Boehmite: Structure, Dynamics, and Yield Stress of Rod Gels. Langmuir 1998, 14 (1), 55-65. doi: 10.1021/la970376z. Honorato Rios, C.; Kuhnhold, A.; Bruckner, J.; Dannert, R.; Schilling, T.; Lagerwall, J. P. F. Equilibrium Liquid Crystal Phase Diagrams and Detection of Kinetic Arrest in Cellulose Nanocrystal Suspensions. Frontiers in Materials 2016, 3. doi: 10.3389/fmats.2016.00021. Araki, J.; Wada, M.; Kuga, S.; Okano, T. Birefringent Glassy Phase of a Cellulose Microcrystal Suspension. Langmuir 2000, 16 (6), 2413-2415. doi: 10.1021/la9911180. Shafiei-Sabet, S.; Hamad, W. Y.; Hatzikiriakos, S. G. Influence of degree of sulfation on the rheology of cellulose nanocrystal suspensions. Rheologica Acta 2013, 52 (8), 741751. doi: 10.1007/s00397-013-0722-6. Shafiei-Sabet, S.; Hamad, W. Y.; Hatzikiriakos, S. G. Rheology of Nanocrystalline Cellulose Aqueous Suspensions. Langmuir 2012, 28 (49), 17124-17133. doi: 10.1021/la303380v. Ureña-Benavides, E. E.; Ao, G.; Davis, V. A.; Kitchens, C. L. Rheology and Phase Behavior of Lyotropic Cellulose Nanocrystal Suspensions. Macromolecules 2011, 44 (22), 8990-8998. doi: 10.1021/ma201649f. Iwamoto, S.; Isogai, A.; Iwata, T. Structure and Mechanical Properties of Wet-Spun Fibers Made from Natural Cellulose Nanofibers. Biomacromolecules 2011, 12 (3), 831836. doi: 10.1021/bm101510r. Hakansson, K. M. O. Online determination of anisotropy during cellulose nanofibril assembly in a flow focusing device. RSC Advances 2015, 5 (24), 18601-18608. doi: 10.1039/c4ra12285b. Wågberg, L.; Decher, G.; Norgren, M.; Lindström, T.; Ankerfors, M.; Axnäs, K. The Build-Up of Polyelectrolyte Multilayers of Microfibrillated Cellulose and Cationic Polyelectrolytes. Langmuir 2008, 24 (3), 784-795. doi: 10.1021/la702481v. Saito, T.; Kimura, S.; Nishiyama, Y.; Isogai, A. Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose. Biomacromolecules 2007, 8 (8), 24852491. doi: 10.1021/bm0703970. Salajkova, M.; Berglund, L. A.; Zhou, Q. Hydrophobic cellulose nanocrystals modified with quaternary ammonium salts. Journal of Materials Chemistry 2012, 22 (37), 1979819805. doi: 10.1039/c2jm34355j. Uhlig, M.; Fall, A.; Wellert, S.; Lehmann, M.; Prevost, S.; Wagberg, L.; von Klitzing, R.; Nystrom, G. Two-Dimensional Aggregation and Semidilute Ordering in Cellulose Nanocrystals. Langmuir 2016, 32 (2), 442-450. doi: 10.1021/acs.langmuir.5b04008. Schütz, C.; Agthe, M.; Fall, A. B.; Gordeyeva, K.; Guccini, V.; Salajková, M.; Plivelic, T. S.; Lagerwall, J. P. F.; Salazar-Alvarez, G.; Bergström, L. Rod Packing in Chiral Nematic Cellulose Nanocrystal Dispersions Studied by Small-Angle X-ray Scattering and Laser Diffraction. Langmuir 2015, 31 (23), 6507-6513. doi: 10.1021/acs.langmuir.5b00924. de la Torre, J. G.; Bloomfield, V. A. Hydrodynamic properties of complex, rigid, biological macromolecules: theory and applications. Quarterly Reviews of Biophysics 1981, 14 (01), 81-139. doi: 10.1017/s0033583500002080.


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(32)

(33) (34)

(35) (36)

(37) (38) (39)

(40)

(41)

(42)

(43)

Page 28 of 29

Fadda, G. C.; Lairez, D.; Pelta, J. Critical behavior of gelation probed by the dynamics of latex spheres. Physical Review E 2001, 63 (6), 061405. doi: 10.1103/PhysRevE.63.061405. Shibayama, M.; Norisuye, T. Gel formation analyses by dynamic light scattering. Bulletin of the Chemical Society of Japan 2002, 75 (4), 641-659. doi: 10.1246/bcsj.75.641. Pusey, P. N.; Van Megen, W. Dynamic light scattering by non-ergodic media. Physica A: Statistical Mechanics and its Applications 1989, 157 (2), 705-741. doi: 10.1016/03784371(89)90063-0. Pusey, P. N.; Van Megen, W. Phase-behavior of concentrated suspensions of nearly hard colloidal spheres. Nature 1986, 320 (6060), 340-342. doi: 10.1038/320340a0. Wilkins, G. M. H.; Spicer, P. T.; Solomon, M. J. Colloidal System To Explore Structural and Dynamical Transitions in Rod Networks, Gels, and Glasses. Langmuir 2009, 25 (16), 8951-8959. doi: 10.1021/la9004196. Solomon, M. J.; Spicer, P. T. Microstructural regimes of colloidal rod suspensions, gels, and glasses. Soft Matter 2010, 6 (7), 1391-1400. doi: 10.1039/b918281k. Israelachvili, J. N. Intermolecular and Surface Forces; 3rd ed.; Burlington : Elsevier Science: Burlington, 2010. Fall, A. B.; Lindstrom, S. B.; Sundman, O.; Odberg, L.; Wagberg, L. Colloidal Stability of Aqueous Nanofibrillated Cellulose Dispersions. Langmuir 2011, 27 (18), 1133211338. doi: 10.1021/la201947x. Fukuzumi, H.; Tanaka, R.; Saito, T.; Isogai, A. Dispersion stability and aggregation behavior of TEMPO-oxidized cellulose nanofibrils in water as a function of salt addition. Cellulose 2014, 21 (3), 1553-1559. doi: 10.1007/s10570-014-0180-z. Shafiei-Sabet, S.; Hamad, W. Y.; Hatzikiriakos, S. G. Ionic strength effects on the microstructure and shear rheology of cellulose nanocrystal suspensions. Cellulose 2014, 21 (5), 3347-3359. doi: 10.1007/s10570-014-0407-z. Hough, L. A.; Islam, M. F.; Janmey, P. A.; Yodh, A. G. Viscoelasticity of Single Wall Carbon Nanotube Suspensions. Phys. Rev. Lett. 2004, 93 (16), 168102. doi: 10.1103/PhysRevLett.93.168102. Konkena, B.; Vasudevan, S. Glass, Gel, and Liquid Crystals: Arrested States of Graphene Oxide Aqueous Dispersions. The Journal of Physical Chemistry C 2014, 118 (37), 21706-21713. doi: 10.1021/jp507266t.


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