Effect of Source on the Properties and Behavior of Cellulose

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Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Effect of Source on the Properties and Behavior of Cellulose Nanocrystal Suspensions Christina Schütz,*,†,⊥ Jonas Van Rie,† Samuel Eyley,† Alican Gençer,† Hans van Gorp,∥ Sabine Rosenfeldt,‡ Kyongok Kang,§ and Wim Thielemans*,† †

Renewable Materials and Nanotechnology Research Group, Department of Chemical Engineering, KU Leuven, Campus Kulak Kortrijk, Etienne Sabbelaan 53, 8500 Kortrijk, Belgium ‡ Physical Chemistry I and Bavarian Polymer Institute, University Bayreuth, Universitätsstrasse 30, 95440 Bayreuth, Germany § Forschungszentrum Jülich, Institute of Complex Systems (ICS-3), 52425 Jülich, Germany ∥ Division of Molecular Imaging and Photonics, Department of Chemistry, KU Leuven, Celestijnenlaan, 200 F, 3001 Leuven, Belgium S Supporting Information *

ABSTRACT: Sulfuric acid hydrolysis of native cellulose fibers results in colloidally stable suspensions of cellulose nanocrystals (CNCs). We have investigated the effect of the cellulose source on the suspension properties of CNCs extracted from cotton and wood sources using a comparable preparation strategy. The structural properties were revealed to be similar within the given standard deviation and prevalent polydispersity, whereas other properties such as liquid crystalline phase behavior, viscosity, diffusion coefficients, and surface tension were found to differ significantly. This study shows that ostensibly similar suspensions may exhibit rather differing behaviors and attempts to interpret this phenomenon. This finding shows that full characterization and a detailed description of the preparation of the nanocrystals used in publications are extremely important and should be reported in detail in all instances.

KEYWORDS: Cellulose nanocrystals, Rheology, Liquid crystals, Phase separation, Diffusion coefficients

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The first time CNCs were isolated by sulfuric acid hydrolysis from cotton and wood was by Nickerson and Habrle in 19477 and Rånby and Ribi in 1950.8 Marchessault et al.9 later reported on the birefringent properties of these materials. However, it was only in 1992 that Revol and co-workers showed the formation of a chiral nematic LC phase of aqueous CNC suspensions.10 Since then the interest in these promising materials and their properties due to their abundance, sustainability, and versatility has constantly increased.11−13 The chiral nematic phase of the CNCs is characterized by the periodicity of the helical modulation in the direction in which the CNCs align locally, and is usually characterized by polarized optical microscopy.14 Studies on the development of the LC phase behavior have been conducted by varying the acid hydrolysis conditions and therefore the resulting aspect ratio of the CNCs,15−19 the surface chemistry of the CNC particles,11 and the ionic strength of the dispersion medium.20−22 As shown already by Shafiei-Sabet and co-workers23 the sonication energy used to obtain a stable suspension, as well as the CNC concentration,24 has a crucial impact on the flow behavior of the CNC suspension. Beck-Candanedo et al.15 published in

INTRODUCTION

Novel nanomaterials based on renewable resources are attracting a rapidly growing interest across international science and technology research fields, largely in the context of developing new composite materials with enhanced properties and functionalities derived from the nanomaterials and the structures they form. An important inspiration is the extraordinary performance of biological composites, like bone, nacre, wood, crustacean shells, and butterfly wings, deriving their unique mechanical and/or optical properties from a regular interior nano-/microstructuring of the constituents.1 Particular attention is focused on cellulose, partially because it is the most versatile and abundant biopolymer in nature, but even more so because of the possibility to isolate and utilize novel forms of cellulose that have at least one dimension in the nanometer range.2 Such nanocellulose features an attractive combination of properties that could result in various applications, exploiting, for example, the stiffness of the cellulose nanocrystals (CNCs), as well as their ability to form a lyotropic chiral nematic liquid crystalline (LC) phase in aqueous suspension.3 However, to fully harness the intrinsic nanoparticle properties4 in the design of novel advanced composite materials,5,6 a better understanding of their colloidal behavior as a function of their source is required. © XXXX American Chemical Society

Received: January 22, 2018 Revised: May 3, 2018

A

DOI: 10.1021/acssuschemeng.8b00334 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

solution with a concentration of 0.013 M, standardized against oxalic acid solution. All titrations were repeated three times, and the average value is reported. The standard deviation of the measurements was 5− 7%. Water content was determined as the mass loss at 125 °C and used to correct the elemental analysis data. Elemental analysis (C, H, N, S) data were collected on a Thermo Flash 2000 elemental analyzer (Thermo Fisher Scientific, Inc.) using 5-tert-butyl-2-[5-(5-tert-butyl1,3-benzoxazol-2-yl)thiophen-2-yl]-1,3-benzoxazole (BBOT) as a calibration standard for C and H content and enriched phenanthrene as a standard for nitrogen and sulfur content with linear calibration. Carbon, hydrogen, nitrogen, and sulfur contents were reported as mass percentage of the total sample. The degree of substitution (DS) was calculated using an iterative procedure whereby the empirical formula of the graft added to cellulose was added to the empirical formula of the anhydroglucose unit until the percentage sulfur was equal in the calculated elemental analysis results and the actual results.25 Atomic Force Microscopy. On a fresh cleaved mica, a drop of poly(L)-lysine (0.01 wt %, 20 μL) was deposited, allowed to react for 3 min, and rinsed with Milli-Q water prior to drying under an Ar gas flow. A droplet of CNC suspension (0.001 wt %, 20 μL) was deposited for 3 min, rinsed with Milli-Q water, and dried under Ar gas flow. AFM images were obtained in tapping mode using an Asylum Research Cypher ES microscope with Olympus AC160TS-R3 probes with a spring constant of 26 (11−54) N m−1 and a frequency of 300 ± 100 kHz. The scan rates were varied between 0.20 and 0.56 Hz dependent on the sample conditions. Size distributions were determined by measuring 250−300 particles using ImageJ software,26 version 1.48v; only clearly distinguishable particles were considered. Determination of Volume Fraction of LC Phase. The dispersions with given CNC content were filled into glass vials with a diameter of 1 cm and a total volume of ca. 4 mL and equilibrated at ambient conditions for the respective times in the sealed vials. The images of the vials were taken between crossed polarizers. The images were analyzed using ImageJ software,26 establishing the ratio between the total height of the sample and the height of the anisotropic phase. The accuracy of the manual measurements was better than 10%. Small-Angle X-ray Scattering. Small-angle X-ray scattering was performed using suspensions of 0.01, 3.5, and 5.5 wt % in water, which were measured in 1 mm glass capillaries (Hilgenberg, code 4007610) at room temperature with the small-angle X-ray system “Double Ganesha AIR” (SAXSLAB). The X-ray source of this laboratory-based system is a rotating anode (copper, MicroMax 007HF, Rigaku Corporation) providing a microfocused beam. The data are recorded with a position-sensitive detector (PILATUS 300 K, Dectris). To cover the range of scattering vectors, different detector positions were used. The circularly averaged data were normalized to the incident beam, sample thickness, and measurement time before subtraction of the solvent. The subsequent analysis was done with Origin 8.5 and SasView 3.0 software.27 Rheology. Aqueous suspensions of CNCs were analyzed using an AR-G2 stress-controlled rotational rheometer (TA Instruments). Steady shear viscosity versus shear rate curves were obtained for each sample at shear rates between 10 and 100 s−1. For each sample, the time required to reach steady state was determined by a transient test whereas amplitude sweep experiments at a frequency of 10 rad s−1 were used to determine the strain for frequency sweep experiments. All of the suspensions were tested at 20 °C using double-wall concentric cylinder geometry with gap sizes of 380 and 420 μm. A solvent trap was used to avoid water evaporation. Depolarized Dynamic Light Scattering. Depolarized dynamic light scattering (DDLS) was performed, in collaboration with Forschungszentrum Juelich (Soft-Condensed Matter group, FZJ, Germany), with an ALV 5000 autocorrelator to determine the scattered light electric field time autocorrelation function for each suspension using a polarized laser (532 nm, Spectra-Physics/ Newport). The samples were filtered with a 5 μm hydrophobic PTFE syringe filter to remove dust particles and were filled in NMR tubes (Wilmad NMR tubes, 5 mm diameter). For removal of air bubbles, the filled NMR tubes were sonicated in an ultrasound bath for 1−2 min.

2005 a study where they compared different wood sources and preparation conditions concluding that the size is directly affected by the chosen conditions whereas the influence on the surface change was not apparent. This is just one example of the tremendous impact of small changes in the CNC production process. Consequently, a comparison between different studies can be difficult, especially when extensive characterization data (such as surface charge, particle size, and elemental composition) on the CNCs employed are not included in the published report. On account of this difficulty, this work focuses on a preparation strategy applicable to both cotton and wood, and studies the resulting CNC suspensions with the aim to investigate the effect of the source material on suspension properties, independent of other variables in the production procedure. The intrinsic properties of the starting CNCs were found to be comparable within the statistical error. However, the ensuing properties of their colloidal suspensions such as viscosity, surface tension, diffusion coefficients, and helical pitch were found to differ significantly. This finding shows how important it is to report full details on the preparation procedure and full characterization results of the CNCs, as well as to follow procedures very accurately when comparing or using different batches of CNCs.

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EXPERIMENTAL SECTION

Preparation of CNCs. CNCs were prepared by sulfuric acid hydrolysis of blended, dried Domsjö dissolving wood pulp (60/40 spruce/pine, 95% cellulose, 4.5% hemicellulose, and 0.3% lignin), which was cut up using a kitchen blender, or of cotton wool (German Pharmacopeia grade, Chem-lab Analytical), which was cut up into approximately 2 × 2 × 0.5 cm3 pieces. Starting materials were used as received. A 50 g portion of the starting material was added over 10 and 25 min, for wood and cotton, respectively (due to different fiber size and mixing dynamics and wetting), in 450 mL of 64% sulfuric acid (acid strength verified by titration) at 45 °C and stirred by a Teflon blade using an overhead stirrer at 400 rpm. The total hydrolysis time from the start of addition was 60 min. The reaction was quenched by 10-fold dilution with deionized (DI) water at room temperature. The resulting dispersion was washed with fresh DI water using a centrifuge at 4 °C for 10 min at 10 000 rpm until a further sedimentation was not possible. The supernatant and precipitate were collected and dialyzed with tap water using a Spectra/Por 4 dialysis membrane with a molecular weight cut off of 12−14 kDa and a width 48 mm for 2−3 days. After the dialysis the dispersion was sonicated for ∼5 min with a pulse program (3 s on and 1 s off and until suspension temperature reached 39 °C) using a 12 mm probe with an output of 70% (Branson Digital Sonifier, 250) resulting in 12.4 kW dm−3 applied to a 200 mL CNC suspension (∼1 wt % CNC content). Immediately after the sonication procedure the suspensions were filtered through a frittedglass filter (porosity 2). The obtained filtrate was mixed with Amberlite MB-6113 mixed bed ion-exchange resin and stirred for 24 h. The suspension was filtered again and centrifuged at 6000 rpm and 4 °C for 20 min to remove remaining aggregates. The resulting suspension was concentrated by magnetically stirred evaporation at room temperature to ∼10 wt % and subsequently diluted with Milli-Q water to the desired concentrations. Samples used in further experiments were thus never dried, unless required for a specific analysis (e.g. elemental analysis). The concentrations were determined by gravimetry performed on a Netzsch F3 Tarsus instrument under nitrogen atmosphere. Samples were heated at 10 °C min−1 to 85 °C, followed by an isothermal period for 30 min, then continued to be heated at 10 °C min−1 to 150 °C. Surface Charge Determination. The number of sulfate half-ester groups on the surface of CNCs was determined by elemental analysis and acid−base titration, performed using an SI Analytics Titroline 6000 autotitrator. Samples were titrated with sodium hydroxide B

DOI: 10.1021/acssuschemeng.8b00334 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering The laser beam was vertically polarized using a Glan prism polarizer (B. Halle, extinction ratio 105). The analyzer (Glan-Thompson prism analyzer, B. Halle, extinction ratio 10−6) was horizontally positioned with respect to the scattering plane. A single mode fiber (OZ) and a focusing collimator (LINOS MB 02) together with a photomultiplier pair detector were used. The photon counts were fed into a correlator (ALV 5000/E Multiple Tau correlator with Fast option). All measurements were carried out between 45° and 120° with an angular step of 5° using the ALV/LSE5001/II controller. A bath of toluene was used to control the temperature and match the refractive index of the glass sample holder. The data were analyzed by fitting two collective modes: one is a fast-mode corresponding to a parallel component of thermal fluctuations of CNCs, whereas the other is a slow-mode for

(magnetic/electrostatic) optics (slot aperture), hemispherical analyzer, multichannel plate, and delay line detector (DLD) with a takeoff angle of 90°. The analyzer was operated in fixed analyzer transmission (FAT) mode with survey scans taken with a pass energy of 160 eV and high-resolution scans with a pass energy of 20 eV. All scans were acquired under charge neutralization conditions using a low-energy electron gun within the field of the magnetic lens. Three areas per sample were analyzed and summed where appropriate to enhance the signal-to-noise ratio. The resulting spectra were processed using CasaXPS software. Binding energy was referenced to aliphatic adventitious carbon at 285 eV. High-resolution spectra were fitted using the “LF(α, β, w, m)” line shape corresponding to a numerical convolution of Lorentzian functions (with exponents α and β for the high-binding-energy and low-binding-energy sides) with a Gaussian (width m) and inclusion of tail-damping (w) to provide finite integration limits. Details of this line shape function are available in the CasaXPS documentation online.28 Fitting parameters and constraints for each of the peaks are given in the SI (Figures S1−S4 and Tables S1−S4). Kratos relative sensitivity factors (RSFs) were used for quantification. Quantification is made based on the assumption of atomically smooth surfaces and uniform depth distribution of elements within the analysis area. Quoted errors are based on Monte Carlo simulations of the stability of the fitted parameters (integration regions and component peaks) with respect to noise and do not take into account other errors due to, e.g., incorrect application of RSFs, and sample assumptions. Polarized Light Microscopy. The samples at different concentrations were prepared by filling capillaries (Vitrotubes 0.40 × 4.00 mm2) by capillary forces and sealing the ends with epoxy glue (Pattex Super Mix Metal) to avoid evaporation. The suspensions were equilibrated for 1 month before visualization. The suspensions were imaged using an Olympus BX 51 microscope with crossed polarizers. The pitch values were extracted by image analysis using ImageJ software,26 version 1.48v, and measuring 20−50 times at different locations within the capillaries a distance m, where m = 5p resulting in the reported pitch values p. Pendant Drop Tensiometry. The surface tension of pendant drops containing 3.5 and 5.5 wt % of CNCs was measured with a CAM200 (KSV NIMA) instrument at room temperature. Measurements were taken continually until no variation in drop shape was recorded anymore. The measurement precision was determined using the Worthington number, Wo, which was calculated to be ∼0.8,29 while the dominance of surface tension forces over gravitational forces was confirmed by the Bond/Eötvös number being lower than 0.3.

Figure 1. Normalized intensity autocorrelation functions for selected scattering angles of a 3.5 wt % wood CNC sample. perpendicular motions. An example is shown in Figure 1. The fitting function was chosen as

gE(t ) = A f exp{− (Γf t )} + A s exp{− (Γst )β } + B

(1)

where gE(t) is the field-correlation function, Af and As are the field amplitudes, Γf and Γs are the relaxation rates for the fast- and slowmodes, β is the stretching exponent power, and B is a background value. Typically, contributions of the amplitude values are higher on the “stretched” slow-mode, obtained with relatively high polydispersity of the value β ∼ 0.5. From the dispersion curves of relaxation rates versus scattering wavevector, both anisotropic translational diffusion coefficients and rotational diffusions are extracted, as Γ ∼ Dt,par,perpq2 + 6Dr, where each of the diffusion constants is presented in Table 2. X-ray Photoelectron Spectroscopy. Spectra were recorded on a Kratos Axis Supra X-ray photoelectron spectrometer employing a monochromated Al Kα (hν = 1486.7 eV, 120 W) X-ray source, hybrid

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RESULTS AND DISCUSSION The effect of varying the cellulose source on the resulting cellulose nanocrystal suspensions was investigated using cotton and wood. On the basis of the literature,11 the structural

Figure 2. Structural properties of CNCs from cotton and wood. (a) Histogram of the length distribution of CNC suspension (0.001 wt %) measured from AFM images of (b) cotton and (c) wood. A minimum of 300 particles were measured for each sample. C

DOI: 10.1021/acssuschemeng.8b00334 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 1. Summary of Intrinsic Properties of the CNCs from Cotton and Wood length L (AFM) average ± STD [nm] median [nm] length L (AFM) log-normal μ [nm] STD (σ) form factora (SAXS) short side a [nm] form factora (SAXS) longer side b [nm] aspect ratio pb surface charge (acid−base titration) [mmol g−1] surface chargec (acid−base titration) [e nm−2] surface charge (EA) [mmol g−1] surface chargec (EA) [e nm−2] a

cotton

wood

130 ± 60 121 125 (0.57) 4.8 ± 2.1 24 ± 12 11.3 0.12 0.22 0.19 0.35

116 ± 67 101 101 (0.49) 3.7 ± 1.8 14.8 ± 7.4 13.6 0.15 0.21 0.20 0.28

Form factor determined by fitting a rectangular prism model to SAXS curves. Polydispersity was fitted with Gaussian distribution, and for the longer

side it was set to 0.5 for simplification. bThe aspect ratio was calculated p =

L3 abL

based on ref 35. cAccording to Araki et al.18 on the basis of

rectangular cross section, obtained from the form factor (SAXS) and the median of the length (AFM).

difference lies within the standard deviation of the measurements. However, XPS confirms as well a higher sulfur content for wood than for cotton with S/O−C−O ratios (equivalent to DS) of 0.04 and 0.03, respectively (see the SI, Tables S1 and S2). This is however reversed when the particle size is taken into account, and the surface charge is reported as a unit charge per surface area, instead of the surface charge in mmol per gram cellulose. These results show the consequences of reporting the surface charge either with respect to the particle size or surface area, which can be quite interesting for the reader with respect to reported properties. The rheological behavior of CNC suspensions in water at 3.5 and 5.5 wt % was established using shear measurements followed by fitting the Cross model to the experimental data enabling the determination of the zero shear viscosity (Figure 3). The resulting zero shear viscosity values were as expected

properties between the two different sources, cotton and wood, vary little; however, detailed comparisons were so far impossible since the preparation conditions and suspension treatments have varied significantly depending on the source material. As compensation for the different morphology and wetting behavior of the starting materials, blended dried wood pulp versus fibrous cotton, the addition time, and thus the final reaction time, was adjusted. Cotton was slowly added over 25 min to provide a proper wetting of the cellulose fibers with the sulfuric acid. This is in contrast to the blended wood pulp, which could be added over 10 min. However, the final hydrolysis time for both materials was kept at 60 min. Both resulting suspensions were treated the same way, as described in detail in the Experimental Section. The structural properties were characterized by atomic force microscopy (Figure 2) and fitting of a rectangular prism model30,31 form factor by using the valid decoupling approximation to small-angle X-ray scattering (SAXS) curves (see Figures S5 and S6). The results are summarized in Table 1. The length shows a typical lognormal distribution in length defined by the location parameter μ and a scale parameter σ. Within the measurement errors and the standard deviations depending on the applied mathematical method, the length distribution is the same. The SAXS intensities could be described by a rectangular prism model resulting in a ratio between the short and wider side of 5:1 and 4:1, for cotton and wood, respectively. This clearly shows that the cross section is not circular, and a more appropriate description of CNCs would be a nanobeam or nanoribbon with a rectangular cross section, as previously reported.32 The surface charge was determined by acid−base titration resulting in 0.12 and 0.15 mmol g−1, for cotton and wood, respectively. The same relation was found in the sulfur content determined by elemental analysis (Table 1), which should be present only in the form of sulfate half-esters on the CNC surface. The lower values found using acid−base titration can be explained by the use of mixed bed resin, which has been shown to result in underestimation of the sulfate content by titration.33,34 For comparability the actual amount of surface charges per square nanometer was calculated assuming a rectangular cross section and using the median dimensions given in Table 1. The resulting surface charges on a weight basis are slightly higher for wood than for cotton when determined via acid− base titration and elemental analysis. One could argue that the

Figure 3. Rheological behavior of cotton and wood CNCs at different concentrations. The lines are the respective fits to the Cross model.

dependent on the concentration: for cotton η0,3.5 = 63 mPa s, η0,5.5 = 391 mPa s, and for wood η0,3.5 = 127 mPa s, η0,5.5 = 207 mPa s. The zero shear viscosity values for each sample were used to calculate the theoretical diffusion coefficients according to36 Dt⊥ = D

⊥ 1 kBT (ln p + C t ) πηsL 4

(2) DOI: 10.1021/acssuschemeng.8b00334 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 2. Calculated and Experimentally Determined Rotational and Translational Diffusion Coefficients cotton

wood

3.5 wt % theor Dsr [s−1] D∥t [μm2 s−1] D⊥t [μm2 s−1] a

a

203 0.17 0.13

5.5 wt % a

exptl

theor

14.7 0.46 0.26

33 0.03 0.02

3.5 wt % a

exptl

theor

3.3 0.06 0.06

292 0.11 0.08

5.5 wt % a

exptl

theor

exptl

25 0.11 0.04

180 0.07 0.05

22 0.01 0.01

The theoretical values are estimated in dilute conditions contrary to the experimental data and should only serve as a guideline.

Dt =

Dr =

1 kBT (ln p + C t ) 2 πηsL

the experimental and calculated values. This discrepancy between theoretical and experimental values can mainly be explained because the calculations require dilute conditions and non-interacting rods in the suspensions. Intriguingly, a much sharper decrease of the rotational diffusion occurs in the case of cotton CNCs, when compared to wood CNCs, for an increase of the weight concentration (Table 2). This is also reflected in a more drastic change in zero shear viscosity for cotton compared to wood CNC dispersions. The macroscopic appearance of suspensions, standing undisturbed for 16 weeks with the respective polarized light microscopy images equilibrated for 1 month in glass capillaries prior to visualization, is shown in Figure 4. The relative volume

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kTp2 A 0πηsL3(1 + Cr)

(4)

where L is the rod length, p the aspect ratio p = L/d, ηs the viscosity of the solvent (for water 8.9 × 10−4 kg m−1 s−2), C⊥t = 0.839 + 0.185/p + 0.233/p2, C∥t = −0.207 + 0.980/p − 0.133/ p2, and Cr = 0.677/p − 0.183/p2.36 The diameter d was calculated on the basis of the average of the short and long side of the respective particles. Use of the zero shear viscosity, rather than the pure water viscosity, allows us to lump the hindrance to movement effects into the viscosity as a mean field approximation. The experimental diffusion coefficients were determined by depolarized dynamic light scattering [vertical−horizontal (VH) positioning of the polarizer and the analyzer], and both experimental and calculated values, of which the latter are either smaller or of the same order of magnitude, are summarized in Table 2. Generally, measurements of diffusion coefficients are performed under dilute conditions to allow free movement of the particles, which have been published for CNCs in aqueous media.37−41 The general trend of decreasing diffusion coefficients with increasing concentration of particles is confirmed. Indeed as more particles are present in the medium, their freedom of movement is hindered by the other particles resulting in longer diffusion times and thus reduced diffusion coefficients. The Brownian motion of negatively charged CNCs is also expected to be slowed down because of the presence of counterions in the double layer, known as the electroviscous effect.42,43 The higher surface charge per unit surface area and the larger size for cotton with respect to wood explain the lower rotational diffusion coefficients for cotton-derived CNCs. By calculating the characteristic times for rotation according to Ortega et al.44 τ=

1 (5Dr⊥ + Dr )

Figure 4. Macroscopic appearance between crossed polarizers of CNC suspensions in 4 mL glass vials from cotton and wood at two different concentrations. The polarized light microscopy images are representative for the anisotropic phase of each respective suspension. The scale bars in all images in the middle row are 100 μm and their zoom-ins 25 μm.

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fraction of the LC phase, where the anisotropic phase densifies at the bottom and where the isotropic phase rests on top, increased for cotton from 0.17 to 0.50, when changing from 3.5 to 5.5 wt %, whereas wood spanning the same CNC concentration gives a larger LC phase fraction range from 0.20 to 0.72 determined after 16 weeks. The helical pitch determined by image analysis of polarized light microscopy revealed the expected trend for the wood suspensions in contrast to the cotton samples. For cotton the pitch barely decreased with increasing concentration, from 16.6 to 16.4 μm, whereas for wood a significant decrease is observed, from 12.7 to 8.1 μm, comparable to pitch values determined by laser diffraction from the same wood source.3 We also studied the

using the experimentally determined rotational diffusion coefficients, the trend seen already in the viscosity values is confirmed. For cotton the rotation time is an order of magnitude longer, 0.03 and 0.3 s, when the concentration changes from 3.5 to 5.5 wt %, respectively, which is also reflected in the change of the viscosity. On the other hand, the viscosity values of wood do not change that drastically with increasing concentration, from 3.5 to 5.5 wt %, and the rotation times change only slightly from 0.04 to 0.05 s. The experimental values for the rotational diffusion coefficients were found to be always about an order of magnitude smaller than the calculated ones, but the trends were similar between E

DOI: 10.1021/acssuschemeng.8b00334 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering separation distance between particles by SAXS and found the expected trend3 confirmed; i.e., the separation distance decreased with increasing concentration. Indeed, with increasing mass content the nanobeams are forced closer together, and the separation distance is expected to decrease as a result. However, when we compare wood and cotton at the same concentrations, the particle−particle distance d was always found to be smaller for wood when compared to cotton; at 3.5 wt % a separation distance of 60 nm was observed for cotton whereas for wood 47 nm was found. The scaling relation d ∼ ϕ−β of the d-spacing as a function of the volume fraction of cellulose in water ϕ (Figure S7) gives information about different topologies in lyotropic LC phases.45−47 We observed nearly the same exponent for both systems, cotton and wood, with varying volume fraction of CNCs in water, confirming similar behavior with a change in concentration. The difference in separation distance for wood found in this work compared to wood CNCs from the same source presented in earlier work at the same concentrations3 can be explained by the longer hydrolysis time used in this study giving rise to a higher surface charge (0.14 e nm−2 versus 0.21 e nm−2).3 Furthermore, the onset of the LC phase ϕonset = 3.3 d*/L (d* is the diameter) based on Onsager48 is expected to occur for cotton at φcel = 0.393 (wcel = 0.52) whereas for wood it should happen at φcel = 0.302 (wcel = 0.41), which is more than 1 order of magnitude higher than experimentally determined values as also reported in the literature.16 However, the qualitative relation for the onset concentration for liquid crystallinity based on the aspect ratio (see Table 1) is confirmed; ϕonset for wood is lower than for cotton. One obvious reason for the discrepancy between theory and experiments is the presence of surface charge, which is not accounted for in Onsager’s model. In principle a higher surface charge density means a greater effective volume fraction for the same mass fraction since the electrostatic repulsion yields an excluded volume that is greater than the physical volume of the cellulose nanocrystal itself. However, the matter is complicated by the fact that the charged CNCs also bring counterions into solution, increasing the ionic strength and thus leading to greater screening of the electric charges on the surface of the CNC particles, which reduces the electrostatic effect on the excluded volume. This also means that the effective volume fraction (CNC size including the electronic double layer) decreases for increasing weight fraction as more CNCs are added to the suspension. Because of this latter aspect, the linear increase for isotropic to liquid crystalline sample fraction, as a function of increasing CNC mass fraction as predicted by Onsager, is never observed independently of the time frame3 in the biphasic region of CNC suspensions.20 The amount of surface charge has an important effect on the onset and end volume fraction of the isotropic−nematic biphasic coexistence region of the CNC suspensions. With more sulfate groups, the CNC suspension has a tendency to develop a LC phase at lower concentrations, a trend we can relate to the increased effective rod volume (here the rod is used as a simplification), which comes with the increased surface charge. However, the increase in effective radius leads to decreased aspect ratio, which would promote the opposite effect, which is disguised by the change in the effective rod volume.49 On the basis of the concentration and the surface charge, the Debye length has been calculated:

κ −1 =

0.304 cσ 2

(6)

where σ in this case is the surface charge, and c is the concentration of the suspension assuming a monovalent salt. The Debye length comes out at 4.7 and 4.2 nm for 3.5 wt % cotton and wood, respectively. Even when we consider the Debye length for the onset concentration, the calculated concentration values for the onset of the LC phase are still too high. Moreover, the particle diameter and the Debye length with respect to the center-to-center separation distance measured by SAXS suggest that even at 5.5 wt % the particles are far enough apart to allow translational diffusion between the particles as seen in the experimental data. However, the translational diffusion coefficient for wood, approaching zero, suggests a transition to limited translational diffusion, which can be confirmed by simple square packing geometrical considerations and a continuous decreasing separation distance with increasing concentration. In this 5.5 wt % wood sample, the CNC particles experience significant hindrance at a measured separation distance of 36 nm. Considering the separation distance between neighboring CNC particles and the helical pitch, the twist3 the particles experience is much higher for wood than it is for the cotton CNCs. With increasing concentration the twist angle, calculated3 as θ = 360dp−1

(7)

where d is the separation distance and p the helical pitch, increases for wood from 1.3° to 1.6° while for cotton the value even decreases, from 1.4° to 1.1°. This in turn explains the lower translational freedom due the higher twist the wood particles undergo, whereas the smaller particle size allows a higher rotational freedom. Furthermore, the size distribution itself also influences the phase separation as shown by Wensink and Vroege50 on the isotropic−nematic phase behavior of different length-polydisperse particles, which could thus also influence the differing results seen in this work. The authors further want to draw the reader’s attention to the meniscus line of the macroscopic appearance of the respective suspension in glass vials, Figure 4. The surface tension measurements by pendant droplet of wood are significantly higher than the one for cotton, 72.9 mN m−1 at 5.5 wt % and 73.3 mN m−1 at 3.5 wt % versus 66.7 mN m−1 at 5.5 wt % and 68.7 mN m−1 at 3.5 wt %. The trend that the surface tension decreases with increasing amount of cellulose was already reported,51 and attributed to adsorption of CNCs, albeit limited, to the air−water interface. An explanation for the significant difference in surface tension between both samples can be the discovery of precipitated oligosaccharides on the surface of CNCs during quenching of the sulfuric acid hydrolysis first reported by Labet et al.,52 and studied more recently in more detail by Bouchard and co-workers.53 Bouchard et al. found a temperature dependence of the amount and degree of polymerization of the respective oligosaccharides deposited on the surface of the CNCs. For the reaction conditions chosen in this work (45 °C) a deposition of oligosaccharides with a degree of polymerization between 7 and 20 could be expected. This fact alone cannot be the explanation for the varying behavior between different cellulose sources, even though the degree of crystallinity is different for both starting materials. Wood cellulose is a mixture of 60/40 spruce/pine, 95% cellulose, 4.5% hemicellulose, and F

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ACS Sustainable Chemistry & Engineering Present Address

0.3% lignin whereas cotton is assumed to be more or less 100% cellulosic material. With that, the amount and nature of the side products will also vary during hydrolysis of the two different materials. Furthermore, cotton undergoes a dewaxing before it is sold, and some of the remaining products used in this process could remain and influence the surface properties of the CNC suspensions.54 One of the reasons for the varying behavior could be a less homogeneous charge distribution due to the preparation process, specifically the difference in addition due to the fibrous cotton or blended wood. Furthermore the size distribution, which is in the case of wood weighted toward smaller particles, is seen in the histogram (Figure 2), which is also clearly visible in the represented AFM images. Moreover, the polydispersity itself impacts the macroscopic behavior. Finally, one can summarize that indeed the source can be called to account for the varying behavior, but for a full comparison of the impact of the sources, exactly the same hydrolysis conditions have to be applied, which only can be achieved with comparable morphologies of the initial source material. Nonetheless, further studies are required to understand the actual acid hydrolysis process and herewith the resulting structural and macroscopic properties.

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⊥

C.S.: Physics and Materials Research Unit, University of Luxembourg, 162 A Avenue de la Fai ë ncerie, 1511 Luxembourg, Luxembourg.

Author Contributions

C.S. and W.T. developed the concept of the study, and C.S. performed the experiments and processed the data. C.S. and W.T. wrote the manuscript. C.S. conducted with the help of K.K. the DDLS measurements; J.V.R. and C.S. did the analysis, and K.K. supported the interpretation. H.v.G. carried out the AFM measurements. A.G. carried out the rheological tests and assisted with measurements of pendant drop tensiometry. S.R. performed the SAXS analysis. S.E. conducted the XPS experiments and interpretation. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this work was provided by Research Foundation−Flanders (FWO) under the Odysseus program Grant G.0C60.13 N, KU Leuven (Grant OT/14/072), and the Province of West-VlaanderenBelgium (W.T.’s Provincial Chair in Advanced Materials). C.S. thanks the Alexander von Humboldt Foundation for financial support through a Feodor Lynen scholarship. The authors are thankful to Prof. Dr. Jan Dhont for fruitful discussions and Dr. Hartmut Kriegs for assistance with the depolarized dynamic light scattering measurements. Prof. Dr. Jan Lagerwall is thanked for the use of the polarized light microscope.

CONCLUSION

Seemingly small variations in the intrinsic properties such as size and distribution as well as surface charge have a major influence on the macroscopic behavior such as viscosity, diffusion coefficients, and LC phase formation and separation. The surface tension is most likely altered due to small impurities or byproducts, i.e., introduced by hydrolysis. These are however not easy to detect and are mostly not reported, but are essential for most interpretations of observed phenomena. These seemingly rather unspectacular differences seem to have a major impact on the macroscopic behavior. Consequently, the analysis of CNCs should always include more detailed structural and macroscopic characterizations, which will help to explain the behavior and properties of the resulting systems, and enable the wider community to better compare experimental work.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b00334. More details about the XPS characterization, fits of a rectangular prism model to the respective SAXS data, and scaling of separation distance in respect to volume fraction of CNCs (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Christina Schütz: 0000-0003-0238-1639 Jonas Van Rie: 0000-0002-2658-3561 Alican Gençer: 0000-0001-5225-7121 Wim Thielemans: 0000-0003-4451-1964 G

DOI: 10.1021/acssuschemeng.8b00334 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssuschemeng.8b00334 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX