Surface Charge Influence on the Phase Separation ... - ACS Publications

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Surface charge influence on the phase separation and viscosity of cellulose nanocrystals Tiffany Abitbol, Doron Kam, Yael Levi-Kalisman, Derek G. Gray, and Oded Shoseyov Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04127 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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Surface charge influence on the phase separation and viscosity of cellulose nanocrystals Tiffany Abitbol,1,4* Doron Kam,1* Yael Levi-Kalisman,2 Derek Gray,3 and Oded Shoseyov1 1

The Hebrew University of Jerusalem Robert H. Smith Faculty of Agriculture, Food and Environment Department of Plant Sciences and Genetics in Agriculture Rehovot, Israel 2

The Hebrew University of Jerusalem Silberman Institute of Life Sciences and The Center for Nanoscience and Nanotechnology Jerusalem, Israel 3

McGill University Department of Chemistry Pulp and Paper Building Montreal, Canada 4

RISE Research Institutes of Sweden Bioscience and Materials Chemistry, Materials and Surfaces Stockholm, Sweden Abstract A series of four cellulose nanocrystal (CNC) suspensions were prepared from bleached softwood kraft pulp using different conditions of sulfuric acid hydrolysis. The CNCs were identical in size (95 nm in length × 5 nm in width) but had different surface charges corresponding to the harshness of the hydrolysis conditions. Consequently, it was possible to isolate the effects of surface charge on the self-assembly and viscosity of the CNC suspensions across surface charges ranging from 0.27 %S to 0.89 %S. The four suspensions (never-dried, free of added electrolyte) all underwent liquid crystalline phase separation but the concentration onset for the emergence of the chiral nematic phase shifted to higher values with increasing surface charge. Similarly, suspension viscosity was also influenced by surface charge, with suspensions of lower surface charge CNCs more viscous and tending to gel at lower concentrations. The properties of the suspensions were interpreted in terms of the increase in effective diameter of the nanocrystals due to the surface electrostatic repulsion of the negative sulfate half-esters, as modified by the screening effects of the H+ counterions in the suspensions. The results suggest that there is a threshold surface charge density (~0.3 %S) above which effective volume considerations are dominant across the concentration range relevant to liquid crystalline phase formation. Above this threshold value, phase separation occurs at the same effective volume fraction of CNCs (~10 vol%), with a corresponding increase in critical concentration due to the decrease in effective diameter that occurs with increasing surface charge. Below or near this threshold value, the formation of end-to-end aggregates may favor gelation and interfere with ordered phase formation. Keywords cellulose nanocrystals, surface charge, viscosity, chiral nematic, liquid crystalline, phase separation, Debye length, end-to-end, aggregates

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Introduction Cellulose nanocrystals (CNCs) are usually prepared from the controlled sulfuric acid hydrolysis of natural sources of cellulose, although other preparatory routes exist.1–3 The sulfuric acid preferentially hydrolyzes amorphous cellulose; however, glycosidic bonds in the crystalline regions are eventually affected. During hydrolysis, the cellulose microfibrils are gradually broken down into rod-shaped nanoparticles (CNCs) that form stable suspensions in water due to electrostatic repulsion between charged sulfate half-ester groups grafted onto the surface of the particles. In the early 90s, Revol et al. were first to observe the spontaneous phase separation that occurs upon standing in aqueous suspensions of CNCs above a critical concentration (c*).4,5 Above c*, the suspension separates into two distinct phases, an upper isotropic phase and a lower anisotropic phase, which is characterized by the chiral nematic liquid crystalline ordering of the particles, with some evidence suggesting accompanying size fractionation.6 Interest in the self-assembly of CNCs and the factors that influence it, including axial ratio,7,8 surface charge,7–9 ionic strength,4,10–12concentration,12,13 and ionic or neutral additives,14–16 continues to intensify. However, these analyses are sometimes complicated by potential experimental inconsistencies that challenge reproducibility and accurate comparison, including deionization post-hydrolysis,17,18 applied sonication energy,6,19 and adsorbed oligosaccharides.20 The concentration onset of phase separation, c*, provides a measure of the drive of a given suspension toward phase separation, where the tendency to self-assemble is mainly dictated by the free volume available to the CNCs.21 Free volume refers to the volume that can be occupied by CNCs, whereas excluded volume indicates volume that is inaccessible as it is already occupied. As the concentration of a suspension increases, the proximity between CNCs increases and the free volume available to CNCs decreases. At c*, entropic gains drive the CNCs to self-assemble as a strategy to increase free volume: loss in orientational entropy is outweighed by gains in translational entropy.1 Thus, factors that decrease excluded volume and allow the CNCs more freedom of translation diminish the tendency toward phase separation and shift c* to higher values. Furthermore, the chiral nematic ordered phase is characterized by a pitch, which describes the fundamental length scale of the twisted structure. Left-handed, chiral nematic CNC phases are thought to arise from the packing of particles with right-handed screw-like geometries, and thus conditions that screen particle geometry seem to dampen chiral interactions.1 Thus, considering particle shape screening and free volume arguments, it is expected that CNCs with extended electrostatic double layers (EDLs) and/or larger axial ratios will begin to self-assemble at lower concentrations and experience comparatively pronounced interparticle interactions at a given concentration, potentially leading to increased viscosity and even gelation. However, although the cellulose particles we now know as CNCs were first reported in the late 1940s by Rånby,22 and have been intensively and increasingly researched since the 1990s, conflicting experimental outcomes concerned with arguably the most fundamental behaviors of CNCs (phase separation and viscosity) continue to emerge, as discussed further below. In this work, four electrolyte-free, never-dried CNC suspensions having essentially identical particle sizes but different surface charges were prepared by varying the conditions of sulfuric acid hydrolysis, making it possible to consider the impact of surface charge on the self-assembly of CNCs as well as on their suspension viscosity. This is infrequently accomplished in the literature as increasing conditions of hydrolysis (time, temperature, acid concentration etc.,) is generally reported to not only increase the surface charge but also to shorten particle lengths.7,8 However, there are published reports that describe the modification of hydrolysis conditions and/or cellulose source to achieve particles of the same size but different surface charge.9,23–26 A more typical approach is to oxidize or desulfate CNCs in order to add or remove surface charge groups, respectively, since these post-hydrolysis modifications do not seem to significantly impact CNC dimensions.27–30 The advantage to the current approach is that the


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CNCs are obtained directly and in parallel without the need of time-consuming post-hydrolysis surface functionalization. CNC phase separation and viscosity are related to many factors including particle size, concentration, surface charge, and electrolyte/additive content. The effect of surface charge on phase separation and viscosity of CNCs of the same or very similar sizes has been previously addressed in the literature but in our opinion, is not yet fully resolved, in particular for electrolyte-free suspensions. The properties of the electrolyte-free suspensions described herein were understood by considering that charged CNCs and their associated counterions exert effects similar to added electrolyte. We also considered that CNCs with lower surface charges are more likely to form aggregates with increasing concentration and at comparatively low concentrations relative to CNCs with higher surface charges. Further understanding of the conditions that dictate the interconnected properties of CNC self-assembly and viscosity is necessary to achieve the lab-scale and industrial-scale consistency and control of CNCbased materials and formulations required for commercialization. Experimental Preparation of CNC suspensions. All suspensions were prepared from unbleached, kraft pulp sheets (Tembec) that were shredded and dried in an oven overnight at 50 °C prior to hydrolysis. Two of the suspensions were generously provided by Melodea Ltd using a proprietary approach, and the other two suspensions were based on a well-known recipe that employs 64 wt% sulfuric acid, acid to cellulose ratios of either 8.75 mL/g or 17.5 mL/g, a 45 °C reaction temperature, and a 45 min reaction duration.6 The suspensions prepared using the traditional recipe (64 wt%, 8.75 or 17.5 mL/g acid to cellulose, 45 °C, 45 min) were deacidified using a combination of 10× dilution in deionized water, centrifugation, and extensive dialysis, whereas the freshly-hydrolyzed Melodea Ltd samples were semi-deacidified upon receipt and were thus directly subjected to dialysis to complete the process. Dialysis was considered complete once the pH inside the dialysis bags stabilized, which occurs within 5-6 water changes, although the process was continued for a total of 2 weeks with twice daily water changes. After dialysis, the suspensions were sonicated on ice (Q500 sonicator by QSonica; 6 mm probe; 1 s on/off pulse; 90% amplitude; 150 mL sample/5 min sonication). An attempt was made to maintain sonication energy inputs the same however small variations in suspension concentration translated to a range of energies from 13– 19 kJ/g. Samples were stored in the refrigerator to prevent bacterial growth. All suspensions were neverdried and in acid-form, which means that the counterions of the sulfate half-ester groups were protons. Surface charge. Surface charge was measured as described elsewhere using conductometric titration (Titrando titrator by Metrohm).6,17 Briefly, an exact mass of acid form suspension (0.1 g CNC per titration) was titrated in the presence of dilute NaCl (1 mM) against dilute NaOH (2 mM concentration calibrated with a standard HCl solution). The equivalence point from titration is quantitatively related to the content of anionic sulfate half-ester surface groups grafted onto the particles during hydrolysis. Results are an average of three independent titrations, and are reported with associated standard errors. X-ray diffraction (XRD). X-ray diffraction measurements were accomplished using a Bruker AXS D8 Advance Diffractometer, with a scanning rate of 5°/min and a Cu Kα radiation source (λ = 1.54060 Å) operating at 40 kV and 30 mA. The samples were measured over the angular range of 2θ = 10–50°. Relative crystallinity indices (RCIs) were calculated as follows, (I200-Imin)/I200 × 100%

(1)

where I200 is the intensity above the baseline at 2θ = 22.5° and Imin is the minimum intensity above the baseline near 2θ = 18°, between the (200) and (110)/(11̅0) peaks. To ensure objective RCI quantification,


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additional calculations were performed as described above on diffractograms that had been smoothed using the least-squares method of Savitzky and Golay.31 Particle size by cryogenic transmission electron microscopy (cryo-TEM). A drop of aqueous suspension (3 µL, 0.1 wt%) was placed onto a TEM grid (300 mesh Cu Lacey substrate, Ted Pella Ltd) and vitrified in liquid ethane using a Vitrobot Mark IV (FEI Company). Vitrified samples were transferred in liquid nitrogen to the cryo-TEM holder (Gatan 626 cryo-transfer holder) and examined in the TEM (FEI Tecnai T12 G2 Spirit) at 120 kV. The temperature of the sample was maintained at approximately -177 °C to prevent ice crystallization. The images were recorded by a 4K × 4K FEI Eagle CCD camera in low dose mode. Particle sizes (length × width) were measured from TEM images and represent an average of 200–600 clearly defined particles per sample. The suspensions were composed of individual particles with well-defined borders as well as apparent aggregates, present in all suspensions at an occurrence of 50–60% and with dimensions of approximately 200 nm × 20 nm (maximal length × maximal width). Cryo-TEM of concentrated suspensions. To quantify particle size, the suspensions were viewed at the relatively low concentration of 0.1 wt% to avoid particle overlap and to facilitate size analysis. However, to better understand how CNCs interact at higher concentrations, cryo-TEM as described above was used to visualize and compare two of the suspensions at a higher concentration of 1 wt%. The suspension with the lowest surface charge (0.27 %S) was compared to a higher surface charge suspension (0.574 %S) using this approach. Dynamic light scattering (DLS) and zeta-potential. DLS and zeta-potential measurements were performed using a Zetasizer Nano ZS (0.01 wt% CNC for DLS, 0.1 wt% CNC for zeta potential; 5 mM NaCl; 25 °C; three independent measurements per sample). Phase separation. Suspensions were concentrated by evaporation in large dishes in the fume hood with magnetic stirring, a process that took 1–2 weeks. Suspension concentration was determined gravimetrically, and a series of dilutions were prepared from the stock concentrate. The dilutions were placed into rectangular cuvettes (10 mm in depth × 10 mm in width × 100 mm in length) and left standing at room temperature to phase separate for a minimum of 24 h. The volume fraction of the anisotropic phase was measured at room temperature by dividing the height of the anisotropic phase by the total height of the sample. The volume fraction of the anisotropic phase was plotted against the volume % of CNCs. The density of CNCs was taken as 1.6 × 10-21 g/nm3 in the conversion from weight % to volume %. The full range of the phase behavior could not be studied for the 0.27 %S and 0.49 %S suspensions due to gelation, whereas for the 0.57 %S and 0.89 %S suspensions, the quantity of material was insufficient to fully explore higher concentrations. Polarized optical microscopy (POM). The texture of the anisotropic liquid crystal phase was observed using a cross polarized optical microscope (Nikon Eclipse 80i×) and images were captured using a Nikon DS-Qi1 digital camera. To observe the chiral nematic pitch, biphasic suspensions were placed into sealed rectangular glass microslides (Vitrocom; 1 mm in depth × 10 mm in width × 100 mm in length). Viscosity. Rheology measurements were performed at room temperature using a Haake Rheostress 600 Rheometer (Thermo Fisher Scientific Inc.) with Rheowin 3.23 software. Steady-state shear viscosity measurements were obtained in cone-plate (C-60/1 Ti) geometry with the rheometer operated in controlled rate mode, where a defined shear rate is set and the shear stress is measured. Three independent runs were obtained per sample (1 mL/run) to ensure reproducibility, however only one representative curve is presented in the results.


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Error analysis. The error bars presented in this work are standard errors calculated using the Student’s ttest at 95% confidence. Results The different suspensions were achieved through variations in the conditions of sulfuric acid hydrolysis such that the surface charge increased according to the harshness of the reaction, from 0.27 %S to 0.89%S, but particle size was largely unaffected (see Table 1). The two suspensions with the lower surface charges (0.27 %S and 0.49 %S) were provided by Melodea Ltd, whereas the two suspensions with the higher surface charges (0.574 %S and 0.89 %S) were prepared as described in the experimental section. A window of conditions exists for the efficient extraction of particles recognizable as CNCs by the sulfuric acid hydrolysis of bleached pulps, which lies between 60–64 wt% sulfuric acid concentrations and temperatures of 45–65 °C.32 All CNCs used in this work were produced within this window of hydrolysis conditions. Furthermore, the CNCs in the four suspensions were highly crystalline (>85%) and characterized by native cellulose crystallinity (See Fig. S1 for XRD spectra and Table S1 for RCI), indicating that the crystal structure remained intact throughout the different hydrolyses. The average particle sizes of the different suspensions were measured from cryo-TEM images (see Fig. 1 for representative cryo-TEM images, Table 1 for average sizes, Fig. S2 for detailed particle size distribution, and Table S3 for particle size fitting parameters and polydispersity indices). As mentioned in the experimental section, this average value is representative of the dimensions of welldefined individual particles. Apparent aggregates were also present in all samples at an occurrence of 50– 60% but there was no indication of suspension instability in the samples by DLS and zeta-potential measurements (see Table S2). As can be seen in Table 1, while the surface charge increased with increasing harshness of the hydrolysis conditions, the size of the particles remained stable. Slight variations in CNC size were observed but they in no way correlated to the reaction conditions and the dimensions themselves overlapped within error; thus, we report a single average size for all suspensions of approximately 95 nm in length × 5 nm in width. Table 1. Surface charge and phase separation behavior of CNC suspensions prepared using different conditions of sulfuric acid hydrolysis. % Sulfur

Average size

Surface charge

Onset of phase

Onset of phase

length × width

density (e/nm2)

separation

separation

(wt%)

(vol%)

(nm) 0.27 ± 0.04

101 ± 6 × 5.7 ± 0.2

0.116

0.5

0.3

0.49 ± 0.03

94 ± 7 × 4.5 ± 0.2

0.166

0.65

0.5

0.574 ± 0.003

100 ± 10 × 5.5 ± 0.5

0.237

2.5

1.6

0.89 ± 0.04

89 ± 8 × 5 ± 0.5

0.337

5.1

3.4


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Figure 1. Representative cryo-TEM images of CNC suspensions in pure water: (a) 0.27 %S, (b) 0.49 %S, (c) 0.57 %S, and (d) 0.89 %S. Average CNC sizes were obtained from analysis of the cryo-TEM images and a minimum count of 200 well-defined individual particles.


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Figure 2. Phase separation of never-dried acid-form CNC suspensions in pure water. The volume fraction of the anisotropic phase (φanisotropic) is plotted against the volume fraction of CNCs for the four samples with different surface charges.

Figure 3. Representative polarized optical microscope image of the anisotropic phase of a biphasic suspension (0.57 %S; 4 wt%) showing the typical fingerprint pattern of light and dark lines spaced at regular intervals that is seen in chiral nematic liquid crystals. The distance between two dark or two bright lines gives the half-pitch (P/2), which is the distance traveled by the chiral nematic director in the direction perpendicular to the long axes of the CNCs over a 180° rotation of the CNC orientation. Figure 2 describes the phase separation behavior of the acid-form CNCs in pure water, where acid-form refers to hydronium counterions associated with sulfate half-ester surface groups. As seen by others, a monotonic increase in the volume fraction of the anisotropic phase (ϕanisotropic) is observed with increasing concentration across the biphasic region.8,12,33 Generally, we found an increasing tendency toward phase separation (c* or equivalently φ* shift to lower values) with decreasing surface charge (see Fig. 2 and Table 1), and a corresponding increase in pitch (Figs. 3 and 4). Figure 3 presents a representative image of a chiral nematic CNC phase viewed by polarized optical microscopy, where the distance between adjacent light/dark lines gives the half-pitch of the suspension. Figure 4 presents the variation in pitch as a function of suspension concentration for three of the samples, and indicates an overall decrease in pitch with increasing CNC concentration. For the 0.57 %S sample, the pitch more or less steadily decreased from 7.7 µm to 3.9 µm in the concentration range from 2.3–3 vol % (3.7–4.7 wt%). The decrease in pitch with increasing concentration was more pronounced for the suspensions with


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the lower surface charges, 0.27 %S and 0.49 %S, where the pitch decreased from 63 µm to 29 µm and 25 µm to 19 µm between 0.6–1.1 vol % (1–1.75 wt%) and 1.6–1.8 vol % (2.5–2.8 wt%), respectively, although the pitch values leveled out toward the end of the concentration range.

Figure 4. The effect of CNC concentration on the pitch for suspensions characterized by particles of the same size but with surface charges in the range of 0.27 %S–0.57 %S. The pitch plotted as a function of suspension volume fraction appears continuous across the different surface charge values. In addition to an impact on phase separation behavior, surface charge density was observed to substantially influence suspension viscosity, in particular at low shear. Figure 5 plots the steady-state viscosity of the suspensions at 3 wt%. (Refer to Fig. S3 for steady-state viscosity plots of each suspension as a function of concentration in the range of 1–3 wt%). All samples exhibited shear-thinning regions related to the shear-alignment of individual CNCs and/or of chiral nematic domains. The overall viscosity increased with decreasing surface charge at low and intermediate shear rates, whereas at high shear rates the viscosities of the different samples approached one another. Figure 5 highlights the significant viscosity differences accessed simply through changes in CNC surface charge; for instance, depending on shear rate, a 2 to 3 order of magnitude decrease in viscosity is achieved when the surface charge increases by ~0.6 %S for electrolyte-free suspensions.

Figure 5. Viscosity as a function of shear rate for electrolyte-free CNC suspensions at 3 wt% plotted on a double logarithmic scale. The low surface charge samples are significantly more viscous across most shear rates compared to the high surface charge samples.


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Finally, cryo-TEM imaging of higher concentration suspensions (1 wt%) was performed in order to better understand how the CNCs interact as concentration increases (Fig. 6). Intriguingly, end-to-end type aggregates were evident in the lower surface charge suspension (0.27 %S) and absent in the higher surface charge suspension (0.57 %S). Furthermore, analysis of the cryo-TEM images of the higher surface charge suspension gave a particle size and population distribution that was very similar to the values obtained from the dilute imaging (see Fig. 1 and Table 1), whereas it was nearly impossible to quantify the lower surface charge suspension in the same manner due to the high interconnectivity of the CNCs.

Figure 6. Cryo-TEM imaging of two of the CNC suspensions at the relatively high concentration of 1 wt%: (a) 0.27 %S suspension, (b) 0.57 %S suspension, and (c)-(e) high resolution views of the end-to-end type assemblies present in the 0.27 %S suspension.


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Discussion The CNC sizes were determined by analysis of cryo-TEM images of dilute suspensions (see Fig. 1 for cryo-TEM imaging and Fig. S2 and Table S3 for detailed size analysis). The proportion of single CNCs to aggregates was consistent across the different surface charges at about 50/50. It unclear whether the aggregates are related to insufficient dispersion post-hydrolysis or to an apparent overlap of particles due to the 2-D representation of a 3-D system. In addition, since the proportion of aggregates was the same for all suspensions, we do not consider their occurrence to be related to hydrolysis conditions. Considering the above and the statistically identical cryo-TEM-derived CNC sizes of the four suspensions, we consider it accurate to describe the suspensions as having very similar size distributions and of being composed of CNCs of the same physical dimensions. Two general properties were explored for the electrolyte-free CNC suspensions: (i) self-assembly, and (ii) viscosity as a function of shear. We observed an increase in c* with increasing surface charge and a corresponding decrease in pitch (Table 1, Figs. 2–4), as well as decrease in suspension viscosity at low shear (Fig. 5) apparently related to overall suspension stability and colloidal integrity. The trend in c* is different from the work of Shafiei-Sabet et al. and Beck and Bouchard,9,25 both of whom reported an increase in c* with decreasing surface charge also for CNC suspensions comprised of particles of nearly the same size. The current work covers a broader range of surface charge, but the reason for this discrepancy is unclear although it may be related to our use of H-form CNCs as opposed to neutralized CNCs or possibly to the use of never-dried CNC compared with redispersed dried CNCs. The trend in pitch (Fig. 4) will be addressed in more detail below but is generally consistent with the literature, i.e., pitch tightens with increasing concentration and surface charge. A main motivation for this work was the difference in suspension viscosity that was observed between the low charge CNCs (~0.3 wt%) and suspensions with the more typical surface charges (~0.5– 0.9 %S), which persisted even in the presence of electrolyte, for instance when the suspensions were neutralized by the addition of sodium hydroxide. For instance, when the concentration of electrolyte was matched between the low charge sample and the high charge sample, the low charge sample remained more viscous although its viscosity was somewhat lowered (data not shown). This difference impacts the scope of applications of the two different materials, in particular the solids content that can be achieved prior to sample gelation. In general, higher charge CNCs remain liquid at solid contents of ~6–10 wt%, whereas the range for the lower charge CNCs is narrower at ~3–4 wt%. Several recent studies have taken an in-depth look at the rheological properties of CNCs and how surface charge affects these properties in both the presence and absence of electrolyte.9,11,23,25,34–39 These studies generally report a decrease in viscosity with increasing surface charge (electrolyte-free conditions); however two recent studies report an opposite trend, possibly related to measurement equipment, which was a capillary rheometer as opposed to the more typical shear rheometer, or else to the colloidal stability of the suspensions in question.25,39 Araki et al. provided the first comprehensive study on the rheological properties of CNC suspensions in 1998.23 This work compared the viscosities of CNC suspensions with different surface charges: standard sulfuric acid-hydrolyzed CNCs vs. weakly charged HCl-hydrolyzed CNC suspensions. The authors applied Simha’s equation, which relates the relative viscosity to the axial ratio of rod-like particles, to the CNCs. For the higher charged sulfated CNCs, the calculated axial ratio correlated well to the value measured from nanoscale imaging, however, it overestimated the axial ratio of the HClhydrolyzed CNCs. Overall, the authors related the results to the formation of aggregates in the HClhydrolyzed sample due to van der Waals attraction. Similarly, Boluk et al. and Wu et al. estimated the axial ratios of CNCs from intrinsic viscosity measurements using somewhat different approaches to minimize electroviscous effects, which otherwise inflate particle size estimates.39,40 Generally, with


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increasing electrolyte, the viscosity of CNC suspensions decreases to a minimum value at some critical electrolyte concentration, after which it increases.11,25,39,41,42 The rationale for this behavior is that at electrolyte-free/low electrolyte conditions viscosity is high due to electroviscous effects stemming from an expanded EDL. With increasing electrolyte concentration, viscosity decreases to a minimum value as the EDL is compressed and electroviscous effects are lessened, after which the viscosity increases due to aggregation related to reduced electrostatic stabilization.11,25,39 Furthermore, the rheology of CNC suspensions has been shown to be closely related to concentration and the state of the colloidal dispersion, specifically whether the CNCs are isotropic, biphasic, ordered, gelled or aggregated.9,11,23,37,41–43 To summarize our results, four main experimental observations were made in the course of this study of acid-form CNCs in pure water having the same dimensions but different surface charges: with increasing surface charge (i) the concentration onset c* of ordered phase formation shifts to higher concentrations (Fig. 2), (ii) the pitch of the chiral nematic phase tightens (Figs. 3 and 4), (iii) suspensions become less viscous (Fig. 5), and (iv) the tendency to gel decreases. To understand these behaviors, we consider two scenarios: as surface charge decreases (1) the EDL expands, and (2) aggregation due to diminished electrostatic repulsion leads to higher axial ratio CNCs. The first option, that the EDL expands with decreasing surface charge, seems counterintuitive. However, calculations of the Debye screening length (κ-1), the effective diameter (Deff), and the repulsive twisting factor (h) of the suspensions according to the formulation presented by Dong et al. and based upon the theory of Stroobants, Lekkerkerker, and Odijk (SLO) seem to validate this option (see Table 2).12 For suspensions in pure water, the following equation was used to evaluate κ-1 κ2= 4πQnczc2

,

(1)

where Q is the Bjerrum length (0.7 nm at 298 K), and nc and zc are the counterion concentration and valency, respectively, and are directly related to the surface charge of the particles. As shown in Table 2, increasing the surface charge (and thus increasing the associated counterion concentration) decreases κ1 and Deff (which scales with κ-1), thereby increasing the free volume available for CNC translation and shifting c* to higher values (see Fig. 2). Thus, it seems that for suspensions in pure water, increasing the surface charge decreases excluded volume and delays the onset of phase separation. As was recently pointed out by Honorato-Rios et al., a decrease in Deff leads to opposing effects in terms of phase separation since ordered phase formation is favored by the increase in axial ratio but disfavored by the decrease in excluded volume, which dominates the behavior overall.8 The twisting factor h = κ-1/ Deff, which describes the magnitude of the perpendicular twisting action between charged rods due to electrostatic repulsion, also decreases with increasing surface charge in this system. This is because κ-1 decreases at a faster rate relative to Deff, indicating that effective geometry (i.e., the effective increase in free volume due to EDL compression) becomes increasingly important relative to electrostatic repulsion. The parallel alignment of CNCs is opposed by h; however, in this case as well, the excluded volume fraction is a better predictor for the propensity of a suspension to phase separate. We note that except for the effective volume fraction at c*, the parameters presented in Table 2 are calculated at 1 wt%, a concentration where the pH of the different suspensions was greater than the sulfate half-ester pKa (pKa ≈ 2), indicating that the CNCs were fully deprotonated. Indeed, this was the case (pH > pKa) across the concentration range studied in the phase separation plot (Fig. 2), with the exception of the highest charged suspension (0.89 %S), whose pH hovered just below the pKa (pH = 1.97-1.83) between 5–7 wt%, indicating somewhat less than complete deprotonation at concentrations above 5 wt%.


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Table 2. Comparison of surface charge density (σ), Debye screening length (κ-1), effective diameter (Deff), twisting factor (h), and the effective volume fraction of CNCs at c* for the different CNC suspensions in pure water. Calculations were arbitrarily performed at a suspension concentration of 1 wt% (unless otherwise noted), assuming a rectangular prism shape with a square cross-section for the CNCs, a CNC density of 1.6 g/cm3, and a Bjerrum length of 0.7 nm. CNC

σ (e/nm2)

κ-1 (nm)

Deff (nm)

suspension

h

(repulsive

effective volume

twist factor)

fraction at c*

0.27 %S

0.10

4.68

28.81

0.16

28%

0.49 %S

0.18

3.48

22.85

0.15

15%

0.57 %S

0.22

3.21

21.51

0.15

9%

0.89 %S

0.33

2.58

18.33

0.14

12%

The effective volume fraction of the CNCs at c* based on Deff gives the total volume occupied by the CNCs (Deff2× length × number of CNCs) relative to the total volume of the system. (As Deff varies with concentration, Deff was recalculated at c* in order to determine the effective volume fraction at c* - see Table 1 for c* values.) As mentioned in the above paragraph, for the highest charged suspension, c* is near 5 wt%, a concentration where the CNCs in this suspension are nearly but not entirely deprotonated. This was not explicitly accounted for in our determination of the effective volume fraction at c*, as the small percentage of protonated charged groups will affect κ-1 minimally. Estimates indicate similar effective volume fractions at the onset of phase separation for the three higher charged samples, occurring at approximately 10 vol %, generally supporting the conclusion that free volume is the key physical parameter controlling onset of phase separation. Although the effective volume fractions are similar, c* increases with decreasing effective particle size/increasing surface charge since more and more particles must be packed into a given volume in order to achieve the effective volume fraction of ~10 vol% required for self-assembly (i.e., actually between 9–15 vol%, with the variation perhaps reflective of accuracy in c* assessment). For the lowest surface charge suspension, the effective volume fraction of the CNCs at c* was roughly 2–3× that of the other suspensions. Although this suspension has the fewest CNCs at c*, its large Deff value dominates the effective volume fraction calculation. This analysis seems to suggest a threshold surface charge (ca. 0.3 %S), above which phase separation begins at a critical packing fraction of rods (ca. 10 vol%). Below this threshold surface charge, the critical packing fraction increases due to the large value of Deff. However, SLO theory may not be appropriate at low surface charge since the particle-particle interactions likely to be relevant to this system (attraction, EDL overlap) are not considered. As the chiral nematic pitch is thought to originate from the underlying shape of the particle, factors which obscure particle geometry are expected to dampen the twist, and vice versa. Indeed, Figure 4 indicates that pitch decreases as the Debye length is compressed, which occurs as the surface charge of the particles increases. Unexpectedly, the three sets of data points presented in Figure 4 fall nearly onto a single continuous curve, which seems to indicate that concentration largely determines the pitch whereas surface charge has a minor impact, if any. However, when the pitch values are plotted against the effective volume fraction instead of concentration (not shown), the data is no longer continuous across the different surface charges. Comparison of the calculated effective volume fractions of the 0.49 %S and the 0.57 %S samples indicates at that at very similar volume fractions (8.5–8.9% and 9.3–9.6%, respectively) the pitch is significantly (~3×) tighter for the particles with the higher surface charge – data from 0.27 %S suspension were not considered in this discussion since this sample seems inadequately described by electrostatic arguments. These limited results for the two intermediate surface charges suggest that surface


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charge does indeed impact pitch, and that a higher surface charge at a given effective volume fraction (and not necessarily at a given concentration) correlates to a tighter pitch, which is consistent with the findings of Schütz et al., who reported a tightening of the chiral nematic pitch due to electrostatic repulsion.13We observed an increase in suspension viscosity with decreasing surface charge in particular at low shear (Figs. 5 and S3). For the current electrolyte-free series, we expect the rheological properties to be dominated by electroviscous effects, leading to comparatively large viscosities due to a distortion of the EDL in a shear flow and also potentially to the overlap of EDLs.25 Thus it seems that the differences in suspension viscosity can be understood at least partly on the basis of the electroviscous effect as the EDL expands with decreasing surface charge (see Table 2). Although it is beyond the scope of the current work to fully assess the effects of ionic strength, we wanted to address whether the large disparity in the viscosities of our suspensions could be resolved by matching suspension ionic strengths (briefly mentioned above). To accomplish this, we measured the viscosity of the highest and lowest charged suspensions at 1 wt% and matched the ionic strength of the lowest surface charge suspension (0.27 %S) to that of the electrolyte-free highest surface charge suspension (0.89 %S) through the addition of NaCl (data not shown). We observed that while the viscosity of the low surface charge suspension did decrease with the addition of electrolyte due to compression of the EDL, it was still more viscous compared with the high charge suspension. This result suggests that the differences in viscosity (and possibly also in the self-assembly behavior) are not solely caused by the effective size of the CNCs (EDL + physical dimensions) and that at least for the lowest charge suspension some other factor(s) are at work. Next, we explore the second scenario, which is that as surface charge decreases the likelihood of higher aspect ratio aggregates increases. Our initial exploration of particle size and colloidal stability by cryo-TEM (Fig. 1) and zeta-potential and DLS analyses (see Table S2), indicated similar size distributions for the four suspensions as well as good dispersion. However, these techniques only probe the sample stability at low concentrations (0.01–0.1 wt%) and have little insight to offer in terms of how the particles interact at the higher concentrations relevant to phase separation and viscosity measurements. At higher concentrations, we had empirical evidence for the stability of the suspensions based upon gelation, which was qualitatively determined when suspensions no longer flowed, even upon inversion of the sample vials. For the 0.27 %S and 0.49 %S suspensions, gelation on standing occurred at 2 wt% and 3 wt%, respectively, whereas the higher surface charge samples did not gel in the concentration range studied, up to approximately 10 wt%. The stability of CNC suspensions can be considered as a balance between stabilizing forces that promote dispersion (e.g. electrostatic repulsion) and attractive interparticle interactions (e.g. van der Waals forces) that destabilize the suspensions and promote agglomeration, aggregation, and gelation. The 0.27 %S and the 0.49 %S suspensions were stable enough at low concentrations but at higher concentrations, within the biphasic region of the phase diagram (~0.5–3 wt %), attractive interparticle interactions seem to outweigh repulsive interactions leading to gelation. Indeed, it is likely that attractive interparticle forces are more pronounced in the current system compared to a suspension with added electrolyte, since in the latter case extraneous ions interfere with particle-particle interactions. Additionally, overlap of the EDLs can result in repulsive gels, which may be relevant to the lower charge suspensions with the most extended EDLs (as inferred from the above assessment of the Debye lengths). To better address the state of the CNC dispersions at higher concentrations, we imaged the 0.27 %S and the 0.574 %S suspensions at 1 wt% by cryo-TEM (Fig. 6). Surprisingly, end-to-end type aggregates of longer axial ratio were evident in the low surface charge suspension but not in the higher surface charge suspension. Analysis of the cryo-TEM images of the higher surface charge suspension (0.574 %S) indicated an individual particle size of approximately 100 nm in length by 5 nm in width and a similar distribution of isolated particles and aggregates as was seen by dilute cryo-imaging (see Fig. 1 and Table 1). A similar analysis of the 0.27 %S suspension was non-trivial as most of the CNCs in this sample were associated to other CNCs in such a way that it was virtually impossible to discern where one


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particle ended and another began. These observations suggest that at low surface charge, CNCs form long axial ratio assemblies at relatively low concentrations of ~1 wt%, possibly lower. We are unsure why an end-to-end arrangement appears to be favored over lateral aggregation but speculate that it may be related to distribution of the surface charge groups, with possibly fewer groups located at the ends of the CNCs. This type of aggregate was previously observed by Jiang et al. in a suspension of sulfated CNCs that had been subjected to solvolytic desulfation to remove charge groups,44 supporting the thought that CNCs with low surface charges may favor end-to-end arrangements. An increase in axial ratio shifts the concentration onset of phase separation to lower values and gives ordered phases with longer pitch values. Thus, these long axial ratio end-to-end assemblies may be implicated in the early onset of phase separation of the 0.27%S suspension (c* of ca. 0.5 wt%) and the large pitch of its ordered phases. Furthermore, a greater axial ratio may also be implicated in the gelation observed at relatively low concentrations and in the exceedingly high viscosity of this suspension, both of which may be due to entanglement or the formation of a network structure.41 Finally, we note that the cryo-TEM images of the low surface charge suspensions are reminiscent of a percolated network, however the percolation threshold predicted for these suspensions (calculated for dimensions of 95 nm in length × 5 nm in width) occurs at significantly higher concentrations (ca. 4 wt%).45 The observation of an apparently interconnected network CNCs at relatively low concentrations may have implications for the application of low surface charge CNCs in nanocomposites as reinforcing agents. Conclusions A series of sulfated CNC suspensions in pure water comprised of CNCs of the same physical dimensions but different surface charges were studied in terms of their self-assembly into chiral nematic phases and their rheology. A decrease in surface charge led to samples that self-assembled at lower concentrations into comparatively long-pitch chiral phases, and that were more viscous, tending to gel within the biphasic region of the chiral nematic phase diagram. To understand these behaviors, we considered the electrostatic interactions of the CNCs, specifically the Debye length calculated from SLO theory, but also the possibility that at low surface charge the formation of higher order CNC assemblies may occur. Considering the results as a whole, it seems that at a surface charge above a threshold of approximately 0.3% S, the properties of the suspensions are reasonably described in terms of the effective volume occupied by the CNCs. Thus, although these CNCs have the same physical dimensions, the effective volume that they occupy changes with surface charge, specifically their effective volumes increase with decreasing surface charge due to the corresponding trends in Debye length and effective diameter. According to this rationale, with decreasing surface charge, accelerated phase separation and a relatively long-pitch ordered phase is expected due to the decrease in available free volume at a given concentration and the screening of the particle shape by an extended EDL, respectively. In fact, the effective volume fraction of CNCs dominates the phase separation behavior of the higher charged suspensions, with all three of these suspensions beginning to self-assemble at the same effective volume fraction of approximately 10 vol%. However, consideration of the Debye length and the effective geometry of the CNCs do not completely address the lowest surface charge suspension (0.27 %S). For this suspension, we considered the possibility of higher order aggregates occurring at higher concentrations than those typically probed in colloidal stability measurements. Indeed, cryo-TEM imaging of this suspension at 1 wt% indicated predominately end-to-end type assemblies that were much less common in the higher surface charge suspension. These long axial ratio end-to-end assemblies are likely implicated in the extremely high


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viscosity of this suspension at low shear rates as well as in the comparatively low concentration onset for ordered phase formation and the long pitch values. Reasonably, the viscosity of CNC suspensions and whether self-assembly, gelation, or gelationarrested-self-assembly occur largely depend on the colloidal stability of the CNCs. As such, low surface charge CNCs tend toward the latter two scenarios and give more viscous suspensions in general. Full understanding of the ways that surface charge impacts self-assembly and rheology is in our opinion necessary for the successful commercialization of CNCs, and although the fundamental nature of these particles is essentially the same, i.e., sulfated CNCs, the results presented here highlight how seemingly minor differences in the surface chemistry of CNCs translate into significant differences in the selfassembly and rheology of the particles.

ASSOCIATED CONTENT Supporting information. XRD and RCI, particle size distribution and fitting parameters, zeta-potential and DLS, viscosity profiles at 1–3 wt%. Corresponding Author

Email: [email protected] Notes The authors declare no competing financial interests. *Tiffany Abitbol and Doron Kam contributed equally to this work. ACKNOWLEDGEMENTS The authors thank Melodea Ltd for CNC suspensions and the use of laboratory space and equipment, Jasmine Seror (Collplant Ltd) for rheometer training, and the Hebrew University Center for Nanoscience and Nanotechnology for training and equipment use. Useful discussion with Joel M. Berry, Xuan Yang, and Agne Swerin, as well as a suggestion made by Wim Thielemens are acknowledged. TA is grateful to the Azrieli Foundation for the award of an Azrieli Fellowship and for the support of a VINNOVA/Marie Curie Fellowship. David Weber is acknowledged for TOC artwork. REFERENCES (1) (2) (3) (4) (5)

Habibi, Y.; Lucia, L. A.; Rojas, O. J. Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications. Chem. Rev. 2010, 110 (6), 3479–3500. Nanomaterials, G.; Klemm, D.; Klemm, D.; Kramer, F.; Moritz, S.; Lindström, T.; Ankerfors, M.; Gray, D.; Dorris, A. Reviews Nanocelluloses : A New Family of Nature-Based Materials Angewandte. 2011, 5438–5466. Moon, R. J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J.; O’Sullivan, A. C.; Samir, M. A. S. A.; Alloin, F.; Dufresne, A.; Nishiyama, Y.; et al. Cellulose Nanomaterials Review: Structure, Properties and Nanocomposites. Chem. Soc. Rev. 2011, 40 (7), 3941. Revol, J.-F.; Godbout, L.; Dong, X.-M.; Gray, D. G.; Chanzy, H.; Maret, G. Chiral Nematic Suspensions of Cellulose Crystallites; Phase Separation and Magnetic Field Orientation. Liq. Cryst. 1994, 16 (1), 127–134. Revol, J.-F.; Bradford, H.; Giasson, J.; Marchessault, R. H.; Gray, D. G. Helicoidal Self-Ordering


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

(6) (7) (8) (9) (10) (11) (12) (13)

(14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25)

of Cellulose Microfibrils in Aqueous Suspension. Int. J. Biol. Macromol. 1992, 14 (3), 170–172. Dong, X. M.; Revol, J.-F.; Gray, D. G. Effect of Microcrystallite Preparation Conditions on the Formation of Colloid Crystals of Cellulose. Cellulose 1998, 5 (1), 19–32. Beck-Candanedo, S.; Roman, M.; Gray, D. G. Effect of Reaction Conditions on the Properties and Behavior of Wood Cellulose Nanocrystal Suspensions. Biomacromolecules 2005, 6 (2), 1048– 1054. Honorato-Rios, C.; Kuhnhold, A.; Bruckner, J. R.; Dannert, R.; Schilling, T.; Lagerwall, J. P. F. Equilibrium Liquid Crystal Phase Diagrams and Detection of Kinetic Arrest in Cellulose Nanocrystal Suspensions. Front. Mater. 2016, 3, 21. Shafeiei-Sabet, S.; Hamad, W. Y.; Hatzikiriakos, S. G. Influence of Degree of Sulfation on the Rheology of Cellulose Nanocrystal Suspensions. Rheol. Acta 2013, 52 (8–9), 741–751. Hirai, A.; Inui, O.; Horii, F.; Tsuji, M. Phase Separation Behavior in Aqueous Suspensions of Bacterial Cellulose Nanocrystals Prepared by Sulfuric Acid Treatment. Langmuir 2009, 25 (1), 497–502. 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. Dong, X. M.; Kimura, T.; Revol, J.-F.; Gray, D. G. Effects of Ionic Strength on the Isotropic−Chiral Nematic Phase Transition of Suspensions of Cellulose Crystallites. Langmuir 1996, 12 (8), 2076–2082. 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. Edgar, C. D.; Gray, D. G. Influence of Dextran on the Phase Behavior of Suspensions of Cellulose Nanocrystals. Macromolecules 2002, 35 (12), 7400–7406. Mu, X.; Gray, D. G. Formation of Chiral Nematic Films from Cellulose Nanocrystal Suspensions Is a Two-Stage Process. Langmuir 2014, 30 (31), 9256–9260. Bardet, R.; Belgacem, N.; Bras, J. Flexibility and Color Monitoring of Cellulose Nanocrystal Iridescent Solid Films Using Anionic or Neutral Polymers. ACS Appl. Mater. Interfaces 2015, 7 (7), 4010–4018. Abitbol, T.; Kloser, E.; Gray, D. G. Estimation of the Surface Sulfur Content of Cellulose Nanocrystals Prepared by Sulfuric Acid Hydrolysis. Cellulose 2013, 20 (2). Beck, S.; Méthot, M.; Bouchard, J. General Procedure for Determining Cellulose Nanocrystal Sulfate Half-Ester Content by Conductometric Titration. Cellulose 2015, 22 (1), 101–116. Beck, S.; Bouchard, J.; Berry, R. Controlling the Reflection Wavelength of Iridescent Solid Films of Nanocrystalline Cellulose. Biomacromolecules 2011, 12 (1), 167–172. Bouchard, J.; Méthot, M.; Fraschini, C.; Beck, S. Effect of Oligosaccharide Deposition on the Surface of Cellulose Nanocrystals as a Function of Acid Hydrolysis Temperature. Cellulose 2016, 23 (6), 3555–3567. Abitbol, T.; Cranston, E. D. Chiral Nematic Self-Assembly of Cellulose Nanocrystals in Suspensions and Solid Films. In Handbook of Green Materials; World Scientific, 2013; pp 37–56. Rånby, B. G.; Banderet, A.; Sillén, L. G. Aqueous Colloidal Solutions of Cellulose Micelles. Acta Chem. Scand. 1949, 3, 649–650. Araki, J.; Wada, M.; Kuga, S.; Okano, T. Flow Properties of Microcrystalline Cellulose Suspension Prepared by Acid Treatment of Native Cellulose. Colloids Surfaces A Physicochem. Eng. Asp. 1998, 142 (1), 75–82. Wu, Q.; Meng, Y.; Wang, S.; Li, Y.; Fu, S.; Ma, L.; Harper, D. Rheological Behavior of Cellulose Nanocrystal Suspension: Influence of Concentration and Aspect Ratio. J. Appl. Polym. Sci. 2014, 131 (15), 40525–40532. Beck, S.; Bouchard, J. Ionic Strength Control of Sulfated Cellulose Nanocrystal Suspension Viscosity. TAPPI J. 2016, 15 (6), 363–372.


Page 16 of 24

Page 17 of 24 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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(26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45)

Wu, Q.; Li, X.; Fu, S.; Li, Q.; Wang, S. Estimation of Aspect Ratio of Cellulose Nanocrystals by Viscosity Measurement: Influence of Surface Charge Density and NaCl Concentration. Cellulose 2017, 24 (8), 3255–3264. Habibi, Y.; Chanzy, H.; Vignon, M. R. TEMPO-Mediated Surface Oxidation of Cellulose Whiskers. Cellulose 2006, 13 (6), 679–687. Jiang, F.; Esker, A. R.; Roman, M. Acid-Catalyzed and Solvolytic Desulfation of H 2 SO 4 Hydrolyzed Cellulose Nanocrystals. Langmuir 2010, 26 (23), 17919–17925. Abitbol, T.; Palermo, A.; Moran-Mirabal, J. M.; Cranston, E. D. Fluorescent Labeling and Characterization of Cellulose Nanocrystals with Varying Charge Contents. Biomacromolecules 2013, 14 (9), 3278–3284. Reid, M. S.; Kedzior, S. A.; Villalobos, M.; Cranston, E. D. Effect of Ionic Strength and Surface Charge Density on the Kinetics of Cellulose Nanocrystal Thin Film Swelling. Langmuir 2017, 33 (30), 7403–7411. Savitzky, A.; Golay, M. J. E. Smoothing and Differentiation of Data by Simplified Least Squares Procedures. Anal. Chem. 1964, 36 (8), 1627–1639. Hamad, W. Y.; Hu, T. Q. Structure-Process-Yield Interrelations in Nanocrystalline Cellulose Extraction. Can. J. Chem. Eng. 2010, 88 (3), n/a-n/a. 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. Boluk, Y.; Lahiji, R.; Zhao, L.; McDermott, M. T. Suspension Viscosities and Shape Parameter of Cellulose Nanocrystals (CNC). Colloids Surfaces A Physicochem. Eng. Asp. 2011, 377 (1–3), 297–303. Shafiei-Sabet, S.; Hamad, W. Y.; Hatzikiriakos, S. G. Rheology of Nanocrystalline Cellulose Aqueous Suspensions. Langmuir 2012, 28, 17124–17133. González-Labrada, E.; Gray, D. G. Viscosity Measurements of Dilute Aqueous Suspensions of Cellulose Nanocrystals Using a Rolling Ball Viscometer. Cellulose 2012, 19 (5), 1557–1565. 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. Wu, Q.; Meng, Y.; Wang, S.; Li, Y.; Fu, S.; Ma, L.; Harper, D. Rheological Behavior of Cellulose Nanocrystal Suspension: Influence of Concentration and Aspect Ratio. J. Appl. Polym. Sci. 2014, 131 (15), 40525–40532. Wu, Q.; Li, X.; Fu, S.; Li, Q.; Wang, S. Estimation of Aspect Ratio of Cellulose Nanocrystals by Viscosity Measurement: Influence of Surface Charge Density and NaCl Concentration. Cellulose 2017, 24 (8), 3255–3264. Boluk, Y.; Lahiji, R.; Zhao, L.; McDermott, M. T. Suspension Viscosities and Shape Parameter of Cellulose Nanocrystals (CNC). Colloids Surfaces A Physicochem. Eng. Asp. 2011, 377 (1–3), 297–303. Xu, Y.; Atrens, A. D.; Stokes, J. R. Rheology and Microstructure of Aqueous Suspensions of Nanocrystalline Cellulose Rods. J. Colloid Interface Sci. 2017, 496, 130–140. Oguzlu, H.; Danumah, C.; Boluk, Y. Colloidal Behavior of Aqueous Cellulose Nanocrystal Suspensions. Curr. Opin. Colloid Interface Sci. 2017, 29, 46–56. Shafiei-Sabet, S.; Hamad, W. Y.; Hatzikiriakos, S. G. Rheology of Nanocrystalline Cellulose Aqueous Suspensions. Langmuir 2012, 28, 17124–17133. Jiang, F.; Esker, A. R.; Roman, M. Acid-Catalyzed and Solvolytic Desulfation of H 2 SO 4 Hydrolyzed Cellulose Nanocrystals. Langmuir 2010, 26 (23), 17919–17925. Garboczi, E. J.; Snyder, K. A.; Douglas, J. F.; Thorpe, M. F. Geometrical Percolation Threshold of Overlapping Ellipsoids. Phys. Rev. E 1995, 52 (1), 819–828.


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Figure 1. Representative cryo-TEM images of CNC suspensions in pure water: (a) 0.27 %S, (b) 0.49 %S, (c) 0.57 %S, and (d) 0.89 %S. Average CNC sizes were obtained from analysis of the cryo-TEM images and a minimum count of 200 well-defined individual particles. 163x152mm (144 x 144 DPI)


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Figure 2. Phase separation of acid-form CNC suspensions in pure water. The volume fraction of the anisotropic phase (φanisotropic) is plotted against the volume fraction of CNCs for the four samples with different surface charges. 160x116mm (144 x 144 DPI)


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Figure 3. Representative polarized optical microscope image of the anisotropic phase of a biphasic suspension (0.57 %S; 4 wt%) showing the typical fingerprint pattern of light and dark lines spaced at regular intervals that is seen in chiral nematic liquid crystals. The distance between two dark or two bright lines gives the half pitch (P/2), which is the distance traveled by the chiral nematic director in the direction perpendicular to the long axes of the CNCs over a 180° rotation of the CNC orientation. 148x120mm (96 x 96 DPI)


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Figure 4. The effect of CNC concentration on the pitch of three of the suspensions characterized by particles of the same size but with surface charges in the range of 0.27 %S–0.57 %S. The pitch plotted as a function of suspension volume fraction appears continuous across the different surface charge values. 158x116mm (144 x 144 DPI)


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Figure 5. Viscosity as a function of shear rate for electrolyte-free CNC suspensions at 3 wt% plotted on a double logarithmic scale. The low surface charge samples are significantly more viscous across most shear rates compared to the high surface charge samples. 250x182mm (144 x 144 DPI)


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Figure 6. Cryo-TEM imaging of two of the CNC suspensions at the relatively high concentration of 1 wt%: (a) 0.27 %S suspension, (b) 0.57 %S suspension, and (c)-(e) high resolution views of the end-to-end type assemblies present in the 0.27 %S suspension. 202x181mm (144 x 144 DPI)


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87x43mm (144 x 144 DPI)


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Surface Charge Influence on the Phase Separation ... - ACS Publications

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