Letter pubs.acs.org/macroletters
Cite This: ACS Macro Lett. 2018, 7, 172−177
Nanoscale Assembly of Cellulose Nanocrystals during Drying and Redispersion Yingxin Liu,#,†,‡ Daniela Stoeckel,#,† Korneliya Gordeyeva,† Michael Agthe,†,§ Christina Schütz,†,∥ Andreas B. Fall,*,†,⊥ and Lennart Bergström*,† †
Department of Materials and Environmental Chemistry, Stockholm University, 106 91 Stockholm, Sweden Wallenberg Wood Science Center, KTH, 100 44 Stockholm, Sweden
‡
S Supporting Information *
ABSTRACT: We have followed the structural evolution during evaporation-induced self-assembly of sulfonated cellulose nanocrystal (CNC) in the presence of H+ and Li+ counterions by small-angle X-ray scattering. Drying of CNC-H dispersions results in ordered films that could not be readily redispersed, while the CNC-Li films were disordered and prone to reswelling and redispersion. The scaling of the separation distance (d) between CNC particles and the particle concentration (c) shows that the CNC-H dispersions display a unidimensional contraction of the nematic structure (d ∝ c−1) during drying, while the CNC-Li dispersions consolidate isotropically (d ∝ c−1/3), which is characteristic for hydrogels with no preferential orientation. Temporal evolution of the structure factor and complementary dynamic light-scattering measurements show that CNC-Li is more aggregated than CNC-H during evaporation-induced assembly. Insights on the structural evolution during CNC assembly and redispersion can promote development of novel and optimized processing routes of nanocellulose-based materials.
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There is a competition between the formation of a liquid crystalline phase and gelation in aqueous CNC dispersions.3,12 Indeed, studies on the phase behavior of CNC dispersions are usually limited to concentrations below the gelation threshold;3 hence, the structural evolution of CNC assembly at higher concentrations is poorly understood. The structural features of nanocellulose dispersions and dry films are influenced by, e.g., the aspect ratio13,14 and surface charge density of the nanocellulose particles17 and the ionic strength and dielectric constant of the solvent.12,18,20−22 Nanocellulose is usually negatively charged, and it has been shown that the type of counterion influences the colloidal stability of nanocellulose particles in aqueous dispersions21 and also the mechanical performance of dry nanocellulose films.20,23 Replacing the counterion of sulfonated CNC from hydrogen to a metal ion enabled, e.g., redispersion of a dry CNC film by the addition of water and mild sonication.9,20 Dong et al. suggested that the intermolecular hydrogen bonding in dry CNC films is weaker with, e.g., sodium as counterion compared to hydrogen,20 and bulkier counterions have been shown to promote redispersion.24 However, while the structural features of dispersions and dry films have been studied in some detail, investigations on the structural evolution during drying of aqueous CNC dispersions and redispersion of films and gels are
anocellulose is a renewable biomaterial that combines a high strength and elastic moduli with a low density, versatile surface chemistry, and aqueous processability.1−3 Utilization of the impressive properties of the nanocellulose particles in fibers, films, coatings, and other bulk materials demands control of the microstructure and interfibrillar bonding during assembly of the rod-like nanoparticles. For example, preparation of highly transparent nanocellulose films requires that aggregation of the nanocellulose particles is suppressed,4−6 and the anisotropic mechanical properties of nanocellulose-based composites are strongly influenced by the orientation of the slender nanocellulosic particles.7,8 It is also essential to develop low-cost and energy efficient routes to process nanocellulose in both wet and dry form to facilitate large-scale production of sustainable materials based on nanocellulose. Controllable drying and redispersion is a challenge for all nanoparticle-based materials, and processes and development of robust routes to dry and redisperse nanocellulose would simplify transportation and reduce the risk of fungal breakdown.9 The concentration-dependent structural features of nanocellulose dispersions of both cellulose nanocrystal (CNC) and cellulose nanofibril (CNF) dispersions3,10−12 have been studied using, e.g., visual inspection of the onset and growth of the anisotropic phase,13,14 dynamic light scattering (DLS),12 polarized optical microscopy (POM),11,15−18 laser diffraction,10 and small-angle scattering [SANS (small-angle neutron scattering) and SAXS (small-angle X-ray scattering)].10,16,19 © XXXX American Chemical Society
Received: December 12, 2017 Accepted: January 11, 2018
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DOI: 10.1021/acsmacrolett.7b00964 ACS Macro Lett. 2018, 7, 172−177
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ACS Macro Letters
We have estimated the weight loss of the CNC droplets by eq 1 (details are given in the Supporting Information). Figure S4 shows that the linear theoretical estimate is in excellent agreement with experimental measurements until an evaporation time of 20−25 min (corresponding to a CNC concentration of about 10 vol %; see Figure S5) when the evaporation rate appears to slow down. We attribute this reduction in evaporation rate to a gradual transition from unhindered to hindered evaporation, when the liquid−vapor interface starts to recede into the pores formed by the particle network and the transport of fluid to the surface of the particle network body becomes rate limiting.29 However, the weight loss measurements at longer evaporation time are approaching the sensitivity of the analytical balance, and it is difficult to analyze the data in detail. Evaporation of water from the pinned droplet is associated with a shift of the primary structural peak toward a higher value for the scattering vector (q), which shows that the average separation distance between the CNC rods is decreasing as the solvent is evaporating. The average center to center distance (d) between CNC particles was estimated by a Kratky plot [I(q)·q2 vs q] of the SAXS curves and Gaussian function fitting using d = 2π/q. The scattering intensity of the Kratky plot profiles (Figure S6) decreased with increasing drying time (particle concentration) and eventually reached a lower limit (error of Gaussian fitting on the peak above 20%) where meaningful estimates were no longer possible. Figure 2c shows that the separation distance, d, between the CNC particles could be probed from an initial value of ca. 22− 23 nm down to 13 and 17 nm for the drying CNC-H and CNC-Li dispersions, respectively. The d value of 22−23 nm for the initial CNC particle concentration of 5 vol % correlates well with the d value of 24.5 nm previously reported for a CNC-H dispersion at a concentration of 6.5 vol %.10 Figure 2c shows that the decrease of the d value with increasing particle concentrations follows distinctly different pathways for the drying CNC-H and CNC-Li dispersions. The analysis suggests that the separation distance in the CNC-H dispersion is inversely proportional to the particle concentration (d ∝ c−1). Similar scaling has been observed for the cholesteric pitch (p) and suggests that the cholesteric phase in CNC-H dispersion contracts unidimensionally as the film thickness decreases.26,30 We found that the CNC-Li dispersion displayed a scaling of d ∝ c−1/3, which indicates an isotropic consolidation of randomly aligned particles, similar to previous studies of metallic glasses under compression.31,32 This scaling implies that the drying CNC-Li dispersion forms a disordered hydrogel at a concentration of 5 vol % and above. The DLS measurements in Figure 2d confirm that CNC-Li displays a larger apparent hydrodynamic size than CNC-H at ionic strengths above 1 mmol L−1, which suggests that CNC-Li particles form clusters and aggregates as the ionic strength increases during drying. Counterion specific effects have been shown to affect the critical coagulation concentration of CNC,21 as well as the ordering/interaction within dry films.23,33 The colloidal stability of CNC in electrolyte solutions was shown to follow the Hofmeister series,21 of which an explanation for this trend is still under debate, and the deviated hydration of the ions is considered as a possible cause.34 The hydration strength increases with the charge density of the ions. Hydrogen has an ∼1010 higher charge density than lithium, thus the observations of bigger sized CNC-Li than CNC-H in Figure 2d appear to follow the predictions based on the
sparse and limited to how the helical pitch evolves with increasing concentrations.25,26 In this study, we have probed the evaporation-induced assembly of negatively charged, sulfonated CNC particles with hydrogen and lithium as counterions, denoted as CNC-H and CNC-Li, respectively, by time-resolved SAXS. The in situ SAXS measurements were performed on CNC aqueous droplets that were placed onto a patterned mica substrate, as depicted in Figure 1. The use of the patterned substrates prevented the
Figure 1. Schematic illustration of the in situ SAXS measurements during evaporation-induced assembly of CNC. The upper part of the illustration shows how the CNC aqueous droplet is pinned onto the patterned substrate. The lower part of the illustration displays the measurement setup highlighting the custom-built humidity cell with the pinned CNC aqueous droplet mounted perpendicularly to the Xray beam path.
droplet from sliding when the mica substrate was positioned in a vertical position. The dimensions of the CNC-H particles were unaffected by the ion exchange process, which was supported by the atomic force microscopy characterization (see Figure S1). We confirmed by observing the CNC dispersions between crossed polarizers (Figure S2) that both the CNC-H and CNC-Li dispersions formed a fully anisotropic phase at a concentration of 5 vol % . Figure 2a and b shows representative time-resolved SAXS curves that were extracted from the data sets recorded during evaporation of the CNC-Li and CNC-H aqueous droplets, respectively. The scattering intensity decreased with evaporation time, which is related to the decrease of volume probed by the X-ray beam. The reproducibility of the structural evolution during evaporation-induced assembly was confirmed by repeated measurements (Figure S3). The evaporation from small sessile droplets with a pinned solid−liquid interface at a constant relative humidity and temperature is essentially a quasi-steady-state process dominated by vapor diffusion, and the evaporation rate has been shown to be proportional to the droplet radius and weakly dependent on the contact angle.27,28 −
dm = 4D(1 − H ) ·c V ·R dt
(1)
where m is the water mass, t the evaporation time, D the vapor diffusivity, H the relative humidity, cV the saturated vapor concentration, and R the droplet radius. 173
DOI: 10.1021/acsmacrolett.7b00964 ACS Macro Lett. 2018, 7, 172−177
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Figure 2. Structural properties of CNC dispersions. Time-resolved SAXS data of drying dispersions of (a) CNC-Li and (b) CNC-H. The SAXS data cover a total time span of 60 and 20 min, and the time step for each curve is 2 and 1 min, respectively. t0 is the starting SAXS measurement time. (c) Time-dependent change of the center to center CNC separation distance d with increasing CNC concentrations during evaporation-induced assembly (●: CNC-H; and ■: CNC-Li). The solid curves describe a power law relation with exponents of −1/3 and −1. (d) Apparent hydrodynamic diameter determined by DLS of CNC-H and CNC-Li (c = 0.03 vol %) as a function of ionic strength (H+ and Li+ concentration, respectively). The solid curves are a guide for the eyes only.
Figure 3. Time-dependent evolution of the structure factor [S(q)] during evaporation-induced assembly of (a) CNC-Li and (b) CNC-H dispersions. The SAXS data cover a total time span of 1 h in (a) and 20 min in (b). The time step for each curve is 2 and 1 min in (a) and (b), respectively. t0 is the starting SAXS measurement time.
assumption of hydration. However, firmly stating that hydration strength is the cause of the observed difference in colloidal stability would require a separate study focusing on this subject. We have evaluated the structural changes in more detail during evaporation-induced assembly of CNC by analyzing the temporal evolution of the structure factor. The structure factor [S(q)] was obtained by dividing I(q, c ≥ 5 vol %) with I(q, c = 0.06 vol %); the scattered intensity of very dilute CNC dispersions (c = 0.06 vol %) is determined by the form factor only. Figure 3a shows that the primary peak and secondary shoulder for the CNC-Li dispersions decreased in intensity and
shifted slightly toward higher q values during drying. The secondary shoulder with an initial value around q = 0.7 nm−1 may be associated with intermediate 2-dimensional CNC aggregates, as suggested by Uhlig et al. and Cherhal et al.19,35 Figure 3b shows that the scattering factor during the first 20 min of evaporation is dominated by the primary peak with an initial value around q = 0.3 nm−1 for the CNC-H dispersions and by the secondary shoulder for the evaporating CNC-Li dispersions, which indicates that CNC-Li is indeed more aggregated than CNC-H during evaporation-induced assembly. 174
DOI: 10.1021/acsmacrolett.7b00964 ACS Macro Lett. 2018, 7, 172−177
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ACS Macro Letters Figure 4 shows that the vertically fractured CNC-H films display a layered microstructure characteristic for chiral nematic
We have monitored the structural evolution during redispersion of a dried CNC film after adding a drop of deionized water by in situ SAXS. Figure 5a shows that, after addition of deionized water onto the dry CNC-Li film, the SAXS peak intensity is initially increasing fast and then slowly approaches a plateau value, accompanied by a shift of the primary structural peak to smaller q. Figure 5b shows that the separation distance between the CNC-Li particles increases with time and eventually reached a value of 21.6 nm, which is similar to the separation distance between the CNC-Li particles in the initial dispersion (5 vol %), prior to evaporation-induced assembly (Figure 2c). Simple swelling experiments were performed in a sealed bottle on dry CNC films (CNC-Li and CNC-H) that were allowed to redisperse/reswell for 24 h after addition of water. Figure 5c shows that the CNC-Li film formed a homogeneous suspension. It should be noted that the redispersed suspension contains aggregates, which cause the suspension to be opaque. The redispersed CNC-Li dispersion shows an increase of the hydrodynamic size (273 ± 55 nm) compared to the value (∼25 nm) prior to drying. In comparison, the CNC-H film was only slightly swollen and still intact, and most of the added water was not absorbed by the film, indicating that the cohesion of the CNC-H film was too high for the CNC particles to redisperse, which may be related to the presence of stronger and more numerous intermolecular hydrogen bonding in CNC-H films compared to CNC-Li films.20 The difference in redispersing behavior between the CNC-H and CNC-Li films correlates well to a previous DLS study, which showed that addition of water and mild sonication resulted in redispersed CNC-Na films but had a minor influence on CNC-H films.9 In summary, we have quantitatively followed the nanoscale assembly during evaporation of anionic CNC with H+ and Li+ counterions using time-resolved small-angle X-ray scattering complemented by scanning electron microscopy and dynamic light scattering. Estimation of the evaporation rate in the temperature- and humidity-controlled cell enabled an analysis of how the center to center separation distance (d) varies with particle concentrations. The CNC-H dispersion displayed a scaling typical for unidirectional consolidation of a fully cholesteric phase, and the CNC-Li dispersion displayed a scaling for isotropic contraction of a disordered or amorphous material, e.g., a hydrogel. Dynamic light scattering confirmed that the CNC-Li becomes more aggregated than CNC-H at ionic strengths related to the ion concentration during drying,
Figure 4. SEM (scanning electron microscopy) images of the crosssectional surface along the CNC film normal. (a) Low-magnification and (b) high-magnification images of CNC-H films. (c) Lowmagnification and (d) high-magnification images of CNC-Li films.
CNC films,3 while the microstructure of the CNC-Li films is much less ordered. The poorer order of the CNC-Li films compared to the CNC-H films supports the conjecture that CNC-Li aggregation during drying impedes the formation of a highly ordered chiral nematic phase. Gravity will result in a slightly pear-shaped droplet for a vertically positioned liquid drop. Comparison of the thickness profile of films assembled from CNC dispersions onto a substrate placed in a vertical and horizontal position shows that both films have a higher thickness close to the edge which is a result of the so-called coffee-stain effect (Figure S7). The anisotropic droplet shape of the vertically placed substrate leads to a dried film that is thicker at one edge. The similarity between the microstructure of CNC-H films assembled on vertically positioned substrates (Figure 4a and b) and previous studies on CNC-H assembly on horizontally positioned substrates suggests that gravity has a negligible effect on the nanoscale self-assembly of the CNC particles. Indeed, estimates of the Péclet number of dilute CNC dispersions showed that sedimentation is of minor importance.36
Figure 5. Structural evolution of CNC films during redispersion after the addition of deionized water. (a) Time-resolved SAXS data after deionized water addition to dry CNC-Li films. The measurements cover a total time span of 40 min, and the time step between each curve is 2 min. (b) Timedependent change of the average center to center separation distance between CNC-Li particles after water addition. The solid curve is a guide for the eyes only. (c) Photographs of the redispersing behavior of dry CNC-Li (left) and CNC-H (right) films after soaking in deionized water for 24 h. 175
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(5) Dumanli, A. G.; Kamita, G.; Landman, J.; van der Kooij, H.; Glover, B. J.; Baumberg, J. J.; Steiner, U.; Vignolini, S. Controlled, BioInspired Self-Assembly of Cellulose-Based Chiral Reflectors. Adv. Opt. Mater. 2014, 2, 646−650. (6) Jung, Y. H.; Chang, T.-H.; Zhang, H.; Yao, C.; Zheng, Q.; Yang, V. W.; Mi, H.; Kim, M.; Cho, S. J.; Park, D.-W.; Jiang, H.; Lee, J.; Qiu, Y.; Zhou, W.; Cai, Z.; Gong, S.; Ma, Z. High-Performance Green Flexible Electronics Based on Biodegradable Cellulose Nanofibril Paper. Nat. Commun. 2015, 6, 7170. (7) Håkansson, K. M. O.; Fall, A. B.; Lundell, F.; Yu, S.; Krywka, C.; Roth, S. V.; Santoro, G.; Kvick, M.; Prahl Wittberg, L.; Wågberg, L.; Sö derberg, L. D. Hydrodynamic Alignment and Assembly of Nanofibrils Resulting in Strong Cellulose Filaments. Nat. Commun. 2014, 5, 4018. (8) Wicklein, B.; Kocjan, A.; Salazar-Alvarez, G.; Carosio, F.; Camino, G.; Antonietti, M.; Bergström, L. Thermally Insulating and FireRetardant Lightweight Anisotropic Foams Based on Nanocellulose and Graphene Oxide. Nat. Nanotechnol. 2015, 10, 277−283. (9) Beck, S.; Bouchard, J.; Berry, R. Dispersibility in Water of Dried Nanocrystalline Cellulose. Biomacromolecules 2012, 13, 1486−1494. (10) 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, 6507−6513. (11) Jativa, F.; Schutz, C.; Bergstrom, L.; Zhang, X.; Wicklein, B. Confined Self-Assembly of Cellulose Nanocrystals in a Shrinking Droplet. Soft Matter 2015, 11, 5374−5380. (12) Nordenström, M.; Fall, A.; Nyström, G.; Wågberg, L. Formation of Colloidal Nanocellulose Glasses and Gels. Langmuir 2017, 33, 9772−9780. (13) Gray, D. G. Chemical Characteristics of Cellulosic Liquid Crystals. Faraday Discuss. Chem. Soc. 1985, 79, 257−264. (14) Araki, J.; Wada, M.; Kuga, S.; Okano, T. Birefringent Glassy Phase of a Cellulose Microcrystal Suspension. Langmuir 2000, 16, 2413−2415. (15) Park, J. H.; Noh, J.; Schütz, C.; Salazar-Alvarez, G.; Scalia, G.; Bergström, L.; Lagerwall, J. P. F. Macroscopic Control of Helix Orientation in Films Dried from Cholesteric Liquid-Crystalline Cellulose Nanocrystal Suspensions. ChemPhysChem 2014, 15, 1477− 1484. (16) 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, 127− 134. (17) 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, 19−32. (18) 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, 2076−2082. (19) Uhlig, M.; Fall, A.; Wellert, S.; Lehmann, M.; Prévost, S.; Wågberg, L.; von Klitzing, R.; Nyströ m, G. Two-Dimensional Aggregation and Semidilute Ordering in Cellulose Nanocrystals. Langmuir 2016, 32, 442−450. (20) Dong, X. M.; Gray, D. G. Effect of Counterions on Ordered Phase Formation in Suspensions of Charged Rodlike Cellulose Crystallites. Langmuir 1997, 13, 2404−2409. (21) Phan-Xuan, T.; Thuresson, A.; Skepö, M.; Labrador, A.; Bordes, R.; Matic, A. Aggregation Behavior of Aqueous Cellulose Nanocrystals: The Effect of Inorganic Salts. Cellulose 2016, 23, 3653−3663. (22) Bruckner, J. R.; Kuhnhold, A.; Honorato-Rios, C.; Schilling, T.; Lagerwall, J. P. F. Enhancing Self-Assembly in Cellulose Nanocrystal Suspensions Using High-Permittivity Solvents. Langmuir 2016, 32, 9854−9862. (23) Benítez, A. J.; Walther, A. Counterion Size and Nature Control Structural and Mechanical Response in Cellulose Nanofibril Nanopapers. Biomacromolecules 2017, 18, 1642−1653.
and analysis of the temporal evolution of the structure factor corroborated that the CNC-Li is more aggregated compared to the CNC-H during evaporation. The dry CNC-H films displayed a layered and spatially ordered microstructure, while the CNC-Li films were disordered. The dense and ordered CNC-H films only display partial reswelling, while the disordered CNC-Li films readily reswelled after addition of water.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00964. Experimental section and additional data (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Yingxin Liu: 0000-0002-6859-6993 Lennart Bergström: 0000-0002-5702-0681 Present Addresses §
M.A.: University of Hamburg, Center for Free-Electron Laser Science, DE-22761 Hamburg, Germany. ∥ Physics and Materials Research Unit, University of Luxembourg, 1511 Luxembourg, Luxembourg. ⊥ A.B.F.: RISE Bioeconomy, Box 5604, SE-114 86 Stockholm, Sweden. Author Contributions #
Yingxin Liu and Daniela Stoeckel contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Wallenberg Wood Science Center, the Swedish Research Council (VR), and the Swedish Foundation for Strategic Research (SSF). German Salazar-Alvarez and Valentina Guccini in Stockholm University are acknowledged for fruitful discussions. The authors acknowledge MAX IV for the provision of beam time at the i911-4 beamline of the former MAX-Lab in Lund. Ana Labrador and Tomás S. Plivelic are acknowledged for the technical assistance during measurement.
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REFERENCES
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