Subscriber access provided by CORNELL UNIVERSITY LIBRARY
Article
Gelation Kinetics and Network Structure of Cellulose Nanocrystals in Aqueous Solution Karthik R. Peddireddy, Isabelle Capron, Taco Nicolai, and Lazhar Benyahia Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.6b01061 • Publication Date (Web): 01 Sep 2016 Downloaded from http://pubs.acs.org on September 5, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Biomacromolecules is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 19
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
Biomacromolecules
Gelation Kinetics and Network Structure of Cellulose Nanocrystals in Aqueous Solution
Karthik R. Peddireddy†, Isabelle Capron‡, Taco Nicolai*,†, Lazhar Benyahia†,
†
LUNAM Université du Maine, IMMM UMR-CNRS 6283, 72085 Le Mans cedex 9, France
‡
UR1268 Biopolymères, Interactions et Assemblages, INRA, 44316 Nantes, France
Abstract
Cellulose nanocrystals (CNC) are rod-like bio-sourced nano-particles that are widely used in a range of applications. Charged CNC was obtained by acid extraction from cotton and dispersed in aqueous solution using ultrasound and characterized by light scattering. Aggregation and gelation of CNC induced by addition of NaCl was investigated by light scattering as a function of the NaCl concentration (30–70 mM), the CNC concentration (0.5–5 g/L) and the temperature (10–60°C). Formation of fractal aggregates was observed that grow with time until they percolate and form a weak system spanning network. The aggregation rate and gel time were found to decrease very steeply with increasing NaCl concentration and more weakly with increasing CNC concentration. A decrease of the gel time was also observed with increasing temperature for T>20°C. The structure of the CNC networks was studied using confocal laser scanning microscopy and light scattering. The local structure of the networks was fractal and reflected that of the constituting aggregates. The gels were homogeneous on length scales larger than the correlation length, which decreased with increasing CNC concentration. The CNC gels flowed when tilted for C12 g/L gels formed that did not flow when titled. The effect of
227
the ionic strength was tested by varying the NaCl concentrations between 0 and 250 mM for
228
suspensions at C= 1 g/L. Sedimentation was observed for C≥30 mM NaCl, but the mechanism was
229
strikingly different for [NaCl]≥50 mM and [NaCl]≤40 mM, see fig. 6. At higher ionic strengths, a distinct
230
layer was formed within a few days with a relative height (h) that slowly decreased with time until it
231
reached a constant value. For [NaCl]≤40 mM a dense layer was formed at the bottom after more than a
232
month that slowly increased with time until it reached a constant value. We suggest that the different
233
behavior at low ionic strength can be explained by the extremely slow aggregation process that allows
234
sedimentation of the aggregates before they percolated. The bottom layer is in this case not the
235
increasingly dense collapsing network, but an increasing layer of sedimented aggregates.
236
11 ACS Paragon Plus Environment
Biomacromolecules
h
237 238
Figure 6. CNC in suspensions at C= 1 g/L at 40 mM NaCl (left) and 50 mM NaCl (right) at different times
239
after preparation between two weeks and 6 months. The numbers on the images represent the age of
240
the samples in months.
241 242
The rate at which h decreased for [NaCl]≥50 mM, depended little on the ionic strength, but it
243
was faster if the CNC concentration was lower. The time dependence of h at [NaCl]=70 mM is shown in
244
fig. 7a. As expected, the steady state values of h increased with increasing CNC concentration, see fig.
245
7b. We checked by light scattering that the amount of CNC in the clear top layer was negligible, which
246
means that the CNC concentration in the sedimented layer (Cn) can be calculated as Cn=C.h. The CNC
247
concentration in the sedimented layer after standing for two months increased with increasing CNC
248
concentration from Cn=1.5 g/L at C=0.25 g/L to Cn=3.5 g/L at C=3.0 g/L.
1.0 a 0.8
0.25 g/L 0.5 g/L 1 g/L 2 g/L 3 g/L
0.6
h
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
Page 12 of 19
0.4
0.2
0.0 1
249
10
100
t (day)
250
12 ACS Paragon Plus Environment
Page 13 of 19
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
Biomacromolecules
251 252
Figure 7a Evolution with time of the relative height of the collapsing network layer for CNC suspensions
253
at [NaCl]=70 mM and different concentrations indicated in the figure.
254
Figure 7b Photographs of the samples 1 month after preparation.
255 256
Gel structure
257
The mesoscopic structure of the gels was studied by CLSM for CNC suspensions at different
258
concentrations at 70 mM NaCl that were aged at room temperature. CLSM images of the suspensions
259
taken 15 h after preparation, show that the CNC networks were increasingly dense with increasing
260
concentration, see fig. 8. No movement was observed on length scales probed by CLSM. The structure of
261
the gels was analyzed quantitatively by calculating the pair correlation function (g(r)) of the fluorescent
262
intensity fluctuations as described in detail by Ako et al. ref.17, see fig. 9a. Since the intensity of Nile blue
263
bound to CNC is proportional to the CNC concentration, g(r) represents the pair correlation function of
264
CNC concentration fluctuations. The decay of g(r) for r>0.2µm was approximately exponential: (g(r)-
265
1)=B.exp(-r/ξ), and the results obtained at different concentrations superimposed after normalization of
266
g(r) by B and r by ξ, see fig. 9b. The correlation length ξ decreased approximately linearly with
267
increasing concentration from ξ=2.5 µm at C=1 g/L to ξ=0.3 µm at C=9 g/L, see inset of fig. 9b. Notice
268
that not all Nile blue was bound to the CNC, which explains why the fluorescence intensity in the pores
269
was not zero. This has no effect on the dependence of g(r) on r, but it reduces the contrast and
270
therefore the prefactor B.
271
13 ACS Paragon Plus Environment
Biomacromolecules
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
Page 14 of 19
160 µm
40 µm
160 µm
40 µm
272 273
Figure 8 CLSM images at two different scales of CNC gels at [NaCl]= 70 mM and different concentrations
274
between 0.5 g/L and 9 g/L as indicated in the figure. The images were taken 15 h after preparation, i.e.
275
when steady state was reached, but before sedimentation was significant. For each concentration
276
images at two different lengths scales are shown
277
14 ACS Paragon Plus Environment
Page 15 of 19
1.0
1.03
1.02
0.8
(g (r)-1)/B
1 g/L 2 g/L 3 g/L 5 g/L 9 g/L
1.01
ξ (µm)
a
100
0.6 10-1 1
2
3
5
7
10
C (g/L)
0.4
0.2
1.00 0.1
278
101
b
1.04
g(r)
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
Biomacromolecules
1
0.0 0.1
10
1
10
r/ξ
r (µm)
279
Figure 9a Pair correlation functions of the concentration fluctuations of CNC networks at [NaCl]=70 mM
280
and different CNC concentrations indicated in the figure.
281
Figure 9b Master curve of the pair correlation function after normalization of g(r) by B and r by ξ. The
282
inset shows the dependence of the correlation length on the CNC concentration. The solid lines
283
represent fit results, see text.
284
The structure on length scales smaller than ξ was characterized by light scattering at C=1 g/L,
285
where the effects of turbidity and multiple scattering on Ir were negligible. The measurements were
286
done 15 h after preparation when steady state was reached, but sedimentation was still negligible.
287
When a network is formed the suspension is no longer ergodic, because concentration fluctuations are
288
partially frozen-in. This means that the temporal average no longer corresponds to the spatial average.
289
Therefore the intensity was spatially averaged by slowly rotating the sample. Fig.10 shows that Ir had a
290
power law dependence on q over the whole accessible q-range: Ir∝q-1.6, which means that local
291
structure was self similar on length scales down to about 10 nm and was characterized by a fractal
292
dimension df=1.6. This value of df is significantly smaller than that obtained at [NaCl]= 200 mM and
293
C=7.8 g/L by SANS measurements (df=2.1). One possibility is that the fractal dimension of the CNC
294
network increases with increasing ionic strength. A weaker q-dependence was observed with the SANS
295
measurements at [NaCl]=50 mM, but the results were not interpreted in terms of a fractal dimension.
296
We also note that the lowest q-value in the SANS measurements was 10-2 nm, whereas with light
297
scattering the q-range extended down to 3.10-3nm.
15 ACS Paragon Plus Environment
Biomacromolecules
109
Ir/KC (g/mol)
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
Page 16 of 19
108
107 -1
298
q (m )
299
Figure 10 Dependence of Ir/KC on q for a CNC suspensions at C= 1 g/L and [NaCl]= 70 mM. The
300
measurement was done 15 h after sample preparation, i.e. when the intensity had reached a steady
301
state value, but before sedimentation was significant. The straight line has slope -1.6.
302 303
Redispersion of the CNC gels
304
The bonds that were formed between the CNC particles were strong enough to resist dilution in
305
pure water. However, the gels could be redispersed using a vortex mixer (Fisher) or by applying ultra
306
sound. The efficacy of these two methods was tested for suspensions at C= 5 g/L and [NaCl]= 50 mM
307
that had been left overnight to form gels. The evolution with time of the redispersed suspensions was
308
characterized with light scattering and the results were compared with those obtained on freshly
309
prepared suspensions. The effect of stirring time is shown in Fig. 11a. The value of Ir just after
310
redispersion decreased with mixing time at 3000 rmp, but remained significantly larger than that of
311
freshly prepared solutions even after mixing during 8 min. The gel time increased with mixing time
312
showing that gels are formed quicker if the CNC is not fully dispersed. These measurements show that it
313
is possible to break-up the gel into small aggregates by stirring, but not down to the original CNC
314
trimers. Fig. 11b shows that the latter can be done very quickly using ultrasound. Sonication during 10 s
315
at 0.3 J/s was enough to fully redisperse the gels into trimers. Sonication was found to be effective also
316
at higher NaCl and CNC concentrations.
16 ACS Paragon Plus Environment
Page 17 of 19
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
Biomacromolecules
6
11 10
10
5
9
Ir
6
11
a
Ir tg
7
4
6
5
9
tg(h)
8
b
Ir
tg(h)
8
4
7 6
3
5 0
100
200
300
400
500
3
5 0
2
mixing time (s)
317
4
6
8
10
12
14
16
sonication time (s)
318
Figure 11 Initial value of Ir and gel time after mixing with a vortex mixer at 3000 rpm or sonication time
319
at 0.3 J/s (b) for gelled CNC solutions at C=5 g/L and [NaCl]=50 mM.
320 321
The results reported here for charged CNC obtained by acid extraction from cotton are
322
qualitatively similar to those reported for other aggregating rod-like colloidal particles in the literature,
323
see for a review ref. 12. Quantitatively, the results will depend on the charge density and the shape of
324
the CNC particles that varies depending on the source and the extraction method. Lower charge density
325
reduces the energy barrier that needs to be overcome to form bonds and therefore gelation will be
326
faster. Indeed, preliminary experiments on cotton CNC with very low charge density showed rapid
327
gelation even in the absence of added salt. The rate of gelation and the strength of the gels will also
328
depend on the length of the CNC. For a given concentration we expect faster gelation and stronger gels
329
if CNC with the same thickness is longer. However, in all cases the mechanism of gelation will be the
330
same and the local structure of gels prepared at low CNC concentrations will reflect the fractal structure
331
of the aggregates.
332 333
Conclusions
334
Aggregation of charged CNC in aqueous suspensions can be induced by addition of salt and leads
335
to the formation of self-similar clusters that grow in size until they percolate into a system spanning
336
network at a critical gel time. Networks can be formed even at very low CNC concentration down to at
337
least 0.5 g/L. However, for C≤ 12 g/L the CNC gels are very weak and flow when the vials are tilted. For
17 ACS Paragon Plus Environment
Biomacromolecules
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
Page 18 of 19
338
C< 4 g/L the network collapses under gravity leading to slow sedimentation. The sedimentation rate
339
increases with decreasing CNC concentration, but does not depend much on the ionic strength. The gel
340
time decreases very strongly with increasing ionic strength, e.g. at C= 5 g/L it varied between 2 months
341
at [NaCl]=30 mM and less than 30 min at [NaCl]=70 mM. tg also decreases increasing CNC concentration,
342
but the effect of the CNC concentration is less strong. The dependence of tg on C and [NaCl] can be
343
described by a power law in the ranges investigated: (tg∝[NaCl]-10, tg∝C-1.7). The aggregation process can
344
be accelerated by increasing temperature above 20°C indicating that hydrophobic interactions play a
345
role in the crosslinking process. The local structure of the gels is insensitive to the NaCl concentration
346
between 30mM and 70 mM and reflects that of the aggregates. CLSM shows frozen heterogeneous
347
networks. The correlation length above which the gels are homogeneous decreases with increasing CNC
348
concentration from 2.5 µm at C=1 g/L to 0.3 µm at C=9 g/L. Gels are broken-up in small aggregates after
349
vigorous stirring and can be fully redispersed by sonication.
350 351 352
ACKNOWLEDGMENT
353
The research was financial supported by the MATIERES project and the “Région Pays de la Loire”.
354 355
References
356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371
1. Habibi, Y.; Lucia, L. A.; Rojas, O. J., Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications. Chem. Rev. 2010, 110 (6), 3479–3500. 2. Klemm, D.; Kramer, F.; Moritz, S.; Lindstrom, T.; Ankerfors, M.; Gray, D.; Dorris, A., Nanocelluloses: A New Family of Nature-Based Materials. Angew. Chem., Int. Ed. 2011, 50 (24), 5438– 5466. 3. 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. 4. Araki, J.; Kuga, S., Effect of trace electrolyte on liquid crystal type of cellulose microcrystals. Langmuir 2001, 17 (15), 4493–4496. 5. Boluk, Y.; Lahiji, R.; Zhao, L.; McDermott, M. T., Suspension viscosities and shape parameter of cellulose nanocrystals (CNC). Colloids Surf., A 2011, 377 (1), 297–303. 6. Qi, W.; Xu, H.-N.; Zhang, L., The aggregation behavior of cellulose micro/nanoparticles in aqueous media. RSC Adv. 2015, 5 (12), 8770–8777. 7. Zhong, L.; Fu, S.; Peng, X.; Zhan, H.; Sun, R., Colloidal stability of negatively charged cellulose nanocrystalline in aqueous systems. Carbohydr. Polym. 2012, 90 (1), 644–649.
18 ACS Paragon Plus Environment
Page 19 of 19
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
Biomacromolecules
372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394
8. Fall, A. B.; Lindström, S. B.; Sundman, O.; Ödberg, L.; Wågberg, L., Colloidal stability of aqueous nanofibrillated cellulose dispersions. Langmuir 2011, 27 (18), 11332–11338. 9. Cherhal, F.; Cousin, F.; Capron, I., Influence of charge density and ionic strength on the aggregation process of cellulose nanocrystals in aqueous suspension, as revealed by SANS. Langmuir 2015, 31 (20), 5596–5602. 10. Kalashnikova, I.; Bizot, H.; Cathala, B.; Capron, I., Modulation of Cellulose Nanocrystals Amphiphilic Properties to Stabilize Oil/Water Interface. Biomacromolecules 2012, 13 (1), 267–275. 11. Araki, J., Electrostatic or steric? - preparations and characterizations of well-dispersed systems containing rod-like nanowhiskers of crystalline polysaccharides. Soft Matter 2013, 9 (16), 4125–4141. 12. Solomon, M. J.; Spicer, P. T., Microstructural regimes of colloidal rod suspensions, gels, and glasses. Soft Matter 6 (7), 1391–1400. 13. Cherhal, F.; Cousin, F.; Capron, I., Structural Description of the Interface of Pickering Emulsions Stabilized by Cellulose Nanocrystals. Biomacromolecules 2016, 17 (2), 496–502. 14. Peddireddy, K. R.; Nicolai, T.; Benyahia, L.; Capron, I., Stabilization of Water-in-Water Emulsions by Nanorods. ACS Macro Lett. 2016, 5, 283–286. 15. Brown, W., Light scattering: principles and developpement. Clarendon Press: Oxford, 1996. 16. Elazzouzi-Hafraoui, S.; Nishiyama, Y.; Putaux, J. L.; Heux, L.; Dubreuil, F.; Rochas, C., The shape and size distribution of crystalline nanoparticles prepared by acid hydrolysis of native cellulose. Biomacromolecules 2008, 9 (1), 57–65. 17. Ako, K.; Durand, D.; Nicolai, T.; Becu, L., Quantitative analysis of confocal laser scanning microscopy images of heat-set globular protein gels. Food Hydrocolloids 2009, 23, 1111–1119.
395
For the table of contents only
396 397 398
399
19 ACS Paragon Plus Environment
