Aggregation behavior of aqueous cellulose nanocrystals: the effect of inorganic salts

Tuan Phan-Xuan,Romain Bordes,Aleksandar Matic,Axel Thuresson,Marie Skepö,Ana Labrador

doi:10.1007/s10570-016-1080-1

Tuan Phan-Xuan, Romain Bordes + Show 4 more

Open Access

https://doi.org/10.1007/s10570-016-1080-1

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Abstract

Natural anisotropic building-blocks such as cellulose nanocrystals (CNCs) have attracted considerable attention due to their biodegradability and nanometer-size. In this work the colloidal behavior of CNCs, obtained from sulfuric acid hydrolysis of microcrystalline cellulose, has been studied in presence of salts of different valences. The influence on the colloidal stability and nature of aggregates has been investigated for monovalent salts (LiCl, NaCl, KCl, CsCl), divalent salts (CaCl2 and MgCl2), and a trivalent salt (AlCl3), both experimentally by means of turbidity and small angle X-ray scattering (SAXS) measurements, as well as by Monte Carlo simulations using a simple coarse-grained model. For the entire salt series, a critical aggregation concentration (CAC) could be determined by turbidity measurements, as a result of the reduction of effective Coulomb repulsions due to the presence of sulfate groups on the CNC surface. The CACs also followed the Schulze–Hardy law, i.e. the critical aggregation concentration decreased with increasing counterion valence. For the monovalent ions, the CACs followed the trend Li+ > Na+ > K+ > Cs+, which could be rationalized in terms of matching affinities between the cation and the sulfate groups present at the surface of CNCs. From the SAXS measurements it was shown that the density of the aggregates increased with increasing salt concentration and ion valence. In addition, these findings were rationalized by means of simulation, which showed a good correlation with experimental data. The combination of the experimental techniques and the simulations offered insight into interaction-aggregation relationship of CNC suspensions, which is of importance for their structural design applications.

Highlights

Anisotropic particles extracted from natural resources, such as cellulose nanocrystals (CNCs) or cellulose nanofibrils (CNFs), have many advantages of interest for material design
This work presents in detail the colloidal stability and aggregation behavior of aqueous CNC suspensions in the presence of salts with different valences
These anisotropic particles were perfectly stabilized at low ionic strength, which gave us a possibility to determine particle dimension using light scattering techniques, while they tended to aggregate with increasing salt content, resulting in an increase of the turbidity

Summary

Introduction

Anisotropic particles extracted from natural resources, such as cellulose nanocrystals (CNCs) or cellulose nanofibrils (CNFs), have many advantages of interest for material design. Recent reports have demonstrated that their use as innovative building blocks can lead to materials with outstanding performances (Hakansson et al 2014). This mainly owes to the inherent characteristics of nanocellulose, such as high crystallinity and aspect ratio of the nanofibers (Peng et al 2011). Studies have focused on the interactions of CNCs in suspension and it is regarded as a colloid rather than as a fiber. Understanding how the interactions of nanocellulose, i.e. its colloidal behavior, depend on e.g. salt and/or polymer concentration is of great importance

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