Abstract
Water quality parameters such as salt content and various pH environments can alter the stability of gels as well as their rheological properties. Here, we investigated the effect of various concentrations of NaCl and different pH environments on the rheological properties of TEMPO-oxidised cellulose nanofibril (OCNF) and starch-based hydrogels. Addition of NaCl caused an increased stiffness of the OCNF:starch (1:1 wt%) blend gels, where salt played an important role in reducing the repulsive OCNF fibrillar interactions. The rheological properties of these hydrogels were unchanged at pH 5.0 to 9.0. However, at lower pH (4.0), the stiffness and viscosity of the OCNF and OCNF:starch gels appeared to increase due to proton-induced fibrillar interactions. In contrast, at higher pH (11.5), syneresis was observed due to the formation of denser and aggregated gel networks. Interactions as well as aggregation behaviour of these hydrogels were explored via ζ-potential measurements. Furthermore, the nanostructure of the OCNF gels was probed using small-angle X-ray scattering (SAXS), where the SAXS patterns showed an increase of slope in the low-q region with increasing salt concentration arising from aggregation due to the screening of the surface charge of the fibrils.
Highlights
Cellulose, a polysaccharide, is the most abundant natural polymer [1]
Cellulose nanofibrils have been extensively used to form hydrogels where the properties can be modulated by various factors such as salts [6,7], pH [8,9], concentration [10,11], temperature [12], and surfactants [13,14,15]
TEMPO oxidised cellulose nanofibrils (OCNF) [4,5] with ~25% degree of oxidation were produced from softwood pulp via a high-pressure homogeniser [36,37]
Summary
It can be treated to make various forms of nanocellulose such as nanofibrils and nanocrystals to expand its applicability in diverse fields including food, cosmetics, packaging, and pharmaceuticals [2,3]. Their nanoscale dimensions as well as the high density of free hydroxyl groups on their surface make them promising materials for surface functionalisation. Cellulose nanofibrils have been extensively used to form hydrogels where the properties can be modulated by various factors such as salts [6,7], pH [8,9], concentration [10,11], temperature [12], and surfactants [13,14,15]. OCNF hydrogels have been reported to have excellent shear-thinning properties, which is desirable for some formulation-based products, especially in health and personal care applications [13]
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