Pickering emulsions stabilised by hydrophobically modified cellulose nanocrystals: Responsiveness to pH and ionic strength

Abstract

The aims of this study were to hydrophobically modify cellulose nanocrystals (CNCs), investigate the ability of such modified CNCs (MCNCs) to stabilise Pickering oil-in-water (O/W) emulsions and understand their stability at different pHs (2.0-7.0) and ionic strengths (0-150 mM NaCl). Structural changes that resulted from esterifying CNCs with octenyl succinic anhydride (OSA) were determined using Fourier transform infrared (FTIR) spectroscopy, X-ray diffractometry (XRD), transmission electron microscopy (TEM) and wettability analysis. The stability of the Pickering O/W emulsions (20 wt% oil, 0.05 1.00 wt% MCNCs) was assessed using droplet sizing, microscopy,-potential, apparent viscosity and oscillatory rheological measurements. FTIR spectroscopy confirmed a decrease in the intensity of the-OHassociated band because of reaction of the hydroxyl group with OSA. XRD indicated a lower (11.5%) crystallinity index in MCNCs. TEM revealed that there was no change in morphology of the needle-shaped CNCs upon OSA modification (length/diameter = 40 100 nm/2 4 nm). Hydrophobic modification of CNCs with OSA was evidenced by an increase in static water contact angle from 56 o (untreated CNCs) to 80.2 o (MCNCs) which allowed the MCNCs to be partially wetted by both the phases and stabilise O/W emulsions. The Pickering emulsions showed droplet flocculation at pH < 4.0 (without addition of NaCl) or ionic strength ≥ 20 mM NaCl (pH 7.0), with a predominant elastic gel-like behaviour observed at ≥ 20 mM NaCl. Resistance of MCNC-based Pickering emulsions to coalescence and responsiveness to flocculation at bio-relevant pHs and ionic strengths show promise in the design of delivery vehicles.

Interest of Pickering Emulsions for Sustainable Micro/Nanocellulose in Food and Cosmetic Applications

“…Generally, chemical modifications of cellulose have been made by grafting or adsorbing molecules on pristine cellulose surface, such as hydroxypropylmethyl groups [ 31 ], sodium carboxymethyl cellulose [ 32 ], dodecylamine [ 33 ], polystyrene [ 34 ], cinnamoyl chloride and butyryl chloride [ 35 ], phenyltrimethoxysilane [ 36 ] or octenyl succinic anhydride [ 30 , 37 ]. Others molecules have been grafted on cellulose to bring specific properties at PEs, as N -isopropylacrylamide, a thermo-responsive molecule [ 38 ], or poly[2-(dimethylamino)ethyl methacrylate], a pH-responsive molecule [ 39 ].…”

“…Another example was the CNCs modification with octenyl succinic anhydride (OSA), a molecule used to modify starch in food industry [ 30 ]. This modification increased CNCs hydrophobicity and improved their emulsifying capacity.…”

“…Counter to Du Le et al [ 30 ], Ni et al [ 49 ] succeeded to stabilize PEs without modifying CNCs. CNCs were extracted from ginkgo seed shells, an agricultural waste usually burned or buried in the soil.…”

“…With 1:1 oil/water ratio ( v / v ), PEs were stable for temperatures varying from 20 to 80 °C. Thus, with weaker CNCs concentration and greater oil/water ratio, PEs were more stable than with modified CNCs with OSA [ 30 ]. Stability were explained by different morphology and ζ-potential between modified and unmodified CNCs.…”

“…Due to numerous surface hydroxyl groups, cellulose is a hydrophilic polymer and stabilizes O/W emulsions. However, several authors mention that it would be necessary to modify nanocellulose surface to obtain more stable PEs, because cellulose is too hydrophilic [ 23 , 30 ]. Thus, authors tried to modify chemically cellulose to improve PEs stability and properties, which makes cellulose even less eco-friendly.…”