Carboxylated celluloses as effective stabilizers for super-stable Pickering emulsions: Effects of different carboxyl moieties and particle morphologies on performance

Carboxyl functionalization of cellulose could enhance its stabilizing ability in Pickering emulsions, though the influences of different carboxylation methods remain largely unknown. In order to fill in this knowledge gap, three typical carboxylated cellulosic materials, TEMPO-oxidized cellulose nanofibrils (TCN), carboxymethylated cellulose nanofibrils (CM-CN) and carboxymethyl cellulose (CMC), have been investigated to stabilize Pickering emulsions. There is a common feature among the three carboxylated cellulosic materials that they were all adsorbed at oil-water interface to form an elastic cellulose particle shell around oil droplets. However, the networks formed in the aqueous phase were quite different. For both TCN and CM-CN, the physical entanglement of the fibers and the interactions between fibers, mainly intramolecular hydrogen bonds, led to strong networks in the aqueous phase, thus contributing to good stability of Pickering emulsions. In contrary, the interaction between the suspended CMC particles was limited, which could not drive them to form a continuous network in the aqueous phase, so that CMC was least effective to stabilize oil droplets as indicated by the clearly observed phase separation even in the freshly prepared Pickering emulsions. Specifically, CM-CN with a larger aspect ratio (length of 499 ± 306 nm and diameter of 7 ± 2 nm), excellent thermal stability and comparatively high three phase contact angle (78.4°) was an effective stabilizer to prepare a super-stable Pickering emulsion, which had best emulsifying index (100%) even after 30-day of storage. This study demonstrated that different carboxyl functionalization would lead to different properties of cellulose, thus affecting their performance in stabilizing Pickering emulsions.