Phase behaviour and stability of nanocellulose stabilized oil-in-water emulsions

The research on nanoscale cellulosic materials as particulate emulsion stabilizer has recently intensified as the interest in renewable and efficient emulsifiers has increased. The characteristics of particle stabilized emulsions, Pickering emulsions, depend strongly on the particle used e.g. particle shape and charge highly affect the emulsion properties (stability and droplet size distribution). Nanoscaled cellulosic materials have shown to act as stabilizers of Pickering emulsions. Many studies have focused on the effects on chemical modification of the cellulosic material and its influence on stability whereas lesser focus has been put on the effects of nanocellulose ratio and volume fractions of the dispersed phase. In addition the effect of the size and structure of nanocellulosic materials on the phase behaviour and emulsion droplet size distribution is not clear. In this study two different types of wood derived nanocellulosic materials, mechanically disintegrated unmodified cellulose nanofibrils (CNF) and TEMPO-oxidized highly anionic cellulose nanofibrils (TEMPO-CNF), were studied as stabilizers in oil-in-water emulsion systems. Emulsions of varying CNF concentration and dodecane volume fractions were prepared by using a high shear mixer. The emulsions were first visually examined, stability and creaming behaviour was evaluated by centrifugation and as a function of temperature. Optical methods were applied to examine the droplet size, and the droplet size distribution was obtained from low field NMR measurements. Choice of CNF had the largest impact on the properties of the formed emulsions. In addition, the oil-to-water ratio affects some characteristics. In the presence of unmodified CNF the formed emulsions showed strong creaming behaviour with smaller droplet size and extreme clustering of the droplets. However, coalescence of the droplets was not detected. Emulsions in the presence of TEMPO oxidized anionic CNF formed two different stable phases. The creaming layer contained larger droplets and simultaneously the separation of turbid phase was observed which contained small emulsion droplets with rather narrow size distribution. These findings are of high relevance when considering applications where high stability and controlled droplet size distribution is needed ranging from chemical industry to food and pharma.