Salt-Responsive Pickering Emulsions Stabilized by Functionalized Cellulose Nanofibrils

Abstract

Oil-in-water emulsions have been stabilized by functionalized cellulose nanofibrils bearing either a negative (oxidized cellulose nanofibrils, OCNF) or a positive (cationic cellulose nanofibrils, CCNF) surface charge. The size of the droplets was measured by laser diffraction, while the structure of the shell of the Pickering emulsion droplets was probed using small-angle neutron scattering (SANS), confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM), and rheology measurements. Both OCNF- and CCNF-stabilized emulsions present a very thick shell (>100 nm) comprised of densely packed CNF. OCNF-stabilized emulsions proved to be salt responsive, influencing the droplet aggregation and ultimately the gel properties of the emulsions, while CCNF emulsions, on the other hand, showed very little salt-dependent behavior.

Recent progress in Pickering emulsions stabilised by bioderived particles

“…Since the anionic CNFs possess a highly negative charge on their surface ( ζ -potential ∼ −60 mV), 50 emulsion droplets stabilised using these particles also experience electrostatic repulsion with the surrounding droplets, which can be screened by the addition of salt. 51 For example, the addition of NaCl salt (0.5 M) to an anionic CNF-stabilised Pickering emulsion lowered the surface charge of the emulsion droplets, as the ζ -potential values decreased from −46 to −5 mV which resulted in droplet aggregation as confirmed via confocal microscopic images. 51 On the other hand, cationic CNFs (functionalised using glycidyl trimethylammonium chloride with degree of substitution ∼23% and ζ -potential ∼ +37 mV) used to stabilise a Pickering emulsion had very little response to salt, as the ζ -potential of the droplets decreased only from +24 to +13 mV for emulsions after addition of NaCl (0.5 M).…”

“… 49 The ζ -potential value of the TEMPO-mediation OCNF (with the degree of substitution ∼25%) is around −60 mV (ref. 50 and 51 ) and their typical dimensions are around 3–5 nm in diameter and several microns in length. 52,53 …”

“…Anionic CNFs are suggested to form an inter-connected fibrillar network at the oil–water interfaces during Pickering emulsion formation. 51,54 In addition, relatively long fibrous CNFs may adsorb slowly to the droplet surfaces during homogenisation and form relatively thick interfacial layers. Shell thicknesses above 100 nm with a hydration layer have been reported.…”

“… 51 On the other hand, cationic CNFs (functionalised using glycidyl trimethylammonium chloride with degree of substitution ∼23% and ζ -potential ∼ +37 mV) used to stabilise a Pickering emulsion had very little response to salt, as the ζ -potential of the droplets decreased only from +24 to +13 mV for emulsions after addition of NaCl (0.5 M). 51 …”

Pickering Emulsions Stabilized by Halloysite Nanotubes: From General Aspects to Technological Applications

“…[57] Starting from this, oppositely charged cellulose nanofibrils were employed for the design of salt responsive Pickering systems whereas cellulose nanocrystals were used for the entrapment of thymol, which is an essential oil immiscible with water, and the resulting stabilized particles showed high larvicidal activity even after being embedded into an alginate based matrix. [58,59] Moreover, in order to overcome some limitations to the use of rigid particles (e.g., difficulty to deform and petrochemical nature), also polymers based materials have been used for the design of solids stabilized emulsions systems. Among them poly(lactic acid) (PLA) and poly(caprolactone) (PCL) meet the requirement for biocompatibility, Polystyrene (PS) and poly(ethylene glycol) (PEG) were used for the fabrication of colloidosomes and for the stabilization of Pickering emulsions with a lipids based core and high coverage degree.…”