Mechanisms Behind the Stabilizing Action of Cellulose Nanofibrils in Wet-Stable Cellulose Foams

Abstract

The principal purpose of the investigation was to clarify the mechanisms behind the stabilizing action of cellulose nanofibrils (CNFs) in wet-stable cellulose foams. Following the basic theories for particle-stabilized foams, the investigation was focused on how the surface energy of the stabilizing CNF particles, their aspect ratio and charge density, and the concentration of CNF particles at the air-water interface affect the foam stability and the mechanical properties of a particle-stabilized air-liquid interface. The foam stability was evaluated from how the foam height changed over time, and the mechanical properties of the interface were evaluated as the complex viscoelastic modulus of the interface using the pendant drop method. The most important results and conclusions are that CNFs can be used as stabilizing particles for aqueous foams already at a concentration as low as 5 g/L. The major reasons for this were the small dimensions of the CNF and their high aspect ratio, which is important for gel-formation and the complex viscoelastic modulus of the particle-filled air-water interface. The influence of the aspect ratio was also demonstrated by a much higher foam stability of foams stabilized with CNFs than of foams stabilized by cellulose nanocrystals (CNC) with the same chemical composition. The charge density of the CNFs affects the level of liberation within larger aggregates and hence also the number of contact points at the interface and the gel formation and complex viscoelastic modulus of the air-water interface. The charges also result in a disjoining pressure related to the long-range repulsive electrostatic pressure between particle-stabilized bubbles and hence contribute to foam stability.

Biopolymer-based particles as stabilizing agents for emulsions and foams

“…The efficiency of foam stabilization was attributed to the high aspect ratio 568 of the nanofibrils (∼2 µm × 3 nm), which facilitated the establishment of a robust gel-like 569 network between bubbles, as well as a large viscoelastic modulus at the particle-laden 570 air−water interface. Consistent with this explanation was the authors' observation that the 571 foam stability with the long fibrils was substantially greater than that with some shorter 572 cellulose nanocrystals (300 nm × 4 nm) of similar chemical composition and charge density 573 (Cervin et al, 2015). Another kind of cellulose-derived ingredient that appears suitable for 574 forming and stabilizing foams consists of charged colloidal particles ( Polysaccharide-based particles are not intrinsically surface-active: they usually require some 607 kind of hydrophobic modification in order to make them adhere to liquid interfaces.…”

“…Analogous rheological evidence for an emulsion-stabilizing network 558 by bacterial cellulose fibrils was reported by Paximada et al (2016). In yet further studies, Foam stabilization by nanofibrillated cellulose particles from wood pulp was investigated 564 by Cervin et al (2013Cervin et al ( , 2015. After the anionic nanofibrils had been hydrophobically 565 modified by adsorption of cationic octylamine, the particles exhibited significant surface 566 activity at the air-water interface, and they could make and stabilize foams at concentrations 567 as low as 0.5 wt%.…”

Exploiting poly(ɛ‐caprolactone) and cellulose nanofibrils modified with latex nanoparticles for the development of biodegradable nanocomposites

“…Never‐dried pulp (60% Norwegian spruce and 40% Scots Pine) was kindly supplied by Domsjö Fabriker AB, Örnsköldsvik, Sweden. The CNF with a charge density of 600 μeq g −1 , a dry content of 0.89 wt%, width of 3–5 nm and length of about 500 nm, were prepared by TEMPO‐oxidation followed by high‐pressure homogenization on a Microfluidizer M‐110EH (Microfluidics Corp.) as described elsewhere .…”

Floating solid cellulose nanofibre nanofoams for sustained release of the poorly soluble model drug furosemide

“…To create a wet‐stable foam, that is, stabilize the air–water interface of dispersed bubbles, Gibbs stability criterion must be fulfilled, which states that the surface elasticity of the coating layer surrounding the air bubble must be equal and above half the surface tension of the liquid/suspension . In a previous study, it was shown that the slender cellulose nanofibres alone (as shown Figure a) do not fulfil this stability criterion, but by changing the surface energy of CNF, the migration of CNF to the air–water interface might be facilitated . The change in surface energy can be accomplished by for example adsorbing a surfactant onto the surface of CNF .…”

“…[14][15][16][17][18][19][20] The amphiphilic properties were recently also used to stabilize the interface of oil in water emulsion via a Pickering stabilization. [21] In addition to the unique colloidal properties, [22] CNF has excellent mechanical properties, [12] oxygen barrier properties in the dry state [23] and a large surface area available for CNF-drug interaction.…”