Adsorption and Interfacial Layer Structure of Unmodified Nanocrystalline Cellulose at Air/Water Interfaces

Abstract

Nanocrystalline cellulose (NCC) is a promising biological nanoparticle for the stabilization of fluid interfaces. However, the adsorption and interfacial layer structure of NCC are poorly understood as it is currently unknown how to form NCC interfacial layers. Herein, we present parameters for the adsorption of unmodified NCC at the air−water (A/W) interface. Initial NCC adsorption is limited by diffusion, followed by monolayer saturation and decrease in surface tension at the time scale of hours. These results confirm the current hypothesis of a Pickering stabilization. NCC interfacial performance can be modulated by salt-induced charge screening, enhancing adsorption kinetics, surface load, and interfacial viscoelasticity. Adsorbed NCC layers were visualized by atomic force microscopy at planar Langmuir films and curved air bubbles, whereat NCC coverage was higher at curved interfaces. Structural analysis by neutron reflectometry revealed that NCC forms a discontinuous monolayer with crystallites oriented in the interfacial plane at a contact angle < 90°, favoring NCC desorption upon area compression. This provides the fundamental framework on the formation and structure of NCC layers at the A/W interface, paving the way for exploiting NCC interfacial stabilization for tailored colloidal materials.

Charge-driven interfacial gelation of cellulose nanofibrils across the water/oil interface

“…Bertsch et al reported that for unmodified CNC, a diffusion limited time lag is observed before adsorption occurrs, 15 whilst, similarly to the results herein described, Scheuble et al showed that the kinetic of adsorption for methylated CNP is not delayed by a time lag. 21 This suggests that CNP hydrophobization or interfacial polyelectrolyte-CNP charge complexation (as herein described) are effective strategies to decrease the energy barrier for CNP adsorption at the W/O interface.…”

“…Recently, Bertsch et al reported on the spontaneous adsorption of CNC at water/air (W/A) interface without the addition of any chemical modification or additional species (e.g., surfactants). 15 In this field, three main approaches have been broadly employed to tune the wettability of CNP, favouring their adsorption to the interface: (i) electrostatic complexation between oppositely charged CNP and water-soluble amphiphilic species (e.g., surfactants), [16][17][18] (ii) covalent hydrophobization of the CNP interface, [19][20][21] and, (iii) interfacial polyelectrolyte-CNP charge complexation between a polyelectrolyte, which is largely soluble in the organic phase (e.g., oil), and an oppositely charged water-dispersible CNP. This last approach relies on the use of two immiscible liquids as scaffolds, delivering two oppositely-charged species and directing assembly across the liquid-liquid interface, analogously to the more extensively studied complex coacervates.…”

Adsorption of charged anisotropic nanoparticles at oil–water interfaces

“…A bulk concentration of 0.5 wt% CNCs was chosen, which corresponds to the maximum CNC surface coverage at air-water (A/W) interfaces. 12 CNCs adsorbed at the n-alkane interfaces with measurable changes in normalized surface pressure p norm for 24 h. CNC adsorption was independent of n-alkane chain length and achieved a maximum p norm of 0.23 AE 0.01 (from asymptotic ts). These adsorption kinetics are in good agreement with previous ndings on CNC adsorption at the A/W interface.…”

“…The employed CNCs are 79 AE 6 nm long and 5 nm in height with a linear charge density of 0.67 nm À1 . 12,13 The detailed Experimental section is provided in the ESI. †…”

“…10,11 The CNCs used here were obtained from wood pulp, yielding needle-like crystallites with a mean length of 79 nm and charged sulfate ester residues. 12 Their surface charge allows the modulation of CNC interactions from repulsive to attractive by targeted salt addition, making CNCs an interesting model system to investigate the effect of particle interactions on adsorbed nanoparticle layers. 13,14 CNC-stabilized emulsions reveal extraordinary stability against environmental inuences like heat, pH, ionic strength, and even gastric conditions.…”