Explaining the Exceptional Wet Integrity of Transparent Cellulose Nanofibril Films in the Presence of Multivalent Ions—Suitable Substrates for Biointerfaces

Abstract

pretreatment to introduce charged groups, such as carboxylates or phosphate esters. This treatment facilitates the liberation and improves the colloidal stability of CNF dispersions. [1][2][3] Materials prepared from CNFs, such as aerogels, [4] films, [5] and filaments, [6,7] have an impressive strength in the dry state, but when exposed to moist air or liquid water they become weak and have poor dimensional integrity. These detrimental effects are caused by interactions of the water molecules with the hydrophilic surface of CNFs, which weaken the interfibrillar joints. In the presence of condensed water, the ionic groups on the surface of the CNF also generate an osmotic swelling pressure inside the material so that it expands and becomes further weakened. For most practical applications of CNF materials, it is crucial to reduce this sensitivity to moisture.Introducing covalent crosslinking is a possibility, but a potentially more convenient way to minimize their sensitivity to water is to treat CNF-based materials with multivalent ions, which induce an attractive interaction between the fibrils. It has been suggested that the mechanism of this attraction is crosslinking via metal-ligand complexes, or ionic bridges ("salt" bridges) between the multivalent ion and the carboxylate groups on the surfaces of two or more CNFs. [8][9][10][11] However, the salt bridge model is simplistic, and the observed data do not fully follow the trend of complex stability for divalent and trivalent ions. [12,13] This indicates that there are additional contributions to the attraction between CNFs in the presence of multivalent ions, and these contributions must be identified and characterized to explain the observed properties.In our previous work, [14] we suggested that the attractive interaction in the presence of multivalent ions can essentially be explained by four different mechanisms. These mechanisms are illustrated in Figure 1 and consist of a fundamental driving force from ion-ion correlation, [15][16][17] which allows for the specific ion effects: dispersion interactions [18,19] and metal-ligand complexes, [10,20] in combination with a local acidic environment that changes the dissociation of the charged groups, to further "lock" the fibril network. The most important parameters that govern these mechanisms are the valency (ion-ion correlation) [21] and polarizability (dispersion interactions) [22] of the counter-ions, and the ability of the ion to form complexes with Cellulose nanofibrils (CNFs) assemble into water-resilient materials in the presence of multivalent counter-ions. The essential mechanisms behind these assemblies are ion-ion correlation and specific ion effects. A network model shows that the interfibril attraction indirectly influences the wet modulus by a fourth power relationship to the solidity of the network (E w ∝ φ 4 ). Ions that induce both ion-ion correlation and specific ion effects significantly reduce the swelling of the films, and due to the nonlinear relationship dramatically increase the wet...

Exploiting Supramolecular Interactions from Polymeric Colloids for Strong Anisotropic Adhesion between Solid Surfaces

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Plant Nanomaterials and Inspiration from Nature: Water Interactions and Hierarchically Structured Hydrogels

“…Similar properties to TEMPO‐oxidized CNF were observed for phosphorylated CNF, In turn, carboxymethylated CNF displayed a significantly higher toughness in the form of wet films, crosslinked with multivalent ions, compared with the corresponding phosphorylated CNF or TO‐CNF. [ 234 ]…”

Ion‐Specific Assembly of Strong, Tough, and Stiff Biofibers

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