Search

Coatings of anisotropic nanoparticles such as cellulose nanocrystals (CNCs) can provide tuneable physicochemical surface properties to a substrate such as modifying wettability. These coatings are often formed using dip coating, with CNCs enriched at the air–water interface transferred to a substrate as a monolayer. This process is commonly facilitated by surfactants, which can remain present in the final product, affecting coating properties. In this work, an "additive free" method for creating CNC coatings by exploiting electrostatic interactions within the pH window between pH 2–4 is demonstrated. Within this pH window, the air–water interface is positively charged and CNCs are negatively charged, with surface pressure tensiometry, X‐ray reflectivity, and Brewster angle microscopy indicating that CNCs are driven to the air–water interface. The optimal condition for monolayer coverage was pH 3; at pH 2 charge screening causes localized flocculation at the air–water interface, and at pH 4 interparticle repulsion leads to incomplete, patchy coverage. These findings successfully translate to dip coated CNC monolayers as characterized by atomic force microscopy, showing that the manipulation of pH can facilitate the surfactant‐free dip coating of CNCs, with advantages over the surfactants that are more typically used. [ABSTRACT FROM AUTHOR]

Coating a substrate with anisotropic nanoparticles such as cellulose nanocrystals (CNCs) confers some of their desirable physicochemical properties, such as strength, wettability, and barrier properties. The formation of monolayer coatings of CNCs via dip coating is affected by the enrichment of CNCs at both air–liquid and substrate–liquid interfaces. In this work, a surfactant‐free method for dip coating CNCs is presented through use of the hydrotrope tetraethylammonium chloride. Hydrotropes demonstrate a different mechanism for facilitating interfacial enrichment, adsorbing to CNCs and rendering them weakly hydrophobic, causing them to adsorb to both solid–liquid and air–liquid interfaces without affecting the surface tension of the system. This new coating mechanism may be more robust as adsorption onto the substrate from the bulk dispersion is less sensitive to the air–liquid interface. Adsorption at the solid–liquid interface showed two distinct CNC layers, with a tightly bound, close‐packed CNC layer at the interface, and a loosely associated outer layer. Adsorption of both layers is shown to be fully reversible after washing with ultra pure water, highlighting the potential of hydrotropes for facilitating new coating mechanisms. [ABSTRACT FROM AUTHOR]