Stabilization of Pickering foams by high-aspect-ratio nano-sheets

Abstract

We developed Pickering foams highly stabilized by high-aspect-ratio (ξ = diameter/thickness) nano-sheets. The effects of particle aspect ratio, concentration, and hydrophobicity were also investigated. To our knowledge, our study provides the first experimental evidence of the effect of particle aspect ratio on particle-stabilized foams. The adsorption properties of these highly anisotropic nano-sheets are strongly affected by their small thickness and large lateral size (i.e., two-dimensional). These high-aspect-ratio nano-sheets were obtained by exfoliation of α-zirconium phosphate (ZrP) crystals with propylamine (C₃H₇NH₂, PA). The hydrophobicity of the nano-sheets was tailored by adjusting the PA : ZrP molar ratio in the suspension. The morphology and stability of the foam depend on the nano-sheet aspect ratio and concentration as well as on the PA : ZrP molar ratio. Here, we found that using low and high aspect ratio nano-sheets having a high and an intermediate degree of hydrophobicity, respectively, is the successful formula to obtain high foam stability. The aqueous foams were characterized by optical and cross-polarized micrographs. Scanning electron microscopy (SEM) micrographs of dried foams revealed the adsorption of the PA–ZrP nano-sheets on the air–water interface. The foam stability was studied by measuring the foam and the water volume as a function of time to obtain the foam decay and water drainage rate, respectively. We also observed that the foams were stabilized by jammed layers of nano-sheets located in the bulk and at the air–water interface. These layers of particles prevent air diffusion between the bubbles, hence arresting Ostwald ripening and coalescence.