Binary Suspensions of Beeswax and Mineral Pigment Nanoparticles in Aqueous Gelled Media: Macroscopic Stability and Potential Applications for Painting Films

Abstract Binary suspensions of beeswax and mineral pigment nanoparticles offer a promising route to highly stable, water-based formulations for coating and painting films. In this work, we systematically characterized the stability and structural organization of binary suspensions of beeswax and various mineral pigment particles, including Armenian bole, glauconite, ultramarine, cardinal purple, golden ochre, and shungite. Scanning electron microscopy (SEM) revealed that all investigated mineral pigments naturally occurred as accreted aggregates of nanometer-sized particles, with separate individual nanoparticles also identified in several samples. These individual pigment nanoparticles ranged from 50 to 100 nm, with ζ-potentials from −16 to −37 mV. Beeswax nanoparticles, produced by the phase-inversion temperature method, exhibited diameters of 20–25 nm and a ζ-potential of −7.3 mV. Because the native surface charges of these constituent nanoparticles were insufficient to provide adequate electrostatic stabilization, macroscopic stability against gravitational separation was achieved by rheologically modifying the continuous phase. Highly stable, nonsedimenting suspensions were successfully formulated by gelling the aqueous phase with 1 mass % Carbopol 940, thereby immobilizing the phase-mismatched nanostructures via polymer-network confinement. Scanning transmission electron microscopy (STEM) coupled with energy-dispersive X-ray spectroscopy (EDS) demonstrated that, upon drying, the mineral pigments were embedded within the wax agglomerates or surrounded by the beeswax nanoparticles. Finally, the film-forming behavior of these binary compositions was evaluated through a preliminary qualitative demonstration of their application in cold encaustic painting.